netdev - Re: [RFC PATCH 28/28] tcp: AccECN sysctl documentation

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Date:   Mon, 23 Mar 2020 15:34:40 +0200 (EET)
From:   "Ilpo Järvinen" <ilpo.jarvinen@...helsinki.fi>
To:     Yuchung Cheng <ycheng@...gle.com>
cc:     Dave Taht <dave.taht@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Olivier Tilmans <olivier.tilmans@...ia-bell-labs.com>
Subject: Re: [RFC PATCH 28/28] tcp: AccECN sysctl documentation

On Fri, 20 Mar 2020, Yuchung Cheng wrote:

> On Fri, Mar 20, 2020 at 3:40 PM Ilpo Järvinen
> <ilpo.jarvinen@...helsinki.fi> wrote:
> >
> > On Thu, 19 Mar 2020, Dave Taht wrote:
> >
> > > On Wed, Mar 18, 2020 at 2:44 AM Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:
> > > >
> > > > From: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>
> > > >
> > > > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>
> > > > ---
> > > >  Documentation/networking/ip-sysctl.txt | 12 +++++++++---
> > > >  1 file changed, 9 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
> > > > index 5f53faff4e25..ecca6e1d6bea 100644
> > > > --- a/Documentation/networking/ip-sysctl.txt
> > > > +++ b/Documentation/networking/ip-sysctl.txt
> > > > @@ -301,15 +301,21 @@ tcp_ecn - INTEGER
> > > >                 0 Disable ECN.  Neither initiate nor accept ECN.
> > > >                 1 Enable ECN when requested by incoming connections and
> > > >                   also request ECN on outgoing connection attempts.
> > > > -               2 Enable ECN when requested by incoming connections
> > > > +               2 Enable ECN or AccECN when requested by incoming connections
> > > >                   but do not request ECN on outgoing connections.
> > >
> > > Changing existing user-behavior for this default seems to be overly
> > > optimistic. Useful for testing, but...
> >
> > I disagree.
> >
> > The kernel default on ECN is/has been "do nothing" like forever. Yet,
> > passively allowing ECN on servers is a low risk operation because nothing
> > will change before client actively asks for it. However, it was obvious
> > that the servers didn't do that. The servers could have set tcp_ecn to 1
> > (before 2 was there) which is low risk for _servers_ (unlike for clients)
> > but only very very few did. I don't believe servers would now
> > intentionally pick 2 when they clearly didn't pick 1 earlier either.
> >
> > Adding 2 is/was an attempt to side-step the need for both ends to make
> > conscious decision by setting the sysctl (which servers didn't want to
> > do). That is, 2 gives decision on what to do into the hands of the client
> > side which was the true intent of 2 (in case you don't know, I made that
> > change).
> What can a server configure to process only RFC3168 ECN if it prefers to?

That's why I suggested the flag-based approach?

> > If "full control" is the way to go, I think it should be made using flags
> > instead, along these lines:
> >
> > 1: Enable RFC 3168 ECN in+out
> > 2: Enable RFC 3168 ECN in (default on)
> > 4: Enable Accurate ECN in (default on)
> > 8: Enable Accurate ECN in+out
> >
> > Note that I intentionally reversed the in and in/out order for 4&8
> > (something that couldn't be done with 1&2 to preserve meaning of 1).

It should address any except "out" but no "in" (the meaning of 1 cannot 
be changed I think). But out w/o in doesn't sound very useful.

--
 i.