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RFC1975 - PPP Magnalink Variable Resource Compression

王朝other·作者佚名  2008-05-31
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Network Working Group D. Schremp

Request for Comments: 1975 J. Black

Category: Informational J. Weiss

Magnalink

August 1996

PPP Magnalink Variable Resource Compression

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 of

encapsulating multiple protocol datagrams over point-to-point links.

The PPP Compression Control Protocol [2] provides a method for

negotiating data compression over PPP links.

The Magnalink Variable Resource Compression Algorithm (MVRCA) allows

a wide range of interoperable compression implementations whose

performance characteristics are a function of available CPU and

memory resources.

IntrodUCtion

The Magnalink variable resource compression algorithm defines a

family of interoperable compression solutions with compression

performance as a function of available CPU and memory resources. It

addresses the need for an algorithm which can be tailored to the

system on which it is implemented without compromising

interoperability.

Licensing

Source licenses are available on a non-discriminatory basis.

The contact person for evaluation under NDA and Licensing is:

Director of OEM Sales

Magnalink Communications Division

Telco Systems Inc.

63 Nahatan Street

Norwood, Mass. 02062

Phone: (617) 255-9400, Fax: (617) 255-5885

oem@magna.telco.com

MVRCA Packets

Before any MVRCA packets may be communicated, PPP must reach the

Network-Layer Protocol phase[1], and the Compression Control Protocol

must reach the Opened state.

The text of a Packet to be compressed begins with PPP Protocol

number. The Packet header including the PPP Protocol number may have

already been compressed when Protocol-Field-Compression has been

negotiated.

Reliability and Sequencing

MVRCA packets may be sent across an unreliable link or may use a

reliable link as described in "PPP Reliable Transmission"[3] if the

reliable link has been negotiated. If frames are delivered out of

order or a frame is dropped, the decompressor will detect this and

requests a resynchronization using the Reset-Req and Reset-Ack types

of the CCP[2], with the compressor for the affected context.

Data EXPansion

Although the compression algorithm may occasionally expand a data

packet, there is no expansion in MVRCA since any expanded data is

instead sent uncompressed. Dictionary synchronization is maintained

across uncompressed packets.

Encapsulation

The encapsulation consists of the PPP Protocol Identifier, a bit to

indicate if the data is compressed, the Context Identifier(CID), a

proprietary flag bit (E), a Packet Integrity Byte(PIB), and the

Compressed data.

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PPP Protocol Identifier

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

CE CID PIB C compressed flag

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 data is compressed

Compressed data ... 0 data is not compressed

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Compressed/Uncompressed Flag (C)

When attempting to compress certain types of Packets or Fragments the

compressor may not be effective. When this occurs the uncompressed

data is added to the compression History Buffer and sent across the

link in frame with the Compressed/Uncompressed Flag(C) set to 0.

Context Identifier (CID)

Since PPP will transport multiple protocol datagrams it may be

advantageous to compress each protocol or each virtual circuit in a

different History Buffer or Context. The CID allows the compressor to

indicate to the decompressor which History Buffer the compressor

decided to use for a given Packet. The basis of this decision is up

to the implementor. The number of buffers and size of each buffer is

negotiated.

A CID of 0 indicates that the Packet by Packet context will be used

if it has been negotiated. The Packet by Packet context is cleared

between Packets so that this History Buffer is not maintained across

Packet boundaries.

Packet Integrity Byte (PIB)

To ensure that Packets are being compressed and decompressed

correctly and to ensure History Buffer synchronization is maintained,

a Packet Integrity Byte is added to the packet header.

The packet integrity byte is defined in the full protocol

specification.

Configuration Option Format

Description

The CCP MVRCA Configuration Option negotiates the use of MVRCA on the

link. By default or ultimate disagreement, no compression is used.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type Length FE P History # Contexts

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

24

Length

4

FE - Features

Negotiates features specific to this compression algorithm.

History

Defines the size of the compression history buffer. Valid values are

defined in the full protocol specification.

# Contexts

This is the number of contexts. Each context implies the creation of

a History Buffer for that context of the size indicated in the

Context History field. Values are 1-63. This value includes both

the Packet by Packet context and the number of contexts for which

history is maintained. Therefore, when this value is 1 and the P

(Packet by Packet) flag is also 1, then only in packet compression is

supported and history context is not retained across packet

boundaries. The Context Identifier (CID) starts with 1 for contexts

where the history is maintained.

P - Packet by Packet flag

When 1, packet by packet compression is enabled for the context whose

context ID is 0. When P is 0, packet by packet compression is not

supported.

Security Considerations

Security issues are not discussed in this memo.

References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,

RFC1661, July 1994.

[2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC1962,

June 1996.

[3] Rand, D., "PPP Reliable Transmission", RFC1663, July 1994.

Acknowledgments

Chair's Address

The working group can be contacted via the current chair:

Karl Fox

Ascend Communications

3518 Riverside Drive, Suite 101

Columbus, Ohio 43221

EMail: karl@ascend.com

Authors' Addresses

Comments about this document may also be directed to the authors.

Doug Schremp

Telco Systems, Inc.

Magnalink Communications Division

63 Nahatan Street

Norwood Ma. 02062

Phone: (617) 255-9400

EMail: dhs@magna.telco.com

Jeffrey Black

Telco Systems, Inc.

Magnalink Communications Division

63 Nahatan Street

Norwood Ma. 02062

Phone: (617) 255-9400

EMail: jtb@magna.telco.com

Jeffrey Weiss

Telco Systems, Inc.

Magnalink Communications Division

63 Nahatan Street

Norwood Ma. 02062

Phone: (617) 255-9400

EMail: jaw@magna.telco.com

 
 
 
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