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rfc:rfc1553

Network Working Group S. Mathur Request for Comments: 1553 M. Lewis Category: Standards Track Telebit Corporation

                                                         December 1993
           Compressing IPX Headers Over WAN Media (CIPX)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state and
 status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document describes a method for compressing the headers of IPX
 datagrams (CIPX).  With this method, it is possible to
 significantly improve performance over lower speed wide area
 network (WAN) media.  For normal IPX packet traffic, CIPX can
 provide a compression ratio of approximately 2:1 including both IPX
 header and data.  This method can be used on various type of WAN
 media, including those supporting PPP and X.25.
 This memo ia a product of the Point-to-Point Protocol Extensions
 (PPPEXT) Working Group of the IETF.  Comments should be sent to
 the authors and the ietf-ppp@ucdavis.edu mailing list.

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.

Mathur & Lewis [Page 1] RFC 1553 CIPX December 1993

  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 should 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.

Introduction

 Internetwork Packet Exchange (IPX) is a protocol defined by the
 Novell Corporation [1].  It is derived from the Internet Datagram
 Protocol (IDP) protocol of the Xerox Network Systems (XNS) family
 of protocols.  IPX is a datagram, connectionless protocol that does
 not require an acknowledgment for each packet sent.  The IPX
 protocol corresponds to the network layer of the ISO model.
 Usually, there is a transport layer protocol above IPX.  The most
 common transport protocol is the Netware Core Protocol (NCP), which
 is used for file server access.  The Sequenced Packet Exchange
 (SPX) is the reliable connection-based transport protocol commonly
 used by applications.
 The IPX packet consists of a 30 octet IPX header, usually followed
 by the transport layer protocol header.  The NCP header is 6 octets
 in length.  The SPX header is 12 octets in length.
 Two strategies are described below for compressing IPX headers.
 This specification requires that implementations of CIPX support
 both IPX header compression strategies.  These header compression
 algorithms are based on those Van Jacobson described [2] for TCP/IP
 packets.
 The first strategy is to compress only the IPX header.  This
 compression algorithm can be used to compress any IPX packet,
 without affecting the transport protocol.  This algorithm
 compresses a 30 octet IPX header into a one to seven octet header.
 The second strategy is to compress the combined IPX and NCP
 headers.  This algorithm compresses only NCP packets with NCP type
 of 0x2222 and 0x3333.  This algorithm compresses a 36 octet NCP/IPX

Mathur & Lewis [Page 2] RFC 1553 CIPX December 1993

 header into a one to eight octet header.
 Lastly, it is possible and many times desirable, to use this type
 of header compression in conjunction with some type of data
 compression.
 Data compression technology takes many forms. Link bit stream
 compression is a common approach over very low speed asynchronous
 links, normally performed by modems transparently.  Transparent bit
 stream compression is also offered in some DSUs, routers and
 bridges.  Data compression can be provided using protocols such as
 CCITT V.42bis[3], MNP 5, Lempel-Ziv, or LAPB[4].
 When using both header and data compression, the sequence of
 compression is important.  When sending packets, data compression
 MUST be done after header compression.  Conversely when receiving
 packets, data decompression MUST be done before header
 decompression.

IPX Compression Algorithm

 The normal IPX header consists of the following fields: checksum,
 packet length, transport control (hop count), packet type,
 destination and source address fields.
                           +-----------------------+
                           |       Checksum        |
                           +-----------------------+
                           |     Packet Length     |
                           +-----------+-----------+
                           |    Hops   |Packet Type|
                           +-----------+-----------+
                           |      Destination      |
                           |        Address        |
                           |      (12 Octets)      |
                           +-----------------------+
                           |        Source         |
                           |        Address        |
                           |      (12 Octets)      |
                           +-----------------------+
                               IPX PACKET HEADER
 The IPX header diagram above is shown without the field alignment
 details.  Consider each field of the IPX header separately, and how
 it typically changes.
 Historically, Novell has not used the Checksum field in the IPX

