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Re: backbone routers' priority settings for ICMP & UDP

  • From: Dave Siegel
  • Date: Wed Feb 04 15:03:44 1998

> Marc, I'd have to agree, ICMP is more for flow control than congestion
> control. A source quench is to slow a fast machine from overrunning a slow
> machine, not preventing all flows from going through one link. 

> One (weak) metaphor is that traffic lights at an intersection are for flow
> control, while the traffic lights to get onto the freeway (common here in
> California) are for congestion control... 

Extremely weak metaphore, since a source quench indicates there weren't
enough buffers available to send your packets.

Now, if the freeway was full, and cars started dropping out of the space/time
continuum, that'd be more like a source quench.  ;-)  The freeway would call
your wife at home and say "sorry, but your husband didn't make it to work
because the freeways were too full."  If wife runs correct a correct
TCP implementation, she would know to initiate "slow start" and would 
send out her husbands at a slower rate until she gets a feel for how
bad the traffic is.

> One then
> wonders how well Win95 implements source quench, if at all. 

Which side of the implementation do you mean?  as a client, or as a gateway?
I suppose it doesn't really matter.  Since source quenches are not supposed
to be used on routers anymore, the expectation of receiving a source
quench on a large network (like the Internet) is a bad one, so the TCP
implementations have to implement congestion controls through other means

TCP/IP Illus. Vol. I by W. Richard Stevens has a pretty good explanation
of what source quenches are.


Dave Siegel				[email protected]
Network Engineer			[email protected] (alpha pager)
					(520)579-0450 (home office)