PPP Deflate Protocol

Network Working Group                                           J. Woods
Request for Comments: 1979                                 Proteon, Inc.
Category: Informational                                      August 1996


                          PPP Deflate 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 PPP Deflate compression
   protocol for compressing PPP encapsulated packets.

Table of Contents

     1.     Introduction ......................................    2
        1.1       Licensing ...................................    2
     2.     PPP Deflate Packets ...............................    3
        2.1       Packet Format ...............................    6
     3.     Configuration Option Format .......................    8
     SECURITY CONSIDERATIONS ..................................    9
     REFERENCES ...............................................    9
     ACKNOWLEDGEMENTS .........................................    9
     CHAIR'S ADDRESS ..........................................   10
     AUTHOR'S ADDRESS .........................................   10

1.  Introduction

The 'deflate' compression format[3], as used by the PKZIP and gzip
compressors and as embodied in the freely and widely distributed
zlib[4] library source code, has the following features:

       - an apparently unencumbered encoding and compression
         algorithm, with an open and publically-available
         specification.

       - low-overhead escape mechanism for incompressible data.  The
         PPP Deflate specification offers options to reduce that
         overhead further.

       - heavily used for many years in networks, on modem and other
         point-to-point links to transfer files for personal computers
         and workstations.

       - easily achieves 2:1 compression on the Calgary corpus[5]
         using less than 64KBytes of memory on both sender and
         receive.

1.1.  Licensing

   The zlib source is widely and freely available, subject to the
   following copyright:

      (C) 1995 Jean-Loup Gailly and Mark Adler

       This software is provided 'as-is', without any express or implied
       warranty.  In no event will the authors be held liable for any
       damages arising from the use of this software.

       Permission is granted to anyone to use this software for any
       purpose, including commercial applications, and to alter it and
       redistribute it freely, subject to the following restrictions:

       1. The origin of this software must not be misrepresented; you
          must not claim that you wrote the original software. If you
          use this software in a product, an acknowledgment in the
          product documentation would be appreciated but is not
          required.

       2. Altered source versions must be plainly marked as such, and
          must not be misrepresented as being the original software.

       3. This notice may not be removed or altered from any source
          distribution.

       Jean-Loup Gailly        Mark Adler
       gzip@prep.ai.mit.edu    madler@alumni.caltech.edu

      If you use the zlib library in a product, we would appreciate
      *not* receiving lengthy legal documents to sign. The sources are
      provided for free but without warranty of any kind.  The library
      has been entirely written by Jean-Loup Gailly and Mark Adler; it
      does not include third-party code.

   The deflate format and compression algorithm are based on Lempel-Ziv
   LZ77 compression; extensive research has been done by the GNU Project
   and the Portable Network Graphics working group supporting its patent
   free status.

2.  PPP Deflate Packets

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

   Exactly one PPP Deflate datagram is encapsulated in the PPP
   Information field, where the PPP Protocol field contains 0xFD or
   0xFB.  0xFD is used when the PPP multilink protocol is not used or
   "above" multilink.  0xFB is used "below" multilink, to compress
   independently on individual links of a multilink bundle.

   The maximum length of the PPP Deflate datagram transmitted over a PPP
   link is the same as the maximum length of the Information field of a
   PPP encapsulated packet.

   Only packets with PPP Protocol numbers in the range 0x0000 to 0x3FFF
   and neither 0xFD nor 0xFB are compressed.  Other PPP packets are
   always sent uncompressed.  Control packets are infrequent and should
   not be compressed for robustness.

   Padding

      PPP Deflate packets require the previous negotiation of the Self-
      Describing-Padding Configuration Option [6] if padding is added to
      packets.  If no padding is added, than Self-Describing-Padding is
      not required.

   Reliability and Sequencing

      PPP Deflate requires the packets to be delivered in sequence.  It
      relies on Reset-Request and Reset-Ack LCP packets or on
      renegotiation of the Compression Control Protocol [2] to indicate
      loss of synchronization between the transmitter and receiver.  The
      LCP FCS detects corrupted packets and the normal mechanisms
      discard them.  Missing or out of order packets are detected by the
      sequence number in each packet.  The packet sequence number ought
      to be checked before decoding the packet.