Mathur & Lewis [Page 3] RFC 1553 CIPX December 1993

 header, and has required that this field be set to 0xFFFF.  Since the
 Checksum field remains constant, it is clear that the value can be
 compressed.
 Where Checksums are implemented (not 0xFFFF), the Checksum MUST be
 included in the compressed packet.  Recalculating the checksum would
 destroy the end-to-end reliability of the connection.  Note that
 Checksums are now implemented in the Fault Tolerant Servers.
 For most links, the Packet Length can be determined from the MAC
 layer.  There are cases in which the length cannot be determined from
 the MAC layer.  For example, some hardware devices pad packets to a
 required minimum length.  For links where it is not possible to
 determine the IPX packet length from the MAC layer, packet length
 needs to be included in the compressed packet.
 The Transport Control (Hops) field usually does not change between
 two end-points.  For the purposes of compression, we will assume that
 it never changes, and will not examine this field when determining a
 connection.
 The Packet Type field is constant for any connection.
 The Destination and Source Address fields are each made up of 12
 octets: Network (4 octets), Node (6 octets), and Socket (2 octets)
 fields.  An IPX connection is the logical association between two
 endpoints known by a given source and destination address pair.  For
 any specific IPX connection, the Destination and Source Address
 fields are constant.
 Hence, the fields that may need to be included in the compressed IPX
 header are the Checksum and the Packet Length.
 While using this IPX header compression algorithm, packets can be
 lost.  The loss of an Initial packet presents a problem.  In this
 case, if the sender later tries to send a compressed packet, the
 receiving end cannot decompress the packet correctly.
 Sufficient information is not available in the IPX header to
 determine when a re-transmission has occured.  For this reason, it is
 necessary that the sender of an Initial packet be guaranteed that the
 packet has been received.  Therefore, we provide a mechanism for
 Confirmation of an Initial packet.

NCP/IPX Header Compression

 Since most IPX packets are Netware Core Protocol packets (packet type
 17), compressing the NCP header will give us added performance.  A

Mathur & Lewis [Page 4] RFC 1553 CIPX December 1993

 minimal CIPX implementation MUST also implement NCP/IPX compression.
                                +------------+
                                |    NCP     |
                                |    Type    |
                                +------------+
                                |  Sequence  |
                                |   Number   |
                                +------------+
                                | Connection |
                                |(low octet) |
                                +------------+
                                |   Task     |
                                |   Number   |
                                +------------+
                                | Connection |
                                |(high octet)|
                                +------------+
                                  NCP HEADER
 The NCP header is 6 octets in length consisting of the following
 fields: NCP type, sequence number, connection number and task number.
 The NCP type field values that are currently defined are:
           1111   Create Connection
           2222   NCP request from workstation
           3333   NCP reply from file server
           5555   Destroy Connection
           7777   Burst Mode Packet
           9999   Server Busy Packet
 This NCP header compression algorithm only compresses packets that
 have a type field value of 0x2222 or 0x3333.  If the NCP type is
 0x2222, this packet is a request from the client to the server.
 Conversely if the NCP type is 0x3333, this is a response from the
 server to the client.  All other types of NCP packets are not
 compressed at the NCP level, but are compressed at the IPX level.
 The Create Connection (0x111), Destroy Connection (0x5555) and Server
 Busy (0x9999) packets are not exchanged frequently enough to justify
 special NCP compression.  The Burst Mode (0x7777) packet is discussed
 below.
 The connection number is a constant for a given connection.
 The sequence number is increased by one for each new request.  Hence
 the sequence number can be determined implicitly.  The decompressor

Mathur & Lewis [Page 5] RFC 1553 CIPX December 1993

 increments the sequence number for each compressed packet it receives
 for a connection.
 The task number can change unpredictably, although it might remain
 constant for several packets.  If the NCP task number is different
 from the last one for this connection, the NCP task number must be
 included.
 If the NCP packet is lost, it will be retransmitted through the
 normal transport layer mechanisms.  The Initial NCP packet does not
 require confirmation, as a re-transmitted packet can be easily
 identified.  This is accomplished by comparing the sequence number of
 the packet to the sequence number of the previous packet.  If the
 sequence number is not exactly one greater than the previous packet,
 a new Initial packet must be sent, although the same connection slot
 may be used.
 In the event of compressed packet loss, the sequence number will be
 too small.  When the IPX Checksum is present, the loss can be
 determined at the destination system by an incorrect checksum.  When
 there is no checksum present, the loss is more likely to be detected
 upon receiving a later retransmission.

NCP Burst Mode Packets

 The burst mode protocol uses the NCP type value of 0x7777.  This type
 of packet does not have the normal NCP header described above.
 Instead, it has a 36 octet burst header.  The above NCP header
 compression algorithm should not be used to compress this packet.
 The IPX header in this packet is still compressible with the IPX
 header compression algorithm described.

SPX Packets

    SPX packets are typically used by applications which require
    reliable service such as print servers.  It is possible to apply a
    similar NCP/IPX technique to SPX/IPX packets.  At this time, we
    have not described such a mechanism.  The IPX header in this
    packet is still compressible with the IPX header compression
    algorithm described.

