UNIT-5 Application Layer
UNIT-5 Application Layer
The
application layer in the OSI model is the closest layer to the end user which
means that the application layer and end user can interact directly with the software
application. The application layer programs are based on client and servers.
The Application layer includes the following
functions:
o
Identifying communication partners: The application layer
identifies the availability of communication partners for an application with
data to transmit.
o
Determining resource availability: The application layer
determines whether sufficient network resources are available for the requested
communication.
o
Synchronizing communication: All the
communications occur between the applications requires cooperation which is
managed by an application layer.
Services
of Application Layers
o
Network Virtual terminal: An application layer allows a user to log on to a
remote host. To do so, the application creates a software emulation of a
terminal at the remote host. The user's computer talks to the software
terminal, which in turn, talks to the host. The remote host thinks that it is
communicating with one of its own terminals, so it allows the user to log on.
o
File Transfer, Access, and Management (FTAM): An application allows a user to access files in a
remote computer, to retrieve files from a computer and to manage files in a
remote computer. FTAM defines a hierarchical virtual file in terms of file
structure, file attributes and the kind of operations performed on the files
and their attributes.
o
Addressing: To obtain communication between client and server,
there is a need for addressing. When a client made a request to the server, the
request contains the server address and its own address. The server response to
the client request, the request contains the destination address, i.e., client
address. To achieve this kind of addressing, DNS is used.
o
Mail Services: An application layer provides Email forwarding and
storage.
o
Directory Services: An application contains a distributed database that
provides access for global information about various objects and services.
Authentication: It
authenticates the sender or receiver's message or both.
Network
Application Architecture
Application
architecture is different from the network architecture. The network
architecture is fixed and provides a set of services to applications. The
application architecture, on the other hand, is designed by the application
developer and defines how the application should be structured over the various
end systems.
Application
architecture is of two types:
o
Client-server architecture: An application program running on the local machine
sends a request to another application program is known as a client, and a program
that serves a request is known as a server. For example, when a web server
receives a request from the client host, it responds to the request to the
client host.
Characteristics Of
Client-server architecture:
o
In Client-server architecture, clients do not
directly communicate with each other. For example, in a web application, two
browsers do not directly communicate with each other.
o
A server is fixed, well-known address known
as IP address because the server is always on while the client can always contact
the server by sending a packet to the sender's IP address.
Network
Application Architecture
Application
architecture is different from the network architecture. The network
architecture is fixed and provides a set of services to applications. The application
architecture, on the other hand, is designed by the application developer and
defines how the application should be structured over the various end systems.
Application
architecture is of two types:
o
Client-server architecture: An application program running on the local machine
sends a request to another application program is known as a client, and a
program that serves a request is known as a server. For example, when a web
server receives a request from the client host, it responds to the request to
the client host.
Characteristics Of
Client-server architecture:
o
In Client-server architecture, clients do not
directly communicate with each other. For example, in a web application, two
browsers do not directly communicate with each other.
o
A server is fixed, well-known address known
as IP address because the server is always on while the client can always
contact the server by sending a packet to the sender's IP address.
DNS
An application layer
protocol defines how the application processes running on different systems,
pass the messages to each other.
o
DNS stands for Domain Name System.
o
DNS is a directory service that provides a
mapping between the name of a host on the network and its numerical address.
o
DNS is required for the functioning of the internet.
o
Each node in a tree has a domain name, and a
full domain name is a sequence of symbols specified by dots.
o
DNS is a service that translates the domain
name into IP addresses. This allows the users of networks to utilize
user-friendly names when looking for other hosts instead of remembering the IP
addresses.
o
For example, suppose the FTP site at EduSoft
had an IP address of 132.147.165.50, most people would reach this site by
specifying ftp.EduSoft.com. Therefore, the domain name is more reliable than IP
address.
DNS is a TCP/IP
protocol used on different platforms. The domain name space is divided into
three different sections: generic domains, country domains, and inverse domain.
