path: root/doc/rfc/rfc1294.txt

Network Working Group                                         T. Bradley
Request for Comments: 1294                                      C. Brown
                                          Wellfleet Communications, Inc.
                                                                A. Malis
                                                      BBN Communications
                                                            January 1992

              Multiprotocol Interconnect over Frame Relay

1.  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.

2.  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.  Systems with the ability to transfer
   both this encapsulation method, 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.

3.  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. This document could not
   have been completed without the expertise of the IP over Large Public
   Data Networks working group of the IETF.

4.  Conventions

   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.



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

5.  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.

6.  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:


















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         +-----------------------------+
         |    flag (7E hexadecimal)    |
         +-----------------------------+
         |       Q.922 Address*        |
         +--                         --+
         |                             |
         +-----------------------------+
         | Control (UI = 0x03)         |
         +-----------------------------+
         | Optional Pad(s)   (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 an optional field used to align the remainder of the
   frame to a convenient boundary for the sender.  There may be zero or
   more pad octets within the pad field and all must have a value of
   zero.

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



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   encapsulation scheme, a NLPID value of 0x00 is invalid under the
   Frame Relay encapsulation. The known NLPID values are listed in the
   Appendix.

   For full interoperability with older Frame Relay encapsulation
   formats, a station may implement section 15, Backward Compatibility.

   There is no commonly implemented 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.

7.  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
   indication that the destination may use to correctly interpret the
   contents of the frame.  This indication 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
   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



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   protocol.  Note that OUI 0x00-00-00 specifies that the following PID
   is an EtherType.

7.1.  Routed Frames

   Some protocols will have an assigned NLPID, but because the NLPID
   numbering space is so limited many protocols do not have a specific
   NLPID 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), OUI 0x00-00-00 (which indicates an
   EtherType follows), and the EtherType of the protocol in use.

             Format of Routed Frames
         +-------------------------------+
         |        Q.922 Address          |
         +-------------------------------+
         |Control  0x03  | pad(s)  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:

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









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   In the particular case of an Internet IP datagram, the NLPID is 0xCC.

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

7.2.  Bridged Frames

   The second type of Frame Relay traffic is bridged packets. These
   packets are encapsulated using the NLPID value of 0x80 indicating
   SNAP and the following SNAP header identifies the format of the
   bridged packet.  The OUI value used for this encapsulation is the
   802.1 organization code 0x00-80-C2.  The following two octets (PID)
   specify 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:

            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
         0x00-05              0x00-0B              802.6

      In addition, the PID value 0x00-0E, when used with OUI 0x00-80-C2,
      identifies Bridged Protocol Data Units (BPDUs).

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









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          Format of Bridged Ethernet/802.3 Frame
         +-------------------------------+
         |        Q.922 Address          |
         +-------------------------------+
         |Control  0x03  | pad(s)  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                           |
         +-------------------------------+

          Format of Bridged 802.4 Frame
         +-------------------------------+
         |        Q.922 Address          |
         +-------------------------------+
         |Control  0x03  | pad(s)  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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          Format of Bridged 802.5 Frame
         +-------------------------------+
         |        Q.922 Address          |
         +-------------------------------+
         |Control  0x03  | pad(s)  0x00  |
         +-------------------------------+
         | NLPID   0x80  | OUI     0x00  |
         +---------------+             --+
         | OUI  0x80-C2                  |
         +-------------------------------+
         | PID 0x00-03 or 0x00-09        |
         +-------------------------------+
         | Access Control| Frame Control |
         +-------------------------------+
         | MAC destination address       |
         |             .                 |
         |             .                 |
         +-------------------------------+
         | (remainder of MAC frame)      |
         +-------------------------------+
         | LAN FCS (if PID is 0x00-03)   |
         |                               |
         +-------------------------------+
         | FCS                           |
         +-------------------------------+


























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


























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           Format of Bridged 802.6 Frame
         +-------------------------------+
         |        Q.922 Address          |
         | Control 0x03  | pad(s)  0x00  |
         +-------------------------------+
         | NLPID   0x80  | OUI     0x00  |
         +---------------+             --+
         | OUI  0x80-C2                  |
         +-------------------------------+
         | PID 0x00-05 or 0x00-0B        |
         +-------------------------------+
         |   Reserved    |     BEtag     |  Common
         +---------------+---------------+  PDU
         |            BAsize             |  Header
         +-------------------------------+
         | MAC destination address       |
         +-------------------------------+
         | (remainder of MAC frame)      |
         +-------------------------------+
         |                               |
         +-    Common PDU Trailer       -+
         |                               |
         +-------------------------------+
         | FCS                           |
         +-------------------------------+

      The Common Protocol Data Unit (PDU) Header and Trailer are
      conveyed to allow pipelining at the egress bridge to an 802.6
      subnetwork.  Specifically, the Common PDU 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 subnetwork because the encapsulated BEtag value may conflict
      with the previous BEtag value transmitted by that bridge.







