Network Working Group                                          I. Friend
Request for Comments: 5024                                        ODETTE
Obsoletes: 2204                                            November 2007
Category: Informational

ODETTE File Transfer Protocol 2

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

Abstract

   This memo updates the ODETTE File Transfer Protocol, an established
   file transfer protocol facilitating electronic data interchange of
   business data between trading partners, to version 2.

   The protocol now supports secure and authenticated communication over
   the Internet using Transport Layer Security, provides file
   encryption, signing, and compression using Cryptographic Message
   Syntax, and provides signed receipts for the acknowledgement of
   received files.

   The protocol supports both direct peer-to-peer communication and
   indirect communication via a Value Added Network and may be used with
   TCP/IP, X.25, and ISDN-based networks.

   1. Introduction ....................................................4
      1.1. Background .................................................4
      1.2. Summary of Features ........................................5
      1.3. General Principles .........................................5
      1.4. Structure ..................................................6
      1.5. Virtual Files ..............................................6
      1.6. Service Description ........................................9
      1.7. Security ...................................................9
   2. Network Service ................................................11
      2.1. Introduction ..............................................11
      2.2. Service Primitives ........................................11
      2.3. Secure ODETTE-FTP Session .................................12
      2.4. Port Assignment ...........................................12
   3. File Transfer Service ..........................................13
      3.1. Model .....................................................13
      3.2. Session Setup .............................................14
      3.3. File Transfer .............................................16
      3.4. Session Take Down .........................................20
      3.5. Service State Automata ....................................23
   4. Protocol Specification .........................................28
      4.1. Overview ..................................................28
      4.2. Start Session Phase .......................................28
      4.3. Start File Phase ..........................................30
      4.4. Data Transfer Phase .......................................34
      4.5. End File Phase ............................................35
      4.6. End Session Phase .........................................36
      4.7. Problem Handling ..........................................36
   5. Commands and Formats ...........................................37
      5.1. Conventions ...............................................37
      5.2. Commands ..................................................37
      5.3. Command Formats ...........................................37
      5.4. Identification Code .......................................68
   6. File Services ..................................................69
      6.1. Overview ..................................................69
      6.2. File Signing ..............................................69
      6.3. File Encryption ...........................................70
      6.4. File Compression ..........................................70
      6.5. V Format Files - Record Lengths ...........................70
   7. ODETTE-FTP Data Exchange Buffer ................................71
      7.1. Overview ..................................................71
      7.2. Data Exchange Buffer Format ...............................71
      7.3. Buffer Filling Rules ......................................72
   8. Stream Transmission Buffer .....................................73
      8.1. Introduction ..............................................73
      8.2. Stream Transmission Header Format .........................73

   9. Protocol State Machine .........................................74
      9.1. ODETTE-FTP State Machine ..................................74
      9.2. Error Handling ............................................75
      9.3. States ....................................................76
      9.4. Input Events ..............................................79
      9.5. Output Events .............................................79
      9.6. Local Variables ...........................................80
      9.7. Local Constants ...........................................81
      9.8. Session Connection State Table ............................82
      9.9. Error and Abort State Table ...............................85
      9.10. Speaker State Table 1 ....................................86
      9.11. Speaker State Table 2 ....................................91
      9.12. Listener State Table .....................................93
      9.13. Example ..................................................96
   10. Miscellaneous .................................................97
      10.1. Algorithm Choice .........................................97
      10.2. Cryptographic Algorithms .................................97
      10.3. Protocol Extensions ......................................97
      10.4. Certificate Services .....................................98
   11. Security Considerations .......................................98
   Appendix A. Virtual File Mapping Example .........................100
   Appendix B. ISO 646 Character Subset .............................103
   Appendix C. X.25 Specific Information ............................104
      C.1. X.25 Addressing Restrictions .............................104
      C.2. Special Logic ............................................105
      C.3. PAD Parameter Profile ....................................116
   Appendix D. OFTP X.25 Over ISDN Recommendation ...................118
      D.1. ODETTE ISDN Recommendation ...............................119
      D.2. Introduction to ISDN .....................................120
      D.3. Equipment Types ..........................................123
      D.4. Implementation ...........................................124
   Acknowledgements .................................................132
   Normative References .............................................132
   Informative References ...........................................133
   ODETTE Address ...................................................134

1. Introduction

1.1. Background

   The ODETTE File Transfer Protocol (ODETTE-FTP) was defined in 1986 by
   working group four of the Organisation for Data Exchange by Tele
   Transmission in Europe (ODETTE) to address the electronic data
   interchange (EDI) requirements of the European automotive industry.

   ODETTE-FTP allows business applications to exchange files on a peer-
   to-peer basis in a standardised, purely automatic manner and provides
   a defined acknowledgement process on successful receipt of a file.

   ODETTE-FTP is not to be confused as a variant of, or similar to, the
   Internet FTP [FTP], which provides an interactive means for
   individuals to share files and which does not have any sort of
   acknowledgement process.  By virtue of its interactive nature, lack
   of file acknowledgements, and client/server design, FTP does not
   easily lend itself to mission-critical environments for the exchange
   of business data.

   Over the last ten years, ODETTE-FTP has been widely deployed on
   systems of all sizes from personal computers to large mainframes
   while the Internet has emerged as the dominant international network,
   providing high-speed communication at low cost.  To match the demand
   for EDI over the Internet, ODETTE has decided to extend the scope of
   its file transfer protocol to incorporate security functions and
   advanced compression techniques to ensure that it remains at the
   forefront of information exchange technology.

   The protocol now supports secure and authenticated communication over
   the Internet using Transport Layer Security, provides file
   encryption, signing, and compression using Cryptographic Message
   Syntax, and provides signed receipts for the acknowledgement of
   received files.

   The protocol supports both direct peer-to-peer communication and
   indirect communication via a Value Added Network and may be used with
   TCP/IP, X.25 and ISDN based networks.

   ODETTE-FTP has been defined by the ODETTE Security Working Group
   which consists of a number of ODETTE member organisations.  All
   members have significant operational experience working with and
   developing OFTP and EDI solutions.

1.2. Summary of Features

   This memo is a development of version 1.4 of ODETTE-FTP [OFTP] with
   these changes/additions:

      Session level encryption
      File level encryption
      Secure authentication
      File compression
      Signed End to End Response (EERP)
      Signed Negative End Response (NERP)
      Maximum permitted file size increased to 9 PB (petabytes)
      Virtual file description added
      Extended error codes

   Version 1.4 of ODETTE-FTP included these changes and additions to
   version 1.3:

      Negative End Response (NERP)
      Extended Date and Time stamp
      New reason code 14 (File direction refused)

1.3. General Principles

   The aim of ODETTE-FTP is to facilitate the transmission of a file
   between one or more locations in a way that is independent of the
   data communication network, system hardware, and software
   environment.

   In designing and specifying the protocol, the following factors were
   considered.

   1. The possible differences of size and sophistication of file
      storage and small and large systems.

2. The necessity to work with existing systems (reduce changes to
existing products and allow easy implementation).

3. Systems of different ages.

4. Systems of different manufactures.

5. The potential for growth in sophistication (limit impact and avoid
changes at other locations).

1.4. Structure

   ODETTE-FTP is modelled on the OSI reference model.  It is designed to
   use the Network Service provided by level 3 of the model and provide
   a File Service to the users.  Thus, the protocol spans levels 4 to 7
   of the model.

   The description of ODETTE-FTP contained in this memo is closely
   related to the original 'X.25' specification of the protocol and in
   the spirit of the OSI model describes:

      1. A File Service provided to a User Monitor.

      2. A protocol for the exchange of information between peer
         ODETTE-FTP entities.

1.5. Virtual Files

   Information is always exchanged between ODETTE-FTP entities in a
   standard representation called a Virtual File.  This allows data
   transfer without regard for the nature of the communicating systems.

   The mapping of a file between a local and virtual representation will
   vary from system to system and is not defined here.

1.5.1. Organisation

   Sequential

      Logical records are presented one after another.  ODETTE-FTP must
      be aware of the record boundaries.

1.5.2. Identification

   Dataset Name

      Dataset name of the Virtual File being transferred, assigned by
      bilateral agreement.

   Time stamp (HHMMSScccc)

      A file qualifier indicating the time the Virtual File was made
      available for transmission.  The counter (cccc=0001-9999) gives
      higher resolution.

   Date stamp (CCYYMMDD)

      A file qualifier indicating the date the Virtual File was made
      available for transmission.

   The Dataset Name, Date, and Time attributes are assigned by the
   Virtual File's originator and are used to uniquely identify a file.
   They are all mandatory and must not be changed by intermediate
   locations.

   The User Monitor may use the Virtual File Date and Time attributes in
   local processes involving date comparisons and calculations.  Any
   such use falls outside the scope of this protocol.

1.5.3. Record Format

   Four record formats are defined:

      Fixed (F)

         Each record in the file has the same length.

      Variable (V)

         The records in the file can have different lengths.

      Unstructured (U)

         The file contains a stream of data.  No structure is defined.

      Text File (T)

         A Text File is defined as a sequence of ASCII characters,
         containing no control characters except CR-LF that delimit
         lines.  A line will not have more than 2048 characters.

1.5.4. Restart

   ODETTE-FTP can negotiate the restart of an interrupted Virtual File
   transmission.  Fixed and Variable format files are restarted on
   record boundaries.  For Unstructured and Text files, the restart
   position is expressed as a file offset in 1K (1024 octet) blocks.

The restart position is always calculated relative to the start of
the Virtual File.

1.6. Service Description

   ODETTE-FTP provides a file transfer service to a User Monitor and in
   turn uses the Internet transport layer stream service to communicate
   between peers.

   These services are specified in this memo using service primitives
   grouped into four classes as follows:

      Request (RQ)       An entity asks the service to do some work.
      Indication (IND)   A service informs an entity of an event.
      Response (RS)      An entity responds to an event.
      Confirm (CF)       A service informs an entity of the response.

   Services may be confirmed, using the request, indication, response,
   and confirm primitives, or unconfirmed using just the request and
   indication primitives.

1.7. Security

   ODETTE-FTP provides a number of security services to protect a
   Virtual File transmission across a hostile network.

   These security services are as follows:

      Confidentiality
      Integrity
      Non-repudiation of receipt
      Non-repudiation of origin
      Secure authentication

   Security services in this specification are implemented as follows:

      Session level encryption
      File level encryption
      Signed files
      Signed receipts
      Session level authentication
      ODETTE-FTP Authentication

   Session level encryption provides data confidentiality by encryption
   of all the protocol commands and data exchanged between two parties,
   preventing a third party from extracting any useful information from
   the transmission.

   This session level encryption is achieved by layering ODETTE-FTP over
   Transport Layer Security [TLS], distinguishing between secure and
   unsecure TCP/IP traffic using different port numbers.

   File encryption provides complementary data confidentiality by
   encryption of the files in their entirety.  Generally, this
   encryption occurs prior to transmission, but it is also possible to
   encrypt and send files while in session.  File encryption has the
   additional benefit of allowing a file to remain encrypted outside of
   the communications session in which it was sent.  The file can be
   received and forwarded by multiple intermediaries, yet only the final
   destination will be able to decrypt the file.  File encryption does
   not encrypt the actual protocol commands, so trading partner EDI
   codes and Virtual File names are still viewable.

   Secure authentication is implemented through the session level
   authentication features available in [TLS] and proves the identity of
   the parties wishing to communicate.

   ODETTE-FTP Authentication also provides an authentication mechanism,
   but one that is integral to ODETTE-FTP and is available on all
   network infrastructures over which ODETTE-FTP is operated (this is in
   contrast to [TLS] which is generally only available over TCP/IP-based
   networks).  Both parties are required to possess certificates when
   ODETTE-FTP Authentication is used.

   The security features in ODETTE-FTP 2 are centred around the use of
   [X.509] certificates.  To take advantage of the complete range of
   security services offered in both directions, each party is required
   to possess an [X.509] certificate.  If the confidentiality of data
   between two parties is the only concern, then [TLS] alone can be
   used, which allows the party accepting an incoming connection (the
   Responder) to be the only partner required to possess a certificate.

   For businesses, this means that session level encryption between a
   hub and its trading partners can be achieved without requiring all
   the trading partners to obtain a certificate, assuming that trading
   partners always connect to the hub.

   With the exception of [TLS], all the security services work with X.25
   and ISDN as transport media.  Although nothing technically precludes
   [TLS] from working with X.25 or ISDN, implementations are rare.

2. Network Service

2.1. Introduction

   ODETTE-FTP peer entities communicate with each other via the OSI
   Network Service or the Transmission Control Protocol Transport
   Service [RFC793].  This is described by service primitives
   representing request, indication, response, and confirmation actions.

   For the Internet environment, the service primitives mentioned below
   for the Network Service have to be mapped to the respective Transport
   Service primitives.  This section describes the Network Service
   primitives used by ODETTE-FTP and their relationship to the TCP
   interface.  In practice, the local transport service application
   programming interface will be used to access the TCP service.

2.2. Service Primitives

All network primitives can be directly mapped to the respective
Transport primitives when using TCP.

2.2.1. Network Connection

      N_CON_RQ   ------>   N_CON_IND
      N_CON_CF   <------   N_CON_RS

   This describes the setup of a connection.  The requesting ODETTE-FTP
   peer uses the N_CON_RQ primitive to request an active OPEN of a
   connection to a peer ODETTE-FTP, the Responder, which has previously
   requested a passive OPEN.  The Responder is notified of the incoming
   connection via N_CON_IND and accepts it with N_CON_RS.  The requester
   is notified of the completion of its OPEN request upon receipt of
   N_CON_CF.

   Parameters

   Request           Indication        Response          Confirmation
   ---------------------------------------------------------------------
   Dest addr ------> same              same              same

2.2.2. Network Data

      N_DATA_RQ  ------>   N_DATA_IND

   Data exchange is an unconfirmed service.  The requester passes data
   for transmission to the Network Service via the N_DATA_RQ primitive.
   The Responder is notified of the availability of data via N_DATA_IND.

   In practice, the notification and receipt of data may be combined,
   such as by the return from a blocking read from the network socket.

   Parameters

   Request                  Indication
   ---------------------------------------------------------------------
   Data ------------------> same

2.2.3. Network Disconnection

      N_DISC_RQ  ------>   N_DISC_IND

   An ODETTE-FTP requests the termination of a connection with the
   N_DISC_RQ service primitive.  Its peer is notified of the CLOSE by a
   N_DISC_IND event.  It is recognised that each peer must issue a
   N_DISC_RQ primitive to complete the TCP symmetric close procedure.

2.2.4. Network Reset

    ------>   N_RST_IND

   An ODETTE-FTP entity is notified of a network error by a N_RST_IND
   event.  It should be noted that N_RST_IND would also be generated by
   a peer RESETTING the connection, but this is ignored here as N_RST_RQ
   is never sent to the Network Service by ODETTE-FTP.

2.3. Secure ODETTE-FTP Session

   [TLS] provides a mechanism for securing an ODETTE-FTP session over
   the Internet or a TCP network.  ODETTE-FTP is layered over [TLS],
   distinguishing between secure and unsecure traffic by using different
   server ports.

   The implementation is very simple.  Layer ODETTE-FTP over [TLS] in
   the same way as layering ODETTE-FTP over TCP/IP.  [TLS] provides both
   session encryption and authentication, both of which may be used by
   the connecting parties.  A party acts as a [TLS] server when
   receiving calls and acts as a [TLS] client when making calls.  When
   the [TLS] handshake has completed, the responding ODETTE-FTP may
   start the ODETTE-FTP session by sending the Ready Message.

2.4. Port Assignment

   An ODETTE-FTP requester will select a suitable local port.

   The responding ODETTE-FTP will listen for connections on Registered
   Port 3305; the service name is 'odette-ftp'.

   The responding ODETTE-FTP will listen for secure TLS connections on
   Registered Port 6619; the service name is 'odette-ftps'.

3.  File Transfer Service

   The File Transfer Service describes the services offered by an
   ODETTE-FTP entity to its User Monitor (generally an application).

   NOTE: The implementation of the service primitives is an application
         issue.

3.1. Model

3.2. Session Setup

3.2.1. Session Connection Service

   These diagrams represent the interactions between two communicating
   ODETTE-FTP entities and their respective User Agents.

   The vertical lines represent the ODETTE-FTP entities.  The User
   Agents are not shown.

                             |            |
           F_CONNECT_RQ ---->|------------|----> F_CONNECT_IND
                             |            |
           F_CONNECT_CF <----|------------|<---- F_CONNECT_RS
                             |            |

   Parameters

   Request           Indication        Response          Confirm
   ---------------------------------------------------------------------
   called-address -> same              ---               ----
   calling-address-> same              ---               ----
   ID1 ------------> same              ID2 ------------> same
   PSW1------------> same              PSW2 -----------> same
   mode1 ----------> mode2 ----------> mode3 ----------> same
   restart1 -------> same -----------> restart2 -------> same
   authentication1-> same -----------> authentication2-> same
   ---------------------------------------------------------------------

   Mode

      Specifies the file transfer capabilities of the entity sending or
      receiving a F_CONNECT primitive for the duration of the session.

      Value:
         Sender-only    The entity can only send files.
         Receiver-only  The entity can only receive files.
         Both           The entity can both send and receive files.

      Negotiation:
        Sender-only    Not negotiable.
        Receiver-only  Not negotiable.
        Both           Can be negotiated down to Sender-only or
                       Receiver-only by the User Monitor or the
                       ODETTE-FTP entity.

   Request           Indication        Response          Confirm
   ---------------------------------------------------------------------
   Sender-only ----> Receiver-only --> Receiver-only --> Sender-only

   Receiver-only --> Sender-only ----> Sender-only ----> Receiver-only

   Both -----+-----> Both ----+------> Both -----------> Both
             |             or +------> Receiver-only --> Sender-only
             |             or +------> Sender-only ----> Receiver-only
             |
          or +-----> Receiver-only --> Receiver-only --> Sender-only
          or +-----> Sender-only ----> Sender-only ----> Receiver-only
   ---------------------------------------------------------------------

   Restart

      Specifies the file transfer restart capabilities of the User
      Monitor.

      Value:
         Y         The entity can restart file transfers.
         N         The entity cannot restart file transfers.

      Negotiation:

   Request           Indication        Response          Confirm
   ---------------------------------------------------------------------
   restart = Y ----> restart = Y --+-> restart = Y ----> restart = Y
                                or +-> restart = N ----> restart = N

   restart = N ----> restart = N ----> restart = N ----> restart = N
   ---------------------------------------------------------------------

   Authentication

      Specifies the authentication requirement of the User Monitor.

      Value:
         Y             Authentication required.
         N             Authentication not required.

      Negotiation:     Not negotiable.

   Request           Indication        Response          Confirm
   ---------------------------------------------------------------------
   auth = Y    ----> auth = Y    ----> auth = Y    ----> auth = Y

   auth = N    ----> auth = N    ----> auth = N    ----> auth = N
   ---------------------------------------------------------------------

3.3. File Transfer

3.3.1. File Opening

                             |            |
        F_START_FILE_RQ ---->|------------|----> F_START_FILE_IND
                             |            |
   F_START_FILE_CF(+|-) <----|------------|<---- F_START_FILE_RS(+|-)
                             |            |

   Parameters

   Request          Ind.   RS(+)          CF(+)   RS(-)         CF(-)
   ------------------------------------------------------------------
   filename-------> same   ----           ----    ----          ----
   date-time------> same   ----           ----    ----          ----
   destination----> same   ----           ----    ----          ----
   originator-----> same   ----           ----    ----          ----
   rec-format-----> same   ----           ----    ----          ----
   rec-size ------> same   ----           ----    ----          ----
   file-size------> same   ----           ----    ----          ----
   org-file-size--> same   ----           ----    ----          ----
   signed-eerp----> same   ----           ----    ----          ----
   cipher---------> same   ----           ----    ----          ----
   sec-services---> same   ----           ----    ----          ----
   compression----> same   ----           ----    ----          ----
   envelope-format> same   ----           ----    ----          ----
   description----> same   ----           ----    ----          ----
   restart-pos1---> same-> restart-pos2-> same    ----          ----
   ----             ----   ----           ----    cause ------> same
   ----             ----   ----           ----    retry-later-> same
   ------------------------------------------------------------------

   Notes:

   1. Retry-later has values "Y" or "N".
   2. Cause is the reason for refusing the transfer (1,..,13,99).
   3. Restart-pos1 not equal 0 is only valid if restart has been
      agreed during initial negotiation.
   4. Restart-pos2 is less than or equal to restart-pos1.

3.3.2. Data Regime

                             |            |
              F_DATA_RQ ---->|------------|----> F_DATA_IND
                             |            |
              F_DATA_CF <----|(---CDT----)|
                             |            |

   Note: Unlike other commands, where the F_XXX_CF signal is a result of
      a corresponding F_XXX_RS command, in this case, the local entity
      layer issues this signal when it is ready for the next data
      request.  This decision is based on the current credit count and
      the reception of CDT (Set Credit) from the receiver.

3.3.3. File Closing

                             |            |
         F_CLOSE_FILE_RQ --->|------------|----> F_CLOSE_FILE_IND
                             |            |
    F_CLOSE_FILE_CF(+|-) <---|------------|<---- F_CLOSE_FILE_RS(+|-)
                             |            |

   Parameters

   Request         Ind    RS(+)          CF(+)     RS(-)         CF(-)
   ---------------------------------------------------------------------
   rec-count --->  same   ----           ----      ----          ----
   unit-count -->  same   ----           ----      ----          ----
   ----            ----   Speaker=Y ---> Speaker=N ----          ----
   ----            ----   Speaker=N ---> Speaker=Y ----          ----
   ----            ----   ----           ----      cause --->    same
   ---------------------------------------------------------------------

   In a positive Close File response (F_CLOSE_FILE_RS(+)) the current
   Listener may either:

      1.  Set Speaker to "Yes" and become the Speaker or
      2.  Set Speaker to "No"  and remain the Listener.

   The File Transfer service will ensure that the setting of the speaker
   parameter is consistent with the capabilities of the peer user.

   The turn is never exchanged in the case of a negative response or
   confirmation.

   Only the Speaker is allowed to issue F_XXX_FILE_RQ primitives.

3.3.4. Exchanging the Turn

3.3.4.1. Initial Turn (First Speaker)

   The Initiator becomes the first Speaker at the end of the Session
   Setup (F_CONNECT_CF received by Initiator and F_CONNECT_RS sent by
   Responder).

3.3.4.2. Following Turns

   Rules:

   1. At each unsuccessful End of File, the turn is not exchanged.

   2. At each successful End of File, the turn is exchanged if requested
      by the Listener:

      - The current Listener receives F_CLOSE_FILE_IND (Speaker =
        choice).

      - If the Listener answers F_CLOSE_FILE_RS(Speaker = YES), it
        becomes the Speaker, the Speaker receives F_CLOSE_FILE_CF
        (Speaker = NO) and becomes the Listener.

      - If the Listener answers F_CLOSE_FILE_RS(Speaker = NO), it
        remains as the Listener, and the Speaker receives
        F_CLOSE_FILE_CF (Speaker = YES) and remains as the Speaker.

   3. The Speaker can issue a Change Direction request (F_CD_RQ) to
      become the Listener.  The Listener receives a Change Direction
      indication (F_CD_IND) and becomes the Speaker.

   4. In order to prevent loops of F_CD_RQ/IND, the Speaker may not send
      an F_CD_RQ after receiving an unsolicited F_CD_IND.  If the
      Listener receives a solicited F_CD_IND as a result of sending
      EFPA(Speaker=Yes), it is acceptable to immediately relinquish the
      right to speak by sending an F_CD_RQ.

3.3.5. End to End Response

   This service is initiated by the current Speaker (if there is no file
   transfer in progress) to send an End to End Response from the final
   destination to the originator of a file.

                             |            |
              F_EERP_RQ ---->|------------|----> F_EERP_IND
                             |            |
              F_RTR_CF  <----|------------|<---- F_RTR_RS
                             |            |

   Parameters

   Request               Indication
   ------------------------------------
   filename -----------> same
   date ---------------> same
   time ---------------> same
   destination --------> same
   originator ---------> same
   hash ---------------> same
   signature ----------> same
   ------------------------------------

   Relationship with Turn:

   -  Only the Speaker may send an End to End Response request.

   -  Invoking the EERP service does not change the turn.

