Internet Engineering Task Force (IETF)                    M. Chadalapaka
Request for Comments: 7143                                     Microsoft
Obsoletes: 3720, 3980, 4850, 5048                              J. Satran
Updates: 3721                                             Infinidat Ltd.
Category: Standards Track                                        K. Meth
ISSN: 2070-1721                                                      IBM
                                                                D. Black
                                                                     EMC
                                                              April 2014

Internet Small Computer System Interface (iSCSI) Protocol
(Consolidated)

Abstract

   This document describes a transport protocol for SCSI that works on
   top of TCP.  The iSCSI protocol aims to be fully compliant with the
   standardized SCSI Architecture Model (SAM-2).  RFC 3720 defined the
   original iSCSI protocol.  RFC 3721 discusses iSCSI naming examples
   and discovery techniques.  Subsequently, RFC 3980 added an additional
   naming format to the iSCSI protocol.  RFC 4850 followed up by adding
   a new public extension key to iSCSI.  RFC 5048 offered a number of
   clarifications as well as a few improvements and corrections to the
   original iSCSI protocol.

   This document obsoletes RFCs 3720, 3980, 4850, and 5048 by
   consolidating them into a single document and making additional
   updates to the consolidated specification.  This document also
   updates RFC 3721.  The text in this document thus supersedes the text
   in all the noted RFCs wherever there is a difference in semantics.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7143.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   1. Introduction ...................................................11
   2. Acronyms, Definitions, and Document Summary ....................11
      2.1. Acronyms ..................................................11
      2.2. Definitions ...............................................13
      2.3. Summary of Changes ........................................19
      2.4. Conventions ...............................................20
   3. UML Conventions ................................................20
      3.1. UML Conventions Overview ..................................20
      3.2. Multiplicity Notion .......................................21
      3.3. Class Diagram Conventions .................................22
      3.4. Class Diagram Notation for Associations ...................23
      3.5. Class Diagram Notation for Aggregations ...................24
      3.6. Class Diagram Notation for Generalizations ................25
   4. Overview .......................................................25
      4.1. SCSI Concepts .............................................25
      4.2. iSCSI Concepts and Functional Overview ....................26
           4.2.1. Layers and Sessions ................................27
           4.2.2. Ordering and iSCSI Numbering .......................28
                  4.2.2.1. Command Numbering and Acknowledging .......28
                  4.2.2.2. Response/Status Numbering and
                           Acknowledging .............................32
                  4.2.2.3. Response Ordering .........................32
                           4.2.2.3.1. Need for Response Ordering .....32
                           4.2.2.3.2. Response Ordering Model
                                      Description ....................33
                           4.2.2.3.3. iSCSI Semantics with
                                      the Interface Model ............33
                           4.2.2.3.4. Current List of Fenced
                                      Response Use Cases .............34
                  4.2.2.4. Data Sequencing ...........................35

           4.2.3. iSCSI Task Management ..............................36
                  4.2.3.1. Task Management Overview ..................36
                  4.2.3.2. Notion of Affected Tasks ..................36
                  4.2.3.3. Standard Multi-Task Abort Semantics .......37
                  4.2.3.4. FastAbort Multi-Task Abort Semantics ......38
                  4.2.3.5. Affected Tasks Shared across
                           Standard and FastAbort Sessions ...........40
                  4.2.3.6. Rationale behind the FastAbort Semantics ..41
           4.2.4. iSCSI Login ........................................42
           4.2.5. iSCSI Full Feature Phase ...........................44
                  4.2.5.1. Command Connection Allegiance .............44
                  4.2.5.2. Data Transfer Overview ....................45
                  4.2.5.3. Tags and Integrity Checks .................46
                  4.2.5.4. SCSI Task Management during iSCSI
                           Full Feature Phase ........................47
           4.2.6. iSCSI Connection Termination .......................47
           4.2.7. iSCSI Names ........................................47
                  4.2.7.1. iSCSI Name Properties .....................48
                  4.2.7.2. iSCSI Name Encoding .......................50
                  4.2.7.3. iSCSI Name Structure ......................51
                  4.2.7.4. Type "iqn." (iSCSI Qualified Name) ........52
                  4.2.7.5. Type "eui." (IEEE EUI-64 Format) ..........53
                  4.2.7.6. Type "naa." (Network Address Authority) ...54
           4.2.8. Persistent State ...................................55
           4.2.9. Message Synchronization and Steering ...............55
                  4.2.9.1. Sync/Steering and iSCSI PDU Length ........56
      4.3. iSCSI Session Types .......................................56
      4.4. SCSI-to-iSCSI Concepts Mapping Model ......................57
           4.4.1. iSCSI Architecture Model ...........................58
           4.4.2. SCSI Architecture Model ............................59
           4.4.3. Consequences of the Model ..........................61
                  4.4.3.1. I_T Nexus State ...........................62
                  4.4.3.2. Reservations ..............................63
      4.5. iSCSI UML Model ...........................................64
      4.6. Request/Response Summary ..................................66
           4.6.1. Request/Response Types Carrying SCSI Payload .......66
                  4.6.1.1. SCSI Command ..............................66
                  4.6.1.2. SCSI Response .............................66
                  4.6.1.3. Task Management Function Request ..........67
                  4.6.1.4. Task Management Function Response .........68
                  4.6.1.5. SCSI Data-Out and SCSI Data-In ............68
                  4.6.1.6. Ready To Transfer (R2T) ...................69
           4.6.2. Requests/Responses Carrying SCSI and iSCSI
                  Payload ............................................69
                  4.6.2.1. Asynchronous Message ......................69

           4.6.3. Requests/Responses Carrying iSCSI-Only Payload .....69
                  4.6.3.1. Text Requests and Text Responses ..........69
                  4.6.3.2. Login Requests and Login Responses ........70
                  4.6.3.3. Logout Requests and Logout Responses ......71
                  4.6.3.4. SNACK Request .............................71
                  4.6.3.5. Reject ....................................71
                  4.6.3.6. NOP-Out Request and NOP-In Response .......71
   5. SCSI Mode Parameters for iSCSI .................................72
   6. Login and Full Feature Phase Negotiation .......................72
      6.1. Text Format ...............................................73
      6.2. Text Mode Negotiation .....................................76
           6.2.1. List Negotiations ..................................80
           6.2.2. Simple-Value Negotiations ..........................80
      6.3. Login Phase ...............................................81
           6.3.1. Login Phase Start ..................................84
           6.3.2. iSCSI Security Negotiation .........................87
           6.3.3. Operational Parameter Negotiation during
                  the Login Phase ....................................87
           6.3.4. Connection Reinstatement ...........................88
           6.3.5. Session Reinstatement, Closure, and Timeout ........89
                  6.3.5.1. Loss of Nexus Notification ................90
           6.3.6. Session Continuation and Failure ...................90
      6.4. Operational Parameter Negotiation outside the
           Login Phase ...............................................90
   7. iSCSI Error Handling and Recovery ..............................92
      7.1. Overview ..................................................92
           7.1.1. Background .........................................92
           7.1.2. Goals ..............................................92
           7.1.3. Protocol Features and State Expectations ...........93
           7.1.4. Recovery Classes ...................................94
                  7.1.4.1. Recovery Within-command ...................95
                  7.1.4.2. Recovery Within-connection ................96
                  7.1.4.3. Connection Recovery .......................96
                  7.1.4.4. Session Recovery ..........................97
           7.1.5. Error Recovery Hierarchy ...........................97
      7.2. Retry and Reassign in Recovery ............................99
           7.2.1. Usage of Retry .....................................99
           7.2.2. Allegiance Reassignment ...........................100
      7.3. Usage of Reject PDU in Recovery ..........................101
      7.4. Error Recovery Considerations for Discovery Sessions .....102
           7.4.1. ErrorRecoveryLevel for Discovery Sessions .........102
           7.4.2. Reinstatement Semantics for Discovery Sessions ....102
                  7.4.2.1. Unnamed Discovery Sessions ...............103
                  7.4.2.2. Named Discovery Sessions .................103
           7.4.3. Target PDUs during Discovery ......................103

      7.5. Connection Timeout Management ............................104
           7.5.1. Timeouts on Transport Exception Events ............104
           7.5.2. Timeouts on Planned Decommissioning ...............104
      7.6. Implicit Termination of Tasks ............................104
      7.7. Format Errors ............................................105
      7.8. Digest Errors ............................................106
      7.9. Sequence Errors ..........................................107
      7.10. Message Error Checking ..................................108
      7.11. SCSI Timeouts ...........................................108
      7.12. Negotiation Failures ....................................109
      7.13. Protocol Errors .........................................110
      7.14. Connection Failures .....................................110
      7.15. Session Errors ..........................................111
   8. State Transitions .............................................112
      8.1. Standard Connection State Diagrams .......................112
           8.1.1. State Descriptions for Initiators and Targets .....112
           8.1.2. State Transition Descriptions for
                  Initiators and Targets ............................114
           8.1.3. Standard Connection State Diagram for an
                  Initiator .........................................118
           8.1.4. Standard Connection State Diagram for a Target ....120
      8.2. Connection Cleanup State Diagram for Initiators
           and Targets ..............................................122
           8.2.1. State Descriptions for Initiators and Targets .....124
           8.2.2. State Transition Descriptions for
                  Initiators and Targets ............................124
      8.3. Session State Diagrams ...................................126
           8.3.1. Session State Diagram for an Initiator ............126
           8.3.2. Session State Diagram for a Target ................127
           8.3.3. State Descriptions for Initiators and Targets .....129
           8.3.4. State Transition Descriptions for
                  Initiators and Targets ............................129
   9. Security Considerations .......................................131
      9.1. iSCSI Security Mechanisms ................................132
      9.2. In-Band Initiator-Target Authentication ..................132
           9.2.1. CHAP Considerations ...............................134
           9.2.2. SRP Considerations ................................136
           9.2.3. Kerberos Considerations ...........................136
      9.3. IPsec ....................................................137
           9.3.1. Data Authentication and Integrity .................137
           9.3.2. Confidentiality ...................................138
           9.3.3. Policy, Security Associations, and
                  Cryptographic Key Management ......................139
      9.4. Security Considerations for the X#NodeArchitecture Key ...141
      9.5. SCSI Access Control Considerations .......................143

   10. Notes to Implementers ........................................143
      10.1. Multiple Network Adapters ...............................143
           10.1.1. Conservative Reuse of ISIDs ......................143
           10.1.2. iSCSI Name, ISID, and TPGT Use ...................144
      10.2. Autosense and Auto Contingent Allegiance (ACA) ..........146
      10.3. iSCSI Timeouts ..........................................146
      10.4. Command Retry and Cleaning Old Command Instances ........147
      10.5. Sync and Steering Layer, and Performance ................147
      10.6. Considerations for State-Dependent Devices and
            Long-Lasting SCSI Operations ............................147
           10.6.1. Determining the Proper ErrorRecoveryLevel ........148
      10.7. Multi-Task Abort Implementation Considerations ..........149
   11. iSCSI PDU Formats ............................................150
      11.1. iSCSI PDU Length and Padding ............................150
      11.2. PDU Template, Header, and Opcodes .......................150
           11.2.1. Basic Header Segment (BHS) .......................152
                  11.2.1.1. I (Immediate) Bit .......................152
                  11.2.1.2. Opcode ..................................152
                  11.2.1.3. F (Final) Bit ...........................154
                  11.2.1.4. Opcode-Specific Fields ..................154
                  11.2.1.5. TotalAHSLength ..........................154
                  11.2.1.6. DataSegmentLength .......................154
                  11.2.1.7. LUN .....................................154
                  11.2.1.8. Initiator Task Tag ......................154
           11.2.2. Additional Header Segment (AHS) ..................155
                  11.2.2.1. AHSType .................................155
                  11.2.2.2. AHSLength ...............................155
                  11.2.2.3. Extended CDB AHS ........................156
                  11.2.2.4. Bidirectional Read Expected Data
                            Transfer Length AHS .....................156
           11.2.3. Header Digest and Data Digest ....................156
           11.2.4. Data Segment .....................................157
      11.3. SCSI Command ............................................158
           11.3.1. Flags and Task Attributes (Byte 1) ...............159
           11.3.2. CmdSN - Command Sequence Number ..................159
           11.3.3. ExpStatSN ........................................160
           11.3.4. Expected Data Transfer Length ....................160
           11.3.5. CDB - SCSI Command Descriptor Block ..............160
           11.3.6. Data Segment - Command Data ......................161
      11.4. SCSI Response ...........................................161
           11.4.1. Flags (Byte 1) ...................................162
           11.4.2. Status ...........................................163
           11.4.3. Response .........................................163
           11.4.4. SNACK Tag ........................................164

           11.4.5. Residual Count ...................................164
                  11.4.5.1. Field Semantics .........................164
                  11.4.5.2. Residuals Concepts Overview .............164
                  11.4.5.3. SCSI REPORT LUNS Command and
                            Residual Overflow .......................165
           11.4.6. Bidirectional Read Residual Count ................166
           11.4.7. Data Segment - Sense and Response Data Segment ...167
                  11.4.7.1. SenseLength .............................167
                  11.4.7.2. Sense Data ..............................168
           11.4.8. ExpDataSN ........................................168
           11.4.9. StatSN - Status Sequence Number ..................168
           11.4.10. ExpCmdSN - Next Expected CmdSN from This
                    Initiator .......................................169
           11.4.11. MaxCmdSN - Maximum CmdSN from This Initiator ....169
      11.5. Task Management Function Request ........................170
           11.5.1. Function .........................................170
           11.5.2. TotalAHSLength and DataSegmentLength .............173
           11.5.3. LUN ..............................................173
           11.5.4. Referenced Task Tag ..............................173
           11.5.5. RefCmdSN .........................................174
           11.5.6. ExpDataSN ........................................174
      11.6. Task Management Function Response .......................175
           11.6.1. Response .........................................176
           11.6.2. TotalAHSLength and DataSegmentLength .............177
      11.7. SCSI Data-Out and SCSI Data-In ..........................178
           11.7.1. F (Final) Bit ....................................180
           11.7.2. A (Acknowledge) Bit ..............................180
           11.7.3. Flags (Byte 1) ...................................181
           11.7.4. Target Transfer Tag and LUN ......................181
           11.7.5. DataSN ...........................................182
           11.7.6. Buffer Offset ....................................182
           11.7.7. DataSegmentLength ................................182
      11.8. Ready To Transfer (R2T) .................................183
           11.8.1. TotalAHSLength and DataSegmentLength .............184
           11.8.2. R2TSN ............................................184
           11.8.3. StatSN ...........................................185
           11.8.4. Desired Data Transfer Length and Buffer Offset ...185
           11.8.5. Target Transfer Tag ..............................185
      11.9. Asynchronous Message ....................................186
           11.9.1. AsyncEvent .......................................187
           11.9.2. AsyncVCode .......................................189
           11.9.3. LUN ..............................................189
           11.9.4. Sense Data and iSCSI Event Data ..................190
                  11.9.4.1. SenseLength .............................190

      11.10. Text Request ...........................................191
           11.10.1. F (Final) Bit ...................................192
           11.10.2. C (Continue) Bit ................................192
           11.10.3. Initiator Task Tag ..............................192
           11.10.4. Target Transfer Tag .............................192
           11.10.5. Text ............................................193
      11.11. Text Response ..........................................194
           11.11.1. F (Final) Bit ...................................194
           11.11.2. C (Continue) Bit ................................195
           11.11.3. Initiator Task Tag ..............................195
           11.11.4. Target Transfer Tag .............................195
           11.11.5. StatSN ..........................................196
           11.11.6. Text Response Data ..............................196
      11.12. Login Request ..........................................196
           11.12.1. T (Transit) Bit .................................197
           11.12.2. C (Continue) Bit ................................197
           11.12.3. CSG and NSG .....................................198
           11.12.4. Version .........................................198
                  11.12.4.1. Version-max ............................198
                  11.12.4.2. Version-min ............................198
           11.12.5. ISID ............................................199
           11.12.6. TSIH ............................................200
           11.12.7. Connection ID (CID) .............................200
           11.12.8. CmdSN ...........................................201
           11.12.9. ExpStatSN .......................................201
           11.12.10. Login Parameters ...............................201
      11.13. Login Response .........................................202
           11.13.1. Version-max .....................................202
           11.13.2. Version-active ..................................203
           11.13.3. TSIH ............................................203
           11.13.4. StatSN ..........................................203
           11.13.5. Status-Class and Status-Detail ..................203
           11.13.6. T (Transit) Bit .................................206
           11.13.7. C (Continue) Bit ................................206
           11.13.8. Login Parameters ................................207
      11.14. Logout Request .........................................207
           11.14.1. Reason Code .....................................209
           11.14.2. TotalAHSLength and DataSegmentLength ............209
           11.14.3. CID .............................................210
           11.14.4. ExpStatSN .......................................210
           11.14.5. Implicit Termination of Tasks ...................210
      11.15. Logout Response ........................................211
           11.15.1. Response ........................................212
           11.15.2. TotalAHSLength and DataSegmentLength ............212
           11.15.3. Time2Wait .......................................212
           11.15.4. Time2Retain .....................................212

      11.16. SNACK Request ..........................................213
           11.16.1. Type ............................................214
           11.16.2. Data Acknowledgment .............................215
           11.16.3. Resegmentation ..................................215
           11.16.4. Initiator Task Tag ..............................216
           11.16.5. Target Transfer Tag or SNACK Tag ................216
           11.16.6. BegRun ..........................................216
           11.16.7. RunLength .......................................216
      11.17. Reject .................................................217
           11.17.1. Reason ..........................................218
           11.17.2. DataSN/R2TSN ....................................219
           11.17.3. StatSN, ExpCmdSN, and MaxCmdSN ..................219
           11.17.4. Complete Header of Bad PDU ......................219
      11.18. NOP-Out ................................................220
           11.18.1. Initiator Task Tag ..............................221
           11.18.2. Target Transfer Tag .............................221
           11.18.3. Ping Data .......................................221
      11.19. NOP-In .................................................222
           11.19.1. Target Transfer Tag .............................223
           11.19.2. StatSN ..........................................223
           11.19.3. LUN .............................................223
   12. iSCSI Security Text Keys and Authentication Methods ..........223
      12.1. AuthMethod ..............................................224
           12.1.1. Kerberos .........................................226
           12.1.2. Secure Remote Password (SRP) .....................226
           12.1.3. Challenge Handshake Authentication
                   Protocol (CHAP) ..................................228
   13. Login/Text Operational Text Keys .............................229
      13.1. HeaderDigest and DataDigest .............................230
      13.2. MaxConnections ..........................................232
      13.3. SendTargets .............................................232
      13.4. TargetName ..............................................232
      13.5. InitiatorName ...........................................233
      13.6. TargetAlias .............................................233
      13.7. InitiatorAlias ..........................................234
      13.8. TargetAddress ...........................................234
      13.9. TargetPortalGroupTag ....................................235
      13.10. InitialR2T .............................................236
      13.11. ImmediateData ..........................................236
      13.12. MaxRecvDataSegmentLength ...............................237
      13.13. MaxBurstLength .........................................238
      13.14. FirstBurstLength .......................................238
      13.15. DefaultTime2Wait .......................................239
      13.16. DefaultTime2Retain .....................................239
      13.17. MaxOutstandingR2T ......................................239
      13.18. DataPDUInOrder .........................................240
      13.19. DataSequenceInOrder ....................................240
      13.20. ErrorRecoveryLevel .....................................241

      13.21. SessionType ............................................241
      13.22. The Private Extension Key Format .......................242
      13.23. TaskReporting ..........................................242
      13.24. iSCSIProtocolLevel Negotiation .........................243
      13.25. Obsoleted Keys .........................................243
      13.26. X#NodeArchitecture .....................................244
           13.26.1. Definition ......................................244
           13.26.2. Implementation Requirements .....................244
   14. Rationale for Revised IANA Considerations ....................245
   15. IANA Considerations ..........................................246
   16. References ...................................................248
      16.1. Normative References ....................................248
      16.2. Informative References ..................................251
   Appendix A. Examples .............................................254
     A.1. Read Operation Example ....................................254
     A.2. Write Operation Example ...................................255
     A.3. R2TSN/DataSN Use Examples .................................256
          A.3.1. Output (Write) Data DataSN/R2TSN Example ...........256
          A.3.2. Input (Read) Data DataSN Example ...................257
          A.3.3. Bidirectional DataSN Example .......................258
          A.3.4. Unsolicited and Immediate Output (Write) Data
                 with DataSN Example ................................259
     A.4. CRC Examples ..............................................259
   Appendix B. Login Phase Examples .................................261
   Appendix C. SendTargets Operation ................................268
   Appendix D. Algorithmic Presentation of Error Recovery
               Classes ..............................................272
     D.1. General Data Structure and Procedure Description ..........273
     D.2. Within-command Error Recovery Algorithms ..................274
          D.2.1. Procedure Descriptions .............................274
          D.2.2. Initiator Algorithms ...............................275
          D.2.3. Target Algorithms ..................................277
     D.3. Within-connection Recovery Algorithms .....................279
          D.3.1. Procedure Descriptions .............................279
          D.3.2. Initiator Algorithms ...............................280
          D.3.3. Target Algorithms ..................................283
     D.4. Connection Recovery Algorithms ............................283
          D.4.1. Procedure Descriptions .............................283
          D.4.2. Initiator Algorithms ...............................284
          D.4.3. Target Algorithms ..................................286
   Appendix E. Clearing Effects of Various Events on Targets ........288
     E.1. Clearing Effects on iSCSI Objects .........................288
     E.2. Clearing Effects on SCSI Objects ..........................293
   Acknowledgments ..................................................294

1. Introduction

   The Small Computer System Interface (SCSI) is a popular family of
   protocols for communicating with I/O devices, especially storage
   devices.  SCSI is a client-server architecture.  Clients of a SCSI
   interface are called "initiators".  Initiators issue SCSI "commands"
   to request services from components -- logical units of a server
   known as a "target".  A "SCSI transport" maps the client-server SCSI
   protocol to a specific interconnect.  An initiator is one endpoint of
   a SCSI transport, and a target is the other endpoint.

   The SCSI protocol has been mapped over various transports, including
   Parallel SCSI, Intelligent Peripheral Interface (IPI), IEEE 1394
   (FireWire), and Fibre Channel.  These transports are I/O-specific and
   have limited distance capabilities.

   The iSCSI protocol defined in this document describes a means of
   transporting SCSI packets over TCP/IP, providing for an interoperable
   solution that can take advantage of existing Internet infrastructure,
   Internet management facilities, and address distance limitations.

2. Acronyms, Definitions, and Document Summary

2.1. Acronyms

   Acronym     Definition
   --------------------------------------------------------------
   3DES        Triple Data Encryption Standard
   ACA         Auto Contingent Allegiance
   AEN         Asynchronous Event Notification
   AES         Advanced Encryption Standard
   AH          Additional Header (not the IPsec AH!)
   AHS         Additional Header Segment
   API         Application Programming Interface
   ASC         Additional Sense Code
   ASCII       American Standard Code for Information Interchange
   ASCQ        Additional Sense Code Qualifier
   ATA         AT Attachment
   BHS         Basic Header Segment
   CBC         Cipher Block Chaining
   CD          Compact Disk
   CDB         Command Descriptor Block
   CHAP        Challenge Handshake Authentication Protocol
   CID         Connection ID
   CO          Connection Only
   CRC         Cyclic Redundancy Check
   CRL         Certificate Revocation List
   CSG         Current Stage

   CSM         Connection State Machine
   DES         Data Encryption Standard
   DNS         Domain Name Server
   DOI         Domain of Interpretation
   DVD         Digital Versatile Disk
   EDTL        Expected Data Transfer Length
   ESP         Encapsulating Security Payload
   EUI         Extended Unique Identifier
   FFP         Full Feature Phase
   FFPO        Full Feature Phase Only
   HBA         Host Bus Adapter
   HMAC        Hashed Message Authentication Code
   I_T         Initiator_Target
   I_T_L       Initiator_Target_LUN
   IANA        Internet Assigned Numbers Authority
   IB          InfiniBand
   ID          Identifier
   IDN         Internationalized Domain Name
   IEEE        Institute of Electrical and Electronics Engineers
   IETF        Internet Engineering Task Force
   IKE         Internet Key Exchange
   I/O         Input-Output
   IO          Initialize Only
   IP          Internet Protocol
   IPsec       Internet Protocol Security
   IPv4        Internet Protocol Version 4
   IPv6        Internet Protocol Version 6
   IQN         iSCSI Qualified Name
   iSCSI       Internet SCSI
   iSER        iSCSI Extensions for RDMA (see [RFC7145])
   ISID        Initiator Session ID
   iSNS        Internet Storage Name Service (see [RFC4171])
   ITN         iSCSI Target Name
   ITT         Initiator Task Tag
   KRB5        Kerberos V5
   LFL         Lower Functional Layer
   LTDS        Logical-Text-Data-Segment
   LO          Leading Only
   LU          Logical Unit
   LUN         Logical Unit Number
   MAC         Message Authentication Code
   NA          Not Applicable
   NAA         Network Address Authority
   NIC         Network Interface Card
   NOP         No Operation
   NSG         Next Stage
   OCSP        Online Certificate Status Protocol
   OS          Operating System

   PDU         Protocol Data Unit
   PKI         Public Key Infrastructure
   R2T         Ready To Transfer
   R2TSN       Ready To Transfer Sequence Number
   RDMA        Remote Direct Memory Access
   RFC         Request For Comments
   SA          Security Association
   SAM         SCSI Architecture Model
   SAM-2       SCSI Architecture Model - 2
   SAN         Storage Area Network
   SAS         Serial Attached SCSI
   SATA        Serial AT Attachment
   SCSI        Small Computer System Interface
   SLP         Service Location Protocol
   SN          Sequence Number
   SNACK       Selective Negative Acknowledgment - also
               Sequence Number Acknowledgement for data
   SPDTL       SCSI-Presented Data Transfer Length
   SPKM        Simple Public-Key Mechanism
   SRP         Secure Remote Password
   SSID        Session ID
   SW          Session-Wide
   TCB         Task Control Block
   TCP         Transmission Control Protocol
   TMF         Task Management Function
   TPGT        Target Portal Group Tag
   TSIH        Target Session Identifying Handle
   TTT         Target Transfer Tag
   UA          Unit Attention
   UFL         Upper Functional Layer
   ULP         Upper Level Protocol
   URN         Uniform Resource Name
   UTF         Universal Transformation Format
   WG          Working Group

2.2. Definitions

   - Alias: An alias string can also be associated with an iSCSI node.
     The alias allows an organization to associate a user-friendly
     string with the iSCSI name.  However, the alias string is not a
     substitute for the iSCSI name.

   - CID (connection ID): Connections within a session are identified by
     a connection ID.  It is a unique ID for this connection within the
     session for the initiator.  It is generated by the initiator and
     presented to the target during Login Requests and during logouts
     that close connections.

- Connection: A connection is a TCP connection. Communication
 between the initiator and target occurs over one or more TCP
 connections. The TCP connections carry control messages, SCSI
 commands, parameters, and data within iSCSI Protocol Data Units
 (iSCSI PDUs).

 - I/O Buffer: An I/O Buffer is a buffer that is used in a SCSI read
 or write operation so SCSI data may be sent from or received into
 that buffer. For a read or write data transfer to take place for a
 task, an I/O Buffer is required on the initiator and at least one
 is required on the target.

 - INCITS: "INCITS" stands for InterNational Committee for Information
 Technology Standards. The INCITS has a broad standardization scope
 within the field of Information and Communications Technologies
 (ICT), encompassing storage, processing, transfer, display,
 management, organization, and retrieval of information. INCITS
 serves as ANSI's Technical Advisory Group for the ISO/IEC Joint
 Technical Committee 1 (JTC 1). See <http://www.incits.org>.

 - InfiniBand: InfiniBand is an I/O architecture originally intended
 to replace Peripheral Component Interconnect (PCI) and address
 high-performance server interconnectivity [IB].

 - iSCSI Device: An iSCSI device is a SCSI device using an iSCSI
 service delivery subsystem. The Service Delivery Subsystem is
 defined by [SAM2] as a transport mechanism for SCSI commands and
 responses.

 - iSCSI Initiator Name: The iSCSI Initiator Name specifies the
 worldwide unique name of the initiator.

 - iSCSI Initiator Node: An iSCSI initiator node is the "initiator"
 device. The word "initiator" has been appropriately qualified as
 either a port or a device in the rest of the document when the
 context is ambiguous. All unqualified usages of "initiator" refer
 to an initiator port (or device), depending on the context.

 - iSCSI Layer: This layer builds/receives iSCSI PDUs and
 relays/receives them to/from one or more TCP connections that form
 an initiator-target "session".

 - iSCSI Name: This is the name of an iSCSI initiator or iSCSI target.

 - iSCSI Node: The iSCSI node represents a single iSCSI initiator or
 iSCSI target, or a single instance of each. There are one or more
 iSCSI nodes within a Network Entity. The iSCSI node is accessible
 via one or more Network Portals. An iSCSI node is identified by

     its iSCSI name.  The separation of the iSCSI name from the
     addresses used by and for the iSCSI node allows multiple iSCSI
     nodes to use the same address and the same iSCSI node to use
     multiple addresses.

   - iSCSI Target Name: The iSCSI Target Name specifies the worldwide
     unique name of the target.

   - iSCSI Target Node: The iSCSI target node is the "target" device.
     The word "target" has been appropriately qualified as either a port
     or a device in the rest of the document when the context is
     ambiguous.  All unqualified usages of "target" refer to a target
     port (or device), depending on the context.

   - iSCSI Task: An iSCSI task is an iSCSI request for which a response
     is expected.

   - iSCSI Transfer Direction: The iSCSI transfer direction is defined
     with regard to the initiator.  Outbound or outgoing transfers are
     transfers from the initiator to the target, while inbound or
     incoming transfers are from the target to the initiator.

   - ISID: The ISID is the initiator part of the session identifier.  It
     is explicitly specified by the initiator during login.

   - I_T Nexus: According to [SAM2], the I_T nexus is a relationship
     between a SCSI initiator port and a SCSI target port.  For iSCSI,
     this relationship is a session, defined as a relationship between
     an iSCSI initiator's end of the session (SCSI initiator port) and
     the iSCSI target's portal group.  The I_T nexus can be identified
     by the conjunction of the SCSI port names; that is, the I_T nexus
     identifier is the tuple (iSCSI Initiator Name + ',i,' + ISID, iSCSI
     Target Name + ',t,' + Target Portal Group Tag).

   - I_T_L Nexus: An I_T_L nexus is a SCSI concept and is defined as the
     relationship between a SCSI initiator port, a SCSI target port, and
     a Logical Unit (LU).

   - NAA: "NAA" refers to Network Address Authority, a naming format
     defined by the INCITS T11 Fibre Channel protocols [FC-FS3].

   - Network Entity: The Network Entity represents a device or gateway
     that is accessible from the IP network.  A Network Entity must have
     one or more Network Portals, each of which can be used to gain
     access to the IP network by some iSCSI nodes contained in that
     Network Entity.

   - Network Portal: The Network Portal is a component of a Network
     Entity that has a TCP/IP network address and that may be used by an
     iSCSI node within that Network Entity for the connection(s) within
     one of its iSCSI sessions.  A Network Portal in an initiator is
     identified by its IP address.  A Network Portal in a target is
     identified by its IP address and its listening TCP port.

   - Originator: In a negotiation or exchange, the originator is the
     party that initiates the negotiation or exchange.

   - PDU (Protocol Data Unit): The initiator and target divide their
     communications into messages.  The term "iSCSI Protocol Data Unit"
     (iSCSI PDU) is used for these messages.

   - Portal Groups: iSCSI supports multiple connections within the same
     session; some implementations will have the ability to combine
     connections in a session across multiple Network Portals.  A portal
     group defines a set of Network Portals within an iSCSI Network
     Entity that collectively supports the capability of coordinating a
     session with connections spanning these portals.  Not all Network
     Portals within a portal group need participate in every session
     connected through that portal group.  One or more portal groups may
     provide access to an iSCSI node.  Each Network Portal, as utilized
     by a given iSCSI node, belongs to exactly one portal group within
     that node.

   - Portal Group Tag: This 16-bit quantity identifies a portal group
     within an iSCSI node.  All Network Portals with the same Portal
     Group Tag in the context of a given iSCSI node are in the same
     portal group.

   - Recovery R2T: A recovery R2T is an R2T generated by a target upon
     detecting the loss of one or more Data-Out PDUs through one of the
     following means: a digest error, a sequence error, or a sequence
     reception timeout.  A recovery R2T carries the next unused R2TSN
     but requests all or part of the data burst that an earlier R2T
     (with a lower R2TSN) had already requested.

   - Responder: In a negotiation or exchange, the responder is the party
     that responds to the originator of the negotiation or exchange.

   - SAS: The Serial Attached SCSI (SAS) standard contains both a
     physical layer compatible with Serial ATA, and protocols for
     transporting SCSI commands to SAS devices and ATA commands to SATA
     devices [SAS] [SPL].

   - SCSI Device: This is the SAM-2 term for an entity that contains one
     or more SCSI ports that are connected to a service delivery
     subsystem and supports a SCSI application protocol.  For example, a
     SCSI initiator device contains one or more SCSI initiator ports and
     zero or more application clients.  A target device contains one or
     more SCSI target ports and one or more device servers and
     associated LUs.  For iSCSI, the SCSI device is the component within
     an iSCSI node that provides the SCSI functionality.  As such, there
     can be at most one SCSI device within a given iSCSI node.  Access
     to the SCSI device can only be achieved in an iSCSI Normal
     operational session.  The SCSI device name is defined to be the
     iSCSI name of the node.

   - SCSI Layer: This builds/receives SCSI CDBs (Command Descriptor
     Blocks) and relays/receives them with the remaining Execute Command
     [SAM2] parameters to/from the iSCSI Layer.

   - Session: The group of TCP connections that link an initiator with a
     target form a session (loosely equivalent to a SCSI I_T nexus).
     TCP connections can be added and removed from a session.  Across
     all connections within a session, an initiator sees one and the
     same target.

   - SCSI Port: This is the SAM-2 term for an entity in a SCSI device
     that provides the SCSI functionality to interface with a service
     delivery subsystem.  For iSCSI, the definitions of the SCSI
     initiator port and the SCSI target port are different.

   - SCSI Initiator Port: This maps to the endpoint of an iSCSI Normal
     operational session.  An iSCSI Normal operational session is
     negotiated through the login process between an iSCSI initiator
     node and an iSCSI target node.  At successful completion of this
     process, a SCSI initiator port is created within the SCSI initiator
     device.  The SCSI initiator port name and SCSI initiator port
     identifier are both defined to be the iSCSI Initiator Name together
     with (a) a label that identifies it as an initiator port
     name/identifier and (b) the ISID portion of the session identifier.

   - SCSI Port Name: This is a name consisting of UTF-8 [RFC3629]
     encoding of Unicode [UNICODE] characters and includes the iSCSI
     name + 'i' or 't' + ISID or Target Portal Group Tag.

   - SCSI-Presented Data Transfer Length (SPDTL): SPDTL is the aggregate
     data length of the data that the SCSI layer logically "presents" to
     the iSCSI layer for a Data-In or Data-Out transfer in the context
     of a SCSI task.  For a bidirectional task, there are two SPDTL
     values -- one for Data-In and one for Data-Out.  Note that the
     notion of "presenting" includes immediate data per the data

transfer model in [SAM2] and excludes overlapping data transfers,
 if any, requested by the SCSI layer.

 - SCSI Target Port: This maps to an iSCSI target portal group.

 - SCSI Target Port Name and SCSI Target Port Identifier: These are
 both defined to be the iSCSI Target Name together with (a) a label
 that identifies it as a target port name/identifier and (b) the
 Target Portal Group Tag.

 - SSID (Session ID): A session between an iSCSI initiator and an
 iSCSI target is defined by a session ID that is a tuple composed of
 an initiator part (ISID) and a target part (Target Portal Group
 Tag). The ISID is explicitly specified by the initiator at session
 establishment. The Target Portal Group Tag is implied by the
 initiator through the selection of the TCP endpoint at connection
 establishment. The TargetPortalGroupTag key must also be returned
 by the target as a confirmation during connection establishment.

 - T10: T10 is a technical committee within INCITS that develops
 standards and technical reports on I/O interfaces, particularly the
 series of SCSI (Small Computer System Interface) standards. See
 <http://www.t10.org>.

 - T11: T11 is a technical committee within INCITS responsible for
 standards development in the areas of Intelligent Peripheral
 Interface (IPI), High-Performance Parallel Interface (HIPPI), and
 Fibre Channel (FC). See <http://www.t11.org>.

 - Target Portal Group Tag: This is a numerical identifier (16-bit)
 for an iSCSI target portal group.

 - Target Transfer Tag (TTT): The TTT is an iSCSI protocol field used
 in a few iSCSI PDUs (e.g., R2T, NOP-In) that is always sent from
 the target to the initiator first and then quoted as a reference in
 initiator-sent PDUs back to the target relating to the same
 task/exchange. Therefore, the TTT effectively acts as an opaque
 handle to an existing task/exchange to help the target associate
 the incoming PDUs from the initiator to the proper execution
 context.

 - Third-party: This term is used in this document as a qualifier to
 nexus objects (I_T or I_T_L) and iSCSI sessions, to indicate that
 these objects and sessions reap the side effects of actions that
 take place in the context of a separate iSCSI session. One example
 of a third-party session is an iSCSI session discovering that its
 I_T_L nexus to a LU got reset due to a LU reset operation
 orchestrated via a separate I_T nexus.

   - TSIH (Target Session Identifying Handle): This is a target-assigned
     tag for a session with a specific named initiator.  The target
     generates it during session establishment.  Other than defining it
     as a 16-bit binary string, its internal format and content are not
     defined by this protocol but for the value with all bits set to 0
     that is reserved and used by the initiator to indicate a new
     session.  It is given to the target during additional connection
     establishment for the same session.

2.3. Summary of Changes

   1)  Consolidated RFCs 3720, 3980, 4850, and 5048, and made the
       necessary editorial changes.

   2)  Specified iSCSIProtocolLevel as "1" in Section 13.24 and added a
       related normative reference to [RFC7144].

   3)  Removed markers and related keys.

   4)  Removed SPKM authentication and related keys.

   5)  Added a new Section 13.25 on responding to obsoleted keys.

   6)  Have explicitly allowed initiator+target implementations
       throughout the text.

   7)  Clarified in Section 4.2.7 that implementations SHOULD NOT rely
       on SLP-based discovery.

   8)  Added Unified Modeling Language (UML) diagrams and related
       conventions in Section 3.

   9)  Made FastAbort implementation a "SHOULD" requirement in
       Section 4.2.3.4, rather than the previous "MUST" requirement.

   10) Required in Section 4.2.7.1 that iSCSI Target Name be the same as
       iSCSI Initiator Name for SCSI (composite) devices with both
       roles.

   11) Changed the "MUST NOT" to "should be avoided" in Section 4.2.7.2
       regarding usage of characters such as punctuation marks in iSCSI
       names.

   12) Updated Section 9.3 to require the following: MUST implement
       IPsec, 2400-series RFCs (IPsec v2, IKEv1); and SHOULD implement
       IPsec, 4300-series RFCs (IPsec v3, IKEv2).

   13) Clarified in Section 10.2 that ACA is a "SHOULD" only for iSCSI
       targets.

   14) Prohibited usage of X# name prefix for new public keys in
       Section 6.2.

   15) Prohibited usage of Y# name prefix for new digest extensions in
       Section 13.1 and Z# name prefix for new authentication method
       extensions in Section 12.1.

   16) Added a "SHOULD" in Section 6.2 that initiators and targets
       support at least six (6) exchanges during text negotiation.

   17) Added a clarification that Appendix C is normative.

   18) Added a normative requirement on [RFC7146] and made a few related
       changes in Section 9.3 to align the text in this document with
       that of [RFC7146].

   19) Added a new Section 9.2.3 covering Kerberos authentication
       considerations.

   20) Added text in Section 9.3.3 noting that OCSP is now allowed for
       checking certificates used with IPsec in addition to the use
       of CRLs.

   21) Added text in Section 9.3.1 specifying that extended sequence
       numbers (ESNs) are now required for ESPv2 (part of IPsec v2).

2.4. Conventions

   In examples, "I->" and "T->" show iSCSI PDUs sent by the initiator
   and target, respectively.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3. UML Conventions

3.1. UML Conventions Overview

   The SCSI Architecture Model (SAM) uses class diagrams and object
   diagrams with notation that is based on the Unified Modeling Language
   [UML].  Therefore, this document also uses UML to model the
   relationships for SCSI and iSCSI objects.

   A treatise on the graphical notation used in UML is beyond the scope
   of this document.  However, given the use of ASCII drawing for UML
   static class diagrams, a description of the notational conventions
   used in this document is included in the remainder of this section.

3.2. Multiplicity Notion

   Not specified   The number of instances of an attribute is not
                   specified.

               1   One instance of the class or attribute exists.

            0..*   Zero or more instances of the class or attribute
                   exist.

            1..*   One or more instances of the class or attribute
                   exist.

            0..1   Zero or one instance of the class or attribute
                   exists.

            n..m   n to m instances of the class or attribute exist
                   (e.g., 2..8).

         x, n..m   Multiple disjoint instances of the class or
                   attribute exist (e.g., 2, 8..15).

3.3. Class Diagram Conventions

3.4. Class Diagram Notation for Associations

+-----------------+
 | Class A |
 +-----------------+ association_name +-----------------+
 | attribute 01[1] |<------------------>| Class B |
 | attribute 02[1] | 1..* 0..1 +-----------------+
 +-----------------+ | attribute 03[1] |
 | operation 1() | +-----------------+
 +-----------------+

 The preceding diagram is an example where Class A knows about
 Class B (i.e., read as "Class A association_name Class B") and
 Class B knows about Class A (i.e., read as "Class B
 association_name Class A"). The use of association_name is
 optional. The multiplicity notation (1..* and 0..1) indicates the
 number of instances of the object.

 +--------------------+
 | Class A |
 +--------------------+ +--------------------+
 | attribute 01[1] |<-------------| Class B |
 | attribute 02[1] | 1 0..1 +--------------------+
 +--------------------+ | attribute 03[1] |
 | operation 1() | +--------------------+
 +--------------------+

 The preceding diagram is an example where Class B knows about
 Class A (i.e., read as "Class B knows about Class A") but Class A
 does not know about Class B.

 +----------------------+
 | Class A |
 +----------------------+ +--------------------+
 | attribute 01[1] |----------->| Class B |
 | attribute 02[1] | 0..* 1 +--------------------+
 +----------------------+ | attribute 03[1] |
 | operation 1() | +--------------------+
 +----------------------+

 The preceding diagram is an example where Class A knows about
 Class B (i.e., read as "Class A knows about Class B") but Class B
 does not know about Class A.

3.5. Class Diagram Notation for Aggregations

     +---------------+             +--------------+
     |  Class whole  |o------------|  Class part  |
     +---------------+             +--------------+

     The preceding diagram is an example where Class whole is an
     aggregate that contains Class part and where Class part may
     continue to exist even if Class whole is removed (i.e., read as
     "the whole contains the part").

     +---------------+             +--------------+
     |  Class whole  |@------------|  Class part  |
     +---------------+             +--------------+

     The preceding diagram is an example where Class whole is an
     aggregate that contains Class part where Class part only belongs
     to one Class whole, and the Class part does not continue to exist
     if the Class whole is removed (i.e., read as "the whole contains
     the part").

     +-------------+
     |             |
     +-------------+
        |       |
        + =(a)= +
        |       |

     The preceding diagram is an example where there is a constraint
     between the associations, where the (a) footnote describes the
     constraint.

3.6. Class Diagram Notation for Generalizations

     +---------------+
     |  Superclass   |
     +-------^-------+
            /_\
             |
     +---------------+
     |    Subclass   |
     +---------------+

     The preceding diagram is an example where the subclass is a kind
     of superclass.  A subclass shares all the attributes and
     operations of the superclass (i.e., the subclass inherits from the
     superclass).

4. Overview

4.1. SCSI Concepts

   The SCSI Architecture Model - 2 [SAM2] describes in detail the
   architecture of the SCSI family of I/O protocols.  This section
   provides a brief background of the SCSI architecture and is intended
   to familiarize readers with its terminology.

   At the highest level, SCSI is a family of interfaces for requesting
   services from I/O devices, including hard drives, tape drives, CD and
   DVD drives, printers, and scanners.  In SCSI terminology, an
   individual I/O device is called a "logical unit" (LU).

   SCSI is a client-server architecture.  Clients of a SCSI interface
   are called "initiators".  Initiators issue SCSI "commands" to request
   services from components -- LUs of a server known as a "target".  The
   "device server" on the LU accepts SCSI commands and processes them.

   A "SCSI transport" maps the client-server SCSI protocol to a specific
   interconnect.  The initiator is one endpoint of a SCSI transport.
   The "target" is the other endpoint.  A target can contain multiple
   LUs.  Each LU has an address within a target called a Logical Unit
   Number (LUN).

   A SCSI task is a SCSI command or possibly a linked set of SCSI
   commands.  Some LUs support multiple pending (queued) tasks, but the
   queue of tasks is managed by the LU.  The target uses an initiator-
   provided "task tag" to distinguish between tasks.  Only one command
   in a task can be outstanding at any given time.

   Each SCSI command results in an optional data phase and a required
   response phase.  In the data phase, information can travel from the
   initiator to the target (e.g., write), from the target to the
   initiator (e.g., read), or in both directions.  In the response
   phase, the target returns the final status of the operation,
   including any errors.

   Command Descriptor Blocks (CDBs) are the data structures used to
   contain the command parameters that an initiator sends to a target.
   The CDB content and structure are defined by [SAM2] and device-type
   specific SCSI standards.

4.2. iSCSI Concepts and Functional Overview

   The iSCSI protocol is a mapping of the SCSI command, event, and task
   management model (see [SAM2]) over the TCP protocol.  SCSI commands
   are carried by iSCSI requests, and SCSI responses and status are
   carried by iSCSI responses.  iSCSI also uses the request-response
   mechanism for iSCSI protocol mechanisms.

   For the remainder of this document, the terms "initiator" and
   "target" refer to "iSCSI initiator node" and "iSCSI target node",
   respectively (see iSCSI), unless otherwise qualified.

   As its title suggests, Section 4 presents an overview of the iSCSI
   concepts, and later sections in the rest of the specification contain
   the normative requirements -- in many cases covering the same
   concepts discussed in Section 4.  Such normative requirements text
   overrides the overview text in Section 4 if there is a disagreement
   between the two.

   In keeping with similar protocols, the initiator and target divide
   their communications into messages.  This document uses the term
   "iSCSI Protocol Data Unit" (iSCSI PDU) for these messages.

   For performance reasons, iSCSI allows a "phase-collapse".  A command
   and its associated data may be shipped together from initiator to
   target, and data and responses may be shipped together from targets.

   The iSCSI transfer direction is defined with respect to the
   initiator.  Outbound or outgoing transfers are transfers from an
   initiator to a target, while inbound or incoming transfers are from a
   target to an initiator.

   An iSCSI task is an iSCSI request for which a response is expected.

   In this document, "iSCSI request", "iSCSI command", request, or
   (unqualified) command have the same meaning.  Also, unless otherwise
   specified, status, response, or numbered response have the same
   meaning.

4.2.1. Layers and Sessions

   The following conceptual layering model is used to specify initiator
   and target actions and the way in which they relate to transmitted
   and received Protocol Data Units:

      - The SCSI layer builds/receives SCSI CDBs (Command Descriptor
        Blocks) and passes/receives them with the remaining Execute
        Command [SAM2] parameters to/from

      - the iSCSI layer that builds/receives iSCSI PDUs and
        relays/receives them to/from one or more TCP connections; the
        group of connections form an initiator-target "session".

   Communication between the initiator and target occurs over one or
   more TCP connections.  The TCP connections carry control messages,
   SCSI commands, parameters, and data within iSCSI Protocol Data Units
   (iSCSI PDUs).  The group of TCP connections that link an initiator
   with a target form a session (equivalent to a SCSI I_T nexus; see
   Section 4.4.2).  A session is defined by a session ID that is
   composed of an initiator part and a target part.  TCP connections can
   be added and removed from a session.  Each connection within a
   session is identified by a connection ID (CID).

   Across all connections within a session, an initiator sees one
   "target image".  All target-identifying elements, such as a LUN, are
   the same.  A target also sees one "initiator image" across all
   connections within a session.  Initiator-identifying elements, such
   as the Initiator Task Tag, are global across the session, regardless
   of the connection on which they are sent or received.

   iSCSI targets and initiators MUST support at least one TCP connection
   and MAY support several connections in a session.  For error recovery
   purposes, targets and initiators that support a single active
   connection in a session SHOULD support two connections during
   recovery.

4.2.2. Ordering and iSCSI Numbering

   iSCSI uses command and status numbering schemes and a data sequencing
   scheme.

   Command numbering is session-wide and is used for ordered command
   delivery over multiple connections.  It can also be used as a
   mechanism for command flow control over a session.

   Status numbering is per connection and is used to enable missing
   status detection and recovery in the presence of transient or
   permanent communication errors.

   Data sequencing is per command or part of a command (R2T-triggered
   sequence) and is used to detect missing data and/or R2T PDUs due to
   header digest errors.

   Typically, fields in the iSCSI PDUs communicate the sequence numbers
   between the initiator and target.  During periods when traffic on a
   connection is unidirectional, iSCSI NOP-Out/NOP-In PDUs may be
   utilized to synchronize the command and status ordering counters of
   the target and initiator.

   The iSCSI session abstraction is equivalent to the SCSI I_T nexus,
   and the iSCSI session provides an ordered command delivery from the
   SCSI initiator to the SCSI target.  For detailed design
   considerations that led to the iSCSI session model as it is defined
   here and how it relates the SCSI command ordering features defined in
   SCSI specifications to the iSCSI concepts, see [RFC3783].

4.2.2.1. Command Numbering and Acknowledging

   iSCSI performs ordered command delivery within a session.  All
   commands (initiator-to-target PDUs) in transit from the initiator to
   the target are numbered.

   iSCSI considers a task to be instantiated on the target in response
   to every request issued by the initiator.  A set of task management
   operations, including abort and reassign (see Section 11.5), may be
   performed on an iSCSI task; however, an abort operation cannot be
   performed on a task management operation, and usage of reassign
   operations has certain constraints.  See Section 11.5.1 for details.

   Some iSCSI tasks are SCSI tasks, and many SCSI activities are related
   to a SCSI task ([SAM2]).  In all cases, the task is identified by the
   Initiator Task Tag for the life of the task.

   The command number is carried by the iSCSI PDU as the CmdSN (command
   sequence number).  The numbering is session-wide.  Outgoing iSCSI
   PDUs carry this number.  The iSCSI initiator allocates CmdSNs with a
   32-bit unsigned counter (modulo 2**32).  Comparisons and arithmetic
   on CmdSNs use Serial Number Arithmetic as defined in [RFC1982] where
   SERIAL_BITS = 32.

   Commands meant for immediate delivery are marked with an immediate
   delivery flag; they MUST also carry the current CmdSN.  The CmdSN
   MUST NOT advance after a command marked for immediate delivery is
   sent.

   Command numbering starts with the first Login Request on the first
   connection of a session (the leading login on the leading
   connection), and the CmdSN MUST be incremented by 1 in a Serial
   Number Arithmetic sense, as defined in [RFC1982], for every
   non-immediate command issued afterwards.

   If immediate delivery is used with task management commands, these
   commands may reach the target before the tasks on which they are
   supposed to act.  However, their CmdSN serves as a marker of their
   position in the stream of commands.  The initiator and target MUST
   ensure that the SCSI task management functions specified in [SAM2]
   act in accordance with the [SAM2] specification.  For example, both
   commands and responses appear as if delivered in order.  Whenever the
   CmdSN for an outgoing PDU is not specified by an explicit rule, the
   CmdSN will carry the current value of the local CmdSN variable (see
   later in this section).

   The means by which an implementation decides to mark a PDU for
   immediate delivery or by which iSCSI decides by itself to mark a PDU
   for immediate delivery are beyond the scope of this document.

   The number of commands used for immediate delivery is not limited,
   and their delivery to execution is not acknowledged through the
   numbering scheme.  An iSCSI target MAY reject immediate commands,
   e.g., due to lack of resources to accommodate additional commands.
   An iSCSI target MUST be able to handle at least one immediate task
   management command and one immediate non-task-management iSCSI
   command per connection at any time.

   In this document, delivery for execution means delivery to the SCSI
   execution engine or an iSCSI protocol-specific execution engine
   (e.g., for Text Requests with public or private extension keys
   involving an execution component).  With the exception of the
   commands marked for immediate delivery, the iSCSI target layer MUST
   deliver the commands for execution in the order specified by the
   CmdSN.  Commands marked for immediate delivery may be delivered by

   the iSCSI target layer for execution as soon as detected.  iSCSI may
   avoid delivering some commands to the SCSI target layer if required
   by a prior SCSI or iSCSI action (e.g., a CLEAR TASK SET task
   management request received before all the commands on which it was
   supposed to act).

   On any connection, the iSCSI initiator MUST send the commands in
   increasing order of CmdSN, except for commands that are retransmitted
   due to digest error recovery and connection recovery.

   For the numbering mechanism, the initiator and target maintain the
   following three variables for each session:

      - CmdSN: the current command sequence number, advanced by 1 on
        each command shipped except for commands marked for immediate
        delivery as discussed above.  The CmdSN always contains the
        number to be assigned to the next command PDU.

      - ExpCmdSN: the next expected command by the target.  The target
        acknowledges all commands up to, but not including, this number.
        The initiator treats all commands with a CmdSN less than the
        ExpCmdSN as acknowledged.  The target iSCSI layer sets the
        ExpCmdSN to the largest non-immediate CmdSN that it can deliver
        for execution "plus 1" per [RFC1982].  There MUST NOT be any
        holes in the acknowledged CmdSN sequence.

      - MaxCmdSN: the maximum number to be shipped.  The queuing
        capacity of the receiving iSCSI layer is
        MaxCmdSN - ExpCmdSN + 1.

   The initiator's ExpCmdSN and MaxCmdSN are derived from target-to-
   initiator PDU fields.  Comparisons and arithmetic on the ExpCmdSN and
   MaxCmdSN MUST use Serial Number Arithmetic as defined in [RFC1982]
   where SERIAL_BITS = 32.

   The target MUST NOT transmit a MaxCmdSN that is less than
   ExpCmdSN - 1.  For non-immediate commands, the CmdSN field can take
   any value from the ExpCmdSN to the MaxCmdSN inclusive.  The target
   MUST silently ignore any non-immediate command outside of this range
   or non-immediate duplicates within the range.  The CmdSN carried by
   immediate commands may lie outside the ExpCmdSN-to-MaxCmdSN range.
   For example, if the initiator has previously sent a non-immediate
   command carrying the CmdSN equal to the MaxCmdSN, the target window
   is closed.  For group task management commands issued as immediate
   commands, the CmdSN indicates the scope of the group action (e.g., an
   ABORT TASK SET indicates which commands are to be aborted).

   MaxCmdSN and ExpCmdSN fields are processed by the initiator as
   follows:

      - If the PDU MaxCmdSN is less than the PDU ExpCmdSN - 1 (in a
        Serial Number Arithmetic sense), they are both ignored.

      - If the PDU MaxCmdSN is greater than the local MaxCmdSN (in a
        Serial Number Arithmetic sense), it updates the local MaxCmdSN;
        otherwise, it is ignored.

      - If the PDU ExpCmdSN is greater than the local ExpCmdSN (in a
        Serial Number Arithmetic sense), it updates the local ExpCmdSN;
        otherwise, it is ignored.

   This sequence is required because updates may arrive out of order
   (e.g., the updates are sent on different TCP connections).

   iSCSI initiators and targets MUST support the command numbering
   scheme.

   A numbered iSCSI request will not change its allocated CmdSN,
   regardless of the number of times and circumstances in which it is
   reissued (see Section 7.2.1).  At the target, the CmdSN is only
   relevant while the command has not created any state related to its
   execution (execution state); afterwards, the CmdSN becomes
   irrelevant.  Testing for the execution state (represented by
   identifying the Initiator Task Tag) MUST precede any other action at
   the target.  If no execution state is found, it is followed by
   ordering and delivery.  If an execution state is found, it is
   followed by delivery if it has not already been delivered.

   If an initiator issues a command retry for a command with CmdSN R on
   a connection when the session CmdSN value is Q, it MUST NOT advance
   the CmdSN past R + 2**31 - 1 unless

      - the connection is no longer operational (i.e., it has returned
        to the FREE state; see Section 8.1.3),

      - the connection has been reinstated (see Section 6.3.4), or

      - a non-immediate command with a CmdSN equal to or greater than Q
        was issued subsequent to the command retry on the same
        connection and the reception of that command is acknowledged by
        the target (see Section 10.4).

   A target command response or Data-In PDU with status MUST NOT precede
   the command acknowledgment.  However, the acknowledgment MAY be
   included in the response or the Data-In PDU.

4.2.2.2. Response/Status Numbering and Acknowledging

   Responses in transit from the target to the initiator are numbered.
   The StatSN (status sequence number) is used for this purpose.  The
   StatSN is a counter maintained per connection.  The ExpStatSN is used
   by the initiator to acknowledge status.  The status sequence number
   space is 32-bit unsigned integers, and the arithmetic operations are
   the regular mod(2**32) arithmetic.

   Status numbering starts with the Login Response to the first Login
   Request of the connection.  The Login Response includes an initial
   value for status numbering (any initial value is valid).

   To enable command recovery, the target MAY maintain enough state
   information for data and status recovery after a connection failure.
   A target doing so can safely discard all of the state information
   maintained for recovery of a command after the delivery of the status
   for the command (numbered StatSN) is acknowledged through the
   ExpStatSN.

   A large absolute difference between the StatSN and the ExpStatSN may
   indicate a failed connection.  Initiators MUST undertake recovery
   actions if the difference is greater than an implementation-defined
   constant that MUST NOT exceed 2**31 - 1.

