Unicast: one stream per viewer
Each viewer’s device opens its own connection to a server and receives its own copy of the stream. A thousand viewers of one channel means a thousand copies leaving the servers.
- Used by: streaming apps, websites, and IPTV player apps with a subscription. HLS and MPEG-TS over HTTP are unicast.
- Strengths: works across the whole internet, on any network, with nothing special in your router. Each viewer can pause, rewind or join at a different point.
- Weakness: the provider has to send, and pay for, a copy per viewer. Big events need large server capacity and content delivery networks.
Multicast: one stream, shared
The source sends a channel once to a multicast group address. Routers copy it only where there are viewers who have asked for it. A thousand viewers of one channel on the same network cost little more than one.
- Used by: managed TV services inside an operator’s network, to the operator’s own set-top boxes.
- How you join: your box tells the nearby router which group it wants. RFC 3376 describes IGMP as the protocol IPv4 hosts and routers use to report their multicast group memberships to neighbouring multicast routers.
- Weakness: it needs every router on the path to support it, which the operator controls on its own network but not across the open internet. That’s why streaming apps don’t rely on it.
Side by side
| Unicast | Multicast | |
|---|---|---|
| Copies sent | One per viewer | One per channel, copied where needed |
| Works across the internet | Yes | Generally only inside managed networks |
| Channel change | New request to the server | Leave one group, join another (IGMP) |
| Pause and rewind | Easy | Needs extra network features |
| Router settings at home | None | IGMP proxy or snooping, sometimes a VLAN |
What it means at home
- If you use IPTV apps, everything is unicast. You don’t need IGMP, VLAN or “IPTV mode” settings, and turning them on won’t help. See what is IPTV mode on a router.
- If your broadband provider’s TV box stops working after you change router, it may rely on multicast settings the new router doesn’t have. That’s a question for the broadband provider.
- If busy evenings affect your IPTV, that’s the cost of unicast over the open internet: every viewer pulls their own stream through the same networks. See how to stop IPTV buffering.