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Internet Draft R. Monsour Expires in six months Hi/fn, Inc. July 29, 1997 IP Payload Compression Using LZS <draft-ietf-ippcp-lzs-00.txt> Status of this Memo This document is an Internet-Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet-Drafts draft documents are valid for a maximum of six months and may be updated, replaced, or obsolete by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this memo is unlimited. It is intended that a future version of this draft be submitted to the IESG for publication as an Informational RFC. Abstract This document describes a IP compression method based on the LZS compression algorithm. This document defines the application of the LZS algorithm to the IP Payload Compression Protocol [Thomas]. [Thomas] 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. Table of Contents R. Monsour [Page 1] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 1997 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......................4 2.4 Compression Encoding Format................................5 2.5 Padding....................................................6 3. Decompression Process..........................................6 4. Security Considerations........................................6 5. References.....................................................6 6. Authors Addresses..............................................8 7. Appendix: Compression Efficiency versus Datagram Size..........8 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. It is to be used in conjunction with the IP Payload Compression Protocol which, at this writing, is under development by the IP Payload Compression Protocol working group. 1.2 Background of LZS Compression Starting with a sliding window compression history, similar to LZ1 [LZ1], Hi/fn developed a new, enhanced compression algorithm identified as LZS. The LZS algorithm is optimized to compress all file types as efficiently as possible. Even string matches as short as two octets are effectively compressed. 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 R. Monsour [Page 2] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 1997 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 typical compression ratio is 2:1. LZS achieves a compression ratio of 2.34:1 on the University of Calgary Text Compression Corpus [Calgary]. 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 author's address. 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 [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%. R. Monsour [Page 3] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 1997 If the size of a compressed IP datagram, including whatever overhead is included to define the compression protocol, is not smaller than the size of the original IP datagram, the IP datagram MUST be sent in the original non-compressed form. This policy ensures saving the decompression processing cycles and avoiding incurring IP datagram fragmentation when the expanded datagram is larger than the MTU. Small IP datagrams are more likely to expand as a result of compression. Therefore, a numeric threshold SHOULD be applied before compression, where IP datagrams of size smaller than the threshold are sent in the original form without attempting compression. The numeric threshold is implementation dependent. An IP datagram with payload, which has been previously compressed, tends not to compress any further. Such previously compressed payload may be the result of external processes, such as compression applied by an upper layer in the communication stack, or by an off-line compression utility. An adaptive algorithm SHOULD be implemented in order to avoid the performance penalty of futile compression attempts. Such as adaptive algorithm is implementation dependent and independent of compression method. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Compression Parameters Index (CPI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |C| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Payload Data (variable) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Compression Parameters Index (CPI) The Compression Parameters Index (CPI) is a 32-bit pseudo-random value identifying the association for this datagram. The details of its use can be found in [Thomas]. C R. Monsour [Page 4] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 1997 This one-bit field, when one, indicates that the datagram payload is compressed; a value of zero indicates that the datagram payload is not compressed. Reserved This 31-bit field is reserved for future use and MUST be zeroed by the sender. It SHOULD be ignored by the receiver. Payload Data This variable-length field contains the optionally compressed datagram payload. 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. If the compressed form is used, the output payload as defined in section 3.2 of [ANSI94], and is repeated here for informational purposes ONLY. <Compressed Stream> := [<Compressed String>] <End Marker> <Compressed String> := 0 <Raw Byte> | 1 <Compressed Bytes> <Raw Byte> := (8-bit byte) <Compressed Bytes> := <Offset> <Length> <Offset> := 1 | (7-bit offset) 0 (11-bit offset) <End Marker> := 110000000 := 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 R. Monsour [Page 5] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 1997 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 ... 2.5 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. 3. Decompression Process If the C bit of the received datagram is a one (i.e., indicating the datagram is compressed), the receiver MUST reset the compression 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 compression history, the receiver decompresses the Payload Data field according to the encoding specified in section 3.2 of [ANSI94]. If the C bit of the received datagram is zero, 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. Security Considerations This memo discusses the use of lossless compression technology in the Internet Protocol. This can include use of IP Security. The proposed use of compression within this protocol is not believed to have an effect on the underlying security functionality provide by the 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. The use of compression does change the length of ESP payloads, in a manner that depends on the data prior to encryption. Thus, the use of compression may have an effect on the ability of an eavesdropper to glean information by analyzing the length of transmitted packets. 5. References [AH] Kent, S. and Atkinson, R., "IP Authentication Header", draft- ietf-ipsec-auth-header-01.txt, Work in Progress, July 1997. R. Monsour [Page 6] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 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. [DOI] Piper, D., "The Internet IP Security Domain of Interpretation for ISAKMP", draft-ietf-ipsec-ipsec-doi-02.txt, Work in Progress, February 1997. [ESP] Kent, S. and Atkinson, R., "IP Encapsulating Security Payload", draft-ietf-ipsec-esp-v2-00.txt, Work in Progress, July 1997. [ISAKMP] Maughan, D., Schertler, M., Schneider, M., and Turner, J., "Internet Security Association and Key Management Protocol (ISAKMP)", draft-ietf-ipsec-isakmp-08.txt, Work in Progress, July 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. [RFC-1700] Reynolds, J., Postel, J., "Assigned Numbers", RFC 1700, October 1994. [RFC-1883] Deering, S., Hinden, R., "Internet Protocol, Version 6 (IPv6) Specification", RFC 1883, April 1996. [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. [Shacham] Shacham, A., "IP Payload Compression Protocol (IPComp)", draft-ietf-ippcp-protocol-00.txt, Work in Progress, July 1997. [Thayer] Thayer, R., "Compression Payload for Use with IP Security", draft-thayer-seccomp-01.txt, Work in Progress, March, 1997. [Thomas] Thomas, M., "The Compressed Payload Header", draft-thomas- R. Monsour [Page 7] Internet Draft draft-ietf-ippcp-lzs-00.txt July 29, 1997 ippcp-compression-00.txt, Work in Progress, July 1997. 6. Authors Addresses Robert Monsour Hi/fn Inc. 5973 Avenida Encinas Suite 110 Carlsbad, CA 92008 Email: rmonsour@hifn.com 7. 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 length of the file prior to compression was 3,278,000 bytes. When the entire file was compressed as a single payload, a compression ratio of 2.34 resulted. 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 | R. Monsour [Page 8]