Subject: [Sipping] comments on draft-worley-redundancy-response

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 13 Jul 2007 14:21:42 -0400

I didn't notice any mail list discussion on this draft. I think this is 
a good topic and worthy of discussion.

Firstly, a big issue that comes into play here is what constitutes a 
path. In particular, a PSTN gateway can be considered a destination 
(from a SIP perspective it is), but its really not, in that you could 
try to reach the target number through a different gateway and succeed. 
So for example if you get a 403 forbidden from one PSTN gateway, it 
might make sense to retry on another.

And thus, my main comment is that I am unconvinced that it is the 
response code which uniquely determines whether to attempt MTR or not. I 
think a bigger issue is whether the response comes from the true target 
of the request (PSTN gateways don't count) or not. So for example, if I 
knew definitively that a 408 came from the actual UAS, you would not 
attempt MTR, whereas your draft says 408 always causes MTR. Maybe we 
could try to restrict usage of response codes so that true targets only 
send some response codes, and intermediaries only send others. But I'm 
not sure thats the right thing.

We have no easy way to know whether a response came from an actual 
target of the request or something in between, and securely making such 
a determination is hard.

-Jonathan R.
-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
Cisco Fellow                                   Parsippany, NJ 07054-2711
Cisco Systems
[email-masked]                              FAX:   (973) 952-5050
http://www.jdrosen.net                         PHONE: (973) 952-5000
http://www.cisco.com

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From: [email-masked]
Date: Fri, 20 Jul 2007 15:27:11 -0400

From: Jonathan Rosenberg <[email-masked]>

   So for example, if I knew definitively that a 408 came from the
   actual UAS, you would not attempt MTR, whereas your draft says 408
   always causes MTR. Maybe we could try to restrict usage of response
   codes so that true targets only send some response codes, and
   intermediaries only send others. But I'm not sure thats the right
   thing.

It depends ... I've only seen 408 used to mean "time out while trying
to contact SIP agent", in which case, it is *never* coming from the
UAS.

If you intend for 408 to indicate "Ring No Answer", that would present
a problem.

My goal is to separate response codes into two sets, one which comes
only from UASs, and one which comes from intermediate agents.  That
requires splitting the semantic space of some current response codes.

Dale