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@xxxxxxxxxxxxxx/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@xxxxxxxxxxxxxx> 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
