Re: [PATCH v3 net-next 3/6] tcp: add SACK compression

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Eric Dumazet <edumazet@google.com> 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 <edumazet@google.com>

Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>