[e2e] [Fwd: RED-->ECN]
misra at newworld.cs.umass.edu
Fri Jan 26 17:44:07 PST 2001
On Fri, 26 Jan 2001, Steven Low wrote:
> > In other words, when there are more sources, loss or delay must be
> > increased in order to produce high enough signal intensity for
> > sources to futher cut back their rates; moreover signal intensity
> > must stay high not only during transient
> > when new soruces first start but also afterwards.
> > REM tries to fix this, not through the exponential form of its
> > marking probabiltiy function, but by using a different congestion
> > measure and update rule, that maintains high utilization and low
> > queue in equilibrium.
AQM in the absence of ECN can be counter productive. If you try to
control the buffer to low levels, then you reduce delays. However that
means that loss must go up almost quadratically (taking the
simplified throughput equation for TCP, sqrt(p)*RTT = Constant since link
capacity is not changing). That increases the probability of redundant
retransmissions as also the probability of flows going into
timeouts. Goodput suffers. Hence, decoupling congestion measure and
performance measure may not help if your feedback mechanism is drops
instead of marks, and will in all likelihood hurt performance. Thus, ECN
is *critical* for the success of AQM schemes.
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