Re: Checksum support: default behaviour

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2021-06-11 at 21:05 -0700, Mat Martineau wrote:
> On Fri, 11 Jun 2021, Matthieu Baerts wrote:
> 
> > Hello,
> > 
> > With the current checksum support series from Geliang and Paolo
> > available in our tree, the default behaviour is not to use this checksum
> > feature.
> > 
> > Should we eventually do the opposite and have it enabled by default?
> > 
> > I do understand this has a cost in terms of performances but this could
> > help detecting nasty middleboxes, i.e. the ones that modify the TCP
> > packets without modifying MPTCP options if needed.
> > 
> > On the other hand, I don't have numbers showing if these middleboxes are
> > rare or not.
> > 
> > But also, the main issue I see if we enable the checksum support by
> > default is that we are no longer able to talk to servers not supporting
> > it (<5.13), no?
> > 
> > WDYT?
> > 
> 
> I lean toward leaving checksums off by default, based on what I've heard 
> from community members. It sounds like large deployments haven't seen 
> checksums catch many problems? Some actual data about the frequency of 
> checksum failures would really help.

+1 on keeping csum disabled by default until we have at least one of
- performance data (delta)
- impact proof (csum corruption in the wild)
  (side note: even with no middle boxes at all, no app-layer mangling,
we can have bad MPTCP csum with correct TCP csum, e.g. due to BER plus
bad luck causing a stream corruption not catched by TCP csum but
catched by the MPTCP csum).

/P