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Versions: 00 01                                                         
Network Working Group                           W A Simpson [DayDreamer]
Internet Draft
expires in six months                                        August 1998

                           PPP in Frame Relay

Status of this Memo

   This document is an Internet-Draft.  Internet Drafts are working doc-
   uments of the Internet Engineering Task Force (IETF), its Areas, and
   its Working Groups.  Note that other groups may also distribute work-
   ing documents as Internet Drafts.

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   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) William Allen Simpson (1993-1994, 1996-1998).  All
   Rights Reserved.


   The Point-to-Point Protocol (PPP) [RFC-1661] 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.

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   This specification is intended for those implementations that desire
   to use facilities which are defined for PPP, such as the Link Control
   Protocol, Network-layer Control Protocols, authentication, and com-
   pression.  These capabilities require a point-to-point relationship
   between peers, and are not designed for multi-point or multi-access

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1.  Introduction

   Frame Relay [Q.922] is a relative newcomer to the serial link commu-
   nity.  Like X.25, the protocol was designed to provide virtual cir-
   cuits 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 con-
   trol, simplifying the service immensely.

   At one time, it had been hoped that "PPP in HDLC-like Framing"
   [RFC-1662] would co-exist with other Frame Relay transmissions on the
   same links.  Unfortunately, the Q.922 method for expanding the
   address from 1 to 2 to 4 octets is not reliably distinguishable from
   the ISO 3309 HDLC method, due to the structure of its Data Link Con-
   nection Identifier (DLCI) subfields.  Co-existance is precluded.

   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 [RFC-1490].

1.1.  Terminology

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [RFC-2119].

   To remain consistent with standard Internet practice, and avoid con-
   fusion for people used to reading RFCs, all binary numbers in the
   following descriptions are in Most Significant Bit to Least Signifi-
   cant Bit order, from Most Significant Byte to Least Significant Byte,
   reading from left to right, unless otherwise indicated.  Note that
   this is contrary to ISO and ITU practice, which orders bits as trans-
   mitted (network bit order).  Keep this in mind when comparing this
   document with the other documents.

2.  Physical Layer Requirements

   PPP is capable of operating across most Frame Relay interfaces.  The
   only absolute requirement imposed by PPP is the provision of a bi-
   directional full-duplex circuit, either dedicated (permanent) or
   frame-switched, that can operate in either a bit-synchronous, or
   octet-synchronous mode, transparent to PPP Data Link Layer frames.

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   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, that indicate when the link has become connected or dis-
      connected.  These in turn provide the Up and Down events to the
      PPP LCP state machine.

      Because PPP does not normally require the use of control signals,
      the failure of such signals MUST NOT affect correct operation of
      PPP.  Implications are discussed in [RFC-1662].

2.1.  Transmission Considerations

   The definition of various encodings is the responsibility of the
   DTE/DCE equipment in use, and is outside the scope of this specifica-

   While PPP will operate without regard to the underlying representa-
   tion of the octet stream, bit-synchronous Frame Relay requires NRZ

   In addition, this specification permits octet-synchronous Frame
   Relay, with the same stuffing conventions as HDLC [RFC-1662].

3.  The Data Link Layer

   This specification uses the principles, terminology, and frame struc-
   ture described in [RFC-1490].

   The purpose of this specification is not to document what is already
   standardized in [RFC-1490].  Instead, this document attempts to give
   a concise summary and point out specific options and features used by

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3.1.  Frame Header

   As described in [RFC-1490], Q.922 header address and control fields
   are followed by a Network Layer Protocol Identifier (NLPID) to iden-
   tify the encapsulated packet.  This specification describes the PPP
   Protocol encapsulation.  These fields are transmitted from left to

   |  Flag (0x7e)  |
   |         Q.922 Address         |    Control    |  NLPID(0xcf)  |
   |         PPP Protocol          |

   The PPP Protocol field and the following Information and Padding
   fields are described in the Point-to-Point Protocol Encapsulation

3.2.  Modification of the Basic Frame

   The Link Control Protocol can negotiate modifications to the basic
   frame structure.  This is not compatible with Frame Relay.


      Since Frame Relay Address and Control field values are not con-
      stant, are variable size, and are modified as the frame is trans-
      ported by the network switching fabric, Address-and-Control-Field-
      Compression cannot affect the frame format.


      Since Frame Relay requires a 16-bit FCS, which is modified as the
      frame is transported by the network switching fabric, FCS-
      Alternatives cannot affect the frame format.

   In general, framing-related LCP Configuration Options are not recog-
   nizable, and are not acceptable for negotiation.  The implementation
   MUST NOT send ineffectual options in a Configure-Request, and SHOULD
   respond to such requested options with a Configure-Reject.  See [RFC-
   ffff] for details.

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3.3.  Modification of the Basic Packet

   The Link Control Protocol can negotiate modifications to the basic
   packet structure.  These are transparent to Frame Relay.


      The default Frame Relay header does not align the PPP Information
      field on a 32-bit boundary.  Alignment to a 32-bit boundary occurs
      when the NLPID is removed and the PPP Protocol field is compressed
      to a single octet.  When this improves throughput, Protocol-Field-
      Compression SHOULD be negotiated.

4.  In-Band Protocol Demultiplexing

   The PPP NLPID (CF hex) and PPP Protocol fields easily distinguish the
   PPP encapsulation from the other NLPID encapsulations described in

   The joining of the PPP and NLPID number space has an added advantage,
   in that the LCP Protocol-Reject can be used to indicate NLPIDs that
   are not recognized.  This can eliminate "black-holes" that occur when
   traffic is not supported.

   For those network-layer protocols that have no PPP Protocol assign-
   ment, or have not yet been implemented under the PPP encapsulation,
   or have not been successfully negotiated by a PPP NCP, another method
   of encapsulation defined under [RFC-1490] SHOULD be used.