Generic
Domains
o
It defines the registered hosts according to
their generic behavior.
o
Each node in a tree defines the domain name,
which is an index to the DNS database.
o
It uses three-character labels, and these
labels describe the organization type.
|
Label |
Description |
|
aero |
Airlines and aerospace companies |
|
biz |
Businesses or firms |
|
com |
Commercial Organizations |
|
coop |
Cooperative business Organizations |
|
edu |
Educational institutions |
|
gov |
Government institutions |
|
info |
Information service providers |
|
int |
International Organizations |
|
mil |
Military groups |
|
museum |
Museum & other nonprofit organizations |
|
name |
Personal names |
|
net |
Network Support centers |
|
org |
Nonprofit Organizations |
|
pro |
Professional individual Organizations |
Country
Domain
The format of country
domain is same as a generic domain, but it uses two-character country abbreviations
(e.g., us for the United States) in place of three character organizational
abbreviations.
Inverse
Domain
The inverse domain is
used for mapping an address to a name. When the server has received a request
from the client, and the server contains the files of only authorized clients.
To determine whether the client is on the authorized list or not, it sends a
query to the DNS server and ask for mapping an address to the name.
Working
of DNS
o
DNS is a client/server network communication protocol.
DNS clients send requests to the. server while DNS servers send responses to
the client.
o
Client requests contain a name which is
converted into an IP address known as a forward DNS lookups while requests
containing an IP address which is converted into a name known as reverse DNS
lookups.
o
DNS implements a distributed database to
store the name of all the hosts available on the internet.
o
If a client like a web browser sends a
request containing a hostname, then a piece of software such as DNS resolver sends a request to the DNS server to obtain the IP
address of a hostname. If DNS server does not contain the IP address associated
with a hostname, then it forwards the request to another DNS server. If IP
address has arrived at the resolver, which in turn completes the request over
the internet protocol.
An application layer
protocol defines how the application processes running on different systems,
pass the messages to each other.
o
DNS stands for Domain Name System.
o
DNS is a directory service that provides a
mapping between the name of a host on the network and its numerical address.
o
DNS is required for the functioning of the
internet.
o
Each node in a tree has a domain name, and a
full domain name is a sequence of symbols specified by dots.
o
DNS is a service that translates the domain
name into IP addresses. This allows the users of networks to utilize
user-friendly names when looking for other hosts instead of remembering the IP
addresses.
o
For example, suppose the FTP site at EduSoft
had an IP address of 132.147.165.50, most people would reach this site by
specifying ftp.EduSoft.com. Therefore, the domain name is more reliable than IP
address.
DNS is a TCP/IP
protocol used on different platforms. The domain name space is divided into
three different sections: generic domains, country domains, and inverse domain.
Generic
Domains
o
It defines the registered hosts according to
their generic behavior.
o
Each node in a tree defines the domain name,
which is an index to the DNS database.
o
It uses three-character labels, and these
labels describe the organization type.
|
Label |
Description |
|
aero |
Airlines and aerospace companies |
|
biz |
Businesses or firms |
|
com |
Commercial Organizations |
|
coop |
Cooperative business Organizations |
|
edu |
Educational institutions |
|
gov |
Government institutions |
|
info |
Information service providers |
|
int |
International Organizations |
|
mil |
Military groups |
|
museum |
Museum & other nonprofit organizations |
|
name |
Personal names |
|
net |
Network Support centers |
|
org |
Nonprofit Organizations |
|
pro |
Professional individual Organizations |
Country
Domain
The format of country
domain is same as a generic domain, but it uses two-character country
abbreviations (e.g., us for the United States) in place of three character
organizational abbreviations.
Inverse
Domain
The inverse domain is
used for mapping an address to a name. When the server has received a request
from the client, and the server contains the files of only authorized clients.
To determine whether the client is on the authorized list or not, it sends a query
to the DNS server and ask for mapping an address to the name.