   Initiators and targets MUST support the response-numbering scheme.

4.2.2.3. Response Ordering

4.2.2.3.1. Need for Response Ordering

   Whenever an iSCSI session is composed of multiple connections, the
   Response PDUs (task responses or TMF Responses) originating in the
   target SCSI layer are distributed onto the multiple connections by
   the target iSCSI layer according to iSCSI connection allegiance
   rules.  This process generally may not preserve the ordering of the
   responses by the time they are delivered to the initiator SCSI layer.

   Since ordering is not expected across SCSI Response PDUs anyway, this
   approach works fine in the general case.  However, to address the
   special cases where some ordering is desired by the SCSI layer, we
   introduce the notion of a "Response Fence": a Response Fence is
   logically the attribute/property of a SCSI response message handed
   off to a target iSCSI layer that indicates that there are special
   SCSI-level ordering considerations associated with this particular
   response message.  Whenever a Response Fence is set or required on a

   SCSI response message, we define the semantics in Section 4.2.2.3.2
   with respect to the target iSCSI layer's handling of such SCSI
   response messages.

4.2.2.3.2. Response Ordering Model Description

   The target SCSI protocol layer hands off the SCSI response messages
   to the target iSCSI layer by invoking the "Send Command Complete"
   protocol data service ([SAM2], Clause 5.4.2) and "Task Management
   Function Executed" ([SAM2], Clause 6.9) service.  On receiving the
   SCSI response message, the iSCSI layer exhibits the Response Fence
   behavior for certain SCSI response messages (Section 4.2.2.3.4
   describes the specific instances where the semantics must be
   realized).

   Whenever the Response Fence behavior is required for a SCSI response
   message, the target iSCSI layer MUST ensure that the following
   conditions are met in delivering the response message to the
   initiator iSCSI layer:

      - A response with a Response Fence MUST be delivered
        chronologically after all the "preceding" responses on the I_T_L
        nexus, if the preceding responses are delivered at all, to the
        initiator iSCSI layer.

      - A response with a Response Fence MUST be delivered
        chronologically prior to all the "following" responses on the
        I_T_L nexus.

   The notions of "preceding" and "following" refer to the order of
   handoff of a response message from the target SCSI protocol layer to
   the target iSCSI layer.

4.2.2.3.3. iSCSI Semantics with the Interface Model

   Whenever the TaskReporting key (Section 13.23) is negotiated to
   ResponseFence or FastAbort for an iSCSI session and the Response
   Fence behavior is required for a SCSI response message, the target
   iSCSI layer MUST perform the actions described in this section for
   that session.

      a) If it is a single-connection session, no special processing is
         required.  The standard SCSI Response PDU build and dispatch
         process happens.

      b) If it is a multi-connection session, the target iSCSI layer
         takes note of the last-sent and unacknowledged StatSN on each
         of the connections in the iSCSI session, and waits for an

         acknowledgment (NOP-In PDUs MAY be used to solicit
         acknowledgments as needed in order to accelerate this process)
         of each such StatSN to clear the fence.  The SCSI Response PDU
         requiring the Response Fence behavior MUST NOT be sent to the
         initiator before acknowledgments are received for each of the
         unacknowledged StatSNs.

      c) The target iSCSI layer must wait for an acknowledgment of the
         SCSI Response PDU that carried the SCSI response requiring the
         Response Fence behavior.  The fence MUST be considered cleared
         only after receiving the acknowledgment.

      d) All further status processing for the LU is resumed only after
         clearing the fence.  If any new responses for the I_T_L nexus
         are received from the SCSI layer before the fence is cleared,
         those Response PDUs MUST be held and queued at the iSCSI layer
         until the fence is cleared.

4.2.2.3.4. Current List of Fenced Response Use Cases

   This section lists the situations in which fenced response behavior
   is REQUIRED in iSCSI target implementations.  Note that the following
   list is an exhaustive enumeration as currently identified -- it is
   expected that as SCSI protocol specifications evolve, the
   specifications will enumerate when response fencing is required on a
   case-by-case basis.

   Whenever the TaskReporting key (Section 13.23) is negotiated to
   ResponseFence or FastAbort for an iSCSI session, the target iSCSI
   layer MUST assume that the Response Fence is required for the
   following SCSI completion messages:

      a) The first completion message carrying the UA after the multi-
         task abort on issuing and third-party sessions.  See
         Section 4.2.3.2 for related TMF discussion.

      b) The TMF Response carrying the multi-task TMF Response on the
         issuing session.

      c) The completion message indicating ACA establishment on the
         issuing session.

      d) The first completion message carrying the ACA ACTIVE status
         after ACA establishment on issuing and third-party sessions.

      e) The TMF Response carrying the CLEAR ACA response on the issuing
         session.

      f) The response to a PERSISTENT RESERVE OUT/PREEMPT AND ABORT
         command.

   Notes:

      - Due to the absence of ACA-related fencing requirements in
        [RFC3720], initiator implementations SHOULD NOT use ACA on
        multi-connection iSCSI sessions with targets complying only with
        [RFC3720].  This can be determined via TaskReporting key
        (Section 13.23) negotiation -- when the negotiation results in
        either "RFC3720" or "NotUnderstood".

      - Initiators that want to employ ACA on multi-connection iSCSI
        sessions SHOULD first assess response-fencing behavior via
        negotiating for the "ResponseFence" or "FastAbort" value for the
        TaskReporting (Section 13.23) key.

4.2.2.4. Data Sequencing

   Data and R2T PDUs transferred as part of some command execution MUST
   be sequenced.  The DataSN field is used for data sequencing.  For
   input (read) data PDUs, the DataSN starts with 0 for the first data
   PDU of an input command and advances by 1 for each subsequent data
   PDU.  For output data PDUs, the DataSN starts with 0 for the first
   data PDU of a sequence (the initial unsolicited sequence or any data
   PDU sequence issued to satisfy an R2T) and advances by 1 for each
   subsequent data PDU.  R2Ts are also sequenced per command.  For
   example, the first R2T has an R2TSN of 0 and advances by 1 for each
   subsequent R2T.  For bidirectional commands, the target uses the
   DataSN/R2TSN to sequence Data-In and R2T PDUs in one continuous
   sequence (undifferentiated).  Unlike command and status, data PDUs
   and R2Ts are not acknowledged by a field in regular outgoing PDUs.
   Data-In PDUs can be acknowledged on demand by a special form of the
   SNACK PDU.  Data and R2T PDUs are implicitly acknowledged by status
   for the command.  The DataSN/R2TSN field enables the initiator to
   detect missing data or R2T PDUs.

   For any read or bidirectional command, a target MUST issue less than
   2**32 combined R2T and Data-In PDUs.  Any output data sequence MUST
   contain less than 2**32 Data-Out PDUs.

4.2.3. iSCSI Task Management

4.2.3.1. Task Management Overview

   iSCSI task management features allow an initiator to control the
   active iSCSI tasks on an operational iSCSI session that it has with
   an iSCSI target.  Section 11.5 defines the task management function
   types that this specification defines -- ABORT TASK, ABORT TASK SET,
   CLEAR ACA, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET,
   TARGET COLD RESET, and TASK REASSIGN.

   Out of these function types, ABORT TASK and TASK REASSIGN functions
   manage a single active task, whereas ABORT TASK SET, CLEAR TASK SET,
   LOGICAL UNIT RESET, TARGET WARM RESET, and TARGET COLD RESET
   functions can each potentially affect multiple active tasks.

4.2.3.2. Notion of Affected Tasks

   This section defines the notion of "affected tasks" in multi-task
   abort scenarios.  Scope definitions in this section apply to both the
   standard multi-task abort semantics (Section 4.2.3.3) and the
   FastAbort multi-task abort semantics behavior (Section 4.2.3.4).

   ABORT TASK SET: All outstanding tasks for the I_T_L nexus identified
      by the LUN field in the ABORT TASK SET TMF Request PDU.

   CLEAR TASK SET: All outstanding tasks in the task set for the LU
      identified by the LUN field in the CLEAR TASK SET TMF Request PDU.
      See [SPC3] for the definition of a "task set".

   LOGICAL UNIT RESET: All outstanding tasks from all initiators for the
      LU identified by the LUN field in the LOGICAL UNIT RESET
      Request PDU.

   TARGET WARM RESET/TARGET COLD RESET: All outstanding tasks from all
      initiators across all LUs to which the TMF-issuing session has
      access on the SCSI target device hosting the iSCSI session.

   Usage: An "ABORT TASK SET TMF Request PDU" in the preceding text is
      an iSCSI TMF Request PDU with the "Function" field set to "ABORT
      TASK SET" as defined in Section 11.5.  Similar usage is employed
      for other scope descriptions.

4.2.3.3. Standard Multi-Task Abort Semantics

   All iSCSI implementations MUST support the protocol behavior defined
   in this section as the default behavior.  The execution of ABORT TASK
   SET, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET, and
   TARGET COLD RESET TMF Requests consists of the following sequence of
   actions in the specified order on the specified party.

   The initiator iSCSI layer:

      a) MUST continue to respond to each TTT received for the affected
         tasks.

      b) SHOULD process any responses received for affected tasks in the
         normal fashion.  This is acceptable because the responses are
         guaranteed to have been sent prior to the TMF Response.

      c) SHOULD receive the TMF Response concluding all the tasks in the
         set of affected tasks, unless the initiator has done something
         (e.g., LU reset, connection drop) that may prevent the TMF
         Response from being sent or received.  The initiator MUST thus
         conclude all affected tasks as part of this step in either case
         and MUST discard any TMF Response received after the affected
         tasks are concluded.

   The target iSCSI layer:

      a) MUST wait for responses on currently valid Target Transfer Tags
         of the affected tasks from the issuing initiator.  MAY wait for
         responses on currently valid Target Transfer Tags of the
         affected tasks from third-party initiators.

      b) MUST wait (concurrent with the wait in Step a) for all commands
         of the affected tasks to be received based on the CmdSN
         ordering.  SHOULD NOT wait for new commands on third-party
         affected sessions -- only the instantiated tasks have to be
         considered for the purpose of determining the affected tasks.
         However, in the case of target-scoped requests (i.e., TARGET
         WARM RESET and TARGET COLD RESET), all of the commands that are
         not yet received on the issuing session in the command stream
         can be considered to have been received with no command waiting
         period -- i.e., the entire CmdSN space up to the CmdSN of the
         task management function can be "plugged".

      c) MUST propagate the TMF Request to, and receive the response
         from, the target SCSI layer.

      d) MUST provide the Response Fence behavior for the TMF Response
         on the issuing session as specified in Section 4.2.2.3.2.

      e) MUST provide the Response Fence behavior on the first post-TMF
         Response on third-party sessions as specified in
         Section 4.2.2.3.3.  If some tasks originate from non-iSCSI
         I_T_L nexuses, then the means by which the target ensures that
         all affected tasks have returned their status to the initiator
         are defined by the specific non-iSCSI transport protocol(s).

   Technically, the TMF servicing is complete in Step d).  Data
   transfers corresponding to terminated tasks may, however, still be in
   progress on third-party iSCSI sessions even at the end of Step e).
   The TMF Response MUST NOT be sent by the target iSCSI layer before
   the end of Step d) and MAY be sent at the end of Step d) despite
   these outstanding data transfers until after Step e).

4.2.3.4. FastAbort Multi-Task Abort Semantics

   Protocol behavior defined in this section SHOULD be implemented by
   all iSCSI implementations complying with this document, noting that
   some steps below may not be compatible with [RFC3720] semantics.
   However, protocol behavior defined in this section MUST be exhibited
   by iSCSI implementations on an iSCSI session when they negotiate the
   TaskReporting (Section 13.23) key to "FastAbort" on that session.
   The execution of ABORT TASK SET, CLEAR TASK SET, LOGICAL UNIT RESET,
   TARGET WARM RESET, and TARGET COLD RESET TMF Requests consists of the
   following sequence of actions in the specified order on the specified
   party.

   The initiator iSCSI layer:

      a) MUST NOT send any more Data-Out PDUs for affected tasks on the
         issuing connection of the issuing iSCSI session once the TMF is
         sent to the target.

      b) SHOULD process any responses received for affected tasks in the
         normal fashion.  This is acceptable because the responses are
         guaranteed to have been sent prior to the TMF Response.

      c) MUST respond to each Async Message PDU with a Task Termination
         AsyncEvent (5) as defined in Section 11.9.

      d) MUST treat the TMF Response as terminating all affected tasks
         for which responses have not been received and MUST discard any
         responses for affected tasks received after the TMF Response is
         passed to the SCSI layer (although the semantics defined in
         this section ensure that such an out-of-order scenario will
         never happen with a compliant target implementation).

   The target iSCSI layer:

      a) MUST wait for all commands of the affected tasks to be received
         based on the CmdSN ordering on the issuing session.  SHOULD NOT
         wait for new commands on third-party affected sessions -- only
         the instantiated tasks have to be considered for the purpose of
         determining the affected tasks.  In the case of target-scoped
         requests (i.e., TARGET WARM RESET and TARGET COLD RESET), all
         the commands that are not yet received on the issuing session
         in the command stream can be considered to have been received
         with no command waiting period -- i.e., the entire CmdSN space
         up to the CmdSN of the task management function can be
         "plugged".

      b) MUST propagate the TMF Request to, and receive the response
         from, the target SCSI layer.

      c) MUST leave all active "affected TTTs" (i.e., active TTTs
         associated with affected tasks) valid.

      d) MUST send an Asynchronous Message PDU with AsyncEvent=5
         (Section 11.9) on:

         1) each connection of each third-party session to which at
            least one affected task is allegiant if
            TaskReporting=FastAbort is operational on that third-party
            session, and

         2) each connection except the issuing connection of the issuing
            session that has at least one allegiant affected task.

            If there are multiple affected LUs (say, due to a target
            reset), then one Async Message PDU MUST be sent for each
            such LU on each connection that has at least one allegiant
            affected task.  The LUN field in the Asynchronous Message
            PDU MUST be set to match the LUN for each such LU.

      e) MUST address the Response Fence flag on the TMF Response on the
         issuing session as defined in Section 4.2.2.3.3.

      f) MUST address the Response Fence flag on the first post-TMF
         Response on third-party sessions as defined in
         Section 4.2.2.3.3.  If some tasks originate from non-iSCSI
         I_T_L nexuses, then the means by which the target ensures that
         all affected tasks have returned their status to the initiator
         are defined by the specific non-iSCSI transport protocol(s).

      g) MUST free up the affected TTTs (and STags for iSER, if
         applicable) and the corresponding buffers, if any, once it
         receives each associated NOP-Out acknowledgment that the
         initiator generated in response to each Async Message.

   Technically, the TMF servicing is complete in Step e).  Data
   transfers corresponding to terminated tasks may, however, still be in
   progress even at the end of Step f).  A TMF Response MUST NOT be sent
   by the target iSCSI layer before the end of Step e) and MAY be sent
   at the end of Step e) despite these outstanding Data transfers until
   Step g).  Step g) specifies an event to free up any such resources
   that may have been reserved to support outstanding data transfers.

4.2.3.5. Affected Tasks Shared across Standard and FastAbort Sessions

   If an iSCSI target implementation is capable of supporting
   TaskReporting=FastAbort functionality (Section 13.23), it may end up
   in a situation where some sessions have TaskReporting=RFC3720
   operational (RFC 3720 sessions) while some other sessions have
   TaskReporting=FastAbort operational (FastAbort sessions) even while
   accessing a shared set of affected tasks (Section 4.2.3.2).  If the
   issuing session is an RFC 3720 session, the iSCSI target
   implementation is FastAbort-capable, and the third-party affected
   session is a FastAbort session, the following behavior SHOULD be
   exhibited by the iSCSI target layer:

      a) Between Steps c) and d) of the target behavior in
         Section 4.2.3.3, send an Asynchronous Message PDU with
         AsyncEvent=5 (Section 11.9) on each connection of each third-
         party session to which at least one affected task is allegiant.
         If there are multiple affected LUs, then send one Async Message
         PDU for each such LU on each connection that has at least one
         allegiant affected task.  When sent, the LUN field in the
         Asynchronous Message PDU MUST be set to match the LUN for each
         such LU.

      b) After Step e) of the target behavior in Section 4.2.3.3, free
         up the affected TTTs (and STags for iSER, if applicable) and
         the corresponding buffers, if any, once each associated NOP-Out
         acknowledgment is received that the third-party initiator
         generated in response to each Async Message sent in Step a).

   If the issuing session is a FastAbort session, the iSCSI target
   implementation is FastAbort-capable, and the third-party affected
   session is an RFC 3720 session, the iSCSI target layer MUST NOT send
   Asynchronous Message PDUs on the third-party session to prompt the
   FastAbort behavior.

   If the third-party affected session is a FastAbort session and the
   issuing session is a FastAbort session, the initiator in the third-
   party role MUST respond to each Async Message PDU with AsyncEvent=5
   as defined in Section 11.9.  Note that an initiator MAY thus receive
   these Async Messages on a third-party affected session even if the
   session is a single-connection session.

4.2.3.6. Rationale behind the FastAbort Semantics

   There are fundamentally three basic objectives behind the semantics
   specified in Sections 4.2.3.3 and 4.2.3.4.

      a) Maintaining an ordered command flow I_T nexus abstraction to
         the target SCSI layer even with multi-connection sessions.

         - Target iSCSI processing of a TMF Request must maintain the
           single flow illusion.  The target behavior in Step b) of
           Section 4.2.3.3 and the target behavior in Step a) of
           Section 4.2.3.4 correspond to this objective.

      b) Maintaining a single ordered response flow I_T nexus
         abstraction to the initiator SCSI layer even with multi-
         connection sessions when one response (i.e., TMF Response)
         could imply the status of other unfinished tasks from the
         initiator's perspective.

         - The target must ensure that the initiator does not see "old"
           task responses (that were placed on the wire chronologically
           earlier than the TMF Response) after seeing the TMF Response.
           The target behavior in Step d) of Section 4.2.3.3 and the
           target behavior in Step e) of Section 4.2.3.4 correspond to
           this objective.

         - Whenever the result of a TMF action is visible across
           multiple I_T_L nexuses, [SAM2] requires the SCSI device
           server to trigger a UA on each of the other I_T_L nexuses.
           Once an initiator is notified of such a UA, the application
           client on the receiving initiator is required to clear its
           task state (Clause 5.5 of [SAM2]) for the affected tasks.  It
           would thus be inappropriate to deliver a SCSI Response for a
           task after the task state is cleared on the initiator, i.e.,
           after the UA is notified.  The UA notification contained in

           the first SCSI Response PDU on each affected third-party
           I_T_L nexus after the TMF action thus MUST NOT pass the
           affected task responses on any of the iSCSI sessions
           accessing the LU.  The target behavior in Step e) of
           Section 4.2.3.3 and the target behavior in Step f) of
           Section 4.2.3.4 correspond to this objective.

      c) Draining all active TTTs corresponding to affected tasks in a
         deterministic fashion.

         - Data-Out PDUs with stale TTTs arriving after the tasks are
           terminated can create a buffer management problem even for
           traditional iSCSI implementations and is fatal for the
           connection for iSCSI/iSER implementations.  Either the
           termination of affected tasks should be postponed until the
           TTTs are retired (as in Step a) of Section 4.2.3.3), or the
           TTTs and the buffers should stay allocated beyond task
           termination to be deterministically freed up later (as in
           Steps c) and g) of Section 4.2.3.4).

   The only other notable optimization is the plugging.  If all tasks on
   an I_T nexus will be aborted anyway (as with a target reset), there
   is no need to wait to receive all commands to plug the CmdSN holes.
   The target iSCSI layer can simply plug all missing CmdSN slots and
   move on with TMF processing.  The first objective (maintaining a
   single ordered command flow) is still met with this optimization
   because the target SCSI layer only sees ordered commands.

4.2.4. iSCSI Login

   The purpose of the iSCSI login is to enable a TCP connection for
   iSCSI use, authentication of the parties, negotiation of the
   session's parameters, and marking of the connection as belonging to
   an iSCSI session.

   A session is used to identify to a target all the connections with a
   given initiator that belong to the same I_T nexus.  (For more details
   on how a session relates to an I_T nexus, see Section 4.4.2.)

   The targets listen on a well-known TCP port or other TCP port for
   incoming connections.  The initiator begins the login process by
   connecting to one of these TCP ports.

   As part of the login process, the initiator and target SHOULD
   authenticate each other and MAY set a security association protocol
   for the session.  This can occur in many different ways and is
   subject to negotiation; see Section 12.

   To protect the TCP connection, an IPsec security association MAY be
   established before the Login Request.  For information on using IPsec
   security for iSCSI, see Section 9, [RFC3723], and [RFC7146].

   The iSCSI Login Phase is carried through Login Requests and
   Responses.  Once suitable authentication has occurred and operational
   parameters have been set, the session transitions to the Full Feature
   Phase and the initiator may start to send SCSI commands.  The
   security policy for whether and by what means a target chooses to
   authorize an initiator is beyond the scope of this document.  For a
   more detailed description of the Login Phase, see Section 6.

   The login PDU includes the ISID part of the session ID (SSID).  The
   target portal group that services the login is implied by the
   selection of the connection endpoint.  For a new session, the TSIH is
   zero.  As part of the response, the target generates a TSIH.

   During session establishment, the target identifies the SCSI
   initiator port (the "I" in the "I_T nexus") through the value pair
   (InitiatorName, ISID).  We describe InitiatorName later in this
   section.  Any persistent state (e.g., persistent reservations) on the
   target that is associated with a SCSI initiator port is identified
   based on this value pair.  Any state associated with the SCSI target
   port (the "T" in the "I_T nexus") is identified externally by the
   TargetName and Target Portal Group Tag (see Section 4.4.1).  The ISID
   is subject to reuse restrictions because it is used to identify a
   persistent state (see Section 4.4.3).

   Before the Full Feature Phase is established, only Login Request and
   Login Response PDUs are allowed.  Login Requests and Responses MUST
   be used exclusively during login.  On any connection, the Login Phase
   MUST immediately follow TCP connection establishment, and a
   subsequent Login Phase MUST NOT occur before tearing down the
   connection.

   A target receiving any PDU except a Login Request before the Login
   Phase is started MUST immediately terminate the connection on which
   the PDU was received.  Once the Login Phase has started, if the
   target receives any PDU except a Login Request, it MUST send a Login
   reject (with Status "invalid during login") and then disconnect.  If
   the initiator receives any PDU except a Login Response, it MUST
   immediately terminate the connection.

4.2.5. iSCSI Full Feature Phase

   Once the two sides successfully conclude the login on the first --
   also called the leading -- connection in the session, the iSCSI
   session is in the iSCSI Full Feature Phase.  A connection is in the
   Full Feature Phase if the session is in the Full Feature Phase and
   the connection login has completed successfully.  An iSCSI connection
   is not in the Full Feature Phase when

      a) it does not have an established transport connection, or

      b) when it has a valid transport connection, but a successful
         login was not performed or the connection is currently
         logged out.

   In a normal Full Feature Phase, the initiator may send SCSI commands
   and data to the various LUs on the target by encapsulating them in
   iSCSI PDUs that go over the established iSCSI session.

4.2.5.1. Command Connection Allegiance

   For any iSCSI request issued over a TCP connection, the corresponding
   response and/or other related PDU(s) MUST be sent over the same
   connection.  We call this "connection allegiance".  If the original
   connection fails before the command is completed, the connection
   allegiance of the command may be explicitly reassigned to a different
   transport connection as described in detail in Section 7.2.

   Thus, if an initiator issues a read command, the target MUST send the
   requested data, if any, followed by the status, to the initiator over
   the same TCP connection that was used to deliver the SCSI command.
   If an initiator issues a write command, the initiator MUST send the
   data, if any, for that command over the same TCP connection that was
   used to deliver the SCSI command.  The target MUST return Ready To
   Transfer (R2T), if any, and the status over the same TCP connection
   that was used to deliver the SCSI command.  Retransmission requests
   (SNACK PDUs), and the data and status that they generate, MUST also
   use the same connection.

   However, consecutive commands that are part of a SCSI linked command-
   chain task (see [SAM2]) MAY use different connections.  Connection
   allegiance is strictly per command and not per task.  During the
   iSCSI Full Feature Phase, the initiator and target MAY interleave
   unrelated SCSI commands, their SCSI data, and responses over the
   session.

4.2.5.2. Data Transfer Overview

   Outgoing SCSI data (initiator-to-target user data or command
   parameters) is sent as either solicited data or unsolicited data.
   Solicited data are sent in response to R2T PDUs.  Unsolicited data
   can be sent as part of an iSCSI Command PDU ("immediate data") or in
   separate iSCSI data PDUs.

   Immediate data are assumed to originate at offset 0 in the initiator
   SCSI write-buffer (outgoing data buffer).  All other data PDUs have
   the buffer offset set explicitly in the PDU header.

   An initiator may send unsolicited data up to FirstBurstLength (see
   Section 13.14) as immediate (up to the negotiated maximum PDU
   length), in a separate PDU sequence, or both.  All subsequent data
   MUST be solicited.  The maximum length of an individual data PDU or
   the immediate-part of the first unsolicited burst MAY be negotiated
   at login.

   The maximum amount of unsolicited data that can be sent with a
   command is negotiated at login through the FirstBurstLength (see
   Section 13.14) key.  A target MAY separately enable immediate data
   (through the ImmediateData key) without enabling the more general
   (separate data PDUs) form of unsolicited data (through the
   InitialR2T key).

   Unsolicited data for a write are meant to reduce the effect of
   latency on throughput (no R2T is needed to start sending data).  In
   addition, immediate data is meant to reduce the protocol overhead
   (both bandwidth and execution time).

   An iSCSI initiator MAY choose not to send unsolicited data, only
   immediate data or FirstBurstLength bytes of unsolicited data with a
   command.  If any non-immediate unsolicited data is sent, the total
   unsolicited data MUST be either FirstBurstLength or all of the data,
   if the total amount is less than the FirstBurstLength.

   It is considered an error for an initiator to send unsolicited data
   PDUs to a target that operates in R2T mode (only solicited data are
   allowed).  It is also an error for an initiator to send more
   unsolicited data, whether immediate or as separate PDUs, than
   FirstBurstLength.

   An initiator MUST honor an R2T data request for a valid outstanding
   command (i.e., carrying a valid Initiator Task Tag) and deliver all
   the requested data, provided the command is supposed to deliver

   outgoing data and the R2T specifies data within the command bounds.
   The initiator action is unspecified for receiving an R2T request that
   specifies data, all or in part, outside of the bounds of the command.

   A target SHOULD NOT silently discard data and then request
   retransmission through R2T.  Initiators SHOULD NOT keep track of the
   data transferred to or from the target (scoreboarding).  SCSI targets
   perform residual count calculation to check how much data was
   actually transferred to or from the device by a command.  This may
   differ from the amount the initiator sent and/or received for reasons
   such as retransmissions and errors.  Read or bidirectional commands
   implicitly solicit the transmission of the entire amount of data
   covered by the command.  SCSI data packets are matched to their
   corresponding SCSI commands by using tags specified in the protocol.

   In addition, iSCSI initiators and targets MUST enforce some ordering
   rules.  When unsolicited data is used, the order of the unsolicited
   data on each connection MUST match the order in which the commands on
   that connection are sent.  Command and unsolicited data PDUs may be
   interleaved on a single connection as long as the ordering
   requirements of each are maintained (e.g., command N + 1 MAY be sent
   before the unsolicited Data-Out PDUs for command N, but the
   unsolicited Data-Out PDUs for command N MUST precede the unsolicited
   Data-Out PDUs of command N + 1).  A target that receives data out of
   order MAY terminate the session.

4.2.5.3. Tags and Integrity Checks

   Initiator tags for pending commands are unique initiator-wide for a
   session.  Target tags are not strictly specified by the protocol.  It
   is assumed that target tags are used by the target to tag (alone or
   in combination with the LUN) the solicited data.  Target tags are
   generated by the target and "echoed" by the initiator.

   These mechanisms are designed to accomplish efficient data delivery
   along with a large degree of control over the data flow.

   As the Initiator Task Tag is used to identify a task during its
   execution, the iSCSI initiator and target MUST verify that all other
   fields used in task-related PDUs have values that are consistent with
   the values used at the task instantiation, based on the Initiator
   Task Tag (e.g., the LUN used in an R2T PDU MUST be the same as the
   one used in the SCSI Command PDU used to instantiate the task).
   Using inconsistent field values is considered a protocol error.

4.2.5.4. SCSI Task Management during iSCSI Full Feature Phase

   SCSI task management assumes that individual tasks and task groups
   can be aborted based solely on the task tags (for individual tasks)
   or the timing of the task management command (for task groups) and
   that the task management action is executed synchronously -- i.e., no
   message involving an aborted task will be seen by the SCSI initiator
   after receiving the task management response.  In iSCSI, initiators
   and targets interact asynchronously over several connections.  iSCSI
   specifies the protocol mechanism and implementation requirements
   needed to present a synchronous SCSI view while using an asynchronous
   iSCSI infrastructure.

4.2.6. iSCSI Connection Termination

   An iSCSI connection may be terminated via a transport connection
   shutdown or a transport reset.  A transport reset is assumed to be an
   exceptional event.

   Graceful TCP connection shutdowns are done by sending TCP FINs.  A
   graceful transport connection shutdown SHOULD only be initiated by
   either party when the connection is not in the iSCSI Full Feature
   Phase.  A target MAY terminate a Full Feature Phase connection on
   internal exception events, but it SHOULD announce the fact through an
   Asynchronous Message PDU.  Connection termination with outstanding
   commands may require recovery actions.

   If a connection is terminated while in the Full Feature Phase,
   connection cleanup (see Section 7.14) is required prior to recovery.
   By doing connection cleanup before starting recovery, the initiator
   and target will avoid receiving stale PDUs after recovery.

4.2.7. iSCSI Names

   Both targets and initiators require names for the purpose of
   identification.  In addition, names enable iSCSI storage resources to
   be managed, regardless of location (address).  An iSCSI Node Name is
   also the SCSI device name contained in the iSCSI node.  The iSCSI
   name of a SCSI device is the principal object used in authentication
   of targets to initiators and initiators to targets.  This name is
   also used to identify and manage iSCSI storage resources.

   iSCSI names must be unique within the operation domain of the end
   user.  However, because the operation domain of an IP network is
   potentially worldwide, the iSCSI name formats are architected to be
   worldwide unique.  To assist naming authorities in the construction
   of worldwide unique names, iSCSI provides three name formats for
   different types of naming authorities.

   iSCSI names are associated with iSCSI nodes, and not iSCSI network
   adapter cards, to ensure that the replacement of network adapter
   cards does not require reconfiguration of all SCSI and iSCSI resource
   allocation information.

   Some SCSI commands require that protocol-specific identifiers be
   communicated within SCSI CDBs.  See Section 2.2 for the definition of
   the SCSI port name/identifier for iSCSI ports.

   An initiator may discover the iSCSI Target Names to which it has
   access, along with their addresses, using the SendTargets Text
   Request, or other techniques discussed in [RFC3721].

   iSCSI equipment that needs discovery functions beyond SendTargets
   SHOULD implement iSNS (see [RFC4171]) for extended discovery
   management capabilities and interoperability.  Although [RFC3721]
   implies an SLP ([RFC2608]) implementation requirement, SLP has not
   been widely implemented or deployed for use with iSCSI in practice.
   iSCSI implementations therefore SHOULD NOT rely on SLP-based
   discovery interoperability.

4.2.7.1. iSCSI Name Properties

   Each iSCSI node, whether it is an initiator, a target, or both, MUST
   have an iSCSI name.  Whenever an iSCSI node contains an iSCSI
   initiator node and an iSCSI target node, the iSCSI Initiator Name
   MUST be the same as the iSCSI Target Name for the contained Nodes
   such that there is only one iSCSI Node Name for the iSCSI node
   overall.  Note the related requirements in Section 9.2.1 on how to
   map CHAP names to iSCSI names in such a scenario.

   Initiators and targets MUST support the receipt of iSCSI names of up
   to the maximum length of 223 bytes.

   The initiator MUST present both its iSCSI Initiator Name and the
   iSCSI Target Name to which it wishes to connect in the first Login
   Request of a new session or connection.  The only exception is if a
   Discovery session (see Section 4.3) is to be established.  In this
   case, the iSCSI Initiator Name is still required, but the iSCSI
   Target Name MAY be omitted.

   iSCSI names have the following properties:

      - iSCSI names are globally unique.  No two initiators or targets
        can have the same name.

      - iSCSI names are permanent.  An iSCSI initiator node or target
        node has the same name for its lifetime.

      - iSCSI names do not imply a location or address.  An iSCSI
        initiator or target can move or have multiple addresses.  A
        change of address does not imply a change of name.

      - iSCSI names do not rely on a central name broker; the naming
        authority is distributed.

      - iSCSI names support integration with existing unique naming
        schemes.

      - iSCSI names rely on existing naming authorities.  iSCSI does not
        create any new naming authority.

   The encoding of an iSCSI name has the following properties:

      - iSCSI names have the same encoding method, regardless of the
        underlying protocols.

      - iSCSI names are relatively simple to compare.  The algorithm for
        comparing two iSCSI names for equivalence does not rely on an
        external server.

      - iSCSI names are composed only of printable ASCII and Unicode
        characters.  iSCSI names allow the use of international
        character sets, but uppercase characters are prohibited.  The
        iSCSI stringprep profile [RFC3722] maps uppercase characters to
        lowercase and SHOULD be used to prepare iSCSI names from input
        that may include uppercase characters.  No whitespace characters
        are used in iSCSI names; see [RFC3722] for details.

      - iSCSI names may be transported using both binary and ASCII-based
        protocols.

   An iSCSI name really names a logical software entity and is not tied
   to a port or other hardware that can be changed.  For instance, an
   Initiator Name should name the iSCSI initiator node, not a particular
   NIC or HBA.  When multiple NICs are used, they should generally all
   present the same iSCSI Initiator Name to the targets, because they
   are simply paths to the same SCSI layer.  In most operating systems,
   the named entity is the operating system image.

   Similarly, a target name should not be tied to hardware interfaces
   that can be changed.  A target name should identify the logical
   target and must be the same for the target, regardless of the
   physical portion being addressed.  This assists iSCSI initiators in
   determining that the two targets it has discovered are really two
   paths to the same target.

   The iSCSI name is designed to fulfill the functional requirements for
   Uniform Resource Names (URNs) [RFC1737].  For example, it is required
   that the name have a global scope, be independent of address or
   location, and be persistent and globally unique.  Names must be
   extensible and scalable with the use of naming authorities.  The name
   encoding should be both human and machine readable.  See [RFC1737]
   for further requirements.

4.2.7.2. iSCSI Name Encoding

   An iSCSI name MUST be a UTF-8 (see [RFC3629]) encoding of a string of
   Unicode characters with the following properties:

      - It is in Normalization Form C (see "Unicode Normalization Forms"
        [UNICODE]).

      - It only contains characters allowed by the output of the iSCSI
        stringprep template (described in [RFC3722]).

      - The following characters are used for formatting iSCSI names:

           dash ('-'=U+002d)

           dot ('.'=U+002e)

           colon (':'=U+003a)

      - The UTF-8 encoding of the name is not larger than 223 bytes.

   The stringprep process is described in [RFC3454]; iSCSI's use of the
   stringprep process is described in [RFC3722].  The stringprep process
   is a method designed by the Internationalized Domain Name (IDN)
   working group to translate human-typed strings into a format that can
   be compared as opaque strings.  iSCSI names are expected to be used
   by administrators for purposes such as system configuration; for this
   reason, characters that may lead to human confusion among different
   iSCSI names (e.g., punctuation, spacing, diacritical marks) should be
   avoided, even when such characters are allowed as stringprep
   processing output by [RFC3722].  The stringprep process also converts
   strings into equivalent strings of lowercase characters.

   The stringprep process does not need to be implemented if the names
   are generated using only characters allowed as output by the
   stringprep processing specified in [RFC3722].  Those allowed
   characters include all ASCII lowercase and numeric characters, as
   well as lowercase Unicode characters as specified in [RFC3722].  Once
   iSCSI names encoded in UTF-8 are "normalized" as described in this
   section, they may be safely compared byte for byte.

4.2.7.3. iSCSI Name Structure

   An iSCSI name consists of two parts -- a type designator followed by
   a unique name string.

   iSCSI uses three existing naming authorities in constructing globally
   unique iSCSI names.  The type designator in an iSCSI name indicates
   the naming authority on which the name is based.  The three iSCSI
   name formats are the following:

      a) iSCSI-Qualified Name: based on domain names to identify a
         naming authority

      b) NAA format Name: based on a naming format defined by [FC-FS3]
         for constructing globally unique identifiers, referred to as
         the Network Address Authority (NAA)

      c) EUI format Name: based on EUI names, where the IEEE
         Registration Authority assists in the formation of worldwide
         unique names (EUI-64 format)

   The corresponding type designator strings currently defined are:

      a) iqn. - iSCSI Qualified name

      b) naa. - Remainder of the string is an INCITS T11-defined Network
         Address Authority identifier, in ASCII-encoded hexadecimal

      c) eui. - Remainder of the string is an IEEE EUI-64 identifier, in
         ASCII-encoded hexadecimal

   These three naming authority designators were considered sufficient
   at the time of writing this document.  The creation of additional
   naming type designators for iSCSI may be considered by the IETF and
   detailed in separate RFCs.

   The following table summarizes the current SCSI transport protocols
   and their naming formats.

        SCSI Transport Protocol       Naming Format
     +----------------------------+-------+-----+----+
     |                            | EUI-64| NAA |IQN |
     |----------------------------|-------|-----|----|
     | iSCSI (Internet SCSI)      |   X   |  X  | X  |
     |----------------------------|-------|-----|----|
     | FCP (Fibre Channel)        |       |  X  |    |
     |----------------------------|-------|-----|----|
     | SAS (Serial Attached SCSI) |       |  X  |    |
     +----------------------------+-------+-----+----+

4.2.7.4. Type "iqn." (iSCSI Qualified Name)

   This iSCSI name type can be used by any organization that owns a
   domain name.  This naming format is useful when an end user or
   service provider wishes to assign iSCSI names for targets and/or
   initiators.

   To generate names of this type, the person or organization generating
   the name must own a registered domain name.  This domain name does
   not have to resolve to an address; it just needs to be reserved to
   prevent others from generating iSCSI names using the same
   domain name.

   Since a domain name can expire, be acquired by another entity, or may
   be used to generate iSCSI names by both owners, the domain name must
   be additionally qualified by a date during which the naming authority
   owned the domain name.  A date code is provided as part of the "iqn."
   format for this reason.

   The iSCSI qualified name string consists of:

      - The string "iqn.", used to distinguish these names from "eui."
        formatted names.

      - A date code, in yyyy-mm format.  This date MUST be a date during
        which the naming authority owned the domain name used in this
        format and SHOULD be the first month in which the domain name
        was owned by this naming authority at 00:01 GMT of the first day
        of the month.  This date code uses the Gregorian calendar.  All
        four digits in the year must be present.  Both digits of the
        month must be present, with January == "01" and December ==
        "12".  The dash must be included.

      - A dot "."

      - The reverse domain name of the naming authority (person or
        organization) creating this iSCSI name.

      - An optional, colon (:)-prefixed string within the character set
        and length boundaries that the owner of the domain name deems
        appropriate.  This may contain product types, serial numbers,
        host identifiers, or software keys (e.g., it may include colons
        to separate organization boundaries).  With the exception of the
        colon prefix, the owner of the domain name can assign everything
        after the reverse domain name as desired.  It is the
        responsibility of the entity that is the naming authority to
        ensure that the iSCSI names it assigns are worldwide unique.
        For example, "Example Storage Arrays, Inc." might own the domain
        name "example.com".

   The following are examples of iSCSI qualified names that might be
   generated by "EXAMPLE Storage Arrays, Inc."

                    Naming     String defined by
      Type  Date     Auth      "example.com" naming authority
      +--++-----+ +---------+ +--------------------------------+
      | ||      | |         | |                                |

      iqn.2001-04.com.example:storage:diskarrays-sn-a8675309
      iqn.2001-04.com.example
      iqn.2001-04.com.example:storage.tape1.sys1.xyz
      iqn.2001-04.com.example:storage.disk2.sys1.xyz

4.2.7.5. Type "eui." (IEEE EUI-64 Format)

   The IEEE Registration Authority provides a service for assigning
   globally unique identifiers [EUI].  The EUI-64 format is used to
   build a global identifier in other network protocols.  For example,
   Fibre Channel defines a method of encoding it into a WorldWideName.
   For more information on registering for EUI identifiers, see [OUI].

   The format is "eui." followed by an EUI-64 identifier (16 ASCII-
   encoded hexadecimal digits).

      Example iSCSI name:

         Type   EUI-64 identifier (ASCII-encoded hexadecimal)
         +--++--------------+
         |  ||              |
         eui.02004567A425678D

   The IEEE EUI-64 iSCSI name format might be used when a manufacturer
   is already registered with the IEEE Registration Authority and uses
   EUI-64 formatted worldwide unique names for its products.

   More examples of name construction are discussed in [RFC3721].

4.2.7.6. Type "naa." (Network Address Authority)

   The INCITS T11 Framing and Signaling Specification [FC-FS3] defines a
   format called the Network Address Authority (NAA) format for
   constructing worldwide unique identifiers that use various identifier
   registration authorities.  This identifier format is used by the
   Fibre Channel and SAS SCSI transport protocols.  As FC and SAS
   constitute a large fraction of networked SCSI ports, the NAA format
   is a widely used format for SCSI transports.  The objective behind
   iSCSI supporting a direct representation of an NAA format Name is to
   facilitate construction of a target device name that translates
   easily across multiple namespaces for a SCSI storage device
   containing ports served by different transports.  More specifically,
   this format allows implementations wherein one NAA identifier can be
   assigned as the basis for the SCSI device name for a SCSI target with
   both SAS ports and iSCSI ports.

   The iSCSI NAA naming format is "naa.", followed by an NAA identifier
   represented in ASCII-encoded hexadecimal digits.

   An example of an iSCSI name with a 64-bit NAA value follows:

      Type  NAA identifier (ASCII-encoded hexadecimal)
      +--++--------------+
      |  ||              |
      naa.52004567BA64678D

   An example of an iSCSI name with a 128-bit NAA value follows:

      Type  NAA identifier (ASCII-encoded hexadecimal)
      +--++------------------------------+
      |  ||                              |
      naa.62004567BA64678D0123456789ABCDEF

   The iSCSI NAA naming format might be used in an implementation when
   the infrastructure for generating NAA worldwide unique names is
   already in place because the device contains both SAS and iSCSI SCSI
   ports.

   The NAA identifier formatted in an ASCII-hexadecimal representation
   has a maximum size of 32 characters (128-bit NAA format).  As a
   result, there is no issue with this naming format exceeding the
   maximum size for iSCSI Node Names.

4.2.8. Persistent State

   iSCSI does not require any persistent state maintenance across
   sessions.  However, in some cases, SCSI requires persistent
   identification of the SCSI initiator port name (see Sections 4.4.2
   and 4.4.3.)

   iSCSI sessions do not persist through power cycles and boot
   operations.

   All iSCSI session and connection parameters are reinitialized on
   session and connection creation.

   Commands persist beyond connection termination if the session
   persists and command recovery within the session is supported.
   However, when a connection is dropped, command execution, as
   perceived by iSCSI (i.e., involving iSCSI protocol exchanges for the
   affected task), is suspended until a new allegiance is established by
   the "TASK REASSIGN" task management function.  See Section 11.5.

4.2.9. Message Synchronization and Steering

   iSCSI presents a mapping of the SCSI protocol onto TCP.  This
   encapsulation is accomplished by sending iSCSI PDUs of varying
   lengths.  Unfortunately, TCP does not have a built-in mechanism for
   signaling message boundaries at the TCP layer.  iSCSI overcomes this
   obstacle by placing the message length in the iSCSI message header.
   This serves to delineate the end of the current message as well as
   the beginning of the next message.

   In situations where IP packets are delivered in order from the
   network, iSCSI message framing is not an issue and messages are
   processed one after the other.  In the presence of IP packet
   reordering (i.e., frames being dropped), legacy TCP implementations
   store the "out of order" TCP segments in temporary buffers until the
   missing TCP segments arrive, at which time the data must be copied to
   the application buffers.  In iSCSI, it is desirable to steer the SCSI
   data within these out-of-order TCP segments into the preallocated
   SCSI buffers rather than store them in temporary buffers.  This
   decreases the need for dedicated reassembly buffers as well as the
   latency and bandwidth related to extra copies.

   Relying solely on the "message length" information from the iSCSI
   message header may make it impossible to find iSCSI message
   boundaries in subsequent TCP segments due to the loss of a TCP
   segment that contains the iSCSI message length.  The missing TCP
   segment(s) must be received before any of the following segments can
   be steered to the correct SCSI buffers (due to the inability to
   determine the iSCSI message boundaries).  Since these segments cannot
   be steered to the correct location, they must be saved in temporary
   buffers that must then be copied to the SCSI buffers.

   Different schemes can be used to recover synchronization.  The
   details of any such schemes are beyond this protocol specification,
   but it suffices to note that [RFC4297] provides an overview of the
   direct data placement problem on IP networks, and [RFC5046] specifies
   a protocol extension for iSCSI that facilitates this direct data
   placement objective.  The rest of this document refers to any such
   direct data placement protocol usage as an example of a "Sync and
   Steering layer".

   Under normal circumstances (no PDU loss or data reception out of
   order), iSCSI data steering can be accomplished by using the
   identifying tag and the data offset fields in the iSCSI header in
   addition to the TCP sequence number from the TCP header.  The
   identifying tag helps associate the PDU with a SCSI buffer address,
   while the data offset and TCP sequence number are used to determine
   the offset within the buffer.

4.2.9.1. Sync/Steering and iSCSI PDU Length

   When a large iSCSI message is sent, the TCP segment(s) that contains
   the iSCSI header may be lost.  The remaining TCP segment(s) up to the
   next iSCSI message must be buffered (in temporary buffers) because
   the iSCSI header that indicates to which SCSI buffers the data are to
   be steered was lost.  To minimize the amount of buffering, it is
   recommended that the iSCSI PDU length be restricted to a small value
   (perhaps a few TCP segments in length).  During login, each end of
   the iSCSI session specifies the maximum iSCSI PDU length it will
   accept.

4.3. iSCSI Session Types

iSCSI defines two types of sessions:

a) Normal operational session - an unrestricted session.

      b) Discovery session - a session only opened for target discovery.
         The target MUST ONLY accept Text Requests with the SendTargets
         key and a Logout Request with reason "close the session".  All
         other requests MUST be rejected.

   The session type is defined during login with the SessionType=value
   parameter in the login command.

4.4. SCSI-to-iSCSI Concepts Mapping Model

   The following diagram shows an example of how multiple iSCSI nodes
   (targets in this case) can coexist within the same Network Entity and
   can share Network Portals (IP addresses and TCP ports).  Other more
   complex configurations are also possible.  For detailed descriptions
   of the components of these diagrams, see Section 4.4.1.

                 +-----------------------------------+
                 | Network Entity (iSCSI Client)     |
                 |                                   |
                 |          +-------------+          |
                 |          | iSCSI Node  |          |
                 |          | (Initiator) |          |
                 |          +-------------+          |
                 |              |      |             |
                 | +--------------+ +--------------+ |
                 | |Network Portal| |Network Portal| |
                 | |   192.0.2.4  | |   192.0.2.5  | |
                 +-+--------------+-+--------------+-+
                          |                  |
                          |   IP Networks    |
                          |                  |
                 +-+--------------+-+--------------+-+
                 | |Network Portal| |Network Portal| |
                 | |198.51.100.21 | |198.51.100.3  | |
                 | | TCP Port 3260| | TCP Port 3260| |
                 | +--------------+ +--------------+ |
                 |        |                  |       |
                 |         ------------------        |
                 |            |          |           |
                 | +-------------+ +--------------+  |
                 | | iSCSI Node  | | iSCSI Node   |  |
                 | | (Target)    | | (Target)     |  |
                 | +-------------+ +--------------+  |
                 |                                   |
                 |   Network Entity (iSCSI Server)   |
                 +-----------------------------------+

4.4.1. iSCSI Architecture Model

   This section describes the part of the iSCSI Architecture Model that
   has the most bearing on the relationship between iSCSI and the SCSI
   Architecture Model.

      - Network Entity - represents a device or gateway that is
        accessible from the IP network.  A Network Entity must have one
        or more Network Portals (see the "Network Portal" item below),
        each of which can be used by some iSCSI nodes (see the next
        item) contained in that Network Entity to gain access to the IP
        network.

      - iSCSI Node - represents a single iSCSI initiator or iSCSI
        target, or an instance of each.  There are one or more iSCSI
        nodes within a Network Entity.  The iSCSI node is accessible via
        one or more Network Portals (see below).  An iSCSI node is
        identified by its iSCSI name (see Sections 4.2.7 and 13).  The
        separation of the iSCSI name from the addresses used by and for
        the iSCSI node allows multiple iSCSI nodes to use the same
        addresses and allows the same iSCSI node to use multiple
        addresses.

      - An alias string may also be associated with an iSCSI node.  The
        alias allows an organization to associate a user-friendly string
        with the iSCSI name.  However, the alias string is not a
        substitute for the iSCSI name.

      - Network Portal - a component of a Network Entity that has a
        TCP/IP network address and that may be used by an iSCSI node
        within that Network Entity for the connection(s) within one of
        its iSCSI sessions.  In an initiator, it is identified by its IP
        address.  In a target, it is identified by its IP address and
        its listening TCP port.

      - Portal Groups - iSCSI supports multiple connections within the
        same session; some implementations will have the ability to
        combine connections in a session across multiple Network
        Portals.  A portal group defines a set of Network Portals within
        an iSCSI node that collectively supports the capability of
        coordinating a session with connections that span these portals.
        Not all Network Portals within a portal group need to
        participate in every session connected through that portal
        group.  One or more portal groups may provide access to an iSCSI
        node.  Each Network Portal, as utilized by a given iSCSI node,
        belongs to exactly one portal group within that node.  Portal
        groups are identified within an iSCSI node by a Portal Group
        Tag, a simple unsigned integer between 0 and 65535 (see

        Section 13.9).  All Network Portals with the same Portal Group
        Tag in the context of a given iSCSI node are in the same portal
        group.

        Both iSCSI initiators and iSCSI targets have portal groups,
        though only the iSCSI target portal groups are used directly in
        the iSCSI protocol (e.g., in SendTargets).  For references to
        the initiator portal Groups, see Section 10.1.2.

      - Portals within a portal group should support similar session
        parameters, because they may participate in a common session.

   The following diagram shows an example of one such configuration on a
   target and how a session that shares Network Portals within a portal
   group may be established.

       ----------------------------IP Network---------------------
              |                |                  |
         +----|----------------|----+        +----|---------+
         | +---------+ +---------+  |        | +---------+  |
         | | Network | | Network |  |        | | Network |  |
         | | Portal  | | Portal  |  |        | | Portal  |  |
         | +---------+ +---------+  |        | +---------+  |
         |    |                |    |        |    |         |
         |    |    Portal      |    |        |    | Portal  |
         |    |    Group 1     |    |        |    | Group 2 |
         +--------------------------+        +--------------+
              |                |                  |
     +--------|----------------|------------------|------------------+
     |        |                |                  |                  |
     | +----------------------------+ +----------------------------+ |
     | | iSCSI Session (Target side)| | iSCSI Session (Target side)| |
     | |                            | |                            | |
     | |        (TSIH = 56)         | |        (TSIH = 48)         | |
     | +----------------------------+ +----------------------------+ |
     |                                                               |
     |                      iSCSI Target Node                        |
     |             (within Network Entity, not shown)                |
     +---------------------------------------------------------------+

4.4.2. SCSI Architecture Model

   This section describes the relationship between the SCSI Architecture
   Model [SAM2] and constructs of the SCSI device, SCSI port and I_T
   nexus, and the iSCSI constructs described in Section 4.4.1.

   This relationship implies implementation requirements in order to
   conform to the SAM-2 model and other SCSI operational functions.

   These requirements are detailed in Section 4.4.3.

   The following list outlines mappings of SCSI architectural elements
   to iSCSI.

      a) SCSI Device - This is the SAM-2 term for an entity that
         contains one or more SCSI ports that are connected to a service
         delivery subsystem and supports a SCSI application protocol.
         For example, a SCSI initiator device contains one or more SCSI
         initiator ports and zero or more application clients.  A SCSI
         target device contains one or more SCSI target ports and one or
         more LUs.  For iSCSI, the SCSI device is the component within
         an iSCSI node that provides the SCSI functionality.  As such,
         there can be at most one SCSI device within an iSCSI node.
         Access to the SCSI device can only be achieved in an iSCSI
         Normal operational session (see Section 4.3).  The SCSI device
         name is defined to be the iSCSI name of the node and MUST be
         used in the iSCSI protocol.

      b) SCSI Port - This is the SAM-2 term for an entity in a SCSI
         device that provides the SCSI functionality to interface with a
         service delivery subsystem or transport.  For iSCSI, the
         definitions of the SCSI initiator port and the SCSI target port
         are different.

         SCSI initiator port: This maps to one endpoint of an iSCSI
         Normal operational session (see Section 4.3).  An iSCSI Normal
         operational session is negotiated through the login process
         between an iSCSI initiator node and an iSCSI target node.  At
         successful completion of this process, a SCSI initiator port is
         created within the SCSI initiator device.  The SCSI initiator
         port Name and SCSI initiator port Identifier are both defined
         to be the iSCSI Initiator Name together with (a) a label that
         identifies it as an initiator port name/identifier and (b) the
         ISID portion of the session identifier.

         SCSI target port: This maps to an iSCSI target portal group.
         The SCSI Target Port Name and the SCSI Target Port Identifier
         are both defined to be the iSCSI Target Name together with (a)
         a label that identifies it as a target port name/identifier and
         (b) the Target Portal Group Tag.

         The SCSI port name MUST be used in iSCSI.  When used in SCSI
         parameter data, the SCSI port name MUST be encoded as:

         1) the iSCSI name in UTF-8 format, followed by

         2) a comma separator (1 byte), followed by

         3) the ASCII character 'i' (for SCSI initiator port) or the
            ASCII character 't' (for SCSI target port) (1 byte),
            followed by

         4) a comma separator (1 byte), followed by

         5) a text encoding as a hex-constant (see Section 6.1) of the
            ISID (for SCSI initiator port) or the Target Portal Group
            Tag (for SCSI target port), including the initial 0X or 0x
            and the terminating null (15 bytes for iSCSI initiator port,
            7 bytes for iSCSI target port).

            The ASCII character 'i' or 't' is the label that identifies
            this port as either a SCSI initiator port or a SCSI target
            port.

      c) I_T nexus - This indicates a relationship between a SCSI
         initiator port and a SCSI target port, according to [SAM2].
         For iSCSI, this relationship is a session, defined as a
         relationship between an iSCSI initiator's end of the session
         (SCSI initiator port) and the iSCSI target's portal group.  The
         I_T nexus can be identified by the conjunction of the SCSI port
         names or by the iSCSI session identifier (SSID).  iSCSI defines
         the I_T nexus identifier to be the tuple (iSCSI Initiator Name
         + ",i,0x" + ISID in text format, iSCSI Target Name + ",t,0x" +
         Target Portal Group Tag in text format).  An uppercase hex
         prefix "0X" may alternatively be used in place of "0x".

         NOTE: The I_T nexus identifier is not equal to the SSID.

4.4.3. Consequences of the Model

   This section describes implementation and behavioral requirements
   that result from the mapping of SCSI constructs to the iSCSI
   constructs defined above.  Between a given SCSI initiator port and a
   given SCSI target port, only one I_T nexus (session) can exist.  No
   more than one nexus relationship (parallel nexus) is allowed by
   [SAM2].  Therefore, at any given time, only one session with the same
   SSID can exist between a given iSCSI initiator node and an iSCSI
   target node.

   These assumptions lead to the following conclusions and requirements:

   ISID RULE: Between a given iSCSI initiator and iSCSI target portal
   group (SCSI target port), there can only be one session with a given
   value for the ISID that identifies the SCSI initiator port.  See
   Section 11.12.5.

   The structure of the ISID that contains a naming authority component
   (see Section 11.12.5 and [RFC3721]) provides a mechanism to
   facilitate compliance with the ISID RULE.  See Section 10.1.1.

   The iSCSI initiator node should manage the assignment of ISIDs prior
   to session initiation.  The "ISID RULE" does not preclude the use of
   the same ISID from the same iSCSI initiator with different target
   portal groups on the same iSCSI target or on other iSCSI targets (see
   Section 10.1.1).  Allowing this would be analogous to a single SCSI
   initiator port having relationships (nexus) with multiple SCSI target
   ports on the same SCSI target device or SCSI target ports on other
   SCSI target devices.  It is also possible to have multiple sessions
   with different ISIDs to the same target portal group.  Each such
   session would be considered to be with a different initiator even
   when the sessions originate from the same initiator device.  The same
   ISID may be used by a different iSCSI initiator because it is the
   iSCSI name together with the ISID that identifies the SCSI initiator
   port.

   NOTE: A consequence of the ISID RULE and the specification for the
   I_T nexus identifier is that two nexuses with the same identifier
   should never exist at the same time.

   TSIH RULE: The iSCSI target selects a non-zero value for the TSIH at
   session creation (when an initiator presents a 0 value at login).
   After being selected, the same TSIH value MUST be used whenever the
   initiator or target refers to the session and a TSIH is required.

4.4.3.1. I_T Nexus State

   Certain nexus relationships contain an explicit state (e.g.,
   initiator-specific mode pages) that may need to be preserved by the
   device server [SAM2] in a LU through changes or failures in the iSCSI
   layer (e.g., session failures).  In order for that state to be
   restored, the iSCSI initiator should reestablish its session
   (re-login) to the same target portal group using the previous ISID.
   That is, it should reinstate the session via iSCSI session
   reinstatement (Section 6.3.5) or continue via session continuation
   (Section 6.3.6).  This is because the SCSI initiator port identifier
   and the SCSI target port identifier (or relative target port) form
   the datum that the SCSI LU device server uses to identify the I_T
   nexus.

