[REBOL] Re: I think I've figured out IGMP...
From: pwoodward:cncdsl at: 23-Aug-2001 13:38
Holger -
thanks for the clarification. I sort of figured that most things don't end
up using true multicasts.
> The problem with multicasting is that there are two parts to it: there is
multicasting
> within a LAN, which is pretty easy, just by mapping multicast IP addresses
to multicast
> Ethernet addresses. Only works in a LAN though. Then there is wide-area
multicasting,
> which requires the propagation of "membership lists" between participating
routers
> ("multicast routers"), so routers can build distributed membership and
propagation
> trees. IGMP is used by clients to tell multicast routers which addresses
they are
> interested in. Routers then collect and forward that information using
their own
> protocols (various versions) to build the distributed trees.
Yup - the LAN multicasting is working great! I setup what I called a
heartbeat
server, which was (broad)multicasting "lub-dub" every 5 seconds.
I then went two two other systems on the LAN and created a UDP "listener"
and used set-modes to get it to join the 224.0.1.89 group. Both of the
clients on the LAN were able to get the message. Pretty cool - and ideal
for some small applications I have in mind (a front-door web-cam, with
multi-cast updates for in house use).
> REBOL's 'multicast-groups set/get-modes option maps to the TCP/IP kernel's
> multicast membership API (for BSD-derived kernels this is
IP_ADD/DROP_MEMBERSHIP),
> and the kernel then translates this into the necessary IGMP requests. IGMP
is never
> something implemented at the application level. It is a kernel-level
protocol
> just like ICMP.
Cool - I figured that out after reading the chapter on it. Most routers
which do manage multicasts poll every 125 seconds or so for new reports.
Unfortunately, as you've observed, not many routers are a) multicast capable
or b) compatible with one another for multicast purposes. In essence, my
set(ting) of multicast-group is ultimately translated by the network stack
into a join - and my network card stops ignoring packets that are addressed
to 224.0.1.89.
> - Your OS supports multicasting and you have a multicast-capable
connection out of
> your LAN, either through a multicast router or through a multicast
tunnel, e.g.
> to the MBone or some other multicast network or LAN. In that case
multicasting works
> within the whole multicast network you are connected to. This is where
multicasting
> gets interesting and more useful than broadcasting, but most users don't
have setups
> like this.
That's me! I sort of do - but the Multicasts are still only effective on a
pretty small portion of the network as a whole. People who are "near" my
network segment with XO seem to be able to get my multicasts. But, once we
leave the XO network - I'm dead. So, I can multicast a little outside my
LAN - but not far enough to be really interesting... :( So I can't adapt my
front-door cam idea to let me watch the front door at work via a multicast.
Sure, I can still push it to an accessible URL, but I thought it'd be cool
to multicast it - especially if I could encrypt it (I'm using View/Pro).
- Porter Woodward
