





Network Working Group                                        T. Bradley
Request for Comments: 1490               Wellfleet Communications, Inc.
Obsoletes: 1294                                                C. Brown
                                         Wellfleet Communications, Inc.
                                                               A. Malis
                                                   Ascom Timeplex, Inc.
                                                              July 1993


              Multiprotocol Interconnect over Frame Relay

Status of this Memo

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

Abstract

   This memo describes an encapsulation method for carrying network
   interconnect traffic over a Frame Relay backbone.  It covers aspects
   of both Bridging and Routing.  Additionally, it describes a simple
   fragmentation procedure for carrying large frames over a frame relay
   network with a smaller MTU.

   Systems with the ability to transfer both the encapsulation method
   described in this document, and others must have a priori knowledge
   of which virtual circuits will carry which encapsulation method and
   this encapsulation must only be used over virtual circuits that have
   been explicitly configured for its use.

Acknowledgements

   Comments and contributions from many sources, especially those from
   Ray Samora of Proteon, Ken Rehbehn of Netrix Corporation, Fred Baker
   and Charles Carvalho of Advanced Computer Communications and Mostafa
   Sherif of AT&T have been incorporated into this document. Special
   thanks to Dory Leifer of University of Michigan for his contributions
   to the resolution of fragmentation issues and Floyd Backes from DEC
   and Laura Bridge from Timeplex for their contributions to the
   bridging descriptions. This document could not have been completed
   without the expertise of the IP over Large Public Data Networks
   working group of the IETF.






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RFC 1490             Multiprotocol over Frame Relay            July 1993


1.  Conventions and Acronyms

   The following language conventions are used in the items of
   specification in this document:

      o Must, Shall or Mandatory -- the item is an absolute
        requirement of the specification.

      o Should or Recommended -- the item should generally be
        followed for all but exceptional circumstances.

      o May or Optional -- the item is truly optional and may be
        followed or ignored according to the needs of the
        implementor.

   All drawings in this document are drawn with the left-most bit as the
   high order bit for transmission.  For example, the dawings might be
   labeled as:

              0   1   2   3   4   5   6   7 bits
              +---+---+---+---+---+---+---+

              +---------------------------+
              |    flag (7E hexadecimal)  |
              +---------------------------+
              |       Q.922 Address*      |
              +--                       --+
              |                           |
              +---------------------------+
              :                           :
              :                           :
              +---------------------------+

   Drawings that would be too large to fit onto one page if each octet
   were presented on a single line are drawn with two octets per line.
   These are also drawn with the left-most bit as the high order bit for
   transmission.  There will be a "+" to distinguish between octets as
   in the following example.













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        |---   octet one     ---|---   octet two  ---|
        0  1  2  3  4  5  6  7  0  1  2  3  4  5  6  7
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

        +--------------------------------------------+
        | Organizationally Unique                    |
        +--                     +--------------------+
        | Identifier            | Protocol           |
        +-----------------------+--------------------+
        | Identifier            |
        +-----------------------+

   The following are common acronyms used throughout this document.

      BECN - Backward Explicit Congestion Notification
      BPDU - Bridge Protocol Data Unit
      C/R  - Command/Response bit
      DCE  - Data Communication Equipment
      DE   - Discard Eligibility bit
      DTE  - Data Terminal Equipment
      FECN - Forward Explicit Congestion Notification
      PDU  - Protocol Data Unit
      PTT  - Postal Telephone & Telegraph
      SNAP - Subnetwork Access Protocol

2.  Introduction

   The following discussion applies to those devices which serve as end
   stations (DTEs) on a public or private Frame Relay network (for
   example, provided by a common carrier or PTT.  It will not discuss
   the behavior of those stations that are considered a part of the
   Frame Relay network (DCEs) other than to explain situations in which
   the DTE must react.

   The Frame Relay network provides a number of virtual circuits that
   form the basis for connections between stations attached to the same
   Frame Relay network.  The resulting set of interconnected devices
   forms a private Frame Relay group which may be either fully
   interconnected with a complete "mesh" of virtual circuits, or only
   partially interconnected.  In either case, each virtual circuit is
   uniquely identified at each Frame Relay interface by a Data Link
   Connection Identifier (DLCI).  In most circumstances, DLCIs have
   strictly local significance at each Frame Relay interface.

   The specifications in this document are intended to apply to both
   switched and permanent virtual circuits.





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3.  Frame Format

   All protocols must encapsulate their packets within a Q.922 Annex A
   frame [1,2].  Additionally, frames shall contain information
   necessary to identify the protocol carried within the protocol data
   unit (PDU), thus allowing the receiver to properly process the
   incoming packet.  The format shall be as follows:

                  +---------------------------+
                  |    flag (7E hexadecimal)  |
                  +---------------------------+
                  |       Q.922 Address*      |
                  +--                       --+
                  |                           |
                  +---------------------------+
                  | Control (UI = 0x03)       |
                  +---------------------------+
                  | Optional Pad      (0x00)  |
                  +---------------------------+
                  | NLPID                     |
                  +---------------------------+
                  |             .             |
                  |             .             |
                  |             .             |
                  |           Data            |
                  |             .             |
                  |             .             |
                  +---------------------------+
                  |   Frame Check Sequence    |
                  +--           .           --+
                  |       (two octets)        |
                  +---------------------------+
                  |   flag (7E hexadecimal)   |
                  +---------------------------+

           * Q.922 addresses, as presently defined, are two octets and
             contain a 10-bit DLCI.  In some networks Q.922 addresses
             may optionally be increased to three or four octets.

   The control field is the Q.922 control field.  The UI (0x03) value is
   used unless it is negotiated otherwise.  The use of XID (0xAF or
   0xBF) is permitted and is discussed later.

   The pad field is used to align the remainder of the frame to a two
   octet boundary. There may be zero or one pad octet within the pad
   field and, if present, must have a value of zero.

   The Network Level Protocol ID (NLPID) field is administered by ISO



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   and CCITT.  It contains values for many different protocols including
   IP, CLNP and IEEE Subnetwork Access Protocol (SNAP)[10]. This field
   tells the receiver what encapsulation or what protocol follows.
   Values for this field are defined in ISO/IEC TR 9577 [3]. A NLPID
   value of 0x00 is defined within ISO/IEC TR 9577 as the Null Network
   Layer or Inactive Set.  Since it cannot be distinguished from a pad
   field, and because it has no significance within the context of this
   encapsulation scheme, a NLPID value of 0x00 is invalid under the
   Frame Relay encapsulation. The Appendix contains a list of some of
   the more commonly used NLPID values.

