Subject: [Sip] Use of Tel URL in To header

From: "Conroy, Lawrence (SMTP)" <[email-masked]>
Date: Sat, 14 Dec 2002 12:57:16 +0000

Hi Folks,
  (potentially dumb question warning):

Section 8.1.1.2 (To Header) of RFC3261 - Use of tel URL.

I imply from this that the tel URL in a To header can be evaluated at 
"each domain through which the request passes". Example given there - 
dialing "411" at an Airport would be evaluated at the outgoing proxy 
at the Airport, not in the user's home domain.

Q:: What is the meaning of the "user's home domain" when dialing out 
to a tel URL?

Follow-up:
When originating a call, one assumes that the INVITE request could go 
via an outgoing proxy. Is this proxy required to route a call onwards 
to a "home server", and if so how is that "home server" indicated in 
the INVITE sent to the outgoing proxy?

The From header and Contact header indicates the (potential) AoR and 
current addressability of the caller.
Is the From header used to indicate the home domain for the caller?

all the best from a confused,
   Lawrence
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---

From: Gonzalo Camarillo <[email-masked]>
Date: Mon, 16 Dec 2002 13:09:13 +0200

Lawrence,

inline:

> Q:: What is the meaning of the "user's home domain" when dialing out

That is what the text in 8.1.1.2 is trying to say. Since the caller does
not have any specific domain in mind, it uses a tel URL.

Alternatively, the caller could have dialed
sip:[email-masked];user=phone

In this situation, 411 would have been evaluated in the domain
ericsson.com

> via an outgoing proxy.

Yes, an outbound proxy may or may not be used.

> Is this proxy required to route a call onwards
> to a "home server"

No. The outbound proxy, like any other proxy, will make routing
decisions based on the Route header field, the Request-URI, and any
other information the proxy wants (e.g., time, day of the week, caller's
name...).

>, and if so how is that "home server" indicated in

If you want your INVITE to traverse an outbound proxy and your home
proxy you should indicate so in a Route header. The SIP loose routing
mechanisms allow you to do that.
 
> Is the From header used to indicate the home domain for the caller?

The From header is mostly used to identify the caller from a human
perspective. In order to send requests back within the same dialog you
would use the Contact header field. To send requests to the caller
*outside* the dialog you would typically use the Reply-To header field.

Regards,

Gonzalo