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RFC 2204

 
 
 

ODETTE File Transfer Protocol

Part 2 of 3, p. 22 to 49
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4. Protocol Specification

4.1  Overview

   The ODETTE-FTP protocol 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
      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
      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.  It's peer becomes the
   Responder.

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4.2.2  Protocol Sequence

   The first message must be sent by the Responder.

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

4.3  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
               <------------ 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.

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   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 it has been successfully delivered to it's final
   destination.  This allows the originator to perform house keeping
   tasks such as deleting copies of the delivered data.

   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 it's 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.

   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

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   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 it's 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  Ready To Receive Command (RTR)

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

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

   After sending an EERP, the Speaker must wait for an RTR before
   sending any other commands.

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 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

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   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
               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.

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

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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 it's own error
   handling.

   ODETTE-FTP can detect protocol errors through the construction of
   it's 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 8, the 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 6.

5.1  Conventions

5.1.1  Representation unit:

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

5.1.2  Values and Characters:

   The ISO 646 IRV 7-bit coded character set [ISO-646] is used to encode
   constants and strings within command exchange buffers.

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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.  Each command has a fixed length and can not be
   compressed.

   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 fixed fields within a Command
      Exchange Buffer.  All values are required.

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 SFIDFSIZ field may contain any integer value
             between 0000000 and 9999999.

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

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

      All attributes are in character format.

      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.

      Numeric fields are always right justified and left padding with
      zeros must be done when needed.

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'.

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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  | Exchange Buffer Size                  | V 9(5)  |
   |  40 | SSIDSR    | Send / Receive Capabilities (S/R/B)   | F X(1)  |
   |  41 | SSIDCMPR  | Compression Indicator (Y/N)           | F X(1)  |
   |  42 | SSIDREST  | Restart Indicator (Y/N)               | F X(1)  |
   |  43 | SSIDSPEC  | Special Logic Indicator (N)           | F X(1)  |
   |  44 | SSIDCRED  | Credit                                | V 9(3)  |
   |  47 | SSIDRSV1  | Reserved                              | F X(5)  |
   |  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)

      Value: '1' ODETTE-FTP protocol release level 1.

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

   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.

   SSIDPSWD  Password                                          String(8)

             Key to authenticate the sender.  Assigned by bilateral
             agreement.

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   SSIDSDEB  Exchange Buffer Size                             Numeric(5)

    Minimum: 128
    Maximum: 99999

             The length, in octets, of the largest 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 sessions will not take place.

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

   SSIDCMPR  Compression Indication                            Character

      Value: 'Y'  The location can handle compressed data.
             'N'  The location can not handle compressed data.

             Compression is only used if supported by both locations.
             The compression mechanism is described in Section 6.2

   SSIDREST  Restart Indication                                Character

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

   SSIDSPEC  Special Logic Indication                          Character

      Value: 'N'  Only valid value for TCP.

             The Special Logic extensions are only useful in an X.25
             environment and are not supported for TCP/IP.

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   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 or equal m)
             Note: negotiated value will be "n".

   SSIDRSV1  Reserved                                          String(5)

             This field is reserved for future use.

   SSIDUSER  User Data                                         String(8)

             May be used by the 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'.

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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(9)  |
   |  36 | SFIDDATE  | Virtual File Date stamp, (YYMMDD)     | V X(6)  |
   |  42 | SFIDTIME  | Virtual File Time stamp, (HHMMSS)     | V X(6)  |
   |  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(7)  |
   | 119 | SFIDREST  | Restart Position                      | V 9(9)  |
   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(9)

             This field is reserved for future use.

   SFIDDATE  Virtual File Date stamp                           String(6)

     Format: 'YYMMDD'  6 decimal digits representing the year, month
             and day respectively [ISO-8601].

             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)

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   SFIDTIME  Virtual File Time stamp                           String(6)

     Format: 'HHMMSS'  6 decimal digits representing hours, minutes
             and seconds respectively [ISO-8601].

             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 the 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 1.5.3)

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   SFIDLRECL Maximum Record Size                              Numeric(5)

    Maximum: 99999

             Length in octets of the longest logical record which 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'.

