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
datatracker.ietf.org