Overview Now that you have seen what happens to data packets as
they travel the presentation layer, its time to look at the last
layer in which data packets travel through before reaching their
final destination. The last layer or Layer 7 of the OSI model is
referred to as the application layer. The application layer is the
closest to you as an end-user, when you are interacting with
software applications such as sending and receiving e-mail over a
network.
Slide 3
Overview You will see how the application layer deals with data
packets from client-server applications, domain name services, and
network applications by examining the following: Client-Server
Redirectors Domain Name System E-mail Telnet FTP HTTP
Slide 4
Basics of the Application Layer : Application processes In the
context of the OSI reference model, the application layer (Layer 7)
supports the communicating component of an application. The
application layer is responsible for the following: identifying and
establishing the availability of intended communication partners
synchronizing cooperating applications establishing agreement on
procedures for error recovery controlling data integrity
Slide 5
Basics of the Application Layer : Application processes The
application layer is the OSI layer closest to the end system. This
determines whether sufficient resources exist for communication
between systems. Without the application layer, there would be no
network communication support. The application layer does not
provide services to any other OSI layer. It does provide services
to application processes lying outside the scope of the OSI
model.
Slide 6
Basics of the Application Layer : Application processes
Examples of such application processes include spreadsheet
programs, word processing programs, and banking terminal programs.
Additionally, the application layer provides a direct interface for
the rest of the OSI model by using network applications (e.g. WWW,
e-mail, FTP, Telnet), or an indirect interface by using standalone
applications (e.g. word processors, spreadsheets, presentation
managers) with a network redirector.
Slide 7
Slide 8
Basics of the Application Layer : Direct network applications
Most applications that work in a networked environment are
classified as client-server applications. These applications, such
as FTP, web browsers, and e- mail, all have two components, which
allow them to function - the client side, and the server side. The
client side is located on the local computer and is the requestor
of the services. The server side is located on a remote computer
and provides services in response to the clients requests.
Slide 9
Slide 10
Basics of the Application Layer : Direct network applications A
client-server application works by constantly repeating the
following looped routine: client- request, server-response;
client-request, server-response; etc. For example, a web browser
accesses a web page by requesting a uniform resource locator (URL),
or web address, on a remote web server.
Slide 11
Basics of the Application Layer : Direct network applications
After it locates the URL, the web server that is identified by that
URL responds to the request. Then, based on the information
received from the web server, the client can request more
information from the same web server, or can access another web
page from a different web server.
Slide 12
Slide 13
Basics of the Application Layer : Direct network applications
The World Wide Web, Netscape Navigator, and Internet Explorer, are
probably the most commonly used network applications. An easy way
to understand a Web browser is to compare it to a television remote
control. A remote control gives you the ability to directly control
a TV's functions: volume, channels, brightness, etc
Slide 14
Slide 15
Basics of the Application Layer : Direct network applications
For the remote control to function properly, you do not need to
understand how the remote control functions electronically. The
same is true of a Web browser, in that the browser gives you the
ability to navigate through the Web by clicking on hyperlinks. For
the Web browser to function properly, it is not necessary for you
to understand how the lower layer OSI protocols work and
interact.
Slide 16
Slide 17
Basics of the Application Layer : Indirect network support
Within a LAN environment, indirect- application network support is
a client-server function. If a client wants to save a file from a
word processor to a network server, the redirector enables the word
processing application to become a network client
Slide 18
Basics of the Application Layer : Indirect network support
Redirector is a protocol that works with computer operating systems
and network clients instead of specific application programs.
Examples of redirectors are: Apple File Protocol NetBIOS Extended
User Interface (NetBEUI) Novell IPX/SPX protocols Network File
System (NSF) of the TCP/IP protocol suite
Slide 19
Basics of the Application Layer : Indirect network support The
redirector process is as follows: The client requests that the
network file server allow the data file to be stored. The server
responds by saving the file to its disk, or by rejecting the
client's request. If the client requests that the network print
server allow the data file to be printed by a remote (network)
printer, the server processes the request by printing the file on
one of its print devices, or by rejecting the request.
Slide 20
Slide 21
Slide 22
Basics of the Application Layer : Indirect network support
Redirector allows a network administrator to assign remote
resources to logical names on the local client. When you select one
of these logical names to perform an operation such as saving a
file, or printing a file, the network redirector sends the selected
file to the proper remote resource on the network for processing.
If the resource is on a local computer, the redirector ignores the
request and allows the local operating system to process the
request.
