[SIP] phone-context and RFC2543bis

[Alan Johnston <[e-mail]>]

Based on reports from Adelaide, it sounds like the consensus of the working
group was to progress the Call Flows I-D (draft-ietf-sip-call-flows-00.txt) to
Informational RFC as soon as possible.

One item that needs to be resolved first is the "phone-context" tag which is
used extensively in the Gateway dialing sections of the document for private
phone numbers (extensions).

In RFC2543, support for phone numbers in SIP URLs was included by including
parameters from the then current version of the Tel URL I-D if "user=phone" was
present.  Since then, there have been additional drafts of this document
(currently draft-antti-telephony-url-12.txt).

One of the critical additions to the Tel draft since RFC2543 was the addition of
the phone-context tag used for local numbers to identify the scope in which the
number is valid.  I believe this tag needs to be included in SIP URLs in the
next SIP draft.

For example, the U.S. directory assistance telephone number can be written in
global form as:

	sip:[e-mail];user=phone

However, if the number was only valid if dialed from within the U.S., the number
could be written as a local phone number as:

	sip:[e-mail];phone-context=+1

Where the phone-context tag indicates that it is only valid from within country
code 1.

Another more compelling use of the phone-context tag is in dealing with private
numbers - numbers that are not part of the public number space, but are part of
a private numbering plan administered by a corporation or organization.  The
examples in the Call Flows document are of this kind:

	sip:[e-mail];phone-context=mycarrier

In this example, it appears that the host portion (gateway address) of the URL
is sufficient to specify the context of the private number.  However, for cases
where a gateway is shared among multiple customers, each with possibly
overlapping private numbering plans, the use of phone-context is required:
	sip:[e-mail];phone-context=mycarrier-customer1

The use of the phone-context tag also allows interdomain private dialing,
something impossible in todays PSTN.  For example, dialed digits for a
particular dialing plan could be sent to a proxy for gateway lookup

	sip:[e-mail];phone-context=carrier-customer2

The proxy would lookup the gateway based on dialed digits and phone-context.  If
the proxy did not have gateway information for that carrier (domain), the
request could be proxied to that domain with the Request-URI becoming:

	sip:[e-mail];phone-context=carrier-customer2

These are just a few examples of the use of this tag.

Are there any reasons why this tag should not be supported in SIP URLs in the
next draft of RFC2543?

Alan Johnston
MCI WorldCom



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