RFC2394 - IP Payload Compression Using DEFLATE

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　　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)", RFC2393,
December 1998.

[Deutsch96] Deutsch, P., "DEFLATE Compressed Data Format
Specification version 1.3", RFC1951, May 1996.

[IPDOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC2407, 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
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.

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