<pre>Network Working Group                                          C. Brown
Request for Comments: 2427                                   Consultant
STD: 55                                                        A. Malis
Obsoletes: <a href="./rfc1490">1490</a>, <a href="./rfc1294">1294</a>                       Ascend Communications, Inc.
Category: Standards Track                                September 1998


              <span class="h1">Multiprotocol Interconnect over Frame Relay</span>

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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 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.

Acknowledgments

   This document could not have been completed without the support of
   Terry Bradley of Avici Systems, Inc..  Comments and contributions
   from many sources, especially those from Ray Samora of Proteon, Ken
   Rehbehn of Visual Networks, Fred Baker and Charles Carvalho of Cisco
   Systems, and Mostafa Sherif of AT&amp;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 (though
   it was deleted in the final version) and Floyd Backes and Laura
   Bridge of 3Com 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 and the IP over NBMA working
   groups of the IETF.




<span class="grey">Brown &amp; Malis               Standards Track                     [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Conventions and Acronyms</span>

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [<a href="#ref-16" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">16</a>].

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

        |---   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



<span class="grey">Brown &amp; Malis               Standards Track                     [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


      DE   - Discard Eligibility bit
      DTE  - Data Terminal Equipment
      FECN - Forward Explicit Congestion Notification
      PDU  - Protocol Data Unit
      PTT  - Postal Telephone &amp; Telegraph
      SNAP - Subnetwork Access Protocol

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Introduction</span>

   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.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Frame Format</span>

   All protocols must encapsulate their packets within a Q.922 Annex A
   frame [<a href="#ref-1" title="&quot;ISDN Data Link Layer Specification for Frame Mode Bearer Services&quot;">1</a>].  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:















<span class="grey">Brown &amp; Malis               Standards Track                     [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


                  +---------------------------+
                  |    flag (7E hexadecimal)  |
                  +---------------------------+
                  |       Q.922 Address*      |
                  +--                       --+
                  |                           |
                  +---------------------------+
                  |    Control (UI = 0x03)    |
                  +---------------------------+
                  | Pad (when required) (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 data portion (beyond the
   encapsulation header) of the frame to a two octet boundary.  If
   present, the pad is a single octet and must have a value of zero.
   Explicit directions of when to use the pad field are discussed later
   in this document.

   The Network Level Protocol ID (NLPID) field is administered by ISO
   and the ITU.  It contains values for many different protocols
   including IP, CLNP, and IEEE Subnetwork Access Protocol (SNAP)[<a href="#ref-10" title="&quot;IEEE Standard for Local and Metropolitan Area Networks: Overview and architecture&quot;">10</a>].
   This field tells the receiver what encapsulation or what protocol
   follows.  Values for this field are defined in ISO/IEC TR 9577 [<a href="#ref-3">3</a>]. 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



<span class="grey">Brown &amp; Malis               Standards Track                     [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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. <a href="#appendix-A">Appendix A</a> 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.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Interconnect Issues</span>

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

   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.





<span class="grey">Brown &amp; Malis               Standards Track                     [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Routed Frames</span>

   Some protocols will have an assigned NLPID, but because the NLPID
   numbering space is 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 the presence of a SNAP header) 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.

   When a SNAP header is present as described above, a one octet pad is
   used to align the protocol data on a two octet boundary as shown
   below.

                       Format of Routed Frames
                         with a SNAP Header
                  +-------------------------------+
                  |         Q.922 Address         |
                  +---------------+---------------+
                  | Control  0x03 | pad     0x00  |
                  +---------------+---------------+
                  | NLPID    0x80 | Organization- |
                  +---------------+               |
                  | ally Unique Identifier (OUI)  |
                  +-------------------------------+
                  |   Protocol Identifier (PID)   |
                  +-------------------------------+
                  |                               |
                  |         Protocol Data         |
                  |                               |
                  +-------------------------------+
                  |              FCS              |
                  +-------------------------------+

   In the few cases when a protocol has an assigned NLPID (see <a href="#appendix-A">Appendix</a>
   <a href="#appendix-A">A</a>), 48 bits can be saved using the format below:

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



<span class="grey">Brown &amp; Malis               Standards Track                     [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   When using the NLPID encapsulation format as described above, the pad
   octet is not used.

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

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Bridged Frames</span>

   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.

   Following the precedent in <a href="./rfc1638">RFC 1638</a> [<a href="#ref-4" title="&quot;PPP Bridging Control Protocol (BCP)&quot;">4</a>], non-canonical MAC
   destination addresses are used for encapsulated IEEE 802.5 and FDDI
   frames, and canonical MAC destination addresses are used for the
   remaining encapsulations defined in this section.

