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Network Working Group                                        R. FriendRequest for Comments: 2395                                  R. MonsourCategory: Informational                                    Hi/fn, Inc.                                                         December 1998IP Payload Compression Using LZSStatus 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 Contents1. Introduction...................................................21.1 General....................................................21.2 Background of LZS Compression..............................21.3 Licensing..................................................31.4 Specification of Requirements..............................32. Compression Process............................................32.1 Compression History........................................32.2 Compression Encoding Format................................32.3 Padding....................................................43. Decompression Process..........................................44. IPComp Association (IPCA) Parameters...........................44.1 ISAKMP Transform ID........................................54.2 ISAKMP Security Association Attributes.....................54.3 Manual configuration.......................................54.4 Minimum packet size threshold..............................54.5 Compressibility test.......................................55. Security Considerations........................................56. Acknowledgements...............................................57. References.....................................................68. Authors' Addresses.............................................7Friend & Monsour             Informational                      [Page 1]

RFC 2395            IP Payload Compression Using LZS       December 19989. Appendix: Compression Efficiency versus Datagram Size..........810. Full Copyright Statement......................................91. Introduction1.1 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 inSection7.Friend & Monsour             Informational                      [Page 2]

RFC 2395            IP Payload Compression Using LZS       December 19981.3 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 Process2.1 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>Friend & Monsour             Informational                      [Page 3]

RFC 2395            IP Payload Compression Using LZS       December 1998   <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].Friend & Monsour             Informational                      [Page 4]

RFC 2395            IP Payload Compression Using LZS       December 19984.1 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.Friend & Monsour             Informational                      [Page 5]

RFC 2395            IP Payload Compression Using LZS       December 19987. 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              atftp://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.Friend & Monsour             Informational                      [Page 6]

RFC 2395            IP Payload Compression Using LZS       December 19988. 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.comFriend & Monsour             Informational                      [Page 7]

RFC 2395            IP Payload Compression Using LZS       December 19989. 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         |Friend & Monsour             Informational                      [Page 8]

RFC 2395            IP Payload Compression Using LZS       December 199810.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Friend & Monsour             Informational                      [Page 9]