Network Working Group                                          J. Satran
Request for Comments: 3720                                       K. Meth
Category: Standards Track                                            IBM
                                                          C. Sapuntzakis
                                                           Cisco Systems
                                                          M. Chadalapaka
                                                     Hewlett-Packard Co.
                                                              E. Zeidner
                                                                     IBM
                                                              April 2004

Internet Small Computer Systems Interface (iSCSI)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Abstract

   This document describes a transport protocol for Internet Small
   Computer Systems Interface (iSCSI) that works on top of TCP.  The
   iSCSI protocol aims to be fully compliant with the standardized SCSI
   architecture model.

   SCSI is a popular family of protocols that enable systems to
   communicate with I/O devices, especially storage devices.  SCSI
   protocols are request/response application protocols with a common
   standardized architecture model and basic command set, as well as
   standardized command sets for different device classes (disks, tapes,
   media-changers etc.).

   As system interconnects move from the classical bus structure to a
   network structure, SCSI has to be mapped to network transport
   protocols.  IP networks now meet the performance requirements of fast
   system interconnects and as such are good candidates to "carry" SCSI.

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   9
   2.  Definitions and Acronyms. . . . . . . . . . . . . . . . . . .  10
       2.1.   Definitions. . . . . . . . . . . . . . . . . . . . . .  10
       2.2.   Acronyms . . . . . . . . . . . . . . . . . . . . . . .  14
       2.3.   Conventions. . . . . . . . . . . . . . . . . . . . . .  16
              2.3.1.    Word Rule. . . . . . . . . . . . . . . . . .  16
              2.3.2.    Half-Word Rule . . . . . . . . . . . . . . .  17
              2.3.3.    Byte Rule. . . . . . . . . . . . . . . . . .  17
   3.  Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .  17
       3.1.   SCSI Concepts. . . . . . . . . . . . . . . . . . . . .  17
       3.2.   iSCSI Concepts and Functional Overview . . . . . . . .  18
              3.2.1.    Layers and Sessions. . . . . . . . . . . . .  19
              3.2.2.    Ordering and iSCSI Numbering . . . . . . . .  19
                        3.2.2.1.   Command Numbering and
                                   Acknowledging . . . . . . . . . .  20
                        3.2.2.2.   Response/Status Numbering and
                                   Acknowledging . . . . . . . . . .  23
                        3.2.2.3.   Data Sequencing   . . . . . . . .  24
              3.2.3.    iSCSI Login. . . . . . . . . . . . . . . . .  24
              3.2.4.    iSCSI Full Feature Phase . . . . . . . . . .  25
                        3.2.4.1.   Command Connection Allegiance . .  26
                        3.2.4.2.   Data Transfer Overview. . . . . .  27
                        3.2.4.3.   Tags and Integrity Checks . . . .  28
                        3.2.4.4.   Task Management . . . . . . . . .  28
              3.2.5.    iSCSI Connection Termination . . . . . . . .  29
              3.2.6.    iSCSI Names. . . . . . . . . . . . . . . . .  29
                        3.2.6.1.   iSCSI Name Properties . . . . . .  30
                        3.2.6.2.   iSCSI Name Encoding . . . . . . .  31
                        3.2.6.3.   iSCSI Name Structure. . . . . . .  32
                                   3.2.6.3.1.  Type "iqn." (iSCSI
                                               Qualified Name) . . .  32
                                   3.2.6.3.2.  Type "eui." (IEEE
                                               EUI-64 format). . . .  34
              3.2.7.    Persistent State . . . . . . . . . . . . . .  34
              3.2.8.    Message Synchronization and Steering . . . .  35
                        3.2.8.1.   Sync/Steering and iSCSI PDU
                                   Length  . . . . . . . . . . . . .  36
       3.3.   iSCSI Session Types. . . . . . . . . . . . . . . . . .  36
       3.4.   SCSI to iSCSI Concepts Mapping Model . . . . . . . . .  37
              3.4.1.    iSCSI Architecture Model . . . . . . . . . .  37
              3.4.2.    SCSI Architecture Model. . . . . . . . . . .  39
              3.4.3.    Consequences of the Model. . . . . . . . . .  41
                        3.4.3.1.   I_T Nexus State . . . . . . . . .  42
       3.5.   Request/Response Summary . . . . . . . . . . . . . . .  42
              3.5.1.    Request/Response Types Carrying SCSI Payload  43
                        3.5.1.1.   SCSI-Command  . . . . . . . . . .  43

                        3.5.1.2.   SCSI-Response   . . . . . . . . .  43
                        3.5.1.3.   Task Management Function Request.  44
                        3.5.1.4.   Task Management Function Response  44
                        3.5.1.5.   SCSI Data-Out and SCSI Data-In. .  44
                        3.5.1.6.   Ready To Transfer (R2T) . . . . .  45
              3.5.2.    Requests/Responses carrying SCSI and iSCSI
                        Payload. . . . . . . . . . . . . . . . . . .  46
                        3.5.2.1.   Asynchronous Message. . . . . . .  46
              3.5.3.    Requests/Responses Carrying iSCSI Only
                        Payload. . . . . . . . . . . . . . . . . . .  46
                        3.5.3.1.   Text Request and Text Response. .  46
                        3.5.3.2.   Login Request and Login Response.  47
                        3.5.3.3.   Logout Request and Response . . .  47
                        3.5.3.4.   SNACK Request . . . . . . . . . .  48
                        3.5.3.5.   Reject. . . . . . . . . . . . . .  48
                        3.5.3.6.   NOP-Out Request and NOP-In
                                   Response  . . . . . . . . . . . .  48
   4.  SCSI Mode Parameters for iSCSI. . . . . . . . . . . . . . . .  48
   5.  Login and Full Feature Phase Negotiation. . . . . . . . . . .  48
       5.1.   Text Format. . . . . . . . . . . . . . . . . . . . . .  50
       5.2.   Text Mode Negotiation. . . . . . . . . . . . . . . . .  53
              5.2.1.    List negotiations. . . . . . . . . . . . . .  56
              5.2.2.    Simple-value Negotiations. . . . . . . . . .  56
       5.3.   Login Phase. . . . . . . . . . . . . . . . . . . . . .  57
              5.3.1.    Login Phase Start. . . . . . . . . . . . . .  60
              5.3.2.    iSCSI Security Negotiation . . . . . . . . .  62
              5.3.3.    Operational Parameter Negotiation During
                        the Login Phase. . . . . . . . . . . . . . .  63
              5.3.4.    Connection Reinstatement . . . . . . . . . .  64
              5.3.5.    Session Reinstatement, Closure, and Timeout.  64
                                   5 5.3.5.1.  Loss of Nexus
                                               Notification. . . . .  65
              5.3.6.    Session Continuation and Failure . . . . . .  65
       5.4.   Operational Parameter Negotiation Outside the Login
              Phase. . . . . . . . . . . . . . . . . . . . . . . . .  66
   6.  iSCSI Error Handling and Recovery . . . . . . . . . . . . . .  67
       6.1.   Overview . . . . . . . . . . . . . . . . . . . . . . .  67
              6.1.1.    Background . . . . . . . . . . . . . . . . .  67
              6.1.2.    Goals. . . . . . . . . . . . . . . . . . . .  67
              6.1.3.    Protocol Features and State Expectations . .  68
              6.1.4.    Recovery Classes . . . . . . . . . . . . . .  69
                        6.1.4.1.   Recovery Within-command . . . . .  69
                        6.1.4.2.   Recovery Within-connection. . . .  70
                        6.1.4.3.   Connection Recovery . . . . . . .  71
                        6.1.4.4.   Session Recovery. . . . . . . . .  72
              6.1.5.  Error Recovery Hierarchy . . . . . . . . . . .  72
       6.2.   Retry and Reassign in Recovery . . . . . . . . . . . .  74
              6.2.1.    Usage of Retry . . . . . . . . . . . . . . .  74

              6.2.2.    Allegiance Reassignment. . . . . . . . . . .  75
       6.3.   Usage Of Reject PDU in Recovery. . . . . . . . . . . .  76
       6.4.   Connection Timeout Management. . . . . . . . . . . . .  76
              6.4.1.    Timeouts on Transport Exception Events . . .  77
              6.4.2.    Timeouts on Planned Decommissioning. . . . .  77
       6.5.   Implicit Termination of Tasks. . . . . . . . . . . . .  77
       6.6.   Format Errors. . . . . . . . . . . . . . . . . . . . .  78
       6.7.   Digest Errors. . . . . . . . . . . . . . . . . . . . .  78
       6.8.   Sequence Errors. . . . . . . . . . . . . . . . . . . .  80
       6.9.   SCSI Timeouts. . . . . . . . . . . . . . . . . . . . .  81
       6.10.  Negotiation Failures . . . . . . . . . . . . . . . . .  81
       6.11.  Protocol Errors. . . . . . . . . . . . . . . . . . . .  82
       6.12.  Connection Failures. . . . . . . . . . . . . . . . . .  82
       6.13.  Session Errors . . . . . . . . . . . . . . . . . . . .  83
   7.  State Transitions . . . . . . . . . . . . . . . . . . . . . .  84
       7.1.   Standard Connection State Diagrams . . . . . . . . . .  84
              7.1.1.    State Descriptions for Initiators and
                        Targets. . . . . . . . . . . . . . . . . . .  84
              7.1.2.    State Transition Descriptions for Initiators
                        and Targets. . . . . . . . . . . . . . . . .  85
              7.1.3.    Standard Connection State Diagram for an
                        Initiator. . . . . . . . . . . . . . . . . .  88
              7.1.4.    Standard Connection State Diagram for a
                        Target . . . . . . . . . . . . . . . . . . .  90
       7.2.   Connection Cleanup State Diagram for Initiators and
              Targets. . . . . . . . . . . . . . . . . . . . . . . .  92
              7.2.1.    State Descriptions for Initiators and
                        Targets. . . . . . . . . . . . . . . . . . .  94
              7.2.2.    State Transition Descriptions for Initiators
                        and Targets. . . . . . . . . . . . . . . . .  94
       7.3.   Session State Diagrams . . . . . . . . . . . . . . . .  95
              7.3.1.    Session State Diagram for an Initiator . . .  95
              7.3.2.    Session State Diagram for a Target . . . . .  96
              7.3.3.    State Descriptions for Initiators and
                        Targets. . . . . . . . . . . . . . . . . . .  97
              7.3.4.    State Transition Descriptions for Initiators
                        and Targets. . . . . . . . . . . . . . . . .  98
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  99
       8.1.   iSCSI Security Mechanisms. . . . . . . . . . . . . . . 100
       8.2.   In-band Initiator-Target Authentication. . . . . . . . 100
              8.2.1.    CHAP Considerations. . . . . . . . . . . . . 101
              8.2.2.    SRP Considerations . . . . . . . . . . . . . 103
       8.3.   IPsec. . . . . . . . . . . . . . . . . . . . . . . . . 104
              8.3.1.    Data Integrity and Authentication. . . . . . 104
              8.3.2.    Confidentiality. . . . . . . . . . . . . . . 105
              8.3.3.    Policy, Security Associations, and
                        Cryptographic Key Management . . . . . . . . 105
   9.  Notes to Implementers . . . . . . . . . . . . . . . . . . . . 106

       9.1.   Multiple Network Adapters. . . . . . . . . . . . . . . 106
              9.1.1.    Conservative Reuse of ISIDs. . . . . . . . . 107
              9.1.2.    iSCSI Name, ISID, and TPGT Use . . . . . . . 107
       9.2.   Autosense and Auto Contingent Allegiance (ACA) . . . . 109
       9.3.   iSCSI Timeouts . . . . . . . . . . . . . . . . . . . . 109
       9.4.   Command Retry and Cleaning Old Command Instances . . . 110
       9.5.   Synch and Steering Layer and Performance . . . . . . . 110
       9.6.   Considerations for State-dependent Devices and
              Long-lasting SCSI Operations . . . . . . . . . . . . . 111
              9.6.1.    Determining the Proper ErrorRecoveryLevel. . 112
   10. iSCSI PDU Formats . . . . . . . . . . . . . . . . . . . . . . 112
       10.1.  iSCSI PDU Length and Padding . . . . . . . . . . . . . 113
       10.2.  PDU Template, Header, and Opcodes. . . . . . . . . . . 113
              10.2.1.   Basic Header Segment (BHS) . . . . . . . . . 114
                        10.2.1.1.  I . . . . . . . . . . . . . . . . 115
                        10.2.1.2.  Opcode. . . . . . . . . . . . . . 115
                        10.2.1.3.  Final (F) bit . . . . . . . . . . 116
                        10.2.1.4.  Opcode-specific Fields. . . . . . 116
                        10.2.1.5.  TotalAHSLength. . . . . . . . . . 116
                        10.2.1.6.  DataSegmentLength . . . . . . . . 116
                        10.2.1.7.  LUN . . . . . . . . . . . . . . . 116
                        10.2.1.8.  Initiator Task Tag. . . . . . . . 117
              10.2.2.  Additional Header Segment (AHS) . . . . . . . 117
                        10.2.2.1.  AHSType . . . . . . . . . . . . . 117
                        10.2.2.2.  AHSLength . . . . . . . . . . . . 117
                        10.2.2.3.  Extended CDB AHS. . . . . . . . . 118
                        10.2.2.4.  Bidirectional Expected Read-Data
                                   Length AHS. . . . . . . . . . . . 118
              10.2.3.   Header Digest and Data Digest. . . . . . . . 118
              10.2.4.   Data Segment . . . . . . . . . . . . . . . . 119
       10.3.  SCSI Command . . . . . . . . . . . . . . . . . . . . . 119
              10.3.1.   Flags and Task Attributes (byte 1) . . . . . 120
              10.3.2.   CmdSN - Command Sequence Number. . . . . . . 120
              10.3.3.   ExpStatSN. . . . . . . . . . . . . . . . . . 120
              10.3.4.   Expected Data Transfer Length. . . . . . . . 121
              10.3.5.   CDB - SCSI Command Descriptor Block. . . . . 121
              10.3.6.   Data Segment - Command Data. . . . . . . . . 121
       10.4.  SCSI Response. . . . . . . . . . . . . . . . . . . . . 122
              10.4.1.   Flags (byte 1) . . . . . . . . . . . . . . . 123
              10.4.2.   Status . . . . . . . . . . . . . . . . . . . 123
              10.4.3.   Response . . . . . . . . . . . . . . . . . . 124
              10.4.4.   SNACK Tag. . . . . . . . . . . . . . . . . . 125
              10.4.5.   Residual Count . . . . . . . . . . . . . . . 125
              10.4.6.   Bidirectional Read Residual Count. . . . . . 125
              10.4.7.   Data Segment - Sense and Response Data
                        Segment. . . . . . . . . . . . . . . . . . . 125
                        10.4.7.1.  SenseLength . . . . . . . . . . . 126
                        10.4.7.2.  Sense Data. . . . . . . . . . . . 126

              10.4.8.   ExpDataSN. . . . . . . . . . . . . . . . . . 127
              10.4.9.   StatSN - Status Sequence Number. . . . . . . 127
              10.4.10.  ExpCmdSN - Next Expected CmdSN from this
                        Initiator. . . . . . . . . . . . . . . . . . 128
              10.4.11.  MaxCmdSN - Maximum CmdSN from this Initiator 128
       10.5.  Task Management Function Request . . . . . . . . . . . 129
              10.5.1.   Function . . . . . . . . . . . . . . . . . . 129
              10.5.2.   TotalAHSLength and DataSegmentLength . . . . 132
              10.5.3.   LUN. . . . . . . . . . . . . . . . . . . . . 132
              10.5.4.   Referenced Task Tag. . . . . . . . . . . . . 132
              10.5.5.   RefCmdSN . . . . . . . . . . . . . . . . . . 132
              10.5.6.   ExpDataSN. . . . . . . . . . . . . . . . . . 133
       10.6.  Task Management Function Response. . . . . . . . . . . 134
              10.6.1.   Response . . . . . . . . . . . . . . . . . . 134
              10.6.2.   Task Management Actions on Task Sets . . . . 136
              10.6.3.   TotalAHSLength and DataSegmentLength . . . . 137
       10.7.  SCSI Data-Out & SCSI Data-In . . . . . . . . . . . . . 137
              10.7.1.   F (Final) Bit. . . . . . . . . . . . . . . . 139
              10.7.2.   A (Acknowledge) Bit. . . . . . . . . . . . . 139
              10.7.3.   Flags (byte 1) . . . . . . . . . . . . . . . 140
              10.7.4.   Target Transfer Tag and LUN. . . . . . . . . 140
              10.7.5.   DataSN . . . . . . . . . . . . . . . . . . . 141
              10.7.6.   Buffer Offset. . . . . . . . . . . . . . . . 141
              10.7.7.   DataSegmentLength. . . . . . . . . . . . . . 141
       10.8.  Ready To Transfer (R2T). . . . . . . . . . . . . . . . 142
              10.8.1.   TotalAHSLength and DataSegmentLength . . . . 143
              10.8.2.   R2TSN. . . . . . . . . . . . . . . . . . . . 143
              10.8.3.   StatSN . . . . . . . . . . . . . . . . . . . 144
              10.8.4.   Desired Data Transfer Length and Buffer
                        Offset . . . . . . . . . . . . . . . . . . . 144
              10.8.5.   Target Transfer Tag. . . . . . . . . . . . . 144
       10.9.  Asynchronous Message . . . . . . . . . . . . . . . . . 145
              10.9.1.   AsyncEvent . . . . . . . . . . . . . . . . . 146
              10.9.2.   AsyncVCode . . . . . . . . . . . . . . . . . 147
              10.9.3.   LUN. . . . . . . . . . . . . . . . . . . . . 147
              10.9.4.   Sense Data and iSCSI Event Data. . . . . . . 148
                        10.9.4.1.  SenseLength . . . . . . . . . . . 148
       10.10. Text Request . . . . . . . . . . . . . . . . . . . . . 149
              10.10.1.  F (Final) Bit. . . . . . . . . . . . . . . . 150
              10.10.2.  C (Continue) Bit . . . . . . . . . . . . . . 150
              10.10.3.  Initiator Task Tag . . . . . . . . . . . . . 150
              10.10.4.  Target Transfer Tag. . . . . . . . . . . . . 150
              10.10.5.  Text . . . . . . . . . . . . . . . . . . . . 151
       10.11. Text Response. . . . . . . . . . . . . . . . . . . . . 152
              10.11.1.  F (Final) Bit. . . . . . . . . . . . . . . . 152
              10.11.2.  C (Continue) Bit . . . . . . . . . . . . . . 153
              10.11.3.  Initiator Task Tag . . . . . . . . . . . . . 153
              10.11.4.  Target Transfer Tag. . . . . . . . . . . . . 153

