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
 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.
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>
 <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 =わ 14
 01 =わ 3 1111 0111 =わ 15
 10 =わ 4 1111 1000 =わ 16
 1100 =わ 5 1111 1001 =わ 17
 1101 =わ 6 1111 1010 =わ 18
 1110 =わ 7 1111 1011 =わ 19
 1111 0000 =わ 8 1111 1100 =わ 20
 1111 0001 =わ 9 1111 1101 =わ 21
 1111 0010 =わ 10 1111 1110 =わ 22
 1111 0011 =わ 11 1111 1111 0000 =わ 23
 1111 0100 =わ 12 1111 1111 0001 =わ 24
 1111 0101 =わ 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].
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.
4.5 Compressibility test
 There is no adaptive algorithm embodied in the LZS algorithm, for
 compressibility testing, as referenced in [IPCOMP].
5. Security Considerations
 This document does not add any further security considerations that
 [IPCOMP] and [Deutsch96] have already declared.
6. Acknowledgments
 The LZS details presented here are similar to those in PPP LZS-DCP
 Compression Protocol (LZS-DCP), [RFC-1967].
 The author wishes to thank the participants of the IPPCP working
 group mailing list whose discussion is currently active and is
 working to generate the protocol specification for integrating
 compression with IP.
7. References
 [AH] Kent, S., and R., Atkinson, "IP Authentication Header",
 RFC 2402, November 1998.
 [ANSI94] American National Standards Institute, Inc., "Data
 Compression Method for Information Systems," ANSI X3.241-
 1994, August 1994.
 [Calgary] Text Compression Corpus, University of Calgary, available
 at ftp://ftp.cpsc.ucalgary.ca/pub/projects/text.
 compression.corpus.
 [IPCOMP] Shacham, A., "IP Payload Compression Protocol (IPComp)",
 RFC 2393, December 1998.
 [LZ1] Lempel, A., and Ziv, J., "A Universal Algorithm for
 Sequential Data Compression", IEEE Transactions On
 Information Theory, Vol. IT-23, No. 3, May 1977.
 [RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)",
 RFC 1962, June 1996.
 [RFC-1967] Schneider, K., and R. Friend, "PPP LZS-DCP Compression
 Protocol (LZS-DCP)", RFC 1967, August 1996.
 [RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
 October 1996.
 [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [SECDOI] Piper, D., "The Internet IP Security Domain of
 Interpretation for ISAKMP", RFC 2407, November 1998.
8. Authors' Addresses
 Robert Friend
 Hi/fn Inc.
 5973 Avenida Encinas
 Suite 110
 Carlsbad, CA 92008
 EMail: rfriend@hifn.com
 Robert Monsour
 Hi/fn Inc.
 2105 Hamilton Avenue
 Suite 230
 San Jose, CA 95125
 EMail: rmonsour@hifn.com
9. Appendix: Compression Efficiency versus Datagram Size
 The following table offers some guidance on the compression
 efficiency that can be achieved as a function of datagram size. Each
 entry in the table shows the compression ratio that was achieved when
 LZS was applied to a test file using datagrams of a specified size.
 The test file was the University of Calgary Text Compression Corpus
 [Calgary]. The Calgary Corpus consists of 18 files with a total size
 (all files) of 3.278MB.
 Datagram size,|
 bytes | 64 128 256 512 1024 2048 4096 8192 16384
 --------------|----------------------------------------------------
 Compression |1.18 1.28 1.43 1.58 1.74 1.91 2.04 2.11 2.14
 ratio |
10. Full Copyright Statement
 Copyright (C) The Internet Society (1998). All Rights Reserved.
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 included on all such copies and derivative works. However, this
 document itself may not be modified in any way, such as by removing
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