Howto find out all members of a multicast group?

Hi,
i need to develop some application that is sending some multicast udp
packets, but which knows all the the recievers(Before sending these
multicast packets i need to negotiate some parameters). What is the
simlest way how to do it? I have one idea but i dont know if there is
some other possibility or if this idea is from scratch very bad....So
my idea is:
1.Sender(the process who need to know all recievers) send some udp
packet to multicast address.
2.All computers listeneng to this multicast traffic sends back some udp
packet to sender address
3.Sender collects all this packets and from source address he knows the
addresses of all multicast listeners and now he may connect to them...

Thanks for all notes to this solution and all messagess.

Jirka

JirkaJ wrote:

> Hi,
> i need to develop some application that is sending some multicast udp
> packets, but which knows all the the recievers(Before sending these
> multicast packets i need to negotiate some parameters). What is the
> simlest way how to do it? I have one idea but i dont know if there is
> some other possibility or if this idea is from scratch very bad....So
> my idea is:
> 1.Sender(the process who need to know all recievers) send some udp
> packet to multicast address.
> 2.All computers listeneng to this multicast traffic sends back some udp
> packet to sender address
> 3.Sender collects all this packets and from source address he knows the
> addresses of all multicast listeners and now he may connect to them...

That is the only way you can do it, however...

That last part where you say "and now he may connect to them" has me worried. If
you think you will be able to directly send a normal UDP packet or attempt a TCP
connection to those IP addresses you will have problems if they are behind
routers. All communications should use multicast to assure all the clients will
get it.

--
Phil Frisbie, Jr.
Hawk Software
http://www.hawksoft.com

"Phil Frisbie, Jr." <(E-Mail Removed)> wrote in message
news:LXx7f.845$(E-Mail Removed)...

> That last part where you say "and now he may connect to them" has me
> worried. If you think you will be able to directly send a normal UDP
> packet or attempt a TCP connection to those IP addresses you will have
> problems if they are behind routers. All communications should use
> multicast to assure all the clients will get it.

You can, of course, allow the client to say that it is available for
normal TCP or UDP packets, but you should be prepared to fall back to
multicast if you get no replies to those TCP or UDP attempts. The most
robust software is fully prepared to work even if only one type of
communication succeeds. Of course, you can support more efficient mechanisms
when they happen to work.

DS

Hi, thanks for answer, is it normal situation that when i use source
address from incoming udp packet to direct (TCP)connect that that
address will be unreachable? Or this situation happens only in rare
cases?

thanks
JIrka

JirkaJ wrote:
> Hi, thanks for answer, is it normal situation that when i use source
> address from incoming udp packet to direct (TCP)connect that that
> address will be unreachable? Or this situation happens only in rare
> cases?

It's absolutely normal that you would not be able to connect. Unless
you know that there is some application listening on a specific port
for a TCP connection, then you will not be able to connect.

However, unreachable is different. If you get an incoming packet and
use the source address to perform a TCP connect then it's highly likely
that the destination will be reachable. It will just refuse the
connection.

WRT your original question about knowing who is in the multicast group
(i.e. getting the multicast packets), you can look at the RFC's related
to IGMP. You can monitor the IGMP packets and derive that information.

Hi
thanks a lot, my application will be listening on a specific port for
tcp connection too so i am glad that i may use that source address(on
that tcp connection will go some control messages-for each listener one
connection before sending multicast packets).
To monitoring packets: when i monitor packets i will only see packets
from multicast listeners on the same network, not behind router, is it
true or not?

JIrka

JirkaJ wrote:
> Hi
> thanks a lot, my application will be listening on a specific port for
> tcp connection too so i am glad that i may use that source address(on
> that tcp connection will go some control messages-for each listener one
> connection before sending multicast packets).

Your logic is not clear. If you are sending multicast packets then why
is the response from any particular TCP connection important as a
criteria for initiating multicast? Only the first person asking for
the multicast should be important in that respect. After that anyone
who joins the multicast group will immediately begin receiving the
transmission.

> To monitoring packets: when i monitor packets i will only see packets
> from multicast listeners on the same network, not behind router, is it
> true or not?

You are probably only going to see IGMP joins from members on the LAN
side of the router. That would be correct. I hope you are going to
use a gateway and not a simple router. A simple router will most
likely not be configurable for passing multicast properly. Got a brand
and model number?

to logic: what i need to do with that tcp connection is to negotiate
some parameters of multicast-size of packets, time schedule, port, and
some other things, and only one way how to do it is a direct connection
to each listener, after sending of all multicast packets i would like
to connect again to each host and download some statistics...
I dont know exactly which brand and models i will use, i leave it on
network admin

JirkaJ wrote:

> to logic: what i need to do with that tcp connection is to negotiate
> some parameters of multicast-size of packets, time schedule, port, and
> some other things, and only one way how to do it is a direct connection
> to each listener, after sending of all multicast packets i would like
> to connect again to each host and download some statistics...
> I dont know exactly which brand and models i will use, i leave it on
> network admin

OK, but everything you say you need can be done within the multicast group. And
as you have already been told, there WILL be some clients that are behind
routers that will be impossible to connect to using TCP.

--
Phil Frisbie, Jr.
Hawk Software
http://www.hawksoft.com