   There is no commonly implemented minimum maximum frame size for Frame
   Relay.  A network must, however, support at least a 262 octet
   maximum.  Generally, the maximum will be greater than or equal to
   1600 octets, but each Frame Relay provider will specify an
   appropriate value for its network.  A Frame Relay DTE, therefore,
   must allow the maximum acceptable frame size to be configurable.

   The minimum frame size allowed for Frame Relay is five octets between
   the opening and closing flags assuming a two octet Q.922 address
   field.  This minimum increases to six octets for three octet Q.922
   address and seven octets for the four octet Q.922 address format.

4.  Interconnect Issues

   There are two basic types of data packets that travel within the
   Frame Relay network: routed packets and bridged packets.  These
   packets have distinct formats and therefore, must contain an
   indicator that the destination may use to correctly interpret the
   contents of the frame.  This indicator is embedded within the NLPID
   and SNAP header information.

   For those protocols that do not have a NLPID already assigned, it is
   necessary to provide a mechanism to allow easy protocol
   identification.  There is a NLPID value defined indicating the
   presence of a SNAP header.

   A SNAP header is of the form:

            +--------------------------------------------+
            | Organizationally Unique                    |
            +--                     +--------------------+
            | Identifier            | Protocol           |
            +-----------------------+--------------------+
            | Identifier            |
            +-----------------------+

   All stations must be able to accept and properly interpret both the



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   NLPID encapsulation and the SNAP header encapsulation for a routed
   packet.

   The three-octet Organizationally Unique Identifier (OUI) identifies
   an organization which administers the meaning of the Protocol
   Identifier (PID) which follows.  Together they identify a distinct
   protocol.  Note that OUI 0x00-00-00 specifies that the following PID
   is an Ethertype.

4.1.  Routed Frames

   Some protocols will have an assigned NLPID, but because the NLPID
   numbering space is so limited, not all protocols have specific NLPID
   values assigned to them. When packets of such protocols are routed
   over Frame Relay networks, they are sent using the NLPID 0x80 (which
   indicates a SNAP follows) followed by SNAP.  If the protocol has an
   Ethertype assigned, the OUI is 0x00-00-00 (which indicates an
   Ethertype follows), and PID is the Ethertype of the protocol in use.
   There will be one pad octet to align the protocol data on a two octet
   boundary as shown below.

                      Format of Routed Frames
                          with Ethertypes

                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | pad     0x00  |
                  +---------------+---------------+
                  | NLPID   0x80  | OUI     0x00  |
                  +---------------+             --+
                  | OUI  0x00-00                  |
                  +-------------------------------+
                  |           Ethertype           |
                  +-------------------------------+
                  |         Protocol Data         |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

   In the few cases when a protocol has an assigned NLPID (see
   appendix), 48 bits can be saved using the format below:









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                   Format of Routed NLPID Protocol
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  |     NLPID     |
                  +---------------+---------------+
                  |         Protocol Data         |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

   The NLPID encapsulation does not require a pad octet for alignment,
   so none is permitted.

   In the case of ISO protocols, the NLPID is considered to be the first
   octet of the protocol data.  It is unnecessary to repeat the NLPID in
   this case.  The single octet serves both as the demultiplexing value
   and as part of the protocol data (refer to "Other Protocols over
   Frame Relay for more details). Other protocols, such as IP, have a
   NLPID defined (0xCC), but it is not part of the protocol itself.

                    Format of Routed IP Datagram
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  |  NLPID  0xCC  |
                  +---------------+---------------+
                  |          IP Datagram          |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

4.2.  Bridged Frames

The second type of Frame Relay traffic is bridged packets. These
packets are encapsulated using the NLPID value of 0x80 indicating
SNAP.  As with other SNAP encapsulated protocols, there will be one
pad octet to align the data portion of the encapsulated frame.  The
SNAP header which follows the NLPID identifies the format of the
bridged packet.  The OUI value used for this encapsulation is the
802.1 organization code 0x00-80-C2.  The PID portion of the SNAP
header (the two bytes immediately following the OUI) specifies the
form of the MAC header, which immediately follows the SNAP header.
Additionally, the PID indicates whether the original FCS is preserved
within the bridged frame.

The 802.1 organization has reserved the following values to be used
with Frame Relay:



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           PID Values for OUI 0x00-80-C2

        with preserved FCS   w/o preserved FCS    Media
        ------------------   -----------------    ----------------
        0x00-01              0x00-07              802.3/Ethernet
        0x00-02              0x00-08              802.4
        0x00-03              0x00-09              802.5
        0x00-04              0x00-0A              FDDI
                             0x00-0B              802.6

      In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2,
      identifies bridged protocol data units (BPDUs) as defined by
      802.1(d) or 802.1(g) [12].

   A packet bridged over Frame Relay will, therefore, have one of the
   following formats:

                   Format of Bridged Ethernet/802.3 Frame
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | pad     0x00  |
                  +---------------+---------------+
                  | NLPID   0x80  | OUI     0x00  |
                  +---------------+             --+
                  | OUI  0x80-C2                  |
                  +-------------------------------+
                  | PID 0x00-01 or 0x00-07        |
                  +-------------------------------+
                  | MAC destination address       |
                  :                               :
                  |                               |
                  +-------------------------------+
                  | (remainder of MAC frame)      |
                  +-------------------------------+
                  | LAN FCS (if PID is 0x00-01)   |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+












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RFC 1490             Multiprotocol over Frame Relay            July 1993


                   Format of Bridged 802.4 Frame
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | pad     0x00  |
                  +---------------+---------------+
                  | NLPID   0x80  | OUI     0x00  |
                  +---------------+             --+
                  | OUI  0x80-C2                  |
                  +-------------------------------+
                  | PID 0x00-02 or 0x00-08        |
                  +---------------+---------------+
                  |  pad  0x00    | Frame Control |
                  +---------------+---------------+
                  | MAC destination address       |
                  :                               :
                  |                               |
                  +-------------------------------+
                  | (remainder of MAC frame)      |
                  +-------------------------------+
                  | LAN FCS (if PID is 0x00-02)   |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+



























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RFC 1490             Multiprotocol over Frame Relay            July 1993


