One to many delivery
Multicast delivers a single packet to a group of receivers that have chosen to listen. Unlike unicast, which sends a separate copy per receiver, the source sends one stream and the network duplicates packets only where paths diverge. This saves bandwidth for live video and market data feeds.
How groups work
- A multicast group address identifies the stream rather than any single host.
- Receivers join a group, and senders simply send to the group address.
- Hosts tell their local router which groups they want using a membership protocol.
- Routers build a distribution tree so each link carries at most one copy of the stream.
On a local network, a host signals interest with the Internet Group Management Protocol, and switches can use snooping to forward the stream only to ports with listeners. Across routers, protocols such as PIM construct the tree between networks. Multicast usually rides on UDP because acknowledging every receiver would not scale, so applications add their own loss handling when needed.
Key idea
Multicast sends one packet to a group and lets the network duplicate it only where paths split, saving bandwidth.