rfc2189.txt

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   all groups for which the link is its child.

   ECHO_REPLY messages are unicast or multicast, as appropriate.

4.6.2.  Receiving ECHO_REPLY messages

   An ECHO_REPLY message must be received via a valid parent interface.

   For each group reported in an ECHO_REPLY, the downstream router
   attempts to match the group with one in its forwarding cache for
   which the arrival interface is the group's parent interface. For each
   successful match, the entry is "refreshed". If however, after
   [GROUP_EXPIRE_TIME] seconds a group has not been "refreshed", a
   QUIT_NOTIFICATION is sent upstream, and a FLUSH_TREE message is sent
   downstream, for the group.

   If this router has directly attached members for any of the flushed
   groups, the receipt of an IGMP host membership report for any of
   those groups will prompt this router to rejoin the corresponding
   tree(s).







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4.7.  FLUSH_TREE Processing

   The FLUSH_TREE (flush) message is the mechanism by which a router
   invokes the tearing down of all its downstream branches for a
   particular group. The flush message is multicast to the "all-cbt-
   routers" group when sent over multicast-capable interfaces, and
   unicast otherwise.

4.7.1.  Sending FLUSH_TREE messages

   A FLUSH_TREE message is sent over each downstream (child) interface
   when a router has lost reachability with its parent router for the
   group (detected via ECHO_REQUEST and ECHO_REPLY messages). All group
   state is removed from an interface over which a flush message is
   sent.  A flush can specify a single group, or all groups
   (INADDR_ANY).

4.7.2.  Receiving FLUSH_TREE messages

   A FLUSH_TREE message must be received over the parent interface for
   the specified group, otherwise the message is discarded.

   The flush message must be forwarded over each child interface for the
   specified group.

   Once the flush message has been forwarded, all state for the group is
   removed from the router's forwarding cache.

5.  Non-Member Sending

   Data can be sent to a CBT tree by a sender not attached to the group
   tree.  The sending host originates native multicast data, which is
   promiscuously received by a local router, which must be CBT capable.
   It is assumed the local CBT router knows about the relevant <core,
   group> mapping, and thus can encapsulate (IP-in-IP) the data packet
   and unicast it to the corresponding core router. On arriving at the
   core router, the data packet is decapsulated and disemminated over
   the group tree in the manner already described.

6.  Timers and Default Values

   This section provides a summary of the timers described above,
   together with their recommended default values. Other values may be
   configured; if so, the values used should be consistent across all
   CBT routers attached to the same network.

   o    [HELLO_INTERVAL]: the interval between sending an HELLO message.
        Default: 60 seconds.



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   o    [HELLO_PREFERENCE]: Default: 255.

   o    [HOLDTIME]: generic response interval. Default: 3 seconds.

   o    [MAX_RTX]: default maximum number of retransmissions. Default 3.

   o    [RTX_INTERVAL]: message retransmission time. Default: 5 seconds.

   o    [JOIN_TIMEOUT]: raise exception due to tree join failure.
        Default: 3.5 times [RTX_INTERVAL].

   o    [TRANSIENT_TIMEOUT]: delete (unconfirmed) transient state.
        Default: (1.5*RTX_INTERVAL) seconds.

   o    [CACHE_DEL_TIMER]: remove child interface from forwarding cache.
        Default: (1.5*HOLDTIME) seconds.

   o    [GROUP_EXPIRE_TIME]: time to send a QUIT_NOTIFICATION to our
        non-responding parent.  Default: (1.5*ECHO_INTERVAL).

   o    [ECHO_INTERVAL]: interval between sending ECHO_REQUEST to parent
        routers.  Default: 60 seconds.

   o    [EXPECTED_REPLY_TIME]: consider parent unreachable. Default: 70
        seconds.

7. CBT Packet Formats and Message Types

   CBT control packets are encapsulated in IP. CBT has been assigned IP
   protocol number 7 by IANA [4].

7.1.  CBT Common Control Packet Header

   All CBT control messages have a common fixed length header.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  vers | type  |  addr len     |         checksum              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 1. CBT Common Control Packet Header


   This CBT specification is version 2.





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   CBT packet types are:

   o    type 0: HELLO

   o    type 1: JOIN_REQUEST

   o    type 2: JOIN_ACK

   o    type 3: QUIT_NOTIFICATION

   o    type 4: ECHO_REQUEST

   o    type 5: ECHO_REPLY

   o    type 6: FLUSH_TREE

   o    type 7: Bootstrap Message (optional)

   o    type 8: Candidate Core Advertisement (optional)


   o    Addr Length: address length in bytes of unicast or multicast
        addresses carried in the control packet.

   o    Checksum: the 16-bit one's complement of the one's complement
        sum of the entire CBT control packet.

7.2.  HELLO Packet Format

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    CBT Control Packet Header                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Preference   |  option type  |  option len   |  option value |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2. HELLO Packet Format


   HELLO Packet Field Definitions:

   o    preference: sender's HELLO preference.

   o    option type: the type of option present in the "option value"
        field.  One option type is currently defined: option type 0
        (zero) = BR_HELLO; option value 0 (zero); option length 0
        (zero). This option type is used with HELLO messages sent by a



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        border router (BR) as part of designated BR election (see [5]).

   o    option len: length of the "option value" field in bytes.

   o    option value: variable length field carrying the option value.

7.3.  JOIN_REQUEST Packet Format


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    CBT Control Packet Header                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          group address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          target router                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        originating router                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  option type  |  option len   |        option value           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3. JOIN_REQUEST Packet Format


      JOIN_REQUEST Field Definitions

   o    group address: multicast group address of the group being joined.
        For a "wildcard" join (see [5]), this field contains the value of
        INADDR_ANY.

   o    target router: target (core) router for the group.

   o    originating router: router that originated this JOIN_REQUEST.

   o    option type, option len, option value: see HELLO packet format,
        section 7.2.

7.4.  JOIN_ACK Packet Format

      JOIN_ACK Field Definitions

   o    group address: multicast group address of the group being joined.

   o    target router: router (DR) that originated the corresponding
        JOIN_REQUEST.




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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    CBT Control Packet Header                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          group address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           target router                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  option type  |  option len   |         option value          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4. JOIN_ACK Packet Format
   o    option type, option len, option value: see HELLO packet format,
        section 7.2.

7.5.  QUIT_NOTIFICATION Packet Format


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    CBT Control Packet Header                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          group address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    originating child router                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 5. QUIT_NOTIFICATION Packet Format


      QUIT_NOTIFICATION Field Definitions

   o    group address: multicast group address of the group being joined.

   o    originating child router: address of the router that
        originates the QUIT_NOTIFICATION.













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7.6.  ECHO_REQUEST Packet Format


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    CBT Control Packet Header                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    originating child router                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+