Internet Draft                                              R. Monsour
Expires in six months                                      Hi/fn, Inc.
                                                         July 29, 1997

                    IP Payload Compression Using LZS

Status of this Memo

   This document is an Internet-Draft.  Internet Drafts are working
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   It is intended that a future version of this draft be submitted to the
   IESG for publication as an Informational RFC.


   This document describes a IP compression method based on the LZS
   compression algorithm. This document defines the application of the
   LZS algorithm to the IP Payload Compression Protocol [Thomas].
   [Thomas] defines a method for applying lossless compression to the
   payloads of Internet Protocol datagrams.


   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

Table of Contents

R. Monsour                                                       [Page 1]

Internet Draft       draft-ietf-ippcp-lzs-00.txt            July 29, 1997

   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......................4
      2.4 Compression Encoding Format................................5
      2.5 Padding....................................................6
   3. Decompression Process..........................................6
   4. Security Considerations........................................6
   5. References.....................................................6
   6. Authors Addresses..............................................8
   7. Appendix: Compression Efficiency versus Datagram Size..........8

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 (
   or to the editor.

   This document specifies the application of LZS compression, a lossless
   compression algorithm, to IP datagram payloads. It is to be used in
   conjunction with the IP Payload Compression Protocol which, at this
   writing, is under development by the IP Payload Compression Protocol
   working group.

1.2 Background of LZS Compression

   Starting with a sliding window compression history, similar to LZ1
   [LZ1], Hi/fn developed a new, enhanced compression algorithm
   identified as LZS. The LZS algorithm is optimized to compress all file
   types as efficiently as possible.  Even string matches as short as two
   octets are effectively compressed.

   The LZS algorithm uses a sliding window of 2,048 bytes.  During
   compression, redundant sequences of data are replaced with tokens that
   represent those sequences. During decompression, the original

R. Monsour                                                      [Page 2]

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   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 typical compression ratio is 2:1.
   LZS achieves a compression ratio of 2.34:1 on the University of
   Calgary Text Compression Corpus [Calgary].

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 author's address.

1.4 Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [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%.

R. Monsour                                                      [Page 3]

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   If the size of a compressed IP datagram, including whatever overhead
   is included to define the compression protocol, is not smaller than
   the size of the original IP datagram, the IP datagram MUST be sent in
   the original non-compressed form.  This policy ensures saving the
   decompression processing cycles and avoiding incurring IP datagram
   fragmentation when the expanded datagram is larger than the MTU.

   Small IP datagrams are more likely to expand as a result of
   compression. Therefore, a numeric threshold SHOULD be applied before
   compression, where IP datagrams of size smaller than the threshold are
   sent in the original form without attempting compression.  The numeric
   threshold is implementation dependent.

   An IP datagram with payload, which has been previously compressed,
   tends not to compress any further.  Such previously compressed payload
   may be the result of external processes, such as compression applied
   by an upper layer in the communication stack, or by an off-line
   compression utility.  An adaptive algorithm SHOULD be implemented in
   order to avoid the performance penalty of futile compression attempts.
   Such as adaptive algorithm is implementation dependent and independent
   of compression method.

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
   |              Compression Parameters Index (CPI)               |
   |C|                        Reserved                             |
   |                                                               |
   ~                    Payload Data (variable)                    ~
   |                                                               |

   Compression Parameters Index (CPI)

     The Compression Parameters Index (CPI) is a 32-bit pseudo-random
     value identifying the association for this datagram. The details of
     its use can be found in [Thomas].


R. Monsour                                                      [Page 4]

Internet Draft       draft-ietf-ippcp-lzs-00.txt            July 29, 1997

     This one-bit field, when one, indicates that the datagram payload is
     compressed; a value of zero indicates that the datagram payload is
     not compressed.


     This 31-bit field is reserved for future use and MUST be zeroed by
     the sender. It SHOULD be ignored by the receiver.

   Payload Data

     This variable-length field contains the optionally compressed
     datagram payload.

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. If the compressed form is used, the output payload
   as defined in section 3.2 of [ANSI94], and 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
   1100      = 5     1111 1001      = 17
   1101      = 6     1111 1010      = 18
   1110      = 7     1111 1011      = 19
   1111 0000 = 8     1111 1100      = 20

R. Monsour                                                      [Page 5]

Internet Draft       draft-ietf-ippcp-lzs-00.txt            July 29, 1997

   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.

3. Decompression Process

   If the C bit of the received datagram is a one (i.e., indicating the
   datagram is compressed), the receiver MUST reset the compression
   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
   compression history, the receiver decompresses the Payload Data field
   according to the encoding specified in section 3.2 of [ANSI94].

   If the C bit of the received datagram is zero, 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. Security Considerations

   This memo discusses the use of lossless compression technology in the
   Internet Protocol. This can include use of IP Security. The proposed
   use of compression within this protocol is not believed to have an
   effect on the underlying security functionality provide by the
   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.

   The use of compression does change the length of ESP payloads, in a
   manner that depends on the data prior to encryption.  Thus, the use of
   compression may have an effect on the ability of an eavesdropper to
   glean information by analyzing the length of transmitted packets.

5. References

   [AH]  Kent, S. and Atkinson, R., "IP Authentication Header", draft-
   ietf-ipsec-auth-header-01.txt, Work in Progress, July 1997.

R. Monsour                                                      [Page 6]

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   [ANSI94] American National Standards Institute, Inc., "Data
   Compression Method for Information Systems," ANSI X3.241-1994, August

   [Calgary]  Text Compression Corpus, University of Calgary, available

   [DOI] Piper, D., "The Internet IP Security Domain of Interpretation
   for ISAKMP", draft-ietf-ipsec-ipsec-doi-02.txt, Work in Progress,
   February 1997.

   [ESP] Kent, S. and Atkinson, R., "IP Encapsulating Security Payload",
   draft-ietf-ipsec-esp-v2-00.txt, Work in Progress, July 1997.

   [ISAKMP] Maughan, D., Schertler, M., Schneider, M., and Turner, J.,
   "Internet Security Association and Key Management Protocol (ISAKMP)",
   draft-ietf-ipsec-isakmp-08.txt, Work in Progress, July 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.

   [RFC-1700] Reynolds, J., Postel, J., "Assigned Numbers", RFC 1700,
   October 1994.

   [RFC-1883] Deering, S., Hinden, R., "Internet Protocol, Version 6
   (IPv6) Specification", RFC 1883, April 1996.

   [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.

   [Shacham] Shacham, A., "IP Payload Compression Protocol (IPComp)",
   draft-ietf-ippcp-protocol-00.txt, Work in Progress, July 1997.

   [Thayer] Thayer, R., "Compression Payload for Use with IP Security",
   draft-thayer-seccomp-01.txt, Work in Progress, March, 1997.

   [Thomas] Thomas, M., "The Compressed Payload Header", draft-thomas-

R. Monsour                                                      [Page 7]

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   ippcp-compression-00.txt, Work in Progress, July 1997.

6. Authors Addresses

   Robert Monsour
   Hi/fn Inc.
   5973 Avenida Encinas
   Suite 110
   Carlsbad, CA  92008

7. 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 length of the file prior to compression was 3,278,000
   bytes.  When the entire file was compressed as a single payload, a
   compression ratio of 2.34 resulted.

    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         |

R. Monsour                                                      [Page 8]