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.

   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
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