Hi everyone,
I am faced with what strikes me as a genuine mystery. In the last few
days, I have encountered several completely unrelated commercial SIP
UAC implementations that all exhibit the same erroneous behaviour.
This behaviour, while certainly incorrect as far as I can tell, is
consistent across three or four completely unrelated stacks, leading
me to wonder what I am missing.
The scenario is that UAC A places call to UAS B through stateful proxy P:
A ---> P ---> B
P inserts Record-Route header on the initial INVITE in order to stay
in the loop on sequential requests. Nothing eccentric in that
inherently.
When final 200 OK response to the initial INVITE transaction is
relayed from P back to A, it contains a normal Contact URI which
should be used as the remote target URI for sequential requests
(absent any requests that would refresh it):
Contact: <sip:x...@a.b.c.d:5060>
It also contains a Record-Route header which has been copied into the
reply by B, of course, which looks something like this:
Record-Route: <sip:ip.of.proxy.p;lr;param1=blah1;param2=blah2>
But bizarrely, the end-to-end ACK sent by A to this 2xx response has a
request line formed not from the Contact in the 2xx reply, but from
the content of the Record-Route header:
ACK sip:ip.of.proxy.p;lr;param1=blah1;param2=blah2 SIP/2.0
Naturally, the proxy is quite bewildered and does not propogate this.
3261 scripture seems quite clear--at least, to me--on why this would
be wrong. 12.2.1.1 ("Generating the [in-dialog] Request") says:
If the route set is not empty, and the first URI in the route
set contains the lr parameter (see Section 19.1.1), the UAC
MUST place the remote target URI into the Request-URI and
MUST include a Route header field containing the route set
values in order, including all parameters.
Other parts of the spec make quite clear that this kind of ACK is an
in-dialog request, and that the Contact in the 2xx response
establishes the "remote target URI" aforementioned. In other words,
pretty commonsensical stuff. I think anyone can agree the request
line of the ACK should be:
ACK sip:x...@a.b.c.d:5060 SIP/2.0
However, the fact that I have encountered this in completely unrelated
situations with an improbable frequency leads me to believe that there
must be some element of this I am failing to grasp. Is it some sort
of legacy behaviour and/or RFC 2543 backward compatibility mode that
is causing this, or perhaps something else?
Thanks,
--
Alex Balashov - Principal
Evariste Systems LLC
1170 Peachtree Street
12th Floor, Suite 1200
Atlanta, GA 30309
Tel: +1-678-954-0670
Fax: +1-404-961-1892
Web: http://www.evaristesys.com/
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