[$] More accurate congestion notification for TCP

The “More Accurate Explicit Congestion Notification” (AccECN) mechanism is
defined by this
RFC draft. The Linux kernel has been gaining support for AccECN with
TCP over the last few releases; the 7.0 release will enable it by default
for general use. AccECN is a subtle change to how TCP works, but it has
the potential to improve how traffic flows over both public and private
networks.