              10.11.5.  StatSN . . . . . . . . . . . . . . . . . . . 154
              10.11.6.  Text Response Data . . . . . . . . . . . . . 154
       10.12. Login Request. . . . . . . . . . . . . . . . . . . . . 154
              10.12.1.  T (Transit) Bit. . . . . . . . . . . . . . . 155
              10.12.2.  C (Continue) Bit . . . . . . . . . . . . . . 155
              10.12.3.  CSG and NSG. . . . . . . . . . . . . . . . . 156
              10.12.4.  Version. . . . . . . . . . . . . . . . . . . 156
                        10.12.4.1.  Version-max. . . . . . . . . . . 156
                        10.12.4.2.  Version-min. . . . . . . . . . . 156
              10.12.5.  ISID . . . . . . . . . . . . . . . . . . . . 157
              10.12.6.  TSIH . . . . . . . . . . . . . . . . . . . . 158
              10.12.7.  Connection ID - CID. . . . . . . . . . . . . 158
              10.12.8.  CmdSN. . . . . . . . . . . . . . . . . . . . 159
              10.12.9.  ExpStatSN. . . . . . . . . . . . . . . . . . 159
              10.12.10. Login Parameters . . . . . . . . . . . . . . 159
       10.13. Login Response . . . . . . . . . . . . . . . . . . . . 160
              10.13.1.  Version-max. . . . . . . . . . . . . . . . . 160
              10.13.2.  Version-active . . . . . . . . . . . . . . . 161
              10.13.3.  TSIH . . . . . . . . . . . . . . . . . . . . 161
              10.13.4.  StatSN . . . . . . . . . . . . . . . . . . . 161
              10.13.5.  Status-Class and Status-Detail . . . . . . . 161
              10.13.6.  T (Transit) Bit. . . . . . . . . . . . . . . 164
              10.13.7.  C (Continue) Bit . . . . . . . . . . . . . . 164
              10.13.8.  Login Parameters . . . . . . . . . . . . . . 164
       10.14. Logout Request . . . . . . . . . . . . . . . . . . . . 165
              10.14.1.  Reason Code. . . . . . . . . . . . . . . . . 167
              10.14.2.  TotalAHSLength and DataSegmentLength . . . . 168
              10.14.3.  CID. . . . . . . . . . . . . . . . . . . . . 168
              10.14.4.  ExpStatSN. . . . . . . . . . . . . . . . . . 168
              10.14.5.  Implicit termination of tasks. . . . . . . . 168
       10.15. Logout Response. . . . . . . . . . . . . . . . . . . . 169
              10.15.1.  Response . . . . . . . . . . . . . . . . . . 170
              10.15.2.  TotalAHSLength and DataSegmentLength . . . . 170
              10.15.3.  Time2Wait. . . . . . . . . . . . . . . . . . 170
              10.15.4.  Time2Retain. . . . . . . . . . . . . . . . . 170
       10.16. SNACK Request. . . . . . . . . . . . . . . . . . . . . 171
              10.16.1.  Type . . . . . . . . . . . . . . . . . . . . 172
              10.16.2.  Data Acknowledgement . . . . . . . . . . . . 173
              10.16.3.  Resegmentation . . . . . . . . . . . . . . . 173
              10.16.4.  Initiator Task Tag . . . . . . . . . . . . . 174
              10.16.5.  Target Transfer Tag or SNACK Tag . . . . . . 174
              10.16.6.  BegRun . . . . . . . . . . . . . . . . . . . 174
              10.16.7.  RunLength. . . . . . . . . . . . . . . . . . 174
       10.17. Reject . . . . . . . . . . . . . . . . . . . . . . . . 175
              10.17.1.  Reason . . . . . . . . . . . . . . . . . . . 176
              10.17.2.  DataSN/R2TSN . . . . . . . . . . . . . . . . 177
              10.17.3.  StatSN, ExpCmdSN and MaxCmdSN. . . . . . . . 177
              10.17.4.  Complete Header of Bad PDU . . . . . . . . . 177

       10.18. NOP-Out. . . . . . . . . . . . . . . . . . . . . . . . 178
              10.18.1.  Initiator Task Tag . . . . . . . . . . . . . 179
              10.18.2.  Target Transfer Tag. . . . . . . . . . . . . 179
              10.18.3.  Ping Data. . . . . . . . . . . . . . . . . . 179
       10.19. NOP-In . . . . . . . . . . . . . . . . . . . . . . . . 180
              10.19.1.  Target Transfer Tag. . . . . . . . . . . . . 181
              10.19.2.  StatSN . . . . . . . . . . . . . . . . . . . 181
              10.19.3.  LUN. . . . . . . . . . . . . . . . . . . . . 181
   11. iSCSI Security Text Keys and Authentication Methods . . . . . 181
       11.1.  AuthMethod . . . . . . . . . . . . . . . . . . . . . . 182
              11.1.1.   Kerberos . . . . . . . . . . . . . . . . . . 184
              11.1.2.   Simple Public-Key Mechanism (SPKM) . . . . . 184
              11.1.3.   Secure Remote Password (SRP) . . . . . . . . 185
              11.1.4.   Challenge Handshake Authentication Protocol
                        (CHAP) . . . . . . . . . . . . . . . . . . . 186
   12. Login/Text Operational Text Keys. . . . . . . . . . . . . . . 187
       12.1.  HeaderDigest and DataDigest. . . . . . . . . . . . . . 188
       12.2.  MaxConnections . . . . . . . . . . . . . . . . . . . . 190
       12.3.  SendTargets. . . . . . . . . . . . . . . . . . . . . . 191
       12.4.  TargetName . . . . . . . . . . . . . . . . . . . . . . 191
       12.5.  InitiatorName. . . . . . . . . . . . . . . . . . . . . 192
       12.6.  TargetAlias. . . . . . . . . . . . . . . . . . . . . . 192
       12.7.  InitiatorAlias . . . . . . . . . . . . . . . . . . . . 193
       12.8.  TargetAddress. . . . . . . . . . . . . . . . . . . . . 193
       12.9.  TargetPortalGroupTag . . . . . . . . . . . . . . . . . 194
       12.10. InitialR2T . . . . . . . . . . . . . . . . . . . . . . 194
       12.11. ImmediateData. . . . . . . . . . . . . . . . . . . . . 195
       12.12. MaxRecvDataSegmentLength . . . . . . . . . . . . . . . 196
       12.13. MaxBurstLength . . . . . . . . . . . . . . . . . . . . 196
       12.14. FirstBurstLength . . . . . . . . . . . . . . . . . . . 197
       12.15. DefaultTime2Wait . . . . . . . . . . . . . . . . . . . 197
       12.16. DefaultTime2Retain . . . . . . . . . . . . . . . . . . 198
       12.17. MaxOutstandingR2T. . . . . . . . . . . . . . . . . . . 198
       12.18. DataPDUInOrder . . . . . . . . . . . . . . . . . . . . 198
       12.19. DataSequenceInOrder. . . . . . . . . . . . . . . . . . 199
       12.20. ErrorRecoveryLevel . . . . . . . . . . . . . . . . . . 199
       12.21. SessionType. . . . . . . . . . . . . . . . . . . . . . 200
       12.22. The Private or Public Extension Key Format . . . . . . 200
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 201
       13.1.  Naming Requirements. . . . . . . . . . . . . . . . . . 203
       13.2.  Mechanism Specification Requirements . . . . . . . . . 203
       13.3.  Publication Requirements . . . . . . . . . . . . . . . 203
       13.4.  Security Requirements. . . . . . . . . . . . . . . . . 203
       13.5.  Registration Procedure . . . . . . . . . . . . . . . . 204
              13.5.1.   Present the iSCSI extension item to the
                        Community. . . . . . . . . . . . . . . . . . 204
              13.5.2.   iSCSI extension item review and IESG
                        approval . . . . . . . . . . . . . . . . . . 204

              13.5.3.   IANA Registration. . . . . . . . . . . . . . 204
              13.5.4.   Standard iSCSI extension item-label format . 204
       13.6.  IANA Procedures for Registering iSCSI extension items. 205
   References. . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
   Appendix A. Sync and Steering with Fixed Interval Markers . . . . 209
       A.1.   Markers At Fixed Intervals . . . . . . . . . . . . . . 209
       A.2.   Initial Marker-less Interval . . . . . . . . . . . . . 210
       A.3.   Negotiation. . . . . . . . . . . . . . . . . . . . . . 210
              A.3.1.    OFMarker, IFMarker . . . . . . . . . . . . . 210
              A.3.2.    OFMarkInt, IFMarkInt . . . . . . . . . . . . 211
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . . 212
       B.1.   Read Operation Example . . . . . . . . . . . . . . . . 212
       B.2.   Write Operation Example. . . . . . . . . . . . . . . . 213
       B.3.   R2TSN/DataSN Use Examples. . . . . . . . . . . . . . . 214
       B.4.   CRC Examples . . . . . . . . . . . . . . . . . . . . . 217
   Appendix C.  Login Phase Examples . . . . . . . . . . . . . . . . 219
   Appendix D.  SendTargets Operation. . . . . . . . . . . . . . . . 229
   Appendix E.  Algorithmic Presentation of Error Recovery Classes . 233
       E.1.   General Data Structure and Procedure Description . . . 233
       E.2.   Within-command Error Recovery Algorithms . . . . . . . 234
              E.2.1.    Procedure Descriptions . . . . . . . . . . . 234
              E.2.2.    Initiator Algorithms . . . . . . . . . . . . 235
              E.2.3.    Target Algorithms. . . . . . . . . . . . . . 237
       E.3.   Within-connection Recovery Algorithms. . . . . . . . . 240
              E.3.1.    Procedure Descriptions . . . . . . . . . . . 240
              E.3.2.    Initiator Algorithms . . . . . . . . . . . . 241
              E.3.3.    Target Algorithms. . . . . . . . . . . . . . 243
       E.4.   Connection Recovery Algorithms . . . . . . . . . . . . 243
              E.4.1.    Procedure Descriptions . . . . . . . . . . . 243
              E.4.2.    Initiator Algorithms . . . . . . . . . . . . 244
              E.4.3.    Target Algorithms. . . . . . . . . . . . . . 246
   Appendix F.  Clearing Effects of Various Events on Targets. . . . 249
       F.1.   Clearing Effects on iSCSI Objects. . . . . . . . . . . 249
       F.2.   Clearing Effects on SCSI Objects . . . . . . . . . . . 253
   Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . 254
   Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 256
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 257

1. Introduction

   The Small Computer Systems 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, 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 (see [RFC791], [RFC793],
   [RFC1035], [RFC1122]), providing for an interoperable solution which
   can take advantage of existing Internet infrastructure, Internet
   management facilities, and address distance limitations.

2. Definitions and Acronyms

2.1. 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).

   - iSCSI Device: A SCSI Device using an iSCSI service delivery
     subsystem.  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: The "initiator".  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: The name of an iSCSI initiator or iSCSI target.

   - iSCSI Node: The iSCSI Node represents a single iSCSI initiator or
     iSCSI target.  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 "target".

   - 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 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,'+ Portal Group Tag).

   - 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 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: 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 party that responds to
     the originator of the negotiation or exchange.

   - SCSI Device: This is the SAM2 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 logical units.  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 command execute
     [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 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: This is the SAM2 term for an entity in a SCSI Device
     that provides the SCSI functionality to interface with a service
     delivery subsystem.  For iSCSI, the definition of the SCSI
     Initiator Port and the SCSI Target Port are different.

   - SCSI Port Name: A name made up as UTF-8 [RFC2279] characters and
     includes the iSCSI Name + 'i' or 't' + ISID or Portal Group Tag.

   - 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
     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
     when TargetName is given.

   - Target Portal Group Tag: A numerical identifier (16-bit) for an
     iSCSI Target Portal Group.

   - TSIH (Target Session Identifying Handle): A target assigned tag for
     a session with a specific named initiator.  The target generates it
     during session establishment.  Its internal format and content are
     not defined by this protocol, except for the value 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.

2.2. 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
   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
   ESP         Encapsulating Security Payload
   EUI         Extended Unique Identifier
   FFP         Full Feature Phase
   FFPO        Full Feature Phase Only
   FIM         Fixed Interval Marker
   Gbps        Gigabits per Second
   HBA         Host Bus Adapter
   HMAC        Hashed Message Authentication Code
   I_T         Initiator_Target
   I_T_L       Initiator_Target_LUN
   IANA        Internet Assigned Numbers Authority

   ID          Identifier
   IDN         Internationalized Domain Name
   IEEE        Institute of Electrical & 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
   ISID        Initiator Session ID
   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 Codes
   NA          Not Applicable
   NIC         Network Interface Card
   NOP         No Operation
   NSG         Next Stage
   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
   SAM         SCSI Architecture Model
   SAM2        SCSI Architecture Model - 2
   SAN         Storage Area Network
   SCSI        Small Computer Systems Interface
   SN          Sequence Number
   SNACK       Selective Negative Acknowledgment - also
               Sequence Number Acknowledgement for data
   SPKM        Simple Public-Key Mechanism
   SRP         Secure Remote Password
   SSID        Session ID
   SW          Session Wide
   TCB         Task Control Block
   TCP         Transmission Control Protocol
   TPGT        Target Portal Group Tag
   TSIH        Target Session Identifying Handle

   TTT         Target Transfer Tag
   UFL         Upper Functional Layer
   ULP         Upper Level Protocol
   URN         Uniform Resource Names [RFC2396]
   UTF         Universal Transformation Format
   WG          Working Group

2.3. 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 BCP 14 [RFC2119].

   iSCSI messages - PDUs - are represented by diagrams as in the
   following example:

    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)                                    |
      +---------------+---------------+---------------+---------------+
    ----------
     +|                                                               |
      +---------------+---------------+---------------+---------------+

   The diagrams include byte and bit numbering.

   The following representation and ordering rules are observed in this
   document:

     - Word Rule
     - Half-word Rule
     - Byte Rule

2.3.1. Word Rule

   A word holds four consecutive bytes.  Whenever a word has numeric
   content, it is considered an unsigned number in base 2 positional
   representation with the lowest numbered byte (e.g., byte 0) bit 0
   representing 2**31 and bit 1 representing 2**30 through lowest
   numbered byte + 3 (e.g., byte 3) bit 7 representing 2**0.

   Decimal and hexadecimal representation of word values map this
   representation to decimal or hexadecimal positional notation.

2.3.2. Half-Word Rule

   A half-word holds two consecutive bytes.  Whenever a half-word has
   numeric content it is considered an unsigned number in base 2
   positional representation with the lowest numbered byte (e.g., byte
   0), bit 0 representing 2**15 and bit 1 representing 2**14 through
   lowest numbered byte + 1 (e.g., byte 1), bit 7 representing 2**0.

   Decimal and hexadecimal representation of half-word values map this
   representation to decimal or hexadecimal positional notation.

2.3.3. Byte Rule

   For every PDU, bytes are sent and received in increasing numbered
   order (network order).

   Whenever a byte has numerical content, it is considered an unsigned
   number in base 2 positional representation with bit 0 representing
   2**7 and bit 1 representing 2**6 through bit 7 representing 2**0.

3. Overview

3.1. 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, logical units, of a server known as a
   "target".  The "device server" on the logical unit accepts SCSI
   commands and processes them.

   A "SCSI transport" maps the client-server SCSI protocol to a specific
   interconnect.  Initiators are one endpoint of a SCSI transport.  The
   "target" is the other endpoint.  A target can contain multiple
   Logical Units (LUs).  Each Logical Unit 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 logical unit.  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 target (e.g., WRITE), target to 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 (CDB) are the data structures used to
   contain the command parameters that an initiator sends to a target.
   The CDB content and structure is defined by [SAM2] and device-type
   specific SCSI standards.

3.2. iSCSI Concepts and Functional Overview

   The iSCSI protocol is a mapping of the SCSI remote procedure
   invocation 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 Section 3.4.1 iSCSI Architecture Model) unless
   otherwise qualified.

   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.

3.2.1. 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:

      a) the SCSI layer builds/receives SCSI CDBs (Command Descriptor
         Blocks) and passes/receives them with the remaining command
         execute parameters ([SAM2]) to/from

      b) 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 (loosely equivalent to a SCSI I_T nexus,
   see Section 3.4.2 SCSI Architecture Model).  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 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.

3.2.2. 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/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 [CORD].

