IP Payload Compression Using LZS
draft-ietf-ippcp-lzs-03

Internet Draft R. Friend
Expires in six months R. Monsour
 Hi/fn, Inc.
 February 6, 1998
 IP Payload Compression Using LZS
 <draft-ietf-ippcp-lzs-03.txt>
Status of this Memo
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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.
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.
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Internet Draft draft-ietf-ippcp-lzs-03.txt February 6, 1998
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 Anti-expansion of Payload Data.............................3
 2.3 Format of Compressed Datagram Payload......................3
 2.4 Compression Encoding Format................................4
 2.5 Padding....................................................5
 3. Decompression Process..........................................5
 4. IPComp Association (IPCA) Parameters...........................5
 4.1 ISAKMP Transform ID........................................5
 4.2 ISAKMP 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. References.....................................................6
 7. Authors Addresses..............................................7
 8. Appendix: Compression Efficiency versus Datagram Size..........7
1. Introduction
1.1 General
 This document is a submission to the IETF IP Payload Compression
 Protocol (IPPCP) Working Group. Comments are solicited and should be
 addressed to the working group mailing list (ippcp@external.cisco.com)
 or to the editor.
 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
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 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 Anti-expansion of Payload Data
 The maximum expansion produced by the LZS algorithm is 12.5%.
 If the size of a compressed IP datagram, including the Next Header,
 Flags, and CPI fields, is not smaller than the size of the original
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 IP datagram, the IP datagram MUST be sent in the original non-
 compressed form, as described in [IPCOMP].
2.3 Format of Compressed Datagram Payload
 The following is an example datagram that results when using LZS as
 the compression algorithm for the IP Payload Control Protocol. Note
 that the IP header has been omitted for clarity.
 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Next Header | Flags | Compression Parameter Index |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 ~ Payload Data (variable) ~
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Next Header, Flags, and Compression Parameter Index fields are
 all described in [IPCOMP].
2.4 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
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 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 ...
2.5 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 0x03, 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 0x03 is used for a manually configured IPComp
 Security Associations.
4.4 Minimum packet size threshold
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 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
 IP payload compression potentially reduces the security of the
 Internet, similar to the effects of IP encapsulation [RFC-2003]. For
 example, IPComp makes it difficult for border routers to filter
 datagrams based on header fields. In particular, the original value
 of the Protocol field in the IP header is not located in its normal
 positions within the datagram, and any transport-layer header fields
 within the datagram, such as port numbers, are neither located in
 their normal positions within the datagram nor presented in their
 original values after compression. A filtering border router can
 filter the datagram only if it shares the IPComp Association used for
 the compression. To allow this sort of compression in environments in
 which all packets need to be filtered (or at least accounted for), a
 mechanism must be in place for the receiving node to securely
 communicate the IPComp Association to the border router. This might,
 more rarely, also apply to the IPComp Association used for outgoing
 datagrams.
 When IPComp is used in the context of IPSec, it is not believed to
 have an effect on the underlying security functionality provide by
 the IPSec protocol; i.e., the use of compression is not known to
 degrade or alter the nature of the underlying security architecture
 or the encryption technologies used to implement it.
6. References
 [AH] Kent, S. and Atkinson, R., "IP Authentication Header", draft-
 ietf-ipsec-auth-header-01.txt, Work in Progress, July 1997.
 [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)",
 draft-ietf-ippcp-protocol-01.txt, Work in Progress, October 1997.
 [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.
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 [RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)",
 RFC-1962, June 1996.
 [RFC-1967] K. Schneider, 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", RFC 2119, March 1997.
 [SECDOI] Piper, D., "The Internet IP Security Domain of 
 Interpretation for ISAKMP", Internet-Draft:
 draft-ietf-ipsec-ipsec-doi-06.txt, Work in Progress, November 1997.
7. 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
8. 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 |
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