The MPTCP protocol is complex, mainly to be able to survive on the Internet where middleboxes such as NATs, firewalls, IDS or proxies can modify parts of the TCP packets. Worst case scenario, an MPTCP connection should fallback to “plain” TCP. Today, such fallbacks are rarer than before – probably because MPTCP has been used since 2013 on millions of Apple smartphones worldwide – but they can still exist, e.g. on some mobile networks using Performance Enhancing Proxies (PEPs) where MPTCP connect...