RFC2394 - IP Payload Compression Using DEFLATE

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.

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:

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

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

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