Transport layer protocols deal with process to process communication. The transport layer is the fourth layer of the OSI model and is responsible for ensuring reliable and efficient data transfer between processes running on different hosts. It provides end-to-end communication services for applications.

Process to process communication:
The transport layer protocols establish communication between two processes, which are running on different hosts. These processes could be applications such as web browsers, email clients, or file transfer programs. The transport layer protocols ensure that the data sent by one process is received correctly by the other process.

Reliable data transfer:
One of the key responsibilities of the transport layer is to provide reliable data transfer. It achieves this by breaking the data into smaller segments and adding sequence numbers to each segment. The receiving process can then reassemble these segments in the correct order and ensure that no data is lost or corrupted during transmission.

Flow control:
Transport layer protocols also handle flow control, which means they regulate the rate at which data is sent from the sender to the receiver. This ensures that the receiver does not get overwhelmed with data and can process it at a manageable pace. Flow control helps prevent congestion and improves the overall efficiency of the network.

Error detection and correction:
Transport layer protocols also include mechanisms for error detection and correction. They use techniques such as checksums to detect errors in the data during transmission. If errors are detected, the protocols can request the sender to retransmit the corrupted or lost segments.

Multiplexing and demultiplexing:
Another important function of the transport layer is multiplexing and demultiplexing. Multiplexing allows multiple processes to share a single network connection, while demultiplexing enables the transport layer to deliver the received data to the correct process on the receiving host.

Conclusion:
In summary, transport layer protocols are responsible for establishing process to process communication, ensuring reliable data transfer, handling flow control, error detection and correction, as well as multiplexing and demultiplexing. They play a crucial role in the successful transmission of data between applications running on different hosts.