[$] More accurate congestion notification for TCP

The "More Accurate Explicit Congestion Notification" (AccECN) mechanism isdefined by thisRFC draft. The Linux kernel has been gaining support for AccECN withTCP over the last few releases; the 7.0 release will enable it by defaultfor general use. AccECN is a subtle change to how TCP works, but it hasthe potential to improve how traffic flows over both public and privatenetworks.