                   Format of Bridged 802.5 Frame
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | pad     0x00  |
                  +---------------+---------------+
                  | NLPID   0x80  | OUI     0x00  |
                  +---------------+             --+
                  | OUI  0x80-C2                  |
                  +-------------------------------+
                  | PID    0x00-03 or 0x00-09     |
                  +---------------+---------------+
                  | pad    0x00   | Frame Control |
                  +---------------+---------------+
                  | MAC destination address       |
                  :                               :
                  |                               |
                  +-------------------------------+
                  | (remainder of MAC frame)      |
                  +-------------------------------+
                  | LAN FCS (if PID is 0x00-03)   |
                  |                               |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+


























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RFC 1490             Multiprotocol over Frame Relay            July 1993


                    Format of Bridged FDDI Frame
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | pad     0x00  |
                  +---------------+---------------+
                  | NLPID   0x80  | OUI     0x00  |
                  +---------------+             --+
                  | OUI  0x80-C2                  |
                  +-------------------------------+
                  | PID 0x00-04 or 0x00-0A        |
                  +---------------+---------------+
                  | pad     0x00  | Frame Control |
                  +---------------+---------------+
                  | MAC destination address       |
                  :                               :
                  |                               |
                  +-------------------------------+
                  | (remainder of MAC frame)      |
                  +-------------------------------+
                  | LAN FCS (if PID is 0x00-04)   |
                  |                               |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+


























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RFC 1490             Multiprotocol over Frame Relay            July 1993


                    Format of Bridged 802.6 Frame
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  | Control 0x03  | pad     0x00  |
                  +---------------+---------------+
                  | NLPID   0x80  | OUI     0x00  |
                  +---------------+             --+
                  | OUI  0x80-C2                  |
                  +-------------------------------+
                  |         PID  0x00-0B          |
                  +---------------+---------------+ -------
                  |   Reserved    |     BEtag     |  Common
                  +---------------+---------------+  PDU
                  |            BAsize             |  Header
                  +-------------------------------+ -------
                  | MAC destination address       |
                  :                               :
                  |                               |
                  +-------------------------------+
                  | (remainder of MAC frame)      |
                  +-------------------------------+
                  |                               |
                  +-    Common PDU Trailer       -+
                  |                               |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

   Note that in bridge 802.6 PDUs, there is only one choice for the PID
   value, since the presence of a CRC-32 is indicated by the CIB bit in
   the header of the MAC frame.

   The Common Protocol Data Unit (CPDU) Header and Trailer are conveyed
   to allow pipelining at the egress bridge to an 802.6 subnetwork.
   Specifically, the CPDU Header contains the BAsize field, which
   contains the length of the PDU.  If this field is not available to
   the egress 802.6 bridge, then that bridge cannot begin to transmit
   the segmented PDU until it has received the entire PDU, calculated
   the length, and inserted the length into the BAsize field.  If the
   field is available, the egress 802.6 bridge can extract the length
   from the BAsize field of the Common PDU Header, insert it into the
   corresponding field of the first segment, and immediately transmit
   the segment onto the 802.6 subnetwork.  Thus, the bridge can begin
   transmitting the 802.6 PDU before it has received the complete PDU.

   One should note that the Common PDU Header and Trailer of the
   encapsulated frame should not be simply copied to the outgoing 802.6



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RFC 1490             Multiprotocol over Frame Relay            July 1993


   subnetwork because the encapsulated BEtag value may conflict with the
   previous BEtag value transmitted by that bridge.

                   Format of BPDU Frame
                  +-------------------------------+
                  |         Q.922 Address         |
                  +-------------------------------+
                  |        Control   0x03         |
                  +-------------------------------+
                  |          PAD    0x00          |
                  +-------------------------------+
                  |          NLPID  0x80          |
                  +-------------------------------+
                  |        OUI 0x00-80-C2         |
                  +-------------------------------+
                  |         PID 0x00-0E           |
                  +-------------------------------+
                  |                               |
                  |      BPDU as defined by       |
                  |     802.1(d) or 802.1(g)[12]  |
                  |                               |
                  +-------------------------------+

4.  Data Link Layer Parameter Negotiation

   Frame Relay stations may choose to support the Exchange
   Identification (XID) specified in Appendix III of Q.922 [1].  This
   XID exchange allows the following parameters to be negotiated at the
   initialization of a Frame Relay circuit: maximum frame size N201,
   retransmission timer T200, and the maximum number of outstanding
   Information (I) frames K.

   A station may indicate its unwillingness to support acknowledged mode
   multiple frame operation by specifying a value of zero for the
   maximum window size, K.

   If this exchange is not used, these values must be statically
   configured by mutual agreement of Data Link Connection (DLC)
   endpoints, or must be defaulted to the values specified in Section
   5.9 of Q.922:











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RFC 1490             Multiprotocol over Frame Relay            July 1993


                       N201: 260 octets

                          K:  3 for a 16 Kbps link,
                              7 for a 64 Kbps link,
                             32 for a 384 Kbps link,
                             40 for a 1.536 Mbps or above link

                      T200: 1.5 seconds [see Q.922 for further details]

   If a station supporting XID receives an XID frame, it shall respond
   with an XID response.  In processing an XID, if the remote maximum
   frame size is smaller than the local maximum, the local system shall
   reduce the maximum size it uses over this DLC to the remotely
   specified value.  Note that this shall be done before generating a
   response XID.

   The following diagram describes the use of XID to specify non-use of
   acknowledged mode multiple frame operation.

































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RFC 1490             Multiprotocol over Frame Relay            July 1993


               Non-use of Acknowledged Mode Multiple Frame Operation
                      +---------------+
                      |    Address    |     (2,3 or 4 octets)
                      |               |
                      +---------------+
                      | Control 0xAF  |
                      +---------------+
                      | format  0x82  |
                      +---------------+
                      | Group ID 0x80 |
                      +---------------+
                      | Group Length  |     (2 octets)
                      |    0x00-0E    |
                      +---------------+
                      |      0x05     |     PI = Frame Size (transmit)
                      +---------------+
                      |      0x02     |     PL = 2
                      +---------------+
                      |    Maximum    |     (2 octets)
                      |   Frame Size  |
                      +---------------+
                      |      0x06     |     PI = Frame Size (receive)
                      +---------------+
                      |      0x02     |     PL = 2
                      +---------------+
                      |    Maximum    |     (2 octets)
                      |   Frame Size  |
                      +---------------+
                      |      0x07     |     PI = Window Size
                      +---------------+
                      |      0x01     |     PL = 1
                      +---------------+
                      |      0x00     |
                      +---------------+
                      |      0x09     |     PI = Retransmission Timer
                      +---------------+
                      |      0x01     |     PL = 1
                      +---------------+
                      |      0x00     |
                      +---------------+
                      |      FCS      |     (2 octets)
                      |               |
                      +---------------+

6.  Fragmentation Issues

   Fragmentation allows the exchange of packets that are greater than
   the maximum frame size supported by the underlying network.  In the



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   case of Frame Relay, the network may support a maximum frame size as
   small as 262 octets.  Because of this small maximum size, it is
   recommended, but not required, to support fragmentation and
   reassembly.

