netdev - Re: Default value of ipv4.tcp_keepalive

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Date:   Mon, 17 May 2021 12:00:38 +0200
From:   Willy Tarreau <w@....eu>
To:     Troels Arvin <troels@...in.dk>
Cc:     netdev@...r.kernel.org
Subject: Re: Default value of ipv4.tcp_keepalive_time

Hello,

On Mon, May 17, 2021 at 10:00:04AM +0200, Troels Arvin wrote:
> Hello,
> 
> At work, we have spent a great deal of work on a situation which ended up
> being resolved by changing the net.ipv4.tcp_keepalive_time sysctl to a value
> much lower than the default (we set it to 300). This was two Linux-based
> systems communicating without any firewalls in-between, where some
> long-running connections would be considered down by one system, while the
> other expected them to still be around.
> 
> The following is the description of the setting:
> "The interval between the last data packet sent (simple ACKs are not
> considered data) and the first keepalive probe; after the connection is
> marked to need keepalive, this counter is not used any further."
> 
> The default value of net.ipv4.tcp_keepalive_time sysctl is 7200 seconds,
> i.e. two hours.
> 
> It seems odd to me to still have such a long period of waiting, before
> keep-alive kicks in. With such a long initial wait, it's questionable how
> much value the keep-alive functionality has, I think.
> 
> Could it be that it's time to change the default? I would suggest a value of
> 10 minutes, i.e. 600 seconds, but I have to admit, that I don't have any
> objective argument for exactly that value.

There is no good value, it's entirely dependent on the application. At
least with a large default value you have little risk to flood your
links when dealing with hundreds of thousands to millions of idle
connections (think WebSocket for example). A 10 minute value would
still be too large for plenty of users but cause excessive traffic
for others.

I strongly suggest that the value must be tunable in the application
instead, that's the only way to get it right for everyone (see
TCP_KEEPCNT, TCP_KEEPINTVL and TCP_KEEPIDLE).

Just my two cents,
Willy