troubleshooting Question

RTP Problem with Firewall

Avatar of Rohit Bajaj
Rohit BajajFlag for India asked on
Networking ProtocolsIP TelephonyNetworking
10 Comments1 Solution1260 ViewsLast Modified:
I saw the following statement in a tutorial on SIP, RTP firewall n NAT. I am unable to understand it - '

When there is a firewall in between the SIP signaling request response is allowed. But there is a problem during RTP.  During RTP the data will be able to go through firewall to User agent but data from user agent won’t be able to reach the other side of the firewall as the firewall does not have information about port to which the User Agent is sending data.

What i dont understand here is that
1) Why the firewall needs to have prior information about the port to which the User Agent will send data ?
why cant it allow data transfer to the port.
2) If there is a problem with RTP transfer when there is a firewall, why is there no problem during SIP signalling.

Also please let me know any good resources or tools to study SIP (session initiation protocol) and RTP, firewalls
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