Spring 2006CS 3321 Multicast Outline Link-state Multicast Distance-vector Multicast Protocol Independent Multicast

Spring 2006CS 3322 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

Spring 2006CS 3323 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

Spring 2006CS 3324 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

Spring 2006CS 3325

Spring 2006CS 3326 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

Spring 2006CS 3327 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

Spring 2006CS 3328 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.

Spring 2006CS 3329 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)

Spring 2006CS 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

Spring 2006CS 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

Spring 2006CS 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.

Spring 2006CS PIM Example

Spring 2006CS 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

Spring 2006CS 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

Spring 2006CS 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