GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1294

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.

Bradley, Brown, Malis [Page 1] RFC 1294 Multiprotocol over Frame Relay January 1992

   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:

Bradley, Brown, Malis [Page 2] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 3] RFC 1294 Multiprotocol over Frame Relay January 1992

 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

Bradley, Brown, Malis [Page 4] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 5] RFC 1294 Multiprotocol over Frame Relay January 1992

 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:

Bradley, Brown, Malis [Page 6] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 7] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 8] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 9] RFC 1294 Multiprotocol over Frame Relay January 1992

         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.

Bradley, Brown, Malis [Page 10] RFC 1294 Multiprotocol over Frame Relay January 1992

        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]

Bradley, Brown, Malis [Page 11] RFC 1294 Multiprotocol over Frame Relay January 1992

 If a station supporting XID receives an XID frame, it shall respond
 with an XID response.  In processing an XID, if the remote maximum
 frame size is smaller than the local maximum, the local system shall
 reduce the maximum size it uses over this DLC to the remotely
 specified value.  Note that this shall be done before generating a
 response XID.
 The following diagram describes the use of XID to specify non-use of
 acknowledged mode multiple frame operation.

Bradley, Brown, Malis [Page 12] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 13] RFC 1294 Multiprotocol over Frame Relay January 1992

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:

Bradley, Brown, Malis [Page 14] RFC 1294 Multiprotocol over Frame Relay January 1992

        +---------------+---------------+
        |         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.

Bradley, Brown, Malis [Page 15] RFC 1294 Multiprotocol over Frame Relay January 1992

                      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

Bradley, Brown, Malis [Page 16] RFC 1294 Multiprotocol over Frame Relay January 1992

 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                  |
       |                       .                       |
       |                       .                       |
       |                       .                       |
       +-----------------------+-----------------------+

Bradley, Brown, Malis [Page 17] RFC 1294 Multiprotocol over Frame Relay January 1992

 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.

Bradley, Brown, Malis [Page 18] RFC 1294 Multiprotocol over Frame Relay January 1992

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

Bradley, Brown, Malis [Page 19] RFC 1294 Multiprotocol over Frame Relay January 1992

 will be invalid.  The address found in the frame header will,
 however, be correct. Though it does violate the purity of layering,
 Frame Relay may use the address in the header as the sender hardware
 address.  It should also be noted that the target hardware address,
 in both ARP request and reply, will also be invalid.  This should not
 cause problems since ARP does not rely on these fields and in fact,
 an implementation may zero fill or ignore the target hardware address
 field entirely.
 As an example of how this address replacement scheme may work, refer
 to figure 1.  If station A (protocol address pA) wished to resolve
 the address of station B (protocol address pB), it would format an
 ARP request with the following values:
       ARP request from A
         ar$op     1 (request)
         ar$sha    unknown
         ar$spa    pA
         ar$tha    undefined
         ar$tpa    pB
 Because station A will not have a source address associated with it,
 the source hardware address field is not valid.  Therefore, when the
 ARP packet is received, it must extract the correct address from the
 Frame Relay header and place it in the source hardware address field.
 This way, the ARP request from A will become:
       ARP request from A as modified by B
         ar$op     1 (request)
         ar$sha    0x1061 (DLCI 70) from Frame Relay header
         ar$spa    pA
         ar$tha    undefined
         ar$tpa    pB
 Station B's ARP will then be able to store station A's protocol
 address and Q.922 address association correctly.  Next, station B
 will form a reply message.  Many implementations simply place the
 source addresses from the ARP request into the target addresses and
 then fills in the source addresses with its addresses.  In this case,
 the ARP response would be:
       ARP response from B
         ar$op     2 (response)
         ar$sha    unknown
         ar$spa    pB
         ar$tha    0x1061 (DLCI 70)
         ar$tpa    pA

Bradley, Brown, Malis [Page 20] RFC 1294 Multiprotocol over Frame Relay January 1992

 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.

Bradley, Brown, Malis [Page 21] RFC 1294 Multiprotocol over Frame Relay January 1992

 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.

Bradley, Brown, Malis [Page 22] RFC 1294 Multiprotocol over Frame Relay January 1992

       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

Bradley, Brown, Malis [Page 23] RFC 1294 Multiprotocol over Frame Relay January 1992

 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

Bradley, Brown, Malis [Page 24] RFC 1294 Multiprotocol over Frame Relay January 1992

 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:

Bradley, Brown, Malis [Page 25] RFC 1294 Multiprotocol over Frame Relay January 1992

       +-----------------------+-----------------------+
       |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.

Bradley, Brown, Malis [Page 26] RFC 1294 Multiprotocol over Frame Relay January 1992

 [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

Bradley, Brown, Malis [Page 27] RFC 1294 Multiprotocol over Frame Relay January 1992

         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

Bradley, Brown, Malis [Page 28]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1294.txt · Last modified: 1992/01/16 21:15 (external edit)