What is Multipath TCP
Multipath TCP (MPTCP) sends data over multiple network paths simultaneously — using both WiFi and cellular at the same time for better throughput and resilience. If one path fails, traffic continues on the others without dropping the connection.
How does it work?
A standard TCP connection uses a single path: one source IP, one destination IP. MPTCP starts with a regular TCP connection, then negotiates additional subflows over different network interfaces. Each subflow is a full TCP connection with its own sequence numbers, congestion control, and ACKs.
A scheduler at the MPTCP layer decides which subflow to send each chunk of data on. It can distribute data across paths for maximum throughput, or use one path as primary and another as backup for resilience.
Who deployed it?
Apple was the first major deployer. Starting with iOS 7, Siri used MPTCP to send voice data over both WiFi and cellular simultaneously. The result: 80% fewer connection errors on unreliable networks. If WiFi dropped mid-sentence, the cellular subflow carried the data without interruption.
The Linux kernel has supported MPTCP since version 5.6. Android added support in Android 12.
What are the limitations?
MPTCP faces a practical problem: middleboxes. Many firewalls, NATs, and load balancers don't understand MPTCP's TCP option extensions and strip them out. When this happens, MPTCP falls back to regular TCP silently. This middlebox interference is one reason QUIC chose to build on UDP instead of extending TCP further.
How is it different from Multipath QUIC?
MPTCP extends TCP at the kernel level, which means it requires OS support and is vulnerable to middlebox interference. Multipath QUIC operates in user space on top of UDP, making it easier to deploy and immune to TCP middlebox issues.
Why it matters
MPTCP proved that multipath networking works in production at massive scale. Its deployment by Apple demonstrated measurable reliability improvements. While Multipath QUIC may eventually supersede it, MPTCP paved the way and remains the most battle-tested multipath transport protocol.