IGMP (Internet Group Management Protocol)

IGMP is used by Hosts to report their group memberships to Multicast Routers.

IGMPv1:  /* RFC1112*/
IGMP message types
General Query
Report (Join Group)

IGMPv2: /* RFC2236*/
The main difference compared with IGMPv1 is that there is a *Leave* Group message on IGMPv2.

IGMPv2 additional message types
Group-Specific Query
Leave Group

幾個default value:

• Robustness Variable

The Robustness Variable allows tuning for the expected packet loss on a subnet. If a subnet is expected to be lossy, the Robustness Variable may be increased. IGMP is robust to (Robustness Variable-1) packet losses. The Robustness Variable MUST NOT be zero, and SHOULD NOT be one. Default: 2

• Query Interval

The Query Interval is the interval between General Queries sent by the Querier. Default: 125 seconds. By varying the [Query Interval], an administrator may tune the number of IGMP messages on the subnet; larger values cause IGMP Queries to be sent less often.

• Query Response Interval

The Max Response Time inserted into the periodic General Queries. Default: 100 (10 seconds) By varying the [Query Response Interval], an administrator may tune the burstiness of IGMP messages on the subnet; larger values make the traffic less bursty, as host responses are spread out over a larger interval. The number of seconds represented by the [Query Response Interval] must be less than the [Query Interval].

• Group Membership Interval

The Group Membership Interval is the amount of time that must pass before a multicast router decides there are no more members of a group on a network. This value MUST be ((the Robustness Variable) times (the Query Interval)) plus (one Query Response Interval).

IGMPv3:  /* RFC3376*/
IGMPv3 enhancements:
IGMPv3 adds support for *source filtering*
*only* from a source
*all but* a source

IGMPv3 used by multicast routing protocols to avoid delivering multicast packets from specific sources to networks where there are no interested receivers.





IGMP v2 support "leave group"，v3則是多了filtering的方式，可以選擇誰可以join group

Immediate leave vs. Normal leave vs. Fast leave

IGMP Router   

   |

   |

Switch (IGMP Snooping)

 |                 |

 |                 |

HOSTA       HOST-B

Normal leave:
當有HOST要Leave group則會發出leave group的message，而switch做IGMP Snooping則是把這訊息forward給IGMP Router，Router收到leave group message後要做check誰還在group的動作所以發出 GSQ(Group Specific Query)，Switch收到後再forwaard給同group的HOST，這時候要離開Group的HOST就不會再發出Query message告知說他還在這個group中；反之要繼續在這個group的HOST就會發出Query給Switch確認還在group中。


Fast leave:
縮短leave group的時間。當HOST要leave時發出leave group message。此時switch會代替IGMP Router發出GSQ訊息給同group的HOST，但同時也forward leave message給Router。這時候仍要在這個group的HOST會發出Query；反之要離開的在收到GSQ時就完全leave


Immediate leave:
當有HOST要leave group時發出leave group的message，則不等收到GSQ就直接脫離這個multicast group。



Reference:
[1]http://ccie11440.blogspot.com/2007/07/igmpv1-vs-igmpv2-vs-igmpv3.html