Slide 23
Slide 24
Basics of the Application Layer : Indirect network support The
advantage of using a network redirector on a local client is that
the applications on the client never have to recognize the network.
In addition, the application that requests service is located on
the local computer and the redirector reroutes the request to the
proper network resource, while the application treats it as a local
request.
Slide 25
Basics of the Application Layer : Indirect network support
Redirectors expand the capabilities of non- network software. They
also allow users to share documents, templates, databases,
printers, and many other resource types, without having to use
special application software.
Slide 26
Basics of the Application Layer : Indirect network support
Networking has had a great influence on the development of programs
like word processors, spreadsheets, presentation managers, database
programs, graphics, and productivity software. Many of these
software packages are now network-integrated or network-aware. They
have the capabilities of launching integrated Web browsers or
Internet tools, and to publish their output to HTML for easy Web
integration.
Slide 27
Basics of the Application Layer : Making and breaking a
connection It is important to note that in each of the previous
examples the connection to the server was maintained only long
enough to process the transaction. In the Web example, the
connection was maintained just long enough to download the current
Web page. In the printer example, the connection was maintained
just long enough to send the document to the print server.
Slide 28
Basics of the Application Layer : Making and breaking a
connection After the processing was completed, the connection was
broken and had to be re- established for the next processing
request to take place. This is one of the two ways that
communication processing takes place.
Slide 29
Slide 30
Slide 31
Slide 32
Slide 33
Slide 34
Slide 35
Slide 36
Slide 37
Basics of the Application Layer : Making and breaking a
connection Later in this chapter, you will learn about the second
method in which communication processing takes place. This is
illustrated by the Telnet and FTP examples, which establish a
connection to the server, and maintains that connection until all
processing has been performed.
Slide 38
Basics of the Application Layer : Making and breaking a
connection The client computer terminates the connection when the
user determines that he/she has finished. All communication
activity falls into one of these two categories. In the next
section, you will learn about the Domain Name System, which is
supported by the application layer processes.
Slide 39
Domain Name System : Problems with using IP addresses In the
network layer chapter, you learned that the Internet is built on a
hierarchical addressing scheme. This allows for routing that is
based on classes of addresses, as opposed to individual addresses.
The problem this creates for the user is associating the correct
address with the Internet site.
Slide 40
Domain Name System : Problems with using IP addresses The only
difference between the address 198.151.11.12 and 198.151.11.21 is
one transposed digit. It is very easy to forget an address to a
particular site, because there is nothing to associate the contents
of the site with its address.
Slide 41
Slide 42
Slide 43
Domain Name System : Problems with using IP addresses In order
to associate the contents of the site with its address, a domain
naming system was developed. 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 and/or numbers,
usually a name or abbreviation, that represents the numeric address
of an Internet site.
Slide 44
Domain Name System : Problems with using IP addresses 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, examples of which include the
following: .edu - educational sites .com - commercial sites .gov -
government sites .org - non-profit sites .net - network
service
Slide 45
Slide 46
Slide 47
Slide 48
Domain Name System : The domain name server The domain name
server (DNS) is a device on a network. It responds to requests from
clients to translate a domain name into the associated IP address.
The DNS system is set up in a hierarchy that creates different
levels of DNS servers.
Slide 49
Domain Name System : The domain name server If a local DNS is
able to translate a domain name into its associated IP address, it
does so, and returns the result to the client. If it cannot
translate the address, it passes the request up to the next
higher-level DNS on the system, which then tries to translate the
address. If the DNS at this level is able to translate the domain
name into an associated IP address, it does so, and returns the
result to the client.
Slide 50
Domain Name System : The domain name server If not, it sends
the request to the next higher level. This process repeats itself
until the domain name has been translated, or the top-level DNS has
been reached. If the domain name cannot be found on the top level
DNS, it is considered to be an error and the corresponding error
message is returned. Any type of application that uses domain names
to represent IP addresses, uses the DNS to translate that name into
its corresponding IP address.
Slide 51
Network Applications : Internet applications You select network
applications based on the type of work you need to accomplish. A
complete set of application layer programs is available to
interface with the Internet. Each application program type is
associated with its own application protocol.
Slide 52
Network Applications : Internet applications Although there are
more programs and protocol types available, the following are the
main focus of this chapter: The World Wide Web uses the HTTP
protocol. Remote access programs use the Telnet protocol for
directly connecting to remote resources. E-mail programs support
the POP3 application layer protocol for electronic mail. File
utility programs use the FTP protocol for copying and moving files
between remote sites. Network data gathering and monitoring use the
SNMP protocol.
