Multicast on the Internet

CSE 6590

1April 19, 2023

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) multicastBoth cases need filtering at IP layer.

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IPv4 Address Formats

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Multicast Addressing at Link Layer

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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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

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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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

IGMP: Internet Group Management Protocol

• 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.• 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

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query report

IGMP• Router: broadcasts Host

Membership Query message on LAN.

• Host: replies with Host Membership Report message to indicate group membership– randomized delay before

responding– cancel its own report if

hearing another– 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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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.

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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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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

Truncated Broadcasting

• Extension of Reverse Path Forwarding.• No members of a group on a subnet leaf router will not

forward packets of this group to the subnet (pruning).• But does not reduce traffic in the core network.• More efficient multicast routing is needed!!!

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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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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: 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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Multicast Routing Approaches

• Minimum cost trees– Steiner trees

• Shortest path trees– Source-based trees– Core-based trees

…we first look at basic approaches, then specific protocols adopting these approaches

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Steiner Trees

• Steiner Tree: minimum cost tree connecting all routers with attached group members.

• Problem is NP-complete.• Excellent heuristics exist.• Not used in practice:

– computational complexity.– information about entire network needed.– monolithic: rerun whenever a router needs to join/leave.

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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 autonomous

system.• 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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Core-Based Trees

• For many-to-many multicast.• Protocols: CBT, PIM-SM, PIM-DM (Protocol Independent

Multicast, sparse/dense mode)• Purpose: to reduce the amount of routing info stored at

routers when a multicast group has a large number of members and multiple senders.

• A multicast group requires a core (rendez-vous point).• Receivers “join” the (shortest-path) tree rooted at the core

only one tree per multicast group (used for multiple senders).• Sources send multicast data to the core, which then

multicasts the data to the tree.

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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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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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