TCP AND UDP
Figure 1-10 in Chapter 1 showed that layer four of the OSI Model is the Transport layer. Layer four is where Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) operate. We shall see that TCP is a connection-oriented protocol, and is responsible for the reliable transfer of user traffic between two computers. Consequently, it uses sequence numbers and acknowledgments to make certain all traffic is delivered safely to the destination endpoint.
UDP is a connectionless protocol and does not provide sequencing or acknowledgments. It is used in place of TCP in situations where the full services of TCP are not needed. For example, telephony traffic, the Trivial File Transfer Protocol (TFTP), and the Remote Procedure ...
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