A network designed to deliver a multicast service using IGMP might use this basic architecture: IGMP operates between a host and a local multicast router. Switches featuring IGMP snooping derive useful information by observing these IGMP transactions. Protocol Independent Multicast is then used between the local and remote multicast routers, to direct multicast traffic from hosts sending multicasts to hosts that have registered through IGMP to receive them. IGMP operates on the network layer, just the same as other network management protocols like ICMP. The IGMP protocol is implemented on a particular host and within a router. A host requests membership to a group through its local router while a router listens for these requests and periodically sends out subscription queries. A single router per subnet is elected to perform this querying function. Some multilayer switches include an IGMP querier capability to allow their IGMP snooping features to work in the absence of an IP multicast capability in the larger network. IGMP is vulnerable to some attacks, and firewalls commonly allow the user to disable it if not needed.
Versions
There are three versions of IGMP. IGMPv1 is defined by, IGMPv2 is defined by and IGMPv3 was initially defined by and has been updated by which defines both IGMPv3 and MLDv2. IGMPv2 improves IGMPv1 by adding the ability for a host to signal desire to leave a multicast group. IGMPv3 improves IGMPv2 by supporting source-specific multicast and introduces membership report aggregation. These versions are backwards compatible. A router supporting IGMPv3 can support clients running IGMPv1, IGMPv2 and IGMPv3. IGMPv1 uses a query-response model. Queries are sent to 224.0.0.1. Membership reports are sent to the group's multicast address. IGMPv2 accelerates the process of leaving a group and adjusts other timeouts. Leave-group messages are sent to 224.0.0.2. A group-specific query is introduced. Group-specific queries are sent to the group's multicast address. A means for routers to select an IGMP querier for the network is introduced. IGMPv3 introduces source-specific multicast capability. Membership reports are sent to 224.0.0.22.
Messages
There are several types of IGMP messages: ;General membership queries ;Group-specific membership queries ;Group-and-source-specific queries ;Membership reports ;Leave group messages IGMP messages are carried in bare IP packets with IP protocol number 2. Similar to the Internet Control Message Protocol, there is no transport layer used with IGMP messaging.
IGMPv2 messages
Where: ; Type : Indicates the message type as follows ; Max Resp Time : Specifies the required responsiveness of replies to a Membership Query. This field is meaningful only in Membership Query; in other messages it is set to 0 and ignored by the receiver. The field specifies time in units of 0.1 second. Larger values reduce IGMP traffic burstiness and smaller values improve protocol responsiveness when the last host leaves a group. ; Group Address : This is the multicast address being queried when sending a Group-Specific or Group-and-Source-Specific Query. The field is zeroed when sending a General Query. The message is sent using the following IP destination addresses:
Message Type
Multicast Address
General Query
All hosts
Group-Specific Query
The group being queried
Membership Report
The group being reported
Leave Group
All routers
IGMPv3 membership query
Where: ; Max Resp Code : This field specifies the maximum time allowed before sending a responding report. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent and mantissa. ; Checksum : This is the 16-bit one's complement of the one's complement sum of the entire IGMP message. ; Group Address : This is the multicast address being queried when sending a Group-Specific or Group-and-Source-Specific Query. The field is zeroed when sending a General Query. ; Resv : This field is reserved. It should be zeroed when sent and ignored when received. ; S Flag : When this flag is set, it indicates to receiving routers that they are to suppress the normal timer updates. ; QRV : If this is non-zero, it contains the Robustness Variable value used by the sender of the query. Routers should update their Robustness Variable to match the most recently received query unless the value is zero. ; QQIC : This code is used to specify the Query Interval value used by the querier. If the number is below 128, the value is used directly. If the value is 128 or more, it is interpreted as an exponent and mantissa. ; Number of Sources : This field specifies the number of source addresses present in the query. For General and Group-Specific Queries, this value is zero. For Group-and-Source-Specific Queries, this value is non-zero, but limited by the network's MTU. ; Source Address : The Source Address fields are a vector of n IP unicast addresses, where n is the value in the Number of Sources field.
