netdev - Re: [PATCH 0/4] Make tcp-metrics source-address aware

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Message-ID: <20131216200115.GA15318@cpaasch-mac>
Date:	Mon, 16 Dec 2013 21:01:15 +0100
From:	Christoph Paasch <christoph.paasch@...ouvain.be>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Julian Anastasov <ja@....bg>
Subject: Re: [PATCH 0/4] Make tcp-metrics source-address aware

On 16/12/13 - 11:53:31, Yuchung Cheng wrote:
> > Agreed, in this case it would be negative. Although I doubt that it happens
> > often that the source changes while the public IP remains the same.
> >
> > But for the WiFi/3G-case it is really bad that parameters like
> > ssthresh/rtt/... are kept the same. We will exit slow-start too early when
> > going from WiFi to 3G because ssthresh is too low.
> >
> > What if we copy the fast-open cookie across the different src/dst tcp_metrics-pairs
> > to handle Yuchung's case?
> > Or, maybe get rid of ssthresh/rtt/... in tcp-metrics? But that would probably be too
> > agressive...
> +1 to get rid of ssthresh caching. It often hurts than helps and
> cubic-hystart already mitigates the SS overshoot. See sec 6.2.4 in a
> recent research study
> http://conferences.sigcomm.org/co-next/2013/program/p303.pdf

I haven't yet read the paper, but in their presentation at CoNEXT they said
that the biggest impact comes from spurious retransmission timeouts and the
subsequent cwnd reduction. I would have thought that the cwnd-reduction
undo-procedure covers this case.

> The only case caching RTT could help is the init RTO. But it's unclear
> adding src-ip along will help like DM said.
> 
> Another metric that may have negative effect is reordering metrics.

So, let's get rid of all these metrics (except for TFO)? ;)


Christoph

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