[SR-Users] Handle '486 busy here' from upstream carrier

[Iñaki Baz Castillo]

2010/11/12 Daniel-Constantin Mierla <miconda at gmail.com>:
>> The idea is to receive the 486 from the carrier and not send the
>> INVITE SDP back to the carrier, this is causing the carrier to send a
>> 482 loop detected.
>
> First, if you create a new branch and send to same SIP gateway and you get
> loop detected, then the gateway is broken. It does not see that there is a
> different branch value in the top Via header.

Hi Daniel, the 482 reply is correct:

----------------------------------------------------------------------------------
RFC 3261 - 8.2.2.2 Merged Requests

   If the request has no tag in the To header field, the UAS core MUST
   check the request against ongoing transactions.  If the From tag,
   Call-ID, and CSeq exactly match those associated with an ongoing
   transaction, but the request does not match that transaction (based
   on the matching rules in Section 17.2.3), the UAS core SHOULD
   generate a 482 (Loop Detected) response and pass it to the server
   transaction.

      The same request has arrived at the UAS more than once, following
      different paths, most likely due to forking.  The UAS processes
      the first such request received and responds with a 482 (Loop
      Detected) to the rest of them.
----------------------------------------------------------------------------------

This is exactly the case the user is experimenting: the INVITE arrives
twice to the same server, with same From tag, Call-ID and CSeq, but
different branch/transaction. So 482 is the correct response.


Regards.


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