Re: [PATCH v2 6/9] tcp: Don't limit window to less-than-MSS values, use zero instead

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

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On Sat, Dec 13, 2025 at 03:24:06PM +0100, Stefano Brivio wrote:
> On Mon, 8 Dec 2025 09:11:07 +0100
> Stefano Brivio <sbrivio@redhat.com> wrote:
> 
> > On Mon, 8 Dec 2025 17:43:14 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> > 
> > > On Mon, Dec 08, 2025 at 01:22:14AM +0100, Stefano Brivio wrote:  
> > > > If the sender uses data clumping (including Nagle's algorithm) for
> > > > Silly Window Syndrome (SWS) avoidance, advertising less than a MSS
> > > > means the sender might stop sending altogether, and window updates
> > > > after a low window condition are just as important as they are in
> > > > a zero-window condition.
> > > > 
> > > > For simplicity, approximate that limit to zero, as we have an
> > > > implementation forcing window updates after zero-sized windows.
> > > > This matches the suggestion from RFC 813, section 4.
> > > > 
> > > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > > > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>    
> > > 
> > > Looking at this again, I'm worrying if it might allow a pathalogical
> > > case here: unlikely to hit, but very bad if it did.
> > > 
> > > Suppose we have:
> > >   1. A receiver that wants to consume its input in fixed largish
> > >      (~64kiB) records
> > >   2. The receiver has locked its SO_RCVBUF to that record length, or
> > >      only slightly more
> > >   3. The receive buf is near full - but not quite a full record's
> > >      worth
> > > 
> > > The receiver doesn't consume anything, because it doesn't have a full
> > > record.  Its rcvbuf is near full, so its kernel advertises only a
> > > small window.  We approximate that to zero, so the sender can't send
> > > anything.  So, the record never gets completed and we stall
> > > completely.  
> > 
> > I don't think it can be a problem because the receiver shouldn't
> > advertise less than a MSS in that case anyway, but I need to look up
> > normative references for this.
> 
> As you predicted, I just got a report of a similar stall, which is
> pretty bad and easy to reproduce.

Sometimes I hate being right :).

-- 
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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