[PATCH net-next 0/2] RTAX_INITRWND should be able to bring the rcv_ssthresh above 64KiB

[Marek Majkowski <marek@cloudflare.com>]

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.

Signed-off-by: Marek Majkowski <marek@cloudflare.com>

Marek Majkowski (2):
  RTAX_INITRWND should be able to bring the rcv_ssthresh above 64KiB
  Tests for RTAX_INITRWND

 include/net/inet_sock.h                       |   1 +
 net/ipv4/tcp_minisocks.c                      |   8 +-
 net/ipv4/tcp_output.c                         |  10 +-
 .../selftests/bpf/prog_tests/tcp_initrwnd.c   | 398 ++++++++++++++++++
 .../selftests/bpf/progs/test_tcp_initrwnd.c   |  30 ++
 5 files changed, 443 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

-- 
2.25.1