After a few attempts of failed MPTCP, directly fallback to TCP for new connections

[matttbe]

The idea would be to imitate what is done in TFO, using a cache: if a destination IP is known not to support TFO (MPTCP here), fallback.

Note that now I think for TFO, they use a global counter because most issues (middleboxes) are in the operator network → most likely that either all connections are "failing" (e.g. with TFO: SYN with data are dropped / blackhole) in the network of the operator or all are OK even when trying different servers. So the "source" (client's network) is more an issue than the destination (server's network).

(Feature from the initial roadmap)