      Instead of transmitting a Reset-Request packet when detecting a
      sequence error, the receiver MAY momentarily force CCP to drop out
      of the Opened state by transmitting a new CCP Configure-Request.
      This method is more expensive than using Reset-Requests.

      When the receiver first encounters an unexpected sequence number
      it SHOULD send a Reset-Request LCP packet as defined in the
      Compression Control Protocol.  When the transmitter sends the
      Reset-Ack or when the receiver receives a Reset-ACK, they must
      reset the sequence number to zero, clear the compression
      dictionary, and resume sending and receiving compressed packets.
      The receiver MUST discard all compressed packets after detecting
      an error and until it receives a Reset-Ack.  This strategy can be
      thought of as abandoning the transmission of one "file" and
      starting the transmission of a new "file."

      The transmitter must clear its compression history and respond
      with a Reset-Ack each time it receives a Reset-Request, because it
      cannot know if previous Reset-Acks reached the receiver.  The
      receiver need not do anything to its history when it receives a
      Reset-Ack, because the transmitter will simply not refer to any
      prior history ('deflate' is a sliding-window compressor).

      When the link is busy, one decompression error is usually followed
      by several more before the Reset-Ack can be received.  It is
      undesirable to transmit Reset-Requests more frequently than the
      round-trip-time of the link, because redundant Reset-Requests
      cause unnecessary compression dictionary clearing.  The receiver
      MAY transmit an additional Reset-Request each time it receives a
      compressed or uncompressed packet until it finally receives a
      Reset-Ack, but the receiver ought not transmit another Reset-
      Request until the Reset-Ack for the previous one is late.  The
      receiver MUST transmit enough Reset-Request packets to ensure that
      the transmitter receives at least one.  For example, the receiver
      might choose to not transmit another Reset-Request until after one
      second (or, of course, a Reset-Ack has been received and
      decompression resumed).

   Data Expansion

      'Deflate', as used in this standard, expands incompressible data
      by approximately 14-18 bytes (8 bytes worst-case at the 'deflate'
      level, two further bytes for the 'deflate' end-of-block and the
      zero-length synchronization block header, two bytes of sequence
      number, and two bytes to account for adding the PPP Protocol Field
      to the transmitted data unit).

      The BSD Compress draft proposal[7] describes an escape mechanism
      for incompressible data that trades off a layering violation for
      the irritating complications of variable and potentially
      unpredictable effective MRU lengths.  That direct escape mechanism
      (and much of the text of its description) is used here as well.

      If an incompressible data packet does not fit within the MRU of
      the link, the packet MUST be sent in its original form without CCP
      encapsulation; PPP packets with significant data expansion that do
      not exceed the MRU of the link SHOULD be sent in their original
      form without CCP encapsulation.  In both of these cases, the
      transmitter must increment the sequence number, as future
      encapsulated packets will depend on the correct reception of some
      number of unencapsulated packets.

      When a PPP packet is received with PPP Protocol numbers in the
      range 0x0000 to 0x3FFF, (except, of course, 0xFD and 0xFB) it is
      assumed that the packet would have caused expansion.  The packet
      is locally added to the compression history.  (Given the
      definition of the 'deflate' format, a convenient method of doing
      this is to locally "decompress" a stored-block header of the
      appropriate length, followed by the actual data block; or the data
      can simply be appended to the receiver's history, depending on
      implementation details.)

      Sending incompressible packets in their native encapsulation
      avoids maximum transmission unit complications.  If uncompressed
      packets could be larger than their native form, then it would be
      necessary for the upper layers of an implementation to treat the
      PPP link as if it had a smaller MTU, to ensure that compressed
      incompressible packets are never larger than the negotiated PPP
      MTU.

      Using native encapsulation for incompressible packets complicates
      the implementation.  The transmitter and the receiver must start
      putting information into the compression dictionary starting with
      the same packets, without relying upon seeing a compressed packet
      for synchronization.  The first few packets after clearing the
      dictionary are usually incompressible, and so are likely to sent
      in their native encapsulation, just like packets before
      compression is turned on.  If CCP or LCP packets are handled
      separately from Network-Layer packets (e.g. a "daemon" for control
      packets and "kernel code" for data packets), care must be taken to
      ensure that the transmitter synchronizes clearing the dictionary
      with the transmission of the configure-ACK or Reset-Ack that
      starts compression, and the receiver must similarly ensure that
      its dictionary is cleared before it processes the next packet.

