RFC1086

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

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  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.

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  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.

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  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    |
        +-+-+-+-+-+-+-+-+

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  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

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     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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  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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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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Authors' Addresses:

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

  EMail: [email protected]

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

  Phone: (415) 962-7100

  EMail: [email protected]

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