Transcript Hour 5

Hour 7
The Application Layer
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What Is the Application Layer?
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The Application layer is the top layer in TCP/IP's
protocol suite
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Some of the components at the Application layer are
simple utilities that collect information about the
network configuration.
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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.
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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
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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
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A server is a computer that provides services
for other computers. Two common types of
servers are file servers and print servers.
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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
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Name resolution is the process of mapping IP address to predefined, userfriendly alphanumeric names.
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The domain name service (DNS) provides name resolution for the Internet
and can also provide name resolution for isolated TCP/IP networks
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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
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Other name resolution systems exist, such as Network Information Service
(NIS), NetBIOS name resolution
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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.
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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
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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.
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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
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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
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An Application Programming Interface
(API) is a predefined collection of
functions that a program can use to
access other parts of the operating
environment.
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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
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IPConfig
–
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Ping
–
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A utility that traces the path of a datagram through the internetwork.RouteA utility that
lets you view, add, or edit entries in a routing table.
Netstat
–
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A utility that lets you view (and possibly modify) the ARP cache of a local or remote
computer. The ARP cache contains the physical address to IP address mappings.
Traceroute (Tracert for windows)
–
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A utility that tests for network connectivity.
Arp
–
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A Windows utility that displays TCP/IP configuration settings. (The Unix utility ifconfig is
similar.)
A utility that displays IP, UDP, TCP, and ICMP statistics
NBTstat
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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
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Ftp
– A basic file transfer utility that uses TCP.
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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.
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Rcp
– A simple remote file transfer utility.
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Remote Utilities
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Telnet
– A remote terminal utility.
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Rexec
– A utility that runs commands on a remote computer through
the rexecd daemon.
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Rsh
– A utility that invokes the shell on a remote computer to
execute a command.
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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
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Browsers
–
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Newsreaders
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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
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Utilities that provide a means of sending and receiving email.
Archie
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Utilities that connect with Internet newsgroups
Email readers
–
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Utilities that provide access to World Wide Web HTML content.
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
Internet white pages.
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Whois (TCP port 43)
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