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Date: Fri, 1 May 2009 13:23:37 +0100
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] PATCH: enabling TCP keepalives - v3
Message-ID: <20090501122337.GC15311@shareable.org>
References: <49F9FEBA.6050901@gmail.com>
	<20090501113204.GA10763@amd.home.annexia.org>
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: qemu-devel@nongnu.org, David Ahern <dsahern@gmail.com>

Richard W.M. Jones wrote:
> On Thu, Apr 30, 2009 at 01:40:42PM -0600, David Ahern wrote:
> > Did not see a response to the last version.
> > 
> > This patch enables TCP keepalives on VNC connections and TCP-based char
> > devices.
> > 
> > Default parameters have keep alive probes sent after 60-seconds of idle
> > time. Probes are sent every 12 seconds with the connection resetting
> > after 5 failed probes (ie., connection is closed if no response received
> > in 60-seconds).
> 
> IMHO this should be optional, and firmly default to _OFF_.  Brief
> network outages shouldn't result in connections failing all over the
> place.  In addition, does this negatively impact migration?

Note that if either side tries to send actual data, such as due to a
mouse movement or screen update, then connections will fail after a
TCP timeout anyway.  There's no getting away from a network outage
tripping it, unless the session is completely idle - static screen, no
input.

Keepalive just causes the connections to drop when there was no other
traffic to detect the connection being down.  It's being added because
without it, QEMU accumulates connections when the network is really is
gone - they never go away.

-- Jamie