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
 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.
 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.
 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
 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.
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.
 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.
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.
 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
 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.
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 =わ 14
 01 =わ 3 1111 0111 =わ 15
 10 =わ 4 1111 1000 =わ 16
 1100 =わ 5 1111 1001 =わ 17
 1101 =わ 6 1111 1010 =わ 18
 1110 =わ 7 1111 1011 =わ 19
 1111 0000 =わ 8 1111 1100 =わ 20
 1111 0001 =わ 9 1111 1101 =わ 21
 1111 0010 =わ 10 1111 1110 =わ 22
 1111 0011 =わ 11 1111 1111 0000 =わ 23
 1111 0100 =わ 12 1111 1111 0001 =わ 24
 1111 0101 =わ 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"),
 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
 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,
 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
 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
 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.
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
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