Re: SIP Tracing Facility

On Thu, 2009-03-05 at 08:16 +0000, Elwell, John wrote:
> What about non-INVITEs? Provisional responses are designed for INVITEs
> and I am not certain about their applicability to non-INVITEs.

Yes, provisional responses to non-INVITEs may be slightly out of
specification, but I don't expect any SIP element to malfunction if it
receives a provisional response to a non-INVITE request.  OTOH, I would
expect most uses of 'trace' to be attached to INVITEs, although that
makes me think that tracing an OPTIONS might be a useful test tool.

(Which leads to an unexpected observation:  RFC 3261 presupposes that
all non-INVITE responses are processed promptly, but in the face of
transport errors, this is not always so.  In sipX, we've adjusted
transaction handling so that if a re-send of a non-INVITE message is
seen the recipient sends a 100 to quench further re-sends.  It seems to
work exactly as expected, and noticeably reduces re-sends in situations
with transport errors.)

> In the case of INVITE, what about Contact? Is it permissible to send a
> 1xx (other than 100) without a Contact URI? RFC 3261 seems to indicate
> it is optional in a 1xx, but that might be solely to cater for omission
> in 100.

The Contact value of a 170 should never be used, so it doesn't really
matter if one is provided, or if it is usable.  OTOH, every SIP element
should be able to create a usable Contact URI.

Dale


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