rfc2118.txt

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Network Working Group                                            G. Pall
Request for Comments: 2118                         Microsoft Corporation
Category: Informational                                       March 1997



          Microsoft Point-To-Point Compression (MPPC) Protocol

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 for
   transporting multi-protocol datagrams over point-to-point links.

   The PPP Compression Control Protocol [2] provides a method to
   negotiate and utilize compression protocols over PPP encapsulated
   links.

   This document describes the use of the Microsoft Point to Point
   Compression protocol (also referred to as MPPC in this document) for
   compressing PPP encapsulated packets.

Table of Contents

   1.     Introduction ..........................................    2
      1.1       Licensing .......................................    2
      1.2.      Specification of Requirements ...................    2
   2.     Configuration Option Format ...........................    3
   3.     MPPC Packets ..........................................    4
      3.1       Packet Format....................................    5
   4. Description of Compressor and Encoding ....................    6
      4.1       Literal Encoding ................................    7
      4.2       Copy Tuple Encoding .............................    7
          4.2.1     Offset Encoding .............................    7
          4.2.2     Length-of-Match Encoding ....................    7
      4.3       Synchronization .................................    8
   SECURITY CONSIDERATIONS ......................................    8
   REFERENCES ...................................................    9
   ACKNOWLEDGEMENTS .............................................    9
   CHAIR'S ADDRESS    ...........................................    9
   AUTHORS' ADDRESS .............................................    9





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RFC 2118                     MPPC Protocol                    March 1997


1.  Introduction


   The Microsoft Point to Point Compression scheme is a means of
   representing arbitrary Point to Point Protocol (PPP) packets in a
   compressed form. The MPPC algorithm is designed to optimize processor
   utilization and bandwidth utilization in order to support large
   number of simultaneous connections. The MPPC algorithm is also
   optimized to work efficiently in typical PPP scenarios
   (1500 byte MTU, etc.).

   The MPPC algorithm uses an LZ [3] based algorithm with a sliding
   window history buffer.

   The MPPC algorithm keeps a continous history so that after 8192 bytes
   of data has been transmitted compressed there is always 8192 bytes of
   history to use for compressing, except when the history is flushed.

1.1.  Licensing

   MPPC can only be used in products that implement the Point to Point
   Protocol AND for the sole purpose of interoperating with other MPPC
   and Point to Point Protocol implementations.

   Source and object licenses are available on a non-discriminatory
   basis from Stac Electronics. Please contact:

         Cheryl Poland
         Stac Electronics
         12636 High Bluff Drive,
         San Deigo, CA 92130
         Phone: [phone]
         Email: [e-mail]

1.2.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.







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RFC 2118                     MPPC Protocol                    March 1997


   SHOULD    This word, or the adjective "recommended", means that there
             may exist valid reasons in particular circumstances to
             ignore this item, but the full implications MUST be
             understood and carefully weighed before choosing a
             different course.

   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST be
             prepared to interoperate with another implementation which
             does include the option.

2.  Configuration Option Format

   Description

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

   A summary of the CCP Configuration Option format is shown below.
   The fields are transmitted from left to right.

    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     |        Supported Bits         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Supported Bits          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      18

   Length

      6

   Supported Bits

      This field is 4 octets, most significant octet first. The least
      significant bit in the least significant octet set to 1 indicates
      desire to negotiate MPPC.

      All other bits MUST be set to 0.





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RFC 2118                     MPPC Protocol                    March 1997


3.  MPPC Packets

   Before any MPPC packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the CCP Control Protocol must reach
   the Opened state.

   Exactly one MPPC datagram is encapsulated in the PPP Information
   field. The PPP Protocol field indicates type hex 00FD for all
   compressed datagrams.

   The maximum length of the MPPC datagram transmitted over a PPP link
   is the same as the maximum length of the Information field of a PPP
   encapsulated packet. Since the history buffer is limited to 8192
   bytes, this length cannot be greater than 8192 bytes.

   Only packets with PPP Protocol numbers in the range hex 0021 to hex
   00FA are compressed.  Other packets are not passed thru the MPPC
   processor and are sent with their original PPP Protocol numbers.

   Padding

      It is recommended that padding not be used with MPPC since it
      defeats the purpose of compression. If the sender must use padding
      it MUST negotiate the Self-Describing-Padding Configuration option
      during LCP phase and use self-describing pads.

   Reliability and Sequencing

      The MPPC scheme does not require a reliable link.  Instead, it
      relies on a 12 bit coherency count in each packet to keep the
      history buffers synchronized.  If the receiver recognizes that the
      coherency count received in the packet does not match the count it
      is expecting, it sends a CCP Reset-Request packet to resynchronize
      its history buffer with the sender's history buffer.

      MPPC expects the packets to be delivered in sequence, otherwise
      history buffer re-synchronization will not occur.

      MPPC MAY be used over a reliable link, as described in "PPP
      Reliable Transmision" [5], but this typically just adds
      unnecessary overhead since only the coherency count is required.

   Data Expansion

      If compressing the data results in data expansion, the original
      data is sent as an uncompressed MPPC packet. The sender must flush
      the history before compressing any more data and set the FLUSHED
      bit on the next outgoing packet.



Pall                         Informational                      [Page 4]

RFC 2118                     MPPC Protocol                    March 1997


3.1.  Packet Format

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         PPP Protocol          |A|B|C|D| Coherency Count       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Compressed Data...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PPP Protocol

      The PPP Protocol field is described in the Point-to-Point Protocol
      Encapsulation [1].

      When the MPPC compression protocol is successfully negotiated by
      the PPP Compression Control Protocol, the value is hex 00FD. This
      value MAY be compressed when Protocol-Field-Compression is
      negotiated.

   Bit A

      This bit indicates that the history buffer has just been
      initialized before this packet was generated.  This packet can