RFC2118 - Microsoft Point-To-Point Compression (MPPC) Protocol

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　　Network Working Group G. Pall
Request for Comments: 2118 Microsoft Corporation
Category: Informational March 1997

Microsoft Point-To-Point Compression (MPPC) 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 Microsoft Point to Point
Compression protocol (also referred to as MPPC in this document) for
compressing PPP encapsulated packets.

Table of Contents

1. Introduction .......................................... 2
1.1 Licensing ....................................... 2
1.2. Specification of Requirements ................... 2
2. Configuration Option Format ........................... 3
3. MPPC Packets .......................................... 4
3.1 Packet Format.................................... 5
4. Description of Compressor and Encoding .................... 6
4.1 Literal Encoding ................................ 7
4.2 Copy Tuple Encoding ............................. 7
4.2.1 Offset Encoding ............................. 7
4.2.2 Length-of-Match Encoding .................... 7
4.3 Synchronization ................................. 8
SECURITY CONSIDERATIONS ...................................... 8
REFERENCES ................................................... 9
ACKNOWLEDGEMENTS ............................................. 9
CHAIR'S ADDRESS ........................................... 9
AUTHORS' ADDRESS ............................................. 9

1. Introduction

The Microsoft Point to Point Compression scheme is a means of
representing arbitrary Point to Point Protocol (PPP) packets in a
compressed form. The MPPC algorithm is designed to optimize processor
utilization and bandwidth utilization in order to support large
number of simultaneous connections. The MPPC algorithm is also
optimized to work efficiently in typical PPP scenarios
(1500 byte MTU, etc.).

The MPPC algorithm uses an LZ [3] based algorithm with a sliding
window history buffer.

The MPPC algorithm keeps a continous history so that after 8192 bytes
of data has been transmitted compressed there is always 8192 bytes of
history to use for compressing, except when the history is flushed.

1.1. Licensing

MPPC can only be used in products that implement the Point to Point
Protocol AND for the sole purpose of interoperating with other MPPC
and Point to Point Protocol implementations.

Source and object licenses are available on a non-discriminatory
basis from Stac Electronics. Please contact:

Cheryl Poland
Stac Electronics
12636 High Bluff Drive,
San Deigo, CA 92130
Phone: (619)794-4534
Email: cherylp@stac.com

1.2. Specification of Requirements

In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.

MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.

MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.

SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications MUST be
understood and carefully weighed before choosing a
different course.

MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.

2. Configuration Option Format

Description

The CCP Configuration Option negotiates the use of MPPC on the
link. By default or ultimate disagreement, no compression is
used.

A summary of the CCP Configuration Option format is shown below.
The fields are transmitted from left to right.

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 | Supported Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Supported Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

18

Length

6

Supported Bits

This field is 4 octets, most significant octet first. The least
significant bit in the least significant octet set to 1 indicates
desire to negotiate MPPC.

All other bits MUST be set to 0.

3. MPPC Packets

Before any MPPC packets may be communicated, PPP must reach the
Network-Layer Protocol phase, and the CCP Control Protocol must reach
the Opened state.

Exactly one MPPC datagram is encapsulated in the PPP Information
field. The PPP Protocol field indicates type hex 00FD for all
compressed datagrams.

The maximum length of the MPPC datagram transmitted over a PPP link
is the same as the maximum length of the Information field of a PPP
encapsulated packet. Since the history buffer is limited to 8192
bytes, this length cannot be greater than 8192 bytes.

Only packets with PPP Protocol numbers in the range hex 0021 to hex
00FA are compressed. Other packets are not passed thru the MPPC
processor and are sent with their original PPP Protocol numbers.

Padding

It is recommended that padding not be used with MPPC since it
defeats the purpose of compression. If the sender must use padding
it MUST negotiate the Self-Describing-Padding Configuration option
during LCP phase and use self-describing pads.

Reliability and Sequencing

The MPPC scheme does not require a reliable link. Instead, it
relies on a 12 bit coherency count in each packet to keep the
history buffers synchronized. If the receiver recognizes that the
coherency count received in the packet does not match the count it
is expecting, it sends a CCP Reset-Request packet to resynchronize
its history buffer with the sender's history buffer.

MPPC expects the packets to be delivered in sequence, otherwise
history buffer re-synchronization will not occur.

MPPC MAY be used over a reliable link, as described in "PPP
Reliable Transmision" [5], but this typically just adds
unnecessary overhead since only the coherency count is required.

Data Expansion

If compressing the data results in data expansion, the original
data is sent as an uncompressed MPPC packet. The sender must flush
the history before compressing any more data and set the FLUSHED
bit on the next outgoing packet.

3.1. Packet Format

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP Protocol |A|B|C|D| Coherency Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Compressed Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PPP Protocol

The PPP Protocol field is described in the Point-to-Point Protocol
Encapsulation [1].

When the MPPC compression protocol is successfully negotiated by
the PPP Compression Control Protocol, the value is hex 00FD. This
value MAY be compressed when Protocol-Field-Compression is
negotiated.

Bit A

This bit indicates that the history buffer has just been
initialized before this packet was generated. This packet can
ALWAYS be decompressed because it is not based on any previous
history. This bit is typically sent to inform the peer that the
sender has initialized its history buffer before compressing the
packet and that the receiving peer must initialize its history
buffer before decompressing the packet. This bit is referred to as
FLUSHED bit in this document.

Implementation Note: Compression and decompression histories are
always initialized with all zeroes.

Bit B

This bit indicates that the packet was moved to the front of the
history buffer typically because there was no room at the end of
the history buffer. This bit is used to tell the decompressor to
set its history pointer to the beginning of the history buffer.