   Unlike IP fragmentation procedures, the scope of Frame Relay
   fragmentation procedure is limited to the boundary (or DTEs) of the
   Frame Relay network.

   The general format of fragmented packets is the same as any other
   encapsulated protocol.  The most significant difference being that
   the fragmented packet will contain the encapsulation header.  That
   is, a packet is first encapsulated (with the exception of the address
   and control fields) as defined above. Large packets are then broken
   up into frames appropriate for the given Frame Relay network and are
   encapsulated using the Frame Relay fragmentation format.  In this
   way, a station receiving fragments may reassemble them and then put
   the reassembled packet through the same processing path as a packet
   that had not been fragmented.

   Within Frame Relay fragments are encapsulated using the SNAP format
   with an OUI of 0x00-80-C2 and a PID of 0x00-0D.  Individual fragments
   will, therefore, have the following format:

                   +---------------+---------------+
                   |         Q.922 Address         |
                   +---------------+---------------+
                   | Control 0x03  | pad     0x00  |
                   +---------------+---------------+
                   | NLPID   0x80  | OUI     0x00  |
                   +---------------+---------------+
                   | OUI                  0x80-C2  |
                   +---------------+---------------+
                   | PID                  0x00-0D  |
                   +---------------+---------------+
                   |        sequence number        |
                   +-+-------+-----+---------------+
                   |F| RSVD  |offset               |
                   +-+-------+-----+---------------+
                   |    fragment data              |
                   |               .               |
                   |               .               |
                   |               .               |
                   +---------------+---------------+
                   |              FCS              |
                   +---------------+---------------+

   The sequence field is a two octet identifier that is incremented



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   every time a new complete message is fragmented.  It allows detection
   of lost frames and is set to a random value at initialization.

   The reserved field is 4 bits long and is not currently defined.  It
   must be set to 0.

   The final bit is a one bit field set to 1 on the last fragment and
   set to 0 for all other fragments.

   The offset field is an 11 bit value representing the logical offset
   of this fragment in bytes divided by 32. The first fragment must have
   an offset of zero.

   The following figure shows how a large IP datagram is fragmented over
   Frame Relay.  In this example, the complete datagram is fragmented
   into two Frame Relay frames.



































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RFC 1490             Multiprotocol over Frame Relay            July 1993


                           Frame Relay Fragmentation Example
                                              +-----------+-----------+
                                              |     Q.922 Address     |
                                              +-----------+-----------+
                                              | Ctrl 0x03 | pad  0x00 |
                                              +-----------+-----------+
                                              |NLPID 0x80 | OUI 0x00  |
                                              +-----------+-----------+
                                              | OUI          0x80-C2  |
            +-----------+-----------+         +-----------+-----------+
            |ctrl 0x03  |NLPID 0xCC |         | PID          0x00-0D  |
            +-----------+-----------+         +-----------+-----------+
            |                       |         | sequence number   n   |
            |                       |         +-+------+--+-----------+
            |                       |         |0| RSVD |offset (0)    |
            |                       |         +-+------+--+-----------+
            |                       |         | ctrl 0x03 |NLPID 0xCC |
            |                       |         +-----------+-----------+
            |                       |         |   first m bytes of    |
            |  large IP datagram    |   ...   |     IP datagram       |
            |                       |         |                       |
            |                       |         +-----------+-----------+
            |                       |         |          FCS          |
            |                       |         +-----------+-----------+
            |                       |
            |                       |         +-----------+-----------+
            |                       |         |     Q.922 Address     |
            |                       |         +-----------+-----------+
            |                       |         | Ctrl 0x03 | pad  0x00 |
            +-----------+-----------+         +-----------+-----------+
                                              |NLPID 0x80 | OUI 0x00  |
                                              +-----------+-----------+
                                              | OUI          0x80-C2  |
                                              +-----------+-----------+
                                              | PID          0x00-0D  |
                                              +-----------+-----------+
                                              | sequence number   n   |
                                              +-+------+--+-----------+
                                              |1| RSVD |offset (m/32) |
                                              +-+------+--+-----------+
                                              |    remainder of IP    |
                                              |        datagram       |
                                              +-----------+-----------+
                                              |          FCS          |
                                              +-----------+-----------+

   Fragments must be sent in order starting with a zero offset and
   ending with the final fragment.  These fragments must not be



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   interrupted with other packets or information intended for the same
   DLC. An end station must be able to re-assemble up to 2K octets and
   is suggested to support up to 8K octet re-assembly.  If at any time
   during this re-assembly process, a fragment is corrupted or a
   fragment is missing, the entire message is dropped.  The upper layer
   protocol is responsible for any retransmission in this case.  Note
   that there is no reassembly timer, nor is one needed.  This is
   because the Frame Relay service is required to deliver frames in
   order.

   This fragmentation algorithm is not intended to reliably handle all
   possible failure conditions.  As with IP fragmentation, there is a
   small possibility of reassembly error and delivery of an erroneous
   packet.  Inclusion of a higher layer checksum greatly reduces this
   risk.

