iOS supports Multipath TCP (MPTCP) and allows an iPhone or iPad to establish a backup TCP connection to a destination host over a cellular data connection.
Network administrators might want to use MPTCP. Customers with a typical home network don't need to turn on MPTCP.
About Multipath TCP
MPTCP is a set of extensions to the Transmission Control Protocol (TCP) specification. With MPTCP, a client can connect to the same destination host with multiple connections over different network adapters This creates strong and efficient data connections between hosts that works with existing networking infrastructures.
Multipath TCP on your iPhone or iPad
iPhone and iPad use MPTCP with an active cellular data connection to make two connections:
- A primary TCP connection over Wi-Fi
- A backup connection over cellular data
If Wi-Fi becomes unavailable or unresponsive, iOS uses the cellular data connection.
MPTCP uses TCP Option field 30, which the Internet Assigned Number Authority (IANA) reserves for this use. If any middleboxes, such as routers or switches, between the iOS device and server don’t support MPTCP, iOS makes a standard TCP connection.
For example, when you ask Siri a question, Siri tries to make an MPTCP connection over Wi-Fi. If successful, Siri creates a backup connection over cellular data. If Wi-Fi becomes unavailable or unreliable, MPTCP immediately and invisibly switches to cellular data.
Turn on MPTCP for a network
MPTCP works with existing networks. If a network doesn’t support MPTCP, the client uses standard TCP connections. However, network administrators must check their firewall policies to make sure that all intervening devices allow TCP Option 30 to pass through without changes.
Many commercial routers replace unknown TCP options with NOOP data. Ask your vendor how to turn on TCP options.