rfc1086.txt

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      The X.25 address, which should be a subaddress of the TP0 bridge's
      X.25 address, on which to listen for incoming X.25 connections is
      now sent by the initiating host to the TP0 bridge.

      Next, the initiating host sends an IP address and TCP port number
      which will service incoming calls for the indicated X.25 address.
      The format of a TCP/IP address is described in Section 6 of this
      memo.

      The TP0 bridge now listens, on behalf of the initiating host, on
      the indicated X.25 address.

      If an incoming call is received, a TCP connection is established
      to the corresponding TCP/IP address.  If this connection is
      successful, then the connection establishment phase has succeeded
      and the data transfer phase is begun.  If the connection fails,
      the incoming call is refused.

      The TCP/IP connection between the initiating host and the TP0
      bridge is a "heartbeat" connection for the registration function.
      If this connection closes, the TP0 bridge assumes hat the
      listening function has been terminated by the initiating host, and
      consequently, the TP0 bridge no longer listens for incoming calls



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RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


      on the indicated X.25 address.  If such a facility were not
      present, then the indicated X.25 address could not be recovered
      for reuse.

5.  Format of X.25 Addresses

   A standardized octet-encoding of X.25 addresses is used by the
   protocol described in this memo.  The encoding has a fixed-length of
   68 octets and contains 10 fields:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        address  type          | X.121 address        ...      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ...      |      ...      |      ...      |      ...      |
   |      ...      |      ...      |      ...      |      ...      |
   |      ...      |      ...      |      ...      |      ...      |
   |      ...      |      ...      | X.121 length  | Protocol ID   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ...      |      ...      |      ...      | PID length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Call User Data field          |      ...      |      ...      |
   |      ...      |      ...      |      ...      |      ...      |
   |      ...      |      ...      |      ...      |      ...      |
   |      ...      |      ...      |      ...      |      ...      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | CUDF length   |  X.25 Facilities     ...      |      ...      |
   |      ...      |      ...      |      ...      |Facility Length|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   address type (2 octets) - a binary-encoded value in network order
   indicating the address type. The value 3 is used for X.25 addressing
   of this format.

   X.121 address (16 octets) - the ascii-encoded value of the X.121
   address.

   address length (1 octet) - a binary-encoded value in network order
   indicating how many octets of the X.121 address are meaningful.

   Protocol ID (4 octets) - meaningful at the remote system.

   Protocol ID length (1 octet) - a binary-encoded value indicating the
   number of protocol ID octets are meaningful.




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RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


   User Data (16 octets) - meaningful at the remote system.

   User Data Length (1 octet) - a binary-encoded value indicating the
   number of User Data octets are meaningful.

   X.25 Facilities (6 octets) - meaningful at the remote system.

   X.25 Facilities length (1 octet) - a binary-encoded value indicating
   the number of Facility octets are meaningful.

6.  Format of TCP/IP Addresses

   A standardized octet-encoding of TCP/IP addresses is used by the
   protocol described in this memo.  The encoding has a fixed-length of
   16 octets and contains 4 fields:


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         address  type         |           TCP  port           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           IP address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    reserved   |      ...      |      ...      |      ...      |
   |      ...      |      ...      |      ...      |      ...      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are:

   address type (2 octets) - a binary-encoded value in network order.
   The value 2 is used.

   TCP port (2 octets) - a binary-encoded value in network order.

   IP address (4 octets) - a binary-encoded value in network order.

   reserved (16 octets) - null-value padding.

Comments

   At present, the structure of the X.25 address and the internet
   address are rather ad-hoc and specific to the UNIX operating system.
   These structures may change in the future as experience is gained in
   the use of the TP0 bridge.






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RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


References

     [ISO8072]  Information processing systems -- Open systems
                interconnection, "Transport Service Definition",
                International Standard, June, 1985.

     [ISO8073]  Information processing systems -- Open systems
                interconnection, "Transport Protocol Specification",
                International Standard, July, 1986.

     [ISO8208]  Information processing systems, "X.25 package level
                protocol for data terminal equipment", Draft
                International Standard, July, 1985.

     [ISO8878]  Information processing systems -- Data communications,
                Use of X.25 to provide the OSI connection-mode network
                service", Draft International Standard, January, 1987.

     [MIL1777]  Military Standard 1777, "Internet Protocol".

     [MIL1778]  Military Standard 1778, "Transmission Control Protocol".

     [RFC791]  Postel, J., "Internet Protocol - DARPA Internet Program
               Protocol Specification", RFC 791, USC/ISI,
               September 1981.

     [RFC793]  Postel, J., "Transmission Control Protocol - DARPA
               Internet Program Protocol Specification", RFC 793,
               USC/ISI, September 1981.

     [RFC983]  Cass, D., and M. Rose, "ISO Transport Services on Top
               of the TCP", RFC 983, NTRC, April 1986.

     [RFC1006]  Rose, M., and D. Cass, "ISO Transport Service on Top
                of the TCP Version: 3", NTRC, May 1987.

     [X.25]  CCITT Recommendation X.25, "Interface Between Data
             Terminal Equipment (DTE) and Data Circuit Terminating
             Equipment (DCE) for Terminals Operating in the Packet
             Mode on Public Data Networks," International Telegraph
             and Telephone Consultative Committee Yellow book, Vol.
             VIII.2, Geneva, 1981.









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RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


Authors' Addresses:

   Julian P. Onions
   Computer Science Department
   Nottingham University
   University Park
   Nottingham, NG7 2RD
   United Kingdom

   EMail: [e-mail]

   Marshall Rose
   The Wollongong Group
   1129 San Antonio Road
   Palo Alto, CA 94303

   Phone: [phone]

   EMail: [e-mail]
































Onions & Rose                                                   [Page 9]