4.4.3.2. Reservations

   There are two reservation management methods defined in the SCSI
   standards: reserve/release reservations, based on the RESERVE and
   RELEASE commands [SPC2]; and persistent reservations, based on the
   PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT commands [SPC3].
   Reserve/release reservations are obsolete [SPC3] and should not be
   used.  Persistent reservations are suggested as an alternative; see
   Annex B of [SPC4].

   State for persistent reservations is required to persist through
   changes and failures at the iSCSI layer that result in I_T nexus
   failures; see [SPC3] for details and specific requirements.

   In contrast, [SPC2] does not specify detailed persistence
   requirements for reserve/release reservation state after an I_T nexus
   failure.  Nonetheless, when reserve/release reservations are
   supported by an iSCSI target, the preferred implementation approach
   is to preserve reserve/release reservation state for iSCSI session
   reinstatement (see Section 6.3.5) or session continuation (see
   Section 6.3.6).

   Two additional caveats apply to reserve/release reservations:

      - Retention of a failed session's reserve/release reservation
        state by an iSCSI target, even after that failed iSCSI session
        is not reinstated or continued, may require an initiator to
        issue a reset (e.g., LOGICAL UNIT RESET; see Section 11.5) in
        order to remove that reservation state.

      - Reserve/release reservations may not behave as expected when
        persistent reservations are also used on the same LU; see the
        discussion of "Exceptions to SPC-2 RESERVE and RELEASE behavior"
        in [SPC4].

4.5. iSCSI UML Model

   This section presents the application of the UML modeling concepts
   discussed in Section 3 to the iSCSI and SCSI Architecture Model
   discussed in Section 4.4.

                       +----------------+
                       | Network Entity |
                       +----------------+
                            @ 1     @ 1
                            |       |
     +----------------------+       |
     |                              |
     |                              | 0..*
     |                   +------------------+
     |                   | iSCSI Node       |
     |                   +------------------+
     |                       @       @
     |                       |       |
     |           +-----------+ =(a)= +-----------+
     |           |                               |
     |           | 0..1                          | 0..1
     | +------------------------+       +----------------------+
     | |    iSCSI Target Node   |       | iSCSI Initiator Node |
     | +------------------------+       +----------------------+
     |             @ 1                            @ 1
     |             +---------------+              |
     |                        1..* |              | 1..*
     |                    +-----------------------------+
     |                    |         Portal Group        |
     |                    +-----------------------------+
     |                                     O 1
     |                                     |
     |                                     | 1..*
     |               1..* +------------------------+
     +--------------------|        Network Portal  |
                          +------------------------+

   (a) Each instance of an iSCSI node class MUST contain one iSCSI
       target node instance, one iSCSI initiator node instance, or both.

+----------------+
 | Network Entity |
 +----------------+
 @ 1 @ 1
 | | +------------------+
 +---------------------+ | | iSCSI Session |
 | | +------------------+
 | | 0..* | SSID[1] |
 | +--------------------+ | ISID[1] |
 | | iSCSI Node | +------------------+
 | +--------------------+ @ 1
 | | iSCSI Node Name[1] | |
 | | Alias [0..1] | | 0..*
 | +--------------------+ +------------------+
 | | | | iSCSI Connection |
 | +--------------------+ +------------------+
 | @ 1 @ 1 | CID[1] |
 | | | +------------------+
 | +-------------+ ==(b)== +---------+ 0..* |
 | | 1 | 1 |
 | +------------------------+ +------------------------+ |
 | | iSCSI Target Node | | iSCSI Initiator Node | |
 | +------------------------+ +------------------------+ |
 | | iSCSI Target Name [1] | |iSCSI Initiator Name [1]| |
 | +------------------------+ +------------------------+ |
 | @ 1 @ 1 |
 | | 1..* | 1..* |
 | +--------------------------+ +------------------------+ |
 | | Target Portal Group | | Initiator Portal Group | |
 | +--------------------------+ +------------------------+ |
 | |Target Portal Group Tag[1]| | Portal Group Tag[1] | |
 | +--------------------------+ +------------------------+ |
 | o 1 o 1 |
 | +------------+ +----------+ |
 | 1..* | | 1..* |
 | +-------------------------+ |
 | | Network Portal | |
 | +-------------------------+ |
 | 1..* | IP Address [1] | 1 |
 +----------------| TCP Port [0..1] |<-----------------------+
 +-------------------------+

 (b) Each instance of an iSCSI node class MUST contain one iSCSI
 target node instance, one iSCSI initiator node instance, or both.
 However, in all scenarios, note that an iSCSI node MUST only have
 a single iSCSI name. Note the related requirement in
 Section 4.2.7.1.

4.6. Request/Response Summary

   This section lists and briefly describes all the iSCSI PDU types
   (requests and responses).

   All iSCSI PDUs are built as a set of one or more header segments
   (basic and auxiliary) and zero or one data segments.  The header
   group and the data segment may each be followed by a CRC (digest).

   The basic header segment has a fixed length of 48 bytes.

4.6.1. Request/Response Types Carrying SCSI Payload

4.6.1.1. SCSI Command

   This request carries the SCSI CDB and all the other SCSI Execute
   Command [SAM2] procedure call IN arguments, such as task attributes,
   Expected Data Transfer Length for one or both transfer directions
   (the latter for bidirectional commands), and a task tag (as part of
   the I_T_L_x nexus).  The I_T_L nexus is derived by the initiator and
   target from the LUN field in the request, and the I_T nexus is
   implicit in the session identification.

   In addition, the SCSI Command PDU carries information required for
   the proper operation of the iSCSI protocol -- the command sequence
   number (CmdSN) and the expected status sequence number (ExpStatSN) on
   the connection it is issued.

   All or part of the SCSI output (write) data associated with the SCSI
   command may be sent as part of the SCSI Command PDU as a data
   segment.

4.6.1.2. SCSI Response

   The SCSI Response carries all the SCSI Execute Command procedure call
   (see [SAM2]) OUT arguments and the SCSI Execute Command procedure
   call return value.

   The SCSI Response contains the residual counts from the operation, if
   any; an indication of whether the counts represent an overflow or an
   underflow; and the SCSI status if the status is valid or a response
   code (a non-zero return value for the Execute Command procedure call)
   if the status is not valid.

   For a valid status that indicates that the command has been processed
   but resulted in an exception (e.g., a SCSI CHECK CONDITION), the PDU
   data segment contains the associated sense data.  The use of
   Autosense ([SAM2]) is REQUIRED by iSCSI.

   Some data segment content may also be associated (in the data
   segment) with a non-zero response code.

   In addition, the SCSI Response PDU carries information required for
   the proper operation of the iSCSI protocol:

      - ExpDataSN - the number of Data-In PDUs that a target has sent
        (to enable the initiator to check that all have arrived)

      - StatSN - the status sequence number on this connection

      - ExpCmdSN - the next expected command sequence number at the
        target

      - MaxCmdSN - the maximum CmdSN acceptable at the target from this
        initiator

4.6.1.3. Task Management Function Request

   The Task Management Function Request provides an initiator with a way
   to explicitly control the execution of one or more SCSI tasks or
   iSCSI functions.  The PDU carries a function identifier (i.e., which
   task management function to perform) and enough information to
   unequivocally identify the task or task set on which to perform the
   action, even if the task(s) to act upon has not yet arrived or has
   been discarded due to an error.

   The referenced tag identifies an individual task if the function
   refers to an individual task.

   The I_T_L nexus identifies task sets.  In iSCSI, the I_T_L nexus is
   identified by the LUN and the session identification (the session
   identifies an I_T nexus).

   For task sets, the CmdSN of the Task Management Function Request
   helps identify the tasks upon which to act, namely all tasks
   associated with a LUN and having a CmdSN preceding the Task
   Management Function Request CmdSN.

   For a task management function, the coordination between responses to
   the tasks affected and the Task Management Function Response is done
   by the target.

4.6.1.4. Task Management Function Response

   The Task Management Function Response carries an indication of
   function completion for a Task Management Function Request, including
   how it completed (response and qualifier) and additional information
   for failure responses.

   After the Task Management Function Response indicates task management
   function completion, the initiator will not receive any additional
   responses from the affected tasks.

4.6.1.5. SCSI Data-Out and SCSI Data-In

   SCSI Data-Out and SCSI Data-In are the main vehicles by which SCSI
   data payload is carried between the initiator and target.  Data
   payload is associated with a specific SCSI command through the
   Initiator Task Tag.  For target convenience, outgoing solicited data
   also carries a Target Transfer Tag (copied from R2T) and the LUN.
   Each PDU contains the payload length and the data offset relative to
   the buffer address contained in the SCSI Execute Command procedure
   call.

   In each direction, the data transfer is split into "sequences".  An
   end-of-sequence is indicated by the F bit.

   An outgoing sequence is either unsolicited (only the first sequence
   can be unsolicited) or consists of all the Data-Out PDUs sent in
   response to an R2T.

   Input sequences enable the switching of direction for bidirectional
   commands as required.

   For input, the target may request positive acknowledgment of input
   data.  This is limited to sessions that support error recovery and is
   implemented through the A bit in the SCSI Data-In PDU header.

   Data-In and Data-Out PDUs also carry the DataSN to enable the
   initiator and target to detect missing PDUs (discarded due to an
   error).

   In addition, the StatSN is carried by the Data-In PDUs.

   To enable a SCSI command to be processed while involving a minimum
   number of messages, the last SCSI Data-In PDU passed for a command
   may also contain the status if the status indicates termination with
   no exceptions (no sense or response involved).

4.6.1.6. Ready To Transfer (R2T)

   R2T is the mechanism by which the SCSI target "requests" the
   initiator for output data.  R2T specifies to the initiator the offset
   of the requested data relative to the buffer address from the Execute
   Command procedure call and the length of the solicited data.

   To help the SCSI target associate the resulting Data-Out with an R2T,
   the R2T carries a Target Transfer Tag that will be copied by the
   initiator in the solicited SCSI Data-Out PDUs.  There are no
   protocol-specific requirements with regard to the value of these
   tags, but it is assumed that together with the LUN, they will enable
   the target to associate data with an R2T.

   R2T also carries information required for proper operation of the
   iSCSI protocol, such as:

      - R2TSN (to enable an initiator to detect a missing R2T)

      - StatSN

      - ExpCmdSN

      - MaxCmdSN

4.6.2. Requests/Responses Carrying SCSI and iSCSI Payload

4.6.2.1. Asynchronous Message

   Asynchronous Message PDUs are used to carry SCSI asynchronous event
   notifications (AENs) and iSCSI asynchronous messages.

   When carrying an AEN, the event details are reported as sense data in
   the data segment.

4.6.3. Requests/Responses Carrying iSCSI-Only Payload

4.6.3.1. Text Requests and Text Responses

   Text Requests and Responses are designed as a parameter negotiation
   vehicle and as a vehicle for future extension.

   In the data segment, Text Requests/Responses carry text information
   using a simple "key=value" syntax.

   Text Requests/Responses may form extended sequences using the same
   Initiator Task Tag.  The initiator uses the F (Final) flag bit in the
   Text Request header to indicate its readiness to terminate a
   sequence.  The target uses the F bit in the Text Response header to
   indicate its consent to sequence termination.

   Text Requests and Responses also use the Target Transfer Tag to
   indicate continuation of an operation or a new beginning.  A target
   that wishes to continue an operation will set the Target Transfer Tag
   in a Text Response to a value different from the default 0xffffffff.
   An initiator willing to continue will copy this value into the Target
   Transfer Tag of the next Text Request.  If the initiator wants to
   restart the current target negotiation (start fresh), it will set the
   Target Transfer Tag to 0xffffffff.

   Although a complete exchange is always started by the initiator,
   specific parameter negotiations may be initiated by the initiator or
   target.

4.6.3.2. Login Requests and Login Responses

   Login Requests and Responses are used exclusively during the Login
   Phase of each connection to set up the session and connection
   parameters.  (The Login Phase consists of a sequence of Login
   Requests and Responses carrying the same Initiator Task Tag.)

   A connection is identified by an arbitrarily selected connection ID
   (CID) that is unique within a session.

   Similar to the Text Requests and Responses, Login Requests/Responses
   carry key=value text information with a simple syntax in the data
   segment.

   The Login Phase proceeds through several stages (security
   negotiation, operational parameter negotiation) that are selected
   with two binary coded fields in the header -- the Current Stage (CSG)
   and the Next Stage (NSG) -- with the appearance of the latter being
   signaled by the "Transit" flag (T).

   The first Login Phase of a session plays a special role, called the
   leading login, which determines some header fields (e.g., the version
   number, the maximum number of connections, and the session
   identification).

   The CmdSN initial value is also set by the leading login.

   The StatSN for each connection is initiated by the connection login.

   A Login Request may indicate an implied logout (cleanup) of the
   connection to be logged in (a connection restart) by using the same
   connection ID (CID) as an existing connection as well as the same
   session-identifying elements of the session to which the old
   connection was associated.

4.6.3.3. Logout Requests and Logout Responses

   Logout Requests and Responses are used for the orderly closing of
   connections for recovery or maintenance.  The Logout Request may be
   issued following a target prompt (through an Asynchronous Message) or
   at an initiator's initiative.  When issued on the connection to be
   logged out, no other request may follow it.

   The Logout Response indicates that the connection or session cleanup
   is completed and no other responses will arrive on the connection (if
   received on the logging-out connection).  In addition, the Logout
   Response indicates how long the target will continue to hold
   resources for recovery (e.g., command execution that continues on a
   new connection) in the Time2Retain field and how long the initiator
   must wait before proceeding with recovery in the Time2Wait field.

4.6.3.4. SNACK Request

   With the SNACK Request, the initiator requests retransmission of
   numbered responses or data from the target.  A single SNACK Request
   covers a contiguous set of missing items, called a run, of a given
   type of items.  The type is indicated in a type field in the PDU
   header.  The run is composed of an initial item (StatSN, DataSN,
   R2TSN) and the number of missed Status, Data, or R2T PDUs.  For long
   Data-In sequences, the target may request (at predefined minimum
   intervals) a positive acknowledgment for the data sent.  A SNACK
   Request with a type field that indicates ACK and the number of
   Data-In PDUs acknowledged conveys this positive acknowledgment.

4.6.3.5. Reject

   Reject enables the target to report an iSCSI error condition (e.g.,
   protocol, unsupported option) that uses a Reason field in the PDU
   header and includes the complete header of the bad PDU in the Reject
   PDU data segment.

4.6.3.6. NOP-Out Request and NOP-In Response

   This request/response pair may be used by an initiator and target as
   a "ping" mechanism to verify that a connection/session is still
   active and all of its components are operational.  Such a ping may be

   triggered by the initiator or target.  The triggering party indicates
   that it wants a reply by setting a value different from the default
   0xffffffff in the corresponding Initiator/Target Transfer Tag.

   NOP-In/NOP-Out may also be used in "unidirectional" fashion to convey
   to the initiator/target command, status, or data counter values when
   there is no other "carrier" and there is a need to update the
   initiator/target.

5. SCSI Mode Parameters for iSCSI

There are no iSCSI-specific mode pages.

6. Login and Full Feature Phase Negotiation

   iSCSI parameters are negotiated at session or connection
   establishment by using Login Requests and Responses (see
   Section 4.2.4) and during the Full Feature Phase (Section 4.2.5) by
   using Text Requests and Responses.  In both cases, the mechanism used
   is an exchange of iSCSI-text-key=value pairs.  For brevity,
   iSCSI-text-keys are called just "keys" in the rest of this document.

   Keys are either declarative or require negotiation, and the key
   description indicates whether the key is declarative or requires
   negotiation.

   For the declarative keys, the declaring party sets a value for the
   key.  The key specification indicates whether the key can be declared
   by the initiator, the target, or both.

   For the keys that require negotiation, one of the parties (the
   proposing party) proposes a value or set of values by including the
   key=value in the data part of a Login or Text Request or Response.
   The other party (the accepting party) makes a selection based on the
   value or list of values proposed and includes the selected value in a
   key=value in the data part of the following Login or Text Response or
   Request.  For most of the keys, both the initiator and target can be
   proposing parties.

   The login process proceeds in two stages -- the security negotiation
   stage and the operational parameter negotiation stage.  Both stages
   are optional, but at least one of them has to be present to enable
   setting some mandatory parameters.

   If present, the security negotiation stage precedes the operational
   parameter negotiation stage.

   Progression from stage to stage is controlled by the T (Transit) bit
   in the Login Request/Response PDU header.  Through the T bit set
   to 1, the initiator indicates that it would like to transition.  The
   target agrees to the transition (and selects the next stage) when
   ready.  A field in the Login PDU header indicates the current stage
   (CSG), and during transition, another field indicates the next stage
   (NSG) proposed (initiator) and selected (target).

   The text negotiation process is used to negotiate or declare
   operational parameters.  The negotiation process is controlled by the
   F (Final) bit in the PDU header.  During text negotiations, the F bit
   is used by the initiator to indicate that it is ready to finish the
   negotiation and by the target to acquiesce the end of negotiation.

   Since some key=value pairs may not fit entirely in a single PDU, the
   C (Continue) bit is used (both in Login and Text) to indicate that
   "more follows".

   The text negotiation uses an additional mechanism by which a target
   may deliver larger amounts of data to an inquiring initiator.  The
   target sets a Target Task Tag to be used as a bookmark that, when
   returned by the initiator, means "go on".  If reset to a "neutral
   value", it means "forget about the rest".

   This section details the types of keys and values used, the syntax
   rules for parameter formation, and the negotiation schemes to be used
   with different types of parameters.

6.1. Text Format

   The initiator and target send a set of key=value pairs encoded in
   UTF-8 Unicode.  All the text keys and text values specified in this
   document are case sensitive; they are to be presented and interpreted
   as they appear in this document without change of case.

   The following character symbols are used in this document for text
   items (the hexadecimal values represent Unicode code points):

   (a-z, A-Z) (0x61-0x7a, 0x41-0x5a) - letters
                   (0-9) (0x30-0x39) - digits
                          " " (0x20) - space
                          "." (0x2e) - dot
                          "-" (0x2d) - minus
                          "+" (0x2b) - plus
                          "@" (0x40) - commercial at
                          "_" (0x5f) - underscore
                          "=" (0x3d) - equal
                          ":" (0x3a) - colon

                          "/" (0x2f) - solidus or slash
                          "[" (0x5b) - left bracket
                          "]" (0x5d) - right bracket
                         null (0x00) - null separator
                          "," (0x2c) - comma
                          "~" (0x7e) - tilde

   Key=value pairs may span PDU boundaries.  An initiator or target that
   sends partial key=value text within a PDU indicates that more text
   follows by setting the C bit in the Text or Login Request or the Text
   or Login Response to 1.  Data segments in a series of PDUs that have
   the C bit set to 1 and end with a PDU that has the C bit set to 0, or
   that include a single PDU that has the C bit set to 0, have to be
   considered as forming a single logical-text-data-segment (LTDS).

   Every key=value pair, including the last or only pair in a LTDS, MUST
   be followed by one null (0x00) delimiter.

   A key-name is whatever precedes the first "=" in the key=value pair.
   The term "key" is used frequently in this document in place of
   "key-name".

   A value is whatever follows the first "=" in the key=value pair up to
   the end of the key=value pair, but not including the null delimiter.

   The following definitions will be used in the rest of this document:

      - standard-label: A string of one or more characters that consists
        of letters, digits, dot, minus, plus, commercial at, or
        underscore.  A standard-label MUST begin with a capital letter
        and must not exceed 63 characters.

      - key-name: A standard-label.

      - text-value: A string of zero or more characters that consists of
        letters, digits, dot, minus, plus, commercial at, underscore,
        slash, left bracket, right bracket, or colon.

      - iSCSI-name-value: A string of one or more characters that
        consists of minus, dot, colon, or any character allowed by the
        output of the iSCSI stringprep template as specified in
        [RFC3722] (see also Section 4.2.7.2).

      - iSCSI-local-name-value: A UTF-8 string; no null characters are
        allowed in the string.  This encoding is to be used for
        localized (internationalized) aliases.

      - boolean-value: The string "Yes" or "No".

      - hex-constant: A hexadecimal constant encoded as a string that
        starts with "0x" or "0X" followed by one or more digits or the
        letters a, b, c, d, e, f, A, B, C, D, E, or F.  Hex-constants
        are used to encode numerical values or binary strings.  When
        used to encode numerical values, the excessive use of leading 0
        digits is discouraged.  The string following 0X (or 0x)
        represents a base16 number that starts with the most significant
        base16 digit, followed by all other digits in decreasing order
        of significance and ending with the least significant base16
        digit.  When used to encode binary strings, hexadecimal
        constants have an implicit byte-length that includes four bits
        for every hexadecimal digit of the constant, including leading
        zeroes.  For example, a hex-constant of n hexadecimal digits has
        a byte-length of (the integer part of) (n + 1)/2.

      - decimal-constant: An unsigned decimal number with the digit 0 or
        a string of one or more digits that starts with a non-zero
        digit.  Decimal-constants are used to encode numerical values or
        binary strings.  Decimal-constants can only be used to encode
        binary strings if the string length is explicitly specified.
        There is no implicit length for decimal strings.
        Decimal-constants MUST NOT be used for parameter values if the
        values can be equal to or greater than 2**64 (numerical) or for
        binary strings that can be longer than 64 bits.

      - base64-constant: Base64 constant encoded as a string that starts
        with "0b" or "0B" followed by 1 or more digits, letters, plus
        sign, slash, or equals sign.  The encoding is done according to
        [RFC4648].

      - numerical-value: An unsigned integer always less than 2**64
        encoded as a decimal-constant or a hex-constant.  Unsigned
        integer arithmetic applies to numerical-values.

      - large-numerical-value: An unsigned integer that can be larger
        than or equal to 2**64 encoded as a hex-constant or
        base64-constant.  Unsigned integer arithmetic applies to large-
        numerical-values.

      - numerical-range: Two numerical-values separated by a tilde,
        where the value to the right of the tilde must not be lower than
        the value to the left.

      - regular-binary-value: A binary string not longer than 64 bits
        encoded as a decimal-constant, hex-constant, or base64-constant.
        The length of the string is either specified by the key
        definition or is the implicit byte-length of the encoded string.

      - large-binary-value: A binary string longer than 64 bits encoded
        as a hex-constant or base64-constant.  The length of the string
        is either specified by the key definition or is the implicit
        byte-length of the encoded string.

      - binary-value: A regular-binary-value or a large-binary-value.
        Operations on binary values are key-specific.

      - simple-value: Text-value, iSCSI-name-value, boolean-value,
        numerical-value, a numerical-range, or a binary-value.

      - list-of-values: A sequence of text-values separated by a comma.

   If not otherwise specified, the maximum length of a simple-value (not
   its encoded representation) is 255 bytes, not including the delimiter
   (comma or zero byte).

   Any iSCSI target or initiator MUST support receiving at least
   8192 bytes of key=value data in a negotiation sequence.  When
   proposing or accepting authentication methods that explicitly require
   support for very long authentication items, the initiator and target
   MUST support receiving at least 64 kilobytes of key=value data.

6.2. Text Mode Negotiation

   During login, and thereafter, some session or connection parameters
   are either declared or negotiated through an exchange of textual
   information.

   The initiator starts the negotiation and/or declaration through a
   Text or Login Request and indicates when it is ready for completion
   (by setting the F bit to 1 and keeping it at 1 in a Text Request, or
   the T bit in the Login Request).  As negotiation text may span PDU
   boundaries, a Text or Login Request or a Text or Login Response PDU
   that has the C bit set to 1 MUST NOT have the F bit or T bit set
   to 1.

   A target receiving a Text or Login Request with the C bit set to 1
   MUST answer with a Text or Login Response with no data segment
   (DataSegmentLength 0).  An initiator receiving a Text or Login
   Response with the C bit set to 1 MUST answer with a Text or Login
   Request with no data segment (DataSegmentLength 0).

   A target or initiator SHOULD NOT use a Text or Login Response or a
   Text or Login Request with no data segment (DataSegmentLength 0)
   unless explicitly required by a general or a key-specific negotiation
   rule.

There MUST NOT be more than one outstanding Text Request, or Text
 Response PDU on an iSCSI connection. An outstanding PDU in this
 context is one that has not been acknowledged by the remote iSCSI
 side.

 The format of a declaration is:

 Declarer-> <key>=<valuex>

 The general format of text negotiation is:

 Proposer-> <key>=<valuex>

 Acceptor-> <key>={<valuey>|NotUnderstood|Irrelevant|Reject}

 Thus, a declaration is a one-way textual exchange (unless the key is
 not understood by the receiver), while a negotiation is a two-way
 exchange.

 The proposer or declarer can be either the initiator or the target,
 and the acceptor can be either the target or initiator, respectively.
 Targets are not limited to respond to key=value pairs as proposed by
 the initiator. The target may propose key=value pairs of its own.

 All negotiations are explicit (i.e., the result MUST only be based on
 newly exchanged or declared values). There are no implicit
 proposals. If a proposal is not made, then a reply cannot be
 expected. Conservative design also requires that default values
 should not be relied upon when the use of some other value has
 serious consequences.

 The value proposed or declared can be a numerical-value, a numerical-
 range defined by the lower and upper value with both integers
 separated by a tilde, a binary value, a text-value, an iSCSI-name-
 value, an iSCSI-local-name-value, a boolean-value (Yes or No), or a
 list of comma-separated text-values. A range, a large-numerical-
 value, an iSCSI-name-value, and an iSCSI-local-name-value MAY ONLY be
 used if explicitly allowed. An accepted value can be a numerical-
 value, a large-numerical-value, a text-value, or a boolean-value.

 If a specific key is not relevant for the current negotiation, the
 acceptor may answer with the constant "Irrelevant" for all types of
 negotiations. However, the negotiation is not considered to have
 failed if the answer is "Irrelevant". The "Irrelevant" answer is
 meant for those cases in which several keys are presented by a
 proposing party but the selection made by the acceptor for one of the

   keys makes other keys irrelevant.  The following example illustrates
   the use of "Irrelevant":

      I->T InitialR2T=No,ImmediateData=Yes,FirstBurstLength=4192
      T->I InitialR2T=Yes,ImmediateData=No,FirstBurstLength=Irrelevant
      I->T X-rdname-vkey1=(bla,alb,None), X-rdname-vkey2=(bla,alb)
      T->I X-rdname-vkey1=None, X-rdname-vkey2=Irrelevant

   Any key not understood by the acceptor may be ignored by the acceptor
   without affecting the basic function.  However, the answer for a key
   that is not understood MUST be key=NotUnderstood.  Note that
   NotUnderstood is a valid answer for both declarative and negotiated
   keys.  The general iSCSI philosophy is that comprehension precedes
   processing for any iSCSI key.  A proposer of an iSCSI key, negotiated
   or declarative, in a text key exchange MUST thus be able to properly
   handle a NotUnderstood response.

   The proper way to handle a NotUnderstood response depends on where
   the key is specified and whether the key is declarative or
   negotiated.  An iSCSI implementation MUST comprehend all text keys
   defined in this document.  Returning a NotUnderstood response on any
   of these text keys therefore MUST be considered a protocol error and
   handled accordingly.  For all other "later" keys, i.e., text keys
   defined in later specifications, a NotUnderstood answer concludes the
   negotiation for a negotiated key, whereas for a declarative key a
   NotUnderstood answer simply informs the declarer of a lack of
   comprehension by the receiver.

   In either case, a NotUnderstood answer always requires that the
   protocol behavior associated with that key not be used within the
   scope of the key (connection/session) by either side.

   The constants "None", "Reject", "Irrelevant", and "NotUnderstood" are
   reserved and MUST ONLY be used as described here.  Violation of this
   rule is a protocol error (in particular, the use of "Reject",
   "Irrelevant", and "NotUnderstood" as proposed values).

   "Reject" or "Irrelevant" are legitimate negotiation options where
   allowed, but their excessive use is discouraged.  A negotiation is
   considered complete when the acceptor has sent the key value pair
   even if the value is "Reject", "Irrelevant", or "NotUnderstood".
   Sending the key again would be a renegotiation and is forbidden for
   many keys.

   If the acceptor sends "Reject" as an answer, the negotiated key is
   left at its current value (or default if no value was set).  If the
   current value is not acceptable to the proposer on the connection or
   to the session in which it is sent, the proposer MAY choose to
   terminate the connection or session.

   All keys in this document MUST be supported by iSCSI initiators and
   targets when used as specified here.  If used as specified, these
   keys MUST NOT be answered with NotUnderstood.

   Implementers may introduce new private keys by prefixing them with X-
   followed by their (reverse) domain name, or with new public keys
   registered with IANA.  For example, the entity owning the domain
   example.com can issue:

      X-com.example.bar.foo.do_something=3

   Each new public key in the course of standardization MUST define the
   acceptable responses to the key, including NotUnderstood as
   appropriate.  Unlike [RFC3720], note that this document prohibits the
   X# prefix for new public keys.  Based on iSCSI implementation
   experience, we know that there is no longer a need for a standard
   name prefix for keys that allow a NotUnderstood response.  Note that
   NotUnderstood will generally have to be allowed for new public keys
   for backwards compatibility, as well as for private X- keys.  Thus,
   the name prefix "X#" in new public key-names does not carry any
   significance.  To avoid confusion, new public key-names MUST NOT
   begin with an "X#" prefix.

   Implementers MAY also introduce new values, but ONLY for new keys or
   authentication methods (see Section 12) or digests (see
   Section 13.1).

   Whenever parameter actions or acceptance are dependent on other
   parameters, the dependency rules and parameter sequence must be
   specified with the parameters.

   In the Login Phase (see Section 6.3), every stage is a separate
   negotiation.  In the Full Feature Phase, a Text Request/Response
   sequence is a negotiation.  Negotiations MUST be handled as atomic
   operations.  For example, all negotiated values go into effect after
   the negotiation concludes in agreement or are ignored if the
   negotiation fails.

   Some parameters may be subject to integrity rules (e.g., parameter-x
   must not exceed parameter-y, or parameter-u not 1 implies that
   parameter-v be Yes).  Whenever required, integrity rules are
   specified with the keys.  Checking for compliance with the integrity

   rule must only be performed after all the parameters are available
   (the existent and the newly negotiated).  An iSCSI target MUST
   perform integrity checking before the new parameters take effect.  An
   initiator MAY perform integrity checking.

   An iSCSI initiator or target MAY terminate a negotiation that does
   not terminate within an implementation-specific reasonable time or
   number of exchanges but SHOULD allow at least six (6) exchanges.

6.2.1. List Negotiations

   In list negotiation, the originator sends a list of values (which may
   include "None"), in order of preference.

   The responding party MUST respond with the same key and the first
   value that it supports (and is allowed to use for the specific
   originator) selected from the originator list.

   The constant "None" MUST always be used to indicate a missing
   function.  However, "None" is only a valid selection if it is
   explicitly proposed.  When "None" is proposed as a selection item in
   a negotiation for a key, it indicates to the responder that not
   supporting any functionality related to that key is legal, and if
   "None" is the negotiation result for such a key, it means that key-
   specific semantics are not operational for the negotiation scope
   (connection or session) of that key.

   If an acceptor does not understand any particular value in a list, it
   MUST ignore it.  If an acceptor does not support, does not
   understand, or is not allowed to use any of the proposed options with
   a specific originator, it may use the constant "Reject" or terminate
   the negotiation.  The selection of a value not proposed MUST be
   handled by the originator as a protocol error.

6.2.2. Simple-Value Negotiations

   For simple-value negotiations, the accepting party MUST answer with
   the same key.  The value it selects becomes the negotiation result.

   Proposing a value not admissible (e.g., not within the specified
   bounds) MAY be answered with the constant "Reject"; otherwise, the
   acceptor MUST select an admissible value.

   The selection, by the acceptor, of a value not admissible under the
   selection rules is considered a protocol error.  The selection rules
   are key-specific.

   For a numerical range, the value selected MUST be an integer within
   the proposed range or "Reject" (if the range is unacceptable).

   For Boolean negotiations (i.e., keys taking the values "Yes" or
   "No"), the accepting party MUST answer with the same key and the
   result of the negotiation when the received value does not determine
   that result by itself.  The last value transmitted becomes the
   negotiation result.  The rules for selecting the value with which to
   answer are expressed as Boolean functions of the value received, and
   the value that the accepting party would have selected if given a
   choice.

   Specifically, the two cases in which answers are OPTIONAL are:

      - The Boolean function is "AND" and the value "No" is received.
        The outcome of the negotiation is "No".

      - The Boolean function is "OR" and the value "Yes" is received.
        The outcome of the negotiation is "Yes".

   Responses are REQUIRED in all other cases, and the value chosen and
   sent by the acceptor becomes the outcome of the negotiation.

6.3. Login Phase

   The Login Phase establishes an iSCSI connection between an initiator
   and a target; it also creates a new session or associates the
   connection to an existing session.  The Login Phase sets the iSCSI
   protocol parameters and security parameters, and authenticates the
   initiator and target to each other.

   The Login Phase is only implemented via Login Requests and Responses.
   The whole Login Phase is considered as a single task and has a single
   Initiator Task Tag (similar to the linked SCSI commands).

   There MUST NOT be more than one outstanding Login Request or Login
   Response on an iSCSI connection.  An outstanding PDU in this context
   is one that has not been acknowledged by the remote iSCSI side.

   The default MaxRecvDataSegmentLength is used during login.

   The Login Phase sequence of requests and responses proceeds as
   follows:

      - Login initial request

      - Login partial response (optional)

      - More Login Requests and Responses (optional)

      - Login Final-Response (mandatory)

   The initial Login Request of any connection MUST include the
   InitiatorName key=value pair.  The initial Login Request of the first
   connection of a session MAY also include the SessionType key=value
   pair.  For any connection within a session whose type is not
   "Discovery", the first Login Request MUST also include the TargetName
   key=value pair.

   The Login Final-Response accepts or rejects the Login Request.

   The Login Phase MAY include a SecurityNegotiation stage and a
   LoginOperationalNegotiation stage and MUST include at least one of
   them, but the included stage MAY be empty except for the mandatory
   names.

   The Login Requests and Responses contain a field (CSG) that indicates
   the current negotiation stage (SecurityNegotiation or
   LoginOperationalNegotiation).  If both stages are used, the
   SecurityNegotiation MUST precede the LoginOperationalNegotiation.

   Some operational parameters can be negotiated outside the login
   through Text Requests and Responses.

   Authentication-related security keys (Section 12) MUST be completely
   negotiated within the Login Phase.  The use of underlying IPsec
   security is specified in Section 9.3, in [RFC3723], and in [RFC7146].
   iSCSI support for security within the protocol only consists of
   authentication in the Login Phase.

   In some environments, a target or an initiator is not interested in
   authenticating its counterpart.  It is possible to bypass
   authentication through the Login Request and Response.

   The initiator and target MAY want to negotiate iSCSI authentication
   parameters.  Once this negotiation is completed, the channel is
   considered secure.

   Most of the negotiation keys are only allowed in a specific stage.
   The keys used during the SecurityNegotiation stage are listed in
   Section 12, and the keys used during the LoginOperationalNegotiation
   stage are discussed in Section 13.  Only a limited set of keys
   (marked as Any-Stage in Section 13) may be used in either of the two
   stages.

   Any given Login Request or Response belongs to a specific stage; this
   determines the negotiation keys allowed with the request or response.
   Sending a key that is not allowed in the current stage is considered
   a protocol error.

   Stage transition is performed through a command exchange
   (request/response) that carries the T bit and the same CSG code.
   During this exchange, the next stage is selected by the target via
   the Next Stage code (NSG).  The selected NSG MUST NOT exceed the
   value stated by the initiator.  The initiator can request a
   transition whenever it is ready, but a target can only respond with a
   transition after one is proposed by the initiator.

   In a negotiation sequence, the T bit settings in one Login Request-
   Login Response pair have no bearing on the T bit settings of the next
   pair.  An initiator that has the T bit set to 1 in one pair and is
   answered with a T bit setting of 0 may issue the next request with
   the T bit set to 0.

   When a transition is requested by the initiator and acknowledged by
   the target, both the initiator and target switch to the selected
   stage.

   Targets MUST NOT submit parameters that require an additional
   initiator Login Request in a Login Response with the T bit set to 1.

   Stage transitions during login (including entering and exit) are only
   possible as outlined in the following table:

     +-----------------------------------------------------------+
     |From      To ->  | Security    | Operational | FullFeature |
     | |               |             |             |             |
     | V               |             |             |             |
     +-----------------------------------------------------------+
     | (start)         | yes         | yes         | no          |
     +-----------------------------------------------------------+
     | Security        | no          | yes         | yes         |
     +-----------------------------------------------------------+
     | Operational     | no          | no          | yes         |
     +-----------------------------------------------------------+

   The Login Final-Response that accepts a Login Request can only come
   as a response to a Login Request with the T bit set to 1, and both
   the request and response MUST indicate FullFeaturePhase as the next
   phase via the NSG field.

   Neither the initiator nor the target should attempt to declare or
   negotiate a parameter more than once during login, except for
   responses to specific keys that explicitly allow repeated key
   declarations (e.g., TargetAddress).  An attempt to
   renegotiate/redeclare parameters not specifically allowed MUST be
   detected by the initiator and target.  If such an attempt is detected
   by the target, the target MUST respond with a Login reject (initiator
   error); if detected by the initiator, the initiator MUST drop the
   connection.

6.3.1. Login Phase Start

   The Login Phase starts with a Login Request from the initiator to the
   target.  The initial Login Request includes:

      - Protocol version supported by the initiator

      - iSCSI Initiator Name and iSCSI Target Name

      - ISID, TSIH, and connection IDs

      - Negotiation stage that the initiator is ready to enter

   A login may create a new session, or it may add a connection to an
   existing session.  Between a given iSCSI initiator node (selected
   only by an InitiatorName) and a given iSCSI target defined by an
   iSCSI TargetName and a Target Portal Group Tag, the login results are
   defined by the following table:

    +----------------------------------------------------------------+
    |ISID    | TSIH        | CID    |   Target Action                |
    +----------------------------------------------------------------+
    |new     | non-zero    | any    |   fail the login               |
    |        |             |        |   ("session does not exist")   |
    +----------------------------------------------------------------+
    |new     | zero        | any    |   instantiate a new session    |
    +----------------------------------------------------------------+
    |existing| zero        | any    |   do session reinstatement     |
    |        |             |        |   (see Section 6.3.5)          |
    +----------------------------------------------------------------+
    |existing| non-zero    | new    |   add a new connection to      |
    |        | existing    |        |   the session                  |
    +----------------------------------------------------------------+
    |existing| non-zero    |existing|   do connection reinstatement  |
    |        | existing    |        |   (see Section 7.1.4.3)        |
    +----------------------------------------------------------------+
    |existing| non-zero    | any    |   fail the login               |
    |        | new         |        |   ("session does not exist")   |
    +----------------------------------------------------------------+

   The determination of "existing" or "new" is made by the target.

   Optionally, the Login Request may include:

      - Security parameters OR

      - iSCSI operational parameters AND/OR

      - The next negotiation stage that the initiator is ready to
        enter

   The target can answer the login in the following ways:

      - Login Response with Login reject.  This is an immediate
        rejection from the target that causes the connection to
        terminate and the session to terminate if this is the first (or
        only) connection of a new session.  The T bit, the CSG field,
        and the NSG field are reserved.

      - Login Response with Login accept as the Final-Response (T bit
        set to 1 and the NSG in both request and response is set to
        FullFeaturePhase).  The response includes the protocol version
        supported by the target and the session ID and may include iSCSI
        operational or security parameters (that depend on the current
        stage).

      - Login Response with Login accept as a partial response (NSG not
        set to FullFeaturePhase in both request and response) that
        indicates the start of a negotiation sequence.  The response
        includes the protocol version supported by the target and either
        security or iSCSI parameters (when no security mechanism is
        chosen) supported by the target.

   If the initiator decides to forego the SecurityNegotiation stage, it
   issues the Login with the CSG set to LoginOperationalNegotiation, and
   the target may reply with a Login Response that indicates that it is
   unwilling to accept the connection (see Section 11.13) without
   SecurityNegotiation and will terminate the connection with a response
   of Authentication failure (see Section 11.13.5).

   If the initiator is willing to negotiate iSCSI security, but is
   unwilling to make the initial parameter proposal and may accept a
   connection without iSCSI security, it issues the Login with the T bit
   set to 1, the CSG set to SecurityNegotiation, and the NSG set to
   LoginOperationalNegotiation.  If the target is also ready to skip
   security, the Login Response only contains the TargetPortalGroupTag
   key (see Section 13.9), the T bit set to 1, the CSG set to
   SecurityNegotiation, and the NSG set to LoginOperationalNegotiation.

   An initiator that chooses to operate without iSCSI security and with
   all the operational parameters taking the default values issues the
   Login with the T bit set to 1, the CSG set to
   LoginOperationalNegotiation, and the NSG set to FullFeaturePhase.  If
   the target is also ready to forego security and can finish its
   LoginOperationalNegotiation, the Login Response has the T bit set to
   1, the CSG set to LoginOperationalNegotiation, and the NSG set to
   FullFeaturePhase in the next stage.

   During the Login Phase, the iSCSI target MUST return the
   TargetPortalGroupTag key with the first Login Response PDU with which
   it is allowed to do so (i.e., the first Login Response issued after
   the first Login Request with the C bit set to 0) for all session
   types.  The TargetPortalGroupTag key value indicates the iSCSI portal
   group servicing the Login Request PDU.  If the reconfiguration of
   iSCSI portal groups is a concern in a given environment, the iSCSI
   initiator should use this key to ascertain that it had indeed
   initiated the Login Phase with the intended target portal group.

6.3.2. iSCSI Security Negotiation

   The security exchange sets the security mechanism and authenticates
   the initiator and the target to each other.  The exchange proceeds
   according to the authentication method chosen in the negotiation
   phase and is conducted using the key=value parameters carried in the
   Login Requests and Responses.

   An initiator-directed negotiation proceeds as follows:

      - The initiator sends a Login Request with an ordered list of the
        options it supports (authentication algorithm).  The options are
        listed in the initiator's order of preference.  The initiator
        MAY also send private or public extension options.

      - The target MUST reply with the first option in the list it
        supports and is allowed to use for the specific initiator,
        unless it does not support any, in which case it MUST answer
        with "Reject" (see Section 6.2).  The parameters are encoded in
        UTF-8 as key=value.  For security parameters, see Section 12.

      - When the initiator considers itself ready to conclude the
        SecurityNegotiation stage, it sets the T bit to 1 and the NSG to
        what it would like the next stage to be.  The target will then
        set the T bit to 1 and set the NSG to the next stage in the
        Login Response when it finishes sending its security keys.  The
        next stage selected will be the one the target selected.  If the
        next stage is FullFeaturePhase, the target MUST reply with a
        Login Response with the TSIH value.

   If the security negotiation fails at the target, then the target MUST
   send the appropriate Login Response PDU.  If the security negotiation
   fails at the initiator, the initiator SHOULD close the connection.

   It should be noted that the negotiation might also be directed by the
   target if the initiator does support security but is not ready to
   direct the negotiation (propose options); see Appendix B for an
   example.

6.3.3. Operational Parameter Negotiation during the Login Phase

   Operational parameter negotiation during the Login Phase MAY be done:

      - starting with the first Login Request if the initiator does not
        propose any security/integrity option.

      - starting immediately after the security negotiation if the
        initiator and target perform such a negotiation.

   Operational parameter negotiation MAY involve several Login Request-
   Login Response exchanges started and terminated by the initiator.
   The initiator MUST indicate its intent to terminate the negotiation
   by setting the T bit to 1; the target sets the T bit to 1 on the last
   response.

   Even when the initiator indicates its intent to switch stages by
   setting the T bit to 1 in a Login Request, the target MAY respond
   with a Login Response with the T bit set to 0.  In that case, the
   initiator SHOULD continue to set the T bit to 1 in subsequent Login
   Requests (even empty requests) that it sends, until the target sends
   a Login Response with the T bit set to 1 or sends a key that requires
   the initiator to set the T bit to 0.

   Some session-specific parameters can only be specified during the
   Login Phase of the first connection of a session (i.e., begun by a
   Login Request that contains a zero-valued TSIH) -- the leading Login
   Phase (e.g., the maximum number of connections that can be used for
   this session).

   A session is operational once it has at least one connection in the
   Full Feature Phase.  New or replacement connections can only be added
   to a session after the session is operational.

   For operational parameters, see Section 13.

6.3.4. Connection Reinstatement

   Connection reinstatement is the process of an initiator logging in
   with an ISID-TSIH-CID combination that is possibly active from the
   target's perspective, which causes the implicit logging out of the
   connection corresponding to the CID and reinstatement of a new Full
   Feature Phase iSCSI connection in its place (with the same CID).
   Thus, the TSIH in the Login Request PDU MUST be non-zero, and the CID
   does not change during a connection reinstatement.  The Login Request
   performs the logout function of the old connection if an explicit
   logout was not performed earlier.  In sessions with a single
   connection, this may imply the opening of a second connection with
   the sole purpose of cleaning up the first.  Targets MUST support
   opening a second connection even when they do not support multiple
   connections in the Full Feature Phase if ErrorRecoveryLevel is 2 and
   SHOULD support opening a second connection if ErrorRecoveryLevel is
   less than 2.

   If the operational ErrorRecoveryLevel is 2, connection reinstatement
   enables future task reassignment.  If the operational
   ErrorRecoveryLevel is less than 2, connection reinstatement is the

   replacement of the old CID without enabling task reassignment.  In
   this case, all the tasks that were active on the old CID must be
   immediately terminated without further notice to the initiator.

   The initiator connection state MUST be CLEANUP_WAIT (Section 8.1.3)
   when the initiator attempts a connection reinstatement.

   In practical terms, in addition to the implicit logout of the old
   connection, reinstatement is equivalent to a new connection login.

6.3.5. Session Reinstatement, Closure, and Timeout

   Session reinstatement is the process of an initiator logging in with
   an ISID that is possibly active from the target's perspective for
   that initiator, thus implicitly logging out the session that
   corresponds to the ISID and reinstating a new iSCSI session in its
   place (with the same ISID).  Therefore, the TSIH in the Login PDU
   MUST be zero to signal session reinstatement.  Session reinstatement
   causes all the tasks that were active on the old session to be
   immediately terminated by the target without further notice to the
   initiator.

   The initiator session state MUST be FAILED (Section 8.3) when the
   initiator attempts a session reinstatement.

   Session closure is an event defined to be one of the following:

      - a successful "session close" logout.

      - a successful "connection close" logout for the last Full Feature
        Phase connection when no other connection in the session is
        waiting for cleanup (Section 8.2) and no tasks in the session
        are waiting for reassignment.

   Session timeout is an event defined to occur when the last connection
   state timeout expires and no tasks are waiting for reassignment.
   This takes the session to the FREE state (see the session state
   diagrams in Section 8.3).

6.3.5.1. Loss of Nexus Notification

   The iSCSI layer provides the SCSI layer with the "I_T nexus loss"
   notification when any one of the following events happens:

      - successful completion of session reinstatement

      - session closure event

      - session timeout event

   Certain SCSI object clearing actions may result due to the
   notification in the SCSI end nodes, as documented in Appendix E.

6.3.6. Session Continuation and Failure

   Session continuation is the process by which the state of a
   preexisting session continues to be used by connection reinstatement
   (Section 6.3.4) or by adding a connection with a new CID.  Either of
   these actions associates the new transport connection with the
   session state.

   Session failure is an event where the last Full Feature Phase
   connection reaches the CLEANUP_WAIT state (Section 8.2) or completes
   a successful recovery logout, thus causing all active tasks (that are
   formerly allegiant to the connection) to start waiting for task
   reassignment.

6.4. Operational Parameter Negotiation outside the Login Phase

   Some operational parameters MAY be negotiated outside (after) the
   Login Phase.

   Parameter negotiation in the Full Feature Phase is done through Text
   Requests and Responses.  Operational parameter negotiation MAY
   involve several Text Request-Text Response exchanges, all of which
   use the same Initiator Task Tag; the initiator always starts and
   terminates each of these exchanges.  The initiator MUST indicate its
   intent to finish the negotiation by setting the F bit to 1; the
   target sets the F bit to 1 on the last response.

   If the target responds to a Text Request with the F bit set to 1 with
   a Text Response with the F bit set to 0, the initiator should keep
   sending the Text Request (even empty requests) with the F bit set to
   1 while it still wants to finish the negotiation, until it receives
   the Text Response with the F bit set to 1.  Responding to a Text
   Request with the F bit set to 1 with an empty (no key=value pairs)
   response with the F bit set to 0 is discouraged.

   Even when the initiator indicates its intent to finish the
   negotiation by setting the F bit to 1 in a Text Request, the target
   MAY respond with a Text Response with the F bit set to 0.  In that
   case, the initiator SHOULD continue to set the F bit to 1 in
   subsequent Text Requests (even empty requests) that it sends, until
   the target sends the final Text Response with the F bit set to 1.
   Note that in the same case of a Text Request with the F bit set to 1,
   the target SHOULD NOT respond with an empty (no key=value pairs) Text
   Response with the F bit set to 0, because such a response may cause
   the initiator to abandon the negotiation.

   Targets MUST NOT submit parameters that require an additional
   initiator Text Request in a Text Response with the F bit set to 1.

   In a negotiation sequence, the F bit settings in one Text Request-
   Text Response pair have no bearing on the F bit settings of the next
   pair.  An initiator that has the F bit set to 1 in a request and is
   being answered with an F bit setting of 0 may issue the next request
   with the F bit set to 0.

   Whenever the target responds with the F bit set to 0, it MUST set the
   Target Transfer Tag to a value other than the default 0xffffffff.

   An initiator MAY reset an operational parameter negotiation by
   issuing a Text Request with the Target Transfer Tag set to the value
   0xffffffff after receiving a response with the Target Transfer Tag
   set to a value other than 0xffffffff.  A target may reset an
   operational parameter negotiation by answering a Text Request with a
   Reject PDU.

   Neither the initiator nor the target should attempt to declare or
   negotiate a parameter more than once during any negotiation sequence,
   except for responses to specific keys that explicitly allow repeated
   key declarations (e.g., TargetAddress).  If such an attempt is
   detected by the target, the target MUST respond with a Reject PDU
   with a reason of "Protocol Error".  The initiator MUST reset the
   negotiation as outlined above.

   Parameters negotiated by a text exchange negotiation sequence only
   become effective after the negotiation sequence is completed.

7. iSCSI Error Handling and Recovery

7.1. Overview

7.1.1. Background

   The following two considerations prompted the design of much of the
   error recovery functionality in iSCSI:

      - An iSCSI PDU may fail the digest check and be dropped, despite
        being received by the TCP layer.  The iSCSI layer must
        optionally be allowed to recover such dropped PDUs.

      - A TCP connection may fail at any time during the data transfer.
        All the active tasks must optionally be allowed to be continued
        on a different TCP connection within the same session.

   Implementations have considerable flexibility in deciding what degree
   of error recovery to support, when to use it, and by which mechanisms
   to achieve the required behavior.  Only the externally visible
   actions of the error recovery mechanisms must be standardized to
   ensure interoperability.

   This section describes a general model for recovery in support of
   interoperability.  See Appendix D for further details on how the
   described model may be implemented.  Compliant implementations do not
   have to match the implementation details of this model as presented,
   but the external behavior of such implementations must correspond to
   the externally observable characteristics of the presented model.

7.1.2. Goals

   The major design goals of the iSCSI error recovery scheme are as
   follows:

      - Allow iSCSI implementations to meet different requirements by
        defining a collection of error recovery mechanisms from which
        implementations may choose.

      - Ensure interoperability between any two implementations
        supporting different sets of error recovery capabilities.

      - Define the error recovery mechanisms to ensure command ordering
        even in the face of errors, for initiators that demand ordering.

      - Do not make additions in the fast path, but allow moderate
        complexity in the error recovery path.

      - Prevent both the initiator and target from attempting to recover
        the same set of PDUs at the same time.  For example, there must
        be a clear "error recovery functionality distribution" between
        the initiator and target.

7.1.3. Protocol Features and State Expectations

   The initiator mechanisms defined in connection with error recovery
   are:

      a) NOP-Out to probe sequence numbers of the target (Section 11.18)

      b) Command retry (Section 7.2.1)

      c) Recovery R2T support (Section 7.8)

      d) Requesting retransmission of status/data/R2T using the SNACK
         facility (Section 11.16)

      e) Acknowledging the receipt of the data (Section 11.16)

      f) Reassigning the connection allegiance of a task to a different
         TCP connection (Section 7.2.2)

      g) Terminating the entire iSCSI session to start afresh
         (Section 7.1.4.4)

   The target mechanisms defined in connection with error recovery are:

      a) NOP-In to probe sequence numbers of the initiator
         (Section 11.19)

      b) Requesting retransmission of data using the recovery R2T
         feature (Section 7.8)

      c) SNACK support (Section 11.16)

      d) Requesting that parts of read data be acknowledged
         (Section 11.7.2)

      e) Allegiance reassignment support (Section 7.2.2)

      f) Terminating the entire iSCSI session to force the initiator to
         start over (Section 7.1.4.4)

   For any outstanding SCSI command, it is assumed that iSCSI, in
   conjunction with SCSI at the initiator, is able to keep enough
   information to be able to rebuild the command PDU and that outgoing

   data is available (in host memory) for retransmission while the
   command is outstanding.  It is also assumed that at the target,
   incoming data (read data) MAY be kept for recovery, or it can be
   reread from a device server.

   It is further assumed that a target will keep the "status and sense"
   for a command it has executed if it supports status retransmission.

   A target that agrees to support data retransmission is expected to be
   prepared to retransmit the outgoing data (i.e., Data-In) on request
   until either the status for the completed command is acknowledged or
   the data in question has been separately acknowledged.

7.1.4. Recovery Classes

   iSCSI enables the following classes of recovery (in the order of
   increasing scope of affected iSCSI tasks):

      - within a command (i.e., without requiring command restart)

      - within a connection (i.e., without requiring the connection to
        be rebuilt, but perhaps requiring command restart)

      - connection recovery (i.e., perhaps requiring connections to be
        rebuilt and commands to be reissued)

      - session recovery

   The recovery scenarios detailed in the rest of this section are
   representative rather than exclusive.  In every case, they detail the
   lowest recovery class that MAY be attempted.  The implementer is left
   to decide under which circumstances to escalate to the next recovery
   class and/or what recovery classes to implement.  Both the iSCSI
   target and initiator MAY escalate the error handling to an error
   recovery class, which impacts a larger number of iSCSI tasks in any
   of the cases identified in the following discussion.

   In all classes, the implementer has the choice of deferring errors to
   the SCSI initiator (with an appropriate response code), in which case
   the task, if any, has to be removed from the target and all the side
   effects, such as ACA, must be considered.

   The use of within-connection and within-command recovery classes MUST
   NOT be attempted before the connection is in the Full Feature Phase.

   In the detailed description of the recovery classes, the mandating
   terms (MUST, SHOULD, MAY, etc.) indicate normative actions to be
   executed if the recovery class is supported (see Section 7.1.5 for
   the related negotiation semantics) and used.

7.1.4.1. Recovery Within-command

   At the target, the following cases lend themselves to within-command
   recovery:

      Lost data PDU - realized through one of the following:

      a) Data digest error - dealt with as specified in Section 7.8,
         using the option of a recovery R2T

      b) Sequence reception timeout (no data or partial-data-and-no-
         F-bit) - considered an implicit sequence error and dealt with
         as specified in Section 7.9, using the option of a recovery R2T

      c) Header digest error, which manifests as a sequence reception
         timeout or a sequence error - dealt with as specified in
         Section 7.9, using the option of a recovery R2T

   At the initiator, the following cases lend themselves to within-
   command recovery:

      Lost data PDU or lost R2T - realized through one of the following:

      a) Data digest error - dealt with as specified in Section 7.8,
         using the option of a SNACK

      b) Sequence reception timeout (no status) or response reception
         timeout - dealt with as specified in Section 7.9, using the
         option of a SNACK

      c) Header digest error, which manifests as a sequence reception
         timeout or a sequence error - dealt with as specified in
         Section 7.9, using the option of a SNACK

   To avoid a race with the target, which may already have a recovery
   R2T or a termination response on its way, an initiator SHOULD NOT
   originate a SNACK for an R2T based on its internal timeouts (if any).
   Recovery in this case is better left to the target.

   The timeout values used by the initiator and target are outside the
   scope of this document.  A sequence reception timeout is generally a
   large enough value to allow the data sequence transfer to be
   complete.

7.1.4.2. Recovery Within-connection

   At the initiator, the following cases lend themselves to within-
   connection recovery:

      a) Requests not acknowledged for a long time.  Requests are
         acknowledged explicitly through the ExpCmdSN or implicitly by
         receiving data and/or status.  The initiator MAY retry
         non-acknowledged commands as specified in Section 7.2.

      b) Lost iSCSI numbered response.  It is recognized by either
         identifying a data digest error on a Response PDU or a Data-In
         PDU carrying the status, or receiving a Response PDU with a
         higher StatSN than expected.  In the first case, digest error
         handling is done as specified in Section 7.8, using the option
         of a SNACK.  In the second case, sequence error handling is
         done as specified in Section 7.9, using the option of a SNACK.

   At the target, the following cases lend themselves to within-
   connection recovery:

      - Status/Response not acknowledged for a long time.  The target
        MAY issue a NOP-In (with a valid Target Transfer Tag or
        otherwise) that carries the next status sequence number it is
        going to use in the StatSN field.  This helps the initiator
        detect any missing StatSN(s) and issue a SNACK for the status.

   The timeout values used by the initiator and the target are outside
   the scope of this document.

