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Distance Vector Multicast Routing Protocol

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• A Note on Multicast Routing on the Internet
  • The Core Based Tree method was the second multicast routing method proposed for the Internet

  • The first multicast routing method for the Internet was the Distance Vector Multicast Routing Protocol (DVMRP) by Steve Deering in 1993.

    It is probably still the most widely used multicast routing (although PIM - also by Steve Deering as a CISCO employee - is available, you have to pay big money for it, while DVMRP is free).

  • DVMRP is based on Reverse Path Broadcasting

    The Reverse Path Broadcasting constructs a broadcast tree for every node in the network

    We obviously do NOT want to send to ALL nodes

  • In order to send a message to a subset of (multicast) receivers , Steve Deering added a feature to prune off the branches of the broadcast tree that do not serve any multicast members.

  • It's very straightforward and I will illustrated the method with the follows set of figures.

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• The DVMRP Prune Table
  • Recall that the RPB method constructs a broadcast tree for EVERY (source) node

    The following figure depicts ONLY the tree rooted at A :

    Other entries (rows) in the Broadcast Routing Table will store entries for the other broadcast trees.

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  • The DVMRP method adds a new Prune Table for EVERY every multicast group

    Obviously, it will require to much memory - the Prune Tables are created when a multicast group exists

    More specifically:

    A router that receives a multicast message for a group G must create a prune table for the multicast group G

    So a prune table only exists when there is actual communication (packets) to the members of that group.

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  • The Prune Tables contains a subset of the list of nodes in the RPB table.

    Nodes appearing in the Prune Tables are edges of the RPB tree that have been pruned away

    The pruning away of edges will prevent the "broadcast message" from reaching all nodes. By pruning away the correct edges, the "broadcast message" will only reach nodes that have multicast receivers

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  • Example
    • Nodes B and D are multicast receivers.

    • Nodes C and E are NOT multicast receivers.

    The following picture shows how the Prune Tables can prevent a "broadcast message" from A from reaching nodes C and E:

  • The question is: how do you get the correct prune entries ?
    Simple, you tell your parent node to prune you away if all your children nodes are not multicast receivers:

    The following figure depicts a full tree with only one multicast receiver:

    1. At first, the leaf nodes sends prune messages to their parent nodes when they are not multicast receivers

    2. When all children of an internal node are prune, the internal node becomes a leaf node and will itself send a prune message to its parent if it is not a multicast receiver

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• The DVMRP Multicast Routing Method

  In the example, I will use node A as the multicast source

  This is the initial situation when there are no multicast receivers !!!

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  • Suppose some hosts served by routers B and D join multicast group G:

    Notice that there is NO WAY for a source to find out who (which node) and where are the multicast receivers

    This is also true for multicast sender served by node A

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  • Now some host served by A transmits a multicast message to group G:

    The multicast message will be forwarded using the Reverse Path Broadcast table (with source = A) as follows:

    Result of a multicast message from A:

    All routers receive the multicast to multicast group G and will create a prune table for G Initially, the prune table is empty (i.e., no links pruned).

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  • Now the pruning begins...
    1. The leaf router C does not serve any multicast member of G

    2. A router that is not interested in the multicast session sends a Non-Membership Report (NMR) to the upstream router, i.e., to the higher-level router in the multicast tree

    3. The NMR report is also known as a prune(G) message

    4. In response to the prune message, node E enters the sender ( C ) in its prune table for group G

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  • The pruning continues...
    1. Now router E has become a leaf node in the multicast tree
       (it can tell from the fact that the list of nodes in the prune table is equal to the list of nodes in the RPB table entry)

    2. Therefore, router E does not serve any multicast receiver of group G.

    3. So router E sends a prune(G) message to its upstream node D:

    4. In response to the prune message, node D enters the sender ( E ) in its prune table for group G

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  • The pruning ends:
    • There are no further prune messages because all leaf routers serve multicast receivers from group G.

    • The resulting multicast tree will forward messages on a "pruned" reverse path broadcast tree.

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• Joining a multicast group:
  • If a host joins a multicast group and its router is part of the multicast tree, then the join operation is easy.

  • But if the router was not part of the multicast tree , then the router may not have information on the senders of the multicast group.

    Without knowning who the senders of the multicast group are, the router cannot know where to send "craft" message

  • Let us handle the simple case first:
    • Suppose some host served by node C joins multicast group G, AND

    • Node C knows that A is a sender of multicast group G.

    • (This may happen if router caches multicast sender IDs)

    Then C can send a "craft" message to its upstream node to A to "unprune" the link:

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  • The most common case is where node C does NOT know that A (or any othe node) is a sender of multicast group G.

  • DVMRP uses a timeout mechanism in the prune tables:

    The entries in the prune table has a time out value When the time out expires, the entries in the prune table are deleted This will RE-establish the initial situation - i.e., the tree used is the Reverse Path Broadcast Tree The newly joined receivers will now get the multicast messages The pruning operation is restarted again, and prune away the unnecessary branches. The branches serving the newly joins receivers will ofcourse NOT pruned away !