   Currently, there are no conflicts between NLPID and PPP Protocol val-
   ues.  If a future implementation is configured to send a NLPID value
   which is the same as a compressed Protocol field, that Protocol field
   MUST NOT be sent compressed.

   On reception, the first octet following the Control field is exam-

   -  If the octet is zero, it MUST be assumed that the packet is for-
      matted according to [RFC-1490].

   -  Initial LCP packets contain the sequence cf-c0-21 following the
      Control field.  When a LCP Configure-Request packet is received
      and recognized, the PPP link enters Link Establishment phase.

   -  If the octet is not the PPP NLPID value, and Protocol-Field-
      Compression is enabled, and the associated NCP has been negoti-
      ated, then it is expected to be a compressed PPP Protocol value.

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   -  Otherwise, it MUST be assumed that the packet is formatted accord-
      ing to [RFC-1490].

   Once PPP has entered the Link Establishment phase, packets with other
   NLPID values MUST NOT be sent, and on receipt such packets MUST be
   silently discarded, until the PPP link enters the Network-Layer Pro-
   tocol phase.

   Once PPP has entered the Network-Layer Protocol phase, and success-
   fully negotiated a particular NCP for a PPP Protocol, if a frame
   arrives using another equivalent data encapsulation defined in
   [RFC-1490], the PPP Link MUST re-enter Link Establishment phase and
   send a new LCP Configure-Request.  This prevents "black-holes" that
   occur when the peer loses state.

   An implementation that requires PPP link configuration, and other PPP
   negotiated features (such as authentication), MAY enter Termination
   phase when configuration fails.  Otherwise, when the Configure-
   Request sender reaches the Max-Configure limit, it MUST fall back to
   send only frames encapsulated according to [RFC-1490].

   Implementation Notes

      The PPP Protocol field value 0x00cf is not allowed (reserved) to
      avoid ambiguity when Protocol-Field-Compression is enabled.  For
      consistency, the NLPID value 0xcf MAY be treated as a compressed
      PPP Protocol which indicates that another PPP Protocol packet fol-

      The accidental connection of a link to feed a multipoint network
      (or multicast group) SHOULD result in a misconfiguration indica-
      tion.  This can be detected by multiple responses to the LCP Con-
      figure-Request with the same Identifier, coming from different
      framing addresses.  Some implementations might be physically
      unable to either log or report such information.

5.  Out-of-Band signaling

   There is no generally agreed method of out-of-band signalling.  Until
   such a method is universally available, an implementation MUST use
   In-Band Protocol Demultiplexing for both Permanent and Switched Vir-
   tual Circuits.

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6.  Configuration Details

   The following Configuration Options are recommended:

      Magic Number
      Protocol Field Compression

   The standard LCP configuration defaults apply to Frame Relay links,
   except Maximum-Receive-Unit (MRU).

   To ensure interoperability with existing Frame Relay implementations,
   the initial MRU is 1600 octets [RFC-1490].  This only affects the
   minimum required buffer space available for receiving packets, not
   the size of packets sent.

   The typical network feeding the link is likely to have a MRU of
   either 1500, or 2048 or greater.  To avoid fragmentation, the Maxi-
   mum-Transmission-Unit (MTU) at the network layer SHOULD NOT exceed
   1500, unless a peer MRU of 2048 or greater is specifically negoti-

   Some Frame Relay switches are only capable of 262 octet frames.  It
   is not recommended that anyone deploy or use a switch that is capable
   of less than 1600 octet frames.  However, PPP implementations MUST be
   configurable to limit the size of LCP packets that are sent to 259
   octets (leaving room for the NLPID and PPP Protocol fields), until
   LCP negotiation is complete.

   XID negotiation is not required to be supported for links that are
   capable of PPP negotiation.

   Inverse ARP is not required to be supported for PPP links.  That
   function is provided by PPP NCP negotiation.

Security Considerations

   This specification introduces no known security vulnerabilities.

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   This design was inspired by the paper "Parameter Negotiation for the
   Multiprotocol Interconnect", Keith Sklower and Clifford Frost, Uni-
   versity of California, Berkeley, 1992, unpublished.

   Use of octet-synchronous interfaces, such as SONET/SDH, was first
   proposed by John Bartell (BellSouth).


   [Q.922]     CCITT Recommendation Q.922, "ISDN Data Link Layer Speci-
               fication for Frame Mode Bearer Services", International
               Telegraph and Telephone Consultative Committee, 1992.

   [RFC-1490]  Bradley, T., Brown, C., and Malis, A., "Multiprotocol
               Interconnect over Frame Relay", July 1993.

   [RFC-1661]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
               STD-51, DayDreamer, July 1994.

   [RFC-1662]  Simpson, W., Editor, "PPP in HDLC-like Framing", STD-51,
               DayDreamer, July 1994.

   [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP-14, Harvard University, March

   [RFC-ffff]  Simpson, W., "PPP with Framing Conversion", work in

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   Comments about this document should be discussed on the ietf-
   ppp@merit.edu mailing list.

   This document was reviewed by the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  The working
   group can be contacted via the current chair:

      Karl Fox
      Ascend Communications
      655 Metro Place South,  Suite 370
      Dublin, Ohio  43017


   Questions about this document can also be directed to:

      William Allen Simpson
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

          wsimpson@GreenDragon.com (preferred)

Full Copyright Statement

   Copyright (C) William Allen Simpson (1993-1994, 1996-1998).  All
   Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this doc-
   ument itself may not be modified in any way, except as required to
   translate it into languages other than English.

   This document and the information contained herein is provided on an
   "AS IS" basis and the author(s) DISCLAIM ALL WARRANTIES, EXPRESS OR

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