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组播
某些应用是一对多或多对多通信，如广播流、电视频道、电话会议和游戏；单播“每个接收者一份分组”的冗余会压垮发送方，因此 IP 组播沿分发树只发送一次分组，该树仅在必要时分叉。主机通过本地路由器借助 IGMP 自由加入和离开组，而发送方只需寻址到该组；地址取自保留范围，并压缩到 23 位以太网组播地址上。早期路由采用逆向路径转发进行洪泛与剪枝；留存下来的设计——稀疏模式 PIM——从加入报文朝汇聚点构建显式树。域间组播几乎从未真正实现，其任务由应用层覆盖网络继承。
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?