tcp-ip IGMP: Internet Group Management Protocol

The Internet Group Management Protocol (IGMP) is used by hosts and routers that support multicasting. It lets all the systems on a physical network know which hosts currently belong to which multicast groups. This information is required by the multicast routers, so they know which multicast datagrams to forward onto which interfaces.

IGMP has a fixed-size message, with no optional data. IGMP messages are specified in the IP datagram with a protocol value of 2.

The IGMP version is 1. An IGMP type of 1 is a query sent by a multicast router, and 2 is a response sent by a host.

The group address is a class D IP address. In a query the group address is set to 0, and in a report it contains the group address being reported.

Joining a Multicast Group

Fundamental to multicasting is the concept of a process joining a multicast group on a given interface on a host. (We use the term process to mean a program being executed by the operating system.) Membership in a multicast group on a given interface is dynamic-it changes over time as processes join and leave the group.

We imply here that a process must have a way of joining a multicast group on a given interface. A process can also leave a multicast group that it previously joined. These are required parts of any API on a host that supports multicasting. We use the qualifier "interface" because membership in a group is associated with an interface. A process can join the same group on multiple interfaces.

Implied here is that a host identifies a group by the group address and the interface. A host must keep a table of all the groups that at least one process belongs to, and a reference count of the number of processes belonging to the group.

IGMP Reports and Queries

IGMP messages are used by multicast routers to keep track of group membership on each of the router's physically attached networks. The following rules apply.

1. A host sends an IGMP report when the first process joins a group. If multiple processes on a given host join the same group, only one report is sent, the first time a process joins that group. This report is sent out the same interface on which the process joined the group.
2. A host does not send a report when processes leave a group, even when the last process leaves a group. The host knows that there are no members in a given group, so when it receives the next query (next step), it won't report the group.
3. A multicast router sends an IGMP query at regular intervals to see if any hosts still have processes belonging to any groups. The router must send one query out each interface. The group address in the query is 0 since the router expects one response from a host for every group that contains one or more members on that host.
4. A host responds to an IGMP query by sending one IGMP report for each group that still contains at least one process.

Using these queries and reports, a multicast router keeps a table of which of its interfaces have one or more hosts in a multicast group. When the router receives a multicast datagram to forward, it forwards the datagram (using the corresponding multicast link-layer address) only out the interfaces that still have hosts with processes belonging to that group.

Time-to-Live Field

The TTL field of the reports and queries is set to 1. This refers to the normal TTL field in the IP header. A multicast datagram with an initial TTL of 0 is restricted to the same host. By default, multicast datagrams are sent with a TTL of 1. This restricts the datagram to the same subnet. Higher TTLs can be forwarded by multicast routers.

An ICMP error is never generated in response to a datagram destined to a multicast address. Multicast routers do not generate ICMP "time exceeded" errors when the TTL reaches 0.

By increasing the TTL an application can perform an expanding ring search for a particular server. The first multicast datagram is sent with a TTL of 1. If no response is received, a TTL of 2 is tried, then 3, and so on. In this way the application locates the closest server, in terms of hops.

The special range of addresses 224.0.0.0 through 224.0.0.255 is intended for applications that never need to multicast further than one hop. A multicast router should never forward a datagram with one of these addresses as the destination, regardless of the TTL.

All-Hosts Group
We indicated that the router's IGMP query is sent to the destination IP address of 224.0.0.1. This is called the all-hosts group address. It refers to all the multicast-capable hosts and routers on a physical network. Each host automatically joins this multicast group on all multicast-capable interfaces, when the interface is initialized. Membership in this group is never reported.