RE: [Sip] INFO considered harmful

["Dean Willis" <dean.willis@softarmor.com>]

Christer said:
> I agree that the semantics of the commands are different. 
> However, one IS allowed to include any kind of message body 
> in the INVITE request, to provide whatever "extra 
> information" about the session (text, HTML, whatever...), so 
> someone could argue that whatever stuff they put into the 
> INVITE does provide "extra information" about the session...
> 
> And, since it IS allowed to send re-INVITE, or UPDATE, 
> without actually updating the session description (the SDP 
> message body) someone could include that whatever stuff also 
> in those messages...

That's actually quite interesting. One can get the same functionality as
INFO using UPDATE methods. For example, SIP-T could encode the mid-session
ISUP and send it using UPDATE.

So, what is the semantic differentiation?

I propose that we've been operating under a logic system where "UPDATE
affects SIP state and INFO doesn't". It might be that this exposes a fallacy
(which one might call "telephone centric" thinking) in our logic. There are
really two different "sessions" in a typical SIP exchange: 1) The SIP-level
session, or "dialog", framed in INVITE and BYE or SUBSCRIBE and BYE, and 2)
the negotiated session, expressed in SDP, which may or may not exist.

Since there may or may not be a negotiated session, and since SIP works just
fine either way, is it reasonable to divide SIP mid-dialog semantics into
"things that we think might affect the negotiated session", and "things that
we think don't"? Perhaps all we're really doing is handing a blob of data to
the application (which exists ABOVE the SIP layer), which then uses that
information to affect its own state, and may consequently affect the SIP
state?

In any case, I think there is a much deeper semantic problem lurking here
that we really haven't thought out.

--
Dean

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