





Network Working Group                                       K. Schneider
Request for Comments: 1967                                  ADTRAN, Inc.
Category: Informational                                        R. Friend
                                                         Stac Technology
                                                             August 1996


               PPP LZS-DCP Compression Protocol (LZS-DCP)

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 Stac LZS data compression
   algorithm for compressing PPP encapsulated packets, using a DCP
   header [6].  This protocol is an enhanced version of the non-DCP
   (Option 17) PPP Stac LZS compression protocol [5], and will be
   referred to as the LZS-DCP Compression Protocol.

Table of Contents

     1.     Introduction ..........................................    2
        1.1       Licensing .......................................    3
        1.2       Specification of Requirements ...................    3
        1.3       Terminology .....................................    3
     2.     LZS-DCP Packets .......................................    4
        2.1       Example LZS-DCP Packets .........................    5
        2.2       Padding .........................................    6
        2.3       Reliabliity and Squencing .......................    6
        2.4       Data Expansion ..................................    6
        2.5       Packet Format ...................................    7
           2.5.1  PPP Protocol ....................................    7
           2.5.2  DCP-Header ......................................    8
           2.5.3  History Number ..................................    9
           2.5.4  Sequence Number .................................    9
           2.5.5  Data ............................................   10
           2.5.6  Longitudinal Check Byte .........................   10



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RFC 1967                        LZS-DCP                      August 1996


           2.5.7  Compressed Data .................................   11
     3.     Sending Compressed Datagrams     .....................    11
        3.1       Transmitter Process .............................   11
        3.2       Receiver Process ................................   12
        3.3       History Maintenance .............................   13
        3.4       Anti-Expansion Mechanism ........................   14
        3.5       History Resynchronization Mechanism .............   14
     4.     Configuration Option Format ...........................   15
     SECURITY CONSIDERATIONS ......................................   16
     REFERENCES ...................................................   17
     CHAIR'S ADDRESS ..............................................   17
     AUTHORS' ADDRESSES ...........................................   18

1.  Introduction

   Starting with a sliding window compression history, similar to LZ1
   [3], Stac Electronics developed a compression algorithm identified as
   Stac LZS.  A PPP Compression Protocol for this compression algorithm
   was developed and published [5].  That protocol was taken as a basis
   for data compression work done in TIA for DSU/CSUs.  As a part of
   that standardization process, the concept of a portable Data
   Compression Protocol (DCP) was introduced [6].  The resulting
   (pending) TIA/EIA-655 standard uses this LZS-DCP protocol, which
   ncorporates DCP into a PPP compression protocol for Stac LZS.  A very
   similar protocol is currently out for ballot in the Frame Relay
   Forum.  (It is identical except for the size of the history number
   field.)

   This publication of the LZS-DCP compression protocol is in the
   interest of providing a common compression protocol for Stac-LZS, and
   to provide features that are not available with the LZS compression
   protocol [5].  Some of the differences between the LZS-DCP and LZS
   (compression type 17) protocols are as follows:

        1) LZS-DCP provides an option which allows packets containing
           uncompressible data to be transferred without requiring the
           compression history to be cleared, potentially allowing a
           higher compression ratio.  A bit is included in the DCP
           header to indicate whether the packet contains compressed or
           uncompressed data.

        2) LZS-DCP uses reset request and acknowledgment bits in the DCP
           header that is included on each packet rather than using
           CCP's reset request and acknowledge packets, which may result
           in fewer discarded data packets during the REQ/ACK handshake.

        3) LZS-DCP allows simultaneous use of both sequence numbers and
           the LCB for compression error detection.



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RFC 1967                        LZS-DCP                      August 1996


   The Stac LZS compression algorithm supports both single and multiple
   compression histories.  A single compression history will require the
   minimum amount of memory to implement, but may not provide as much
   compression as a multiple history implementation.

   Often, many streams of information are interleaved over the same
   physical link.  Each virtual connection will transmit data that is
   independent of other virtual connections.  Using multiple compression
   histories can improve the compression ratio of a communication link
   by associating separate compression histories with separate virtual
   links of communication.

