may bea bad choice with asymmetric links R1 R2 R3 R4 R5 R6 R7 router with

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.