Q: connection from clients with vpn IPaddress to a server with publicIP address.

Hi,
I have a question on connection of UDP and TCP.

As I know, some video conferencing programs or network camera systems
support network connections between public and VPN IP addresses through
TCP or UDP (I am not sure they used TCP or UDP).

I would connect UDP between a public IP address and at least one private
IP address.
If in TCP connection private IP addresss holder tried first to connect
to the public IP address holder, it is OK like "ping". But on UDP
connection, some router should support a port designated to a private
address holder. then is is OK. However, like this TCP connection, if
private IP holder requsts connection to the public IP address the public
IP address can get the private IP address holder's or VPN gateway's IP
address from "recvfrom()". I did not succeed on this way.

Therefore, I have some questions on this.
1. Should I use only TCP?
if so, should the private IP address holder request connection to
the public IP address holder?
if not so, any other way instead of UDP?
2. Is it possible to connect a private IP holder to a public IP holder
through UDP connection?
If so, how to?
3. Are there any good example codes?

Regards,
bkna.