Comment by floam
4 months ago
If anyone is unaware:
MPTCP being enabled on a server is what makes clients running on Apple devices magically not time out when you walk out of Wi-Fi range or switch to LTE (and is behind the “Wi-Fi Assist” setting/feature). iOS and macOS have had it quietly on by default for years: at first just iCloud etc. used it under the hood but for instance WeChat started enabling it like a decade ago for the improved performance.
With MPTCP, the same TCP session hops networks mid-flight. Without it, these seamless handoffs are at best fast reconnects. It’s one of those Apple things that “just works”; on your Linux server you need to flip it on in `socket()` or look into `mptcpize` last I checked but it’s no big deal. I dont think it’s well supported as a client yet and who knows if Android will ever.
(the “Wi-Fi Assist” toggle in Settings doesn’t enable/disable MPTCP, it is on regardless, but it decides if iOS will spin up a cellular subflow when Wi-Fi starts flaking out. It will use some metered data, hence the user-facing toggle.)
More and more apps (mostly server apps) have a dedicated option to enable MPTCP. Some server apps have even decided to enable MPTCP support by default, which makes sense: if MPTCP is not requested, TCP is used like before. Note that server apps written in Go usually have MPTCP enabled by default (if supported by the OS/kernel). See: https://www.mptcp.dev/apps.html
mptcp.io monitors servers supporting MPTCP.
> I dont think it’s well supported as a client yet
It is: by default, NetworkManager will configure MPTCP endpoints, so app can use multiple interfaces (if any). See: https://www.mptcp.dev/pm.html
> who knows if Android will ever
Sadly, it is difficult to talk to people in charge there. A few years ago, they were interested in MPTCP, but it was not available in the official Linux kernel. Now it is, and easily accessible (especially for small actors)... but Google has enough resources to find and use alternatives they fully control.