📄 rfc2684.txt
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In LLC Encapsulation, bridged PDUs are encapsulated by identifying
the type of the bridged media in the SNAP header. The presence of
the SNAP header MUST be indicated by the LLC header value 0xAA-AA-03.
The OUI value in the SNAP header MUST be the 802.1 organization code
0x00-80-C2. The type of the bridged media MUST be specified by the
two octet PID. The PID MUST also indicate whether the original Frame
Check Sequence (FCS) is preserved within the bridged PDU. Appendix B
provides a list of media type (PID) values that can be used in ATM
encapsulation.
Grossman & Heinanen Standards Track [Page 6]
RFC 2684 Multiprotocol Over AALS September 1999
The AAL5 CPCS-PDU Payload field carrying a bridged PDU MUST have one
of the following formats. The necessary number of padding octets
MUST be added after the PID field in order to align the
Ethernet/802.3 LLC Data field, 802.4 Data Unit field, 802.5 Info
field, FDDI Info field or 802.6 Info field (respectively) of the
bridged PDU to begin at a four octet boundary. The bit ordering of
the MAC address MUST be the same as it would be on the LAN or MAN
(e.g., in canoncial form for bridged Ethernet/IEEE 802.3 PDUs, but in
802.5/FDDI format for bridged 802.5 PDUs).
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) |
+-------------------------------+
The Ethernet/802.3 physical layer requires padding of frames to a
minimum size. A bridge that uses uses the Bridged Ethernet/802.3
encapsulation format with the preserved LAN FCS MUST include padding.
A bridge that uses the Bridged Ethernet/802.3 encapsulation format
without the preserved LAN FCS MAY either include padding, or omit it.
When a bridge receives a frame in this format without the LAN FCS, it
MUST be able to insert the necessary padding (if none is already
present) before forwarding to an Ethernet/802.3 subnetwork.
Grossman & Heinanen Standards Track [Page 7]
RFC 2684 Multiprotocol Over AALS September 1999
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) |
+-------------------------------+
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) |
+-------------------------------+
Since the 802.5 Access Control (AC) field has no significance outside
the local 802.5 subnetwork, it is treated by this encapsulation as
the last octet of the three octet PAD field. It MAY be set to any
value by the sending bridge and MUST be ignored by the receiving
bridge.
Grossman & Heinanen Standards Track [Page 8]
RFC 2684 Multiprotocol Over AALS September 1999
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) |
+-------------------------------+
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 |
| |
+-------------------------------+
In bridged 802.6 PDUs, the presence of a CRC-32 is indicated by the
CIB bit in the header of the MAC frame. Therefore, the same PID
value is used regardless of the presence or absence of the CRC-32 in
the PDU.
Grossman & Heinanen Standards Track [Page 9]
RFC 2684 Multiprotocol Over AALS September 1999
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
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.
Payload Format for BPDUs
+-------------------------------+
| LLC 0xAA-AA-03 |
+-------------------------------+
| OUI 0x00-80-C2 |
+-------------------------------+
| PID 0x00-0E |
+-------------------------------+
| |
| BPDU as defined by |
| 802.1(d) or 802.1(g) |
| |
+-------------------------------+
6. VC Multiplexing
VC Multiplexing creates a binding between an ATM VC and the type of
the network protocol carried on that VC. Thus, there is no need for
protocol identification information to be carried in the payload of
each AAL5 CPCS-PDU. This reduces payload overhead and can reduce
per-packet processing. VC multiplexing can improve efficiency by
reducing the number of cells needed to carry PDUs of certain lengths.
Grossman & Heinanen Standards Track [Page 10]
RFC 2684 Multiprotocol Over AALS September 1999
For ATM PVCs, the type of the protocol to be carried over each PVC
MUST be determined by configuration. For ATM SVCs, the negotiations
specified in RFC 1755 [5] MUST be used.
6.1. VC Multiplexing of Routed Protocols
PDUs of routed protocols MUST be carried as the only content of 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) |
| . |
| . |
+-------------------------------+
6.2. VC Multiplexing of Bridged Protocols
PDUs of bridged protocols MUST be carried in the Payload of the AAL5
CPCS-PDU exactly as described in section 5.2, except that only the
fields after the PID field MUST be included. The AAL5 CPCS-PDU
Payload field carrying a bridged PDU MUST, 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) |
+-------------------------------+
Grossman & Heinanen Standards Track [Page 11]
RFC 2684 Multiprotocol Over AALS September 1999
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) |
| |
+-------------------------------+
| LAN FCS (VC dependent option) |
+-------------------------------+
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 in case of 802.5 can
be set to any value (XX).
Payload Format for Bridged 802.6 PDUs
+---------------+---------------+ -------
| Reserved | BEtag | Common
+---------------+---------------+ PDU
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