Final answer:
Applications use the Application Programming Interface (API) within the operating system to communicate the need to establish a TCP connection. The networking API includes system calls that, when utilized by applications, initiate the OS to establish the connection.
Step-by-step explanation:
Applications communicate the need to establish a TCP connection through the operating system by using a part known as the Application Programming Interface (API), specifically the networking API which typically includes functions or system calls provided by the operating system. An application will make a call to a networking API function like socket() followed by connect(), which are part of the operating system's network stack. This request is then processed by the operating system and if all requirements are satisfied, such as the availability of a network interface and correct parameters (like the destination IP address and port), the operating system establishes a TCP connection.
Example:
Let's say an application wants to establish a TCP connection with a web server. It will create a socket using the socket API and provide the IP address and port number of the server. The operating system will handle the process of establishing the connection, including the TCP handshake.