Multicast on the Internet

1
Multicast on the Internet

• CSE 6590

2
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 1s)
• 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.

3
IPv4 Address Formats
4
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.

5
Mapping from Class D IP adress to Ethernet
multicast adress
6
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.

7
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.

8
Multicast Protocols

• Transport layer
• UDP
• Real-time Transport Protocol (RTP) for
  multimedia content
• ReSerVation Protocol (RSVP) for bandwidth
  reservation in a multicast distribution

9
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)

10
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
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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

report
query
12
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)

• rely on routers 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

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Reverse Path Forwarding Example
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Spanning-Tree Broadcast
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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,
  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
S source
LEGEND
R1
R4
router with attached group member
R2
router with no attached group member
R5
link used for forwarding, i indicates order
link added by algorithm
R3
R7
R6
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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.
• Dijkstras algorithm.

S source
LEGEND
R1
R4
router with attached group member
R2
router with no attached group member
R5
link used for forwarding, i indicates order
link added by algorithm
R3
R7
R6
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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, ltR1, R2, gt.
• 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.