Re: [sipcore] Proxy feature tag use-case: capability to use Path header field information in both directions

["Worley, Dale R (Dale)" <[e-mail]>]

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From: [e-mail] [[e-mail]] On Behalf Of Christer Holmberg [[e-mail]]

PROBLEM:
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Many of you may remember the discussions we've had every now and then regarding issues of building a Service-Route.

The RFC recommends against intermediaries inserting themselves in the Service-Route of the REGISTER response.

In addition, the registrar cannot assume that the Path header(s) received in the REGISTER request can be used by the registrar to build the Service-Route, so often the only instane in the Service-Route is the registrar itself.

In addition, the UA cannot assume that the Path header(s) received in the REGISTER response can be used to build a route for initial dialog or stand-alone SIP requests. The reasons is that Path by definition only provides routing information for the registrar-to-UA direction.

So, today, if initial dialog or stand-alone SIP requests are to be routed via specific intermediaries, it has to be done by configuration, since neither the Service-Route or Path can be used to build the route.
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It seems to me that no manipulation of the Path header (as seen by the registrar) is useful for solving the problem of building a suitable Service-Route.

There are two cases:  In the first case, the registrar, if it receives an INVITE (for example) is capable of processing it correctly or sending it to some element that can.  In this case, the UA does not need a Service-Route, as it can send the INVITE by the same mechanism that it used to send the REGISTER.

In the second case, where the INVITE must be routed to a different element than the REGISTER, the incoming Path value seen by the registrar is not useful for constructing a suitable Service-Route, because the Path values describe how a request is routed to the registrar, not the service proxy.

Knowing that the Path can be used in the UA-to-registrar direction (as well as the registrar-to-UA direction) tells the UA something that it *already knows*, viz., how to get requests to the registrar.

Dale