7.1.4.3. Connection Recovery

   At an iSCSI initiator, the following cases lend themselves to
   connection recovery:

      a) TCP connection failure: The initiator MUST close the
         connection.  It then MUST either implicitly or explicitly log
         out the failed connection with the reason code "remove the
         connection for recovery" and reassign connection allegiance for
         all commands still in progress associated with the failed
         connection on one or more connections (some or all of which MAY
         be newly established connections) using the "TASK REASSIGN"
         task management function (see Section 11.5.1).  For an
         initiator, a command is in progress as long as it has not
         received a response or a Data-In PDU including status.

         Note: The logout function is mandatory.  However, a new
         connection establishment is only mandatory if the failed
         connection was the last or only connection in the session.

      b) Receiving an Asynchronous Message that indicates that one or
         all connections in a session have been dropped.  The initiator
         MUST handle it as a TCP connection failure for the
         connection(s) referred to in the message.

   At an iSCSI target, the following cases lend themselves to connection
   recovery:

      - TCP connection failure: The target MUST close the connection
        and, if more than one connection is available, the target SHOULD
        send an Asynchronous Message that indicates that it has dropped
        the connection.  Then, the target will wait for the initiator to
        continue recovery.

7.1.4.4. Session Recovery

   Session recovery should be performed when all other recovery attempts
   have failed.  Very simple initiators and targets MAY perform session
   recovery on all iSCSI errors and rely on recovery on the SCSI layer
   and above.

   Session recovery implies the closing of all TCP connections,
   internally aborting all executing and queued tasks for the given
   initiator at the target, terminating all outstanding SCSI commands
   with an appropriate SCSI service response at the initiator, and
   restarting a session on a new set of connection(s) (TCP connection
   establishment and login on all new connections).

   For possible clearing effects of session recovery on SCSI and iSCSI
   objects, refer to Appendix E.

7.1.5. Error Recovery Hierarchy

   The error recovery classes described so far are organized into a
   hierarchy for ease in understanding and to limit the complexity of
   the implementation.  With a few well-defined recovery levels,
   interoperability is easier to achieve.  The attributes of this
   hierarchy are as follows:

      a) Each level is a superset of the capabilities of the previous
         level.  For example, Level 1 support implies supporting all
         capabilities of Level 0 and more.

b) As a corollary, supporting a higher error recovery level means
 increased sophistication and possibly an increase in resource
 requirements.

 c) Supporting error recovery level "n" is advertised and
 negotiated by each iSCSI entity by exchanging the text key
 "ErrorRecoveryLevel=n". The lower of the two exchanged values
 is the operational ErrorRecoveryLevel for the session.

 The following diagram represents the error recovery hierarchy.

 +
 / \
 / 2 \ <-- Connection recovery
 +-----+
 / 1 \ <-- Digest failure recovery
 +---------+
 / 0 \ <-- Session failure recovery
 +-------------+

 The following table lists the error recovery (ER) capabilities
 expected from the implementations that support each error recovery
 level.

 +-------------------+--------------------------------------------+
 |ErrorRecoveryLevel | Associated Error Recovery Capabilities |
 +-------------------+--------------------------------------------+
 | 0 | Session recovery class |
 | | (Session Recovery) |
 +-------------------+--------------------------------------------+
 | 1 | Digest failure recovery (see Note below) |
 | | plus the capabilities of ER Level 0 |
 +-------------------+--------------------------------------------+
 | 2 | Connection recovery class |
 | | (Connection Recovery) |
 | | plus the capabilities of ER Level 1 |
 +-------------------+--------------------------------------------+

 Note: Digest failure recovery is comprised of two recovery classes:
 the Within-connection recovery class (recovery within-connection) and
 the Within-command recovery class (recovery within-command).

 When a defined value of ErrorRecoveryLevel is proposed by an
 originator in a text negotiation, the originator MUST support the
 functionality defined for the proposed value and, additionally,
 functionality corresponding to any defined value numerically less
 than the proposed value. When a defined value of ErrorRecoveryLevel

   is returned by a responder in a text negotiation, the responder MUST
   support the functionality corresponding to the ErrorRecoveryLevel it
   is accepting.

   When either party attempts to use error recovery functionality beyond
   what is negotiated, the recovery attempts MAY fail, unless an
   a priori agreement outside the scope of this document exists between
   the two parties to provide such support.

   Implementations MUST support error recovery level "0", while the rest
   are OPTIONAL to implement.  In implementation terms, the above
   striation means that the following incremental sophistication with
   each level is required:

    +-------------------+--------------------------------------------+
    | Level Transition  | Incremental Requirement                    |
    +-------------------+--------------------------------------------+
    |        0->1       | PDU retransmissions on the same connection |
    +-------------------+--------------------------------------------+
    |        1->2       | Retransmission across connections and      |
    |                   | allegiance reassignment                    |
    +-------------------+--------------------------------------------+

7.2. Retry and Reassign in Recovery

This section summarizes two important and somewhat related iSCSI
protocol features used in error recovery.

7.2.1. Usage of Retry

   By resending the same iSCSI Command PDU ("retry") in the absence of a
   command acknowledgment (by way of an ExpCmdSN update) or a response,
   an initiator attempts to "plug" (what it thinks are) the
   discontinuities in CmdSN ordering on the target end.  Discarded
   command PDUs, due to digest errors, may have created these
   discontinuities.

   Retry MUST NOT be used for reasons other than plugging command
   sequence gaps and, in particular, cannot be used for requesting PDU
   retransmissions from a target.  Any such PDU retransmission requests
   for a currently allegiant command in progress may be made using the
   SNACK mechanism described in Section 11.16, although the usage of
   SNACK is OPTIONAL.

   If initiators, as part of plugging command sequence gaps as described
   above, inadvertently issue retries for allegiant commands already in
   progress (i.e., targets did not see the discontinuities in CmdSN
   ordering), the duplicate commands are silently ignored by targets as
   specified in Section 4.2.2.1.

   When an iSCSI command is retried, the command PDU MUST carry the
   original Initiator Task Tag and the original operational attributes
   (e.g., flags, function names, LUN, CDB, etc.) as well as the original
   CmdSN.  The command being retried MUST be sent on the same connection
   as the original command, unless the original connection was already
   successfully logged out.

7.2.2. Allegiance Reassignment

   By issuing a "TASK REASSIGN" task management request
   (Section 11.5.1), the initiator signals its intent to continue an
   already active command (but with no current connection allegiance) as
   part of connection recovery.  This means that a new connection
   allegiance is requested for the command, which seeks to associate it
   to the connection on which the task management request is being
   issued.  Before the allegiance reassignment is attempted for a task,
   an implicit or explicit Logout with the reason code "remove the
   connection for recovery" (see Section 11.14.1) MUST be successfully
   completed for the previous connection to which the task was
   allegiant.

   In reassigning connection allegiance for a command, the target SHOULD
   continue the command from its current state.  For example, when
   reassigning read commands, the target SHOULD take advantage of the
   ExpDataSN field provided by the Task Management Function Request
   (which must be set to 0 if there was no data transfer) and bring the
   read command to completion by sending the remaining data and sending
   (or resending) the status.  The ExpDataSN acknowledges all data sent
   up to, but not including, the Data-In PDU and/or R2T with the DataSN
   (or R2TSN) equal to the ExpDataSN.  However, targets may choose to
   send/receive all unacknowledged data or all of the data on a
   reassignment of connection allegiance if unable to recover or
   maintain accurate state.  Initiators MUST NOT subsequently request
   data retransmission through Data SNACK for PDUs numbered less than
   the ExpDataSN (i.e., prior to the acknowledged sequence number).  For
   all types of commands, a reassignment request implies that the task
   is still considered in progress by the initiator, and the target must
   conclude the task appropriately if the target returns the "Function
   complete" response to the reassignment request.  This might possibly
   involve retransmission of data/R2T/status PDUs as necessary but MUST
   involve the (re)transmission of the status PDU.

   It is OPTIONAL for targets to support the allegiance reassignment.
   This capability is negotiated via the ErrorRecoveryLevel text key
   during the login time.  When a target does not support allegiance
   reassignment, it MUST respond with a task management response code of
   "Task allegiance reassignment not supported".  If allegiance
   reassignment is supported by the target but the task is still
   allegiant to a different connection, or a successful recovery Logout
   of the previously allegiant connection was not performed, the target
   MUST respond with a task management response code of "Task still
   allegiant".

   If allegiance reassignment is supported by the target, the task
   management response to the reassignment request MUST be issued before
   the reassignment becomes effective.

   If a SCSI command that involves data input is reassigned, any SNACK
   Tag it holds for a final response from the original connection is
   deleted, and the default value of 0 MUST be used instead.

7.3. Usage of Reject PDU in Recovery

   Targets MUST NOT implicitly terminate an active task by sending a
   Reject PDU for any PDU exchanged during the life of the task.  If the
   target decides to terminate the task, a Response PDU (SCSI, Text,
   Task, etc.) must be returned by the target to conclude the task.  If
   the task had never been active before the Reject (i.e., the Reject is
   on the command PDU), targets should not send any further responses
   because the command itself is being discarded.

   The above rule means that the initiator can eventually expect a
   response on receiving Rejects, if the received Reject is for a PDU
   other than the command PDU itself.  The non-command Rejects only have
   diagnostic value in logging the errors, and they can be used for
   retransmission decisions by the initiators.

   The CmdSN of the rejected command PDU (if it is a non-immediate
   command) MUST NOT be considered received by the target (i.e., a
   command sequence gap must be assumed for the CmdSN), even though the
   CmdSN of the rejected command PDU may be reliably ascertained.  Upon
   receiving the Reject, the initiator MUST plug the CmdSN gap in order
   to continue to use the session.  The gap may be plugged by either
   transmitting a command PDU with the same CmdSN or aborting the task
   (see Section 7.11 for information regarding how an abort may plug a
   CmdSN gap).

   When a data PDU is rejected and its DataSN can be ascertained, a
   target MUST advance the ExpDataSN for the current data burst if a
   recovery R2T is being generated.  The target MAY advance its
   ExpDataSN if it does not attempt to recover the lost data PDU.

7.4. Error Recovery Considerations for Discovery Sessions

7.4.1. ErrorRecoveryLevel for Discovery Sessions

   The negotiation of the key ErrorRecoveryLevel is not required for
   Discovery sessions -- i.e., for sessions that negotiated
   "SessionType=Discovery" -- because the default value of 0 is
   necessary and sufficient for Discovery sessions.  It is, however,
   possible that some legacy iSCSI implementations might attempt to
   negotiate the ErrorRecoveryLevel key on Discovery sessions.  When
   such a negotiation attempt is made by the remote side, a compliant
   iSCSI implementation MUST propose a value of 0 (zero) in response.
   The operational ErrorRecoveryLevel for Discovery sessions thus MUST
   be 0.  This naturally follows from the functionality constraints that
   Section 4.3 imposes on Discovery sessions.

7.4.2. Reinstatement Semantics for Discovery Sessions

Discovery sessions are intended to be relatively short-lived.
 Initiators are not expected to establish multiple Discovery sessions
 to the same iSCSI Network Portal. An initiator may use the same
 iSCSI Initiator Name and ISID when establishing different unique
 sessions with different targets and/or different portal groups. This
 behavior is discussed in Section 10.1.1 and is, in fact, encouraged
 as conservative reuse of ISIDs.

 The ISID RULE in Section 4.4.3 states that there must not be more
 than one session with a matching 4-tuple: <InitiatorName, ISID,
 TargetName, TargetPortalGroupTag>. While the spirit of the ISID RULE
 applies to Discovery sessions the same as it does for Normal
 sessions, note that some Discovery sessions differ from the Normal
 sessions in two important aspects:

 a) Because Appendix C allows a Discovery session to be established
 without specifying a TargetName key in the Login Request PDU
 (let us call such a session an "Unnamed" Discovery session),
 there is no target node context to enforce the ISID RULE.

 b) Portal groups are defined only in the context of a target node.
 When the TargetName key is NULL-valued (i.e., not specified),
 the TargetPortalGroupTag thus cannot be ascertained to enforce
 the ISID RULE.

The following two sections describe Unnamed Discovery sessions and
Named Discovery sessions, respectively.

7.4.2.1. Unnamed Discovery Sessions

For Unnamed Discovery sessions, neither the TargetName nor the
 TargetPortalGroupTag is available to the targets in order to enforce
 the ISID RULE. Therefore, the following rule applies.

 UNNAMED ISID RULE: Targets MUST enforce the uniqueness of the
 following 4-tuple for Unnamed Discovery sessions: <InitiatorName,
 ISID, NULL, TargetAddress>. The following semantics are implied by
 this uniqueness requirement.

 Targets SHOULD allow concurrent establishment of one Discovery
 session with each of its Network Portals by the same initiator port
 with a given iSCSI Node Name and an ISID. Each of the concurrent
 Discovery sessions, if established by the same initiator port to
 other Network Portals, MUST be treated as independent sessions --
 i.e., one session MUST NOT reinstate the other.

 A new Unnamed Discovery session that has a matching <InitiatorName,
 ISID, NULL, TargetAddress> to an existing Discovery session MUST
 reinstate the existing Unnamed Discovery session. Note thus that
 only an Unnamed Discovery session may reinstate another Unnamed
 Discovery session.

7.4.2.2. Named Discovery Sessions

For Named Discovery sessions, the TargetName key is specified by the
 initiator, and thus the target can unambiguously ascertain the
 TargetPortalGroupTag as well. Since all the four elements of the
 4-tuple are known, the ISID RULE MUST be enforced by targets with no
 changes from Section 4.4.3 semantics. A new session with a matching
 <InitiatorName, ISID, TargetName, TargetPortalGroupTag> thus will
 reinstate an existing session. Note in this case that any new iSCSI
 session (Discovery or Normal) with the matching 4-tuple may reinstate
 an existing Named Discovery iSCSI session.

7.4.3. Target PDUs during Discovery

   Targets SHOULD NOT send any responses other than a Text Response and
   Logout Response on a Discovery session, once in the Full Feature
   Phase.

   Implementation Note: A target may simply drop the connection in a
   Discovery session when it would have requested a Logout via an Async
   Message on Normal sessions.

7.5. Connection Timeout Management

   iSCSI defines two session-global timeout values (in seconds) --
   Time2Wait and Time2Retain -- that are applicable when an iSCSI Full
   Feature Phase connection is taken out of service either intentionally
   or by an exception.  Time2Wait is the initial "respite time" before
   attempting an explicit/implicit Logout for the CID in question or
   task reassignment for the affected tasks (if any).  Time2Retain is
   the maximum time after the initial respite interval that the task
   and/or connection state(s) is/are guaranteed to be maintained on the
   target to cater to a possible recovery attempt.  Recovery attempts
   for the connection and/or task(s) SHOULD NOT be made before
   Time2Wait seconds but MUST be completed within Time2Retain seconds
   after that initial Time2Wait waiting period.

7.5.1. Timeouts on Transport Exception Events

   A transport connection shutdown or a transport reset without any
   preceding iSCSI protocol interactions informing the endpoints of the
   fact causes a Full Feature Phase iSCSI connection to be abruptly
   terminated.  The timeout values to be used in this case are the
   negotiated values of DefaultTime2Wait (Section 13.15) and
   DefaultTime2Retain (Section 13.16) text keys for the session.

7.5.2. Timeouts on Planned Decommissioning

   Any planned decommissioning of a Full Feature Phase iSCSI connection
   is preceded by either a Logout Response PDU or an Async Message PDU.
   The Time2Wait and Time2Retain field values (Section 11.15) in a
   Logout Response PDU, and the Parameter2 and Parameter3 fields of an
   Async Message (AsyncEvent types "drop the connection" or "drop all
   the connections"; see Section 11.9.1), specify the timeout values to
   be used in each of these cases.

   These timeout values are only applicable for the affected connection
   and the tasks active on that connection.  These timeout values have
   no bearing on initiator timers (if any) that are already running on
   connections or tasks associated with that session.

7.6. Implicit Termination of Tasks

   A target implicitly terminates the active tasks due to iSCSI protocol
   dynamics in the following cases:

      a) When a connection is implicitly or explicitly logged out with
         the reason code "close the connection" and there are active
         tasks allegiant to that connection.

      b) When a connection fails and eventually the connection state
         times out (state transition M1 in Section 8.2.2), and there are
         active tasks allegiant to that connection.

      c) When a successful Logout with the reason code "remove the
         connection for recovery" is performed while there are active
         tasks allegiant to that connection, and those tasks eventually
         time out after the Time2Wait and Time2Retain periods without
         allegiance reassignment.

      d) When a connection is implicitly or explicitly logged out with
         the reason code "close the session" and there are active tasks
         in that session.

   If the tasks terminated in cases a), b), c), and d) above are SCSI
   tasks, they must be internally terminated as if with CHECK CONDITION
   status.  This status is only meaningful for appropriately handling
   the internal SCSI state and SCSI side effects with respect to
   ordering, because this status is never communicated back as a
   terminating status to the initiator.  However, additional actions may
   have to be taken at the SCSI level, depending on the SCSI context as
   defined by the SCSI standards (e.g., queued commands and ACA; UA for
   the next command on the I_T nexus in cases a), b), and c); etc. --
   see [SAM2] and [SPC3]).

7.7. Format Errors

   The following two explicit violations of PDU layout rules are format
   errors:

      a) Illegal contents of any PDU header field except the Opcode
         (legal values are specified in Section 11).

      b) Inconsistent field contents (consistent field contents are
         specified in Section 11).

   Format errors indicate a major implementation flaw in one of the
   parties.

   When a target or an initiator receives an iSCSI PDU with a format
   error, it MUST immediately terminate all transport connections in the
   session with either a connection close or a connection reset, and
   escalate the format error to session recovery (see Section 7.1.4.4).

   All initiator-detected PDU construction errors MUST be considered as
   format errors.  Some examples of such errors are:

      - NOP-In with a valid TTT but an invalid LUN

      - NOP-In with a valid ITT (i.e., a NOP-In response) and also a
        valid TTT

      - SCSI Response PDU with Status=CHECK CONDITION, but
        DataSegmentLength = 0

7.8. Digest Errors

   The discussion below regarding the legal choices in handling digest
   errors excludes session recovery as an explicit option, but either
   party detecting a digest error may choose to escalate the error to
   session recovery.

   When a target or an initiator receives any iSCSI PDU with a header
   digest error, it MUST either discard the header and all data up to
   the beginning of a later PDU or close the connection.  Because the
   digest error indicates that the length field of the header may have
   been corrupted, the location of the beginning of a later PDU needs to
   be reliably ascertained by other means, such as the operation of a
   Sync and Steering layer.

   When a target receives any iSCSI PDU with a payload digest error, it
   MUST answer with a Reject PDU with a reason code of Data-Digest-Error
   and discard the PDU.

   - If the discarded PDU is a solicited or unsolicited iSCSI data PDU
     (for immediate data in a command PDU, the non-data PDU rule below
     applies), the target MUST do one of the following:

     a) Request retransmission with a recovery R2T.

     b) Terminate the task with a SCSI Response PDU with a CHECK
        CONDITION Status and an iSCSI Condition of "Protocol Service CRC
        error" (Section 11.4.7.2).  If the target chooses to implement
        this option, it MUST wait to receive all the data (signaled by a
        data PDU with the Final bit set for all outstanding R2Ts) before
        sending the SCSI Response PDU.  A task management command (such
        as an ABORT TASK) from the initiator during this wait may also
        conclude the task.

   - No further action is necessary for targets if the discarded PDU is
     a non-data PDU.  In the case of immediate data being present on a
     discarded command, the immediate data is implicitly recovered when
     the task is retried (see Section 7.2.1), followed by the entire
     data transfer for the task.

   When an initiator receives any iSCSI PDU with a payload digest error,
   it MUST discard the PDU.

      - If the discarded PDU is an iSCSI data PDU, the initiator MUST do
        one of the following:

        a) Request the desired data PDU through SNACK.  In response to
           the SNACK, the target MUST either resend the data PDU or
           reject the SNACK with a Reject PDU with a reason code of
           "SNACK reject", in which case:

           a.1) If the status has not already been sent for the command,
                the target MUST terminate the command with a CHECK
                CONDITION Status and an iSCSI Condition of "SNACK
                rejected" (Section 11.4.7.2).

           a.2) If the status was already sent, no further action is
                necessary for the target.  The initiator in this case
                MUST wait for the status to be received and then discard
                it, so as to internally signal the completion with CHECK
                CONDITION Status and an iSCSI Condition of "Protocol
                Service CRC error" (Section 11.4.7.2).

        b) Abort the task and terminate the command with an error.

      - If the discarded PDU is a response PDU or an unsolicited PDU
        (e.g., Async, Reject), the initiator MUST do one of the
        following:

        a) Request PDU retransmission with a status of SNACK.

        b) Log out the connection for recovery, and continue the tasks
           on a different connection instance as described in
           Section 7.2.

        c) Log out to close the connection (abort all the commands
           associated with the connection).

      Note that an unsolicited PDU carries the next StatSN value on an
      iSCSI connection, thereby advancing the StatSN.  When an initiator
      discards one of these PDUs due to a payload digest error, the
      entire PDU, including the header, MUST be discarded.
      Consequently, the initiator MUST treat the exception like a loss
      of any other solicited response PDU.

7.9. Sequence Errors

   When an initiator receives an iSCSI R2T/data PDU with an out-of-order
   R2TSN/DataSN or a SCSI Response PDU with an ExpDataSN that implies
   missing data PDU(s), it means that the initiator must have detected a
   header or payload digest error on one or more earlier R2T/data PDUs.

   The initiator MUST address these implied digest errors as described
   in Section 7.8.  When a target receives a data PDU with an out-of-
   order DataSN, it means that the target must have hit a header or
   payload digest error on at least one of the earlier data PDUs.  The
   target MUST address these implied digest errors as described in
   Section 7.8.

   When an initiator receives an iSCSI status PDU with an out-of-order
   StatSN that implies missing responses, it MUST address the one or
   more missing status PDUs as described in Section 7.8.  As a side
   effect of receiving the missing responses, the initiator may discover
   missing data PDUs.  If the initiator wants to recover the missing
   data for a command, it MUST NOT acknowledge the received responses
   that start from the StatSN of the relevant command until it has
   completed receiving all the data PDUs of the command.

   When an initiator receives duplicate R2TSNs (due to proactive
   retransmission of R2Ts by the target) or duplicate DataSNs (due to
   proactive SNACKs by the initiator), it MUST discard the duplicates.

7.10. Message Error Checking

   In iSCSI implementations to date, there has been some uncertainty
   regarding the extent to which incoming messages have to be checked
   for protocol errors, beyond what is strictly required for processing
   the inbound message.  This section addresses this question.

   Unless this document requires it, an iSCSI implementation is not
   required to do an exhaustive protocol conformance check on an
   incoming iSCSI PDU.  The iSCSI implementation in particular is not
   required to double-check the remote iSCSI implementation's
   conformance to protocol requirements.

7.11. SCSI Timeouts

   An iSCSI initiator MAY attempt to plug a command sequence gap on the
   target end (in the absence of an acknowledgment of the command by way
   of the ExpCmdSN) before the ULP timeout by retrying the
   unacknowledged command, as described in Section 7.2.

   On a ULP timeout for a command (that carried a CmdSN of n), if the
   iSCSI initiator intends to continue the session it MUST abort the
   command by using either an appropriate Task Management Function
   Request for the specific command or a "close the connection" logout.

   When using an ABORT TASK, if the ExpCmdSN is still less than (n + 1),
   the target may see the abort request while missing the original
   command itself, due to one of the following reasons:

      - The original command was dropped due to digest error.

      - The connection on which the original command was sent was
        successfully logged out.  On logout, the unacknowledged commands
        issued on the connection being logged out are discarded.

   If the abort request is received and the original command is missing,
   targets MUST consider the original command with that RefCmdSN as
   received and issue a task management response with the response code
   "Function complete".  This response concludes the task on both ends.
   If the abort request is received and the target can determine (based
   on the Referenced Task Tag) that the command was received and
   executed, and also that the response was sent prior to the abort,
   then the target MUST respond with the response code "Task Does Not
   Exist".

7.12. Negotiation Failures

   Text Request and Response sequences, when used to set/negotiate
   operational parameters, constitute the negotiation/parameter setting.
   A negotiation failure is considered to be one or more of the
   following:

      - For a negotiated key, none of the choices are acceptable to one
        of the sides in the negotiation.

      - For a declarative key, the declared value is not acceptable to
        the other side in the negotiation.

      - The Text Request timed out and possibly terminated.

      - The Text Request was answered with a Reject PDU.

   The following two rules should be used to address negotiation
   failures:

      a) During login, any failure in negotiation MUST be considered a
         login process failure; the Login Phase, along with the
         connection, MUST be terminated.  If the target detects the
         failure, it must terminate the login with the appropriate Login
         response code.

      b) A failure in negotiation during the Full Feature Phase will
         terminate the entire negotiation sequence, which may consist of
         a series of Text Requests that use the same Initiator Task Tag.
         The operational parameters of the session or the connection
         MUST continue to be the values agreed upon during an earlier
         successful negotiation (i.e., any partial results of this
         unsuccessful negotiation MUST NOT take effect and MUST be
         discarded).

7.13. Protocol Errors

   Mapping framed messages over a "streaming" connection such as TCP
   makes the proposed mechanisms vulnerable to simple software framing
   errors.  On the other hand, the introduction of framing mechanisms to
   limit the effects of these errors may be onerous on performance for
   simple implementations.  Command sequence numbers and the mechanisms
   for dropping and reestablishing connections (discussed earlier in
   Section 7 and its subsections) help handle this type of mapping
   errors.

   All violations of iSCSI PDU exchange sequences specified in this
   document are also protocol errors.  This category of errors can only
   be addressed by fixing the implementations; iSCSI defines Reject and
   response codes to enable this.

7.14. Connection Failures

   iSCSI can keep a session in operation if it is able to keep/establish
   at least one TCP connection between the initiator and the target in a
   timely fashion.  Targets and/or initiators may recognize a failing
   connection by either transport-level means (TCP), a gap in the
   command sequence number, a response stream that is not filled for a
   long time, or a failing iSCSI NOP (acting as a ping).  The latter MAY
   be used periodically to increase the speed and likelihood of
   detecting connection failures.  As an example for transport-level
   means, initiators and targets MAY also use the keep-alive option (see
   [RFC1122]) on the TCP connection to enable early link failure
   detection on otherwise idle links.

   On connection failure, the initiator and target MUST do one of the
   following:

      a) Attempt connection recovery within the session (Connection
         Recovery).

      b) Log out the connection with the reason code "close the
         connection" (Section 11.14.5), reissue missing commands, and
         implicitly terminate all active commands.  This option requires
         support for the Within-connection recovery class (recovery
         within-connection).

      c) Perform session recovery (Session Recovery).

   Either side may choose to escalate to session recovery (via the
   initiator dropping all the connections or via an Async Message that
   announces the similar intent from a target), and the other side MUST
   give it precedence.  On a connection failure, a target MUST terminate
   and/or discard all of the active immediate commands, regardless of
   which of the above options is used (i.e., immediate commands are not
   recoverable across connection failures).

7.15. Session Errors

   If all of the connections of a session fail and cannot be
   reestablished in a short time, or if initiators detect protocol
   errors repeatedly, an initiator may choose to terminate a session and
   establish a new session.

   In this case, the initiator takes the following actions:

      - Resets or closes all the transport connections.

      - Terminates all outstanding requests with an appropriate response
        before initiating a new session.  If the same I_T nexus is
        intended to be reestablished, the initiator MUST employ session
        reinstatement (see Section 6.3.5).

   When the session timeout (the connection state timeout for the last
   failed connection) happens on the target, it takes the following
   actions:

      - Resets or closes the TCP connections (closes the session).

      - Terminates all active tasks that were allegiant to the
        connection(s) that constituted the session.

A target MUST also be prepared to handle a session reinstatement
request from the initiator that may be addressing session errors.

8. State Transitions

   iSCSI connections and iSCSI sessions go through several well-defined
   states from the time they are created to the time they are cleared.

   The connection state transitions are described in two separate but
   dependent sets of state diagrams for ease in understanding.  The
   first set of diagrams, "standard connection state diagrams",
   describes the connection state transitions when the iSCSI connection
   is not waiting for, or undergoing, a cleanup by way of an explicit or
   implicit logout.  The second set, "connection cleanup state diagram",
   describes the connection state transitions while performing the iSCSI
   connection cleanup.  While the first set has two diagrams -- one each
   for initiator and target -- the second set has a single diagram
   applicable to both initiators and targets.

   The "session state diagram" describes the state transitions an iSCSI
   session would go through during its lifetime, and it depends on the
   states of possibly multiple iSCSI connections that participate in the
   session.

   States and transitions are described in text, tables, and diagrams.
   The diagrams are used for illustration.  The text and the tables are
   the governing specification.

8.1. Standard Connection State Diagrams

8.1.1. State Descriptions for Initiators and Targets

   State descriptions for the standard connection state diagram are as
   follows:

   S1: FREE

       - initiator: State on instantiation, or after successful
         connection closure.

       - target: State on instantiation, or after successful
         connection closure.

   S2: XPT_WAIT

       - initiator: Waiting for a response to its transport
         connection establishment request.

       - target: Illegal.

   S3: XPT_UP

       - initiator: Illegal.

       - target: Waiting for the login process to commence.

   S4: IN_LOGIN

       - initiator: Waiting for the login process to conclude,
         possibly involving several PDU exchanges.

       - target: Waiting for the login process to conclude,
         possibly involving several PDU exchanges.

   S5: LOGGED_IN

       - initiator: In the Full Feature Phase, waiting for all
         internal, iSCSI, and transport events.

       - target: In the Full Feature Phase, waiting for all internal,
         iSCSI, and transport events.

   S6: IN_LOGOUT

       - initiator: Waiting for a Logout Response.

       - target: Waiting for an internal event signaling completion
         of logout processing.

   S7: LOGOUT_REQUESTED

       - initiator: Waiting for an internal event signaling
         readiness to proceed with Logout.

       - target: Waiting for the Logout process to start after
         having requested a Logout via an Async Message.

   S8: CLEANUP_WAIT

       - initiator: Waiting for the context and/or resources to
         initiate the cleanup processing for this CSM.

       - target: Waiting for the cleanup process to start for this CSM.

8.1.2. State Transition Descriptions for Initiators and Targets

   T1:

       - initiator: Transport connect request was made (e.g., TCP SYN
         sent).

       - target: Illegal.

   T2:

       - initiator: Transport connection request timed out, a
         transport reset was received, or an internal event of
         receiving a Logout Response (success) on another connection
         for a "close the session" Logout Request was received.

       - target: Illegal.

   T3:

       - initiator: Illegal.

       - target: Received a valid transport connection request that
         establishes the transport connection.

   T4:

       - initiator: Transport connection established, thus
         prompting the initiator to start the iSCSI Login.

       - target: Initial iSCSI Login Request was received.

   T5:

       - initiator: The final iSCSI Login Response with a Status-Class
         of zero was received.

       - target: The final iSCSI Login Request to conclude the
         Login Phase was received, thus prompting the target to send
         the final iSCSI Login Response with a Status-Class of zero.

   T6:

       - initiator: Illegal.

       - target: Timed out waiting for an iSCSI Login, transport
         disconnect indication was received, transport reset was
         received, or an internal event indicating a transport
         timeout was received.  In all these cases, the connection is
         to be closed.

   T7:

       - initiator: One of the following events caused the transition:

         a) The final iSCSI Login Response was received with a
            non-zero Status-Class.

         b) Login timed out.

         c) A transport disconnect indication was received.

         d) A transport reset was received.

         e) An internal event indicating a transport timeout was
            received.

         f) An internal event of receiving a Logout Response
            (success) on another connection for a "close the
            session" Logout Request was received.

       In all these cases, the transport connection is closed.

       - target: One of the following events caused the transition:

         a) The final iSCSI Login Request to conclude the Login
            Phase was received, prompting the target to send the
            final iSCSI Login Response with a non-zero Status-Class.

         b) Login timed out.

         c) A transport disconnect indication was received.

         d) A transport reset was received.

         e) An internal event indicating a transport timeout was
            received.

         f) On another connection, a "close the session" Logout Request
            was received.

       In all these cases, the connection is to be closed.

   T8:

       - initiator: An internal event of receiving a Logout
         Response (success) on another connection for a "close the
         session" Logout Request was received, thus closing this
         connection and requiring no further cleanup.

       - target: An internal event of sending a Logout Response
         (success) on another connection for a "close the session"
         Logout Request was received, or an internal event of a
         successful connection/session reinstatement was received,
         thus prompting the target to close this connection cleanly.

   T9, T10:

       - initiator: An internal event that indicates the readiness
         to start the Logout process was received, thus prompting an
         iSCSI Logout to be sent by the initiator.

       - target: An iSCSI Logout Request was received.

   T11, T12:

       - initiator: An Async PDU with AsyncEvent "Request Logout"
         was received.

       - target: An internal event that requires the decommissioning
         of the connection was received, thus causing an Async PDU with
         an AsyncEvent "Request Logout" to be sent.

   T13:

       - initiator: An iSCSI Logout Response (success) was received,
         or an internal event of receiving a Logout Response (success)
         on another connection for a "close the session" Logout Request
         was received.

       - target: An internal event was received that indicates
         successful processing of the Logout, which prompts an iSCSI
         Logout Response (success) to be sent; an internal event of
         sending a Logout Response (success) on another connection
         for a "close the session" Logout Request was received; or

         an internal event of a successful connection/session
         reinstatement was received.  In all these cases, the
         transport connection is closed.

   T14:

       - initiator: An Async PDU with AsyncEvent "Request Logout"
         was received again.

       - target: Illegal.

   T15, T16:

       - initiator: One or more of the following events caused this
         transition:

         a) An internal event that indicates a transport connection
            timeout was received, thus prompting a transport reset
            or transport connection closure.

         b) A transport reset was received.

         c) A transport disconnect indication was received.

         d) An Async PDU with AsyncEvent "Drop connection" (for this
            CID) was received.

         e) An Async PDU with AsyncEvent "Drop all connections" was
            received.

       - target: One or more of the following events caused this
         transition:

         a) Internal event that indicates that a transport connection
            timeout was received, thus prompting a transport reset
            or transport connection closure.

         b) An internal event of a failed connection/session
            reinstatement was received.

         c) A transport reset was received.

         d) A transport disconnect indication was received.

         e) An internal emergency cleanup event was received, which
            prompts an Async PDU with AsyncEvent "Drop connection" (for
            this CID), or event "Drop all connections".

   T17:

       - initiator: One or more of the following events caused this
         transition:

         a) A Logout Response (failure, i.e., a non-zero status)
            was received, or Logout timed out.

         b) Any of the events specified for T15 and T16 occurred.

       - target: One or more of the following events caused this
         transition:

         a) An internal event that indicates a failure of the
            Logout processing was received, which prompts a
            Logout Response (failure, i.e., a non-zero status)
            to be sent.

         b) Any of the events specified for T15 and T16 occurred.

   T18:

       - initiator: An internal event of receiving a Logout
         Response (success) on another connection for a "close the
         session" Logout Request was received.

       - target: An internal event of sending a Logout Response
         (success) on another connection for a "close the session"
         Logout Request was received, or an internal event of a
         successful connection/session reinstatement was received.
         In both these cases, the connection is closed.

   The CLEANUP_WAIT state (S8) implies that there are possible iSCSI
   tasks that have not reached conclusion and are still considered
   busy.

8.1.3. Standard Connection State Diagram for an Initiator

   Symbolic names for states:

      S1: FREE

      S2: XPT_WAIT

      S4: IN_LOGIN

      S5: LOGGED_IN

S6: IN_LOGOUT

 S7: LOGOUT_REQUESTED

 S8: CLEANUP_WAIT

 States S5, S6, and S7 constitute the Full Feature Phase operation of
 the connection.

 The state diagram is as follows:

 -------<-------------+
 +--------->/ S1 \<----+ |
 T13| +->\ /<-+ \ |
 | / ---+--- \ \ |
 | / | T2 \ | |
 | T8 | |T1 | | |
 | | | / |T7 |
 | | | / | |
 | | | / | |
 | | V / / |
 | | ------- / / |
 | | / S2 \ / |
 | | \ / / |
 | | ---+--- / |
 | | |T4 / |
 | | V / | T18
 | | ------- / |
 | | / S4 \ |
 | | \ / |
 | | ---+--- | T15
 | | |T5 +--------+---------+
 | | | /T16+-----+------+ |
 | | | / -+-----+--+ | |
 | | | / / S7 \ |T12| |
 | | | / +->\ /<-+ V V
 | | | / / -+----- -------
 | | | / /T11 |T10 / S8 \
 | | V / / V +----+ \ /
 | | ---+-+- ----+-- | -------
 | | / S5 \T9 / S6 \<+ ^
 | +-----\ /--->\ / T14 |
 | ------- --+---+---------+T17
 +---------------------------+

   The following state transition table represents the above diagram.
   Each row represents the starting state for a given transition, which,
   after taking a transition marked in a table cell, would end in the
   state represented by the column of the cell.  For example, from
   state S1, the connection takes the T1 transition to arrive at
   state S2.  The fields marked "-" correspond to undefined transitions.

      +----+---+---+---+---+----+---+
      |S1  |S2 |S4 |S5 |S6 |S7  |S8 |
   ---+----+---+---+---+---+----+---+
    S1| -  |T1 | - | - | - | -  | - |
   ---+----+---+---+---+---+----+---+
    S2|T2  |-  |T4 | - | - | -  | - |
   ---+----+---+---+---+---+----+---+
    S4|T7  |-  |-  |T5 | - | -  | - |
   ---+----+---+---+---+---+----+---+
    S5|T8  |-  |-  | - |T9 |T11 |T15|
   ---+----+---+---+---+---+----+---+
    S6|T13 |-  |-  | - |T14|-   |T17|
   ---+----+---+---+---+---+----+---+
    S7|T18 |-  |-  | - |T10|T12 |T16|
   ---+----+---+---+---+---+----+---+
    S8| -  |-  |-  | - | - | -  | - |
   ---+----+---+---+---+---+----+---+

8.1.4. Standard Connection State Diagram for a Target

   Symbolic names for states:

      S1: FREE

      S3: XPT_UP

      S4: IN_LOGIN

      S5: LOGGED_IN

      S6: IN_LOGOUT

      S7: LOGOUT_REQUESTED

      S8: CLEANUP_WAIT

   States S5, S6, and S7 constitute the Full Feature Phase operation of
   the connection.

The state diagram is as follows:

 -------<-------------+
 +--------->/ S1 \<----+ |
 T13| +->\ /<-+ \ |
 | / ---+--- \ \ |
 | / | T6 \ | |
 | T8 | |T3 | | |
 | | | / |T7 |
 | | | / | |
 | | | / | |
 | | V / / |
 | | ------- / / |
 | | / S3 \ / |
 | | \ / / | T18
 | | ---+--- / |
 | | |T4 / |
 | | V / |
 | | ------- / |
 | | / S4 \ |
 | | \ / |
 | | ---+--- T15 |
 | | |T5 +--------+---------+
 | | | /T16+-----+------+ |
 | | | / -+-----+---+ | |
 | | | / / S7 \ |T12| |
 | | | / +->\ /<-+ V V
 | | | / / -+----- -------
 | | | / /T11 |T10 / S8 \
 | | V / / V \ /
 | | ---+-+- ------- -------
 | | / S5 \T9 / S6 \ ^
 | +-----\ /--->\ / |
 | ------- --+---+---------+T17
 +---------------------------+

   The following state transition table represents the above diagram and
   follows the conventions described for the initiator diagram.

      +----+---+---+---+---+----+---+
      |S1  |S3 |S4 |S5 |S6 |S7  |S8 |
   ---+----+---+---+---+---+----+---+
    S1| -  |T3 | - | - | - | -  | - |
   ---+----+---+---+---+---+----+---+
    S3|T6  |-  |T4 | - | - | -  | - |
   ---+----+---+---+---+---+----+---+
    S4|T7  |-  |-  |T5 | - | -  | - |
   ---+----+---+---+---+---+----+---+
    S5|T8  |-  |-  | - |T9 |T11 |T15|
   ---+----+---+---+---+---+----+---+
    S6|T13 |-  |-  | - |-  |-   |T17|
   ---+----+---+---+---+---+----+---+
    S7|T18 |-  |-  | - |T10|T12 |T16|
   ---+----+---+---+---+---+----+---+
    S8| -  |-  |-  | - | - | -  | - |
   ---+----+---+---+---+---+----+---+

8.2. Connection Cleanup State Diagram for Initiators and Targets

   Symbolic names for states:

      R1: CLEANUP_WAIT (same as S8)

      R2: IN_CLEANUP

      R3: FREE (same as S1)

   Whenever a connection state machine in cleanup (let's call it CSM-C)
   enters the CLEANUP_WAIT state (S8), it must go through the state
   transitions described in the connection cleanup state diagram, using
   either a) a separate Full Feature Phase connection (let's call it
   CSM-E, for explicit) in the LOGGED_IN state in the same session or
   b) a new transport connection (let's call it CSM-I, for implicit) in
   the FREE state that is to be added to the same session.  In the CSM-E
   case, an explicit logout for the CID that corresponds to CSM-C (as
   either a connection or session logout) needs to be performed to
   complete the cleanup.  In the CSM-I case, an implicit logout for the
   CID that corresponds to CSM-C needs to be performed by way of
   connection reinstatement (Section 6.3.4) for that CID.  In either
   case, the protocol exchanges on CSM-E or CSM-I determine the state
   transitions for CSM-C.  Therefore, this cleanup state diagram is only
   applicable to the instance of the connection in cleanup (i.e.,
   CSM-C).  In the case of an implicit logout, for example, CSM-C

reaches FREE (R3) at the time CSM-I reaches LOGGED_IN. In the case
 of an explicit logout, CSM-C reaches FREE (R3) when CSM-E receives a
 successful Logout Response while continuing to be in the LOGGED_IN
 state.

 An initiator must initiate an explicit or implicit connection logout
 for a connection in the CLEANUP_WAIT state, if the initiator intends
 to continue using the associated iSCSI session.

 The following state diagram applies to both initiators and targets.
 (M1, M2, M3, and M4 are defined in Section 8.2.2.)

 ---------
 / R1 \
 +---\ /<-+
 / ----+---- \
 / | \ M3
 M1 | |M2 |
 | | /
 | | /
 | | /
 | V /
 | ---------/
 | / R2 \
 | \ /
 | ---------
 | |
 | |M4
 | |
 | |
 | |
 | V
 | --------
 | / R3 \
 +----->\ /
 --------

   The following state transition table represents the above diagram and
   follows the same conventions as in earlier sections.

        +----+----+----+
        |R1  |R2  |R3  |
   -----+----+----+----+
    R1  | -  |M2  |M1  |
   -----+----+----+----+
    R2  |M3  | -  |M4  |
   -----+----+----+----+
    R3  | -  | -  | -  |
   -----+----+----+----+

8.2.1. State Descriptions for Initiators and Targets

   R1: CLEANUP_WAIT (same as S8)

       - initiator: Waiting for the internal event to initiate the
         cleanup processing for CSM-C.

       - target: Waiting for the cleanup process to start for CSM-C.

   R2: IN_CLEANUP

       - initiator: Waiting for the connection cleanup process to
         conclude for CSM-C.

       - target: Waiting for the connection cleanup process to conclude
         for CSM-C.

   R3: FREE (same as S1)

       - initiator: End state for CSM-C.

       - target: End state for CSM-C.

8.2.2. State Transition Descriptions for Initiators and Targets

   M1: One or more of the following events was received:

       - initiator:

         * An internal event that indicates connection state timeout.

         * An internal event of receiving a successful Logout Response
           on a different connection for a "close the session" Logout.

       - target:

         * An internal event that indicates connection state timeout.

         * An internal event of sending a Logout Response (success) on a
           different connection for a "close the session" Logout
           Request.

   M2: An implicit/explicit logout process was initiated by the
       initiator.

       - In CSM-I usage:

         * initiator: An internal event requesting the connection (or
           session) reinstatement was received, thus prompting a
           connection (or session) reinstatement Login to be sent,
           transitioning CSM-I to state IN_LOGIN.

         * target: A connection/session reinstatement Login was received
           while in state XPT_UP.

       - In CSM-E usage:

         * initiator: An internal event was received that indicates that
           an explicit logout was sent for this CID in state LOGGED_IN.

         * target: An explicit logout was received for this CID in state
           LOGGED_IN.

   M3: Logout failure was detected.

       - In CSM-I usage:

         * initiator: CSM-I failed to reach LOGGED_IN and arrived into
           FREE instead.

         * target: CSM-I failed to reach LOGGED_IN and arrived into FREE
           instead.

       - In CSM-E usage:

         * initiator: either CSM-E moved out of LOGGED_IN, or Logout
           timed out and/or aborted, or Logout Response (failure) was
           received.

         * target: either CSM-E moved out of LOGGED_IN, Logout timed out
           and/or aborted, or an internal event that indicates that a
           failed Logout processing was received.  A Logout Response
           (failure) was sent in the last case.

   M4: Successful implicit/explicit logout was performed.

       - In CSM-I usage:

         * initiator: CSM-I reached state LOGGED_IN, or an internal
           event of receiving a Logout Response (success) on another
           connection for a "close the session" Logout Request was
           received.

         * target: CSM-I reached state LOGGED_IN, or an internal event
           of sending a Logout Response (success) on a different
           connection for a "close the session" Logout Request was
           received.

       - In CSM-E usage:

         * initiator: CSM-E stayed in LOGGED_IN and received a Logout
           Response (success), or an internal event of receiving a
           Logout Response (success) on another connection for a "close
           the session" Logout Request was received.

         * target: CSM-E stayed in LOGGED_IN and an internal event
           indicating a successful Logout processing was received, or an
           internal event of sending a Logout Response (success) on a
           different connection for a "close the session" Logout Request
           was received.

8.3. Session State Diagrams

8.3.1. Session State Diagram for an Initiator

   Symbolic names for states:

      Q1: FREE

      Q3: LOGGED_IN

      Q4: FAILED

   State Q3 represents the Full Feature Phase operation of the session.

The state diagram is as follows. (N1, N3, N4, N5, and N6 are defined
 in Section 8.3.4.)

 ---------
 / Q1 \
 +---------->\ /<-+
 / ----+---- |
 / | |N3
 N6 | |N1 |
 | | |
 | N4 | |
 | +------------+ | /
 | | | | /
 | | | | /
 | | V V /
 --+-+--- -------+-
 / Q4 \ N5 / Q3 \
 \ /<------\ /
 -------- ---------

 The state transition table is as follows:

 +---+---+---+
 |Q1 |Q3 |Q4 |
 -----+---+---+---+
 Q1 | - |N1 | - |
 -----+---+---+---+
 Q3 |N3 | - |N5 |
 -----+---+---+---+
 Q4 |N6 |N4 | - |
 -----+---+---+---+

8.3.2. Session State Diagram for a Target

   Symbolic names for states:

      Q1: FREE

      Q2: ACTIVE

      Q3: LOGGED_IN

      Q4: FAILED

      Q5: IN_CONTINUE

   State Q3 represents the Full Feature Phase operation of the session.

The state diagram is as follows:

 ---------
 +------------------->/ Q1 \
 / +-------------->\ /<-+
 | | ---+----- |
 | | ^ | |N3
 N6 | |N11 N9| V N1 |
 | | +-------- |
 | | / Q2 \ |
 | | \ / |
 | ---+----- +--+----- |
 | / Q5 \ | |
 | \ / N10 | |
 | -+-+----+-----------+ | N2 /
 | ^ | | | /
 | N7| |N8 | | /
 | | | | V /
 --+---+-V V------+-
 / Q4 \ N5 / Q3 \
 \ /<-------------\ /
 --------- ---------

 The state transition table is as follows:

 +----+----+----+----+----+
 |Q1 |Q2 |Q3 |Q4 |Q5 |
 -----+----+----+----+----+----+
 Q1 | - |N1 | - | - | - |
 -----+----+----+----+----+----+
 Q2 |N9 | - |N2 | - | - |
 -----+----+----+----+----+----+
 Q3 |N3 | - | - |N5 | - |
 -----+----+----+----+----+----+
 Q4 |N6 | - | - | - |N7 |
 -----+----+----+----+----+----+
 Q5 |N11 | - |N10 |N8 | - |
 -----+----+----+----+----+----+

8.3.3. State Descriptions for Initiators and Targets

   Q1: FREE

       - initiator: State on instantiation or after cleanup.

       - target: State on instantiation or after cleanup.

   Q2: ACTIVE

       - initiator: Illegal.

       - target: The first iSCSI connection in the session transitioned
         to IN_LOGIN, waiting for it to complete the login process.

   Q3: LOGGED_IN

       - initiator: Waiting for all session events.

       - target: Waiting for all session events.

   Q4: FAILED

       - initiator: Waiting for session recovery or session
         continuation.

       - target: Waiting for session recovery or session continuation.

   Q5: IN_CONTINUE

       - initiator: Illegal.

       - target: Waiting for session continuation attempt to reach a
         conclusion.

8.3.4. State Transition Descriptions for Initiators and Targets

   N1:

       - initiator: At least one transport connection reached the
         LOGGED_IN state.

       - target: The first iSCSI connection in the session had reached
         the IN_LOGIN state.

   N2:

       - initiator: Illegal.

       - target: At least one iSCSI connection reached the LOGGED_IN
         state.

   N3:

       - initiator: Graceful closing of the session via session closure
         (Section 6.3.6).

       - target: Graceful closing of the session via session closure
         (Section 6.3.6) or a successful session reinstatement cleanly
         closed the session.

   N4:

       - initiator: A session continuation attempt succeeded.

       - target: Illegal.

   N5:
       - initiator: Session failure (Section 6.3.6) occurred.

       - target: Session failure (Section 6.3.6) occurred.

   N6:

       - initiator: Session state timeout occurred, or a session
         reinstatement cleared this session instance.  This results in
         the freeing of all associated resources, and the session state
         is discarded.

       - target: Session state timeout occurred, or a session
         reinstatement cleared this session instance.  This results in
         the freeing of all associated resources, and the session state
         is discarded.

   N7:

       - initiator: Illegal.

       - target: A session continuation attempt was initiated.

   N8:

       - initiator: Illegal.

       - target: The last session continuation attempt failed.

   N9:

       - initiator: Illegal.

       - target: Login attempt on the leading connection failed.

   N10:

       - initiator: Illegal.

       - target: A session continuation attempt succeeded.

   N11:

       - initiator: Illegal.

       - target: A successful session reinstatement cleanly closed the
         session.

9. Security Considerations

   Historically, native storage systems have not had to consider
   security, because their environments offered minimal security risks.
   That is, these environments consisted of storage devices either
   directly attached to hosts or connected via a Storage Area Network
   (SAN) distinctly separate from the communications network.  The use
   of storage protocols, such as SCSI, over IP networks requires that
   security concerns be addressed.  iSCSI implementations must provide
   means of protection against active attacks (e.g., pretending to be
   another identity; message insertion, deletion, modification, and
   replaying) and passive attacks (e.g., eavesdropping, gaining
   advantage by analyzing the data sent over the line).

   Although technically possible, iSCSI SHOULD NOT be configured without
   security, specifically in-band authentication; see Section 9.2.
   iSCSI configured without security should be confined to closed
   environments that have very limited and well-controlled security
   risks.  [RFC3723] specifies the mechanisms that must be used in order
   to mitigate risks fully described in that document.

   The following section describes the security mechanisms provided by
   an iSCSI implementation.

9.1. iSCSI Security Mechanisms

   The entities involved in iSCSI security are the initiator, target,
   and the IP communication endpoints.  iSCSI scenarios in which
   multiple initiators or targets share a single communication endpoint
   are expected.  To accommodate such scenarios, iSCSI supports two
   separate security mechanisms: in-band authentication between the
   initiator and the target at the iSCSI connection level (carried out
   by exchange of iSCSI Login PDUs), and packet protection (integrity,
   authentication, and confidentiality) by IPsec at the IP level.  The
   two security mechanisms complement each other.  The in-band
   authentication provides end-to-end trust (at login time) between the
   iSCSI initiator and the target, while IPsec provides a secure channel
   between the IP communication endpoints.  iSCSI can be used to access
   sensitive information for which significant security protection is
   appropriate.  As further specified in the rest of this security
   considerations section, both iSCSI security mechanisms are mandatory
   to implement (MUST).  The use of in-band authentication is strongly
   recommended (SHOULD).  In contrast, the use of IPsec is optional
   (MAY), as the security risks that it addresses may only be present
   over a subset of the networks used by an iSCSI connection or a
   session; a specific example is that when an iSCSI session spans data
   centers, IPsec VPN gateways at the data center boundaries to protect
   the WAN connectivity between data centers may be appropriate in
   combination with in-band iSCSI authentication.

   Further details on typical iSCSI scenarios and the relationship
   between the initiators, targets, and the communication endpoints can
   be found in [RFC3723].

9.2. In-Band Initiator-Target Authentication

   During login, the target MAY authenticate the initiator and the
   initiator MAY authenticate the target.  The authentication is
   performed on every new iSCSI connection by an exchange of iSCSI Login
   PDUs using a negotiated authentication method.

   The authentication method cannot assume an underlying IPsec
   protection, because IPsec is optional to use.  An attacker should
   gain as little advantage as possible by inspecting the authentication
   phase PDUs.  Therefore, a method using cleartext (or equivalent)
   passwords MUST NOT be used; on the other hand, identity protection is
   not strictly required.

   The authentication mechanism protects against an unauthorized login
   to storage resources by using a false identity (spoofing).  Once the
   authentication phase is completed, if the underlying IPsec is not
   used, all PDUs are sent and received in the clear.  The

   authentication mechanism alone (without underlying IPsec) should only
   be used when there is no risk of eavesdropping or of message
   insertion, deletion, modification, and replaying.

   Section 12 defines several authentication methods and the exact steps
   that must be followed in each of them, including the iSCSI-text-keys
   and their allowed values in each step.  Whenever an iSCSI initiator
   gets a response whose keys, or their values, are not according to the
   step definition, it MUST abort the connection.

   Whenever an iSCSI target gets a request or response whose keys, or
   their values, are not according to the step definition, it MUST
   answer with a Login reject with the "Initiator Error" or "Missing
   Parameter" status.  These statuses are not intended for
   cryptographically incorrect values such as the CHAP response, for
   which the "Authentication Failure" status MUST be specified.  The
   importance of this rule can be illustrated in CHAP with target
   authentication (see Section 12.1.3), where the initiator would have
   been able to conduct a reflection attack by omitting its response key
   (CHAP_R), using the same CHAP challenge as the target and reflecting
   the target's response back to the target.  In CHAP, this is prevented
   because the target must answer the missing CHAP_R key with a
   Login reject with the "Missing Parameter" status.

   For some of the authentication methods, a key specifies the identity
   of the iSCSI initiator or target for authentication purposes.  The
   value associated with that key MAY be different from the iSCSI name
   and SHOULD be configurable (CHAP_N: see Section 12.1.3; SRP_U: see
   Section 12.1.2).  For this reason, iSCSI implementations SHOULD
   manage authentication in a way that impersonation across iSCSI names
   via these authentication identities is not possible.  Specifically,
   implementations SHOULD allow configuration of an authentication
   identity for a Name if different, and authentication credentials for
   that identity.  During the login time, implementations SHOULD verify
   the Name-to-identity relationship in addition to authenticating the
   identity through the negotiated authentication method.

   When an iSCSI session has multiple TCP connections, either
   concurrently or sequentially, the authentication method and
   identities should not vary among the connections.  Therefore, all
   connections in an iSCSI session SHOULD use the same authentication
   method, iSCSI name, and authentication identity (for authentication
   methods that use an authentication identity).  Implementations SHOULD
   check this and cause an authentication failure on a new connection
   that uses a different authentication method, iSCSI name, or
   authentication identity from those already used in the session.  In

   addition, implementations SHOULD NOT support both authenticated and
   unauthenticated TCP connections in the same iSCSI session, added
   either concurrently or sequentially to the session.

9.2.1. CHAP Considerations

   Compliant iSCSI initiators and targets MUST implement the CHAP
   authentication method [RFC1994] (according to Section 12.1.3,
   including the target authentication option).

   When CHAP is performed over a non-encrypted channel, it is vulnerable
   to an off-line dictionary attack.  Implementations MUST support the
   use of up to 128-bit random CHAP secrets, including the means to
   generate such secrets and to accept them from an external generation
   source.  Implementations MUST NOT provide secret generation (or
   expansion) means other than random generation.

   An administrative entity of an environment in which CHAP is used with
   a secret that has less than 96 random bits MUST enforce IPsec
   encryption (according to the implementation requirements in
   Section 9.3.2) to protect the connection.  Moreover, in this case,
   IKE authentication with group pre-shared cryptographic keys SHOULD
   NOT be used unless it is not essential to protect group members
   against off-line dictionary attacks by other members.

   CHAP secrets MUST be an integral number of bytes (octets).  A
   compliant implementation SHOULD NOT continue with the login step in
   which it should send a CHAP response (CHAP_R; see Section 12.1.3)
   unless it can verify that the CHAP secret is at least 96 bits or that
   IPsec encryption is being used to protect the connection.

   Any CHAP secret used for initiator authentication MUST NOT be
   configured for authentication of any target, and any CHAP secret used
   for target authentication MUST NOT be configured for authentication
   of any initiator.  If the CHAP response received by one end of an
   iSCSI connection is the same as the CHAP response that the receiving
   endpoint would have generated for the same CHAP challenge, the
   response MUST be treated as an authentication failure and cause the
   connection to close (this ensures that the same CHAP secret is not
   used for authentication in both directions).  Also, if an iSCSI
   implementation can function as both initiator and target, different
   CHAP secrets and identities MUST be configured for these two roles.
   The following is an example of the attacks prevented by the above
   requirements:

      a) "Rogue" wants to impersonate "Storage" to Alice and knows that
         a single secret is used for both directions of Storage-Alice
         authentication.

      b) Rogue convinces Alice to open two connections to itself and
         identifies itself as Storage on both connections.

      c) Rogue issues a CHAP challenge on Connection 1, waits for Alice
         to respond, and then reflects Alice's challenge as the initial
         challenge to Alice on Connection 2.

      d) If Alice doesn't check for the reflection across connections,
         Alice's response on Connection 2 enables Rogue to impersonate
         Storage on Connection 1, even though Rogue does not know the
         Alice-Storage CHAP secret.

