RFC1962 - The PPP Compression Control Protocol (CCP)

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　　Network Working Group D. Rand
Request for Comments: 1962 Novell
Category: Standards Track June 1996

The PPP Compression Control Protocol (CCP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. 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. PPP
also defines an extensible Link Control Protocol.

This document defines a method for negotiating data compression over
PPP links.

Table of Contents

1. Introduction .......................................... 1
2. Compression Control Protocol (CCP) .................... 2
2.1 Sending Compressed Datagrams .................... 3
3. Additional Packets .................................... 4
3.1 Reset-Request and Reset-Ack ..................... 4
4. CCP Configuration Options ............................. 5
4.1 Proprietary Compression OUI ..................... 7
4.2 Other Compression Types ......................... 8
SECURITY CONSIDERATIONS ...................................... 9
REFERENCES ................................................... 9
ACKNOWLEDGEMENTS ............................................. 9
CHAIR'S ADDRESS .............................................. 9
AUTHOR'S ADDRESS ............................................. 9

1. Introduction

In order to establish communications over a PPP link, each end of the
link must first send LCP packets to configure and test the data link
during Link Establishment phase. After the link has been
established, optional facilities may be negotiated as needed.

One such facility is data compression. A wide variety of compression
methods may be negotiated, although typically only one method is used
in each direction of the link.

A different compression algorithm may be negotiated in each
direction, for speed, cost, memory or other considerations, or only
one direction may be compressed.

2. Compression Control Protocol (CCP)

The Compression Control Protocol (CCP) is responsible for
configuring, enabling, and disabling data compression algorithms on
both ends of the point-to-point link. It is also used to signal a
failure of the compression/decompression mechanism in a reliable
manner.

CCP uses the same packet exchange mechanism as the Link Control
Protocol (LCP). CCP packets may not be exchanged until PPP has
reached the Network-Layer Protocol phase. CCP packets received
before this phase is reached should be silently discarded.

The Compression Control Protocol is exactly the same as the Link
Control Protocol [1] with the following exceptions:

Frame Modifications

The packet may utilize any modifications to the basic frame format
which have been negotiated during the Link Establishment phase.

Data Link Layer Protocol Field

Exactly one CCP packet is encapsulated in the PPP Information
field, where the PPP Protocol field indicates type hex 80FD
(Compression Control Protocol).

When individual link data compression is used in a multiple link
connection to a single destination, the PPP Protocol field
indicates type hex 80FB (Individual link Compression Control
Protocol).

Code field

In addition to Codes 1 through 7 (Configure-Request, Configure-
Ack, Configure-Nak, Configure-Reject, Terminate-Request,
Terminate-Ack and Code-Reject), two additional Codes 14 and 15
(Reset-Request and Reset-Ack) are defined for this protocol.
Other Codes should be treated as unrecognized and should result in
Code-Rejects.

Timeouts

CCP packets may not be exchanged until PPP has reached the
Network-Layer Protocol phase. An implementation should be
prepared to wait for Authentication and Link Quality Determination
to finish before timing out waiting for a Configure-Ack or other
response. It is suggested that an implementation give up only
after user intervention or a configurable amount of time.

Configuration Option Types

CCP has a distinct set of Configuration Options.

2.1. Sending Compressed Datagrams

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

One or more compressed packets are encapsulated in the PPP
Information field, where the PPP Protocol field indicates type hex
00FD (Compressed datagram). Each of the compression algorithms may
use a different mechanism to indicate the inclusion of more than one
uncompressed packet in a single Data Link Layer frame.

When using multiple PPP links to a single destination, there are two
methods of employing data compression. The first method is to
compress the data prior to sending it out through the multiple links.
The second is to treat each link as a separate connection, that may
or may not have compression enabled. In the second case, the PPP
Protocol field MUST be type hex 00FB (Individual link compressed
datagram).

Only one primary algorithm in each direction is in use at a time, and
that is negotiated prior to sending the first compressed frame. The
PPP Protocol field of the compressed datagram indicates that the
frame is compressed, but not the algorithm with which it was
compressed.

The maximum length of a compressed packet transmitted over a PPP link
is the same as the maximum length of the Information field of a PPP
encapsulated packet. Larger datagrams (presumably the result of the
compression algorithm increasing the size of the message in some
cases) may be sent uncompressed, using its standard form, or may be
sent in multiple datagrams, if the compression algorithm supports it.

Each of the compression algorithms must supply a way of determining
if they are passing data reliably, or they must require the use of a

reliable transport such as LAPB [3]. Vendors are strongly encouraged
to employ a method of validating the compressed data, or recognizing
out-of-sync compressor/decompressor pairs.

3. Additional Packets

The Packet format and basic facilities are already defined for LCP
[1].

Up-to-date values of the CCP Code field are specified in the most
recent "Assigned Numbers" RFC[2]. This specification concerns the
following values:

14 Reset-Request
15 Reset-Ack

3.1. Reset-Request and Reset-Ack

Description

CCP includes Reset-Request and Reset-Ack Codes in order to provide
a mechanism for indicating a decompression failure in one
direction of a compressed link without affecting traffic in the
other direction. A decompression failure may be determined by
periodically passing a hash value, performing a CRC check on the
decompressed data, or other mechanism. It is strongly suggested
that some mechanism be available in all compression algorithms to
validate the decompressed data before passing the data on to the
rest of the system.

