Hour 7

The Application Layer

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What Is the Application Layer?

The Application layer is the top layer in TCP/IP's protocol suite

Some of the components at the Application layer are simple utilities that collect information about the network configuration.

Some Application layer components might be a user interface system (such as the X Window interface) or an Application Program Interface (API) that supports a desktop operating environment.

Some Application layer components provide services for the network

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The TCP/IP Application Layer and OSI

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Network Services

Example of Network Services

– File and print services

– Name resolution services

– Redirector services

– Other important network services, such as mail services and network management services, are discussed in other chapters

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File and Print Services

A server is a computer that provides services for other computers. Two common types of servers are file servers and print servers.

A print server operates a printer and fulfills requests to print documents on that printer. A file server operates a data storage device, such as a hard drive, and fulfills requests to read or write data to that device

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File and Print Services (continue)

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Name Resolution Services Name resolution is the process of mapping IP address to predefined, user-friendly

alphanumeric names.

The domain name service (DNS) provides name resolution for the Internet and can also provide name resolution for isolated TCP/IP networks

DNS uses name servers to resolve DNS name queries. A name server service runs at the Application layer of the name server computer and communicates with other name servers to exchange name resolution information

Other name resolution systems exist, such as Network Information Service (NIS), NetBIOS name resolution

Name resolution is an example of an Application layer service that functions integrally with lower protocol layers and actively participates in the interactions of the protocol stack.

DNS or WINS queries are initiated by the protocol software of the client machine, rather than by a user or user application.

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Redirector Services

To integrate the local environment with the network, some network operating systems use a service called a redirector. A redirector is sometimes called a requester.

A redirector intercepts service requests in the local computer and checks to see whether the request should be fulfilled locally or forwarded to another computer on the network. If the request is addressed to a service on another machine, the redirector redirects the request to the network

A redirector enables the user to access network resources as if they were part of the local environment. For instance, a remote disk drive could appear as a local disk drive on the client machine

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Redirector Services

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APIs and the Application Layer

An Application Programming Interface (API) is a predefined collection of functions that a program can use to access other parts of the operating environment.

Figure 7.4 show network API.

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TCP/IP Utilities

Classifies into four categories

– Connectivity

– File Transfer Utility

– Remote Utility

– Internet Utility

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Connectivity Utilities IPConfig

– A Windows utility that displays TCP/IP configuration settings. (The Unix utility ifconfig is similar.)

Ping– A utility that tests for network connectivity.

Arp– A utility that lets you view (and possibly modify) the ARP cache of a local or remote computer. Th

e ARP cache contains the physical address to IP address mappings.

Traceroute (Tracert for windows)– A utility that traces the path of a datagram through the internetwork.RouteA utility that lets you vi

ew, add, or edit entries in a routing table.

Netstat– A utility that displays IP, UDP, TCP, and ICMP statistics

NBTstat– A utility that displays statistics on NetBIOS and NBT.HostnameA utility that returns the hostname

of the local host.

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File Transfer Utilities

Ftp– A basic file transfer utility that uses TCP.

Tftp (Trivial File Transfer Protocol )

– A basic file transfer utility that uses UDP port 69. Tftp offers very little security and is used for tasks such as downloading code to network devices.

Rcp– A simple remote file transfer utility.

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Remote Utilities

Telnet– A remote terminal utility.

Rexec– A utility that runs commands on a remote computer through th

e rexecd daemon.

Rsh– A utility that invokes the shell on a remote computer to execut

e a command.

Finger– A utility that displays user information.

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Telnet (TCP port 23)

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Rexec (TCP port 512) Rsh (TCP port 514)

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Finger (TCP port 79)

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Internet Utilities Browsers

– Utilities that provide access to World Wide Web HTML content.

Newsreaders– Utilities that connect with Internet newsgroups

Email readers– Utilities that provide a means of sending and receiving email.

Archie– A once-popular Internet utility that provides access to indexes of anonymous FTP sites. The

World Wide Web and its search engines have reduced the importance of Archie.

Gopher– A menu-based Internet information utility. Like Archie, Gopher looks old-fashioned next to the

World Wide Web, and is no longer popular.

Whois– A utility that provides access to directories with personal contact information, similar to Intern

et white pages.

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Whois (TCP port 43)

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