Transmission Control Protocol (TCP) and Internet Protocol (IP) are the native protocols governing most Internet communications. TCP has its roots in the Department of Defense (DoD) Arpanet project and is fully defined in the RFC do*censored*ent #793: Transmission Control Protocol. You are using TCP, along with IP, each time an e-mail is sent or received, a file is downloaded, or a Web site is visited. In this article, I’ll cover the details of TCP and explain how to use UNIX tools to monitor the network and apply security.