3.2.2.1. 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 10.5 Task
   Management Function Request) may be performed on any iSCSI task.

   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 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 CmdSN 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.  CmdSN does
   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 command numbers are incremented by 1 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 task management commands act as specified by [SAM2].
   For example, both commands and responses appear as if delivered in
   order.  Whenever CmdSN for an outgoing PDU is not specified by an
   explicit rule, 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 for execution is not acknowledged through the
   numbering scheme.  Immediate commands MAY be rejected by the iSCSI
   target layer due to a lack of resources.  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 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., 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.  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 CmdSN less than
         ExpCmdSN as acknowledged.  The target iSCSI layer sets the
         ExpCmdSN to the largest non-immediate CmdSN that it can deliver
         for execution plus 1 (no holes in the 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
   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 ExpCmdSN to 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 MaxCmdSN, the target window is
   closed.  For group task management commands issued as immediate
   commands, CmdSN indicates the scope of the group action (e.g., on
   ABORT TASK SET indicates which commands are aborted).

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

      -  If the PDU MaxCmdSN is less than the PDU ExpCmdSN-1 (in Serial
         Arithmetic Sense), they are both ignored.
      -  If the PDU MaxCmdSN is greater than the local MaxCmdSN (in
         Serial Arithmetic Sense), it updates the local MaxCmdSN;
         otherwise, it is ignored.
      -  If the PDU ExpCmdSN is greater than the local ExpCmdSN (in
         Serial 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 6.2.1 Usage of Retry).  At the target, CmdSN is
   only relevant when the command has not created any state related to
   its execution (execution state); afterwards, 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 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
   7.1.3 Standard Connection State Diagram for an Initiator), the
   connection has been reinstated (see Section 5.3.4 Connection
   Reinstatement), or a non-immediate command with CmdSN equal 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 9.4 Command Retry and Cleaning Old Command
   Instances).

   A target MUST NOT issue a command response or Data-In PDU with status
   before acknowledging the command.  However, the acknowledgement can
   be included in the response or Data-In PDU.

3.2.2.2. 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.  StatSN
   is a counter maintained per connection.  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 ExpStatSN.

   A large absolute difference between StatSN and 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.

3.2.2.3. 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, 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, 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.

3.2.3. 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 3.4.2 SCSI
   Architecture Model).

   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.

   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 Chapter 8 and [RFC3723].

   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 Chapter 5.

   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 portal group tag (see Section 3.4.1 iSCSI Architecture
   Model).  ISID is subject to reuse restrictions because it is used to
   identify a persistent state (see Section 3.4.3 Consequences of the
   Model).

   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 a 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.

3.2.4. iSCSI Full Feature Phase

   Once the initiator is authorized to do so, the iSCSI session is in
   the iSCSI Full Feature Phase.  A session is in Full Feature Phase
   after successfully finishing the Login Phase on the first (leading)

   connection of a session.  A connection is in Full Feature Phase if
   the session is in Full Feature Phase and the connection login has
   completed successfully.  An iSCSI connection is not in Full Feature
   Phase

      a) when 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.

3.2.4.1. 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 6.2 Retry and
   Reassign in Recovery.

   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.

3.2.4.2. 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 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 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 on 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 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.

3.2.4.3. 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.

3.2.4.4. Task Management

   SCSI task management assumes that individual tasks and task groups
   can be aborted solely based 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 view while using an asynchronous
   infrastructure.

3.2.5. iSCSI Connection Termination

   An iSCSI connection may be terminated by use of a transport
   connection shutdown or a transport reset.  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 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 Full Feature Phase, connection
   cleanup (see section 7) is required prior to recovery.  By doing
   connection cleanup before starting recovery, the initiator and target
   will avoid receiving stale PDUs after recovery.

3.2.6. 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 of an iSCSI device.  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 operational domain of the end
   user.  However, because the operational 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 two 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 3.4.2 SCSI Architecture
   Model 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].

3.2.6.1. iSCSI Name Properties

   Each iSCSI node, whether an initiator or target, MUST have an iSCSI
   name.

   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 2.3 iSCSI Session Types) 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:

      a) iSCSI names are globally unique.  No two initiators or targets
         can have the same name.
      b) iSCSI names are permanent.  An iSCSI initiator node or target
         node has the same name for its lifetime.
      c) 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.
      d) iSCSI names do not rely on a central name broker; the naming
         authority is distributed.
      e) iSCSI names support integration with existing unique naming
         schemes.
      f) 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:

      a) iSCSI names have the same encoding method regardless of the
         underlying protocols.
      b) iSCSI names are relatively simple to compare.  The algorithm
         for comparing two iSCSI names for equivalence does not rely on
         an external server.

      c) iSCSI names are composed only of displayable characters.  iSCSI
         names allow the use of international character sets but are not
         case sensitive.  No whitespace characters are used in iSCSI
         names.
      d) 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 (URN) [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.

3.2.6.2. iSCSI Name Encoding

   An iSCSI name MUST be a UTF-8 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].  Stringprep 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.  Strings MUST NOT include punctuation, spacing,
   diacritical marks, or other characters that could get in the way of
   readability.  The stringprep process also converts strings into
   equivalent strings of lower-case characters.

   The stringprep process does not need to be implemented if the names
   are only generated using numeric and lower-case (any character set)
   alphabetic characters.

   Once iSCSI names encoded in UTF-8 are "normalized" they may be safely
   compared byte-for-byte.

3.2.6.3. iSCSI Name Structure

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

   The iSCSI name does not define any new naming authorities.  Instead,
   it supports two existing ways of designating naming authorities: an
   iSCSI-Qualified Name, using domain names to identify a naming
   authority, and the EUI format, where the IEEE Registration Authority
   assists in the formation of worldwide unique names (EUI-64 format).

   The type designator strings currently defined are:

     iqn.       - iSCSI Qualified name
     eui.       - Remainder of the string is an IEEE EUI-64
                  identifier, in ASCII-encoded hexadecimal.

   These two 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.

3.2.6.3.1. 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 be active, and 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.  For this reason, a date code is provided as
   part of the "iqn." format.

   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 reversed 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 reversed 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

3.2.6.3.2. 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].

3.2.7. 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 Section 3.4.2
   SCSI Architecture Model and Section 3.4.3 Consequences of the Model).

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

   All iSCSI session and connection parameters are re-initialized upon
   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 10.5 Task
   Management Function Request.)

3.2.8. 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, upon which 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 pre-allocated
   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.  To make
   these schemes work, iSCSI implementations have to make sure that the
   appropriate protocol layers are provided with enough information to
   implement a synchronization and/or data steering mechanism.  One of
   these schemes is detailed in Appendix A.  - Sync and Steering with
   Fixed Interval Markers -.

   The Fixed Interval Markers (FIM) scheme works by inserting markers in
   the payload stream at fixed intervals that contain the offset for the
   start of the next iSCSI PDU.

   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.

   When the part of the TCP data stream containing an iSCSI PDU header
   is delayed or lost, markers may be used to minimize the damage as
   follows:

     - Markers indicate where the next iSCSI PDU starts and enable
       continued processing when iSCSI headers have to be dropped due to
       data errors discovered at the iSCSI level (e.g., iSCSI header CRC
       errors).

     - Markers help minimize the amount of data that has to be kept by
       the TCP/iSCSI layer while waiting for a late TCP packet arrival
       or recovery, because later they might help find iSCSI PDU headers
       and use the information contained in those to steer data to SCSI
       buffers.

3.2.8.1. Sync/Steering and iSCSI PDU Length

   When a large iSCSI message is sent, the TCP segment(s) that contain
   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.

3.3. 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 the reason "close
          the session".  All other requests MUST be rejected.

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

3.4. 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 3.4.1 iSCSI
   Architecture Model.

                  +-----------------------------------+
                  |  Network Entity (iSCSI Client)    |
                  |                                   |
                  |         +-------------+           |
                  |         | iSCSI Node  |           |
                  |         | (Initiator) |           |
                  |         +-------------+           |
                  |            |       |              |
                  | +--------------+ +--------------+ |
                  | |Network Portal| |Network Portal| |
                  | |   10.1.30.4  | |   10.1.40.6  | |
                  +-+--------------+-+--------------+-+
                           |               |
                           |  IP Networks  |
                           |               |
                  +-+--------------+-+--------------+-+
                  | |Network Portal| |Network Portal| |
                  | |  10.1.30.21  | |   10.1.40.3  | |
                  | | TCP Port 3260| | TCP Port 3260| |
                  | +--------------+ +--------------+ |
                  |        |               |          |
                  |        -----------------          |
                  |           |         |             |
                  |  +-------------+ +--------------+ |
                  |  | iSCSI Node  | | iSCSI Node   | |
                  |  |  (Target)   | |  (Target)    | |
                  |  +-------------+ +--------------+ |
                  |                                   |
                  |   Network Entity (iSCSI Server)   |
                  +-----------------------------------+

3.4.1. 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.

      a)  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 item d), each of which can be
          used by some iSCSI Nodes (see item (b)) contained in that
          Network Entity to gain access to the IP network.

      b)  iSCSI Node - represents a single iSCSI initiator or iSCSI
          target.  There are one or more iSCSI Nodes within a Network
          Entity.  The iSCSI Node is accessible via one or more Network
          Portals (see item d).  An iSCSI Node is identified by its
          iSCSI Name (see Section 3.2.6 iSCSI Names and Chapter 12).
          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 the same iSCSI node to use multiple
          addresses.

      c)  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.

      d)  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.

      e)  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 12.3 SendTargets).  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 9.1.1 Conservative
          Reuse of ISIDs.

      f)  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)                   |
   +------------------------------------------------------------------+

3.4.2. SCSI Architecture Model

   This section describes the relationship between the SCSI Architecture
   Model [SAM2] and the constructs of the SCSI device, SCSI port and I_T
   nexus, and the iSCSI constructs described in Section 3.4.1 iSCSI
   Architecture Model.

   This relationship implies implementation requirements in order to
   conform to the SAM2 model and other SCSI operational functions.
   These requirements are detailed in Section 3.4.3 Consequences of the
   Model.

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

      a)  SCSI Device - the SAM2 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 logical units.  For iSCSI, the SCSI Device is the
          component within an iSCSI Node that provides the SCSI
          functionality.  As such, there can be one SCSI Device, at
          most, within an iSCSI Node.  Access to the SCSI Device can
          only be achieved in an iSCSI normal operational session (see
          Section 3.3 iSCSI Session Types).  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 - the SAM2 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 definition of
          SCSI Initiator Port and SCSI Target Port are different.

          SCSI Initiator Port: This maps to one endpoint of an iSCSI
          normal operational session (see Section 3.3 iSCSI Session
          Types).  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 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:
           - The iSCSI Name in UTF-8 format, followed by
           - a comma separator (1 byte), followed by
           - the ASCII character 'i' (for SCSI Initiator Port) or the
             ASCII character 't' (for SCSI Target Port) (1 byte),
             followed by

           - a comma separator (1 byte), followed by
           - a text encoding as a hex-constant (see Section 5.1 Text
             Format) of the ISID (for SCSI initiator port) or the portal
             group tag (for SCSI target port) including the initial 0X
             or 0x and the terminating null (15 bytes).

          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 - 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' + ISID,
          iSCSI Target Name + 't' + Portal Group Tag).

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

3.4.3. 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 can exist
   between a given iSCSI initiator node and an iSCSI target node, with
   the same session identifier (SSID).

   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 ISID that identifies the SCSI initiator port.  See Section
   10.12.5 ISID.

   The structure of the ISID that contains a naming authority component
   (see Section 10.12.5 ISID and [RFC3721]) provides a mechanism to
   facilitate compliance with the ISID rule.  (See Section 9.1.1
   Conservative Reuse of ISIDs.)

   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 9.1.1 Conservative Reuse of ISIDs).  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 nexus 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.

3.4.3.1. 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 logical unit 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 perform session recovery as described in Chapter
   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 logical unit device server uses to identify the I_T nexus.

3.5. Request/Response Summary

   This section lists and briefly describes all the iSCSI PDU types
   (request 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.

3.5.1. Request/Response Types Carrying SCSI Payload

3.5.1.1. SCSI-Command

   This request carries the SCSI CDB and all the other SCSI execute
   command procedure call (see [SAM2]) IN arguments such as task
   attributes, Expected Data Transfer Length for one or both transfer
   directions (the latter for bidirectional commands), and 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) for the session and the expected status number
   (ExpStatSN) for the connection.

   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.

3.5.1.2. 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:

     - 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.

3.5.1.3 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 (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.

3.5.1.4. Task Management Function Response

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

   After the Task Management response indicates Task Management function
   completion, the initiator will not receive any additional responses
   from the affected tasks.

3.5.1.5. 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 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 are built to enable the direction switching for
   bidirectional commands.

   For input, the target may request positive acknowledgement 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, 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).

3.5.1.6. 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

3.5.2. Requests/Responses carrying SCSI and iSCSI Payload

3.5.2.1. Asynchronous Message

   Asynchronous Messages are used to carry SCSI asynchronous events
   (AEN) and iSCSI asynchronous messages.

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

3.5.3. Requests/Responses Carrying iSCSI Only Payload

3.5.3.1. Text Request and Text Response

   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 Request/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 (Final) flag bit in the text
   response header to indicate its consent to sequence termination.

   Text Request 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) 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.

3.5.3.2. Login Request and Login Response

   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.

   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.

3.5.3.3. Logout Request and Response

   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 initiators 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 text key Time2Retain and how long the
   initiator must wait before proceeding with recovery in the text key
   Time2Wait.

3.5.3.4. 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 acknowledgement 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 acknowledgement.

3.5.3.5. 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.

3.5.3.6. 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 "unidirectional" 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.

4. SCSI Mode Parameters for iSCSI

There are no iSCSI specific mode pages.

5. Login and Full Feature Phase Negotiation

   iSCSI parameters are negotiated at session or connection
   establishment by using Login Requests and Responses (see Section
   3.2.3 iSCSI Login) and during the Full Feature Phase (Section 3.2.4
   iSCSI Full Feature Phase) 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 if the key is declarative or requires
   negotiation.

   For the declarative keys, the declaring party sets a value for the
   key.  The key specification indicates if the key can be declared by
   the initiator, 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
   PDUs.  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 one of the following Login
   or Text Response or Request PDUs.  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 the
   setting of 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 (Transition)
   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 (continuation) 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 enquiring 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 chapter details types of keys and values used, the syntax rules
   for parameter formation, and the negotiation schemes to be used with
   different types of parameters.

5.1. 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 to be presented and interpreted in the case in which
   they appear in this document.  They are case sensitive.

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

   (a-z, A-Z) - letters
   (0-9) - 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 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 have the C bit set to 0, or
   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 consist 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 consist 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 consist
       of minus, dot, colon, or any character allowed by the output of
       the iSCSI string-prep template as specified in [RFC3722] (see
       also Section 3.2.6.2 iSCSI Name Encoding).

     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 start 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-constant MUST
       NOT be used for parameter values if the values can be equal 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 or letters or plus
       or slash or equal.  The encoding is done according to [RFC2045]
       and each character, except equal, represents a base64 digit or a
       6-bit binary string.  Base64-constants are used to encode
       numerical-values or binary strings.  When used to encode
       numerical values, the excessive use of leading 0 digits (encoded
       as A) is discouraged.  The string following 0B (or 0b) represents
       a base64 number that starts with the most significant base64
       digit, followed by all other digits in decreasing order of
       significance and ending with the least-significant base64 digit;
       the least significant base64 digit may be optionally followed by
       pad digits (encoded as equal) that are not considered as part of
       the number.  When used to encode binary strings, base64-constants
       have an implicit
       byte-length that includes six bits for every character of the
       constant, excluding trailing equals (i.e., a base64-constant of n
       base64 characters excluding the trailing equals has a byte-length
       of ((the integer part of) (n*3/4)).  Correctly encoded base64
       strings cannot have n values of 1, 5 ... k*4+1.

     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-numeric-values.

     numeric-range: Two numerical-values separated by a tilde where the
       value to the right of 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,
       numeric-value, a numeric-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 of at least 64 kilobytes of key=value data (see Appendix
   11.1.2 - Simple Public-Key Mechanism (SPKM) - that require support
   for public key certificates).

5.2. 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 to 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 Text or Login Response PDU
   that has the C bit set to 1 MUST NOT have the F/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 Text
   or Login Request with no data segment (DataSegmentLength 0) unless
   explicitly required by a general or a key-specific negotiation rule.

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 while a negotiation
 is a two-way exchange.

 The proposer or declarer can either be the initiator or the target,
 and the acceptor can either be 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 use of some other value has serious
 consequences.

 The value proposed or declared can be a numerical-value, a
 numerical-range defined by lower and upper values 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 it is 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
 negotiation. However the negotiation is not considered as 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 OFMarker=Yes,OFMarkInt=2048~8192
 T->I OFMarker=No,OFMarkInt=Irrelevant

 I->T X#vkey1=(bla,alb,None),X#vkey2=(bla,alb)
 T->I X#vkey1=None,X#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
   not understood MUST be key=NotUnderstood.

   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 re-negotiation 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 it is sent, the proposer MAY choose to terminate the
   connection or session.

   All keys in this document, except for the X extension formats, 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 keys by prefixing them with
   "X-", followed by their (reversed) domain name, or with new keys
   registered with IANA prefixing them with X#.  For example, the entity
   owning the domain example.com can issue:

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

   or a new registered key may be used as in:

         X#SuperCalyPhraGilistic=Yes

   Implementers MAY also introduce new values, but ONLY for new keys or
   authentication methods (see Section 11 iSCSI Security Text Keys and
   Authentication Methods), or digests (see Section 12.1 HeaderDigest
   and DataDigest).

