Eric Dumazet <> writes:

> When TCP receives an out-of-order packet, it immediately sends
> a SACK packet, generating network load but also forcing the
> receiver to send 1-MSS pathological packets, increasing its
> RTX queue length/depth, and thus processing time.
> Wifi networks suffer from this aggressive behavior, but generally
> speaking, all these SACK packets add fuel to the fire when networks
> are under congestion.
> This patch adds a high resolution timer and tp->compressed_ack counter.
> Instead of sending a SACK, we program this timer with a small delay,
> based on RTT and capped to 1 ms :
>       delay = min ( 5 % of RTT, 1 ms)
> If subsequent SACKs need to be sent while the timer has not yet
> expired, we simply increment tp->compressed_ack.
> When timer expires, a SACK is sent with the latest information.
> Whenever an ACK is sent (if data is sent, or if in-order
> data is received) timer is canceled.
> Note that tcp_sack_new_ofo_skb() is able to force a SACK to be sent
> if the sack blocks need to be shuffled, even if the timer has not
> expired.
> A new SNMP counter is added in the following patch.
> Two other patches add sysctls to allow changing the 1,000,000 and 44
> values that this commit hard-coded.
> Signed-off-by: Eric Dumazet <>

Acked-by: Toke Høiland-Jørgensen <>

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