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From: "Louis A. Mamakos" <louie@TransSys.COM>
Subject: Re: IGMP queries 
References: <001f01c07286$9a055a00$0e00a8c0@neland.dk> <20001230215241.M253@speedy.gsinet> 
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> 
> > Btw, can I use IGMP to something useful/interesting/funny?
> 
> AFAIK it's some kind of dynamic route establishment (learning
> about topology by listening to what your neighbour knows about
> the network).  Home users and small LANs won't need it IMHO,
> maybe WAN links will benefit?  But I'm definitely not keen on
> having "the world" tell me where to send my packets to.  I just
> hand the traffic to my provider's dialin port. :>

IGMP is the protocol used between a multicast router, and end-hosts on
a subnetwork; much like ICMP is used between a router and an end-host
to help manage unicast traffic.

So, when your hosts joins a multicast group, it sends (to a multicast group)
an IGMP message announcing this.  This is supposed to cause a multicast
router on the subnetwork to begin forwarding the traffic onto the
subnet.  The multicast router will also periodically send IGMP
group membership queries onto the subnetwork to see if there are
any hosts still interested/subscribed to a group.  If it gets no replies
after a while, it will stop forwarding unsubscribed groups onto
that subnetwork.

So that's why you're seeing IGMP queries; the multicast router is
trying to see if you've joined any multicast groups so it can
try to send you traffic.  It's got nothing to do with "having the
world tell you where to send your packets to."

louie


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