Internet Core Protocols: The Definitive Guide

Chapter 4. Multicasting and the Internet Group Management Protocol

Summary	IP multicasting allows IP datagrams to be sent to explicit hosts on multiple networks simultaneously. This is different from traditional broadcasting in that not all hosts have to process the data, and hosts on remote networks can also receive the datagrams. The Internet Group Management Protocol provides a mechanism for IP devices to register with multicast routers, indicating the multicast groups they are interested in participating in.
Protocol ID	2
Relevant STDs	2 (http://www.iana.org/); 3 (includes RFCs 1122 and 1123); 5 (includes RFCs 791, 792, 919, 922, 950 and 1112)
Relevant RFCs	1112 (IP Multicasting and IGMPv1); 1122 (Host Network Requirements); 1469 (Token Ring Multicasting); 2236 (IMGP v2)
Related RFCs	1075 (Distance Vector Multicast Routing Protocol); 1256 (Router Discovery); 1584 (Multicast Extensions to OSPF); 2365 (Administratively Scoped IP Multicast); 2588 (IP Multicast and Firewalls)

When a system needs to communicate with other hosts on the local network, it typically does so either by broadcasting a packet to every host on the local network or by sending unicast datagrams to a specific destination.

Another alternative that can be used is “multicasting,” a technique that lets a system send data to a group address. Any hosts that are monitoring that group address will receive the data, while every other device on the network will simply ignore it.

This model is particularly useful for applications that require a host to send data to multiple destinations simultaneously, but would be hindered by the limitations found with broadcasts.

For example, applications such as streaming audio require that a host be able to send data to multiple hosts simultaneously. While it may seem that general-purpose broadcasting can be used for these types of applications, it is not necessarily the most efficient model to use. For one thing, broadcasts are meant for every host on the local network. As such, they will not reach hosts that are on remote networks (most routers filter out broadcast traffic). But if a host on a remote network needed to receive that data, then the broadcast traffic would have to be re-broadcast to the remote network, getting sent to every host on that remote network, regardless of whether or not they wanted the traffic.

Furthermore, with broadcast traffic, every device on the local network has to monitor all of the packets and then decide if the data is needed by any local applications. If the data wasn’t useful, then it still had to be processed before this fact was discovered. With bandwidth-intensive applications like network multimedia, this traffic results in every system on the network spending a large amount of computing resources examining and then discarding unwanted messages. But by using multicasts instead of broadcasts, hosts can choose which network streams they want to monitor, and higher-layer protocols will only have to process the packets they are specifically interested in. Hosts that do not want any data from a particular multicast group will not have to process anything extra.

In short, multicasts allow for broadcast-like activity, while providing explicit support for remote hosts and networks on a selective basis. This is a particularly important service with bandwidth-intensive applications like network multimedia, but also is useful for any application that requires a host be able to communicate with multiple systems simultaneously.

For example, the Router Discovery protocol (defined in RFC 1256) makes extensive use of multicasting to allow devices to automatically locate the routers on a network, and there is a draft proposal that proposes to imbue DNS clients with the ability to locate DNS servers automatically using this same model. There are even proposals for using multicast transfers with TFTP to send data to multiple hosts simultaneously, and for replicating Usenet newsfeeds to multiple servers with a single stream (instead of many large unicast transfers).

Multicasts are useful for all of these applications, allowing data to be sent to multiple systems simultaneously, rather than sending it on a point-to-point basis to each of them discreetly. In fact, anything that can benefit from a single feed to multiple, distributed destinations can benefit from IP multicasts.