Understanding Multicast in Internetworking: Challenges and Solutions, Slides of Computer Networks

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2

Lecture No. 27

3

Multicast

5

Internet Multicast Outline• Motivation and challenges• Support strategy• IP multicast service model• Multicast in the Internet• Routing

• Review of ELAN techniques– Multicast routing
  • Limitations

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Multicast

-^

Unicast

: one destination

-^

Broadcast

: all destinations

-^

Multicast

: subset of destinations

• When is multicast useful?
  • Send data to

multiple

receivers at once

• Videoconferencing, video-on-demand,

telecollaboration

• Software update to group of customers
  • Limited broadcast/self-defined multicast
    • Send question to unknown receiver• Resource discovery; Distributed database

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Multicast Support Strategy

• IPv4 used as basis for experimental solutions
  • Use

class D

addresses (1110 <28 bits>)

• Demonstrated with MBone– Uses tunneling
  • Multicast integrated into IPv6•^

Internet Group Management Protocol

IGMP

• Several routing/forwarding schemes:
  • Distance-vector– Link-state– Protocol-independent

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IP Multicast Service Model

•^

Each

group

uses a single address

• Class D addresses (1110 <28 bits>)– Some are well-known, some are dynamically assigned -^

Group membership– Members located

anywhere

in the Internet

• Number of receivers is arbitrary– Members can

join/leave

dynamically

• Hosts can belong to more than one group

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IP Multicast Service Model

• Multicast reception requires

membership

in

group– Internet Group Management Protocol

( IGMP

), RFC 1112

• New operations to join and leave group– LAN routers track local membership– Forwarding depends on routing scheme– Last hop typically uses LAN broadcast
  • Packet reception same as IP

unicast

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Internet Multicast Backbone - MBone

• Existing infrastructure for multicast in the Internet• Multicast route propagation using

DVMRP

• Problem: most IP routers do not support multicast• Solution:

tunneling

by multicast-capable routers

• Encapsulate multicast traffic in IP packets– Send to other multicast-capable routers– Recipients unpack & forward original multicast packet
  • Passes through multicast-incapable areas of

Internet

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ELAN Multicast Techniques

• Spanning tree selection
  • Elect a leader; spanning tree is shortest path to leader

( Perlman

• Distribute topology everywhere, compute in parallel

( link-state

• Problems with spanning trees
  • Bandwidth wasted for groups with few receivers;

Solution: prune LAN’s with no receivers from tree

• For very large ELAN’s, no single tree is efficient;

Solution: define tree per group or tree per source

• The same solutions are used in the Internet!

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Spanning Tree Tradeoffs

• Tree per group or tree per source ?•

Per group

advantage

• One routing entry per group

Per source

advantages

• More efficient distribution– Spreads load better across links– Leverage unicast routing tables

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Multicast Routing in the Internet

• How do senders find receivers?
  • Receivers

inform

all senders of interest (MOSPF)

• Send to

all

receivers; uninterested receivers prune

(DVMRP, PIM-DM)

• Agree on set of rendezvous points (PIM-SM)
  • Types of distribution trees

Separate

tree from each sender (DVMRP,

MOSPF, PIM-DM, PIM-SM)

• Tree rooted at rendezvous point (PIM-SM)

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Link State Multicast (MOSPF)

• Each host on a LAN
  • Periodically announces its group memberships, via

Internet Group Management Protocol (

IGMP

• Extend

LSP

to include set of groups with

members on a given LAN

• MOSPF routing extends OSPF
  • Uses Dijkstra’s algorithm– Computes shortest-path spanning tree for source-

group pairs

• Forward packet on

local

portion of tree

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Distance Vector Multicast (DVMRP)

-^

Idea– Graph of directed next-hop edges to a destination S form

a

tree

• Use

reverse

edges to broadcast from S

-^

Implementation (Reverse Path Broadcast, or

RPB

)

• Forward multicast packet on all links– If and only if packet came from

next hop

for packet

source

-^

Avoid repetition on LAN’s– Assign

parent

router for each LAN

• Has shortest path to source, ties broken by ID– Track parenthood via vector exchanges

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RPB and RPM

M

M

M

M

Member ofmulticastgroup G

RPM from S to GRPB from SUnicast route to SPruned

G

SS