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

   In the Login Phase (see Section 5.3 Login Phase), every stage is a
   separate negotiation.  In the FullFeaturePhase, 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 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 end within a reasonable time or number of exchanges.

5.2.1. List negotiations

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

   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 as a protocol error.

5.2.2. 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" or the acceptor
   MAY 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).

   In Boolean negotiations (i.e., those that result in 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 to answer
   with 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.

5.3. 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, security parameters, and authenticates the
   initiator and target to each other.

   The Login Phase is only implemented via Login Request 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).

   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 or both, but MUST include at least
   one of them.  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.

   Security MUST be completely negotiated within the Login Phase.  The
   use of underlying IPsec security is specified in Chapter 8 and in
   [RFC3723].  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 SecurityNegotiation keys appear in Chapter 11 and the
   LoginOperationalNegotiation keys appear in Chapter 12.  Only a
   limited set of keys (marked as Any-Stage in Chapter 12) may be used
   in any 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.
   It is considered to be a protocol error to send a key that is not
   allowed in the current stage.

   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 through 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 pair of Login
   Request-Responses have no bearing on the T bit settings of the next
   pair.  An initiator that has a 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 Login reject (initiator
   error); if detected by the initiator, the initiator MUST drop the
   connection.

5.3.1. Login Phase Start

   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 and the CSG and NSG fields are
       reserved.
     - Login Response with Login Accept as a final response (T bit set
       to 1 and the NSG in both request and response are 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 10.13 Login Response)
   without SecurityNegotiation and will terminate the connection with a
   response of Authentication failure (see Section 10.13.5 Status-Class
   and Status-Detail).

   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 12.9 TargetPortalGroupTag), 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, 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 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 when TargetName is given and the response is not a redirection.
   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.

5.3.2. iSCSI Security Negotiation

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

   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 5.2 Text Mode Negotiation).  The parameters
       are encoded in UTF8 as key=value.  For security parameters, see
       Chapter 11.

     - When the initiator considers that it is 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 respond 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).

5.3.3. Operational Parameter Negotiation During the Login Phase

   Operational parameter negotiation during the login 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-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.

   If the target responds to a Login Request that has the T bit set to 1
   with a Login Response that has the T bit set to 0, the initiator
   should keep sending the Login Request (even empty) with the T bit set
   to 1, while it still wants to switch stage, until it receives the
   Login Response that has the T bit set to 1 or it receives a key that
   requires it 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
   FullFeaturePhase.  New or replacement connections can only be added
   to a session after the session is operational.

   For operational parameters, see Chapter 12.

5.3.4. 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 reinstating a new Full
   Feature Phase iSCSI connection in its place (with the same CID).
   Thus, the TSIH in the Login 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 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 7.1.3)
   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.

5.3.5. Session Reinstatement, Closure, and Timeout

   Session reinstatement is the process of the initiator logging in with
   an ISID that is possibly active from the target's perspective.  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 7.3 Session State
   Diagrams) 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 7.2 Connection Cleanup State Diagram
       for Initiators and Targets) 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 (N6 transition in the
   session state diagram).

5.3.5.1. 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:

      a)  Successful completion of session reinstatement.
      b)  Session closure event.
      c)  Session timeout event.

   Certain SCSI object clearing actions may result due to the
   notification in the SCSI end nodes, as documented in Appendix F.
   - Clearing Effects of Various Events on Targets -.

5.3.6. 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 5.3.4 Connection Reinstatement), 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 7.2 Connection
   Cleanup State Diagram for Initiators and Targets), or completes a
   successful recovery logout, thus causing all active tasks (that are
   formerly allegiant to the connection) to start waiting for task
   reassignment.

5.4. Operational Parameter Negotiation Outside the Login Phase

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

   Parameter negotiation in Full Feature Phase is done through Text
   requests and responses.  Operational parameter negotiation MAY
   involve several Text request-response exchanges, which the initiator
   always starts and terminates using the same Initiator Task Tag.  The
   initiator MUST indicate its intent to terminate 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 and
   with a Text response with the F bit set to 0, the initiator should
   keep sending the Text request (even empty) 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.

   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 pair of Text
   request-responses 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
   without an intervening operational parameter negotiation reset,
   except for responses to specific keys that explicitly allow repeated
   key declarations (e.g., TargetAddress).  If detected by the target,
   this MUST result in 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.

6. iSCSI Error Handling and Recovery

6.1. Overview

6.1.1. Background

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

      i)  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.
      ii) A TCP connection may fail at any time during the data
          transfer.  All the active tasks must optionally be allowed to
          continue 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 chapter describes a general model for recovery in support of
   interoperability.  See Appendix E.  - Algorithmic Presentation of
   Error Recovery Classes - for further detail 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.

6.1.2. Goals

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

      a)  Allow iSCSI implementations to meet different requirements by
          defining a collection of error recovery mechanisms that
          implementations may choose from.
      b)  Ensure interoperability between any two implementations
          supporting different sets of error recovery capabilities.
      c)  Define the error recovery mechanisms to ensure command
          ordering even in the face of errors, for initiators that
          demand ordering.

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

6.1.3. 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
          10.18)
      b)  Command retry (section 6.2.1)
      c)  Recovery R2T support (section 6.7)
      d)  Requesting retransmission of status/data/R2T using the SNACK
          facility (section 10.16)
      e)  Acknowledging the receipt of the data (section 10.16)
      f)  Reassigning the connection allegiance of a task to a different
          TCP connection (section 6.2.2)
      g)  Terminating the entire iSCSI session to start afresh (section
          6.1.4.4)

   The target mechanisms defined in connection with error recovery are:

      a)  NOP-IN to probe sequence numbers of the initiator (section
          10.19)
      b)  Requesting retransmission of data using the recovery R2T
          feature (section 6.7)
      c)  SNACK support (section 10.16) d)  Requesting that parts of
          read data be acknowledged (section 10.7.2)
      e)  Allegiance reassignment support (section 6.2.2)
      f)  Terminating the entire iSCSI session to force the initiator to
          start over (section 6.1.4.4)

   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 & 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.

6.1.4. 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 class recovery 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.

   Use of within-connection and within-command recovery classes MUST NOT
   be attempted before the connection is in 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 and used.

6.1.4.1. 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 6.7
           Digest Errors, 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 6.8 Sequence
           Errors, 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 6.8 Sequence Errors, 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 6.7
           Digest Errors, using the option of a SNACK.
       b)  Sequence reception timeout (no status) or response reception
           timeout - dealt with as specified in Section 6.8 Sequence
           Errors, 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 6.8 Sequence Errors, 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.  Sequence reception timeout is generally a
   large enough value to allow the data sequence transfer to be
   complete.

6.1.4.2. Recovery Within-connection

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

    -  Requests not acknowledged for a long time.  Requests are
       acknowledged explicitly through ExpCmdSN or implicitly by
       receiving data and/or status.  The initiator MAY retry
       non-acknowledged commands as specified in Section 6.2 Retry and
       Reassign in Recovery.

    -  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 by receiving a Response PDU with a
       higher StatSN than expected.  In the first case, digest error
       handling is done as specified in Section 6.7 Digest Errors using
       the option of a SNACK.  In the second case, sequence error
       handling is done as specified in Section 6.8 Sequence Errors,
       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.

6.1.4.3. Connection Recovery

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

    - TCP connection failure: The initiator MUST close the connection.
      It then MUST either implicitly or explicitly logout 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 10.5.1 Function). 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.

    - Receiving an Asynchronous Message that indicates one or all
      connections in a session has 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 it has dropped the
      connection. Then, the target will wait for the initiator to
      continue recovery.

6.1.4.4. 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 F. - Clearing Effects of Various Events on
   Targets -.

6.1.5. Error Recovery Hierarchy

   The error recovery classes described so far are organized into a
   hierarchy for ease in understanding and to limit the implementation
   complexity. With few and 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 capabilities expected
 from the implementations that support each error recovery level.

 +-------------------+--------------------------------------------+
 |ErrorRecoveryLevel | Associated Error recovery capabilities |
 +-------------------+--------------------------------------------+
 | 0 | Session recovery class |
 | | (Section 6.1.4.4 Session Recovery) |
 +-------------------+--------------------------------------------+
 | 1 | Digest failure recovery (See Note below.) |
 | | plus the capabilities of ER Level 0 |
 +-------------------+--------------------------------------------+
 | 2 | Connection recovery class |
 | | (Section 6.1.4.3 Connection Recovery) |
 | | plus the capabilities of ER Level 1 |
 +-------------------+--------------------------------------------+

 Note: Digest failure recovery is comprised of two recovery classes:
 Within-Connection recovery class (Section 6.1.4.2 Recovery Within-
 connection) and Within-Command recovery class (Section 6.1.4.1
 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, the
 functionality corresponding to any defined value numerically less
 than the proposed. 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                    |
   +-------------------+---------------------------------------------+

6.2. Retry and Reassign in Recovery

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

6.2.1. Usage of Retry

   By resending the same iSCSI command PDU ("retry") in the absence of a
   command acknowledgement (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 10.16, 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 3.2.2.1.

   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.

6.2.2. Allegiance Reassignment

   By issuing a "task reassign" task management request (Section 10.5.1
   Function), 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 10.14) MUST be successfully
   completed for the previous connection to which the task was
   allegiant.

   In reassigning connection allegiance for a command, the targets
   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 zero if there was no data transfer) and bring
   the read command to completion by sending the remaining data and
   sending (or resending) the status.  ExpDataSN acknowledges all data
   sent up to, but not including, the Data-In PDU and or R2T with DataSN
   (or R2TSN) equal to 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 an accurate state.  Initiators MUST not subsequently request
   data retransmission through Data SNACK for PDUs numbered less than
   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
   "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.

6.3. 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 either by
   transmitting a command PDU with the same CmdSN, or by aborting the
   task (see section 6.9 on how an abort may plug a CmdSN gap).

   When a data PDU is rejected and its DataSN can be ascertained, a
   target MUST advance 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.

6.4. 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.

6.4.1. Timeouts on Transport Exception Events

   A transport connection shutdown or a transport reset without any
   preceding iSCSI protocol interactions informing the end-points 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 12.15
   DefaultTime2Wait) and DefaultTime2Retain (Section 12.16
   DefaultTime2Retain) text keys for the session.

6.4.2. 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 10.15) 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"; section 10.9.1) 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.

6.5. 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 of "Close the connection" and there are active
          tasks allegiant to that connection.

      b)  When a connection fails and the connection state eventually
          times out (state transition M1 in Section 7.2.2 State
          Transition Descriptions for Initiators and Targets) and there
          are active tasks allegiant to that connection.

      c)  When a successful Logout with the reason code of "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 of "Close the session" and there are active
          tasks in that session.

   If the tasks terminated in the above cases a), b, c) and d)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 SCSI level depending on the SCSI context as
   defined by the SCSI standards (e.g., queued commands and ACA, in
   cases a), b), and c), 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, and the ASC/ASCQ value of 47h/7Fh - "SOME COMMANDS CLEARED
   BY ISCSI PROTOCOL EVENT" , etc. - see [SAM2] and [SPC3]).

6.6. 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 10 iSCSI PDU Formats).
      b)  Inconsistent field contents (consistent field contents are
          specified in Section 10 iSCSI PDU Formats).

   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 either with a connection close or with a connection reset and
   escalate the format error to session recovery (see Section 6.1.4.4
   Session Recovery).

6.7. Digest Errors

   The discussion of the legal choices in handling digest errors below
   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, 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 response PDU with a CHECK
            CONDITION Status and an iSCSI Condition of "protocol service
            CRC error" (Section 10.4.7.2 Sense Data).  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 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 case of immediate data being present on
         a discarded command, the immediate data is implicitly recovered
         when the task is retried (see section 6.2.1), 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:
         i)  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 10.4.7.2 Sense Data).
         ii) 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 10.4.7.2 Sense Data).
      b) Abort the task and terminate the command with an error.

   -  If the discarded PDU is a response PDU, the initiator MUST do one
      of the following:

      a) Request PDU retransmission with a status SNACK.
      b) Logout the connection for recovery and continue the tasks on a
         different connection instance as described in Section 6.2 Retry
         and Reassign in Recovery.
      c) Logout to close the connection (abort all the commands
         associated with the connection).

   -  No further action is necessary for initiators if the discarded PDU
      is an unsolicited PDU (e.g., Async, Reject).  Task timeouts as in
      the initiator waiting for a command completion, or process
      timeouts, as in the target waiting for a Logout, will ensure that
      the correct operational behavior will result in these cases
      despite the discarded PDU.

6.8. 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 6.7 Digest Errors.  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 6.7 Digest Errors.

   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 6.7 Digest Errors.
   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.

6.9. SCSI Timeouts

   An iSCSI initiator MAY attempt to plug a command sequence gap on the
   target end (in the absence of an acknowledgement of the command by
   way of ExpCmdSN) before the ULP timeout by retrying the
   unacknowledged command, as described in Section 6.2 Retry and
   Reassign in Recovery.

   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 either using 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:

      -  Original command was dropped due to digest error.
      -  Connection on which the original command was sent was
         successfully logged out.  Upon 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 to be
   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 of "Task Does Not
   Exist".

6.10. 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:

      -  None of the choices, or the stated value, is acceptable to one
         of the sides 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:

      -  During Login, any failure in negotiation MUST be considered a
         login process failure and the Login Phase must be terminated,
         and with it, the connection.  If the target detects the
         failure, it must terminate the login with the appropriate Login
         Response code.

      -  A failure in negotiation, while in the Full Feature Phase, will
         terminate the entire negotiation sequence that 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).

6.11. Protocol Errors

   Mapping framed messages over a "stream" 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 above
   mechanisms for connection drop and reestablishment 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.

6.12. 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 by a failing iSCSI NOP (acting as a ping).
   The latter MAY be used periodically to increase the speed and
   likelihood of detecting connection failures.  Initiators and targets
   MAY also use the keep-alive option 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:

      -  Attempt connection recovery within the session (Section 6.1.4.3
         Connection Recovery).

      -  Logout the connection with the reason code "closes the
         connection" (Section 10.14.5 Implicit termination of tasks),
         re-issue missing commands, and implicitly terminate all active
         commands.  This option requires support for the
         within-connection recovery class (Section 6.1.4.2 Recovery
         Within-connection).

      -  Perform session recovery (Section 6.1.4.4 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).

6.13. 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 5.3.5).

   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.

7. 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 state diagrams for ease in understanding.  The first
   diagram, "standard connection state diagram", 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 diagram, "connection cleanup state diagram",
   describes the connection state transitions while performing the iSCSI
   connection cleanup.

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

7.1. Standard Connection State Diagrams

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

7.1.2. 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:
            - The final iSCSI Login Response was received with a
              non-zero Status-Class.
            - Login timed out.
            - A transport disconnect indication was received.
            - A transport reset was received.
            - An internal event was received indicating a transport
              timeout.
            - 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:
            - 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.
            - Login timed out.
            - Transport disconnect indication was received.
            - Transport reset was received.
            - An internal event indicating a transport timeout was
              received.
            - 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
            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 is 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: Async PDU with AsyncEvent "Request Logout" was
            received.
        -target: An internal event that requires the decommissioning of
            the connection is 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 is received.  In all these cases, the
            transport connection is closed.

   -T14:
        -initiator: Async PDU with AsyncEvent "Request Logout" was
            received again.
        -target: Illegal
   -T15, T16:
        -initiator: One or more of the following events caused this
            transition:
            -Internal event that indicates a transport connection
               timeout was received thus prompting transport RESET or
               transport connection closure.
            -A transport RESET.
            -A transport disconnect indication.
            -Async PDU with AsyncEvent "Drop connection" (for this CID).
            -Async PDU with AsyncEvent "Drop all connections".
        -target: One or more of the following events caused this
            transition:
            -Internal event that indicates a transport connection
               timeout was received, thus prompting transport RESET or
               transport connection closure.
            -An internal event of a failed connection/session
               reinstatement is received.
            -A transport RESET.
            -A transport disconnect indication.

            -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:
            -Logout response, (failure i.e., a non-zero status) was
               received, or Logout timed out.
            -Any of the events specified for T15 and T16.
        -target:  One or more of the following events caused this
            transition:
            -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.
            -Any of the events specified for T15 and T16.
   -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 is 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.

7.1.3. 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| -  |-  |-  | - | - | -  | - |
      ---+----+---+---+---+---+----+---+

7.1.4. 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| -  |-  |-  | - | - | -  | - |
   ---+----+---+---+---+---+----+---+

7.2. 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 (e.g., CSM-C) enters the
   CLEANUP_WAIT state (S8), it must go through the state transitions
   described in the connection cleanup state diagram either a) using a
   separate full-feature phase connection (let's call it CSM-E) in the
   LOGGED_IN state in the same session, or b) using a new transport
   connection (let's call it CSM-I) 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 (either as 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 5.3.4)
   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.

 -------
 / 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 | - | - | - |
 -----+----+----+----+

7.2.1. 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.

7.2.2. 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 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 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: CSM-E either moved out of LOGGED_IN, or Logout
               timed out and/or aborted, or Logout response (failure)
               was received.
            -target: CSM-E either moved out of LOGGED_IN,  Logout timed
               out and/or aborted, or an internal event that indicates 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.

7.3. Session State Diagrams

7.3.1. 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:

 -------
 / 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 | - |
 -----+----+----+----+

7.3.2. 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 | - |
 -----+----+----+----+----+----+

7.3.3. 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.

