[Ekiga-list] no video: is video transferred on separate connection?

[Zhang Weiwu <zhangweiwu realss com>]

My home office network is behind a single NAT firewall (DSL router). I
set up port-fowarding on the router to forward the a few ports
<http://wiki.ekiga.org/index.php/Internet_ports_used_by_Ekiga> needed by
ekiga, and tell my
colleague to call me using public IP address sip:123.116.115.109

When I am in home office receiving incoming video phone call from
colleague, my video is not transmitted to my colleague. My colleague is
behind a very strict NAT (in fact, two layers of ordinary NAT firewall),
seeing only moving ekiga logo, "remote video" option in his "view" menu
is disabled during the phone call. With his complicated NAT firewall, he
only wish to call me and see me on the video, not thinking about
receiving a call.

Question: is it necessary that my colleague should be able to receive
incoming tcp/udp connection in order to see my video? With my limited
knowledge of network, by actually dialed me successfully, he already
established a connection to my PC, so if my video is transferred to him
with the same connection, there should be nothing to worry about
firewall. But, if I should initialize a separate connection back to my
colleague in order to deliver video to him, like old time
non-passive-mode FTP protocol, then we must have hit a problem with the
firewall.

FYI: Within my home office video phoning from my notebook computer to my
desktop PC is okay, indicating the video camera is compatible with Linux
and ekiga can work in case there is no firewall.