The present disclosure relates to methods and arrangements for multipath traffic aggregation using a multipath Transmission Control Protocol, MPTCP, proxy. A method of relaying data between an MPTCP capable wireless device and a server is performed in a multipath Transmission Control Protocol, MPTCP, proxy, configured with a unique Internet Protocol, IP, address. The comprises establishing (S33) an MPTCP session between the MPTCP proxy and the wireless device, the MPTCP session comprising a first MPTCP subflow mapped on a first network path for the wireless device using a default traffic flow tuple, and establishing (S35) a TCP session with the server. The method further comprises initiating (S37) a further MPTCP subflow in the MPTCP session between the MPTCP proxy and the wireless device based on a mapping of the further MPTCP subflow to a second network path for the wireless device using a filtering traffic flow tuple comprising the unique IP-address configured for the MPTCP proxy. Data is relayed (S39) between the wireless device and the server, wherein data between the MPTCP proxy and the wireless device is exchanged in the MPTCP session comprising the first MPTCP subflow on the first network path and the further MPTCP subflow on the second network path and wherein data between the MPTCP proxy and the server is exchanged in the TCP session.