netdev - Re: [PATCH net-next v2 0/2] RTAX_INITRWND should be able to bring the rcv

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Date:   Fri, 29 Jul 2022 17:12:57 +0200
From:   Marek Majkowski <marek@...udflare.com>
To:     network dev <netdev@...r.kernel.org>
Cc:     bpf <bpf@...r.kernel.org>,
        kernel-team <kernel-team@...udflare.com>,
        Ivan Babrou <ivan@...udflare.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, brakmo@...com
Subject: Re: [PATCH net-next v2 0/2] RTAX_INITRWND should be able to bring the
 rcv_ssthresh above 64KiB

On Fri, Jul 29, 2022 at 4:39 PM Marek Majkowski <marek@...udflare.com> wrote:
>
> Among many route options we support initrwnd/RTAX_INITRWND path
> attribute:
>
>  $ ip route change local 127.0.0.0/8 dev lo initrwnd 1024
>
> This sets the initial receive window size (in packets). However, it's
> not very useful in practice. For smaller buffers (<128KiB) it can be
> used to bring the initial receive window down, but it's hard to
> imagine when this is useful. The same effect can be achieved with
> TCP_WINDOW_CLAMP / RTAX_WINDOW option.
>
> For larger buffers (>128KiB) the initial receive window is usually
> limited by rcv_ssthresh, which starts at 64KiB. The initrwnd option
> can't bring the window above it, which limits its usefulness
>
> This patch changes that. Now, by setting RTAX_INITRWND path attribute
> we bring up the initial rcv_ssthresh in line with the initrwnd
> value. This allows to increase the initial advertised receive window
> instantly, after first TCP RTT, above 64KiB.
>
> With this change, the administrator can configure a route (or skops
> ebpf program) where the receive window is opened much faster than
> usual. This is useful on big BDP connections - large latency, high
> throughput - where it takes much time to fully open the receive
> window, due to the usual rcv_ssthresh cap.
>
> However, this feature should be used with caution. It only makes sense
> to employ it in limited circumstances:
>
>  * When using high-bandwidth TCP transfers over big-latency links.
>  * When the truesize of the flow/NIC is sensible and predictable.
>  * When the application is ready to send a lot of data immediately
>    after flow is established.
>  * When the sender has configured larger than usual `initcwnd`.
>  * When optimizing for every possible RTT.
>
> This patch is related to previous work by Ivan Babrou:
>
>   https://lore.kernel.org/bpf/CAA93jw5+LjKLcCaNr5wJGPrXhbjvLhts8hqpKPFx7JeWG4g0AA@mail.gmail.com/T/
>
> Please note that due to TCP wscale semantics, the TCP sender will need
> to receive first ACK to be informed of the large opened receive
> window. That is: the large window is advertised only in the first ACK
> from the peer. When the TCP client has large window, it is advertised
> in the third-packet (ACK) of the handshake. When the TCP sever has
> large window, it is advertised only in the first ACK after some data
> has been received.
>
> Syncookie support will be provided in subsequent patchet, since it
> requires more changes.
>
> *** BLURB HERE ***
>
> Marek Majkowski (2):
>   RTAX_INITRWND should be able to set the rcv_ssthresh above 64KiB
>   Tests for RTAX_INITRWND
>
>  include/linux/tcp.h                           |   1 +
>  net/ipv4/tcp_minisocks.c                      |   9 +-
>  net/ipv4/tcp_output.c                         |   7 +-
>  .../selftests/bpf/prog_tests/tcp_initrwnd.c   | 420 ++++++++++++++++++
>  .../selftests/bpf/progs/test_tcp_initrwnd.c   |  30 ++
>  5 files changed, 463 insertions(+), 4 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/tcp_initrwnd.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_tcp_initrwnd.c

Changelog:
 - moved proposed rcv_ssthresh from `struct inet_request_soct` into
`struct tcp_request_sock` as per Eric's suggestion
 - extended tests to be more explicit about syncookies