   SFIDFSIZ  File Size                                        Numeric(7)

    Maximum: 9999999

             Space in 1K (1024 octet) blocks required at the Originator
             location to store the Virtual File.

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

   SFIDREST  Restart Position                                 Numeric(9)

    Maximum: 999999999

             Virtual File restart position.

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

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

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

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(9)  |
   o-------------------------------------------------------------------o

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   SFPACMD   Command Code                                      Character

      Value: '2'  SFPA Command identifier.

   SFPAACNT  Answer Count                                     Numeric(9)

             The Listener must enter a count lower 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)  |
   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.
             '99'  Unspecified reason.

             Reason why transmission can not proceed.

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   SFNARRTR  Retry Indicator                                   Character

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

             This parameter is used to advise the Speaker if it should
             retry at a latter point in 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 it therefore recommended that when a SFNA with Retry = Y
             is received the User Monitor attempts to retransmit the
             relevant file in a subsequent session.

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 X(n)  |
   o-------------------------------------------------------------------o

   DATACMD   Command Code                                      Character

      Value: 'D'  DATA Command identifier.

   DATABUF   Data Exchange Buffer payload                      String(n)

             Variable length buffer containing the data payload.  The
             Data Exchange Buffer is described in Section 6.

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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(9)  |
   |  10 | EFIDUCNT  | Unit Count                            | V 9(12) |
   o-------------------------------------------------------------------o

   EFIDCMD   Command Code                                      Character

      Value: 'T'  EFID Command identifier.

   EFIDRCNT  Record Count                                     Numeric(9)

    Maximum: 999999999

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

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

   EFIDUCNT  Unit Count                                      Numeric(12)

    Maximum: 999999999999

             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)  |
   o-------------------------------------------------------------------o

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   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.
             '99'  Unspecified reason.

             Reason why transmission can not proceed.

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 | 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).

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             '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 is different to 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

             '99'  Unspecified Abort code

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

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   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(9)  |
   |  36 | EERPDATE  | Virtual File Date stamp, (YYMMDD)     | V X(6)  |
   |  42 | EERPTIME  | Virtual File Time stamp, (HHMMSS)     | V X(6)  |
   |  48 | EERPUSER  | User Data                             | V X(8)  |
   |  56 | EERPDEST  | Destination                           | V X(25) |
   |  81 | EERPORIG  | Originator                            | V X(25) |
   o-------------------------------------------------------------------o

   EERPCMD   Command Code                                      Character

      Value: 'E'  EERP Command identifier.

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   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(9)

             This field is reserved for future use.

   EERPDATE  Virtual File Date stamp                           String(6)

     Format: 'YYMMDD'  6 decimal digits representing the year, month
             and day respectively [ISO-8601].

             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                           String(6)

      Format: 'HHMMSS'  6 decimal digits representing hours, minutes
             and seconds respectively [ISO-8601].

             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 the 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 (mapped) the data for
             transmission.

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   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 perform mapping functions.  It is also the
             location that creates the EERP for the received file.

5.3.14  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.

5.4  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 Sub-Address                  | 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.

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   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,
             apace and hyphen characters.

   SIOCSA    Computer Sub-Address                              String(6)

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

6. ODETTE-FTP Data Exchange Buffer

6.1  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.  The format is based on the Network Independent File
   Transfer Protocol [NIFTP].

   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.

6.2  Data Exchange Buffer Format

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

   The Data Exchange Buffer is made up of the initial Command character,

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

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   CMD

      The Data Exchange Buffer Command Character, 'D'.

   HDR

      A one 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 six bits are available, the next subrecord may
             represent between 0 and 63 octets of the Virtual File.

6.3  Buffer Filling Rules

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

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   Virtual File records may be concatenated within one 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.

7. Stream Transmission Buffer (TCP only)

7.1  Introduction

   The ODETTE-FTP was originally designed to utilise the ISO Network
   Service, specifically the X.25 specification.  It relies 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.  The TCP offers a stream based connection which does not
   provide these functions.

   In order to utilise the TCP stream without disruption to the existing
   ODETTE-FTP 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.

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

   The Stream Transmission Buffer comprises of a STH and OEB.

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

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7.2  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 a STB length of 100003.

      The length is expressed as a binary number with the most
      significant bit on the left.

   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.


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