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Network Working Group                                          J. Onions
Request for Comments: 1086                                    Nottingham
                                                                 M. Rose
                                                                     TWG
                                                           December 1988

                  ISO-TP0 bridge between TCP and X.25



Status of this Memo

   This memo proposes a standard for the Internet community.  Hosts on
   the Internet that choose to implement ISO TP0 transport connectivity
   between TCP and X.25 based hosts are expected to experiment with this
   proposal.  TCP port 146 is reserved for this proposal.  Distribution
   of this memo is unlimited and comments are highly encouraged.

Introduction

   This memo specifies a protocol that is used to bridge ISO TP0 packets
   between X.25 and TCP networks.  This technique is useful when
   interconnecting a DDN IP internet to an X.25 subnetwork.  This is not
   a "magic bullet" solution to the DDN/ISO interoperability problem.
   Rather, if one is running higher-layer ISO protocols in both networks
   (namely ISO TP0), then a TP0 bridge can be used to achieve
   connectivity.

   The protocol itself is fairly simple as the method of operation for
   running TP0 over the TCP and X.25 protocols have previously been
   defined.  A bridge offering ISO-TP0 gateway services simply applies
   both methods as appropriate.  The protocol works by TP0/TCP hosts
   "registering" an X.25 subaddress (and corresponding TCP port/IP
   address) with the bridge.  TP0/X.25 hosts use the standard method for
   establishing, maintaining, and releasing connections.  When a
   connection is established, the bridge establishes the corresponding
   TCP connection and simply shuffles TP0 packets between the two.  When
   a TP0/TCP host initiates a connection, it establishes a TCP
   connection to the bridge using port number 146 and communicates the
   desired X.25 address.  The bridge establishes a connection to the
   native X.25 host and simply shuffles TP0 packets between the two.

1.  Introduction and Motivation

   The migratory protocol described in [RFC1006] makes possible the
   transmission of TP0 packets between hosts on TCP/IP internets.  With
   the addition of a small protocol converter, a TCP/IP host can be made
   to appear in the X.25 addressing space and be able to accept and make



Onions & Rose                                                   [Page 1]

RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


   connections using the TP0 protocol.

   This procedure is particularly useful in the following cases:

      1.  A host on an IP based internet can communicate with hosts on
      X.25 based networks providing the hosts are running ISO protocols.
      This also assumes a friendly gateway willing to run the actual TP0
      bridge and make available to the IP host part of its X.25 address
      space.

      2.  A site having sparse connections to an X.25 network and using
      a TCP/IP based local area network for local communications.  In
      this case all hosts on the LAN can have access to hosts on the
      X.25 network running ISO TP0.


   Pictorially, this memo describes interoperation in the following
   environment:

          +---------------------------------+
          |                                 |
          |                   +-----------------------------------+
          |  +----+           |     +----+  |           +----+    |
          |  |    |           |     |    |  |           |    |    |
          |  |    +-----------|-----+    +--------------+    |    |
          |  |    |     TP0   |     |    |  |  TP0      |    |    |
          |  +----+           |     +----+  |           +----+    |
          | TCP Host          |  Bridge Host|         X.25 Host   |
          |                   |             |                     |
          |                   |             |                     |
          |                   |             |                     |
          +-------------------|-------------+                     |
            TCP/IP Network    |                                   |
                              |                                   |
                              +-----------------------------------+
                                           X.25 Network

2.  Definitions and Philosophy

   Some modest terminology and philosophy is introduced to aid
   readability and stir interest.

   The ISO Transport Service (TS) provides a reliable, packet-stream to
   its users [ISO8072].  The ISO Transport Protocol (TP) implements this
   service [ISO8073].  There are five classes of this protocol.  The
   class is selected on the basis of the services offered by the
   underlying network service.  Transport class 0 (TP0) is used when the
   network service offered is connection-oriented and error-detecting.



Onions & Rose                                                   [Page 2]

RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


   As should be expected, TP0 is a rather simple protocol, since the
   underlying network service actually provides most of the qualities
   offered by the transport service.

   CCITT Recommendation X.25 [ISO8208,X.25] offers such a network
   service.  It is beyond the scope of this memo to describe X.25 in any
   detail, but two observations are pertinent:  First, X.25 is offered
   as a wide-area network service by many commercial and (non-U.S.)
   government carriers.  Second, the TP0/X.25 combination is very
   popular in Europe and other communities with a strong PTT-oriented
   market.