Working
of DNS
o
DNS is a client/server network communication
protocol. DNS clients send requests to the. server while DNS servers send
responses to the client.
o
Client requests contain a name which is
converted into an IP address known as a forward DNS lookups while requests
containing an IP address which is converted into a name known as reverse DNS
lookups.
o
DNS implements a distributed database to
store the name of all the hosts available on the internet.
o
If a client like a web browser sends a
request containing a hostname, then a piece of software such as DNS resolver sends a request to the DNS server to obtain the IP
address of a hostname. If DNS server does not contain the IP address associated
with a hostname, then it forwards the request to another DNS server. If IP
address has arrived at the resolver, which in turn completes the request over
the internet protocol.
Telnet
o
The main task of the internet is to provide
services to users. For example, users want to run different application
programs at the remote site and transfers a result to the local site. This
requires a client-server program such as FTP, SMTP. But this would not allow us
to create a specific program for each demand.
o
The better solution is to provide a general
client-server program that lets the user access any application program on a
remote computer. Therefore, a program that allows a user to log on to a remote
computer. A popular client-server program Telnet is used to meet such demands.
Telnet is an abbreviation for Terminal Network.
o
Telnet provides a connection to the remote
computer in such a way that a local terminal appears to be at the remote side.
There
are two types of login:
Local Login
o
When a user logs into a local computer, then it is known
as local login.
o
When the workstation running terminal emulator, the
keystrokes entered by the user are accepted by the terminal driver. The
terminal driver then passes these characters to the operating system which in
turn, invokes the desired application program.
o
However, the operating system has special meaning to
special characters. For example, in UNIX some combination of characters have
special meanings such as control character with "z" means suspend.
Such situations do not create any problem as the terminal driver knows the
meaning of such characters. But, it can cause the problems in remote login.
Remote login
o
When the user wants to access an application
program on a remote computer, then the user must perform remote login.
How
remote login occurs
At the local site
The user sends the keystrokes to the terminal driver, the characters are
then sent to the TELNET client. The TELNET client which in turn, transforms the
characters to a universal character set known as network virtual terminal
characters and delivers them to the local TCP/IP stack
At the remote site
The commands in NVT forms are transmitted to the TCP/IP at the remote
machine. Here, the characters are delivered to the operating system and then
pass to the TELNET server. The TELNET server transforms the characters which
can be understandable by a remote computer. However, the characters cannot be
directly passed to the operating system as a remote operating system does not
receive the characters from the TELNET server. Therefore it requires some piece
of software that can accept the characters from the TELNET server. The
operating system then passes these characters to the appropriate application
program.
Network
Virtual Terminal (NVT)
o
The network virtual terminal is an interface
that defines how data and commands are sent across the network.
o
In today's world, systems are heterogeneous.
For example, the operating system accepts a special combination of characters
such as end-of-file token running a DOS operating system ctrl+z while
the token running a UNIX operating system is ctrl+d.
o
TELNET solves this issue by defining a
universal interface known as network virtual interface.
o
The TELNET client translates the characters
that come from the local terminal into NVT form and then delivers them to the
network. The Telnet server then translates the data from NVT form into a form
which can be understandable by a remote computer.
SMTP
o
SMTP stands for Simple Mail Transfer
Protocol.
o
SMTP is a set of communication guidelines
that allow software to transmit an electronic mail over the internet is
called Simple
Mail Transfer Protocol.
o
It is a program used for sending messages to
other computer users based on e-mail addresses.
o
It provides a mail exchange between users on
the same or different computers, and it also supports:
o
It can send a single message to one or more
recipients.
o
Sending message can include text, voice,
video or graphics.
o
It can also send the messages on networks
outside the internet.
o
The main purpose of SMTP is used to set up
communication rules between servers. The servers have a way of identifying
themselves and announcing what kind of communication they are trying to
perform. They also have a way of handling the errors such as incorrect email
address. For example, if the recipient address is wrong, then receiving server
reply with an error message of some kind.
