Re: SIP Tracing Facility

> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf Of Dale
> Worley
> Sent: Thursday, March 05, 2009 10:26 AM
>
> 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.

OPTIONS doesn't always get routed through the same path as INVITE, fwiw.


> (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.)

I agree for most methods, but we've seen problems with 100 for the OPTIONS.  Some devices use the OPTIONS as a reachability ping/test, so sometimes the 100 makes them think the far-end is reachable even if they get a 408 later; in other cases some devices look for specific response codes to detect administrative out-of-service, for example a 500/503 to an OPTIONS sometimes is used for that, and sometimes they didn't code for the 100 Trying as being a possibility, so they see it and assume it's not out-of-service, and the 503 sent later doesn't get processed the same.  I consider these bugs/broken implementations, but just noting sending the 100 is not bug-free.

 -hadriel
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