Re: FW: I-D Action:draft-kaplan-sip-session-id-00.txt

Hi Ian,
Comments inline...

> -----Original Message-----
> From: Ian Elz [mailto:[email protected]]
> Sent: Monday, December 01, 2008 4:35 AM
>
> I know that we have to work with B2BUAs, and I understand the sentiment
> behind getting rid of them all together. The ietf work tends to preclude
> handling by B2BUAs but based upon the RFC3261 definition of a B2BUA as a
> concatenation of the UAC and UAS then all the ietf specifications have
> to ensure is that each Request can be routed to the correct B2BUA and
> then it is up to the B2BUA to ensure that it performs the correct
> actions for the end-to-end service.

This requires 2 things:
(1) that every B2BUA constantly be upgraded to handle whatever new mechanism's syntax is for the identifiers, every time a new one is defined. (e.g., new XML schema or new header)  I realize that's the nature of "you break it, you fix it", but if people want to have their new thing work without needing B2BUA's to be upgraded or re-configured, then there needs to be some generic way of handling this.
(2) that the mechanism using the call-id+tags gets sent to the B2BUA that changed them.  Often it will.  Sometimes it can't.  For example, the mediactrl-framework model.  Stick a b2bua between the UA and the Media-Control-Client, and the application doesn't work - or wouldn't have, except they changed it to use a new identifier different from call-id to avoid this problem.


> Based upon RFC3261 a B2BUA MUST map the call-id as a Call-id MUST be
> globally unique. As a B2BUA is a UAS/UAC the dialogs on either side of
> the B2BUA are different dialogs and must have different Call-ids.

Actually that's debatable.  I don't think we'd want to say B2BUA's MUST change the Call-ID.  3261 did say a B2BUA is the logical concatenation of a UAS and UAC, but clearly not much text in 3261 was given to how a B2BUA should operate.

> The major issue which currently occurs is that a PUI is used to try and
> route a request which should be directed to specific UA, e.g. when
> referencing an extant dialog. To reach the specific UA the Contact
> should be used and if a B2BUA maps the Contact as well as the Call-id
> then the new Request should route to the B2BUA which can perform its
> 'magic' to provide the end-to-end service.

Some folks (from 3gpp) have asked that B2BAU's not change the contact-uri if it's a GRUU, though I'm not exactly sure why.  There are also some cases in which having a B2BUA change the contact to be its specific instance doesn't work, because that address is not globally reachable; so leaving it as a GRUU (or something like a GRUU), is necessary but means out-of-dialog requests don't always reach the same B2BUA instance.  This has been found in REFER transfer cases, where the referred party can't reach the same B2BUA as the referrer can.

-hadriel
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