2.1.  Packet Format

   A summary of the PPP Deflate packet 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         PPP Protocol          |           Sequence            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Data ...
   +-+-+-+-+-+-+-+-+


   PPP Protocol

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

      When the PPP Deflate compression protocol is successfully
      negotiated by the PPP Compression Control Protocol [2], the value
      of the protocol field is 0xFD or 0xFB.  This value MAY be
      compressed when Protocol-Field-Compression is negotiated.

   Sequence

      The sequence number is sent most significant octet first.  It
      starts at 0 when the dictionary is cleared, and is incremented by
      1 for each packet, including uncompressed packets.  The sequence
      number after 65535 is zero.  In other words, the sequence number
      "wraps" in the usual way.

      The sequence number ensures that lost or out of order packets do
      not cause the compression databases of the peers to become
      unsynchronized.  When an unexpected sequence number is
      encountered, the dictionaries must be resynchronized with a CCP
      Reset-Request or Configure-Request.  The packet sequence number
      can be checked before a compressed packet is decoded.

   Data

      The compressed PPP encapsulated packet, consisting of the Protocol
      and Data fields of the original, uncompressed packet follows.

      The Protocol field compression MUST be applied to the protocol
      field in the original packet before the sequence number is
      computed or the entire packet is compressed, regardless of whether
      the PPP protocol field compression has been negotiated.  Thus, if
      the original protocol number was less than 0x100, it must be
      compressed to a single byte.

      The basic format of the compressed data is precisely described by
      the 'Deflate' Compressed Data Format Specification[3].  Each
      transmitted packet must begin at a 'deflate' block boundary, to
      ensure synchronization when incompressible data resets the
      transmitter's state; to ensure this, each transmitted packet must
      be terminated with a zero-length 'deflate' non-compressed block
      (BTYPE of 00).  This means that the last four bytes of the
      compressed format must be 0x00 0x00 0xFF 0xFF.  These bytes MUST
      be removed before transmission; the receiver can reinsert them if
      required by the implementation.

3.  Configuration Option Format


   Description

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

   A summary of the PPP Deflate 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     |Window | Method|    MBZ    |Chk|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      26 for PPP Deflate.

   Length

      3

   Window

      Represents the maximum window size the decompressor is willing to
      allocate; expressed as the base-2 logarithm of the LZ77 window
      size, minus 8.  'Deflate' compliant decompressors must be willing
      to accept the maximum 32KB window size, represented by a value of
      7.  A 'deflate' compliant compressor is at liberty to use a
      reduced window size, so a PPP Deflate compressor MUST either honor
      the restriction requested or reject the option.

   Method

      Must be the binary number 1000.  Represents the 'zlib' Compression
      Method identifier of '8' for 'deflate' compression with up to 32K
      window size.

   MBZ

      Must be all 0 bits.

   Chk

      Must be 00 to specify sequence number check method.

Security Considerations

   Security issues are not discussed in this memo.

References

   [1]   Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
         RFC 1661, July 1994.

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

   [3]   Deutsch, L.P., "'Deflate' Compressed Data Format
         Specification", draft available in
         ftp.uu.net:/pub/archiving/zip/doc/deflate-1.1.doc.

   [4]   Gailly, J.-L., "Zlib 0.95 beta".

   [5]   Bell, T.C., Cleary, G. G. and Witten, I.H., "Text Compression",
         Prentice_Hall, Englewood Cliffs NJ, 1990.  The compression
         corpus itself can be found in ftp.uu.net:/pub/archiving/zip/.

   [6]   Simpson, W., "PPP LCP Extensions", RFC 1570, January 1994.

   [7]   Schryver, V., "PPP BSD Compression Protocol", RFC 1977,
         August 1996.

Acknowledgments

   William Simpson provided the very valuable idea of not using any
   additional header bytes for incompressible packets.

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:

   John Woods
   Proteon, Inc.
   9 Technology Drive
   Westborough MA 01581-1799

   (508) 898-2800 ext. 2651
   EMail: jfw@funhouse.com