   It has been argued that the DoD Transmission Control Protocol (TCP)
   [MIL1778, RFC793] can also be seen as providing a connection-oriented
   and error-detecting network service.  This remark is controversial in
   the sense that the TCP is actually an end-to-end transport protocol
   and not a network protocol; the DoD Internet Protocol (IP) [MIL1777,
   RFC791] is the network protocol in the DoD Protocol Suite.  However,
   one of the advantages of layering is that, when properly architected,
   it enhances flexibility.  This notion led to the development of
   [RFC983] and its successor [RFC1006], which described how to provide
   the ISO transport service on top of TCP/IP internetworks.

3.  The Model

   The model is simple.  The method for transmitting TP0 packets using
   TCP is defined in [RFC1006].  The method for transmitting TP0 packets
   using X.25 is defined in [ISO8878].  The TP0 bridge merely has to
   convert between the two forms.  As with most protocols, there are
   three well-defined phases of interaction:  connection establishment,
   data transfer, and connection release.  The method of operation for
   the data transfer and connection release phases are quite similar
   when using TP0 over either network service.  Hence the resulting
   protocol mapping functions are quite simple.

   The difficult part is in managing connection establishment.  A small
   "registration" protocol is used to aid the protocol mapping function
   for the connection establishment phase.  The protocol performs one of
   two operations: an X.25 address is specified for an outgoing call, or
   an X.25 address is specified to accept incoming calls.

   This memo ignores the problems of authentication and authorization.
   These areas are presumed to be a local matter.  It is worth pointing
   out that running such a TP0 bridge with unrestricted access allows
   any TCP/IP host to lay claim to part of the TP0 bridge host's X.25
   address space.  This address space is limited and will not support
   many foreign hosts registering listening addresses.




Onions & Rose                                                   [Page 3]

RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


   The protocol makes no attempt to report errors other than those
   transmitted by the TP0 protocol.  To attempt such additions would
   require other mechanism such as a new protocol layer or equivalent.
   The chosen model is kept as simple as possible with network errors
   being ignored if recoverable, and resulting in disconnection
   otherwise.  This actually enhances the transparency of the gateway,
   in that the only gateway specific functions are collected together in
   the connection phase.  The resultant circuit, once established, is
   indistinguishable from an [RFC1006] implementation.

4.  The Protocol

   The protocol is quite simple.  A successful connection establishment
   phase results in two network connections being established.  TP0 is
   used over each network connection, though one network connection is
   provided by X.25 and the other by the TCP.

   During the data transfer phase, the TP0 bridge reads TPDUs (transport
   protocol data units) from one network connection and writes them to
   the other network connection.  During the connection release phase,
   when one network indicates a disconnect, the bridge disconnects the
   other network connection; or in the case of simultaneous network
   disconnects, no action is taken by the bridge.

   As expected, the method of operation for the connection establishment
   phase is more complex.  Connection establishment is driven by a
   registration procedure which is initiated by a TCP/IP host initiating
   a connection with the TP0 bridge.  This procedure takes on one of two
   "flavors" depending on whether the initiating host wishes to
   establish a connection to a particular X.25 address or listen for
   connections on a particular X.25 address.

   The initiating host initiates the registration procedure by
   establishing a connection to TCP port 146 on the TP0 bridge.  It then
   sends one octet which indicates the flavor the registration procedure
   will take:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |   function    |
         +-+-+-+-+-+-+-+-+










Onions & Rose                                                   [Page 4]

RFC 1086          ISO-TP0 bridge between TCP and X.25      December 1988


   The value of this octet is a binary-encoded value:

           value   meaning
           -----   -------
             0     illegal
             1     connect to a particular X.25 host
             2     listen for incoming X.25 connections
           3-255   reserved

   The method of operation for the registration procedure now diverges,
   based on the function chosen.

   FUNCTION 1: CONNECTION THROUGH THE TP0 BRIDGE

      The X.25 address to call is now sent by the initiating host to the
      TP0 bridge.  The format of an X.25 address is described in Section
      5 of this memo.

      The TP0 bridge now attempts to call the specified address.  If
      this succeeds, the connection establishment phase has succeeded
      and the data transfer phase is begun.  If the call fails, then the
      TP0 bridge closes the TCP connection.

   FUNCTION 2: ESTABLISHING A LISTENING ADDRESS

      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



Onions & Rose                                                   [Page 5]

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.




Onions & Rose                                                   [Page 6]

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.






Onions & Rose                                                   [Page 7]

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.









Onions & Rose                                                   [Page 8]

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: JPO@CS.NOTT.AC.UK

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

   Phone: (415) 962-7100

   EMail: mrose@TWG.COM
































Onions & Rose                                                   [Page 9]