rfc2395.txt

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Network Working Group                                        R. Friend
Request for Comments: 2395                                  R. Monsour
Category: Informational                                    Hi/fn, Inc.
                                                         December 1998


                    IP Payload Compression Using LZS

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 (1998).  All Rights Reserved.

Abstract

   This document describes a compression method based on the LZS
   compression algorithm. This document defines the application of the
   LZS algorithm to the IP Payload Compression Protocol [IPCOMP].
   [IPCOMP] defines a method for applying lossless compression to the
   payloads of Internet Protocol datagrams.

Table of Contents

   1. Introduction...................................................2
      1.1 General....................................................2
      1.2 Background of LZS Compression..............................2
      1.3 Licensing..................................................3
      1.4 Specification of Requirements..............................3
   2. Compression Process............................................3
      2.1 Compression History........................................3
      2.2 Compression Encoding Format................................3
      2.3 Padding....................................................4
   3. Decompression Process..........................................4
   4. IPComp Association (IPCA) Parameters...........................4
      4.1 ISAKMP Transform ID........................................5
      4.2 ISAKMP Security Association Attributes.....................5
      4.3 Manual configuration.......................................5
      4.4 Minimum packet size threshold..............................5
      4.5 Compressibility test.......................................5
   5. Security Considerations........................................5
   6. Acknowledgements...............................................5
   7. References.....................................................6
   8. Authors' Addresses.............................................7



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   9. Appendix: Compression Efficiency versus Datagram Size..........8
   10. Full Copyright Statement......................................9

1. Introduction

1.1 General

   This document specifies the application of LZS compression, a
   lossless compression algorithm, to IP datagram payloads. This
   document is to be used in conjunction with the IP Payload Compression
   Protocol [IPCOMP].  This specification assumes a thorough
   understanding of the IPComp protocol.

1.2 Background of LZS Compression

   Starting with a sliding window compression history, similar to [LZ1],
   Hi/fn developed a new, enhanced compression algorithm identified as
   LZS. The LZS algorithm is a general purpose lossless compression
   algorithm for use with a wide variety of data types.  Its encoding
   method is very efficient, providing compression for strings as short
   as two octets in length.

   The LZS algorithm uses a sliding window of 2,048 bytes.  During
   compression, redundant sequences of data are replaced with tokens
   that represent those sequences. During decompression, the original
   sequences are substituted for the tokens in such a way that the
   original data is exactly recovered. LZS differs from lossy
   compression algorithms, such as those often used for video
   compression, that do not exactly reproduce the original data.

   The details of LZS compression can be found in [ANSI94].

   The efficiency of the LZS algorithm depends on the degree of
   redundancy in the original data.  A table of compression ratios for
   the [Calgary] Corpus file set is provided in the appendix in Section
   7.















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1.3 Licensing

   Hi/fn, Inc. holds patents on the LZS algorithm. Licenses for a
   reference implementation are available for use in IPPCP, IPSec, TLS
   and PPP applications at no cost.  Source and object licenses are
   available on a non-discriminatory basis. Hardware implementations are
   also available.  For more information, contact Hi/fn at the address
   listed with the authors' addresses.

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 [RFC-2119].

2. Compression Process

2.1 Compression History

   The sender MUST reset the compression history prior to processing
   each datagram's payload. This ensures that each datagram's payload
   can be decompressed independently of any other, as is needed when
   datagrams are received out of order.

   The sender MUST flush the compressor each time it transmits a
   compressed datagram.  Flushing means that all data going into the
   compressor is included in the output, i.e., no data is held back in
   the hope of achieving better compression.  Flushing is necessary to
   prevent a datagram's data from spilling over into a later datagram.

2.2 Compression Encoding Format

   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 [ANSI94], which is repeated
   here for informational purposes ONLY.

   <Compressed Stream> := [<Compressed String>] <End Marker>
   <Compressed String> := 0 <Raw Byte> | 1 <Compressed Bytes>
   <Raw Byte> := <b><b><b><b><b><b><b><b>          (8-bit byte)
   <Compressed Bytes> := <Offset> <Length>



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   <Offset> := 1 <b><b><b><b><b><b><b> |           (7-bit offset)
               0 <b><b><b><b><b><b><b><b><b><b><b> (11-bit offset)
   <End Marker> := 110000000

   <b> := 1 | 0

   <Length> :=
   00        = 2     1111 0110      = 14
   01        = 3     1111 0111      = 15
   10        = 4     1111 1000      = 16
   1100      = 5     1111 1001      = 17
   1101      = 6     1111 1010      = 18
   1110      = 7     1111 1011      = 19
   1111 0000 = 8     1111 1100      = 20
   1111 0001 = 9     1111 1101      = 21
   1111 0010 = 10    1111 1110      = 22
   1111 0011 = 11    1111 1111 0000 = 23
   1111 0100 = 12    1111 1111 0001 = 24
   1111 0101 = 13     ...

2.3 Padding

   A datagram payload compressed using LZS always ends with the last
   compressed data byte (also known as the <end marker>), which is used
   to disambiguate padding.  This allows trailing bits as well as bytes
   to be considered padding.

   The size of a compressed payload MUST be in whole octet units.

3. Decompression Process

   If the received datagram is compressed, the receiver MUST reset the
   decompression history prior to processing the datagram. This ensures
   that each datagram can be decompressed independently of any other, as
   is needed when datagrams are received out of order. Following the
   reset of the decompression history, the receiver decompresses the
   Payload Data field according to the encoding specified in section 3.2
   of [ANSI94].

   If the received datagram is not compressed, the receiver needs to
   perform no decompression processing and the Payload Data field of the
   datagram is ready for processing by the next protocol layer.

4. IPComp Association (IPCA) Parameters

   ISAKMP MAY be used to negotiate the use of the LZS compression method
   to establish an IPCA, as defined in [IPCOMP].




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4.1 ISAKMP Transform ID

   The LZS Transform ID as IPCOMP_LZS, as specified in The Internet IP
   Security Domain of Interpretation [SECDOI].  This value is used to
   negotiate the LZS compression algorithm under the ISAKMP protocol.

4.2 ISAKMP Security Association Attributes

   There are no other parameters required for LZS compression negotiated
   under ISAKMP.

4.3 Manual configuration

   The CPI value IPCOMP_LZS is used for a manually configured IPComp
   Compression Associations.

4.4 Minimum packet size threshold

   As stated in [IPCOMP], small packets may not compress well.  Informal
   tests using the LZS algorithm over the Calgary Corpus data set show
   that the average payload size that may produce expanded data is
   approximately 90 bytes.  Thus implementations may not want to attempt
   to compress payloads smaller than 90 bytes.