1 Internet Networking Spring 2006 Tutorial 7 DVMRP

2 Introduction When we discuss multicast routing protocols 3 issues should be addressed: How hosts can join to a multicast group (usually performed by IGMP). How routers distribute between them information about registered multicast subscriptions. How a router performs routing of a multicast packet.

3 A simple solution Consider the following solution: multicast packets are flooded from a source to all the routers in the AS. Advantages: Simplicity. Disadvantages: Packets are unnecessarily received by all routers. Routers receive duplicate packets. A filtering mechanism should be employed. Each router should remember what packets it already forwarded.

4 Avoiding the need to remember To avoid the need to remember, a packet is accepted only if it arrives on the port that corresponds to the shortest path from S. Can it be done in Distance Vector routing? Symmetric paths are assumed. Duplicate packets are not avoided. A BDC EF An accepted packet Sent but not accepted packet

5 Reverse Path Forwarding To avoid duplications a packet is sent to a neighbor only if it will be accepted. A router sends a packet to a neighbor only if it is on the shortest path from it to the source. The router needs to know which of its neighbors use it as a “ next hop ” to the source. This information is advertised to the router as a regular route report having a cost of more than infinity. This technique is called “ Poison Reverse ”. RPF guarantees that every router receives every packet exactly once.

6 Reverse Path Forwarding A B DC EF

7 Pruning Problem: Flooding still occurs throughout the AS. Solution: Flood & Prune method. The first multicast packet from a source S is propagated to all the network nodes (flooding). When a leaf router (at the specific tree) receives a multicast message and doesn ’ t have group members for it, it sends PRUNE message to its father node. When an intermediate router gets PRUNE messages from all its children then it sends PRUNE message to its father node.

8 Pruning example B before pruning after pruning A DC EF PRUNE A DC EF B B Only C and E are group members.

9 Re-joining the Tree After a period of time the PRUNE effect vanishes and the messages are flooded again. Provides robustness to topology changes. Each PRUNE message has a lifetime value associated with it. The lifetime of a PRUNE message sent to an upstream node must be no more than the minimum of the remaining lifetimes of the PRUNE messages received from the downstream nodes. This is an example of a soft state concept. Send an explicit GRAFT (join) request, which will propagate upwards. A GRAFT message must be acknowledged to ensure the reception of the message. Used only to undo the effect of a PRUNE message.

10 RPF with pruning - summary Advantages: Simplicity Robustness Disadvantages: Packets are flooded to the whole AS on a periodic basis. All routers must keep state on a per-group and per-source basis. In principle each GRANT and PRUNE message for a group must be sent per-source. Does not scale for large multicast network.

11 DVMRP Protocol Protocol for multicast routing inside of ASs that use Distance Vector Routing (e.g. RIP). Defined in RFC Revised by Internet Draft: draft-ietf-idmr-dvmrp-v3-11. May become an RFC in the future. Uses IGMP-like messages for exchanging multicast information between routers. Based on RPF and flood & prune algorithms. Suitable for dense multicast trees. Uses its own routing tables. Allows the multicast routes to be independent of the unicast routes.

12 DVMRP Forwarding Table Represents the local router ’ s understanding of the shortest path delivery tree for each (source, group) pair. Example: a prune message has been sent to the upstream router the router has received a prune message from a downstream router.

13 Tunneling A method for sending datagram between routers separated by gateways that do not support multicast. Acts as a virtual network between two routers. Example: Host on net 1 wants to send a multicast message to a host on net 2 Internet with no support for multicast R1R2 net 1net 2

14 Tunneling Tunneling is done by encapsulating the original multicast datagram with an unicast IP datagram. The source and the destination of the unicast IP packets are the end point of the tunnel. The encapsulation of the datagram is done by the source. The destination address in the unicast header is the address of the next router which supports multicast (it is considered the egress of the tunnel).

15 Tunneling Example: Source:R1 Dest:R2 Protocol: IP in IP Source:S Dest:G Protocol: UDP UDP header and data Internet with no support for multicast R1R2 S member of G

16 Comparison to CBT & PIM CBT and PIM are also used by ASs which employ distance vector routing. CBT defines a core router from which the multicast packets are propagated, hence: There is no need for the routers to keep a state per-source. Only GRAFT messages are used to join a tree. Since there is one tree, only one GRAFT message needs to be sent to accept packets from all possible sources. The flood & prune method is not needed. Suitable for sparse multicast trees. PIM uses a hybrid approach. For sparse multicast trees a CBT-like protocol is used. For dense multicast trees a DVMRP-like protocol is used. PIM is considered the multicast protocol of choice.