Subject: [Sip] Number URIs in Contact of 302 response

From: "Elwell, John" <[email-masked]>
Date: Fri, 09 May 2008 16:48:56 +0100

In the recent enormous thread "E.164 - who owns it" there was a lot of
discussion on number URIs in 302 responses, and whether the UAS includes
a tel URI, a SIP URI with the destination domain (B.com) or a SIP URI
with the originating domain (A.com), and in the last two cases whether
it is with or without user=phone.

What that thread failed to capture was the user's involvement in
choosing the contact URI. Basically the UA either has to be
pre-configured with the redirection URI to use if certain conditions
apply to an incoming request, or the user takes some action when the
request arrives. In either case, the user has two ways of selecting the
URI:

1. From some sort of address book, call log or whatever. In this case
the user reuses some URI that has been received by some other means
(e.g., From URI from some previous communication, or vcard). Such a URI
could be in any of the forms that are valid for making a call, including
tel URI, email-style sip/sips URI, e.164-based SIP URI with/without
user=phone. In the last case, the domain part will be whatever was
received, which may be the local domain (B.com) but is unlikely to be
A.com. At least if it is B.com, the B.com proxy could, if necessary,
convert to something else (e.g., sip:[email-masked]) before passing the
302 onwards.

2. By keying in the URI. The keying procedure is likely to be similar to
what is used for keying in a URI when making a call. On a device with
only a numeric keypad, the user will generally just enter a number, and
the UA will convert this to some URI. This typically results in a URI of
the form sip:[email-masked], where number presumably should have some
significance within domain B.com. In practice the number is probably a
dial string, which would suggest inclusion of user=dialstring, although
in practice this is probably rare. It probably doesn't matter. In any
case, before passing the 302 onwards, the B.com proxy would need to
covert this "dial string" URI into a URI meaningful outside its domain.
This could be a tel URI or a SIP URI, although it is unclear how it
could populate a SIP URI for passing to another domain.

My point is, the UAS is typically not in a position to choose the URI
format, but the B.com proxy, when passing the 302 onwards outside its
domain, will have to take care to use an appropriate format, i.e., it
would have to convert a SIP URI with the B.com domain into something
else, e.g., a TEL URI. Any other form of URI probably has to be passed
on unchanged by the B.com proxy.

John
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