RFC 1967 (rfc1967) - Page 2 of 18

PPP LZS-DCP Compression Protocol (LZS-DCP)

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
           header that is included on each packet rather than using
           CCP's reset request and acknowledge packets, which may result
           in fewer discarded data packets during the REQ/ACK handshake.

        3) LZS-DCP allows simultaneous use of both sequence numbers and
           the LCB for compression error detection.



Schneider & Friend           Informational