Multiprotocol Interconnect on X.25 and ISDN in the Packet Mode
RFC 1356

Document Type RFC - Draft Standard (August 1992; No errata)
Obsoletes RFC 877
Last updated 2013-03-02
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Network Working Group                                           A. Malis
Request for Comments: 1356                            BBN Communications
Obsoletes: RFC 877                                           D. Robinson
                                      Computervision Systems Integration
                                                              R. Ullmann
                                            Process Software Corporation
                                                             August 1992

                       Multiprotocol Interconnect
                  on X.25 and ISDN in the Packet Mode

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.


   This document specifies the encapsulation of IP and other network
   layer protocols over X.25 networks, in accordance and alignment with
   ISO/IEC and CCITT standards.  It is a replacement for RFC 877, "A
   Standard for the Transmission of IP Datagrams Over Public Data
   Networks" [1].

   It was written to correct several ambiguities in the Internet
   Standard for IP/X.25 (RFC 877), to align it with ISO/IEC standards
   that have been written following RFC 877, to allow interoperable
   multiprotocol operation between routers and bridges over X.25, and to
   add some additional remarks based upon practical experience with the
   specification over the 8 years since that RFC.

   The substantive change to the IP encapsulation is an increase in the
   allowed IP datagram Maximum Transmission Unit from 576 to 1600, to
   reflect existing practice.

   This document also specifies the Internet encapsulation for
   protocols, including IP, on the packet mode of the ISDN.  It applies
   to the use of Internet protocols on the ISDN in the circuit mode only
   when the circuit is established as an end-to-end X.25 connection.

Malis, Robinson, & Ullmann                                      [Page 1]
RFC 1356           Multiprotocol Interconnect on X.25        August 1992


   RFC 877 was written by J. T. Korb of Purdue University, and this
   document follows that RFC's format and builds upon its text as
   appropriate.  This document was produced under the auspices of the IP
   over Large Public Data Networks Working Group of the IETF.

1. Conventions

   The following language conventions are used in the items of
   specification in this document:

   o MUST -- the item is an absolute requirement of the specification.
     MUST is only used where it is actually required for interoperation,
     not to try to impose a particular method on implementors where not
     required for interoperability.

   o SHOULD -- the item should be followed for all but exceptional

   o MAY or optional -- the item is truly optional and may be followed
     or ignored according to the needs of the implementor.

   The words "should" and "may" are also used, in lower case, in their
   more ordinary senses.

2. Introduction

   RFC 877 was written to document the method CSNET and the VAN Gateway
   had adopted to transmit IP datagrams over X.25 networks.  Its success
   is evident in its current wide use and the inclusion of its IP
   protocol identifier in ISO/IEC TR 9577, "Protocol Identification in
   the Network Layer" [2], which is administered by ISO/IEC and CCITT.

   However, due to changes in the scope of X.25 and the protocols that
   it can carry, several inadequacies have become evident in the RFC,
   especially in the areas of IP datagram Maximum Transmission Unit
   (MTU) size, X.25 maximum data packet size, virtual circuit
   management, and the interoperable encapsulation, over X.25, of
   protocols other than IP between multiprotocol routers and bridges.

   As with RFC 877, one or more X.25 virtual circuits are opened on
   demand when datagrams arrive at the network interface for
   transmission.  A virtual circuit is closed after some period of
   inactivity (the length of the period depends on the cost associated
   with an open virtual circuit).  A virtual circuit may also be closed
   if the interface runs out of virtual circuits.

Malis, Robinson, & Ullmann                                      [Page 2]
RFC 1356           Multiprotocol Interconnect on X.25        August 1992

3. Standards

3.1 Protocol Data Units (PDUs) are sent as X.25 "complete packet
    sequences".  That is, PDUs begin on X.25 data packet boundaries and
    the M bit ("more data") is used to fragment PDUs that are larger
    than one X.25 data packet in length.

    In the IP encapsulation the PDU is the IP datagram.

3.2 The first octet in the Call User Data (CUD) Field (the first data
    octet in the Call Request packet) is used for protocol
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