[OpenSIPS-Users] Receiving requests via registration TCP connection

[Iñaki Baz Castillo]

Hi, in case a TCP client behind NAT sends a REGISTER toa proxy with
public IP, the proxy/registrar can "solve" NAT issues by adding
"received:REAL_SOURCE_IP:REAL_SOURCE_PORT" to the Contact in REGISTER.
So when the proxy receives a request for this AoR it will forward it
to the public IP and port from where it received the REGISTER.

Of course, for this to work is needed the TCP connection remains open
(it could be achieved by requiring REGISTER every 32 seconds or using
PING-PONG TCP keepalive if both client and server implement it).

But I wonder how "legal/valid" is this in RFC3261. I mean: is really
valid for a proxy to send *new* requests to a registered UA via the
TCP connection used for UA registration?

Imagine UA1 sends a TCP INVITE to proxy P2 responsible for UA2 AoR. So
UA1 starts a TCP connection from a random port (maybe 22222) to P2.
This connection is just supposed for requests sent **by UA1** and
responses sent from UA2.
Now imagine that P2 sends a request to UA1 against port 22222 using
the previous connection. AFAIK this is not defined in RFC3261 and UA1
could just drop these requests.

So I wonder which is the difference between this example and the case
in with a registrar/proxy sends requests to a registered user via the
TCP connection used for UA registration.

Opinions? With the TCP UA's I've tryed (i-e- Twinkle) it seems to work
(Twinkle accepts request from registrar/proxy coming via the TCP
connection used for registration).


-- 
Iñaki Baz Castillo
<ibc at aliax.net>