[Sip] What good is SIPS:?

[Dean Willis <dean.willis at softarmor.com>]

What does sips: do for us?

It allows the end-user to ask the proxies to apply hop-by-hop  
cryptography and authentication, with the assurance that proxies that  
support the spec will honor that request.

It's not a high level of security as it says nothing about and cannot  
detect non-compliant proxies, but it is a property that we don't have  
with any other SIP mechanism at this time.

Many people believe this is a useful property despite its  
limitations. People are apparently trying to use it today, with  
inconsistent results. Francois' draft is intended to at least help  
them produce consistent results.

As we think about alternatives or changes in specification, let's  
keep that one useful property in-mind.

This leaves us with two questions, and we should try and  
differentiate further discussion by which question we're talking about:

1) Is it worth helping people get what they can out of SIPS: as  
roughly described in 3261, or is it so broken we should just suggest  
not bothering?


2) Do we need to do something beyond what sips:-as-per-RFC 3261 does,  
and if so, what properties does that something need to have? I  
personally suspect that there are three useful categories of "beyond":

	a. Fix the last-hop exception and first-hop lack-of-exception,  
either by eliminating it/them or more clearly codifying what we're  
talking about.

	b. Clarify the usage for non-TLS alternatives, if any.

	c. Provide an end-to-end alternative that is fully verifiable by the  
UAC (and maybe the UAS). Of course, this leaves open the question of  
what happens if either node is a B2BUA. 

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