Hannes,
I'm still worried that because we're using the routing fields for
marking, there's a risk that proxies will do things to those headers
that will remove the marking and cause the emergency call to fail.
For instance, I've heard tell that some proxies like to strip all Route
headers from INVITE messages passing through them. In your example
below, that would cause the call to need another LoST lookup, which may
be impossible if no subsequent proxy supports LoST, or if there's no
proxy-readable location in the INVITE. Moreover, if a proxy changes the
To field or the Request URI, then downstream proxies might not even
recognize that a LoST lookup is required and thus would cause the call
to fail.
I don't disagree with the three abstract states you've defined
(unrecognized, recognized but not routed, and routed), but I would
prefer that (1) The Service URN were carried in an explicit marking
field even if a new one has to be created (say, Requested-Service), and
that (2) after routing (LoST) a call looks like any other call to a PSAP
URI, but with an explicit marking. I think this would be much more in
the spirit of minimal surprise.
--Richard
Hannes Tschofenig wrote:
Hi all,
after some discussions on the usage of the Service URN it seems that
most people in ECRIT agreed to move forward with the initially planned
approach. Below, you can find a description of how the Service URN would
be used in various scenarios.
Are there any objections?
Ciao
Hannes
---------------------
== End Host Procedures ==
* UA does not recognize the emergency call.
To: Dial String
Request URI: Dial String
No Route header
* UA runs LoST and determines the PSAP URI:
Request URI: Service URN
To: Service URN
Route Header: PSAP URI
* UA does not run LoST but recognizes the emergency call.
Request URI: Service URN
To: Service URN
No Route header
== Proxy Procedures ==
* Incoming request contains Service URN; Proxy runs LoST
and determines the PSAP URI
--- Input:
Request URI: Service URN
To: Service URN
No Route header
--- Output:
Request URI: Service URN
To: Service URN
Route Header: PSAP URI
* Incoming request contains Dial String; Proxy runs LoST
and determines the PSAP URI:
--- Input:
Request URI: Dial String
To: Dial String
No Route header
--- Output:
Request URI: Service URN
To: Dial String
Route Header: PSAP URI
Remark: "Dial String" refers to http://tools.ietf.org/html/rfc4967
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