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Date: Tue, 14 Nov 2023 16:04:32 -0500
From: Anup Agarwal <anupa@...rew.cmu.edu>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org
Subject: Re: Potential bug in linux TCP pacing implementation

Got it. Thanks.

On Tue, Nov 14, 2023 at 3:58 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Nov 14, 2023 at 9:39 PM Anup Agarwal <anupa@...rew.cmu.edu> wrote:
> >
> > Thanks for your response.
> >
> > Yeah, I think for the currently deployed CCAs, the current pacing
> > implementation works fine.
> >
> > I wanted to clarify, the issue is caused due to temporal change in
> > sk_pacing_rate and is independent of pkt sizes or network parameters
> > (bandwidth, rtt, etc.). If the sk_pacing_rate goes from r1=0.1 pkt per
> > ms (~1.2 Mbps for ~1500B MTU) to r2=10 pkts per ms (~120 Mbps), then
> > opportunity to send 99 pkts (=(r2/r1)-1) is missed. This is because
> > tcp_wstamp_ns was computed as =10ms using r1, even though
> > sk_pacing_rate changed to r2 (immediately after tcp_wstamp_ns
> > computation) and a pkt could have been sent at 0.1ms.
> >
> > The ratio of the new and old rate matters, not the pkt sizes, or other
> > network params. Typical CCAs perhaps only change rate by ~2 times so
> > only 1 pkt (=r2/r1-1 = 2-1) worth of sending opportunity is lost. This
> > is why I guess the issue has not been observed in practice.
> >
> > Yeah I did see there is an option to specify "skb_mstamp_ns", that
> > might allow CCAs to enforce rates better. I don't know how easy or
> > difficult it would be for CCAs to set skb_mstamp_ns. Because CCAs may
> > not look at individual skbuffs and also given tcp_congestion_ops only
> > has callbacks on ACK events and not pkt send events. I guess BPFs are
> > to be used? (https://netdevconf.info//0x14/pub/papers/55/0x14-paper55-talk-paper.pdf)
> >
> > Also, to clarify, the reason for the conscious choice is that the fix
> > would require more state in TCP socket? Or are there more reasons, any
> > pointers? I imagine, for the fix, the state would increase by ~2-3 u64
> > values, e.g., credits in units of bytes, the time the credits was
> > updated, and the time the credits expire). Is this too much? Or will
> > the fix require more state than this?
>
> It is too much, yes, and not needed currently.

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