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









<span class="grey">Brown &amp; Malis               Standards Track                     [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


           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 Bridge Protocol Data Units (BPDUs) as defined by
      802.1(d) or 802.1(g) [<a href="#ref-12" title="&quot;IEEE Standard for Local and Metropolitan Networks: Media Access Control (MAC) Bridges&quot;">12</a>], and the PID value 0x00-0F identifies
      Source Routing BPDUs.

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











<span class="grey">Brown &amp; Malis               Standards Track                     [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


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



























<span class="grey">Brown &amp; Malis               Standards Track                     [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


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


























<span class="grey">Brown &amp; Malis               Standards Track                    [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


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


























<span class="grey">Brown &amp; Malis               Standards Track                    [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


                    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.






<span class="grey">Brown &amp; Malis               Standards Track                    [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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.

                         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)[<a href="#ref-12" title="&quot;IEEE Standard for Local and Metropolitan Networks: Media Access Control (MAC) Bridges&quot;">12</a>]  |
                  |                               |
                  +-------------------------------+
                  |              FCS              |
                  +-------------------------------+

                 Format of Source Routing BPDU Frame
                  +-------------------------------+
                  |         Q.922 Address         |
                  +-------------------------------+
                  |        Control   0x03         |
                  +-------------------------------+
                  |          PAD     0x00         |
                  +-------------------------------+
                  |         NLPID    0x80         |
                  +-------------------------------+
                  |        OUI 0x00-80-C2         |
                  +-------------------------------+
                  |          PID 0x00-0F          |
                  +-------------------------------+
                  |                               |
                  |      Source Routing BPDU      |
                  |                               |
                  |                               |
                  +-------------------------------+
                  |              FCS              |
                  +-------------------------------+



<span class="grey">Brown &amp; Malis               Standards Track                    [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Data Link Layer Parameter Negotiation</span>

   Frame Relay stations may choose to support the Exchange
   Identification (XID) specified in <a href="#appendix-I">Appendix I</a>II of Q.922 [<a href="#ref-1" title="&quot;ISDN Data Link Layer Specification for Frame Mode Bearer Services&quot;">1</a>].  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 <a href="#section-5.9">Section</a>
   <a href="#section-5.9">5.9</a> 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]

   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.















<span class="grey">Brown &amp; Malis               Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


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

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Address Resolution for PVCs</span>

   This document only describes address resolution as it applies to
   PVCs.  SVC operation will be discussed in future documents.



<span class="grey">Brown &amp; Malis               Standards Track                    [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   There are situations in which a Frame Relay station may wish to
   dynamically resolve a protocol address over PVCs.  This may be
   accomplished using the standard Address Resolution Protocol (ARP) [<a href="#ref-6" title="&quot;An Ethernet Address Resolution Protocol - or - Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware&quot;">6</a>]
   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$hln    8 bits     Octet length of hardware address (n)
           ar$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) [<a href="#ref-7" title="&quot;Assigned Numbers&quot;">7</a>].

           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).




<span class="grey">Brown &amp; Malis               Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


           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.

                              ~~~~~~~~~~~~~~~
                             (                )
           +-----+          (                  )             +-----+
           |     |-50------(--------------------)---------70-|     |
           |  A  |        (                      )           |  B  |
           |     |-60-----(---------+            )           |     |
           +-----+         (        |           )            +-----+
                            (       |          )
                             (      |         )  &lt;---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.



<span class="grey">Brown &amp; Malis               Standards Track                    [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


              DLCI to Q.922 Address Table for Figure 1

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

      For authoritative description of the correlation between DLCI and
      Q.922 [<a href="#ref-1" title="&quot;ISDN Data Link Layer Specification for Frame Mode Bearer Services&quot;">1</a>] addresses, the reader should consult that specification.
      A summary of the correlation is included here for convenience. 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
   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






<span class="grey">Brown &amp; Malis               Standards Track                    [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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

   Again, the source hardware address is unknown and when the response
   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) [<a href="#ref-8" title="&quot;A Reverse Address Resolution Protocol&quot;">8</a>] works in exactly the same way.  Still using
   figure 1, if we assume station C is an address server, the following
   RARP exchanges will occur:






<span class="grey">Brown &amp; Malis               Standards Track                    [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


          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, as
   specified by [<a href="#ref-19" title="&quot;Frame Relay PVC Multicast Service and Protocol Implementation Agreement&quot;">19</a>], is presently being considered by Frame Relay
   providers.  In time, multicast addressing may become useful in
   sending ARP requests and other "broadcast" messages.