Slide 53
Network Applications : Internet applications It is important to
re-emphasize the fact that the application layer is just another
protocol layer in the OSI or TCP/IP models. The programs interface
with application layer protocols.
Slide 54
Network Applications : Internet applications E-mail client
applications (i.e. Eudora, Microsoft Mail, Pegasus, and Netscape
Mail) work with the POP3 protocol. The same is true with Web
browsers. The two most popular Web browsers are Microsoft Internet
Explorer and Netscape Communicator. The appearance and operation of
these two programs is very different, but they both work with the
application layer HTTP protocol.
Slide 55
Network Applications : E-mail message Electronic mail (e-mail)
enables you to send messages between connected computers. The
procedure for sending an e-mail document involves two separate
processes. The first is to send the e-mail to the users post
office, and the second is to deliver the e- mail from that post
office to the users e-mail client (i.e. the recipient)
Slide 56
Slide 57
Network Applications : E-mail message The following steps will
help you understand the process of sending an e-mail: Start your
e-mail program. Type in a recipient's e-mail address. Type in a
subject. Type a letter.
Slide 58
Network Applications : E-mail message Now, examine the e-mail
address. This is an example of what it may look like:
[email protected]. It consists of two parts: the recipients name
(located before the @ sign); and the recipients post office address
(after the @ sign). The recipients name is only important after the
message arrives at the post office address, which is a DNS entry
that represents the IP address of the post office server.
Slide 59
Slide 60
Network Applications : DNS function Whenever e-mail clients
send letters, they request that a DNS connected to the network
translate the domain names into their associated IP addresses. If
the DNS is able to translate the names, it returns the IP addresses
to the clients, thus enabling proper transport layer segmentation
and encapsulation. If the DNS cannot translate the names, the
requests are passed on until the names can be translated
Slide 61
Slide 62
Network Applications : DNS function The part of the e-mail
address that contains the recipient's name becomes important at
this point. The server extracts it from the e-mail message and
checks to see if the he/she is a member of its post office. If the
recipient is a member, it stores the message in his/her mailbox
until someone retrieves it. If the recipient is not a member, the
post office generates an error message and sends the e-mail back to
the sender
Slide 63
Slide 64
Network Applications : DNS function The second part of the
e-mailing process is the receiving process. E-mail message
recipients must use the e-mail client software on their computers
to establish requests to the e-mail post offices. When message
recipients click the "Get Mail" or "Retrieve Mail" buttons on the
e-mail client, they are usually prompted for a password. After they
enter the password and click "OK", the e-mail software builds a
request for the post office servers.
Slide 65
Network Applications : DNS function It then extracts the post
office addresses from the configuration data that was entered when
their e-mail software was configured. The process then uses another
DNS search to find the IP addresses of the servers. Finally, the
requests are segmented and sequenced by the transport layer.
Slide 66
Network Applications : DNS function Data packets travel through
the rest of the OSI model layers (i.e. network, data link,
physical) and are then transmitted across the Internet to the
destination e-mail post office. At this post office the packets are
reassembled, in the proper sequence, and are checked for any data
transmission errors
Slide 67
Slide 68
Network Applications : DNS function At the post office,
requests are examined, and user names and passwords are verified.
If everything is correct, the post office server transmits all
e-mail messages to computers. Here the messages are again
segmented, sequenced, and encapsulated as data frames, to be sent
to the client's or the e-mail recipient's computer.
Slide 69
Slide 70
Slide 71
Network Applications : DNS function After e-mail messages
arrive at a computer, you may open them and read them. If you click
on the "Reply", or the "Forward" button, to send a response to a
messages, the whole process starts over again. E-mail messages,
themselves, are normally sent as ASCII text, but the attachments
that are sometimes included with them, can be audio, video,
graphic, or many other types of data.
Slide 72
Network Applications : DNS function To correctly send and
receive attachments, the encoding schemes must be the same on both
the sending and the receiving computer. The two most common formats
for e-mail attachments are the Multipurpose Internet Mail Extension
(MIME) and UUencode (a Unix utility program).
Slide 73
Application Layer Examples : Telnet Terminal emulation (Telnet)
software provides the ability to remotely access another computer.
It allows you to log in to an Internet host and execute commands. 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.
Slide 74
Application Layer Examples : Telnet To make a connection from a
Telnet client, you must select a connection option. A dialog box
prompts you for a "Host Name" and "Terminal Type". 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.
Slide 75
Application Layer Examples : Telnet The Telnet operation uses
none of the transmitting computers 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
take place on the remote computer.
