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Multicasting (19.1)

CSE 3213

Fall 2011

128 November 2011

Multicast Addressing

• Class D address (see next slide)

• Source: unicast IP address S

Receivers: multicast group ID G, a class-D address

• Each group is identified by (S, G)

• Ethernet broadcast address (all 1’s)

• 2 ways of doing IP multicast at the link layer:

– Link-layer (Ethernet) broadcast

– Link-layer (Ethernet) multicast

Both cases need filtering at IP layer.

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2

IPv4 Address Formats

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Multicast Addressing (2)

1. Link-layer (Ethernet) broadcast

• IP multicast packet is encapsulated in an Ethernet broadcast frame and

transmitted on the bus.

• Every host picks up the Ethernet frame and does filtering at the IP layer to

decide whether to keep or discard the frame.

2. Link-layer (Ethernet) multicast

• Requires a mapping of IP multicast address to an Ethernet multicast

address (see next slide).

• There are up to 32 IP class-D addresses mapped to the same Ethernet

multicast address.

• The IP module still has to filter out packets for non-member hosts.

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3

Mapping from Class D IP adress to Ethernet

multicast adress

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Internet Multicast Service Model

Multicast group concept: use of indirection

• a host “sends” IP datagrams to multicast group.

• routers forward multicast datagrams to hosts that have “joined”

that multicast group.

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128.119.40.186

128.59.16.12

128.34.108.63

128.34.108.60

multicast group

226.17.30.197

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Multicast groups

� Class D Internet addresses reserved for multicast:

� Host group semantics:

o anyone can “join” (receive from) multicast group.

o anyone can send to multicast group.

o no network-layer identification to hosts of members.

� Needed: infrastructure to deliver multicast-addressed

datagrams to all hosts that have joined that multicast group.

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Multicast Protocols

Transport layer

• UDP

• Real-time Transport Protocol (RTP): for multimedia

content

• ReSerVation Protocol (RSVP): for bandwidth

reservation in a multicast distribution

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5

Multicast Protocols (2)

Routing, delivery

• On a local network (join/leave):

– Internet Group Management Protocol (IGMP)

– Multicast Listener Discovery (MLD): similar to IGMP but for

IPv6

• Intra-domain (routing):

– MOSPF, PIM, DVMRP

• Inter-domain (routing):

– Multicast Border Gateway Protocol (MBGP)

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Joining a multicast group: 2-step process

• Local: host informs local multicast router of desire to join

group: IGMP (Internet Group Management Protocol)

• Wide area: local router interacts with other routers to receive

multicast datagram flow

– many protocols (e.g., DVMRP, MOSPF, PIM)

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IGMPIGMP

IGMP

wide-areamulticast routing

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IGMP

Internet Group Management

Protocol (IGMP)

• RFC 3376 used to exchange multicast group

information between hosts & routers on a LAN

• hosts send messages to routers to subscribe and

unsubscribe from multicast group

• routers check which multicast groups are of interest

to which hosts

• IGMP currently at version 3

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IGMP (2)

• Host: sends IGMP report (reply) when application wishes to

join a multicast group.

– IP_ADD_MEMBERSHIP socket option

– hosts need not explicitly “unjoin” group when leaving

• Router: sends IGMP query at regular intervals

– hosts belonging to a multicast group must reply to query if

wishing to join or stay in the group.

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query report

IGMP (3)

• Router: broadcasts Host

Membership Query

message on LAN.

• Host: replies with Host

Membership Report

message to indicate group

membership

– randomized delay

before responding

– implicit leave via no

reply to Query

• Group-specific Query

• Leave Group message

– Last host replying to

Query can send explicit

Leave Group message

– Router performs group-

specific query to see if any

hosts left in group

– Introduced in RFC 2236

• IGMP v3: current version

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8

Operation of IGMP v1 and v2

• IGMPv1

– hosts could join group

– routers used timer to unsubscribe members

• IGMPv2 enabled hosts to unsubscribe

• operational model:

– receivers have to subscribe to groups

– sources do not have to subscribe to groups

– any host can send traffic to any multicast group

• problems:

– spamming of multicast groups

– establishment of distribution trees is problematic

– finding globally unique multicast addresses difficult

IGMP v3

• addresses weaknesses by:

– allowing hosts to specify list from which they want

to receive traffic

– blocking traffic from other hosts at routers

– allowing hosts to block packets from sources that

send unwanted traffic

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IGMP Message Formats

Membership Query

• sent by multicast router

• three subtypes: general query, group-specific query, group-

and-source specific query

Membership Query Fields

type

max response time

checksum

group address

S flag

QRV (querier's robustness variable)

QQIC (querier's querier interval code)

number of sources

source addresses

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IGMP Message Formats

Membership Report

IGMP Message Formats

Group Record

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IGMP Operation - Joining

• IGMP host wants to make itself known as group member to other hosts and routers on LAN

• IGMPv3 can signal group membership with filtering capabilities with respect to sources

– EXCLUDE mode – all members except those listed

– INCLUDE mode – only from group members listed

to join send IGMP membership report message

• address field multicast address of group

• sent in IP datagram

• current group members receive & learn new member

• routers listen to all IP multicast addresses to hear all reports

IGMP Operation –

Keeping Lists Valid

routers periodically issue IGMP general query

message

• in datagram with all-hosts multicast address

• hosts must read such datagrams

• hosts respond withreport message

router doesn’t know every host in a group

• needs to know at least one group member still active

• each host in group sets timer with random delay

• host hearing another report cancels own

• if timer expires, host sends report

• only one member of each group reports to router

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IGMP Operation - Leaving

• host leaves group by sending a leave group message

to the all-routers static multicast address

– sends a membership report message with EXCLUDE option

and null list of source addresses

• router determines if have any remaining group

members using group-specific query message

Group Membership with IPv6

• IGMP defined for IPv4

– uses 32-bit addresses

• IPv6 internets need functionality

• IGMP functions included in Internet Control

Message Protocol v6 (ICMPv6)

– ICMPv6 has functionality of ICMPv4 & IGMP

• ICMPv6 includes group-membership query

and group-membership report message

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IGMP: Summary• For membership management.