7.3.4. 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 5.3.6 Session Continuation and Failure).
        -target: Graceful closing of the session via session closure
            (Section 5.3.6 Session Continuation and Failure) or a
            successful session reinstatement cleanly closed the session.

   -N4:
        -initiator: A session continuation attempt succeeded.
        -target: Illegal.

   -N5:
        -initiator: Session failure (Section 5.3.6 Session Continuation
            and Failure) occurred.
        -target: Session failure (Section 5.3.6 Session Continuation and
            Failure) 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 is 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.

8. 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.  iSCSI configured without security should be confined, in
   extreme cases, to closed environments without any security risk.
   [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.

8.1. iSCSI Security Mechanisms

   The entities involved in iSCSI security are the initiator, target,
   and the IP communication end points.  iSCSI scenarios in which
   multiple initiators or targets share a single communication end point
   are expected.  To accommodate such scenarios, iSCSI uses 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 end points.

   Further details on typical iSCSI scenarios and the relation between
   the initiators, targets, and the communication end points can be
   found in [RFC3723].

8.2. 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 clear text (or equivalent)
   passwords is not acceptable; 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 clear.  The authentication

   mechanism alone (without underlying IPsec) should only be used when
   there is no risk of eavesdropping, message insertion, deletion,
   modification, and replaying.

   Section 11 iSCSI Security Text Keys and Authentication Methods
   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 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 "Authentication Failure" status MUST be
   specified.  The importance of this rule can be illustrated in CHAP
   with target authentication (see Section 11.1.4 Challenge Handshake
   Authentication Protocol (CHAP)) where the initiator would have been
   able to conduct a reflection attack by omitting his 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 11.1.4 Challenge
   Handshake Authentication Protocol (CHAP) and SRP_U, see Section
   11.1.3 Secure Remote Password (SRP)).

8.2.1. CHAP Considerations

   Compliant iSCSI initiators and targets MUST implement the CHAP
   authentication method [RFC1994] (according to Section 11.1.4
   Challenge Handshake Authentication Protocol (CHAP) 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 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

   8.3.2 Confidentiality) 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, Section 11.1.4
   Challenge Handshake Authentication Protocol (CHAP)) 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:

     Rogue wants to impersonate Storage to Alice, and knows that a
      single secret is used for both directions of Storage-Alice
      authentication.

     Rogue convinces Alice to open two connections to Rogue, and Rogue
      identifies itself as Storage on both connections.

     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.

     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 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) 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 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.

8.2.2. 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-German 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].

8.3. IPsec

   iSCSI uses the IPsec mechanism for packet protection (cryptographic
   integrity, authentication, and confidentiality) at the IP level
   between the iSCSI communicating end points.  The following sections
   describe the IPsec protocols that must be implemented for data
   integrity and authentication, 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].

8.3.1. Data Integrity and Authentication

   Data authentication and integrity is 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 integrity
   and authentication by implementing IPsec [RFC2401] with ESP [RFC2406]
   in tunnel mode and MAY provide data integrity and authentication by
   implementing IPsec with ESP in transport mode.  The IPsec
   implementation MUST fulfill the following iSCSI specific
   requirements:

     - HMAC-SHA1 MUST be implemented [RFC2404].
     - AES CBC MAC with XCBC extensions SHOULD be implemented
       [RFC3566].

   The ESP anti-replay service MUST also be implemented.

   At the high speeds iSCSI is expected to operate, a single IPsec SA
   could rapidly cycle through the 32-bit IPsec sequence number space.
   In view of this, it may be desirable in the future for an iSCSI
   implementation that operates at speeds of 1 Gbps or greater to
   implement the IPsec sequence number extension [SEQ-EXT].

8.3.2. Confidentiality

   Confidentiality is provided by encrypting the data in every packet.
   When confidentiality is used it MUST be accompanied by data integrity
   and authentication to provide comprehensive protection against
   eavesdropping, message insertion, deletion, modification, and
   replaying.

   An iSCSI compliant initiator or target MUST provide confidentiality
   by implementing IPsec [RFC2401] with ESP [RFC2406] in tunnel mode and
   MAY provide confidentiality by implementing IPsec with ESP in
   transport mode, with the following iSCSI specific requirements:

     - 3DES in CBC mode MUST be implemented [RFC2451].
     - AES in Counter mode SHOULD be implemented [RFC3686].

   DES in CBC mode SHOULD NOT be used due to its inherent weakness.  The
   NULL encryption algorithm MUST also be implemented.

8.3.3. 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] with the following iSCSI specific requirements:

    -  Peer authentication using a pre-shared cryptographic key MUST be
       supported.  Certificate-based peer authentication using digital
       signatures MAY be supported.  Peer authentication using the
       public key encryption methods outlined in IKE sections 5.2 and
       5.3[7] SHOULD NOT be used.

    -  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
       the pertinent Certificate Revocation List (CRL) before accepting
       a PKI certificate for use in IKE authentication procedures.

    -  Conformant iSCSI implementations MUST support IKE Main Mode and
       SHOULD support Aggressive Mode.  IKE main mode with pre-shared
       key authentication method SHOULD NOT be used when either the
       initiator or the target uses dynamically assigned IP 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.

    -  In the IKE Phase 2 Quick Mode, exchanges for creating the Phase 2
       SA, the Identity Payload, fields MUST be present.  ID_IPV4_ADDR,
       ID_IPV6_ADDR (if the protocol stack supports IPv6) and ID_FQDN
       Identity payloads MUST be supported; ID_USER_FQDN SHOULD be
       supported.  The IP Subnet, IP Address Range, ID_DER_ASN1_DN, and
       ID_DER_ASN1_GN formats SHOULD NOT be used.  The ID_KEY_ID
       Identity Payload MUST NOT be used.

   Manual cryptographic keying MUST NOT be used because it does not
   provide the necessary re-keying support.

   When IPsec is used, the receipt of an IKE Phase 2 delete message
   SHOULD NOT be interpreted as a reason for tearing down the iSCSI TCP
   connection.  If additional traffic is sent on it, a new IKE Phase 2
   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.

   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. 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 3.4.3 Consequences of the Model.

9.1. 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.

9.1.1. 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 clauses (particularly, Section 3.4.2
   SCSI Architecture Model) 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 to 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
   to 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 3.4.3 Consequences of the Model)
   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.

9.1.2. iSCSI Name, ISID, and TPGT Use

   The designers of the iSCSI protocol envisioned there being one iSCSI
   Initiator Node Name per operating system image on a machine.  This
   enables SAN resource configuration and authentication schemes based
   on a  system's identity.  It 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 (ISID for initiators) to enforce both
   the ISID and TSIH RULES (see Section 3.4.3 Consequences of the
   Model).

   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
   anytime 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 either by pointing the
   component to a vendor-specific location for this datum or to a
   system-wide location.  The structure of the ISID namespace (see
   Section 10.12.5 ISID 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 3.4.3 Consequences of the Model) 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 unique worldwide.  It is therefore important that when one
   chooses to reuse the iSCSI Node Name of a disabled unit, not to
   re-assign 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 pre-assigned 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 via private APIs driven by the
   vendor's own software.

9.2. Autosense and Auto Contingent Allegiance (ACA)

   Autosense refers to the automatic return of sense data to the
   initiator in case 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 iSCSI can have many commands in-flight between initiator
   and target, iSCSI initiators and targets SHOULD support ACA.

9.3. iSCSI Timeouts

   iSCSI recovery actions are often dependent on iSCSI time-outs being
   recognized and acted upon before SCSI time-outs.  Determining the
   right time-outs to use for various iSCSI actions (command
   acknowledgements expected, status acknowledgements, etc.) is very
   much dependent on infrastructure (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 the
   network load variability.  For connection teardown the implementer
   may want to consider also the 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 SHOULD be generous enough to avoid
   affecting interoperability (e.g., allowing each key to be negotiated
   on a separate exchange).

   The relation 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.

9.4. 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 wrap around, the
   protocol requires (see Section 3.2.2.1 Command Numbering and
   Acknowledging) that on every connection on which a retry has been
   issued, a non-immediate command be issued and acknowledged within a
   2**31-1 commands interval 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.  As 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.

9.5. Synch and Steering Layer and Performance

   While a synch and steering layer is optional, an initiator/target
   that does not have it working against a target/initiator that demands
   synch and steering may experience performance degradation caused by
   packet reordering and loss.  Providing a synch and steering mechanism
   is recommended for all high-speed implementations.

9.6. 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 is 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 re-issued
   due to no status received for the command.  If the first SPACE
   command was actually processed, the re-issued 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 re-issued due
   to no status received for the command.  If the first EXCHANGE MEDIUM
   command was actually processed, the re-issued EXCHANGE MEDIUM
   command, if processed, will perform the swap again.  The net effect
   is that a swap was not performed thus leaving a data integrity
   exposure.

   All commands that change the state of the device (as in SPACE
   commands for sequential access devices, and EXCHANGE MEDIUM for
   medium changer device), MUST be issued as non-immediate commands for
   deterministic and in order 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 SCSI level is difficult and
   error recovery at 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).

9.6.1. 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".  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 the connection
   failure is also of high likelihood during a backup/retrieval.

   For extended copy operations, implementations SHOULD use
   ErrorRecoveryLevel=2 whenever there is a relatively high likelihood
   of connection failure.

10. 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 zero 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.

10.1. iSCSI PDU Length and Padding

iSCSI PDUs are padded to the closest integer number of four byte
words. The padding bytes SHOULD be sent as 0.

10.2. 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 four byte words.  For example, all PDU segments and digests start
   at a four 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.

10.2.1. 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

10.2.1.1 I

For request PDUs, the I bit set to 1 is an immediate delivery marker.

10.2.1.2. 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 reserved.

10.2.1.3. Final (F) bit

When set to 1 it indicates the final (or only) PDU of a sequence.

10.2.1.4. Opcode-specific Fields

These fields have different meanings for different opcode types.

10.2.1.5. TotalAHSLength

   Total length of all AHS header segments in units of four 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.

10.2.1.6. DataSegmentLength

This is the data segment payload length in bytes (excluding padding).
The DataSegmentLength MUST be 0 whenever the PDU has no data segment.

10.2.1.7. LUN

   Some opcodes operate on a specific Logical Unit.  The Logical Unit
   Number (LUN) field identifies which Logical Unit.  If the opcode does
   not relate to a Logical Unit, 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.

10.2.1.8. 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.

10.2.2. 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

10.2.2.1. AHSType

   The AHSType field is coded as follows:

       bit 0-1 - Reserved

       bit 2-7 - AHS code

        0 - Reserved
        1 - Extended CDB
        2 - Expected Bidirectional Read Data Length
        3 - 63 Reserved

10.2.2.2. 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).

10.2.2.3. 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.

10.2.2.4. Bidirectional Expected Read-Data 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| Expected Read-Data Length                                     |
     +---------------+---------------+---------------+---------------+
    8

10.2.3. 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.

10.2.4. 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.

10.3. 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)                                        /
     +---------------+---------------+---------------+---------------+

10.3.1. Flags and Task Attributes (byte 1)

   The flags for a SCSI Command 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

   Setting both the W and the F bit to 0 is an error.  Either or both of
   R and W MAY be 1 when either the Expected Data Transfer Length and/or
   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).

10.3.2. CmdSN - Command Sequence Number

Enables ordered delivery across multiple connections in a single
session.

10.3.3. ExpStatSN

Command responses up to ExpStatSN-1 (mod 2**32) have been received
(acknowledges status) on the connection.

10.3.4. 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 SAM2 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 SAM2 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.

10.3.5. 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.

10.3.6. 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 3.2.4.2 Data
   Transfer Overview.

10.4. 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)                                        |
     +---------------+---------------+---------------+---------------+

10.4.1. 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 Expected Bidirectional Read 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.

10.4.2. 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.

10.4.3. 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 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 PDU that includes SCSI status (Response PDU or Data-In PDU
   including status) 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.

10.4.4. 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 an 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 10.16 SNACK
   Request.

10.4.5. Residual Count

   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 is reserved.  Targets may set the residual count and initiators
   may use it when the response code is "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.

10.4.6. 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 "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 Expected Bidirectional Read 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.

10.4.7. 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                                                 /
     /                                                               /
     +---------------+---------------+---------------+---------------+
    z|

10.4.7.1. SenseLength

Length of Sense Data.

10.4.7.2. Sense Data

   The Sense Data contains detailed information about a check condition
   and [SPC3] specifies the format and content of the Sense Data.

   Certain iSCSI conditions result in the command being terminated at
   the target (response Command Completed at Target) with a SCSI Check
   Condition Status as outlined in the next table:

10.4.8. ExpDataSN

   The number of R2T and 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.

10.4.9. StatSN - Status Sequence Number

   StatSN is a Sequence Number that the target iSCSI layer generates per
   connection and that in turn, enables the initiator to acknowledge
   status reception.  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.

10.4.10. ExpCmdSN - Next Expected CmdSN from this Initiator

   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.

10.4.11. MaxCmdSN - Maximum CmdSN from this Initiator

   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 MaxCmdSN
   is equal to ExpCmdSN-1, this indicates to the initiator that the
   target cannot receive any additional commands.  When 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.

10.5. 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)                                      |
     +---------------+---------------+---------------+---------------+

10.5.1. 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
        logical unit.

   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 Referenced Task Tag field to this connection,
        thus resuming the iSCSI exchanges for the task.

   For all these functions, the Task Management function response MUST
   be returned as detailed in Section 10.6 Task Management Function
   Response.  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 with CmdSN equal or exceeding CmdSN.

   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 CmdSN lower
   than the task management command CmdSN) but except 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, 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 on its part MUST wait for responses on all affected target

   transfer tags before acting on either of these two task management
   requests.  In case 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 within-command error recovery class (see Section
   6.1.4.1 Recovery Within-command) 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 RefCmdSN MUST be that of the Task Management request itself
   (i.e., CmdSN and RefCmdSN are equal); otherwise RefCmdSN MUST be set
   to the CmdSN of the task to be aborted (lower than CmdSN).

   If the connection is still active (it is not undergoing an implicit
   or explicit logout), 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 issued but not acknowledged, will be
   reissued 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 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 10.6 Task Management Function Response 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 Logical Units 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.

   The target MUST treat the TARGET COLD RESET function additionally 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 6.2 Retry and Reassign in
   Recovery.

   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 Task Management response of "Function
   rejected".

   TASK REASSIGN MUST be issued as an immediate command.

10.5.2. TotalAHSLength and DataSegmentLength

For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

10.5.3. 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.

10.5.4. Referenced Task Tag

   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.

10.5.5. RefCmdSN

   If an ABORT TASK is issued for a task created by an immediate command
   then RefCmdSN MUST be that of the Task Management request itself
   (i.e., CmdSN and RefCmdSN are equal).

   For an ABORT TASK of a task created by non-immediate command 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
   10.6.1 when the task identified by the Referenced Task Tag field is
   not with the target.

   Otherwise, this field is reserved.

10.5.6. ExpDataSN

   For recovery purposes, the iSCSI target and initiator maintain a data
   acknowledgement 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, ExpDataSN will contain  an updated data
   acknowledgement reference number or the value 0; the latter
   indicating that the data acknowledgement 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
   acknowledgement 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.

10.6. 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.

10.6.1. Response

   The target provides a Response, which may take on the following
   values:

      a)    0 - Function complete.
      b)    1 - Task does not exist.
      c)    2 - LUN does not exist.
      d)    3 - Task still allegiant.
      e)    4 - Task allegiance reassignment not supported.

      f)    5 - Task management function not supported.
      g)    6 - Function authorization failed.
      h)  255 - Function rejected.

   All other values are reserved.

   For a discussion on usage of response codes 3 and 4, see Section
   6.2.2 Allegiance Reassignment.

   For the TARGET COLD RESET and TARGET WARM RESET functions, the target
   cancels all pending operations across all Logical Units 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.  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 have been
   confirmed (acknowledged through ExpStatSN) by the initiator on all
   connections of this session.  For the exact timeline of events, refer
   to Section 10.6.2 Task Management Actions on Task Sets.

   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 if 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 received
          and return the "Function complete" response.

      c)  If the Referenced Task Tag does not identify an existing task
          and if 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.

10.6.2. Task Management Actions on Task Sets

   The execution of ABORT TASK SET and CLEAR TASK SET Task Management
   function requests consists of the following sequence of events in the
   specified order on each of the entities.

   The initiator:

         a) Issues ABORT TASK SET/CLEAR TASK SET request.
         b) Continues to respond to each target transfer tag received
            for the affected task set.
         c) Receives any responses for the tasks in the affected task
            set (may process them as usual because they are guaranteed
            to be valid).
         d) Receives the task set management response, thus concluding
            all the tasks in the affected task set.

   The target:

         a) Receives the ABORT TASK SET/CLEAR TASK SET request.
         b) Waits for all target transfer tags to be responded to and
            for all affected tasks in the task set to be received.
         c) Propagates the command to and receives the response from the
            target SCSI layer.
         d) Takes note of last-sent StatSN on each of the connections in
            the iSCSI sessions (one or more) sharing the affected task
            set, and waits for acknowledgement of each StatSN (may
            solicit for acknowledgement by way of a NOP-In).  If some
            tasks originate from non-iSCSI I_T_L nexi then the means by
            which the target insures that all affected tasks have
            returned their status to the initiator are defined by the
            specific protocol.

         e) Sends the task set management response to the issuing
            initiator.

10.6.3. TotalAHSLength and DataSegmentLength

For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

10.7. SCSI Data-Out & 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 though 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.

