may bea bad choice with asymmetric links R1 R2 R3 R4 R5 R6 R7 router with attached group member router with no attached group member datagram will be forwarded LEGEND S: source datagram will not be forwarded
Reverse Path Forwarding: pruning ❒ forwarding tree contains subtrees with no mcast group members ❍ no need to forward datagrams down subtree ❍ “prune” msgs sent upstream by router with no downstream group members R1 R2 R3 R4 R5 R6 R7 router with attached group member router with no attached group member prunemessage LEGEND S: source links with multicast forwarding P P P
Shared-Tree: Steiner Tree ❒ Steiner Tree: minimumcost treeconnecting all routers with attached group members ❒ problem is NP-complete ❒ excellent heuristics exists ❒ not used in practice: ❍ computational complexity ❍ information about entirenetwork needed ❍ monolithic: rerun whenever a router needs to join/leave
Center-based trees ❒ singledelivery treeshared by all ❒ onerouter identified as “center” of tree ❒ to join: ❍ edgerouter sends unicast join-msg addressed to center router ❍ join-msg “processed” by intermediaterouters and forwarded towards center ❍ join-msg either hits existing treebranch for this center, or arrives at center ❍ path taken by join-msg becomes new branch of tree for this router
Center-based trees: an example Suppose R6 chosen as center: R1 R2 R3 R4 R5 R6 R7 router with attached group member router with no attached group member path order in which join messages generated LEGEND 2 1 3 1
Internet Multicasting Routing: DVMRP ❒ DVMRP: distancevector multicast routing protocol, RFC1075 ❒ flood and prune: reversepath forwarding, source-based tree ❍ RPF treebased on DVMRP’s own routing tables constructed by communicating DVMRP routers ❍ no assumptions about underlying unicast ❍ initial datagram to mcast group flooded everywhere via RPF ❍ routers not wanting group: send upstream prunemsgs
DVMRP: continued… ❒ soft state: DVMRP router periodically (1 min.) “forgets” branches arepruned: ❍ mcast data again flows down unpruned branch ❍ downstream router: repruneor elsecontinueto receive data ❒ routers can quickly regraft to tree ❍ following IGMP join at leaf ❒ odds and ends ❍ commonly implemented in commercial routers ❍ Mbonerouting done using DVMRP
Tunneling Q: How to connect “islands” of multicast routers in a “sea” of unicast routers? mcast datagram encapsulated inside “normal” (non-multicast-addressed) datagram normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router receiving mcast router unencapsulates to get mcast datagram physical topology logical topology
PIM: Protocol Independent Multicast ❒ not dependent on any specific underlying unicast routing algorithm (works with all) ❒ two different multicast distribution scenarios : Dense : group members densely packed, in “close” proximity.
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- Fall '09
- routing protocol, twork Laye