Tuesday 24 February 2009

Flavours of Multicast

The reason why multicast is not prevalent on the Internet today is due to two main reasons, first is lack of support in network infrastructure.  Multicast is an optional part of the IPv4 protocol and so not every vendor has implemented support.  Second is filtering, as in what control is there over different parties sending data to any multicast group.  This is an issue as multicast uses a separate range of IP addresses for its communication of which is a limited number.

As an example, imagine the US President Obama’s inauguration is being being multicast live on the Internet, what happens if at the same time a radio station in New Zealand is broadcasting live news, the Hong Kong Stock Exchange is publishing stock prices, and Wembley stadium sending live match details from London?  The answer is a mess, wasted routing and link resources forwarding packets from all around to world to parties simply not interested.

This method of multicast, the default operation, is called any-source multicast (ASM), and is more suited to controlled environments such as private networks in which applications and network topology can be arranged to suit the expected usage and conflicts from other applications is not going to occur.

Source-specific multicast (SSM) was then created to limit the source of packets to a selected range of addresses.  This requires end-point router support, IGMPv3 for IPv4, and MLDv2 for IPv6, together with operating system support for the matching API and filtering without router support.