netdev - Re: TCP connection issues against Amazon S3

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Date:	Wed, 07 Jan 2015 07:58:04 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Erik Grinaker <erik@...gler.no>
Cc:	Yuchung Cheng <ycheng@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>
Subject: Re: TCP connection issues against Amazon S3

On Wed, 2015-01-07 at 13:31 +0000, Erik Grinaker wrote:
> On 06 Jan 2015, at 22:00, Yuchung Cheng <ycheng@...gle.com> wrote:
> > On Tue, Jan 6, 2015 at 1:04 PM, Erik Grinaker <erik@...gler.no> wrote:
> >> 
> >>> On 06 Jan 2015, at 20:26, Erik Grinaker <erik@...gler.no> wrote:
> >> This still doesn’t explain why it works with older kernels, but not newer ones. I’m thinking it’s
> > probably some minor change, which gets amplified by the lack of SACKs
> > on the loadbalancer. Anyway, I’ll bring it up with Amazon.
> > can you post traces with the older kernels?
> 
> Here is a dump using 3.11.10 against a non-SACK-enabled loadbalancer:
> 
> http://abstrakt.bengler.no/tcp-issues-s3-nosack-3.11.10.pcap.bz2
> 
> The transfer shows lots of DUPACKs and retransmits, but this does not
> seem to have as bad an effect as it did with the failing transfer we
> saw on newer kernels:
> 
> http://abstrakt.bengler.no/tcp-issues-s3-failure.pcap.bz2
> 
> One big difference, which Rick touched on earlier, is that the newer
> kernels keep sending TCP window updates as it’s going through the
> retransmits. The older kernel does not do this.

The new kernel is the receiver : It does no retransmits.

Increasing window in ACK packets should not prevent sender into
retransmitting missing packets.

Sender is not a linux host and is very buggy IMO : If receiver
advertises a too big window, sender decides to not retransmit in some
cases.

You can play with /proc/sys/net/ipv4/tcp_rmem and adopt very low values
to work around the sender bug.

( Or use SO_RCVBUF in receiver application)


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