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Versions: 00 01                                                         
Internet Draft                          November 1996 (Expires May 1997)

                                                    M. Sabin, Consultant
                                                  R. Monsour, Hi/fn Inc.


               LZS Payload Compression Transform for ESP
                    <draft-sabin-lzs-payload-00.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,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
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   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   It is intended that a future version of this draft be submitted to
   the IESG for publication as an Informational RFC.  Comments about
   this draft should be submitted to the authors or to the IPSEC mailing
   list (ipsec@tis.com).


Abstract

   This memo proposes a "payload compression transform" based on the LZS
   compression algorithm.  The transform can be used to compress the
   payload field of an IP datagram that uses the Encapsulating Security
   Payload (ESP) format.  The compression transform proposed here is
   stateless, meaning that a datagram can be decompressed independently
   of any other datagram.  Compression is performed prior to the
   encryption operation of ESP, which has the side benefit of reducing
   the amount of data that must be encrypted.

   This memo anticipates a forthcoming draft of ESP that will supercede
   [Atkins96].  The forthcoming draft will allow for ESP payloads to be
   compressed via transforms such as the one described in this memo.






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Acknowledgments

   The LZS details presented here are similar to those in "PPP Stac LZS
   Compression Protocol," by R. Friend and W. A. Simpson [RFC-1974].

   The authors wish to thank the many participants of the IPSEC mailing
   list whose discussion made possible the architecture for integrating
   compression with ESP.


Table of Contents

   1.  Introduction
   2.  Format of Transformed Payload
   3.  Compression Procedure
   4.  Decompression Procedure
   5.  Security Considerations
   6.  References
   7.  Author's Addresses
   8.  Appendix:  Compression Efficiency versus Datagram Size


1.  Introduction

   Encrypted data is random in nature and not compressible.  When an IP
   datagram is encrypted, compression methods used at lower protocol
   layers -- e.g., PPP compression [RFC-1962] -- no longer work.  If
   both compression and encryption are desired, compression must be
   performed first.

   A side benefit of compressing the data first is that the amount of
   data which must be encrypted is reduced.  In some implementations,
   compression is done in hardware and encryption is done in software,
   and this can represent a significant reduction in software overhead.

   The Encapsulating Security Payload (ESP) format is well suited to
   combining compression with encryption, for a variety of reasons:

      -  ESP is the place were encryption is applied to an IP datagram.
      It is straightforward to precede the encryption step by an
      optional compression step.  The compression step transforms an
      uncompressed ESP payload into a compressed ESP payload.  This
      "payload compression transform" can be independently defined and
      used with any ESP transform.

      -  ESP provides a security parameters index (SPI) that links a
      datagram to security parameters negotiated elsewhere.  A
      destination uses the SPI to look up the ESP transform needed to
      decode an incoming datagram.  If compression details are included
      among the negotiated parameters, a destination can also use the
      SPI to look up the compression transform needed to decode the ESP



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      payload.

   This memo proposes a payload compression transform based on the LZS
   compression algorithm.  The transform can be used to compress any ESP
   payload.  The transform is stateless, meaning that the payload of a
   datagram can be decompressed independently of any other datagram.
   This is important in IP, where the delivery of individual datagrams
   is not guaranteed.


   1.1  Background of LZS Compression

      The LZS algorithm is a lossless compression method that 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 sequences are
      substituted for the tokens, in such a way that the original data
      is exactly recovered.  LZS differs from lossy schemes, such as
      those often used for video compression, that do not exactly
      reproduce the original data.

      Details of LZS formatting 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].


   1.2  Licensing

      Source and object licenses for LZS 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.


   1.3  Requirements Terminology

      In this document, the words that are used to define the
      significance of each particular requirement are usually
      capitalized.  These words are:

         - MUST:  This word, or the adjective "REQUIRED," means that the
         item is an absolute requirement of the specification.

         - SHOULD:  This word, or the adjective "RECOMMENDED," means
         that there might exist valid reasons in particular
         circumstances to ignore this item, but the full implications
         should be understood and the case carefully weighed before
         taking a different course.



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         - MAY:  This word, or the adjective "OPTIONAL," means that the
         item is truly optional.  One vendor might choose to include the
         item because of a particular marketplace requirement or because
         it enhances the product, while another vendor might omit the
         item.


