Re: [Sipping] UPDATE during re-INVITE and related issues

[Paul Kyzivat <pkyzivat at cisco.com>]

Anton,

I don't think I understand you. Comments inline.

Anton Tveretin wrote:
> Hi,
> First, preconditions could (and IMHO SHOULD) be satisfied before 
> re-INVITE. So anyway, they will be rolled back explicitly if re-INVITE 
> fails. IMHO normal diagram for re-INVITE would look like this, am I 
> right? (Adding a new stream)

IIUC, you are assuming that a session has already been established with 
one or more media streams before the below?

> -->RSVP:Path
> <--RSVP:Resv
> -->RSVP:ResvConf

If these RSVP messages are for a contemplated new media stream, where 
are you sending them? You don't know where the media stream will terminate.

> -->SIP:INVITE(offer);
> <--SIP:180(no SDP)

I think the UAS can't return 180 until it knows the resources are 
available to establish the new stream.

> <--RSVP:Path
> -->SIP:PRACK
> <--SIP:200 PRACK
> -->RSVP:Resv
> <--RSVP:ResvConf
> ---waiting for user to accept or reject new media---
> <--SIP:200 INVITE(answer)
> -->SIP:ACK
> I mean, 1xx responses to re-INVITE should not contain SDP, as there is 
> no reason.

The reason is so that the UAC will know if there will indeed be a new 
media stream, and where it will terminate. Until it knows that, how can 
it do the RSVP to reserve resources for the path?

I'll stop commenting here because without agreement on this its hard to 
talk about more.

	Thanks,
	Paul

> Second, UPDATE easily goes into race conditions with re-INVITE. Recall 
> Gao's diagram:
> -->re-INVITE (F1)
> <--1xx (F2)
> <--UPDATE (F3)
> F2 and F3 may be delivered out of order. So there will be no way to tell 
> whether F1 or F3 came out first. This is also true even both re-INVITE 
> and UPDATE were sent by the same UA. Only SDP versioning helps.
> So UPDATE is not a part of re-INVITE transaction if it contained no SDP.
> IMO everything that clearly is a part of re-INVITE transaction, must be 
> rolled back.
> Consider the following:
> -->re-INVITE(offer1)
> <--180(answer1)
> -->PRACK
> <--200 PRACK
> -->UPDATE(offer2)
> <--200 UPDATE(answer2)
> <--603 INVITE
> -->ACK
> Still, UA at left might not commited the second O/A pair (if responses 
> to the re-INVITE and UPDATE were re-ordered).
> Regards
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