At 03:12 PM 11/8/00, jack wrote:
>Hi all,
>I don't know if someone has passed the BCMSN test but there is
>something strange in the book about IGMP:
>It says that the host sends a Host Membership Report to the all-router
>group address 224.0.0.2
>which I believe is wrong. The host sends a report to its Group address,
>not to 224.0.0.2
I did the Foundation test so I could avoid BCMSN! &;-) But, I have
researched IGMP, and you are right. 224.0.0.2 is for Leave messages in
IGMPv2. In IGMPv1, routers query to find members and hosts can also send
unsolicited Membership Reports. The hosts send Membership Reports to the
group address, not to the routers. The routers are listening for the groups
they know about, so this works.
RFC 1112, which specifies IGMPv1, says this:
"Multicast routers send Host Membership Query messages (hereinafter called
Queries) to discover which host groups have members on their attached local
networks. Queries are addressed to the all-hosts group (address 224.0.0.1).
Hosts respond to a Query by generating Host Membership Reports (hereinafter
called Reports), reporting each host group to which they belong on the
network interface from which the Query was received.
In order to avoid an 'implosion' of concurrent Reports and to reduce the
total number of Reports transmitted ... a Report is sent with an IP
destination address equal to the host group address being reported, so that
other members of the same group on the same network can overhear the
Report. Thus, in the normal case, only one Report will be generated for
each group present on the network, by the member host whose delay timer
expires first. Note that the multicast routers receive all IP multicast
datagrams, and therefore need not be addressed explicitly. Further note
that the routers need not know which hosts belong to a group, only that at
least one host belongs to a group on a particular network."
I also checked Beau Williamson's book, "Developing IP Multicast Networks,"
which is a great book, BTW. He mentions a feature that is commonplace, that
in fact I thought was in RFC 1112, but it doesn't actually seem to be. He
says that, to reduce join latency, particularly when a host is the first to
join a group, a host can immediately send an unsolicited Membership Report.
The Report goes to the group address. Williamson says that people assume
that the Report goes to routers, but it actually goes to the group address.
Also, see the Cisco page here, which confirms this:
http://www.ieng.com/warp/public/473/22.html
The main feature of IGMPv2 is that hosts can send an Leave Message. When a
host wants to leave a group, it should send a Leave Group message to
destination 224.0.0.2, instead of leaving silently like in IGMPv1. That
lets routers more easily learn when there are no hosts left for a group, so
they can stop multicasting to that group. Good thing.
I think a lot of books get IGMP slightly wrong. It's a shame.
Priscilla
>Can any one verify this?
>And does the same thing happen with IGMPv2 ?
>
>
>Thanks!
>Jack Svolakis
>
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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