   -  If an F_CD_IND has been received just before F_EERP_RQ is issued,
      this results in leaving the special condition created by the
      reception of F_CD_IND; i.e., while it was possible to issue
      F_RELEASE_RQ and not possible to issue F_CD_RQ just after the
      reception of F_CD_IND, after having issued F_EERP_RQ the normal
      Speaker status is entered again (F_CD_RQ valid, but F_RELEASE_RQ
      not valid).

   Notes:

   1. The F_EERP_RQ (and also F_NERP_RQ) is confirmed with an F_RTR_CF
      signal.  The F_RTR_CF signal is common to both F_EERP_RQ and
      F_NERP_RQ.  There should be no ambiguity, since there can only be
      one such request pending at any one time.

   2. The signature is optional and is requested when sending the
      F_START_FILE_RQ.

   3. If it is not possible to sign the EERP, then an unsigned EERP
      should still be sent.

   4. It is an application implementation issue to validate the contents
      of the EERP and its signature and to decide what action to take on
      receipt of an EERP that fails validation or is not signed when a
      signed EERP was requested.

3.3.6. Negative End Response

   This service is initiated by the current speaker (if there is no file
   transfer in progress) to send a Negative End Response when a file
   could not be transmitted to the next destination.  It is sent only if
   the problem is of a non-temporary kind.

   This service may also be initiated by the final destination instead
   of sending an End to End Response when a file could not be processed,
   after having successfully received the file.

                             |            |
              F_NERP_RQ ---->|------------|----> F_NERP_IND
                             |            |
              F_RTR_CF  <----|------------|----- F_RTR_RS
                             |            |

   Parameters

   Request                          Indication
   ---------------------------------------------------
   filename ----------------------> same
   date --------------------------> same
   time --------------------------> same
   destination -------------------> same
   originator --------------------> same
   creator of negative response --> same
   reason ------------------------> same
   reason text -------------------> same
   hash --------------------------> same
   signature ---------------------> same
   ---------------------------------------------------

   Relationship with Turn:

   The same as for the End-To-End response (see Section 3.3.5).

3.4. Session Take Down

3.4.1. Normal Close

                             |            |
           F_RELEASE_RQ ---->|------------|----> F_RELEASE_IND
                             |            |

   Parameters

   Request                  Indication
   ---------------------------------------------------------------------
   reason = normal -------> ----
   ---------------------------------------------------------------------

   The Release service can only be initiated by the Speaker.

   The Speaker can only issue a Release request (F_RELEASE_RQ) just
   after receiving an unsolicited Change Direction indication
   (F_CD_IND).  This ensures that the other partner doesn't want to send
   any more files in this session.

   Peer ODETTE-FTP entities action a normal session release by
   specifying Reason = Normal in an End Session (ESID) command.

3.4.2. Abnormal Close

                             |            |
           F_RELEASE_RQ ---->|------------|----> F_ABORT_IND
                             |            |

   Parameters

   Request                  Indication
   ---------------------------------------------------------------------
   reason = error value --> same (or equivalent)
                              AO (Abort Origin) = (L)ocal or (D)istant
   ---------------------------------------------------------------------

   Abnormal session release can be initiated by either the Speaker or
   the Listener and also by the user or provider.

   Abnormal session release can occur at any time within the session.

   Peer ODETTE-FTP entities action an abnormal session release by
   specifying Reason = Error-value in an End Session (ESID) command.

   The abnormal session release deals with the following types of error:

   1. The service provider will initiate an abnormal release in the
      following cases:

      1. Protocol error.
      2. Failure of the Start Session (SSID) negotiation.
      3. Command not recognised.
      4. Data Exchange Buffer size error.
      5. Resources not available.
      6. Other unspecified abort code (with Reason = unspecified).

   2. The User Monitor will initiate an abnormal release in the
      following cases:

      1. Local site emergency close down.
      2. Resources not available.
      3. Other unspecified abort code (with Reason = unspecified).

   Other error types may be handled by an abort of the connection.

3.4.3. Abort

                             |            |
             F_ABORT_RQ ---->|------------|----> F_ABORT_IND
                             |            |
             User-Initiated Abort

                             |            |
            F_ABORT_IND <----|------------|----> F_ABORT_IND
                             |            |
            Provider-Initiated Abort

   Parameters

   Request                  Indication
   ---------------------------------------------------------------------
   --                       R  (Reason): specified or unspecified
   --                       AO (Abort Origin): (L)ocal or (D)istant
   ---------------------------------------------------------------------

   The Abort service may be invoked by either entity at any time.

   The service provider may initiate an abort in case of error
   detection.

3.4.4. Explanation of Session Take Down Services

            User  | OFTP |        Network       | OFTP |  User
   ---------------|------|----------------------|------|---------------
                  |      |                      |      |

   1. Normal Release

     F_RELEASE_RQ |      | ESID(R=normal)       |      | F_RELEASE_IND
   *--------------|->  ==|======================|=>  --|-------------->
     (R=normal)   |      |                      |      |

   2. User-Initiated Abnormal Release

     F_RELEASE_RQ |      | ESID(R=error)        |      | F_ABORT_IND
   *--------------|->  ==|======================|=>   -|-------------->
   (R=error value)|      |                      |      | (R=error,AO=D)

   3. Provider-Initiated Abnormal Release

     F_ABORT_IND  |      | ESID(R=error)        |      | F_ABORT_IND
   <--------------|-*  *=|======================|=>  --|-------------->
                  |      |                      |      |

   4. User-Initiated Connection Abort

    F_ABORT_RQ    |      | N_DISC_RQ            |      | F_ABORT_IND
   *--------------|->  --|--------->..----------|->  --|-------------->
                  |      |           N_DISC_IND |      | (R=unsp.,AO=D)

   5. Provider-Initiated Connection Abort

     F_ABORT_IND  |      | N_DISC_RQ            |      | F_ABORT_IND
   <--------------|-*  *-|--------->..----------|->  --|-------------->
   (R=error,AO=L) |      |           N_DISC_IND |      | (R=unsp.,AO=D)

           Key:  *        Origin of command flow
                 F_ --->  File Transfer Service primitive
                 N_ --->  Network Service primitive
                    ===>  ODETTE-FTP (OFTP) protocol message

3.5. Service State Automata

   These state automata define the service as viewed by the User
   Monitor.  Events causing a state transition are shown in lower case
   and the resulting action in upper case where appropriate.

3.5.1. Idle State Diagram

                              o------------o
                  decision    |            |  f_connect_ind
            +-----------------|    IDLE    |-----------------+
            |   F_CONNECT_RQ  |    (0)     |  F_CONNECT_RS   |
            |                 o------------o                 |
            V                                                |
   o-----------------o                                       |
   |                 |                                       |
   | I_WF_FCONNECTCF |                                       |
   |                 |                                       |
   o--------+--------o                                       |
            |                                                |
            | F_CONNECT_CF                                   |
            V                                                V
   o-----------------o                            o-----------------o
   |                 |                            |                 |
   |  IDLE  SPEAKER  |                            | IDLE  LISTENER  |
   |       (1)       |                            |       (2)       |
   |   See Speaker   |                            |  See Listener   |
   |  State Diagram  |                            |  State Diagram  |
   |                 |                            |                 |
   o-----------------o                            o-----------------o

3.5.2. Speaker State Diagram

   o-----------------o                              o-----------------o
   |  IDLE LISTENER  |                              |      IDLE       |
   | CD_RQ just sent |                              |     see (0)     |
   | see (3), Listen |                              |      Idle       |
   |  State Diagram  |                              |  State Diagram  |
   o-----------------o                              o-----------------o
            A                                                A
            |                                                |
        decision                                          decision
        F_CD_RQ                                         F_RELEASE_RQ
            |                                                |
   o================o decision  o----------o decision  o---------------o
   |                |---------->| WAIT FOR |<----------|               |
   |                | F_EERP_RQ |          | F_EERP_RQ |               |
   |     IDLE       |           | EERP/    |           |    IDLE       |
   |   SPEAKER      | decision  | NERP     | decision  |   SPEAKER     |
   |     (1)        |---------->| CONFIRM. |<----------|     (4)       |
   |                | F_NERP_RQ |          | F_NERP_RQ |               |
   |                |           |          |           |               |
   |                |           |          |           |    CD_IND     |
   |                | f_rtr_cf  |          |           | just received |
   |                |<----------|          |           |               |
   |                |           o----------o           |               |
   |                |                                  |               |
   |                |                                  |               |
   o================o                                  o---------------o
     A  A        |                                               |
     |  |        | decision and P2              decision and P2  |
     |  |        +-----------------+       +---------------------+
     |  |         F_START_FILE_RQ  |       |    F_START_FILE_RQ
     |  |                          V       V
     |  |                      o---------------o
     |  |  f_file_start_cf(-)  |               |
     |  +----------------------|    OPENING    |
     |                         |               |
     |                         o---------------o
     |                                 |
   f_file_close_cf(-) or          f_start_file_cf(+)
   f_file_close_cf(+) and not P1       |
     |                                 V

   o---------------o     o---------------o  record to send   o---------o
   |               |     |               |------------------>|         |
   |    CLOSING    |     | DATA TRANSFER |     F_DATA_RQ     | NEXT    |
   |               |     |               |                   | RECORD  |
   |               |     |               |     f_data_cf     |         |
   |               |     |               |<------------------|         |
   o---------------o     o---------------o                   o---------o
     |         A                   |
     |         |    end of file    |
     |         +-------------------+
     |            F_CLOSE_FILE_RQ
     |                                              o-----------------o
     |                f_file_close_cf(+) and P1     |  IDLE LISTENER  |
     +--------------------------------------------->| see (2), Listen |
                                                    |  State Diagram  |
   Predicates:                                      o-----------------o
   P1: Positive confirmation and Speaker = YES
   P2: Mode = Both or (Mode = Sender-only)

3.5.3 Listener State Diagram

   o-----------------o                              o-----------------o
   |  IDLE SPEAKER   |                              |      IDLE       |
   |   CD_IND just   |                              |                 |
   | received see(4) |                              |     see (0)     |
   |  Speaker State  |                              |      Idle       |
   |     Diagram     |                              |  State Diagram  |
   o-----------------o                              o-----------------o
            A                                                A
            |                                                |
         decision      f_eerp_ind                         decision
         F_CD_IND  +--------------+                    F_RELEASE_IND
            |      |   F_RTR_RS   |                          |
   o=================o            |                 o-----------------o
   |                 |<-----------+                 |                 |
   |                 |                              |                 |
   |                 | f_nerp_ind                   |                 |
   |                 |------------+                 |                 |
   |                 | F_RTR_RS   |                 |                 |
   |                 |            |                 |                 |
   |                 |<-----------+                 |                 |
   |  IDLE LISTENER  |                 f_eerp_ind   |  IDLE LISTENER  |
   |       (2)       |<-----------------------------|       (3)       |
   |                 |                 F_RTR_RS     |      CD_RQ      |
   |                 |                              |    just sent    |
   |                 |                 f_nerp_ind   |                 |
   |                 |<-----------------------------|                 |

   |                 |                 F_RTR_RS     |                 |
   |                 |                              |                 |
   |                 | f_start_file_ind             |                 |
   |                 |    and not P1                |                 |
   |                 |---------------------+        |                 |
   o=================o F_START_FILE_RS(-)  |        o-----------------o
     A A    |   A  A                       |           |          |
     | |    |   |  +-----------------------+           |          |
     | |    |   |                                      |          |
     | |    |   | f_start_file_ind and not P1          |          |
     | |    |   +--------------------------------------+          |
     | |    |            F_START_FILE_RS(-)                       |
     | |    |                                                     |
     | |    |        f_start_file_ind           f_start_file_ind  |
     | |    |           and P1                        and P1      |
     | |    +----------------------------+     +------------------+
     | |             F_START_FILE_RS(+)  |     | F_START_FILE_RS(+)
     | |                                 V     V
     | |                            o---------------o
     | |f_close_file_ind and not P3 |               |
     | +----------------------------|               |
     |    F_CLOSE_FILE_RS(+,N)      |               |
     |                              |     DATA      |
     |                              |   TRANSFER    |
     |  f_close_file_ind and not P2 |               |-------------+
     +------------------------------|               |             |
          F_CLOSE_FILE_RS(-)        |               |<------------+
                                    o---------------o  F_DATA_IND
   o---------------o                           |
   | IDLESPEAKER  |  f_close_file_ind and P3  |
   | see (1), Spkr |<--------------------------+
   | State Diagram |    F_CLOSE_FILE_RS(+,Y)
   o---------------o

   Predicates:
   P1: Decision to send F_START_FILE_RS(+)
   P2: Decision to send F_CLOSE_FILE_RS(+)
   P3: Decision to become Speaker

4. Protocol Specification

4.1. Overview

   ODETTE-FTP is divided into five operating phases.

      Start Session
      Start File
      Data Transfer
      End File
      End Session

   After the End File phase, an ODETTE-FTP entity may enter a new Start
   File phase or terminate the session via the End Session phase.

   ODETTE-FTP peers communicate by sending and receiving messages in
   Exchange Buffers via the Network Service.  Each Exchange Buffer
   contains one of the following commands.

      SSRM    Start Session Ready Message
      SSID    Start Session
      SECD    Security Change Direction
      AUCH    Authentication Challenge
      AURP    Authentication Response
      SFID    Start File
      SFPA    Start File Positive Answer
      SFNA    Start File Negative Answer
      DATA    Data
      CDT     Set Credit
      EFID    End File
      EFPA    End File Positive Answer
      EFNA    End File Negative Answer
      ESID    End Session
      CD      Change Direction
      EERP    End to End Response
      NERP    Negative End Response
      RTR     Ready To Receive

   The remainder of this section describes the protocol flows.  Section
   five details the command formats.

4.2. Start Session Phase

The Start Session phase is entered immediately after the network
connection has been established.

4.2.1. Entity Definition

   The ODETTE-FTP entity that took the initiative to establish the
   network connection becomes the Initiator.  Its peer becomes the
   Responder.

4.2.2. Protocol Sequence

   The first message must be sent by the Responder.

   1. Initiator <-------------SSRM -- Responder   Ready Message
                -- SSID ------------>             Identification
                <------------ SSID --             Identification

4.2.3. Secure Authentication

   Having exchanged SSIDs, the Initiator may optionally begin an
   authentication phase, in which each party proves its identity to the
   other.

4.2.4. Protocol Sequence

   The first authentication message must be sent by the Initiator.

   1. Initiator -- SECD ------------> Responder   Change Direction
                <------------ AUCH --             Challenge
                -- AURP ------------>             Response
                <------------ SECD --             Change Direction
                -- AUCH ------------>             Challenge
                <------------ AURP --             Response

   The Initiator sends a Security Change Direction (SECD) to which the
   Responder replies with an Authentication Challenge (AUCH).

   The Responder looks up the public certificate that is linked to the
   purported identity of the Initiator (located in the SSID).  If the
   Responder is unable to locate a suitable certificate then
   authentication fails.  The Responder uses the public key contained in
   the certificate to encrypt a random challenge, unique for each
   session, for the Initiator.  This encrypted challenge is sent as a
   [CMS] envelope to the Initiator as part of the AUCH.

   The Initiator decrypts the challenge using their private key and
   sends the decrypted challenge back to the Responder in the
   Authentication Response (AURP).

   The Responder checks that the data received in the AURP matches the
   random challenge that was sent to the Initiator.

   If the data matches, then the Initiator has authenticated
   successfully and the Responder replies with a Security Change
   Direction (SECD) beginning the complementary process of verifying the
   Responder to the Initiator.  If the data does not match, then the
   Initiator fails authentication.

4.3. Start File Phase

4.3.1. Entity Definition

   The Initiator from the Start Session phase is designated the Speaker
   while the Responder becomes the Listener.  The roles are reversed by
   the Speaker sending a Change Direction command to the Listener.

4.3.2. Protocol Sequence

   1. Speaker  -- SFID ------------> Listener   Start File
               <------------ SFPA --            Answer YES

   2. Speaker  -- SFID ------------> Listener   Start File
               <------------ SFNA --            Answer NO
                     Go To 1

      Note: The User Monitor should take steps to prevent a loop
            situation occurring.

   2. Speaker  -- CD --------------> Listener   Change Direction
      Listener <------------ EERP -- Speaker    End to End Response
               -- RTR ------------->            Ready to Receive
               <------------ NERP --            Negative End Response
               -- RTR ------------->            Ready to Receive
               <------------ SFID --            Start File

4.3.3. Restart Facilities

   The Start File command includes a count allowing the restart of an
   interrupted transmission to be negotiated.  If restart facilities are
   not available, the restart count must be set to zero.  The sender
   will start with the lowest record count + 1.

4.3.4. Broadcast Facilities

   The destination in a Start File command can be specified as follows.

   1.  An explicitly defined destination.

   2.  A group destination that allows an intermediate location to
       broadcast the Virtual File to multiple destinations.

The Listener will send a negative answer to the Speaker when the
destination is not known.

4.3.5. Priority

   The prioritisation of files for transmission is left to the local
   implementation.  To allow some flexibility, a change direction
   mechanism is available in the End File phase.

4.3.6. End to End Response (EERP)

   The End to End Response (EERP) command notifies the originator of a
   Virtual File that the Virtual File has been successfully delivered to
   its final destination.  This allows the originator to perform house
   keeping tasks such as deleting copies of the delivered data.

   If the originator of the Virtual File requested a signed EERP in the
   SFID, the EERP must be signed.  Signing allows the originator of the
   file to prove that the EERP was generated by the final destination.
   If the final destination is unable to sign the EERP, it may send back
   an unsigned EERP.  It is an implementation issue to allow the
   acceptance of an unsigned EERP if a signed EERP is requested.

   A Response Command must be sent from the location performing the
   final processing or distribution of the data to the originator.  The
   Response is mandatory and may be sent in the same or in any
   subsequent session.

   When an intermediate location broadcasts or distributes a Virtual
   File, it must receive a Response command from all the locations to
   which it forwarded the data before sending its own Response.  This
   ensures that the Response received by the Virtual File's originator
   accounts for all the destination locations.  An intermediate location
   therefore needs to track the status of files it processes over time.

   The requesting of a signed EERP is incompatible with the use of
   broadcast facilities because an EERP can be signed by only one
   destination.  If this scenario occurs, the intermediate broadcast
   location may continue and ignore the request for a signed EERP or
   send back a NERP.

   Example: Point to Point

      Location A sends file Ba to location B, which will send an EERP to
      location A after it successfully receives the file.

         o----------o                          o-----------o
         | Loc. A   |----------- S1 ---------->| Loc. B    |
         |          |                          |           |
         | [Ba]     |<---------- R2 -----------| [Ba]      |
         +----------o                          o-----------o

                     Key:   S - File Transfer
                            R - Response EERP
                         [Ba] - File for B from A

   Example: Data distribution

      Location A sends a Virtual File containing data for distribution
      to locations B and C via clearing centres E1 and E2.  Clearing
      centre E1 must wait for a response from E2 (for file Ba) and
      location C before it sends its response, R8, to location A.
      Clearing centre E2 can only send response R7 to E1 when location B
      acknowledges file Ba with response R6.

   o---------o        o---------o        o---------o        o---------o
   | Loc. A  |-- S1 ->| Loc. E1 |-- S2 ->| Loc. E2 |-- S5 ->| Loc. B  |
   |         |        |         |        |         |        |         |
   | [Ba,Ca] |<- R8 --| [Ba,Ca] |<- R7 --| [Ba]    |<- R6 --| [Ba]    |
   o---------o        o---------o        o---------o        o---------o
                         A   |
                         |   |           o---------o
                         |   +----- S3 ->| Loc. C  |
                         |               |         |
                         +--------- R4 --| [Ca]    |
                                         o---------o

   Example: Data collection

      Locations A and B send files Ca and Cb to clearing centre E1,
      which forwards both files to location C in a single Virtual File.
      When it receives response R4 from C, clearing centre E1 sends
      response R5 to location A and R6 to location B.

         o---------o        o---------o        o---------o
         | Loc. A  |-- S1 ->| Loc. E1 |-- S3 ->| Loc. C  |
         |         |        |         |        |         |
         | [Ca]    |<- R5 --| [Ca,Cb] |<- R4 --| [Ca,Cb] |
         o---------o        o---------o        o---------o
                               A   |
         o---------o           |   |
         | Loc. B  |-- S2 -----+   |
         |         |               |
         | [Cb]    |<- R6 ---------+
         o---------o

4.3.7. Negative End Response (NERP)

   In addition to the EERP, which allows control over successful
   transmission of a file, a Negative End Response signals that a file
   could not be delivered to the final destination or that the final
   destination could not process the received file.

   It may be created by an intermediate node that could not transmit the
   file any further because the next node refuses to accept the file.
   The cause of the refusal has to be non-temporary, otherwise the
   intermediate node has to try the transmission again.

   It may also be created by the final node that is unable to process
   the file because of non-recoverable syntax or semantic errors in the
   file, or because of the failure of any other processing performed on
   the file.

   The NERP will be sent back to the originator of the file.

   The parameters are equal to the ones of the EERP, but with additional
   information about the creator of the NERP and the abort reason.
   Where the NERP is created due to a failure to transmit, the abort
   reason is taken from the refusal reason that was sent by the node
   refusing the file.  Because of the NERP, it is possible for the
   intermediate node to stop trying to send the non-deliverable file and
   to delete the file.

   The NERP allows the originator of the file to react to the
   unsuccessful transmission or processing, depending on the reason code
   and the creator of the NERP.

   If the originator of the Virtual File requested a signed EERP in the
   SFID, the NERP must be signed.  Signing allows the originator of the
   file to prove by whom the NERP was generated.  If the location

   generating the NERP is unable to sign the NERP, it may send back an
   unsigned NERP.  It is an implementation issue to allow the acceptance
   of an unsigned EERP if a signed NERP is requested.

4.3.8. Ready To Receive Command (RTR)

   In order to avoid congestion between two adjacent nodes caused by a
   continuous flow of EERPs and NERPs, a Ready To Receive (RTR) command
   is provided.  The RTR acts as an EERP/NERP acknowledgement for flow
   control but has no end-to-end significance.

      Speaker  -- EERP ------------> Listener   End to End Response
               <------------- RTR --            Ready to Receive
               -- EERP ------------>            End to End Response
               <------------- RTR --            Ready to Receive
               -- NERP ------------>            Negative End Response
               <------------- RTR --            Ready to Receive
               -- SFID ------------>            Start File
                         or
               -- CD -------------->            Exchange the turn

   After sending an EERP or NERP, the Speaker must wait for an RTR
   before sending any other commands.  The only acceptable commands to
   follow are:

        EERP
        NERP
        SFID or CD (if there are no more EERPs or NERPs to be sent)

4.4. Data Transfer Phase

Virtual File data flows from the Speaker to the Listener during the
Data Transfer phase, which is entered after the Start File phase.

4.4.1. Protocol Sequence

   To avoid congestion at the protocol level, a flow control mechanism
   is provided via the Set Credit (CDT) command.

   A Credit limit is negotiated in the Start Session phase; this
   represents the number of Data Exchange Buffers that the Speaker may
   send before it is obliged to wait for a Credit command from the
   Listener.

   The available credit is initially set to the negotiated value by the
   Start File positive answer, which acts as an implicit Credit command.
   The Speaker decreases the available credit count by one for each data
   buffer sent to the Listener.

   When the available credit is exhausted, the Speaker must wait for a
   Credit command from the Listener; otherwise, a protocol error will
   occur and the session will be aborted.

   The Listener should endeavour to send the Credit command without
   delay to prevent the Speaker blocking.

   1. Speaker  -- SFID ------------> Listener   Start File
               <------------ SFPA --            Answer YES

   2. If the credit value is set to 2

      Speaker  -- Data ------------> Listener   Start File
               -- Data ------------>
               <------------- CDT --            Set Credit
               -- Data ------------>
               -- EFID ------------>            End File

4.5. End File Phase

4.5.1. Protocol Sequence

   The Speaker notifies the Listener that it has finished sending a
   Virtual File by sending an End File (EFID) command.  The Listener
   replies with a positive or negative End File command and has the
   option to request a Change Direction command from the Speaker.

   1. Speaker  -- EFID ------------> Listener   End File
               <------------ EFPA --            Answer YES

   2. Speaker  -- EFID ------------> Listener   End File
               <------------ EFPA --            Answer YES + CD
               -- CD -------------->            Change Direction
      Listener <------------ EERP -- Speaker    End to End Response
               -------------- RTR ->            Ready to Receive
      Listener <------------ NERP -- Speaker    Negative End Response
               -------------- RTR ->            Ready to Receive
               Go to Start File Phase

   3. Speaker  -- EFID ------------> Listener   End File
               <------------ EFNA --            Answer NO

4.6. End Session Phase

4.6.1. Protocol Sequence

   The Speaker terminates the session by sending an End Session (ESID)
   command.  The Speaker may only do this if the Listener has just
   relinquished its role as speaker.