1.1.  Licensing

   Source and object licenses are available on a non-discriminatory
   basis.  Hardware implementations are also available.  Contact Stac
   Electronics (hardware.sales@stac.com) for further information.

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.

1.3.  Terminology

   This document frequently uses the following terms:

   datagram  The unit of transmission in the network layer (such as IP).
             A datagram may be encapsulated in one or more packets
             passed to the data link layer.



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   frame     The unit of transmission at the data link layer.  A frame
             may include a header and/or a trailer, along with some
             number of units of data.

   packet    The basic unit of encapsulation, which is passed across the
             interface between the network layer and the data link
             layer.  A packet is usually mapped to a frame; the
             exceptions are when data link layer fragmentation is being
             performed, or when multiple packets are incorporated into a
             single frame.

   peer      The other end of the point-to-point link.

   silently discard

             This means the implementation discards the packet without
             further processing.  The implementation SHOULD provide the
             capability of logging the error, including the contents of
             the silently discarded packet, and SHOULD record the event
             in a statistics counter.

2.  LZS-DCP Packets

   Before any LZS-DCP packets are communicated, PPP MUST reach the
   Network-Layer Protocol phase, and the CCP Control Protocol MUST reach
   the Opened state.

   Exactly one LZS-DCP datagram is encapsulated in the PPP Information
   field, where the PPP Protocol field indicates type hex 00FD
   (compressed datagram) or type hex 00FB (Individual link compressed
   datagram).  Type hex 00FD is used when compression is negotiated over
   a single physical link or when compression is negotiated over a
   single bundle consisting of multiple physical links.  Type hex 00FB
   is used when compression is negotiated separately over individual
   physical links to the same destination.  For more information, please
   refer to PPP Compression Control Protocol.

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

   Prior to compression, the uncompressed data begins with the PPP
   Protocol ID Field.  Protocol-Field-Compression MAY be used on this
   value, if has been successfully negotiated for the link.

   The PPP Protocol ID Field is followed by the original Information
   field. The length of the uncompressed data field is limited only by
   the allowed size of the compressed data field and the higher protocol



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RFC 1967                        LZS-DCP                      August 1996


   layers.

   PPP Link Control Protocol packets MUST NOT be sent within LZS-DCP
   packets.  PPP Network Control Protocol packets MUST NOT be sent
   within LZS-DCP packets.

2.1.  Example LZS-DCP packets (shown using PPP in HDLC-like framing,
      using Address-and-Control-Field-Compression and Protocol-Field-
      Compression. - RFC 1662 )

   Compressed Packet:

        PPP |                                        | PPP
        PID | HDR   SEQ           DATA           LCB | FCS
      +-----+-----+-----+---................---+-----+-----+
      | F D | C 0 | n n |   Compressed Data    | y y | z z |
      +-----+-----+-----+---................---+-----+-----+
                        /                      \
                       /      Compression       \
                      /      Transformation      \
                     /                            \
                    /PPP                           \
                   / PID   PPP Information Field    \
                  +-----+----....................----+
                  | x x | upper layer protocol data  |
                  +-----+----....................----+


   Uncompressed Packet

        PPP |                                  | PPP
        PID | HDR   SEQ           DATA         | FCS
      +-----+-----+-----+---................---+-----+
      | F D | 8 0 | n n |   Un-compressed Data | z z |
      +-----+-----+-----+---................---+-----+
                        /                      \
                       /                        \
                      /                          \
                     /                            \
                    /PPP                           \
                   / PID   PPP Information Field    \
                  +-----+----....................----+
                  | x x | upper layer protocol data  |
                  +-----+----....................----+

      where:  C0 and 80 are representative LZS-DCP headers; nn, xx, yy,
              and zz are values determined by the packet's context.




