Multicast

1. Multicast Outline Link-state Multicast Distance-vector Multicast Protocol Independent Multicast CS 332

2. Multicast • LANs support hardware multicast (at worst, just broadcast) • We’re concerned here with multicast in an internetwork • What we don’t want to have to do: have the source(s) send individual packets to each receiver CS 332

3. Multicast • Source sends packet to a single multicast address (class D in IPv4) associated with a specific multicast group • Internet Group Multicast Protocol (IGMP): used to join and leave groups • Informs router on local network of desire to receive packets addressed for specific multicast group • How to learn about addresses? Out-of-band CS 332

4. Link-state Multicast • Extend link-state unicast (easy) • Add the set of groups that have members on a particular link (LAN) to “state” for that link • Use Dijkstra as before to create shortest-path multicast tree • Local router knows above “state” from IGMP messages received from local host(s). • Each host periodically announces group memberships to LAN • If announcements stop (I.e. group not listed) router assumes host has left particular group. • Router keeps cache of all active source/group pairs CS 332

5. CS 332

6. Distance-Vector Multicast • More involved than link-state, since routers do not know entire network topology • Extend DVR in two stages: • Broadcast method allowing sending of packet to all networks on Internet • Prune back above mechanism so that packets don’t get flooded to networks that have no interest CS 332

7. Reverse-Path Broadcast (RPB) • Recall (Destination, Cost, NextHop) tuples • If router receives multicast packet from source S: • Forward packet on all outgoing links (except one on which packet arrived) if and only if packet arrived on link that is the shortest path to S (thus the “reverse”) • Floods packets from S without looping them toward S • Two problems • Flooding even to uninterested groups • Packet forwarded to LAN over each router on LAN CS 332

8. A Partial Solution • “Parent” router for each link (LAN) for each source • Router with shortest path to S is parent for S • Tie goes to router with smallest address • Routers learn about parenthood from LSAs • Routers keep, for each source, a bit for each incident link (LAN) indicating whether it is parent for that source/link pair. CS 332

9. Reverse-Path Multicast (RPM) • Goal: exclude packets for group G from networks that have no members of G • Again, two stages: • Recognize when leaf network (I.e. only one router) has no group members • Propogate “no group G members here” message up shortest-path tree. • Augment (Destination,Cost) pairs leaf sends with the set of groups for which leaf network is interested in receiving packets • Work way back up tree • Lots of additional info in routing updates, so pruning done only when source starts sending packets to group (use RPB until then) CS 332

10. The MBone • Multicast is not supported at present by many routers in Internet. Multicast made available by multicast backbone (MBone) • Logical internet layered over Internet • Class D addresses • Tunnelling (to connect multicast supporting routers) • Distance-Vector Multicast Routing Protocol (DVMRP) • Exponential growth: thousands of networks on four continents CS 332

11. Protocol Independent Multicast (PIM) • Other methods don’t scale well in situations where relatively small proportion of routers want to receive traffic for a given group • PIM has “sparse-mode” and “dense-mode”, we discuss PIM-SM (sparse mode) • Explicit Join and Prune messages sent to a router designated as the rendezvous point (RP) for that domain (complex protocol to handle network partitioning,crash of RP, etc.) • Construct shared or source-specific trees CS 332

12. PIM Tree Construction • Router sends Join message to RP for group using IP unicast (with sender field “wildcarded”) • Each router along path sees Join, notes the interface on which it was received, and makes forwarding table entry that indicates that packets destined for this group go through this interface (called an “all senders” entry) • Router then determines interface on which to forward the Join. This becomes only interface on which it will receive packets for this group • If new Join arrives for same group on different link, router notes change, but no need to forward the Join. CS 332

13. PIM Example CS 332

14. Sending with PIM • Host sends packet with group multicast address to designated router (DR) on local network • DR tunnels packet to RP using IP unicast • RP receives packet, unpacks multicast packet, and forwards it along the multicast tree CS 332

15. PIM Optimizations • Tunneling is inefficient, so based on data rate, RP can force group knowledge into intervening routers • Send Join toward the sending host • Routers create sender-specific state in routers, creating a sender-specific tree distinct from the all senders tree • Replace shared tree with source-specific tree • Router at downstream end of tree sends Join message toward source (not necessarily RP). Router must remain on shared tree (in case new senders show up), but source-specific trees may be much shorter than those through RP CS 332

16. PIM Final Remarks • PIM is protocol independent in sense that it can use any unicast routing protocol (DVR, link-state, etc), but is not independent of IP • Note again tradeoff between scalability and optimality • Shared tree reduces state in routers to order of number of groups, so more scalable than source-specific trees, which provide optimality CS 332