TCP/IP Application Layer 499 Telnet Terminal emulation (Telnet) software provides the capability to remotely access another computer. It enables you to log in to an Internet host and execute commands. Telnet commonly is used for remote administration of servers and network equipment such as routers and switches. A Telnet client is referred to as a local host, and a Telnet server, which uses special software called a daemon, is referred to as a remote host, as shown in Figure 9-14. Figure 9-14 Telnet To make a connection from a Telnet client, you must select a connection option. The host name and terminal type will need to be entered on the command-line prompt. The host name is the IP address (DNS) of the remote computer to which you connect. The terminal type describes the type of terminal emulation that you want the computer to perform. The Telnet operation uses none of the transmitting computer’s processing power. Instead, it transmits the keystrokes to the remote host and sends the resulting screen output back to the local monitor. All processing and storage takes place on the remote computer. When a DNS name for a Telnet location is entered, the name must be translated into its associated IP address before a connection can be established. The Telnet application works mainly at the top three layers of the OSI model—the application layer (commands), the presentation layer (formats, usually ASCII), and the session layer (transmits). The data then passes to the transport layer, where it is segmented, and the port address and error checking are added. The data then passes to the network layer, where the IP header (containing the source and destination IP addresses) is added. Next, the packet travels to the data link layer, which encapsulates the packet in a data frame, adds the source and destination MAC address, and adds a frame trailer. If the source computer doesn’t have the MAC address of the destination computer, it performs an ARP request. When the MAC address has been determined, the frame travels across the physical medium (in binary form) to the next device. Telnet is a good network troubleshooting tool because it tests all seven layers of the OSI model and allows remote diagnostics to be performed. 1102.book Page 499 Tuesday, May 20, 2003 2:53 PM 500 Chapter 9: TCP/IP Transport and Application Layer When the data reaches the remote host computer, the data link layer, the network layer, and the transport layer reassemble the original data commands. The remote host com- puter executes the commands and transmits the results back to the local client computer by using the same process of encapsulation that delivered the original commands. This whole process repeats itself, sending commands and receiving results until the local client has completed the work that needs to be done. When the work is done, the client terminates the session. Summary In this chapter, you learned about the functions of the TCP/IP application layer and transport layer. You also learned about the different processes that occur as data pack- ets travel through this layer. More specifically, you learned that the application layer is responsible for these actions: ■ Identifies and establishes the availability of intended communication partners ■ Synchronizes cooperating applications ■ Establishes agreement on procedures for error recovery ■ Controls data integrity In addition, you learned that the application layer supports the following: ■ Direct and indirect network applications ■ The Domain Name System ■ Telnet, FTP, and HTTP ■ TCP, UDP, SMTP, and SNMP To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book. 1102.book Page 500 Tuesday, May 20, 2003 2:53 PM Key Terms 501 Key Terms acknowledgment Notification sent from one network device to another to acknowledge that some event (for example, receipt of a message) occurred. Sometimes abbreviated ACK. application A program that performs a function directly for a user. FTP and Telnet clients are examples of network applications. application layer Layer 7 of the OSI reference model. This layer provides services to application processes (such as e-mail, file transfer, and terminal emulation) that are outside the OSI reference model. The application layer identifies and establishes the availability of intended communication partners (and the resources required to con- nect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. ASCII (American Standard Code for Information Interchange) An 8-bit code (7 bits plus parity) for character representation. DNS (Domain Name System) The system used in the Internet for translating names of network nodes into addresses. flow control A technique for ensuring that a transmitting entity does not overwhelm a receiving entity with data. When the buffers on the receiving device are full, a mes- sage is sent to the sending device to suspend the transmission until the data in the buffers has been processed. In IBM networks, this technique is called pacing. FTP (File Transfer Protocol) An application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes. FTP is defined in RFC 959. HTML (Hypertext Markup Language) A simple hypertext document formatting lan- guage that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a web browser. HTTP (Hypertext Transfer Protocol) The protocol used by web browsers and web servers to transfer files, such as text and graphics files. port In IP terminology, an upper-layer process that receives information from lower layers. Ports are numbered, and many are associated with a specific process. For exam- ple, SMTP is associated with port 25. A port number of this type is called a well-known port or address. TCP (Transmission Control Protocol) A connection-oriented transport-layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP proto- col stack. 