   Because of the inefficiencies of emulating broadcasting in a Frame
   Relay environment, a new address resolution variation was developed.
   It is called Inverse ARP [<a href="#ref-11" title="&quot;Inverse Address Resolution Protocol&quot;">11</a>] 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.  Support
   for Inverse ARP is not required to implement this specification, but
   it has proven useful for Frame Relay interface autoconfiguration.
   See [<a href="#ref-11" title="&quot;Inverse Address Resolution Protocol&quot;">11</a>] for its description and an example of its use with Frame
   Relay.

   Stations must be able to map more than one IP address in the same IP
   subnet (CIDR address prefix) to a particular DLCI on a Frame Relay
   interface. This need arises from applications such as remote access,
   where servers must act as ARP proxies for many dial-in clients, each
   assigned a unique IP address while sharing bandwidth on the same DLC.
   The dynamic nature of such applications result in frequent address
   association changes with no affect on the DLC's status as reported by
   Frame Relay PVC Status Signaling.




<span class="grey">Brown &amp; Malis               Standards Track                    [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   As with any other interface that utilizes ARP, stations may learn the
   associations between IP addresses and DLCIs by processing unsolicited
   ("gratuitous") ARP requests that arrive on the DLC.  If one station
   (perhaps a terminal server or remote access server) wishes to inform
   its peer station on the other end of a Frame Relay DLC of a new
   association between an IP address and that PVC, it should send an
   unsolicited ARP request with the source IP address equal to the
   destination IP address, and both set to the new IP address being used
   on the DLC.  This allows a station to "announce" new client
   connections on a particular DLCI.  The receiving station must store
   the new association, and remove any old existing association, if
   necessary, from any other DLCI on the interface.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IP over Frame Relay</span>

   Internet Protocol [<a href="#ref-9" title="&quot;A Standard for the Transmission of IP Datagrams over IEEE 802 Networks&quot;">9</a>] (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.

         1.  NLPID value indicating IP

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




















<span class="grey">Brown &amp; Malis               Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


         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.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Other Protocols over Frame Relay</span>

   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
   will follow immediately.  The frame would be as follows:

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




<span class="grey">Brown &amp; Malis               Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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 (UI)  0x03   |      pad  0x00       |
                  +----------------------+----------------------+
                  | NLPID    0x80 (SNAP) | OUI - 0x00 00 00     |
                  +----------------------+                      |
                  |                                             |
                  +---------------------------------------------+
                  |                PID    0x8137                |
                  +---------------------------------------------+
                  |                 IPX packet                  |
                  |                      .                      |
                  |                      .                      |
                  +---------------------------------------------+

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Bridging Model for Frame Relay</span>

   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" [<a href="#ref-13" title="March 12">13</a>] 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.

   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.




<span class="grey">Brown &amp; Malis               Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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 [<a href="#ref-13" title="March 12">13</a>].  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
   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 <a href="#section-4.2">section 4.2</a>.  In this way, the receiving Frame Relay
   interface will know to look into the bridged packet to gather the
   appropriate information.





<span class="grey">Brown &amp; Malis               Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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
   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.





<span class="grey">Brown &amp; Malis               Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   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.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Security Considerations</span>

   This document defines mechanisms for identifying the multiprotocol
   encapsulation of datagrams over Frame Relay.  There is obviously an
   element in trust in any encapsulation protocol - a receiver must
   trust that the sender has correctly identified the protocol being
   encapsulated.  In general, there is no way for a receiver to try to
   ascertain that the sender did indeed use the proper protocol
   identification, nor would this be desired functionality.

   It also specifies the use of ARP and RARP with Frame Relay, and is
   subject to the same security constraints that affect ARP and similar
   address resolution protocols.  Because authentication is not a part
   of ARP, there are known security issues relating to its use (e.g.,
   host impersonation).  No additional security mechanisms have been
   added to ARP or RARP for use with Frame Relay networks.




















<span class="grey">Brown &amp; Malis               Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  <a href="#appendix-A">Appendix A</a> - NLPIDS and PIDs</span>

   List of Commonly Used NLPIDs

   0x00    Null Network Layer or Inactive Set
           (not used with Frame Relay)
   0x08    Q.933 [<a href="#ref-2" title="&quot;Signalling Specifications for Frame Mode Switched and Permanent Virtual Connection Control and Status Monitoring&quot;">2</a>]
   0x80    SNAP
   0x81    ISO CLNP
   0x82    ISO ESIS
   0x83    ISO ISIS
   0x8E    IPv6
   0xB0    FRF.9 Data Compression [<a href="#ref-14" title="&quot;Data Compression Over Frame Relay Implementation Agreement&quot;">14</a>]
   0xB1    FRF.12 Fragmentation [<a href="#ref-18" title="&quot;Frame Relay Fragmentation Implementation Agreement&quot;">18</a>]
   0xCC    IPv4
   0xCF    PPP in Frame Relay [<a href="#ref-17" title="&quot;PPP in Frame Relay&quot;">17</a>]

