<myself_rajat@yahoo.com> wrote:

> I have some quries regarding MULTICASTING. As recently I was looking
> out net for building a simple multicast application, using
> setsockopt(IP_ADD_MEMBERSHIP). By using that application I thought
> there will be some multicast routers, and we will be subscribing to
> them using their IP addresses. But while using sniffer I was surprised
> to see the results.
>
> However I was sending multicast join report through
> setsockopt(IP_ADD_MEMBERSHIP) to a IP address 234.5.6.7 (assuming the
> IP address of Multicast router), but actually outgoing IP datagram had
> 224.0.0.2 as its destination IP address........Seeing this I could
> make
>
> out that the actual ip address of multi-router would be 224.0.0.22
> only, whereas it will be holding some groups like 234.5.6.7 etc. But
> later I found this 224.0.0.2 is a special class D address, which is to
> make sure delivery of this mutlicast join request to each router, with
> TTL of 1. But still I am not clear with the whole process, like what
> are the things each router will do one receipt of this join request,
> as
>
> well as until when this packet will be circulated in the net, to make
> sure its delivery for each router??

First of all, you need to determine which version of IGMP the host is
using.

If it's IGMP versions 1 or 2, the reports a host sends to its
multicast-enabled router, to join a multicast group, are addressed to
the multicast address (Class D address) of the multicast group. That
way, the router knows that someone has requested membership to that
particular multicast group. (See RFCs 1112 and 2236). And when one host
of a given multicast group renews its membership, in response to a query
from the router, all local members of the multicast group receive that
host's report, so they don't need to send one of their own.

If it's IGMPv3, then the host wanting to join a multicast group sends
reports to IP Class D address 224.0.0.22, as stipulated in RFC 3376
section 4.2.14.

Multicast address 224.0.0.2 is the "all routers" multicast address. It
is used by hosts to send "leave group" reports in IGMPv2 only.

> Next how we could make sure of a exclusive group, say A and B wanted
> to
> use a special multicast address 234.5.6.7 exclusively for their use
> only, how would they make sure that this multicast wont be in use by
> others??

That particular address you mentioned is in a "reserved" range. So it
shouldn't be used.

In general, IP multicast addresses have to be managed. The network
admninistrator and the IANA keep track of these, to make sure there are
no conflicts. You can't just randomly pick an address and use it.
(Although if people do this, ususally no harm is done. That's because
ISPs typically block any multicast they aren't controlling themselves.)

Some IP multicast address ranges are registered with the IANA and are of
global scope, some are dedicated to specific purposes, and some are
meant to remain within the boundaries of individual networks only, so
they don't need to be registered with the IANA. You need to read RFC
2365 and 3171 to see how the IP Class D address range is allocated.

To get the address group information out to hosts of a network, there
are various ways of doing this. And this keeps changing still. Some
possible schemes are Multicast Source Discovery Protocol (RFC 3618),
Session Announcement Protocol (RFC 2974), use of a local web server, or
static configuration. And other schemes that have been proposed at one
time or other, but not survived the test of time.

> And how the routing will take place for IP 234.5.6.7, from multicast
> router to subscribed clients. Like way so many question are going
> around in my mind, could any body put some light over basics.

From the last router to the subscribed clients, the router simply sends
the multicast frames using the MAC multicast address derived from the IP
multicast address. When a host "joins" a multicast group, it must also
set its MAC layer to accept frames from a MAC multicast address that is
algorithmically derived from the IP Class D address. This is explained
in RFC 1112.

In the local Ethernet, the layer 2 switches can do either of two things.
They can flood all ports in the spanning tree with multicast frames
(default), or they can route the MAC frames only to ports that have
membership in the multicast group of the frame. Layer 2 switches can
figure out where the members are either with IGMP "snooping" or with a
layer 2 protocol called GMRP (Clause 10 of IEEE 802.1D).

To download the RFCs, just go to http://www.ietf.org/rfc.html

To download IEEE 802 standards, go to
http://standards.ieee.org/getieee802/802.1.html

Bert