Network Working Group R. Pereira
Request for Comments: 2394 TimeStep Corporation
Category: Informational December 1998
 IP Payload Compression Using DEFLATE
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 DEFLATE
 compression algorithm. This document defines the application of the
 DEFLATE algorithm to the IP Payload Compression Protocol.
Table of Contents
 1. Introduction...................................................2
 1.1 The DEFLATE Compression Algorithm...........................2
 1.2 Licensing...................................................2
 1.3 Specification of Requirements...............................3
 2. DEFLATE Algorithm Implementation...............................3
 2.1 Compression.................................................3
 2.2 Decompression...............................................4
 3. Thresholds.....................................................4
 4. IPSec Transform Identifier.....................................4
 5. Security Considerations........................................4
 6. References.....................................................5
 7. Acknowledgments................................................5
 8. Editor's Address...............................................5
 9. Full Copyright Statement.......................................6
1. Introduction
 The IP Payload Compression Protocol allows the compression of IP
 datagrams by supporting different compression algorithms. This
 document describes how to integrate the DEFLATE compression algorithm
 [Deutsch96] into IPCOMP [IPCOMP].
 This document SHOULD be read in conjunction with [IPCOMP] and MUST be
 taken in its context.
1.1 The DEFLATE Compression Algorithm
 The 'deflate' compression format [Deutsch96], as used by the PKZIP
 and gzip compressors and as embodied in the freely and widely
 distributed zlib [Gailly95] library source code, has the following
 features:
 o an apparently unencumbered encoding and compression algorithm,
 with an open and publicly-available specification.
 o low-overhead escape mechanism for incompressible data. The PPP
 Deflate specification offers options to reduce that overhead
 further.
 o heavily used for many years in networks, on modem and other point-
 to-point links to transfer files for personal computers and
 workstations.
 o easily achieves 2:1 compression on the Calgary corpus [Corpus90]
 using less than 64KBytes of memory on both sender and receive.
1.2 Licensing
 The zlib source is widely and freely available, subject to the
 following copyright:
 (C) 1995 Jean-Loup Gailly and Mark Adler
 This software is provided 'as-is', without any express or implied
 warranty. In no event will the authors be held liable for any
 damages arising from the use of this software.
 Permission is granted to anyone to use this software for any
 purpose, including commercial applications, and to alter it and
 redistribute it freely, subject to the following restrictions:
 1. The origin of this software must not be misrepresented; you
 must not claim that you wrote the original software. If you use
 this software in a product, an acknowledgment in the product
 documentation would be appreciated but is not required.
 2. Altered source versions must be plainly marked as such, and
 must not be misrepresented as being the original software.
 3. This notice may not be removed or altered from any source
 distribution.
 Jean-Loup Gailly Mark Adler
 gzip@prep.ai.mit.edu madler@alumni.caltech.edu
 If you use the zlib library in a product, we would appreciate
 *not* receiving lengthy legal documents to sign. The sources are
 provided for free but without warranty of any kind. The library
 has been entirely written by Jean-Loup Gailly and Mark Adler; it
 does not include third-party code.
 The deflate format and compression algorithm are based on Lempel-Ziv
 LZ77 compression; extensive research has been done by the GNU Project
 and the Portable Network Graphics working group supporting its patent
 free status.
1.3 Specification of Requirements
 The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
 and "MAY" that appear in this document are to be interpreted as
 described in [Bradner97].
2. DEFLATE Algorithm Implementation
 The DEFLATE compression algorithm was designed by Phil Katz and its
 details are publicly available in [Deutsch96]. Thus it is a good
 freely available algorithm to implement within IPCOMP.
 Compression and decompression algorithm details should be followed as
 outlined in [Deutsch96] or the use of a software library may be
 preferable. Since IPComp is a stateless protocol, history MUST be
 cleared between packets when either compressing or decompressing.
2.1 Compression
 As defined in [IPCOMP], the compression process is determined by the
 IP Compression Association (IPCA). The IPCA MUST define the DEFLATE
 algorithm for the process within this document to take place.
 The compression process entails compressing the data from the IP
 datagram and placing the result after the IPComp header. For
 example, compressing a TCP datagram;
 Before: IP TCP ...
 After: IP IPCOMP (TCP ...)
 Please note how everything after the IPCOMP header is compressed.
 DEFLATE allows for a number of compression levels ranging from best
 compression but slow to fast compression. The level that one
 compresses data is implementation dependant since it is always
 compatible with the decompression algorithm.
2.2 Decompression
 As in the compression process, the IPCA defines the parameters and
 algorithm to utilize for the decompression process.
 As defined in [IPCOMP] the data after the IPComp header is
 decompressed and replaces the IPComp header within the IP header.
 Decompression using the DEFLATE algorithm follows the decompression
 process defined in [Deutsch96].
3. Thresholds
 As stated in [IPCOMP], compression on small buffers does not usually
 work as well as on fast links since the time it takes to compress is
 slower than the time to transport the data. Informal tests show that
 the average buffer size that produces larger results is around 90
 bytes. Thus implementations SHOULD NOT attempt to compress buffers
 smaller than 90 bytes.
 Other than a packet size limit, no compressibility test as defined in
 [IPCOMP] is outlined in this document.
4. IPSec Transform Identifier
 [IPDOI] states that the ISAKMP IPCOMP transform ID for the DEFLATE
 compression algorithm is IPCOMP_DEFLATE. No other ISAKMP parameters
 are required for the IPCOMP DEFLATE algorithm.
5. Security Considerations
 This document does not add any further security considerations that
 [IPCOMP] and [Deutsch96] have already declared.
6. References
 [IPCOMP] Shacham, A., Monsour, R., Pereira, R., and M. Thomas, "IP
 Payload Compression Protocol (IPComp)", RFC 2393,
 December 1998.
 [Deutsch96] Deutsch, P., "DEFLATE Compressed Data Format
 Specification version 1.3", RFC 1951, May 1996.
 [IPDOI] Piper, D., "The Internet IP Security Domain of
 Interpretation for ISAKMP", RFC 2407, November 1998.
 [Corpus90] Bell, T.C., Cleary, G. G. and Witten, I.H., "Text
 Compression", Prentice_Hall, Englewood Cliffs NJ, 1990.
 The compression corpus itself can be found in
 ftp://ftp.uu.net/pub/archiving/zip/
 [Gailly95] Gailly, J.-L., "Zlib 0.95 beta"
7. Acknowledgments
 The author wishes to thank all of the active members of the IPPCP
 working group especially Abraham Shacham and Naganand Doraswamy.
8. Editor's Address
 Roy Pereira
 TimeStep Corporation
 Phone: +1 (613) 599-3610 x 4808
 EMail: rpereira@timestep.com
 The IP Payload Compression Protocol (IPPCP) working group can be
 contacted via email (ippcp@cisco.com) or through its chair:
 Naganand Dorswamy
 Bay Networks
 EMail: naganand@baynetworks.com
9. 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.
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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