   1. Speaker  -- EFID ------------> Listener   End File
               <------------ EFPA --            Answer YES
               -- CD -------------->            Change Direction
      Listener <------------ ESID -- Speaker    End Session

4.7. Problem Handling

   Error detection and handling should be done as close as possible to
   the problem.  This aids problem determination and correction.  Each
   layer of the reference model is responsible for its own error
   handling.

   ODETTE-FTP can detect protocol errors by virtue of its state machine
   and uses activity timers to detect session hang conditions.  These
   mechanisms are separate from the End to End controls.

4.7.1. Protocol Errors

If a protocol error occurs, the session will be terminated and
application activity aborted. Both locations enter the IDLE state.

4.7.2. Timers

   To protect against application and network hang conditions, ODETTE-
   FTP uses activity timers for all situations where a response is
   required.  The timers and actions to be taken if they expire are
   described in Section 9, "Protocol State Machine".

4.7.3. Clearing Centres

   The use of clearing centres introduces the possibility of errors
   occurring as a result of data processing activities within the
   centre.  Such errors are not directly related to ODETTE-FTP or the
   communication network and are therefore outside the scope of this
   specification.

5.  Commands and Formats

   ODETTE-FTP entities communicate via Exchange Buffers.  The Command
   Exchange Buffers are described below.  Virtual File data is carried
   in Data Exchange Buffers, which are described in Section 7.

5.1. Conventions

5.1.1. Representation Unit

The basic unit of information is an octet, containing 8 bits.

5.1.2. Values and Characters

   The ISO 646 IRV 7-bit coded character set [ISO-646], according to
   Appendix B, is used to encode constants and strings within Command
   Exchange Buffers except where [UTF-8] is explicitly indicated against
   a field.

5.2. Commands

   A Command Exchange Buffer contains a single command starting at the
   beginning of the buffer.  Commands and data are never mixed within an
   Exchange Buffer.  Commands cannot be compressed.  Variable-length
   parameters may be omitted entirely if not required and the associated
   length indicator field set to zero.

   Components:

   1. Command identifier:

      The first octet of an Exchange Buffer is the Command Identifier
      and defines the format of the buffer.

   2. Parameter(s):

      Command parameters are stored in fields within a Command Exchange
      Buffer.  Where variable-length fields are used, they are preceded
      with a header field indicating the length.  All values are
      required except where explicitly indicated.

5.3. Command Formats

The ODETTE-FTP commands are described below using the following
definitions.

   Position (Pos)

      Field offset within the Command Exchange Buffer, relative to a
      zero origin.

   Field

      The name of the field.

   Description

      A description of the field.

   Format

      F    - A field containing fixed values.  All allowable values for
             the field are enumerated in the command definition.

      V    - A field with variable values within a defined range.  For
             example, the SFIDLRECL field may contain any integer value
             between 00000 and 99999.

      X(n) - An alphanumeric field of length n octets.

        A String contains alphanumeric characters from the following
        set:

         The numerals:               0 to 9
         The upper case letters:     A to Z
         The following special set:  / - . & ( ) space.

        Space is not allowed as an embedded character.

      9(n) - A numeric field of length n octets.

      U(n) - A binary field of length n octets.

             Numbers encoded as binary are always unsigned and in
             network byte order.

      T(n) - An field of length n octets, encoded using [UTF-8].

      String and alphanumeric fields are always left justified and right
      padded with spaces where needed.

      Numeric fields are always right justified and left padded with
      zeros where needed.

Reserved fields should be padded with spaces.

5.3.1. SSRM - Start Session Ready Message

   o-------------------------------------------------------------------o
   |       SSRM        Start Session Ready Message                     |
   |                                                                   |
   |       Start Session Phase     Initiator <---- Responder           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SSRMCMD   | SSRM Command, 'I'                     | F X(1)  |
   |   1 | SSRMMSG   | Ready Message, 'ODETTE FTP READY '    | F X(17) |
   |  18 | SSRMCR    | Carriage Return                       | F X(1)  |
   o-------------------------------------------------------------------o

   SSRMCMD   Command Code                                      Character

      Value: 'I'  SSRM Command identifier.

   SSRMMSG   Ready Message                                    String(17)

      Value: 'ODETTE FTP READY '

   SSRMCR    Carriage Return                                   Character

      Value: Character with hex value '0D' or '8D'.

5.3.2. SSID - Start Session

   o-------------------------------------------------------------------o
   |       SSID        Start Session                                   |
   |                                                                   |
   |       Start Session Phase     Initiator <---> Responder           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SSIDCMD   | SSID Command 'X'                      | F X(1)  |
   |   1 | SSIDLEV   | Protocol Release Level                | F 9(1)  |
   |   2 | SSIDCODE  | Initiator's Identification Code       | V X(25) |
   |  27 | SSIDPSWD  | Initiator's Password                  | V X(8)  |
   |  35 | SSIDSDEB  | Data Exchange Buffer Size             | V 9(5)  |
   |  40 | SSIDSR    | Send / Receive Capabilities (S/R/B)   | F X(1)  |
   |  41 | SSIDCMPR  | Buffer Compression Indicator (Y/N)    | F X(1)  |
   |  42 | SSIDREST  | Restart Indicator (Y/N)               | F X(1)  |
   |  43 | SSIDSPEC  | Special Logic Indicator (Y/N)         | F X(1)  |
   |  44 | SSIDCRED  | Credit                                | V 9(3)  |
   |  47 | SSIDAUTH  | Secure Authentication (Y/N)           | F X(1)  |
   |  48 | SSIDRSV1  | Reserved                              | F X(4)  |
   |  52 | SSIDUSER  | User Data                             | V X(8)  |
   |  60 | SSIDCR    | Carriage Return                       | F X(1)  |
   o-------------------------------------------------------------------o

      SSIDCMD   Command Code
      Character

      Value: 'X'  SSID Command identifier.

   SSIDLEV   Protocol Release Level                           Numeric(1)

             Used to specify the level of the ODETTE-FTP protocol

      Value: '1' for Revision 1.2
             '2' for Revision 1.3
             '4' for Revision 1.4
             '5' for Revision 2.0

             Future release levels will have higher numbers.  The
             protocol release level is negotiable, with the lowest level
             being selected.

             Note: ODETTE File Transfer Protocol 1.3 (RFC 2204)
                   specifies '1' for the release level, despite adhering
                   to revision 1.3.

   SSIDCODE  Initiator's Identification Code                  String(25)

    Format:  See Identification Code (Section 5.4)

             Uniquely identifies the Initiator (sender) participating in
             the ODETTE-FTP session.

             It is an application implementation issue to link the
             expected [X.509] certificate to the SSIDCODE provided.

   SSIDPSWD  Initiator's Password                              String(8)

             Key to authenticate the sender.  Assigned by bilateral
             agreement.

   SSIDSDEB  Data Exchange Buffer Size                        Numeric(5)

    Minimum: 128
    Maximum: 99999

             The length, in octets, of the largest Data Exchange Buffer
             that can be accepted by the location.  The length includes
             the command octet but does not include the Stream
             Transmission Header.

             After negotiation, the smallest size will be selected.

   SSIDSR    Send / Receive Capabilities                       Character

      Value: 'S'  Location can only send files.
             'R'  Location can only receive files.
             'B'  Location can both send and receive files.

             Sending and receiving will be serialised during the
             session, so parallel transmissions will not take place in
             the same session.

             An error occurs if adjacent locations both specify the send
             or receive capability.

   SSIDCMPR  Buffer Compression Indicator                      Character

      Value: 'Y'  The location can handle OFTP data buffer compression
             'N'  The location cannot handle OFTP buffer compression

             Compression is only used if supported by both locations.

             The compression mechanism referred to here applies to each
             individual OFTP data buffer.  This is different from the
             file compression mechanism in OFTP, which involves the
             compression of whole files.

   SSIDREST  Restart Indicator                                 Character

      Value: 'Y'  The location can handle the restart of a partially
                  transmitted file.
             'N'  The location cannot restart a file.

   SSIDSPEC  Special Logic Indicator                           Character

      Value: 'Y'  Location can handle Special Logic
             'N'  Location cannot handle Special Logic

             Special Logic is only used if supported by both locations.

             The Special Logic extensions are only useful to access an
             X.25 network via an asynchronous entry and are not
             supported for TCP/IP connections.

   SSIDCRED  Credit                                           Numeric(3)

    Maximum: 999

             The number of consecutive Data Exchange Buffers sent by the
             Speaker before it must wait for a Credit (CDT) command from
             the Listener.

             The credit value is only applied to Data flow in the Data
             Transfer phase.

             The Speaker's available credit is initialised to SSIDCRED
             when it receives a Start File Positive Answer (SFPA)
             command from the Listener.  It is zeroed by the End File
             (EFID) command.

             After negotiation, the smallest size must be selected in
             the answer of the Responder, otherwise a protocol error
             will abort the session.

             Negotiation of the "credit-window-size" parameter.

             Window Size m  -- SSID ------------>
                            <------------ SSID --  Window Size n
                                                  (n less than or
                                                   equal to m)
             Note: negotiated value will be "n".

   SSIDAUTH  Secure Authentication                             Character

      Value: 'Y'  The location requires secure authentication.  'N'  The
             location does not require secure authentication.

             Secure authentication is only used if agreed by both
             locations.

             If the answer of the Responder does not match with the
             authentication requirements of the Initiator, then the
             Initiator must abort the session.

             No negotiation of authentication is allowed.

             authentication p  -- SSID ------------>
                               <------------ SSID --  authentication q

             p == q -> continue.
             p != q -> abort.

   SSIDRSV1  Reserved                                          String(4)

             This field is reserved for future use.

   SSIDUSER  User Data                                         String(8)

             May be used by ODETTE-FTP in any way.  If unused, it should
             be initialised to spaces.  It is expected that a bilateral
             agreement exists as to the meaning of the data.

   SSIDCR    Carriage Return                                   Character

      Value: Character with hex value '0D' or '8D'.

5.3.3. SFID - Start File

   o-------------------------------------------------------------------o
   |       SFID        Start File                                      |
   |                                                                   |
   |       Start File Phase           Speaker ----> Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SFIDCMD   | SFID Command, 'H'                     | F X(1)  |
   |   1 | SFIDDSN   | Virtual File Dataset Name             | V X(26) |
   |  27 | SFIDRSV1  | Reserved                              | F X(3)  |
   |  30 | SFIDDATE  | Virtual File Date stamp, (CCYYMMDD)   | V 9(8)  |
   |  38 | SFIDTIME  | Virtual File Time stamp, (HHMMSScccc) | V 9(10) |
   |  48 | SFIDUSER  | User Data                             | V X(8)  |
   |  56 | SFIDDEST  | Destination                           | V X(25) |
   |  81 | SFIDORIG  | Originator                            | V X(25) |
   | 106 | SFIDFMT   | File Format (F/V/U/T)                 | F X(1)  |
   | 107 | SFIDLRECL | Maximum Record Size                   | V 9(5)  |
   | 112 | SFIDFSIZ  | File Size, 1K blocks                  | V 9(13) |
   | 125 | SFIDOSIZ  | Original File Size, 1K blocks         | V 9(13) |
   | 138 | SFIDREST  | Restart Position                      | V 9(17) |
   | 155 | SFIDSEC   | Security Level                        | F 9(2)  |
   | 157 | SFIDCIPH  | Cipher suite selection                | F 9(2)  |
   | 159 | SFIDCOMP  | File compression algorithm            | F 9(1)  |
   | 160 | SFIDENV   | File enveloping format                | F 9(1)  |
   | 161 | SFIDSIGN  | Signed EERP request                   | F X(1)  |
   | 162 | SFIDDESCL | Virtual File Description length       | V 9(3)  |
   | 165 | SFIDDESC  | Virtual File Description              | V T(n)  |
   o-------------------------------------------------------------------o

   SFIDCMD   Command Code                                      Character

      Value: 'H'  SFID Command identifier.

   SFIDDSN   Virtual File Dataset Name                        String(26)

             Dataset name of the Virtual File being transferred,
             assigned by bilateral agreement.

             No general structure is defined for this attribute.

             See Virtual Files - Identification (Section 1.5.2)

   SFIDRSV1  Reserved                                          String(3)

             This field is reserved for future use.

   SFIDDATE  Virtual File Date stamp                          Numeric(8)

     Format: 'CCYYMMDD'  8 decimal digits representing the century,
             year, month, and day.

             Date stamp assigned by the Virtual File's Originator
             indicating when the file was made available for
             transmission.

             See Virtual Files - Identification (Section 1.5.2)

   SFIDTIME  Virtual File Time stamp                         Numeric(10)

     Format: 'HHMMSScccc'  10 decimal digits representing hours,
             minutes, seconds, and a counter (0001-9999), which gives
             higher resolution.

             Time stamp assigned by the Virtual File's Originator
             indicating when the file was made available for
             transmission.

             See Virtual Files - Identification (Section 1.5.2)

   SFIDUSER  User Data                                         String(8)

             May be used by ODETTE-FTP in any way.  If unused, it should
             be initialised to spaces.  It is expected that a bilateral
             agreement exists as to the meaning of the data.

   SFIDDEST  Destination                                      String(25)

     Format: See Identification Code (Section 5.4)

             The Final Recipient of the Virtual File.

             This is the location that will look into the Virtual File
             content and perform mapping functions.  It is also the
             location that creates the End to End Response (EERP)
             command for the received file.

   SFIDORIG  Originator                                       String(25)

     Format: See Identification Code (Section 5.4)

             Originator of the Virtual File.

             It is the location that created (mapped) the data for
             transmission.

   SFIDFMT   File Format                                       Character

      Value: 'F'  Fixed format binary file
             'V'  Variable format binary file
             'U'  Unstructured binary file
             'T'  Text

             Virtual File format.  Used to calculate the restart
             position (Section 1.5.4).

             Once a file has been signed, compressed, and/or encrypted,
             in file format terms it becomes unstructured, format U.
             The record boundaries are no longer discernable until the
             file is decrypted, decompressed, and/or verified.  SFID
             File Format Field in this scenario indicates the format of
             the original file, and the transmitted file must be treated
             as U format.

   SFIDLRECL Maximum Record Size                              Numeric(5)

    Maximum: 99999

             Length in octets of the longest logical record that may be
             transferred to a location.  Only user data is included.

             If SFIDFMT is 'T' or 'U', then this attribute must be set
             to '00000'.

             If SFIDFMT is 'V' and the file is compressed, encrypted, or
             signed, then the maximum value of SFIDRECL is '65536'.

   SFIDFSIZ  Transmitted File Size                           Numeric(13)

    Maximum: 9999999999999

             Space in 1K (1024 octet) blocks required at the Originator
             location to store the actual Virtual File that is to be
             transmitted.

             For example, if a file is compressed before sending, then
             this is the space required to store the compressed file.

             This parameter is intended to provide only a good estimate
             of the Virtual File size.

             Using 13 digits allows for a maximum file size of
             approximately 9.3 PB (petabytes) to be transmitted.

   SFIDOSIZ  Original File Size                              Numeric(13)

    Maximum: 9999999999999

             Space in 1K (1024 octet) blocks required at the Originator
             location to store the original before it was signed,
             compressed, and/or encrypted.

             If no security or compression services have been used,
             SFIDOSIZ should contain the same value as SFIDFSIZ.

             If the original file size is not known, the value zero
             should be used.

             This parameter is intended to provide only a good estimate
             of the original file size.

             The sequence of events in file exchange are:

              (a) raw data file ready to be sent
                   SFIDOSIZ = Original File Size

              (b) signing/compression/encryption

              (c) transmission
                   SFIDFSIZ = Transmitted File Size

              (d) decryption/decompression/verification

              (e) received raw data file for in-house applications
                   SFIDOSIZ = Original File Size

             The Transmitted File Size at (c) indicates to the receiver
             how much storage space is needed to receive the file.

             The Original File Size at (e) indicates to the in-house
             application how much storage space is needed to process the
             file.

   SFIDREST  Restart Position                                Numeric(17)

    Maximum: 99999999999999999

             Virtual File restart position.

             The count represents the:
                - Record Number if SSIDFMT is 'F' or 'V'.
                - File offset in 1K (1024 octet) blocks if SFIDFMT is
                  'U' or 'T'.

             The count will express the transmitted user data (i.e.,
             before ODETTE-FTP buffer compression, header not included).

             After negotiation between adjacent locations,
             retransmission will start at the lowest value.

             Once a file has been signed, compressed, and/or encrypted,
             in file format terms, it has become unstructured, like
             format U.  The file should be treated as format U for the
             purposes of restart, regardless of the actual value in
             SFIDFMT.

   SFIDSEC   Security Level                                   Numeric(2)

      Value: '00'  No security services
             '01'  Encrypted
             '02'  Signed
             '03'  Encrypted and signed

             Indicates whether the file has been signed and/or encrypted
             before transmission. (See Section 6.2.)

   SFIDCIPH  Cipher suite selection                           Numeric(2)

      Value: '00'  No security services
             '01'  See Section 10.2

             Indicates the cipher suite used to sign and/or encrypt the
             file and also to indicate the cipher suite that should be
             used when a signed EERP or NERP is requested.

   SFIDCOMP  File compression algorithm                       Numeric(1)

      Value: '0'  No compression
             '1'  Compressed with [ZLIB] algorithm

             Indicates the algorithm used to compress the file.
             (See Section 6.4.)

   SFIDENV   File enveloping format                           Numeric(1)

      Value: '0'  No envelope
             '1'  File is enveloped using [CMS]

             Indicates the enveloping format used in the file.

             If the file is encrypted/signed/compressed or is an
             enveloped file for the exchange and revocation of
             certificates, this field must be set accordingly.

   SFIDSIGN  Signed EERP request                               Character

      Value: 'Y'  The EERP returned in acknowledgement of the file
                  must be signed
             'N'  The EERP must not be signed

             Requests whether the EERP returned for the file must be
             signed.

   SFIDDESCL Virtual File Description length                  Numeric(3)

             Length in octets of the field SFIDDESC.

             A value of 0 indicates that no description is present.

   SFIDDESC  Virtual File Description                         [UTF-8](n)

             May be used by ODETTE-FTP in any way.  If not used,
             SFIDDESCL should be set to zero.

             No general structure is defined for this attribute, but it
             is expected that a bilateral agreement exists as to the
             meaning of the data.

             It is encoded using [UTF-8] to support a range of national
             languages.

             Maximum length of the encoded value is 999 octets.

5.3.4. SFPA - Start File Positive Answer

   o-------------------------------------------------------------------o
   |       SFPA        Start File Positive Answer                      |
   |                                                                   |
   |       Start File Phase           Speaker <---- Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SFPACMD   | SFPA Command, '2'                     | F X(1)  |
   |   1 | SFPAACNT  | Answer Count                          | V 9(17) |
   o-------------------------------------------------------------------o

   SFPACMD   Command Code                                      Character

      Value: '2'  SFPA Command identifier.

   SFPAACNT  Answer Count                                    Numeric(17)

             The Listener must enter a count lower than or equal to the
             restart count specified by the Speaker in the Start File
             (SFID) command.  The count expresses the received user
             data.  If restart facilities are not available, a count of
             zero must be specified.

5.3.5. SFNA - Start File Negative Answer

   o-------------------------------------------------------------------o
   |       SFNA        Start File Negative Answer                      |
   |                                                                   |
   |       Start File Phase           Speaker <---- Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SFNACMD   | SFNA Command, '3'                     | F X(1)  |
   |   1 | SFNAREAS  | Answer Reason                         | F 9(2)  |
   |   3 | SFNARRTR  | Retry Indicator, (Y/N)                | F X(1)  |
   |   4 | SFNAREASL | Answer Reason Text Length             | V 9(3)  |
   |   7 | SFNAREAST | Answer Reason Text                    | V T(n)  |
   o-------------------------------------------------------------------o

   SFNACMD   Command Code                                      Character

      Value: '3'  SFNA Command identifier.

   SFNAREAS  Answer Reason                                    Numeric(2)

      Value: '01'  Invalid filename.
             '02'  Invalid destination.
             '03'  Invalid origin.
             '04'  Storage record format not supported.
             '05'  Maximum record length not supported.
             '06'  File size is too big.
             '10'  Invalid record count.
             '11'  Invalid byte count.
             '12'  Access method failure.
             '13'  Duplicate file.
             '14'  File direction refused.
             '15'  Cipher suite not supported.
             '16'  Encrypted file not allowed.
             '17'  Unencrypted file not allowed.
             '18'  Compression not allowed.
             '19'  Signed file not allowed.
             '20'  Unsigned file not allowed.
             '99'  Unspecified reason.

             Reason why transmission cannot proceed.

   SFNARRTR  Retry Indicator                                   Character

      Value: 'N'  Transmission should not be retried.
             'Y'  The transmission may be retried later.

             This parameter is used to advise the Speaker if it should
             retry at a later time due to a temporary condition at the
             Listener site, such as a lack of storage space.  It should
             be used in conjunction with the Answer Reason code
             (SFNAREAS).

             An invalid file name error code may be the consequence of a
             problem in the mapping of the Virtual File on to a real
             file.  Such problems cannot always be resolved immediately.
             It is therefore recommended that when an SFNA with Retry =
             Y is received the User Monitor attempts to retransmit the
             relevant file in a subsequent session.

   SFNAREASL Answer Reason Text Length                        Numeric(3)

             Length in octets of the field SFNAREAST.

0 indicates that no SFNAREAST field follows.

   SFNAREAST Answer Reason Text                               [UTF-8](n)

             Reason why transmission cannot proceed in plain text.

             It is encoded using [UTF-8].

             Maximum length of the encoded reason is 999 octets.

             No general structure is defined for this attribute.

5.3.6. DATA - Data Exchange Buffer

   o-------------------------------------------------------------------o
   |       DATA        Data Exchange Buffer                            |
   |                                                                   |
   |       Data Transfer Phase        Speaker ----> Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | DATACMD   | DATA Command, 'D'                     | F X(1)  |
   |   1 | DATABUF   | Data Exchange Buffer payload          | V U(n)  |
   o-------------------------------------------------------------------o

   DATACMD   Command Code                                      Character

      Value: 'D'  DATA Command identifier.

   DATABUF   Data Exchange Buffer payload                      Binary(n)

             Variable-length buffer containing the data payload.  The
             Data Exchange Buffer is described in Section 7.

5.3.7. CDT - Set Credit

   o-------------------------------------------------------------------o
   |       CDT         Set Credit                                      |
   |                                                                   |
   |       Data Transfer Phase        Speaker <---- Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | CDTCMD    | CDT Command, 'C'                      | F X(1)  |
   |   1 | CDTRSV1   | Reserved                              | F X(2)  |
   o-------------------------------------------------------------------o

   CDTCMD    Command Code                                      Character

      Value: 'C'  CDT Command identifier.

CDTRSV1 Reserved String(2)

This field is reserved for future use.

5.3.8. EFID - End File

   o-------------------------------------------------------------------o
   |       EFID        End File                                        |
   |                                                                   |
   |       End File Phase             Speaker ----> Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | EFIDCMD   | EFID Command, 'T'                     | F X(1)  |
   |   1 | EFIDRCNT  | Record Count                          | V 9(17) |
   |  18 | EFIDUCNT  | Unit Count                            | V 9(17) |
   o-------------------------------------------------------------------o

   EFIDCMD   Command Code                                      Character

      Value: 'T'  EFID Command identifier.

   EFIDRCNT  Record Count                                    Numeric(17)

    Maximum: 99999999999999999

             For SSIDFMT 'F' or 'V', the exact record count.
             For SSIDFMT 'U' or 'T', zeros.

             The count will express the real size of the file (before
             buffer compression, header not included).  The total count
             is always used, even during restart processing.

   EFIDUCNT  Unit Count                                      Numeric(17)

    Maximum: 99999999999999999

             Exact number of units (octets) transmitted.

             The count will express the real size of the file.  The
             total count is always used, even during restart processing.

5.3.9. EFPA - End File Positive Answer

   o-------------------------------------------------------------------o
   |       EFPA        End File Positive Answer                        |
   |                                                                   |
   |       End File Phase             Speaker <---- Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | EFPACMD   | EFPA Command, '4'                     | F X(1)  |
   |   1 | EFPACD    | Change Direction Indicator, (Y/N)     | F X(1)  |
   o-------------------------------------------------------------------o

   EFPACMD   Command Code                                      Character

      Value: '4'  EFPA Command identifier.

