netdev - Re: [RFC] TCP illinois max rtt aging

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Message-ID: <aa7d2c6d0712031723t314da471n8f033e8d1b523e1a@mail.gmail.com>
Date:	Mon, 3 Dec 2007 17:23:35 -0800
From:	"Lachlan Andrew" <lachlan.andrew@...il.com>
To:	"Stephen Hemminger" <shemminger@...ux-foundation.org>
Cc:	shaoliu@...nceton.edu, "David S. Miller" <davem@...emloft.net>,
	"Herbert Xu" <herbert@...dor.apana.org.au>,
	"Douglas Leith" <doug.leith@...m.ie>,
	"Robert Shorten" <robert.shorten@...m.ie>, netdev@...r.kernel.org
Subject: Re: [RFC] TCP illinois max rtt aging

Greetings,

On 03/12/2007, Stephen Hemminger <shemminger@...ux-foundation.org> wrote:
> On Mon, 3 Dec 2007 15:59:23 -0800
> "Shao Liu" <shaoliu@...nceton.EDU> wrote:
> > 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
Ph: +1 (626) 395-8820    Fax: +1 (626) 568-3603
http://netlab.caltech.edu/~lachlan
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