Outline 11: ❒ IP Multicast ❒ Multicast routing IP Multicast ❍ Design choices ❍ Distance Vector Multicast Routing Protocol (DVMRP) Last Modified: ❍ Core Based Trees (CBT) ❍ Protocol Independent Multicast (PIM) 4/9/2003 1:15:00 PM ❍ Border Gateway Multicast Protocol (BGMP) ❒ Issues in IP Multicast Deplyment Based on slides by Gordon Chaffee Berkeley Multimedia Research Center URL: http://bmrc.berkeley.edu/people/chaffee 4: Network Layer 4: Network Layer 4a-1 4a-2 What is multicast? Unicast ❒ Problem ❒ 1 to N communication Sender ❍ Sending same data to ❒ Nandwidth-conserving technology that many receivers via reduces traffic by simultaneously unicast is inefficient delivering a single stream of information to ❒ Example multiple recipients R ❍ Popular WWW sites ❒ Examples of Multicast become serious ❍ Network hardware efficiently supports bottlenecks 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 4: Network Layer 4a-3 4a-4 Multicast IP Multicast Introduction ❒ Efficient one to many ❒ Efficient one to many data distribution Sender data distribution ❍ Tree style data distribution ❍ Packets traverse network links only once ❒ Location independent addressing R ❍ 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 4: Network Layer 4a-5 4a-6
IP Multicast Internet Group Management Protocol (IGMP) ❒ Service ❒ Protocol for managing group membership ❍ All senders send at the same time to the same ❍ IP hosts report multicast group memberships to group neighboring routers ❍ Receivers subscribe to any group ❍ Messages in IGMPv2 (RFC 2236) ❍ Routers find receivers • Membership Query (from routers) • Membership Report (from hosts) ❒ Unreliable delivery • Leave Group (from hosts) ❒ Reserved IP addresses ❒ Announce-Listen protocol with Suppression ❍ 224.0.0.0 to 239.255.255.255 reserved for ❍ Hosts respond only if no other hosts has multicast responded ❍ Static addresses for popular services (e.g. ❒ Soft State protocol Session Announcement Protocol) 4: Network Layer 4: Network Layer 4a-7 4a-8 IGMP Example (1) IGMP Example (2) Membership Report Leave Group 1 3 3 1 3 Network 1 Network 2 Router Network 1 Network 2 Router 4 2 2 4 ❒ Host 3 joins conference ❍ Sends IGMP Membership Report message ❒ Host 1 begins sending packets ❒ Router begins forwarding packets onto Network 2 ❍ No IGMP messages sent ❒ Host 3 leaves conference ❍ Packets remain on Network 1 ❍ Sends IGMP Leave Group message ❒ Router periodically sends IGMP Membership Query ❍ Only sent if it was the last host to send an IGMP Membership Report message 4: Network Layer 4: Network Layer 4a-10 4a-9 Multicast Routing Discussion Source Specific Filtering: IGMPv3 ❒ Adds Source Filtering to group selection ❒ What is the problem? ❍ Receive packets only from specific source ❍ Need to find all receivers in a multicast group addresses ❍ Need to create spanning tree of receivers ❍ Receive packets from all but specific source ❒ Design goals addresses ❍ Minimize unwanted traffic ❒ Benefits ❍ Minimize router state ❍ Helps prevent denial of service attacks ❍ Scalability ❍ Better use of bandwidth ❍ Reliability ❒ Status: Internet Draft? 4: Network Layer 4a-11 4: Network Layer 4a-12
Data Flooding Reverse Path Forwarding (RPF) ❒ Send data to all nodes in network ❒ Simple technique for building trees ❒ Problem ❒ Send out all interfaces except the one with ❍ Need to prevent cycles the shortest path to the sender ❍ Need to send only once to all nodes in network ❒ In unicast routing, routers send to the ❍ Could keep track of every packet and check if it had destination via the shortest path previously visited node, but means too much state ❒ In multicast routing, routers send away from the shortest path to the sender R2 R1 R3 Sender 4: Network Layer 4a-13 4: Network Layer 4a-14 Data Distribution Choices Reverse