netdev - Re: [PATCH v3 2/2] tcp: Add rcv_wnd to TCP

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Date:   Fri, 13 Sep 2019 14:29:43 +0000
From:   Thomas Higdon <tph@...com>
To:     Dave Taht <dave.taht@...il.com>
CC:     Neal Cardwell <ncardwell@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "Dave Jones" <dsj@...com>, Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH v3 2/2] tcp: Add rcv_wnd to TCP_INFO

On Thu, Sep 12, 2019 at 10:14:33AM +0100, Dave Taht wrote:
> On Thu, Sep 12, 2019 at 1:59 AM Neal Cardwell <ncardwell@...gle.com> wrote:
> >
> > On Wed, Sep 11, 2019 at 6:32 PM Thomas Higdon <tph@...com> wrote:
> > >
> > > Neal Cardwell mentioned that rcv_wnd would be useful for helping
> > > diagnose whether a flow is receive-window-limited at a given instant.
> > >
> > > This serves the purpose of adding an additional __u32 to avoid the
> > > would-be hole caused by the addition of the tcpi_rcvi_ooopack field.
> > >
> > > Signed-off-by: Thomas Higdon <tph@...com>
> > > ---
> >
> > Thanks, Thomas.
> >
> > I know that when I mentioned this before I mentioned the idea of both
> > tp->snd_wnd (send-side receive window) and tp->rcv_wnd (receive-side
> > receive window) in tcp_info, and did not express a preference between
> > the two. Now that we are faced with a decision between the two,
> > personally I think it would be a little more useful to start with
> > tp->snd_wnd. :-)
> >
> > Two main reasons:
> >
> > (1) Usually when we're diagnosing TCP performance problems, we do so
> > from the sender, since the sender makes most of the
> > performance-critical decisions (cwnd, pacing, TSO size, TSQ, etc).
> > From the sender-side the thing that would be most useful is to see
> > tp->snd_wnd, the receive window that the receiver has advertised to
> > the sender.
> 
> I am under the impression, that particularly in the mobile space, that
> network behavior
> is often governed by rcv_wnd. At least, there's been so many papers on
> this that I'd
> tended to assume so.
> 
> Given a desire to do both vars, is there a *third* u32 we could add to
> fill in the next hole? :)
> ecn marks?

Neal makes some good points -- there is a fair amount of existing
information for deriving receive window. It seems like snd_wnd would be
more valuable at this moment. For the purpose of pairing up these __u32s
to get something we can commit, I propose that we go with
the rcv_ooopack/snd_wnd pair for now, and when something comes up later,
one might consider pairing up rcv_wnd.