rfc1356.txt

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    networks are free to internally combine or split X.25 data packets
    as long as the complete packet sequence is preserved.

    The optimum X.25 data packet size is, therefore, dependent on the
    network, and is not necessarily the largest size offered by that
    network.

4.5 Another method of increasing performance is to open multiple virtual
    circuits to the same destination, specifying the same CUD.  Like
    packet size, this is not always the best method.

    When the throughput limitation is due to X.25 window size, opening
    multiple circuits effectively multiplies the window, and may
    increase performance.

    However, opening multiple circuits also competes more effectively
    for the physical path, by taking more shares of the available
    bandwidth.  While this may be desirable to the user of the
    encapsulation, it may be somewhat less desirable to the other users
    of the path.

    Opening multiple circuits may also cause datagram sequencing and
    reordering problems in end systems with limited buffering (e.g., at
    the TCP level, receiving segments out of order, when a single
    circuit would have delivered them in order). This will only affect
    performance, not correctness of operation.

    Opening multiple circuits may also increase the cost of delivering
    datagrams across a public data network.






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RFC 1356           Multiprotocol Interconnect on X.25        August 1992


4.6 This document does not specify any method of dynamic IP to X.25 (or
    X.121) address resolution.  The problem is left for further study.

    Typical present-day implementations use static tables of varying
    kinds, or an algorithmic transformation between IP and X.121
    addresses [7,9].  There are proposals for other methods.  In
    particular, RFC 1183 [10] describes Domain Name System (DNS)
    resource records that may be useful either for automatic resolution
    or for maintenance of static tables.  Use of these method(s) is
    entirely experimental at this time.

5. Packet Formats

   For each protocol encoding, the diagrams outline the call request and
   the data packet format. The data packet shown is the first of a
   complete packet (M bit) sequence.

5.1 IP Encapsulation

    Call Request:

    +----------------+-----------+------------+----+
    | GFI, LCN, type | addresses | facilities | CC |
    +----------------+-----------+------------+----+

    X.25 data packets:

    +----------------+------------------------+
    | GFI, LCN, I    | IP datagram            |
    +----------------+------------------------+

5.2 CLNP, ES-IS, IS-IS Encapsulation

    Call Request:

    +----------------+-----------+------------+----+
    | GFI, LCN, type | addresses | facilities | 81 |
    +----------------+-----------+------------+----+

    X.25 data packets:

    +----------------+--------------------------------+
    | GFI, LCN, I    | CLNP, ES-IS, or IS-IS datagram |
    +----------------+--------------------------------+

    (Note that these datagrams are self-identifying in their
    first octet).




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RFC 1356           Multiprotocol Interconnect on X.25        August 1992


5.3 SNAP Encapsulation

    Call Request:

    +----------------+-----------+------------+----+-----------------+
    | GFI, LCN, type | addresses | facilities | 80 | SNAP (5 octets) |
    +----------------+-----------+------------+----+-----------------+

    X.25 data packets:

    +----------------+-------------------------------------+
    | GFI, LCN, I    | Protocol Data Unit (no SNAP header) |
    +----------------+-------------------------------------+

5.4 Null (Multiplexed) Encapsulation

    Call Request:

    +----------------+-----------+------------+----+
    | GFI, LCN, type | addresses | facilities | 00 |
    +----------------+-----------+------------+----+

    X.25 data packets:

    +----------------+-----------------+---------------------+
    | GFI, LCN, I    | NLPID (1 octet) | Protocol Data Unit  |
    +----------------+-----------------+---------------------+

    Examples of data packets:

    Multiplexed IP datagram:

    +----------------+----+-----------------------+
    | GFI, LCN, I    | CC | IP datagram           |
    +----------------+----+-----------------------+

    Multiplexed CLNP datagram:

    +----------------+----+-------------------------+
    | GFI, LCN, I    | 81 | CLNP datagram           |
    +----------------+----+-------------------------+

    Multiplexed SNAP PDU:

    +----------------+----+-----------------+--------------------+
    | GFI, LCN, I    | 80 | SNAP (5 octets) | Protocol Data Unit |
    +----------------+----+-----------------+--------------------+




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RFC 1356           Multiprotocol Interconnect on X.25        August 1992


6. Security Considerations

   Security issues are not discussed in this memo.

7. References

   [1]  Korb, J., "A Standard for the Transmission of IP Datagrams Over
        Public Data Networks", RFC 877, Purdue University, September
        1983.

   [2]  ISO/IEC TR 9577, Information technology - Telecommunications and
        Information exchange between systems - Protocol Identification
        in the network layer, 1990 (E) 1990-10-15.

   [3]  IEEE, "IEEE Standard for Local and Metropolitan Area Networks:
        Overview and Architecture", IEEE Standards 802-1990.

   [4]  ISO/IEC 8473, Information processing systems - Data
        communications - Protocol for providing the connectionless- mode
        network service, 1988.

   [5]  ISO/IEC 9542, Information processing systems -
        Telecommunications and information exchange between systems -
        End system to intermediate system routeing protocol for use in
        conjunction with the protocol for providing the connectionless-
        mode network service (ISO/IEC 8473), 1988.

   [6]  Postel, J., Editor., "Internet Protocol - DARPA Internet Program
        Protocol Specification", RFC 791, USC/Information Sciences
        Institute, September 1981.

   [7]  Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1340,
        USC/Information Sciences Institute, July 1992.

   [8]  Bradley, T., Brown, C., and A. Malis, "Multiprotocol
        Interconnect over Frame Relay", RFC 1294, Wellfleet
        Communications and BBN Communications, January 1992.

   [9]  "Defense Data Network X.25 Host Interface Specification",
        contained in "DDN Protocol Handbook", Volume 1, DDN Network
        Information Center 50004, December 1985.

  [10]  Everhart, C., Mamakos, L., Ullmann, R, and P. Mockapetris,
        Editors, "New DNS RR Definitions", RFC 1183, Transarc,
        University of Maryland, Prime Computer, USC/Information Sciences
        Institute, October 1990.

  [11]  ISO/IEC 8208, Information processing systems - Data



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RFC 1356           Multiprotocol Interconnect on X.25        August 1992


        communications - X.25 Packet Level Protocol for Data Terminal
        Equipment, 1987.

8. Authors' Addresses

   Andrew G. Malis
   BBN Communications
   150 CambridgePark Drive
   Cambridge, MA 02140
   USA

   Phone: [phone]
   Email: [e-mail]


   David Robinson
   Computervision Systems Integration
   201 Burlington Road
   Bedford, MA 01730
   USA

   Phone: [phone] x2774
   Email: [e-mail]


   Robert L. Ullmann
   Process Software Corporation
   959 Concord Street
   Framingham, MA 01701
   USA

   Phone: [phone]
   Email: [e-mail]


















Malis, Robinson, & Ullmann                                     [Page 14]