Subject: [Sip] Questions on subscription dialogs

From: "Christer Holmberg \(JO/LMF\)" <[email-masked]>
Date: Mon, 27 Feb 2006 09:45:30 +0100

Hi,

A few questions on subscription dialogs.

Q1: Is it allowed to send non-NOTIFY request within a subscription
dialog (initiated by SUBSCRIBE), assuming the users support it?

Q2: If Q1==TRUE, is it allowed to add media during a subscription
dialog, by sending an SDP offer in an UPDATE/re-INVITE?

Q3: If Q2==TRUE, is it allowed to use re-INVITE, eventhough the dialog
was not initiated using INVITE?

Q4: If Q1==TRUE, if I send an empty re-INVITE within a subscription
dialog, e.g. to update the remote target, I assume the receiver MUST
create and include an SDP offer in the 200 OK response, eventhough no
media has previously been used within the dialog?

Regards,

Christer

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From: Paul Kyzivat <[email-masked]>
Date: Mon, 27 Feb 2006 16:30:15 -0500

Christer - see comments inline.

> dialog (initiated by SUBSCRIBE), assuming the users support it?

Yes, with limits. In general, a request that is legal outside a dialog 
is also legal within a dialog. (E.g. OPTIONS.) In some cases (e.g. 
MESSAGE) other limitations have been applied. (You aren't supposed to 
use MESSAGE within a dialog as a way to avoid session mode messaging.)

It is legal, though generally discouraged, to send requests within an 
existing dialog that establish a new dialog usage. Sending an INVITE 
within a dialog established by a SUBSCRIBE falls into this category.

> dialog, by sending an SDP offer in an UPDATE/re-INVITE?

An INVITE in this case (at least the first one) would not be a reINVITE. 
It would establish a new invite dialog usage within an already 
established dialog.

IMO an UPDATE is only permitted within an existing invite dialog usage. 
So you would have to send an INVITE to establish the usage before you 
could use UPDATE to do a subsequent offer/answer.

> was not initiated using INVITE?

See above. Beware that I have heard of no implementation that actually 
support this.

> media has previously been used within the dialog?

Again see above. The invite must follow all the normal rules.

Note that as defined, all the usages of the dialog share the same 
targets. If you have a dialog shared by an invite usage and a subscribe 
usage, then either a reinvite or a resubscribe will update the target 
they share in common.

If you have just a subscription, and what to change the target, you can 
use a resubscribe. No need to use UPDATE or INVITE to accomplish that.

	Paul