rfc3051.txt

最新的RFC

Network Working Group                                           J. Heath
Request for Comments: 3051                                     J. Border
Category: Informational                           Hughes Network Systems
                                                            January 2001


         IP Payload Compression Using ITU-T V.44 Packet Method

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document describes a compression method based on the data
   compression algorithm described in International Telecommunication
   Union (ITU-T) Recommendation V.44.  Recommendation V.44 is a modem
   standard but Annex B, Clause B.1, of the recommendation describes the
   implementation of V.44 in packet networks (e.g., V.44 Packet Method).
   This document defines the application of V.44 Packet Method to the
   Internet Protocol (IP) Payload Compression Protocol (RFC 2393).  RFC
   2393 defines a method for applying lossless compression to the
   payload portion of IP datagrams.

   V.44 Packet Method is based upon the LZJH data compression algorithm.
   Throughout the remainder of this document the terms V.44 Packet
   Method and LZJH are synonymous.


















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RFC 3051        IP Payload Compression Using ITU-T V.44     January 2001


Table of Contents

    1. Introduction...................................................2
       1.1 General....................................................2
       1.2 Background of LZJH Data Compression........................2
       1.3 Intellectual Property Rights...............................3
       1.4 Specification of Requirements..............................4
    2. Compression Process............................................4
       2.1 Encoder Dictionary.........................................4
       2.2 Encoder Output.............................................4
       2.3 Padding....................................................4
    3. Decompression Process..........................................5
       3.1 Compressed Datagram........................................5
       3.2 Original Uncompressed Datagram.............................5
    4. IPComp Association (IPCA) Parameters...........................5
       4.1 Transform ID...............................................5
       4.2 Security Association Attributes............................5
       4.3 Manual configuration.......................................5
       4.4 Minimum packet size threshold..............................6
       4.5 Compressibility test.......................................6
    5. Security Considerations........................................6
    6. IANA Considerations............................................6
    7. Acknowledgements...............................................6
    8. References.....................................................6
    9. Authors' Addresses.............................................7
   10. Full Copyright Statement.......................................8

1. Introduction

1.1 General

   This document specifies the application of LZJH data compression, a
   lossless data compression algorithm, to IP datagram payloads.  LZJH
   data compression is to be used in conjunction with the IP Payload
   Compression Protocol (IPComp) [RFC2393].  This document is written
   with the assumption that the reader has an understanding of the
   IPComp protocol.

1.2 Background of LZJH Data Compression

   LZJH is similar to the algorithm described in [LZ78] although it also
   has aspects which are similar to the algorithm described in [LZ77].
   As such, it provides the execution speed and low memory requirements
   of [LZ78] with compression ratios that are better than [LZ77].
   Originally developed for the satellite industry to compress IP
   datagrams independently, it is ideal for the IPComp application.  The
   LZJH algorithm was modified to compress a continuous stream of data
   for a modem environment and this modified version is the basis for



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RFC 3051        IP Payload Compression Using ITU-T V.44     January 2001


   Recommendation V.44.  LZJH is an adaptive, general purpose, lossless
   data compression algorithm.  It was selected by the ITU-T as the
   basis for Recommendation V.44 based on its performance across a wide
   variety of data types, particularly web HTML's, and based on its
   compression ratio characteristics, per MIP and memory utilized (as
   compared to other candidate algorithms).  Its encoder is extremely
   efficient and can encode a two character string with 3 bits the
   second time that string is encountered in the data.

   A typical [LZ78] compression algorithm, such as V.42bis, is not
   suitable for an IPComp application since it takes too long to build
   up its dictionary, resulting in poor compression ratios on IP
   datagrams that are compressed independently.  It also requires too
   many cycles to reset an [LZ78] dictionary between datagrams which
   adversely affects execution times.

   Similarly, a typical [LZ77] compression algorithm suffers in the
   IPComp application due to poor execution times.  Hash tables, that
   help improve execution times when compressing continuous data, may
   cause deterioration of execution times in an IPComp application since
   they must be reset to an initial state between each datagram.

   LZJH not only has superior execution times when encoding or decoding
   packet data, but the reset of the dictionary between IP datagrams is
   trivial.  The encoder requires only the initialization of a 256 word
   array and a handful of variables while the decoder requires only the
   initialization of a handful of variables.

   The LZJH algorithm uses a dictionary of 1525 entries, a total of only
   16K of dictionary memory, for the IPComp application.  During the
   encode process unmatched characters are encoded as ordinals and
   matched redundant strings of characters are encoded as codewords or
   string-extension lengths that represent the redundant strings.
   During the decode process the ordinals, codewords, and string-
   extension lengths are interpreted to re-create exactly the original
   datagram payload.

   The details of LZJH data compression can be found in [V44].

1.3 Intellectual Property Rights

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specifications contained in this
   document.  For more information, consult the online list of claimed
   rights.






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RFC 3051        IP Payload Compression Using ITU-T V.44     January 2001


1.4 Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. Compression Process

   The compression of datagrams is performed by a function called the
   Encoder.

2.1 Encoder Dictionary

   The transmitting entity MUST reset the encoder dictionary prior to
   processing each datagram's payload, as specified in clause 7.5.1 of
   [V44].  This ensures that each datagram's payload can be correctly
   decompressed independently of any other, as is required in an
   environment where datagrams may be lost or received out of order.

   The transmitting entity MUST flush unprocessed encoder data after the
   last byte of the datagram has been passed into the encoder such that
   the compressed datagram can be transmitted as a unit.  The flush
   ensures that all data is processed and included in the output, i.e.,
   the compressed datagram is complete and no data from the current
   datagram will be processed with the next datagram.

2.2 Encoder Output

   The input to the payload compression algorithm is an IP datagram
   payload.  The output of the algorithm is a new (and hopefully
   smaller) payload.  The output payload contains the input payload's
   data in either compressed or uncompressed format.  The input and
   output payloads are each an integral number of bytes in length.

   If the uncompressed form is used, the output payload is identical to
   the input payload and the IPComp header is omitted.  If the
   compressed form is used, the output payload is prepended with the
   IPComp header and encoded as defined in clause 6.3 of [V44].

2.3 Padding

   A datagram payload compressed using LZJH always ends with a FLUSH
   codeword in the last one or two compressed data bytes.  The FLUSH
   codeword may start in the 2nd to the last compressed data byte and
   end in the last compressed data byte or be totally within the last
   data byte.  The FLUSH codeword is used to signal the end of the





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