A CCP implementation wishing to indicate a decompression failure
SHOULD transmit a CCP packet with the Code field set to 14
(Reset-Request), and the Data field filled with any desired data.
Once a Reset-Request has been sent, any Compressed packets
received are discarded, and another Reset-Request is sent with the
same Identifier, until a valid Reset-Ack is received.

Upon reception of a Reset-Request, the transmitting compressor is
reset to an initial state. This may include clearing a
dictionary, resetting hash codes, or other mechanisms. A CCP
packet MUST be transmitted with the Code field set to 15 (Reset-
Ack), the Identifier field copied from the Reset-Request packet,
and the Data field filled with any desired data.

On receipt of a Reset-Ack, the receiving decompressor is reset to
an initial state. This may include clearing a dictionary,
resetting hash codes, or other mechanisms. Since there may be
several Reset-Acks in the pipe, the decompressor MUST be reset for

each Reset-Ack which matches the currently expected identifier.

A summary of the Reset-Request and Reset-Ack packet formats 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+

Code

14 for Reset-Request;

15 for Reset-Ack.

Identifier

On transmission, the Identifier field MUST be changed whenever the
content of the Data field changes, and whenever a valid reply has
been received for a previous request. For retransmissions, the
Identifier MAY remain unchanged.

On reception, the Identifier field of the Reset-Request is copied
into the Identifier field of the Reset-Ack packet.

Data

The Data field is zero or more octets and contains uninterpreted
data for use by the sender. The data may consist of any binary
value and may be of any length from zero to the peer's established
MRU minus four.

4. CCP Configuration Options

CCP Configuration Options allow negotiation of compression algorithms
and their parameters. CCP uses the same Configuration Option format
defined for LCP [1], with a separate set of Options.

Configuration Options, in this protocol, indicate algorithms that the
receiver is willing or able to use to decompress data sent by the
sender. As a result, it is to be expected that systems will offer to
accept several algorithms, and negotiate a single one that will be
used.

There is the possibility of not being able to agree on a compression
algorithm. In that case, no compression will be used, and the link
will continue to operate without compression. If link reliability
has been separately negotiated, then it will continue to be used,
until the LCP is re-negotiated.

We expect that many vendors will want to use proprietary compression
algorithms, and have made a mechanism available to negotiate these
without encumbering the Internet Assigned Number Authority with
proprietary number requests.

The LCP option negotiation techniques are used. If an option is
unrecognized, a Configure-Reject MUST be sent. If all protocols the
sender implements are Configure-Rejected by the receiver, then no
compression is enabled in that direction of the link.

If an option is recognized, but not acceptable due to values in the
request (or optional parameters not in the request), a Configure-NAK
MUST be sent with the option modified appropriately. The Configure-
NAK MUST contain only those options that will be acceptable. A new
Configure-Request SHOULD be sent with only the single preferred
option, adjusted as specified in the Configure-Nak.

Up-to-date values of the CCP Option Type field are specified in the
most recent "Assigned Numbers" RFC[2]. Current values are assigned
as follows:

CCP Option Compression type
0 OUI
1 Predictor type 1
2 Predictor type 2
3 Puddle Jumper
4-15 unassigned
16 Hewlett-Packard PPC
17 Stac Electronics LZS
18 Microsoft PPC
19 Gandalf FZA
20 V.42bis compression
21 BSD LZW Compress
255 Reserved

The unassigned values 4-15 are intended to be assigned to other
freely available compression algorithms that have no license fees.

4.1. Proprietary Compression OUI

Description

This Configuration Option provides a way to negotiate the use of a
proprietary compression protocol.

Since the first matching compression will be used, it is
recommended that any known OUI compression options be transmitted
first, before the common options are used.

Before accepting this option, the implementation must verify that
the Organization Unique Identifier identifies a proprietary
algorithm that the implementation can decompress, and that any
vendor specific negotiation values are fully understood.

A summary of the Proprietary Compression OUI 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 | OUI ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OUI | Subtype | Values...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Type

0

Length

>= 6

IEEE OUI

The vendor's IEEE Organization Unique Identifier (OUI), which is
the most significant three octets of an Ethernet Physical Address,
assigned to the vendor by IEEE 802. This identifies the option as
being proprietary to the indicated vendor. The bits within the
octet are in canonical order, and the most significant octet is
transmitted first.

Subtype

This field is specific to each OUI, and indicates a compression
type for that OUI. There is no standardization for this field.
Each OUI implements its own values.

Values

This field is zero or more octets, and contains additional data as
determined by the vendor's compression protocol.

4.2. Other Compression Types

Description

These Configuration Options provide a way to negotiate the use of
a publicly defined compression algorithm. Many compression
algorithms are specified. No particular compression technique has
arisen as an Internet Standard.

These protocols will be made available to all interested parties,
but may have certain licensing restrictions associated with them.
For additional information, refer to the compression protocol
documents that define each of the compression types.

A summary of the Compression Type 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 | Values...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Type

1 to 254

Length

>= 2

Values

This field is zero or more octets, and contains additional data as
determined by the compression protocol.

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] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, RFC
1700, USC/Information Sciences Institute, October 1994.

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

Acknowledgments

Bill Simpson helped with the document formatting.

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

Author's Address

Questions about this memo can also be directed to:

Dave Rand
Novell, Inc.
2180 Fortune Drive
San Jose, CA 95131

+1 408 321-1259

EMail: dlr@daver.bungi.com

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