rfc1490.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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


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

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

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

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

4.2.  Bridged Frames

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

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



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


           PID Values for OUI 0x00-80-C2

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

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

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

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












Bradley, Brown & Malis                                          [Page 8]

RFC 1490             Multiprotocol over Frame Relay            July 1993


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



























Bradley, Brown & Malis                                          [Page 9]

RFC 1490             Multiprotocol over Frame Relay            July 1993


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


























Bradley, Brown & Malis                                         [Page 10]

RFC 1490             Multiprotocol over Frame Relay            July 1993


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


























Bradley, Brown & Malis                                         [Page 11]

RFC 1490             Multiprotocol over Frame Relay            July 1993


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

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

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