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Introduction to IGMP V1

Introduction to IGMP V1

Posted on Jan 24, 2020 (0)

Introduction to IGMP V1

IGMP: IGMP is internet group management protocol which is primarily used by multicast host to inform their local multicast router that when they wish to join specific multicast group and begin receiving group traffic.

There are various extension made in IGMP and there are three versions of IGMP currently used.

  • IGMP v1
  • IGMP v2
  • IGMP v3

Here we are going to discuss about IGMP v1 and in next section we will be discussing about other versions also.

IGMP v1:

IGMP is used to inform local router that a host is intended to receive the multicast traffic for particular group.

IGMP v1 Message Format:

Now here we are going to learn about working of IGMP v1.

Working of IGMP v1:

In working of IGMP v1 there are core concepts about following process.

  • IGMPv1 Query-Response Process
  • IGMPv1 Report Suppression method
  • IGMPv1 Querier Selection
  • IGMPv1 Join and Leave Process

IGMPv1 Query-Response Process

IGMP v1 uses Query-Response Model that helps multicast router to determine which multicast groups are active. Let’s understand this by below example:

  • Router 1 (the IGMP Querier) periodically sends an IGMPv1 Membership Query to the All-Hosts multicast group (224.0.0.1) on the local subnet. The default timer is (the default is every 60 seconds)
  • All hosts will receive this IGMPv1 Membership Query, and one host H1 who is member of multicast group 224.2.2.2 will responds first by multicasting an IGMPv1 Membership Report to All-Router ( 224.0.0.2 ) in the multicast group. By IGMPv1 Membership report it informs the routers on the subnet that a host is interested in receiving multicast traffic for group 224.2.2.2
  • As Host H2 is also listening to multicast group 224.2.2.2, and it will hear the IGMPv1 Membership Report that was multicast by Host H1. Host H1 will now suppresses the sending of its report for group 224.2.2.2.This Report Suppression mechanism helps reduce the amount of traffic on the local network.
  • Host H3 will also receive the IGMPv1 Membership Query, and it will responds by multicasting an IGMPv1 Membership Report to the multicast group 224.4.4.4. By this report routers will be aware that on the subnet a host is interested in receiving multicast traffic for group 224.4.4.4

IGMPv1 Report Suppression Method:

This method reduces the amount of IGMPv1 traffic on local subnet and its working is described as follows:

  • As soon as any host receives an IGMP Membership Query from IGMPv1 Querier, the host starts a timer called report-timer for each multicast group it has joined. Each report-timer has a random value between zero and the maximum response interval. The default is 10 seconds.
  • If a report-timer expires, the host multicasts an IGMP Membership report for the active multicast group associated with the report-timer.
  • If the host hears that any another host has sent the IGMP Membership report Report, it cancels its own report-timer associated with the received Membership Report, and suppress the sending of a Membership Report for the group.

IGMPv1 Querier:

If there are multiple multicast routers on a subnet, an IGMPv1 Querier which is a router from any one of them is responsible for sending all IGMPv1 Queries on a subnet.IGMPv1 with the help of Layer 3 IP Multicast Routing protocol (PIM, DVMRP) elects a Designated Router for the subnet. Router with lowest IP address will become Querier.

The multicast routing protocol selects the Designated Router to handle certain multicast forwarding duties on the subnet and also by RFC 1112 it is assumed that this Designated Router would also perform the tasks of sending IGMP Query messages. But in IGMPv2 these two functions were later separated.

IGMPv1 Join Process:

 When any host wants to receive any multicast stream or wants to join a multicast group, the host immediately will send unsolicited Membership Reports for the specific multicast group.

In the above figure Host H1 wants to receive traffic for multicast group 224.2.2.2 it will not wait for next Membership Query from Router A, and it will immediately multicasts an unsolicited IGMPv1 Membership Report to group 224.2.2.2 to inform the routers on the subnet of its desire to join this group.

IGMP Leave Process:

In IGMPv1 there is not specific leave process for any host to leave any multicast group, if any Host want to receive any multicast group, it saliently leaves the group without informing any one.

Let’s suppose if this is last host to leave the Multicast group, than in this case the Local router will keep sending multicast stream by 3 mints, because timeout of Query packet is three times of Query interval. 

Different Timer:

Following are the different timer used : 

  • IGMP query interval is 60 seconds
  • IGMP querier timeout is 120 seconds
  • IGMP max query response time is 10 seconds

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