them here briefly, because we will describe the functions and relation- ships of these protocols in a later chapter in great detail. The Internet Protocol Layer, or IP, is roughly functionally equivalent to the OSI Model's Network Layer. It performs routing and congestion con- trol functions and, as the diagram illustrates, includes some of the pro- tocols we mentioned earlier: RIP, OSPF, and a variety of address conversion protocols. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols Figure 2-38 The TCP/IP Application Services Layer protocol suite Transmission Control Protocol (TCP) Datagram Protocol (UDP) RIP, OSPF, ICMP InARP Internet Protocol (IP) Network Interface Layer The Transmission Control Protocol Layer, or TCP, is responsible for message integrity, similar to the service provided by OSI's Transport Layer. It is extremely capable and has the ability to recover from virtu- ally any network failure imaginable to ensure the integrity of the mes- sages it is designed to protect. For situations in which the high degree of protection provided by TCP (and its attendant overhead) is considered to be overkill, a corollary protocol, called User Datagram Protocol (UDP), is also available at this layer. It provides a connectionless network service and is used in situations in which the transported traffic is less critical and in which the overhead inherent in TCP poses a potential problem due to congestion. The uppermost layer in the TCP/IP stack is called the Application Ser- vices Layer. This is where the utility of the stack becomes obvious, because this is where the actual applications are found such as HTTP, FTP, Telnet, and the other utilities that make the Internet useful to the user. We will discuss the TCP/IP protocols stack in more detail later in the book, but for now the information provided will suffice. The poin