rfc1483.txt

<VC++网络游戏建摸与实现>源代码

   This is compatible with RFC 1042 [7].  Any changes in the header
   format specified in RFC 1042 should be followed by this memo.

4.2.  LLC Encapsulation for Bridged Protocols

   In LLC Encapsulation bridged PDUs are encapsulated by identifying the
   type of the bridged media in the SNAP header.  As with routed non-ISO
   protocols, the presence of the SNAP header is indicated by the LLC
   header value 0xAA-AA-03.  With bridged protocols the OUI value in the
   SNAP header is the 802.1 organization code 0x00-80-C2 and the actual
   type of the bridged media is specified by the two octet PID.
   Additionally, the PID indicates whether the original Frame Check
   Sequence (FCS) is preserved within the bridged PDU.  The media type
   (PID) values that can be used in ATM encapsulation are listed in
   Appendix B.

   The AAL5 CPCS-PDU Payload field carrying a bridged PDU shall,
   therefore, have one of the following formats.  Padding is added after
   the PID field if necessary in order to align the user information
   field of the bridged PDU at a four octet boundary.

















Heinanen                                                        [Page 6]

RFC 1483                Multiprotocol over AAL5                July 1993


               Payload Format for Bridged Ethernet/802.3 PDUs
               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |    PID 0x00-01 or 0x00-07     |
               +-------------------------------+
               |         PAD 0x00-00           |
               +-------------------------------+
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               |  LAN FCS (if PID is 0x00-01)  |
               +-------------------------------+


                Payload Format for Bridged 802.4 PDUs
               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |    PID 0x00-02 or 0x00-08     |
               +-------------------------------+
               |        PAD 0x00-00-00         |
               +-------------------------------+
               |    Frame Control (1 octet)    |
               +-------------------------------+
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               |  LAN FCS (if PID is 0x00-02)  |
               +-------------------------------+











Heinanen                                                        [Page 7]

RFC 1483                Multiprotocol over AAL5                July 1993


                Payload Format for Bridged 802.5 PDUs
               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |    PID 0x00-03 or 0x00-09     |
               +-------------------------------+
               |        PAD 0x00-00-XX         |
               +-------------------------------+
               |    Frame Control (1 octet)    |
               +-------------------------------+
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               |  LAN FCS (if PID is 0x00-03)  |
               +-------------------------------+

   Note that the 802.5 Access Control (AC) field has no significance
   outside the local 802.5 subnetwork.  It can thus be regarded as the
   last octet of the three octet PAD field, which can be set to any
   value (XX).

                Payload Format for Bridged FDDI PDUs
               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |    PID 0x00-04 or 0x00-0A     |
               +-------------------------------+
               |        PAD 0x00-00-00         |
               +-------------------------------+
               |    Frame Control (1 octet)    |
               +-------------------------------+
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               |  LAN FCS (if PID is 0x00-04)  |
               +-------------------------------+





Heinanen                                                        [Page 8]

RFC 1483                Multiprotocol over AAL5                July 1993


                Payload Format for Bridged 802.6 PDUs
               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |         PID 0x00-0B           |
               +---------------+---------------+ ------
               |   Reserved    |     BEtag     |  Common
               +---------------+---------------+  PDU
               |            BAsize             |  Header
               +-------------------------------+ -------
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               |                               |
               |      Common PDU Trailer       |
               |                               |
               +-------------------------------+

   Note that in bridged 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 (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.

   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.

   An ingress 802.6 bridge can abort an AAL5 CPCS-PDU by setting its
   Length field to zero.  If the egress bridge has already begun
   transmitting segments of the PDU to an 802.6 subnetwork and then



Heinanen                                                        [Page 9]

RFC 1483                Multiprotocol over AAL5                July 1993


   notices that the AAL5 CPCS-PDU has been aborted, it may immediately
   generate an EOM cell that causes the 802.6 PDU to be rejected at the
   receiving bridge.  Such an EOM cell could, for example, contain an
   invalid value in the Length field of the Common PDU Trailer.

               +-------------------------------+
               |       LLC  0xAA-AA-03         |
               +-------------------------------+
               |        OUI 0x00-80-C2         |
               +-------------------------------+
               |         PID 0x00-0E           |
               +-------------------------------+
               |                               |
               |      BPDU as defined by       |
               |     802.1(d) or 802.1(g)      |
               |                               |
               +-------------------------------+

5.  VC Based Multiplexing

   In VC Based Multiplexing, the carried network interconnect protocol
   is identified implicitly by the VC connecting the two ATM stations,
   i.e.  each protocol must be carried over a separate VC.  There is
   therefore no need to include explicit multiplexing information in the
   Payload of the AAL5 CPCS-PDU.  This results in minimal bandwidth and
   processing overhead.

   As indicated above, the carried protocol can be either manually
   configured or negotiated dynamically during call establishment using
   signalling procedures.  The signalling details will be defined later
   in other RFCs when the relevant standards have become available.


5.1.  VC Based Multiplexing of Routed Protocols

   PDUs of routed protocols shall be carried as such in the Payload of
   the AAL5 CPCS-PDU.  The format of the AAL5 CPCS-PDU Payload field
   thus becomes:

               Payload Format for Routed PDUs
               +-------------------------------+
               |             .                 |
               |         Carried PDU           |
               |    (up to 2^16 - 1 octets)    |
               |             .                 |
               |             .                 |
               +-------------------------------+




Heinanen                                                       [Page 10]

RFC 1483                Multiprotocol over AAL5                July 1993


5.2.  VC Based Multiplexing of Bridged Protocols

   PDUs of bridged protocols shall be carried in the Payload of the AAL5
   CPCS-PDU exactly as described in section 4.2 except that only the
   fields after the PID field are included.  The AAL5 CPCS-PDU Payload
   field carrying a bridged PDU shall, therefore, have one of the
   following formats.

                Payload Format for Bridged Ethernet/802.3 PDUs
               +-------------------------------+
               |         PAD 0x00-00           |
               +-------------------------------+
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+
               | LAN FCS (VC dependent option) |
               +-------------------------------+


                Payload Format for Bridged 802.4/802.5/FDDI PDUs
               +-------------------------------+
               | PAD 0x00-00-00 or 0x00-00-XX  |
               +-------------------------------+
               |    Frame Control (1 octet)    |
               +-------------------------------+
               |    MAC destination address    |
               +-------------------------------+
               |                               |
               |   (remainder of MAC frame)    |
               |                               |
               +-------------------------------+