Compression Header

    IPX compression should be negotiated by some means (eg. IPXCP or
    IPXWAN).  Each end must negotiate the desired options, such as the
    maximum number of concurrent connections which will be maintained
    in each direction.  Once IPX compression is negotiated, all IPX
    packets sent over that link have a CIPX header added to the

Mathur & Lewis [Page 6] RFC 1553 CIPX December 1993

    beginning of the packet.  The CIPX header is variable in length.
    The one octet CIPX header is added even when a regular IPX packet
    is sent over the link.  By including the CIPX header on every
    packet, we support the ability to run CIPX over various WAN links
    as if it were a normal IPX packet.  It does not rely on any new
    link specific packet demultiplexing.
    Implementations of this compression protocol must maintain send
    and receive tables indicating the state of each connection.  The
    original header for each connection is stored in a "slot".
    Typically, each client-server connection will use a separate slot.
    Both sides keep a copy of the full IPX header corresponding to
    each slot.  The sending side (compressor) uses this information to
    determine the fields that have changed.  The receiving side
    (decompressor) uses this information to reconstruct the original
    packet header.
    The CIPX packet header specifies the type of the packet and any
    options for that packet.  The minimum CIPX header is one octet in
    length.
       7   6   5   4   3   2   1   0
     +---+---+---+---+---+---+---+---+
     |   |   |   |   |   |   |   |   |
     +---+---+---+---+---+---+---+---+
       ^   ^   ^   ^   ^   ^   ^   ^
       |   |   |   |   |   |   |   |
       |   |   |   |   |___|___|___|___ Packet Type
       |   |   |   |                    0    Compressed
       |   |   |   |                    1    Regular
       |   |   |   |                    3    Confirmed Initial
       |   |   |   |                    5    Confirm
       |   |   |   |                    7    Unconfirmed Initial
       |   |   |   |                    9    Reject
       |   |   |   |                   11-15 Reserved
       |   |   |   |
       |__ |__ |__ |___________________ Packet Type Dependent Flags
                              FLAGS OCTET
    As can be seen above, the low order bits specify the packet type.
    All Compressed packets have a lowest bit of zero.  The other
    packet types are odd numbers.
    Note that the Flags octet MUST NOT contain the value 0xFF.  This
    is necessary to distinguish the CIPX flags octet from a normal IPX
    header with a 0xFFFF checksum field.  It is important to be able

Mathur & Lewis [Page 7] RFC 1553 CIPX December 1993

    to recognize a normal IPX header regardless of the state of
    compression.  It is possible with some link layer protocols such
    as X.25 Permanent Virtual Circuits that one end of the link may
    fail and start sending regular IPX packets without the CIPX
    header.  CIPX implementations MUST recognize this situation and
    renegotiate the use of CIPX.
    Each packet type has associated flag bits, which are called Packet
    Type Dependent Flags.  Different packet types have different
    Packet Type Dependent Flags.  All bits that are reserved or are
    not specified must be set to zero.
    Since none of the packet types other than Compressed currently
    uses any of the flag bits, the packet type field could easily be
    expanded.  Any future expansion must ensure that at least one of
    the bits in the Flags octet remains zero so the value cannot be
    0xFF.

Compressed Packet

 This type of packet does not contain a packet header (either 30 byte
 IPX, or 36 byte NCP).  A slot number indicates to the receiver which
 saved header to use to formulate the original packet header before
 passing the packet up to IPX.

Mathur & Lewis [Page 8] RFC 1553 CIPX December 1993

    ________________________________ Slot Number
    |                                0    Assume same as last packet
    |                                1    Included in packet
    |
    |   ____________________________ Checksum
    |   |                            0    Assume 0xFFFF
    |   |                            1    Included in packet
    |   |
    |   |   ________________________ Length
    |   |   |                        0    Determine from MAC length
    |   |   |                        1    Included in packet
    |   |   |
    |   |   |   ____________________ Task Number (NCP only)
    |   |   |   |                    0    Assume same as last packet
    |   |   |   |                    1    Included in packet
    |   |   |   |
    |   |   |   |   ________________ Reserved (Must be zero)
    |   |   |   |   |   |   |
    |   |   |   |   |   |   |   ____ Packet Type
    |   |   |   |   |   |   |   |    0    Compressed Packet
    v   v   v   v   v   v   v   v
  +---+---+---+---+---+---+---+---+
  |   |   |   |   | 0 | 0 | 0 | 0 |
  +---+---+---+---+---+---+---+---+
    7   6   5   4   3   2   1   0
 Consider each flag in the flags octet, looking at the high order bits
 working toward the lower order bits.  Each of the fields is optional,
 but if present will be found in the same order in the compressed
 packet header.

