netdev - Re: Doubts about listen backlog and tcp_max_syn

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Message-ID: <20130124122223.GQ4608@sociomantic.com>
Date:	Thu, 24 Jan 2013 13:22:23 +0100
From:	Leandro Lucarella <leandro.lucarella@...iomantic.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Doubts about listen backlog and tcp_max_syn_backlog

On Wed, Jan 23, 2013 at 11:28:08AM -0800, Rick Jones wrote:
> >Then if syncookies are enabled, the time spent in connect() shouldn't be
> >bigger than 3 seconds even if SYNs are being "dropped" by listen, right?
> 
> Do you mean if "ESTABLISHED" connections are dropped because the
> listen queue is full?  I don't think I would put that as "SYNs being
> dropped by listen" - too easy to confuse that with an actual
> dropping of a SYN segment.

I was just kind of quoting the name given by netstat: "SYNs to LISTEN
sockets dropped" (for kernel 3.0, I noticed newer kernels don't have
this stat anymore, or the name was changed). I still don't know if we
are talking about the same thing.

> But yes, I would not expect a connect() call to remain incomplete
> for any longer than it took to receive an SYN|ACK from the other
> end.

So the only reason to experience these high times spent in connect()
should be because a SYN or SYN|ACK was actually loss in a lower layer,
like an error in the network device or a transmission error?

> That would be 3 (,9, 21, etc...) seconds on a kernel with 3
> seconds as the initial retransmission timeout.

Which can't be changed without recompiling, right?

Thanks!

-- 
Leandro Lucarella
sociomantic labs GmbH
http://www.sociomantic.com
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