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rfc:rfc1490

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.

Bradley, Brown & Malis [Page 1] 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.

Bradley, Brown & Malis [Page 2] RFC 1490 Multiprotocol over Frame Relay July 1993

      |---   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.

Bradley, Brown & Malis [Page 3] RFC 1490 Multiprotocol over Frame Relay July 1993

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

Bradley, Brown & Malis [Page 4] RFC 1490 Multiprotocol over Frame Relay July 1993

 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

Bradley, Brown & Malis [Page 5] RFC 1490 Multiprotocol over Frame Relay July 1993

 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:

Bradley, Brown & Malis [Page 6] RFC 1490 Multiprotocol over Frame Relay July 1993

                 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:

Bradley, Brown & Malis [Page 7] RFC 1490 Multiprotocol over Frame Relay July 1993

         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                           |
                +-------------------------------+

Bradley, Brown & Malis [Page 8] 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                           |
                +-------------------------------+

Bradley, Brown & Malis [Page 9] 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                           |
                +-------------------------------+

Bradley, Brown & Malis [Page 10] 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                           |
                +-------------------------------+

Bradley, Brown & Malis [Page 11] 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

Bradley, Brown & Malis [Page 12] 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:

Bradley, Brown & Malis [Page 13] 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.

Bradley, Brown & Malis [Page 14] 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

Bradley, Brown & Malis [Page 15] RFC 1490 Multiprotocol over Frame Relay July 1993

 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

Bradley, Brown & Malis [Page 16] RFC 1490 Multiprotocol over Frame Relay July 1993

 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.

Bradley, Brown & Malis [Page 17] 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

Bradley, Brown & Malis [Page 18] RFC 1490 Multiprotocol over Frame Relay July 1993

 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)

Bradley, Brown & Malis [Page 19] RFC 1490 Multiprotocol over Frame Relay July 1993

         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.

Bradley, Brown & Malis [Page 20] RFC 1490 Multiprotocol over Frame Relay July 1993

                                ~~~~~~~~~~~~~~~
                               (                )
             +-----+          (                  )             +-----+
             |     |-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

Bradley, Brown & Malis [Page 21] RFC 1490 Multiprotocol over Frame Relay July 1993

 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

Bradley, Brown & Malis [Page 22] RFC 1490 Multiprotocol over Frame Relay July 1993

 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

Bradley, Brown & Malis [Page 23] RFC 1490 Multiprotocol over Frame Relay July 1993

 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.

Bradley, Brown & Malis [Page 24] RFC 1490 Multiprotocol over Frame Relay July 1993

         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).

Bradley, Brown & Malis [Page 25] RFC 1490 Multiprotocol over Frame Relay July 1993

 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.

Bradley, Brown & Malis [Page 26] RFC 1490 Multiprotocol over Frame Relay July 1993

 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

Bradley, Brown & Malis [Page 27] RFC 1490 Multiprotocol over Frame Relay July 1993

 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

Bradley, Brown & Malis [Page 28] RFC 1490 Multiprotocol over Frame Relay July 1993

 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.

Bradley, Brown & Malis [Page 29] RFC 1490 Multiprotocol over Frame Relay July 1993

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
  1. end-to-end systems or
  2. LAN to LAN bride and routers or
  3. a combination of the above.
   over frame relay networks.

Bradley, Brown & Malis [Page 30] RFC 1490 Multiprotocol over Frame Relay July 1993

                            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.

Bradley, Brown & Malis [Page 31] RFC 1490 Multiprotocol over Frame Relay July 1993

                    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)

Bradley, Brown & Malis [Page 32] RFC 1490 Multiprotocol over Frame Relay July 1993

 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.

Bradley, Brown & Malis [Page 33] RFC 1490 Multiprotocol over Frame Relay July 1993

 [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.

Bradley, Brown & Malis [Page 34] RFC 1490 Multiprotocol over Frame Relay July 1993

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

Bradley, Brown & Malis [Page 35]

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