IGMPv3 enhancements

IGMPv3 is based on and is compatible with IGMPv1 and IGMPv2. It enhances the control capabilities of hosts and the query and report capabilities of IGMP routers.

IGMPv3 introduced two source filtering modes (Include and Exclude). These modes allow a host to choose whether to receive or reject multicast data from the specified multicast sources. When a host joins a multicast group, one of the following occurs:

• If the host expects to receive multicast data from specific sources like S1, S2, …, it sends a report with the Filter-Mode denoted as "Include Sources (S1, S2, …)."

• If the host expects to reject multicast data from specific sources like S1, S2, …, it sends a report with the Filter-Mode denoted as "Exclude Sources (S1, S2, …)".

As shown in Figure 30, the network has two multicast sources: Source 1 (S1) and Source 2 (S2). Both of these sources can send multicast data to the multicast group G. Host B wants to receive multicast data addressed to G from Source 1 but not from Source 2.

In IGMPv1 or IGMPv2, Host B cannot select multicast sources when it joins the multicast group G. The multicast streams from both Source 1 and Source 2 flow to Host B whether or not it needs them.

In IGMPv3, Host B can explicitly express that it needs to receive the multicast data addressed to G from Source 1 but not from Source 2. As a result, Host B receives only multicast data from Source 1.

IGMPv3 introduces IGMP group-and-source queries and IGMP reports carrying group records.

• Query message carrying the source addresses

  Compatible with IGMPv1 and IGMPv2, IGMPv3 supports general queries and group-specific queries. In addition, it introduces group-and-source-specific queries.

  • A general query does not carry a group address or a source address.

  • A group-specific query carries a group address, but no source address.

  • A group-and-source-specific query carries a group address and one or more source addresses.

• Reports containing multiple group records

  Unlike an IGMPv1 or IGMPv2 report, an IGMPv3 report is destined to 224.0.0.22 and contains one or more group records. Each group record contains a multicast group address and a multicast source address list.

  Group records include the following categories:

  • IS_IN—The current filtering mode is Include. The report sender requests the multicast data from only the sources specified in the Source Address field.

  • IS_EX—The current filtering mode is Exclude. The report sender requests the multicast data from any sources except those specified in the Source Address field.

  • TO_IN—The filtering mode has changed from Exclude to Include.

  • TO_EX—The filtering mode has changed from Include to Exclude.

  • ALLOW—The Source Address field contains a list of additional sources from which the receiver wants to obtain data. If the current filtering mode is Include, these sources are added to the INCLUDE source address list. If the current filtering mode is Exclude, these sources are deleted from the EXCLUDE source address list.

  • BLOCK—The Source Address field contains a list of the sources from which the receiver no longer wants to obtain data. If the current filtering mode is Include, these sources are deleted from the INCLUDE source address list. If the current filtering mode is Exclude, these sources are added to the EXCLUDE source address list.