[brenns10]

My understanding is that it was basically a condition enforced by the maintainers of the Linux TCP / networking subsystems. If you look at the initial upstreaming discussions[1], this was setup as a ground rule.

If you look at the older multipath TCP implementation, prior to the upstreaming, it was intended to be fully transparent to the application, which I think makes more sense for the intent of the protocol. Sure, in many cases MPTCP may be better with application-guided logic, but having a standard system approach (e.g. establish sub-flows on an LTE connection for automatic failover, but don't send any data along those sub-flows) would have worked for 95% of cases.

[1] https://lore.kernel.org/all/alpine.OSX.2.21.1707181728570.11...

[matttbe]

Yes, that's correct. In the Linux kernel, it would not be possible to switch to MPTCP by default.

But apps can use it by default. For the server case, it really makes sense: https://www.mptcp.dev/faq.html#why--when-should-mptcp-be-ena...

GNU/Linux distributions could even switch MPTCP on by default (via eBPF).