PPP LZS-DCP Compression Protocol (LZS-DCP)
RFC 1967

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Network Working Group                                       K. Schneider
Request for Comments: 1967                                  ADTRAN, Inc.
Category: Informational                                        R. Friend
                                                         Stac Technology
                                                             August 1996

               PPP LZS-DCP Compression Protocol (LZS-DCP)

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   The PPP Compression Control Protocol [2] provides a method to
   negotiate and utilize compression protocols over PPP encapsulated
   links.

   This document describes the use of the Stac LZS data compression
   algorithm for compressing PPP encapsulated packets, using a DCP
   header [6].  This protocol is an enhanced version of the non-DCP
   (Option 17) PPP Stac LZS compression protocol [5], and will be
   referred to as the LZS-DCP Compression Protocol.

Table of Contents

     1.     Introduction ..........................................    2
        1.1       Licensing .......................................    3
        1.2       Specification of Requirements ...................    3
        1.3       Terminology .....................................    3
     2.     LZS-DCP Packets .......................................    4
        2.1       Example LZS-DCP Packets .........................    5
        2.2       Padding .........................................    6
        2.3       Reliabliity and Squencing .......................    6
        2.4       Data Expansion ..................................    6
        2.5       Packet Format ...................................    7
           2.5.1  PPP Protocol ....................................    7
           2.5.2  DCP-Header ......................................    8
           2.5.3  History Number ..................................    9
           2.5.4  Sequence Number .................................    9
           2.5.5  Data ............................................   10
           2.5.6  Longitudinal Check Byte .........................   10

Schneider & Friend           Informational                      [Page 1]
RFC 1967                        LZS-DCP                      August 1996

           2.5.7  Compressed Data .................................   11
     3.     Sending Compressed Datagrams     .....................    11
        3.1       Transmitter Process .............................   11
        3.2       Receiver Process ................................   12
        3.3       History Maintenance .............................   13
        3.4       Anti-Expansion Mechanism ........................   14
        3.5       History Resynchronization Mechanism .............   14
     4.     Configuration Option Format ...........................   15
     SECURITY CONSIDERATIONS ......................................   16
     REFERENCES ...................................................   17
     CHAIR'S ADDRESS ..............................................   17
     AUTHORS' ADDRESSES ...........................................   18

1.  Introduction

   Starting with a sliding window compression history, similar to LZ1
   [3], Stac Electronics developed a compression algorithm identified as
   Stac LZS.  A PPP Compression Protocol for this compression algorithm
   was developed and published [5].  That protocol was taken as a basis
   for data compression work done in TIA for DSU/CSUs.  As a part of
   that standardization process, the concept of a portable Data
   Compression Protocol (DCP) was introduced [6].  The resulting
   (pending) TIA/EIA-655 standard uses this LZS-DCP protocol, which
   ncorporates DCP into a PPP compression protocol for Stac LZS.  A very
   similar protocol is currently out for ballot in the Frame Relay
   Forum.  (It is identical except for the size of the history number
   field.)

   This publication of the LZS-DCP compression protocol is in the
   interest of providing a common compression protocol for Stac-LZS, and
   to provide features that are not available with the LZS compression
   protocol [5].  Some of the differences between the LZS-DCP and LZS
   (compression type 17) protocols are as follows:

        1) LZS-DCP provides an option which allows packets containing
           uncompressible data to be transferred without requiring the
           compression history to be cleared, potentially allowing a
           higher compression ratio.  A bit is included in the DCP
           header to indicate whether the packet contains compressed or
           uncompressed data.

        2) LZS-DCP uses reset request and acknowledgment bits in the DCP