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

On Thu, May 17, 2018 at 2:57 PM, Neal Cardwell [off-list ref] wrote:

On Thu, May 17, 2018 at 5:47 PM Eric Dumazet [off-list ref] wrote:

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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.

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Wifi networks suffer from this aggressive behavior, but generally
speaking, all these SACK packets add fuel to the fire when networks
are under congestion.

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This patch adds a high resolution timer and tp->compressed_ack counter.

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Instead of sending a SACK, we program this timer with a small delay,
based on RTT and capped to 1 ms :

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         delay = min ( 5 % of RTT, 1 ms)

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If subsequent SACKs need to be sent while the timer has not yet
expired, we simply increment tp->compressed_ack.

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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.

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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.

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A new SNMP counter is added in the following patch.

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Two other patches add sysctls to allow changing the 1,000,000 and 44
values that this commit hard-coded.

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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Very nice. I like the constants and the min(rcv_rtt, srtt).

Acked-by: Neal Cardwell <ncardwell@google.com>

Acked-by: Yuchung Cheng <redacted>

Great work. Hopefully this would save middle-boxes' from handling
TCP-ACK themselves.

Thanks!

neal