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Multicast Route construction in the Core Base Tree (CBT) method

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• Overview of the Core Base Tree multicast routing method
  • Some node in the network is designated as the core node of a multicast group:

    The core node need not be a MEMBER of the multicast group A node can be the core node of many different multicast groups The identity of the core node of each multicast group is known to all nodes

  • Any node in the network can send multicast messages to the multicast group.

    (So a node that is not a member of the multicast group can send messages to the nodes in a multicast group)

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• Joining a multicast group

  • How a node join a multicast group:

    A host computer wants to join a multicast group G, it will send a Join control message (called Internet Group Management Protocol (IGMP) Join message) This message is forwarded to the router serving the host computer How this IGMP Join(G) will be processed is discussed next...

  • Processing IGMP Join message:

    Each router has (in addition to the "ordinary" routing table) a multicast routing (forwarding) table (see: click here) When a router receives a IGMP join(G) message and there is already an existing forwarding entry for multicast group G in its multicast routing (forwarding) table , then: The router adds the interface on which the IGMP join(G) message was received to the multicast route entry When a router receives a IGMP join(G) message and there is NO forwarding entry for multicast group G in its multicast routing (forwarding) table , then: The router adds the interface on which the IGMP join(G) message was received to the multicast route entry Add the next hop router for the core node to the multicast route entry. Forward a Craft message towards the core node of the multicast group.

  • Processing a Craft message:

    A router that receives a craft message for a multicast group G and there is already an existing forwarding entry for multicast group G in its multicast routing (forwarding) table , then:: add the ID of the sending router (this is the neighboring node who sent the craft message - it is NOT the original source) to the multicast entry of group G. A router that receives a craft message for a multicast group G and there is NO forwarding entry for multicast group G in its multicast routing (forwarding) table , then:: Create a new entry in the multicast table for group G and add the ID of the sending router (this is the neighboring node who sent the craft message - it is NOT the original source) to the multicast entry of group G. Add the next hop router for the core node to the multicast route entry. Forward the craft message towards the core node of multicast group G

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• Example:
  • Initial state...
    

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  • A host computer at C joins multicast group G:
    
    1. C creates a new entry for multicast group G
    2. C adds the interface #0 to the entry for multicast group G
    3. C adds the next hop node (B) to core node (A) to the entry for multicast group G
    4. C forwards a Craft(G) message towards code node (A)

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    5. B receives Craft(G) message
    6. B creates a new entry for multicast group G
    7. B adds the sending node (C) to the entry for multicast group G
    8. B adds the next hop node (A) to core node (A) to the entry for multicast group G
    9. B forwards a Craft(G) message towards code node (A)

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    10. The core node A receives Craft(G) message (done - no processing is needed)

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    See how multicast packets are routed:

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    1. The sequence of blue arrows shows how a multicast message from a host at node E will be forwarded to the members of the multicast group (a host at C)

    2. The sequence of red arrows shows how a multicast message from a host at node B will be forwarded to the members of the multicast group (a host at C)

       Notice that there are some extraneous messages sent towards the core node

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  • Continue with example... a host at D joins multicast group G:
    

    (The processing is similar to the previous example)

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  • And finally, a host at F joins multicast group G:
    
    • Note that since router F is part of the core base tree, after processing the join/craft message, it does not need to forward the join/craft message toward the core

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• Some Comments
  • Difference between Multicast and Unicast Routing
    • In unicast, there is always a route in the unicast routing table for any (valid) destination

    • In multicast, a multicast route to a receiver is only created (i.e., information is added to enable routing) when that receiver joins a multicast group.

      I.e., the multicast routing tables in routes are being manipulated by multicast receivers !!!

  • Requirements to perform Multicasting
    • In order to provide multicast capability, a multicast enabled router must be used.

      Most routers on the Internet today are multicast capable

    • Such a multicast capable router can process IGMP (Internet Group Management Protocol) messages

    • A host that wants to join a multicast group will announce their multicast group memberships to the multicast router using a special IGMP-JOIN protocol message:

      

      The multicast enable router receives the IGMP join message and forward special route update messages to other nodes to extend the multicast forwarding tree to accommodate new receivers on its LAN.