Components
of SMTP
o
First, we will break the SMTP client and SMTP
server into two components such as user agent (UA) and mail transfer agent
(MTA). The user agent (UA) prepares the message, creates the envelope and then
puts the message in the envelope. The mail transfer agent (MTA) transfers this
mail across the internet.
o
SMTP allows a more complex system by adding a
relaying system. Instead of just having one MTA at sending side and one at
receiving side, more MTAs can be added, acting either as a client or server to
relay the email.
o
The relaying system without TCP/IP protocol
can also be used to send the emails to users, and this is achieved by the use
of the mail gateway. The mail gateway is a relay MTA that can be used to
receive an email.
Working
of SMTP
1.
Composition of Mail: A user sends an e-mail by composing an electronic
mail message using a Mail User Agent (MUA). Mail User Agent is a program which
is used to send and receive mail. The message contains two parts: body and
header. The body is the main part of the message while the header includes
information such as the sender and recipient address. The header also includes
descriptive information such as the subject of the message. In this case, the
message body is like a letter and header is like an envelope that contains the
recipient's address.
2.
Submission of Mail: After composing an email, the mail client then
submits the completed e-mail to the SMTP server by using SMTP on TCP port 25.
3.
Delivery of Mail: E-mail addresses contain two parts: username of the
recipient and domain name. For example, vivek@gmail.com, where
"vivek" is the username of the recipient and "gmail.com" is
the domain name.
If the domain name of the recipient's email address is different from the
sender's domain name, then MSA will send the mail to the Mail Transfer Agent
(MTA). To relay the email, the MTA will find the target domain. It checks the
MX record from Domain Name System to obtain the target domain. The MX record
contains the domain name and IP address of the recipient's domain. Once the
record is located, MTA connects to the exchange server to relay the message.
4.
Receipt and Processing of Mail: Once the incoming message is received, the exchange
server delivers it to the incoming server (Mail Delivery Agent) which stores
the e-mail where it waits for the user to retrieve it.
5.
Access and Retrieval of Mail: The stored email in MDA can be retrieved by using
MUA (Mail User Agent). MUA can be accessed by using login and password.
SNMP
o
SNMP stands for Simple Network
Management Protocol.
o
SNMP is a framework used for managing devices
on the internet.
o
It provides a set of operations for
monitoring and managing the internet.
SNMP
Concept
o
SNMP has two components Manager and agent.
o
The manager is a host that controls and
monitors a set of agents such as routers.
o
It is an application layer protocol in which
a few manager stations can handle a set of agents.
o
The protocol designed at the application
level can monitor the devices made by different manufacturers and installed on
different physical networks.
o
It is used in a heterogeneous network made of
different LANs and WANs connected by routers or gateways.
Managers
& Agents
o
A manager is a host that runs the SNMP client
program while the agent is a router that runs the SNMP server program.
o
Management of the internet is achieved
through simple interaction between a manager and agent.
o
The agent is used to keep the information in
a database while the manager is used to access the values in the database. For
example, a router can store the appropriate variables such as a number of
packets received and forwarded while the manager can compare these variables to
determine whether the router is congested or not.
o
Agents can also contribute to the management
process. A server program on the agent checks the environment, if something
goes wrong, the agent sends a warning message to the manager.
Management
with SNMP has three basic ideas:
o
A manager checks the agent by requesting the
information that reflects the behavior of the agent.
o
A manager also forces the agent to perform a
certain function by resetting values in the agent database.
o
An agent also contributes to the management
process by warning the manager regarding an unusual condition.
Management
Components
o
Management is not achieved only through the
SNMP protocol but also the use of other protocols that can cooperate with the
SNMP protocol. Management is achieved through the use of the other two
protocols: SMI (Structure of management information) and MIB(management
information base).
o
Management is a combination of SMI, MIB, and
SNMP. All these three protocols such as abstract syntax notation 1 (ASN.1) and
basic encoding rules (BER).
SMI
The SMI (Structure of
management information) is a component used in network management. Its main
function is to define the type of data that can be stored in an object and to
show how to encode the data for the transmission over a network.
