In message <[email protected]>, "Robert C. Durst" w
rites:
>
> Actually, in an environment that involves a geostationary satellite
> hop, the existing mechanism for signaling congestion, Source Quench,
> provides some added advantage over a bit in the header. If the congested
> site is on the "near" side of the satellite hop, the source quench message
> will arrive much sooner than if a congestion bit is required to propagate
> to the destination and then be fed back (you get both satellite hops in
> that case). The problem with source quench in the past has been when
> to send them. Cisco sends source quenches, but not more than once per
> second, which seems OK. With RED, one can mark packets (by sending a
> source quench) when operating in the probabilistic marking region, but
> not discard these packets. With a tighter feedback loop, the information
> might get there in time to prevent loss.
Source quench is currently sent by some routers but universally
ignored. I don't know that there are any exceptions. This is because
early routers sent source quench far too often and the use of source
quench is now strongly discouraged.
Sending one source quench per second would have no affect at all on
typical Internet traffic which has hundreds to hundreds of thousands
of TCP flows. In congested multiple bottleneck situations with lots
of flows, sending more traffic (source quench) has been proven to be a
bad idea. Links that are congested with a very high contribution from
small duration flows can need 5-15% loss. This would mean you'd need
to add 5-15% more packets (small ones) to get the equivalent slow down.
You can't propose something that will help the lossy wireless and
satellite cases and break the rest of the Internet. (Actually you can
propose anything, you just can't expect the proposal to get anywhere).
When Sally Floyd put out the TCP-ECN paper in 1994, the source quench
idea was soundly rejected but there has been interest in the ECN part
as a means to suppliment loss as a congestion indication, not replace
it.
The idea of an "experiencing loss" bit where loss should be ignored is
also dangerous if you assume that some routers will be congestion bit
aware and some still use drop as the means to indicate congestion.
Such a scheme would face serious obstacles in being deployed.
Curtis
This archive was generated by hypermail 2b29 : Mon Feb 14 2000 - 16:14:31 EST