7.  Address Resolution

   There are situations in which a Frame Relay station may wish to
   dynamically resolve a protocol address.  Address resolution may be
   accomplished using the standard Address Resolution Protocol (ARP) [6]
   encapsulated within a SNAP encoded Frame Relay packet as follows:

           +-----------------------+-----------------------+
           | Q.922 Address                                 |
           +-----------------------+-----------------------+
           | Control (UI)  0x03    |     pad     0x00      |
           +-----------------------+-----------------------+
           |  NLPID = 0x80         |                       |  SNAP Header
           +-----------------------+  OUI = 0x00-00-00     +  Indicating
           |                                               |  ARP
           +-----------------------+-----------------------+
           |  PID = 0x0806                                 |
           +-----------------------+-----------------------+
           |                   ARP packet                  |
           |                       .                       |
           |                       .                       |
           |                       .                       |
           +-----------------------+-----------------------+


     Where the ARP packet has the following format and values:


         Data:
           ar$hrd   16 bits     Hardware type
           ar$pro   16 bits     Protocol type
           ar$hln    8 bits     Octet length of hardware address (n)



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           ar$pln    8 bits     Octet length of protocol address (m)
           ar$op    16 bits     Operation code (request or reply)
           ar$sha   noctets     source hardware address
           ar$spa   moctets     source protocol address
           ar$tha   noctets     target hardware address
           ar$tpa   moctets     target protocol address

           ar$hrd - assigned to Frame Relay is 15 decimal
                     (0x000F) [7].

           ar$pro - see assigned numbers for protocol ID number for
                    the protocol using ARP. (IP is 0x0800).

           ar$hln - length in bytes of the address field (2, 3, or 4)

           ar$pln - protocol address length is dependent on the
                    protocol (ar$pro) (for IP ar$pln is 4).

           ar$op -  1 for request and 2 for reply.

           ar$sha - Q.922 source hardware address, with C/R, FECN,
                    BECN, and DE set to zero.

           ar$tha - Q.922 target hardware address, with C/R, FECN,
                    BECN, and DE set to zero.

   Because DLCIs within most Frame Relay networks have only local
   significance, an end station will not have a specific DLCI assigned
   to itself.  Therefore, such a station does not have an address to put
   into the ARP request or reply.  Fortunately, the Frame Relay network
   does provide a method for obtaining the correct DLCIs. The solution
   proposed for the locally addressed Frame Relay network below will
   work equally well for a network where DLCIs have global significance.

   The DLCI carried within the Frame Relay header is modified as it
   traverses the network.  When the packet arrives at its destination,
   the DLCI has been set to the value that, from the standpoint of the
   receiving station, corresponds to the sending station.  For example,
   in figure 1 below, if station A were to send a message to station B,
   it would place DLCI 50 in the Frame Relay header.  When station B
   received this message, however, the DLCI would have been modified by
   the network and would appear to B as DLCI 70.









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                                  ~~~~~~~~~~~~~~~
                                 (                )
               +-----+          (                  )             +-----+
               |     |-50------(--------------------)---------70-|     |
               |  A  |        (                      )           |  B  |
               |     |-60-----(---------+            )           |     |
               +-----+         (        |           )            +-----+
                                (       |          )
                                 (      |         )  <---Frame Relay
                                  ~~~~~~~~~~~~~~~~         network
                                        80
                                        |
                                     +-----+
                                     |     |
                                     |  C  |
                                     |     |
                                     +-----+
                                Figure 1

      Lines between stations represent data link connections (DLCs).
      The numbers indicate the local DLCI associated with each
      connection.

              DLCI to Q.922 Address Table for Figure 1

              DLCI (decimal)  Q.922 address (hex)
                   50              0x0C21
                   60              0x0CC1
                   70              0x1061
                   80              0x1401

      If you know about frame relay, you should understand the
      correlation between DLCI and Q.922 address.  For the uninitiated,
      the translation between DLCI and Q.922 address is based on a two
      byte address length using the Q.922 encoding format.  The format
      is:

                8   7   6   5   4   3    2   1
              +------------------------+---+--+
              |  DLCI (high order)     |c/r|ea|
              +--------------+----+----+---+--+
              | DLCI (lower) |FECN|BECN|DE |EA|
              +--------------+----+----+---+--+

      For ARP and its variants, the FECN, BECN, C/R and DE bits are
      assumed to be 0.

   When an ARP message reaches a destination, all hardware addresses



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   will be invalid.  The address found in the frame header will,
   however, be correct. Though it does violate the purity of layering,
   Frame Relay may use the address in the header as the sender hardware
   address.  It should also be noted that the target hardware address,
   in both ARP request and reply, will also be invalid.  This should not
   cause problems since ARP does not rely on these fields and in fact,
   an implementation may zero fill or ignore the target hardware address
   field entirely.

   As an example of how this address replacement scheme may work, refer
   to figure 1.  If station A (protocol address pA) wished to resolve
   the address of station B (protocol address pB), it would format an
   ARP request with the following values:

              ARP request from A
                ar$op     1 (request)
                ar$sha    unknown
                ar$spa    pA
                ar$tha    undefined
                ar$tpa    pB

   Because station A will not have a source address associated with it,
   the source hardware address field is not valid.  Therefore, when the
   ARP packet is received, it must extract the correct address from the
   Frame Relay header and place it in the source hardware address field.
   This way, the ARP request from A will become:

              ARP request from A as modified by B
                ar$op     1 (request)
                ar$sha    0x1061 (DLCI 70) from Frame Relay header
                ar$spa    pA
                ar$tha    undefined
                ar$tpa    pB

   Station B's ARP will then be able to store station A's protocol
   address and Q.922 address association correctly.  Next, station B
   will form a reply message.  Many implementations simply place the
   source addresses from the ARP request into the target addresses and
   then fills in the source addresses with its addresses.  In this case,
   the ARP response would be:

              ARP response from B
                ar$op     2 (response)
                ar$sha    unknown
                ar$spa    pB
                ar$tha    0x1061 (DLCI 70)
                ar$tpa    pA




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   Again, the source hardware address is unknown and when the request is
   received, station A will extract the address from the Frame Relay
   header and place it in the source hardware address field.  Therefore,
   the response will become:

              ARP response from B as modified by A
                ar$op     2 (response)
                ar$sha    0x0C21 (DLCI 50)
                ar$spa    pB
                ar$tha    0x1061 (DLCI 70)
                ar$tpa    pA


   Station A will now correctly recognize station B having protocol
   address pB associated with Q.922 address 0x0C21 (DLCI 50).

   Reverse ARP (RARP) [8] will work in exactly the same way.  Still
   using figure 1, if we assume station C is an address server, the
   following RARP exchanges will occur:

          RARP request from A             RARP request as modified by C
             ar$op  3 (RARP request)         ar$op  3  (RARP request)
             ar$sha unknown                  ar$sha 0x1401 (DLCI 80)
             ar$spa undefined                ar$spa undefined
             ar$tha 0x0CC1 (DLCI 60)         ar$tha 0x0CC1 (DLCI 60)
             ar$tpa pC                       ar$tpa pC

   Station C will then look up the protocol address corresponding to
   Q.922 address 0x1401 (DLCI 80) and send the RARP response.

