[jeroenhd]

I think that's great until those middleboxes start trying to parse TLS headers. TLS 1.3 looks like TLS 1.2 because of the same protocol ossification problem.

There will be issues with middleboxes, but in my opinion middleboxes have been given more than enough time to stop freaking out about network interception after the problem with TLS 1.3 was discovered.

The Red Hat people primarily seem to focus on this as a backend/server-to-server communication method which means that MPTCP (or SCTP) can be rolled out without too much trouble. A big problem SCTP has is that there is no good implementation outside Linux, and even on Linux performance is suboptimal. MPTCP can be linked into the application at runtime (https://github.com/ngi-mptcp/curl/wiki/Multipath-TCP-on-Wind...) so that's not necessarily an issue here, which helps.