The Application Layer

Introduction to the TCP/IP application layer

When the TCP/IP model was designed, the session and presentation layers from the OSI model were bundled into the application layer of the TCP model. This means that issues of representation, encoding, and dialog control are handled in the application layer rather than in separate lower layers as in the OSI model. This design assures that the TCP/IP model provides maximum flexibility at the application layer for developers of software. The TCP/IP protocols that support file transfer, e-mail, and remote login are probably the most familiar to users of the Internet. These protocols include the following applications: Domain Name System (DNS) File Transfer Protocol (FTP) Hypertext Transfer Protocol (HTTP) Simple Mail Transfer Protocol (SMTP) Simple Network Management Protocol (SNMP) Telnet

DNS

The Internet is built on a hierarchical addressing scheme. This scheme allows for routing to be based on classes of addresses rather than based on individual addresses. The problem this creates for the user is associating the correct address with the Internet site. It is very easy to forget an IP address to a particular site because there is nothing to associate the contents of the site with the address. Imagine the difficulty of remembering the IP addresses of tens, hundreds, or even thousands of Internet sites.

A domain naming system was developed in order to associate the contents of the site with the address of that site. The Domain Name System (DNS) is a system used on the Internet for translating names of domains and their publicly advertised network nodes into IP addresses. A domain is a group of computers that are associated by their geographical location or their business type. A domain name is a string of characters, number, or both. Usually a name or abbreviation that represents the numeric address of an Internet site will make up the domain name. There are more than 200 top-level domains on the Internet, examples of which include the following:

.us – United States
.uk – United Kingdom

There are also generic names, which examples include the following:

.edu – educational sites
.com – commercial sites
.gov – government sites
.org – non-profit sites
.net – network service

See Figure for a detailed explanation of these domains.

FTP and TFTP

FTP is a reliable, connection-oriented service that uses TCP to transfer files between systems that support FTP. The main purpose of FTP is to transfer files from one computer to another by copying and moving files from servers to clients, and from clients to servers. When files are copied from a server, FTP first establishes a control connection between the client and the server. Then a second connection is established, which is a link between the computers through which the data is transferred. Data transfer can occur in ASCII mode or in binary mode. These modes determine the encoding used for data file, which in the OSI model is a presentation layer task. After the file transfer has ended, the data connection terminates automatically. When the entire session of copying and moving files is complete, the command link is closed when the user logs off and ends the session. TFTP is a connectionless service that uses User Datagram Protocol (UDP). TFTP is used on the router to transfer configuration files and Cisco IOS images and to transfer files between systems that support TFTP. TFTP is designed to be small and easy to implement. Therefore, it lacks most of the features of FTP. TFTP can read, write, or mail files to or from a remote server but it cannot list directories and currently has no provisions for user authentication. It is useful in some LANs because it operates faster than FTP and in a stable environment it works reliably.

HTTP

Hypertext Transfer Protocol (HTTP) works with the World Wide Web, which is the fastest growing and most used part of the Internet. One of the main reasons for the extraordinary growth of the Web is the ease with which it allows access to information. A Web browser is a client-server application, which means that it requires both a client and a server component in order to function. A Web browser presents data in multimedia formats on Web pages that use text, graphics, sound, and video. The Web pages are created with a format language called Hypertext Markup Language (HTML). HTML directs a Web browser on a particular Web page to produce the appearance of the page in a specific manner. In addition, HTML specifies locations for the placement of text, files, and objects that are to be transferred from the Web server to the Web browser.

Hyperlinks make the World Wide Web easy to navigate. A hyperlink is an object, word, phrase, or picture, on a Web page. When that hyperlink is clicked, it directs the browser to a new Web page. The Web page contains, often hidden within its HTML description, an address location known as a Uniform Resource Locator (URL).

In the URL http://www.cisco.com/edu/, the "http://" tells the browser which protocol to use. The second part, "www", is the hostname or name of a specific machine with a specific IP address. The last part, /education identifies the specific folder location on the server that contains the default web page.

