[Users] Use STUN always - is there any downside?

[Klaus Darilion]

Barry Flanagan wrote:
> Hi,
> 
> We are using SIPPS as the softphone for our ISP customers, all of whom
> are on DSL and behind some sort of NAT'ed firewall.
> 
> SIPPS does not always pick up the need for STUN when it checks the
> network, and then the user has problems until STUN is enabled.
> 
> My question is - would there be any downside in us just hard coding the
> client to use STUN, given that the end user is a home user on DSL?

There are 2 usages of STUN.
1. to detect if the client is behind NAT and which version of NAT

2. if in 1. is detected that the client is behind NAT and this NAT can 
be traversed by STUN, then STUN will be used to detect public IP:port 
and to make the keep-alive.

Usually, if the SIP client has a STUN server configured, it will do step 
1, and according to the result of step 1 it will do step 2.

Thus, a STUN server should always be configured. Then, the client is 
able to do step 1 and if necessary step 2.

If you have problems, then either the client is buggy in step 1 or there 
are bad NATs which change behaviour and SIPPS thinks that step 2 is not 
necessary although it is necessary.

By "always enable STUN" it depends on what the client exactly does, 
because if the client is behind symmetric NAT (detected in step 1), step 
2 MUST NOT be executed as it will fail.

Thus, it depends on the exact implementation of "always use STUN" in the 
client.

regards
klaus