Re: [PATCH net-next v2] tcp: enable mid stream window clamp

[Eric Dumazet <edumazet@google.com>]

On Wed, Aug 25, 2021 at 9:57 AM Neil Spring <ntspring@fb.com> wrote:
>
>
> Eric Dumazet wrote:
> > On 8/19/21 12:54 PM, Neil Spring wrote:
> > > The TCP_WINDOW_CLAMP socket option is defined in tcp(7) to "Bound the size of
> > > the advertised window to this value."  Window clamping is distributed across two
> > > variables, window_clamp ("Maximal window to advertise" in tcp.h) and rcv_ssthresh
> > > ("Current window clamp").
> > >
> > > This patch updates the function where the window clamp is set to also reduce the current
> > > window clamp, rcv_sshthresh, if needed.  With this, setting the TCP_WINDOW_CLAMP option
> > > has the documented effect of limiting the window.
> > >
> > > Signed-off-by: Neil Spring <ntspring@fb.com>
> > > ---
> > > v2: - fix email formatting
> > >
> > >
> > >  net/ipv4/tcp.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > > index f931def6302e..2dc6212d5888 100644
> > > --- a/net/ipv4/tcp.c
> > > +++ b/net/ipv4/tcp.c
> > > @@ -3338,6 +3338,8 @@ int tcp_set_window_clamp(struct sock *sk, int val)
> > >        } else {
> > >                tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
> > >                        SOCK_MIN_RCVBUF / 2 : val;
> > > +             tp->rcv_ssthresh = min(tp->rcv_ssthresh,
> > > +                                    tp->window_clamp);
>
> > This fits in a single line I think.
> >                 tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
>
> I'll fix in v3 in a moment, thanks!
>
> > >        }
> > >        return 0;
> > >  }
> > >
>
> > Hi Neil
> >
> > Can you provide a packetdrill test showing the what the new expected behavior is ?
>
> Sure.  I submitted a pull request on packetdrill -
> https://github.com/google/packetdrill/pull/56 - to document the intended behavior.
>
> > It is not really clear why you need this.
>
> The observation is that this option is currently ineffective at limiting once the
> connection is exchanging data. I interpret this as a result of only looking at
> the window clamp when growing rcv_ssthresh, not as a key to reduce this
> limit.
>
> The packetdrill example will fail at the point where an ack should have a reduced
> window due to the clamp.
>
> > Also if we are unable to increase tp->rcv_ssthresh, this means the following sequence
> > will not work as we would expect :
>
> > +0 setsockopt(5, IPPROTO_TCP, TCP_WINDOW_CLAMP, [10000], 4) = 0
> > +0 setsockopt(5, IPPROTO_TCP, TCP_WINDOW_CLAMP, [100000], 4) = 0
>
> The packetdrill shows that raising the window clamp works:
> tcp_grow_window takes over and raises the window quickly, but I'll add
> a specific test for this sequence (with no intervening data) to confirm.

Sure, raising the window clamping is working (even before your patch)

But after your patch, rcv_ssthresh will still be 10000, instead of
something maybe bigger ?

>
> > Thanks.
>
> Thanks Eric!
>
> -neil