Computer Networks · free preview
Multicast
Some applications speak one-to-many or many-to-many, radio streams, television channels, teleconferences and games, and unicast's one-packet-per-receiver redundancy would crush the sender, so IP multicast delivers each packet once along a distribution tree that branches only where it must, hosts join and leave groups freely through their local router with IGMP while senders simply address the group, addresses come from a reserved range squeezed onto 23-bit Ethernet multicast addresses, early routing flooded and pruned with reverse path forwarding, and the surviving design, sparse-mode PIM, builds explicit trees from joins toward a rendezvous point, with interdomain multicast never quite happening and application-layer overlays inheriting its job.
1. Why Not Send N Copies
Why Not Send N Copies
2. Flood and Prune
Flood and Prune
3. Sparse Mode and the Rendezvous
Sparse Mode and the Rendezvous
4. The Domain Boundary
The Domain Boundary
5. The sending cost
A radio station streams to ten thousand listeners. Why is plain unicast the wrong tool?
6. The group abstraction
In IP multicast, what does a group address name, and who may send to it?
7. The 23-bit squeeze
Why can two different IP multicast groups share one Ethernet multicast address, and what handles the overlap?
8. Reverse path forwarding
A router receives a flooded multicast packet from a source. How does it decide whether to forward the flood?
9. Sparse versus dense
Why does sparse-mode multicast replace flood-and-prune with explicit joins?
10. The boundary it never crossed
What happened to multicast between domains?
11. Group management
Which protocol does a host speak to its local router to join or leave a multicast group?
12. The pruned message
In flood-and-prune multicast, what message does a router send upstream when no hosts downstream belong to the group?
13. The meeting point
In sparse-mode multicast, each group has a designated router where senders and receivers' join messages meet. What is it called?
14. The denser default
PIM's dense mode assumes most networks want the group and therefore uses which older algorithmic approach?
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