What is Multicast?
Multicast IP Routing protocols are used to distribute data
(for example, audio/video streaming broadcasts) to multiple recipients. Using
multicast, a source can send a single copy of data to a single multicast address, which is then distributed to an entire group of recipients.
A multicast group identifies a set of recipients that are interested in a
particular data stream, and is represented by an IP address from a well-defined
range. Data sent to this IP address is forwarded to all members of the multicast
Routers between the source and recipients duplicate data packets and forward multiple copies wherever the path to recipients diverges. Group membership
information is used to calculate the best routers at which to duplicate the
packets in the data stream to optimize the use of the network.
A source host sends data to a multicast group by simply setting the destination
IP address of the datagram to be the multicast group address. Any host can
become a source and send data to a multicast group. Sources do not need to
register in any way before they can begin sending data to a group, and do
not need to be members of the group themselves.
There are many different multicast protocols and modes of operation, each
optimized for a particular scenario. Many of these are still at an early stage of standardization. However, they all operate
in the same general way, as follows.
The multicast distribution tree of receiving hosts holds the route to every recipient that has
joined the multicast group, and is optimized so that
- multicast traffic does not reach networks that do not have any
such recipients (unless the network is a transit network on the way to other recipients)
- duplicate copies of packets are kept to a minimum.
More information about multicast routing protocols can be found in Metaswitch's White Paper, IP Multicast Explained.
For more information about Metaswitch's
Multicast IP Routing products and expertise contact