Slot Number

 The slot number flag indicates the slot number field is included in
 the compressed packet.  The slot number field is one octet in length
 and specifies the number of the slot which corresponds to the Initial
 packet header.  Slots are numbered starting at zero and continue to
 the maximum number of slots minus one.
 By default, slot compression is disabled.  If negotiated, slot
 compression can be enabled for those slots which were created by the
 Unconfirmed Initial packet.  When set to one (1), the slot number
 flag indicates the inclusion of the the slot number in the compressed
 packet.  When set to zero (0), the slot number flag indicates the
 omission of the the slot number and specifies the use of the same
 slot number as for the last packet.

Mathur & Lewis [Page 9] RFC 1553 CIPX December 1993

    Implementation Note:
       Slot compression MUST only be enabled in a receiver which can
       account for all erroneous and discarded packets.  When a packet
       has been discarded, the slot number is indeterminate for future
       packets.  The decompressor MUST discard all further packets
       until a slot number is received.

Checksum

 When set to one (1), the checksum flag indicates the compressed
 packet will include the 2 octet checksum.  When set to zero (0),
 this flag indicates the omission of the checksum and the decompressor
 is to assume the checksum is 0xFFFF.  Note that meaningful checksums
 must be included in the packet with the checksum flag set to one (1).

Length

 When set to one (1), the length flag indicates the inclusion of the
 IPX data length field in the compressed packet.  When set to zero
 (0), the length flag indicates the omission of the IPX data length
 field in the compressed packet.
 This is the Length field from the original IPX packet header.  It
 specifies the length of IPX header and data in the packet prior to
 compression.  It does not include the CIPX compression field such as
 flags, slot number, checksum, length field, or the NCP task number.
 Note that it is preferable to determine the length field from the MAC
 layer whenever possible, by subtracting the length of the compression
 header fields and adding the length of the saved packet header.
 Since every octet is significant over lower speed WAN links, an
 optimization is used in the specification of the length.  It can be
 specified as a one, two or three octet field.  If the length is in
 the range 0 to 127, then it is specified as a one octet field.  If
 the length is in the range 128 to 16383, it is specified as a two
 octet field in high to low order, with the first octet in the range
 128 to 191.  Otherwise, if the length is greater than 16383, the
 first octet contains 192, and the second and third octets contain the
 full length.  (This scheme is extensible to 8 octets, but currently
 is not required in the IPX protocol suite.)

Mathur & Lewis [Page 10] RFC 1553 CIPX December 1993

 +-+-+-+-+-+-+-+-+
 |0|   length    |   length < 128
 +-+-+-+-+-+-+-+-+
 ONE OCTET LENGTH FIELD
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 0|          length           |   length < 16384
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 TWO OCTET LENGTH FIELD
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 1 0 0 0 0 0 0|            length             |  length < 65535
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 THREE OCTET LENGTH FIELD

Task Number

 When set to one (1), the NCP task number flag indicates the NCP task
 number is included in the compressed packet (see NCP/IPX compression
 above).  When set to zero (0), the NCP task number flag indicates the
 omission of the NCP task number in the compressed packet.  When the
 NCP task number is not included in the compressed packet, we use the
 same NCP task number as that of last packet.
 Based upon the bits set in the flags octet, optional portions are
 included in the compressed IPX header.  The minimum compressed IPX
 header contains only the Flags octet.  All fields in the original IPX
 header have been compressed out of the header.  The maximum
 compressed IPX header can include up to 7 octets, the Flags, Slot,
 Checksum (2 octets), and Length (3 octets) fields, or 8 octets if the
 NCP Task Number is included.  The minimum and maximum compressed IPX
 packets are shown below.  Header fields are one octet in length
 except where noted.

Mathur & Lewis [Page 11] RFC 1553 CIPX December 1993

      +--------+---------
      | Flags  | DATA ...
      |  0x00  |
      +--------+---------
      MINIMUM COMPRESSED IPX PACKET
      +--------+--------+---------+---------+---------
      | Flags  |  Slot  |Checksum | Length  | DATA ...
      |  0xE0  | Number |2 octets |3 octets |
      +--------+--------+---------+---------+---------
      MAXIMUM COMPRESSED IPX PACKET
      +--------+--------+---------+---------+--------+---------
      | Flags  |  Slot  |Checksum | Length  |NCP Task| DATA ...
      |  0xF0  | Number |2 octets |3 octets | Number |
      +--------+--------+---------+---------+--------+---------
      MAXIMUM COMPRESSED NCP/IPX PACKET