   EFPACD    Change Direction Indicator                        Character

      Value: 'N'  Change direction not requested.
             'Y'  Change direction requested.

             This parameter allows the Listener to request a Change
             Direction (CD) command from the Speaker.

5.3.10. EFNA - End File Negative Answer

   o-------------------------------------------------------------------o
   |       EFNA        End File Negative Answer                        |
   |                                                                   |
   |       End File Phase             Speaker <---- Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | EFNACMD   | EFNA Command, '5'                     | F X(1)  |
   |   1 | EFNAREAS  | Answer Reason                         | F 9(2)  |
   |   3 | EFNAREASL | Answer Reason Text Length             | V 9(3)  |
   |   6 | EFNAREAST | Answer Reason Text                    | V T(n)  |
   o-------------------------------------------------------------------o

   EFNACMD   Command Code                                      Character

      Value: '5'  EFNA Command identifier.

   EFNAREAS  Answer Reason                                    Numeric(2)

      Value: '01'  Invalid filename.
             '02'  Invalid destination.
             '03'  Invalid origin.
             '04'  Storage record format not supported.
             '05'  Maximum record length not supported.
             '06'  File size is too big.
             '10'  Invalid record count.
             '11'  Invalid byte count.
             '12'  Access method failure.
             '13'  Duplicate file.
             '14'  File direction refused.
             '15'  Cipher suite not supported.
             '16'  Encrypted file not allowed.
             '17'  Unencrypted file not allowed.
             '18'  Compression not allowed.
             '19'  Signed file not allowed.
             '20'  Unsigned file not allowed.
             '21'  Invalid file signature.
             '22'  File decryption failure.
             '23'  File decompression failure.
             '99'  Unspecified reason.

             Reason why transmission failed.

   EFNAREASL Answer Reason Text Length                        Numeric(3)

             Length in octets of the field EFNAREAST.

             0 indicates that no EFNAREAST field follows.

   EFNAREAST Answer Reason Text                               [UTF-8](n)

             Reason why transmission failed in plain text.

             It is encoded using [UTF-8].

             Maximum length of the encoded reason is 999 octets.

             No general structure is defined for this attribute.

5.3.11. ESID - End Session

   o-------------------------------------------------------------------o
   |       ESID        End Session                                     |
   |                                                                   |
   |       End Session Phase          Speaker ----> Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | ESIDCMD   | ESID Command, 'F'                     | F X(1)  |
   |   1 | ESIDREAS  | Reason Code                           | F 9(2)  |
   |   3 | ESIDREASL | Reason Text Length                    | V 9(3)  |
   |   6 | ESIDREAST | Reason Text                           | V T(n)  |
   |     | ESIDCR    | Carriage Return                       | F X(1)  |
   o-------------------------------------------------------------------o

   ESIDCMD   Command Code                                      Character

      Value: 'F'  ESID Command identifier.

   ESIDREAS  Reason Code                                      Numeric(2)

      Value: '00'  Normal session termination

             '01'  Command not recognised

                   An Exchange Buffer contains an invalid command code
                   (1st octet of the buffer).

             '02'  Protocol violation

                   An Exchange Buffer contains an invalid command for
                   the current state of the receiver.

             '03'  User code not known

                   A Start Session (SSID) command contains an unknown or
                   invalid Identification Code.

             '04'  Invalid password

                   A Start Session (SSID) command contained an invalid
                   password.

             '05'  Local site emergency close down

                   The local site has entered an emergency close down
                   mode.  Communications are being forcibly terminated.

             '06'  Command contained invalid data

                   A field within a Command Exchange Buffer contains
                   invalid data.

             '07'  Exchange Buffer size error

                   The length of the Exchange Buffer as determined by
                   the Stream Transmission Header differs from the
                   length implied by the Command Code.

             '08'  Resources not available

                   The request for connection has been denied due to a
                   resource shortage.  The connection attempt should be
                   retried later.

             '09'  Time out

             '10'  Mode or capabilities incompatible

             '11'  Invalid challenge response

             '12'  Secure authentication requirements incompatible

             '99'  Unspecified Abort code

                   An error was detected for which no specific code is
                   defined.

   ESIDREASL Reason Text Length                               Numeric(3)

             Length in octets of the field ESIDREAST.

             0 indicates that no ESIDREAST field is present.

   ESIDREAST Reason Text                                      [UTF-8](n)

             Reason why session ended in plain text.

             It is encoded using [UTF-8].

             Maximum length of the encoded reason is 999 octets.

             No general structure is defined for this attribute.

ESIDCR Carriage Return Character

Value: Character with hex value '0D' or '8D'.

5.3.12. CD - Change Direction

   o-------------------------------------------------------------------o
   |       CD          Change Direction                                |
   |                                                                   |
   |       Start File Phase           Speaker ----> Listener           |
   |       End File Phase             Speaker ----> Listener           |
   |       End Session Phase        Initiator <---> Responder          |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | CDCMD     | CD Command, 'R'                       | F X(1)  |
   o-------------------------------------------------------------------o

   CDCMD     Command Code                                      Character

      Value: 'R'  CD Command identifier.

5.3.13. EERP - End to End Response

   o-------------------------------------------------------------------o
   |       EERP        End to End Response                             |
   |                                                                   |
   |       Start File Phase           Speaker ----> Listener           |
   |       End File Phase             Speaker ----> Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | EERPCMD   | EERP Command, 'E'                     | F X(1)  |
   |   1 | EERPDSN   | Virtual File Dataset Name             | V X(26) |
   |  27 | EERPRSV1  | Reserved                              | F X(3)  |
   |  30 | EERPDATE  | Virtual File Date stamp, (CCYYMMDD)   | V 9(8)  |
   |  38 | EERPTIME  | Virtual File Time stamp, (HHMMSScccc) | V 9(10) |
   |  48 | EERPUSER  | User Data                             | V X(8)  |
   |  56 | EERPDEST  | Destination                           | V X(25) |
   |  81 | EERPORIG  | Originator                            | V X(25) |
   | 106 | EERPHSHL  | Virtual File hash length              | V U(2)  |
   | 108 | EERPHSH   | Virtual File hash                     | V U(n)  |
   |     | EERPSIGL  | EERP signature length                 | V U(2)  |
   |     | EERPSIG   | EERP signature                        | V U(n)  |
   o-------------------------------------------------------------------o

   EERPCMD   Command Code                                      Character

      Value: 'E'  EERP Command identifier.

   EERPDSN   Virtual File Dataset Name                        String(26)

             Dataset name of the Virtual File being transferred,
             assigned by bilateral agreement.

             No general structure is defined for this attribute.

             See Virtual Files - Identification (Section 1.5.2)

   EERPRSV1  Reserved                                          String(3)

             This field is reserved for future use.

   EERPDATE  Virtual File Date stamp                          Numeric(8)

     Format: 'CCYYMMDD'  8 decimal digits representing the century,
             year, month, and day, respectively.

             Date stamp assigned by the Virtual File's Originator
             indicating when the file was made available for
             transmission.

             See Virtual Files - Identification (Section 1.5.2)

   EERPTIME  Virtual File Time stamp                         Numeric(10)

     Format: 'HHMMSScccc'  10 decimal digits representing hours,
             minutes, seconds, and a counter (0001-9999), which gives
             higher resolution.

             Time stamp assigned by the Virtual File's Originator
             indicating when the file was made available for
             transmission.

             See Virtual Files - Identification (Section 1.5.2)

   EERPUSER  User Data                                         String(8)

             May be used by ODETTE-FTP in any way.  If unused, it should
             be initialised to spaces.  It is expected that a bilateral
             agreement exists as to the meaning of the data.

   EERPDEST  Destination                                      String(25)

     Format: See Identification Code (Section 5.4)

             Originator of the Virtual File.

             This is the location that created the data for
             transmission.

   EERPORIG  Originator                                       String(25)

     Format: See Identification Code (Section 5.4)

             Final Recipient of the Virtual File.

             This is the location that will look into the Virtual File
             content and process it accordingly.  It is also the
             location that creates the EERP for the received file.

   EERPHSHL  Virtual File hash length                          Binary(2)

             Length in octets of the field EERPHSH.

             A binary value of 0 indicates that no hash is present.
             This is always the case if the EERP is not signed.

   EERPHSH   Virtual File hash                                 Binary(n)

             Hash of the transmitted Virtual File, i.e., not the hash of
             the original file.

             The algorithm used is determined by the bilaterally agreed
             cipher suite specified in the SFIDCIPH.

             It is an application implementation issue to validate the
             EERPHSH to ensure that the EERP is acknowledging the exact
             same file as was originally transmitted.

   EERPSIGL  EERP signature length                             Binary(2)

             0 indicates that this EERP has not been signed.

             Any other value indicates the length of EERPSIG in octets
             and indicates that this EERP has been signed.

   EERPSIG   EERP signature                                    Binary(n)

             Contains the [CMS] enveloped signature of the EERP.

             Signature = Sign{EERPDSN
                              EERPDATE
                              EERPTIME
                              EERPDEST
                              EERPORIG
                              EERPHSH}

             Each field is taken in its entirety, including any padding.
             The envelope must contain the original data, not just the
             signature.

             The [CMS] content type used is SignedData.

             The encapsulated content type used is id-data.

             It is an application issue to validate the signature with
             the contents of the EERP.

5.3.14. NERP - Negative End Response

   o-------------------------------------------------------------------o
   |       NERP        Negative End Response                           |
   |                                                                   |
   |       Start File Phase           Speaker ----> Listener           |
   |       End File Phase             Speaker ----> Listener           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | NERPCMD   | NERP Command, 'N'                     | F X(1)  |
   |   1 | NERPDSN   | Virtual File Dataset Name             | V X(26) |
   |  27 | NERPRSV1  | Reserved                              | F X(6)  |
   |  33 | NERPDATE  | Virtual File Date stamp, (CCYYMMDD)   | V 9(8)  |
   |  41 | NERPTIME  | Virtual File Time stamp, (HHMMSScccc) | V 9(10) |
   |  51 | NERPDEST  | Destination                           | V X(25) |
   |  76 | NERPORIG  | Originator                            | V X(25) |
   | 101 | NERPCREA  | Creator of NERP                       | V X(25) |
   | 126 | NERPREAS  | Reason code                           | F 9(2)  |
   | 128 | NERPREASL | Reason text length                    | V 9(3)  |
   | 131 | NERPREAST | Reason text                           | V T(n)  |
   |     | NERPHSHL  | Virtual File hash length              | V U(2)  |
   |     | NERPHSH   | Virtual File hash                     | V U(n)  |
   |     | NERPSIGL  | NERP signature length                 | V U(2)  |
   |     | NERPSIG   | NERP signature                        | V U(n)  |
   o-------------------------------------------------------------------o

   NERPCMD   Command Code                                      Character

      Value: 'N'  NERP Command identifier.

   NERPDSN   Virtual File Dataset Name                        String(26)

             Dataset name of the Virtual File being transferred,
             assigned by bilateral agreement.

             No general structure is defined for this attribute.

             See Virtual Files - Identification (Section 1.5.2)

   NERPRSV1  Reserved                                          String(6)

             This field is reserved for future use.

   NERPDATE  Virtual File Date stamp                          Numeric(8)

     Format: 'CCYYMMDD'  8 decimal digits representing the century,
             year, month, and day, respectively.

             Date stamp assigned by the Virtual File's Originator
             indicating when the file was made available for
             transmission.

             See Virtual Files - Identification (Section 1.5.2)

   NERPTIME  Virtual File Time stamp                         Numeric(10)

     Format: 'HHMMSScccc'  10 decimal digits representing hours,
             minutes, seconds, and a counter (0001-9999), which gives
             higher resolution.

             Time stamp assigned by the Virtual File's Originator
             indicating when the file was made available for
             transmission.

             See Virtual Files - Identification (Section 1.5.2)

   NERPDEST  Destination                                      String(25)

     Format: See Identification Code (Section 5.4)

             Originator of the Virtual File.

             This is the location that created the data for
             transmission.

   NERPORIG  Originator                                       String(25)

     Format: See Identification Code (Section 5.4)

             The Final Recipient of the Virtual File.

             This is the location that will look into the Virtual File
             content and perform mapping functions.

   NERPCREA  Creator of the NERP                              String(25)

     Format: See Identification Code (Section 5.4)

             It is the location that created the NERP.

   NERPREAS  Reason code                                      Numeric(2)

             This attribute will specify why transmission cannot proceed
             or why processing of the file failed.

             "SFNA(RETRY=N)" below should be interpreted as "EFNA or
             SFNA(RETRY=N)" where appropriate.

      Value  '03'  ESID received with reason code '03'
                    (user code not known)
             '04'  ESID received with reason code '04'
                    (invalid password)
             '09'  ESID received with reason code '99'
                    (unspecified reason)
             '11'  SFNA(RETRY=N) received with reason code '01'
                    (invalid file name)
             '12'  SFNA(RETRY=N) received with reason code '02'
                    (invalid destination)
             '13'  SFNA(RETRY=N) received with reason code '03'
                    (invalid origin)
             '14'  SFNA(RETRY=N) received with reason code '04'
                    (invalid storage record format)
             '15'  SFNA(RETRY=N) received with reason code '05'
                    (maximum record length not supported)
             '16'  SFNA(RETRY=N) received with reason code '06'
                    (file size too big)
             '20'  SFNA(RETRY=N) received with reason code '10'
                    (invalid record count)
             '21'  SFNA(RETRY=N) received with reason code '11'
                    (invalid byte count)
             '22'  SFNA(RETRY=N) received with reason code '12'
                    (access method failure)

             '23'  SFNA(RETRY=N) received with reason code '13'
                    (duplicate file)
             '24'  SFNA(RETRY=N) received with reason code '14'
                    (file direction refused)
             '25'  SFNA(RETRY=N) received with reason code '15'
                    (cipher suite not supported)
             '26'  SFNA(RETRY=N) received with reason code '16'
                    (encrypted file not allowed)
             '27'  SFNA(RETRY=N) received with reason code '17'
                    (unencrypted file not allowed)
             '28'  SFNA(RETRY=N) received with reason code '18'
                    (compression not allowed)
             '29'  SFNA(RETRY=N) received with reason code '19'
                    (signed file not allowed)
             '30'  SFNA(RETRY=N) received with reason code '20'
                    (unsigned file not allowed)
             '31'  File signature not valid.
             '32'  File decompression failed.
             '33'  File decryption failed.
             '34'  File processing failed.
             '35'  Not delivered to recipient.
             '36'  Not acknowledged by recipient.
             '50'  Transmission stopped by the operator.
             '90'  File size incompatible with recipient's
                    protocol version.
             '99'  Unspecified reason.

   NERPREASL Reason Text Length                              Numeric(3)

             Length in octets of the field NERPREAST.

             0 indicates that no NERPREAST field follows.

   NERPREAST Reason Text                                     [UTF-8](n)

             Reason why transmission cannot proceed in plain text.

             It is encoded using [UTF-8].

             Maximum length of the encoded reason is 999 octets.

             No general structure is defined for this attribute.

   NERPHSHL  Virtual File hash length                          Binary(2)

             Length in octets of the field NERPHSH.

             A binary value of 0 indicates that no hash is present.
             This is always the case if the NERP is not signed.

   NERPHSH   Virtual File hash                                 Binary(n)

             Hash of the Virtual File being transmitted.

             The algorithm used is determined by the bilaterally agreed
             cipher suite specified in the SFIDCIPH.

   NERPSIGL  NERP Signature length                             Binary(2)

             0 indicates that this NERP has not been signed.

             Any other value indicates the length of NERPSIG in octets
             and indicates that this NERP has been signed.

   NERPSIG   NERP Signature                                    Binary(n)

             Contains the [CMS] enveloped signature of the NERP.

             Signature = Sign{NERPDSN
                              NERPDATE
                              NERPTIME
                              NERPDEST
                              NERPORIG
                              NERPCREA
                              NERPHSH}

             Each field is taken in its entirety, including any padding.
             The envelope must contain the original data, not just the
             signature.

             The [CMS] content type used is SignedData.

             The encapsulated content type used is id-data.

             It is an application issue to validate the signature with
             the contents of the NERP.

5.3.15. RTR - Ready To Receive

   o-------------------------------------------------------------------o
   |       RTR         Ready To Receive                                |
   |                                                                   |
   |       Start File Phase         Initiator <---- Responder          |
   |       End File Phase           Initiator <---- Responder          |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | RTRCMD    | RTR Command, 'P'                      | F X(1)  |
   o-------------------------------------------------------------------o

   RTRCMD    Command Code                                      Character

      Value: 'P'  RTR Command identifier.

5.3.16. SECD - Security Change Direction

   o-------------------------------------------------------------------o
   |       SECD        Security Change Direction                       |
   |                                                                   |
   |       Start Session Phase     Initiator <---> Responder           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SECDCMD   | SECD Command, 'J'                     | F X(1)  |
   o-------------------------------------------------------------------o

   SECDCMD   Command Code                                      Character

      Value: 'J'  SECD Command identifier.

5.3.17. AUCH - Authentication Challenge

   o-------------------------------------------------------------------o
   |       AUCH        Authentication Challenge                        |
   |                                                                   |
   |       Start Session Phase     Initiator <---> Responder           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | AUCHCMD   | AUCH Command, 'A'                     | F X(1)  |
   |   1 | AUCHCHLL  | Challenge Length                      | V U(2)  |
   |   3 | AUCHCHAL  | Challenge                             | V U(n)  |
   o-------------------------------------------------------------------o

   AUCHCMD   Command Code                                      Character

      Value: 'A'  AUCH Command identifier.

   AUCHCHLL  Challenge length                                  Binary(2)

             Indicates the length of AUCHCHAL in octets.

             The length is expressed as an unsigned binary number using
             network byte order.

   AUCHCHAL  Challenge                                         Binary(n)

             A [CMS] encrypted 20-byte random number uniquely generated
             each time an AUCH is sent.

   NOTE:

   Any encryption algorithm that is available through a defined cipher
   suite (Section 10.2) may be used.  See Section 10.1 regarding the
   choice of a cipher suite.

5.3.18. AURP - Authentication Response

   o-------------------------------------------------------------------o
   |       AURP        Authentication Response                         |
   |                                                                   |
   |       Start Session Phase     Initiator <---> Responder           |
   |-------------------------------------------------------------------|
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | AURPCMD   | AURP Command, 'S'                     | F X(1)  |
   |   1 | AURPRSP   | Response                              | V U(20) |
   o-------------------------------------------------------------------o

   AURPCMD   Command Code                                      Character

      Value: 'S'  AURP Command identifier.

   AURPRSP   Response                                         Binary(20)

             Contains the decrypted challenge (AUCHCHAL).

   IMPORTANT:

   It is an application implementation issue to validate a received AURP
   to ensure that the response matches the challenge.  This validation
   is extremely important to ensure that a party is correctly
   authenticated.

5.4. Identification Code

   The Initiator (sender) and Responder (receiver) participating in an
   ODETTE-FTP session are uniquely identified by an Identification Code
   based on [ISO-6523], Structure for the Identification of
   Organisations (SIO).  The locations are considered to be adjacent for
   the duration of the transmission.

   The SIO has the following format.

   o-------------------------------------------------------------------o
   | Pos | Field     | Description                           | Format  |
   |-----+-----------+---------------------------------------+---------|
   |   0 | SIOOID    | ODETTE Identifier                     | F X(1)  |
   |   1 | SIOICD    | International Code Designator         | V 9(4)  |
   |   5 | SIOORG    | Organisation Code                     | V X(14) |
   |  19 | SIOCSA    | Computer Subaddress                   | V X(6)  |
   o-------------------------------------------------------------------o

   SIOOID    ODETTE Identifier                                 Character

      Value: 'O' Indicates ODETTE assigned Organisation Identifier.
                 Other values may be used for non-ODETTE codes.

   SIOICD    International Code Designator                     String(4)

             A code forming part of the Organisation Identifier.

   SIOORG    Organisation Code                                String(14)

             A code forming part of the Organisation Identifier.  This
             field may contain the letters A to Z, the digits 0 to 9,
             and space and hyphen characters.

   SIOCSA    Computer Subaddress                               String(6)

             A locally assigned address that uniquely identifies a
             system within an organisation (defined by an Organisation
             Identifier).

6. File Services

6.1. Overview

   ODETTE-FTP provides services for compressing, encrypting, and signing
   files.  These services should generally be performed off line,
   outside of the ODETTE-FTP communications session for performance
   reasons, although this is not a strict requirement.

   ODETTE-FTP requires that the following steps must be performed in
   this exact sequence, although any of steps 2, 3, or 4 may be omitted.
   Step 1 is required only if any of steps 2, 3, or 4 are performed:

   1. Insert record length indicators (V format files only; see Section
      6.5)
   2. Sign
   3. Compress
   4. Encrypt

   The cipher suite for the encryption and signing algorithms is
   assigned by bilateral agreement.

   Secured and/or compressed files must be enveloped.  The envelope
   contains additional information about the service used that is
   necessary for a receiving party to fully process the file.

   The [CMS] content types used are:

   EnvelopedData  - Indicates encrypted data
   CompressedData - Indicates compressed data
   SignedData     - Indicates signed content
   Data           - Indicates unstructured data

   For signed or encrypted data, the encapsulated content type
   (eContentType field) is id-data.

6.2. File Signing

   Files that are to be signed are enveloped according to the file
   enveloping format (SFIDENV).  Generally, this will be as a [CMS]
   package.

   A file may be signed more than once to ease the changeover between
   old and new certificates.

   It is recommended that the envelope does not contain the public
   certificate of the signer.  Where files are sent to the same
   recipient continuously, it would serve no benefit to repeatedly send
   the same certificate.  Both the original file data and signature are
   stored within the [CMS] package.

6.3. File Encryption

   Files that are to be encrypted are enveloped according to the file
   enveloping format (SFIDENV).  Generally, this will be as a [CMS]
   package.

   It is recommended that encryption should be performed before the
   ODETTE-FTP session starts because a large file takes a long time to
   encrypt and could cause session time outs, even on high-performance
   machines.

   Likewise, decryption of the file should occur outside of the session.
   However, an application may choose to allow in-session encryption and
   decryption for very small files.

6.4. File Compression

   Files that are to be compressed are enveloped according to the file
   enveloping format (SFIDENV).  Generally, this will be as a [CMS]
   package using the [CMS-Compression] data type, which uses the [ZLIB]
   compression algorithm by default.

   Unlike the buffer compression method, this method operates on a whole
   file.  Because of the increased levels of compression, file level
   compression essentially deprecates the older buffer compression
   inside ODETTE-FTP.  The buffer compression is kept for backwards
   compatibility.

6.5. V Format Files - Record Lengths

   A file that has been signed, compressed, and/or encrypted will have
   lost its record structure, so ODETTE-FTP will not be able to insert
   the End of Record Flag in subrecord headers in Data Exchange Buffers.
   To preserve the record structure, V format files must have record
   headers inserted into them prior to signing, compression, or
   encryption.  These 2-byte binary numbers, in network byte order,
   indicate the length of each record, allowing the receiving system,
   where appropriate, to recreate the files complete with the original
   variable-length records.  Note that the header bytes hold the number
   of data bytes in the record and don't include themselves.

This is only applicable to V format files, which themselves are
typically only of concern for mainframes.

7. ODETTE-FTP Data Exchange Buffer

7.1. Overview

   Virtual Files are transmitted by mapping the Virtual File records
   into Data Exchange Buffers, the maximum length of which was
   negotiated between the ODETTE-FTP entities via the Start Session
   (SSID) commands exchanged during the Start Session phase of the
   protocol.

   Virtual File records may be of arbitrary length.  A simple
   compression scheme is defined for strings of repeated characters.

   An example of the use of the Data Exchange Buffer can be found in
   Appendix A.

7.2. Data Exchange Buffer Format

   For transmission of Virtual File records, data is divided into
   subrecords, each of which is preceded by a 1-octet Subrecord Header.

   The Data Exchange Buffer is made up of the initial Command Character
   followed by pairs of Subrecord Headers and subrecords, as follows.

      o--------------------------------------------------------
      | C | H |           | H |           | H |           |   /
      | M | D | SUBRECORD | D | SUBRECORD | D | SUBRECORD |  /_
      | D | R |           | R |           | R |           |   /
      o-------------------------------------------------------

   CMD

      The Data Exchange Buffer Command Character, 'D'.

