On Sep 26, 2007, at 10:25 AM, Brian Rosen wrote:
Yeah, but I think you are starting with an incorrect assumption
that if you
improve the average response, you will get a better global result.
The
assumption usually made in these systems is that the network is
horribly
overloaded; nearly all low priority messages are timing out and the
system
is choked. You are way past the point where anything like what you
are
talking about can help.
Perhaps. But if you have to receive, enqueue, and parse the messages
to find out they have high priority or not, and if that's a
substantial part of the workload, and NOT forwarding the low-priority
responses is going to dramatically increase your workload (due to
cascade), then you gain nothing and lose much by using RPH to
prioritize responses. This needs to be tied to some lower-layer
mechanism (like a diffserv marking) that can allow for discard
without so much parsing.
Or it needs to be tied to a pushback "congestion notification" kind
of approach, and we have SIP set up with no way to push back against
responses, just against requests. So if a proxy is having to discard
a response because of congestion, the proxy needs to inform the
sender of the request that generated the response to back off.
Perhaps a proxy could MODIFY a response, turning it into a terminal
"retry after" or something like that. Of course, then it wouldn't be
a proxy -- it would be a B2BUA.
Maybe a header that says "I don't care if this looks like a 200 OK,
but some proxy in the path has decided an overload applies, so
abandon this request and don't retry" would work, but then we're in
the middle-to-end authentication realm, and that's a mess all on its
own.
Nope, I think SIP is just set up where trying to prioritize responses
doesn't work.
--
Dean
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