The Illustrated Network- P10:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 2 TCP IP Protocols and Devices 59 TCP contains all the functions and mechanisms needed to make up for the best-effort connectionless delivery provided by the IP layer. Packets could arrive at a host with errors out of their correct sequence duplicated or with gaps in sequence due to lost or discarded packets. TCP must guarantee that the data stream is delivered to the destination application error-free with all data in sequence and complete. Following the practice used in connection-oriented networks TCP uses acknowledgments that periodically flow from the destination to the source to assure the sender that all is well with the data received to that point in time. On the sending side TCP passes segments to the IP layer for encapsulation in packets which the IP layer in hosts and routers route connectionlessly to the destination host. On the receiving side TCP accepts the incoming segments from the IP layer and delivers the data they represent to the proper application running above TCP in the exact order in which the data were sent. User Datagram Protocol The TCP IP transport layer has another major protocol. UDP is as connectionless as IP. When applications use UDP instead of TCP there is no need to establish maintain or tear down a connection between a source and destination before sending data. Connection management adds overhead and some initial delay to the network. UDP is a way to send data quickly and simply. However UDP offers none of the reliability services that TCP does. UDP applications cannot rely on TCP to ensure error-free guaranteed via acknowledgments in-sequence delivery of data to the destination. For some simple applications purely connectionless data delivery is good enough. Single request-response message pairs between applications are sent more efficiently with UDP because there is no need to exchange a flurry of initial TCP segments to establish a connection. Many applications will not be satisfied with this mode of operation however
