Multicast on the Internet
CSE 6590
6 April 2015
1
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.
2
IPv4 Address Formats
3
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.
4
Mapping from Class D IP adress to Ethernet
multicast adress
5
Internet Multicast Service Model
128.59.16.12
128.119.40.186
multicast
group
226.17.30.197
128.34.108.63
128.34.108.60
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.
6
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.
7
Multicast Protocols
Transport layer
• UDP
• Real-time Transport Protocol (RTP): for multimedia
content
• ReSerVation Protocol (RSVP): for bandwidth
reservation in a multicast distribution
8
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)
9
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)
IGMP
IGMP
wide-area
multicast
routing
IGMP
10
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
query
report
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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 groupspecific 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 hostmembership 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.
13
Reverse Path Forwarding
• Building a loop-free broadcast tree
• No knowledge of group membership
14
Reverse Path Forwarding (2)
 rely on router’s knowledge of unicast shortest
path from it to sender
 each router has simple forwarding behavior:
if (multicast datagram received on incoming link on
shortest path back to sender)
then flood datagram onto all outgoing links
else ignore datagram
15
Reverse Path Forwarding: Example
A
B
c
F
E
D
G
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Spanning-Tree Broadcast
A
B
c
F
A
E
B
c
D
F
G
(a) Broadcast initiated at A
E
D
G
(b) Broadcast initiated at D
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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, sourcebased 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
S: source
LEGEND
R1
1
2
R4
R2
3
R3
router with attached
group member
5
4
R6
router with no attached
group member
R5
6
R7
i
link used for forwarding,
i indicates order link
added by algorithm
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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.
S: source
LEGEND
R1
1
2
R4
R2
3
R3
router with attached
group member
5
4
R6
router with no attached
group member
R5
6
R7
i
link used for forwarding,
i indicates order link
added by algorithm
25
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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