MIB
o
The MIB (Management information base) is a
second component for the network management.
o
Each agent has its own MIB, which is a
collection of all the objects that the manager can manage. MIB is categorized
into eight groups: system, interface, address translation, ip, icmp, tcp, udp,
and egp. These groups are under the mib object.
SNMP
SNMP defines five
types of messages: GetRequest, GetNextRequest, SetRequest, GetResponse, and
Trap.
GetRequest: The GetRequest
message is sent from a manager (client) to the agent (server) to retrieve the
value of a variable.
GetNextRequest: The
GetNextRequest message is sent from the manager to agent to retrieve the value
of a variable. This type of message is used to retrieve the values of the
entries in a table. If the manager does not know the indexes of the entries,
then it will not be able to retrieve the values. In such situations, GetNextRequest
message is used to define an object.
GetResponse: The GetResponse
message is sent from an agent to the manager in response to the GetRequest and
GetNextRequest message. This message contains the value of a variable requested
by the manager.
SetRequest: The SetRequest
message is sent from a manager to the agent to set a value in a variable.
Trap: The Trap
message is sent from an agent to the manager to report an event. For example,
if the agent is rebooted, then it informs the manager as well as sends the time
of rebooting.
HTTP
o
HTTP stands for HyperText
Transfer Protocol.
o
It is a protocol used to access the data on
the World Wide Web (www).
o
The HTTP protocol can be used to transfer the
data in the form of plain text, hypertext, audio, video, and so on.
o
This protocol is known as HyperText Transfer
Protocol because of its efficiency that allows us to use in a hypertext
environment where there are rapid jumps from one document to another document.
o
HTTP is similar to the FTP as it also
transfers the files from one host to another host. But, HTTP is simpler than
FTP as HTTP uses only one connection, i.e., no control connection to transfer
the files.
o
HTTP is used to carry the data in the form of
MIME-like format.
o
HTTP is similar to SMTP as the data is
transferred between client and server. The HTTP differs from the SMTP in the
way the messages are sent from the client to the server and from server to the
client. SMTP messages are stored and forwarded while HTTP messages are
delivered immediately.
Features
of HTTP:
o
Connectionless protocol: HTTP is a connectionless protocol. HTTP client
initiates a request and waits for a response from the server. When the server
receives the request, the server processes the request and sends back the
response to the HTTP client after which the client disconnects the connection.
The connection between client and server exist only during the current request
and response time only.
o
Media independent: HTTP protocol is a media independent as data can be
sent as long as both the client and server know how to handle the data content.
It is required for both the client and server to specify the content type in
MIME-type header.
o
Stateless: HTTP is a stateless protocol as both the client and
server know each other only during the current request. Due to this nature of
the protocol, both the client and server do not retain the information between
various requests of the web pages.
HTTP
Transactions
PlayNext
The above figure
shows the HTTP transaction between client and server. The client initiates a
transaction by sending a request message to the server. The server replies to
the request message by sending a response message.
Messages
HTTP messages are of
two types: request and response. Both the message types follow the same message
format.
Request Message: The request
message is sent by the client that consists of a request line, headers, and
sometimes a body.
Response Message: The response
message is sent by the server to the client that consists of a status line,
headers, and sometimes a body.
Uniform
Resource Locator (URL)
o
A client that wants to access the document in
an internet needs an address and to facilitate the access of documents, the
HTTP uses the concept of Uniform Resource Locator (URL).
o
The Uniform Resource Locator (URL) is a
standard way of specifying any kind of information on the internet.
o
The URL defines four parts: method, host
computer, port, and path.
o
Method: The method is the protocol used to retrieve the
document from a server. For example, HTTP.
o
Host: The
host is the computer where the information is stored, and the computer is given
an alias name. Web pages are mainly stored in the computers and the computers
are given an alias name that begins with the characters "www". This
field is not mandatory.
o
Port: The
URL can also contain the port number of the server, but it's an optional field.
