Greetings,

On 03/12/2007, Stephen Hemminger [off-list ref] wrote:

On Mon, 3 Dec 2007 15:59:23 -0800
"Shao Liu" [off-list ref] wrote:

quoted

And also a question, why the samples when SACK is active are outliers?

Any sample with SACK is going to mean a loss or reordering has occurred.
So shouldn't the SACK values be useful, but RTT values from retransmits
are not useful.

When SACK is active, the per-packet processing becomes more involved,
tracking the list of lost/SACKed packets.  This causes a CPU spike
just after a loss, which increases the RTTs, at least in my
experience.  This is a separate issue from the fact that it is hard to
get RTT measurements from lost/retransmitted packets themselves.

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
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http://netlab.caltech.edu/~lachlan