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          Format of BPDU Frame
      +-------------------------------+
      |        Q.922 Address          |
      +-------------------------------+
      |Control  0x03  | pad(s)  0x00  |
      +-------------------------------+
      | NLPID   0x80  | OUI     0x00  |
      +---------------+             --+
      | OUI  0x80-C2                  |
      +-------------------------------+
      | PID 0x00-0E                   |
      +-------------------------------+  ----
      | 802.1(d) Protocol Identifier  |  BPDU, as defined
      +-------------------------------+  by 802.1(d),
      | Version = 00  |  BPDU Type    |  section 5.3
      +-------------------------------+
      | (remainder of BPDU)           |
      +-------------------------------+  ----
      | FCS                           |
      +-------------------------------+

8.  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 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:

                  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]




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   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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      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)
             |               |
             +---------------+








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9.  Fragmentation Issues

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

























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          +---------------+---------------+
          |         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
   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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                        Frame Relay Fragmentation Example
                                           +-----------+-----------+
                                           |     Q.922 Address     |
                                           +-----------+-----------+
                                           | Ctrl 0x03 | pad  0x00 |
                                           +-----------+-----------+
                                           |NLPID 0x80 | OUI 0x00  |
                                           +-----------+-----------+
                                           | OUI          0x80-C2  |
         +-----------+-----------+         +-----------+-----------+
         | pad 0x00  |NLPID 0xCC |         | PID          0x00-0D  |
         +-----------+-----------+         +-----------+-----------+
         |                       |         | sequence number   n   |
         |                       |         +-----------+-----------+
         |                       |         |0| RSVD |offset (0)    |
         |                       |         +-----------+-----------+
         |                       |         | pad 0x00  |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.

   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.

10.  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(s)  0x00      |
         +-----------------------+-----------------------+
         |  NLPID = 0x80         |                       |  SNAP Header
         +-----------------------+  OUI = 0x00-00-00     +  Indicating
         |                                               |  ARP
         +-----------------------+-----------------------+
         |  PID = 0x0806                                 |
         +-----------------------+-----------------------+
         |                   ARP packet                  |
         |                       .                       |
         |                       .                       |
         |                       .                       |
         +-----------------------+-----------------------+















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   Where the ARP packet has the following format and values:

      Data:
        ar$hrd   16 bits     Hardware type
        ar$pro   16 bits     Protocol type
        ar$hln    8 bits     Octet length of hardware address (n)
        ar$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
      corrolation 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
   addressing may become useful in sending ARP requests and other
   "broadcast" messages.



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   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.

11.  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(s)  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.

12.  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).
   Therefore, the NLPID field will indicate ISO CLNP and the data packet



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   will follow immediately.  The frame would be as follows:

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

13.  Bridging in a Frame Relay network

   A Frame Relay interface acting as a bridge must be able to flood,
   forward, and filter packets.

   Flooding is performed by sending the packet to all possible
   destinations.  In the Frame Relay environment this means sending the
   packet through each relevant DLC.

   To forward a packet, a bridge must be able to associate a destination
   MAC address with a DLC.  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
   bridges must provide enough information to allow a Frame Relay
   interface to dynamically learn about foreign destinations beyond the
   set of Frame Relay stations.

   To accomplish 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 and
   learn the association between foreign destination and Frame Relay
   station.

14. For Future Study

   It may be desirable for the two ends of a connection to have the
   capability to negotiate end-to-end configuration and service
   parameters.  The actual protocol and parameters to be negotiated will
   be a topic of future RFCs.

15.  Backward Compatibility

   This section is included in this RFC for completeness only.  It is
   not intended to suggest additional requirements.

   Some existing Frame Relay stations use the NLPID value of 0xCE to



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   indicate an escape to Ethernet Packet Types as defined in the latest
   version of the Assigned Numbers (RFC-1060) [7].  In this case, the
   frame will have the following format:

         +-----------------------------+
         | Q.922 Address               |
         +--                         --+
         |                             |
         +-----------------------------+
         | Control (UI = 0x03)         |
         +-----------------------------+
         | Optional Pad(s)   (0x00)    |
         +-----------------------------+
         | NLPID    (0xCE)             |
         +-----------------------------+
         | Ethertype                   |
         +-                           -+
         |                             |
         +-----------------------------+
         |             .               |
         |             .               |
         |           Data              |
         |             .               |
         |             .               |
         +-----------------------------+
         |    Frame Check Sequence     |
         +--           .             --+
         |       (two octets)          |
         +-----------------------------+

   The Ethertype field is a 16-bit value used to identify a protocol
   type for the following PDU.

   In order to be fully interoperable with stations that use this
   encoding, Frame Relay stations may recognize the NLPID value of 0xCE
   and interpret the following two byte Ethertype.  It is never
   necessary to generate this encapsulation format only to properly
   interpret it's meaning.

   For example, IP encapsulated with this NLPID value will have the
   following format:










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         +-----------------------+-----------------------+
         |Q.922 Address                                  |
         +-----------------------+-----------------------+
         |Control (UI)  0x03     | NLPID    0xCE         |
         +-----------------------+-----------------------+
         |Ethertype [7]                         0x0800   |
         +-----------------------+-----------------------+
         |  IP Packet                                    |
         |                       .                       |
         |                       .                       |
         +-----------------------+-----------------------+

16.  Appendix

   List of Known 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
      0xCE    EtherType - unofficial temporary use

   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
      0x00-05              0x00-0B              802.6
      0x00-0D                                   Fragments
      0x00-0E                                   BPDUs

17.  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, Fred, "Point to Point Protocol Extensions for Bridging",
        Point to Point Working Group, RFC-1220, 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, David C., An Ethernet Address Resolution Protocol",
        RFC-826, November 1982.

   [7]  Reynolds, J. and Postel, J., "Assigned Numbers", RFC-1060, ISI,
        March 1990.

   [8]  Finlayson, Mann, Mogul, Theimer, "A Reverse Address Resolution
        Protocol", 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, ISI, 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.

18.  Security Considerations

        Security issues are not addressed in this memo.

19.  Authors' Addresses

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

           Phone:  (617) 275-2400

           Email:  tbradley@wellfleet.com






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

           Phone:  (617) 275-2400

           Email:  cbrown@wellfleet.com


           Andrew G. Malis
           BBN Communications
           150 CambridgePark Drive
           Cambridge, MA  02140

           Phone:  (617) 873-3419

           Email: malis@bbn.com

































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