[signa11]

apllications use just the socket api for their interactions with the n/w stack. there is no change there, and hence the "unmodified applications" part.

from the mptcp paper, we have the following :

In brief, here is how MPTCP works. MPTCP is nego- tiated via new TCP options in SYN packets, and the end- points exchange connection identifiers; these are used later to add new paths—subflows—to an existing con- nection. Subflows resemble TCP flows on the wire, but they all share a single send and receive buffer at the end- points. MPTCP uses per subflow sequence numbers to detect losses and drive retransmissions, and connection- level sequence numbers to allow reordering at the re- ceiver. Connection-level acknowledgements are used to implement proper flow control.

so yes, both endpoints need to support mptcp for this to work.