An introduction to IGMP

Now that we know why do we need multicast and how multicast addressing works, let us look at how hosts/receivers indicate their interest in receiving contents for a given multicast group.

As described in my earlier post on multicast addressing, Applications/Servers uses a IP Multicast Address (Class D address) as the destination address in the IP data packet while sending out the contents.

Let me take a simple test. Please take a look at figure 1 below, what do you think who all will receive the multicast traffic sent by the Multicast Server?

Did you say 'all-of-them'? If yes, you got it wrong. None of them will receive the multicast traffic as they did not indicated their interest in receiving the multicast traffic.

So how do receivers indicate their interest? They use a simple protocol known as IGMP (Internet Group Management Protocol). IGMP has two parts. One part runs in the hosts (receivers) which is used to indicate host's interest while other part runs in routers which is used to collect the interests of all local hosts.  IGMP runs directly on top of IP protocol and so is unreliable in nature. Routers running IGMP sends queries and all hosts send their current multicast state in the form of IGMP reports.

In a LAN environment, when a host indicate its interest in a IP multicast address, its corresponding ethernet MAC gets configured in the NIC to receive multicast data traffic. So if you look back at figure-1 and if PC-1 has indiacated its interest in the group, only PC-1's NIC will be configured and so only PC-1 will receive the multicast traffic.

Let me give you a little run down on the two parts of IGMP.

Host part:

Host part, as the name suggest, runs in a host. When the host is interested in receiving traffic for a multicast group or a combination of group and source, it sends out an unsolicited IGMP membership report.

Host usually sends two memberrship report messages. This is to make sure that the routers receive at least one. Can you guess why is this done?

As I mentioned IGMP is unreliable and this is one of the simplest way to make sure that all routers receive at least one of the report. Now you will ask, what if both the reports are lost? I will say 'good thinking'. To just take care of such a scenario, this count, known as 'robustness count' can be configured. So if you think your network can loose two reports, configure this as three or so on. If you are increasing this count, you must know that there will be more control packets on the network.

Now let us assume that all unsolicited reports are lost, how will this work? It may not work till the next query comes in.  Once the next query comes, host will generate a solicited report which router can use to update its state.

Once host does not want to receive multicast traffic, it generates a leave.

When host receives a general query, it sends out solicited membership report with all the multicast groups it is interested in.

When host receives a group or group-source specific query, it sends out membership report with this group or group-source information.

Router part:

Router part, as the name suggest, runs in a router. All routers on a LAN elect one router as querier. This router sends general queries periodically.

All the routers on a LAN does querier election and once a querier is selected, only that router sends queries. The router with the lowest IP address is selected as the querier.

When routers receives the unsolicited membership report, it creates local forwarding state and refresh this state when solicited membership reports are received.

When routers receive leave message, it does not leave the group immediately, it first send group or group-and-source specific query and if no host is interested in this group or group-and-source, router removes the local state.

Usually IGMP is configured either in IGMP Proxy mode or along with a multicast routing protocol like PIM (i.e both IGMP router part and a multicast routing protocol is enabled). In case of IGMP proxy, there is usually one single upstream router. In case of multicast routing protocol, routing protocol defines which router should handle the locally connected host membership. For example: in PIM-SM, Designated Router (DR) or the Assert Winner takes care of local host memberships. Please look at figure-2 where router R1 acts as IGMP proxy while router R2 and R3 on LAN-2 are configured with PIM-SM. In LAN-2, router R2 is DR and so handles the IGMP joins sent by R1 on upstream interface.

Whats next:

I have explained the basic IGMP functionality here. I will cover some of the finer points of IGMP in other posts. Now that we understand how IGMP works, we will next look at IGMP proxying.

If you want me to explain any specific behaviour of IGMP, please do let me know.