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rfc:rfc1975

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
         oem@magna.telco.com

Schremp, Black & Weiss Informational [Page 1] 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Schremp, Black & Weiss Informational [Page 2] RFC 1975 PPP Magnalink Variable Resource Compression August 1996

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   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Schremp, Black & Weiss Informational [Page 3] 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.

Schremp, Black & Weiss Informational [Page 4] 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: karl@ascend.com

Schremp, Black & Weiss Informational [Page 5] 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: dhs@magna.telco.com
 Jeffrey Black
 Telco Systems, Inc.
 Magnalink Communications Division
 63 Nahatan Street
 Norwood Ma. 02062
 Phone: (617) 255-9400
 EMail: jtb@magna.telco.com
 Jeffrey Weiss
 Telco Systems, Inc.
 Magnalink Communications Division
 63 Nahatan Street
 Norwood Ma. 02062
 Phone: (617) 255-9400
 EMail: jaw@magna.telco.com

Schremp, Black & Weiss Informational [Page 6]

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