Multipath TCP (mptcp) support is not enabled in the kernel.
I am not sure whether it is the right place to talk about this issue, but alpine is the only linux distribution I have found recently which disables mptcp in its default kernel. I am personally a heavy user of MPTCP, and it has made it very difficult to choose alpine as a VM host, or really anything where I'd use alpine's kernel, as I would always have to compile my own kernel to get mptcp support.
Every other linux distribution I have tried (void, suse, fedora, debian, ubuntu, even enterprise linux) have enabled mptcp support.
I am sure it is a decision made when trying to keep the kernel light, but it is an important feature, I think, and should be enabled. I haven't found anywhere it has been discussed so I hope to start the discussion.