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Re: Question about propagation and queuing delays

  • From: Andre Oppermann
  • Date: Mon Aug 22 12:41:36 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




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