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2.2.  Padding

      PPP padding is not allowed in a LZS-DCP packet.  However, on
      compressed packets, padding may be accomplished by extending the
      data field with zeros following the last compressed data octet
      (see Section 2.1.1).  This is referred to as LZS Padding.  The
      LCB, if present, MUST be the octet preceding the frame CRC.

2.3.  Reliability and Sequencing

      When no Compression History is kept, the algorithm does not depend
      on a reliable link, and does not require that packets be delivered
      in sequence.  However, per packet compression results in a lower
      compression ratio than it could be on a stream.

      Some reasons for clearing the history on a per packet basis
      include:

      -  The link has a high error rate.
      -  The resources of the transmitter or receiver limit the ability
         to maintain a compression history between packets.

      When one or more compression Histories are negotiated, the packet
      sequence MUST be preserved within specific History Numbers.  There
      is no sequence requirement between different History Numbers.

      When using one or more compression histories, the implementation
      MUST rely on either a lower layer reliable link protocol (RFC
      1663), use a technique to keep the compressor and decompressor
      histories in synchronization, or both.  The LZS-DCP protocol
      provides the Request-Req and Request-Ack bits in the DCP header
      for this purpose.  Since this synchronization is done on a per
      history basis, the history number fields are required to be the
      same size in both directions of the link.  Any data contained in
      the packet is processed after the signaling bits are processed.

      The transmitter MAY clear a Compression History at any time.

      The transmitter MUST clear a history after a receiving a Reset-
      Request for a given History Number.

2.4.  Data Expansion

      The maximum expansion of Stac LZS is 12.5%.

      A Maximum Receive Unit (MRU) MAY be negotiated that is 12.5%
      larger than the size of a normal packet.  Then, packets can always
      be sent compressed regardless of expansion.



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      The transmitter MAY send an uncompressed LZS-DCP packet at any
      time, although the typical use of uncompressed LZS-DCP packets is
      as an anti-expansion mechanism.

      When the expansion plus compression header exceeds the size of the
      peer's MRU for the link, the data MUST be sent as an uncompressed
      LZS-DCP packet.

      An uncompressed LZS-DCP packet is transmitted according to the
      format shown in Section 2.1, with the C/U bit set to 0
      (Uncompressed-Data).  If the Configuration Option Field 'Process
      Mode', is set to a value of 1 (Process-Uncompressed), uncompressed
      LZS-DCP packets are processed by both the compressor and the
      decompressor, updating the histories of each. If the Process Mode
      Field is set to a value of 0 (None), and the compressor has
      modified its history before sending the uncompressed packet, the
      compressor history MUST be clear.

2.5.  Packet Format

   A summary of the LZS-DCP packet format is shown below.  The fields
   are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          PPP Protocol         |   DCP-Header  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       (History Number)        |  (Seq Num)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     (LCB)     |
   +-+-+-+-+-+-+-+-+

2.5.1.  PPP Protocol

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

      When the LZS-DCP compression protocol is successfully negotiated
      by the PPP Compression Control Protocol [2], the value is 00FD or
      00FB hex.  This value MAY be compressed when Protocol-Field-
      Compression is negotiated.







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2.5.2.  DCP-Header

      The DCP-Header is nominally one octet in length, but may be
      extended through the use of the extension bit.

      The format of the DCP-Header is as follows:

         0     1     2     3     4     5     6     7
      +-----+-----+-----+-----+-----+-----+-----+-----+
      |  E  | C/U | R-A | R-R | Res | Res | Res | C/D |
      +-----+-----+-----+-----+-----+-----+-----+-----+

      E - Extension Bit

         The E bit is the extension bit.  If set to 0, it indicates that
         another octet of the DCP-Header is present.  Currently, this
         bit is always set to 1, since the DCP-Header field is only one
         octet long.

      C/U - Compressed/Uncompressed Bit

         The C/U indicates whether the data field contains compressed or
         uncompressed data.  A value of 1 indicates compressed data
         (often referred to as a compressed packet), and a value of 0
         indicates uncompressed data (or an uncompressed packet).