Regular Packet

 The Regular packet type designates an IPX packet for which no
 compression is desired.  This type of packet is sent when a packet
 cannot be compressed, or a decision is made not to compress it.
        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
      +---+---+---+---+---+---+---+---+
        ^   ^   ^   ^   ^   ^   ^   ^
        |   |   |   |   |   |   |   |
        |   |   |   |   |___|___|___|___ Packet Type
        |   |   |   |                    1    Regular
        |   |   |   |
        |__ |__ |__ |___________________ Reserved (must be zero)
 The Regular packet is rarely sent.  Usually, the Regular packet is
 sent when there is not enough memory for the overhead of a new
 compression slot.  Also, this type is included for future unforeseen
 changes to the IPX protocol which defeat the effectiveness of
 compression.
    Implementation Note:
       The Regular Packet can be used for packets that are sporadic,
       which are not worth setting-up a compression slot.  This may be

Mathur & Lewis [Page 12] RFC 1553 CIPX December 1993

       hard to determine for specific protocols.  Various methods such
       as hold-down and least-recently-used timers are currently being
       used.
    On receipt, the 1 octet header is simply removed and the packet
    passed up to IPX.
    The entire IPX packet follows the single Flags octet.  Note for a
    Regular Packet (not compressed or uncompressed), the slot number
    field is not included.

Confirmed Initial Packet

 The Confirmed Initial packet type is used by the compressor to inform
 the decompressor of the original packet header which will be used for
 subsequent compression, and to request Confirmation.  The high order
 4 bits are reserved for expansion to support additional protocols.
        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 |
      +---+---+---+---+---+---+---+---+
        ^   ^   ^   ^   ^   ^   ^   ^
        |   |   |   |   |   |   |   |
        |   |   |   |   |___|___|___|___ Packet Type
        |   |   |   |                    3     Confirmed Initial
        |   |   |   |
        |__ |__ |__ |___________________ 0     IPX Protocol
                                         1-15  Reserved
 This type of packet is sent to inform the receiver to associate the
 IPX packet header with a slot number.  This packet is sent each time
 a different header format is sent for a given slot, or when the
 sender has not received a Confirmation Packet from the receiver.
 The Flags octet lower 4 bits indicate the Confirmed Initial CIPX
 packet type.  The high order 4 bits are reserved for expansion to
 support additional protocols.  The Flags octet is always followed by
 the Slot Number and an ID field.  The ID field is one octet in
 length.
 For each slot, the ID will increment with every new header sent.
 Different slots may have the same ID.  The combination of slot and ID
 uniquely identify a header.  In practice, the ID octet can be any
 number which is unique for a "reasonably long period" of time.  A
 reasonably long period is a function of transmission speed, round
 trip delays, and network load.  There must be very little chance of
 duplicate slot and ID combinations within this period.  Otherwise,

Mathur & Lewis [Page 13] RFC 1553 CIPX December 1993

 there is ambiguity as to which header is being identified.
    Implementation Note:
       There is no requirement to hold or resend the Confirmed Initial
       packet until confirmation.  When a new packet with the same IPX
       header is to be sent, another Confirmed Initial packet should
       be sent using the same slot, the same ID, and the new packet
       data.
       When a new packet with a different IPX header is to be sent, it
       may be sent using a slot which has not received confirmation.
       A Confirmed Initial packet is sent with the same slot, an
       incremented ID, and the new packet data.  Assuming a least-
       recently-used policy for selecting a slot for a new IPX header,
       this provides the ability to reuse slots when a Confirmed
       Initial packet has been sent but not confirmed.
            +---------+---------+---------+-/       /-+----------
            |  Flags  |   Slot  |   ID    |    IPX    |  DATA ...
            |   0x03  |  Number |         |   Header  |
            +---------+---------+---------+-/       /-+----------

CONFIRMED INITIAL PACKET

 Note that a Confirmed Initial header is followed by a complete IPX
 packet.

