[GnomeMeeting-list] Call to Gnomemeeting from endpoint behind NAT

[Conrad Beckert <conrad_b yahoo com>]

Hi

can anyone help me with this:

My gnomemeeting is on an public IP - visible to
everyone. My partner was on Netmeeting and sits behind
 a NAT Router he has no control of. (Some DSL Router
out of the box - so I don'k know if it natively
supports H323 address translation)

Netmeeting works - my partner can call me using my
dyndns. 

Now I installed openphone on his machine (based on
OpenH323 - for using Speex codecs) The network is
still the same.

Openphone connects and can send images and sound to me
but I'm invisible and silent on the other side.

I set up the ports using UDP for RTP/RCTP 5000-5020
and 30000-30010 to get through my packet filter - so I
think the packets go through my firewall.

I analyzed the protocol between Openphone and
Gnomemeeting on my little internal network (without
NAT) and it looks like this: (Call initiated from
Openphone)

1. Openphone calls GM on 1720 (sender port is 1085)
2. negociations take place between the endpoints (TCP
and Q.931 packets)

3. Gnomemeeting starts sending packets to Openphone
port 5008 with sender 5000
4. Openphone returns using the same ports (happens
quite a few times - obviously that's the RTP
audio/video data yet)
5. Gnomemeeting starts sending packets to Openphone on
port 5009 with sender 5001
6. Openphone returns using the same ports - obiously
that's the RTCP data
7 later there is a conversation from GM port 5001 to
Openphone 5009 and another from GM 5003 to 5011 that
in the same direction that gets answered. 

My conclusion: I don't know if I'm right as I'm not an
H323 expert:
What happened:
- Openphone calls Gnomemeeting from behind the NAT
router and gets its answer because the router knows
where to send the returning packages to.
- Gnomemeeting starts sending to 5008 - the Nat does't
know what to do with it and drops the packets.
- Openphone starts sending and is hence visible.

I suppose that Netmeeting issues the connections from
behind the NAT and thus the NAT knows how to handle
the returning Gnomemeeting packets. (Though I have
another Netmeeting partner of which I have the same
phenomenon as with Openphone)

Can anybody help me:
- How can I set up the Openphone (or OpenH323
respectively) to initiate the RTP/RTCP connection and
get Gnomemeeting to reply its data from/to the same
ports?
- Is H323 capable of doing so after all? Perhaps I've
misinterpreted my observations and all my efforts were
futile from the beginning.
- I've read about Master-Slave Determination. Is this
how to tell both endpoints who should start sending on
which port? Is there a way to control this to get the
endpoint behind NAT to start?


Thank you in advance for help
Conrad





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