[e2e] Can feedback be generated more fast in ECN?

[Neil Spring]

On Wed, Feb 14, 2001 at 04:45:31PM -0800, Eric A. Hall wrote:
> "Reduces" is probably an accurate term since a greater distance between
> the sender and the congestion point means higher latencies for the alert
> itself. But it never goes to zero benefit unless the alert itself is lost
> (which, btw, is equally possible with SQ and ACKs if you've really got
> that much congestion on the return path). IE, there is always >0 benefit,
> except in those cases where it is 0 for everybody.

If an ACK carrying the ECN-echo bit is lost, this matters
very little, since subsequent ACKs will also contain
the ECN-echo bit until the sender acknowledges it with
the Congestion Window Reduced (CWR) bit.  So, while the
probability of loss on the return path is the same for
SQ and an ACK, the consequences of loss are significant
for SQ and insignificant for ECN-ACKs.  In this sense,
ECN provides reliable contestion signalling, while source
quench does not.

Knowing that all congestion signals are delivered seems
very important.  To provide reliable congestion signals
using SQ, the router would have to drop the packet,
with obvious disadvantages, to force the receiver to
(implicitly) tell the sender to begin loss recovery and
a congestion response.

-neil
an SQ infidel