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Computer Networks

IP Multicast

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Recall

UnicastBroadcastMulticast

sends to a specific group

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Unicast

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Multicast

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Introduction to IP Multicast

Class D group address (224-239.x.x.x)First bits must be 1110 of first octet

How to join/leave a multicast group?How to map to hardware address?How to route to a IP multicast group?

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IP Multicast Addressing

Some reserved addresses224.0.0.0/24 Link local reserved

e.g. 224.0.0.9 - RIPv2 routers

239.0.0.0/8 Private (Administratively Scoped)

IP Multicast to ethernet multicast mappingplace low-order 23 bits of IP address into low-

order 23 bits of special Ethernet address 01:00:5e:00:00:00

problem... IP multicast has 28 significant bits!

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Internet Group Multicast Protocol (IGMP)

Standardized in RFC 1112 and 2236Host/Router signalling protocolHosts send IGMP join messagesRouter periodically queriesIGMPv3 in draft

Hosts only listen to specific group sources

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Multicast routing

Backwards from unicast routingReverse Path Forwarding (RFP)

Route away from sourceCould follow the shortest (source tree)

pathCould build a shared distribution tree

Rendevous Point (RP)

Dense (Push) and Sparse (Pull) Modes

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Distance Vector Multicast Routing Protocol (DVMRP)

Similar to RIPInfinity = 32 hopsFlood and Prune builds distribution treeGrafting adds back a linkWas widely used in early MBONEDoes not scale

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Multicast OSPF (MOSPF)

LSAs carry multicast route informationEach router learns entire multicast treeOnly works with OSPF networksDoes not scale wellNot popular

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Protocol Independent Multicast Dense Mode (PIM-DM)

Uses any underlying routing protocole.g. RIP, OSPF, IS-IS, BGP, etc.

Very similar to DVMRPFloods and prunes back based on

membershipDoes not scale to large networks

(MBONE)

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Protocol Independent Multicast Sparse Mode (PIM-SM)

Supports source and shared treesUses a Rendevous Point (RP)Can use any underlying routing

protocolTraffic only sent down joined branchesCan dynamically switch to optimal

source tree depending on traffic pattern

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PIM-SM Illustrated

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Internet MBONE

Multicast backbone overlay on the Internet

Originally used DVMRP, now mostly uses PIM-SM

Few commercial organizations using itMany educational and research

institutions using it

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Common MBONE Tools

SDRVAT/RATVICWBIP/TVMtrace