WARNING- VERBOSITY ALERT...Delete now if you are
not interested in Multicast. Excellent discussion
by Priscilla on IGMP membership. Comments
below.
________________
> 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.
___________________
This explanation of IGMP joins, leaves, and group
membership is right on the mark. Part of the
confusion exists on who is sending what to whom
and what is in each header. For example, an IGMP
query report mechanism in action can be seen with
the following packet capture(watch wrap):
http://www.west-
point.org/users/usma1983/40768/Chesinc/docs/DENSE.
txt
You will note that the Query is sent to the MAC
address of 01005E000001 and the IP destination
address is 224.0.0.1 (which jives with the MAC
address). Most importantly, note the group
address on the IGMP header: 0.0.0.0
This is referred to as a general (non-group
specific) query. There was no mechanism or
thought put into making group specific queries
(mainly because there was no advanced method of
announcing explicit leaves). While some might
view this as a shortcoming, it ranks no higher
than reserving an entire Class A address for
loopback and testing (127.x.x.x) or reserving
only 32 bits for an IP address. Note also that
the query is sent to the "all hosts this subnet
address of 224.0.0.1" All reports(IGMPv1 and v2)
are sent to the group address (in my captures
they are 224.1.1.1,224.2.2.2, and 224.3.3.3
respectively).
Steve Deering's work from RFC 1112 was improved
upon when scalability and maintenance issues
arose as more groups joined the network. This
was the basis for making explicit leaves included
in IGMP v2(RFC 2236). Of course, this would not
have nearly the impact on performance unless some
method of IGMP snooping (or CGMP) was used to
communicate intelligence to the switches to shut
off a multicast feed o a given switch port. That
drove the need for a more refined group
maintenance mechanism. This evolved into a group
specific query. A group specific query can be
seen in action with the following capture:
http://www.west-
point.org/users/usma1983/40768/Chesinc/docs/IGMPv2
GSQ.txt
Note that on frame number four, a host leaves the
group 224.1.1.1 (Note- it was a sample group I
created. Other groups were 224.2.2.2 and
224.3.3.3) As soon as the host left the group
from unicast address 192.168.1.150, it triggered
a group specific query from the designated
querier. Since IGMPv2 is running on all routers,
this designated querier is the low IP address on
the segment (192.168.1.1). Notice the desination
MAC/IP/IGMP address that is used. They are
respectively:
01005E010101
224.1.1.1
224.1.1.1
This is very different of an IGMP group address
of 0.0.0.0. This allows the designated querier
to maintain state on specific groups rather than
a shotgun blast on all groups. Note however that
the next query update interval has a query to the
following addresses(frame 21):
01005E000001
224.0.0.1
0.0.0.0
This makes sense given that no other IGMPv2 host
has announced an explicit leave report.
Final notes on IGMP, multicast, PIM, etc. I
wholeheartedly agree with and endorse Priscilla's
recommendation on Beau Williamson's
book, "Deploying IP Multicast Networks". It is
an excellent read and an informative reference
manual on multicast. It doesn't get much better
than his book. If you attend Networkers, I would
recommend attending a multicast brief by Beau.
He makes one of the driest topics (multicast)
into an entertaining and understandable session.
HTH,
Paul Werner
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