STUN over symmetric NAT, how come it works for me?

Hi all,

I have a simple setup - a Granstream Budgetone BT-100 connected via NAT on a D-Link DSL-G604T.

According to the STUN server (at sipdiscount), this router uses symmetric NAT, and everything I've read on the internet says that STUN and symmetric NAT don't work together.

But I can make both incoming and outgoing calls just fine.

Can someone shed some light on this? Has the STUN server incorrectly identified the router as symmetric?
(I have no UDP port ranges mapped to my phone, either).

Tony

You may have a router that supports uPnP and this is enabled. Your Windows is requesting the ports to be opened via this protocol.

telocho :

You may have a router that supports uPnP and this is enabled. Your Windows is requesting the ports to be opened via this protocol.

This is what's confusing me - uPnP is turned off on the router, and my Windows PC doesn't even know about the standalone phone's existence.

My understanding of SIP and keepalives is that the phone will keep registering (or sending SIP "option" messages) to the STUN server to keep the SIP UDP port open on a NAT router. But when a media stream is initialised, symmetric NAT won't route other ports opened on the WAN to the IP address on the phone (even if the packets have the phone's IP address). The three other types of NAT do route other ports for the same destination IP, even if they are opened on different ports.

So I don't see how it can work if it really is symmetric NAT, as reported by the STUN server?

The following could be valid in your case: Hosted NAT traversal.

The session border controller (SBC) if they have one, at sipdiscount, might detect that you use NAT. Then it will work as well. Reregister times then are shortened to 30 sec so that the NAT/firewall will not close ports.

(Hosted NAT traversal and STUN are no good combination, my recommendation always to switch off STUN and test first if it works without)

Do you have either outbound proxy or regiatrar server options enabled on your ATA?

Yes, I suspect telocho is correct, you're using their proxy.

Disable STUN. If it still works, that's the reason.

If that is the case, port forwarding is likely to improve call quality for you (one less thing in the chain).

Dean