The main function of the transport layer is end to end delivery. Let's break this down into further detail:

End to End Delivery:
The transport layer is responsible for ensuring that data is delivered from the sender to the receiver without any errors or loss. It breaks down the data into smaller packets and sends them over the network. The transport layer at the receiving end then reassembles these packets to form the original data.

Reliability:
The transport layer guarantees reliable delivery of data. It ensures that all packets are delivered in sequence and that there is no data loss. It also handles any errors that may occur during transmission.

Flow Control:
The transport layer regulates the flow of data sent over the network. It ensures that the sender does not overwhelm the receiver with too much data. This prevents congestion on the network and ensures that data is delivered efficiently.

Multiplexing:
The transport layer allows for multiple applications to use the network simultaneously. It assigns each application a unique port number, allowing the network to differentiate between different applications.

Connection Establishment and Termination:
The transport layer establishes and terminates connections between applications. It ensures that the sender and receiver are ready to exchange data before data transmission begins. It also ensures that the connection is properly terminated once data transmission is complete.

In conclusion, the transport layer plays a crucial role in ensuring that data is delivered reliably and efficiently over the network. It provides end to end delivery, reliability, flow control, multiplexing, and connection establishment and termination.