Multipath TCP (MPTCP) is a TCP extension specified in RFC8684 that allows a TCP connection to use different paths such as Wi-Fi and cellular on smartphones or Wi-Fi and Ethernet on laptops. Multipath TCP is supported by Linux kernel versions >=5.6 and enabled on many popular distributions. It is also possible for applications to use it on iOS >= 11 for the client side only.
The utilization of Multipath TCP is negotiated during the TCP three-way handshake. This implies that if a Multipath TCP client contacts a regular TCP server, they will use a regular TCP connection. Similarly, if a regular TCP client contacts a Multipath TCP server, they will use a regular TCP connection. Multipath TCP will only be used if both the client and the server support Multipath TCP.
Enabling Multipath TCP in an applications on an operating system supporting it is easy: MPTCP has to be enabled before creating the connection. On Linux, applications must pass IPPROTO_MPTCP as the third parameter of the socket() system call. On iOS, the MultiPath Service should be enabled. For more details about that: please check the mptcp.dev website.
The following examples show how enable Multipath TCP with different programming languages:
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On Linux:
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On macOS/iOS:
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On iOS only:
If you do not have access to the application's source code, you can use mptcpize on Linux to automatically transform the TCP socket system calls into Multipath TCP sockets.
The implementation of Multipath TCP in the Linux kernel is regularly improved. You can track the changes here.