Subject: RE: [Sip] another modest proposal: change call-leg ID

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Thu, 12 Jul 2001 07:38:47 -0700

> 
> Well, the proposal is not meant to support proxies modifying 
> the To/From,
> but rather, allow endpoints to change the To/From in the 
> middle of the call.
> 
Hi Jonathan,

Perhaps not but I think it could it be used for such a purpose with _huge_
advantage (especially in telephony applications) and yet without being too
open to wholesale abuse. It may also deprecate some of the reasons that
people are using B2BUA-like devices merely so that they can rewrite the
To/From fields (for subscriber identity hiding). [Where transport-layer
identity screening is not important, eg, on a multi-user server or gateway.]

For instance, in telephony applications where it is desired that the caller
can be determined from the From URL, the ingress & egress gateway be able to
map the From URL into a global number context (eg local source number
567-1111 must be mapped to +1-203-567-1111 at the ingress gateway and then
the egress gateway must map this back into the appropriate representation
for the egress domain). This means that there must be a lot of routing
knowledge embedded in each and every endpoint for the correct number format
of the caller at the egress. Heaven forbid when you have internal numbering
and DID lines, etc,etc in the picture. 

This is not a problem for the request-uri because it changes hop-by-hop. For
instance, are request-uri of tel:5672222 can be automatically changed to
tel:+12035672222 when a proxy routes it to an international domain. This is
has previously not been possible for a pure proxy because it changes the
call-leg id. Now with this proposal a proxy can also remap the To & From
numbers into an appropriate format as the leave a number domain. As with all
routing problems they are all a function of local policy. 

Example using a local extension-based dial plan

request-uri: tel:95672222
From: tel:1111

could be remapped to 

request-uri: sip:[email-masked]
From: sip:[email-masked]

In this example the From tel:1111 has no significance outside of internal
number domain. Unless every phone is configured with logic to determine the
external globally reachable domain to insert - the From becomes useless.
What if there are multiple external domains involved? !!!This removes a huge
burden from the configuration of endpoints!!!

Now some might argue that this is the purpose of Remote-Party-Id. I still
believe it has its purpose - there may often be multiple representations of
the Remote-Party-Id. The way I see it is that the From URL provides the
preferred representation and something that still makes sense to a UA that
does not implement Remote-Party-Id extension. It will also be helpful in
actually providing a From URL that can be used to contact/callback the
caller (from the callee's domain).

I can only see this good for encouraging people to solving these real
requirements the right way.

My two cents.

Robert.

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---

From: "Jo Hornsby" <[email-masked]>
Date: Fri, 13 Jul 2001 12:21:52 +0100

Robert wrote:
> Jonathan wrote:
[...]

Eeek!  I think this might be A Bridge Too Far.  Okay,
I guess that proxies could unwind any From (/ To) changes
they made as responses swam back, but this is going to
break loop detection and request-merge detection and
I can imagine lots more with proxies that don't know these
new rules, if nothing else.  Just seems a little bit too
much of a leap to me.

 - Jo.