10.7.1. 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
   MaxBurstLength (or FirstBurstLength) to MaxRecvDataSegmentLength
   ratio (as PDUs may be limited in length by the sender capabilities).
   Using 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.

10.7.2. A (Acknowledge) Bit

   For sessions with ErrorRecoveryLevel 1 or higher, the target sets
   this bit to 1 to indicate that it requests a positive acknowledgement
   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 ExpStatSN on other outbound PDUs if the status for
   the task is also received.  In the latter case (acknowledgement
   through 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 acknowledgement through 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 acknowledgement for a
   task that generated the Data-In PDUs is considered by the target as
   an implicit acknowledgement of the Data-In PDUs if such an
   acknowledgement was requested by the target.

10.7.3. 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.  As 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).  If Status is sent with the data, then a SCSI Response PDU
   MUST NOT be sent as this would violate SCSI rules (a single status).
   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
                10.4.1 Flags (byte 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 and their values are defined in
   Section 10.4 SCSI Response.

10.7.4. 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.

10.7.5. 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 3.2.2.3 Data Sequencing).

   For output (write) data PDUs, the DataSN is the Data-Out PDU number
   within the current output sequence.  The current output sequence is
   either identified by the Initiator Task Tag (for unsolicited data) or
   is a data sequence generated for one R2T (for data solicited through
   R2T).

10.7.6. 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.

10.7.7. 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.

10.8. 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 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 "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
   are limited by the value of the negotiated key MaxOutstandingR2T.
   Within a connection, 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.

10.8.1. TotalAHSLength and DataSegmentLength

For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

10.8.2. 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 3.2.2.3 Data Sequencing).

10.8.3. StatSN

The StatSN field will contain the next StatSN. The StatSN for this
connection is not advanced after this PDU is sent.

10.8.4. 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.

10.8.5. 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.

10.9. Asynchronous Message

   An Asynchronous Message may be sent from the target to the initiator
   without correspondence 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].

   StatSN counts this PDU as an acknowledgeable event (StatSN is
   advanced), which allows for initiator and target state
   synchronization.

10.9.1. AsyncEvent

   The codes used for iSCSI Asynchronous Messages (events) are:

      0 - 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 - 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.
          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 Logout 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 - target indicates 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 - target indicates it will drop all the connections of this
          session.

          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 - 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.

255 - vendor specific iSCSI Event. The AsyncVCode details the
vendor code, and data MAY accompany the report.

All other event codes are reserved.

10.9.2. 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.

10.9.3. LUN

The LUN field MUST be valid if AsyncEvent is 0. Otherwise, this
field is reserved.

10.9.4. 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|

10.9.4.1. 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.

10.10. 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 to 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 have at most 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 must cause such a task
   to be implicitly terminated by the target.

10.10.1. F (Final) Bit

   When set to 1,  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.

10.10.2. C (Continue) Bit

   When set to 1, 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.

10.10.3. Initiator Task Tag

   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.

10.10.4. 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 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)

10.10.5. Text

   The data lengths of a text request MUST NOT exceed the iSCSI target
   MaxRecvDataSegmentLength (a per connection and per direction
   negotiated parameter).  The text format is specified in Section 5.2
   Text Mode Negotiation.

   Chapter 11 and Chapter 12 list some basic Text key=value pairs, some
   of which can be used in Login Request/Response and some in Text
   Request/Response.

   A key=value pair can span Text request or 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.

   Chapter 5 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.

10.11. 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)                                        |
     +---------------+---------------+---------------+---------------+

10.11.1. 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 of 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 0xffffffff.

10.11.2. C (Continue) Bit

   When set to 1, 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.

10.11.3. Initiator Task Tag

The Initiator Task Tag matches the tag used in the initial Text
Request.

10.11.4. 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 of
   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 of 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), state 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.

10.11.5. StatSN

The target StatSN variable is advanced by each Text Response sent.

10.11.6. Text Response Data

   The data lengths of a text response MUST NOT exceed the iSCSI
   initiator MaxRecvDataSegmentLength (a per connection and per
   direction negotiated parameter).

   The text in the Text Response Data is governed by the same rules as
   the text in the Text Request Data (see Section 10.10.5 Text).

   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.

10.12. 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 Chapter 5) consists of a sequence of Login
   Requests and 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         /
    +/                                                               /
     +---------------+---------------+---------------+---------------+

10.12.1. T (Transit) Bit

   If set to 1, indicates that the initiator is ready to transit to the
   next stage.

   If the T bit is set to 1 and NSG is FullFeaturePhase, then this also
   indicates that the initiator is ready for the Final Login Response
   (see Chapter 5).

10.12.2. C (Continue) Bit

   When set to 1,  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.

10.12.3. 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 Chapter 5).  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.

10.12.4. Version

The version number of the current draft is 0x00. As such, all
devices MUST carry version 0x00 for both Version-min and Version-max.

10.12.4.1. Version-max

   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.

10.12.4.2. 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.

10.12.5. ISID

   This is an initiator-defined component of the session identifier and
   is structured as follows (see [RFC3721] and Section 9.1.1
   Conservative Reuse of ISIDs 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&B are a 22 bit OUI
             (the I/G & U/L bits are omitted)
             C&D 24 bit qualifier
     01b     EN - Format (IANA Enterprise Number)
             A - Reserved
             B&C EN (IANA Enterprise Number)
             D - Qualifier
     10b     "Random"
             A - Reserved
             B&C Random
             D - Qualifier
     11b     A,B,C&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 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 Section 3.4.3 Consequences of
   the Model and Section 9.1.1 Conservative Reuse of ISIDs).

   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 Section 9.1.1
   Conservative Reuse of ISIDs and Section 9.1.2 iSCSI Name, ISID, and
   TPGT Use).  The resultant ISID MUST also be persistent over power
   cycles, reboot, card swap, etc.

10.12.6. TSIH

   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 5.3.1 Login Phase Start.

10.12.7. Connection ID - CID

   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 5.3.4 Connection Reinstatement).  For the details
   of the implicit Logout Request, see Section 10.14 Logout Request.

10.12.8. CmdSN

   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
         FullFeaturePhase 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 CmdSN.

   If the Login Request is a leading Login Request, the target MUST use
   the value presented in CmdSN as the target value for ExpCmdSN.

10.12.9. ExpStatSN

   For the first Login Request on a connection this is ExpStatSN for the
   old connection and this field is only valid if the Login Request
   restarts a connection (see Section 5.3.4 Connection Reinstatement).

   For subsequent Login Requests it is used to acknowledge the Login
   Responses with their increasing StatSN values.

10.12.10. 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 10.10.5 Text for text requests
   also hold for Login Requests.  Keys and their explanations are listed
   in Chapter 11 (security negotiation keys) and Chapter 12 (operational
   parameter negotiation keys).  All keys in Chapter 12, except for the
   X extension formats, MUST be supported by iSCSI initiators and
   targets.  Keys in Chapter 11 only need to be supported when the
   function to which they refer is mandatory to implement.

10.13. 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         /
    +/                                                               /
     +---------------+---------------+---------------+---------------+

10.13.1. 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.

10.13.2. 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.

10.13.3. 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 that 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 5.3 Login Phase).

10.13.4. 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.

10.13.5. 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

   0 Status-Class 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 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 re-try 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 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 |      | of 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 1 in both the request and the matching
   response, and the NSG is 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.

10.13.6. 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 NSG is FullFeaturePhase, then this is also
   the Final Login Response (see Chapter 5).  A T bit of 0 indicates a
   "partial" response, which means "more negotiation needed".

   A Login Response with a 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.

10.13.7. C (Continue) Bit

   When set to 1,  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.

10.13.8. 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 10.11.6 Text Response Data for
   text responses also hold for Login Responses.  Keys and their
   explanations are listed in Chapter 11 (security negotiation keys) and
   Chapter 12 (operational parameter negotiation keys).  All keys in
   Chapter 12, except for the X extension formats, MUST be supported by
   iSCSI initiators and targets.  Keys in Chapter 11, only need to be
   supported when the function to which they refer is mandatory to
   implement.

10.14. 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 of "close the
   connection" or "close the session", the target MUST terminate all
   pending commands, whether acknowledged via ExpCmdSN or not, on that
   connection or session respectively.

   When receiving a Logout Request with the reason code "remove
   connection for recovery", the target MUST discard all requests not
   yet acknowledged via 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 the Login Request
   with a non-zero TSIH and the same CID on a new connection for the
   same effect (see Section 10.14.3 CID).  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 5.3.4 Connection Reinstatement).

A successful completion of a Logout Request with the reason code of
 "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 have not been
 delivered to SCSI because one or more commands with a smaller CmdSN
 has not been received by iSCSI. See Section 3.2.2.1 Command
 Numbering and Acknowledging. The resulting holes the in command
 sequence numbers will have to be handled by appropriate recovery (see
 Chapter 6) unless the session is also closed.

 The entire logout discussion in this section is also applicable for
 an implicit Logout realized via 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)                                      |
     +---------------+---------------+---------------+---------------+

10.14.1. Reason Code

   Reason Code 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 connection
          (if any) are terminated.

      2 - remove the connection for recovery.  Connection is closed and
          all commands associated with it, if any, are to be prepared
          for a new allegiance.

   All other values are reserved.

10.14.2. TotalAHSLength and DataSegmentLength

For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

10.14.3. 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".

10.14.4. ExpStatSN

This is the last ExpStatSN value for the connection to be closed.

10.14.5. 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 of "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 7.2.2 State
          Transition Descriptions for Initiators and Targets) 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 of "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 SCSI level depending on the SCSI context as defined by the
   SCSI standards (e.g., queued commands and ACA, in cases a), b), and
   c), 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, and

the ASC/ASCQ value of 47h/7Fh - "SOME COMMANDS CLEARED BY ISCSI
PROTOCOL EVENT" - etc. - see [SAM2] and [SPC3]).

10.15. 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)                                      |
     +---------------+---------------+---------------+---------------+

10.15.1. Response

   Logout Response:

      0 - connection or session closed successfully.

      1 - CID not found.

      2 - connection recovery is not supported.  If Logout reason code
         was recovery and target does not support it as indicated by the
         ErrorRecoveryLevel.

      3 - cleanup failed for various reasons.

10.15.2. TotalAHSLength and DataSegmentLength

For this PDU TotalAHSLength and DataSegmentLength MUST be 0.

10.15.3. Time2Wait

   If the Logout Response code is 0 and if 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 if 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.

10.15.4. Time2Retain

   If the Logout response code is 0 and if the operational
   ErrorRecoveryLevel is 2, this is the maximum amount of time, in
   seconds, after the initial wait (Time2Wait), 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 if 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), 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.

10.16. 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 by the target, 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 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
   10.16.3 Resegmentation).

   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".

10.16.1. 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 acknowledgement for the given command.

10.16.2. Data Acknowledgement

   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.

10.16.3. 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
   10.16.1 Type).  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 responses must
   carry the same StatSN (see Section 10.4.4 SNACK Tag).  If an
   initiator attempts to recover a lost SCSI Response (with a
   Status SNACK, see Section 10.16.1 Type) 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 6.2.2 Allegiance Reassignment.

10.16.4. Initiator Task Tag

   For 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.

10.16.5. Target Transfer Tag or SNACK Tag

   For an 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 of 0xffffffff.

10.16.6. BegRun

   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).

   BegRun 0 when used in conjunction with RunLength 0 means resend all
   unacknowledged Data-In, R2T or Response PDUs.

   BegRun MUST be 0 for a R-Data SNACK.

10.16.7. RunLength

   The number of PDUs whose retransmission is requested.

   RunLength 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
   R-Data SNACK.

10.17. 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.).

10.17.1. Reason

   The reject Reason is coded as follows:

   +------+----------------------------------------+------------------+
   | Code | Explanation                            | Can the original |
   | (hex)|                                        | PDU be re-sent?  |
   +------+----------------------------------------+------------------+
   | 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 Operation Reject - Can't generate | yes              |
   |      | Target Transfer Tag - out of resources |                  |
   |      |                                        |                  |
   | 0x0b | Negotiation Reset                      | no               |
   |      |                                        |                  |
   | 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.

   All other values for Reason are reserved.

   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 10.4.3
   Response.  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 Reject PDU, see Section 6.3 Usage
   Of Reject PDU in Recovery.

10.17.2. 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 10.16 SNACK
   Request).  The DataSN/R2TSN is the next Data/R2T sequence number that
   the target would send for the task, if any.

10.17.3. StatSN, ExpCmdSN and MaxCmdSN

These fields carry their usual values and are not related to the
rejected command. StatSN is advanced after a Reject.

10.17.4. Complete Header of Bad PDU

The target returns the header (not including digest) of the PDU in
error as the data of the response.

10.18. 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)                                        |
     +---------------+---------------+---------------+---------------+

   A 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 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.

10.18.1. 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 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 10.19 NOP-In).

   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.

10.18.2. Target Transfer Tag

   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.

10.18.3. Ping Data

   Ping data are reflected in the NOP-In Response.  The length of the
   reflected data are limited to MaxRecvDataSegmentLength.  The length
   of ping data are indicated by the DataSegmentLength.  0 is a valid
   value for the DataSegmentLength and indicates the absence of ping
   data.

10.19. 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 either sent by a target as a response to a NOP-Out, as a
   "ping" to an initiator, or as 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.

   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).

10.19.1. Target Transfer Tag

   If the target is responding to a NOP-Out, this 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 0xffffffff).

   If the target is initiating a NOP-In without wanting to receive a
   corresponding NOP-Out, this field MUST hold the reserved value of
   0xffffffff.

10.19.2. StatSN

   The StatSN field will always contain the next StatSN.  However, when
   the Initiator Task Tag is set to 0xffffffff, StatSN for the
   connection is not advanced after this PDU is sent.

10.19.3. LUN

A LUN MUST be set to a correct value when the Target Transfer Tag is
valid (not the reserved value 0xffffffff).

11. 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 under Section 11.1 AuthMethod 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 Chapter 12 as they can be
   used also in the OperationalNegotiation stage.

   All security keys have connection-wide applicability.

11.1. 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 those listed in the following table or are
 vendor-unique methods:

 +------------------------------------------------------------+
 | Name | Description |
 +------------------------------------------------------------+
 | KRB5 | Kerberos V5 - defined in [RFC1510] |
 +------------------------------------------------------------+
 | SPKM1 | Simple Public-Key GSS-API Mechanism |
 | | defined in [RFC2025] |
 +------------------------------------------------------------+
 | SPKM2 | Simple Public-Key GSS-API Mechanism |
 | | defined in [RFC2025] |
 +------------------------------------------------------------+
 | 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 keys(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 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:

 a) Z-reversed.vendor.dns_name.do_something=
 b) Z<#><IANA-registered-string>=

 Authentication methods named using the Z- format are used as private
 extensions. Authentication methods named using the Z# format are
 used as public extensions that must be registered with IANA and MUST
 be described by an informational RFC.

 For all of the public or private extension authentication methods,
 the method specific keys MUST conform to the format specified in
 Section 5.1 Text Format for standard-label.

 To identify the vendor for private extension authentication methods,
 we suggest you use the reversed DNS-name as a prefix to the proper
 digest names.

 The part of digest-name following Z- and Z# MUST conform to the
 format for standard-label specified in Section 5.1 Text Format.

 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.

11.1.1. Kerberos

For KRB5 (Kerberos V5) [RFC1510] and [RFC1964], the initiator MUST
 use:

 KRB_AP_REQ=<KRB_AP_REQ>

 where KRB_AP_REQ is the client message as defined in [RFC1510].

 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
 [RFC1510].

 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.

11.1.2. Simple Public-Key Mechanism (SPKM)

For SPKM1 and SPKM2 [RFC2025], the initiator MUST use:

 SPKM_REQ=<SPKM-REQ>

 where SPKM-REQ is the first initiator token as defined in [RFC2025].

 [RFC2025] defines situations where each side may send an error token
 that may cause the peer to re-generate and resend its last token.
 This scheme is followed in iSCSI, and the error token syntax is:

 SPKM_ERROR=<SPKM-ERROR>

However, SPKM-DEL tokens that are defined by [RFC2025] for fatal
 errors will not be used by iSCSI. If the target needs to send a
 SPKM-DEL token, it will, instead, send a Login "login reject" message
 with the "Authentication Failure" status and terminate the
 connection. If the initiator needs to send a SPKM-DEL token, it will
 close the connection.

 In the following sections, we assume that no SPKM-ERROR tokens are
 required.

 If the initiator authentication fails, the target MUST return an
 error. Otherwise, if the AuthMethod is SPKM1 or if the initiator has
 selected the mutual authentication option (by setting mutual-state
 bit in the options field of the REQ-TOKEN in the SPKM-REQ), the
 target MUST reply with:

 SPKM_REP_TI=<SPKM-REP-TI>

 where SPKM-REP-TI is the target token as defined in [RFC2025].

 If mutual authentication was selected and target authentication
 fails, the initiator MUST close the connection. Otherwise, if the
 AuthMethod is SPKM1, the initiator MUST continue with:

 SPKM_REP_IT=<SPKM-REP-IT>

 where SPKM-REP-IT is the second initiator token as defined in
 [RFC2025]. If the initiator authentication fails, the target MUST
 answer with a Login reject with "Authentication Failure" status.

 SPKM requires support for very long authentication items.

 All the SPKM-* tokens 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.

11.1.3. 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.

 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.

11.1.4. Challenge Handshake Authentication Protocol (CHAP)

For CHAP [RFC1994], in the first step, the initiator MUST send:

 CHAP_A=<A1,A2...>

 Where A1,A2... are proposed algorithms, in order of preference.

In the second step, 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.

