Re: [PATCH v3 4/4] tcp: Update data retransmission timeout

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Oct 29, 2025 at 12:13:30AM +0100, Stefano Brivio wrote:
> On Mon, 20 Oct 2025 20:17:10 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Mon, Oct 20, 2025 at 07:11:07AM +0200, Stefano Brivio wrote:
> > > On Mon, 20 Oct 2025 11:20:19 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
[snip]
> > > > > Rather than the local link I was thinking of whatever monitor or
> > > > > liveness probe in KubeVirt which might have a 60-second period, or some
> > > > > firewall agent, or how long it typically takes for guests to stop and
> > > > > resume again in KubeVirt.    
> > > > 
> > > > Right, I hadn't considered those.  Although.. do those actually re-use
> > > > a single connection?  I would have guessed they use a new connection
> > > > each time, making the timeouts here irrelevant.  
> > > 
> > > It depends on the definition of "each time", because we don't time out
> > > host-side connections immediately.  
> > 
> > Hm, ok.  Is your concern that getting a negative answer from the probe
> > will take too long?
> 
> More like getting a positive answer taking too long, because we retry
> so infrequently.

Right, but it will only be slow if we lose the first probe, which
should be very rare.

> > > Pretending passt isn't there, the timeout would come from the default
> > > values for TCP connections. It looks like there's no specific
> > > SO_SNDTIMEO value set for those probes, and you can't configure the
> > > timeout, at least according to:
> > > 
> > >   https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-a-tcp-liveness-probe  
> > 
> > My guess would be that the probe would probably time out at the
> > application level long before the TCP layer times out, but I don't
> > know for sure.
> 
> I don't think so. What I was pointing out is that I couldn't find any
> place in the implementation of those probes where a particular
> *handshake timeout* (not probe interval) is set on top of Linux's
> defaults, so timeouts at TCP layer and application level should be the
> same (no additional timeout in application logic).

Huh, that's mildly surprising to me.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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