         RARP response from C            RARP response as modified by A
                 ar$op  4  (RARP response)       ar$op  4 (RARP response)
                 ar$sha unknown                  ar$sha 0x0CC1 (DLCI 60)
                 ar$spa pC                       ar$spa pC
                 ar$tha 0x1401 (DLCI 80)         ar$tha 0x1401 (DLCI 80)
                 ar$tpa pA                       ar$tpa pA


   This means that the Frame Relay interface must only intervene in the
   processing of incoming packets.

   In the absence of suitable multicast, ARP may still be implemented.
   To do this, the end station simply sends a copy of the ARP request
   through each relevant DLC, thereby simulating a broadcast.

   The use of multicast addresses in a Frame Relay environment is
   presently under study by Frame Relay providers.  At such time that
   the issues surrounding multicasting are resolved, multicast



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   addressing may become useful in sending ARP requests and other
   "broadcast" messages.

   Because of the inefficiencies of broadcasting in a Frame Relay
   environment, a new address resolution variation was developed.  It is
   called Inverse ARP [11] and describes a method for resolving a
   protocol address when the hardware address is already known.  In
   Frame Relay's case, the known hardware address is the DLCI.  Using
   Inverse ARP for Frame Relay follows the same pattern as ARP and RARP
   use.  That is the source hardware address is inserted at the
   receiving station.

   In our example, station A may use Inverse ARP to discover the
   protocol address of the station associated with its DLCI 50.  The
   Inverse ARP request would be as follows:

              InARP Request from A (DLCI 50)
              ar$op   8       (InARP request)
              ar$sha  unknown
              ar$spa  pA
              ar$tha  0x0C21  (DLCI 50)
              ar$tpa  unknown

   When Station B receives this packet, it will modify the source
   hardware address with the Q.922 address from the Frame Relay header.
   This way, the InARP request from A will become:

              ar$op   8       (InARP request)
              ar$sha  0x1061
              ar$spa  pA
              ar$tha  0x0C21
              ar$tpa  unknown.

   Station B will format an Inverse ARP response and send it to station
   A as it would for any ARP message.

8.  IP over Frame Relay

   Internet Protocol [9] (IP) datagrams sent over a Frame Relay network
   conform to the encapsulation described previously.  Within this
   context, IP could be encapsulated in two different ways.










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           1.  NLPID value indicating IP

           +-----------------------+-----------------------+
           | Q.922 Address                                 |
           +-----------------------+-----------------------+
           | Control (UI)  0x03    | NLPID = 0xCC          |
           +-----------------------+-----------------------+
           | IP Packet             .                       |
           |                       .                       |
           |                       .                       |
           +-----------------------+-----------------------+


           2.  NLPID value indicating SNAP

           +-----------------------+-----------------------+
           | Q.922 Address                                 |
           +-----------------------+-----------------------+
           | Control (UI)  0x03    |     pad     0x00      |
           +-----------------------+-----------------------+
           |  NLPID = 0x80         |                       |  SNAP Header
           +-----------------------+  OUI = 0x00-00-00     +  Indicating
           |                                               |  IP
           +-----------------------+-----------------------+
           |  PID = 0x0800                                 |
           +-----------------------+-----------------------+
           |                   IP packet                   |
           |                       .                       |
           |                       .                       |
           |                       .                       |
           +-----------------------+-----------------------+

   Although both of these encapsulations are supported under the given
   definitions, it is advantageous to select only one method as the
   appropriate mechanism for encapsulating IP data.  Therefore, IP data
   shall be encapsulated using the NLPID value of 0xCC indicating IP as
   shown in option 1 above.  This (option 1) is more efficient in
   transmission (48 fewer bits), and is consistent with the
   encapsulation of IP in X.25.

9.  Other Protocols over Frame Relay

   As with IP encapsulation, there are alternate ways to transmit
   various protocols within the scope of this definition.  To eliminate
   the conflicts, the SNAP encapsulation is only used if no NLPID value
   is defined for the given protocol.

   As an example of how this works, ISO CLNP has a NLPID defined (0x81).



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   Therefore, the NLPID field will indicate ISO CLNP and the data packet
   will follow immediately.  The frame would be as follows:

                  +---------------------------------------------+
                  |               Q.922 Address                 |
                  +----------------------+----------------------+
                  | Control     (0x03)   | NLPID  - 0x81 (CLNP) |
                  +----------------------+----------------------+
                  | remainder of CLNP packet                    |
                  |                   .                         |
                  |                   .                         |
                  +---------------------------------------------+

   In this example, the NLPID is used to identify the data packet as
   CLNP.  It is also considered part of the CLNP packet and as such, the
   NLPID should not be removed before being sent to the upper layers for
   processing.  The NLPID is not duplicated.

   Other protocols, such as IPX, do not have a NLPID value defined.  As
   mentioned above, IPX would be encapsulated using the SNAP header.  In
   this case, the frame would be as follows:

                  +---------------------------------------------+
                  |               Q.922 Address                 |
                  +----------------------+----------------------+
                  | Control       0x03   | pad  0x00            |
                  +----------------------+----------------------+
                  | NLPID  - 0x80 (SNAP) | OUI - 0x00 00 00     |
                  +----------------------+                      |
                  |                                             |
                  +---------------------------------------------+
                  | PID = 0x8137                                |
                  +---------------------------------------------+
                  |   IPX packet                                |
                  |                   .                         |
                  |                   .                         |
                  +---------------------------------------------+

10.  Bridging Model for Frame Relay

   The model for bridging in a Frame Relay network is identical to the
   model for remote bridging as described in IEEE P802.1g "Remote MAC
   Bridging" [13] and supports the concept of "Virtual Ports". Remote
   bridges with LAN ports receive and transmit MAC frames to and from
   the LANS to which they are attached. They may also receive and
   transmit MAC frames through virtual ports to and from other remote
   bridges.  A virtual port may represent an abstraction of a remote
   bridge's point of access to one, two or more other remote bridges.



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   Remote Bridges are statically configured as members of a remote
   bridge group by management. All members of a remote bridge group are
   connected by one or more virtual ports. The set of remote MAC bridges
   in a remote bridge group provides actual or *potential* MAC layer
   interconnection between a set of LANs and other remote bridge groups
   to which the remote bridges attach.