   HDR

      A 1-octet Subrecord Header defined as follows:

          0   1   2   3   4   5   6   7
        o-------------------------------o
        | E | C |                       |
        | o | F | C O U N T             |
        | R |   |                       |
        o-------------------------------o

      Bits

       0     End of Record Flag

             Set to indicate that the next subrecord is the last
             subrecord of the current record.

             Unstructured files are transmitted as a single record; in
             this case, the flag acts as an end-of-file marker.

       1     Compression Flag

             Set to indicate that the next subrecord is compressed.

      2-7    Subrecord Count

             The number of octets in the Virtual File represented by the
             next subrecord expressed as a binary value.

             For uncompressed data, this is simply the length of the
             subrecord.

             For compressed data, this is the number of times that the
             single octet in the following subrecord must be inserted in
             the Virtual File.

             As 6 bits are available, the next subrecord may represent
             between 0 and 63 octets of the Virtual File.

7.3. Buffer Filling Rules

   A Data Exchange Buffer may be any length up to the value negotiated
   in the Start Session exchange.

   Virtual File records may be concatenated within one Data Exchange
   Buffer or split across a number of buffers.

   A subrecord is never split between two Exchange Buffers.  If the
   remaining space in the current Exchange Buffer is insufficient to
   contain the next 'complete' subrecord, one of the following
   strategies should be used:

   1. Truncate the Exchange Buffer, and put the complete subrecord
      (preceded by its header octet) in a new Exchange Buffer.

   2. Split the subrecord into two, filling the remainder of the
      Exchange Buffer with the first new subrecord and starting a new
      Exchange Buffer with the second.

   A record of length zero may appear anywhere in the Exchange Buffer.

   A subrecord of length zero may appear anywhere in the record and/or
   the Exchange Buffer.

8. Stream Transmission Buffer

8.1. Introduction

   To utilise the TCP stream, a Stream Transmission Buffer (STB) is
   created by adding a Stream Transmission Header (STH) to the start of
   all Command and Data Exchange Buffers before they are passed to the
   TCP transport service.  This allows the receiving ODETTE-FTP to
   recover the original Exchange Buffers.

   Note: The Stream Transmission Buffer is not used when using ODETTE-
         FTP over an X.25 network.

   This is because ODETTE-FTP can rely on the fact that the Network
   Service will preserve the sequence and boundaries of data units
   transmitted through the network and that the Network Service will
   pass the length of the data unit to the receiving ODETTE-FTP.  TCP
   offers a stream-based connection that does not provide these
   functions.

   The Stream Transmission Buffer is composed of an STH and an OEB.

   o-----+-----------------+-----+--------------------+-----+------
   | STH | OEB             | STH |  OEB               | STH | OEB/
   o-----+-----------------+-----+--------------------+-----+----

      STH - Stream Transmission Header
      OEB - ODETTE-FTP Exchange Buffer

8.2. Stream Transmission Header Format

   The Stream Transmission Header is shown below.  The fields are
   transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Version| Flags | Length                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version

      Value: 0001 (binary)

             Stream Transmission Header version number.

   Flags

      Value: 0000 (binary)

             Reserved for future use.

   Length

      Range: 5 - 100003 (decimal)

      The length of the Stream Transmission Buffer (STH+OEB).

      The smallest STB is 5 octets consisting of a 4-octet header
      followed by a 1-octet Exchange Buffer such as a Change Direction
      (CD) command.

      The maximum Exchange Buffer length that can be negotiated is 99999
      octets (Section 5.3.2) giving an STB length of 100003.

      The length is expressed as a binary number in network byte order.

   It is expected that implementations of this protocol will follow the
   Internet robustness principle of being conservative in what is sent
   and liberal in what is accepted.

9. Protocol State Machine

9.1. ODETTE-FTP State Machine

   The operation of an ODETTE-FTP entity is formally defined by the
   State Machine presented below.  There are five State and Transition
   tables, and for each table additional information is given in the
   associated Predicate and Action lists.

   The response of an ODETTE-FTP entity to the receipt of an event is
   defined by a Transition table entry indexed by the Event/State
   intersection within the appropriate state table.

   Each Transition table entry defines the actions taken, events
   generated, and new state entered.  Predicates may be used within a
   table entry to select the correct response on the basis of local
   information held by the entity.

   A Transition table contains the following fields:

     Index (I)   State transition index.

     Predicate   A list of predicates used to select between different
                 possible transitions.  The predicates are defined in
                 the Predicate and Action lists.

     Actions     A list of actions taken by the entity.  The actions are
                 defined in the Predicate and Action lists.

     Events      Output events generated by the entity.

     Next State  The new state of the entity.

9.2. Error Handling

   The receipt of an event in a given state may be invalid for three
   reasons.

   1.  The case is impossible by design of the state automata, denoted
       'X' in the state tables.  For example, a timer that has not been
       set cannot run out.

   2.  The event is the result of an error in the Network Service
       implementation, also denoted 'X' in the state tables.  The
       Network Service implementation is considered to be correct.

   3.  For all other cases, the event is considered to be a User Error,
       denoted "U" in the state tables.

   The state tables define the conditions under which a User event is
   valid, thus preventing the generation of a protocol error by the
   ODETTE-FTP entity as a result of a User Monitor error.  The reaction
   of the entity to such errors is undefined and regarded as a local
   implementation issue.

   The state tables also allow protocol errors due to the receipt of
   invalid Exchange Buffers, to be detected.  In such cases, the
   reaction of the entity to the error is defined.

9.3. States

   The Command Mode is strictly a half-duplex flip-flop mode.

   A_NC_ONLY   Responder, Network Connection opened

               The Responder has sent its Ready Message (SSRM) and is
               waiting for Start Session (SSID) from the Initiator.

   A_WF_CONRS  Responder Waiting for F_CONNECT_RS

               The Responder has received the Initiator's Start Session
               (SSID) and is waiting for a response (F_CONNECT_RS) from
               its User Monitor.

   CDSTWFCD    CD_RQ stored in WF_CD state

               Since the User Monitor doesn't see the WF_CD state, it
               may send a Change Direction request (F_CD_RQ) before the
               ODETTE-FTP receives a Change Direction (CD) command.

   CLIP        Close Input Pending

               The Listener has received an End File (EFID) command and
               is waiting for the Close File response (F_CLOSE_FILE_RS)
               from its User Monitor.

   CLOP        Close Out Pending

               The Speaker has sent an End File (EFID) command and is
               waiting for an End File Answer (EFPA or EFNA).

   ERSTWFCD    End to End Response stored in WF_CD state

               Since the User Monitor doesn't see the WF_CD state, it
               may send F_EERP_RQ, before ODETTE-FTP receives a Change
               Direction (CD) command.

   IDLE        Connection IDLE

   IDLELI      Idle Listener

   IDLELICD    Idle Listener, F_CD_RQ Received

               The ODETTE-FTP entity has become the Listener after
               receiving a Change Direction request (F_CD_RQ) from the
               User Monitor.  The receipt of an End Session (ESID) is
               valid in this state.

   IDLESP      Idle Speaker

   IDLESPCD    Idle Speaker, F_CD_IND Sent

               The ODETTE-FTP entity has sent a Change Direction
               indication (F_CD_IND) to the User Monitor.  A Change
               Direction request (F_CD_RQ) is invalid in this state.

   I_WF_NC     Initiator Waiting for Network Connection

               The Initiator has requested a new network connection and
               is waiting for a Connection confirmation (N_CON_CF) from
               the Network Service.

   I_WF_RM     Initiator Waiting for Ready Message

               Before sending Start Session (SSID), the Initiator must
               wait for a Ready Message (SSRM) from the Responder.

   I_WF_SSID   Initiator Waiting for SSID

               The Initiator has sent a Start Session (SSID) command and
               is waiting for Start Session from the Responder.

   NRSTWFCD    Negative End Response stored in WF_CD state

               Since the User Monitor doesn't see the WF_CD state, it
               may send F_NERP_RQ, before ODETTE-FTP receives a Change
               Direction (CD) command.

   OPI         Open Input (Data Transfer Phase)

               The Listener is waiting for the Speaker to send a Data
               Exchange Buffer.

   OPIP        Open Input Pending

               The Listener has received a Start File (SFID) command and
               is waiting for the Start File response (F_START_FILE_RS)
               from its User Monitor.

   OPO         Open Out (Data Transfer Phase)

               The Speaker has received a Start File Positive Answer
               (SFPA) and is waiting for a Data (F_DATA_RQ) or Close
               File (F_CLOSE_FILE) request from its User Monitor.

   OPOP        Open Out Pending

               The Speaker has sent a Start File (SFID) command and is
               waiting for a Start File Answer (SFPA or SFNA).

   OPOWFC      Open Out Wait for Credit

               The Speaker is waiting for a Set Credit (CDT) command
               before sending further Data Exchange buffers.

   RTRP        Ready to Receive (RTR) Pending

               The Listener has received an EERP or a NERP and is
               waiting for the Ready to Receive response (F_RTR_RS) from
               its User Monitor.

   SFSTWFCD    Start File Request stored in WF_CD state.

               Since the User Monitor doesn't see the WF_CD state, it
               may send a Start File request (F_START_FILE_RQ) before
               the ODETTE-FTP receives a Change Direction (CD) command.

   WF_CD       Wait for Change Direction

               The Listener wishes to become the Speaker and is waiting
               for a Change Direction (CD) command after sending an End
               File Positive Answer (EFPA) requesting change direction.

   WF_RTR      Wait for Ready To Receive

               The Speaker has sent an End to End Response (EERP) or a
               Negative End Response (NERP) command and must wait for
               Ready To Receive (RTR) from the Listener.

   WF_NDISC    Wait for N_DISC_IND

               ODETTE-FTP has sent an End Session (ESID) command and is
               waiting for a Disconnection indication (N_DISC_IND) from
               the Network Service.

   WF_SECD     Wait for Security Change Direction

               The Speaker is expecting a Security Change Direction
               (SECD) from the Listener.

   WF_AUCH     Wait for Authentication Challenge

               The Speaker has sent a Security Change Direction (SECD)
               command and must wait for Authentication Challenge (AUCH)
               from the Listener.

   WF_AURP     Wait for Authentication Response

               The Speaker has sent an Authentication Challenge (AUCH)
               command and must wait for Authentication Response (AURP)
               from the Listener.

9.4. Input Events

   User Monitor Input Events (Section 3)

     F_DATA_RQ   F_CONNECT_RQ   F_START_FILE_RQ      F_CLOSE_FILE_RQ
     F_EERP_RQ   F_CONNECT_RS   F_START_FILE_RS(+)   F_CLOSE_FILE_RS(+)
     F_NERP_RQ   F_ABORT_RQ     F_START_FILE_RS(-)   F_CLOSE_FILE_RS(-)
     F_CD_RQ     F_RELEASE_RQ   F_RTR_RS

   Network Input Events (Section 2.2)

      N_CON_IND   N_CON_CF   N_DATA_IND   N_DISC_IND   N_RST_IND

   Peer ODETTE-FTP Input Events (Section 4)

      SSID   SFID   SFPA   SFNA   EFID   EFPA   EFNA
      DATA   ESID   EERP   RTR    CD     CDT    SSRM
      NERP   SECD   AUCH   AURP

   Internal Input Events

      TIME-OUT - Internal ODETTE-FTP timer expires.

   Input event parameters are denoted I.Event-name.Parameter-name within
   the state table action and predicate lists.  Their value can be
   examined but not changed by the ODETTE-FTP entity.

9.5. Output Events

   User Monitor Output Events (Section 3)

     F_DATA_IND  F_CONNECT_IND  F_START_FILE_IND     F_CLOSE_FILE_IND
     F_EERP_IND  F_CONNECT_CF   F_START_FILE_CF(+)   F_CLOSE_FILE_CF(+)
     F_CD_IND    F_ABORT_IND    F_START_FILE_CF(-)   F_CLOSE_FILE_CF(-)
     F_NERP_IND  F_RELEASE_IND  F_DATA_CF            F_RTR_CF

   Network Output Events (Section 2.2)

      N_CON_RQ   N_CON_RS   N_DATA_RQ   N_DISC_RQ

   Peer ODETTE-FTP Output Events (Section 4)

      SSID   SFID   SFPA   SFNA   EFID   EFPA   EFNA
      DATA   ESID   EERP   RTR    CD     CDT    SSRM
      NERP   SECD   AUCH   AURP

   Output event parameters are denoted O.Event-name.Parameter-name
   within the state table action and predicate lists.  Their values can
   be examined and changed by the ODETTE-FTP entity.

9.6. Local Variables

   The following variables are maintained by the ODETTE-FTP entity to
   assist the operation of the protocol.  They are denoted V.Variable-
   name within the state table action and predicate lists.  Their value
   can be examined and changed by the ODETTE-FTP entity.  The initial
   value of each variable is undefined.

   Variable       Type       Comments
   ---------------------------------------------------------------------
   Buf-size       Integer    Negotiated Data Exchange Buffer size.
   Called-addr    Address    Used to build O.F_CONNECT_IND.Called-addr
   Calling-addr   Address    To build O.F_CONNECT_IND.Calling-addr
   Compression    Yes/No     Compression in use as agreed.
   Credit_L       Integer    Listener's credit counter.
   Credit_S       Integer    Speaker's credit counter.
   Id             String     Used to build O.SSID.Id
   Mode                      Sender-only, Receiver-only, Both.
   Pswd           String     Password, used to build O.SSID.Pswd
   Req-buf        Primitive  Input event (F_XXX_RQ) stored in WF_CD
                              state.
   Restart        Yes/No     Restart in used as agreed.
   Restart-pos    Integer    Used only during file opening.
   Window         Integer    The credit value negotiated for the
                              session.
   Caller         Yes/No     This entity initiated the ODETTE-FTP
                              session.
   Authentication Yes/No     Secure authentication in use as agreed
   Challenge      Binary     Random challenge
   ---------------------------------------------------------------------

9.7. Local Constants

   The following constants define the capabilities of a given ODETTE-FTP
   entity.  They are denoted C.Constant-name within the state table
   action and predicate lists.  Their value can be examined but not
   changed by the ODETTE-FTP entity.

   Constant         Value               Comments
   ---------------------------------------------------------------------
   Cap-compression  Yes/No              Compression supported?
   Cap-init         Initiator           Must be Initiator.
                    Responder           Must be Responder.
                    Both                Can be Initiator or Responder.
   Cap-mode         Sender-only         Must be sender.
                    Receiver-only       Must be receiver.
                    Both                Can be sender or receiver.
   Max-buf-size     127 < Int < 100000  Maximum Data Exchange Buffer
                                         size supported.
   Max-window       0 < Int < 1000      Local maximum credit value.
   Cap-restart      Yes/No              Restart supported?
   Cap-logic        0, 1, 2             0 = does not support special
                                            logic
                                        1 = supports special logic
                                        2 = needs special logic
   ---------------------------------------------------------------------

9.8. Session Connection State Table

9.8.1. State Table

   o----------------------------------------------------------o
   |   | Other States                                         |
   |   |--------------------------------------------------o   |
   |   | WF_SECD                                          |   |
   |   |----------------------------------------------o   |   |
   |   | WF_AURP                                      |   |   |
   |   |------------------------------------------o   |   |   |
   |   | WF_AUCH                                  |   |   |   |
   |   |--------------------------------------o   |   |   |   |
   | S | A_WF_CONRS                           |   |   |   |   |
   |   |----------------------------------o   |   |   |   |   |
   | T | A_NC_ONLY                        |   |   |   |   |   |
   |   |------------------------------o   |   |   |   |   |   |
   | A | I_WF_SSID                    |   |   |   |   |   |   |
   |   |--------------------------o   |   |   |   |   |   |   |
   | T | I_WF_RM                  |   |   |   |   |   |   |   |
   |   |----------------------o   |   |   |   |   |   |   |   |
   | E | I_WF_NC              |   |   |   |   |   |   |   |   |
   |   |------------------o   |   |   |   |   |   |   |   |   |
   |   | IDLE             |   |   |   |   |   |   |   |   |   |
   |==================o---+---+---+---+---+---+---+---+---+---|
   |   | F_CONNECT_RQ | A | X | X | X | X | X | X | X | X | X |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   | E | N_CON_CF     | X | C | X | X | X | X | X | X | X | X |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   | V | SSRM         | X | X | H | X | X | X | L | L | L | X |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   | E | SSID         | X | X | X | D | E | F | L | L | L | F |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   | N | N_CON_IND    | B | X | X | X | X | X | X | X | X | X |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   | T | F_CONNECT_RS | X | U | U | U | U | G | X | X | X | U |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   |   | ESID         | X | X | X | F | X | X | F | F | F | X |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   |   | AUCH         | X | X | U | U | X | X | I | L | L | U |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   |   | AURP         | X | X | U | U | X | X | L | K | L | U |
   |   |--------------+---+---+---+---+---+---+---+---+---+---|
   |   | SECD         | X | X | U | U | X | X | L | L | J | U |
   o----------------------------------------------------------o

9.8.2. Transition Table

    I | Predicate    Actions     Output Events            Next State
   ===o=============================================================
    A | P1:                      F_ABORT_IND              IDLE
      | !P1:            1,2      N_CON_RQ                 I_WF_NC
   ---+-------------------------------------------------------------
    B | P3:                      N_DISC_RQ                IDLE
      | !P3:            2        N_CON_RS
      |                          SSRM                     A_NC_ONLY
   ---+-------------------------------------------------------------
    C |                 4,2                               I_WF_RM
   ---+-------------------------------------------------------------
    D | P2 & P8 & P11:  4,2,5    SECD                     WF_AUCH
      | P2 & P8 & !P11: 4,2,5    F_CONNECT_CF             IDLESP
      | P2 & !P8:       4,2      ESID(R=12)
      |                          F_ABORT_IND(R,AO=L)      WF_NDISC
      | else:           4,2      ESID(R=10)
      |                          F_ABORT_IND(R,AO=L)      WF_NDISC
   ---+-------------------------------------------------------------
    E | P4:             4        N_DISC_RQ                IDLE
      | !P4:            4,2      F_CONNECT_IND            A_WF_CONRS
   ---+-------------------------------------------------------------
    F |                 4        F_ABORT_IND
      |                          N_DISC_RQ                IDLE
   ---+-------------------------------------------------------------
    G | P2 &  P9 & P10: 4,2,5    SSID                     WF_SECD
      | P2 & !P9 & P10: 4,2,5    SSID                     IDLELI
      | !P10:           4,2      ESID(R=12)
      |                          F_ABORT_IND(R,AO=L)      WF_NDISC
      | else:           4,2      ESID(R=10)
      |                          F_ABORT_IND(R,AO=L)      WF_NDISC
   ---+-------------------------------------------------------------
    H |                 4,2,3    SSID                     I_WF_SSID
   ---+-------------------------------------------------------------
    I | P5:             4,2      AURP                     WF_SECD
      | !P5:            4,2      AURP                     IDLELI
   ---+-------------------------------------------------------------
    J |                 4,2      AUCH                     WF_AURP
   ---+-------------------------------------------------------------
    K | P6:             4,2      F_CONNECT_CF             IDLESP
      | P7:             4,2      SECD                     WF_AUCH
      | else:           4,2      ESID(R=11)
      |                          F_ABORT_IND(R,AO=L)      WF_NDISC
   ---+-------------------------------------------------------------
    L |                 4,2      ESID(R=02)
      |                          F_ABORT_IND(R,AO=L)      WF_NDISC
   ---+-------------------------------------------------------------

9.8.3. Predicates and Actions

   Predicate P1:  (No resources available) OR
                  (C.Cap-init = Responder) OR
                  (C.Cap-mode = Sender-only AND
                     I.F_CONNECT_RQ.Mode = Receiver-only) OR
                  (C.Cap-mode = Receiver-only AND
                     I.F_CONNECT_RQ.Mode = Sender-only)

   Predicate P2:  SSID negotiation is successful
                  (for these, Buf-size, Restart, Compression, Mode,
                   Special logic, and Window, compare the inbound SSID
                   with the local constants to set the local variables.
                   Any incompatibilities result in failure of the
                   negotiation.)

   Predicate P3:  C.Cap-init = Initiator

   Predicate P4:  Mode in SSID incompatible with C.Cap-mode

   Predicate P5:  V.Caller = Yes

   Predicate P6:  (V.Caller = Yes) AND (AURP.Signature verifies with
                   V.Challenge)

   Predicate P7:  (V.Caller = No)  AND (AURP.Signature verifies with
                   V.Challenge)

   Predicate P8:  V.Authentication = I.SSID.Authentication

   Predicate P9:  I.F_CONNECT_RS.Authentication = Yes

  Predicate P10:  O.F_CONNECT_IND.Authentication =
                   I.F_CONNECT_RS.Authentication

  Predicate P11:  V.Authentication = Yes

       Action 1:  Set V.Mode from (C.Cap-mode, I.F_CONNECT_RQ.Mode)
                  Set V.Pswd, V.Id, V.Restart, and
                   V.Authentication from I.F_CONNECT_RQ
                  Set V.Buf-size = C.Max-buf-size
                  Set V.Compression = C.Cap-compression
                  Set V.Caller = Yes
                  Build O.N_CON_RQ

       Action 2:  Start inactivity timer

       Action 3:  Set parameters in O.SSID = from local variables

       Action 4:  Stop timer

       Action 5:  Set V.Mode, V.Restart, V.Compression, V.Buf-size,
                      V.Window, V.Authentication = from SSID

       Action 6:  Set V.Challenge = A random number unique to the
                   session

9.9. Error and Abort State Table

9.9.1. State Table

   o--------------------------------------o
   |   | Other States                     |
   | S |------------------------------o   |
   | T | WF_NDISC                     |   |
   | A |--------------------------o   |   |
   | T | I_WF_NC                  |   |   |
   | E |----------------------o   |   |   |
   |   | IDLE                 |   |   |   |
   |======================o---+---+---+---|
   |   | TIME-OUT         | X | X | A | B |
   |   |------------------+---+---+---+---|
   | E | F_ABORT_RQ       | X | A | X | C |
   | V |------------------+---+---+---+---|
   | E | N_RST_IND        | X | X | A | D |
   | N |------------------+---+---+---+---|
   | T | N_DISC_IND       | X | E | F | G |
   |   |------------------+---+---+---+---|
   |   | Invalid Buffer   | X | X | H | I |
   o--------------------------------------o

9.9.2. Transition Table

    I | Predicate    Actions     Output Events             Next State
   ===o=================================================================
    A |                          N_DISC_RQ                 IDLE
   ---+-----------------------------------------------------------------
    B |                          F_ABORT_IND
      |                          N_DISC_RQ                 IDLE
   ---+-----------------------------------------------------------------
    C |              1           N_DISC_RQ                 IDLE
   ---+-----------------------------------------------------------------
    D |              1           N_DISC_RQ
      |                          F_ABORT_IND               IDLE
   ---+-----------------------------------------------------------------
    E |                          F_ABORT_IND               IDLE
   ---+-----------------------------------------------------------------
    F |              1                                     IDLE
   ---+-----------------------------------------------------------------
    G |              1           F_ABORT_IND               IDLE
   ---+-----------------------------------------------------------------
    H |                                                    WF_NDISC
   ---+-----------------------------------------------------------------
    I |              1,2         ESID(R=01)
      |                          F_ABORT_IND(R,AO=L)       WF_NDISC
   ---------------------------------------------------------------------

9.9.3. Predicates and Actions

Action 1: Stop inactivity timer

Action 2: Start inactivity timer

9.10. Speaker State Table 1

9.10.1. State Table

   The following abbreviations are used in the Speaker state table.

