1. 4: Network Layer4-1 Chapter 4: Network Layer Last time: r Internet routing protocols m RIP m OSPF m IGRP m BGP r Router architectures r IPv6 Today: r IPv6 Headers r Multicast Routing r Assign HW7, due Mon m Ch 4 Review #25 m Ch 4 Problems #13-16,18,19 r Proj 2 due Wednesday r Start Ch5 on Wednesday

2. 4: Network Layer4-2 IPv6 Header Revisited Next header: identify upper layer protocol for data …really, who interprets the next header? Could be options, ICMP, upper layer protocols, and more. Fragmentation: permitted, but not performed on the fly by routers.

3. 4: Network Layer4-3 Multicast Routing r Three types of communication supported under IPv4: m Unicast – one to one m Broadcast – one to all m Multicast – one to many r In multicasting, messages are sent to a set of receivers that could be anywhere on the Internet. r Many applications would benefit from multicast support m Live media transfer, distributed data feeds, cache updating, interactive gaming

4. 4: Network Layer4-4 Multicast Abstraction r Two approaches toward implementing the multicast abstraction m Separate unicast transport to each receiver m Explicit multicast support at network layer

5. 4: Network Layer4-5 Multicasting Issues r Efficiency m Messages should be delivered only to intended recipients m Messages should travel the “best” paths m Messages should not be duplicated over common links m Approach should scale r Reliability m Any guarantees?

6. 4: Network Layer4-6 Multicast: Connection-oriented r Unlike unicast service, Internet multicast is not a connectionless service. r State must be maintained in multicast routers. r Protocols are needed to set up, maintain, and tear down connection state in the routers. r Each router must know how to route packets for multicast addresses. r How does a router know anyone is listening?

7. 4: Network Layer4-7 IGMP: Host to Router r Internet Group Management Protocol m Host can inform router of desire to join specific multicast group m Router doesn’t care how many hosts join group m Other protocols needed between routers

8. 4: Network Layer4-8 IGMP Messages IGMP Message TypesSent byPurpose Membership query: generalRouter Query multicast groups joined by attached hosts Membership query: specific Router Query if specific multicast group joined by attached hosts Membership reportHost Report host wants to join or is joined to given multicast group Leave groupHostReport leaving multicast group

9. 4: Network Layer4-9 Multicast Routing r How to route multicast data only to appropriate routers and hosts? r Need to find a tree of links that connects all routers that have hosts belonging to multicast group. r Two approaches: m Single shared tree m Source-specific tree

10. 4: Network Layer4-10 Multicast Routing Trees r Left: A single, shared tree r Right: Two source-based trees (A and B) r Efficiency vs. Complexity m Finding minimum-cost trees is complex.

11. 4: Network Layer4-11 Minimum-cost Multicast Trees r A minimum-cost (group-shared) multicast tree. r Finding minimum cost tree (Steiner tree) is NP-complete, but good heuristics exist. r Not used in practice. Why? m Need to know about all links in network m Need to re-run algorithm whenever link costs change

12. 4: Network Layer4-12 Group-shared Multicast Trees r A center-based tree. r Center node (E) is first identified. How? r Routers wanting to join tree send unicast message to center. r Message is forwarded to center unless received by router already present in tree.

13. 4: Network Layer4-13 Source-based Multicast Routing r Least unicast-cost path routing tree (for A). r We know Dijkstra’s algorithm can calculate such trees. m A link state algorithm! r Minimal cost source-based trees vary depending on source.

14. 4: Network Layer4-14 Reverse Path Forwarding (RPF) r A simpler multicast routing algorithm m Less link state info r Router forwards multicast packet to all outgoing links m Except link that first carried packet m Only if source link is on shortest path back to sender m Otherwise, drop r Note that ALL routers get multicast packets!

15. 4: Network Layer4-15 RPF Pruning r Multicast router w/o hosts in group sends prune msg to upstream router. r Routers in which all downstream routers send prune messages also sends prune to upstream router. r Complications: m Router must know which routers are downstream. m What happens when router later wants to join group?

16. 4: Network Layer4-16 Multicast Routing in the Internet r DVMRP: Distance Vector Multicast Routing Protocol m Source-based trees with RPF Pruning Grafting m Distance vector algorithm to compute shortest path back to every source m De facto inter-AS multicast routing algorithm

17. 4: Network Layer4-17 Multicast Tunneling r Small fraction of routers in Internet are multicast capable. r What happens when all neighbors of a routers are not capable? m Tunneling (just like IPv6) is used.

18. 4: Network Layer4-18 Spanning Tree Problem r Show the minimal-cost spanning tree rooted at A that includes C, D, E, and G. G F E D C A B H 2 1 2 4 3 1 1 1 3 6 1 4 9 14

19. 4: Network Layer4-19 Spanning Tree Answer r Show the minimal-cost spanning tree rooted at A that includes C, D, E, and G. G F E D C A B H 2 1 2 4 3 1 1 1 3 6 1 4 9 14

20. 4: Network Layer4-20 Discussion Questions r TRUE or FALSE: m When a host joins a multicast group, it must change its IP address to be that of the multicast group it is joining. m In RPF, a node will receive multiple copies of the same packet. m In RPF, a node may forward multiple copies of a packet over the same outgoing link. r How can multicast apps learn the identities of the hosts that are participating in a multicast group? r How can the application be certain that only authorized recipients get the multicast data? r If 1000 multicast groups each choose a multicast group address at random, how likely is it that there will be conflicts?