   In a Frame Relay network there must be a full mesh of Frame Relay VCs
   between bridges of a remote bridge group.  If the frame relay network
   is not a full mesh, then the bridge network must be divided into
   multiple remote bridge groups.

   The frame relay VCs that interconnect the bridges of a remote bridge
   group may be combined or used individually to form one or more
   virtual bridge ports.  This gives flexibility to treat the Frame
   Relay interface either as a single virtual bridge port, with all VCs
   in a group, or as a collection of bridge ports (individual or grouped
   VCs).

   When a single virtual bridge port provides the interconnectivity for
   all bridges of a given remote bridge group (i.e. all VCs are combined
   into a single virtual port), the standard Spanning Tree Algorithm may
   be used to determine the state of the virtual port.  When more than
   one virtual port is configured within a given remote bridge group
   then an "extended" Spanning Tree Algorithm is required.  Such an
   extended algorithm is defined in IEEE 802.1g [13].  The operation of
   this algorithm is such that a virtual port is only put into backup if
   there is a loop in the network external to the remote bridge group.

   The simplest bridge configuration for a Frame Relay network is the
   LAN view where all VCs are combined into a single virtual port.
   Frames, such as BPDUs,  which would be broadcast on a LAN, must be
   flooded to each VC (or multicast if the service is developed for
   Frame Relay services). Flooding is performed by sending the packet to
   each relevant DLC associated with the Frame Relay interface. The VCs
   in this environment are generally invisible to the bridge.  That is,
   the bridge sends a flooded frame to the frame relay interface and
   does not "see" that the frame is being forwarded to each VC
   individually.  If all participating bridges are fully connected (full
   mesh) the standard Spanning Tree Algorithm will suffice in this
   configuration.

   Typically LAN bridges learn which interface a particular end station
   may be reached on by associating a MAC address with a bridge port.
   In a Frame Relay network configured for the LAN-like single bridge
   port (or any set of VCs grouped together to form a single bridge
   port), however, the bridge must not only associated a MAC address
   with a bridge port, but it must also associate it with a connection



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   identifier.  For Frame Relay networks, this connection identifier is
   a DLCI.  It is unreasonable and perhaps impossible to require bridges
   to statically configure an association of every possible destination
   MAC address with a DLC.  Therefore, Frame Relay LAN-modeled bridges
   must provide a mechanism to allow the Frame Relay bridge port to
   dynamically learn the associations.  To accomplish this dynamic
   learning, a bridged packet shall conform to the encapsulation
   described within section 7.  In this way, the receiving Frame Relay
   interface will know to look into the bridged packet to gather the
   appropriate information.

   A second Frame Relay bridging approach, the point-to-point view,
   treats each Frame Relay VC as a separate bridge port.  Flooding and
   forwarding packets are significantly less complicated using the
   point-to-point approach because each bridge port has only one
   destination.  There is no need to perform artificial flooding or to
   associate DLCIs with destination MAC addresses.  Depending upon the
   interconnection of the VCs, an extended Spanning Tree algorithm may
   be required to permit all virtual ports to remain active as long as
   there are no true loops in the topology external to the remote bridge
   group.

   It is also possible to combine the LAN view and the point-to-point
   view on a single Frame Relay interface.  To do this, certain VCs are
   combined to form a single virtual bridge port while other VCs are
   independent bridge ports.

   The following drawing illustrates the different possible bridging
   configurations.  The dashed lines between boxes represent virtual
   circuits.

                                                 +-------+
                              -------------------|   B   |
                             /            -------|       |
                            /            /       +-------+
                           /             |
                 +-------+/              \       +-------+
                 |   A   |                -------|   C   |
                 |       |-----------------------|       |
                 +-------+\                      +-------+
                           \
                            \                    +-------+
                             \                   |   D   |
                              -------------------|       |
                                                 +-------+

   Since there is less than a full mesh of VCs between the bridges in
   this example, the network must be divided into more than one remote



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   bridge group.  A reasonable configuration is to have bridges A, B,
   and C in one group, and have bridges A and D in a second.

   Configuration of the first bridge group combines the VCs
   interconnection the three bridges (A, B, and C) into a single virtual
   port.  This is an example of the LAN view configuration.  The second
   group would also be a single virtual port which simply connects
   bridges A and D.  In this configuration the standard Spanning Tree
   Algorithm is sufficient to detect loops.

   An alternative configuration has three individual virtual ports in
   the first group corresponding to the VCs interconnecting bridges A, B
   and C.  Since the application of the standard Spanning Tree Algorithm
   to this configuration would detect a loop in the topology, an
   extended Spanning Tree Algorithm would have to be used in order for
   all virtual ports to be kept active.  Note that the second group
   would still consist of a single virtual port and the standard
   Spanning Tree Algorithm could be used in this group.

   Using the same drawing, one could construct a remote bridge scenario
   with three bridge groups.  This would be an example of the point-to-
   point case.  Here, the VC connecting A and B, the VC connecting A and
   C, and the VC connecting A and D are all bridge groups with a single
   virtual port.



























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11.  Appendix A

        List of Commonly Used NLPIDs

           0x00    Null Network Layer or Inactive Set
                   (not used with Frame Relay)
           0x80    SNAP
           0x81    ISO CLNP
           0x82    ISO ESIS
           0x83    ISO ISIS
           0xCC    Internet IP

        List of PIDs of OUI 00-80-C2

           with preserved FCS   w/o preserved FCS    Media
           ------------------   -----------------    --------------
           0x00-01              0x00-07              802.3/Ethernet
           0x00-02              0x00-08              802.4
           0x00-03              0x00-09              802.5
           0x00-04              0x00-0A              FDDI
                                0x00-0B              802.6
                                0x00-0D              Fragments
                                0x00-0E              BPDUs as defined by
                                                       802.1(d) or
                                                       802.1(g)[12].

12.  Appendix B - Connection Oriented procedures.

   This appendix contains additional information and instructions for
   using CCITT Q.933 and other CCITT standards for encapsulating data
   over frame relay.  The information contained here is similar (and in
   some cases identical) to that found in Annex F to ANSI T1.617 written
   by Rao Cherukuri of IBM.  The authoritative source for this
   information is in Annex F and is repeated here only for convenience.

