[e2e] [Fwd: RED-->ECN]

[Steven Low]

> i used to think that measuring average queue size was really just a proxy for
> measuring average arrival rate.
> 
> however, V. Jacobson, K. Nichols, and K. Poduri have pointed out that the
> arrival rate might equal the link bandwidth (their example uses TCP ACK
> clocking, so no new packets show up until old ones leave), but there may still
> be a standing queue.
> 
> thus, if i had been measuring arrival rate, i would have decided "no need to
> drop [mark] packets".  whereas, i should have been measuring (and seeing) a
> standing queue and decided "i need to drop [mark] packets".
> 
> cheers,  Greg Minshall

Indeed.  When input rate equals the output rate in equilibrium, any
(equilibrium)
queue length is compatible, because 
	rate of change of queue = input rate - output rate = 0
This means one can attmpt to match input to output rate, and achieve, in
equilibrium,
full utilizaiton when the queue length stabilizes.   The value at which
the queue
stabilizes depends on the *process*, i.e., depends on
how we do congestion control (TCP/AQM), especially AQM.

In REM, the congestion measure (a number we call 'price') is incremented
when
the weighted sum of rate mismatch (input rate - output rate) and queue
mismatch (current queue - target queue, target may be 0) is positive,
and
decremented otherwise.  When the weighted sum is postive, it means
either input
rate exceeds output rate, or there is a large queue to be cleared, or
both.
So REM is striving to drive this weighted sum of mismatches to zero, and
it can 
be zero (in equilibrium) *if and only if* the both mismatches are zero,
i.e.
	input rate = output rate and queue = target.    
This is how REM achieves high utilization and low queue in equilibrium.

Steven

-- 
__________________________________________________________________
Steven Low, Assoc Prof of CS & EE 
slow at caltech.edu			netlab.caltech.edu
Tel: (626) 395-6767			Caltech MC256-80
Fax: (626) 792-4257			Pasadena CA 91125