On Mon, 2008-01-28 at 13:04 +0000, SCG2 wrote:

> Is there any circumstance at all where it makes sense to challenge an INVITE
> which is putting a call on hold?

A UAS or a Proxy is allowed to challenge any request (except an ACK,
since it isn't supposed to get any response) for any reason at any time,
and the UAC should be prepared to retry with authentication.  It doesn't
make sense to explicitly mention this fact in every discussion of every
SIP usage - it's just true unless there is a specific exception called
out by some spec.

Paul Kyzivat responds:

> [...] practically speaking authentication is 
> rarely done on in-dialog requests. Bypassing it assumes that the dialog 
> id is only known to the parties on the signaling path, even though this 
> might not be true.

We've recently started challenging some in-dialog requests:
specifically, any REFER without Replaces is challenged by our proxy so
that we can use the identity of the transfer controller to authorize the
INVITE generated by the transferee.


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