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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Multicast Routing on the Internet

Jason Chu

Svetlana Kotchenova

Can Le

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

What is multicast?

Individual hosts are configured as members of different

multicast groups

Point-to-multipoint transmission

Multicasting is not connection oriented

An IP multicast group is identified by a Class D address

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Multicast IP Delivery Service

The Internet Management Protocol runs between host and their

immediately neighboring multicast routers

A Multicast Routing Protocol is responsible for the construction ofmulticast delivery trees and forwarding of multicast datagramsacross an internetwork

Group MembershipProtocol

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Internet Group Management Protocol (IGMP)

A multicast router periodically transmits Host Membership Querymessages

When a host wants to join a group, it immediately transmits a HostMembership Report for the group

Election of the querier (The router with the lowest IP address)

A Group-Specific Query Message

A Leave Group message

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Multicast forwarding algorithms

Flooding

Spanning Trees

Reverse Path Broadcasting

Truncated Reverse Path Broadcasting Reverse Path Multicasting

Core-Based Trees

Multicast Routing Protocols Distance Vector Multicast Routing protocol

Multicast OSPF

Protocol Independent Multicast (Dense and Sparse modes)

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Reverse Path Multicasting

Every time a router receives a packet

on one of its links, it checks whether

this link belongs to the shortest path

towards the source

yes no

Forward the

packet to all

downstream

neighbors

Discard the

packet

Poison reversethe route

The parent link the interface over

which a router expects to receive multicastpackets from a particular source

The child links the outgoing links over

which a router forwards multicast packets

S

A

B

D

C

E

1

4

3

2

5

8

(RPB)

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Reverse Path Multicasting

Multicast Group

Membership should be

taken into account ->

use of the IGMP (TRPB)

A router can transmit a

prune message on its

parent link (RPM)

Prune messages aresent only one hop back

toward the source

Result: a source-rooted, shortest-path tree for each (source-group) pair

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Drawbacks of RPM

1. The prune information in routers must be removedperiodically and the next packet for a (source, group) isforwarded to all leaf routers

2. Relatively big memory space is required for maintainingstate information for all (source, group) pairs -> it makesthe algorithm not scalable

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Project goals

Simulation of unicast forwarding since in real life bothunicast and multicast communication pattern areused in the same network

Simulation of Reverse Path Multicasting using aspecial data structure to store the information

Evaluation of the algorithm efficiency in the case of

the large internetworks (up to 10000 nodes)

Comparison with another multicast forwardingalgorithm, Core-Based Trees, which was preliminary

created to overcome the limitations of Reverse Path

Multicasting

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Core-Based Trees

A single router or a set of routers acts as the core

A host that wishes to receive traffic for a multicast group isrequired to send a join message to one of the groups corerouters.

Result: a single delivery tree that is shared by all members of a group

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Core-Based Trees

Benefits

Efficient use of router

resources (maintenance of

information for each group,not for each (source, group)

pair)

Conserves network

bandwidth

Limitations

Traffic concentration near

core routers

Possible delays

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Where can multicast be used?

Sharing audio and video (multiparty video-conferencing)

Web caching (the temporary storage of web objects for

later retrieval)

Web clustering

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SC546 Computer Communications and Networks, Boston University, Spring, 2001

Thank you!

Any questions?..