2. Transmission Control Protocol

The TCP/IP protocol has two parts: TCP (Transmission Control Protocol) and IP (Internet Protocol).

The first part, TCP, governs how messages are prepared and, once they're received, it governs how they are decoded.

So the TCP itself can be broken down into two parts: the rules for sending data, and the rules for receiving data.

When sending data the TCP rules (protocol) include

• Divide the message into chunks of data called "packets". For more information on packets, see the section on packet switching.
• Add a sequence number to each packet. This lets the receiving computer know what order the packets are supposed to be re-assembled in.
• Add error-correction data, so errors introduced during transmission can be spotted and fixed

When receiving data the TCP rules (protocol) include

• Examine each packet for errors by using the extra information that was added to it
• Fix errors (if possible) or request that the packet be re-sent
• Identify missing packets and request them to be re-sent
• Reassemble the message in the correct order, using the sequence number of each packet
• Send the message on to the correct application. Emails go to the email application, web pages go to the browser, etc.