Question about propagation and queuing delays

Mon Aug 22 16:40:30 UTC 2005

Tony Finch wrote:
> On Mon, 22 Aug 2005, Petri Helenius wrote:
>>David Hagel wrote:
>>>This is interesting. This may sound like a naive question. But if
>>>queuing delays are so insignificant in comparison to other fixed delay
>>>components then what does it say about the usefulness of all the
>>>extensive techniques for queue management and congestion control
>>>(including TCP congestion control, RED and so forth) in the context of
>>>today's backbone networks? Any thoughts? What do the people out there
>>>in the field observe? Are all the congestion control researchers out
>>>of touch with reality?
>>
>>Co-operative congestion control is like many other things where you're better
>>off without it if most of "somebody else" is using it. TCP does not give you
>>optimal performance but tries to make sure everybody gets along.
> 
> TCP performs much better if queueing delays are short, because that
> means it gets feedback from packet drops more promptly, and its RTT
> measurements are more accurate so the retransmission timeout doesn't get
> artificially inflated.

A combination of WRED and friends doing tail- and head-drop would seem
to be ideal for this.

OTOH you rather want some queueing delays instead of packet drops if
your RTT is high.  Recovering from packet loss at >100ms is far slower
than having 10ms added to the RTT.

-- 
Andre