Confirm Packet

 The Confirm packet type is used by the decompressor to tell the
 compressor that it has received the Confirmed Initial packet.
 When the compressor receives this, it can start sending Compressed
 frames.
        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 |
      +---+---+---+---+---+---+---+---+
        ^   ^   ^   ^   ^   ^   ^   ^
        |   |   |   |   |   |   |   |
        |   |   |   |   |___|___|___|___ Packet Type
        |   |   |   |                    5    Confirm
        |   |   |   |
        |__ |__ |__ |___________________ Reserved (must be zero)
 A Confirm Packet is exactly 3 octets in length.  It consists of the

Mathur & Lewis [Page 14] RFC 1553 CIPX December 1993

 Flags, Slot Number and ID fields.  The Slot Number field contains the
 number of the slot which is being acknowledged.  The ID field
 contains the ID of the Confirmed Initial Packet which is being
 acknowledged.
      +---------+---------+----------+
      |  Flags  |   Slot  |    ID    |
      |   0x05  |  Number |          |
      +---------+---------+----------+

CONFIRM PACKET

Unconfirmed Initial Packet

 The Unconfirmed Initial packet type is used by the compressor to
 inform the decompressor of the original packet header which will be
 used for subsequent compression while not requesting confirmation.
 After sending an Unconfirmed Initial packet, the compressor may
 immediately send Compressed packets without confirmation.
        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 |
      +---+---+---+---+---+---+---+---+
        ^   ^   ^   ^   ^   ^   ^   ^
        |   |   |   |   |   |   |   |
        |   |   |   |   |___|___|___|___ Packet Type
        |   |   |   |                    7     Unconfirmed Initial
        |   |   |   |
        |__ |__ |__ |___________________ 0     NCP Protocol
                                         1-15  Reserved
 This type of packet is sent to inform the receiver to associate the
 IPX packet header with a slot number.  This packet is sent each time
 a different header format is sent for a given slot.
 The Flags octet lower 4 bits indicate the Unconfirmed Initial CIPX
 packet type.  The high order 4 bits are reserved for expansion to
 support additional protocols.  The Flags octet is always followed by
 the Slot Number.
      +---------+---------+-/        /-+-/       /-+---------
      |  Flags  |   Slot  |    IPX     |    NCP    | NCP
      |   0x07  |  Number |   Header   |   Header  | DATA ...
      +---------+---------+-/        /-+-/       /-+---------

Mathur & Lewis [Page 15] RFC 1553 CIPX December 1993

UNCONFIRMED INITIAL PACKET

 Note that an Unconfirmed Initial header is followed by a complete IPX
 packet.

Reject Packet

 The Reject packet type is used by the decompressor to tell the
 compressor that it has received a CIPX packet with a header which it
 does not support.  This is provided to regulate future extensions to
 CIPX.
        7   6   5   4   3   2   1   0
      +---+---+---+---+---+---+---+---+
      | 0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 |
      +---+---+---+---+---+---+---+---+
        ^   ^   ^   ^   ^   ^   ^   ^
        |   |   |   |   |   |   |   |
        |   |   |   |   |___|___|___|___ Packet Type
        |   |   |   |                    9    Reject
        |   |   |   |
        |__ |__ |__ |___________________ Reserved (must be zero)
 A Reject Packet is exactly 3 octets in length.  It consists of the
 Flags, Slot Number and Rejected Flags fields.
 The Slot Number field contains the number of the slot of the packet
 which is being rejected.  Since the actual packet type may be unknown
 or misunderstood, this field actually contains the second octet of
 the rejected packet.  In the normal case of a known CIPX packet type,
 this will be the slot number of an initial packet.
 The Rejected Flags field contains the first octet of the packet being
 rejected.  The packet type field is left untouched.  Any flags which
 are correctly recognized should be cleared.  The remaining flags
 indicate specific features that are being rejected.  This information
 should be sufficient for implementations to adjust the use of certain
 packet types or dependent flags.
    Implementation Note:
       The Flags value of 0xFF is not a valid CIPX packet type.
       Hence, such a packet type should be recognized as a standard
       IPX header and forwarded without CIPX processing to the
       appropriate routines.  Under no circumstances should a Flags
       value of 0xFF be rejected in a Reject Packet.

Mathur & Lewis [Page 16] RFC 1553 CIPX December 1993

            +---------+---------+----------+
            |  Flags  |   Slot  | Rejected |
            |   0x09  |  Number |  Flags   |
            +---------+---------+----------+
            REJECT PACKET

Compression Negotiation over PPP Links

 For PPP links [5], the use of header compression can be negotiated by
 IPXCP [6].  By default, no compression is enabled.
 The IPX-Compression-Protocol Configuration Option is used to indicate
 the ability to receive compressed packets.  Each end of the link must
 separately request this option if bi-directional compression is
 desired.
 The PPP Protocol field is set to the same value as the usual IPX
 packets, and all IPX packets sent over the link MUST conform to the
 compressed format.
 A summary of the IPX-Compression-Protocol Configuration Option format
 to negotiate Telebit IPX header compression (CIPX) 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     |    IPX-Compression-Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Max-Slot-Id  |    Options    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Type
         3
     Length
         6
     IPX-Compression-Protocol
         0002 (hex) for Telebit Compressed IPX headers (CIPX).
      Max-Slot-Id
         The Max-Slot-Id field is one octet and indicates the maximum

Mathur & Lewis [Page 17] RFC 1553 CIPX December 1993

         slot identifier.  This is one less than the actual number of
         slots; the slot identifier has values from zero to Max-Slot-
         Id.

