RFC 793 is a document that defines the Transmission Control Protocol (TCP), which is a fundamental protocol used for communication over the Internet. Published in September 1981, it outlines how data is transmitted between computers, ensuring reliable and ordered delivery of packets. TCP is essential for applications like web browsing and email, where data integrity is crucial. The document details the mechanisms for establishing connections, managing data flow, and handling errors. It also describes how TCP segments are structured and how they interact with other protocols in the Internet Protocol Suite. Overall, RFC 793 serves as a foundational reference for network communication.