1102.book Page 501 Tuesday, May 20, 2003 2:53 PM 502 Chapter 9: TCP/IP Transport and Application Layer Telnet A standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is used for remote terminal connection, enabling users to log in to remote systems and use resources as if they were connected to a local system. Telnet is defined in RFC 854. TFTP (Trivial File Transfer Protocol) A simplified version of FTP that allows files to be transferred from one computer to another over a network. three-way handshake A sequence of messages exchanged between two or more net- work devices to ensure transmission synchronization before sending user data. UDP (User Datagram Protocol) A connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retrans- mission be handled by other protocols. UDP is defined in RFC 768. windowing A flow-control mechanism requiring that the source device receive an acknowledgment from the destination after transmitting a certain amount of data. Check Your Understanding Complete all the review questions to test your understanding of the topics and con- cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding Answer Key.” 1. When conversing with an individual whose primary language is different than yours, you might need to repeat your words and speak more slowly. Repeating your words can be compared to , and the need to speak slowly can be compared to the functions of the transport layer. A. Reliability; flow control B. Flow control; reliability C. Transport; acknowledgment D. Flow control; transport 2. The following characteristics describe what TCP/IP protocol: connection-oriented; resends anything not received; divides outgoing messages into segments. A. IPX B. TCP C. UDP D. SPS 1102.book Page 502 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 503 3. What does the window field in a TCP segment indicate? A. Number of 32-bit words in the header B. Number of the called port C. Number used to ensure correct sequencing of the arriving data D. Number of octets that the device is willing to accept 4. What transport protocol exchanges datagrams without acknowledgments or guaranteed delivery? A. UDP B. TCP C. IRQ D. LLC 5. What do TCP and UDP use to keep track of different conversations crossing a network at the same time? A. Port numbers B. IP addresses C. MAC addresses D. Route numbers 6. How does TCP synchronize a connection between the source and the destination before data transmission? A. Two-way handshake B. Three-way handshake C. Four-way handshake D. Holton functions 7. Which range of port numbers is unregulated? A. Below 255 B. Between 256 and 512 C. Between 256 and 1023 D. Above 1023 1102.book Page 503 Tuesday, May 20, 2003 2:53 PM 504 Chapter 9: TCP/IP Transport and Application Layer 8. With TCP transmission, what occurs if a segment is not acknowledged in a certain time period? A. UDP takes over the transmission. B. The virtual circuit is terminated. C. Nothing happens. D. Retransmission occurs. 9. Which best describes flow control? A. A method of managing limited bandwidth B. A method of connecting two hosts synchronously C. A method of preventing buffer overrun D. A method of checking data for viruses before transmission 10. Which of the following best describes the purpose of the TCP/IP protocol stack? A. Maps closely to the OSI reference model’s upper layers B. Supports all standard physical and data link protocols C. Transfers information from one host to another in a sequence of datagrams D. Reassembles datagrams into complete messages at the receiving location 11. Which of the following is one of the protocols found in the transport layer? A. UCP B. UDP C. TDP D. TDC 12. What is the purpose of port numbers? A. They keep track of different upper-layer conversations crossing the network at the same time. B. Source systems use them to keep a session organized. C. End systems use them to assign end users dynamically to a particular session, depending on their application use. D. Source systems generate them to predict destination addresses. 1102.book Page 504 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 505 13. Why are TCP three-way handshake/open connections used? Select all that apply. A. To ensure that lost data can be recovered if problems occur later B. To determine how much data the receiving station can accept at one time C. To provide efficient use of bandwidth by users D. To change binary ping responses into information in the upper layers 14. What does a dynamic TCP window field do? A. It makes the window larger so that more data can come through at once, which results in more efficient use of bandwidth. B. The window size slides to each section of the datagram to receive data, which results in more efficient use of bandwidth. C. It allows the window size to be negotiated dynamically during the TCP session, which results in more efficient use of bandwidth. D. It limits the incoming data so that each segment must be sent one by one, which is an inefficient use of bandwidth. 15. UDP segments use what protocols to provide reliability? A. Network layer protocols B. Application layer protocols C. Internet protocols D. Transmission control protocols 16. A network redirector enables data to travel . A. Only to a network print server B. Only to a network file server C. In a single direction D. None of the above 17. An example of a client/server application is . A. E-mail B. A spreadsheet C. A NIC D. Hard-drive utilities 1102.book Page 505 Tuesday, May 20, 2003 2:53 PM 506 Chapter 9: TCP/IP Transport and Application Layer 18. The client side of the client/server relationship is . A. Located on the remote computer B. The requestor of services C. The most important D. Always located on the server 19. Which of the following best describes a domain name? A. It represents the numeric address of an Internet site. B. It is the same as the name you give your primary server. C. It represents the specific location where your LAN is located. D. It is an IP address used to represent a print server. 20. .com is the domain typically assigned to . A. Client machines B. Customers C. Network provider companies D. Corporations 21. During a Telnet connection, the remote computer is responsible for . A. Nothing B. Processing C. Client-side Telnet application D. Client-side printing 22. At which three layers of the OSI model does Telnet primarily work? A. Application layer, session layer, transport layer B. Presentation layer, session layer, transport layer C. Data link layer, transport layer, presentation layer D. Application layer, presentation layer, session layer 1102.book Page 506 Tuesday, May 20, 2003 2:53 PM Check Your Understanding 507 23. Typical anonymous FTP sessions use as the login ID and as the password. A. Anonymous; the user e-mail address B. The user e-mail address; FTP C. FTP; FTP D. Guest; anonymous 24. Instead of working with specific application programs, redirectors work with . A. Computer operating systems B. Spreadsheets C. E-mail D. Web browsers 1102.book Page 507 Tuesday, May 20, 2003 2:53 PM . unregulated? A. Below 25 5 B. Between 25 6 and 5 12 C. Between 25 6 and 10 23 D. Above 10 23 11 02. book Page 503 Tuesday, May 20 , 20 03 2: 53 PM 504 Chapter 9: TCP/IP Transport and Application Layer 8. With. transmission. TCP is part of the TCP/IP proto- col stack. 11 02. book Page 5 01 Tuesday, May 20 , 20 03 2: 53 PM 5 02 Chapter 9: TCP/IP Transport and Application Layer Telnet A standard terminal emulation. presentation layer, session layer 11 02. book Page 506 Tuesday, May 20 , 20 03 2: 53 PM Check Your Understanding 507 23 . Typical anonymous FTP sessions use as the login ID and as the password. A. Anonymous;