• Between a host on a subnet (Ethernet) and the router for the subnet.

• The router periodically broadcast an IGMP host-membership query message on its subnet.

• A host subscribes to a group replies by multicasting a host-membership report message.

– Note: feedback implosion → uses a random timer.

• The report is sent 3 times (for reliability).

• IGMP-1: hosts send no report → leaving the group

IGMP-2: hosts send explicit host-membership leave messages to reduce leave latency.

IGMP-3: filtering and blocking

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Multicast Routing

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Shortest Path Tree

• Multicast cast forwarding tree: tree of shortest path routes

from source to all receivers.

– Dijkstra’s algorithm.

R1

R2

R3

R4

R5

R6 R7

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6

3 4

5

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

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MOSPF

• Extends OSPF for multicasting.

• Every router has the complete topology of its own network.

• A receiver joins a multicast group G by exchanging IGMP messages with its end-router R.

• The end-router R broadcasts its group membership to the whole network in the form (G, R).

• Every router in the network maintains a group membership table with each entry being a tuple [S, G, <R1, R2, …>].

• A sender simply sends data packets as they are available.

• Each router uses the network topology, the group membership table, and the multicast group ID in the data packets to compute the route(s) to the destination(s).

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Reverse Path Forwarding

• Building a loop-free broadcast tree

• No knowledge of group membership

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Reverse Path Forwarding (2)

if (multicast datagram received on incoming link on

shortest path back to sender)

then flood datagram onto all outgoing links

else ignore datagram

� rely on router’s knowledge of unicast shortest path

from it to sender

� each router has simple forwarding behavior:

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16

Reverse Path Forwarding: Example

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A

B

G

DE

c

F

Spanning-Tree Broadcast

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A

B

G

DE

c

F

A

B

G

DE

c

F

(a) Broadcast initiated at A (b) Broadcast initiated at D

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Internet Multicasting Routing: DVMRP

• DVMRP: distance vector multicast routing protocol,

RFC1075.

• Flood and prune: reverse path forwarding, source-

based tree.

– initial datagram to multicast group is flooded

everywhere via RPF

– routers not wanting the multicast data: send

prune messages to upstream neighbors

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DVMRP Example

R1

R2

R3

R4

R5

R6 R7

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6

3 4

5

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

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How DVMRP Fits into Multicasting

• IGMP - used for discovery of hosts in multicast

groups, messages exchanged between

neighbors

• DVMRP - routes multicast datagrams within an

AS

• MBGP (Multicast BGP) - routes multicast

datagrams between ASs

DVMRP Details

• Soft state: DVMRP router periodically (1 min.) “forgets” that branches are pruned:

– multicast data again flows down unpruned branches.

– downstream routers: reprune or else continue to receive data.

• Routers can quickly re-graft to tree following an IGMP join at a leaf router by sending a “graft” message upstream.

• Deployment:

– commonly implemented in commercial routers.

– Mbone routing done using DVMRP.

• Works well in small autonomous domains.

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DVMRP Prune

• Sent from routers receiving multicast traffic

for which they have no downstream group

members

• “Prunes” the tree created by DVMRP

• Stops needless data from being sent

DVMRP Graft

• Used after a branch has been pruned back

• Sent upstream by a router that has a host that joins a multicast group

• Goes from router to router until a router active on the multicast group is reached

• Sent for the following cases

– A new host member joins a group

– A new dependent router joins a pruned branch

– A dependent router restarts on a pruned branch

– If a Graft Ack is not received before the timeout

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DVMRP Graft ACK

• Used to acknowledge receipt of Graft message

• Allows sending downstream router to know

upstream router has received and processed

its request

• If not received within 5 sec. Graft message

sent again

DVMRP Issues

• Hop count used as metric

• Scalability issues

– Performs periodic flooding

– Maintains its own routing table

• Older versions lack pruning

• Even though it is a main part of MBone, MBone is being phased out

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DVMRP: Summary

• Distance Vector Multicast Routing Protocol

• Leaf router sends a prune message to neighbouring routers

when there is no group member on the subnet.

• Intermediate routers perform pruning whenever possible.

• Flooding and pruning are repeated periodically, when the

current state times out.

• Between flooding rounds, a leaf router can re-join a group by

sending a graft message upstream.

• Intermediate routers propagates the graft message upstream

until the path is re-connected.

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MBone

• Multicast backbone of the Internet.

• Not all routers support multicast routing protocols and IGMP.

• Connecting multicast-capable routers using (virtual) IP

tunnels.

• Was a long-running experimental approach to enabling

multicast between sites through the use of tunnels.

• No longer operational.

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Reading

• Section 19.1, Stallings

More references:

• Multicasting on the Internet and Its Applications, Sanjoy Paul,

Kluwer Academic Publishers, 1998, chapters 2, 4, 5.

• Computer Networking: A Top-Down Approach, 5th edition,

Kurose and Ross.

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