Good news: #5 exactly the mechanism RFC 5022 uses. Brilliant (or
toasted) minds think alike.
Bad news: it does not solve the interoperability problem in its
entirety. It means that the MSCML endpoints are sure they understand
each other. However, if some one *also* does Cisco DTMF INFO, there is
no way of signaling that, or that a particular INFO message is MSCML
or DTMF, other than for the fact the Content-Type *hopefully* will be
different.
Note Content-Type is not enough to solve the problem in the abstract,
although it works for the known usages. That means I would NOT
advocate relying on Content-Type to disambiguate INFO messages.
On Jul 15, 2008, at 7:19 PM, Dean Willis wrote:
We had a large, long, and lengthy thread of discussion that I will try
to summarize.
So far, almost everybody who has voiced an opinions says we need to
fix,
rather than deprecate, INFO.
Opinions on fixing it vary.
Most people seem to think we need a registry of INFO usages that will,
at the very least, give us a table of the existing usages and the RFCs
or other specs that define them.
The sticky point of discussion is a negotiation and discovery
mechanism.
We've had on-list proposals for a strong mechanism based on the mode
used in RFC 3265.
Some folks think we need to do this, and others thing there's no
reason
to bother since it is not likely to get implemented because just
inventing a non-standard INFO use is easier.
Is there a middle ground?
What if we were to:
1) Establish an INFO registry and register our existing usages, policy
either "first come, first served" or "Specification required".
2) Define an Info-type header field and register (in the regoistry of
#1) a value for each known Info usage. Fully-compliant implementations
would send an Info-type header field with the appropriate value in
every
INFO message.
3) Require registration of an Info-type for each future usage into the
registry of #1 above.
4) Define an "Info-type not supported" error response message. This
handles the use case of a UAS that receives an INFO with an Info-
type it
does not understand.
5) For Info-types for which discovery is required, use a standards-
track
RFC to define a SIP extension and option tag, and use the usual
OPTIONS/Require negotiation mechanism for discovery. We might consider
revising each INFO-using RFC to define an appropriate option tag.
This lets people easily register INFO usages and a corresponding
Info-type tag. It lets nodes that don't understand an Info-type usage
reject the message (at the expense of whacking the dialog, which
arguably was already in need of whacking). And it provides a standard
(although heavy) mechanism for discovery and negotiation when those
are
required.
I think the above addresses the major concerns, is reasonably
implementable, is reasonably operable, and requires the smallest
possible effort to document. It's also pretty much backward
compatible.
--
Dean
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