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Versions: 00 01 02 03 RFC 2395

Internet Draft                                               R. Friend
Expires in six months                                       R. Monsour
                                                           Hi/fn, Inc.
                                                      February 6, 1998



                    IP Payload Compression Using LZS
                      <draft-ietf-ippcp-lzs-03.txt>



Status of this Memo

   This document is an Internet-Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   Distribution of this memo is unlimited.

   It is intended that a future version of this draft be submitted to
   the IESG for publication as an Informational RFC.

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.

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                                                  [Page 1]


Internet Draft       draft-ietf-ippcp-lzs-03.txt         February 6, 1998


Table of Contents

   1. Introduction...................................................2
      1.1 General....................................................2
      1.2 Background of LZS Compression..............................2
      1.3 Licensing..................................................3
      1.4 Specification of Requirements..............................3
   2. Compression Process............................................3
      2.1 Compression History........................................3
      2.2 Anti-expansion of Payload Data.............................3
      2.3 Format of Compressed Datagram Payload......................3
      2.4 Compression Encoding Format................................4
      2.5 Padding....................................................5
   3. Decompression Process..........................................5
   4. IPComp Association (IPCA) Parameters...........................5
      4.1 ISAKMP Transform ID........................................5
      4.2 ISAKMP Security Association Attributes.....................5
      4.3 Manual configuration.......................................5
      4.4 Minimum packet size threshold..............................6
      4.5 Compressibility test.......................................6
   5. Security Considerations........................................6
   6. References.....................................................6
   7. Authors Addresses..............................................7
   8. Appendix: Compression Efficiency versus Datagram Size..........7


1. Introduction

1.1 General

   This document is a submission to the IETF IP Payload Compression
   Protocol (IPPCP) Working Group. Comments are solicited and should be
   addressed to the working group mailing list (ippcp@external.cisco.com)
   or to the editor.

   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



Friend, Monsour                                                  [Page 2]


Internet Draft       draft-ietf-ippcp-lzs-03.txt         February 6, 1998


   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 in
   Section 7.

1.3 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 Process


2.1 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 Anti-expansion of Payload Data

   The maximum expansion produced by the LZS algorithm is 12.5%.

   If the size of a compressed IP datagram, including the Next Header,
   Flags, and CPI fields, is not smaller than the size of the original



Friend, Monsour                                                  [Page 3]


Internet Draft       draft-ietf-ippcp-lzs-03.txt         February 6, 1998


   IP datagram, the IP datagram MUST be sent in the original non-
   compressed form, as described in [IPCOMP].

2.3 Format of Compressed Datagram Payload

   The following is an example datagram that results when using LZS as
   the compression algorithm for the IP Payload Control Protocol.  Note
   that the IP header has been omitted for clarity.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |     Flags     | Compression Parameter Index   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                    Payload Data (variable)                    ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Next Header, Flags, and Compression Parameter Index fields are
   all described in [IPCOMP].

2.4 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>

   <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



Friend, Monsour                                                  [Page 4]


Internet Draft       draft-ietf-ippcp-lzs-03.txt         February 6, 1998


   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     ...

2.5 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].

4.1 ISAKMP Transform ID
   The LZS Transform ID as 0x03, 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 0x03 is used for a manually configured IPComp
   Security Associations.

4.4 Minimum packet size threshold



Friend, Monsour                                                  [Page 5]


Internet Draft       draft-ietf-ippcp-lzs-03.txt         February 6, 1998


   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

   IP payload compression potentially reduces the security of the
   Internet, similar to the effects of IP encapsulation [RFC-2003].  For
   example, IPComp makes it difficult for border routers to filter
   datagrams based on header fields.  In particular, the original value
   of the Protocol field in the IP header is not located in its normal
   positions within the datagram, and any transport-layer header fields
   within the datagram, such as port numbers, are neither located in
   their normal positions within the datagram nor presented in their
   original values after compression.  A filtering border router can
   filter the datagram only if it shares the IPComp Association used for
   the compression.  To allow this sort of compression in environments in
   which all packets need to be filtered (or at least accounted for), a
   mechanism must be in place for the receiving node to securely
   communicate the IPComp Association to the border router.  This might,
   more rarely, also apply to the IPComp Association used for outgoing
   datagrams.

   When IPComp is used in the context of IPSec, it is not believed to
   have an effect on the underlying security functionality provide by
   the IPSec protocol; i.e., the use of compression is not known to
   degrade or alter the nature of the underlying security architecture
   or the encryption technologies used to implement it.

6. References

   [AH]  Kent, S. and Atkinson, R., "IP Authentication Header", draft-
   ietf-ipsec-auth-header-01.txt, Work in Progress, July 1997.
   [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
   at ftp://ftp.cpsc.ucalgary.ca/pub/projects/text.compression.corpus.

   [IPCOMP] Shacham, A., "IP Payload Compression Protocol (IPComp)",
   draft-ietf-ippcp-protocol-01.txt, Work in Progress, October 1997.

   [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.


Friend, Monsour                                                  [Page 6]


Internet Draft       draft-ietf-ippcp-lzs-03.txt         February 6, 1998


   [RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)",
   RFC-1962, June 1996.

   [RFC-1967] K. Schneider, 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", RFC 2119, March 1997.

   [SECDOI] Piper, D., "The Internet IP Security Domain of
   Interpretation for ISAKMP", Internet-Draft:
   draft-ietf-ipsec-ipsec-doi-06.txt, Work in Progress, November 1997.

7. 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.com

8. 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                                                  [Page 7]


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