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Transcript
Distance Vector Multicast Routing Protocol (DVMRP) • Distributed protocol that generates IP Multicast delivery tree per source-group • Shortest path from Source to hosts – based on Number of hops metric • Derived from Routing Information Protocol – RIP forwards the unicast packets based on the the next-hop towards a destination – DVMRP constructs delivery trees based on shortest previous-hop back to the source • Supports hierarchical routing 1 Algorithm • Per-source broadcast trees built based on routing exchanges ( using DVRP) • Reverse Path Multicasting – Initially, assume that every host on the network is part of the Multicast group (TRPB) – Per Source-Group Multicast delivery tree – Reverse Path Forwarding check – Poison Reverse • Determine downstream interfaces to forward the packet on – Prune and Graft messages 2 Tunnel Encapsulation • Encapsulated in IP packets 3 Neighbor Discovery • Neighbor Probe messages with TTL = 1 • Addressed to “ALL_DVMRP_ROUTERS” • Contains a list of Neighbor DVMRP routers for which a Probe has been received on that interface – Establish “Two-Way adjacency” – Know capabilities of routers (Version no) – Keep-alive function • Sent every 10 secs • Neighbor time-out: 35secs 4 DVMRP Probe Message Format 5 Source Location • When a multicast datagram is received at the router – Look up the source network in the routing table – RPF check – Forwarding cache entry created • Provide consistent view of shortest path to the source – Propagate routing table to all routers 6 • Poison Reverse – To determine if any downstream interfaces depend on them for forwarding data • Designated Forwarder – When two or more Mrouters connected to a multi-access network – Both routers may forward packets on the LAN – Elect one router per source Router with lowest metric back to the source – Equal metrics, router with lowest IP address • Route report interval of 60 secs 7 Routing Table • Does not consider group memberships • Source Subnet – The subnetwork containing the source host Source subnet 128.1.0.0 128.2.0.0 128.3.0.0 128.4.0.0 Subnet mask From Gateway Metric 255.255.0.0 128.7.5.2 3 255.255.0.0 128.7.5.2 5 255.255.0.0 128.6.3.1 2 255.255.0.0 128.6.3.1 4 Status Up Up Up Up TTL 200 150 150 200 InPort OutPorts 1 2,3 2 1 2 1,3 1 2 8 Building Multicast Trees • Determine upstream interface : RPF • Forward on downstream interfaces – Initially, all downstream interfaces determined by Poison Reverse are part of tree – If downstream interface is a Leaf network • Consult IGMP Local database – Non-Leaf Networks • Delete interface if a Prune is received 9 Forwarding Cache Entries • Separate entries for each (Source network, Destination group) pair • Created on demand based on Routing table, Group memberships and Prunes Source subnet Multicast Group 128.1.0.0 224.1.1.1 224.2.2.2 224.3.3.3 128.2.0.0 224.1.1.1 TTL 200 150 150 200 InPort 1Pr 1 1 1 OutPorts 2p 3p 2p 3 2 2p 3 10 Pruning Multicast Trees • If a router has no dependent downstream interfaces, a Prune sent up to delete that interface from list of dependent interfaces – Leaf networks without any host members – Non-leaf networks, all downstream interfaces send a Prune 11 Prune message Packet Format 12 Grafting • To support dynamic host memberships • To cancel previously pruned interfaces – When a new host joins the group – Or a graft message received from downstream • Separate messages sent for each source network pruned • Acknowledge each Graft with a “Graft ACK” hop by hop 13 Graft / Graft ACK Packet Format 14