How do VoIP devices punch holes for RTP?

[fredtheman]

Hi

I was wondering: How do SIP devices open the ports they need for RTP?

I'm about to send an IP phone to someone who doesn't know much about things computers, so I need to find out if his router can handle SIP/RTP automagically, or if he'll need to map incoming ports to this device.

Is there a way to check this easily? Some special words to look for in the router's documentation ("UPnP?")? Some utility I can run from the outside or that he can download and run on his computer to check if his router will open ports dynamically?

Thank you.

[chandave]

Most SIP devices don't support UPnP.

What you need to look for in a router is the capability known as Full/Restricted Cone or destination address independent behavior.

This refers to the fact that some routers can be configured to forward communication from any Internet Address into a device behind the router so long as the device behind the router sends some communication out first.

On some routers, this is the "Gaming Mode". On other routers this means to turn off Stateful Packet Inspection (SPI). If you see a router that has "SIP ALG" (Session Initiation Protocol Application Layer Gateway) support, I would suggest you turn that off.

There's a Windows and Linux STUN (Simple Transversal of UDP through NAT) client application available on Sourceforge (http://sourceforge.net/projects/stun/). You can run that to determine what type of behavior your router exhibits.

What you want the STUN client to return back is a Full cone or Restricted cone behavior. If you get Symmetric then the router will not support RTP traffic without manually configuring the router for port-forwarding a range of ports to the SIP device.

See ya...

d.c.

[fredtheman]

Originally Posted by chandave

What you need to look for in a router is the capability known as Full/Restricted Cone or destination address independent behavior.

Thanks a bunch. I'll read up on UPnP, STUN, and the different types of firewalls so it begins to make sense.