Implementation Notes:
1. It is implied that this bit must be set at least once for every
8192 bytes of data that is sent compressed.
2. It is also implied that this bit can be set even if the
sender's history buffer is not full. Initialized history that
has not been used for compressing data must not be referred to
in the compressed packets.

Bit C

This bit (if set) is used to indicate that the packet is
compressed.

Bit D

This bit must be set to 0.

Coherency Count

The coherency count is used to assure that the packets are sent in
proper order and that no packet has been dropped. This count
starts at 0 and is always increased by 1 and NEVER decreases or
goes back. When all bits are 1, the count returns to 0.

Compressed Data

The compressed data begins with the protocol field. For example,
in case of an IP packet (0021 followed by an IP header), the
compressor will first try to compress the 0021 protocol field and
then compress the IP header.

If the packet contains header compression, the MPPC compressor is
applied AFTER header compression is preformed and MUST be applied
to the compressed header as well. For example, if a packet
contained the protocol 002d for a compressed TCP/IP header, the
compressor would first attempt to compress 002d and then it
would attempt to compress the compressed Van-Jacobsen TCP/IP
header.

4. Description of Compressor and Encoding

The compressor runs through the length of the frame producing as
output a Literal (byte to be sent uncompressed) or a <Offset,
Length-of-Match> Copy tuple, where Offset is the number of bytes
before in the history where the match lies and Length-of-Match is the
number of bytes to copy from the location indicated by Offset.

For example, comsider the following string:

0 1 2 3 4
012345678901234567890123456789012345678901234567890
for whom the bell tolls, the bell tolls for thee.

The compressor would produce:

for whom the bell tolls,<16,15> <40,4><19,3>e.

The Literal and Copy tuple tokens are then encoded according to the
MPPC encoding scheme.

4.1 Literal Encoding

Literals are bytes sent uncompressed. If the value of the Literal is
below hex 80, it is encoded with its value itself. If the Literal has
value greater than hex 7F it is sent as bits 10 followed by the lower
7 bits of the Literal.

Example: Literal hex 56 is transmitted as 01010110
Literal hex E7 is transmitted as 101100111

4.2 Copy Tuple Encoding

Copy tuples represent compressed data. A tuple has two elements: the
Offset and Length-of-Match. The Offset is encoded before the Length-
of-Match.

4.2.1 Offset Encoding

Offset values less than 64 are encoded as bits 1111 followed by the
lower 6 bits of the value.

Offset values between 64 and 320 are encoded as bits 1110 followed by
the lower 8 bits of the computation (value - 64).

Offset values between 320 and 8191 are encoded as bits 110 followed
by the lower 13 bits of the computation (value - 320).

Examples: Offset value of 3 is encoded as: 1111 000011
Offset value of 128 is encoded as: 1110 01000000
Offset value of 1024 is encoded as: 110 0001011000000

4.2.2 Length-of-Match Encoding

Length of 3 is encoded with bit 0.

Length values from 4 to 7 are encoded as 10 followed by lower 2 bits
of the value.

Length values from 8 to 15 are encoded as 110 followed by lower 3
bits of the value.

Length values from 16 to 31 are encoded as 1110 followed by lower 4
bits of the value.

Length values from 32 to 63 are encoded as 11110 followed by lower 5
bits of the value.

Length values from 64 to 127 are encoded as 111110 followed by lower
6 bits of the value.

Length values from 128 to 255 are encoded as 1111110 followed by
lower 7 bits of the value.

Length values from 256 to 511 are encoded as 11111110 followed by
lower 8 bits of the value.

Length values from 512 to 1023 are encoded as 111111110 followed by
lower 9 bits of the value.

Length values from 1024 to 2047 are encoded as 1111111110 followed by
lower 10 bits of the value.

Length values from 2048 to 4095 are encoded as 11111111110 followed
by lower 11 bits of the value.

Length values from 4096 to 8191 are encoded as 111111111110 followed
by lower 12 bits of the value.

Examples: Length of 15 is encoded as: 110 111
Length of 120 is encoded as: 111110 111000
Length of 4097 is encoded as:111111111110 000000000001

The largest Length value that can be encoded is 8191.

4.3 Synchronization

Packets may be lost during transfer. If the decompressor maintained
coherency count does not match the coherency count received in the
compressed packet, the decompressor drops the packet and sends a CCP
Reset-Request packet. The compressor on receiving this packet flushes
the history buffer and sets the FLUSHED bit in the next packet it
sends. The decompressor on receiving a packet with its FLUSHED bit
set flushes its history buffer and sets its coherency count to the
one transmitted by the compressor in that packet. Thus
synchronization is achieved without a CCP Reset-Ack packet.

Security Considerations

Security issues are not discussed in this memo.

References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC1661, Daydreamer, July 1994.

[2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC
1962, Novell, June 1996.

[3] Lempel, A. and Ziv, J., "A Universal Algorithm for Sequential
Data Compression", IEEE Transactions On Information Theory,
Vol. IT-23, No. 3, May 1977.

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

Acknowledgments

Thomas Dimitri made significant contributions towards the design and
development of Microsoft Point-To-Point Compression Protocol. Robert
Friend of Stac Technology provided editoral input.

Chair's Address

The working group can be contacted via the current chair:

Karl F. Fox
Ascend Communications
3518 Riverside Dr., Suite 101
Columbus, Ohio 43221

(614) 451-1883

EMail: karl@ascend.Com

Author's Address

Questions about this memo can also be directed to:

Gurdeep Singh Pall
1, Microsoft Way,
Redmond, WA 98052

(206) 882-8080

Email: gurdeep@microsoft.com

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