RFC 1973 (rfc1973) - Page 1 of 8

PPP in Frame Relay

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Network Working Group                                         W. Simpson
Request for Comments: 1973                                    Daydreamer
Category: Standards Track                                      June 1996


                           PPP in Frame Relay



Status of this Memo

   This document specifies an Internet standards track protocol for
   the Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.


Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   This document describes the use of Frame Relay for framing PPP
   encapsulated packets.


Applicability

   This specification is intended for those implementations which desire
   to use facilities which are defined for PPP, such as the Link Control






















Simpson                      Standards Track                    [Page i]

RFC 1973                     PPP in Frame Relay                June 1996


   Protocol, Network-layer Control Protocols, authentication, and
   compression.  These capabilities require a point-to-point
   relationship between peers, and are not designed for multi-point or
   multi-access environments.


Table of Contents


     1.     Introduction ..........................................    1

     2.     Physical Layer Requirements ...........................    1

     3.     The Data Link Layer ...................................    2
        3.1       Frame Format ....................................    2
        3.2       Modification of the Basic Frame .................    3

     4.     In-Band Protocol Demultiplexing .......................    4

     5.     Out-of-Band signaling .................................    5

     6.     Configuration Details .................................    5

     SECURITY CONSIDERATIONS ......................................    7

     REFERENCES ...................................................    7

     ACKNOWLEDGEMENTS .............................................    7

     CHAIR'S ADDRESS ..............................................    8

     AUTHOR'S ADDRESS .............................................    8



















Simpson                      Standards Track                   [Page ii]

RFC 1973                     PPP in Frame Relay                June 1996



1.  Introduction

   Frame Relay [2] is a relative newcomer to the serial link community.
   Like X.25, the protocol was designed to provide virtual circuits for
   connections between stations attached to the same Frame Relay
   network.  The improvement over X.25 is that Q.922 is restricted to
   delivery of packets, and dispenses with sequencing and flow control,
   simplifying the service immensely.

   PPP uses ISO 3309 HDLC as a basis for its framing [3].

   When Frame Relay is configured as a point-to-point circuit, PPP can
   use Frame Relay as a framing mechanism, ignoring its other features.
   This is equivalent to the technique used to carry SNAP headers over
   Frame Relay [4].

   At one time, it had been hoped that PPP in HDLC-like frames and Frame
   Relay would co-exist on the same links.  Unfortunately, the Q.922
   method for expanding the address from 1 to 2 to 4 octets is not
   indistinguishable from the ISO 3309 method, due to the structure of
   its Data Link Connection Identifier (DLCI) subfields.  Co-existance
   is precluded.



2.  Physical Layer Requirements

   PPP treats Frame Relay framing as a bit-synchronous link.  The link
   MUST be full-duplex, but MAY be either dedicated (permanent) or
   switched.

   Interface Format

      PPP presents an octet interface to the physical layer.  There is
      no provision for sub-octets to be supplied or accepted.

   Transmission Rate

      PPP does not impose any restrictions regarding transmission rate,
      other than that of the particular Frame Relay interface.

   Control Signals

      Implementation of Frame Relay requires the provision of control
      signals, which indicate when the link has become connected or
      disconnected.  These in turn provide the Up and Down events to the
      LCP state machine.



Simpson                      Standards Track
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