> UE -------------------------------  N/W(Registrar)
> 
> -------> Registration (UDP)  ---------------- -->
> <------- 401 challenge  (UDP)    <------------
> --------> Registration with response (UDP)-->
> <-------- 200 OK (TCP)  <------------------------------
> --------> Subscribe  for reg event (UDP)    -->
> <-------- 200 OK (TCP)  <------------------------------
> 
> I have never seen it before, Is it possible to 
> have both UDP and TCP communication happening 
> simultaneously? 

Yes; however within this situation the UAC or UAS is non compliant to RFC 3261.

Per section 18.1.1 (and elsewhere), the via entry must indicate the transport 
used to the send the request.  And per section 18.2.2, the response must be 
sent using the transport indicated within the via entry; however some vendors 
will send a 400 on the received transport to indicate that the via entry 
contained the wrong transport.


> what are the consequences of such communication?

UAC might not receive or successfully process the 200 OK.


<snip>

> will that transaction be closed ever as the UE 
> is not receiving a response on the UDP port?

Yes; see Timer F within RFC 3261.


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