If the port number is included, then it must come between the host and path and
it should be separated from the host by a colon.
o
Path: Path
is the pathname of the file where the information is stored. The path itself
contain slashes that separate the directories from the subdirectories and
files.
WWW
Working of WWW:
The World Wide Web is based
on several different technologies: Web browsers, Hypertext Markup Language
(HTML) and Hypertext Transfer Protocol (HTTP).
A Web browser is used to
access web pages. Web browsers can be defined as programs which display text,
data, pictures, animation and video on the Internet. Hyperlinked resources on
the World Wide Web can be accessed using software interfaces provided by Web
browsers. Initially, Web browsers were used only for surfing the Web but now
they have become more universal. Web browsers can be used for several tasks
including conducting searches, mailing, transferring files, and much more. Some
of the commonly used browsers are Internet Explorer, Opera Mini, and Google
Chrome.
Features of WWW:
HyperText Information
System
Cross-Platform
Distributed
Open Standards and Open
Source
Uses Web Browsers to
provide a single interface for many services
Dynamic, Interactive and
Evolving.
“Web 2.0”
Components of the Web:
There are 3 components of the web:
Uniform Resource Locator
(URL): serves as a system for resources on the web.
HyperText Transfer Protocol
(HTTP): specifies communication of browser and server.
Hyper Text Markup Language
(HTML): defines the structure, organisation and content of a webpage.
we have understood that WWW
is a collection of websites connected to the internet so that people can search
and share information. Now, let us understand how it works!
o
o
The
Web works as per the internet's basic client-server format as shown in the
following image. The servers store and transfer web pages or information to
user's computers on the network when requested by the users. A web server is a
software program which serves the web pages requested by web users using a
browser. The computer of a user who requests documents from a server is known
as a client. Browser, which is installed on the user' computer, allows users to
view the retrieved documents.
o
o
All
the websites are stored in web servers. Just as someone lives on rent in a
house, a website occupies a space in a server and remains stored in it. The
server hosts the website whenever a user requests its WebPages, and the website
owner has to pay the hosting price for the same.
o
The
moment you open the browser and type a URL in the address bar or search
something on Google, the WWW starts working. There are three main technologies
involved in transferring information (web pages) from servers to clients
(computers of users). These technologies include Hypertext Markup Language
(HTML), Hypertext Transfer Protocol (HTTP) and Web browsers.
Cryptography refers to the
science and art of transforming messages to make them secure and immune to
attacks. It is a method of storing and transmitting data in a particular form
so that only those for whom it is intended can read and process it.
Cryptography not only protects data from theft or alteration but can also be
used for user authentication.
Components
There are various components
of cryptography which are as follows −
Plaintext and Ciphertext
The original message, before
being transformed, is called plaintext. After the message is transformed, it is
called ciphertext. An encryption algorithm transforms the plaintext into ciphertext;
a decryption algorithm transforms the ciphertext back into plaintext. The
sender uses an encryption algorithm, and the receiver uses a decryption
algorithm.
Cipher
We refer to encryption and
decryption algorithms as ciphers. The term cipher is also used to refer to
different categories of algorithms in cryptography. This is not to say that
every sender-receiver pair needs their very own unique cipher for secure
communication. On the contrary, one cipher can serve millions of communicating
pairs.
Key
A key is a number (or a set
of numbers) that the cipher, as an algorithm, operates on. To encrypt a
message, we need an encryption algorithm, an encryption key, and plaintext.
These create the ciphertext. To decrypt a message, we need a decryption algorithm,
a decryption key, and the ciphertext. These reveal the original plaintext.
Types
There are two types of
cryptography which are as follows −
Symmetric Key Cryptography
In symmetric-key
cryptography, the same key is used by both parties. The sender uses this key
and an encryption algorithm to encrypt data; the receiver uses the same key and
the corresponding decryption algorithm to decrypt the data.
Asymmetric-Key Cryptography
In asymmetric or public-key
cryptography, there are two keys: a private key and a public key. The private
key is kept by the receiver. The public key is announced to the public.
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