The PPP Compression Control Protocol (CCP) (original) (raw)

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PROPOSED STANDARD
Updated by: 2153

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](#ref-1 ""The Point-to-Point Protocol (PPP)"")] 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.

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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](#ref-1 ""The Point-to-Point Protocol (PPP)"")] 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.

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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

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reliable transport such as LAPB [[3](#ref-3 ""PPP Reliable Transmission"")]. 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](#ref-1 ""The Point-to-Point Protocol (PPP)"")].

Up-to-date values of the CCP Code field are specified in the most recent "Assigned Numbers" RFC [[2](#ref-2 ""Assigned Numbers"")]. 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

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  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

  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](#ref-1 ""The Point-to-Point Protocol (PPP)"")], 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.

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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](#ref-2 ""Assigned Numbers"")]. 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.

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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

  >= 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.

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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.

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Security Considerations

Security issues are not discussed in this memo.

References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, 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", RFC 1663, 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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