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

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.

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: 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

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?

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?

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

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

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

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.

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

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.

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.

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!

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?

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

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.

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

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

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?