Computer Networks: Understanding Multicasting in ECS 152B, Study notes of Computer Systems Networking and Telecommunications

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ECS 152B Computer Networks

ECS 152B Computer Network

Lecture 16

Demet Aksoy

ECS 152B Computer Networks

Multicasting^ „^ Packets delivered to a subset ofnetwork nodes^ „^ Many applications

„^ Real-time media delivery „^ Software upgrades

ECS 152B Computer Networks

Multicast: one sender to manyreceivers

Network multicast „^ Router activelyparticipate inmulticast, makingcopies of packets asneeded andforwarding towardsmulticast receivers

Multicastrouters (red) duplicate andforward multicast datagrams

ECS 152B Computer Networks

Multicast: one sender to manyreceivers

Application-layermulticast „^ end systems involvedin multicast copy andforward unicastdatagrams amongthemselves

ECS 152B Computer Networks

Multicast groups

‰^ class D Internet addresses reserved for multicast: ‰^ host group semantics:

o^ anyone can “join” (receive) multicast groupo^ anyone can send to multicast groupo^ no network-layer identification to hosts ofmembers

‰^ needed: infrastructure to deliver mcast-addresseddatagrams to all hosts that have joined thatmulticast group

ECS 152B Computer Networks

Joining a mcast group: two-stepprocess^ „^ local: host informs local mcast router of desireto join group: IGMP (Internet GroupManagement Protocol)^ „^ wide area: local router interacts with otherrouters to receive mcast datagram flow

„^ many protocols (e.g., DVMRP, MOSPF, PIM)

IGMP

IGMP IGMP wide-areamulticastrouting

ECS 152B Computer Networks

IGMP: Internet Group ManagementProtocol^ „

host: sends IGMP report when application joinsmcast group^ „^ IP_ADD_MEMBERSHIP socket option^ „^ host need not explicitly “unjoin” group whenleaving „ router: sends IGMP query at regular intervals^ „^ host belonging to a mcast group must reply toquery

query

report

ECS 152B Computer Networks ApplicationTCP

UDP IP

ICMP

ARP^

RARP

Physicalnetwork

Application

Figure 8.

IGMP

Internet Control MessageProtocol (IGMP)^ „^

Like a transport protocol (more or less) [RFC 1112][RFC 2236]

ECS 152B Computer Networks

Multicast Routing

• 223.1.1. „ How are messagesdelivered to aspecific multicastgroup?
  • 223.1.1.
• 223.1.1. - 223.1.2. - 223.1.2. - 223.1.2. - 223.1.3. - 223.1.3. - 223.1.3.
• 223.1.1.2 223.1.7. - 223.1.7. - 223.1.8. - 223.1.8. - 223.1.9. - 223.1.9.2 Multicast group237.1. 9.

ECS 152B Computer Networks

Multicast Routing

„^ goal: find a tree oflinks that connectsall routers that haveattached hostsbelonging to amulticast group

A^

B

E

F

D

C

Source-based

Group-shared tree

G

router with attachedgroup memberrouter with no attachedgroup member LEGEND

ECS 152B Computer Networks

16

Source-based Tree

„^ An individual routingtree is constructed foreach sender in themulticast group „^ In a multicast groupwith N hosts, Ndifferent routing treeswill be constructed foreach sender in themulticast group

A^

B

E

F

D

C

G

ECS 152B Computer Networks

17

Group-shared Tree

„^ A single routing treeis constructed forthe entire multicastgroup

A^

B

E

F

D

C

G

ECS 152B Computer Networks

19

Reverse Path Forwarding

„^ When a router receives amulticast packet with agiven source address, ittransmits the packet on allof its outgoing links(except the one on which itwas received) only if thepacket arrived on the linkthat is on its own shortestpath back to the sender;Otherwise discards thepacket.

A^

B

E

F

D

C

G

source

ECS 152B Computer Networks

20

Pruning

„^ When a router receivesunwanted multicastpackets and has noattached hosts joined tothat multicast group willsend a prune message toits upstream router „^ If a router receives aprune message from oneof its downstreamrouters, then it canforward a prune messageupstream

A^

B

E

F

D

C

G

source prune