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Multicast Forwarding

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• Overview
  • I will split the discussion on Multicasting into:
    1. Multicast forwarding - what information do you need to forward multicast packet to their destinations

    2. Multicast routing (route construction) - how do you obtain/build the multicast forwarding information.

  • From the previous discussion, it is clear that the best strategy to forward multicast packets to their destinations is by means of:
    • a tree structure that spans the sender and all receivers

  • Problem Statement: (by means of an example)

    Consider the following network with network with 3 multicast receivers in multicast group G:

  What information do you keep in the routers so that IP multicast messages can be forwarded to all receivers in the multicast group ?

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• Approaches of multicast forwarding:
  • Single tree approach - the routers will construct one multicast forwarding tree structure for one multicast group.

  • Per source tree approach - the routers will construct one multicast forwarding tree structure for each multicast sender

• Characteristics of single tree approach:
  • Use less memory storage for multicast routes
  • Minimize the total link cost

  • Cannot minimize the delay from source to destinations

• Characteristics of the per source tree approach:
  • Use more memory storage for multicast routes

  • Minimize the delay from source to destinations

• Representative methods of each of the approaches:
  • Single tree:
    1. Core-base tree
    2. PIM - dense operation mode

  • Per source tree:
    1. DVMRP (Distance Vector Multicast Routing Protocol)
    2. PIM - sparse operation mode

   

   

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  The Single Tree Approach

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• Single Tree Multicast Forwarding: Core Based Tree

  • (The PIM multucast routing protocol operating in the dense mode works in a similar manner...)

  • Special designated node called core node.

    (In PIM, this node is called the rendez-vous node).)

  • The core node is well-known to all routers throughout the Internet

  • A multicast tree always include the core node

    but... that does not imply that the core node must always have multicast receivers

    As you will see, the core is used only for multicast forwarding

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• Single Tree Multicast Forwarding Table
  • Multicast enabled routers have (in addition to the unicast routing table) multicast forwarding information in a multicast forwarding table

  • The information stored in the multicast forwarding table in the single tree approach consists of:

    Multicast Group ID | Next hop routers (a list !) --------------------+------------------------------- MC group 1 | ....... MC group 2 | ....... and so on | .......

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• Multicast Forwarding in the Single Tree approach

  • The multicast forwarding algorithm in the single tree approach is as follows:

    Suppose the router receives a multicast packet for multicast group G First, the router looks up the group G in its multicast forwarding table. If the multicast group G is found in the multicast forwarding table: then the node forwards the multicast packet to all the routers given in the "next hop routers" list but excluding the router from which the multicast packet eas received Otherwise (the multicast group G is not found): then the node uses the IP routing table and forwards the multicast packet towards the CORE node

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  • Example 1: forwarding a multicast packet starting from a node ON the multicast tree

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  • Example 2: forwarding a multicast packet starting from a node OUTSIDE the multicast tree

   

   

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  The Per Source (Multiple) Tree Approach

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• Per Source Tree Multicast Forwarding Table
  • Per Source Tree Multicast Forwarding constructs one multicast tree for each multicast source

  • In most IP multicast applications (although the number receivers can be large) the number of senders are usually limited

    In fact, in many applications, there is just one sender, e.g., tele-conferencing (only the conferencing site transmits video and audio)

  • The first implementation of multicast forwarding on the Internet was DVMRP (Distance Vector Multicast Routing protocol) and it was a per-source multicast forwarding method.

  • The information stored in the multicast forwarding table in the per-source tree approach consists of:

    Source IP addr | Multicast Group ID | Next hop routers (a list !) ---------------+--------------------+----------------------------- IP1 | MC group 1 | ....... IP2 | MC group 1 | ....... IP3 | MC group 1 | ....... IP8 | MC group 2 | ....... IP9 | MC group 2 | ....... .....

    • There are 2 multicast groups currently in the table: MC group 1 and MC group 2

    • There are 3 senders in MC group 1 and 2 senders in MC group 2

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• Per Source Multicast Tree Forwarding Algorithm

  • The multicast forwarding algorithm in the per source tree approach is as follows:

    Suppose the router receives a multicast packet for multicast group G from the source S It looks up the entry for (S, G) in its multicast forwarding table. If the entry for (S, G) is found in the table: the node forwards the multicast packet to all nodes in the next hop list. Otherwise ((S, G) is NOT found ), the node drops the packet Note: This is very similar to unicast (single destination) routing technique

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  • Example 1:

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  • Example 2:

    NOTE: a different tree is used for each source node !!!

    NOTE: every source MUST have its own multicast tree

    Therefore, a source CANNOT lie outside a multicast tree !!!