The Internet chosen connectionless service at the network layer. Datagram approach to packet switching is used. Route for each packet is calculated independently.
TCP is connection oriented in the way that data transfer happens in 3 phases:
- Connection Establishment
- Data Transfer
- Connection Termination
TCP makes sure packet reach the destination host. In TCP, connection oriented is related to reliability not path followed by packets.
Therefore, Routing happens at the Network Layer which is an unreliable and connectionless service.
A page from A. Forouzon (Data Communication & Networking)