 In the third step, 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 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, and C and
 R are large-binary-values and their binary length (not the length of
 the character string that represents them in encoded form) MUST not
 exceed 1024 bytes.

 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.

12. 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 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 chapter 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).

12.1. 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 that 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 for this digest is given in
 hex-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 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 considered 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 in bit 7 of the lowest numbered byte of the digest
 continuing through to the byte up to the x^24 coefficient in
 bit 0 of the lowest numbered byte, continuing with the x^23
 coefficient in bit 7 of next byte through x^0 in bit 0 of the
 highest numbered byte.

- 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 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:

 a) Y-reversed.vendor.dns_name.do_something=
 b) Y<#><IANA-registered-string>=

 Digests named using the Y- format are used for private purposes
 (unregistered). Digests named using the Y# format (public extension)
 must be registered with IANA and MUST be described by an
 informational RFC.

 For private extension digests, to identify the vendor, we suggest
 you use the reversed DNS-name as a prefix to the proper digest
 names.

 The part of digest-name following Y- and Y# MUST conform to the
 format for standard-label specified in Section 5.1 Text Format.

 Support for public or private extension digests is OPTIONAL.

12.2. 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.

Initiator and target negotiate the maximum number of connections
requested/acceptable.

12.3. SendTargets

   Use: FFPO
   Senders: Initiator
   Scope: SW

   For a complete description, see Appendix D.  - SendTargets
   Operation -.

12.4. 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

 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).

 TargetName MUST not be redeclared within the login phase.

12.5. 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

 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.

 InitiatorName MUST not be redeclared within the login phase.

12.6. 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 12.21
 SessionType). 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.

12.7. 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 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.

12.8. 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 [RFC2732].

   If the TCP port is not specified, it is assumed to be the
   IANA-assigned default port for iSCSI (see Section 13 IANA
   Considerations).

   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

   Use of the portal-group-tag is described in Appendix D.
   - SendTargets Operation -.  The formats for the port and
   portal-group-tag are the same as the one specified in Section 12.9
   TargetPortalGroupTag.

12.9. TargetPortalGroupTag

Use: IO by target, Declarative, Any-Stage
 Senders: Target
 Scope: SW

 TargetPortalGroupTag=<16-bit-binary-value>

 Examples:
 TargetPortalGroupTag=1

 The target portal group tag 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.

 For the complete usage expectations of this key see Section 5.3 Login
 Phase.

12.10. 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 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).

12.11. 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   |Immediate Data|
   |          |             |   Data Out PDUs  |              |
   +----------+-------------+------------------+--------------+
   | No       | No          | Yes              | No           |
   +----------+-------------+------------------+--------------+
   | No       | Yes         | Yes              | Yes          |
   +----------+-------------+------------------+--------------+
   | Yes      | No          | No               | No           |
   +----------+-------------+------------------+--------------+
   | Yes      | Yes         | No               | Yes          |
   +----------+-------------+------------------+--------------+

12.12. 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).

12.13. 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 Kbytes).
   Result function is Minimum.

   The initiator and target negotiate 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 one.

12.14. 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 Kbytes).
 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.

12.15. 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.

12.16. 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.

12.17. 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.

 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 6.1.4.1 Recovery Within-command) is encountered for
 that data sequence.

12.18. 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.

12.19. 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 one. 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 MaxOustandingR2T
 MUST be set to 1.

12.20. 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 6.1.5 Error Recovery Hierarchy describes the
   mapping between the classes and the levels.

12.21. 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.

12.22. The Private or Public Extension Key Format

Use: ALL
 Senders: Initiator and Target
 Scope: specific key dependent

 X-reversed.vendor.dns_name.do_something=

 or

 X<#><IANA-registered-string>=

   Keys with this format are used for public or private extension
   purposes.  These keys always start with X- if unregistered with IANA
   (private) or X# if registered with IANA (public).

   For unregistered keys, to identify the vendor, we suggest you use the
   reversed DNS-name as a prefix to the key-proper.

   The part of key-name following X- and X# MUST conform to the format
   for key-name specified in Section 5.1 Text Format.

   For IANA registered keys the string following X# must be registered
   with IANA and the use of the key MUST be described by an
   informational RFC.

   Vendor specific keys MUST ONLY be used in normal sessions.

   Support for public or private extension keys is OPTIONAL.

13. IANA Considerations

   This section conforms to [RFC2434].

   The well-known user 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
   use of port 860, as 3260 is the only allowed default.

   Extension keys, authentication methods, or digest types for which a
   vendor or group of vendors intend to provide publicly available
   descriptions MUST be described by an RFC and MUST be registered with
   IANA.

   The IANA has set up the following three registries:

         a)  iSCSI extended key registry
         b)  iSCSI authentication methods registry
         c)  iSCSI digests registry

   [RFC3723] also instructs IANA to maintain a registry for the values
   of the SRP_GROUP key.  The format of these values must conform to the
   one specified for iSCSI extension item-label in Section 13.5.4
   Standard iSCSI extension item-label format.

   For the iSCSI authentication methods registry and the iSCSI digests
   registry, IANA MUST also assign a 16-bit unsigned integer number (the
   method number for the authentication method and the digest number for
   the digest).

   The following initial values for the registry for authentication
   methods are specified by the standards action of this document:

    Authentication Method                   | Number |
   +----------------------------------------+--------+
   | CHAP                                   |     1  |
   +----------------------------------------+--------+
   | SRP                                    |     2  |
   +----------------------------------------+--------+
   | KRB5                                   |     3  |
   +----------------------------------------+--------+
   | SPKM1                                  |     4  |
   +----------------------------------------+--------+
   | SPKM2                                  |     5  |
   +----------------------------------------+--------+

   All other record numbers from 0 to 255 are reserved.  IANA will
   register numbers above 255.

   Authentication methods with numbers above 255 MUST be unique within
   the registry and MUST be used with the prefix Z#.

   The following initial values for the registry for digests are
   specified by the standards action of this document:

    Digest                                  | Number |
   +----------------------------------------+--------+
   | CRC32C                                 |     1  |
   +----------------------------------------+--------+

   All other record numbers from 0 to 255 are reserved.  IANA will
   register numbers above 255.

   Digests with numbers above 255 MUST be unique within the registry and
   MUST be used with the prefix Y#.

   The RFC that describes the item to be registered MUST indicate in the
   IANA Considerations section the string and iSCSI registry to which it
   should be recorded.

   Extension Keys, Authentication Methods, and digests (iSCSI extension
   items) must conform to a number of requirements as described below.

13.1. Naming Requirements

   Each iSCSI extension item must have a unique name in its category.
   This name will be used as a standard-label for the key, access
   method, or digest and must conform to the syntax specified in Section
   13.5.4 Standard iSCSI extension item-label format for iSCSI extension
   item-labels.

13.2. Mechanism Specification Requirements

   For iSCSI extension items all of the protocols and procedures used by
   a given iSCSI extension item must be described, either in the
   specification of the iSCSI extension item itself or in some other
   publicly available specification, in sufficient detail for the iSCSI
   extension item to be implemented by any competent implementor.  Use
   of secret and/or proprietary methods in iSCSI extension items are
   expressly prohibited.  In addition, the restrictions imposed by
   [RFC1602] on the standardization of patented algorithms must be
   respected.

13.3. Publication Requirements

   All iSCSI extension items must be described by an RFC.  The RFC may
   be informational rather than Standards-Track, although Standards
   Track review and approval are encouraged for all iSCSI extension
   items.

13.4. Security Requirements

   Any known security issues that arise from the use of the iSCSI
   extension item must be completely and fully described.  It is not
   required that the iSCSI extension item be secure or that it be free
   from risks, but that the known risks be identified.  Publication of a
   new iSCSI extension item does not require an exhaustive security
   review, and the security considerations section is subject to
   continuing evaluation.

   Additional security considerations should be addressed by publishing
   revised versions of the iSCSI extension item specification.

   For each of these registries, IANA must record the registered string,
   which MUST conform to the format rules described in Section 13.5.4
   Standard iSCSI extension item-label format for iSCSI extension
   item-labels, and the RFC number that describes it.  The key prefix
   (X#, Y# or Z#) is not part of the recorded string.

13.5. Registration Procedure

Registration of a new iSCSI extension item starts with the
construction of an Internet Draft to become an RFC.

13.5.1. Present the iSCSI extension item to the Community

   Send a proposed access type specification to the IPS WG mailing list,
   or if the IPS WG is disbanded at the registration time, to a mailing
   list designated by the IETF Transport Area Director for a review
   period of a month.  The intent of the public posting is to solicit
   comments and feedback on the iSCSI extension item specification and a
   review of any security considerations.

13.5.2. iSCSI extension item review and IESG approval

   When the one month period has passed, the IPS WG chair or a person
   nominated by the IETF Transport Area Director (the iSCSI extension
   item reviewer) forwards the Internet Draft to the IESG for
   publication as an informational RFC or rejects it.  If the
   specification is a standards track document, the usual IETF
   procedures for such documents are followed.

   Decisions made by the iSCSI extension item reviewer must be published
   within two weeks after the month-long review period.  Decisions made
   by the iSCSI extension item reviewer can be appealed through the IESG
   appeal process.

13.5.3. IANA Registration

   Provided that the iSCSI extension item has either passed review or
   has been successfully appealed to the IESG, and the specification is
   published as an RFC, then IANA will register the iSCSI extension item
   and make the registration available to the community.

13.5.4. Standard iSCSI extension item-label format

   The following character symbols are used iSCSI extension item-labels
   (the hexadecimal values represent Unicode code points):

   (a-z, A-Z) - letters
   (0-9) - digits
   "."  (0x2e) - dot
   "-"  (0x2d) - minus
   "+"  (0x2b) - plus
   "@"  (0x40) - commercial at
   "_"  (0x5f) - underscore

   An iSCSI extension item-label is a string of one or more characters
   that consist of letters, digits, dot, minus, plus, commercial at, or
   underscore.  An iSCSI extension item-label MUST begin with a capital
   letter and must not exceed 63 characters.

13.6. IANA Procedures for Registering iSCSI extension items

   The identity of the iSCSI extension item reviewer is communicated to
   the IANA by the IESG.  Then, the IANA only acts in response to iSCSI
   extension item definitions that are approved by the iSCSI extension
   item reviewer and forwarded by the reviewer to the IANA for
   registration, or in response to a communication from the IESG that an
   iSCSI extension item definition appeal has overturned the iSCSI
   extension item reviewer's ruling.

References

Normative References

   [CAM]          ANSI X3.232-199X, Common Access Method-3.

   [EUI]          "Guidelines for 64-bit Global Identifier (EUI-64)",
                  http:
                  //standards.ieee.org/regauth/oui/tutorials/EUI64.html

   [OUI]          "IEEE OUI and Company_Id Assignments",
                  http://standards.ieee.org/regauth/oui

   [RFC791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                  September 1981.

   [RFC793]       Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

   [RFC1035]      Mockapetris, P., "Domain Names - Implementation and
                  Specification", STD 13, RFC 1035, November 1987.

   [RFC1122]      Braden, R., Ed., "Requirements for Internet Hosts-
                  Communication Layer", STD 3, RFC 1122, October 1989.

   [RFC1510]      Kohl, J. and C. Neuman, "The Kerberos Network
                  Authentication Service (V5)", RFC 1510, September
                  1993.

   [RFC1737]      Sollins, K. and L. Masinter "Functional Requirements
                  for Uniform Resource Names"RFC 1737, December 1994.

   [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.

   [RFC2025]      Adams, C., "The Simple Public-Key GSS-API Mechanism
                  (SPKM)", RFC 2025, October 1996.

   [RFC2045]      Borenstein, N. and N. Freed, "MIME (Multipurpose
                  Internet Mail Extensions) Part One: Mechanisms for
                  Specifying and Describing the Format of Internet
                  Message Bodies", RFC 2045, November 1996.

   [RFC2119]      Bradner, S. "Key Words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2279]      Yergeau, F., "UTF-8, a Transformation Format of ISO
                  10646", RFC 2279 October 1996.

   [RFC2373]      Hinden, R. and S. Deering, "IP Version 6 Addressing
                  Architecture", RFC 2373, July 1998.

   [RFC2396]      Berners-Lee, T., Fielding, R. and L. Masinter "Uniform
                  Resource Identifiers", RFC 2396, August 1998.

   [RFC2401]      Kent, S. and R. Atkinson, "Security Architecture for
                  the Internet Protocol", RFC 2401, November 1998.

   [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.

   [RFC2407]      Piper, D., "The Internet IP Security Domain of
                  Interpretation of ISAKMP", RFC 2407, November 1998.

   [RFC2409]      Harkins, D. and D. Carrel, "The Internet Key Exchange
                  (IKE)", RFC2409, November 1998.

   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs.", BCP 26, RFC
                  2434, October 1998.

   [RFC2451]      Pereira, R. and R. Adams " The ESP CBC-Mode Cipher
                  Algorithms", RFC 2451, November 1998.

   [RFC2732]      Hinden, R., Carpenter, B. and L. Masinter, "Format for
                  Literal IPv6 Addresses in URL's", RFC 2451, December
                  1999.

   [RFC2945]      Wu, T., "The SRP Authentication and Key Exchange
                  System", RFC 2945, September 2000.

   [RFC3066]      Alvestrand, H., "Tags for the Identification of
                  Languages", STD 47, RFC 3066, January 2001.

   [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.

   [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, March
                  2004.

   [RFC3723]      Aboba, B., Tseng, J., Walker, J., Rangan, V. and F.
                  Travostino, "Securing Block Storage Protocols over
                  IP", RFC 3723, March 2004.

   [SAM2]         T10/1157D, SCSI Architecture Model - 2 (SAM-2).

   [SBC]          NCITS.306-1998, SCSI-3 Block Commands (SBC).

   [SPC3]         T10/1416-D, SCSI Primary Commands-3.

   [UNICODE]      Unicode Standard Annex #15, "Unicode Normalization
                  Forms", http://www.unicode.org/unicode/reports/tr15

Informative References

   [BOOT]         P. Sarkar, et al., "Bootstrapping Clients using the
                  iSCSI Protocol", Work in Progress, July 2003.

   [Castagnoli93] G. Castagnoli, S. Braeuer and M. Herrman "Optimization
                  of Cyclic Redundancy-Check Codes with 24 and 32 Parity
                  Bits", IEEE Transact. on Communications, Vol. 41, No.
                  6, June 1993.

   [CORD]          Chadalapaka, M. and R. Elliott, "SCSI Command
                  Ordering Considerations with iSCSI", Work in Progress.

   [RFC3347]      Krueger, M., Haagens, R., Sapuntzakis, C. and M.
                  Bakke, "Small Computer Systems Interface protocol over
                  the Internet (iSCSI) Requirements and Design
                  Considerations", RFC 3347, July 2002.

   [RFC3385]      Sheinwald, D., Staran, J., Thaler, P. and V. Cavanna,
                  "Internet Protocol Small Computer System Interface
                  (iSCSI) Cyclic Redundancy Check (CRC)/Checksum
                  Considerations", RFC 3385, September 2002.

   [RFC3721]      Bakke M., Hafner, J., Hufferd, J., Voruganti, K. and
                  M. Krueger, "Internet Small Computer Systems Interface
                  (iSCSI) Naming and Discovery, RFC 3721, March 2004.

   [SEQ-EXT]      Kent, S., "IP Encapsulating Security Payload (ESP)",
                  Work in Progress, July 2002.

Appendix A. Sync and Steering with Fixed Interval Markers

   This appendix presents a simple scheme for synchronization (PDU
   boundary retrieval).  It uses markers that include synchronization
   information placed at fixed intervals in the TCP stream.

   A Marker consists of:

   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| Next-iSCSI-PDU-start pointer - copy #1                        |
     +---------------+---------------+---------------+---------------+
    4| Next-iSCSI-PDU-start pointer - copy #2                        |
     +---------------+---------------+---------------+---------------+

   The Marker scheme uses payload byte stream counting that includes
   every byte placed by iSCSI in the TCP stream except for the markers
   themselves.  It also excludes any bytes that TCP counts but are not
   originated by iSCSI.

   Markers MUST NOT be included in digest calculation.

   The Marker indicates the offset to the next iSCSI PDU header.  The
   Marker is eight bytes in length and contains two 32-bit offset fields
   that indicate how many bytes to skip in the TCP stream in order to
   find the next iSCSI PDU header.  The marker uses two copies of the
   pointer so that a marker that spans a TCP packet boundary should
   leave at least one valid copy in one of the packets.

   The structure and semantics of an inserted marker are independent of
   the marker interval.

   The use of markers is negotiable.  The initiator and target MAY
   indicate their readiness to receive and/or send markers during login
   separately for each connection.  The default is No.

A.1. Markers At Fixed Intervals

   A marker is inserted at fixed intervals in the TCP byte stream.
   During login, each end of the iSCSI session specifies the interval at
   which it is willing to receive the marker, or it disables the marker
   altogether.  If a receiver indicates that it desires a marker, the
   sender MAY agree (during negotiation) and provide the marker at the
   desired interval.  However, in certain environments, a sender that
   does not provide markers to a receiver that wants markers may suffer
   an appreciable performance degradation.

   The marker interval and the initial marker-less interval are counted
   in terms of the bytes placed in the TCP stream data by iSCSI.

   When reduced to iSCSI terms, markers MUST indicate the offset to a
   4-byte word boundary in the stream.  The least significant two bits
   of each marker word are reserved and are considered 0 for offset
   computation.

   Padding iSCSI PDU payloads to 4-byte word boundaries simplifies
   marker manipulation.