Options

 The Options field is one octet, and is comprised of the "logical or"
 of the following values:
    0  No options.
    1  The slot identifer may be compressed.
       The slot identifier must not be compressed if there is no
       ability for the PPP link level to indicate an error in
       reception to the decompression module.  Synchronization after
       errors depends on receiving a packet with the slot identifier.
    2  Redefine Compressed Packet type bits 1-3.
       It was noted earlier that packet types have been chosen such
       that only the Compressed Packet type is an even number value
       with the lowest order bit of zero.  All other packet types are
       odd values with a lowest order bit of one.  The reason for this
       assignment was to make it possible to determine the Compressed
       Packet type by examining only one bit.  This make it possible
       to use all the other 7 bits to indicate status in the
       Compressed Packet.  The 7 bits are composed of the upper 4 bits
       which are permanently defined to indicate packet dependent
       flags, plus bits 1-3 which are otherwise part of the Packet
       Type.  The upper 4 bits are defined above.  The redefinition of
       bits 1-3 of the Compressed Packet type is left for future
       expansion.
             7   6   5   4   3   2   1   0
           +---+---+---+---+---+---+---+---+
           |   |   |   |   |   |   |   | 0 |
           +---+---+---+---+---+---+---+---+
             ^   ^   ^   ^   ^   ^   ^   ^
             |   |   |   |   |   |   |   |___ Packet Type
             |   |   |   |   |   |   |        0    Compressed Packet
             |   |   |   |   |   |   |
             |   |   |   |   |___|___|_______ Redefined bits
             |   |   |   |
             |___|___|___|___________________ Compressed Packet flags
       By default, this feature in not enabled and this flag is
       set to zero.  When this flag is set to one, it indicates

Mathur & Lewis [Page 18] RFC 1553 CIPX December 1993

       the desire to use this feature.

Compression Negotiation over IPXWAN Links

 "IPXWAN" is the protocol Novell uses to exchange necessary router
 to router information prior to exchanging standard IPX routing
 information and traffic over WAN datalinks [7].  To negotiate the
 Telebit compression option, we use Novell's allocated option number
 for CIPX (00) in the IPXWAN timer request/response packet.
 The Timer Request packet contains the following Telebit compression
 option:
   WOption Number       80        - Define compression type
   WAccept Option       01        - 0=No, 1=Yes, 3=N/A
   WOption Data Len     00 03     - Length of option
   WOption Data         00        - Telebit's compression (CIPX)
   WOption Data         XX        - Compression options
   WOption Data         NN        - Compression slots
 Where the WOption Data fields are:
   00   Telebit's compression option described in this
        document (CIPX).
   XX   Compression options as defined below:
           0x01   Compress slot ID when possible
           0x02   Redefine Compressed Packet type bits 1-3.
   NN   The requested # of compression slots.
   Accept Option (for compression type) must be set to YES if the
   option is supported and NO if the option is not supported.  A Timer
   Response must respond with only one header compression type set to
   YES.
   The Timer Response packet that accepts the option will look like
   this:
   WOption Number       80        - Define compression type
   WAccept Option       01        - 0=No, 1=Yes, 3=N/A
   WOption Data Len     00 03     - Length of option
   WOption Data         00        - Telebit's compression (CIPX)
   WOption Data         XX        - Compression options
   WOption Data         NN        - Compression slots

Mathur & Lewis [Page 19] RFC 1553 CIPX December 1993

 Where the WOption Data fields are:
   00   Telebit's compression option described in this
        document (CIPX).
   XX   Compression options as defined below:
           0x01   Compress slot ID when possible
           0x02   Redefine Compressed Packet type bits 1-3.
   NN   The negotiated # of slots (The lower of each side's
        requested number of slots)
 IPX packets (except of course IPXWAN packets) are not sent over the
 link until the IPXWAN negotiations are completed.  Once IPXWAN
 negotiations are completed, regular IPX packets can be sent over the
 link.
 If both ends of the link agree on the compression options, then the
 IPX packets are sent using the specified options.  If either end of
 the link does not accept a compression option, then this compression
 option will not be used.  Compression will be done using any
 remaining options.  Options, by definition, are not required.
 Implementations MUST support CIPX without any options.
 It is the responsibility of the router sending the IPXWAN Timer
 Response to inform the other router of the options that will be used.
 The Timer Response MUST contain a subset of the options received in a
 Timer Request.
 To be clear, IPXWAN is used to set up a symmetrical compression link.
 Compression is configured identically in both directions.  Each end
 will use the same number of slots and same compression options.  It
 is illegal for link ends to use different number of slots or
 different options.

