rfc2878.txt

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   Implementations SHOULD implement a backward compatibility mode.

4.2.  Bridged LAN Traffic (IEEE 802 Untagged Frame)

   For Bridging LAN traffic, the format of the frame on the line is
   shown below. This format is used if the traffic does not include VLAN
   ID and priority.

   The fields are transmitted from left to right.







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RFC 2878          PPP Bridging Control Protocol (BCP)          July 2000


   802.3 Frame format (IEEE 802 Un-tagged Frame)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+
     |   HDLC FLAG   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Address and Control      |      0x00     |      0x31     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |F|0|Z|0| Pads  |    MAC Type   |      Destination MAC Address  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Destination MAC Address                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Source MAC Address                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Source MAC Address        |      Length/Type              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               LLC data       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   LAN FCS (optional)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                potential line protocol pad                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Frame FCS            |   HDLC FLAG   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


























Higashiyama & Baker         Standards Track                    [Page 13]

RFC 2878          PPP Bridging Control Protocol (BCP)          July 2000


   802.4/802.5/FDDI Frame format (IEEE 802 Un-tagged Frame)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+
     |   HDLC FLAG   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Address and Control      |      0x00     |      0x31     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |F|0|Z|0| Pads  |    MAC Type   |   Pad Byte    | Frame Control |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Destination MAC Address                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Destination MAC Address   |  Source MAC Address           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Source MAC Address                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               LLC data       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   LAN FCS (optional)                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              optional Data Link Layer padding                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Frame FCS            |   HDLC FLAG   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Address and Control

      As defined by the framing in use.

   PPP Protocol

      0x0031 for PPP Bridging

   Flags

      bit F:  Set if the LAN FCS Field is present
      bit Z:  Set if IEEE 802.3 Pad must be zero filled to minimum size
      bit 0:  reserved, must be zero

   Pads

      Any PPP frame may have padding inserted in the "Optional Data Link
      Layer Padding" field.  This number tells the receiving system how
      many pad octets to strip off.






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RFC 2878          PPP Bridging Control Protocol (BCP)          July 2000


   MAC Type

      Up-to-date values of the MAC Type field are specified in the most
      recent "Assigned Numbers" RFC [4].  Current values are assigned as
      follows:

          0: reserved
          1: IEEE 802.3/Ethernet  with canonical addresses
          2: IEEE 802.4           with canonical addresses
          3: IEEE 802.5           with non-canonical addresses
          4: FDDI                 with non-canonical addresses
       5-10: reserved
         11: IEEE 802.5           with canonical addresses
         12: FDDI                 with canonical addresses

      "Canonical" is the address format defined as standard address
      representation by the IEEE.  In this format, the bit within each
      byte that is to be transmitted first on a LAN is represented as
      the least significant bit.  In contrast, in non-canonical form,
      the bit within each byte that is to be transmitted first is
      represented as the most-significant bit.  Many LAN interface
      implementations use non-canonical form.  In both formats, bytes
      are represented in the order of transmission.

      If an implementation supports a MAC Type that is the higher-
      numbered format of that MAC Type, then it MUST also support the
      lower-numbered format of that MAC Type.  For example, if an
      implementation supports FDDI with canonical address format, then
      it MUST also support FDDI with non-canonical address format.  The
      purpose of this requirement is to provide backward compatibility
      with earlier versions of this specification.

      A system MUST NOT transmit a MAC Type numbered higher than 4
      unless it has received from its peer a MAC-Support Configuration
      Option indicating that the peer is willing to receive frames of
      that MAC Type.

   Frame Control

      On 802.4, 802.5, and FDDI LANs, there are a few octets preceding
      the Destination MAC Address, one of which is protected by the FCS.

      The MAC Type of the frame determines the contents of the Frame
      Control field.  A pad octet is present to provide 32-bit packet
      alignment.






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RFC 2878          PPP Bridging Control Protocol (BCP)          July 2000


   Destination MAC Address

      As defined by the IEEE.  The MAC Type field defines the bit
      ordering.

   Source MAC Address

      As defined by the IEEE.  The MAC Type field defines the bit
      ordering.

   LLC data

      This is the remainder of the MAC frame which is (or would be were
      it present) protected by the LAN FCS.

      For example, the 802.5 Access Control field, and Status Trailer
      are not meaningful to transmit to another ring, and are omitted.

   LAN FCS

      If present, this is the LAN FCS which was calculated by (or which
      appears to have been calculated by) the originating station.  If
      the LAN FCS flag is not set, then this field is not present, and
      the PDU is four octets shorter.

   Optional Data Link Layer Padding

      Any PPP frame may have padding inserted between the Information
      field and the Frame FCS.  The Pads field contains the length of
      this padding, which may not exceed 15 octets.

      The PPP LCP Extensions [5] specify a self-describing pad.
      Implementations are encouraged to set the Pads field to zero, and
      use the self-describing pad instead.

   Frame FCS

      Mentioned primarily for clarity.  The FCS used on the PPP link is
      separate from and unrelated to the LAN FCS.

4.3.  Bridged LAN Traffic in IEEE 802 Tagged Frame

   To connect two or more Virtual LAN segments, the frame MUST include
   its VLAN ID and priority. An IEEE 802 Tagged Frame may be used if the
   IEEE-802-Tagged-Frame Option is accepted by the peer. The format of
   the frame on the line is shown below.

   The fields are transmitted from left to right.



Higashiyama & Baker         Standards Track                    [Page 16]

RFC 2878          PPP Bridging Control Protocol (BCP)          July 2000


   802.3 Frame format (IEEE 802 Tagged Frame)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+
      |   HDLC FLAG   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Address and Control      |      0x00     |      0x31     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|0|Z|0| Pads  |    MAC Type   |      Destination MAC Address  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Destination MAC Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Source MAC Address                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Source MAC Address        |     0x81       |     0x00     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Pri  |C| VLAN ID               |      Length/Type              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               LLC data       ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   LAN FCS (optional)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                potential line protocol pad                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Frame FCS            |   HDLC FLAG   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
























Higashiyama & Baker         Standards Track                    [Page 17]

RFC 2878          PPP Bridging Control Protocol (BCP)          July 2000


   802.4/802.5/FDDI Frame format (IEEE 802 Tagged Frame)

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+
     |   HDLC FLAG   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Address and Control      |      0x00     |      0x31     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |F|0|Z|0| Pads  |    MAC Type   |   Pad Byte    | Frame Control |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Destination MAC Address                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Destination MAC Address   |  Source MAC Address           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Source MAC Address                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   SNAP-encoded TPID                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   SNAP-encoded TPID                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Pri  |C| VLAN ID               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               LLC data       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   LAN FCS (optional)                          |