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A PRACTICAL GUIDE TO TCP/IP PROTOCOLS

by Tracy Thomas

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Itýs difficult for embedded systems programmers to sift through the wealth of reference material regarding networking and TCP/IP protocols. Yet, an increasing number of embedded applications benefit from the addition of networking capability. This article is a practical guide to the basics of TCP/IP and contains information that is most often misunderstood by programmers who are new to networking.

THE BASICS

The first point to understand is that TCP/IP often refers to the family of protocols, which includes TCP (Transport Control Protocol), UDP (User Datagram Protocol), IP (Internet Protocol), and some underlying link layers such as Ethernet. Data sent using the TCP protocol is called a segment, and data sent using the UDP protocol is referred to as a packet. IP is the network layer that lies under TCP and UDP. IP provides connectionless packet delivery to a specified host address, making it unreliable because IP packets (called datagrams) can be lost, duplicated, or delivered out of order. The advantage of IP is that a datagram provides a universal method for delivering data, independent of the underlying network technology.

UDP and TCP sit on top of IP at the transport layer. Both use port numbers to de-mulitplex data sent to a host. A port number is specific to an application. The use of multiple port numbers allows a single host to run multiple networking applications. Each UDP packet and TCP segment has a source and destination port number.

The host that waits for incoming connections is called the server, and the host that initiates a connection is the client. Servers "listen" to well-known port numbers for common applications such as FTP (File Transfer Protocol), e-mail, and HTTP. Clients generally choose a random source port number and connect to a server at a well-known port. Custom applications should use a port number greater than 1024, because port numbers less than 1024 are reserved for well-known applications.

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