EE 122 Multicast Ion Stoica TAs Junda Liu DK Moon David Zats http inst eecs berkeley edu ee122 fa09 Materials with thanks to Vern Paxson Jennifer Rexford and colleagues at UC Berkeley 1 Motivation Example Internet Radio Live 8 concert Send 1 000 Kb s video streams Peak usage 100 000 simultaneous users Consumes 100 Gbps If 1000 people are in Berkeley and if the concert were broadcast from a single location 1000 unicast streams are sent from that location to Berkeley 2 This approach does not scale Broadcast Center Backbone ISP 3 Instead build trees Copy data at routers At most one copy of a data packet per link Broadcast Center Routers keep track of groups in real time Routers compute trees and forward packets along them Backbone ISP LANs implement link layer multicast by broadcasting 4 Multicast Service Model S G data Net as G R t a n i d o 0 G R1 j G data G ns R1 R 0 jo i RG n 1 d joaita ns G Rn 1 Receivers join a multicast group which is identified by a multicast address e g G Sender s send data to address G Network routes data to each of the receivers Note multicast vs broadcast Broadcast packets are delivered to all end hosts in the network Multicast packets are delivered only to end hosts that are in have joined the multicast group 5 Multicast Service Model cont d Membership access control Open group anyone can join Closed group restrictions on joining Sender access control Anyone can send to group Anyone in group can send to group Restrictions on which host can send to group 6 Multicast and Layering Multicast can be implemented at different layers data link layer network layer e g IP multicast application layer e g Ethernet multicast e g End system multicast Which layer is best 7 Multicast Implementation Issues How are multicast packets addressed How is join implemented How is send implemented How much state is kept and who keeps it 8 Data Link Layer Multicast Recall end hosts in the same local area network LAN can hear from each other at the data link layer e g Ethernet Reserve some data link layer addresses for multicast Join group at multicast address G Send to group G Network interface card NIC normally only listens for packets sent to unicast address A and broadcast address B To join group G NIC also listens for packets sent to multicast address G NIC limits number of groups joined Implemented in hardware thus efficient Packet is flooded on all LAN segments like broadcast Can waste bandwidth but LANs should not be very large Only host NICs keep state about who has joined scalable to large number of receivers groups 9 Problems with Data Link Layer Multicast Single data link technology Single LAN Limited to small number of hosts Limited to low diameter latency Essentially all the limitations of LANs compared to internetworks 10 Network Layer IP Multicast Overcomes limitations of data link layer multicast Performs inter network multicast routing Relies on data link layer multicast for intra network routing Portion of IP address space defined as multicast addresses 228 addresses for entire Internet Open group membership Anyone can send to group Flexible but leads to problems 11 IP Multicast Routing Intra domain Source Specific Tree Distance Vector Multicast Routing Protocol DVRMP Shared Tree Core Based Tree CBT Inter domain Protocol Independent Multicast Single Source Multicast 12 Distance Vector Multicast Routing Protocol DVRMP An elegant extension to DV routing Use shortest path DV routes to determine if link is on the source rooted spanning tree Three steps in developing DVRMP Reverse Path Flooding Reverse Path Broadcasting Truncated Reverse Path Broadcasting 13 Reverse Path Flooding RPF Extension to DV unicast routing Packet forwarding If incoming link is shortest path to source Send on all links except incoming Packets always take shortest path assuming delay is symmetric Issues Some links LANs may receive multiple copies Every link receives each multicast packet even if no interested hosts s 3 s 3 s 2 s 2 s 3 s 3 s 1 s 1 s 2 s 2 ss rr 14 Example Flooding can cause a given packet to be sent multiple times over the same link SS xx yy a duplicate packet zz b Solution Reverse Path Broadcasting 15 Reverse Path Broadcasting RPB Chose parent of each link along reverse shortest path to source Only parent forward to a link child link Identify Child Links 1 Routing updates identify parent 2 Since distances are known each router can easily figure out if it s the parent for a given link 3 In case of tie lower address wins SS Parent of z on reverse path 5 xx forward only to child link child link of x for S 6 yy a zz b 16 Don t Really Want to Flood This is still a broadcast algorithm the traffic goes everywhere Need to Prune the tree when there are subtrees with no group members Solution Truncated Reverse Path Broadcasting 17 Truncated Reverse Path Broadcasting TRPB Extend DV RPB to eliminate unneeded forwarding Identify leaves Routers announce that a link is their next link to source S Parent router can determine that it is not a leaf Explicit group joining on LAN Members periodically with random offset multicast report locally Hear and report then suppress own Packet forwarding If not a leaf router or have members Out all links except incoming SS NL NL NL L L r2 r2 r1 r1 L leaf node NL Non leaf node 18 Pruning Details Prune Source Group at leaf if no members If all children of router R send NRM prune S G Send Non Membership Report NMR up tree Propagate prune for S G to parent R On timeout Prune dropped Flow is reinstated Down stream routers re prune Note a soft state approach 19 Pruning Details How to pick prune timers Too long large join time Too short high control overhead What do you do when a member of a group re joins Issue prune cancellation message grafts 20 Distance Vector Multicast Scaling State requirements O Sources Groups active state How to get better scaling Hierarchical Multicast Core based Trees 21 Core Based Trees CBT Pick a rendevouz point for the group called the core Shared tree Unicast packet to core and bounce it back to multicast group Tree construction is receiver based Joins can be tunneled if required Only nodes on One tree per group tree involved Reduce routing table state from O S x G to O G 22 Example Group members M1 M2 M3 M1 sends data root M1 M2 M3 control join messages data 23 Disadvantages Sub optimal delay Need good core selection Optimal choice computing topological center is NP hard 24 Problems with Network Layer Multicast NLM Scales poorly
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