1 Spring Semester 2009, Dept. of Computer Science, Technion Internet Networking recitation #7 DVMRP

2 Internet Networking Introduction  When we discuss multicast routing protocols 3 issues should be addressed:  How hosts can join 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 Internet Networking 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 Internet Networking 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 Internet Networking 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 infinity. This technique is called “Poison Reverse”.  RPF guarantees that every router receives every packet exactly once.

6 Internet Networking Reverse Path Forwarding A B DC EF

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

9 Internet Networking 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 by a node to an upstream node must be no more than the minimum of the remaining lifetimes of the PRUNE messages received from its downstream nodes.  Send an explicit JOIN request, which will propagate upwards.  A JOIN message must be acknowledged to ensure the reception of the message.  Used only to undo the effect of a PRUNE message.

10 Internet Networking 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 JOIN and PRUNE message for a group must be sent per-source.  Does not scale for large multicast network.

11 Internet Networking 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 Internet Networking 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 Internet Networking 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 Internet Networking 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 Internet Networking 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