      R-A - Reset-Ack

         The R-A bit is used to inform the decompressing peer that
         the history buffer specified by the history number in the
         packet was in the cleared state just before the data contained
         in the packet was processed by the compression transformation
         (see section 3., Sending Compressed Datagrams).  This bit MUST
         be set to a value of "1" to indicate a Reset-Ack, and to
         acknowledge a receive failure (R-R) (see section 3., Sending
         Compressed Datagrams).  This bit is specific to the history
         number of the packet containing it.

      R-R - Reset-Request

         The R-R bit is used to request that the compressing peer
         clear the history buffer specified by the history number in the
         packet.  This bit MUST be set to a value of "1" to indicate a
         Reset-Request, and to respond to a receive failure (R-R) (see
         section 3., Sending Compressed Datagrams).  This bit is
         specific to the history number of the packet containing it.





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      Res - Reserved

         These bits are reserved and MUST be set to 0

      C/D - Control/Data

         This bit is used by DCP to provide in-band negotiation in
         applications where out-of-band negotiation methods are not
         provided (i.e. Frame Relay).  Since CCP provides an out of band
         negotiating mechanism, this feature is not used in this
         application.  All packets MUST set this bit to a value of 0,
         which signifies that the packet is a data packet.  (Packets
         containing only Reset- Requests are classified as data
         packets.)

2.5.3.  History Number

      The number of the compression history which was used, ranging from
      1 to the negotiated value in the History Count field.

      If the negotiated History Count is less than 2, this field is
      removed.  If the negotiated History Count is 2 or more, but less
      than 256, this field is 1 octet.  If 256 or more histories are
      negotiated, this field is 2 octets, most significant octet first.

      If multiple histories are used in one direction on a link, the
      history number field MUST be present on all packets in both
      directions, and sized according to the largest number of histories
      in either direction.

      If multiple histories are used, this field MUST be present in
      uncompressed as well as compressed packets.

2.5.4.  Sequence Number

      The sequence number field is one octet in length.  When the check
      mode field is set to the "Sequence Number" or "Sequence Number +
      LCB" options, the sequence number field MUST be present in all
      data compression packets that contain a data field.

      The value of the sequence number field (the sequence number of the
      packet) MUST begin with "1" and increment modulo 256 on successive
      packets that contain data fields.  This number is relative to the
      history number used.

      On receipt of a packet with the R-A bit set to "0", if the
      sequence number of the packet is any number other than (N+1) mod
      256, where N is the sequence number of the last packet received



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      for the same history, or an initial value of "0", a receive
      failure for that history has occurred.  The receive failure MUST
      be handled according to the synchronization procedure in section
      3.5.

      The sequence number MUST NOT be reset by the transmitter when a
      packet containing a Reset-Ack is sent. The decompressor MUST
      resynchronize its sequence number reference for the indicated
      history when a packet containing a Reset-Ack is received.

2.5.5.  Data

      The data field MUST contain a single datagram in either compressed
      or uncompressed form, depending on the state of the C/U bit in the
      Header.  This length of this field is always be an integer number
      of octets.  This field is required in all packets that do not have
      the R-R bit set to "1".

      If the C/U bit is set to "0", the data field contains the
      uncompressed form of the datagram.

      If the C/U bit is set to "1", the form of the data field is one
      block of compressed data as defined in 3.2 of X3.241-1994, with
      the following exceptions:  1) the end marker may be followed with
      additional octets containing only zeros;  2) if the final octet in
      the block of compressed data has a value of "0", then it MAY be
      removed from the data field.

      There is only one end marker per block of compressed data.

2.5.6.  Longitudinal Check Byte

      The LCB field is one octet in length, and if present MUST be the
      last octet in the data compression packet.  When the check-mode
      field is set to "LCB" or "Sequence Number + LCB", this field MUST
      be present in all packets where the data field contains compressed
      data.  This field MUST NOT be present in data compression packets
      where the data field contains uncompressed data.  This field
      contains the result of the LCB calculation, in accordance with the
      following paragraph.