A.2. Initial Marker-less Interval

   To enable the connection setup including the Login Phase negotiation,
   marking (if any) is only started at the first marker interval after
   the end of the Login Phase.  However, in order to enable the marker
   inclusion and exclusion mechanism to work without knowledge of the
   length of the Login Phase, the first marker will be placed in the TCP
   stream as if the Marker-less interval had included markers.

   Thus, all markers appear in the stream at locations conforming to the
   formula: [(MI + 8) * n - 8] where MI = Marker Interval, n = integer
   number.

   For example, if the marker interval is 512 bytes and the login ended
   at byte 1003 (first iSCSI placed byte is 0), the first marker will be
   inserted after byte 1031 in the stream.

A.3. Negotiation

   The following operational key=value pairs are used to negotiate the
   fixed interval markers.  The direction (output or input) is relative
   to the initiator.

A.3.1. OFMarker, IFMarker

Use: IO
 Senders: Initiator and Target
 Scope: CO

 OFMarker=<boolean-value>
 IFMarker=<boolean-value>

 Default is No.

 Result function is AND.

   OFMarker is used to turn on or off the initiator to target markers
   on the connection.  IFMarker is used to turn on or off the target to
   initiator markers on the connection.

   Examples:

     I->OFMarker=Yes,IFMarker=Yes
     T->OFMarker=Yes,IFMarker=Yes

   Results in the Marker being used in both directions while:

     I->OFMarker=Yes,IFMarker=Yes
     T->OFMarker=Yes,IFMarker=No

   Results in Marker being used from the initiator to the target, but
   not from the target to initiator.

A.3.2. OFMarkInt, IFMarkInt

Use: IO
 Senders: Initiator and Target
 Scope: CO
 OFMarkInt is Irrelevant when: OFMarker=No
 IFMarkInt is Irrelevant when: IFMarker=No

 Offering:

 OFMarkInt=<numeric-range-from-1-to-65535>
 IFMarkInt=<numeric-range-from-1-to-65535>

 Responding:

 OFMarkInt=<numeric-value-from-1-to-65535>|Reject
 IFMarkInt=<numeric-value-from-1-to-65535>|Reject

 OFMarkInt is used to set the interval for the initiator to target
 markers on the connection. IFMarkInt is used to set the interval for
 the target to initiator markers on the connection.

 For the offering, the initiator or target indicates the minimum to
 maximum interval (in 4-byte words) it wants the markers for one or
 both directions. In case it only wants a specific value, only a
 single value has to be specified. The responder selects a value
 within the minimum and maximum offered or the only value offered or
 indicates through the xFMarker key=value its inability to set and/or
 receive markers. When the interval is unacceptable the responder
 answers with "Reject". Reject is resetting the marker function in
 the specified direction (Output or Input) to No.

   The interval is measured from the end of a marker to the beginning of
   the next marker.  For example, a value of 1024 means 1024 words (4096
   bytes of iSCSI payload between markers).

   The default is 2048.

Appendix B. Examples

B.1. Read Operation Example

B.2. Write Operation Example

B.3. R2TSN/DataSN Use Examples

B.4. CRC Examples

N.B. 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 C. 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 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 SPKM1:

 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=SPKM1,KRB5,None

 T-> Login (CSG,NSG=0,0 T=0)
 AuthMethod=SPKM1

 I-> Login (CSG,NSG=0,0 T=0)
 SPKM_REQ=<spkm-req>

 (spkm-req is the SPKM-REQ token with the mutual-state bit in the
 options field of the REQ-TOKEN set)

 T-> Login (CSG,NSG=0,0 T=0)
 SPKM_REP_TI=<spkm-rep-ti>

 If the authentication is successful, the initiator proceeds:

 I-> Login (CSG,NSG=0,1 T=1)
 SPKM_REP_IT=<spkm-rep-it>

 If the authentication is successful, the target proceeds with:

 T-> Login (CSG,NSG=0,1 T=1)

 The initiator may proceed:

 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 target's authentication by the initiator is not
 successful, the initiator terminates the connection (without
 responding to the Login SPKM_REP_TI message).

If the initiator's authentication by the target is not
 successful, the target responds with:

 T-> Login "login reject"

 instead of the Login "proceed and change stage" message, and
 terminates the connection.

 In the next example, the initiator and target authenticate each
 other via SPKM2:

 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=SPKM1,SPKM2

 T-> Login-PR (CSG,NSG=0,0 T=0)
 AuthMethod=SPKM2

 I-> Login (CSG,NSG=0,1 T=1)
 SPKM_REQ=<spkm-req>

 (spkm-req is the SPKM-REQ token with the mutual-state bit in the
 options field of the REQ-TOKEN not set)

 If the authentication is successful, the target proceeds with:

 T-> Login (CSG,NSG=0,1 T=1)

 The initiator may proceed:

 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 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_GROUP=SRP-1536,SRP-1024
 SRP_s=<s>

 I-> Login (CSG,NSG=0,0 T=0)
 SRP_GROUP=SRP-1536
 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:

 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
 terminates the connection.

 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=No

 T-> Login (CSG,NSG=0,0 T=0)
 SRP_GROUP=SRP-1536
 SRP_s=<s>

 I-> Login (CSG,NSG=0,0 T=0)
 SRP_GROUP=SRP-1536
 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:

 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:

 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 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:

 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 D. SendTargets Operation

   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 (target name and
   IP address-port pairs and portal group tags) for the targets on the
   target network entity which 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 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 or 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 had 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 12.4 TargetName.

 In a discovery session, a target 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, 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 portal group tag (as in Section 12.9 TargetPortalGroupTag).
   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 target 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.

   Initiator sends text request that contains:

         SendTargets=All

   Target sends a text response that contains:

         TargetName=iqn.1993-11.com.example:diskarray.sn.8675309

   All the target had to return in the 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 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 portal group tag; they do not support spanning
   multiple-connection sessions with each other.  Keep in mind that the
   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
   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 E. 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:

      -  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.

E.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-a-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);

E.2. Within-command Error Recovery Algorithms

E.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: Handle-Status-SNACK-
         request is defined in Within-connection recovery algorithms.

      -  The Response processing pseudo-code, shown in the target
         algorithms, applies to all solicited PDUs that carry StatSN -
         SCSI Response, Text Response etc.

E.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-a-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);
  }
}

E.2.3. Target Algorithms

Receive-a-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 upto 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);
      }
  }
}

E.3. Within-connection Recovery Algorithms

E.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 tunables:
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, which, when out of order, could trigger the
         out of order StatSN handling in Within-command algorithms,
         again leading to Recover-Status-if-Possible.

      -  The pseudo-code 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.

E.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-a-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);
      }
  }
}

E.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);
  }
}

E.4. Connection Recovery Algorithms

E.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);

Notes:
      - 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.

E.4.2. Initiator Algorithms

         Receive-a-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;
           } }

E.4.3. Target Algorithms

         Receive-a-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, "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 F. Clearing Effects of Various Events on Targets

F.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    |Y    |Y    |Y    |Y    |
   +---------------------+-----+-----+-----+-----+-----+
   |target warm reset(16)|Y    |Y    |Y    |Y    |Y    |
   +---------------------+-----+-----+-----+-----+-----+
   |LU reset(19)         |Y    |Y    |Y    |Y    |Y    |
   +---------------------+-----+-----+-----+-----+-----+
   |powercycle(16)       |Y    |Y    |Y    |Y    |Y    |
   +---------------------+-----+-----+-----+-----+-----+

   1.  Incomplete TTTs - 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 - immediate commands, but waiting for
   execution on a target.  For example, Abort Task Set.

   5.  Connection Tasks - tasks that are active on the iSCSI connection
   in question.

   6.  Session Tasks - tasks that are active on the entire iSCSI
   session.  A union of "connection tasks" on all participating
   connections.

   7.  Partial PDUs (if any) - 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 shutdown, 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
   that 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).

   10.  These are defined in Section 5.3.5 Session Reinstatement,
   Closure, and Timeout.

   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 5.3.6 Session
   Continuation and Failure.

   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 5.3.6 Session Continuation
and Failure.

19. This operation affects all logged-in initiators and the clearing
effects are only applicable to the LU being reset.

                         +-----+-----+-----+-----+-----+
                         |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    |
   +---------------------+-----+-----+-----+-----+-----+
   |powercycle           |Y    |Y    |Y    |Y(10)|NA   |
   +---------------------+-----+-----+-----+-----+-----+

   1.  Discontiguous Commands - 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 - data PDUs received for the task in question
   and waiting to be reordered due to prior discontiguities in DataSN.

   3.  StatSN

   4.  CmdSN

   5.  DataSN

   7.  It 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.

F.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 4.3.5.1
   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 [SBC]) 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.

Acknowledgements

   This protocol was developed by a design team that, in addition to the
   authors, included Daniel Smith, Ofer Biran, Jim Hafner and John
   Hufferd (IBM), Mark Bakke (Cisco), Randy Haagens (HP), Matt Wakeley
   (Agilent, now Sierra Logic), Luciano Dalle Ore (Quantum), and Paul
   Von Stamwitz (Adaptec, now TrueSAN Networks).

   Furthermore, a large group of people contributed to this work through
   their review, comments, and valuable insights.  We are grateful to
   all of them.  We especially thank those people who found the time and
   patience to take part in our weekly phone conferences and
   intermediate meetings in Almaden and Haifa, which helped shape this
   document: Prasenjit Sarkar, Meir Toledano, John Dowdy, Steve Legg,
   Alain Azagury (IBM), Dave Nagle (CMU), David Black (EMC), John Matze
   (Veritas - now Okapi Software), Steve DeGroote, Mark Schrandt
   (Cisco), Gabi Hecht (Gadzoox), Robert Snively and Brian Forbes
   (Brocade), Nelson Nachum (StorAge), and Uri Elzur (Broadcom).  Many
   others helped edit and improve this document within the IPS working
   group.  We are especially grateful to David Robinson and Raghavendra
   Rao (Sun), Charles Monia, Joshua Tseng (Nishan), Somesh Gupta
   (Silverback), Michael Krause, Pierre Labat, Santosh Rao, Matthew
   Burbridge, Bob Barry, Robert Elliott, Nick Martin (HP), Stephen
   Bailey (Sandburst), Steve Senum, Ayman Ghanem, Dave Peterson (Cisco),
   Barry Reinhold (Trebia Networks), Bob Russell (UNH), Eddy Quicksall
   (iVivity, Inc.), Bill Lynn and Michael Fischer (Adaptec), Vince
   Cavanna, Pat Thaler (Agilent), Jonathan Stone (Stanford), Luben
   Tuikov (Splentec), Paul Koning (EqualLogic), Michael Krueger
   (Windriver), Martins Krikis (Intel), Doug Otis (Sanlight), John
   Marberg (IBM), Robert Griswold and Bill Moody (Crossroads), Bill
   Studenmund (Wasabi Systems), Elizabeth Rodriguez (Brocade) and Yaron
   Klein (Sanrad).  The recovery chapter was enhanced with the help of
   Stephen Bailey (Sandburst), Somesh Gupta (Silverback), and Venkat
   Rangan (Rhapsody Networks).  Eddy Quicksall contributed some examples
   and began the Definitions section.  Michael Fischer and Bob Barry
   started the Acronyms section.  Last, but not least, we thank Ralph
   Weber for keeping us in line with T10 (SCSI) standardization.

   We would like to thank Steve Hetzler for his unwavering support and
   for coming up with such a good name for the protocol, and Micky
   Rodeh, Jai Menon, Clod Barrera, and Andy Bechtolsheim for helping
   make this work happen.

   In addition to this document, we recommend you acquaint yourself with
   the following in order to get a full understanding of the iSCSI
   specification: "iSCSI Naming & Discovery"[RFC3721], "Bootstrapping
   Clients using the iSCSI Protocol" [BOOT], "Securing Block Storage
   Protocols over IP" [RFC3723] documents, "iSCSI Requirements and

   Design Considerations" [RFC3347] and "SCSI Command Ordering
   Considerations with iSCSI" [CORD].

   The "iSCSI Naming & Discovery" document is authored by:

      Mark Bakke (Cisco), Jim Hafner, John Hufferd, Kaladhar Voruganti
         (IBM), and Marjorie Krueger (HP).

   The "Bootstrapping Clients using the iSCSI Protocol" document is
   authored by:

      Prasenjit Sarkar (IBM), Duncan Missimer (HP), and Costa
         Sapuntzakis (Cisco).

   The "Securing Block Storage Protocols over IP" document is authored
   by:

      Bernard Aboba (Microsoft), Joshua Tseng (Nishan), Jesse Walker
         (Intel), Venkat Rangan (Rhapsody Networks), and Franco
         Travostino (Nortel Networks).

   The "iSCSI Requirements and Design Considerations" document is
   authored by:

      Marjorie Krueger, Randy Haagens (HP), Costa Sapuntzakis, and Mark
      Bakke (Cisco).

   The "SCSI Command Ordering Considerations with iSCSI" document is
   authored by:

      Mallikarjun Chadalapaka, Rob Elliot (HP)

   We are grateful to all of them for their good work and for helping us
   correlate this document with the ones they produced.

Authors' Addresses

   Julian Satran
   IBM Research Laboratory in Haifa
   Haifa University Campus - Mount Carmel
   Haifa 31905, Israel

   Phone +972.4.829.6264
   EMail: Julian_Satran@il.ibm.com

   Kalman Meth
   IBM Research Laboratory in Haifa
   Haifa University Campus - Mount Carmel
   Haifa 31905, Israel

   Phone +972.4.829.6341
   EMail: meth@il.ibm.com

   Costa Sapuntzakis
   Stanford University
   353 Serra Mall Dr #407
   Stanford, CA 94305

   Phone: +1.650.723.2458
   EMail: csapuntz@alum.mit.edu

   Efri Zeidner
   XIV Ltd.
   1 Azrieli Center,
   Tel-Aviv 67021, Israel

   Phone: +972.3.607.4722
   EMail: efri@xiv.co.il

   Mallikarjun Chadalapaka
   Hewlett-Packard Company
   8000 Foothills Blvd.
   Roseville, CA 95747-5668, USA

   Phone: +1.916.785.5621
   EMail: cbm@rose.hp.com

Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
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Abstract

Table of Contents

1. Introduction

2. Definitions and Acronyms

2.1. Definitions

2.2. Acronyms

2.3. Conventions

2.3.1. Word Rule

2.3.2. Half-Word Rule

2.3.3. Byte Rule

3. Overview

3.1. SCSI Concepts

3.2. iSCSI Concepts and Functional Overview

3.2.1. Layers and Sessions

3.2.2. Ordering and iSCSI Numbering

3.2.2.1. Command Numbering and Acknowledging

3.2.2.2. Response/Status Numbering and Acknowledging

3.2.2.3. Data Sequencing

3.2.3. iSCSI Login

3.2.4. iSCSI Full Feature Phase

3.2.4.1. Command Connection Allegiance

3.2.4.2. Data Transfer Overview

3.2.4.3. Tags and Integrity Checks

3.2.4.4. Task Management

3.2.5. iSCSI Connection Termination

3.2.6. iSCSI Names

3.2.6.1. iSCSI Name Properties

3.2.6.2. iSCSI Name Encoding

3.2.6.3. iSCSI Name Structure

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

3.2.6.3.2. Type "eui." (IEEE EUI-64 format)

3.2.7. Persistent State

3.2.8. Message Synchronization and Steering

3.2.8.1. Sync/Steering and iSCSI PDU Length

3.3. iSCSI Session Types

3.4. SCSI to iSCSI Concepts Mapping Model

3.4.1. iSCSI Architecture Model

3.4.2. SCSI Architecture Model

3.4.3. Consequences of the Model

3.4.3.1. I_T Nexus State

3.5. Request/Response Summary

3.5.1. Request/Response Types Carrying SCSI Payload

3.5.1.1. SCSI-Command

3.5.1.2. SCSI-Response

3.5.1.3 Task Management Function Request

3.5.1.4. Task Management Function Response

3.5.1.5. SCSI Data-Out and SCSI Data-In

3.5.1.6. Ready To Transfer (R2T)

3.5.2. Requests/Responses carrying SCSI and iSCSI Payload

3.5.2.1. Asynchronous Message

3.5.3. Requests/Responses Carrying iSCSI Only Payload

3.5.3.1. Text Request and Text Response

3.5.3.2. Login Request and Login Response

3.5.3.3. Logout Request and Response

3.5.3.4. SNACK Request

3.5.3.5. Reject

3.5.3.6. NOP-Out Request and NOP-In Response

4. SCSI Mode Parameters for iSCSI

5. Login and Full Feature Phase Negotiation

5.1. Text Format

5.2. Text Mode Negotiation

5.2.1. List negotiations

5.2.2. Simple-value Negotiations

5.3. Login Phase

5.3.1. Login Phase Start

5.3.2. iSCSI Security Negotiation

5.3.3. Operational Parameter Negotiation During the Login Phase

5.3.4. Connection Reinstatement

5.3.5. Session Reinstatement, Closure, and Timeout

5.3.5.1. Loss of Nexus Notification

5.3.6. Session Continuation and Failure

5.4. Operational Parameter Negotiation Outside the Login Phase

6. iSCSI Error Handling and Recovery

6.1. Overview

6.1.1. Background

6.1.2. Goals

6.1.3. Protocol Features and State Expectations

6.1.4. Recovery Classes

9.6. Considerations for State-dependent Devices and Long-lasting SCSI
Operations

0 - Reserved
1 - Extended CDB
2 - Expected Bidirectional Read Data Length
3 - 63 Reserved