Other Multicast Technologies
As of 2006, most effort at scaling multicast up to large networks have concentrated on the simpler case of single-source multicast, which seems to be more computationally tractable.
Still, the large state requirements in routers make applications using a large number of trees unworkable using IP multicast. Take presence information as an example where each person needs to keep at least one tree of its subscribers, if not several. No mechanism has yet been demonstrated that would allow the IP multicast model to scale to millions of senders and millions of multicast groups and, thus, it is not yet possible to make fully general multicast applications practical. For these reasons, and also reasons of economics, IP multicast is not, in general, used in the commercial Internet backbone. The increasing availability of WiFi Access Points that support multicast IP is facilitating the emergence of WiCast WiFi Multicast that allows the binding of data to geographical locations.
Explicit Multi-Unicast (XCAST) is an alternate multicast strategy to IP multicast that provides reception addresses of all destinations with each packet. As such, since the IP packet size is limited in general, XCAST cannot be used for multicast groups of large number of destinations. The XCAST model generally assumes that the stations participating in the communication are known ahead of time, so that distribution trees can be generated and resources allocated by network elements in advance of actual data traffic.
Other multicast technologies not based on IP multicast are more widely used. Notably the Internet Relay Chat (IRC), which is more pragmatic and scales better for large numbers of small groups. IRC implements a single spanning tree across its overlay network for all conference groups. However, this leads to suboptimal routing for some of these groups. Additionally, IRC keeps a large amount of distributed states that limit growth of an IRC network, leading to fractioning into several non-interconnected networks. The lesser known PSYC technology uses custom multicast strategies per conference. Also some peer-to-peer technologies employ the multicast concept when distributing content to multiple recipients.
Read more about this topic: Multicast