2. Format of Transformed Payload

The input to the payload compression transform is a payload to be
encapsulated by ESP.  The output of the transform is a new payload of
following format:


  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      CC       |                                               |
  +-+-+-+-+-+-+-+-+                                               |
  |                                                               |
  |           Payload Data (compressed or uncompressed)           |
  |                                                               |
  |                                               +-+-+-+-+-+-+-+-|
  |                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   2.1  Compression Control

      The Compression Control (CC) field is a single, bit-mapped byte.
      The bits are numbered 7 (most significant) to 0 (least
      significant).  The following bits are defined:


         - COMPRESSED (bit 7)

         This bit is set to 1 to indicate the payload is compressed.  It
         is cleared to 0 to indicate the payload is not compressed.


         - HIST_RESET (bit 6)

         This bit is set to 1 to indicate that the compression history
         associated with this datagram's SPI was reset prior to
         processing this datagram's payload.  It is cleared to 0 to
         indicate the compression history was not reset.

         In order to make the transform stateless between datagrams, the
         sender MUST reset the compression history prior to processing
         each datagram's payload.  Thus, the HIST_RESET bit MUST be set
         to 1 in every datagram.  (The HIST_RESET bit is defined here
         for upwards compatibility with future transforms that may allow



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         statefulness.)

         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  Payload Data

      The Payload Data is either compressed or uncompressed.  The value
      of the COMPRESSED bit of the CC field is set accordingly.  The
      Payload Data field is an integral number of bytes in length.


3.  Compression Procedure

   The compression procedure consists of the following steps:

      i)  The sender resets the compression history and sets the
      HIST_RESET bit of the CC field to 1.

      ii)  The sender decides whether or not to compress the payload.

         - If the sender chooses to compress the payload, the LZS
         algorithm is applied.  The resulting compressed data is
         formatted according to [ANSI94].  The COMPRESSED bit of the CC
         field is set to 1.

         - If the sender chooses not to compress the payload, the
         COMPRESSED bit of the CC field is set to 0.

   An implementation SHOULD monitor the results of the payload
   compression operation and reject the operation if it results in
   expansion.  In such a case, the uncompressed payload SHOULD be
   transmitted with the COMPRESSED bit set to 1.

   After the payload has been transformed by these steps, the
   transformed payload is submitted to the encode procedure of the
   selected ESP transform.


4.  Decompression Procedure

   Prior to applying the decompression procedure, the decode procedure
   of the selected ESP transform is applied to extract the payload.

   The decompression procedure consists of the following steps:




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      i)  The receiver checks the HIST_RESET bit of the CC field.  If
      HIST_RESET = 1, the decompression history is reset.  If HIST_RESET
      = 0, the datagram is discarded.

      ii)  The receiver checks the COMPRESSED bit of the CC field.  If
      COMPRESSED = 1, the LZS decompression algorithm is applied to the
      payload data.  If COMPRESSED = 0, decompression is not applied.


5.  Security Considerations

   Security issues are not discussed in this memo.


6.  References

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

   [Atkins96]  Atkinson, R., "IP Encapsulating Security Protocol,"
      RFC-xxxx, June 1996.

   [Calgary]  Text Compression Corpus, University of Calgary, available
      at
      ftp://ftp.cpsc.ucalgary.ca/pub/projects/text.compression.corpus.

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

   [RFC-1974] Friend, R., and Simpson, W.A., "PPP Stac LZS Compression
      Protocol," RFC-1974, August 1996.


7.  Authors' Addresses

   Michael Sabin
   883 Mango Avenue
   Sunnyvale, CA  94087
   Email:  mike.sabin@worldnet.att.net

   Robert Monsour
   Hi/fn Inc.
   12636 High Bluff Drive
   San Diego, CA  92130
   Email: rmonsour@earthlink.net


8.  Appendix:  Compression Efficiency versus Datagram Size

   The following table offers some guidance on the compression



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   efficiency that can be achieved as a function of datagram size.  Each
   entry in the table shows the compression ratio that was achieved when
   the proposed transform 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,|  64   128   256   512  1024  2048  4096  8192 16384
    bytes         |
    --------------|----------------------------------------------------
    Compression   |1.18  1.28  1.43  1.58  1.74  1.91  2.04  2.11  2.14
    ratio         |






































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