Does QUIC/HTTP3 have benifit from udp when network switches?

Question

We commonly know QUIC use UDP as its transport layer. UDP has connectless propertey in compartion to TCP.
. So could I draw a conclusion that we can freely switch network connection between wifi and 4G mobile?
If it does, the cost of switch is lower than TCP. TCP need 3-way handshake to build connection and UDP doesnot need it.

Answer 1

Yes, this has been built in to the QUIC protocol under the Connection Migration section, though this can be disabled with the disable_active_migration transport parameter so not all QUIC connections may support this.

TCP connections are basically identified and handled by the 4-tuple of client IP, client port, server IP, and server port. This means if any of these change the connection has to be reestablished.

QUIC instead creates a Connection Id, and this can be sent over different connections to reuse the same connection, so benefiting from not having to do the expensive initial 3-way TCP handshake and then the even more expensive TLS handshake, as you say (though these are combined so already smaller in QUIC so there may not be as much benefit in this as there would be if it worked as it currently does in the TCP world).

Perhaps more interestingly this also allows the potential use of multipath where you could use your WiFi connection and your mobile connection at the same time, to increase your bandwidth and download speed. It should be noted however that, for simplicity sake, multipath will not be in the first RFC version of QUIC (due to be formally released and say now), and will be one of the next things the QUIC Working Group work on.