longer one: with mptcp, there needs to be some way to distinguish connection-teardown vs subflow-teardown. with, RST the subflow is terminated. FIN is more subtle (since it occupies the sequence space in normal tcp) however. FIN is handled via an explicit DATA-FIN within TCP option to indicate the end of data-sequence-space (which maps subflow sequence numbers to data sequence numbers in TCP options). gracefully terminating a connection, thus involves sending DATA-FIN on all subflows together with a FIN.