   Originators MUST NOT reuse the CHAP challenge sent by the responder
   for the other direction of a bidirectional authentication.
   Responders MUST check for this condition and close the iSCSI TCP
   connection if it occurs.

   The same CHAP secret SHOULD NOT be configured for authentication of
   multiple initiators or multiple targets, as this enables any of them
   to impersonate any other one of them, and compromising one of them
   enables the attacker to impersonate any of them.  It is recommended
   that iSCSI implementations check for the use of identical CHAP
   secrets by different peers when this check is feasible and take
   appropriate measures to warn users and/or administrators when this is
   detected.

   When an iSCSI initiator or target authenticates itself to
   counterparts in multiple administrative domains, it SHOULD use a
   different CHAP secret for each administrative domain to avoid
   propagating security compromises across domains.

   Within a single administrative domain:

      - A single CHAP secret MAY be used for authentication of an
        initiator to multiple targets.

      - A single CHAP secret MAY be used for an authentication of a
        target to multiple initiators when the initiators use an
        external server (e.g., RADIUS [RFC2865]) to verify the target's
        CHAP responses and do not know the target's CHAP secret.

   If an external response verification server (e.g., RADIUS) is not
   used, employing a single CHAP secret for authentication of a target
   to multiple initiators requires that all such initiators know that
   target's secret.  Any of these initiators can impersonate the target
   to any other such initiator, and compromise of such an initiator
   enables an attacker to impersonate the target to all such initiators.
   Targets SHOULD use separate CHAP secrets for authentication to each

   initiator when such risks are of concern; in this situation, it may
   be useful to configure a separate logical iSCSI target with its own
   iSCSI Node Name for each initiator or group of initiators among which
   such separation is desired.

   The above requirements strengthen the security properties of CHAP
   authentication for iSCSI by comparison to the basic CHAP
   authentication mechanism [RFC1994].  It is very important to adhere
   to these requirements, especially the requirements for strong (large
   randomly generated) CHAP secrets, as iSCSI implementations and
   deployments that fail to use strong CHAP secrets are likely to be
   highly vulnerable to off-line dictionary attacks on CHAP secrets.

   Replacement of CHAP with a better authentication mechanism is
   anticipated in a future version of iSCSI.  The FC-SP-2 standard
   [FC-SP-2] has specified the Extensible Authentication Protocol -
   Generalized Pre-Shared Key (EAP-GPSK) authentication mechanism
   [RFC5433] as an alternative to (and possible future replacement for)
   Fibre Channel's similar usage of strengthened CHAP.  Another possible
   replacement for CHAP is a secure password mechanism, e.g., an updated
   version of iSCSI's current SRP authentication mechanism.

9.2.2. SRP Considerations

   The strength of the SRP authentication method (specified in
   [RFC2945]) is dependent on the characteristics of the group being
   used (i.e., the prime modulus N and generator g).  As described in
   [RFC2945], N is required to be a Sophie Germain prime (of the form
   N = 2q + 1, where q is also prime) and the generator g is a primitive
   root of GF(N).  In iSCSI authentication, the prime modulus N MUST be
   at least 768 bits.

   The list of allowed SRP groups is provided in [RFC3723].

9.2.3. Kerberos Considerations

   iSCSI uses raw Kerberos V5 [RFC4120] for authenticating a client
   (iSCSI initiator) principal to a service (iSCSI target) principal.
   Note that iSCSI does not use the Generic Security Service Application
   Program Interface (GSS-API) [RFC2743] or the Kerberos V5 GSS-API
   security mechanism [RFC4121].  This means that iSCSI implementations
   supporting the KRB5 AuthMethod (Section 12.1) are directly involved
   in the Kerberos protocol.  When Kerberos V5 is used for
   authentication, the following actions MUST be performed as specified
   in [RFC4120]:

      - The target MUST validate KRB_AP_REQ to ensure that the initiator
        can be trusted.

      - When mutual authentication is selected, the initiator MUST
        validate KRB_AP_REP to determine the outcome of mutual
        authentication.

   As Kerberos V5 is capable of providing mutual authentication,
   implementations SHOULD support mutual authentication by default for
   login authentication.

   Note, however, that Kerberos authentication only assures that the
   server (iSCSI target) can be trusted by the Kerberos client
   (initiator) and vice versa; an initiator should employ appropriately
   secured service discovery techniques (e.g., iSNS; see Section 4.2.7)
   to ensure that it is talking to the intended target principal.

   iSCSI does not use Kerberos v5 for either integrity or
   confidentiality protection of the iSCSI protocol.  iSCSI uses IPsec
   for those purposes as specified in Section 9.3.

9.3. IPsec

   iSCSI uses the IPsec mechanism for packet protection (cryptographic
   integrity, authentication, and confidentiality) at the IP level
   between the iSCSI communicating endpoints.  The following sections
   describe the IPsec protocols that must be implemented for data
   authentication and integrity; confidentiality; and cryptographic key
   management.

   An iSCSI initiator or target may provide the required IPsec support
   fully integrated or in conjunction with an IPsec front-end device.
   In the latter case, the compliance requirements with regard to IPsec
   support apply to the "combined device".  Only the "combined device"
   is to be considered an iSCSI device.

   Detailed considerations and recommendations for using IPsec for iSCSI
   are provided in [RFC3723] as updated by [RFC7146].  The IPsec
   requirements are reproduced here for convenience and are intended to
   match those in [RFC7146]; in the event of a discrepancy, the
   requirements in [RFC7146] apply.

9.3.1. Data Authentication and Integrity

   Data authentication and integrity are provided by a cryptographic
   keyed Message Authentication Code in every sent packet.  This code
   protects against message insertion, deletion, and modification.
   Protection against message replay is realized by using a sequence
   counter.

   An iSCSI-compliant initiator or target MUST provide data
   authentication and integrity by implementing IPsec v2 [RFC2401] with
   ESPv2 [RFC2406] in tunnel mode, SHOULD provide data authentication
   and integrity by implementing IPsec v3 [RFC4301] with ESPv3 [RFC4303]
   in tunnel mode, and MAY provide data authentication and integrity by
   implementing either IPsec v2 or v3 with the appropriate version of
   ESP in transport mode.  The IPsec implementation MUST fulfill the
   following iSCSI-specific requirements:

      - HMAC-SHA1 MUST be implemented in the specific form of
        HMAC-SHA-1-96 [RFC2404].

      - AES CBC MAC with XCBC extensions using 128-bit keys SHOULD be
        implemented [RFC3566].

      - Implementations that support IKEv2 [RFC5996] SHOULD also
        implement AES Galois Message Authentication Code (GMAC)
        [RFC4543] using 128-bit keys.

   The ESP anti-replay service MUST also be implemented.

   At the high speeds at which iSCSI is expected to operate, a single
   IPsec SA could rapidly exhaust the ESP 32-bit sequence number space,
   requiring frequent rekeying of the SA, as rollover of the ESP
   sequence number within a single SA is prohibited for both ESPv2
   [RFC2406] and ESPv3 [RFC4303].  In order to provide the means to
   avoid this potentially undesirable frequent rekeying, implementations
   that are capable of operating at speeds of 1 gigabit/second or higher
   MUST implement extended (64-bit) sequence numbers for ESPv2 (and
   ESPv3, if supported) and SHOULD use extended sequence numbers for all
   iSCSI traffic.  Extended sequence number negotiation as part of
   security association establishment is specified in [RFC4304] for
   IKEv1 and [RFC5996] for IKEv2.

9.3.2. Confidentiality

   Confidentiality is provided by encrypting the data in every packet.
   When confidentiality is used, it MUST be accompanied by data
   authentication and integrity to provide comprehensive protection
   against eavesdropping and against message insertion, deletion,
   modification, and replaying.

   An iSCSI-compliant initiator or target MUST provide confidentiality
   by implementing IPsec v2 [RFC2401] with ESPv2 [RFC2406] in tunnel
   mode, SHOULD provide confidentiality by implementing IPsec v3
   [RFC4301] with ESPv3 [RFC4303] in tunnel mode, and MAY provide

   confidentiality by implementing either IPsec v2 or v3 with the
   appropriate version of ESP in transport mode, with the following
   iSCSI-specific requirements that apply to IPsec v2 and IPsec v3:

      - 3DES in CBC mode MAY be implemented [RFC2451].

      - AES in CBC mode with 128-bit keys MUST be implemented [RFC3602];
        other key sizes MAY be supported.

      - AES in Counter mode MAY be implemented [RFC3686].

      - Implementations that support IKEv2 [RFC5996] SHOULD also
        implement AES Galois/Counter Mode (GCM) with 128-bit keys
        [RFC4106]; other key sizes MAY be supported.

   Due to its inherent weakness, DES in CBC mode MUST NOT be used.

   The NULL encryption algorithm MUST also be implemented.

9.3.3. Policy, Security Associations, and Cryptographic Key Management

   A compliant iSCSI implementation MUST meet the cryptographic key
   management requirements of the IPsec protocol suite.  Authentication,
   security association negotiation, and cryptographic key management
   MUST be provided by implementing IKE [RFC2409] using the IPsec DOI
   [RFC2407] and SHOULD be provided by implementing IKEv2 [RFC5996],
   with the following iSCSI-specific requirements:

      a) Peer authentication using a pre-shared cryptographic key MUST
         be supported.  Certificate-based peer authentication using
         digital signatures MAY be supported.  For IKEv1 ([RFC2409]),
         peer authentication using the public key encryption methods
         outlined in Sections 5.2 and 5.3 of [RFC2409] SHOULD NOT be
         used.

      b) When digital signatures are used to achieve authentication, an
         IKE negotiator SHOULD use IKE Certificate Request Payload(s) to
         specify the certificate authority.  IKE negotiators SHOULD
         check certificate validity via the pertinent Certificate
         Revocation List (CRL) or via the use of the Online Certificate
         Status Protocol (OCSP) [RFC6960] before accepting a PKI
         certificate for use in IKE authentication procedures.  OCSP
         support within the IKEv2 protocol is specified in [RFC4806].
         These checks may not be needed in environments where a small
         number of certificates are statically configured as trust
         anchors.

      c) Conformant iSCSI implementations of IKEv1 MUST support Main
         Mode and SHOULD support Aggressive Mode.  Main Mode with a
         pre-shared key authentication method SHOULD NOT be used when
         either the initiator or the target uses dynamically assigned
         addresses.  While in many cases pre-shared keys offer good
         security, situations in which dynamically assigned addresses
         are used force the use of a group pre-shared key, which creates
         vulnerability to a man-in-the-middle attack.

      d) In the IKEv1 Phase 2 Quick Mode, in exchanges for creating the
         Phase 2 SA, the Identification Payload MUST be present.

      e) The following identification type requirements apply to IKEv1:
         ID_IPV4_ADDR, ID_IPV6_ADDR (if the protocol stack supports
         IPv6), and ID_FQDN Identification Types MUST be supported;
         ID_USER_FQDN SHOULD be supported.  The IP Subnet, IP Address
         Range, ID_DER_ASN1_DN, and ID_DER_ASN1_GN Identification Types
         SHOULD NOT be used.  The ID_KEY_ID Identification Type MUST NOT
         be used.

      f) If IKEv2 is supported, the following identification
         requirements apply:  ID_IPV4_ADDR, ID_IPV6_ADDR (if the
         protocol stack supports IPv6), and ID_FQDN Identification Types
         MUST be supported; ID_RFC822_ADDR SHOULD be supported.  The
         ID_DER_ASN1_DN and ID_DER_ASN1_GN Identification Types SHOULD
         NOT be used.  The ID_KEY_ID Identification Type MUST NOT be
         used.

   The reasons for the "MUST NOT" and "SHOULD NOT" for identification
   type requirements in preceding bullets e) and f) are:

      - IP Subnet and IP Address Range are too broad to usefully
        identify an iSCSI endpoint.

      - The DN and GN types are X.500 identities; it is usually better
        to use an identity from subjectAltName in a PKI certificate.

      - ID_KEY_ID is not interoperable as specified.

   Manual cryptographic keying MUST NOT be used, because it does not
   provide the necessary rekeying support.

   When Diffie-Hellman (DH) groups are used, a DH group of at least
   2048 bits SHOULD be offered as a part of all proposals to create
   IPsec security associations to protect iSCSI traffic, with both IKEv1
   and IKEv2.

   When IPsec is used, the receipt of an IKEv1 Phase 2 delete message or
   an IKEv2 INFORMATIONAL exchange that deletes the SA SHOULD NOT be
   interpreted as a reason for tearing down the iSCSI TCP connection.
   If additional traffic is sent on it, a new IKE SA will be created to
   protect it.

   The method used by the initiator to determine whether the target
   should be connected using IPsec is regarded as an issue of IPsec
   policy administration and thus not defined in the iSCSI standard.

   The method used by an initiator that supports both IPsec v2 and v3 to
   determine which versions of IPsec are supported by the target is also
   regarded as an issue of IPsec policy administration and thus not
   defined in the iSCSI standard.  If both IPsec v2 and v3 are supported
   by both the initiator and target, the use of IPsec v3 is recommended.

   If an iSCSI target is discovered via a SendTargets request in a
   Discovery session not using IPsec, the initiator should assume that
   it does not need IPsec to establish a session to that target.  If an
   iSCSI target is discovered using a Discovery session that does use
   IPsec, the initiator SHOULD use IPsec when establishing a session to
   that target.

9.4. Security Considerations for the X#NodeArchitecture Key

   The security considerations in this section are specific to the
   X#NodeArchitecture discussed in Section 13.26.

   This extension key transmits specific implementation details about
   the node that sends it; such details may be considered sensitive in
   some environments.  For example, if a certain software or firmware
   version is known to contain security weaknesses, announcing the
   presence of that version via this key may not be desirable.  The
   countermeasures for this security concern are:

      a) sending less detailed information in the key values,

      b) not sending the extension key, or

      c) using IPsec ([RFC4303]) to provide confidentiality for the
         iSCSI connection on which the key is sent.

   To support the first and second countermeasures, all implementations
   of this extension key MUST provide an administrative mechanism to
   disable sending the key.  In addition, all implementations SHOULD
   provide an administrative mechanism to configure a verbosity level of
   the key value, thereby controlling the amount of information sent.

   For example, a lower verbosity level might enable transmission of
   node architecture component names only, but no version numbers.  The
   choice of which countermeasure is most appropriate depends on the
   environment.  However, sending less detailed information in the key
   values may be an acceptable countermeasure in many environments,
   since it provides a compromise between sending too much information
   and the other more complete countermeasures of not sending the key at
   all or using IPsec.

   In addition to security considerations involving transmission of the
   key contents, any logging method(s) used for the key values MUST keep
   the information secure from intruders.  For all implementations, the
   requirements to address this security concern are as follows:

      a) Display of the log MUST only be possible with administrative
         rights to the node.

      b) Options to disable logging to disk and to keep logs for a fixed
         duration SHOULD be provided.

   Finally, it is important to note that different nodes may have
   different levels of risk, and these differences may affect the
   implementation.  The components of risk include assets, threats, and
   vulnerabilities.  Consider the following example iSCSI nodes, which
   demonstrate differences in assets and vulnerabilities of the nodes,
   and, as a result, differences in implementation:

      a) One iSCSI target based on a special-purpose operating system:
         Since the iSCSI target controls access to the data storage
         containing company assets, the asset level is seen as very
         high.  Also, because of the special-purpose operating system,
         in which vulnerabilities are less well known, the vulnerability
         level is viewed as low.

      b) Multiple iSCSI initiators in a blade farm, each running a
         general-purpose operating system: The asset level of each node
         is viewed as low, since blades are replaceable and low cost.
         However, the vulnerability level is viewed as high, since there
         may be many well-known vulnerabilities to that general-purpose
         operating system.  For this target, an appropriate
         implementation might be the logging of received key values but
         no transmission of the key.  For this initiator, an appropriate
         implementation might be transmission of the key but no logging
         of received key values.

9.5. SCSI Access Control Considerations

   iSCSI is a SCSI transport protocol and as such does not apply any
   access controls on SCSI-level operations such as SCSI task management
   functions (e.g., LU reset; see Section 11.5.1).  SCSI-level access
   controls (e.g., ACCESS CONTROL OUT; see [SPC3]) have to be
   appropriately deployed in practice to address SCSI-level security
   considerations, in addition to security via iSCSI connection and
   packet protection mechanisms that were already discussed in preceding
   sections.

10. Notes to Implementers

   This section notes some of the performance and reliability
   considerations of the iSCSI protocol.  This protocol was designed to
   allow efficient silicon and software implementations.  The iSCSI task
   tag mechanism was designed to enable Direct Data Placement (DDP -- a
   DMA form) at the iSCSI level or lower.

   The guiding assumption made throughout the design of this protocol is
   that targets are resource constrained relative to initiators.

   Implementers are also advised to consider the implementation
   consequences of the iSCSI-to-SCSI mapping model as outlined in
   Section 4.4.3.

10.1. Multiple Network Adapters

   The iSCSI protocol allows multiple connections, not all of which need
   to go over the same network adapter.  If multiple network connections
   are to be utilized with hardware support, the iSCSI protocol command-
   data-status allegiance to one TCP connection ensures that there is no
   need to replicate information across network adapters or otherwise
   require them to cooperate.

   However, some task management commands may require some loose form of
   cooperation or replication at least on the target.

10.1.1. Conservative Reuse of ISIDs

   Historically, the SCSI model (and implementations and applications
   based on that model) has assumed that SCSI ports are static, physical
   entities.  Recent extensions to the SCSI model have taken advantage
   of persistent worldwide unique names for these ports.  In iSCSI,
   however, the SCSI initiator ports are the endpoints of dynamically
   created sessions, so the presumptions of "static and physical" do not
   apply.  In any case, the "model" sections (particularly,

   Section 4.4.1) provide for persistent, reusable names for the
   iSCSI-type SCSI initiator ports even though there does not need to be
   any physical entity bound to these names.

   To both minimize the disruption of legacy applications and better
   facilitate the SCSI features that rely on persistent names for SCSI
   ports, iSCSI implementations SHOULD attempt to provide a stable
   presentation of SCSI initiator ports (both to the upper OS layers and
   the targets to which they connect).  This can be achieved in an
   initiator implementation by conservatively reusing ISIDs.  In other
   words, the same ISID should be used in the login process to multiple
   target portal groups (of the same iSCSI target or different iSCSI
   targets).  The ISID RULE (Section 4.4.3) only prohibits reuse to the
   same target portal group.  It does not "preclude" reuse to other
   target portal groups.  The principle of conservative reuse
   "encourages" reuse to other target portal groups.  When a SCSI target
   device sees the same (InitiatorName, ISID) pair in different sessions
   to different target portal groups, it can identify the underlying
   SCSI initiator port on each session as the same SCSI port.  In
   effect, it can recognize multiple paths from the same source.

10.1.2. iSCSI Name, ISID, and TPGT Use

   The designers of the iSCSI protocol are aware that legacy SCSI
   transports rely on initiator identity to assign access to storage
   resources.  Although newer techniques that simplify access control
   are available, support for configuration and authentication schemes
   that are based on initiator identity is deemed important in order to
   support legacy systems and administration software.  iSCSI thus
   supports the notion that it should be possible to assign access to
   storage resources based on "initiator device" identity.

   When there are multiple hardware or software components coordinated
   as a single iSCSI node, there must be some (logical) entity that
   represents the iSCSI node that makes the iSCSI Node Name available to
   all components involved in session creation and login.  Similarly,
   this entity that represents the iSCSI node must be able to coordinate
   session identifier resources (the ISID for initiators) to enforce
   both the ISID RULE and the TSIH RULE (see Section 4.4.3).

   For targets, because of the closed environment, implementation of
   this entity should be straightforward.  However, vendors of iSCSI
   hardware (e.g., NICs or HBAs) intended for targets SHOULD provide
   mechanisms for configuration of the iSCSI Node Name across the portal
   groups instantiated by multiple instances of these components within
   a target.

   However, complex targets making use of multiple Target Portal Group
   Tags may reconfigure them to achieve various quality goals.  The
   initiators have two mechanisms at their disposal to discover and/or
   check reconfiguring targets -- the Discovery session type and a key
   returned by the target during login to confirm the TPGT.  An
   initiator should attempt to "rediscover" the target configuration
   whenever a session is terminated unexpectedly.

   For initiators, in the long term, it is expected that operating
   system vendors will take on the role of this entity and provide
   standard APIs that can inform components of their iSCSI Node Name and
   can configure and/or coordinate ISID allocation, use, and reuse.

   Recognizing that such initiator APIs are not available today, other
   implementations of the role of this entity are possible.  For
   example, a human may instantiate the (common) node name as part of
   the installation process of each iSCSI component involved in session
   creation and login.  This may be done by pointing the component to
   either a vendor-specific location for this datum or a system-wide
   location.  The structure of the ISID namespace (see Section 11.12.5
   and [RFC3721]) facilitates implementation of the ISID coordination by
   allowing each component vendor to independently (of other vendor's
   components) coordinate allocation, use, and reuse of its own
   partition of the ISID namespace in a vendor-specific manner.
   Partitioning of the ISID namespace within initiator portal groups
   managed by that vendor allows each such initiator portal group to act
   independently of all other portal groups when selecting an ISID for a
   login; this facilitates enforcement of the ISID RULE (see
   Section 4.4.3) at the initiator.

   A vendor of iSCSI hardware (e.g., NICs or HBAs) intended for use in
   initiators MUST implement a mechanism for configuring the iSCSI Node
   Name.  Vendors and administrators must ensure that iSCSI Node Names
   are worldwide unique.  It is therefore important that when one
   chooses to reuse the iSCSI Node Name of a disabled unit one does not
   reassign that name to the original unit unless its worldwide
   uniqueness can be ascertained again.

   In addition, a vendor of iSCSI hardware must implement a mechanism to
   configure and/or coordinate ISIDs for all sessions managed by
   multiple instances of that hardware within a given iSCSI node.  Such
   configuration might be either permanently preassigned at the factory
   (in a necessarily globally unique way), statically assigned (e.g.,
   partitioned across all the NICs at initialization in a locally unique
   way), or dynamically assigned (e.g., on-line allocator, also in a
   locally unique way).  In the latter two cases, the configuration may

   be via public APIs (perhaps driven by an independent vendor's
   software, such as the OS vendor) or private APIs driven by the
   vendor's own software.

   The process of name assignment and coordination has to be as
   encompassing and automated as possible, as years of legacy usage have
   shown that it is highly error-prone.  It should be mentioned that
   today SCSI has alternative schemes of access control that can be used
   by all transports, and their security is not dependent on strict
   naming coordination.

10.2. Autosense and Auto Contingent Allegiance (ACA)

   "Autosense" refers to the automatic return of sense data to the
   initiator in cases where a command did not complete successfully.
   iSCSI initiators and targets MUST support and use Autosense.

   ACA helps preserve ordered command execution in the presence of
   errors.  As there can be many commands in-flight between an initiator
   and a target, SCSI initiator functionality in some operating systems
   depends on ACA to enforce ordered command execution during error
   recovery, and hence iSCSI initiator implementations for those
   operating systems need to support ACA.  In order to support error
   recovery for these operating systems and iSCSI initiators, iSCSI
   targets SHOULD support ACA.

10.3. iSCSI Timeouts

   iSCSI recovery actions are often dependent on iSCSI timeouts being
   recognized and acted upon before SCSI timeouts.  Determining the
   right timeouts to use for various iSCSI actions (command
   acknowledgments expected, status acknowledgments, etc.) is very much
   dependent on infrastructure (e.g., hardware, links, TCP/IP stack,
   iSCSI driver).  As a guide, the implementer may use an average
   NOP-Out/NOP-In turnaround delay multiplied by a "safety factor"
   (e.g., 4) as a good estimate for the basic delay of the iSCSI stack
   for a given connection.  The safety factor should account for network
   load variability.  For connection teardown, the implementer may want
   to also consider TCP common practice for the given infrastructure.

   Text negotiations MAY also be subject to either time limits or limits
   in the number of exchanges.  Those limits SHOULD be generous enough
   to avoid affecting interoperability (e.g., allowing each key to be
   negotiated on a separate exchange).

   The relationship between iSCSI timeouts and SCSI timeouts should also
   be considered.  SCSI timeouts should be longer than iSCSI timeouts
   plus the time required for iSCSI recovery whenever iSCSI recovery is

   planned.  Alternatively, an implementer may choose to interlock iSCSI
   timeouts and recovery with SCSI timeouts so that SCSI recovery will
   become active only where iSCSI is not planned to, or failed to,
   recover.

   The implementer may also want to consider the interaction between
   various iSCSI exception events -- such as a digest failure -- and
   subsequent timeouts.  When iSCSI error recovery is active, a digest
   failure is likely to result in discovering a missing command or data
   PDU.  In these cases, an implementer may want to lower the timeout
   values to enable faster initiation for recovery procedures.

10.4. Command Retry and Cleaning Old Command Instances

   To avoid having old, retried command instances appear in a valid
   command window after a command sequence number wraparound, the
   protocol requires (see Section 4.2.2.1) that on every connection on
   which a retry has been issued a non-immediate command be issued and
   acknowledged within an interval of 2**31 - 1 commands from the CmdSN
   of the retried command.  This requirement can be fulfilled by an
   implementation in several ways.

   The simplest technique to use is to send a (non-retry) non-immediate
   SCSI command (or a NOP if no SCSI command is available for a while)
   after every command retry on the connection on which the retry was
   attempted.  Because errors are deemed rare events, this technique is
   probably the most effective, as it does not involve additional checks
   at the initiator when issuing commands.

10.5. Sync and Steering Layer, and Performance

   While a Sync and Steering layer is optional, an initiator/target that
   does not have it working against a target/initiator that demands sync
   and steering may experience performance degradation caused by packet
   reordering and loss.  Providing a sync and steering mechanism is
   recommended for all high-speed implementations.

10.6. Considerations for State-Dependent Devices and Long-Lasting SCSI
Operations

   Sequential access devices operate on the principle that the position
   of the device is based on the last command processed.  As such,
   command processing order, and knowledge of whether or not the
   previous command was processed, are of the utmost importance to
   maintain data integrity.  For example, inadvertent retries of SCSI
   commands when it is not known if the previous SCSI command was
   processed is a potential data integrity risk.

   For a sequential access device, consider the scenario in which a SCSI
   SPACE command to backspace one filemark is issued and then reissued
   due to no status received for the command.  If the first SPACE
   command was actually processed, the reissued SPACE command, if
   processed, will cause the position to change.  Thus, a subsequent
   write operation will write data to the wrong position, and any
   previous data at that position will be overwritten.

   For a medium changer device, consider the scenario in which an
   EXCHANGE MEDIUM command (the SOURCE ADDRESS and DESTINATION ADDRESS
   are the same, thus performing a swap) is issued and then reissued due
   to no status received for the command.  If the first EXCHANGE MEDIUM
   command was actually processed, the reissued EXCHANGE MEDIUM command,
   if processed, will perform the swap again.  The net effect is that no
   swap was performed, thus putting data integrity at risk.

   All commands that change the state of the device (e.g., SPACE
   commands for sequential access devices and EXCHANGE MEDIUM commands
   for medium changer devices) MUST be issued as non-immediate commands
   for deterministic and ordered delivery to iSCSI targets.

   For many of those state-changing commands, the execution model also
   assumes that the command is executed exactly once.  Devices
   implementing READ POSITION and LOCATE provide a means for SCSI-level
   command recovery, and new tape-class devices should support those
   commands.  In their absence, a retry at the SCSI level is difficult,
   and error recovery at the iSCSI level is advisable.

   Devices operating on long-latency delivery subsystems and performing
   long-lasting SCSI operations may need mechanisms that enable
   connection replacement while commands are running (e.g., during an
   extended copy operation).

10.6.1. Determining the Proper ErrorRecoveryLevel

   The implementation and use of a specific ErrorRecoveryLevel should be
   determined based on the deployment scenarios of a given iSCSI
   implementation.  Generally, the following factors must be considered
   before deciding on the proper level of recovery:

      a) Application resilience to I/O failures.

      b) Required level of availability in the face of transport
         connection failures.

      c) Probability of transport-layer "checksum escape" (message error
         undetected by TCP checksum -- see [RFC3385] for related
         discussion).  This in turn decides the iSCSI digest failure
         frequency and thus the criticality of iSCSI-level error
         recovery.  The details of estimating this probability are
         outside the scope of this document.

   A consideration of the above factors for SCSI tape devices as an
   example suggests that implementations SHOULD use ErrorRecoveryLevel=1
   when transport connection failure is not a concern and SCSI-level
   recovery is unavailable, and ErrorRecoveryLevel=2 when there is a
   high likelihood of connection failure during a backup/retrieval.

   For extended copy operations, implementations SHOULD use
   ErrorRecoveryLevel=2 whenever there is a relatively high likelihood
   of connection failure.

10.7. Multi-Task Abort Implementation Considerations

   Multi-task abort operations are typically issued in emergencies, such
   as clearing a device lock-up, HA failover/failback, etc.  In these
   circumstances, it is desirable to rapidly go through the error-
   handling process as opposed to the target waiting on multiple third-
   party initiators that may not even be functional anymore --
   especially if this emergency is triggered because of one such
   initiator failure.  Therefore, both iSCSI target and initiator
   implementations SHOULD support FastAbort multi-task abort semantics
   (Section 4.2.3.4).

   Note that in both standard semantics (Section 4.2.3.3) and FastAbort
   semantics (Section 4.2.3.4) there may be outstanding data transfers
   even after the TMF completion is reported on the issuing session.  In
   the case of iSCSI/iSER [RFC7145], these would be tagged data
   transfers for STags not owned by any active tasks.  Whether or not
   real buffers support these data transfers is implementation
   dependent.  However, the data transfers logically MUST be silently
   discarded by the target iSCSI layer in all cases.  A target MAY, on
   an implementation-defined internal timeout, also choose to drop the
   connections on which it did not receive the expected Data-Out
   sequences (Section 4.2.3.3) or NOP-Out acknowledgments
   (Section 4.2.3.4) so as to reclaim the associated buffer, STag, and
   TTT resources as appropriate.

11. iSCSI PDU Formats

   All multi-byte integers that are specified in formats defined in this
   document are to be represented in network byte order (i.e.,
   big-endian).  Any field that appears in this document assumes that
   the most significant byte is the lowest numbered byte and the most
   significant bit (within byte or field) is the lowest numbered bit
   unless specified otherwise.

   Any compliant sender MUST set all bits not defined and all reserved
   fields to 0, unless specified otherwise.  Any compliant receiver MUST
   ignore any bit not defined and all reserved fields unless specified
   otherwise.  Receipt of reserved code values in defined fields MUST be
   reported as a protocol error.

   Reserved fields are marked by the word "reserved", some abbreviation
   of "reserved", or by "." for individual bits when no other form of
   marking is technically feasible.

11.1. iSCSI PDU Length and Padding

iSCSI PDUs are padded to the closest integer number of 4-byte words.
The padding bytes SHOULD be sent as 0.

11.2. PDU Template, Header, and Opcodes

   All iSCSI PDUs have one or more header segments and, optionally, a
   data segment.  After the entire header segment group, a header digest
   MAY follow.  The data segment MAY also be followed by a data digest.

   The Basic Header Segment (BHS) is the first segment in all of the
   iSCSI PDUs.  The BHS is a fixed-length 48-byte header segment.  It
   MAY be followed by Additional Header Segments (AHS), a Header-Digest,
   a Data Segment, and/or a Data-Digest.

   The overall structure of an iSCSI PDU is as follows:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0/ Basic Header Segment (BHS)                                    /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48/ Additional Header Segment 1 (AHS) (optional)                  /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     / Additional Header Segment 2 (AHS) (optional)                  /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     +---------------+---------------+---------------+---------------+
     / Additional Header Segment n (AHS) (optional)                  /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
    k/ Header-Digest (optional)                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
    l/ Data Segment (optional)                                       /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
    m/ Data-Digest (optional)                                        /
    +/                                                               /
     +---------------+---------------+---------------+---------------+

   All PDU segments and digests are padded to the closest integer number
   of 4-byte words.  For example, all PDU segments and digests start at
   a 4-byte word boundary, and the padding ranges from 0 to 3 bytes.
   The padding bytes SHOULD be sent as 0.

   iSCSI Response PDUs do not have AH Segments.

11.2.1. Basic Header Segment (BHS)

   The BHS is 48 bytes long.  The Opcode and DataSegmentLength fields
   appear in all iSCSI PDUs.  In addition, when used, the Initiator Task
   Tag and Logical Unit Number always appear in the same location in the
   header.

   The format of the BHS is:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|I| Opcode    |F| Opcode-specific fields                      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Opcode-specific fields                                 |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20/ Opcode-specific fields                                        /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48

11.2.1.1. I (Immediate) Bit

For Request PDUs, the I bit set to 1 is an immediate delivery marker.

11.2.1.2. Opcode

   The Opcode indicates the type of iSCSI PDU the header encapsulates.

   The Opcodes are divided into two categories: initiator Opcodes and
   target Opcodes.  Initiator Opcodes are in PDUs sent by the initiator
   (Request PDUs).  Target Opcodes are in PDUs sent by the target
   (Response PDUs).

   Initiators MUST NOT use target Opcodes, and targets MUST NOT use
   initiator Opcodes.

   Initiator Opcodes defined in this specification are:

      0x00 NOP-Out

      0x01 SCSI Command (encapsulates a SCSI Command Descriptor
           Block)

      0x02 SCSI Task Management Function Request

      0x03 Login Request

      0x04 Text Request

      0x05 SCSI Data-Out (for write operations)

      0x06 Logout Request

      0x10 SNACK Request

      0x1c-0x1e Vendor-specific codes

   Target Opcodes are:

      0x20 NOP-In

      0x21 SCSI Response - contains SCSI status and possibly sense
           information or other response information

      0x22 SCSI Task Management Function Response

      0x23 Login Response

      0x24 Text Response

      0x25 SCSI Data-In (for read operations)

      0x26 Logout Response

      0x31 Ready To Transfer (R2T) - sent by target when it is ready
           to receive data

      0x32 Asynchronous Message - sent by target to indicate certain
           special conditions

      0x3c-0x3e Vendor-specific codes

      0x3f Reject

All other Opcodes are unassigned.

11.2.1.3. F (Final) Bit

When set to 1 it indicates the final (or only) PDU of a sequence.

11.2.1.4. Opcode-Specific Fields

These fields have different meanings for different Opcode types.

11.2.1.5. TotalAHSLength

   This is the total length of all AHS header segments in units of
   4-byte words, including padding, if any.

   The TotalAHSLength is only used in PDUs that have an AHS and MUST be
   0 in all other PDUs.

11.2.1.6. DataSegmentLength

This is the data segment payload length in bytes (excluding padding).
The DataSegmentLength MUST be 0 whenever the PDU has no data segment.

11.2.1.7. LUN

   Some Opcodes operate on a specific LU.  The Logical Unit Number (LUN)
   field identifies which LU.  If the Opcode does not relate to a LU,
   this field is either ignored or may be used in an Opcode-specific
   way.  The LUN field is 64 bits and should be formatted in accordance
   with [SAM2].  For example, LUN[0] from [SAM2] is BHS byte 8 and so on
   up to LUN[7] from [SAM2], which is BHS byte 15.

11.2.1.8. Initiator Task Tag

   The initiator assigns a task tag to each iSCSI task it issues.  While
   a task exists, this tag MUST uniquely identify the task session-wide.
   SCSI may also use the Initiator Task Tag as part of the SCSI task
   identifier when the timespan during which an iSCSI Initiator Task Tag
   must be unique extends over the timespan during which a SCSI task tag
   must be unique.  However, the iSCSI Initiator Task Tag must exist and
   be unique even for untagged SCSI commands.

   An ITT value of 0xffffffff is reserved and MUST NOT be assigned for a
   task by the initiator.  The only instance in which it may be seen on
   the wire is in a target-initiated NOP-In PDU (Section 11.19) and in
   the initiator response to that PDU, if necessary.

11.2.2. Additional Header Segment (AHS)

   The general format of an AHS is:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0| AHSLength                     | AHSType       | AHS-Specific  |
     +---------------+---------------+---------------+---------------+
    4/ AHS-Specific                                                  /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
    x

11.2.2.1. AHSType

   The AHSType field is coded as follows:

      bit 0-1 - Reserved

      bit 2-7 - AHS code

0 - Reserved

      1 - Extended CDB

      2 - Bidirectional Read Expected Data Transfer Length

      3 - 63 Reserved

11.2.2.2. AHSLength

   This field contains the effective length in bytes of the AHS,
   excluding AHSType and AHSLength and padding, if any.  The AHS is
   padded to the smallest integer number of 4-byte words (i.e., from 0
   up to 3 padding bytes).

11.2.2.3. Extended CDB AHS

   The format of the Extended CDB AHS is:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0| AHSLength (CDBLength - 15)    | 0x01          |  Reserved     |
     +---------------+---------------+---------------+---------------+
    4/ ExtendedCDB...+padding                                        /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
    x

   This type of AHS MUST NOT be used if the CDBLength is less than 17.

   The length includes the reserved byte 3.

11.2.2.4. Bidirectional Read Expected Data Transfer Length AHS

   The format of the Bidirectional Read Expected Data Transfer Length
   AHS is:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0| AHSLength (0x0005)            | 0x02          | Reserved      |
     +---------------+---------------+---------------+---------------+
    4| Bidirectional Read Expected Data Transfer Length              |
     +---------------+---------------+---------------+---------------+
    8

11.2.3. Header Digest and Data Digest

   Optional header and data digests protect the integrity of the header
   and data, respectively.  The digests, if present, are located,
   respectively, after the header and PDU-specific data and cover,
   respectively, the header and the PDU data, each including the padding
   bytes, if any.

   The existence and type of digests are negotiated during the Login
   Phase.

   The separation of the header and data digests is useful in iSCSI
   routing applications, in which only the header changes when a message
   is forwarded.  In this case, only the header digest should be
   recalculated.

   Digests are not included in data or header length fields.

   A zero-length Data Segment also implies a zero-length Data-Digest.

11.2.4. Data Segment

   The (optional) Data Segment contains PDU-associated data.  Its
   payload effective length is provided in the BHS field --
   DataSegmentLength.  The Data Segment is also padded to an integer
   number of 4-byte words.

11.3. SCSI Command

   The format of the SCSI Command PDU is:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|I| 0x01      |F|R|W|. .|ATTR | Reserved                      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| Logical Unit Number (LUN)                                     |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Expected Data Transfer Length                                 |
     +---------------+---------------+---------------+---------------+
   24| CmdSN                                                         |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32/ SCSI Command Descriptor Block (CDB)                           /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48/ AHS (optional)                                                /
     +---------------+---------------+---------------+---------------+
    x/ Header-Digest (optional)                                      /
     +---------------+---------------+---------------+---------------+
    y/ (DataSegment, Command Data) (optional)                        /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
    z/ Data-Digest (optional)                                        /
     +---------------+---------------+---------------+---------------+

11.3.1. Flags and Task Attributes (Byte 1)

   The flags for a SCSI Command PDU are:

      bit 0    (F) is set to 1 when no unsolicited SCSI Data-Out PDUs
               follow this PDU.  When F = 1 for a write and if Expected
               Data Transfer Length is larger than the
               DataSegmentLength, the target may solicit additional data
               through R2T.

      bit 1    (R) is set to 1 when the command is expected to input
               data.

      bit 2    (W) is set to 1 when the command is expected to output
               data.

      bit 3-4  Reserved.

      bit 5-7  contains Task Attributes.

   Task Attributes (ATTR) have one of the following integer values (see
   [SAM2] for details):

        0 - Untagged

        1 - Simple

        2 - Ordered

        3 - Head of queue

        4 - ACA

      5-7 - Reserved

   At least one of the W and F bits MUST be set to 1.

   Either or both of R and W MAY be 1 when the Expected Data Transfer
   Length and/or the Bidirectional Read Expected Data Transfer Length
   are 0, but they MUST NOT both be 0 when the Expected Data Transfer
   Length and/or Bidirectional Read Expected Data Transfer Length are
   not 0 (i.e., when some data transfer is expected, the transfer
   direction is indicated by the R and/or W bit).

11.3.2. CmdSN - Command Sequence Number

The CmdSN enables ordered delivery across multiple connections in a
single session.

11.3.3. ExpStatSN

Command responses up to ExpStatSN - 1 (modulo 2**32) have been
received (acknowledges status) on the connection.

11.3.4. Expected Data Transfer Length

   For unidirectional operations, the Expected Data Transfer Length
   field contains the number of bytes of data involved in this SCSI
   operation.  For a unidirectional write operation (W flag set to 1 and
   R flag set to 0), the initiator uses this field to specify the number
   of bytes of data it expects to transfer for this operation.  For a
   unidirectional read operation (W flag set to 0 and R flag set to 1),
   the initiator uses this field to specify the number of bytes of data
   it expects the target to transfer to the initiator.  It corresponds
   to the SAM-2 byte count.

   For bidirectional operations (both R and W flags are set to 1), this
   field contains the number of data bytes involved in the write
   transfer.  For bidirectional operations, an additional header segment
   MUST be present in the header sequence that indicates the
   Bidirectional Read Expected Data Transfer Length.  The Expected Data
   Transfer Length field and the Bidirectional Read Expected Data
   Transfer Length field correspond to the SAM-2 byte count.

   If the Expected Data Transfer Length for a write and the length of
   the immediate data part that follows the command (if any) are the
   same, then no more data PDUs are expected to follow.  In this case,
   the F bit MUST be set to 1.

   If the Expected Data Transfer Length is higher than the
   FirstBurstLength (the negotiated maximum amount of unsolicited data
   the target will accept), the initiator MUST send the maximum amount
   of unsolicited data OR ONLY the immediate data, if any.

   Upon completion of a data transfer, the target informs the initiator
   (through residual counts) of how many bytes were actually processed
   (sent and/or received) by the target.

11.3.5. CDB - SCSI Command Descriptor Block

   There are 16 bytes in the CDB field to accommodate the commonly used
   CDBs.  Whenever the CDB is larger than 16 bytes, an Extended CDB AHS
   MUST be used to contain the CDB spillover.

11.3.6. Data Segment - Command Data

   Some SCSI commands require additional parameter data to accompany the
   SCSI command.  This data may be placed beyond the boundary of the
   iSCSI header in a data segment.  Alternatively, user data (e.g., from
   a write operation) can be placed in the data segment (both cases are
   referred to as immediate data).  These data are governed by the rules
   for solicited vs. unsolicited data outlined in Section 4.2.5.2.

11.4. SCSI Response

   The format of the SCSI Response PDU is:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x21      |1|. .|o|u|O|U|.| Response      | Status        |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| Reserved                                                      |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| SNACK Tag or Reserved                                         |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| ExpDataSN or Reserved                                         |
     +---------------+---------------+---------------+---------------+
   40| Bidirectional Read Residual Count or Reserved                 |
     +---------------+---------------+---------------+---------------+
   44| Residual Count or Reserved                                    |
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / Data Segment (optional)                                       /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

11.4.1. Flags (Byte 1)

   bit 1-2     Reserved.

   bit 3 - (o) set for Bidirectional Read Residual Overflow.  In this
               case, the Bidirectional Read Residual Count indicates the
               number of bytes that were not transferred to the
               initiator because the initiator's Bidirectional Read
               Expected Data Transfer Length was not sufficient.

   bit 4 - (u) set for Bidirectional Read Residual Underflow.  In this
               case, the Bidirectional Read Residual Count indicates the
               number of bytes that were not transferred to the
               initiator out of the number of bytes expected to be
               transferred.

   bit 5 - (O) set for Residual Overflow.  In this case, the Residual
               Count indicates the number of bytes that were not
               transferred because the initiator's Expected Data
               Transfer Length was not sufficient.  For a bidirectional
               operation, the Residual Count contains the residual for
               the write operation.

   bit 6 - (U) set for Residual Underflow.  In this case, the Residual
               Count indicates the number of bytes that were not
               transferred out of the number of bytes that were expected
               to be transferred.  For a bidirectional operation, the
               Residual Count contains the residual for the write
               operation.

   bit 7 - (0) Reserved.

   Bits O and U and bits o and u are mutually exclusive (i.e., having
   both o and u or O and U set to 1 is a protocol error).

   For a response other than "Command Completed at Target", bits 3-6
   MUST be 0.

11.4.2. Status

   The Status field is used to report the SCSI status of the command (as
   specified in [SAM2]) and is only valid if the response code is
   Command Completed at Target.

   Some of the status codes defined in [SAM2] are:

      0x00 GOOD

      0x02 CHECK CONDITION

      0x08 BUSY

      0x18 RESERVATION CONFLICT

      0x28 TASK SET FULL

      0x30 ACA ACTIVE

      0x40 TASK ABORTED

   See [SAM2] for the complete list and definitions.

   If a SCSI device error is detected while data from the initiator is
   still expected (the command PDU did not contain all the data and the
   target has not received a data PDU with the Final bit set), the
   target MUST wait until it receives a data PDU with the F bit set in
   the last expected sequence before sending the Response PDU.

11.4.3. Response

   This field contains the iSCSI service response.

   iSCSI service response codes defined in this specification are:

      0x00 - Command Completed at Target

      0x01 - Target Failure

      0x80-0xff - Vendor specific

   All other response codes are reserved.

   The Response field is used to report a service response.  The mapping
   of the response code into a SCSI service response code value, if
   needed, is outside the scope of this document.  However, in symbolic
   terms, response value 0x00 maps to the SCSI service response (see

   [SAM2] and [SPC3]) of TASK COMPLETE or LINKED COMMAND COMPLETE.  All
   other Response values map to the SCSI service response of SERVICE
   DELIVERY OR TARGET FAILURE.

   If a SCSI Response PDU does not arrive before the session is
   terminated, the SCSI service response is SERVICE DELIVERY OR TARGET
   FAILURE.

   A non-zero response field indicates a failure to execute the command,
   in which case the Status and Flag fields are undefined and MUST be
   ignored on reception.

11.4.4. SNACK Tag

   This field contains a copy of the SNACK Tag of the last SNACK Tag
   accepted by the target on the same connection and for the command for
   which the response is issued.  Otherwise, it is reserved and should
   be set to 0.

   After issuing a R-Data SNACK, the initiator must discard any SCSI
   status unless contained in a SCSI Response PDU carrying the same
   SNACK Tag as the last issued R-Data SNACK for the SCSI command on the
   current connection.

   For a detailed discussion on R-Data SNACK, see Section 11.16.3.

11.4.5. Residual Count

11.4.5.1. Field Semantics

   The Residual Count field MUST be valid in the case where either the U
   bit or the O bit is set.  If neither bit is set, the Residual Count
   field MUST be ignored on reception and SHOULD be set to 0 when
   sending.  Targets may set the residual count, and initiators may use
   it when the response code is Command Completed at Target (even if the
   status returned is not GOOD).  If the O bit is set, the Residual
   Count indicates the number of bytes that were not transferred because
   the initiator's Expected Data Transfer Length was not sufficient.  If
   the U bit is set, the Residual Count indicates the number of bytes
   that were not transferred out of the number of bytes expected to be
   transferred.

11.4.5.2. Residuals Concepts Overview

   "SCSI-Presented Data Transfer Length (SPDTL)" is the term this
   document uses (see Section 2.2 for definition) to represent the
   aggregate data length that the target SCSI layer attempts to transfer
   using the local iSCSI layer for a task.  "Expected Data Transfer

Length (EDTL)" is the iSCSI term that represents the length of data
 that the iSCSI layer expects to transfer for a task. EDTL is
 specified in the SCSI Command PDU.

 When SPDTL = EDTL for a task, the target iSCSI layer completes the
 task with no residuals. Whenever SPDTL differs from EDTL for a task,
 that task is said to have a residual.

 If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in the
 SCSI Response PDU as specified in Section 11.4.5.1. The Residual
 Count MUST be set to the numerical value of (SPDTL - EDTL).

 If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in the
 SCSI Response PDU as specified in Section 11.4.5.1. The Residual
 Count MUST be set to the numerical value of (EDTL - SPDTL).

 Note that the Overflow and Underflow scenarios are independent of
 Data-In and Data-Out. Either scenario is logically possible in
 either direction of data transfer.

11.4.5.3. SCSI REPORT LUNS Command and Residual Overflow

   This section discusses the residual overflow issues, citing the
   example of the SCSI REPORT LUNS command.  Note, however, that there
   are several SCSI commands (e.g., INQUIRY) with ALLOCATION LENGTH
   fields following the same underlying rules.  The semantics in the
   rest of the section apply to all such SCSI commands.

   The specification of the SCSI REPORT LUNS command requires that the
   SCSI target limit the amount of data transferred to a maximum size
   (ALLOCATION LENGTH) provided by the initiator in the REPORT LUNS CDB.

   If the Expected Data Transfer Length (EDTL) in the iSCSI header of
   the SCSI Command PDU for a REPORT LUNS command is set to at least as
   large as that ALLOCATION LENGTH, the SCSI-layer truncation prevents
   an iSCSI Residual Overflow from occurring.  A SCSI initiator can
   detect that such truncation has occurred via other information at the
   SCSI layer.  The rest of the section elaborates on this required
   behavior.

   The SCSI REPORT LUNS command requests a target SCSI layer to return a
   LU inventory (LUN list) to the initiator SCSI layer (see Clause 6.21
   of [SPC3]).  The size of this LUN list may not be known to the
   initiator SCSI layer when it issues the REPORT LUNS command; to avoid
   transferring more LUN list data than the initiator is prepared for,
   the REPORT LUNS CDB contains an ALLOCATION LENGTH field to specify
   the maximum amount of data to be transferred to the initiator for
   this command.  If the initiator SCSI layer has underestimated the

   number of LUs at the target, it is possible that the complete LU
   inventory does not fit in the specified ALLOCATION LENGTH.  In this
   situation, Clause 4.3.4.6 of [SPC3] requires that the target SCSI
   layer "shall terminate transfers to the Data-In Buffer" when the
   number of bytes specified by the ALLOCATION LENGTH field have been
   transferred.

   Therefore, in response to a REPORT LUNS command, the SCSI layer at
   the target presents at most ALLOCATION LENGTH bytes of data (LU
   inventory) to iSCSI for transfer to the initiator.  For a REPORT LUNS
   command, if the iSCSI EDTL is at least as large as the ALLOCATION
   LENGTH, the SCSI truncation ensures that the EDTL will accommodate
   all of the data to be transferred.  If all of the LU inventory data
   presented to the iSCSI layer -- i.e., the data remaining after any
   SCSI truncation -- is transferred to the initiator by the iSCSI
   layer, an iSCSI Residual Overflow has not occurred and the iSCSI (O)
   bit MUST NOT be set in the SCSI Response or final SCSI Data-Out PDU.
   Note that this behavior is implied in Section 11.4.5.1, along with
   the specification of the REPORT LUNS command in [SPC3].  However, if
   the iSCSI EDTL is larger than the ALLOCATION LENGTH in this scenario,
   note that the iSCSI Underflow MUST be signaled in the SCSI Response
   PDU.  An iSCSI Underflow MUST also be signaled when the iSCSI EDTL is
   equal to the ALLOCATION LENGTH but the LU inventory data presented to
   the iSCSI layer is smaller than the ALLOCATION LENGTH.

   The LUN LIST LENGTH field in the LU inventory (the first field in the
   inventory) is not affected by truncation of the inventory to fit in
   ALLOCATION LENGTH; this enables a SCSI initiator to determine that
   the received inventory is incomplete by noticing that the LUN LIST
   LENGTH in the inventory is larger than the ALLOCATION LENGTH that was
   sent in the REPORT LUNS CDB.  A common initiator behavior in this
   situation is to reissue the REPORT LUNS command with a larger
   ALLOCATION LENGTH.

11.4.6. Bidirectional Read Residual Count

   The Bidirectional Read Residual Count field MUST be valid in the case
   where either the u bit or the o bit is set.  If neither bit is set,
   the Bidirectional Read Residual Count field is reserved.  Targets may
   set the Bidirectional Read Residual Count, and initiators may use it
   when the response code is Command Completed at Target.  If the o bit
   is set, the Bidirectional Read Residual Count indicates the number of
   bytes that were not transferred to the initiator because the
   initiator's Bidirectional Read Expected Data Transfer Length was not
   sufficient.  If the u bit is set, the Bidirectional Read Residual
   Count indicates the number of bytes that were not transferred to the
   initiator out of the number of bytes expected to be transferred.

11.4.7. Data Segment - Sense and Response Data Segment

   iSCSI targets MUST support and enable Autosense.  If Status is CHECK
   CONDITION (0x02), then the data segment MUST contain sense data for
   the failed command.

   For some iSCSI responses, the response data segment MAY contain some
   response-related information (e.g., for a target failure, it may
   contain a vendor-specific detailed description of the failure).

   If the DataSegmentLength is not 0, the format of the data segment is
   as follows:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|SenseLength                    | Sense Data                    |
     +---------------+---------------+---------------+---------------+
    x/ Sense Data                                                    /
     +---------------+---------------+---------------+---------------+
    y/ Response Data                                                 /
     /                                                               /
     +---------------+---------------+---------------+---------------+

11.4.7.1. SenseLength

This field indicates the length of Sense Data.

11.4.7.2. Sense Data

   The Sense Data contains detailed information about a CHECK CONDITION.
   [SPC3] specifies the format and content of the Sense Data.

   Certain iSCSI conditions result in the command being terminated at
   the target (response code of Command Completed at Target) with a SCSI
   CHECK CONDITION Status as outlined in the next table:

   +--------------------------+-----------+---------------------------+
   | iSCSI Condition          |Sense      | Additional Sense Code and |
   |                          |Key        | Qualifier                 |
   +--------------------------+-----------+---------------------------+
   | Unexpected unsolicited   |Aborted    | ASC = 0x0c ASCQ = 0x0c    |
   | data                     |Command-0B | Write Error               |
   +--------------------------+-----------+---------------------------+
   | Incorrect amount of data |Aborted    | ASC = 0x0c ASCQ = 0x0d    |
   |                          |Command-0B | Write Error               |
   +--------------------------+-----------+---------------------------+
   | Protocol Service CRC     |Aborted    | ASC = 0x47 ASCQ = 0x05    |
   | error                    |Command-0B | CRC Error Detected        |
   +--------------------------+-----------+---------------------------+
   | SNACK rejected           |Aborted    | ASC = 0x11 ASCQ = 0x13    |
   |                          |Command-0B | Read Error                |
   +--------------------------+-----------+---------------------------+

   The target reports the "Incorrect amount of data" condition if,
   during data output, the total data length to output is greater than
   FirstBurstLength and the initiator sent unsolicited non-immediate
   data but the total amount of unsolicited data is different than
   FirstBurstLength.  The target reports the same error when the amount
   of data sent as a reply to an R2T does not match the amount
   requested.

11.4.8. ExpDataSN

   This field indicates the number of Data-In (read) PDUs the target has
   sent for the command.

   This field MUST be 0 if the response code is not Command Completed at
   Target or the target sent no Data-In PDUs for the command.

11.4.9. StatSN - Status Sequence Number

   The StatSN is a sequence number that the target iSCSI layer generates
   per connection and that in turn enables the initiator to acknowledge
   status reception.  The StatSN is incremented by 1 for every
   response/status sent on a connection, except for responses sent as a

   result of a retry or SNACK.  In the case of responses sent due to a
   retransmission request, the StatSN MUST be the same as the first time
   the PDU was sent, unless the connection has since been restarted.

11.4.10. ExpCmdSN - Next Expected CmdSN from This Initiator

   The ExpCmdSN is a sequence number that the target iSCSI returns to
   the initiator to acknowledge command reception.  It is used to update
   a local variable with the same name.  An ExpCmdSN equal to
   MaxCmdSN + 1 indicates that the target cannot accept new commands.

11.4.11. MaxCmdSN - Maximum CmdSN from This Initiator

   The MaxCmdSN is a sequence number that the target iSCSI returns to
   the initiator to indicate the maximum CmdSN the initiator can send.
   It is used to update a local variable with the same name.  If the
   MaxCmdSN is equal to ExpCmdSN - 1, this indicates to the initiator
   that the target cannot receive any additional commands.  When the
   MaxCmdSN changes at the target while the target has no pending PDUs
   to convey this information to the initiator, it MUST generate a
   NOP-In to carry the new MaxCmdSN.

11.5. Task Management Function Request

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|I| 0x02      |1| Function    | Reserved                      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| Logical Unit Number (LUN) or Reserved                         |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Referenced Task Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| CmdSN                                                         |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32| RefCmdSN or Reserved                                          |
     +---------------+---------------+---------------+---------------+
   36| ExpDataSN or Reserved                                         |
     +---------------+---------------+---------------+---------------+
   40/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+

11.5.1. Function

   The task management functions provide an initiator with a way to
   explicitly control the execution of one or more tasks (SCSI and iSCSI
   tasks).  The task management function codes are listed below.  For a
   more detailed description of SCSI task management, see [SAM2].

      1  ABORT TASK - aborts the task identified by the Referenced Task
         Tag field.

      2  ABORT TASK SET - aborts all tasks issued via this session on
         the LU.

      3  CLEAR ACA - clears the Auto Contingent Allegiance condition.

      4  CLEAR TASK SET - aborts all tasks in the appropriate task set
         as defined by the TST field in the Control mode page
         (see [SPC3]).

      5  LOGICAL UNIT RESET

      6  TARGET WARM RESET

      7  TARGET COLD RESET

      8  TASK REASSIGN - reassigns connection allegiance for the task
         identified by the Initiator Task Tag field to this connection,
         thus resuming the iSCSI exchanges for the task.

   Values 9-12 are assigned in [RFC7144].  All other possible values for
   the Function field are unassigned.