      F_REL_RQ(Ok)   -  F_RELEASE_RQ Reason = Normal
      F_REL_RQ(Err)  -  F_RELEASE_RQ Reason = Error

  o--------------------------------------------------------------------o
  | | Other States                                                     |
  | |--------------------------------------------------------------o   |
  | | WF_NDISC                                                     |   |
  | |----------------------------------------------------------o   |   |
  | | OPOWFC                                                   |   |   |

  | |------------------------------------------------------o   |   |   |
  | | OPO                                                  |   |   |   |
  |S|--------------------------------------------------o   |   |   |   |
  | | OPOP                                             |   |   |   |   |
  |T|----------------------------------------------o   |   |   |   |   |
  | | CDSTWFCD                                     |   |   |   |   |   |
  |A|------------------------------------------o   |   |   |   |   |   |
  | | SFSTWFCD                                 |   |   |   |   |   |   |
  |T|--------------------------------------o   |   |   |   |   |   |   |
  | | NRSTWFCD                             |   |   |   |   |   |   |   |
  |E|----------------------------------o   |   |   |   |   |   |   |   |
  | | ERSTWFCD                         |   |   |   |   |   |   |   |   |
  | |------------------------------o   |   |   |   |   |   |   |   |   |
  | | WF_CD                        |   |   |   |   |   |   |   |   |   |
  | |--------------------------o   |   |   |   |   |   |   |   |   |   |
  | | WF_RTR                   |   |   |   |   |   |   |   |   |   |   |
  | |----------------------o   |   |   |   |   |   |   |   |   |   |   |
  | | IDLESPCD             |   |   |   |   |   |   |   |   |   |   |   |
  | |------------------o   |   |   |   |   |   |   |   |   |   |   |   |
  | | IDLESP           |   |   |   |   |   |   |   |   |   |   |   |   |
  |=+==============o---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | F_EERP_RQ    | A | A | W | F | W | W | U | U | U | U | U | U | U |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | F_NERP_RQ    | Y | Y | W | Z | W | W | U | U | U | U | U | U | U |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | F_START_     | B | B | W | G | W | W | U | U | U | U | U | X | U |
  | |   FILE_RQ    |   |   |   |   |   |   |   |   |   |   |   |   |   |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | SFPA         | C | C | C | C | C | C | C | C | K | C | C | S | C |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  |E| SFNA         | C | C | C | C | C | C | C | C | L | C | C | S | C |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  |V| CD           | C | C | C | H | R | Z1| I | J | C | C | C | S | C |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  |E| F_DATA_RQ    | U | U | U | U | U | U | U | U | U | M | U | S | U |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  |N| CDT          | C | C | C | C | C | C | C | C | C | P | O | S | C |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  |T| F_CD_RQ      | D | U | W | T | W | W | U | U | U | U | U | X | U |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | F_REL_RQ(Ok) | U | E | U | U | U | U | U | U | U | U | U | X | U |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | F_REL_RQ(Err)| Q | Q | Q | Q | Q | Q | Q | Q | Q | Q | Q | S | Q |
  | |--------------+---+---+---+---+---+---+---+---+---+---+---+---+---|
  | | RTR          | C | C | N | C | C | C | C | C | C | C | C | S | C |
  o--------------------------------------------------------------------o

9.10.2. Transition Table

    I | Predicate    Actions      Output Events              Next State
   ===o=================================================================
    A | P5:          1,2,3,18     EERP                       WF_RTR
      | !P5:         1,2,3        EERP                       WF_RTR
   ---+-----------------------------------------------------------------
    B | P1:                                                  UE
      | !P1:         1,2,5        SFID                       OPOP
   ---+-----------------------------------------------------------------
    C |              1,2          ESID(R=02)
      |                           F_ABORT_IND(R,AO=L)        WF_NDISC
   ---+-----------------------------------------------------------------
    D |              1,2          CD                         IDLELICD
   ---+-----------------------------------------------------------------
    E |              1,2          ESID(R=00)                 WF_NDISC
   ---+-----------------------------------------------------------------
    F |              4                                       ERSTWFCD
   ---+-----------------------------------------------------------------
    G | P1:                                                  UE
      | !P1:         6                                       SFSTWFCD
   ---+-----------------------------------------------------------------
    H |              1,2                                     IDLESP
   ---+-----------------------------------------------------------------
    I |              1,2,10       SFID                       OPOP
   ---+-----------------------------------------------------------------
    J |              1,2          CD                         IDLELICD
   ---+-----------------------------------------------------------------
    K | P2:          1,2          ESID(R=02)
      |                           F_ABORT_IND(R,AO=L)        WF_NDISC
      | !P2:         1,2,7,12     F_START_FILE_CF(+)         OPO
   ---+-----------------------------------------------------------------
    L |              1,2,8        F_START_FILE_CF(-)         IDLESP
   ---+-----------------------------------------------------------------
    M | P3:          1,2,11,13    DATA                       OPOWFC
      | !P3:         1,2,11,13    DATA
      |                           F_DATA_CF                  OPO
   ---+-----------------------------------------------------------------
    N |                           F_RTR_CF                   IDLESP
   ---+-----------------------------------------------------------------
    O |              12           F_DATA_CF                  OPO
   ---+-----------------------------------------------------------------
    P | Protocol     1,2          ESID(R=02)
      | Error                     F_ABORT_IND(R,AO=L)        WF_NDISC
   ---+-----------------------------------------------------------------
    Q |              1,2          ESID(R)                    WF_NDISC
   ---+-----------------------------------------------------------------
                                                           Continued -->

    I | Predicate    Actions      Output Events              Next State
   ===o=================================================================
    R |              1,2,9        EERP                       WF_RTR
   ---+-----------------------------------------------------------------
    S |                                                      WF_NDISC
   ---+-----------------------------------------------------------------
    T |                                                      CDSTWFCD
   ---+-----------------------------------------------------------------
    U |                           User Error                 UE
   ---+-----------------------------------------------------------------
    W |                           User Error - Note 1        UE
   ---+-----------------------------------------------------------------
    X |                           Error
   ---+-----------------------------------------------------------------
    Y |  P4 &  P5:   1,2,15,18    NERP                       WF_RTR
      | !P4 & !P5:   1,2,15,14    NERP                       WF_RTR
      |  P4 & !P5:   1,2,15       NERP                       WF_RTR
      | !P4 &  P5:   1,2,15,14,18 NERP                       WF_RTR
   ---+-----------------------------------------------------------------
    Z |              16                                      NRSTWFCD
   ---------------------------------------------------------------------
    Z1| P4:          1,2,17       NERP                       WF_RTR
      | !P4:         1,2,17,14    NERP                       WF_RTR
   ---------------------------------------------------------------------

9.10.3. Predicates and Actions

   Predicate P1:  (I.F_START_FILE_RQ.Restart-pos > 0 AND V.Restart = No)
                  OR (V.Mode = Receiver-only)

           Note:  Restart requested and not supported for this session.

   Predicate P2:  I.SFPA.Restart-pos > V.Restart-pos

           Note:  Protocol error due to the restart position in the SFPA
                  acknowledgement being greater than the position
                  requested in the SFID request.

   Predicate P3:  V.Credit_S - 1 = 0

           Note:  Speaker's Credit is exhausted.

   Predicate P4:  No special logic is in use

   Predicate P5:  Signed EERP/NERP requested

       Action 1:  Stop inactivity timer

       Action 2:  Start inactivity timer

       Action 3:  Build an EERP from F_EERP_RQ

       Action 4:  Store F_EERP_RQ in V.Req-buf

       Action 5:  Build SFID from F_START_FILE_RQ
                  V.Restart-pos = I.F_START_FILE_RQ.Restart-pos

       Action 6:  Store F_START_FILE_RQ in V.Req-buf

       Action 7:  Build F_START_FILE_CF(+) from I.SFPA

       Action 8:  Build F_START_FILE_CF(-) from I.SFNA

       Action 9:  Build EERP from F_EERP_RQ stored in V.Req-buf

       Action 10: Build SFID from F_START_FILE_RQ stored in V.Req-buf
                  Set V.Restart-pos

       Action 11: Build Exchange Buffer

       Action 12: V.Credit_S = V.Window

       Action 13: V.Credit_S = V.Credit_S - 1

       Action 14: Activate CRC-calculus function.  Wrap Exchange buffer
                  in special logic

       Action 15: Build a NERP from F_NERP_RQ

       Action 16: Store F_NERP_RQ in V.Req-buf

       Action 17: Build NERP from F_NERP_RQ stored in V.Req-buf

       Action 18: Sign the contents of NERP/EERP

          Note 1: Whether to accept this "Request/Event" while in this
                  state is a matter of local implementation.  The ODETTE
                  state tables are based on the assumption that this
                  event cannot occur in this state and is considered to
                  be a user error (UE).

9.11. Speaker State Table 2

9.11.1. State Table

   o---------------------------------o
   | S | CLOP                        |
   | T |-------------------------o   |
   | A | OPOWFC                  |   |
   | T |---------------------o   |   |
   | E | OPO                 |   |   |
   |=====================o---+---+---|
   | E | F_CLOSE_FILE_RQ | A | E | U |
   | V |-----------------+---+---+---|
   | E | EFPA            | B | B | C |
   | N |-----------------+---+---+---|
   | T | EFNA            | B | B | D |
   o---------------------------------o

9.11.2. Transition Table

    I | Predicate    Actions     Output Events              Next State
   ===o=================================================================
    A |              1,2,5,7     EFID                       CLOP
   ---+-----------------------------------------------------------------
    B |              1,2         ESID(R=02)
      |                          F_ABORT_IND(R,AO=L)        WF_NDISC
   ---+-----------------------------------------------------------------
    C | P1:          1,2,3       F_CLOSE_FILE_CF(+,SP=No)
      |                          CD                         IDLELI
      | !P1:         1,2,4       F_CLOSE_FILE_CF(+,SP=Yes)  IDLESP
   ---+-----------------------------------------------------------------
    D |              1,2,6       F_CLOSE_FILE_CF(-)         IDLESP
   ---+-----------------------------------------------------------------
    E |                          See Note 1
   ---+-----------------------------------------------------------------
    U |                          User Error                 UE
   ---------------------------------------------------------------------

9.11.3. Predicates and Actions

   Predicate P1: (I.EFPA.CD-Request = Yes)

   Predicate P2:  No special logic is in use

       Action 1:  Stop inactivity timer

       Action 2:  Start inactivity timer

       Action 3:  O.F_CLOSE_FILE_CF(+).Speaker = No

       Action 4:  O.F_CLOSE_FILE_CF(+).Speaker = Yes

       Action 5:  Build EFID from F_CLOSE_FILE_RQ

       Action 6:  Build F_CLOSE_FILE_CF(-) from EFNA

       Action 7:  Set V.Credit_S = 0

       Action 8:  Wrap Exchange buffer in special logic

         Note 1:  In order to respect the "half duplex" property of
                  ODETTE-FTP, it is forbidden to send EFID while in the
                  OPOWFC state.  EFID can be sent only in the OPO state.

                  The ODETTE-FTP implementation must avoid sending EFID
                  (or receiving F_CLOSE_FILE_RQ) while in the OPOWFC
                  state.

9.12. Listener State Table

9.12.1. State Table

   o---------------------------------------------o
   |   | RTRP                                    |
   |   |-------------------------------------o   |
   |   | CLIP                                |   |
   |   |---------------------------------o   |   |
   |   | OPI                             |   |   |
   | S |-----------------------------o   |   |   |
   | T | OPIP                        |   |   |   |
   | A |-------------------------o   |   |   |   |
   | T | IDLELICD                |   |   |   |   |
   | E |---------------------o   |   |   |   |   |
   |   | IDLELI              |   |   |   |   |   |
   |=====================o---+---+---+---+---+---+
   |   | SFID            | A | A | B | B | B | B |
   |   |-----------------+---+---+---+---+---+---+
   | E | DATA            | B | B | B | I | B | B |
   | V |-----------------+---+---+---+---+---+---+
   | E | EFID            | B | B | B | J | B | B |
   | N |-----------------+---+---+---+---+---+---+
   | T | F_START_FILE_RS | U | U | H | U | U | U |
   |   |-----------------+---+---+---+---+---+---+
   |   | F_CLOSE_FILE_RS | U | U | U | U | K | U |
   |   |-----------------+---+---+---+---+---+---+
   |   | CD              | C | B | B | B | B | B |
   |   |-----------------+---+---+---+---+---+---+
   |   | ESID R=Normal   | D | F | D | D | D | D |
   |   |-----------------+---+---+---+---+---+---+
   |   | ESID R=Error    | D | D | D | D | D | D |
   |   |-----------------+---+---+---+---+---+---+
   |   | EERP            | E | E | B | B | B | B |
   |   |-----------------+---+---+---+---+---+---+
   |   | NERP            | L | L | B | B | B | B |
   |   |-----------------+---+---+---+---+---+---+
   |   | F_RTR_RS        | U | U | U | U | U | M |
   o---------------------------------------------o

9.12.2. Transition Table

    I | Predicate          Actions    Output Events           Next State
   ===o=================================================================
    A | P1:                1,2        ESID(R=02)
      |                               F_ABORT_IND(R,AO=L)       WF_NDISC
      | !P1:               1,2,3      F_START_FILE_IND          OPIP
   ---+-----------------------------------------------------------------
    B |                    1,2        ESID(R=02)
      |                               F_ABORT_IND(R,AO=L)       WF_NDISC
   ---+-----------------------------------------------------------------
    C |                    1,2        F_CD_IND                  IDLESPCD
   ---+-----------------------------------------------------------------
    D |                    1          F_ABORT_IND(Received
      |                               ESID Reason,AO=D)
      |                               N_DISC_RQ                 IDLE
   ---+-----------------------------------------------------------------
    E |                    1,2,4      F_EERP_IND                RTRP
   ---+-----------------------------------------------------------------
    F |                    1          F_RELEASE_IND
      |                               N_DISC_RQ                 IDLE
   ---+-----------------------------------------------------------------
    H |  P4:                          User Error                UE
      |  P2 & !P4 & !P5:   1,2,8      SFPA                      OPI
      | !P2 & !P4 & !P5:   1,2        SFNA                      IDLELI
      |  P2 & !P4 &  P5:   1,2,5,8    SFPA                      OPI
      | !P2 & !P4 &  P5:   1,2,5      SFNA                      IDLELI
   ---+-----------------------------------------------------------------
    I | P6:                1,2        ESID(R=02)
      |                               F_ABORT_IND(R,A0=L)       WF_NDISC
      | !P5 & !P6 & !P7:   1,2,7      F_DATA_IND (See Note 1)   OPI
      | !P5 & !P6 &  P7:   1,2,8      F_DATA_IND
      |                               CDT (See Note 1)          OPI
      |  P5 & !P6 &  P8:   1,2        ESID(R=07)
      |                               F_ABORT_IND(R,A0=L)       WF_NDISC
      |  P5 & !P6 & !P7 :  1,2,6,7    F_DATA_IND (See Note 1)   OPI
      |   & !P8
      |  P5 & !P6 &  P7 :  1,2,5,6,8  F_DATA_IND                OPI
      |   & !P8                       CDT (See Note 1)
   ---+-----------------------------------------------------------------
    J |                    1,2        F_CLOSE_FILE_IND          CLIP
   ---+-----------------------------------------------------------------
    K |  P2 &  P3 & !P5:   1,2        EFPA(CD-Req)              WF_CD
      |  P2 & !P3 & !P5:   1,2        EFPA(no CD)               IDLELI
      | !P2 & !P5:         1,2        EFNA                      IDLELI
      |  P2 & !P3 & P5:    1,2,5      EFPA(no CD)               IDLELI
      | !P2 &  P5:         1,2,5      EFNA                      IDLELI
      |  P2 &  P3 & P5:    1,2,5      EFPA(CD-Req)              WF_CD

   ---+-----------------------------------------------------------------
    L |                    1,2,10     F_NERP_IND                RTRP
   ---+-----------------------------------------------------------------
    M |                    1,2        RTR                       IDLELI
   ---+-----------------------------------------------------------------
    U |                               User Error                UE
   ---------------------------------------------------------------------

9.12.3. Predicates and Actions

   Predicate P1:  (I.SFID.Restart-pos > 0 AND V.Restart = No) OR (V.Mode
                  = Sender-only)

           Note:  Invalid Start File command.

   Predicate P2:  Positive Response

   Predicate P3:  I.F_CLOSE_FILE_RS(+).Speaker = Yes

   Predicate P4:  I.F_START_FILE_RS(+).Restart-pos > V.Restart

   Predicate P5:  Special logic is used

   Predicate P6:  V.Credit_L - 1 < 0

           Note:  Protocol Error because the Speaker has exceeded its
                  available transmission credit.

   Predicate P7:  V.Credit_L - 1 = 0

           Note:  The Speaker's credit must be reset before it can send
                  further Data Exchange Buffers.

   Predicate P8:  The calculus of the received CRC indicates an error

       Action 1:  Stop inactivity timer

       Action 2:  Start inactivity timer

       Action 3:  Build F_START_FILE_IND from I.SFID
                  V.Restart-pos = I.SFID.Restart-pos

       Action 4:  Build F_EERP_IND from I.EERP

       Action 5:  Add special logic header to the command to be sent to
                  the Speaker

       Action 6:  Suppress the special logic header from the data buffer
                  before giving it to the user

       Action 7:  V.Credit_L = V.Credit_L - 1

       Action 8:  V.Credit_L = V.Window

       Action 10: Build F_NERP_IND from I.NERP

         Note 1:  Flow control in case of reception.

                  The ODETTE-FTP Listener must periodically send new
                  credit to the Speaker.  The timing of this operation
                  will depend on:

                  1. The User Monitor's capacity to receive data.
                  2. The number of buffers available to ODETTE-FTP.
                  3. The Speaker's available credit, which must be
                     equal to zero.

9.13. Example

   Consider an ODETTE-FTP entity that has sent a Start File (SFID)
   command and entered the Open Out Pending (OPOP) state.  Its response
   on receiving a Positive Answer (SFPA) is documented in Speaker State
   Table 1, which shows that transition 'K' should be applied and is
   interpreted as follows:

      if (I.SFPA.Restart-pos > V.Restart-pos) then
      begin                                       // invalid restart
         Actions:   Stop inactivity timer,        // reset timer
                    Start inactivity timer;
         Output:    ESID(R=02),                   // to peer ODETTE-FTP
                    F_ABORT_IND(R,AO=L);          // to User Monitor
         New State: WF_NDISC;
      end
      else begin
         Actions:   Stop inactivity timer,        // reset timer
                    Start inactivity timer;
                    Build F_START_FILE_CF(+) from I.SFPA
                    V.Credit_S = V.Window         // initialise credit
         Output:    F_START_FILE_CF(+);           // to User Monitor
         New State: OPO;
      end

   ODETTE-FTP checks the restart position in the received Start File
   Positive Answer (SFPA) command.  If it is invalid, it aborts the
   session by sending an End Session (ESID) command to its peer and an
   Abort indication (F_ABORT_IND) to its User Monitor.  If the restart
   position is valid, a Start File confirmation (F_START_FILE_CF) is
   built and sent to the User Monitor, the credit window is initialised,
   and the Open Out (OPO) state is entered.

10. Miscellaneous

10.1. Algorithm Choice

The choice of algorithms to use for security or compression between
partners is for bilateral agreement outside of ODETTE-FTP.

10.2. Cryptographic Algorithms

   The algorithms for symmetric and asymmetric cryptography and hashing
   are represented by a coded value, the cipher suite:

       Cipher Suite  Symmetric          Asymmetric    Hashing
       ------------  -----------------  ------------  -------

01 3DES_EDE_CBC_3KEY RSA_PKCS1_15 SHA-1
02 AES_256_CBC RSA_PKCS1_15 SHA-1

   Support of all cipher suites listed here is mandatory.

   The certificates used must be [X.509] certificates.

   TripleDES is using Cipher Block Chaining (CBC) mode for added
   security and uses the Encryption Decryption Encryption (EDE) process
   with 3 different 64-bit keys.

   RSA padding is as defined in [PKCS#1].

   AES is using a 256-bit key in CBC mode.

   An extended list of optional cipher suites may be used (Section
   10.3), but there is no guarantee that two communicating ODETTE-FTP
   entities would both support these optional cipher suites.

10.3. Protocol Extensions

The algorithms and file enveloping formats available in ODETTE-FTP
may be extended outside of this document.

   An up-to-date list of cipher suite values for use in ODETTE-FTP is
   maintained by ODETTE International, and published on their website at
   www.odette.org.

10.4. Certificate Services

   Certificates and certificate revocation lists may be exchanged as
   [CMS] enveloped files.  It is therefore valid to exchange a [CMS]
   file that is neither encrypted, compressed, nor signed.  It is an
   application implementation issue to determine the correct course of
   action on receipt of such a file.

11. Security Considerations

   ODETTE-FTP security requires the use of [X.509] certificates.  If no
   security options are agreed for use, the send and receive passwords
   are sent in plain text.  Whilst this is acceptable over X.25 and ISDN
   networks, this is a risky practice over insecure public networks such
   as the Internet.

   All, some, or none of the security options available in ODETTE-FTP
   may be used.  No recommendations for the use of these options are
   provided in this specification.  Whilst use of the highest-strength
   encryption algorithms may seem admirable, there is often a
   performance tradeoff to be made, and signing all files and
   acknowledgements has potential legal implications that should be
   considered.

   It should be noted that whilst the security measures ensure that an
   ODETTE-FTP partner is authenticated, it does not necessarily mean
   that the partner is authorised.  Having proven the identity of a
   partner, it is an application issue to decide whether that partner is
   allowed to connect or exchange files.

   Extracted from [RFC3850]:

   "When processing certificates, there are many situations where the
   processing might fail.  Because the processing may be done by a user
   agent, a security gateway, or other program, there is no single way
   to handle such failures.  Just because the methods to handle the
   failures have not been listed, however, the reader should not assume
   that they are not important.  The opposite is true: if a certificate
   is not provably valid and associated with the message, the processing
   software should take immediate and noticeable steps to inform the end
   user about it.

   Some of the many situations in which signature and certificate
   checking might fail include the following:

     No certificate chain leads to a trusted CA
     No ability to check the Certificate Revocation List (CRL) for a
      certificate
     An invalid CRL was received
     The CRL being checked is expired
     The certificate is expired
     The certificate has been revoked

   There are certainly other instances where a certificate may be
   invalid, and it is the responsibility of the processing software to
   check them all thoroughly, and to decide what to do if the check
   fails.  See RFC 3280 for additional information on certificate path
   validation."

   The push / pull nature of ODETTE-FTP means that a party can make an
   outbound connection from behind a firewall to another party and
   exchange files in both directions.  There is no need for both
   partners to open ports on their firewalls to allow incoming
   connections; only one party needs to allow incoming connections.

   See Section 1.7 for a discussion of the benefits of session security
   [TLS] versus file security.

Appendix A. Virtual File Mapping Example

   This example demonstrates the mapping of a Virtual File into a
   sequence of ODETTE-FTP Data Exchange Buffers.

   Each line in this extract from 'The Rime of the Ancient Mariner' by
   Coleridge [RIME] is separated by CR-LFs in a file that is being
   transmitted as a T format file.

             It is an ancient Mariner,
             And he stoppeth one of three.
             "By thy long grey beard and glittering eye,
             Now wherefore stopp'st thou me?

             "The Bridegroom's doors are opened wide,
             And I am next of kin;
             The guests are met, the feast is set:
             May'st hear the merry din."

             He holds him with his skinny hand,
             "There was a ship," quoth he.
             "Hold off! unhand me, grey-beard loon!"
             Eftsoons his hand dropt he.

             He holds him with his glittering eye--
             The Wedding-Guest stood still,
             And listens like a three years; child:
             The Mariner hath his will.

             The Wedding-Guest sat on a stone:
             He cannot chuse but hear;
             And thus spake on that ancient man,
             The bright-eyed Mariner.

             The ship was cheered, the harbour cleared,
             Merrily did we drop
             Below the kirk, below the hill,
             Below the light-house top.

   The Exchange Buffers below were built from the above.  The top line
   of each represents the ASCII code, while the two lines below give the
   hexadecimal value.

   Note that:

     . The "D" at the beginning of each Exchange Buffer is the command
       code.

     . The "?" preceding each subrecord is the header octet (see the
       hexadecimal value).