   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)[<a href="#ref-12" title="&quot;IEEE Standard for Local and Metropolitan Networks: Media Access Control (MAC) Bridges&quot;">12</a>].
                        0x00-0F              Source Routing BPDUs




















<span class="grey">Brown &amp; Malis               Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  <a href="#appendix-B">Appendix B</a> - Connection Oriented Procedures</span>

   This Appendix contains additional information and instructions for
   using ITU Recommendation Q.933 [<a href="#ref-2" title="&quot;Signalling Specifications for Frame Mode Switched and Permanent Virtual Connection Control and Status Monitoring&quot;">2</a>] and other ITU standards for
   encapsulating data over frame relay.  The information contained here
   is similar (and in some cases identical) to that found in Annex E to
   ITU Q.933.  The authoritative source for this information is in Annex
   E and is repeated here only for convenience.

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

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

   over frame relay networks.

                              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   |0x50
        |     |     |     |       |
       7776  Q.922 Others 802.2  User
                                 Specified



<span class="grey">Brown &amp; Malis               Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   For those protocols which do not have a NLPID assigned or do not have
   a SNAP encapsulation, the NLPID value of 0x08, indicating ITU
   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 ITU Q.933 low
   layer compatibility information element.  The code points for "User
   Specified" are described in Frame Relay Forum FRF.3.1 [<a href="#ref-15" title="&quot;Multiprotocol Encapsulation Implementation Agreement&quot;">15</a>].  There is
   also an escape for defining non-standard protocols.

                      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 1    |   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              |
                  +-------------------------------+




<span class="grey">Brown &amp; Malis               Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


            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)

   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 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....".







<span class="grey">Brown &amp; Malis               Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  <a href="#appendix-C">Appendix C</a> - Modifications from <a href="./rfc1490">RFC 1490</a></span>

   <a href="./rfc1490">RFC 1490</a> has been widely implemented and used, and has been adopted
   by the Frame Relay Forum in FRF.3.1 [<a href="#ref-15" title="&quot;Multiprotocol Encapsulation Implementation Agreement&quot;">15</a>] and by the ITU in Q.933 [<a href="#ref-2" title="&quot;Signalling Specifications for Frame Mode Switched and Permanent Virtual Connection Control and Status Monitoring&quot;">2</a>].
   This section describes updates to <a href="./rfc1490">RFC 1490</a> that have been made as a
   result of this implementation and interoperability experience, and
   which reflect current implementation practice.

   Some language changes were necessary to clarify <a href="./rfc1490">RFC 1490</a>.  None of
   these changes impacted the technical aspects of this document, but
   were required to keep diagrams and language specific and consistent.
   Specifics of these changes will not be listed here.  Below are listed
   those changes which were significant.

   a) The requirement for stations to accept SNAP encapsulated protocols
      for which a NLPID was available, was removed.  <a href="./rfc1490">RFC 1490</a> indicated
      that, if a protocol, such as IP, had a designated NLPID value, it
      must be used.  Later the document required stations to accept a
      SNAP encapsulated version of this same protocol.  This is clearly
      inconsistent.  A compliant station must send and accept the NLPID
      encapsulated version of such a protocol.  It MAY accept the SNAP
      encapsulation but should not be required to do so as these frames
      are noncompliant.

   b) Fragmentation was removed.  To date there are no interoperable
      implementations of the fragmentation algorithm presented in <a href="./rfc1490">RFC</a>
      <a href="./rfc1490">1490</a>.  Additionally, there have been several suggestions that the
      proposed mechanisms are insufficient for some frame relay
      applications.  To this end, fragmentation was removed from this
      document, and has been replaced by the fragmentation specified in
      FRF.12 [<a href="#ref-18" title="&quot;Frame Relay Fragmentation Implementation Agreement&quot;">18</a>].

   c) The address resolution presented in <a href="./rfc1490">RFC 1490</a> referred only to PVC
      environments and is insufficient for SVC environments.  Therefore
      the section title was changed to reflect this.  Further work on
      SVC address resolution will take place in the ION working group.

   d) The encapsulation for Source Routing BPDUs was added, and the
      lists in <a href="#appendix-A">Appendix A</a> were augmented.

   e) The use of canonical and non-canonical MAC destination addresses
      in the bridging encapsulations was clarified.

   f) The Inverse ARP description was moved to the Inverse ARP
      specification [<a href="#ref-11" title="&quot;Inverse Address Resolution Protocol&quot;">11</a>].

   g) A new security section was added.




