MPTCP for Linux

Summary

Introduction Multipath TCP or MPTCP is an extension to the standard TCP and is described in RFC 8684. It allows a device to make use of multiple interfaces at once to send and receive TCP packets over a single MPTCP connection. MPTCP can aggregate the bandwidth of multiple interfaces or prefer the one with the lowest latency. It also allows a fail-over if one path is down, and the traffic is seamlessly reinjected on other paths.graph TD; subgraph MPTCP direction LR C_1(<div style="display: inline-block; min-width: 32px"><font size="7">fa:fa-mobile</font></div>) S_1((<div style="display: inline-block; min-width: 55px"><font size="7">fa:fa-cloud</font></div>)) end subgraph TCP direction LR C_2(<div style="display: inline-block; min-width: 32px"><font size="7">fa:fa-mobile</font></div>) S_2((<div style="display: inline-block; min-width: 55px"><font size="7">fa:fa-cloud</font></div>)) end C_1 <== "5G" ==> S_1 C_1 <== "Wi-Fi<br /><br />Multiple paths (<i>subflows</i>)<br />at the same time" ==> S_1 C_2 x-. "5G" .-x S_2 C_2 <== "Wi-Fi<br /><br />One path at a time" ==> S_2 linkStyle 0 stroke:green; linkStyle 1 stroke:green; linkStyle 2 stroke:red; linkStyle 3 stroke:green; Use cases Thanks to MPTCP, being able to use multiple paths in parallel or simultaneously brings new use-cases, compared to TCP: Seamless handovers: switching from one path to another while preserving established connections, e.g. Apple is using Multipath TCP on smartphones mainly for this reason since 2013. Best network selection: using the “best” available path depending on some conditions, e.g. latency, losses, cost, bandwidth, etc. Network aggregation: using multiple paths at the same time to have a higher throughput, e.g. to combine fixed and mobile networks to send files faster. Concepts Technically, when a new socket is created with the IPPROTO_MPTCP protocol (Linux-specific), a subflow (or path) is created. This subflow consists of a regular TCP connection that is used to transmit data through o...