Internet Networking recitation #6

1. Internet Networkingrecitation #6 DVMRP Spring Semester 2008, Dept. of Computer Science, Technion

2. 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. Internet Networking

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. Internet Networking

4. A B D C E F An accepted packet Sent but not accepted packet 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. Internet Networking

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 infinity. • This technique is called “Poison Reverse”. • RPF guarantees that every router receives every packet exactly once. Internet Networking

6. A B C D E F Reverse Path Forwarding Internet Networking

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. Internet Networking

8. A A PRUNE B C D C D PRUNE E F E F Pruning example Only C and E are group members. before pruning after pruning B B Internet Networking

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 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. Internet Networking

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

11. DVMRP Protocol • Protocol for multicast routing inside of ASs that use Distance Vector Routing (e.g. RIP). • Defined in RFC 1075. • 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. Internet Networking

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. Internet Networking

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 net 1 net 2 Internet with no support for multicast R1 R2 Internet Networking

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). Internet Networking

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