   For all these functions, the Task Management Function Response MUST
   be returned as detailed in Section 11.6.  All these functions apply
   to the referenced tasks, regardless of whether they are proper SCSI
   tasks or tagged iSCSI operations.  Task management requests must act
   on all the commands from the same session having a CmdSN lower than
   the task management CmdSN.  LOGICAL UNIT RESET, TARGET WARM RESET,
   and TARGET COLD RESET may affect commands from other sessions or
   commands from the same session, regardless of their CmdSN value.

   If the task management request is marked for immediate delivery, it
   must be considered immediately for execution, but the operations
   involved (all or part of them) may be postponed to allow the target
   to receive all relevant tasks.  According to [SAM2], for all the
   tasks covered by the task management response (i.e., with a CmdSN
   lower than the task management command CmdSN), except for the task
   management response to a TASK REASSIGN, additional responses MUST NOT
   be delivered to the SCSI layer after the task management response.
   The iSCSI initiator MAY deliver to the SCSI layer all responses
   received before the task management response (i.e., it is a matter of
   implementation if the SCSI responses that are received before the
   task management response but after the task management request was
   issued are delivered to the SCSI layer by the iSCSI layer in the
   initiator).  The iSCSI target MUST ensure that no responses for the
   tasks covered by a task management function are delivered to the
   iSCSI initiator after the task management response, except for a task
   covered by a TASK REASSIGN.

   For ABORT TASK SET and CLEAR TASK SET, the issuing initiator MUST
   continue to respond to all valid Target Transfer Tags (received via
   R2T, Text Response, NOP-In, or SCSI Data-In PDUs) related to the
   affected task set, even after issuing the task management request.

   The issuing initiator SHOULD, however, terminate (i.e., by setting
   the F bit to 1) these response sequences as quickly as possible.  The
   target for its part MUST wait for responses on all affected Target
   Transfer Tags before acting on either of these two task management
   requests.  If all or part of the response sequence is not received
   (due to digest errors) for a valid TTT, the target MAY treat it as a
   case of a within-command error recovery class (see Section 7.1.4.1)
   if it is supporting ErrorRecoveryLevel >= 1 or, alternatively, may
   drop the connection to complete the requested task set function.

   If an ABORT TASK is issued for a task created by an immediate
   command, then the RefCmdSN MUST be that of the task management
   request itself (i.e., the CmdSN and RefCmdSN are equal); otherwise,
   the RefCmdSN MUST be set to the CmdSN of the task to be aborted
   (lower than the CmdSN).

   If the connection is still active (i.e., it is not undergoing an
   implicit or explicit logout), an ABORT TASK MUST be issued on the
   same connection to which the task to be aborted is allegiant at the
   time the task management request is issued.  If the connection is
   implicitly or explicitly logged out (i.e., no other request will be
   issued on the failing connection and no other response will be
   received on the failing connection), then an ABORT TASK function
   request may be issued on another connection.  This task management
   request will then establish a new allegiance for the command to be
   aborted as well as abort it (i.e., the task to be aborted will not
   have to be retried or reassigned, and its status, if sent but not
   acknowledged, will be resent followed by the task management
   response).

   At the target, an ABORT TASK function MUST NOT be executed on a task
   management request; such a request MUST result in a task management
   response of "Function rejected".

   For the LOGICAL UNIT RESET function, the target MUST behave as
   dictated by the Logical Unit Reset function in [SAM2].

   The implementation of the TARGET WARM RESET function and the TARGET
   COLD RESET function is OPTIONAL and, when implemented, should act as
   described below.  The TARGET WARM RESET is also subject to SCSI
   access controls on the requesting initiator as defined in [SPC3].
   When authorization fails at the target, the appropriate response as
   described in Section 11.6.1 MUST be returned by the target.  The
   TARGET COLD RESET function is not subject to SCSI access controls,
   but its execution privileges may be managed by iSCSI mechanisms such
   as login authentication.

   When executing the TARGET WARM RESET and TARGET COLD RESET functions,
   the target cancels all pending operations on all LUs known by the
   issuing initiator.  Both functions are equivalent to the TARGET RESET
   function specified by [SAM2].  They can affect many other initiators
   logged in with the servicing SCSI target port.

   Additionally, the target MUST treat the TARGET COLD RESET function as
   a power-on event, thus terminating all of its TCP connections to all
   initiators (all sessions are terminated).  For this reason, the
   service response (defined by [SAM2]) for this SCSI task management
   function may not be reliably delivered to the issuing initiator port.

   For the TASK REASSIGN function, the target should reassign the
   connection allegiance to this new connection (and thus resume iSCSI
   exchanges for the task).  TASK REASSIGN MUST ONLY be received by the
   target after the connection on which the command was previously
   executing has been successfully logged out.  The task management
   response MUST be issued before the reassignment becomes effective.

   For additional usage semantics, see Section 7.2.

   At the target, a TASK REASSIGN function request MUST NOT be executed
   to reassign the connection allegiance of a Task Management Function
   Request, an active text negotiation task, or a Logout task; such a
   request MUST result in a task management response of "Function
   rejected".

   TASK REASSIGN MUST be issued as an immediate command.

11.5.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.5.3. LUN

   This field is required for functions that address a specific LU
   (ABORT TASK, CLEAR TASK SET, ABORT TASK SET, CLEAR ACA, LOGICAL UNIT
   RESET) and is reserved in all others.

11.5.4. Referenced Task Tag

   This is the Initiator Task Tag of the task to be aborted for the
   ABORT TASK function or reassigned for the TASK REASSIGN function.
   For all the other functions, this field MUST be set to the reserved
   value 0xffffffff.

11.5.5. RefCmdSN

   If an ABORT TASK is issued for a task created by an immediate
   command, then the RefCmdSN MUST be that of the task management
   request itself (i.e., the CmdSN and RefCmdSN are equal).

   For an ABORT TASK of a task created by a non-immediate command, the
   RefCmdSN MUST be set to the CmdSN of the task identified by the
   Referenced Task Tag field.  Targets must use this field as described
   in Section 11.6.1 when the task identified by the Referenced Task Tag
   field is not with the target.

   Otherwise, this field is reserved.

11.5.6. ExpDataSN

   For recovery purposes, the iSCSI target and initiator maintain a data
   acknowledgment reference number -- the first input DataSN number
   unacknowledged by the initiator.  When issuing a new command, this
   number is set to 0.  If the function is TASK REASSIGN, which
   establishes a new connection allegiance for a previously issued read
   or bidirectional command, the ExpDataSN will contain an updated data
   acknowledgment reference number or the value 0; the latter indicates
   that the data acknowledgment reference number is unchanged.  The
   initiator MUST discard any data PDUs from the previous execution that
   it did not acknowledge, and the target MUST transmit all Data-In PDUs
   (if any) starting with the data acknowledgment reference number.  The
   number of retransmitted PDUs may or may not be the same as the
   original transmission, depending on if there was a change in
   MaxRecvDataSegmentLength in the reassignment.  The target MAY also
   send no more Data-In PDUs if all data has been acknowledged.

   The value of ExpDataSN MUST be 0 or higher than the DataSN of the
   last acknowledged Data-In PDU, but not larger than DataSN + 1 of the
   last Data-IN PDU sent by the target.  Any other value MUST be ignored
   by the target.

   For other functions, this field is reserved.

11.6. Task Management Function Response

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x22      |1| Reserved    | Response      | Reserved      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------------------------------------------------------+
    8/ Reserved                                                      /
     /                                                               /
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+

   For the functions ABORT TASK, ABORT TASK SET, CLEAR ACA, CLEAR TASK
   SET, LOGICAL UNIT RESET, TARGET COLD RESET, TARGET WARM RESET, and
   TASK REASSIGN, the target performs the requested task management
   function and sends a task management response back to the initiator.
   For TASK REASSIGN, the new connection allegiance MUST ONLY become
   effective at the target after the target issues the task management
   response.

11.6.1. Response

   The target provides a response, which may take on the following
   values:

       0 - Function complete
       1 - Task does not exist
       2 - LUN does not exist
       3 - Task still allegiant
       4 - Task allegiance reassignment not supported
       5 - Task management function not supported
       6 - Function authorization failed
     255 - Function rejected

   In addition to the above values, the value 7 is defined by [RFC7144].

   For a discussion on the usage of response codes 3 and 4, see
   Section 7.2.2.

   For the TARGET COLD RESET and TARGET WARM RESET functions, the target
   cancels all pending operations across all LUs known to the issuing
   initiator.  For the TARGET COLD RESET function, the target MUST then
   close all of its TCP connections to all initiators (terminates all
   sessions).

   The mapping of the response code into a SCSI service response code
   value, if needed, is outside the scope of this document.  However, in
   symbolic terms, Response values 0 and 1 map to the SCSI service
   response of FUNCTION COMPLETE.  Response value 2 maps to the SCSI
   service response of INCORRECT LOGICAL UNIT NUMBER.  All other
   Response values map to the SCSI service response of FUNCTION
   REJECTED.  If a Task Management Function Response PDU does not arrive
   before the session is terminated, the SCSI service response is
   SERVICE DELIVERY OR TARGET FAILURE.

   The response to ABORT TASK SET and CLEAR TASK SET MUST only be issued
   by the target after all of the commands affected have been received
   by the target, the corresponding task management functions have been
   executed by the SCSI target, and the delivery of all responses
   delivered until the task management function completion has been
   confirmed (acknowledged through the ExpStatSN) by the initiator on
   all connections of this session.  For the exact timeline of events,
   refer to Sections 4.2.3.3 and 4.2.3.4.

   For the ABORT TASK function,

      a) if the Referenced Task Tag identifies a valid task leading to a
         successful termination, then targets must return the "Function
         complete" response.

      b) if the Referenced Task Tag does not identify an existing task
         but the CmdSN indicated by the RefCmdSN field in the Task
         Management Function Request is within the valid CmdSN window
         and less than the CmdSN of the Task Management Function Request
         itself, then targets must consider the CmdSN as received and
         return the "Function complete" response.

      c) if the Referenced Task Tag does not identify an existing task
         and the CmdSN indicated by the RefCmdSN field in the Task
         Management Function Request is outside the valid CmdSN window,
         then targets must return the "Task does not exist" response.

   For response semantics on function types that can potentially impact
   multiple active tasks on the target, see Section 4.2.3.

11.6.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.7. SCSI Data-Out and SCSI Data-In

   The SCSI Data-Out PDU for write operations has the following format:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x05      |F| Reserved                                    |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   36| DataSN                                                        |
     +---------------+---------------+---------------+---------------+
   40| Buffer Offset                                                 |
     +---------------+---------------+---------------+---------------+
   44| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment                                                   /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   The SCSI Data-In PDU for read operations has the following format:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x25      |F|A|0 0 0|O|U|S| Reserved      |Status or Rsvd |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| StatSN or Reserved                                            |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| DataSN                                                        |
     +---------------+---------------+---------------+---------------+
   40| Buffer Offset                                                 |
     +---------------+---------------+---------------+---------------+
   44| Residual Count                                                |
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment                                                   /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   Status can accompany the last Data-In PDU if the command did not end
   with an exception (i.e., the status is "good status" -- GOOD,
   CONDITION MET, or INTERMEDIATE-CONDITION MET).  The presence of
   status (and of a residual count) is signaled via the S flag bit.
   Although targets MAY choose to send even non-exception status in
   separate responses, initiators MUST support non-exception status in
   Data-In PDUs.

11.7.1. F (Final) Bit

   For outgoing data, this bit is 1 for the last PDU of unsolicited data
   or the last PDU of a sequence that answers an R2T.

   For incoming data, this bit is 1 for the last input (read) data PDU
   of a sequence.  Input can be split into several sequences, each
   having its own F bit.  Splitting the data stream into sequences does
   not affect DataSN counting on Data-In PDUs.  It MAY be used as a
   "change direction" indication for bidirectional operations that need
   such a change.

   DataSegmentLength MUST NOT exceed MaxRecvDataSegmentLength for the
   direction it is sent, and the total of all the DataSegmentLength of
   all PDUs in a sequence MUST NOT exceed MaxBurstLength (or
   FirstBurstLength for unsolicited data).  However, the number of
   individual PDUs in a sequence (or in total) may be higher than the
   ratio of MaxBurstLength (or FirstBurstLength) to
   MaxRecvDataSegmentLength (as PDUs may be limited in length by the
   capabilities of the sender).  Using a DataSegmentLength of 0 may
   increase beyond what is reasonable for the number of PDUs and should
   therefore be avoided.

   For bidirectional operations, the F bit is 1 for both the end of the
   input sequences and the end of the output sequences.

11.7.2. A (Acknowledge) Bit

   For sessions with ErrorRecoveryLevel=1 or higher, the target sets
   this bit to 1 to indicate that it requests a positive acknowledgment
   from the initiator for the data received.  The target should use the
   A bit moderately; it MAY only set the A bit to 1 once every
   MaxBurstLength bytes, or on the last Data-In PDU that concludes the
   entire requested read data transfer for the task from the target's
   perspective, and it MUST NOT do so more frequently.  The target MUST
   NOT set to 1 the A bit for sessions with ErrorRecoveryLevel=0.  The
   initiator MUST ignore the A bit set to 1 for sessions with
   ErrorRecoveryLevel=0.

   On receiving a Data-In PDU with the A bit set to 1 on a session with
   ErrorRecoveryLevel greater than 0, if there are no holes in the read
   data until that Data-In PDU, the initiator MUST issue a SNACK of type
   DataACK, except when it is able to acknowledge the status for the
   task immediately via the ExpStatSN on other outbound PDUs if the
   status for the task is also received.  In the latter case
   (acknowledgment through the ExpStatSN), sending a SNACK of type
   DataACK in response to the A bit is OPTIONAL, but if it is done, it
   must not be sent after the status acknowledgment through the

   ExpStatSN.  If the initiator has detected holes in the read data
   prior to that Data-In PDU, it MUST postpone issuing the SNACK of type
   DataACK until the holes are filled.  An initiator also MUST NOT
   acknowledge the status for the task before those holes are filled.  A
   status acknowledgment for a task that generated the Data-In PDUs is
   considered by the target as an implicit acknowledgment of the Data-In
   PDUs if such an acknowledgment was requested by the target.

11.7.3. Flags (Byte 1)

   The last SCSI data packet sent from a target to an initiator for a
   SCSI command that completed successfully (with a status of GOOD,
   CONDITION MET, INTERMEDIATE, or INTERMEDIATE-CONDITION MET) may also
   optionally contain the Status for the data transfer.  In this case,
   Sense Data cannot be sent together with the Command Status.  If the
   command is completed with an error, then the response and sense data
   MUST be sent in a SCSI Response PDU (i.e., MUST NOT be sent in a SCSI
   data packet).  For bidirectional commands, the status MUST be sent in
   a SCSI Response PDU.

      bit 2-4          - Reserved.

      bit 5-6          - used the same as in a SCSI Response.  These
                         bits are only valid when S is set to 1.  For
                         details, see Section 11.4.1.

      bit 7 S (status) - set to indicate that the Command Status field
                         contains status.  If this bit is set to 1, the
                         F bit MUST also be set to 1.

   The fields StatSN, Status, and Residual Count only have meaningful
   content if the S bit is set to 1.  The values for these fields are
   defined in Section 11.4.

11.7.4. Target Transfer Tag and LUN

   On outgoing data, the Target Transfer Tag is provided to the target
   if the transfer is honoring an R2T.  In this case, the Target
   Transfer Tag field is a replica of the Target Transfer Tag provided
   with the R2T.

   On incoming data, the Target Transfer Tag and LUN MUST be provided by
   the target if the A bit is set to 1; otherwise, they are reserved.
   The Target Transfer Tag and LUN are copied by the initiator into the
   SNACK of type DataACK that it issues as a result of receiving a SCSI
   Data-In PDU with the A bit set to 1.

   The Target Transfer Tag values are not specified by this protocol,
   except that the value 0xffffffff is reserved and means that the
   Target Transfer Tag is not supplied.  If the Target Transfer Tag is
   provided, then the LUN field MUST hold a valid value and be
   consistent with whatever was specified with the command; otherwise,
   the LUN field is reserved.

11.7.5. DataSN

   For input (read) or bidirectional Data-In PDUs, the DataSN is the
   input PDU number within the data transfer for the command identified
   by the Initiator Task Tag.

   R2T and Data-In PDUs, in the context of bidirectional commands, share
   the numbering sequence (see Section 4.2.2.4).

   For output (write) data PDUs, the DataSN is the Data-Out PDU number
   within the current output sequence.  Either the current output
   sequence is identified by the Initiator Task Tag (for unsolicited
   data) or it is a data sequence generated for one R2T (for data
   solicited through R2T).

11.7.6. Buffer Offset

   The Buffer Offset field contains the offset of this PDU payload data
   within the complete data transfer.  The sum of the buffer offset and
   length should not exceed the expected transfer length for the
   command.

   The order of data PDUs within a sequence is determined by
   DataPDUInOrder.  When set to Yes, it means that PDUs have to be in
   increasing buffer offset order and overlays are forbidden.

   The ordering between sequences is determined by DataSequenceInOrder.
   When set to Yes, it means that sequences have to be in increasing
   buffer offset order and overlays are forbidden.

11.7.7. DataSegmentLength

   This is the data payload length of a SCSI Data-In or SCSI Data-Out
   PDU.  The sending of 0-length data segments should be avoided, but
   initiators and targets MUST be able to properly receive 0-length data
   segments.

   The data segments of Data-In and Data-Out PDUs SHOULD be filled to
   the integer number of 4-byte words (real payload), unless the F bit
   is set to 1.

11.8. Ready To Transfer (R2T)

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x31      |1| Reserved                                    |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN                                                           |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag                                           |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| R2TSN                                                         |
     +---------------+---------------+---------------+---------------+
   40| Buffer Offset                                                 |
     +---------------+---------------+---------------+---------------+
   44| Desired Data Transfer Length                                  |
     +---------------------------------------------------------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+

   When an initiator has submitted a SCSI command with data that passes
   from the initiator to the target (write), the target may specify
   which blocks of data it is ready to receive.  The target may request
   that the data blocks be delivered in whichever order is convenient
   for the target at that particular instant.  This information is
   passed from the target to the initiator in the Ready To Transfer
   (R2T) PDU.

   In order to allow write operations without an explicit initial R2T,
   the initiator and target MUST have negotiated the key InitialR2T to
   No during login.

   An R2T MAY be answered with one or more SCSI Data-Out PDUs with a
   matching Target Transfer Tag.  If an R2T is answered with a single
   Data-Out PDU, the buffer offset in the data PDU MUST be the same as

   the one specified by the R2T, and the data length of the data PDU
   MUST be the same as the Desired Data Transfer Length specified in the
   R2T.  If the R2T is answered with a sequence of data PDUs, the buffer
   offset and length MUST be within the range of those specified by the
   R2T, and the last PDU MUST have the F bit set to 1.  If the last PDU
   (marked with the F bit) is received before the Desired Data Transfer
   Length is transferred, a target MAY choose to reject that PDU with
   the "Protocol Error" reason code.  DataPDUInOrder governs the
   Data-Out PDU ordering.  If DataPDUInOrder is set to Yes, the buffer
   offsets and lengths for consecutive PDUs MUST form a continuous
   non-overlapping range, and the PDUs MUST be sent in increasing offset
   order.

   The target may send several R2T PDUs.  It therefore can have a number
   of pending data transfers.  The number of outstanding R2T PDUs is
   limited by the value of the negotiated key MaxOutstandingR2T.  Within
   a task, outstanding R2Ts MUST be fulfilled by the initiator in the
   order in which they were received.

   R2T PDUs MAY also be used to recover Data-Out PDUs.  Such an R2T
   (Recovery-R2T) is generated by a target upon detecting the loss of
   one or more Data-Out PDUs due to:

      - Digest error

      - Sequence error

      - Sequence reception timeout

   A Recovery-R2T carries the next unused R2TSN but requests part of or
   the entire data burst that an earlier R2T (with a lower R2TSN) had
   already requested.

   DataSequenceInOrder governs the buffer offset ordering in consecutive
   R2Ts.  If DataSequenceInOrder is Yes, then consecutive R2Ts MUST
   refer to continuous non-overlapping ranges, except for Recovery-R2Ts.

11.8.1. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.8.2. R2TSN

   R2TSN is the R2T PDU input PDU number within the command identified
   by the Initiator Task Tag.

   For bidirectional commands, R2T and Data-In PDUs share the input PDU
   numbering sequence (see Section 4.2.2.4).

11.8.3. StatSN

The StatSN field will contain the next StatSN. The StatSN for this
connection is not advanced after this PDU is sent.

11.8.4. Desired Data Transfer Length and Buffer Offset

   The target specifies how many bytes it wants the initiator to send
   because of this R2T PDU.  The target may request the data from the
   initiator in several chunks, not necessarily in the original order of
   the data.  The target therefore also specifies a buffer offset that
   indicates the point at which the data transfer should begin, relative
   to the beginning of the total data transfer.  The Desired Data
   Transfer Length MUST NOT be 0 and MUST NOT exceed MaxBurstLength.

11.8.5. Target Transfer Tag

   The target assigns its own tag to each R2T request that it sends to
   the initiator.  This tag can be used by the target to easily identify
   the data it receives.  The Target Transfer Tag and LUN are copied in
   the outgoing data PDUs and are only used by the target.  There is no
   protocol rule about the Target Transfer Tag except that the value
   0xffffffff is reserved and MUST NOT be sent by a target in an R2T.

11.9. Asynchronous Message

   An Asynchronous Message may be sent from the target to the initiator
   without corresponding to a particular command.  The target specifies
   the reason for the event and sense data.

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x32      |1| Reserved                                    |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| 0xffffffff                                                    |
     +---------------+---------------+---------------+---------------+
   20| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| AsyncEvent    | AsyncVCode    | Parameter1 or Reserved        |
     +---------------+---------------+---------------+---------------+
   40| Parameter2 or Reserved        | Parameter3 or Reserved        |
     +---------------+---------------+---------------+---------------+
   44| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment - Sense Data and iSCSI Event Data                 /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   Some Asynchronous Messages are strictly related to iSCSI, while
   others are related to SCSI [SAM2].

   The StatSN counts this PDU as an acknowledgeable event (the StatSN is
   advanced), which allows for initiator and target state
   synchronization.

11.9.1. AsyncEvent

   The codes used for iSCSI Asynchronous Messages (events) are:

        0 (SCSI Async Event) - a SCSI asynchronous event is reported in
          the sense data.  Sense Data that accompanies the report, in
          the data segment, identifies the condition.  The sending of a
          SCSI event ("asynchronous event reporting" in SCSI
          terminology) is dependent on the target support for SCSI
          asynchronous event reporting (see [SAM2]) as indicated in the
          standard INQUIRY data (see [SPC3]).  Its use may be enabled by
          parameters in the SCSI Control mode page (see [SPC3]).

        1 (Logout Request) - the target requests Logout.  This Async
          Message MUST be sent on the same connection as the one
          requesting to be logged out.  The initiator MUST honor this
          request by issuing a Logout as early as possible but no later
          than Parameter3 seconds.  The initiator MUST send a Logout
          with a reason code of "close the connection" OR "close the
          session" to close all the connections.  Once this message is
          received, the initiator SHOULD NOT issue new iSCSI commands on
          the connection to be logged out.  The target MAY reject any
          new I/O requests that it receives after this message with the
          reason code "Waiting for Logout".  If the initiator does not
          log out in Parameter3 seconds, the target should send an Async
          PDU with iSCSI event code "Dropped the connection" if possible
          or simply terminate the transport connection.  Parameter1 and
          Parameter2 are reserved.

        2 (Connection Drop Notification) - the target indicates that it
          will drop the connection.

          The Parameter1 field indicates the CID of the connection that
          is going to be dropped.

          The Parameter2 field (Time2Wait) indicates, in seconds, the
          minimum time to wait before attempting to reconnect or
          reassign.

          The Parameter3 field (Time2Retain) indicates the maximum time
          allowed to reassign commands after the initial wait (in
          Parameter2).

          If the initiator does not attempt to reconnect and/or reassign
          the outstanding commands within the time specified by
          Parameter3, or if Parameter3 is 0, the target will terminate

          all outstanding commands on this connection.  In this case, no
          other responses should be expected from the target for the
          outstanding commands on this connection.

          A value of 0 for Parameter2 indicates that reconnect can be
          attempted immediately.

        3 (Session Drop Notification) - the target indicates that it
          will drop all the connections of this session.

          The Parameter1 field is reserved.

          The Parameter2 field (Time2Wait) indicates, in seconds, the
          minimum time to wait before attempting to reconnect.

          The Parameter3 field (Time2Retain) indicates the maximum time
          allowed to reassign commands after the initial wait (in
          Parameter2).

          If the initiator does not attempt to reconnect and/or reassign
          the outstanding commands within the time specified by
          Parameter3, or if Parameter3 is 0, the session is terminated.
          In this case, the target will terminate all outstanding
          commands in this session; no other responses should be
          expected from the target for the outstanding commands in this
          session.  A value of 0 for Parameter2 indicates that reconnect
          can be attempted immediately.

        4 (Negotiation Request) - the target requests parameter
          negotiation on this connection.  The initiator MUST honor this
          request by issuing a Text Request (that can be empty) on the
          same connection as early as possible, but no later than
          Parameter3 seconds, unless a Text Request is already pending
          on the connection, or by issuing a Logout Request.  If the
          initiator does not issue a Text Request, the target may
          reissue the Asynchronous Message requesting parameter
          negotiation.

        5 (Task Termination) - all active tasks for a LU with a matching
          LUN field in the Async Message PDU are being terminated.  The
          receiving initiator iSCSI layer MUST respond to this message
          by taking the following steps, in order:

          - Stop Data-Out transfers on that connection for all active
            TTTs for the affected LUN quoted in the Async Message PDU.

          - Acknowledge the StatSN of the Async Message PDU via a
            NOP-Out PDU with ITT=0xffffffff (i.e., non-ping flavor),
            while copying the LUN field from the Async Message to
            NOP-Out.

          This value of AsyncEvent, however, MUST NOT be used on an
          iSCSI session unless the new TaskReporting text key defined in
          Section 13.23 was negotiated to FastAbort on the session.

    248-255 (Vendor-unique) - vendor-specific iSCSI event.  The
          AsyncVCode details the vendor code, and data MAY accompany the
          report.

   All other event codes are unassigned.

11.9.2. AsyncVCode

   AsyncVCode is a vendor-specific detail code that is only valid if the
   AsyncEvent field indicates a vendor-specific event.  Otherwise, it is
   reserved.

11.9.3. LUN

The LUN field MUST be valid if AsyncEvent is 0. Otherwise, this
field is reserved.

11.9.4. Sense Data and iSCSI Event Data

   For a SCSI event, this data accompanies the report in the data
   segment and identifies the condition.

   For an iSCSI event, additional vendor-unique data MAY accompany the
   Async event.  Initiators MAY ignore the data when not understood,
   while processing the rest of the PDU.

   If the DataSegmentLength is not 0, the format of the DataSegment is
   as follows:

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|SenseLength                    | Sense Data                    |
     +---------------+---------------+---------------+---------------+
    x/ Sense Data                                                    /
     +---------------+---------------+---------------+---------------+
    y/ iSCSI Event Data                                              /
     /                                                               /
     +---------------+---------------+---------------+---------------+
    z|

11.9.4.1. SenseLength

This is the length of Sense Data. When the Sense Data field is empty
(e.g., the event is not a SCSI event), SenseLength is 0.

11.10. Text Request

   The Text Request is provided to allow for the exchange of information
   and for future extensions.  It permits the initiator to inform a
   target of its capabilities or request some special operations.

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|I| 0x04      |F|C| Reserved                                  |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| CmdSN                                                         |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment (Text)                                            /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   An initiator MUST NOT have more than one outstanding Text Request on
   a connection at any given time.

   On a connection failure, an initiator must either explicitly abort
   any active allegiant text negotiation task or cause such a task to be
   implicitly terminated by the target.

11.10.1. F (Final) Bit

   When set to 1, this bit indicates that this is the last or only Text
   Request in a sequence of Text Requests; otherwise, it indicates that
   more Text Requests will follow.

11.10.2. C (Continue) Bit

   When set to 1, this bit indicates that the text (set of key=value
   pairs) in this Text Request is not complete (it will be continued on
   subsequent Text Requests); otherwise, it indicates that this Text
   Request ends a set of key=value pairs.  A Text Request with the C bit
   set to 1 MUST have the F bit set to 0.

11.10.3. Initiator Task Tag

   This is the initiator-assigned identifier for this Text Request.  If
   the command is sent as part of a sequence of Text Requests and
   responses, the Initiator Task Tag MUST be the same for all the
   requests within the sequence (similar to linked SCSI commands).  The
   I bit for all requests in a sequence also MUST be the same.

11.10.4. Target Transfer Tag

   When the Target Transfer Tag is set to the reserved value 0xffffffff,
   it tells the target that this is a new request, and the target resets
   any internal state associated with the Initiator Task Tag (resets the
   current negotiation state).

   The target sets the Target Transfer Tag in a Text Response to a value
   other than the reserved value 0xffffffff whenever it indicates that
   it has more data to send or more operations to perform that are
   associated with the specified Initiator Task Tag.  It MUST do so
   whenever it sets the F bit to 0 in the response.  By copying the
   Target Transfer Tag from the response to the next Text Request, the
   initiator tells the target to continue the operation for the specific
   Initiator Task Tag.  The initiator MUST ignore the Target Transfer
   Tag in the Text Response when the F bit is set to 1.

   This mechanism allows the initiator and target to transfer a large
   amount of textual data over a sequence of text-command/text-response
   exchanges or to perform extended negotiation sequences.

   If the Target Transfer Tag is not 0xffffffff, the LUN field MUST be
   sent by the target in the Text Response.

A target MAY reset its internal negotiation state if an exchange is
 stalled by the initiator for a long time or if it is running out of
 resources.

 Long Text Responses are handled as shown in the following example:

 I->T Text SendTargets=All (F = 1, TTT = 0xffffffff)

 T->I Text <part 1> (F = 0, TTT = 0x12345678)

 I->T Text <empty> (F = 1, TTT = 0x12345678)

 T->I Text <part 2> (F = 0, TTT = 0x12345678)

 I->T Text <empty> (F = 1, TTT = 0x12345678)

 ...

 T->I Text <part n> (F = 1, TTT = 0xffffffff)

11.10.5. Text

   The data lengths of a Text Request MUST NOT exceed the iSCSI target
   MaxRecvDataSegmentLength (a parameter that is negotiated per
   connection and per direction).  The text format is specified in
   Section 6.2.

   Sections 12 and 13 list some basic Text key=value pairs, some of
   which can be used in Login Requests/Responses and some in Text
   Requests/Responses.

   A key=value pair can span Text Request or Text Response boundaries.
   A key=value pair can start in one PDU and continue on the next.  In
   other words, the end of a PDU does not necessarily signal the end of
   a key=value pair.

   The target responds by sending its response back to the initiator.
   The response text format is similar to the request text format.  The
   Text Response MAY refer to key=value pairs presented in an earlier
   Text Request, and the text in the request may refer to earlier
   responses.

   Section 6.2 details the rules for the Text Requests and Responses.

   Text operations are usually meant for parameter setting/negotiations
   but can also be used to perform some long-lasting operations.

Text operations that take a long time should be placed in their own
Text Request.

11.11. Text Response

   The Text Response PDU contains the target's responses to the
   initiator's Text Request.  The format of the Text field matches that
   of the Text Request.

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x24      |F|C| Reserved                                  |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment (Text)                                            /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

11.11.1. F (Final) Bit

   When set to 1, in response to a Text Request with the Final bit set
   to 1, the F bit indicates that the target has finished the whole
   operation.  Otherwise, if set to 0 in response to a Text Request with
   the Final Bit set to 1, it indicates that the target has more work to

   do (invites a follow-on Text Request).  A Text Response with the
   F bit set to 1 in response to a Text Request with the F bit set to 0
   is a protocol error.

   A Text Response with the F bit set to 1 MUST NOT contain key=value
   pairs that may require additional answers from the initiator.

   A Text Response with the F bit set to 1 MUST have a Target Transfer
   Tag field set to the reserved value 0xffffffff.

   A Text Response with the F bit set to 0 MUST have a Target Transfer
   Tag field set to a value other than the reserved value 0xffffffff.

11.11.2. C (Continue) Bit

   When set to 1, this bit indicates that the text (set of key=value
   pairs) in this Text Response is not complete (it will be continued on
   subsequent Text Responses); otherwise, it indicates that this Text
   Response ends a set of key=value pairs.  A Text Response with the
   C bit set to 1 MUST have the F bit set to 0.

11.11.3. Initiator Task Tag

The Initiator Task Tag matches the tag used in the initial Text
Request.

11.11.4. Target Transfer Tag

   When a target has more work to do (e.g., cannot transfer all the
   remaining text data in a single Text Response or has to continue the
   negotiation) and has enough resources to proceed, it MUST set the
   Target Transfer Tag to a value other than the reserved value
   0xffffffff.  Otherwise, the Target Transfer Tag MUST be set to
   0xffffffff.

   When the Target Transfer Tag is not 0xffffffff, the LUN field may be
   significant.

   The initiator MUST copy the Target Transfer Tag and LUN in its next
   request to indicate that it wants the rest of the data.

   When the target receives a Text Request with the Target Transfer Tag
   set to the reserved value 0xffffffff, it resets its internal
   information (resets state) associated with the given Initiator Task
   Tag (restarts the negotiation).

   When a target cannot finish the operation in a single Text Response
   and does not have enough resources to continue, it rejects the Text
   Request with the appropriate Reject code.

   A target may reset its internal state associated with an Initiator
   Task Tag (the current negotiation state) as expressed through the
   Target Transfer Tag if the initiator fails to continue the exchange
   for some time.  The target may reject subsequent Text Requests with
   the Target Transfer Tag set to the "stale" value.

11.11.5. StatSN

The target StatSN variable is advanced by each Text Response sent.

11.11.6. Text Response Data

   The data lengths of a Text Response MUST NOT exceed the iSCSI
   initiator MaxRecvDataSegmentLength (a parameter that is negotiated
   per connection and per direction).

   The text in the Text Response Data is governed by the same rules as
   the text in the Text Request Data (see Section 11.11.2).

   Although the initiator is the requesting party and controls the
   request-response initiation and termination, the target can offer
   key=value pairs of its own as part of a sequence and not only in
   response to the initiator.

11.12. Login Request

   After establishing a TCP connection between an initiator and a
   target, the initiator MUST start a Login Phase to gain further access
   to the target's resources.

   The Login Phase (see Section 6.3) consists of a sequence of Login
   Requests and Login Responses that carry the same Initiator Task Tag.

   Login Requests are always considered as immediate.

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|1| 0x03      |T|C|.|.|CSG|NSG| Version-max   | Version-min   |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| ISID                                                          |
     +                               +---------------+---------------+
   12|                               | TSIH                          |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| CID                           | Reserved                      |
     +---------------+---------------+---------------+---------------+
   24| CmdSN                                                         |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN or Reserved                                         |
     +---------------+---------------+---------------+---------------+
   32| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   36| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   40/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48/ DataSegment - Login Parameters in Text Request Format         /
    +/                                                               /
     +---------------+---------------+---------------+---------------+

11.12.1. T (Transit) Bit

   When set to 1, this bit indicates that the initiator is ready to
   transit to the next stage.

   If the T bit is set to 1 and the NSG is set to FullFeaturePhase, then
   this also indicates that the initiator is ready for the Login
   Final-Response (see Section 6.3).

11.12.2. C (Continue) Bit

   When set to 1, this bit indicates that the text (set of key=value
   pairs) in this Login Request is not complete (it will be continued on
   subsequent Login Requests); otherwise, it indicates that this Login
   Request ends a set of key=value pairs.  A Login Request with the
   C bit set to 1 MUST have the T bit set to 0.

11.12.3. CSG and NSG

   Through these fields -- Current Stage (CSG) and Next Stage (NSG) --
   the Login negotiation requests and responses are associated with a
   specific stage in the session (SecurityNegotiation,
   LoginOperationalNegotiation, FullFeaturePhase) and may indicate the
   next stage to which they want to move (see Section 6.3).  The Next
   Stage value is only valid when the T bit is 1; otherwise, it is
   reserved.

   The stage codes are:

      0 - SecurityNegotiation

      1 - LoginOperationalNegotiation

      3 - FullFeaturePhase

   All other codes are reserved.

11.12.4. Version

The version number for this document is 0x00. Therefore, both
Version-min and Version-max MUST be set to 0x00.

11.12.4.1. Version-max

   Version-max indicates the maximum version number supported.

   All Login Requests within the Login Phase MUST carry the same
   Version-max.

   The target MUST use the value presented with the first Login Request.

11.12.4.2. Version-min

   All Login Requests within the Login Phase MUST carry the same
   Version-min.  The target MUST use the value presented with the first
   Login Request.

11.12.5. ISID

   This is an initiator-defined component of the session identifier and
   is structured as follows (see Section 10.1.1 for details):

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    8| T |     A     |              B                |      C        |
     +---------------+---------------+---------------+---------------+
   12|               D               |
     +---------------+---------------+

   The T field identifies the format and usage of A, B, C, and D as
   indicated below:

      T

      00b    OUI-Format

             A and B: 22-bit OUI

             (the I/G and U/L bits are omitted)

             C and D: 24-bit Qualifier

      01b    EN: Format (IANA Enterprise Number)

A: Reserved

B and C: EN (IANA Enterprise Number)

D: Qualifier

      10b    "Random"

             A: Reserved

             B and C: Random

             D: Qualifier

      11b    A, B, C, and D: Reserved

   For the T field values 00b and 01b, a combination of A and B (for
   00b) or B and C (for 01b) identifies the vendor or organization whose
   component (software or hardware) generates this ISID.  A vendor or

   organization with one or more OUIs, or one or more Enterprise
   Numbers, MUST use at least one of these numbers and select the
   appropriate value for the T field when its components generate ISIDs.
   An OUI or EN MUST be set in the corresponding fields in network byte
   order (byte big-endian).

   If the T field is 10b, B and C are set to a random 24-bit unsigned
   integer value in network byte order (byte big-endian).  See [RFC3721]
   for how this affects the principle of "conservative reuse".

   The Qualifier field is a 16-bit or 24-bit unsigned integer value that
   provides a range of possible values for the ISID within the selected
   namespace.  It may be set to any value within the constraints
   specified in the iSCSI protocol (see Sections 4.4.3 and 10.1.1).

   The T field value of 11b is reserved.

   If the ISID is derived from something assigned to a hardware adapter
   or interface by a vendor as a preset default value, it MUST be
   configurable to a value assigned according to the SCSI port behavior
   desired by the system in which it is installed (see Sections 10.1.1
   and 10.1.2).  The resultant ISID MUST also be persistent over power
   cycles, reboot, card swap, etc.

11.12.6. TSIH

   The TSIH must be set in the first Login Request.  The reserved value
   0 MUST be used on the first connection for a new session.  Otherwise,
   the TSIH sent by the target at the conclusion of the successful login
   of the first connection for this session MUST be used.  The TSIH
   identifies to the target the associated existing session for this new
   connection.

   All Login Requests within a Login Phase MUST carry the same TSIH.

   The target MUST check the value presented with the first Login
   Request and act as specified in Section 6.3.1.

11.12.7. Connection ID (CID)

   The CID provides a unique ID for this connection within the session.

   All Login Requests within the Login Phase MUST carry the same CID.

   The target MUST use the value presented with the first Login Request.

   A Login Request with a non-zero TSIH and a CID equal to that of an
   existing connection implies a logout of the connection followed by a
   login (see Section 6.3.4).  For details regarding the implicit Logout
   Request, see Section 11.14.

11.12.8. CmdSN

   The CmdSN is either the initial command sequence number of a session
   (for the first Login Request of a session -- the "leading" login) or
   the command sequence number in the command stream if the login is for
   a new connection in an existing session.

   Examples:

   - Login on a leading connection: If the leading login carries the
     CmdSN 123, all other Login Requests in the same Login Phase carry
     the CmdSN 123, and the first non-immediate command in the Full
     Feature Phase also carries the CmdSN 123.

   - Login on other than a leading connection: If the current CmdSN at
     the time the first login on the connection is issued is 500, then
     that PDU carries CmdSN=500.  Subsequent Login Requests that are
     needed to complete this Login Phase may carry a CmdSN higher than
     500 if non-immediate requests that were issued on other connections
     in the same session advance the CmdSN.

   If the Login Request is a leading Login Request, the target MUST use
   the value presented in the CmdSN as the target value for the
   ExpCmdSN.

11.12.9. ExpStatSN

   For the first Login Request on a connection, this is the ExpStatSN
   for the old connection, and this field is only valid if the Login
   Request restarts a connection (see Section 6.3.4).

   For subsequent Login Requests, it is used to acknowledge the Login
   Responses with their increasing StatSN values.

11.12.10. Login Parameters

   The initiator MUST provide some basic parameters in order to enable
   the target to determine if the initiator may use the target's
   resources and the initial text parameters for the security exchange.

   All the rules specified in Section 11.10.5 for Text Requests also
   hold for Login Requests.  Keys and their explanations are listed in
   Section 12 (security negotiation keys) and in Section 13 (operational

   parameter negotiation keys).  All keys listed in Section 13, except
   for the X extension formats, MUST be supported by iSCSI initiators
   and targets.  Keys listed in Section 12 only need to be supported
   when the function to which they refer is mandatory to implement.

11.13. Login Response

   The Login Response indicates the progress and/or end of the Login
   Phase.

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x23      |T|C|.|.|CSG|NSG| Version-max   |Version-active |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| ISID                                                          |
     +                               +---------------+---------------+
   12|                               | TSIH                          |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| Status-Class  | Status-Detail | Reserved                      |
     +---------------+---------------+---------------+---------------+
   40/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48/ DataSegment - Login Parameters in Text Request Format         /
    +/                                                               /
     +---------------+---------------+---------------+---------------+

11.13.1. Version-max

   This is the highest version number supported by the target.

   All Login Responses within the Login Phase MUST carry the same
   Version-max.

The initiator MUST use the value presented as a response to the first
Login Request.

11.13.2. Version-active

   Version-active indicates the highest version supported by the target
   and initiator.  If the target does not support a version within the
   range specified by the initiator, the target rejects the login and
   this field indicates the lowest version supported by the target.

   All Login Responses within the Login Phase MUST carry the same
   Version-active.

   The initiator MUST use the value presented as a response to the first
   Login Request.

11.13.3. TSIH

   The TSIH is the target-assigned session-identifying handle.  Its
   internal format and content are not defined by this protocol, except
   for the value 0, which is reserved.  With the exception of the Login
   Final-Response in a new session, this field should be set to the TSIH
   provided by the initiator in the Login Request.  For a new session,
   the target MUST generate a non-zero TSIH and ONLY return it in the
   Login Final-Response (see Section 6.3).

11.13.4. StatSN

   For the first Login Response (the response to the first Login
   Request), this is the starting status sequence number for the
   connection.  The next response of any kind -- including the next
   Login Response, if any, in the same Login Phase -- will carry this
   number + 1.  This field is only valid if the Status-Class is 0.

11.13.5. Status-Class and Status-Detail

   The Status returned in a Login Response indicates the execution
   status of the Login Phase.  The status includes:

      Status-Class

      Status-Detail

   A Status-Class of 0 indicates success.

   A non-zero Status-Class indicates an exception.  In this case,
   Status-Class is sufficient for a simple initiator to use when
   handling exceptions, without having to look at the Status-Detail.

   The Status-Detail allows finer-grained exception handling for more
   sophisticated initiators and for better information for logging.

   The Status-Classes are as follows:

      0  Success - indicates that the iSCSI target successfully
         received, understood, and accepted the request.  The numbering
         fields (StatSN, ExpCmdSN, MaxCmdSN) are only valid if Status-
         Class is 0.

      1  Redirection - indicates that the initiator must take further
         action to complete the request.  This is usually due to the
         target moving to a different address.  All of the redirection
         Status-Class responses MUST return one or more text key
         parameters of the type "TargetAddress", which indicates the
         target's new address.  A redirection response MAY be issued by
         a target prior to or after completing a security negotiation if
         a security negotiation is required.  A redirection SHOULD be
         accepted by an initiator, even without having the target
         complete a security negotiation if any security negotiation is
         required, and MUST be accepted by the initiator after the
         completion of the security negotiation if any security
         negotiation is required.

      2  Initiator Error (not a format error) - indicates that the
         initiator most likely caused the error.  This MAY be due to a
         request for a resource for which the initiator does not have
         permission.  The request should not be tried again.

      3  Target Error - indicates that the target sees no errors in the
         initiator's Login Request but is currently incapable of
         fulfilling the request.  The initiator may retry the same Login
         Request later.

   The table below shows all of the currently allocated status codes.
   The codes are in hexadecimal; the first byte is the Status-Class, and
   the second byte is the status detail.

     -----------------------------------------------------------------
     Status        | Code | Description
                   |(hex) |
     -----------------------------------------------------------------
     Success       | 0000 | Login is proceeding OK (*1).
     -----------------------------------------------------------------
     Target moved  | 0101 | The requested iSCSI Target Name (ITN)
     temporarily   |      | has temporarily moved
                   |      | to the address provided.
     -----------------------------------------------------------------
     Target moved  | 0102 | The requested ITN has permanently moved
     permanently   |      | to the address provided.
     -----------------------------------------------------------------
     Initiator     | 0200 | Miscellaneous iSCSI initiator
     error         |      | errors.
     -----------------------------------------------------------------
     Authentication| 0201 | The initiator could not be
     failure       |      | successfully authenticated or target
                   |      | authentication is not supported.
     -----------------------------------------------------------------
     Authorization | 0202 | The initiator is not allowed access
     failure       |      | to the given target.
     -----------------------------------------------------------------
     Not found     | 0203 | The requested ITN does not
                   |      | exist at this address.
     -----------------------------------------------------------------
     Target removed| 0204 | The requested ITN has been removed, and
                   |      | no forwarding address is provided.
     -----------------------------------------------------------------
     Unsupported   | 0205 | The requested iSCSI version range is
     version       |      | not supported by the target.
     -----------------------------------------------------------------
     Too many      | 0206 | Too many connections on this SSID.
     connections   |      |
     -----------------------------------------------------------------
     Missing       | 0207 | Missing parameters (e.g., iSCSI
     parameter     |      | Initiator Name and/or Target Name).
     -----------------------------------------------------------------
     Can't include | 0208 | Target does not support session
     in session    |      | spanning to this connection (address).
     -----------------------------------------------------------------
     Session type  | 0209 | Target does not support this type of
     not supported |      | session or not from this initiator.
     -----------------------------------------------------------------

     Session does  | 020a | Attempt to add a connection
     not exist     |      | to a non-existent session.
     -----------------------------------------------------------------
     Invalid during| 020b | Invalid request type during login.
     login         |      |
     -----------------------------------------------------------------
     Target error  | 0300 | Target hardware or software error.
     -----------------------------------------------------------------
     Service       | 0301 | The iSCSI service or target is not
     unavailable   |      | currently operational.
     -----------------------------------------------------------------
     Out of        | 0302 | The target has insufficient session,
     resources     |      | connection, or other resources.
     -----------------------------------------------------------------

   (*1) If the response T bit is set to 1 in both the request and the
        matching response, and the NSG is set to FullFeaturePhase in
        both the request and the matching response, the Login Phase is
        finished, and the initiator may proceed to issue SCSI commands.

   If the Status-Class is not 0, the initiator and target MUST close the
   TCP connection.

   If the target wishes to reject the Login Request for more than one
   reason, it should return the primary reason for the rejection.

11.13.6. T (Transit) Bit

   The T bit is set to 1 as an indicator of the end of the stage.  If
   the T bit is set to 1 and the NSG is set to FullFeaturePhase, then
   this is also the Login Final-Response (see Section 6.3).  A T bit of
   0 indicates a "partial" response, which means "more negotiation
   needed".

   A Login Response with the T bit set to 1 MUST NOT contain key=value
   pairs that may require additional answers from the initiator within
   the same stage.

   If the Status-Class is 0, the T bit MUST NOT be set to 1 if the T bit
   in the request was set to 0.

11.13.7. C (Continue) Bit

   When set to 1, this bit indicates that the text (set of key=value
   pairs) in this Login Response is not complete (it will be continued
   on subsequent Login Responses); otherwise, it indicates that this
   Login Response ends a set of key=value pairs.  A Login Response with
   the C bit set to 1 MUST have the T bit set to 0.

11.13.8. Login Parameters

   The target MUST provide some basic parameters in order to enable the
   initiator to determine if it is connected to the correct port and the
   initial text parameters for the security exchange.

   All the rules specified in Section 11.11.6 for Text Responses also
   hold for Login Responses.  Keys and their explanations are listed in
   Section 12 (security negotiation keys) and in Section 13 (operational
   parameter negotiation keys).  All keys listed in Section 13, except
   for the X extension formats, MUST be supported by iSCSI initiators
   and targets.  Keys listed in Section 12 only need to be supported
   when the function to which they refer is mandatory to implement.

11.14. Logout Request

   The Logout Request is used to perform a controlled closing of a
   connection.

   An initiator MAY use a Logout Request to remove a connection from a
   session or to close an entire session.

   After sending the Logout Request PDU, an initiator MUST NOT send any
   new iSCSI requests on the closing connection.  If the Logout Request
   is intended to close the session, new iSCSI requests MUST NOT be sent
   on any of the connections participating in the session.

   When receiving a Logout Request with the reason code "close the
   connection" or "close the session", the target MUST terminate all
   pending commands, whether acknowledged via the ExpCmdSN or not, on
   that connection or session, respectively.

   When receiving a Logout Request with the reason code "remove the
   connection for recovery", the target MUST discard all requests not
   yet acknowledged via the ExpCmdSN that were issued on the specified
   connection and suspend all data/status/R2T transfers on behalf of
   pending commands on the specified connection.

   The target then issues the Logout Response and half-closes the TCP
   connection (sends FIN).  After receiving the Logout Response and
   attempting to receive the FIN (if still possible), the initiator MUST
   completely close the logging-out connection.  For the terminated
   commands, no additional responses should be expected.

   A Logout for a CID may be performed on a different transport
   connection when the TCP connection for the CID has already been
   terminated.  In such a case, only a logical "closing" of the iSCSI
   connection for the CID is implied with a Logout.

All commands that were not terminated or not completed (with status)
 and acknowledged when the connection is closed completely can be
 reassigned to a new connection if the target supports connection
 recovery.

 If an initiator intends to start recovery for a failing connection,
 it MUST use the Logout Request to "clean up" the target end of a
 failing connection and enable recovery to start, or use the Login
 Request with a non-zero TSIH and the same CID on a new connection for
 the same effect. In sessions with a single connection, the
 connection can be closed and then a new connection reopened. A
 connection reinstatement login can be used for recovery (see
 Section 6.3.4).

 A successful completion of a Logout Request with the reason code
 "close the connection" or "remove the connection for recovery"
 results at the target in the discarding of unacknowledged commands
 received on the connection being logged out. These are commands that
 have arrived on the connection being logged out but that have not
 been delivered to SCSI because one or more commands with a smaller
 CmdSN have not been received by iSCSI. See Section 4.2.2.1. The
 resulting holes in the command sequence numbers will have to be
 handled by appropriate recovery (see Section 7), unless the session
 is also closed.

 The entire logout discussion in this section is also applicable for
 an implicit Logout realized by way of a connection reinstatement or
 session reinstatement. When a Login Request performs an implicit
 Logout, the implicit Logout is performed as if having the reason
 codes specified below:

 Reason Code Type of Implicit Logout
 -------------------------------------------------------------

 0 session reinstatement

 1 connection reinstatement when the operational
 ErrorRecoveryLevel < 2

 2 connection reinstatement when the operational
 ErrorRecoveryLevel = 2

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|I| 0x06      |1| Reason Code | Reserved                      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------------------------------------------------------+
    8/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| CID or Reserved               | Reserved                      |
     +---------------+---------------+---------------+---------------+
   24| CmdSN                                                         |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+

11.14.1. Reason Code

   The Reason Code field indicates the reason for Logout as follows:

      0 - close the session.  All commands associated with the
          session (if any) are terminated.

      1 - close the connection.  All commands associated with the
          connection (if any) are terminated.

      2 - remove the connection for recovery.  The connection is
          closed, and all commands associated with it, if any, are
          to be prepared for a new allegiance.

   All other values are reserved.

11.14.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.14.3. CID

   This is the connection ID of the connection to be closed (including
   closing the TCP stream).  This field is only valid if the reason code
   is not "close the session".

11.14.4. ExpStatSN

This is the last ExpStatSN value for the connection to be closed.

11.14.5. Implicit Termination of Tasks

   A target implicitly terminates the active tasks due to the iSCSI
   protocol in the following cases:

      a) When a connection is implicitly or explicitly logged out with
         the reason code "close the connection" and there are active
         tasks allegiant to that connection.

      b) When a connection fails and eventually the connection state
         times out (state transition M1 in Section 8.2.2) and there are
         active tasks allegiant to that connection.

      c) When a successful recovery Logout is performed while there are
         active tasks allegiant to that connection and those tasks
         eventually time out after the Time2Wait and Time2Retain periods
         without allegiance reassignment.

      d) When a connection is implicitly or explicitly logged out with
         the reason code "close the session" and there are active tasks
         in that session.

   If the tasks terminated in any of the above cases are SCSI tasks,
   they must be internally terminated as if with CHECK CONDITION status.
   This status is only meaningful for appropriately handling the
   internal SCSI state and SCSI side effects with respect to ordering,
   because this status is never communicated back as a terminating
   status to the initiator.  However, additional actions may have to be
   taken at the SCSI level, depending on the SCSI context as defined by
   the SCSI standards (e.g., queued commands and ACA; UA for the next
   command on the I_T nexus in cases a), b), and c) above).  After the
   tasks are terminated, the target MUST report a Unit Attention
   condition on the next command processed on any connection for each
   affected I_T_L nexus with the status of CHECK CONDITION, the ASC/ASCQ
   value of 47h/7Fh ("SOME COMMANDS CLEARED BY ISCSI PROTOCOL EVENT"),
   etc.; see [SPC3].

11.15. Logout Response

   The Logout Response is used by the target to indicate if the cleanup
   operation for the connection(s) has completed.

   After Logout, the TCP connection referred by the CID MUST be closed
   at both ends (or all connections must be closed if the logout reason
   was session close).

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x26      |1| Reserved    | Response      | Reserved      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------------------------------------------------------+
    8/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag                                            |
     +---------------+---------------+---------------+---------------+
   20| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| Reserved                                                      |
     +---------------------------------------------------------------+
   40| Time2Wait                     | Time2Retain                   |
     +---------------+---------------+---------------+---------------+
   44| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+

11.15.1. Response

   Response field settings are as follows:

      0 - connection or session closed successfully.

      1 - CID not found.

      2 - connection recovery is not supported (i.e., the Logout reason
          code was "remove the connection for recovery" and the target
          does not support it as indicated by the operational
          ErrorRecoveryLevel).

      3 - cleanup failed for various reasons.

11.15.2. TotalAHSLength and DataSegmentLength

For this PDU, TotalAHSLength and DataSegmentLength MUST be 0.

11.15.3. Time2Wait

   If the Logout response code is 0 and the operational
   ErrorRecoveryLevel is 2, this is the minimum amount of time, in
   seconds, to wait before attempting task reassignment.  If the Logout
   response code is 0 and the operational ErrorRecoveryLevel is less
   than 2, this field is to be ignored.

   This field is invalid if the Logout response code is 1.

   If the Logout response code is 2 or 3, this field specifies the
   minimum time to wait before attempting a new implicit or explicit
   logout.

   If Time2Wait is 0, the reassignment or a new Logout may be attempted
   immediately.

11.15.4. Time2Retain

   If the Logout response code is 0 and the operational
   ErrorRecoveryLevel is 2, this is the maximum amount of time, in
   seconds, after the initial wait (Time2Wait) that the target waits for
   the allegiance reassignment for any active task, after which the task
   state is discarded.  If the Logout response code is 0 and the
   operational ErrorRecoveryLevel is less than 2, this field is to be
   ignored.

   This field is invalid if the Logout response code is 1.

   If the Logout response code is 2 or 3, this field specifies the
   maximum amount of time, in seconds, after the initial wait
   (Time2Wait) that the target waits for a new implicit or explicit
   logout.

   If it is the last connection of a session, the whole session state is
   discarded after Time2Retain.

   If Time2Retain is 0, the target has already discarded the connection
   (and possibly the session) state along with the task states.  No
   reassignment or Logout is required in this case.

11.16. SNACK Request

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x10      |1|.|.|.| Type  | Reserved                      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag or 0xffffffff                              |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or SNACK Tag or 0xffffffff                |
     +---------------+---------------+---------------+---------------+
   24| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   40| BegRun                                                        |
     +---------------------------------------------------------------+
   44| RunLength                                                     |
     +---------------------------------------------------------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+

   If the implementation supports ErrorRecoveryLevel greater than zero,
   it MUST support all SNACK types.

   The SNACK is used by the initiator to request the retransmission of
   numbered responses, data, or R2T PDUs from the target.  The SNACK
   Request indicates the numbered responses or data "runs" whose
   retransmission is requested, where the run starts with the first
   StatSN, DataSN, or R2TSN whose retransmission is requested and
   indicates the number of Status, Data, or R2T PDUs requested,
   including the first.  0 has special meaning when used as a starting
   number and length:

      - When used in RunLength, it means all PDUs starting with the
        initial.

      - When used in both BegRun and RunLength, it means all
        unacknowledged PDUs.

   The numbered response(s) or R2T(s) requested by a SNACK MUST be
   delivered as exact replicas of the ones that the target transmitted
   originally, except for the fields ExpCmdSN, MaxCmdSN, and ExpDataSN,
   which MUST carry the current values.  R2T(s)requested by SNACK MUST
   also carry the current value of the StatSN.

