Igmp

IGMP(internet group management

Protocol)

6th June 2014

Raghavendra Hamilpure

Index

Why IGMP

Basic of IGMP

IGMP messages Membership Query

Membership Report

Leave Report

IGMP header

Different version of IGMP

IGMP snooping

MLD (Multicast Listener Discovery-IPv6) MLDv1 use IGMPv2.

MLDv2 use IGMPv3

Why IGMP ?

IGMP is enhancement in IP multicast.

Disadvantages of Multicast.

Multicast is UDP-based.

Efficiency issues in filtering and in security.

And many more…

Basic of IGMP

An IGMP-enabled router sends out several General Membership Queries at startup(224.0.0.1)

Hosts that are members of specific multicast groups send Membership Reports back to the router to report their membership.

The router receives the Membership Reports and builds lists of multicast group memberships for each attached network.

If there are two router in the network then lowest IP address will be Querier.

If you check the above output, it shows When the router(.3) is sending general Membership query, host .2 has sent Membership report with specific 224.7.7.7 group address. Here only one membership report need to be sent on behalf of other hosts. Host .2 must have lowest response time to send membership report.

IGMP messages

MEMBERSHIP QUERY [ 0x11 ] Membership Query messages are used by multicast enabled routers running IGMP to

discover which hosts on attached networks are members of which multicast groups. Membership Query messages are sent to the 'all-systems' multicast group address of 224.0.0.1.

There are two sub types of Membership Query:

General Query(every 60s) - used to learn which groups have members on an attached network.

Group-Specific Query - used to learn if a specific group has any members on an attached network.

Router sends query membership message to a single group rather than all hosts(reduce traffic)

MEMBERSHIP REPORT (v1/v2) [ 0x12 / 0x16 ] A membership report message is sent by a host whenever it joins a multicast group, and

when responding to Membership Queries sent by an IGMP router that is functioning as a Querier.

LEAVE GROUP [ 0x17 ] This message is sent when a host leaves a multicast group. This message is sent to the

'all-routers' multicast address of 224.0.0.2. The router then sends out a group-specific membership query to the network to verify if the last member of a group has left.

General Membership Query: Router is sending MQ to all multicast systems i.e. 224.0.0.1

Membership Report

Leave Report When a host sees that no process is interested in a specific group, it

sends leave report. If router receives a leave report it won’t purge the list if there are still other hosts interested in that group. For that purpose router sends a special query message with a specified response time for the group in question to see if there is anyone interested in that group. If there is no response a membership report, it purges the list.

The last host will sent Leave report as per RFC.

Any host sends a leave message. Router will send Group specific query 139/140 and host has to respond within 1 sec. if it does not respond within one second group specific query is sent again.

If the standby router does not see 2x Query interval then it will take the lead.

Encapsulation of IGMP message(IGMP messages are encapsulated in IP datagrams)

IGMP header Version: Number of the IGMP version used.

Type: Type of the IGMP message.

Max. Response This depends upon the type of version used.

Checksum: Checksum of the IGMP message.

Different versions of IGMP

IGMPv1(RFC 1112)

IGMPv2(RFC 2236)

IGMPv3(RFC 3376)

IGMPv1 There are no messages for leaving the group. They use timeout to

detect groups of no concern for the interface. If Host to leave the group it has to wait for 3 minutes (60 sec General queryx3) to remove the host from the group.

IGMPv2

They came up with a message for leaving group.

Host will send leave message with destination 224.0.0.2(All routers) with specifying specific group information Ex. 224.7.7.7 (VLC player).

Querier will send Group specific query to 224.7.7.7(1sec). Every host who still interested in receiving 224.7.7.7, they have to send immediately membership report to Querier to check the group is active or not.

IGMP v2 requires separate report/ leave messages.

IGMv3 It adds support for “source filtering” that is, the ability for a system to

report interest in receiving packets *only* from specific source addresses or from *all but* specific source addresses, sent to a particular multicast address. Version 3 is designed to interoperable with v1/v2.

Host can join the group and leave the group in the same transaction.

Optimized bandwidth utilization—Receiver may request to receive traffic only from explicitly known sources.

Improved security—No denial of service attacks from unknown sources.

IGMP snooping Normally (without IGMP snooping) a switch will forward a multicast

frame to all switch ports (except incoming port).

IGMP snooping allows the switch to send multicast frames only to those receivers that join a particular group by listening for report/leave messages from the hosts.

By default this feature is enabled on a layer 2 switch. If you want to enable is only on particular VLAN you can disable it globally & configure “ip igmp snooping vlan vlan-id” with required vlan-id.

Multicast Listener Discovery (MLD)

MLD is used in a similar way by IPv6 systems.

MLDv1 implements the functionality of IGMPv2

MLDv2 implements the functionality of IGMPv3

References

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli_rel_4_0_1a/CLIConfigurationGuide/IGMPSnooping.html

http://technet.microsoft.com/en-us/library/cc787925(v=ws.10).aspx

http://www.networksorcery.com/enp/protocol/igmp.htm

https://www.youtube.com/watch?v=GGqcwdDW1a8

http://mrncciew.com/2012/12/25/igmp-basics/

http://tools.ietf.org/html/rfc3376

http://www.inetdaemon.com/tutorials/internet/igmp/messages.shtml

http://www.slideserve.com/vitalis/ip-multicasting-explaining-multicast