Network Working Group                                         D. Schremp
Request for Comments: 1975                                      J. Black
Category: Informational                                         J. Weiss
                                                              Magnalink
                                                            August 1996

             PPP Magnalink Variable Resource Compression

Status of This Memo

  This memo provides information for the Internet community.  This memo
  does not specify an Internet standard of any kind.  Distribution of
  this memo is unlimited.

Abstract

  The Point-to-Point Protocol (PPP) [1] provides a standard method of
  encapsulating multiple protocol datagrams over point-to-point links.
  The PPP Compression Control Protocol [2] provides a method for
  negotiating data compression over PPP links.

  The Magnalink Variable Resource Compression Algorithm (MVRCA) allows
  a wide range of interoperable compression implementations whose
  performance characteristics are a function of available CPU and
  memory resources.

Introduction

  The Magnalink variable resource compression algorithm defines a
  family of interoperable compression solutions with compression
  performance as a function of available CPU and memory resources.  It
  addresses the need for an algorithm which can be tailored to the
  system on which it is implemented without compromising
  interoperability.

Licensing

  Source licenses are available on a non-discriminatory basis.

  The contact person for evaluation under NDA and Licensing is:

          Director of OEM Sales
          Magnalink Communications Division
          Telco Systems Inc.
          63 Nahatan Street
          Norwood, Mass. 02062
          Phone: (617) 255-9400, Fax: (617) 255-5885
          [email protected]



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RFC 1975      PPP Magnalink Variable Resource Compression    August 1996


MVRCA Packets

  Before any MVRCA packets may be communicated, PPP must reach the
  Network-Layer Protocol phase[1], and the Compression Control Protocol
  must reach the Opened state.

  The text of a Packet to be compressed begins with PPP Protocol
  number.  The Packet header including the PPP Protocol number may have
  already been compressed when Protocol-Field-Compression has been
  negotiated.

Reliability and Sequencing

  MVRCA packets may be sent across an unreliable link or may use a
  reliable link as described in "PPP Reliable Transmission"[3] if the
  reliable link has been negotiated. If frames are delivered out of
  order or a frame is dropped, the decompressor will detect this and
  requests a resynchronization using the Reset-Req and Reset-Ack types
  of the CCP[2], with the compressor for the affected context.

Data Expansion

  Although the compression algorithm may occasionally expand a data
  packet, there is no expansion in MVRCA since any expanded data is
  instead sent uncompressed. Dictionary synchronization is maintained
  across uncompressed packets.

Encapsulation

  The encapsulation consists of the PPP Protocol Identifier, a bit to
  indicate if the data is compressed, the Context Identifier(CID), a
  proprietary flag bit (E), a Packet Integrity Byte(PIB), and the
  Compressed data.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | PPP Protocol Identifier       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |C|E|   CID     |     PIB       |  C compressed flag
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  1 data is compressed
     | Compressed data  ...             0 data is not compressed
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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Compressed/Uncompressed Flag (C)

  When attempting to compress certain types of Packets or Fragments the
  compressor may not be effective. When this occurs the uncompressed
  data is added to the compression History Buffer and sent across the
  link in frame with the Compressed/Uncompressed Flag(C) set to 0.

Context Identifier (CID)

  Since PPP will transport multiple protocol datagrams it may be
  advantageous to compress each protocol or each virtual circuit in a
  different History Buffer or Context. The CID allows the compressor to
  indicate to the decompressor which History Buffer the compressor
  decided to use for a given Packet. The basis of this decision is up
  to the implementor. The number of buffers and size of each buffer is
  negotiated.

  A CID of 0 indicates that the Packet by Packet context will be used
  if it has been negotiated. The Packet by Packet context is cleared
  between Packets so that this History Buffer is not maintained across
  Packet boundaries.

Packet Integrity Byte (PIB)

  To ensure that Packets are being compressed and decompressed
  correctly and to ensure History Buffer synchronization is maintained,
  a Packet Integrity Byte is added to the packet header.

  The packet integrity byte is defined in the full protocol
  specification.

Configuration Option Format

  Description

  The CCP MVRCA Configuration Option negotiates the use of MVRCA on the
  link. By default or ultimate disagreement, no compression is used.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |FE |P| History |  # Contexts   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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RFC 1975      PPP Magnalink Variable Resource Compression    August 1996


  Type

     24

  Length

      4


  FE  - Features

  Negotiates features specific to this compression algorithm.


  History

  Defines the size of the compression history buffer.  Valid values are
  defined in the full protocol specification.


  # Contexts

  This is the number of contexts. Each context implies the  creation of
  a History Buffer for that context of the size  indicated in the
  Context History field. Values are  1-63.  This value includes both
  the Packet by Packet context and the number of contexts for which
  history is maintained.  Therefore, when this value is 1 and the P
  (Packet by Packet) flag is also 1, then only in packet compression is
  supported and history context is not retained across packet
  boundaries.  The Context Identifier (CID) starts with 1 for contexts
  where the history is maintained.


  P - Packet by Packet flag

  When 1, packet by packet compression is enabled for the context whose
  context ID is 0.  When P is 0, packet by packet compression is not
  supported.













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RFC 1975      PPP Magnalink Variable Resource Compression    August 1996


Security Considerations

  Security issues are not discussed in this memo.

References

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

  [2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962,
      June 1996.

  [3] Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994.

Acknowledgments

Chair's Address

  The working group can be contacted via the current chair:

  Karl Fox
  Ascend Communications
  3518 Riverside Drive, Suite 101
  Columbus, Ohio 43221

  EMail: [email protected]

























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RFC 1975      PPP Magnalink Variable Resource Compression    August 1996


Authors' Addresses

  Comments about this document may also be directed to the authors.

  Doug Schremp
  Telco Systems, Inc.
  Magnalink Communications Division
  63 Nahatan Street
  Norwood Ma. 02062

  Phone: (617) 255-9400
  EMail: [email protected]


  Jeffrey Black
  Telco Systems, Inc.
  Magnalink Communications Division
  63 Nahatan Street
  Norwood Ma. 02062

  Phone: (617) 255-9400
  EMail: [email protected]


  Jeffrey Weiss
  Telco Systems, Inc.
  Magnalink Communications Division
  63 Nahatan Street
  Norwood Ma. 02062

  Phone: (617) 255-9400
  EMail: [email protected]



















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