     Exchange Buffer 1

     D?It is an ancient Mariner,..And he stoppeth one of three..."By
     4347267266266666672467666720046626627767767626662662767662002472
     4F9409301E01E395E40D129E52CDA1E4085034F005480FE50F6048255EDA2290

     t?hy long grey beard and glittering eye,..Now wherefore stopp'st
     7367266662676726667626662666776766626762004672766766676277677277
     4F890CFE70725902512401E407C944529E70595CDAEF70785256F25034F00734

      ?thou me?...."The Bridegroom's doors are opened wide,..And I am
     2376672663000025662476666766627266677267626766662766620046624266
     0F48F50D5FDADA248502294572FFD7304FF2301250F05E5407945CDA1E40901D

      ?next of kin;..The guests are met, the feast is set:..May'st he
     2366772662666300566267677726762667227662666772672767300467277266
     0FE5840F60B9EBDA485075534301250D54C04850651340930354ADAD19734085

     a?r the merry din."....He holds him with his skinny hand,.."Ther
     6372766266777266622000046266667266627676266727666672666620025667
     1F204850D5229049EE2DADA8508FC43089D07948089303B9EE9081E4CDA24852

     e? was a ship," quoth he..."Hold off! unhand me, grey-beard loon
     6327672627667222776762662002466626662276666626622676726667626666
     5F07130103890C2015F48085EDA28FC40F66105E81E40D5C07259D251240CFFE

     !?"..Eftsoons his hand dropt he.....He holds him with his glitte
     2320046776667266726666267677266200004626666726662767626672666776
     1F2DA5643FFE30893081E4042F04085EDADA8508FC43089D07948089307C9445

     r?ing eye--..The Wedding-Guest stood still,..And listens like a
     7366626762200566256666662476772776662776662004662667766726666262
     2F9E70595DDDA485075449E7D75534034FF40349CCCDA1E40C9345E30C9B5010

     t?hree years; child:..The Mariner hath his will.....The Wedding-
     7367662766773266666300566246766672667626672766620000566256666662
     4F8255095123B0389C4ADA4850D129E52081480893079CCEDADA485075449E7D

     G?uest sat on a stone:..He cannot chuse but hear;..And thus spak
     4376772767266262776663004626666672667762677266673004662767727766
     7F553403140FE01034FE5ADA85031EEF4038535025408512BDA1E4048530301B

     e? on that ancient man,..The bright-eyed Mariner.....The ship wa
     6326627667266666672666200566267666726766246766672000056627667276
     5F0FE0481401E395E40D1ECDA4850229784D59540D129E52EDADA48503890071

     s? cheered, the harbour cleared,..Merrily did we drop..Below the
     7326666766227662667667726666766200467766726662762676700466672766
     3F03855254C048508122F5203C51254CDAD5229C90494075042F0DA25CF70485

      .kirk, below the hill,..Below the light-house top...
     2B667622666672766266662004666727662666672667762767200
     03B92BC025CF70485089CCCDA25CF704850C9784D8F53504F0EDA

Appendix B. ISO 646 Character Subset

   o-----------------------------------------------------------------o
   |            |   7| 0   | 0   | 0   | 0   | 1   | 1   | 1   | 1   |
   |            | B -+-----+-----+-----+-----+-----+-----+-----+-----|
   |            | I 6|  0  |  0  |  1  |  1  |  0  |  0  |  1  |  1  |
   |            | T -+-----+-----+-----+-----+-----+-----+-----+-----|
   |            |   5|   0 |   1 |   0 |   1 |   0 |   1 |   0 |   1 |
   |            |----+-----+-----+-----+-----+-----+-----+-----+-----|
   |            |    |     |     |     |     |     |     |     |     |
   |            |    |     |     |     |     |     |     |     |     |
   |------------|    |  0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |
   |    BIT     |    |     |     |     |     |     |     |     |     |
   | 4  3  2  1 |    |     |     |     |     |     |     |     |     |
   |============o====o=====+=====+=====+=====+=====+=====+=====+=====|
   | 0  0  0  0 |  0 |     |     | SP  |  0  |     |  P  |     |     |
   |------------|----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  0  0  1 |  1 |     |     |     |  1  |  A  |  Q  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  0  1  0 |  2 |     |     |     |  2  |  B  |  R  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  0  1  1 |  3 |     |     |     |  3  |  C  |  S  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  1  0  0 |  4 |     |     |     |  4  |  D  |  T  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  1  0  1 |  5 |     |     |     |  5  |  E  |  U  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  1  1  0 |  6 |     |     |  &  |  6  |  F  |  V  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 0  1  1  1 |  7 |     |     |     |  7  |  G  |  W  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  0  0  0 |  8 |     |     |  (  |  8  |  H  |  X  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  0  0  1 |  9 |     |     |  )  |  9  |  I  |  Y  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  0  1  0 | 10 |     |     |     |     |  J  |  Z  |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  0  1  1 | 11 |     |     |     |     |  K  |     |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  1  0  0 | 12 |     |     |     |     |  L  |     |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  1  0  1 | 13 |     |     |  -  |     |  M  |     |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  1  1  0 | 14 |     |     |  .  |     |  N  |     |     |     |
   |------------+----|-----+-----+-----+-----+-----+-----+-----+-----|
   | 1  1  1  1 | 15 |     |     |  /  |     |  O  |     |     |     |
   o-----------------------------------------------------------------o

Appendix C. X.25 Specific Information

   The International Organization for Standardization (ISO) Open Systems
   Interconnection (OSI) model is the basis for ODETTE-FTP.

   ODETTE-FTP covers levels 4 to 7, and originally CCITT X.25 was the
   only recommended telecommunication protocol for OSI's layers 1, 2, 3.

    ISO Reference Model:

          +------------------------------+  <====  File Service
          | Level-7  FTP    application  |
          |------------------------------|
          | Level-6  FTP    presentation |
          |------------------------------|
          | Level-5  FTP    session      |
          |------------------------------|
          | Level-4  FTP    transport    |
          |------------------------------|  <====  Network Service
          | Level-3         X.25         |
          |------------------------------|
          | Level-2         X.25         |
          |------------------------------|
          | Level-1         X.25         |
          +------------------------------+

C.1. X.25 Addressing Restrictions

   When an X.25 call is made over a PSDN, the Network User Address (NUA)
   of the destination must be specified in order that the PTT may route
   the call.  The call placed is directed to the termination equipment
   upon the user's premises.

   It is possible to provide extra information in the Call Request
   Packet in addition to the mandatory NUA required by the PTT.

   This extra information may be of 2 kinds:

    (a) A subaddress:

       It is simply an extension to the address and it is put into the
       called address field of the Call Request Packet.  This
       information (Address + Subaddress) is taken from the destination
       address field of the F_CONNECT_RQ; therefore, from the user's
       point of view, there is no distinction between the main address
       and subaddress parts.

    (b) User data:

       There is no standard for user data.  Moreover, there is no
       information in the F_CONNECT_RQ from which the ODETTE-entity may
       derive user data to be put in the N_CONNECT_RQ; therefore, user
       data shall not be used.

C.2. Special Logic

   The SSID field SSIDSPEC specifies whether special logic must be
   applied (Y (yes) or N (no)) to the Data Exchange Buffer before the
   ODETTE-FTP moves the data into the NSDU (Network Service Data Unit)
   and passes control to the Network Service.

C.2.1. When Special Logic Is Not To Be Used

This logic is not applied to SSRM and SSID commands.

C.2.2. The Need for "Enveloping" Exchange Buffers

   The "special-logic" parameter was created in order to allow the use
   of ODETTE-FTP over asynchronous links.  The "special-logic" could be
   needed to enable terminals to access an X.25 network via an
   asynchronous entry (through a PAD: Packet Assembly / Disassembly).
   The "special-logic" is not needed in case of a whole X.25 connection.
   This "special-logic" realises a CRC function in order to detect
   errors due to the asynchronous medium.

   Negotiation of the "special-logic" parameter in the SSID command is
   as follows:

           SSID                                       SSID
           -----------------------------------------------

           special-logic=yes --------------------->

              <------------------------------------   special-logic=yes
                                                  or
              <------------------------------------   special-logic=no

           special-logic=no ---------------------->

              <------------------------------------   special-logic=no

   This logic is activated when the "special-logic" parameter in the
   SSID specifies Y (yes).

   Special logic processing, when activated, will function within level
   4 of the OSI model.

          +------------------------------+  <====  File Service
          | Level-7  FTP    application  |
          |------------------------------|
          | Level-6  FTP    presentation |
          |------------------------------|
          | Level-5  FTP    session      |
          |------------------------------|
          | Level-4  FTP    transport    |
          |  SPECIAL LOGIC PROCESSING    |
          |------------------------------|  <====  Network Service
          | Level-3         X.25         |
          |------------------------------|
          | Level-2         X.25         |
          |------------------------------|
          | Level-1         X.25         |
          +------------------------------+

C.2.3. Responsibilities of Special Logic

   When transmitting an Exchange Buffer and special logic is active,
   layer 4 will wrap the Exchange Buffer in synchronization and
   delineation characters, then protect the data integrity by means of a
   block checksum (BCS).  When receiving an Exchange Buffer and special
   logic is active, layer 4 will remove such things as synchronization
   and delineation characters, etc., before passing the Exchange Buffer
   to the higher layers.

C.2.4. Extended Exchange Buffer Format

   Each envelope has a 1-byte header prefixed to it, and a 2-byte
   checksum appended to the end.  The checksum is derived in a manner
   specified in the ISO DIS 8073 TRANSPORT LAYER documentation.

   The layout of the data buffer will be structured as follows:

   +------------------------------------------------------------------+
   | S | B |                                                  | B | C |
   | T | S |         COMPLETE EXCHANGE BUFFER (CEB)           | C | / |
   | X | N |                                                  | S | R |
   +------------------------------------------------------------------+
     A   A                                                      A   A
     |   |                                                      |   |
     |   +-------------  Block sequence number                  |   |
     |                                                          |   |
     +-----------------  Synchronization character              |   |
                                                                |   |
                         Block checksum  -----------------------+   |
                                                                    |
                         Delineation character  --------------------+

   The envelope is initialised with an STX and the checksum variables
   are set to 0.  The leading STX is not protected by the checksum
   calculation but is explicitly protected by a character compare at the
   receiver's end.  The Exchange Buffer is processed character by
   character.  As each character is removed from the Exchange Buffer, it
   is put through the checksum calculation and then, prior to its
   insertion in the envelope, it is put through the Shift-out
   transparency logic, which will result in either one or two characters
   being inserted.  When the contents of the Exchange Buffer have been
   entirely processed, then the checksum variables are brought up to
   date by inserting two X'00's through the checksum calculator and the
   two resultant checksum characters forwarded to the Shift-out
   transparency logic for insertion into the envelope.  Finally, a
   carriage return (CR) is appended to the envelope.  The segment is now
   ready for transmission to line.

   Upon receipt of a valid envelope that has the correct sequence
   number, the host should increment his sequence number register ready
   for the next transmission.

   The receiver will initialise his receiving buffer area upon receipt
   of an STX character, place the STX at the beginning of the buffer,
   and reset checksum variables.  All subsequent characters are
   processed using Shift-out logic before they are inserted into the
   buffer, at which point they will NOT be processed by the checksum
   calculator, although the character following the Shift-out (after
   subtracting X'20') will be.  The checksum characters themselves will
   be processed by the checksum calculator by virtue of the design of
   the checksum algorithm.

C.2.5. Error recovery

C.2.5.1. Mechanism

   The error correction scheme is implemented by the definition of three
   timers and the use of an ASCII NAK (Negative Acknowledgement)
   character followed by a C/R.  The <NAK><C/R> will flow between the
   two session partners, but only as a consequence of previous bad data.

   A user of the error recovery correcting extension must always work
   with a credit value of 1.  This can be forced upon any session
   partner at SSID negotiation.  The effect will be to force a simple
   half-duplex flip-flop protocol.

   Upon receipt of a bad block, send <NAK><C/R> to the session partner.

   Upon receipt of a <NAK><C/R>, a session partner should retransmit the
   last block in its entirety.

C.2.5.2. Timers

   The majority of error conditions will be detected by a bad BCS
   sequence.  However, certain conditions cannot be so detected.  For
   example, a corrupt C/R will mean that the receiver will not know that
   the end of a block has been reached.  No matter how long he waits, no
   more data will come from the sender.  Thus, a timer is the only way
   to detect this type of corruption.  There are three timers needed to
   detect all possible malignant conditions of this type.

     T1 - Exchange Buffer Time Out (Inactivity or Response)
     T2 - Inter Character Time Out
     T3 - Data Carrier Detect Loss Time Out

   The three timers are in addition to the timer defined in the original
   protocol.

    TIMER T1 - RESPONSE TIME OUT (DEFAULT = 45 SECONDS):

     Used to detect a high-level block Time Out, e.g., the Time Out
     between an SFID and its associated SFPA or SFNA response.

      Started - It is started after the last character of an exchange
                buffer has been sent to the line.

      Stopped - It is stopped when an STX has been received.

      Expiry  - Retransmit the whole block again, until such time as the
                retry limit has been reached.

    TIMER T2 - INTER CHARACTER TIME OUT (DEFAULT = 7 SECONDS):

     Used to detect errors in the reception of individual characters.

      Started - For an asynchronous entity, it is started upon receipt
                of each character while in synchronisation mode.  For an
                X.25 entity, it is started after a received block that
                did not terminate an Exchange Buffer.

      Stopped - Upon receipt of the next character.

      Expiry  - Send a <NAK><C/R>, drop out of synchronised mode, and go
                back and listen to line.

    TIMER T3 - DATA CARRIER TEMPORARY LOSS (DEFAULT = 1 SECOND):

     Used by an asynchronous entity only and is used to detect a
     temporary carrier failure.

      Started - When DCD (Data Carrier Detect) is lost.

      Stopped - When DCD is regained.

      Expiry  - Disconnect the session.

C.2.5.3. Types of Error

   Data corruption when it occurs can be categorised in one of five
   ways:

   (1) CORRUPT STX (START OF TEXT)

    In this situation the STX is not seen and synchronisation is not
    achieved.  The terminating C/R is received out of synchronisation
    and hence the block is not seen by the receiver.  A <NAK><C/R> is
    transmitted to the sender to indicate this.  The sender should then
    retransmit the last block (each implementation will need to set a
    retry limit to be used for the number of consecutive times it
    attempts to retransmit a block -- a default limit of 5 is
    recommended).  All data received outside synchronisation (except
    <NAK><C/R>) are ignored.

        (A)                                    (B)

    Dropped Start of Text (STX)

          +-------------------------+
          |   | B |         | B | C |
     -----|   | S |  CEB    | C | / |----->  Not sync
          |   | N |         | S | R |
          +-------------------------+

                   +-------+
                   | N | C |
             <-----| A | / |-----            Not sync
                   | K | R |
                   +-------+

    Exchange Buffer Resent

          +-------------------------+
          | S | B |         | B | C |
     -----| T | S |  CEB    | C | / |----->  Sync
          | X | N |         | S | R |
          +-------------------------+

   (2) CORRUPT TERMINATION (C/R)

    This situation manifests itself as an extended period of
    synchronisation with no activity.  The T2 timer will detect this
    condition.

        (A)                                    (B)

    Corrupt Carriage Return

          +-------------------------+
          | S | B |         | B |   |
     -----| T | S |  CEB    | C |   |----->  No activity
          | X | N |         | S |   |
          +-------------------------+

                   +-------+
                   | N | C |                 T2
             <-----| A | / |-----            Timed out
                   | K | R |
                   +-------+

    Exchange Buffer Resent

          +-------------------------+
          | S | B |         | B | C |
     -----| T | S |  CEB    | C | / |----->  Sync
          | X | N |         | S | R |
          +-------------------------+

   (3) BAD DATA
   (4) BAD BCS (BLOCK CHECK SUM)

    In this situation, the receiver is unable to tell whether the error
    is bad data or bad BCS.  In either case, the response is to discard
    the Exchange Buffer and send a <NAK><C/R>.

        (A)                                    (B)

    Bad Data/BCS

          +-------------------------+
          | S | B |         | B | C |        Bad data
     -----| T | S |  "%!    | C | / |----->  detected
          | X | N |         | S | R |
          +-------------------------+

                   +-------+
                   | N | C |
             <-----| A | / |-----            Discard Block
                   | K | R |
                   +-------+

    Exchange Buffer Resent

          +-------------------------+
          | S | B |         | B | C |
     -----| T | S |  CEB    | C | / |----->  Data OK
          | X | N |         | S | R |
          +-------------------------+

   (5) BAD BLOCK SEQUENCE NUMBER (BSN)

    A circular sequential number (0 up to and including 9) is assigned
    to transmitted Exchange Buffers.  This is to aid detection of
    duplicate or out-of-sequence Exchange Buffers.  Once a duplicate
    block is detected, the Exchange Buffer in question is discarded.
    Once an out of sequence block is detected, this should result in a
    protocol violation.

    Example protocol sequence:

        (A)                                    (B)

    Exchange Buffer Being Sent

          +-------------------------+
          | S |   |         | B | C |        Expecting
     -----| T | 0 |  EERP   | C | / |----->  BSN=0
          | X |   |         | S | R |        Transmission
          +-------------------------+

    Exchange Buffer Being Sent

          +-------------------------+
          | S |   |         | B | C |        Response to
     <----| T | 0 |  RTR    | C | / |-----   Previous
          | X |   |         | S | R |        Block
          +-------------------------+

    Exchange Buffer Being Sent

          +-------------------------+        Expecting
          | S |   |         | B | C |        BSN=1 (Block
     -----| T | 1 |  SFID   | C | / |- // -> lost in
          | X |   |         | S | R |        Transmission)
          +-------------------------+        T1 Timed Out

    Exchange Buffer Being Sent

          +-------------------------+
          | S |   |         | B | C |        Send last
     <----| T | 0 |  RTR    | C | / |-----   Block
          | X |   |         | S | R |        again
          +-------------------------+

    Discard Block
    and start
    Timer T1

    T1 Timed Out

    Exchange Buffer Resent

          +-------------------------+
          | S |   |         | B | C |        Expecting
     -----| T | 1 |  SFID   | C | / |----->  BSN=1
          | X |   |         | S | R |        Block OK
          +-------------------------+

    Exchange Buffer Being Sent

          +-------------------------+
          | S |   |         | B | C |        Response
     <----| T | 1 |  SFPA   | C | / |-----   BSN=1
          | X |   |         | S | R |        Block OK
          +-------------------------+

    Exchange Buffer Being Sent

          +-------------------------+
          | S |   |         | B | C |
     -----| T | 2 |  DATA   | C | / |----->  Data OK
          | X |   |         | S | R |
          +-------------------------+

   Note: A credit value of 1 must be used to guarantee half-duplex
   flip-flop.

C.2.6. Sequence of Events for Special Logic Processing

   The following functions will be executed in sequence:

   1. Calculation of the Block Sequence Number (BSN):

      BSN is set to zero by SSID.  First block will be sent with value
      zero.  Value of BSN is increased by one for each data buffer to be
      transmitted.  When BSN value exceeds 9, counter will be reset to
      zero.

      Format: numeric/1 pos.

   2. Calculation of the Block Checksum (BCS):

      Calculation is done as specified in the ISO DIS 8073 TRANSPORT
      LAYER document.

      Format: binary/2 pos.

   3. Shift-out transparency  (See TRANSMIT/RECEIVE logic.)

      To avoid appearance of any control characters in the data stream,
      all the characters of the extended Exchange Buffer (with exception
      of the STX and carriage return characters enveloping the buffer)
      are put through a Shift-out logic, which result in a character
      being inserted (SO) and adding hex value '20' to the control
      character.

   4. The carriage return is inserted at the end of the data buffer.

   Note: After adding STX, BSN, BCS, CR, and SO-logic, the data buffer
         may exceed the Data Exchange Buffer size.

C.2.7. Checksum Creation Algorithm

   These follow the ISO DIS 8073 TRANSPORT LAYER standard.

    SYMBOLS:

     The following symbols are used:

     C0,C1   Variables used in the algorithm
     L       Length of the complete NSDU
     X       Value of the first octet of the checksum parameter
     Y       Value of the second octet of the checksum parameter

    ARITHMETIC CONVENTIONS:

     Addition is performed in one of the two following modes:

       a) modulo 255 arithmetic
       b) one's complement arithmetic in which if any of the variables
          has the value minus zero (i.e., 255) it shall be regarded as
          though if was plus zero (i.e., 0).

    ALGORITHM FOR GENERATING CHECKSUM PARAMETERS:

     . Set up the complete NSDU with the value of the checksum parameter
       field set to zero.

     . Initialise C0 and C1 to zero.

     . Process each octet sequentially from i=1 to L by

         a) adding the value of the octet to C0, then
         b) adding the value of C0 to C1.

     . Calculate X and Y such that

            X = C0 - C1
            Y = C1 - 2*C0

     . Place the values X and Y in the checksum bytes 1 and 2,
       respectively.

C.2.8. Algorithm for checking checksum parameters

     . Initialise parameters C0 and C1 to zero.

     . Process each octet of NSDU sequentially from i=1 to L by

         a) adding the value of the octet to C0, then
         b) adding the value of C0 to C1.

     . If, when all the octets have been processed, either or both C0
       and C1 does not have the value zero, then the checksum formulas
       have not been satisfied.

     Note that the nature of the algorithm is such that it is not
     necessary to compare explicitly the stored checksum bytes.

C.2.9. Shift-out Processing

       (Transparency for all control characters)

    TRANSMIT LOGIC  (values SO: X'0E' or X'8E')

     Buffer(1), ... , (n) is a character in the buffer to be sent.

     FOR i=1 to n                    /* for all octets of the buffer */

         IF    ((buffer(i)  &  X'7F')  <  X'20')

         THEN  output (SO)
               output (buffer(i)  +  X'20')

         ELSE  output (buffer(i))

     NEXT:

    RECEIVE  LOGIC  (values SO: X'0E' or X'8E')

     Buffer(1), ... , (n) is a character in the received buffer.

     drop = false
     FOR i=1 to n                    /* for all octets of the buffer */

         IF    drop = true

         THEN  output (buffer(i)  -  X'20')
               drop = false

         ELSE  IF    buffer(i) = (X'0D'  or  X'8D')
               THEN  Stop
               ELSE  IF    buffer(i) = SO
                     THEN  drop = true
                     ELSE  output (buffer(i))

     NEXT:

C.3. PAD Parameter Profile

   Before an (ODETTE-FTP) asynchronous entity --> Modem --> PAD -->
   (ODETTE-FTP) native X.25 link can be established, the target PAD
   parameters must be set such that correct communication is
   established.  It is strongly recommended that the PAD parameters are
   set by the X.25 entity.  CCITT recommendations X.3, X.28, and X.29
   define the PAD parameters and procedures for exchange of control
   information and user data between a PAD and a packet mode Data
   Terminal Equipment (DTE).

   Following is the Parameter list and values used to set the PAD for
   ODETTE-FTP communication.  For further detailed information see the
   specification for CCITT X.25, X.28, X.29 and X.3.

   No. Description                    Value  Meaning
   ---------------------------------------------------------------
   1   Escape from Data Transfer       0     Controlled by host
   2   Echo                            0     No Echo
   3   Data Forwarding Signal          2     Carriage Return
   4   Selection of Idle Timer Delay   20    1 second
   5   Ancillary Device Control        0     X-ON, X-OFF not used
   6   PAD Service Signals             1     All except prompt
   7   Procedure on Break              2     Reset
   8   Discard Output                  0     Do not discard
   9   Padding after Carriage Return   0     No padding

   10  Line Folding                    0     No line folding
   11  Terminal Data Rate              -     Read only
   12  Flow Control of the PAD         0     No flow control used
   13  Linefeed Insertion after C/R    0     No linefeed
   14  Linefeed Padding                0     No linefeed padding
   15  Editing                         0     No editing
   16  Character Delete                127   Delete
   17  Line Delete                     24    <CTRL>X
   18  Line Display                    18    <CTRL>R
   19  Editing PAD Service Signals     0     No service signal
   20  Echo Mask                       0     No echo mask
   21  Parity Treatment                0     No parity check
   22  Page Wait                       0     No page wait

   Note 1:

   Refer to CCITT (1984)
   - Parameters 1 - 12 are mandatory and available internationally.
   - Parameters 13 - 22 may be available on certain networks and may
     also be available internationally.
   - A parameter value may be mandatory or optional.

   The ODETTE profile refers only to parameter values which must be
   internationally implemented if the parameter is made available
   internationally.

   The ODETTE-FTP "special-logic" parameter may be impossible on some
   PADs because they do not support of some of the parameters (13 - 22).
   (If the PAD is supporting parity check (21) by default, ODETTE-FTP
   special logic would be impossible.)

   It is a user responsibility to ensure special logic consistency when
   making the PAD subscription.

   Note 2:

   Some parameters may have to be set differently depending on:
   - Make and function of the start-stop mode DTE entity.
   - Start-stop mode DTE entity ODETTE-FTP monitor function.
   - PAD services implemented.
   - Packet mode DTE entity ODETTE-FTP monitor function.

Appendix D. OFTP X.25 over ISDN Recommendation

   This appendix describes the recommendation of ODETTE Group 4 (1) for
   the use of OFTP (2) over X.25 over ISDN.

      (1) ODETTE Group 4 is responsible for the specification of
          Telecommunications standards and recommendations for use
          within the Automotive Industry.

