Subject: Re: [sipcore] Review request: Dial String syntax for the "tel" URI Scheme

From: "Worley, Dale R (Dale)" <[email-masked]>
Date: Mon, 3 May 2010 13:58:05 -0400

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From: [email-masked] [[email-masked]] On Behalf Of Zehler, Peter [[email-masked]]

The draft title is "Dial String syntax for the "tel" URI Scheme" and it can be retrieved from <http://tools.ietf.org/html/draft-zehler-teldial-info-00>.   It proposes three “tel” URI schemes parameters that cover pre-dialing, post-dialing  and T.33 subaddressing.  The post-dialing and T.33 subaddressing are adopted from the now obsolete rfc2806 but updated for tfc3966 syntax.  Pre-dial was created to avoid obfuscation of the resource identified by the URI.  Rfc2806 allowed the pre-dial string to be included in the tel: URI.
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In regard to subaddressing, this is a good idea, since the subaddressed destination is a descrete entity with a name that is unique within the context of the E.164 number.

In regard to "dial strings", I think this is not a good idea.  A "dial string" is the specific routing from one location to one destination.  It isn't a name for the destination, and it really isn't a URL, as there is the implication that a URL is globally valid.  What you are attempting to do is create a URL-like string which is the dialing instructions to reach a particular destination from a particular origin and which also makes explicit the E.164 address of the destination.  It has no portability at all.

Also, the commonest transformation that is applied to convert E.164 numbers to dial strings is the removal of the country and city codes from the beginning of the E.164 number, and the draft has no mechanism for specifying that.

I think a better approach would be to see if the context concept of RFC 3966 solves the problem, that is, the device which is acting on the tel URI should be provisioned with an appropriate converstion table from telephone numbers to dialing actions.

If you really need to transport a string of dialing instructions from one element to another, it would be much safer to define a new namespace.  E.g., "dial:w9w01118005551212".

Dale