Transport Layer $($Layer $4)$ functionality for protocols such as Transmission Control Protocol $($TCP$)$ and User Datagram Protocol $($UDP$)$ is provided or implemented in:

Transport Layer $($Layer $4)$ functionality for protocols such as Transmission Control Protocol $($TCP$)$ and User Datagram Protocol $($UDP$)$ is provided or implemented in:

The operating system of the internetworking capable device.

The application software $($i$.$e$.$ coded by developers of the application software$)$

The network hardware of the internetworking capable device

Routers and Switches