[datastoat]

MPTCP is a drop-in replacement -- but its original designers were paranoid about e.g. packet-sniffing middleboxes, so they made it opt-in.

Suppose for example there's a packet-sniffing security appliance along the path, and it only sees packets on one path of the MPTCP flow, and it says "This stream of packets doesn't look like a legitimate TCP flow, so it's probably an attack, so I'll block it."

[londons_explore]

I think the real answer is to test a lot of middleboxes, come up with some fallback behaviour that will at least work with 99% of them, and then just break the last 1%.

The same happened with Max Segment Size detection - and that mostly works now.

[pid-1]

Tangencial, but that's also the reason QUIC (used by HTTP/3) was built upon UDP.

[usrnm]

Wasn't it used to avoid head of line blocking?

[tracnar]

Built over UDP rather than directly over IP. Introducing a new layer 4 protocol on the internet is not realistic for the foreseeable future.