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Multicast on the Internet
CSE 6590Fall 2009
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Addressing• Class D address• Ethernet broadcast address (all 1’s)• IP multicast using
– Link-layer (Ethernet) broadcast– Link-layer (Ethernet) multicastBoth cases need filtering at IP layer.
• Source: unicast IP address SReceivers: multicast group ID G
• Each group is identified by (S, G)
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IPv4 Address Formats
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Mapping from Class D IP adress to Ethernet multicast adress
Reverse Path Forwarding
if (multicast datagram received on incoming link on shortest path back to center)
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 (2)
• Building a loop-free broadcast tree• No knowledge of group membership
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A
B
G
DE
c
F
Reverse Path Forwarding
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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
Spanning-Tree Broadcast
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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
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 groupo anyone can send to multicast groupo 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 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:
– Internet Group Management Protocol (IGMP)– Multicast Listener Discovery (MLD): similar to IGMP
but for IPv6• Intra-domain:
– MOSPF, PIM, DVMRP• Inter-domain:
– 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)
IGMPIGMP
IGMP
wide-areamulticast
routing
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IGMP: Internet Group Management Protocol
• Host: sends IGMP report when application joins 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
query report
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IGMP• Router: Host
Membership Query message broadcast on LAN to all hosts.
• Host: 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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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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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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Multicast Routing Approaches
• Minimum cost trees– Minimum Steiner trees
• Shortest path trees– Source-based trees– Core-based trees
…we first look at basic approaches, then specific protocols adopting these approaches
Minimum 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.
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 45
i
router with attachedgroup member
router with no attachedgroup memberlink used for forwarding,i indicates order linkadded by algorithm
LEGENDS: source
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 flooded
everywhere via RPF– routers not wanting the multicast data: send
prune messages to upstream neighbors
DVMRP Example
R1
R2
R3
R4
R5
R6 R7
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6
3 45
i
router with attachedgroup member
router with no attachedgroup memberlink used for forwarding,i indicates order linkadded by algorithm
LEGENDS: source
DVMRP (2)• Soft state: DVMRP router periodically (1 min.) “forgets”
that branches are pruned: – mcast data again flows down unpruned branches– downstream routers: reprune or else continue to
receive data• Routers can quickly regraft to tree
– following IGMP join at leaf• Odds and ends
– 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 host can re-join a group by
sending a graft message.• Intermediate routers propagates the graft message
upstream until the path is re-connected.
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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 by exchanging IGMP
messages with its end-router.• The end-router broadcasts the presence of this
destination (group membership) to the whole network.• Each router maintains a group membership table [S,G,
<d1, d2, …>]• 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• CBT, PIM-SM, PIM-DM• 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• Was a long-running experimental approach to enabling
multicast between sites through the use of tunnels• No longer operational• Not all routers support multicast routing protocols and
IGMP.• Connecting multicast-capable routers using (virtual) IP
tunnels
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References• Multicasting on the Internet and Its Applications, Sanjoy
Paul, Kluwer Academic Publishers, 1998.