messenger 6.0 Audio/video

Hi netfilter friends:

How can I do to support Msn Messenger Audio/Video features behind a Iptables
Firewall/Nat?
Is there any ALG to support SIP, SDP and RTP? or what patch I must use to get work AV features in Msn Messenger?

I'm using redhat 7.3 with iptables

Thanks in advance.

Genner