Subject: [sipcore] location-conveyance - used-for-routing parameter

From: "DRAGE, Keith (Keith)" <[email-masked]>
Date: Sun, 2 Aug 2009 13:02:40 +0200

I see in the document a number of sentences that say:

This also gives the "used-for-routing" header 
      field parameter added meaning - as the receiving SIP entity knows
      which location URI the message was routed upon.

and

In such a 
   case, proxies MUST NOT remove any existing "used-for-routing" header
   field parameter.  In this instance, the SIP server retargeting based
   on another locationValue MUST add the "used-for-routing" header 
   field parameter to the locationValue used for retargeting by this 
   server. This will result in a Geolocation header field indicating 
   that the request has been retargeted more than once, which is 
   allowed

As currently written, one implication of this text is that at the recipient, I can audit how the call was routed based on locationValue. I do not believe this is true. All I know is that at some point along the call path, this locationValue was used for routeing. It doesn't tell me which was the last one, and it does not tell me which ones got used more that once (as a header field parameter can only appear once).

This mail is therefore a sanity check to see that we are getting what we want out of the procedures for the use of this parameter, i.e. that we want to know only that at some point in the call path this locationValue out of several has been used for routeing, and not that we want to know for example the last locationValue used for routeing.

When considering this issue, note that for example, these parameter values will survive a change of Request-URI, e.g. due to call forwarding, so "used-for-routing" may apply for how I got to the forwarding point on the old Request-URI.

I'll make proposals to change the text to more accurately reflect what I understand the intentions to be when I see some response to this message giving me some more background to the thinking.

regards

Keith