CORNELL CS 5190 - IP Multicast (69 pages)

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IP Multicast



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IP Multicast

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Pages:
69
School:
Cornell University
Course:
Cs 5190 - Computer Networking
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17 IP Multicast Last Modified 01 14 19 12 02 AM Based on slides by Gordon Chaffee Berkeley Multimedia Research Center URL http bmrc berkeley edu people chaffee 4 Network Layer 4a 1 Outline IP Multicast Multicast routing Design choices Distance Vector Multicast Routing Protocol DVMRP Core Based Trees CBT Protocol Independent Multicast PIM Border Gateway Multicast Protocol BGMP Issues in IP Multicast Deplyment 4 Network Layer 4a 2 What is multicast 1 to N communication Nandwidth conserving technology that reduces traffic by simultaneously delivering a single stream of information to multiple recipients Examples of Multicast Network hardware efficiently supports multicast transport Example Ethernet allows one packet to be received by many hosts Many different protocols and service models Examples IETF IP Multicast ATM Multipoint 4 Network Layer 4a 3 Unicast Problem Sender Sending same data to many receivers via unicast is inefficient Example R Popular WWW sites become serious bottlenecks 4 Network Layer 4a 4 Multicast Efficient one to Sender many data distribution R 4 Network Layer 4a 5 IP Multicast Introduction Efficient one to many data distribution Tree style data distribution Packets traverse network links only once Location independent addressing IP address per multicast group Receiver oriented service model Applications can join and leave multicast groups Senders do not know who is listening Similar to television model Contrasts with telephone network ATM 4 Network Layer 4a 6 IP Multicast Service All senders send at the same time to the same group Receivers subscribe to any group Routers find receivers Unreliable delivery Reserved IP addresses 224 0 0 0 to 239 255 255 255 reserved for multicast Static addresses for popular services e g Session Announcement Protocol 4 Network Layer 4a 7 Internet Group Management Protocol IGMP Protocol for managing group membership IP hosts report multicast group memberships to neighboring routers Messages in IGMPv2 RFC 2236 Membership Query from routers Membership Report from hosts Leave Group from hosts Announce Listen protocol with Suppression Hosts respond only if no other hosts has responded Soft State protocol 4 Network Layer 4a 8 IGMP Example 1 1 3 Network 1 Network 2 Router 2 4 Host 1 begins sending packets No IGMP messages sent Packets remain on Network 1 Router periodically sends IGMP Membership Query 4 Network Layer 4a 9 IGMP Example 2 Membership Leave Report Group 1 3 Network 1 Network 2 Router 2 4 Host 3 joins conference Sends IGMP Membership Report message Router begins forwarding packets onto Network 2 Host 3 leaves conference Sends IGMP Leave Group message Only sent if it was the last host to send an IGMP Membership Report message 4 Network Layer 4a 10 Source Specific Filtering IGMPv3 Adds Source Filtering to group selection Receive packets only from specific source addresses Receive packets from all but specific source addresses Benefits Helps prevent denial of service attacks Better use of bandwidth Status Internet Draft 4 Network Layer 4a 11 Multicast Routing Discussion What is the problem Need to find all receivers in a multicast group Need to create spanning tree of receivers Design goals Minimize unwanted traffic Minimize router state Scalability Reliability 4 Network Layer 4a 12 Data Flooding Send data to all nodes in network Problem Need to prevent cycles Need to send only once to all nodes in network Could keep track of every packet and check if it had previously visited node but means too much state R2 R1 R3 Sender 4 Network Layer 4a 13 Reverse Path Forwarding RPF Simple technique for building trees Send out all interfaces except the one with the shortest path to the sender In unicast routing routers send to the destination via the shortest path In multicast routing routers send away from the shortest path to the sender 4 Network Layer 4a 14 Reverse Path Forwarding Example 1 Router R1 checks Did the data packet arrive on the interface with the shortest path to the Sender Yes so it accepts the packet duplicates it and forwards the packet out all other interfaces except the interface that is the shortest path to the sender i e the interface the packet arrived on Sender 2 Router R2 accepts packets sent from Router R1 because that is the shortest path to the Sender The packet gets sent out all interfaces R1 Drop R2 3 Router R2 drops packets that arrive from Router R3 because that is not the shortest path to the sender Avoids cycles R3 Drop R4 R5 R6 R7 4 Network Layer 4a 15 Data Distribution Choices Source rooted trees State in routers for each sender Forms shortest path tree from each sender to receivers Minimal delays from sources to destinations Shared trees All senders use the same distribution tree State in routers only for wanted groups No per sender state until IGMPv3 Greater latency for data distribution 4 Network Layer 4a 16 Source Rooted vs Shared Trees A B A C B D Source Rooted Trees C D Routers maintain state for each sender in a group Often does not use optimal path from source to destination Shared Tree Traffic is heavily concentrated on some links while others get little utilization 4 Network Layer 4a 17 Distance Vector Multicast Routing DVMRP Steve Deering 1988 Source rooted spanning trees Shortest path tree Minimal hops latency from source to receivers Extends basic distance vector routing Flood and prune algorithm Initial data sent to all nodes in network using Reverse Path Forwarding Prunes remove unwanted branches State in routers for all unwanted groups Periodic flooding since prune state times out soft state 4 Network Layer 4a 18 DVMRP Algorithm Truncated Reverse Path Multicast Optimized version of Reverse Path Forwarding Truncating No packets sent onto leaf networks with no receivers Still how truncated is this Pruning Prune messages sent if no downstream receivers State maintained for each unwanted group Grafting On join or graft remove prune state and propagate graft message 4 Network Layer 4a 19 Truncated Reverse Path Multicast Example Sender Router R2 accepts packets sent from Router R1 because that is the shortest path to the Sender Unlike Reverse Path Forwarding which simply forwards out all but the incoming interface DVMRP s Reverse Path Multicast maintains a list of child links for each sender It sends packets only out child links not parent or sibling links This means Router R2 will not forward data from the Sender to Router R3 R1 R2 R4 Receiver Siblings R5 R3 R6 R7 Truncation no packets


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