I'm searching for standard method/gateway/protocol/configuration/setup etc
that enable direct connection between stations
(PC/Server or whatever it call), that all resided behind firewalls.

As TCPIP protocol defined, at least one of the stations must have able to
"listening" on public-IP.

One way is a center public-accessible server like what provide by chatting
like icq, yahoo and msn (hereby, refer it to "messenger") that provide a
gateway for these purpose,
(I have found that a permanent connection is established with our station
and messenger site while the session is on).

But, what-about if the traffic is high (for example, video conferencing),
with all the parties behind firewalls, does this mean the "messenger" server
have to relay every bytes ?
won't this impose a very high band-width usage/workload to the (center
messenger) server ?

I can't think of other method to by-bass center server under this situation,
Is there any I have not awared of ?

Also, if the center gateways is unavoidable (at least for initial state),
what is best equipment,
commercial or open-source software and configuration can be deployed or used

How is the implementation/setup on "typical" messenger provider ? How 'SIP'
help in this context ?

1. I have try to search this topic for some time, but still not find usefull
site, most is "junk" site).
2. I know port-forwarding through gateway might help, BUT it is not viable
on my requirement,
I want something that not need any router setup among all the station.

Thank for any infor, I think this is interesting for many people as well :-)