      The LCB octet is the Exclusive-OR of FF(hex) and each octet of the
      uncompressed datagram (prior to the compression transformation).
      On receipt, the receiver computes the Exclusive-OR of FF(hex) and
      each octet of the decompressed packet.  If this value does not
      match the received LCB, then a receive failure for that history
      has occurred.  The receive failure is handled according to the
      history synchronization procedure in section 3.5.



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2.5.7.  Compressed Data

   The Stac LZS compression algorithm is Defined in ANSI X3.241-1994
   [7]. The format of the compressed data is repeated here for
   informational purposes ONLY.

   <Compressed Stream> := [<Compressed String>] <End Marker>
   <Compressed String> := 0 <Raw Byte> | 1 <Compressed Bytes>

   <Raw Byte> := <b><b><b><b><b><b><b><b>          (8-bit byte)
   <Compressed Bytes> := <Offset> <Length>

   <Offset> := 1 <b><b><b><b><b><b><b> |           (7-bit offset)
               0 <b><b><b><b><b><b><b><b><b><b><b> (11-bit offset)
   <End Marker> := 110000000
   <b> := 1 | 0

   <Length> :=
   00        = 2     1111 0110      = 14
   01        = 3     1111 0111      = 15
   10        = 4     1111 1000      = 16
   1100      = 5     1111 1001      = 17
   1101      = 6     1111 1010      = 18
   1110      = 7     1111 1011      = 19
   1111 0000 = 8     1111 1100      = 20
   1111 0001 = 9     1111 1101      = 21
   1111 0010 = 10    1111 1110      = 22
   1111 0011 = 11    1111 1111 0000 = 23
   1111 0100 = 12    1111 1111 0001 = 24
   1111 0101 = 13     ...

3.  Sending Compressed Datagrams

   The reliable and efficient transport of datagrams on the data link
   depends on the following processes.

3.1.  Transmitter Process

      The compression operation results in either compressed or
      uncompressed data.  When a network datagram is received, it is
      assigned to a particular history buffer and processed according to
      ANSI X3.241-1994 to form compressed data or used as is to form
      uncompressed data.  Prior to the compression operation, if a
      Reset-Request is outstanding for the history buffer to be used,
      the buffer is cleared.  In performing the compression operation,
      if the process mode field is set to the value None ("0"), the
      history MUST only be updated if the result is compressed data.  If
      process mode field is set to the value Process-Uncompressed ("1"),



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      the history MUST be updated when either compressed data or
      uncompressed data is produced.  Uncompressed data MAY be sent at
      any time.  Uncompressed data MUST be sent if compression causes
      enough expansion to cause the data compression datagram size to
      exceed the Information field's MRU.

      If the Process Mode field is set to the value None ("0") and the
      compressor has modified the history buffer before sending an
      uncompressed datagram, the history buffer MUST be cleared before
      the next datagram is processed.

      The output of the compression operation is placed in the
      information field of the datagram.  The C/U bit is set according
      to whether the data field contains compressed or uncompressed
      data.  If the sequence number field is present according the value
      of the check mode field, the sequence number counter for the
      applicable history number MUST be incremented and its value placed
      in the sequence number field.  If the data field contains
      compressed data, and Check Mode field is set accordingly, the LCB
      field is present and its value is computed as specified in section
      2.2.6.

      Upon reception of a packet containing a Reset-Request, the
      transmitting compressor MUST be cleared to an initial state, which
      includes clearing the history buffer.  If the data field of the
      packet containing the Reset-Request contains data, it is delivered
      to the local receiver as a normal data packet.  In addition to the
      reset of the compressor, a packet MUST be transmitted with Reset-
      Ack bit set to 1.  The data field of this packet MUST be filled
      with data.  If no data is ready for transmission, the transmitter
      MUST wait until data is ready before sending the Reset-Ack.

      If the history buffer is in the clear state (the history buffer
      contains no data bytes) prior to performing the compression
      operation, the resulting compressed or uncompressed packet MUST be
      sent with the R-A bit set to "1".