A Web browser usually opens to a starting or "home" page. The URL of the home page has already been stored in the configuration area of the Web browser and can be changed at any time. From the starting page, click on one of the Web page hyperlinks, or type a URL in the address bar of the browser. The Web browser examines the protocol to determine if it needs to open another program, and then determines the IP address of the Web server using DNS. Then the transport layer, network layer, data link layer, and physical layer work together to initiate a session with the Web server. The data that is transferred to the HTTP server contains the folder name of the Web page location. The data can also contain a specific file name for an HTML page. If no name is given, then the default name as specified in the configuration on the server is used.

The server responds to the request by sending to the Web client all of the text, audio, video, and graphic files specified in the HTML instructions. The client browser reassembles all the files to create a view of the Web page, and then terminates the session. If another page that is located on the same or a different server is clicked, the whole process begins again.

SMTP

Email servers communicate with each other using the Simple Mail Transfer Protocol (SMTP) to send and receive mail. The SMTP protocol transports email messages in ASCII format using TCP. When a mail server receives a message destined for a local client, it stores that message and waits for the client to collect the mail. There are several ways for mail clients to collect their mail. They can use programs that access the mail server files directly or collect their mail using one of many network protocols. The most popular mail client protocols are POP3 and IMAP4, which both use TCP to transport data. Even though mail clients use these special protocols to collect mail, they almost always use SMTP to send mail. Since two different protocols, and possibly two different servers, are used to send and receive mail, it is possible that mail clients can perform one task and not the other. Therefore, it is usually a good idea to troubleshoot e-mail sending problems separately from e-mail receiving problems. When checking the configuration of a mail client, verify that the SMTP and POP or IMAP settings are correctly configured. A good way to test if a mail server is reachable is to Telnet to the SMTP port (25) or to the POP3 port (110). The following command format is used at the Windows command line to test the ability to reach the SMTP service on the mail server at IP address 192.168.10.5: C:\>telnet 192.168.10.5 25 The SMTP protocol does not offer much in the way of security and does not require any authentication. Administrators often do not allow hosts that are not part of their network to use their SMTP server to send or relay mail. This is to prevent unauthorized users from using their servers as mail relays.

SNMP.

The Simple Network Management Protocol (SNMP) is an application layer protocol that facilitates the exchange of management information between network devices. SNMP enables network administrators to manage network performance, find and solve network problems, and plan for network growth. SNMP uses UDP as its transport layer protocol. An SNMP managed network consists of the following three key components: Network management system (NMS) – NMS executes applications that monitor and control managed devices. The bulk of the processing and memory resources required for network management are provided by NMS. One or more NMSs must exist on any managed network. Managed devices – Managed devices are network nodes that contain an SNMP agent and that reside on a managed network. Managed devices collect and store management information and make this information available to NMSs using SNMP. Managed devices, sometimes called network elements, can be routers, access servers, switches, and bridges, hubs, computer hosts, or printers. Agents – Agents are network-management software modules that reside in managed devices. An agent has local knowledge of management information and translates that information into a form compatible with SNMP.

Telnet

Telnet client software provides the ability to login to a remote Internet host that is running a Telnet server application and then to execute commands from the command line. A Telnet client is referred to as a local host. Telnet server, which uses special software called a daemon, is referred to as a remote host. To make a connection from a Telnet client, the connection option must be selected. A dialog box typically prompts for a host name and terminal type. The host name is the IP address or DNS name of the remote computer. The terminal type describes the type of terminal emulation that the Telnet client should perform. The Telnet operation uses none of the processing power from the transmitting computer. Instead, it transmits the keystrokes to the remote host and sends the resulting screen output back to the local monitor. All processing and storage take place on the remote computer. Telnet works at the application layer of the TCP/IP model. Therefore, Telnet works at the top three layers of the OSI model. The application layer deals with commands. The presentation layer handles formatting, usually ASCII. The session layer transmits. In the TCP/IP model, all of these functions are considered to be part of the application layer.\ Cisco Systems, Inc.