Path Forwarding Example 1. Router R1 checks: Did the data ❒ Source rooted trees Sender packet arrive on the interface with the shortest path to the Sender? Yes, so it accepts the ❍ State in routers for each sender 2. Router R2 accepts packets packet, duplicates it, and sent from Router R1 because forwards the packet out all other ❍ Forms shortest path tree from each sender to that is the shortest path to the interfaces except the interface Sender. The packet gets sent receivers that is the shortest path to the out all interfaces. R1 sender (i.e the interface the ❍ Minimal delays from sources to destinations packet arrived on). ❒ Shared trees Drop 3. Router R2 drops R2 R3 ❍ All senders use the same distribution tree packets that arrive from Drop Router R3 because that ❍ State in routers only for wanted groups is not the shortest path to the sender. Avoids cycles. ❍ No per sender state (until IGMPv3) R4 R5 R6 R7 ❍ Greater latency for data distribution 4: Network Layer 4a-15 4: Network Layer 4a-16 Source Rooted vs Shared Trees Distance Vector Multicast Routing (DVMRP) Often does not use ❒ Steve Deering, 1988 optimal path from A A source to destination. ❒ Source rooted spanning trees ❍ Shortest path tree ❍ Minimal hops (latency) from source to receivers B C B C ❒ Extends basic distance vector routing ❒ Flood and prune algorithm ❍ Initial data sent to all nodes in network(!) using Reverse D D Path Forwarding Shared Tree Source Routers maintain ❍ Prunes remove unwanted branches Rooted Trees state for each sender Traffic is heavily ❍ State in routers for all unwanted groups in a group. concentrated on ❍ Periodic flooding since prune state times out (soft state) some links while others get little utilization. 4: Network Layer 4a-17 4: Network Layer 4a-18
DVMRP Algorithm Truncated Reverse Path Multicast Example Sender ❒ Truncated Reverse Path Multicast Router R2 accepts packets sent from ❍ Optimized version of Reverse Path Forwarding Router R1 because that is the shortest path to the Sender. ❍ Truncating Unlike Reverse Path Forwarding, • No packets sent onto leaf networks with no receivers which simply forwards out all but R1 the incoming interface, DVMRP’s ❍ Still how “truncated” is this? Reverse Path Multicast maintains a list of child links for each sender. It ❒ Pruning sends packets only out child links, not parent or sibling links. This Siblings ❍ Prune messages sent if no downstream receivers means Router R2 will not forward R2 R3 data from the Sender to Router R3. ❍ State maintained for each unwanted group ❒ Grafting ❍ On join or graft, remove prune state and propagate graft R4 R5 R6 R7 message Receiver Truncation: no packets forwarded onto leaf networks with no receivers 4: Network Layer 4a-19 4: Network Layer 4a-20 DVMRP Pruning Example DVMRP Grafting Example Sender Sender R1 R1 Prune Graft Join from Receiver 2 causes router to remove R2 R3 R2 R3 its prune state and send a Join message up Prune State toward the Sender. Prune Prune Prune Graft R4 R5 R6 R7 R4 R5 R6 R7 Receiver 2 joins Receiver Receiver 1 multicast Membership group Report 4: Network Layer 4a-21 4: Network Layer 4a-22 Receiver 2 DVMRP Problems Core Based Trees (CBT) ❒ State maintained for unwanted groups ❒ Attributes ❍ Single shared tree per group => sparse trees ❒ Bandwidth intensive ❍ Large number of senders ❍ Periodic data flooding per group ❍ Routing tables scale well, size = O(Groups) • No explicit joins, and prune state times out ❍ Not suitable for heterogeneous networks ❍ Bi-directional tree ❒ Poorly handles large number of senders ❍ Scaling = O(Senders, Groups) ❒ Problems of distance vector routing ❍ slow convergence ❍ cycles due to lack of global knowledge 4: Network Layer 4a-23 4: Network Layer 4a-24
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