Subject: [Iptel] FW: SIP - TEL URI Implementation Query

From: "Gaurav.Kheterpal" <[email-masked]>
Date: Tue, 14 Dec 2004 12:41:06 +0530

Hello,

I am currently implementing support for <tel:> URI's in our User Agent as
per RFC 3966/ RFC 2806-bis standards. I had the following doubts regarding
the same. Any inputs will be highly appreciated.

1. Contact Header: As per RFC 3261 Section 8.1.1.8, it mentions that this
URI must be in either SIP/ SIPS format. In Section 10.2.1, it is mentioned
that for registrations, the Contact header field can contain any URI Scheme
(e.g the tel URL, RFC 2806 [9]) or email addresses (with a mailto URL, RFC
2368 [32]) as Contacts for an address-of-record.

Does this mean that the 'Contact' field in an INVITE message must always be
a SIP/SIPS URI while in a REGISTER message, it can be a SIP/SIPS/TEL/Mailto
URI ?

2. Besides 'Request URI' and 'To:' header fields, can the TEL URI be present
in any other INVITE header values (e.g. Via, Route Header, Contact etc). In
other words, which fields in the INVITE request does the proxy scan for
possible tel->sip translation using ENUM/ any other mechanism ? In case,
there is no other field which contains this information, how is the TEL URI
information propagated across intermediate proxies when the UA is configured
with a outbound proxy having a defined route set. If the defined route set
is proxy 1-> proxy 2 in the below diagram, the request will be formed using:

INVITE sip:proxy1
Route: <sip:proxy2>
To: <tel:1234>

User A          Proxy 1          Proxy 2          User B
     |                |                |                |
     |   INVITE F1    |                |                |
     |--------------->|                |                |

Does this mean that in such a case, the only parameter which contains the
TEL URI is the 'To' field. As I understand, this field is never processed by
the proxies unless they implement privacy/ RFC 3324(5) ??

I am current not aware of any proxy which is capable of handling TEL URI's,
any  information/ links regarding the same will be highly useful.

Thanks in advance for your help.

regards,

Gaurav

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From: Cullen Jennings <[email-masked]>
Date: Thu, 23 Dec 2004 05:47:15 -0800

This question would probably be better on sip-implementors so if you reply,
please remove iptel. Bit of an answer inline.

> a SIP/SIPS URI while in a REGISTER message, it can be a SIP/SIPS/TEL/Mailto

Yes. Pretty weird Eh? The contact header in SIP got overloaded for too many
different uses but that is the way it is. Also, a 302 can have a Contact
header field value that is any URI (such as tel, email).

> in any other INVITE header values (e.g. Via, Route Header, Contact etc).

It can be in a From header and the RFC 3325 P-Asserted-Identity header. It
should never be in a Via.

> In
> possible tel->sip translation using ENUM/ any other mechanism ?

Only the request URI. The others should never be changed by a proxy.

> In case,

Proxies never route based on the To. Again, weird but true. So to make
something like this work the path below happen, you would have. The routes
are used before the R-URI. (I'm assuming that proxy 2 will route 1234 to
user B)

INVITE tel:1234
Route: <sip:proxy1;lr>, <sip:proxy2;lr>
 
> TEL URI is the 'To' field.
No the request URI should have had the tel

> As I understand, this field is never processed by
Yes - even then the the To field is never changed by a proxy.

> any  information/ links regarding the same will be highly useful.

I think there several proxies that can do tel and ENUM - there are also
several GWs capable of tel.

> Thanks in advance for your help.

Hope that helps, Cullen