3.2.  Receiver Process

      When a data compression datagram is received from the peer, the
      R-R and R-A bits MUST be checked.  If the R-R bit is set, the
      local compression engine MUST be signaled that a Reset-Request has
      been received for the history specified by the history number
      field.  If the R-A bit is set, any outstanding receive failure for
      the specified history MUST be cleared.  If no receive failure is
      outstanding, and the sequence number field is present, its value
      checked. If a receive failure has occurred, it MUST be handled
      according to the history resynchronization mechanism described



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      below, and the remainder of the datagram is discarded.  If no
      receive failure is detected, the data is assigned to the indicated
      decompression history buffer and processed according to process
      mode field and C/U bit.

      If the C/U bit is set to "1", a single octet containing the value
      0x00 MUST be appended to the data field and the resulting
      compressed data block MUST be decompressed according to ANSI
      X3.241-1994.  If the LCB field is present on the received
      datagram, an LCB for the uncompressed data MUST be computed and
      checked against the received LCB according to section 2.1.  If a
      receive failure has occurred, it MUST be handled according to the
      History Resynchronization Mechanism described below.

      If the C/U bit is set to "0" and the process mode field is set to
      the value Process-Uncompressed ("1"), the specified decompression
      history buffer MUST be updated with the received uncompressed
      data.

      If the C/U bit is set to "0" and process mode field is set to the
      value None ("0"), the specified decompression history buffer MUST
      NOT be modified.

      If the R-A bit is set to "1", the receiving decompressor MAY be
      reset to an initial state.  (However, due to the characteristics
      of the Stac LZS algorithm, a decompressor reset is not required).
      After reset, any compressed or uncompressed data contained in the
      packet is processed.

      On the occurrence of a receive failure, an implementation MUST
      transmit a packet with the R-R bit set to "1" (a Reset-Request)
      and with the history number matching the history that had the
      failure.  The data field may be present if data is waiting to be
      transported for that history, or the R-R bit may be set in a
      packet transmitted without sequence number, data, or LCB fields.
      Once a receive failure has occurred, the data in any subsequent
      packets received for that history MUST be discarded until a packet
      containing a Reset-Ack is received.  It is the responsibility of
      the receiver to ensure the reliability of the reset request-
      acknowledge mechanism.  This may require the transmission of an
      additional Reset-Request before a Reset-Ack will be received.

3.3.  History Maintenance

      The History Count field determines the number of history buffers
      to be maintained for the compression protocol.  For example, each
      history buffer could represent a separate logical connection
      between the data compression peers.  When maintaining a history,



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      the peers MUST use link error detection and signaling to ensure
      that both the compressor and decompressor copies of each history
      buffer are always identical.

      Setting the History Count field to the value "0" indicates that
      the compression is to be on a connectionless basis.  In this case,
      a single history buffer is used and MUST be cleared at the
      beginning of every datagram.  The compressing entity MUST set the
      R-A bit on all outgoing datagrams.

      When the History Count field is set to the value "1", a single
      history buffer is maintained by each of the data compression
      peers. (A single logical connection.)

      When the History Count field is set to a value greater than "1",
      separate history buffers, error detection states, and signaling
      states are maintained by the decompressing entity for each
      history.  The compressing peer may transmit data on any number of
      separate histories, up to the value of the History Count field.

3.4.  Anti-Expansion Mechanism

      When one or more histories are negotiated and the Process Mode
      field is set to None ("0"), there are 2 options on how to handle
      packets that expand:

         1) Send the expanded data and keep the history, thus allowing
            loss of current bandwidth but preserving future bandwidth on
            the link.
         2) Send the uncompressed data and clear the history, thus
            conserving current bandwidth, but allowing possible loss of
            future bandwidth on the link.

      When 1 or more histories are negotiated and the Process Mode field
      is set to Process-Uncompressed ("1"), there is an additional
      option:

         3) Send the uncompressed data and do not clear the compression
            history; the decompressor will update its history, thus
            conserving the current bandwidth and future bandwidth on the
            link.