      (2) OFTP (ODETTE File Transfer Protocol) is the communications
          standard specified by ODETTE Group 4 designed for the transfer
          of both EDI and non-EDI data.

   This document offers an introductory overview of a technical subject.
   It is structured to contain the ODETTE recommendation, together with
   introductory information for the person not familiar with ISDN, and
   notes on the issues associated with the implementation of the
   recommendation.

   The first section provides the detailed ODETTE recommendation, which
   is followed by a general discussion.  If you are not familiar with
   the terminology, please read the subsequent sections first.

   How far an existing X.25 Line adapter may be replaced by an ISDN line
   adapter in an installation depends on the opportunities in view of
   connections (X.25 or ISDN) of the involved partners for file
   transfer.

   Companies that keep many connections to external partners (for
   example, car manufacturing companies) may use the OFTP file transfer
   in view of compatibility, which must always be considered anyway only
   in parallel to the X.25 network.

   It is not the aim of this recommendation to remove the OFTP file
   transfer generally from the X.25 network to the ISDN network.  This
   will not always be possible for international connections because of
   technical reasons, and this does not always make sense for
   connections with a low size of data to be transmitted.

   Certainly, the use of ISDN, when exchanging a high volume of data
   (for example, CAD/CAM files), is very much cheaper than the use of an
   X.25 network.  For such cases, this recommendation shall provide a
   cost-effective possibility for file transfer.

   This appendix is organized as follows.  D.1 defines the ODETTE
   recommendation in these terms, D.2 introduces the ISDN environment to
   the unfamiliar reader, D.3 describes the various methods of
   connecting to ISDN, and D.4 covers implementation issues.

D.1. ODETTE ISDN Recommendation

X.25: Level 2 ISO 7776
Protocol

                       Level 3          ISO 8208
                       Protocol

                       Packet Size      128

                       Level 2          7
                       Window Size

                       Level 3          7
                       Window Size

                       First LCN        1

                       Number of LCNs   1

                       Facilities       Window Size and Packet Size
                                        negotiation shall be supported
                                        by everybody.  Call User Data
                                        should not be required.

                       Calling NUA      Optionally provided by the call
                                        initiator.

                       Called NUA       Should be set to a value where
                                        the last 'n' digits can be
                                        specified by the called party.

   ISDN:               Apart from requesting a 64K unrestricted digital
                       call, no ISDN features shall be required.

   Timeout control:    To avoid connections (B channels) within the
                       circuit-switched ISDN network remaining active
                       but unused for a long time, the adapter should
                       include a timeout control.

                       An ISDN connection (B channel) should be released
                       if no X.25 packets have been transmitted on this
                       connection for a longer time.  For flexibility a
                       variable user definable timer should be
                       incorporated into the adapter.

                       In the event of a timeout situation the adapter
                       has to release the ISDN connection and notify the
                       local OFTP by the transmission of a clear packet.

   The pages that follow are informational and do not form part of this
   recommendation.

D.2. Introduction to ISDN

   The use of digital encoding techniques over such high-quality,
   error-free backbone networks has allowed the PTTs to offer high
   bandwidths to the end user.  The service is named ISDN (Integrated
   Services Digital Network).

   The increasing need to transfer larger volumes of EDI data, in
   particular CAD/CAM drawings, has focused attention upon high-speed,
   low-cost communication.  The traditional X.25 over a Packet Switched
   Data Network (PSDN) has been a good general purpose communications
   subsystem.  Unfortunately, its cost and transfer speed make PSDN
   expensive for the new requirement.

   X.25 over the new ISDN provides both the transfer speed and cost
   benefits to satisfy the new requirements.

   We include the following terminology because for us to make sense of
   ISDN and X.25, it is important that we use definitions precisely and
   avoid the abuses of the past.

   ISDN:        Integrated Services Digital Network

   X.25:        X.25 is a communications protocol.  It defines the
                structure of data packets that comprise the protocol and
                the manner in which they are used.

   PSDN:        A PSDN (Packet Switched Data Network) is a network over
                which the X.25 protocol is operated.

   PSPDN:       A PSPDN (Packet Switched Public Data Network) is a PSDN
                operated by the PTTs.  PSPDNs are given trade names,
                such as PSS in the UK, Datex-P in Germany, and Transpac
                in France.

   BRI:         Basic Rate Interface, also known as Basic Rate Access,
                defines an ISDN facility with 2 x 64 K B channels.

   PRI:         Primary Rate Interface, also known as Primary Rate
                Access, defines an ISDN facility with 30 x 64 K B
                channels.

   Channels:    ISDN is typically brought into a consumer's premises
                using a twisted pair of wire.  Over this wire, data can
                be transmitted in frequency bands.  These frequency
                bands are allocated as channels.

   B channels:  The B channels are the data channels and operate at 64
                Kb.  The two end users of a connection will communicate
                over a B channel.

   D channel:   Signalling on ISDN is performed over the D channel.
                Signalling is used to set up and release connections on
                the B channels.  In some countries, the D channel can
                also be used for limited X.25 access to the PTTs' PSDN.

                The D channel operates at the lower speed of 16 Kb as it
                is normally used only at the beginning and end of a
                connection.

                                         Bandwidth Allocation:
                     2 Wire                 B2 - 64 Kb
                     Twisted Pair           B1 - 64 Kb
                                         D Channel - 16 Kb

                The standard for the operation of the D channel is
                called ETSI and is used in most European countries.
                However, some countries that started the introduction
                very early used proprietary standards, for example:

                     1TR6 - Used in Germany
                     BTNR - Used in the UK

                Although there are D channel variations, this will not
                affect communications over the B channels as the
                communication over the D channel is between the
                subscriber and the ISDN service provider.

                However, the consumer's equipment must be able to handle
                the channel D signalling operated by the ISDN service
                provider and so there may be a problem of equipment
                availability and certification.

                All the PTTs have committed to migrate to ETSI (also
                known as EURO-ISDN and Q.931) and many are currently
                supporting both their national variant and ETSI.  It is
                advisable that in this situation the subscriber select
                the ETSI variant to avoid unnecessary equipment
                obsolescence.

   Services:    The high-speed service is provided in two forms, Basic
                and Primary.

                Basic: 2+D, the D 2B channel operates at 16 Kb.  The
                Basic Rate access is normally provided to the subscriber
                over simple twisted pair cable.

                Primary: 30B+D, the D channel operates at 64 Kb.
                Primary Rate access is normally provided to the
                subscriber over shielded coaxial cable.  Note that the
                bandwidth for Primary is 2.048 Mbit/s.

   Protocols:   The B channel is a binary channel and is transparent to
                the flow of data.  Therefore, all of the currently
                available protocols can operate over a B channel.  The
                most common protocol is X.25.

   X.25:        The X.25 protocol is a primary protocol for open
                computer-to-computer communication.

   Passive Bus: It is possible to have an ISDN service enter a building
                and then have an 8-core cable laid within the building
                with multiple ISDN junction points, in the same way as
                one would have multiple telephone points (extensions)
                for a particular external telephone line.

   Connection Setup

      The adapter is responsible for analysing the outgoing X.25 call
      request and making an ISDN call to a derived ISDN address,
      establishing a new X.25 level-2 and level-3, and then propagating
      the X.25 Call Request Packet.

   Connection Termination

      The termination phase of the X.25 call is made with a Clear
      Request and finalised with a Clear Confirmation.  The recipient of
      the Clear Confirm should then close down the ISDN connection.

      The clear down of the ISDN connection should only be made if there
      are no other Switched Virtual Circuits (SVCs) active on the ISDN
      connection; note that the usage of multiple simultaneous SVCs is
      only by virtue of bilateral agreement.

D.3. Equipment Types

   There are a number of ways in which ISDN/X.25 access can be made.

      Integrated Adapter

       This is normally a PC-based ISDN adapter inside a PC.  It is
       normal in such an environment that the OFTP application has the
       ability to manipulate the ISDN and X.25 aspects of the session
       independently and therefore have complete control.

       Equally important is that the speed of communication between the
       adapter and the application are at PC BUS speeds.  It is
       therefore more likely that the effective transmission speed will
       be nearer the 64K limit.

       The other benefit of such a direct linkage is that both 64K B
       channels may be used in parallel and both able to operate at
       64Kb.

      Elementary Terminal Adapter

       In this scenario, the computer has an integral X.25 adapter
       communicating X.21 with a Terminal Adapter that fronts the ISDN
       network.  This allows a host with a X.25 capability to interface
       to ISDN, normally on a one-to-one basis.

       The interface between the Terminal Adapter and the PC will
       typically only support one 64K B channel.  This is obviously an
       inefficient usage of the ISDN service.

       Because the linkage between the computer and the Terminal Adapter
       is only X.25, then some modification/configuration may be needed
       inside the Terminal Adapter when new users are added.

      X.25 Switch

       This solution is normally found inside the larger corporates
       where an internal X.25 network is operated or where dual X.25 and
       ISDN is required.

       The main benefit of a switch is to support both PSDN and ISDN
       simultaneously.  Also, multiple X.21 lines may be implemented
       between the X.25 switch and the computer.

       This solution normally requires more effort to configure and may
       require obligations to be placed upon how incoming callers
       specify routing.

D.4. Implementation

   The adoption of ISDN as an additional subsystem to support OFTP
   communications has associated implementation problems, which can be
   categorised as below:

      X.25/ISDN Addressing
      Making a Call
      Receiving a Call
      Logical Channel Assignment
      Facilities Negotiation
      ISDN Call Attributes
      Homologation Issues
      Growth
      Performance

D.4.1. X.25/ISDN Addressing

   The original OFTP was designed to work over the X.25 networks
   provided by the PTTs (PSPDNs).  The national X.25 networks were
   interconnected to provide a global X.25 network, and a common
   addressing scheme was adopted by all.  Although there were a few
   differences in addressing within a national network, the interface to
   other countries was quite rigid and normalised.

   PSPDN Numbering

      The addressing scheme adopted in X.25 is a 15-digit number
      (Network User Address, NUA) where the first three identify the
      country, the fourth digit identifies the network within the
      country, and the remainder specify the individual subscriber plus
      an optional subaddress.  In the UK where a full X.25 numbering
      scheme is adopted, a NUA is, e.g., 234221200170, where 2342 is the
      DNIC (Data Network Identification Code) and 21200170 is the
      subscriber number.

   ISDN Numbering

      ISDN is an extension of the normal telephone system; consequently,
      it adopts (or rather is) the same numbering scheme as the
      telephone system (PSTN).

   The Numbering Conflict

      The PSDN and PSTN numbering schemes are two totally different
      numbering schemes.  There is no relationship between them.  It is
      this conflict that is at the heart of the matter.

D.4.2. Making a Call

   It is a consequence of PSDN and PSTN being based upon different and
   unconnected numbering schemes that the key problem arises.

   For X.25 to work over ISDN, three main methods of addressing are
   available:

     Un-mapped:   The X.25 called NUA is used as the PSTN number.  Thus,
                  an X.25 call to 0733394023 will result in a PSTN call
                  to 0733394023 and the call request that consequently
                  flows will also be to 0733394023.

     Manipulated: The X.25 called NUA is manipulated by the subtraction
                  and/or addition of digits to derive a resultant PSTN
                  number.  Thus, 2394023 could be manipulated to derive
                  a PSTN number of 00944733394023, where the prefix 2 is
                  deleted and replaced by 00944733.

     Mapped:      The X.25 called NUA is used as a look-up into a table
                  of PSTN numbers.  Thus, an X.25 call to 234221200170
                  could be mapped to and result in a PSTN call to
                  0733394023 and the call request that consequently
                  flows will remain as 234221200170.

   Un-mapped Calls

      Un-mapped calls are where the host-specified X.25 NUA is converted
      directly to the corresponding ISDN number.

      Thus, an X.25 call issued by the host to X.25 NUA 0733394023 will
      result in an ISDN call to the PSTN number 0733394023.  After the
      call has been established, then HDLC/X.25 protocol setup will be
      established after which an X.25 call request will be transferred
      with the NUA 0733394023.

      When a PSTN call is made, the number of digits in the called
      number vary depending upon the location of the called party.

      When a number is called, it may be local, national, or
      international.

         local: 394023
         national: 0733 394023
         international: 009 44 733 394023

      Depending upon where a call originates, the corresponding X.25 NUA
      in the call request packet will vary dramatically.

      Such variation of X.25 NUA, in particular the changing prefix, can
      be difficult to be accommodated by X.25 routing logic in many
      products.

      When an international PSTN call is being made, then it is likely
      that the PSTN number exceeds 15 digits, which is the maximum
      length of an X.25 NUA.  Therefore, using un-mapped addressing may
      make some international calls impossible to make.

   Manipulated Calls

      The X.25 called NUA is manipulated by the subtraction and/or
      addition of digits to derive a resultant PSTN number.

      Let us assume that by internal convention we have identified the
      prefix '2' to indicate an international ISDN call.  Thus, an X.25
      call request of 244733394023 could be manipulated to derive a PSTN
      number of 00944733394023, where the prefix '2' is deleted and
      replaced by '009' (the international prefix).

      The X.25 called NUA would typically be left in its un-manipulated
      state.  As individual internal conventions vary, the X.25 called
      NUA will vary.  In the case above, it would be 244733394023, but
      another installation might have the convention where a prefix of
      '56' specifies the UK and so the NUA will be 56733394023, where
      the '56' is deleted and replaced with '00944' to derive the PSTN
      number.

   Mapped Calls

      The mapped method offers maximum flexibility in that:

         The PSTN number can exceed 15 digits.

         The X.25 NUA and PSTN number can be totally different.

      The problem with mapped calls is administrative.  IBM mainframes
      can't handle X.25 over ISDN at all, let alone support mapping.
      For the mainframe solution to work, an external X.25/ISDN router
      box is required and it is the responsibility of the external box
      to provide any mapping necessary.

      This means that any changes or addition of OFTP partners over ISDN
      will require access to the computer room or special configuration
      equipment to change the tables inside the external X.25/ISDN
      router box.

D.4.3. Receiving a Call

      We have seen from the previous section that the called X.25 NUA
      from an ISDN incoming call may vary considerably.  If ISDN/X.25 is
      confined to a national boundary, then such variation will not be
      so great as most calls will have matching called X.25 NUA and PSTN
      numbers.

      X.25 switches and X.25 adapters normally route/accept/reject calls
      based upon their X.25 called NUA.  In particular, routing is made
      upon the X.25 called NUA subaddress.

      To derive this subaddress, there are 2 methods:

      1) the last 'n' digits are analysed.

      2) the base X.25 NUA of the line is removed from the called NUA.
         For example, if the called X.25 NUA is 23422120017010 and the
         PSDN subscriber NUA is 234221200170, then the subaddress
         derived from subtraction is 10.

   Obviously, the second method will not work if the incoming NUA
   varies.

   ISDN Features

      ISDN, like X.25, has a core set of features that are then enriched
      with options.  In the original OFTP X.25 specification, it was
      decided that the Q-bit and D-bit options were not common to all
      networks or applications; they were therefore positively excluded
      from the specification.

      It is proposed that apart from the core ISDN features necessary to
      establish a call, no other features be used.

   Subaddressing

    There are two forms of ISDN subaddressing, overdialled and specific.

    The overdial method allows an ISDN number to be artificially
    extended.  A typical case would be where a private exchange has been
    installed in a larger company.  Assume that the base number is
    394023 and the computer is on internal extension 1234, then by
    specifying an ISDN number of 3940231234, direct access may be made
    to the internal extension.

    The problem with this method is that it extends to called number and
    may, especially for international access, exceed the ISDN numbering
    limits between countries.

    The other method of subaddressing is where a discrete subaddress is
    placed in a specific field in the ISDN call setup.

    The problem with this method, is that it requires the caller to
    place the subaddress in the ISDN call setup.  Not all ISDN
    implementations will allow this insertion.

    In conclusion, subaddressing of any kind should be avoided.

D.4.4. Logical Channel Assignment

    An X.25 dataline will have associated with it a number of logical
    channels.

    The number of channels is a part of the agreement between the PTT
    and the subscriber.  The number of channels subscribed to is
    important; call failure and similar problems will result if the
    number of logical channels defined at the two remote ends are
    different.

    If a DTE makes a call out, then the highest defined logical channel
    number will be selected.  If the remote Data Communications
    Equipment (DCE) does not have the same number of logical channels
    defined, then an invalid logical channel is being used from the
    perspective of the recipient DCE and the call will be rejected.

D.4.5. Facilities Negotiation

    In the PSPDN environment, it is possible to subscribe to negotiation
    of window size and packet size.  Although this negotiation requested
    by the originator's DTE may be propagated to the remote DTE at the
    discretion of the originator's DCE, it is a local responsibility
    between the DTE and DCE pair.

    In the ISDN scenario where it is a DTE-DTE type connection, the
    window size and packet size may be left at the default value and
    consequently the values may be omitted from the call request.  If no
    values are specified, then it is vital that both DTEs have
    configured themselves to the recommended defaults.

    The symptom of a window size mismatch is a hang situation without
    any informational error codes.

    The symptoms of a packet size mismatch could work in some scenarios,
    but would otherwise issue error codes indicating invalid packet
    sizes.

   Window Size

      The CCITT X.25 window size has a default value of '2', although
      subscribers may have other default window sizes, e.g., '7', by
      virtue of agreement with the PTT.

      Window size negotiation can be explicitly requested by specifying
      the requested window size in the Facilities fields in the Call
      Request packet.

   Packet Size

      The CCITT X.25 packet size has a default value of '128' octets,
      although subscribers may have other default values, e.g., '1024',
      agreed with the PTT.

D.4.6. ISDN Call Setup

      The initial setup of an ISDN call is initiated with the
      transmission of a Q.931 SETUP command.  Apart from requesting that
      a call be established, the SETUP command can optionally carry
      information about the calling party, the called party, routing
      information, the type of circuit required (e.g., voice or data),
      and information about the protocols that are requested to be
      established.

      Setup Parameters:

      Bearer capability            Information transfer and
                                   access attributes

      Called Party number          Destination's network address

      Called Party subaddress      Destination's complete
                                   address

      Calling Party number         Source's network address

      Low-layer compatibility      Layer 1-3 indication

      High-layer compatibility     Layer 4-7 indication

D.4.7. Homologation Issues

   Homologation procedures were adopted and vigorously enforced by the
   PTTs with respect to the quality and conformance of communications
   equipment connected to the services provided by the PTTs.

   In particular, commercial X.25 products had to be tested and approved
   before they could be connected to the PTTs' PSPDN.  The advantage of
   this to the subscriber was that there was very little chance of the
   approved equipment not working.

   With ISDN, similar approval standards are still enforced.  So the
   subscriber has the same confidence in their ISDN equipment.  Wrong,
   the ISDN equipment itself is approved, but the X.15 protocol that
   operates on top of ISDN is now outside of the scope of approval
   services.

   This means that quality of conformance to standards of X.25 over ISDN
   is subject to the variable quality procedures within the various ISDN
   equipment manufacturers.

   Although it is likely that commercial reputation will place pressure
   upon the manufacturers with a programming bug to correct such errors,
   it still requires the subscribers that do not communicate well to put
   time and effort into finding the party with the error.

   So far, tests have shown a number of subtle errors, such as timing
   problems, that have taken many days to find, prove, and fix.

D.4.8. Growth

   Primary Rate Access

      If a user decides to plan for growth from the beginning, then the
      Primary Rate Access has apparent financial benefits.  Such
      apparent savings are usually lost due to the increased cost of
      user hardware to support such an interface.  The BRI for data
      usage is very common and cards/adapters are low in cost, whereas
      the PRI cards/adapters are few and far between and consequently
      highly priced.

   Basic Rate Access

      One way to grow with ISDN is to buy multiple BRI lines, increasing
      slowly in units of 2 x B channels.  The PTTs will be able to
      provide the same subscriber number for all the lines provided in a
      similar way to the traditional hunting group associated with PSTN
      type working.

D.4.9. Performance

      The obvious benefit of ISDN is speed; unfortunately, the majority
      of computer systems in use today have a finite amount of computing
      power available.  The attachment of multiple active high-speed
      communication lines used in file transfer mode could take a
      significant amount of CPU resource to the detriment of other users
      on the system.

      Connecting an ISDN line with the default 2 B channels to your
      computer using an X.21 interface is going to give a consistent 64
      Kb throughput only if one of the B channels is active at any one
      time.

      If there are two 64 Kb channels active and contending for a single
      64 Kb X.21 interface, then effective throughput will be reduced
      significantly to just over 50%.

   Mainframe issues:

      Users with a mainframe front-end are also going to find cost an
      issue.  The scanners that scan the communications interfaces are
      based upon aggregate throughput.  A 64 Kb interface takes up a lot
      of cycles.

   Determining 'DTE' or 'DCE' Characteristics

      The following section is an extract from the ISO/IEC 8208
      (International Standards Organization, International
      Electrotechnical Commission) (1990-03-15) standard, which is an
      ISO extension of the CCITT X.25 standard.

      The restart procedure can be used to determine whether the DTE
      acts as a DCE or maintains its role as a DTE with respect to the
      logical channel selection during Virtual Call establishment and
      resolution of Virtual Call collision.

      When prepared to initialise the Packet Layer, the DTE shall
      initiate the restart procedure (i.e., transmit a RESTART REQUEST
      packet).  The determination is based on the response received from
      the data exchange equipment (DXE) as outlined below.

     a) If the DTE receives a RESTART INDICATION packet with a
        restarting cause code that is not 'DTE Originated' (i.e., it
        came from a DCE), then the DTE shall maintain its role as a DTE.

     b) If the DTE receives a RESTART INDICATION packet with a
        restarting cause code of 'DTE Originated' (i.e., it came from
        another DTE), then the DTE shall confirm the restart and act as
        a DCE.

     c) If the DTE receives a RESTART INDICATION packet with a
        restarting cause code of 'DTE Originated' (i.e., it came from
        another DTE) and it does not have an unconfirmed RESTART REQUEST
        packet outstanding (i.e., a restart collision), then the DTE
        shall consider this restart procedure completed but shall take
        no further action except to transmit another RESTART REQUEST
        packet after some randomly chosen time delay.

     d) If the DTE issues a RESTART REQUEST packet that is subsequently
        confirmed with a RESTART CONFIRMATION packet, then the DTE shall
        maintain its role as a DTE.

Acknowledgements

   This document draws extensively on revision 1.4 of the ODETTE File
   Transfer Specification [OFTP].

   Many people have contributed to the development of this protocol and
   their work is hereby acknowledged.

Normative References

   [CMS-Compression]
              Gutmann, P., "Compressed Data Content Type for
              Cryptographic Message Syntax (CMS)", RFC 3274, June 2002.

   [CMS]      Housley, R., "Cryptographic Message Syntax (CMS)", RFC
              3852, July 2004.

   [ISO-646]  International Organisation for Standardisation, ISO
              Standard 646:1991, "Information technology -- ISO 7-bit
              coded character set for information interchange", 1991.

   [PKCS#1]   Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [UTF-8]    Yergeau, F., "UTF-8, A Transformation Format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [ZLIB]     Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950, May 1996.

Informative References

   [ISO-6523] International Organisation for Standardisation, ISO
              Standard 6523:1984, "Data interchange -- Structures for
              the identification of organisations", 1984.

   [OFTP]     Organisation for Data Exchange by Tele Transmission in
              Europe, Odette File Transfer Protocol, Revision 1.4, April
              2000.

   [FTP]      Postel, J. and J. Reynolds, "File Transfer Protocol", STD
              9, RFC 959, October 1985.

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

   [RIME]     Coleridge, Samuel Taylor, "The Rime of the Ancient
              Mariner", 1817.

   [X.509]    Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3850]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Certificate Handling", RFC
              3850, July 2004.

ODETTE Address

   The ODETTE File Transfer Protocol is a product of the Technology
   Committee of Odette International.  The Technology Committee can be
   contacted via the ODETTE Central Office:

   ODETTE INTERNATIONAL Limited
   Forbes House
   Halkin Street
   London
   SW1X 7DS
   United Kingdom

   Phone: +44 (0)171 344 9227
   Fax:   +44 (0)171 235 7112
   EMail: info@odette.org
   URL:   http://www.odette.org

Author's Address

   Ieuan Friend
   Data Interchange Plc
   Rhys House
   The Minerva Business Park
   Lynchwood
   Peterborough
   PE2 6FT
   United Kingdom

   Phone: +44 (0)1733 371 311
   EMail: ieuan.friend@dip.co.uk

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