<span class="grey">Brown &amp; Malis               Standards Track                    [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>.  References</span>

   [<a id="ref-1">1</a>] International Telecommunication Union, "ISDN Data Link Layer
       Specification for Frame Mode Bearer Services", ITU-T
       Recommendation Q.922, 1992.

   [<a id="ref-2">2</a>] International Telecommunication Union, "Signalling Specifications
       for Frame Mode Switched and Permanent Virtual Connection Control
       and Status Monitoring", ITU-T Recommendation Q.933, 1995.

   [<a id="ref-3">3</a>] Information technology - Telecommunications and Information
       Exchange between systems - Protocol Identification in the Network
       Layer, ISO/IEC TR 9577: 1992.

   [<a id="ref-4">4</a>] Baker, F., and R. Bowen, "PPP Bridging Control Protocol (BCP)",
       <a href="./rfc1638">RFC 1638</a>, June 1994.

   [<a id="ref-5">5</a>] International Standard, Information Processing Systems - Local
       Area Networks - Logical Link Control, ISO 8802-2, ANSI/IEEE,
       Second Edition, 1994-12-30.

   [<a id="ref-6">6</a>] Plummer, D., "An Ethernet Address Resolution Protocol - or -
       Converting Network Protocol Addresses to 48.bit Ethernet Address
       for Transmission on Ethernet Hardware", STD 37, <a href="./rfc826">RFC 826</a>, November
       1982.

   [<a id="ref-7">7</a>] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, <a href="./rfc1700">RFC 1700</a>,
       October 1994.  See also: <a href="http://www.iana.org/numbers.html">http://www.iana.org/numbers.html</a>

   [<a id="ref-8">8</a>] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse
       Address Resolution Protocol", STD 38, <a href="./rfc903">RFC 903</a>, June 1984.

   [<a id="ref-9">9</a>] Postel, J., and J. Reynolds, "A Standard for the Transmission of
       IP Datagrams over IEEE 802 Networks", <a href="./rfc1042">RFC 1042</a>, February 1988.

   [<a id="ref-10">10</a>] IEEE, "IEEE Standard for Local and Metropolitan Area Networks:
        Overview and architecture", IEEE Standard 802-1990.

   [<a id="ref-11">11</a>] Bradley, T., Brown, C., and A. Malis, "Inverse Address
        Resolution Protocol", <a href="./rfc2390">RFC 2390</a>, September 1998.

   [<a id="ref-12">12</a>] IEEE, "IEEE Standard for Local and Metropolitan Networks:  Media
        Access Control (MAC) Bridges", IEEE Standard 802.1D-1990.

   [<a id="ref-13">13</a>] ISO/IEC 15802-5 : 1998 (IEEE Standard 802.1G), Remote Media
        Access Control (MAC) Bridging, March 12, 1997.





<span class="grey">Brown &amp; Malis               Standards Track                    [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


   [<a id="ref-14">14</a>] Frame Relay Forum, "Data Compression Over Frame Relay
        Implementation Agreement", FRF.9, January 22, 1996.

   [<a id="ref-15">15</a>] Frame Relay Forum, "Multiprotocol Encapsulation Implementation
        Agreement", FRF.3.1, June 22, 1995.

   [<a id="ref-16">16</a>] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-17">17</a>] Simpson, W., "PPP in Frame Relay", <a href="./rfc1973">RFC 1973</a>, June 1996.

   [<a id="ref-18">18</a>] Frame Relay Forum, "Frame Relay Fragmentation Implementation
        Agreement", FRF.12, December 1997.

   [<a id="ref-19">19</a>] Frame Relay Forum, "Frame Relay PVC Multicast Service and
        Protocol Implementation Agreement", FRF.7, October 21, 1994.

<span class="h2"><a class="selflink" id="section-15" href="#section-15">15</a>.  Authors' Addresses</span>

   Caralyn Brown
   Consultant

   EMail:  cbrown@juno.com


   Andrew Malis
   Ascend Communications, Inc.
   1 Robbins Road
   Westford, MA  01886

   Phone: (978) 952-7414
   EMail:  malis@ascend.com



















<span class="grey">Brown &amp; Malis               Standards Track                    [Page 33]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc2427">RFC 2427</a>             Multiprotocol over Frame Relay       September 1998</span>


<span class="h2"><a class="selflink" id="section-16" href="#section-16">16</a>.  Full Copyright Statement</span>

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























Brown &amp; Malis               Standards Track                    [Page 34]
</pre>