   The numbered Data-In PDUs requested by a Data SNACK MUST be delivered
   as exact replicas of the ones that the target transmitted originally,
   except for the fields ExpCmdSN and MaxCmdSN, which MUST carry the
   current values; and except for resegmentation (see Section 11.16.3).

   Any SNACK that requests a numbered response, data, or R2T that was
   not sent by the target or was already acknowledged by the initiator
   MUST be rejected with a reason code of "Protocol Error".

11.16.1. Type

   This field encodes the SNACK function as follows:

      0 - Data/R2T SNACK: requesting retransmission of one or more
          Data-In or R2T PDUs.

      1 - Status SNACK: requesting retransmission of one or more
          numbered responses.

      2 - DataACK: positively acknowledges Data-In PDUs.

      3 - R-Data SNACK: requesting retransmission of Data-In PDUs with
          possible resegmentation and status tagging.

   All other values are reserved.

Data/R2T SNACK, Status SNACK, or R-Data SNACK for a command MUST
precede status acknowledgment for the given command.

11.16.2. Data Acknowledgment

   If an initiator operates at ErrorRecoveryLevel 1 or higher, it MUST
   issue a SNACK of type DataACK after receiving a Data-In PDU with the
   A bit set to 1.  However, if the initiator has detected holes in the
   input sequence, it MUST postpone issuing the SNACK of type DataACK
   until the holes are filled.  An initiator MAY ignore the A bit if it
   deems that the bit is being set aggressively by the target (i.e.,
   before the MaxBurstLength limit is reached).

   The DataACK is used to free resources at the target and not to
   request or imply data retransmission.

   An initiator MUST NOT request retransmission for any data it had
   already acknowledged.

11.16.3. Resegmentation

   If the initiator MaxRecvDataSegmentLength changed between the
   original transmission and the time the initiator requests
   retransmission, the initiator MUST issue a R-Data SNACK (see
   Section 11.16.1).  With R-Data SNACK, the initiator indicates that it
   discards all the unacknowledged data and expects the target to resend
   it.  It also expects resegmentation.  In this case, the retransmitted
   Data-In PDUs MAY be different from the ones originally sent in order
   to reflect changes in MaxRecvDataSegmentLength.  Their DataSN starts
   with the BegRun of the last DataACK received by the target if any was
   received; otherwise, it starts with 0 and is increased by 1 for each
   resent Data-In PDU.

   A target that has received a R-Data SNACK MUST return a SCSI Response
   that contains a copy of the SNACK Tag field from the R-Data SNACK in
   the SCSI Response SNACK Tag field as its last or only Response.  For
   example, if it has already sent a response containing another value
   in the SNACK Tag field or had the status included in the last Data-In
   PDU, it must send a new SCSI Response PDU.  If a target sends more
   than one SCSI Response PDU due to this rule, all SCSI Response PDUs
   must carry the same StatSN (see Section 11.4.4).  If an initiator
   attempts to recover a lost SCSI Response (with a Status-SNACK; see
   Section 11.16.1) when more than one response has been sent, the
   target will send the SCSI Response with the latest content known to
   the target, including the last SNACK Tag for the command.

   For considerations in allegiance reassignment of a task to a
   connection with a different MaxRecvDataSegmentLength, refer to
   Section 7.2.2.

11.16.4. Initiator Task Tag

   For a Status SNACK and DataACK, the Initiator Task Tag MUST be set to
   the reserved value 0xffffffff.  In all other cases, the Initiator
   Task Tag field MUST be set to the Initiator Task Tag of the
   referenced command.

11.16.5. Target Transfer Tag or SNACK Tag

   For a R-Data SNACK, this field MUST contain a value that is different
   from 0 or 0xffffffff and is unique for the task (identified by the
   Initiator Task Tag).  This value MUST be copied by the iSCSI target
   in the last or only SCSI Response PDU it issues for the command.

   For DataACK, the Target Transfer Tag MUST contain a copy of the
   Target Transfer Tag and LUN provided with the SCSI Data-In PDU with
   the A bit set to 1.

   In all other cases, the Target Transfer Tag field MUST be set to the
   reserved value 0xffffffff.

11.16.6. BegRun

   This field indicates the DataSN, R2TSN, or StatSN of the first PDU
   whose retransmission is requested (Data/R2T and Status SNACK), or the
   next expected DataSN (DataACK SNACK).

   A BegRun of 0, when used in conjunction with a RunLength of 0, means
   "resend all unacknowledged Data-In, R2T or Response PDUs".

   BegRun MUST be 0 for a R-Data SNACK.

11.16.7. RunLength

   This field indicates the number of PDUs whose retransmission is
   requested.

   A RunLength of 0 signals that all Data-In, R2T, or Response PDUs
   carrying the numbers equal to or greater than BegRun have to be
   resent.

   The RunLength MUST also be 0 for a DataACK SNACK in addition to a
   R-Data SNACK.

11.17. Reject

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x3f      |1| Reserved    | Reason        | Reserved      |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   16| 0xffffffff                                                    |
     +---------------+---------------+---------------+---------------+
   20| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36| DataSN/R2TSN or Reserved                                      |
     +---------------+---------------+---------------+---------------+
   40| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   44| Reserved                                                      |
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
   xx/ Complete Header of Bad PDU                                    /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   yy/Vendor-specific data (if any)                                  /
     /                                                               /
     +---------------+---------------+---------------+---------------+
   zz| Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   Reject is used to indicate an iSCSI error condition (protocol,
   unsupported option, etc.).

11.17.1. Reason

   The reject Reason is coded as follows:

   +------+----------------------------------------+----------------+
   | Code | Explanation                            |Can the original|
   | (hex)|                                        |PDU be resent?  |
   +------+----------------------------------------+----------------+
   | 0x01 | Reserved                               | no             |
   |      |                                        |                |
   | 0x02 | Data (payload) digest error            | yes (Note 1)   |
   |      |                                        |                |
   | 0x03 | SNACK Reject                           | yes            |
   |      |                                        |                |
   | 0x04 | Protocol Error (e.g., SNACK Request for| no             |
   |      | a status that was already acknowledged)|                |
   |      |                                        |                |
   | 0x05 | Command not supported                  | no             |
   |      |                                        |                |
   | 0x06 | Immediate command reject - too many    | yes            |
   |      | immediate commands                     |                |
   |      |                                        |                |
   | 0x07 | Task in progress                       | no             |
   |      |                                        |                |
   | 0x08 | Invalid data ack                       | no             |
   |      |                                        |                |
   | 0x09 | Invalid PDU field                      | no (Note 2)    |
   |      |                                        |                |
   | 0x0a | Long op reject - Can't generate Target | yes            |
   |      | Transfer Tag - out of resources        |                |
   |      |                                        |                |
   | 0x0b | Deprecated; MUST NOT be used           | N/A (Note 3)   |
   |      |                                        |                |
   | 0x0c | Waiting for Logout                     | no             |
   +------+----------------------------------------+----------------+

   Note 1: For iSCSI, Data-Out PDU retransmission is only done if the
           target requests retransmission with a recovery R2T.  However,
           if this is the data digest error on immediate data, the
           initiator may choose to retransmit the whole PDU, including
           the immediate data.

   Note 2: A target should use this reason code for all invalid values
           of PDU fields that are meant to describe a task, a response,
           or a data transfer.  Some examples are invalid TTT/ITT,
           buffer offset, LUN qualifying a TTT, and an invalid sequence
           number in a SNACK.

   Note 3: Reason code 0x0b ("Negotiation Reset") as defined in
           Section 10.17.1 of [RFC3720] is deprecated and MUST NOT be
           used by implementations.  An implementation receiving reason
           code 0x0b MUST treat it as a negotiation failure that
           terminates the Login Phase and the TCP connection, as
           specified in Section 7.12.

   All other values for Reason are unassigned.

   In all the cases in which a pre-instantiated SCSI task is terminated
   because of the reject, the target MUST issue a proper SCSI command
   response with CHECK CONDITION as described in Section 11.4.3.  In
   these cases in which a status for the SCSI task was already sent
   before the reject, no additional status is required.  If the error is
   detected while data from the initiator is still expected (i.e., the
   command PDU did not contain all the data and the target has not
   received a Data-Out PDU with the Final bit set to 1 for the
   unsolicited data, if any, and all outstanding R2Ts, if any), the
   target MUST wait until it receives the last expected Data-Out PDUs
   with the F bit set to 1 before sending the Response PDU.

   For additional usage semantics of the Reject PDU, see Section 7.3.

11.17.2. DataSN/R2TSN

   This field is only valid if the rejected PDU is a Data/R2T SNACK and
   the Reject reason code is "Protocol Error" (see Section 11.16).  The
   DataSN/R2TSN is the next Data/R2T sequence number that the target
   would send for the task, if any.

11.17.3. StatSN, ExpCmdSN, and MaxCmdSN

These fields carry their usual values and are not related to the
rejected command. The StatSN is advanced after a Reject.

11.17.4. Complete Header of Bad PDU

The target returns the header (not including the digest) of the PDU
in error as the data of the response.

11.18. NOP-Out

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|I| 0x00      |1| Reserved                                    |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag or 0xffffffff                              |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| CmdSN                                                         |
     +---------------+---------------+---------------+---------------+
   28| ExpStatSN                                                     |
     +---------------+---------------+---------------+---------------+
   32/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment - Ping Data (optional)                            /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   NOP-Out may be used by an initiator as a "ping request" to verify
   that a connection/session is still active and all its components are
   operational.  The NOP-In response is the "ping echo".

   A NOP-Out is also sent by an initiator in response to a NOP-In.

   A NOP-Out may also be used to confirm a changed ExpStatSN if another
   PDU will not be available for a long time.

   Upon receipt of a NOP-In with the Target Transfer Tag set to a valid
   value (not the reserved value 0xffffffff), the initiator MUST respond
   with a NOP-Out.  In this case, the NOP-Out Target Transfer Tag MUST
   contain a copy of the NOP-In Target Transfer Tag.  The initiator

   SHOULD NOT send a NOP-Out in response to any other received NOP-In,
   in order to avoid lengthy sequences of NOP-In and NOP-Out PDUs sent
   in response to each other.

11.18.1. Initiator Task Tag

   The NOP-Out MUST have the Initiator Task Tag set to a valid value
   only if a response in the form of a NOP-In is requested (i.e., the
   NOP-Out is used as a ping request).  Otherwise, the Initiator Task
   Tag MUST be set to 0xffffffff.

   When a target receives the NOP-Out with a valid Initiator Task Tag,
   it MUST respond with a NOP-In Response (see Section 4.6.3.6).

   If the Initiator Task Tag contains 0xffffffff, the I bit MUST be set
   to 1, and the CmdSN is not advanced after this PDU is sent.

11.18.2. Target Transfer Tag

   The Target Transfer Tag is a target-assigned identifier for the
   operation.

   The NOP-Out MUST only have the Target Transfer Tag set if it is
   issued in response to a NOP-In with a valid Target Transfer Tag.  In
   this case, it copies the Target Transfer Tag from the NOP-In PDU.
   Otherwise, the Target Transfer Tag MUST be set to 0xffffffff.

   When the Target Transfer Tag is set to a value other than 0xffffffff,
   the LUN field MUST also be copied from the NOP-In.

11.18.3. Ping Data

   Ping data is reflected in the NOP-In Response.  The length of the
   reflected data is limited to MaxRecvDataSegmentLength.  The length of
   ping data is indicated by the DataSegmentLength.  0 is a valid value
   for the DataSegmentLength and indicates the absence of ping data.

11.19. NOP-In

   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0|.|.| 0x20      |1| Reserved                                    |
     +---------------+---------------+---------------+---------------+
    4|TotalAHSLength | DataSegmentLength                             |
     +---------------+---------------+---------------+---------------+
    8| LUN or Reserved                                               |
     +                                                               +
   12|                                                               |
     +---------------+---------------+---------------+---------------+
   16| Initiator Task Tag or 0xffffffff                              |
     +---------------+---------------+---------------+---------------+
   20| Target Transfer Tag or 0xffffffff                             |
     +---------------+---------------+---------------+---------------+
   24| StatSN                                                        |
     +---------------+---------------+---------------+---------------+
   28| ExpCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   32| MaxCmdSN                                                      |
     +---------------+---------------+---------------+---------------+
   36/ Reserved                                                      /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
   48| Header-Digest (optional)                                      |
     +---------------+---------------+---------------+---------------+
     / DataSegment - Return Ping Data                                /
    +/                                                               /
     +---------------+---------------+---------------+---------------+
     | Data-Digest (optional)                                        |
     +---------------+---------------+---------------+---------------+

   NOP-In is sent by a target as either a response to a NOP-Out, a
   "ping" to an initiator, or a means to carry a changed ExpCmdSN and/or
   MaxCmdSN if another PDU will not be available for a long time (as
   determined by the target).

   When a target receives the NOP-Out with a valid Initiator Task Tag
   (not the reserved value 0xffffffff), it MUST respond with a NOP-In
   with the same Initiator Task Tag that was provided in the NOP-Out
   request.  It MUST also duplicate up to the first
   MaxRecvDataSegmentLength bytes of the initiator-provided Ping Data.
   For such a response, the Target Transfer Tag MUST be 0xffffffff.  The

   target SHOULD NOT send a NOP-In in response to any other received
   NOP-Out in order to avoid lengthy sequences of NOP-In and NOP-Out
   PDUs sent in response to each other.

   Otherwise, when a target sends a NOP-In that is not a response to a
   NOP-Out received from the initiator, the Initiator Task Tag MUST be
   set to 0xffffffff, and the data segment MUST NOT contain any data
   (DataSegmentLength MUST be 0).

11.19.1. Target Transfer Tag

   If the target is responding to a NOP-Out, this field is set to the
   reserved value 0xffffffff.

   If the target is sending a NOP-In as a ping (intending to receive a
   corresponding NOP-Out), this field is set to a valid value (not the
   reserved value 0xffffffff).

   If the target is initiating a NOP-In without wanting to receive a
   corresponding NOP-Out, this field MUST hold the reserved value
   0xffffffff.

11.19.2. StatSN

   The StatSN field will always contain the next StatSN.  However, when
   the Initiator Task Tag is set to 0xffffffff, the StatSN for the
   connection is not advanced after this PDU is sent.

11.19.3. LUN

A LUN MUST be set to a correct value when the Target Transfer Tag is
valid (not the reserved value 0xffffffff).

12. iSCSI Security Text Keys and Authentication Methods

   Only the following keys are used during the SecurityNegotiation stage
   of the Login Phase:

      SessionType

      InitiatorName

      TargetName

      TargetAddress

      InitiatorAlias

      TargetAlias

      TargetPortalGroupTag

      AuthMethod and the keys used by the authentication methods
         specified in Section 12.1, along with all of their associated
         keys, as well as Vendor-Specific Authentication Methods.

   Other keys MUST NOT be used.

   SessionType, InitiatorName, TargetName, InitiatorAlias, TargetAlias,
   and TargetPortalGroupTag are described in Section 13 as they can be
   used in the OperationalNegotiation stage as well.

   All security keys have connection-wide applicability.

12.1. AuthMethod

Use: During Login - Security Negotiation
 Senders: Initiator and target
 Scope: connection

 AuthMethod = <list-of-values>

 The main item of security negotiation is the authentication method
 (AuthMethod).

 The authentication methods that can be used (appear in the list-of-
 values) are either vendor-unique methods or those listed in the
 following table:

 +--------------------------------------------------------------+
 | Name | Description |
 +--------------------------------------------------------------+
 | KRB5 | Kerberos V5 - defined in [RFC4120] |
 +--------------------------------------------------------------+
 | SRP | Secure Remote Password - |
 | | defined in [RFC2945] |
 +--------------------------------------------------------------+
 | CHAP | Challenge Handshake Authentication Protocol - |
 | | defined in [RFC1994] |
 +--------------------------------------------------------------+
 | None | No authentication |
 +--------------------------------------------------------------+

 The AuthMethod selection is followed by an "authentication exchange"
 specific to the authentication method selected.

   The authentication method proposal may be made by either the
   initiator or the target.  However, the initiator MUST make the first
   step specific to the selected authentication method as soon as it is
   selected.  It follows that if the target makes the authentication
   method proposal, the initiator sends the first key(s) of the exchange
   together with its authentication method selection.

   The authentication exchange authenticates the initiator to the target
   and, optionally, the target to the initiator.  Authentication is
   OPTIONAL to use but MUST be supported by the target and initiator.

   The initiator and target MUST implement CHAP.  All other
   authentication methods are OPTIONAL.

   Private or public extension algorithms MAY also be negotiated for
   authentication methods.  Whenever a private or public extension
   algorithm is part of the default offer (the offer made in the absence
   of explicit administrative action), the implementer MUST ensure that
   CHAP is listed as an alternative in the default offer and "None" is
   not part of the default offer.

   Extension authentication methods MUST be named using one of the
   following two formats:

      1) Z-reversed.vendor.dns_name.do_something=

      2) New public key with no name prefix constraints

   Authentication methods named using the Z- format are used as private
   extensions.  New public keys must be registered with IANA using the
   IETF Review process ([RFC5226]).  New public extensions for
   authentication methods MUST NOT use the Z# name prefix.

   For all of the public or private extension authentication methods,
   the method-specific keys MUST conform to the format specified in
   Section 6.1 for standard-label.

   To identify the vendor for private extension authentication methods,
   we suggest using the reversed DNS-name as a prefix to the proper
   digest names.

   The part of digest-name following Z- MUST conform to the format for
   standard-label specified in Section 6.1.

   Support for public or private extension authentication methods is
   OPTIONAL.

   The following subsections define the specific exchanges for each of
   the standardized authentication methods.  As mentioned earlier, the
   first step is always done by the initiator.

12.1.1. Kerberos

For KRB5 (Kerberos V5) [RFC4120] [RFC1964], the initiator MUST use:

 KRB_AP_REQ=<KRB_AP_REQ>

 where KRB_AP_REQ is the client message as defined in [RFC4120].

 The default principal name assumed by an iSCSI initiator or target
 (prior to any administrative configuration action) MUST be the iSCSI
 Initiator Name or iSCSI Target Name, respectively, prefixed by the
 string "iscsi/".

 If the initiator authentication fails, the target MUST respond with a
 Login reject with "Authentication Failure" status. Otherwise, if the
 initiator has selected the mutual authentication option (by setting
 MUTUAL-REQUIRED in the ap-options field of the KRB_AP_REQ), the
 target MUST reply with:

 KRB_AP_REP=<KRB_AP_REP>

 where KRB_AP_REP is the server's response message as defined in
 [RFC4120].

 If mutual authentication was selected and target authentication
 fails, the initiator MUST close the connection.

 KRB_AP_REQ and KRB_AP_REP are binary-values, and their binary length
 (not the length of the character string that represents them in
 encoded form) MUST NOT exceed 65536 bytes. Hex or Base64 encoding
 may be used for KRB_AP_REQ and KRB_AP_REP; see Section 6.1.

12.1.2. Secure Remote Password (SRP)

For SRP [RFC2945], the initiator MUST use:

 SRP_U= TargetAuth=Yes /* or TargetAuth=No */

 The target MUST answer with a Login reject with the "Authorization
 Failure" status or reply with:

 SRP_GROUP=<G1,G2...> SRP_s=<s>

 where G1,G2... are proposed groups, in order of preference.

The initiator MUST either close the connection or continue with:

 SRP_A=<A> SRP_GROUP=<G>

 where G is one of G1,G2... that were proposed by the target.

 The target MUST answer with a Login reject with the "Authentication
 Failure" status or reply with:

 SRP_B=

 The initiator MUST close the connection or continue with:

 SRP_M=<M>

 If the initiator authentication fails, the target MUST answer with a
 Login reject with "Authentication Failure" status. Otherwise, if the
 initiator sent TargetAuth=Yes in the first message (requiring target
 authentication), the target MUST reply with:

 SRP_HM=<H(A | M | K)>

 If the target authentication fails, the initiator MUST close the
 connection:

 where U, s, A, B, M, and H(A | M | K) are defined in [RFC2945] (using
 the SHA1 hash function, such as SRP-SHA1)

 and

 G,Gn ("Gn" stands for G1,G2...) are identifiers of SRP groups
 specified in [RFC3723].

 G, Gn, and U are text strings; s,A,B,M, and H(A | M | K) are
 binary-values. The length of s,A,B,M and H(A | M | K) in binary form
 (not the length of the character string that represents them in
 encoded form) MUST NOT exceed 1024 bytes. Hex or Base64 encoding may
 be used for s,A,B,M and H(A | M | K); see Section 6.1.

 See Appendix B for the related login example.

 For the SRP_GROUP, all the groups specified in [RFC3723] up to
 1536 bits (i.e., SRP-768, SRP-1024, SRP-1280, SRP-1536) must be
 supported by initiators and targets. To guarantee interoperability,
 targets MUST always offer "SRP-1536" as one of the proposed groups.

12.1.3. Challenge Handshake Authentication Protocol (CHAP)

For CHAP [RFC1994], the initiator MUST use:

 CHAP_A=<A1,A2...>

 where A1,A2... are proposed algorithms, in order of preference.

 The target MUST answer with a Login reject with the "Authentication
 Failure" status or reply with:

 CHAP_A=<A> CHAP_I= CHAP_C=<C>

 where A is one of A1,A2... that were proposed by the initiator.

 The initiator MUST continue with:

 CHAP_N=<N> CHAP_R=<R>

 or, if it requires target authentication, with:

 CHAP_N=<N> CHAP_R=<R> CHAP_I= CHAP_C=<C>

 If the initiator authentication fails, the target MUST answer with a
 Login reject with "Authentication Failure" status. Otherwise, if the
 initiator required target authentication, the target MUST either
 answer with a Login reject with "Authentication Failure" or reply
 with:

 CHAP_N=<N> CHAP_R=<R>

 If the target authentication fails, the initiator MUST close the
 connection:

 where N, (A,A1,A2), I, C, and R are (correspondingly) the Name,
 Algorithm, Identifier, Challenge, and Response as defined in
 [RFC1994].

 N is a text string; A,A1,A2, and I are numbers; C and R are
 binary-values. Their binary length (not the length of the character
 string that represents them in encoded form) MUST NOT exceed
 1024 bytes. Hex or Base64 encoding may be used for C and R; see
 Section 6.1.

 See Appendix B for the related login example.

   For the Algorithm, as stated in [RFC1994], one value is required to
   be implemented:

      5     (CHAP with MD5)

   To guarantee interoperability, initiators MUST always offer it as one
   of the proposed algorithms.

13. Login/Text Operational Text Keys

   Some session-specific parameters MUST only be carried on the leading
   connection and cannot be changed after the leading connection login
   (e.g., MaxConnections -- the maximum number of connections).  This
   holds for a single connection session with regard to connection
   restart.  The keys that fall into this category have the "use: LO"
   (Leading Only).

   Keys that can only be used during login have the "use: IO"
   (Initialize Only), while those that can be used in both the Login
   Phase and Full Feature Phase have the "use: ALL".

   Keys that can only be used during the Full Feature Phase use FFPO
   (Full Feature Phase Only).

   Keys marked as Any-Stage may also appear in the SecurityNegotiation
   stage, while all other keys described in this section are
   operational keys.

   Keys that do not require an answer are marked as Declarative.

   Key scope is indicated as session-wide (SW) or connection-only (CO).

   "Result function", wherever mentioned, states the function that can
   be applied to check the validity of the responder selection.
   "Minimum" means that the selected value cannot exceed the offered
   value.  "Maximum" means that the selected value cannot be lower than
   the offered value.  "AND" means that the selected value must be a
   possible result of a Boolean "and" function with an arbitrary Boolean
   value (e.g., if the offered value is No the selected value must be
   No).  "OR" means that the selected value must be a possible result of
   a Boolean "or" function with an arbitrary Boolean value (e.g., if the
   offered value is Yes the selected value must be Yes).

13.1. HeaderDigest and DataDigest

Use: IO
 Senders: Initiator and target
 Scope: CO
 HeaderDigest = <list-of-values>
 DataDigest = <list-of-values>

 Default is None for both HeaderDigest and DataDigest.

 Digests enable the checking of end-to-end, non-cryptographic data
 integrity beyond the integrity checks provided by the link layers and
 the covering of the whole communication path, including all elements
 that may change the network-level PDUs, such as routers, switches,
 and proxies.

 The following table lists cyclic integrity checksums that can be
 negotiated for the digests and MUST be implemented by every iSCSI
 initiator and target. These digest options only have error detection
 significance.

 +---------------------------------------------+
 | Name | Description | Generator |
 +---------------------------------------------+
 | CRC32C | 32-bit CRC |0x11edc6f41|
 +---------------------------------------------+
 | None | no digest |
 +---------------------------------------------+

 The generator polynomial G(x) for this digest is given in hexadecimal
 notation (e.g., "0x3b" stands for 0011 1011, and the polynomial is
 x**5 + x**4 + x**3 + x + 1).

 When the initiator and target agree on a digest, this digest MUST be
 used for every PDU in the Full Feature Phase.

 Padding bytes, when present in a segment covered by a CRC, SHOULD be
 set to 0 and are included in the CRC.

 The CRC MUST be calculated by a method that produces the same results
 as the following process:

 - The PDU bits are considered as the coefficients of a polynomial
 M(x) of degree n - 1; bit 7 of the lowest numbered byte is
 considered the most significant bit (x**n - 1), followed by bit 6
 of the lowest numbered byte through bit 0 of the highest numbered
 byte (x**0).

- The most significant 32 bits are complemented.

 - The polynomial is multiplied by x**32, then divided by G(x). The
 generator polynomial produces a remainder R(x) of degree <= 31.

 - The coefficients of R(x) are formed into a 32-bit sequence.

 - The bit sequence is complemented, and the result is the CRC.

 - The CRC bits are mapped into the digest word. The x**31
 coefficient is mapped to bit 7 of the lowest numbered byte of the
 digest, and the mapping continues with successive coefficients and
 bits so that the x**24 coefficient is mapped to bit 0 of the lowest
 numbered byte. The mapping continues further with the x**23
 coefficient mapped to bit 7 of the next byte in the digest until
 the x**0 coefficient is mapped to bit 0 of the highest numbered
 byte of the digest.

 - Computing the CRC over any segment (data or header) extended to
 include the CRC built using the generator 0x11edc6f41 will always
 get the value 0x1c2d19ed as its final remainder (R(x)). This value
 is given here in its polynomial form (i.e., not mapped as the
 digest word).

 For a discussion about selection criteria for the CRC, see [RFC3385].
 For a detailed analysis of the iSCSI polynomial, see [Castagnoli93].

 Private or public extension algorithms MAY also be negotiated for
 digests. Whenever a private or public digest extension algorithm is
 part of the default offer (the offer made in the absence of explicit
 administrative action), the implementer MUST ensure that CRC32C is
 listed as an alternative in the default offer and "None" is not part
 of the default offer.

 Extension digest algorithms MUST be named using one of the following
 two formats:

 1) Y-reversed.vendor.dns_name.do_something=

 2) New public key with no name prefix constraints

 Digests named using the Y- format are used for private purposes
 (unregistered). New public keys must be registered with IANA using
 the IETF Review process ([RFC5226]). New public extensions for
 digests MUST NOT use the Y# name prefix.

 For private extension digests, to identify the vendor we suggest
 using the reversed DNS-name as a prefix to the proper digest names.

   The part of digest-name following Y- MUST conform to the format for
   standard-label specified in Section 6.1.

   Support for public or private extension digests is OPTIONAL.

13.2. MaxConnections

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery

 MaxConnections=<numerical-value-from-1-to-65535>

 Default is 1.
 Result function is Minimum.

 The initiator and target negotiate the maximum number of connections
 requested/acceptable.

13.3. SendTargets

   Use: FFPO
   Senders: Initiator
   Scope: SW

   For a complete description, see Appendix C.

13.4. TargetName

Use: IO by initiator, FFPO by target -- only as response to a
 SendTargets, Declarative, Any-Stage
 Senders: Initiator and target
 Scope: SW

 TargetName=<iSCSI-name-value>

 Examples:

 TargetName=iqn.1993-11.com.disk-vendor:diskarrays.sn.45678

 TargetName=eui.020000023B040506

 TargetName=naa.62004567BA64678D0123456789ABCDEF

   The initiator of the TCP connection MUST provide this key to the
   remote endpoint in the first Login Request if the initiator is not
   establishing a Discovery session.  The iSCSI Target Name specifies
   the worldwide unique name of the target.

   The TargetName key may also be returned by the SendTargets Text
   Request (which is its only use when issued by a target).

   The TargetName MUST NOT be redeclared within the Login Phase.

13.5. InitiatorName

Use: IO, Declarative, Any-Stage
 Senders: Initiator
 Scope: SW

 InitiatorName=<iSCSI-name-value>

 Examples:

 InitiatorName=iqn.1992-04.com.os-vendor.plan9:cdrom.12345

 InitiatorName=iqn.2001-02.com.ssp.users:customer235.host90

 InitiatorName=naa.52004567BA64678D

 The initiator of the TCP connection MUST provide this key to the
 remote endpoint at the first login of the Login Phase for every
 connection. The InitiatorName key enables the initiator to identify
 itself to the remote endpoint.

 The InitiatorName MUST NOT be redeclared within the Login Phase.

13.6. TargetAlias

Use: ALL, Declarative, Any-Stage
 Senders: Target
 Scope: SW

 TargetAlias=<iSCSI-local-name-value>

 Examples:

 TargetAlias=Bob-s Disk

 TargetAlias=Database Server 1 Log Disk

 TargetAlias=Web Server 3 Disk 20

   If a target has been configured with a human-readable name or
   description, this name SHOULD be communicated to the initiator during
   a Login Response PDU if SessionType=Normal (see Section 13.21).  This
   string is not used as an identifier, nor is it meant to be used for
   authentication or authorization decisions.  It can be displayed by
   the initiator's user interface in a list of targets to which it is
   connected.

13.7. InitiatorAlias

Use: ALL, Declarative, Any-Stage
 Senders: Initiator
 Scope: SW

 InitiatorAlias=<iSCSI-local-name-value>

 Examples:

 InitiatorAlias=Web Server 4

 InitiatorAlias=spyalley.nsa.gov

 InitiatorAlias=Exchange Server

 If an initiator has been configured with a human-readable name or
 description, it SHOULD be communicated to the target during a Login
 Request PDU. If not, the host name can be used instead. This string
 is not used as an identifier, nor is it meant to be used for
 authentication or authorization decisions. It can be displayed by
 the target's user interface in a list of initiators to which it is
 connected.

13.8. TargetAddress

   Use: ALL, Declarative, Any-Stage
   Senders: Target
   Scope: SW

   TargetAddress=domainname[:port][,portal-group-tag]

   The domainname can be specified as either a DNS host name, a dotted-
   decimal IPv4 address, or a bracketed IPv6 address as specified in
   [RFC3986].

   If the TCP port is not specified, it is assumed to be the IANA-
   assigned default port for iSCSI (see Section 14).

   If the TargetAddress is returned as the result of a redirect status
   in a Login Response, the comma and portal-group-tag MUST be omitted.

   If the TargetAddress is returned within a SendTargets response, the
   portal-group-tag MUST be included.

   Examples:

      TargetAddress=10.0.0.1:5003,1

      TargetAddress=[1080:0:0:0:8:800:200C:417A],65

      TargetAddress=[1080::8:800:200C:417A]:5003,1

      TargetAddress=computingcenter.example.com,23

   The use of the portal-group-tag is described in Appendix C.  The
   formats for the port and portal-group-tag are the same as the one
   specified in TargetPortalGroupTag.

13.9. TargetPortalGroupTag

Use: IO by target, Declarative, Any-Stage
 Senders: Target
 Scope: SW

 TargetPortalGroupTag=<16-bit-binary-value>

 Example:

 TargetPortalGroupTag=1

 The TargetPortalGroupTag key is a 16-bit binary-value that uniquely
 identifies a portal group within an iSCSI target node. This key
 carries the value of the tag of the portal group that is servicing
 the Login Request. The iSCSI target returns this key to the
 initiator in the Login Response PDU to the first Login Request PDU
 that has the C bit set to 0 when TargetName is given by the
 initiator.

 [SAM2] notes in its informative text that the TPGT value should be
 non-zero; note that this is incorrect. A zero value is allowed as a
 legal value for the TPGT. This discrepancy currently stands
 corrected in [SAM4].

 For the complete usage expectations of this key, see Section 6.3.

13.10. InitialR2T

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery

 InitialR2T=<boolean-value>

 Examples:

 I->InitialR2T=No

 T->InitialR2T=No

 Default is Yes.
 Result function is OR.

 The InitialR2T key is used to turn off the default use of R2T for
 unidirectional operations and the output part of bidirectional
 commands, thus allowing an initiator to start sending data to a
 target as if it has received an initial R2T with Buffer
 Offset=Immediate Data Length and Desired Data Transfer
 Length=(min(FirstBurstLength, Expected Data Transfer Length) -
 Received Immediate Data Length).

 The default action is that R2T is required, unless both the initiator
 and the target send this key-pair attribute specifying InitialR2T=No.
 Only the first outgoing data burst (immediate data and/or separate
 PDUs) can be sent unsolicited (i.e., not requiring an explicit R2T).

13.11. ImmediateData

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery

 ImmediateData=<boolean-value>

 Default is Yes.
 Result function is AND.

 The initiator and target negotiate support for immediate data. To
 turn immediate data off, the initiator or target must state its
 desire to do so. ImmediateData can be turned on if both the
 initiator and target have ImmediateData=Yes.

   If ImmediateData is set to Yes and InitialR2T is set to Yes
   (default), then only immediate data are accepted in the first burst.

   If ImmediateData is set to No and InitialR2T is set to Yes, then the
   initiator MUST NOT send unsolicited data and the target MUST reject
   unsolicited data with the corresponding response code.

   If ImmediateData is set to No and InitialR2T is set to No, then the
   initiator MUST NOT send unsolicited immediate data but MAY send one
   unsolicited burst of Data-OUT PDUs.

   If ImmediateData is set to Yes and InitialR2T is set to No, then the
   initiator MAY send unsolicited immediate data and/or one unsolicited
   burst of Data-OUT PDUs.

   The following table is a summary of unsolicited data options:

     +----------+-------------+------------------+-------------+
     |InitialR2T|ImmediateData|    Unsolicited   |ImmediateData|
     |          |             |   Data-Out PDUs  |             |
     +----------+-------------+------------------+-------------+
     | No       | No          | Yes              | No          |
     +----------+-------------+------------------+-------------+
     | No       | Yes         | Yes              | Yes         |
     +----------+-------------+------------------+-------------+
     | Yes      | No          | No               | No          |
     +----------+-------------+------------------+-------------+
     | Yes      | Yes         | No               | Yes         |
     +----------+-------------+------------------+-------------+

13.12. MaxRecvDataSegmentLength

Use: ALL, Declarative
 Senders: Initiator and target
 Scope: CO

 MaxRecvDataSegmentLength=<numerical-value-512-to-(2**24 - 1)>

 Default is 8192 bytes.

 The initiator or target declares the maximum data segment length in
 bytes it can receive in an iSCSI PDU.

 The transmitter (initiator or target) is required to send PDUs with a
 data segment that does not exceed MaxRecvDataSegmentLength of the
 receiver.

   A target receiver is additionally limited by MaxBurstLength for
   solicited data and FirstBurstLength for unsolicited data.  An
   initiator MUST NOT send solicited PDUs exceeding MaxBurstLength nor
   unsolicited PDUs exceeding FirstBurstLength (or FirstBurstLength-
   Immediate Data Length if immediate data were sent).

13.13. MaxBurstLength

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery

 MaxBurstLength=<numerical-value-512-to-(2**24 - 1)>

 Default is 262144 (256 KB).
 Result function is Minimum.

 The initiator and target negotiate the maximum SCSI data payload in
 bytes in a Data-In or a solicited Data-Out iSCSI sequence. A
 sequence consists of one or more consecutive Data-In or Data-Out PDUs
 that end with a Data-In or Data-Out PDU with the F bit set to 1.

13.14. FirstBurstLength

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 Irrelevant when: ( InitialR2T=Yes and ImmediateData=No )

 FirstBurstLength=<numerical-value-512-to-(2**24 - 1)>

 Default is 65536 (64 KB).
 Result function is Minimum.

 The initiator and target negotiate the maximum amount in bytes of
 unsolicited data an iSCSI initiator may send to the target during the
 execution of a single SCSI command. This covers the immediate data
 (if any) and the sequence of unsolicited Data-Out PDUs (if any) that
 follow the command.

 FirstBurstLength MUST NOT exceed MaxBurstLength.

13.15. DefaultTime2Wait

Use: LO
 Senders: Initiator and target
 Scope: SW

 DefaultTime2Wait=<numerical-value-0-to-3600>

 Default is 2.
 Result function is Maximum.

 The initiator and target negotiate the minimum time, in seconds, to
 wait before attempting an explicit/implicit logout or an active task
 reassignment after an unexpected connection termination or a
 connection reset.

 A value of 0 indicates that logout or active task reassignment can be
 attempted immediately.

13.16. DefaultTime2Retain

Use: LO
 Senders: Initiator and target
 Scope: SW

 DefaultTime2Retain=<numerical-value-0-to-3600>

 Default is 20.
 Result function is Minimum.

 The initiator and target negotiate the maximum time, in seconds,
 after an initial wait (Time2Wait), before which an active task
 reassignment is still possible after an unexpected connection
 termination or a connection reset.

 This value is also the session state timeout if the connection in
 question is the last LOGGED_IN connection in the session.

 A value of 0 indicates that connection/task state is immediately
 discarded by the target.

13.17. MaxOutstandingR2T

Use: LO
 Senders: Initiator and target
 Scope: SW

 MaxOutstandingR2T=<numerical-value-from-1-to-65535>

   Irrelevant when: SessionType=Discovery

   Default is 1.
   Result function is Minimum.

   The initiator and target negotiate the maximum number of outstanding
   R2Ts per task, excluding any implied initial R2T that might be part
   of that task.  An R2T is considered outstanding until the last data
   PDU (with the F bit set to 1) is transferred or a sequence reception
   timeout (Section 7.1.4.1) is encountered for that data sequence.

13.18. DataPDUInOrder

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery

 DataPDUInOrder=<boolean-value>

 Default is Yes.
 Result function is OR.

 "No" is used by iSCSI to indicate that the data PDUs within sequences
 can be in any order. "Yes" is used to indicate that data PDUs within
 sequences have to be at continuously increasing addresses and
 overlays are forbidden.

13.19. DataSequenceInOrder

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery

 DataSequenceInOrder=<boolean-value>

 Default is Yes.
 Result function is OR.

 A data sequence is a sequence of Data-In or Data-Out PDUs that end
 with a Data-In or Data-Out PDU with the F bit set to 1. A Data-Out
 sequence is sent either unsolicited or in response to an R2T.
 Sequences cover an offset-range.

 If DataSequenceInOrder is set to No, data PDU sequences may be
 transferred in any order.

   If DataSequenceInOrder is set to Yes, data sequences MUST be
   transferred using continuously non-decreasing sequence offsets (R2T
   buffer offset for writes, or the smallest SCSI Data-In buffer offset
   within a read data sequence).

   If DataSequenceInOrder is set to Yes, a target may retry at most the
   last R2T, and an initiator may at most request retransmission for the
   last read data sequence.  For this reason, if ErrorRecoveryLevel is
   not 0 and DataSequenceInOrder is set to Yes, then MaxOutstandingR2T
   MUST be set to 1.

13.20. ErrorRecoveryLevel

Use: LO
 Senders: Initiator and target
 Scope: SW

 ErrorRecoveryLevel=<numerical-value-0-to-2>

 Default is 0.
 Result function is Minimum.

 The initiator and target negotiate the recovery level supported.

 Recovery levels represent a combination of recovery capabilities.
 Each recovery level includes all the capabilities of the lower
 recovery levels and adds some new ones to them.

 In the description of recovery mechanisms, certain recovery classes
 are specified. Section 7.1.5 describes the mapping between the
 classes and the levels.

13.21. SessionType

Use: LO, Declarative, Any-Stage
 Senders: Initiator
 Scope: SW

 SessionType=<Discovery|Normal>

 Default is Normal.

 The initiator indicates the type of session it wants to create. The
 target can either accept it or reject it.

   A Discovery session indicates to the target that the only purpose of
   this session is discovery.  The only requests a target accepts in
   this type of session are a Text Request with a SendTargets key and a
   Logout Request with reason "close the session".

   The Discovery session implies MaxConnections = 1 and overrides both
   the default and an explicit setting.  As Section 7.4.1 states,
   ErrorRecoveryLevel MUST be 0 (zero) for Discovery sessions.

   Depending on the type of session, a target may decide on resources to
   allocate, the security to enforce, etc., for the session.  If the
   SessionType key is thus going to be offered as "Discovery", it SHOULD
   be offered in the initial Login Request by the initiator.

13.22. The Private Extension Key Format

   Use: ALL
   Senders: Initiator and target
   Scope: specific key dependent

   X-reversed.vendor.dns_name.do_something=

   Keys with this format are used for private extension purposes.  These
   keys always start with X- if unregistered with IANA (private).  New
   public keys (if registered with IANA via an IETF Review [RFC5226]) no
   longer have an X# name prefix requirement; implementers may propose
   any intuitive unique name.

   For unregistered keys, to identify the vendor we suggest using the
   reversed DNS-name as a prefix to the key-proper.

   The part of key-name following X- MUST conform to the format for
   key-name specified in Section 6.1.

   Vendor-specific keys MUST ONLY be used in Normal sessions.

   Support for public or private extension keys is OPTIONAL.

13.23. TaskReporting

Use: LO
 Senders: Initiator and target
 Scope: SW
 Irrelevant when: SessionType=Discovery
 TaskReporting=<list-of-values>

 Default is RFC3720.

   This key is used to negotiate the task completion reporting semantics
   from the SCSI target.  The following table describes the semantics
   that an iSCSI target MUST support for respective negotiated key
   values.  Whenever this key is negotiated, at least the RFC3720 and
   ResponseFence values MUST be offered as options by the negotiation
   originator.

     +--------------+------------------------------------------+
     | Name         |             Description                  |
     +--------------+------------------------------------------+
     | RFC3720      | RFC 3720-compliant semantics.  Response  |
     |              | fencing is not guaranteed, and fast      |
     |              | completion of multi-task aborting is not |
     |              | supported.                               |
     +--------------+------------------------------------------+
     | ResponseFence| Response Fence (Section 4.2.2.3.3)       |
     |              | semantics MUST be supported in reporting |
     |              | task completions.                        |
     +--------------+------------------------------------------+
     | FastAbort    | Updated fast multi-task abort semantics  |
     |              | defined in Section 4.2.3.4 MUST be       |
     |              | supported.  Support for the Response     |
     |              | Fence is implied -- i.e., semantics as   |
     |              | described in Section 4.2.2.3.3 MUST be   |
     |              | supported as well.                       |
     +--------------+------------------------------------------+

   When TaskReporting is not negotiated to FastAbort, the standard
   multi-task abort semantics in Section 4.2.3.3 MUST be used.

13.24. iSCSIProtocolLevel Negotiation

   The iSCSIProtocolLevel associated with this document is "1".  As a
   responder or an originator in a negotiation of this key, an iSCSI
   implementation compliant to this document alone, without any future
   protocol extensions, MUST use this value as defined by [RFC7144].

13.25. Obsoleted Keys

   This document obsoletes the following keys defined in [RFC3720]:
   IFMarker, OFMarker, OFMarkInt, and IFMarkInt.  However, iSCSI
   implementations compliant to this document may still receive these
   obsoleted keys -- i.e., in a responder role -- in a text negotiation.

   When an IFMarker or OFMarker key is received, a compliant iSCSI
   implementation SHOULD respond with the constant "Reject" value.  The
   implementation MAY alternatively respond with a "No" value.

   However, the implementation MUST NOT respond with a "NotUnderstood"
   value for either of these keys.

   When an IFMarkInt or OFMarkInt key is received, a compliant iSCSI
   implementation MUST respond with the constant "Reject" value.  The
   implementation MUST NOT respond with a "NotUnderstood" value for
   either of these keys.

13.26. X#NodeArchitecture

13.26.1. Definition

Use: LO, Declarative
 Senders: Initiator and target
 Scope: SW

 X#NodeArchitecture=<list-of-values>

 Default is None.

 Examples:

 X#NodeArchitecture=ExampleOS/v1234,ExampleInc_SW_Initiator/1.05a

 X#NodeArchitecture=ExampleInc_HW_Initiator/4010,Firmware/2.0.0.5

 X#NodeArchitecture=ExampleInc_SW_Initiator/2.1,CPU_Arch/i686

 This document does not define the structure or content of the list of
 values.

 The initiator or target declares the details of its iSCSI node
 architecture to the remote endpoint. These details may include, but
 are not limited to, iSCSI vendor software, firmware, or hardware
 versions; the OS version; or hardware architecture. This key may be
 declared on a Discovery session or a Normal session.

 The length of the key value (total length of the list-of-values) MUST
 NOT be greater than 255 bytes.

 X#NodeArchitecture MUST NOT be redeclared during the Login Phase.

13.26.2. Implementation Requirements

   Functional behavior of the iSCSI node (this includes the iSCSI
   protocol logic -- the SCSI, iSCSI, and TCP/IP protocols) MUST NOT
   depend on the presence, absence, or content of the X#NodeArchitecture
   key.  The key MUST NOT be used by iSCSI nodes for interoperability or

   for exclusion of other nodes.  To ensure proper use, key values
   SHOULD be set by the node itself, and there SHOULD NOT be provisions
   for the key values to contain user-defined text.

   Nodes implementing this key MUST choose one of the following
   implementation options:

      - only transmit the key,

      - only log the key values received from other nodes, or

      - both transmit and log the key values.

   Each node choosing to implement transmission of the key values MUST
   be prepared to handle the response of iSCSI nodes that do not
   understand the key.

   Nodes that implement transmission and/or logging of the key values
   may also implement administrative mechanisms that disable and/or
   change the logging and key transmission details (see Section 9.4).
   Thus, a valid behavior for this key may be that a node is completely
   silent (the node does not transmit any key value and simply discards
   any key values it receives without issuing a NotUnderstood response).

14. Rationale for Revised IANA Considerations

   This document makes rather significant changes in this area, and this
   section outlines the reasons behind the changes.  As previously
   specified in [RFC3720], iSCSI had used text string prefixes, such as
   X- and X#, to distinguish extended login/text keys, digest
   algorithms, and authentication methods from their standardized
   counterparts.  Based on experience with other protocols, [RFC6648],
   however, strongly recommends against this practice, in large part
   because extensions that use such prefixes may become standard over
   time, at which point it can be infeasible to change their text string
   names due to widespread usage under the existing text string name.

   iSCSI's experience with public extensions supports the
   recommendations in [RFC6648], as the only extension item ever
   registered with IANA, the X#NodeArchitecture key, was specified as a
   standard key in a Standards Track RFC [RFC4850] and hence did not
   require the X# prefix.  In addition, that key is the only public
   iSCSI extension that has been registered with IANA since RFC 3720 was
   originally published, so there has been effectively no use of the X#,
   Y#, and Z# public extension formats.

   Therefore, this document makes the following changes to the IANA
   registration procedures for iSCSI:

      1) The separate registries for X#, Y#, and Z# public extensions
         are removed.  The single entry in the registry for X#
         login/text keys (X#NodeArchitecture) is transferred to the main
         "iSCSI Login/Text Keys" registry.  IANA has never created the
         latter two registries because there have been no registration
         requests for them.  These public extension formats (X#, Y#, Z#)
         MUST NOT be used, with the exception of the existing
         X#NodeArchitecture key.

      2) The registration procedures for the main "iSCSI Login/Text
         Keys", "iSCSI digests", and "iSCSI authentication methods" IANA
         registries are changed to IETF Review [RFC5226] for possible
         future extensions to iSCSI.  This change includes a deliberate
         decision to remove the possibility of specifying an IANA-
         registered iSCSI extension in an RFC published via an RFC
         Editor Independent Submission, as the level of review in that
         process is insufficient for iSCSI extensions.

      3) The restriction against registering items using the private
         extension formats (X-, Y-, Z-) in the main IANA registries is
         removed.  Extensions using these formats MAY be registered
         under the IETF Review registration procedures, but each format
         is restricted to the type of extension for which it is
         specified in this RFC and MUST NOT be used for other types.
         For example, the X- extension format for extension login/text
         keys MUST NOT be used for digest algorithms or authentication
         methods.

15. IANA Considerations

   The well-known TCP port number for iSCSI connections assigned by IANA
   is 3260, and this is the default iSCSI port.  Implementations needing
   a system TCP port number may use port 860, the port assigned by IANA
   as the iSCSI system port; however, in order to use port 860, it MUST
   be explicitly specified -- implementations MUST NOT default to the
   use of port 860, as 3260 is the only allowed default.

   IANA has replaced the references for ports 860 and 3260, both TCP and
   UDP, with references to this document.  Please see
   http://www.iana.org/assignments/service-names-port-numbers.

   IANA has updated all references to RFC 3720, RFC 4850, and RFC 5048
   to instead reference this RFC in all of the iSCSI registries that are
   part of the "Internet Small Computer System Interface (iSCSI)
   Parameters" set of registries.  This change reflects the fact that

   those three RFCs are obsoleted by this RFC.  References to other RFCs
   that are not being obsoleted (e.g., RFC 3723, RFC 5046) should not be
   changed.

   IANA has performed the following actions on the "iSCSI Login/Text
   Keys" registry:

      - Changed the registration procedure to IETF Review from Standard
        Required.

      - Changed the RFC 5048 reference for the registry to reference
        this RFC.

      - Added the X#NodeArchitecture key from the "iSCSI extended key"
        registry, and changed its reference to this RFC.

      - Changed all references to RFC 3720 and RFC 5048 to instead
        reference this RFC.

   IANA has changed the registration procedures for the "iSCSI
   authentication methods" and "iSCSI digests" registries to IETF Review
   from RFC Required.

   IANA has removed the "iSCSI extended key" registry, as its one entry
   has been added to the "iSCSI Login/Text Keys" registry.

   IANA has marked as obsolete the values 4 and 5 for SPKM1 and SPKM2,
   respectively, in the "iSCSI authentication methods" subregistry of
   the "Internet Small Computer System Interface (iSCSI) Parameters" set
   of registries.

   IANA has added this document to the "iSCSI Protocol Level" registry
   with value 1, as mentioned in Section 13.24.

   All the other IANA considerations stated in [RFC3720] and [RFC5048]
   remain unchanged.  The assignments contained in the following
   subregistries are not repeated in this document:

      - iSCSI authentication methods (from Section 13 of [RFC3720])

      - iSCSI digests (from Section 13 of [RFC3720])

   This document obsoletes the SPKM1 and SPKM2 key values for the
   AuthMethod text key.  Consequently, the SPKM_ text key prefix MUST be
   treated as obsolete and not be reused.

16. References

16.1. Normative References

[EUI] "Guidelines for 64-bit Global Identifier (EUI-64(TM))",
 <http://standards.ieee.org/regauth/oui/tutorials/
 EUI64.html>.

 [FC-FS3] INCITS Technical Committee T11, "Fibre Channel - Framing
 and Signaling - 3 (FC-FS-3)", ANSI INCITS 470-2011, 2011.

 [OUI] "IEEE OUI and "company_id" Assignments",
 <http://standards.ieee.org/regauth/oui>.

 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
 Communication Layers", STD 3, RFC 1122, October 1989.

 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
 RFC 1964, June 1996.

 [RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
 August 1996.

 [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
 Protocol (CHAP)", RFC 1994, August 1996.

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.

 [RFC2404] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
 ESP and AH", RFC 2404, November 1998.

 [RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
 Payload (ESP)", RFC 2406, November 1998.

 [RFC2451] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
 Algorithms", RFC 2451, November 1998.

 [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
 RFC 2945, September 2000.

 [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
 Internationalized Strings ("stringprep")", RFC 3454,
 December 2002.

 [RFC3566] Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96 Algorithm
 and Its Use With IPsec", RFC 3566, September 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
              ISO 10646", STD 63, RFC 3629, November 2003.

   [RFC3686]  Housley, R., "Using Advanced Encryption Standard (AES)
              Counter Mode With IPsec Encapsulating Security Payload
              (ESP)", RFC 3686, January 2004.

   [RFC3722]  Bakke, M., "String Profile for Internet Small Computer
              Systems Interface (iSCSI) Names", RFC 3722, April 2004.

   [RFC3723]  Aboba, B., Tseng, J., Walker, J., Rangan, V., and F.
              Travostino, "Securing Block Storage Protocols over IP",
              RFC 3723, April 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
              (GCM) in IPsec Encapsulating Security Payload (ESP)",
              RFC 4106, June 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC4171]  Tseng, J., Gibbons, K., Travostino, F., Du Laney, C., and
              J. Souza, "Internet Storage Name Service (iSNS)",
              RFC 4171, September 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4304]  Kent, S., "Extended Sequence Number (ESN) Addendum to
              IPsec Domain of Interpretation (DOI) for Internet Security
              Association and Key Management Protocol (ISAKMP)",
              RFC 4304, December 2005.

   [RFC4543]  McGrew, D. and J. Viega, "The Use of Galois Message
              Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
              May 2006.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
 Encodings", RFC 4648, October 2006.

 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
 IANA Considerations Section in RFCs", BCP 26, RFC 5226,
 May 2008.

 [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
 "Internet Key Exchange Protocol Version 2 (IKEv2)",
 RFC 5996, September 2010.

 [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
 Galperin, S., and C. Adams, "X.509 Internet Public Key
 Infrastructure Online Certificate Status Protocol - OCSP",
 RFC 6960, June 2013.

 [RFC7144] Knight, F. and M. Chadalapaka, "Internet Small Computer
 System Interface (iSCSI) SCSI Features Update", RFC 7144,
 April 2014.

 [RFC7145] Ko, M. and A. Nezhinsky, "Internet Small Computer System
 Interface (iSCSI) Extensions for the Remote Direct Memory
 Access (RDMA) Specification", RFC 7145, April 2014.

 [RFC7146] Black, D. and P. Koning, "Securing Block Storage Protocols
 over IP: RFC 3723 Requirements Update for IPsec v3",
 RFC 7146, April 2014.

 [SAM2] INCITS Technical Committee T10, "SCSI Architecture Model -
 2 (SAM-2)", ANSI INCITS 366-2003, ISO/IEC 14776-412, 2003.

 [SAM4] INCITS Technical Committee T10, "SCSI Architecture Model -
 4 (SAM-4)", ANSI INCITS 447-2008, ISO/IEC 14776-414, 2008.

 [SPC2] INCITS Technical Committee T10, "SCSI Primary Commands -
 2", ANSI INCITS 351-2001, ISO/IEC 14776-452, 2001.

 [SPC3] INCITS Technical Committee T10, "SCSI Primary Commands -
 3", ANSI INCITS 408-2005, ISO/IEC 14776-453, 2005.

 [UML] ISO, "Unified Modeling Language (UML) Version 1.4.2",
 ISO/IEC 19501:2005.

 [UNICODE] The Unicode Consortium, "Unicode Standard Annex #15:
 Unicode Normalization Forms", 2013,
 <http://www.unicode.org/unicode/reports/tr15>.

16.2. Informative References

[Castagnoli93]
 Castagnoli, G., Brauer, S., and M. Herrmann, "Optimization
 of Cyclic Redundancy-Check Codes with 24 and 32 Parity
 Bits", IEEE Transact. on Communications, Vol. 41, No. 6,
 June 1993.

 [FC-SP-2] INCITS Technical Committee T11, "Fibre Channel Security
 Protocols 2", ANSI INCITS 496-2012, 2012.

 [IB] InfiniBand, "InfiniBand(TM) Architecture Specification",
 Vol. 1, Rel. 1.2.1, InfiniBand Trade Association,
 <http://www.infinibandta.org>.

 [RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for
 Uniform Resource Names", RFC 1737, December 1994.

 [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
 Internet Protocol", RFC 2401, November 1998.

 [RFC2407] Piper, D., "The Internet IP Security Domain of
 Interpretation for ISAKMP", RFC 2407, November 1998.

 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
 (IKE)", RFC 2409, November 1998.

 [RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
 "Service Location Protocol, Version 2", RFC 2608,
 June 1999.

 [RFC2743] Linn, J., "Generic Security Service Application Program
 Interface Version 2, Update ", RFC 2743, January 2000.

 [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
 "Remote Authentication Dial In User Service (RADIUS)",
 RFC 2865, June 2000.

 [RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
 "Internet Protocol Small Computer System Interface (iSCSI)
 Cyclic Redundancy Check (CRC)/Checksum Considerations",
 RFC 3385, September 2002.

 [RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
 Algorithm and Its Use with IPsec", RFC 3602,
 September 2003.

   [RFC3720]  Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M.,
              and E. Zeidner, "Internet Small Computer Systems Interface
              (iSCSI)", RFC 3720, April 2004.

   [RFC3721]  Bakke, M., Hafner, J., Hufferd, J., Voruganti, K., and M.
              Krueger, "Internet Small Computer Systems Interface
              (iSCSI) Naming and Discovery", RFC 3721, April 2004.

   [RFC3783]  Chadalapaka, M. and R. Elliott, "Small Computer Systems
              Interface (SCSI) Command Ordering Considerations with
              iSCSI", RFC 3783, May 2004.

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.

   [RFC4297]  Romanow, A., Mogul, J., Talpey, T., and S. Bailey, "Remote
              Direct Memory Access (RDMA) over IP Problem Statement",
              RFC 4297, December 2005.

   [RFC4806]  Myers, M. and H. Tschofenig, "Online Certificate Status
              Protocol (OCSP) Extensions to IKEv2", RFC 4806,
              February 2007.

   [RFC4850]  Wysochanski, D., "Declarative Public Extension Key for
              Internet Small Computer Systems Interface (iSCSI) Node
              Architecture", RFC 4850, April 2007.

