RE: [Sip] INFO considered harmful

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

> The point I was making in the draft was - why bother? INFO by itself 
> provides nothing. If you want differentiable, negotiable and 
> well-documented things, define them as new methods. What does 
> INFO give 
> you beyond that?

The ability to define a new usage without having to make code changes to
one's SIP stack, just as Events allows.

SIP, as you may have noticed, is a fairly complicated protocol. This has
given rise to the distribution of reusable (even open source) SIP stacks.
Personally I'm not up to hacking a new method (into, say, the Vovida stack)
everytime I want to deliver a new type of blob. INFO, constructed as I've
proposed, would let me implement a new INFO package on a stack without
having to rewrite the stack. We'd just need an API to tell the stack what
INFO packages we handled at the application layer, and it could do the rest
of the work without caring what data is being transported in the INFOs.
Events, you may have noticed, has exactly this property.

At some level, this is comparable to the "Why do we use MIME instead of
defining a new transport protocol for each and every possible type of data"
argument.

At its heart, INFO is for delivering APPLICATION SPECIFIC DATA within a
dialog. The SIP state machine does not have to care about that data, just
its delivery (although, as with SIP-T, the application itself may use the
data to influence the SIP state machine via the stacks's APIs). If SIP were
a proper transport protocol, we wouldn't have to define protocol extensions
for every application-specific datum.

I suspect that this working group has gotten very muddled between
"application" and "transport" layers. We have atendency to try and define
application semantics at the protocol level, and that's just wrong. The
protocol should define only protocol semantics.

--
Dean

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