What is the most commonly used protocol in the transport layer?

The Transmission Control Protocol (TCP) is the most commonly used protocol in the transport layer due to its reliable transmission capabilities. TCP is designed to provide a connection-oriented communication channel, which means it establishes a connection between the sender and receiver before data is transmitted. This connection ensures that data packets arrive in the correct order and that lost packets are retransmitted, maintaining data integrity.

TCP is widely utilized in various applications where reliability is critical, such as web browsing (HTTP), email (SMTP), and file transfers (FTP). Its ability to manage data flow and control congestion makes it an essential protocol for many internet-based services.

In contrast, other protocols serve different purposes. For instance, the User Datagram Protocol (UDP) is also a transport layer protocol but is connectionless and does not guarantee delivery or ordering of packets. This makes UDP suitable for applications where speed is more important than reliability, such as video streaming or online gaming. The Internet Protocol (IP) functions at the network layer and is responsible for addressing and routing packets but does not handle the transport of messages. File Transfer Protocol (FTP) is an application layer protocol used specifically for transferring files and relies on underlying transport protocols like TCP to ensure reliable delivery.