Slide 76
Application Layer Examples : Telnet Telnet begins with the
e-mail process. When you enter a DNS name for a Telnet location,
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).
Slide 77
Application Layer Examples : Telnet 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 a frame trailer.
Slide 78
Application Layer Examples : Telnet If the source computer
doesnt 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.
Slide 79
Application Layer Examples : Telnet When the data reaches the
remote host computer, the data link, network, and transport layers,
reassemble the original data commands. The remote host computer
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.
Slide 80
Slide 81
Application Layer Examples : File transfer protocol File
transfer protocol (FTP) is designed to download files (e.g. receive
from the Internet) or upload files (e.g. send to the Internet). The
ability to upload and download files on it is one of the most
valuable features the Internet has to offer. This is especially
helpful for those people who rely on computers for many purposes
and who may need software drivers and upgrades immediately
Slide 82
Application Layer Examples : File transfer protocol Network
administrators can rarely wait even a few days to get the necessary
drivers that enable their network servers to function again. The
Internet can provide these files immediately by using FTP. FTP is a
client-server application just like e-mail and Telnet. It requires
server software running on a host that can be accessed by client
software.
Slide 83
Slide 84
Application Layer Examples : File transfer protocol An FTP
session is established the same way in which a Telnet session is
established. Just like Telnet, the FTP session is maintained until
the client terminates it, or there is some sort of communication
error. Once you establish a connection to an FTP daemon, you must
supply a login ID and a password. Normally, you would use
"anonymous" as the login ID, and your e-mail address as the
password. This type of connection is known as anonymous FTP.
Slide 85
Application Layer Examples : File transfer protocol Upon
establishing your identity, a command link opens between your
client machine and the FTP server. This is similar to a Telnet
session, in which commands are sent and executed on the server and
the results returned to the client. This feature allows you to
create and change folders, erase and rename files, or execute many
other functions associated with file management.
Slide 86
Application Layer Examples : File transfer protocol 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 you copy files from a server, FTP establishes a
second connection, a data link between the computers, across which
the data is transferred. Data transfer can occur in ASCII mode or
in binary mode. These two modes determine how the data file is to
be transferred between the stations.
Slide 87
Application Layer Examples : File transfer protocol After the
file transfer has ended, the data connection terminates
automatically. After you have completed the entire session of
copying and moving files, you may log off, thus closing the command
link, and ending the session. Another protocol that has the ability
to download files is Hypertext Transfer Protocol (HTTP), which you
will learn about in the next section. One limitation of HTTP is
that you can only use it to download files, and not upload
them.
Slide 88
Application Layer Examples : Hypertext transfer protocol
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 in which it allows access to information. A Web browser
(along with all the other network applications covered in this
chapter) is a client- server application, which means that it
requires both a client and a server component in order to
function.
Slide 89
Application Layer Examples : Hypertext transfer protocol 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
Slide 90
Application Layer Examples : Hypertext transfer protocol
Hyperlinks make the World Wide Web easy to navigate. A hyperlink is
an object (word, phrase, or picture) on a Web page that, when
clicked, transfers you 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).
Slide 91
Application Layer Examples : Hypertext transfer protocol In the
following example, the "http://" tells the browser which protocol
to use. The second part, "www", tells the browser what type of
resource it wishes to contact. The third part, "cisco.com,"
identifies the domain of the Web server IP address. The last part,
"edu" identifies the specific folder location (on the server) that
contains the Web page.
Slide 92
Application Layer Examples : Hypertext transfer protocol
Example: http://www.cisco.com/edu/ When you open a Web browser, the
first thing you usually see is a starting (or "home") page. The URL
of the home page has already been stored in the configuration area
of your Web browser and can be changed at any time. From the
starting page you can click on one of the Web page hyperlinks, or
type a URL in the browsers address bar.
Slide 93
Application Layer Examples : Hypertext transfer protocol The
Web browser then examines the protocol to determine if it needs to
open another program, and determines the IP address of the Web
server. After that, the transport layer, network layer, data link
layer, and physical layer 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. (Note: The data can also contain a
specific file name for an HTML page.). If no name is given, the
server uses a default name (as specified in the servers
configuration).
Slide 94
Application Layer Examples : Hypertext transfer protocol The
server responds to the request by sending all of the text, audio,
video, and graphic files, as specified in the HTML instructions, to
the Web client. The client browser reassembles all the files to
create a view of the Web page, and then terminates the session. If
you click on another page that is located on the same, or a
different server, the whole process begins again.