In a packet-switched network, each router keeps state about each ongoing connection (communication from a given source to a given destination) that goes through this router.

The Internet network layer uses packet switching.

The TCP protocol can only run on top of a circuit-switched network (a packet-switched network cannot support TCP connections).

In a circuit-switched network, each router keeps state about each ongoing connection (communication from a given source to a given destination) that goes through this router.

The Internet network layer uses circuit switching, because it provides performance guarantees.