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Date:   Mon, 24 Aug 2020 11:39:10 -0700
From:   Priyaranjan Jha <>
To:     Eric Dumazet <>
Cc:     Robert Bengtsson-Ölund 
        Neal Cardwell <>,
        Yuchung Cheng <>
Subject: Re: Request for backport of 78dc70ebaa38aa303274e333be6c98eef87619e2
 to 4.19.y

Thank you, Eric, Robert.
We will try to provide the backport for the patch soon.


(resending since previous reply bounced back)
On Mon, Aug 24, 2020 at 9:14 AM Eric Dumazet <> wrote:
> On 8/24/20 7:35 AM, Robert Bengtsson-Ölund wrote:
> > Hi everyone
> >
> > We stumbled upon a TCP BBR throughput issue that the following change fixes.
> > git: 78dc70ebaa38aa303274e333be6c98eef87619e2
> >
> > Our issue:
> > We have a transmission that is application limited to 20Mbps on an
> > ethernet connection that has ~1Gbps capacity.
> > Without this change our transmission seems to settle at ~3.5Mbps.
> >
> > We have seen the issue on a slightly different network setup as well
> > between two fiber internet connections.
> >
> > Due to what the mentioned commit changes we suspect some middlebox
> > plays with the ACK frequency in both of our cases.
> >
> > Our transmission is basically an RTMP feed through ffmpeg to MistServer.
> >
> > Best regards
> > /Robert
> >
> Please always CC patch authors in this kind of requests.
> Thanks.
> Patch was :
> commit 78dc70ebaa38aa303274e333be6c98eef87619e2
> Author: Priyaranjan Jha <>
> Date:   Wed Jan 23 12:04:54 2019 -0800
>     tcp_bbr: adapt cwnd based on ack aggregation estimation
>     Aggregation effects are extremely common with wifi, cellular, and cable
>     modem link technologies, ACK decimation in middleboxes, and LRO and GRO
>     in receiving hosts. The aggregation can happen in either direction,
>     data or ACKs, but in either case the aggregation effect is visible
>     to the sender in the ACK stream.
>     Previously BBR's sending was often limited by cwnd under severe ACK
>     aggregation/decimation because BBR sized the cwnd at 2*BDP. If packets
>     were acked in bursts after long delays (e.g. one ACK acking 5*BDP after
>     5*RTT), BBR's sending was halted after sending 2*BDP over 2*RTT, leaving
>     the bottleneck idle for potentially long periods. Note that loss-based
>     congestion control does not have this issue because when facing
>     aggregation it continues increasing cwnd after bursts of ACKs, growing
>     cwnd until the buffer is full.
>     To achieve good throughput in the presence of aggregation effects, this
>     algorithm allows the BBR sender to put extra data in flight to keep the
>     bottleneck utilized during silences in the ACK stream that it has evidence
>     to suggest were caused by aggregation.
>     A summary of the algorithm: when a burst of packets are acked by a
>     stretched ACK or a burst of ACKs or both, BBR first estimates the expected
>     amount of data that should have been acked, based on its estimated
>     bandwidth. Then the surplus ("extra_acked") is recorded in a windowed-max
>     filter to estimate the recent level of observed ACK aggregation. Then cwnd
>     is increased by the ACK aggregation estimate. The larger cwnd avoids BBR
>     being cwnd-limited in the face of ACK silences that recent history suggests
>     were caused by aggregation. As a sanity check, the ACK aggregation degree
>     is upper-bounded by the cwnd (at the time of measurement) and a global max
>     of BW * 100ms. The algorithm is further described by the following
>     presentation:
>     In our internal testing, we observed a significant increase in BBR
>     throughput (measured using netperf), in a basic wifi setup.
>     - Host1 (sender on ethernet) -> AP -> Host2 (receiver on wifi)
>     - 2.4 GHz -> BBR before: ~73 Mbps; BBR after: ~102 Mbps; CUBIC: ~100 Mbps
>     - 5.0 GHz -> BBR before: ~362 Mbps; BBR after: ~593 Mbps; CUBIC: ~601 Mbps
>     Also, this code is running globally on YouTube TCP connections and produced
>     significant bandwidth increases for YouTube traffic.
>     This is based on Ian Swett's max_ack_height_ algorithm from the
>     QUIC BBR implementation.
>     Signed-off-by: Priyaranjan Jha <>
>     Signed-off-by: Neal Cardwell <>
>     Signed-off-by: Yuchung Cheng <>
>     Signed-off-by: David S. Miller <>

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