Re: [PATCH bpf-next] tcp: bpf: Add TCP_BPF_RCV_SSTHRESH for bpf_setsockopt

[Dave Taht <dave.taht@gmail.com>]

On Wed, Jan 12, 2022 at 11:59 AM Ivan Babrou <ivan@cloudflare.com> wrote:
>
> This patch adds bpf_setsockopt(TCP_BPF_RCV_SSTHRESH) to allow setting
> rcv_ssthresh value for TCP connections. Combined with increased
> window_clamp via tcp_rmem[1], it allows to advertise initial scaled
> TCP window larger than 64k. This is useful for high BDP connections,
> where it allows to push data with fewer roundtrips, reducing latency.

I would not use the word "latency" in this way, I would just say
potentially reducing
roundtrips...

and potentially massively increasing packet loss, oversaturating
links, and otherwise
hurting latency for other applications sharing the link, including the
application
that advertised an extreme window like this.

This overall focus tends to freak me out somewhat, especially when
faced with further statements that cloudflare is using an initcwnd of 250!???

The kind of damage just IW10 can do to much slower bandwidth
connections has to be
experienced to be believed.

https://tools.ietf.org/id/draft-gettys-iw10-considered-harmful-00.html

>
> For active connections the larger window is advertised in the first
> non-SYN ACK packet as the part of the 3 way handshake.
>
> For passive connections the larger window is advertised whenever
> there's any packet to send after the 3 way handshake.
>
> See: https://lkml.org/lkml/2021/12/22/652
>
> Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
> ---
>  include/uapi/linux/bpf.h       | 1 +
>  net/core/filter.c              | 6 ++++++
>  tools/include/uapi/linux/bpf.h | 1 +
>  3 files changed, 8 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 791f31dd0abe..36ebf87278bd 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5978,6 +5978,7 @@ enum {
>         TCP_BPF_SYN             = 1005, /* Copy the TCP header */
>         TCP_BPF_SYN_IP          = 1006, /* Copy the IP[46] and TCP header */
>         TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
> +       TCP_BPF_RCV_SSTHRESH    = 1008, /* Set rcv_ssthresh */
>  };
>
>  enum {
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 2e32cee2c469..aafb6066b1a6 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4904,6 +4904,12 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
>                                         return -EINVAL;
>                                 inet_csk(sk)->icsk_rto_min = timeout;
>                                 break;
> +                       case TCP_BPF_RCV_SSTHRESH:
> +                               if (val <= 0)
> +                                       ret = -EINVAL;
> +                               else
> +                                       tp->rcv_ssthresh = min_t(u32, val, tp->window_clamp);
> +                               break;
>                         case TCP_SAVE_SYN:
>                                 if (val < 0 || val > 1)
>                                         ret = -EINVAL;
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 791f31dd0abe..36ebf87278bd 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -5978,6 +5978,7 @@ enum {
>         TCP_BPF_SYN             = 1005, /* Copy the TCP header */
>         TCP_BPF_SYN_IP          = 1006, /* Copy the IP[46] and TCP header */
>         TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
> +       TCP_BPF_RCV_SSTHRESH    = 1008, /* Set rcv_ssthresh */
>  };
>
>  enum {
> --
> 2.34.1
>


-- 
I tried to build a better future, a few times:
https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org

Dave Täht CEO, TekLibre, LLC