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> If any middlebox in between does not support it, the returned SYN+ACK packet will not contain MPTCP options in the TCP option field.

That sounds .. quite restrictive. Is the only requirement on a middlebox to just forward the MPTCP options as-is?

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Before we finalized the specs we did a lot of testing to make sure that enabling MPTCP wouldn't break connectivity. Either it would be passed correctly, or it would fall back to single-path TCP safely. Generally, so long as a middlebox passes through unknown options unmodified, and does not try to enforce that the TCP sequence space it sees is contiguous, MPTCP should work through that middlebox.

If you're interested, we wrote a couple of papers on this:

[1] https://www.usenix.org/conference/nsdi12/technical-sessions/...

[2] https://www.researchgate.net/publication/229002024_Is_it_sti...

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