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

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Message-ID: <CAK6E8=cRCrKVrXOO-3gZMNiw6nQ=6Cq36RnRYrGoOt-Wir-iSQ@mail.gmail.com>
Date:   Thu, 17 May 2018 15:14:08 -0700
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v3 net-next 3/6] tcp: add SACK compression

On Thu, May 17, 2018 at 2:57 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Thu, May 17, 2018 at 5:47 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
>> 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@...gle.com>
>> ---
>
> Very nice. I like the constants and the min(rcv_rtt, srtt).
>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>

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

>
> Thanks!
>
> neal