   The Network Level Protocol ID (NLPID) field is administered by ISO
   and CCITT.  It contains values for many different protocols including
   IP, CLNP (ISO 8473) CCITT Q.933, and ISO 8208.  A figure summarizing
   a generic encapsulation technique over frame relay networks follows.
   The scheme's flexibility consists in the identification of multiple
   alternative to identify different protocols used either by

       - end-to-end systems or
       - LAN to LAN bride and routers or
       - a combination of the above.

     over frame relay networks.




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                              Q.922 control
                                   |
                                   |
              --------------------------------------------
              |                                          |
             UI                                       I Frame
              |                                          |
        ---------------------------------         --------------
        | 0x08    | 0x81      |0xCC     | 0x80    |..01....    |..10....
        |         |           |         |         |            |
       Q.933     CLNP        IP        SNAP     ISO 8208    ISO 8208
        |                               |       Modulo 8    Modulo 128
        |                               |
        --------------------           OUI
        |                  |            |
       L2 ID              L3 ID      -------
        |               User         |     |
        |               specified    |     |
        |               0x70        802.3 802.6
        |
        -------------------
        |0x51 |0x4E |     |0x4C
        |     |     |     |
       7776  Q.922 Others 802.2

   For those protocols which do not have a NLPID assigned or do not have
   a SNAP encapsulation, the NLPID value of 0x08, indicating CCITT
   Recommendation Q.933 should be used.  The four octets following the
   NLPID include both layer 2 and layer 3 protocol identification.  The
   code points for most protocols are currently defined in ANSI T1.617
   low layer compatibility information element.  There is also an escape
   for defining non-standard protocols.



















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                      Format of Other Protocols
                          using Q.933 NLPID
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | NLPID   0x08  |
                  +---------------+---------------+
                  |          L2 Protocol ID       |
                  | octet 1       |  octet 2      |
                  +-------------------------------+
                  |          L3 Protocol ID       |
                  | octet 2       |  octet 2      |
                  +-------------------------------+
                  |         Protocol Data         |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+


                      ISO 8802/2 with user specified
                              layer 3
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  |Control  0x03  | NLPID   0x08  |
                  +---------------+---------------+
                  | 802/2   0x4C  |      0x80     |
                  +-------------------------------+
                  |User Spec. 0x70|     Note 1    |
                  +-------------------------------+
                  |  DSAP         |     SSAP      |
                  +-------------------------------+
                  | Control  (Note 2)             |
                  +-------------------------------+
                  |      Remainder of PDU         |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

                 Note 1: Indicates the code point for user specified
                         layer 3 protocol.

                 Note 2: Control field is two octets for I-format and
                         S-format frames (see 88002/2)


   Encapsulations using I frame (layer 2)




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   The Q.922 I frame is for supporting layer 3 protocols which require
   acknowledged data link layer (e.g., ISO 8208).  The C/R bit (T1.618
   address) will be used for command and response indications.

                      Format of ISO 8208 frame
                              Modulo 8
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  | ....Control I frame           |
                  +---------------+---------------+
                  | 8208 packet (modulo 8) Note 3 |
                  |                               |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

                 Note 3: First octet of 8208 packet also identifies the
                         NLPID which is "..01....".


                      Format of ISO 8208 frame
                              Modulo 128
                  +-------------------------------+
                  |        Q.922 Address          |
                  +---------------+---------------+
                  | ....Control I frame           |
                  +---------------+---------------+
                  | 8208 packet (modulo 128)      |
                  |          Note 4               |
                  +-------------------------------+
                  | FCS                           |
                  +-------------------------------+

                 Note 4: First octet of 8208 packet also identifies the
                         NLPID which is "..10....".

13.  References

   [1] International Telegraph and Telephone Consultative Committee,
       "ISDN Data Link Layer Specification for Frame Mode Bearer
       Services", CCITT Recommendation Q.922, 19 April 1991.

   [2] American National Standard For Telecommunications - Integrated
       Services Digital Network - Core Aspects of Frame Protocol for Use
       with Frame Relay Bearer Service, ANSI T1.618-1991, 18 June 1991.





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   [3] Information technology - Telecommunications and Information
       Exchange between systems - Protocol Identification in the Network
       Layer, ISO/IEC  TR 9577: 1990 (E)  1990-10-15.

   [4] Baker, F., Editor, "Point to Point Protocol Extensions for
       Bridging", RFC 1220, ACC, April 1991.

   [5] International Standard, Information Processing Systems - Local
       Area Networks - Logical Link Control, ISO 8802-2: 1989 (E), IEEE
       Std 802.2-1989, 1989-12-31.

   [6] Plummer, D., "An Ethernet Address Resolution Protocol - or -
       Converting Network Protocol Addresses to 48.bit Ethernet Address
       for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT,
       November 1982.

   [7] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.

   [8] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse
       Address Resolution Protocol", STD 38, RFC 903, Stanford
       University, June 1984.

   [9] Postel, J. and Reynolds, J., "A Standard for the Transmission of
       IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
       Sciences Institute, February 1988.

  [10] IEEE, "IEEE Standard for Local and Metropolitan Area Networks:
       Overview and architecture", IEEE Standards 802-1990.

  [11] Bradley, T., and C. Brown, "Inverse Address Resolution Protocol",
       RFC 1293, Wellfleet Communications, Inc., January 1992.

  [12] IEEE, "IEEE Standard for Local and Metropolitan Networks: Media
       Access Control (MAC) Bridges", IEEE Standard 802.1D-1990.

  [13] PROJECT 802 - LOCAL AND METROPOLITAN AREA NETWORKS, Draft
       Standard 802.1G: Remote MAC Bridging, Draft 6, October 12, 1992.

14.  Security Considerations

   Security issues are not discussed in this memo.









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15.  Authors' Addresses

   Terry Bradley
   Wellfleet Communications, Inc.
   15 Crosby Drive
   Bedford, MA  01730

   Phone:  (617) 280-2401
   Email:  tbradley@wellfleet.com


   Caralyn Brown
   Wellfleet Communications, Inc.
   15 Crosby Drive
   Bedford, MA  01730

   Phone:  (617) 280-2335
   Email:  cbrown@wellfleet.com


   Andrew G. Malis
   Ascom Timeplex, Inc.
   Advanced Products Business Unit
   289 Great Road   Suite 205
   Acton, MA  01720

   Phone:  (508) 266-4500
   Email: malis_a@timeplex.com























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