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Multicasting. CSE 6590 Winter 2012. Internet Multicast Service Model. multicast group Multicast group concept: use of indirection a host “sends” IP datagrams to multicast group.

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CSE 6590

Winter 2012

Internet multicast service model

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.


Multicast groups

Multicast Groups

  • Class D Internet addresses reserved for multicast:

  • Host group semantics:

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

    • anyone can send to multicast group.

    • no network-layer identification to hosts of members.

  • Needed: infrastructure to deliver multicast-addressed datagrams to all hosts that have joined that multicast group.


Multicast addressing

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.

Ipv4 address formats

IPv4 Address Formats

Multicast addressing 2

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.

  • Redundant reception by many hosts.

    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.

Mapping from class d ip adress to ethernet multicast adress

Mapping from Class D IP adress to Ethernet multicast adress

Multicast protocols

Multicast Protocols

Transport layer

  • UDP

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

  • ReSerVation Protocol (RSVP): for bandwidth reservation in a multicast distribution

Multicast protocols 2

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):


  • Inter-domain (routing):

    • Multicast Border Gateway Protocol (MBGP)

Joining a multicast group 2 step process

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)









Internet group management protocol 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 to (join)and unsubscribe from (leave) multicast group

  • routers check which multicast groups are of interest to which hosts

  • IGMP currently at version 3

Igmp 2

IGMP (2)

  • 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 “unsubscribe” when leaving



Igmp 3

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

    • may send multiple times

    • 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

Operation of igmp v1 and v2

Operation of IGMP v1 and v2

  • IGMPv1

    • hosts could join group

    • routers used timer to unsubscribe members

  • IGMPv2enabled 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


  • spamming of multicast groups

  • establishment of distribution trees is problematic (source location unknown)

  • finding globally unique multicast addresses difficult (duplicate addresses)

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

Igmp message formats membership query

IGMP Message FormatsMembership Query

  • sent by multicast router

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

Membership query fields

Membership Query Fields

Igmp message formats membership report

IGMP Message FormatsMembership Report

Igmp message formats group record

IGMP Message FormatsGroup Record

Igmp operation joining

IGMP Operation - Joining

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

  • IGMPv3 allows hosts to specify wanted or unwanted sources (filtering capabilities)

    • EXCLUDE mode – all sources except those listed

    • INCLUDE mode – only from sources listed

Igmp operation keeping lists valid

IGMP Operation – Keeping Lists Valid

Igmp operation leaving

IGMP Operation - Leaving

  • host leaves group by sending a leave group message to the all-routers static multicast address

    • sends a membership report message withEXCLUDE option and null list of source addresses

  • router determines if have any remaining group members using group-specific query message

Group membership with ipv6

Group Membership with IPv6

  • IGMP defined for IPv4

    • uses 32-bit addresses

  • IPv6 internets need the same 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 messages as IGMP.

Igmp summary

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

Multicast routing

Multicast Routing

Shortest path tree








Shortest Path Tree

  • Multicast cast forwarding tree: tree of shortest path routes from source to all receivers.

    • Dijkstra’s algorithm.

S: source




router with attached

group member


router with no attached

group member


link used for forwarding,

i indicates order link

added by algorithm






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).


Reverse path forwarding

Reverse Path Forwarding

  • Building a loop-free broadcast tree

  • No knowledge of group membership

Reverse path forwarding 2

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:

Reverse path forwarding example








Reverse Path Forwarding: Example

Spanning tree broadcast















(b) Broadcast initiated at D

(a) Broadcast initiated at A

Spanning-Tree Broadcast

Internet multicasting routing dvmrp

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

Dvmrp example








DVMRP Example

S: source




router with attached

group member


router with no attached

group member


link used for forwarding,

i indicates order link

added by algorithm




How dvmrp fits into multicasting

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

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.

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

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

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

Dvmrp summary

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.



  • 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.



  • 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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