What is Connection Migration
Connection migration lets an active connection move between networks without restarting — switching from WiFi to cellular without dropping the connection or losing data. QUIC is the first major transport protocol to support it.
Why can't TCP migrate?
TCP identifies connections by the combination of source IP address, source port, destination IP, and destination port. When you walk out of WiFi range and your phone switches to cellular, your IP address changes. TCP has no way to associate the new address with the old connection. The connection dies, and the application has to reconnect, re-authenticate, and restart any in-progress transfers.
How does QUIC make it work?
QUIC identifies connections by connection IDs — random identifiers that are independent of the network path. When the client's IP address changes, it sends a packet to the server with the same connection ID from the new address. The server recognizes the connection and continues.
Before accepting data from the new address, the server performs a path validation — it sends a challenge token that the client must echo back. This prevents an attacker from hijacking a connection by spoofing a migration.
What does it look like in practice?
You're on a video call over WiFi. You walk outside and your phone switches to cellular. With TCP, the call drops and reconnects — you lose a few seconds. With QUIC connection migration, the transport layer detects the path change, validates the new address, and continues the call. The application doesn't even know the network changed.
How is it different from multipath?
Connection migration moves a connection from one path to another. Multipath QUIC uses multiple paths simultaneously. Migration is a fallback mechanism — you switch when one network fails. Multipath is an optimization — you use both networks at the same time for better throughput.
Why it matters
Connection migration is essential for mobile devices, which constantly move between networks. It turns network changes from connection-breaking events into seamless transitions, making QUIC fundamentally better suited to how people actually use the internet today.