   [RFC5046]  Ko, M., Chadalapaka, M., Hufferd, J., Elzur, U., Shah, H.,
              and P. Thaler, "Internet Small Computer System Interface
              (iSCSI) Extensions for Remote Direct Memory Access
              (RDMA)", RFC 5046, October 2007.

   [RFC5048]  Chadalapaka, M., Ed., "Internet Small Computer System
              Interface (iSCSI) Corrections and Clarifications",
              RFC 5048, October 2007.

   [RFC5433]  Clancy, T. and H. Tschofenig, "Extensible Authentication
              Protocol - Generalized Pre-Shared Key (EAP-GPSK) Method",
              RFC 5433, February 2009.

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

   [SAS]      INCITS Technical Committee T10, "Serial Attached SCSI -
              2.1 (SAS-2.1)", ANSI INCITS 457-2010, 2010.

   [SBC2]     INCITS Technical Committee T10, "SCSI Block Commands - 2
              (SBC-2)", ANSI INCITS 405-2005, ISO/IEC 14776-322, 2005.

   [SPC4]     INCITS Technical Committee T10, "SCSI Primary Commands -
              4", ANSI INCITS 513-201x.

   [SPL]      INCITS Technical Committee T10, "SAS Protocol Layer - 2
              (SPL-2)", ANSI INCITS 505-2013, ISO/IEC 14776-262, 2013.

Appendix A. Examples

A.1. Read Operation Example

A.2. Write Operation Example

A.3. R2TSN/DataSN Use Examples

A.3.1. Output (Write) Data DataSN/R2TSN Example

A.3.2. Input (Read) Data DataSN Example

A.3.3. Bidirectional DataSN Example

A.3.4. Unsolicited and Immediate Output (Write) Data with DataSN
Example

A.4. CRC Examples

Note: All values are hexadecimal.

32 bytes of zeroes:

      Byte:        0  1  2  3

         0:       00 00 00 00
       ...
        28:       00 00 00 00

       CRC:       aa 36 91 8a

   32 bytes of ones:

      Byte:        0  1  2  3

         0:       ff ff ff ff
       ...
        28:       ff ff ff ff

       CRC:       43 ab a8 62

   32 bytes of incrementing 00..1f:

      Byte:        0  1  2  3

         0:       00 01 02 03
       ...
        28:       1c 1d 1e 1f

       CRC:       4e 79 dd 46

   32 bytes of decrementing 1f..00:

      Byte:        0  1  2  3

         0:       1f 1e 1d 1c
       ...
        28:       03 02 01 00

       CRC:       5c db 3f 11

   An iSCSI - SCSI Read (10) Command PDU:

     Byte:        0     1    2    3

        0:       01    c0   00   00
        4:       00    00   00   00
        8:       00    00   00   00
       12:       00    00   00   00
       16:       14    00   00   00
       20:       00    00   04   00
       24:       00    00   00   14
       28:       00    00   00   18
       32:       28    00   00   00
       36:       00    00   00   00
       40:       02    00   00   00
       44:       00    00   00   00

      CRC:       56    3a   96   d9

Appendix B. Login Phase Examples

In the first example, the initiator and target authenticate each
 other via Kerberos:

 I-> Login (CSG,NSG=0,1 T=1)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 AuthMethod=KRB5,SRP,None

 T-> Login (CSG,NSG=0,0 T=0)
 AuthMethod=KRB5

 I-> Login (CSG,NSG=0,1 T=1)
 KRB_AP_REQ=<krb_ap_req>

 (krb_ap_req contains the Kerberos V5 ticket and authenticator with
 MUTUAL-REQUIRED set in the ap-options field)

 If the authentication is successful, the target proceeds with:

 T-> Login (CSG,NSG=0,1 T=1)
 KRB_AP_REP=<krb_ap_rep>

 (krb_ap_rep is the Kerberos V5 mutual authentication reply)

 If the authentication is successful, the initiator may proceed
 with:

 I-> Login (CSG,NSG=1,0 T=0) FirstBurstLength=8192

 T-> Login (CSG,NSG=1,0 T=0) FirstBurstLength=4096
 MaxBurstLength=8192

 I-> Login (CSG,NSG=1,0 T=0) MaxBurstLength=8192
 ... more iSCSI Operational Parameters

 T-> Login (CSG,NSG=1,0 T=0)
 ... more iSCSI Operational Parameters

 And at the end:

 I-> Login (CSG,NSG=1,3 T=1)
 optional iSCSI parameters

 T-> Login (CSG,NSG=1,3 T=1) "login accept"

   If the initiator's authentication by the target is not successful,
   the target responds with:

      T-> Login "login reject"

   instead of the Login KRB_AP_REP message, and it terminates the
   connection.

   If the target's authentication by the initiator is not successful,
   the initiator terminates the connection (without responding to the
   Login KRB_AP_REP message).

   In the next example, only the initiator is authenticated by the
   target via Kerberos:

      I-> Login (CSG,NSG=0,1 T=1)
          InitiatorName=iqn.1999-07.com.os:hostid.77
          TargetName=iqn.1999-07.com.example:diskarray.sn.88
          AuthMethod=SRP,KRB5,None

      T-> Login-PR (CSG,NSG=0,0 T=0)
          AuthMethod=KRB5

      I-> Login (CSG,NSG=0,1 T=1)
          KRB_AP_REQ=krb_ap_req

   (MUTUAL-REQUIRED not set in the ap-options field of krb_ap_req)

   If the authentication is successful, the target proceeds with:

      T-> Login (CSG,NSG=0,1 T=1)

      I-> Login (CSG,NSG=1,0 T=0)
          ... iSCSI parameters

      T-> Login (CSG,NSG=1,0 T=0)
          ... iSCSI parameters

      . . .

      T-> Login (CSG,NSG=1,3 T=1)"login accept"

In the next example, the initiator and target authenticate each other
 via SRP:

 I-> Login (CSG,NSG=0,1 T=1)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 AuthMethod=KRB5,SRP,None

 T-> Login-PR (CSG,NSG=0,0 T=0)
 AuthMethod=SRP

 I-> Login (CSG,NSG=0,0 T=0)
 SRP_U=<user>
 TargetAuth=Yes

 T-> Login (CSG,NSG=0,0 T=0)
 SRP_N=<N>
 SRP_g=<g>
 SRP_s=<s>

 I-> Login (CSG,NSG=0,0 T=0)
 SRP_A=<A>

 T-> Login (CSG,NSG=0,0 T=0)
 SRP_B=

 I-> Login (CSG,NSG=0,1 T=1)
 SRP_M=<M>

 If the initiator authentication is successful, the target proceeds
 with:

 T-> Login (CSG,NSG=0,1 T=1)
 SRP_HM=<H(A | M | K)>

 where N, g, s, A, B, M, and H(A | M | K) are defined in [RFC2945].

 If the target authentication is not successful, the initiator
 terminates the connection; otherwise, it proceeds.

 I-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 T-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

And at the end:

 I-> Login (CSG,NSG=1,3 T=1)
 optional iSCSI parameters

 T-> Login (CSG,NSG=1,3 T=1) "login accept"

 If the initiator authentication is not successful, the target
 responds with:

 T-> Login "login reject"

 instead of the T-> Login SRP_HM=<H(A | M | K)> message, and it
 terminates the connection.

 In the next example, only the initiator is authenticated by the
 target via SRP:

 I-> Login (CSG,NSG=0,1 T=1)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 AuthMethod=KRB5,SRP,None

 T-> Login-PR (CSG,NSG=0,0 T=0)
 AuthMethod=SRP

 I-> Login (CSG,NSG=0,0 T=0)
 SRP_U=<user>
 TargetAuth=No

 T-> Login (CSG,NSG=0,0 T=0)
 SRP_N=<N>
 SRP_g=<g>
 SRP_s=<s>

 I-> Login (CSG,NSG=0,0 T=0)
 SRP_A=<A>

 T-> Login (CSG,NSG=0,0 T=0)
 SRP_B=

 I-> Login (CSG,NSG=0,1 T=1)
 SRP_M=<M>

If the initiator authentication is successful, the target proceeds
 with:

 T-> Login (CSG,NSG=0,1 T=1)

 I-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 T-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 And at the end:

 I-> Login (CSG,NSG=1,3 T=1)
 optional iSCSI parameters

 T-> Login (CSG,NSG=1,3 T=1) "login accept"

 In the next example, the initiator and target authenticate each other
 via CHAP:

 I-> Login (CSG,NSG=0,0 T=0)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 AuthMethod=KRB5,CHAP,None

 T-> Login-PR (CSG,NSG=0,0 T=0)
 AuthMethod=CHAP

 I-> Login (CSG,NSG=0,0 T=0)
 CHAP_A=<A1,A2>

 T-> Login (CSG,NSG=0,0 T=0)
 CHAP_A=<A1>
 CHAP_I=
 CHAP_C=<C>

 I-> Login (CSG,NSG=0,1 T=1)
 CHAP_N=<N>
 CHAP_R=<R>
 CHAP_I=
 CHAP_C=<C>

If the initiator authentication is successful, the target proceeds
 with:

 T-> Login (CSG,NSG=0,1 T=1)
 CHAP_N=<N>
 CHAP_R=<R>

 If the target authentication is not successful, the initiator aborts
 the connection; otherwise, it proceeds.

 I-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 T-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 And at the end:

 I-> Login (CSG,NSG=1,3 T=1)
 optional iSCSI parameters

 T-> Login (CSG,NSG=1,3 T=1) "login accept"

 If the initiator authentication is not successful, the target
 responds with:

 T-> Login "login reject"

 instead of the Login CHAP_R=<response> "proceed and change stage"
 message, and it terminates the connection.

 In the next example, only the initiator is authenticated by the
 target via CHAP:

 I-> Login (CSG,NSG=0,1 T=0)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 AuthMethod=KRB5,CHAP,None

 T-> Login-PR (CSG,NSG=0,0 T=0)
 AuthMethod=CHAP

 I-> Login (CSG,NSG=0,0 T=0)
 CHAP_A=<A1,A2>

T-> Login (CSG,NSG=0,0 T=0)
 CHAP_A=<A1>
 CHAP_I=
 CHAP_C=<C>

 I-> Login (CSG,NSG=0,1 T=1)
 CHAP_N=<N>
 CHAP_R=<R>

 If the initiator authentication is successful, the target proceeds
 with:

 T-> Login (CSG,NSG=0,1 T=1)

 I-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 T-> Login (CSG,NSG=1,0 T=0)
 ... iSCSI parameters

 And at the end:

 I-> Login (CSG,NSG=1,3 T=1)
 optional iSCSI parameters

 T-> Login (CSG,NSG=1,3 T=1) "login accept"

 In the next example, the initiator does not offer any security
 parameters. It therefore may offer iSCSI parameters on the Login PDU
 with the T bit set to 1, and the target may respond with a final
 Login Response PDU immediately:

 I-> Login (CSG,NSG=1,3 T=1)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 ... iSCSI parameters

 T-> Login (CSG,NSG=1,3 T=1) "login accept"
 ... ISCSI parameters

 In the next example, the initiator does offer security parameters on
 the Login PDU, but the target does not choose any (i.e., chooses the
 "None" values):

 I-> Login (CSG,NSG=0,1 T=1)
 InitiatorName=iqn.1999-07.com.os:hostid.77
 TargetName=iqn.1999-07.com.example:diskarray.sn.88
 AuthMethod=KRB5,SRP,None

      T-> Login-PR (CSG,NSG=0,1 T=1)
          AuthMethod=None

      I-> Login (CSG,NSG=1,0 T=0)
          ... iSCSI parameters

      T-> Login (CSG,NSG=1,0 T=0)
          ... iSCSI parameters

      And at the end:

      I-> Login (CSG,NSG=1,3 T=1)
          optional iSCSI parameters

      T-> Login (CSG,NSG=1,3 T=1) "login accept"

Appendix C. SendTargets Operation

   The text in this appendix is a normative part of this document.

   To reduce the amount of configuration required on an initiator, iSCSI
   provides the SendTargets Text Request.  The initiator uses the
   SendTargets request to get a list of targets to which it may have
   access, as well as the list of addresses (IP address and TCP port) on
   which these targets may be accessed.

   To make use of SendTargets, an initiator must first establish one of
   two types of sessions.  If the initiator establishes the session
   using the key "SessionType=Discovery", the session is a Discovery
   session, and a target name does not need to be specified.  Otherwise,
   the session is a Normal operational session.  The SendTargets command
   MUST only be sent during the Full Feature Phase of a Normal or
   Discovery session.

   A system that contains targets MUST support Discovery sessions on
   each of its iSCSI IP address-port pairs and MUST support the
   SendTargets command on the Discovery session.  In a Discovery
   session, a target MUST return all path information (IP address-port
   pairs and Target Portal Group Tags) for the targets on the target
   Network Entity that the requesting initiator is authorized to access.

   A target MUST support the SendTargets command on operational
   sessions; these will only return path information about the target to
   which the session is connected and do not need to return information
   about other target names that may be defined in the responding
   system.

   An initiator MAY make use of the SendTargets command as it sees fit.

A SendTargets command consists of a single Text Request PDU. This
 PDU contains exactly one text key and value. The text key MUST be
 SendTargets. The expected response depends upon the value, as well
 as whether the session is a Discovery session or an operational
 session.

 The value must be one of:

 All

 The initiator is requesting that information on all relevant
 targets known to the implementation be returned. This value
 MUST be supported on a Discovery session and MUST NOT be
 supported on an operational session.

 <iSCSI-target-name>

 If an iSCSI Target Name is specified, the session should
 respond with addresses for only the named target, if possible.
 This value MUST be supported on Discovery sessions. A
 Discovery session MUST be capable of returning addresses for
 those targets that would have been returned had value=All been
 designated.

 <nothing>

 The session should only respond with addresses for the target
 to which the session is logged in. This MUST be supported on
 operational sessions and MUST NOT return targets other than the
 one to which the session is logged in.

 The response to this command is a Text Response that contains a list
 of zero or more targets and, optionally, their addresses. Each
 target is returned as a target record. A target record begins with
 the TargetName text key, followed by a list of TargetAddress text
 keys, and bounded by the end of the Text Response or the next
 TargetName key, which begins a new record. No text keys other than
 TargetName and TargetAddress are permitted within a SendTargets
 response.

 For the format of the TargetName, see Section 13.4.

 A Discovery session MAY respond to a SendTargets request with its
 complete list of targets, or with a list of targets that is based on
 the name of the initiator logged in to the session.

 A SendTargets response MUST NOT contain target names if there are no
 targets for the requesting initiator to access.

Each target record returned includes zero or more TargetAddress
 fields.

 Each target record starts with one text key of the form:

 TargetName=<target-name-goes-here>

 followed by zero or more address keys of the form:

 TargetAddress=<hostname-or-ipaddress>[:<tcp-port>],
 <portal-group-tag>

 The hostname-or-ipaddress contains a domain name, IPv4 address, or
 IPv6 address ([RFC4291]), as specified for the TargetAddress key.

 A hostname-or-ipaddress duplicated in TargetAddress responses for a
 given node (the port is absent or equal) would probably indicate that
 multiple address families are in use at once (IPv6 and IPv4).

 Each TargetAddress belongs to a portal group, identified by its
 numeric Target Portal Group Tag (see Section 13.9). The iSCSI Target
 Name, together with this tag, constitutes the SCSI port identifier;
 the tag only needs to be unique within a given target's name list of
 addresses.

 Multiple-connection sessions can span iSCSI addresses that belong to
 the same portal group.

 Multiple-connection sessions cannot span iSCSI addresses that belong
 to different portal groups.

 If a SendTargets response reports an iSCSI address for a target, it
 SHOULD also report all other addresses in its portal group in the
 same response.

 A SendTargets Text Response can be longer than a single Text Response
 PDU and makes use of the long Text Responses as specified.

 After obtaining a list of targets from the Discovery session, an
 iSCSI initiator may initiate new sessions to log in to the discovered
 targets for full operation. The initiator MAY keep the Discovery
 session open and MAY send subsequent SendTargets commands to discover
 new targets.

   Examples:

   This example is the SendTargets response from a single target that
   has no other interface ports.

   The initiator sends a Text Request that contains:

      SendTargets=All

   The target sends a Text Response that contains:

      TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

   All the target had to return in this simple case was the target name.
   It is assumed by the initiator that the IP address and TCP port for
   this target are the same as those used on the current connection to
   the default iSCSI target.

   The next example has two internal iSCSI targets, each accessible via
   two different ports with different IP addresses.  The following is
   the Text Response:

      TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

      TargetAddress=10.1.0.45:3000,1

      TargetAddress=10.1.1.45:3000,2

      TargetName=iqn.1993-11.com.example:diskarray.sn.1234567

      TargetAddress=10.1.0.45:3000,1

      TargetAddress=10.1.1.45:3000,2

   Both targets share both addresses; the multiple addresses are likely
   used to provide multi-path support.  The initiator may connect to
   either target name on either address.  Each of the addresses has its
   own Target Portal Group Tag; they do not support spanning multiple-
   connection sessions with each other.  Keep in mind that the Target
   Portal Group Tags for the two named targets are independent of one
   another; portal group "1" on the first target is not necessarily the
   same as portal group "1" on the second target.

   In the above example, a DNS host name or an IPv6 address could have
   been returned instead of an IPv4 address.

   The next Text Response shows a target that supports spanning sessions
   across multiple addresses and further illustrates the use of the
   Target Portal Group Tags:

      TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

      TargetAddress=10.1.0.45:3000,1

      TargetAddress=10.1.1.46:3000,1

      TargetAddress=10.1.0.47:3000,2

      TargetAddress=10.1.1.48:3000,2

      TargetAddress=10.1.1.49:3000,3

   In this example, any of the target addresses can be used to reach the
   same target.  A single-connection session can be established to any
   of these TCP addresses.  A multiple-connection session could span
   addresses .45 and .46 or .47 and .48 but cannot span any other
   combination.  A TargetAddress with its own tag (.49) cannot be
   combined with any other address within the same session.

   This SendTargets response does not indicate whether .49 supports
   multiple connections per session; it is communicated via the
   MaxConnections text key upon login to the target.

Appendix D.  Algorithmic Presentation of Error Recovery Classes

   This appendix illustrates the error recovery classes using a
   pseudo-programming language.  The procedure names are chosen to be
   obvious to most implementers.  Each of the recovery classes described
   has initiator procedures as well as target procedures.  These
   algorithms focus on outlining the mechanics of error recovery classes
   and do not exhaustively describe all other aspects/cases.  Examples
   of this approach are as follows:

      - Handling for only certain Opcode types is shown.

      - Only certain reason codes (e.g., Recovery in Logout command) are
        outlined.

      - Resultant cases, such as recovery of Synchronization on a header
        digest error, are considered out of scope in these algorithms.
        In this particular example, a header digest error may lead to
        connection recovery if some type of Sync and Steering layer is
        not implemented.

These algorithms strive to convey the iSCSI error recovery concepts
in the simplest terms and are not designed to be optimal.

D.1. General Data Structure and Procedure Description

   This section defines the procedures and data structures that are
   commonly used by all the error recovery algorithms.  The structures
   may not be the exhaustive representations of what is required for a
   typical implementation.

   Data structure definitions:

   struct TransferContext {
           int TargetTransferTag;
           int ExpectedDataSN;
   };

   struct TCB {              /* task control block */
           Boolean SoFarInOrder;
           int ExpectedDataSN; /* used for both R2Ts and Data */
           int MissingDataSNList[MaxMissingDPDU];
           Boolean FbitReceived;
           Boolean StatusXferd;
           Boolean CurrentlyAllegiant;
           int ActiveR2Ts;
           int Response;
           char *Reason;
           struct TransferContext
                       TransferContextList[MaxOutstandingR2T];
           int InitiatorTaskTag;
           int CmdSN;
           int SNACK_Tag;
   };

   struct Connection {
           struct Session SessionReference;
           Boolean SoFarInOrder;
           int CID;
           int State;
           int CurrentTimeout;
           int ExpectedStatSN;
           int MissingStatSNList[MaxMissingSPDU];
           Boolean PerformConnectionCleanup;
   };

   struct Session {
           int NumConnections;
           int CmdSN;
           int Maxconnections;
           int ErrorRecoveryLevel;
           struct iSCSIEndpoint OtherEndInfo;
           struct Connection ConnectionList[MaxSupportedConns];
   };

   Procedure descriptions:

   Receive-an-In-PDU(transport connection, inbound PDU);
   check-basic-validity(inbound PDU);
   Start-Timer(timeout handler, argument, timeout value);
   Build-And-Send-Reject(transport connection, bad PDU, reason code);

D.2. Within-command Error Recovery Algorithms

D.2.1. Procedure Descriptions

   Recover-Data-if-Possible(last required DataSN, task control block);
   Build-And-Send-DSnack(task control block);
   Build-And-Send-RDSnack(task control block);
   Build-And-Send-Abort(task control block);
   SCSI-Task-Completion(task control block);
   Build-And-Send-A-Data-Burst(transport connection, data-descriptor,
      task control block);
   Build-And-Send-R2T(transport connection, data-descriptor,
      task control block);
   Build-And-Send-Status(transport connection, task control block);
   Transfer-Context-Timeout-Handler(transfer context);

   Notes:

   - One procedure used in this section: the Handle-Status-SNACK-request
     is defined in Appendix D.3.

   - The response-processing pseudocode shown in the target algorithms
     applies to all solicited PDUs that carry the StatSN -- SCSI
     Response, Text Response, etc.

D.2.2. Initiator Algorithms

   Recover-Data-if-Possible(LastRequiredDataSN, TCB)
   {
       if (operational ErrorRecoveryLevel > 0) {
            if (# of missing PDUs is trackable) {
                  Note the missing DataSNs in TCB.
                  if (the task spanned a change in
                             MaxRecvDataSegmentLength) {
                       if (TCB.StatusXferd is TRUE)
                           drop the status PDU;
                       Build-And-Send-RDSnack(TCB);
                  } else {
                       Build-And-Send-DSnack(TCB);
                  }

            } else {
                TCB.Reason = "Protocol Service CRC error";
                     }
       } else {
             TCB.Reason = "Protocol Service CRC error";
       }
       if (TCB.Reason == "Protocol Service CRC error") {
             Clear the missing PDU list in the TCB.
             if (TCB.StatusXferd is not TRUE)
                Build-And-Send-Abort(TCB);
       }
   }

   Receive-an-In-PDU(Connection, CurrentPDU)
   {
    check-basic-validity(CurrentPDU);
    if (Header-Digest-Bad) discard, return;
    Retrieve TCB for CurrentPDU.InitiatorTaskTag.
    if ((CurrentPDU.type == Data)
                or (CurrentPDU.type = R2T)) {
       if (Data-Digest-Bad for Data) {
                 send-data-SNACK = TRUE;
         LastRequiredDataSN = CurrentPDU.DataSN;
               } else {
             if (TCB.SoFarInOrder = TRUE) {
                 if (current DataSN is expected) {
                      Increment TCB.ExpectedDataSN.
                 } else {
                         TCB.SoFarInOrder = FALSE;
                         send-data-SNACK = TRUE;
                        }

             } else {
                     if (current DataSN was considered missing) {
                        remove current DataSN from missing PDU list.
                    } else if (current DataSN is higher than expected) {
                                send-data-SNACK = TRUE;
                         } else {
                               discard, return;
                         }
                         Adjust TCB.ExpectedDataSN if appropriate.
                }
                LastRequiredDataSN = CurrentPDU.DataSN - 1;
                  }
                  if (send-data-SNACK is TRUE and
                    task is not already considered failed) {
                Recover-Data-if-Possible(LastRequiredDataSN, TCB);
       }
               if (missing data PDU list is empty) {
                  TCB.SoFarInOrder = TRUE;
               }
       if (CurrentPDU.type == R2T) {
          Increment ActiveR2Ts for this task.
          Create a data-descriptor for the data burst.
          Build-And-Send-A-Data-Burst(Connection, data-descriptor, TCB);
        }
     } else if (CurrentPDU.type == Response) {
        if (Data-Digest-Bad) {
                   send-status-SNACK = TRUE;
                } else {
           TCB.StatusXferd = TRUE;
           Store the status information in TCB.
           if (ExpDataSN does not match) {
                TCB.SoFarInOrder = FALSE;
                Recover-Data-if-Possible(current DataSN, TCB);
           }
                   if (missing data PDU list is empty) {
                        TCB.SoFarInOrder = TRUE;
                   }
        }
     } else { /* REST UNRELATED TO WITHIN-COMMAND-RECOVERY, NOT SHOWN */
     }
     if ((TCB.SoFarInOrder == TRUE) and
                           (TCB.StatusXferd == TRUE)) {
             SCSI-Task-Completion(TCB);
      }
   }

D.2.3. Target Algorithms

   Receive-an-In-PDU(Connection, CurrentPDU)
   {
     check-basic-validity(CurrentPDU);
     if (Header-Digest-Bad) discard, return;
     Retrieve TCB for CurrentPDU.InitiatorTaskTag.
     if (CurrentPDU.type == Data) {
         Retrieve TContext from CurrentPDU.TargetTransferTag;
         if (Data-Digest-Bad) {
                     Build-And-Send-Reject(Connection, CurrentPDU,
                                  Payload-Digest-Error);
            Note the missing data PDUs in MissingDataRange[].
                     send-recovery-R2T = TRUE;
                  } else {
            if (current DataSN is not expected) {
                Note the missing data PDUs in MissingDataRange[].
                         send-recovery-R2T = TRUE;
                     }
            if (CurrentPDU.Fbit == TRUE) {
                if (current PDU is solicited) {
                        Decrement TCB.ActiveR2Ts.
                }
                if ((current PDU is unsolicited and
                        data received is less than I/O length and
                          data received is less than FirstBurstLength)
                     or (current PDU is solicited and the length of
                          this burst is less than expected)) {
                     send-recovery-R2T = TRUE;
                     Note the missing data in MissingDataRange[].
                }
                     }
                  }
                  Increment TContext.ExpectedDataSN.
         if (send-recovery-R2T is TRUE and
                   task is not already considered failed) {
            if (operational ErrorRecoveryLevel > 0) {
                Increment TCB.ActiveR2Ts.
                Create a data-descriptor for the data burst
                           from MissingDataRange.
                Build-And-Send-R2T(Connection, data-descriptor, TCB);
            } else {
                 if (current PDU is the last unsolicited)
                     TCB.Reason = "Not enough unsolicited data";
                 else
                     TCB.Reason = "Protocol Service CRC error";
            }
         }

         if (TCB.ActiveR2Ts == 0) {
            Build-And-Send-Status(Connection, TCB);
         }
     } else if (CurrentPDU.type == SNACK) {
         snack-failure = FALSE;
         if (operational ErrorRecoveryLevel > 0) {
            if (CurrentPDU.type == Data/R2T) {
                if (the request is satisfiable) {
                   if (request for Data) {
                      Create a data-descriptor for the data burst
                          from BegRun and RunLength.
                      Build-And-Send-A-Data-Burst(Connection,
                         data-descriptor, TCB);
                   } else { /* R2T */
                      Create a data-descriptor for the data burst
                          from BegRun and RunLength.
                      Build-And-Send-R2T(Connection, data-descriptor,
                         TCB);
                    }
                 } else {
                       snack-failure = TRUE;
                 }
            } else if (CurrentPDU.type == status) {
                 Handle-Status-SNACK-request(Connection, CurrentPDU);
            } else if (CurrentPDU.type == DataACK) {
                   Consider all data up to CurrentPDU.BegRun as
                   acknowledged.
                   Free up the retransmission resources for that data.
              } else if (CurrentPDU.type == R-Data SNACK) {
                            Create a data descriptor for a data burst
                            covering all unacknowledged data.
                  Build-And-Send-A-Data-Burst(Connection,
                     data-descriptor, TCB);
                  TCB.SNACK_Tag = CurrentPDU.SNACK_Tag;
                  if (there's no more data to send) {
                     Build-And-Send-Status(Connection, TCB);
                  }
            }
         } else { /* operational ErrorRecoveryLevel = 0 */
                  snack-failure = TRUE;
         }
         if (snack-failure == TRUE) {
              Build-And-Send-Reject(Connection, CurrentPDU,
                  SNACK-Reject);
              if (TCB.StatusXferd != TRUE) {
                  TCB.Reason = "SNACK rejected";
                  Build-And-Send-Status(Connection, TCB);
              }

         }

     } else { /* REST UNRELATED TO WITHIN-COMMAND-RECOVERY, NOT SHOWN */
     }
   }

   Transfer-Context-Timeout-Handler(TContext)
   {
     Retrieve TCB and Connection from TContext.
     Decrement TCB.ActiveR2Ts.
     if (operational ErrorRecoveryLevel > 0 and
                   task is not already considered failed) {
         Note the missing data PDUs in MissingDataRange[].
         Create a data-descriptor for the data burst
                           from MissingDataRange[].
         Build-And-Send-R2T(Connection, data-descriptor, TCB);

       } else {
           TCB.Reason = "Protocol Service CRC error";
           if (TCB.ActiveR2Ts = 0) {
              Build-And-Send-Status(Connection, TCB);
           }
       }
   }

D.3. Within-connection Recovery Algorithms

D.3.1. Procedure Descriptions

   Procedure descriptions:

   Recover-Status-if-Possible(transport connection,
      currently received PDU);
   Evaluate-a-StatSN(transport connection, currently received PDU);
   Retransmit-Command-if-Possible(transport connection, CmdSN);
   Build-And-Send-SSnack(transport connection);
   Build-And-Send-Command(transport connection,
      task control block);
   Command-Acknowledge-Timeout-Handler(task control block);
   Status-Expect-Timeout-Handler(transport connection);
   Build-And-Send-NOP-Out(transport connection);
   Handle-Status-SNACK-request(transport connection,
      Status SNACK PDU);
   Retransmit-Status-Burst(Status SNACK, task control block);
   Is-Acknowledged(beginning StatSN, run length);

   Implementation-specific parameters that are tunable:

   InitiatorProactiveSNACKEnabled

   Notes:

   - The initiator algorithms only deal with unsolicited NOP-In PDUs for
     generating Status SNACKs.  A solicited NOP-In PDU has an assigned
     StatSN that, when out of order, could trigger the out-of-order
     StatSN handling in within-command algorithms, again leading to
     Recover-Status-if-Possible.

   - The pseudocode shown may result in the retransmission of
     unacknowledged commands in more cases than necessary.  This will
     not, however, affect the correctness of the operation because the
     target is required to discard the duplicate CmdSNs.

   - The procedure Build-And-Send-Async is defined in the connection
     recovery algorithms.

   - The procedure Status-Expect-Timeout-Handler describes how
     initiators may proactively attempt to retrieve the Status if they
     so choose.  This procedure is assumed to be triggered much before
     the standard ULP timeout.

D.3.2. Initiator Algorithms

     Recover-Status-if-Possible(Connection, CurrentPDU)
     {
         if ((Connection.state == LOGGED_IN) and
                     connection is not already considered failed) {
            if (operational ErrorRecoveryLevel > 0) {
               if (# of missing PDUs is trackable) {
                     Note the missing StatSNs in Connection
                     that were not already requested with SNACK;
                 Build-And-Send-SSnack(Connection);
                       } else {
                         Connection.PerformConnectionCleanup = TRUE;
               }
            } else {
                       Connection.PerformConnectionCleanup = TRUE;
            }
            if (Connection.PerformConnectionCleanup == TRUE) {
               Start-Timer(Connection-Cleanup-Handler, Connection, 0);
                     }
         }

     }

     Retransmit-Command-if-Possible(Connection, CmdSN)
     {
         if (operational ErrorRecoveryLevel > 0) {
            Retrieve the InitiatorTaskTag, and thus TCB for the CmdSN.
            Build-And-Send-Command(Connection, TCB);
         }
     }

     Evaluate-a-StatSN(Connection, CurrentPDU)
     {
         send-status-SNACK = FALSE;
         if (Connection.SoFarInOrder == TRUE) {
            if (current StatSN is the expected) {
                 Increment Connection.ExpectedStatSN.
            } else {
                          Connection.SoFarInOrder = FALSE;
                          send-status-SNACK = TRUE;
                     }
         } else {
            if (current StatSN was considered missing) {
                 remove current StatSN from the missing list.
            } else {
                          if (current StatSN is higher than expected){
                              send-status-SNACK = TRUE;
                          } else {
                              send-status-SNACK = FALSE;
                      discard the PDU;
                 }
            }
            Adjust Connection.ExpectedStatSN if appropriate.
            if (missing StatSN list is empty) {
                 Connection.SoFarInOrder = TRUE;
                     }
         }
         return send-status-SNACK;
     }

     Receive-an-In-PDU(Connection, CurrentPDU)
     {
         check-basic-validity(CurrentPDU);
         if (Header-Digest-Bad) discard, return;
         Retrieve TCB for CurrentPDU.InitiatorTaskTag.
         if (CurrentPDU.type == NOP-In) {
               if (the PDU is unsolicited) {
                     if (current StatSN is not expected) {
                          Recover-Status-if-Possible(Connection,
                                       CurrentPDU);
                     }

                     if (current ExpCmdSN is not Session.CmdSN) {
                          Retransmit-Command-if-Possible(Connection,
                                       CurrentPDU.ExpCmdSN);
                     }
               }
         } else if (CurrentPDU.type == Reject) {
               if (it is a data digest error on immediate data) {
                     Retransmit-Command-if-Possible(Connection,
                                       CurrentPDU.BadPDUHeader.CmdSN);
               }
         } else if (CurrentPDU.type == Response) {
              send-status-SNACK = Evaluate-a-StatSN(Connection,
                                             CurrentPDU);
              if (send-status-SNACK == TRUE)
                  Recover-Status-if-Possible(Connection, CurrentPDU);
         } else { /* REST UNRELATED TO WITHIN-CONNECTION-RECOVERY,
                   * NOT SHOWN */
         }
     }

     Command-Acknowledge-Timeout-Handler(TCB)
     {
         Retrieve the Connection for TCB.
         Retransmit-Command-if-Possible(Connection, TCB.CmdSN);
     }

     Status-Expect-Timeout-Handler(Connection)
     {

         if (operational ErrorRecoveryLevel > 0) {
             Build-And-Send-NOP-Out(Connection);
         } else if (InitiatorProactiveSNACKEnabled){
             if ((Connection.state == LOGGED_IN) and
                          connection is not already considered failed) {
                  Build-And-Send-SSnack(Connection);
             }
         }
     }

D.3.3. Target Algorithms

   Handle-Status-SNACK-request(Connection, CurrentPDU)
     {
         if (operational ErrorRecoveryLevel > 0) {
            if (request for an acknowledged run) {
                Build-And-Send-Reject(Connection, CurrentPDU,
                                              Protocol-Error);
            } else if (request for an untransmitted run) {
                discard, return;
            } else {
                Retransmit-Status-Burst(CurrentPDU, TCB);
            }
         } else {
            Build-And-Send-Async(Connection, DroppedConnection,
                                  DefaultTime2Wait, DefaultTime2Retain);
         }
     }

D.4. Connection Recovery Algorithms

D.4.1. Procedure Descriptions

   Build-And-Send-Async(transport connection, reason code,
      minimum time, maximum time);
   Pick-A-Logged-In-Connection(session);
   Build-And-Send-Logout(transport connection,
      logout connection identifier, reason code);
   PerformImplicitLogout(transport connection,
      logout connection identifier, target information);
   PerformLogin(transport connection, target information);
   CreateNewTransportConnection(target information);
   Build-And-Send-Command(transport connection, task control block);
   Connection-Cleanup-Handler(transport connection);
   Connection-Resource-Timeout-Handler(transport connection);
   Quiesce-And-Prepare-for-New-Allegiance(session, task control block);
   Build-And-Send-Logout-Response(transport connection,
      CID of connection in recovery, reason code);
   Build-And-Send-TaskMgmt-Response(transport connection,
      task mgmt command PDU, response code);
   Establish-New-Allegiance(task control block, transport connection);
   Schedule-Command-To-Continue(task control block);

   Note:

   - Transport exception conditions such as unexpected connection
     termination, connection reset, and hung connection while the
     connection is in the Full Feature Phase are all assumed to be
     asynchronously signaled to the iSCSI layer using the
     Transport_Exception_Handler procedure.

D.4.2. Initiator Algorithms

     Receive-an-In-PDU(Connection, CurrentPDU)
     {
         check-basic-validity(CurrentPDU);
         if (Header-Digest-Bad) discard, return;
         Retrieve TCB from CurrentPDU.InitiatorTaskTag.
         if (CurrentPDU.type == Async) {
             if (CurrentPDU.AsyncEvent == ConnectionDropped) {
                Retrieve the AffectedConnection for
                   CurrentPDU.Parameter1.
                AffectedConnection.CurrentTimeout =
                   CurrentPDU.Parameter3;
               AffectedConnection.State = CLEANUP_WAIT;
               Start-Timer(Connection-Cleanup-Handler,
                            AffectedConnection, CurrentPDU.Parameter2);
             } else if (CurrentPDU.AsyncEvent == LogoutRequest)) {
               AffectedConnection = Connection;
               AffectedConnection.State = LOGOUT_REQUESTED;
               AffectedConnection.PerformConnectionCleanup = TRUE;
                        AffectedConnection.CurrentTimeout =
                           CurrentPDU.Parameter3;
               Start-Timer(Connection-Cleanup-Handler,
                             AffectedConnection, 0);
             } else if (CurrentPDU.AsyncEvent == SessionDropped)) {
               for (each Connection) {
                   Connection.State = CLEANUP_WAIT;
                   Connection.CurrentTimeout = CurrentPDU.Parameter3;
                   Start-Timer(Connection-Cleanup-Handler,
                             Connection, CurrentPDU.Parameter2);
               }
               Session.state = FAILED;
             }

         } else if (CurrentPDU.type == LogoutResponse) {
             Retrieve the CleanupConnection for CurrentPDU.CID.
             if (CurrentPDU.Response = failure) {
                CleanupConnection.State = CLEANUP_WAIT;

             } else {
                 CleanupConnection.State = FREE;
             }
         } else if (CurrentPDU.type == LoginResponse) {
              if (this is a response to an implicit Logout) {
                 Retrieve the CleanupConnection.
                 if (successful) {
                     CleanupConnection.State = FREE;
                     Connection.State = LOGGED_IN;
                 } else {
                      CleanupConnection.State = CLEANUP_WAIT;
                      DestroyTransportConnection(Connection);
                 }
              }
         } else { /* REST UNRELATED TO CONNECTION-RECOVERY,
                   * NOT SHOWN */
         }
         if (CleanupConnection.State == FREE) {
            for (each command that was active on CleanupConnection) {
            /* Establish new connection allegiance */
                 NewConnection = Pick-A-Logged-In-Connection(Session);
                 Build-And-Send-Command(NewConnection, TCB);
             }
         }
     }

     Connection-Cleanup-Handler(Connection)
     {
         Retrieve Session from Connection.
         if (Connection can still exchange iSCSI PDUs) {
             NewConnection = Connection;
         } else {
             Start-Timer(Connection-Resource-Timeout-Handler,
                   Connection, Connection.CurrentTimeout);
             if (there are other logged-in connections) {
                  NewConnection = Pick-A-Logged-In-Connection(Session);
             } else {
                  NewConnection =
                     CreateTransportConnection(Session.OtherEndInfo);
                  Initiate an implicit Logout on NewConnection for
                     Connection.CID.
                  return;
             }
         }
         Build-And-Send-Logout(NewConnection, Connection.CID,
                                             RecoveryRemove);
     }

     Transport_Exception_Handler(Connection)
     {
         Connection.PerformConnectionCleanup = TRUE;
         if (the event is an unexpected transport disconnect) {
             Connection.State = CLEANUP_WAIT;
             Connection.CurrentTimeout = DefaultTime2Retain;
             Start-Timer(Connection-Cleanup-Handler, Connection,
                            DefaultTime2Wait);
         } else {
             Connection.State = FREE;
         }
     }

D.4.3. Target Algorithms

     Receive-an-In-PDU(Connection, CurrentPDU)
     {
         check-basic-validity(CurrentPDU);
         if (Header-Digest-Bad) discard, return;
         else if (Data-Digest-Bad) {
                   Build-And-Send-Reject(Connection, CurrentPDU,
                                            Payload-Digest-Error);
                   discard, return;
         }
         Retrieve TCB and Session.
         if (CurrentPDU.type == Logout) {
            if (CurrentPDU.ReasonCode = RecoveryRemove) {
                Retrieve the CleanupConnection from CurrentPDU.CID).
                for (each command active on CleanupConnection) {
                     Quiesce-And-Prepare-for-New-Allegiance(Session,
                        TCB);
                     TCB.CurrentlyAllegiant = FALSE;
                }
                Cleanup-Connection-State(CleanupConnection);
                if ((quiescing successful) and (cleanup successful))
     {
                     Build-And-Send-Logout-Response(Connection,
                                       CleanupConnection.CID, Success);
                } else {
                     Build-And-Send-Logout-Response(Connection,
                                       CleanupConnection.CID, Failure);
                }

             }

} else if ((CurrentPDU.type == Login) and
 operational ErrorRecoveryLevel == 2) {
 Retrieve the CleanupConnection from CurrentPDU.CID).
 for (each command active on CleanupConnection) {
 Quiesce-And-Prepare-for-New-Allegiance(Session,
 TCB);
 TCB.CurrentlyAllegiant = FALSE;
 }
 Cleanup-Connection-State(CleanupConnection);
 if ((quiescing successful) and (cleanup successful))
 {
 Continue with the rest of the login processing;
 } else {
 Build-And-Send-Login-Response(Connection,
 CleanupConnection.CID, Target Error);
 }
 }
 } else if (CurrentPDU.type == TaskManagement) {
 if (CurrentPDU.function == "TaskReassign") {
 if (Session.ErrorRecoveryLevel < 2) {
 Build-And-Send-TaskMgmt-Response(Connection,
 CurrentPDU,
 "Task allegiance reassignment not
 supported");
 } else if (task is not found) {
 Build-And-Send-TaskMgmt-Response(Connection,
 CurrentPDU, "Task not in task set");
 } else if (task is currently allegiant) {
 Build-And-Send-TaskMgmt-Response(Connection,
 CurrentPDU, "Task still allegiant");
 } else {
 Establish-New-Allegiance(TCB, Connection);
 TCB.CurrentlyAllegiant = TRUE;
 Schedule-Command-To-Continue(TCB);
 }
 }
 } else { /* REST UNRELATED TO CONNECTION-RECOVERY,
 * NOT SHOWN */
 }

 }

     Transport_Exception_Handler(Connection)
     {
         Connection.PerformConnectionCleanup = TRUE;
         if (the event is an unexpected transport disconnect) {
             Connection.State = CLEANUP_WAIT;
              Start-Timer(Connection-Resource-Timeout-Handler,
                 Connection, (DefaultTime2Wait+DefaultTime2Retain));
               if (this Session has Full Feature Phase connections
                     left) {
                   DifferentConnection =
                      Pick-A-Logged-In-Connection(Session);
                    Build-And-Send-Async(DifferentConnection,
                          DroppedConnection, DefaultTime2Wait,
                            DefaultTime2Retain);
             }
         } else {
               Connection.State = FREE;
         }
     }

Appendix E. Clearing Effects of Various Events on Targets

E.1. Clearing Effects on iSCSI Objects

   The following tables describe the target behavior on receiving the
   events specified in the rows of the table.  The second table is an
   extension of the first table and defines clearing actions for more
   objects on the same events.  The legend is:

    Y = Yes (cleared/discarded/reset on the event specified in the row).
        Unless otherwise noted, the clearing action is only applicable
        for the issuing initiator port.

    N = No (not affected on the event specified in the row, i.e., stays
        at previous value).

   NA = Not Applicable or Not Defined.

                            +------+------+------+------+------+
                            |IT (1)|IC (2)|CT (5)|ST (6)|PP (7)|
     +----------------------+------+------+------+------+------+
     |connection failure (8)|Y     |Y     |N     |N     |Y     |
     +----------------------+------+------+------+------+------+
     |connection state      |NA    |NA    |Y     |N     |NA    |
     |timeout (9)           |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |session timeout/      |Y     |Y     |Y     |Y     |Y (14)|
     |closure/reinstatement |      |      |      |      |      |
     |(10)                  |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |session continuation  |NA    |NA    |N (11)|N     |NA    |
     |(12)                  |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |successful connection |Y     |Y     |Y     |N     |Y (13)|
     |close logout          |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |session failure (18)  |Y     |Y     |N     |N     |Y     |
     +----------------------+------+------+------+------+------+
     |successful recovery   |Y     |Y     |N     |N     |Y (13)|
     |Logout                |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |failed Logout         |Y     |Y     |N     |N     |Y     |
     +----------------------+------+------+------+------+------+
     |connection Login      |NA    |NA    |NA    |Y (15)|NA    |
     |(leading)             |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |connection Login      |NA    |NA    |N (11)|N     |Y     |
     |(non-leading)         |      |      |      |      |      |
     +----------------------+------+------+------+------+------+
     |TARGET COLD RESET (16)|Y (20)|Y     |Y     |Y     |Y     |
     +----------------------+------+------+------+------+------+
     |TARGET WARM RESET (16)|Y (20)|Y     |Y     |Y     |Y     |
     +----------------------+------+------+------+------+------+
     |LU reset (19)         |Y (20)|Y     |Y     |Y     |Y     |
     +----------------------+------+------+------+------+------+
     |power cycle (16)      |Y     |Y     |Y     |Y     |Y     |
     +----------------------+------+------+------+------+------+

     (1)  Incomplete TTTs (IT) are Target Transfer Tags on which the
          target is still expecting PDUs to be received.  Examples
          include TTTs received via R2T, NOP-In, etc.

     (2)  Immediate Commands (IC) are immediate commands, but waiting
          for execution on a target (for example, ABORT TASK SET).

     (5)  Connection Tasks (CT) are tasks that are active on the iSCSI
          connection in question.

     (6)  Session Tasks (ST) are tasks that are active on the entire
          iSCSI session.  A union of "connection tasks" on all
          participating connections.

     (7)  Partial PDUs (PP) (if any) are PDUs that are partially sent
          and waiting for transport window credit to complete the
          transmission.

     (8)  Connection failure is a connection exception condition - one
          of the transport connections shut down, transport connections
          reset, or transport connections timed out, which abruptly
          terminated the iSCSI Full Feature Phase connection.  A
          connection failure always takes the connection state machine
          to the CLEANUP_WAIT state.

     (9)  Connection state timeout happens if a connection spends more
          time than agreed upon during login negotiation in the
          CLEANUP_WAIT state, and this takes the connection to the FREE
          state (M1 transition in connection cleanup state diagram; see
          Section 8.2).

     (10) Session timeout, closure, and reinstatement are defined in
          Section 6.3.5.

     (11) This clearing effect is "Y" only if it is a connection
          reinstatement and the operational ErrorRecoveryLevel is less
          than 2.

     (12) Session continuation is defined in Section 6.3.6.

     (13) This clearing effect is only valid if the connection is being
          logged out on a different connection and when the connection
          being logged out on the target may have some partial PDUs
          pending to be sent.  In all other cases, the effect is "NA".

     (14) This clearing effect is only valid for a "close the session"
          logout in a multi-connection session.  In all other cases, the
          effect is "NA".

     (15) Only applicable if this leading connection login is a session
          reinstatement.  If this is not the case, it is "NA".

     (16) This operation affects all logged-in initiators.

     (18) Session failure is defined in Section 6.3.6.

     (19) This operation affects all logged-in initiators, and the
          clearing effects are only applicable to the LU being reset.

     (20) With standard multi-task abort semantics (Section 4.2.3.3), a
          TARGET WARM RESET or a TARGET COLD RESET or a LU reset would
          clear the active TTTs upon completion.  However, the FastAbort
          multi-task abort semantics defined by Section 4.2.3.4 do not
          guarantee that the active TTTs are cleared by the end of the
          reset operations.  In fact, the FastAbort semantics are
          designed to allow clearing the TTTs in a "lazy" fashion after
          the TMF Response is delivered.  Thus, when
          TaskReporting=FastAbort (Section 13.23) is operational on a
          session, the clearing effects of reset operations on
          "Incomplete TTTs" is "N".

                           +------+-------+------+------+-------+
                           |DC (1)|DD (2) |SS (3)|CS (4)|DS (5) |
     +---------------------+------+-------+------+------+-------+
     |connection failure   |N     |Y      |N     |N     |N      |
     +---------------------+------+-------+------+------+-------+
     |connection state     |Y     |NA     |Y     |N     |NA     |
     |timeout              |      |       |      |      |       |
     +---------------------+------+-------+------+------+-------+
     |session timeout/     |Y     |Y      |Y (7) |Y     |NA     |
     |closure/reinstatement|      |       |      |      |       |
     +---------------------+------+-------+------+------+-------+
     |session continuation |N (11)|NA (12)|NA    |N     |NA (13)|
     +---------------------+------+-------+------+------+-------+
     |successful connection|Y     |Y      |Y     |N     |NA     |
     |close Logout         |      |       |      |      |       |
     +---------------------+------+-------+------+------+-------+
     |session failure      |N     |Y      |N     |N     |N      |
     +---------------------+------+-------+------+------+-------+
     |successful recovery  |Y     |Y      |Y     |N     |N      |
     |Logout               |      |       |      |      |       |
     +---------------------+------+-------+------+------+-------+
     |failed Logout        |N     |Y (9)  |N     |N     |N      |
     +---------------------+------+-------+------+------+-------+
     |connection Login     |NA    |NA     |N (8) |N (8) |NA     |
     |(leading             |      |       |      |      |       |
     +---------------------+------+-------+------+------+-------+
     |connection Login     |N (11)|NA (12)|N (8) |N     |NA (13)|
     |(non-leading)        |      |       |      |      |       |
     +---------------------+------+-------+------+------+-------+
     |TARGET COLD RESET    |Y     |Y      |Y     |Y (10)|NA     |
     +---------------------+------+-------+------+------+-------+
     |TARGET WARM RESET    |Y     |Y      |N     |N     |NA     |
     +---------------------+------+-------+------+------+-------+
     |LU reset             |N     |Y      |N     |N     |N      |
     +---------------------+------+-------+------+------+-------+
     |power cycle          |Y     |Y      |Y     |Y (10)|NA     |
     +---------------------+------+-------+------+------+-------+

     (1)  Discontiguous Commands (DC) are commands allegiant to the
          connection in question and waiting to be reordered in the
          iSCSI layer.  All "Y"s in this column assume that the task
          causing the event (if indeed the event is the result of a
          task) is issued as an immediate command, because the
          discontiguities can be ahead of the task.

     (2)  Discontiguous Data (DD) are data PDUs received for the task in
          question and waiting to be reordered due to prior
          discontiguities in the DataSN.

     (3)  "SS" refers to the StatSN.

     (4)  "CS" refers to the CmdSN.

     (5)  "DS" refers to the DataSN.

     (7)  This action clears the StatSN on all the connections.

     (8)  This sequence number is instantiated on this event.

     (9)  A logout failure drives the connection state machine to the
          CLEANUP_WAIT state, similar to the connection failure event.
          Hence, it has a similar effect on this and several other
          protocol aspects.

     (10) This is cleared by virtue of the fact that all sessions with
          all initiators are terminated.

     (11) This clearing effect is "Y" if it is a connection
          reinstatement.

     (12) This clearing effect is "Y" only if it is a connection
          reinstatement and the operational ErrorRecoveryLevel is 2.

     (13) This clearing effect is "N" only if it is a connection
          reinstatement and the operational ErrorRecoveryLevel is 2.

E.2. Clearing Effects on SCSI Objects

   The only iSCSI protocol action that can effect clearing actions on
   SCSI objects is the "I_T nexus loss" notification (Section 6.3.5.1
   ("Loss of Nexus Notification")).  [SPC3] describes the clearing
   effects of this notification on a variety of SCSI attributes.  In
   addition, SCSI standards documents (such as [SAM2] and [SBC2]) define
   additional clearing actions that may take place for several SCSI
   objects on SCSI events such as LU resets and power-on resets.

   Since iSCSI defines a TARGET COLD RESET as a "protocol-equivalent" to
   a target power-cycle, the iSCSI TARGET COLD RESET must also be
   considered as the power-on reset event in interpreting the actions
   defined in the SCSI standards.

   When the iSCSI session is reconstructed (between the same SCSI ports
   with the same nexus identifier) reestablishing the same I_T nexus,
   all SCSI objects that are defined to not clear on the "I_T nexus
   loss" notification event, such as persistent reservations, are
   automatically associated to this new session.

Acknowledgments

   Several individuals on the original IPS Working Group made
   significant contributions to the original RFCs 3720, 3980, 4850,
   and 5048.

   Specifically, the authors of the original RFCs -- which herein are
   consolidated into a single document -- were the following:

      RFC 3720: Julian Satran, Kalman Meth, Costa Sapuntzakis,
      Mallikarjun Chadalapaka, Efri Zeidner

      RFC 3980: Marjorie Krueger, Mallikarjun Chadalapaka, Rob Elliott

      RFC 4850: David Wysochanski

      RFC 5048: Mallikarjun Chadalapaka

   Many thanks to Fred Knight for contributing to the UML notations and
   drawings in this document.

   We would in addition like to acknowledge the following individuals
   who contributed to this revised document: David Harrington, Paul
   Koning, Mark Edwards, Rob Elliott, and Martin Stiemerling.

   Thanks to Yi Zeng and Nico Williams for suggesting and/or reviewing
   Kerberos-related security considerations text.

   The authors gratefully acknowledge the valuable feedback during the
   Last Call review process from a number of individuals; their feedback
   significantly improved this document.  The individuals were Stephen
   Farrell, Brian Haberman, Barry Leiba, Pete Resnick, Sean Turner,
   Alexey Melnikov, Kathleen Moriarty, Fred Knight, Mike Christie, Qiang
   Wang, Shiv Rajpal, and Andy Banta.

   Finally, this document also benefited from significant review
   contributions from the Storm Working Group at large.

   Comments may be sent to Mallikarjun Chadalapaka.

Authors' Addresses

   Mallikarjun Chadalapaka
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   EMail: cbm@chadalapaka.com

   Julian Satran
   Infinidat Ltd.

   EMail: julians@infinidat.com, julian@satran.net

   Kalman Meth
   IBM Haifa Research Lab
   Haifa University Campus - Mount Carmel
   Haifa 31905, Israel

   Phone +972.4.829.6341
   EMail: meth@il.ibm.com

   David L. Black
   EMC Corporation
   176 South St.
   Hopkinton, MA  01748
   USA

   Phone +1 (508) 293-7953
   EMail: david.black@emc.com

Abstract

Status of This Memo

Copyright Notice

Table of Contents

1. Introduction

2. Acronyms, Definitions, and Document Summary

2.1. Acronyms

2.2. Definitions

2.3. Summary of Changes

2.4. Conventions

3. UML Conventions

3.1. UML Conventions Overview

3.2. Multiplicity Notion

3.3. Class Diagram Conventions

3.4. Class Diagram Notation for Associations

3.5. Class Diagram Notation for Aggregations

3.6. Class Diagram Notation for Generalizations

4. Overview

4.1. SCSI Concepts

4.2. iSCSI Concepts and Functional Overview

4.2.1. Layers and Sessions

4.2.2. Ordering and iSCSI Numbering

4.2.2.1. Command Numbering and Acknowledging

4.2.2.2. Response/Status Numbering and Acknowledging

4.2.2.3. Response Ordering

4.2.2.3.1. Need for Response Ordering

4.2.2.3.2. Response Ordering Model Description

4.2.2.3.3. iSCSI Semantics with the Interface Model

4.2.2.3.4. Current List of Fenced Response Use Cases

4.2.2.4. Data Sequencing

4.2.3. iSCSI Task Management

4.2.3.1. Task Management Overview

4.2.3.2. Notion of Affected Tasks

4.2.3.3. Standard Multi-Task Abort Semantics

4.2.3.4. FastAbort Multi-Task Abort Semantics

4.2.3.5. Affected Tasks Shared across Standard and FastAbort Sessions

4.2.3.6. Rationale behind the FastAbort Semantics

4.2.4. iSCSI Login

4.2.5. iSCSI Full Feature Phase

4.2.5.1. Command Connection Allegiance

4.2.5.2. Data Transfer Overview

4.2.5.3. Tags and Integrity Checks

4.2.5.4. SCSI Task Management during iSCSI Full Feature Phase

4.2.6. iSCSI Connection Termination

4.2.7. iSCSI Names

4.2.7.1. iSCSI Name Properties

4.2.7.2. iSCSI Name Encoding

4.2.7.3. iSCSI Name Structure

4.2.7.4. Type "iqn." (iSCSI Qualified Name)

4.2.7.5. Type "eui." (IEEE EUI-64 Format)

4.2.7.6. Type "naa." (Network Address Authority)

4.2.8. Persistent State

4.2.9. Message Synchronization and Steering

4.2.9.1. Sync/Steering and iSCSI PDU Length

4.3. iSCSI Session Types

4.4. SCSI-to-iSCSI Concepts Mapping Model

4.4.1. iSCSI Architecture Model

4.4.2. SCSI Architecture Model

4.4.3. Consequences of the Model

4.4.3.1. I_T Nexus State

4.4.3.2. Reservations

4.5. iSCSI UML Model

4.6. Request/Response Summary

4.6.1. Request/Response Types Carrying SCSI Payload

4.6.1.1. SCSI Command

4.6.1.2. SCSI Response

4.6.1.3. Task Management Function Request

4.6.1.4. Task Management Function Response

4.6.1.5. SCSI Data-Out and SCSI Data-In

4.6.1.6. Ready To Transfer (R2T)

4.6.2. Requests/Responses Carrying SCSI and iSCSI Payload

4.6.2.1. Asynchronous Message

4.6.3. Requests/Responses Carrying iSCSI-Only Payload

4.6.3.1. Text Requests and Text Responses

4.6.3.2. Login Requests and Login Responses

4.6.3.3. Logout Requests and Logout Responses

4.6.3.4. SNACK Request

4.6.3.5. Reject

4.6.3.6. NOP-Out Request and NOP-In Response

5. SCSI Mode Parameters for iSCSI