echo

Hi!

I'm new in SIP technology. I want to write a simple echo application 
(Call a remote sip server user and echo back all reply).
I see the shootist example and I have some questions.

- How can I create a SessionDescription (I see in the example created by 
hand, but this is the only one, and simplest way?)
- After the ACK how can I access/send back the RTP stream data?
- It's work behind a firewall? (I see I need to open a reply port, I 
think it's work with tcp but how?)

Thanks, cow

ps: If somebody have simple echo example please send to me.