IPX Compression Performance

 The performance of this algorithm will depend on the number of active
 connections and the number of slots negotiated.  If the number of
 slots is greater than the number of connections, the hit rate should
 be very high giving a very high compression ratio.  The performance
 also depends on the average size of the IPX packets.  If the average
 size of packets is small, then compression will result in a more
 noticeable performance improvement.

Mathur & Lewis [Page 20] RFC 1553 CIPX December 1993

                          avg_data_len + uncomp_header_len
      Compression ratio = ----------------------------------
                          avg_data_len + avg_comp_header_len
 Where 'avg_data_len' is the average length of data in the IPX packet,
 and 'uncomp_head_len' is the uncompressed header length which is
 fixed at 30 octets.  Where 'avg_comp_header_len' is the average
 length of the compressed IPX header.  The length of the minimum
 compressed IPX header is 1 octet.  The length of the maximum
 compressed NCP/IPX header is 8 octets (including the NCP task
 number), but since no implementation yet sends packets with a length
 greater than 16K, 7 octets is the commonly encountered maximum.
 Perhaps a reasonable 'avg_comp_header_len' is 2, assuming the
 inclusion of the flag and slot number octets.
 The maximum length of the data in an IPX packet is 546 octets (576
 octets - 30 octet IPX header), although newer implementations may
 send packets of up to 4096 octets.  The minimum length of the data in
 an IPX packet is 1 octet.  Within the normal distribution of small
 NCP packets, perhaps a reasonable 'avg_data_len' is 26 octets.
                               546 + 30
      Minimal Compression    = -------- =  1.04
                               546 + 6
                               1 + 30
      Maximal Compression    = ------   = 15.50
                               1 + 1
                               26 + 30
      Likely Compression     = -------  =  2.00
                               26 + 2

Security Considerations

 IPX provides some security features, which are fully applicable to
 CIPX.  CIPX does not significantly alter the basic security of IPX.

Mathur & Lewis [Page 21] RFC 1553 CIPX December 1993

References

 [1] Novell Inc., "IPX Router Specification", September 1992, Part
     Number: 107-000029-001
 [2] Jacobson, Van, "Compressing TCP/IP Headers for Low-Speed Serial
     Links", RFC 1144, February 1990
 [3] CCITT Recommendation V.42bis Error Correcting Procedures for DCEs
     using Error Correction Procedures
 [4] ISO 7776, Information Processing Systems - Data Communication -
     High Level Data Link Control Procedures - Description of the X.25
     LAPB-Compatible DTE Data Link Procedures
 [5] Simpson, W. A., "The Point-to-Point Protocol (PPP)", RFC 1548,
     December 1993
 [6] Simpson, W. A., "The PPP Internet Packet Exchange Control
     Protocol (IPXCP)", RFC 1552, December 1993
 [7] Allen, Michael, "Novell IPX Over Various WAN Media [IPXWAN]",
     RFC 1551, December 1993

Acknowledgements

 This compression algorithm incorporates many ideas from the Van
 Jacobson TCP/IP header compression algorithm.
 Michael Allen from Novell provided a lot of valuable feedback in the
 design of this algorithm.  David Piscitello from Bellcore and Marty
 Del Vecchio at Shiva Corp.  made several good suggestions.  Bill
 Simpson was very helpful in driving PPP, and specifically IPXCP, on
 the standards course.

Chair's Address

    Fred Baker
    Advanced Computer Communications
    315 Bollay Drive
    Santa Barbara, California 93117
    EMail: fbaker@acc.com

Mathur & Lewis [Page 22] RFC 1553 CIPX December 1993

Authors' Addresses

    Saroop Mathur
    Telebit Corp.
    1315 Chesapeake Terrace
    Sunnyvale, CA 94089-1100
    EMail: mathur@telebit.com
    Mark S. Lewis
    Telebit Corp.
    1315 Chesapeake Terrace
    Sunnyvale, CA 94089-1100
    EMail: Mark.S.Lewis@telebit.com

Mathur & Lewis [Page 23]

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