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rfc:rfc1967

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

Schneider & Friend Informational [Page 1] 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.

Schneider & Friend Informational [Page 2] 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.

Schneider & Friend Informational [Page 3] RFC 1967 LZS-DCP August 1996

 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

Schneider & Friend Informational [Page 4] 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.

Schneider & Friend Informational [Page 5] RFC 1967 LZS-DCP August 1996

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:
  1. The link has a high error rate.
  2. 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.

Schneider & Friend Informational [Page 6] RFC 1967 LZS-DCP August 1996

    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.

Schneider & Friend Informational [Page 7] RFC 1967 LZS-DCP August 1996

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.

Schneider & Friend Informational [Page 8] RFC 1967 LZS-DCP August 1996

    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

Schneider & Friend Informational [Page 9] RFC 1967 LZS-DCP August 1996

    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.

Schneider & Friend Informational [Page 10] RFC 1967 LZS-DCP August 1996

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"),

Schneider & Friend Informational [Page 11] RFC 1967 LZS-DCP August 1996

    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

Schneider & Friend Informational [Page 12] RFC 1967 LZS-DCP August 1996

    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,

Schneider & Friend Informational [Page 13] RFC 1967 LZS-DCP August 1996

    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

Schneider & Friend Informational [Page 14] RFC 1967 LZS-DCP August 1996

    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

Schneider & Friend Informational [Page 15] RFC 1967 LZS-DCP August 1996

 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.

Schneider & Friend Informational [Page 16] RFC 1967 LZS-DCP August 1996

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

Schneider & Friend Informational [Page 17] RFC 1967 LZS-DCP August 1996

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

Schneider & Friend Informational [Page 18]

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