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Multicast
In multicast applications, Internet datagrams (IP packets) are normally sent as unicast packets i.e., from one source to one destination. However when multiple receivers require the same data, replicating the data from the source to all the receivers as individual unicast packets increases the network load, resulting in network congestion and loss of expected QoS. IP multicast enables efficient transfer of data from a set of sources to a dynamically formed set of receivers.
But, IP multicast also has its
challenges. these include:
 | Maintaining dynamic group member information at a router which forwards the multicast data. |
| Determining efficient paths to forward data from a set of sources to a set of receivers. |
| Ensuring duplication-free data forwarding. |
| Ensuring committed service-level agreements (SLAs). |
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Multicast routing protocols such as distance vector multicast routing protocol (DVMRP), protocol independent multicast (PIM), multicast border gateway protocol (MBGP) enabled in multicast routers ensure shortest path forwarding between the multicast source(s) and the multicast receivers/hosts. Group membership information is propagated in the network from the hosts towards to the routers that are directly attached to the sources in a bottom-up approach with the help of Internet group management protocol (IGMP) and other multicast protocols (PIM, MOSPF etc.,). These multicast protocols with improved hardware functionality ensure duplication free forwarding and quality of service.
IGMPv1 (IETF RFC 1122) supports two types of protocol messages: membership query
and membership report messages. An IGMPv1 host sends a report when it joins a
multicast group. An IGMPv1 router (also known as querier) queries periodically
using query messages to determine the active members of a group. Whenever a host
receives a query message, it responds with report messages (one report per
group) for all its associated multicast groups.
A host delays its report to a query by a random period (maximum of ten seconds) for each of its associated groups in order to check for a report sent by other members on the LAN. The report is delayed to avoid a burst of report messages from the hosts to the querier.
When the host does not receive a report from other hosts during the delay period, it generates the report by itself. The host suppresses report generation when it receives reports from other hosts during the delay period. The generated report message contains the same IP group host address in the IP destination address field and in the IGMP group membership field.
The querier starts a timer for the group membership information it receives in reports. If the group membership is not refreshed by subsequent reports (in response to general queries), the group information is removed.
The waiting period, which is comprised of few (2 or 3) query intervals to determine membership validity, is typically large (2 or 3 minutes). During this waiting period, the multicast data continues to be forwarded over the interfaces, occupying unnecessary bandwidth. This delay limitation is removed in IGMP version 2, which we'll discuss.
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