Network Working Group R. Friend
Request for Comments: 1974 Stac Electronics
Category: Informational W. Simpson
DayDreamer
August 1996
PPP Stac LZS Compression Protocol
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, with single or multiple compression histories, for
compressing PPP encapsulated packets.
Table of Contents
1. Introduction .......................................... 2
1.1 Licensing ....................................... 2
1.2 Specification of Requirements ................... 3
2. LZS Packets ........................................... 3
2.1 Padding ......................................... 4
2.2 Zero Deletion/Insertion ......................... 4
2.3 Reliability and Sequencing ...................... 4
2.3.1 Reset-Request and Reset-Ack Packet Formats....... 5
2.4 Data Expansion .................................. 6
2.5 Packet Format ................................... 6
2.5.1 PPP Protocol .................................... 7
2.5.2 History Number .................................. 7
2.5.3 Check Value ..................................... 7
2.5.3.1 LCB ........................................ 7
2.5.3.2 CRC ........................................ 7
2.5.3.3 Sequence Number ............................ 8
2.5.3.3.1 History Synchronization with Sequence
Numbers Example ...................... 9
Friend & Simpson Informational [Page 1]
RFC 1974 Stac LZS August 1996
2.5.4 History Synchronization Procedure ............... 10
2.5.5 Compressed Data ................................. 11
3. Sending Compressed Datagrams .......................... 12
3.1 Transmitter Process ............................. 12
3.2 Receiver Process ................................ 12
3.3 History Maintenance ............................. 13
3.4 History Resynchronization Mechanism ............. 14
4. Configuration Option Format ........................... 14
5. Definition of Extended Mode ........................... 16
5.1 Extended Mode Packet Format ..................... 16
5.2 Extended Mode Transmitter Process ............... 18
5.3 Extended Mode Receiver Process .................. 18
5.4 Extended Mode Synchronization ................... 19
SECURITY CONSIDERATIONS ...................................... 19
REFERENCES ................................................... 20
CHAIR'S ADDRESS ........................................... 20
AUTHORS' ADDRESSES............................................ 20
1. Introduction
Starting with a sliding window compression history, similar to LZ1
[3], Stac Electronics developed a new, enhanced compression algorithm
identified as Stac 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 Stac LZS compression algorithm supports both single compression
history communication and multiple compression history communication.
A single compression history will require the minimum amount of
memory to implement, but may not provide as much compression as a
multiple history implementation.
Often, many streams of information are interleaved over the same
link. Each virtual link will transmit data that is independent of
other virtual links. Using multiple compression histories can
improve the compression ratio of a communication link by associating
separate compression histories with separate virtual links of
communication.
1.1. Licensing
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