RFC tcp: Improve mtu probe preconditions

According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes in order to accumulate enough data" but in practice linux only sends probes when a lot of data accumulates on the send side.

Another improvement is to rely on TCP RACK performing timely loss detection with fewer outstanding packets. If this is enabled the size required for a probe can be shrunk.

Successive successful mtu probes will result in reducing the cwnd since it's measured in packets and we send bigger packets. The cwnd value can get stuck below 11 on low-latency links and this prevents further probing. The cwnd logic in tcp_mtu_probe can be reworked to be based on the the number of packets that we actually need to send instead of arbitrary constants.

It is difficult to improve this behavior without introducing unreasonable delays or even stalls. Looking at the current behavior of tcp_mtu_probe it already waits in some scenarios: when there is not enough room inside cwnd or when there is a gap of unacklowledged data between snd_una and snd_nxt. It appears that it is safe to wait if packets_in_flight() != 0.

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Previous RFC: https://lore.kernel.org/netdev/[email protected]/

This series seems to be "correct" this time, I would appreciate any feedback. It possible my understanding of when it is safe to return 0 from tcp_mtu_probe is incorrect. It's possible that even current code would interact poorly with delayed acks in some circumstances?

The tcp_xmit_size_goal changes were dropped. It's still possible to see strange interactions between tcp_push_one and mtu probing: If the receiver window is small (60k) the sender will do a "push_one" when half a window is accumulated (30k) and that would prevent mtu probing even if the sender is writing more than enough data in a single syscall.

Leonard Crestez (3):
tcp: Use smaller mtu probes if RACK is enabled
tcp: Adjust congestion window handling for mtu probe
tcp: Wait for sufficient data in tcp_mtu_probe

 Documentation/networking/ip-sysctl.rst | 10 ++++
 include/net/netns/ipv4.h               |  2 +
 net/ipv4/sysctl_net_ipv4.c             | 14 ++++++
 net/ipv4/tcp_ipv4.c                    |  2 +
 net/ipv4/tcp_output.c                  | 70 +++++++++++++++++++++-----
 5 files changed, 86 insertions(+), 12 deletions(-)

base-commit: e4e92ee78702b13ad55118d8b66f06e1aef62586

2.25.1