3.5.  History Resynchronization Mechanism

      The DCP-Header includes R-R (Reset-Request) and R-A (Reset-Ack)
      bits in order to provide a mechanism for indicating a receiver
      failure in one direction of a compressed link without affecting
      traffic in the other direction.  A receive failure is determined



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      using the sequence number and/or LCB mechanism, according to the
      value of the check mode field.

      Reset-Requests and Reset-Acks are specific to the history number
      of the packet containing them.

      Reset-Request/Reset-Ack history synchronization signaling is
      provided to recover from a loss of synchronization between peers,
      especially in unreliable transport layers.  As with all
      compression algorithms, the decompressor can not recover from
      dropped, erroneous, or mis-ordered datagrams, and will propagate
      errors catastrophically until both peers are reset to an initial
      state.

      The LZS-DCP protocol provides a means to detect these error
      conditions: LCB for erroneous datagrams, and sequence number for
      dropped or mis-ordered datagrams.  There is a means for correcting
      a loss of synchronization: clear both the failing compression and
      decompression histories, and follow the transmitter and receiver
      processes in sections 3.1. and 3.2.

4.  Configuration Option Format

   The LZS-DCP Configuration Option negotiates the use of LZS-DCP on the
   link.  By default or ultimate disagreement, no compression is used.
   This Configuration Option is used in CCP, and can be used in other
   negotiation mechanisms [2].

   All implementations MUST support the default values.

   A summary of the LZS-DCP 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     |        History Count          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Check Mode  | Process Mode  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      23

   Length

      6



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   History Count

      The History Count field is two octets, most significant octet
      first, and specifies the maximum number of Compression Histories.

      The value 0 indicates that the implementation expects the peer to
      clear the Compression History at the beginning of every packet.
      If this value is selected, the transmitter MUST set the Reset-Ack
      bit of every packet that contains compressed data.

      The value 1 is the default value and is used to indicate that only
      one history is maintained.

      Other valid values range from 2 to 65535.  The peer is not
      required to send as many histories as the implementation indicates
      that it can accept.  However, it should be noted that resources
      are allocated in each peer to support the number of negotiated
      histories in this field.

Check Mode

      The Check Mode indicates support of LCB and/or Sequence checking.
      The use of check mode None (0) MUST NOT be used for history counts
      greater than zero.

         0    None
         1    LCB
         2    Sequence Number
         3    Sequence Number + LCB (default)

   Process Mode

      The Process Mode specifies how uncompressed packets are handled.
      A value of None (0) indicates that uncompressed packets are not
      processed by the decompressor.  A value of Process-Uncompressed

      (1) indicates that uncompressed packets are processed by the
      decompressor to update the history.

         0    None (default)
         1    Process-Uncompressed

Security Considerations

   Security issues are not discussed in this memo.






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Acknowledgments

   This document is based on, and uses much of the text of [5].

References

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

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

   [3]    Lempel, A., and J. Ziv, "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", RFC 1663, Novell, July
          1994.

   [5]    Friend, R., and W. Simpson, "PPP Stac LZS Compression
          Protocol", RFC 1974, August 1996.

   [6]    Motorola Information Systems Group, "Data Compression Protocol
          (DCP) Proposal", TR-30.1 ad hoc contribution (email
          reflector), September 21, 1995.

   [7]    ANSI X3.241-1994, "American National Standard Data Compression
          Method, Adaptive Coding with Sliding Window of Information
          Interchange".

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











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Authors' Addresses

   Questions about this memo can also be directed to:

   Kevin Schneider
   Adtran, Inc.
   901 Explorer Blvd.
   Huntsville, AL 25806

   Phone: (205) 971-8024
   EMail: kschneider@adtran.com


   Robert Friend
   Stac Technology
   12636 High Bluff Drive
   San Diego, CA 92130-2093

   Phone: (619) 794-4542
   EMail: rfriend@stac.com































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