There are several sub-protocols within TCP/IP:
1. Internet Protocol (IP), which covers fundamentals like IP addresses and routing of packets of data from one place to another, but doesn't address issues like reliability and delivery in the correct order.
2. Transmission Control Protocol (TCP), which adds the idea of a reliable connection that always delivers a stream of data in the correct order. Telephone modems, Ethernet networks and other physical connections used on the Internet might not be 100% reliable, and some types of connections don't guarantee that the second packet won't arrive before the first one. TCP provides rules for checking the order of the data and for resending anything that is not received. This is the protocol that HTTP, FTP most other Internet protocols you are familiar with are built on top of.
3. User Datagram Protocol (UDP) is a simple wrapper around the basic features of Internet Protocl (IP). UDP is useful when you don't care about reliability or in-order delivery, and you can't afford the extra time that TCP takes to ensure them. When you browse the World Wide Web, you are using the DNS protocol to look up the names of websites. DNS is layered on top of UDP. Online gaming is another popular application of UDP.
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