UNIT – V

Application Layer: Domain name space, DNS in Internet, Electronic Mail,

SMTP, FTP, WWW, HTTP, SNMP.
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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
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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.
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LABEL PURPOSE
AER AIRLINES AND AEROSPACE COMPANIES
O
biz Business firms

com Commercial sites

coop Cooperatives

edu Educational sites

gov Government sites

info Information providers

int International organizations

mil Military groups

museum Museums

name Personal names

net Networking services

org Nonprofits

pro Professionals
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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.

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Introduction:
Electronic mail, commonly known as email, is a method of exchanging messages over the
internet. Here are the basics of email:
1. An email address: This is a unique identifier for each user, typically in the format of
name@domain.com.
2. An email client: This is a software program used to send, receive and manage emails,
such as Gmail, Outlook, or Apple Mail.
3. An email server: This is a computer system responsible for storing and forwarding
emails to their intended recipients.
To send an email:
1. Compose a new message in your email client.
2. Enter the recipient’s email address in the “To” field.
3. Add a subject line to summarize the content of the message.
4. Write the body of the message.
5. Attach any relevant files if needed.
6. Click “Send” to deliver the message to the recipient’s email server.
7. Emails can also include features such as cc (carbon copy) and bcc (blind carbon copy)
to send copies of the message to multiple recipients, and reply, reply all, and forward
options to manage the conversation.
Electronic Mail (e-mail) is one of most widely used services of Internet. This service allows
an Internet user to send a message in formatted manner (mail) to the other Internet user
in any part of world. Message in mail not only contain text, but it also contains images,
audio and videos data. The person who is sending mail is called sender and person who
receives mail is called recipient. It is just like postal mail service. Components of E-Mail
System : The basic components of an email system are : User Agent (UA), Message Transfer
Agent (MTA), Mail Box, and Spool file. These are explained as following below.
 1. User Agent (UA) : The UA is normally a program which is used to send and receive
mail. Sometimes, it is called as mail reader. It accepts variety of commands for
composing, receiving and replying to messages as well as for manipulation of the
mailboxes.
2. Message Transfer Agent (MTA) : MTA is actually responsible for transfer of mail from
one system to another. To send a mail, a system must have client MTA and system
MTA. It transfer mail to mailboxes of recipients if they are connected in the same
machine. It delivers mail to peer MTA if destination mailbox is in another machine.
The delivery from one MTA to another MTA is done by Simple Mail Transfer Protocol.

3. Mailbox : It is a file on local hard drive to collect mails. Delivered mails are present in
this file. The user can read it delete it according to his/her requirement. To use e-mail
system each user must have a mailbox . Access to mailbox is only to owner of
mailbox.
4. Spool file : This file contains mails that are to be sent. User agent appends outgoing
mails in this file using SMTP. MTA extracts pending mail from spool file for their
delivery. E-mail allows one name, an alias, to represent several different e-mail
addresses. It is known as mailing list, Whenever user have to sent a message, system
checks recipient’s name against alias database. If mailing list is present for defined
alias, separate messages, one for each entry in the list, must be prepared and handed
to MTA. If for defined alias, there is no such mailing list is present, name itself
becomes naming address and a single message is delivered to mail transfer entity.
Services provided by E-mail system :
 Composition – The composition refer to process that creates messages and answers.
For composition any kind of text editor can be used.
 Transfer – Transfer means sending procedure of mail i.e. from the sender to recipient.
  Reporting – Reporting refers to confirmation for delivery of mail. It help user to check
whether their mail is delivered, lost or rejected.
 Displaying – It refers to present mail in form that is understand by the user.
 Disposition – This step concern with recipient that what will recipient do after
receiving mail i.e save mail, delete before reading or delete after reading.
Advantages Or Disadvantages:
Advantages of email:
1. Convenient and fast communication with individuals or groups globally.
2. Easy to store and search for past messages.
3. Ability to send and receive attachments such as documents, images, and videos.
4. Cost-effective compared to traditional mail and fax.
5. Available 24/7.
Disadvantages of email:
1. Risk of spam and phishing attacks.
2. Overwhelming amount of emails can lead to information overload.
3. Can lead to decreased face-to-face communication and loss of personal touch.
4. Potential for miscommunication due to lack of tone and body language in written
messages.
5. Technical issues, such as server outages, can disrupt email service.
6. It is important to use email responsibly and effectively, for example, by keeping the
subject line clear and concise, using proper etiquette, and protecting against security
threats.
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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.
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FTP
o FTP stands for File transfer protocol.
o FTP is a standard internet protocol provided by TCP/IP used for transmitting the files
from one host to another.
o It is mainly used for transferring the web page files from their creator to the
computer that acts as a server for other computers on the internet.
o It is also used for downloading the files to computer from other servers.
Objectives of FTP
o It provides the sharing of files.
o It is used to encourage the use of remote computers.
o It transfers the data more reliably and efficiently.
Why FTP?
Although transferring files from one system to another is very simple and straightforward,
but sometimes it can cause problems. For example, two systems may have different file
conventions. Two systems may have different ways to represent text and data. Two systems
may have different directory structures. FTP protocol overcomes these problems by
establishing two connections between hosts. One connection is used for data transfer, and
another connection is used for the control connection.
Mechanism of FTP
The above figure shows the basic model of the FTP. The FTP client has three components:
the user interface, control process, and data transfer process. The server has two
components: the server control process and the server data transfer process.
There are two types of connections in FTP:

o Control Connection: The control connection uses very simple rules for
communication. Through control connection, we can transfer a line of command or
line of response at a time. The control connection is made between the control
processes. The control connection remains connected during the entire interactive
FTP session.
o Data Connection: The Data Connection uses very complex rules as data types may
vary. The data connection is made between data transfer processes. The data
connection opens when a command comes for transferring the files and closes when
the file is transferred.
FTP Clients
o FTP client is a program that implements a file transfer protocol which allows you to
transfer files between two hosts on the internet.
o It allows a user to connect to a remote host and upload or download the files.
o It has a set of commands that we can use to connect to a host, transfer the files
between you and your host and close the connection.
 o The FTP program is also available as a built-in component in a Web browser. This GUI
based FTP client makes the file transfer very easy and also does not require to
remember the FTP commands.
Advantages of FTP:
o Speed: One of the biggest advantages of FTP is speed. The FTP is one of the fastest
way to transfer the files from one computer to another computer.
o Efficient: It is more efficient as we do not need to complete all the operations to get
the entire file.
o Security: To access the FTP server, we need to login with the username and
password. Therefore, we can say that FTP is more secure.
o Back & forth movement: FTP allows us to transfer the files back and forth. Suppose
you are a manager of the company, you send some information to all the employees,
and they all send information back on the same server.
Disadvantages of FTP:
o The standard requirement of the industry is that all the FTP transmissions should be
encrypted. However, not all the FTP providers are equal and not all the providers
offer encryption. So, we will have to look out for the FTP providers that provides
encryption.
o FTP serves two operations, i.e., to send and receive large files on a network. However,
the size limit of the file is 2GB that can be sent. It also doesn't allow you to run
simultaneous transfers to multiple receivers.
o Passwords and file contents are sent in clear text that allows unwanted
eavesdropping. So, it is quite possible that attackers can carry out the brute force
attack by trying to guess the FTP password.
o It is not compatible with every system.
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World Wide Web

In this tutorial, we will be covering the concept of the World Wide Web in Computer
Networks.
The World Wide Web or Web is basically a collection of information that is linked together
from points all over the world. It is also abbreviated as WWW.
 World wide web provides flexibility, portability, and user-friendly features.
 It mainly consists of a worldwide collection of electronic documents (i.e, Web Pages).
  It is basically a way of exchanging information between computers on the Internet.
 The WWW is mainly the network of pages consists of images, text, and sounds on the
Internet which can be simply viewed on the browser by using the browser software.
 It was invented by Tim Berners-Lee.

Components of WWW
The Components of WWW mainly falls into two categories:
1. Structural Components
2. Semantic Components

Architecture of WWW
The WWW is mainly a distributed client/server service where a client using the browser can
access the service using a server. The Service that is provided is distributed over many
different locations commonly known as sites/websites.
 Each website holds one or more documents that are generally referred to as web
pages.
 Where each web page contains a link to other pages on the same site or at other
sites.
 These pages can be retrieved and viewed by using browsers.
In the above case, the client sends some information that belongs to site A. It generally
sends a request through its browser (It is a program that is used to fetch the documents on
the web).
and also the request generally contains other information like the address of the site, web
page(URL).
The server at site A finds the document then sends it to the client. after that when the user
or say the client finds the reference to another document that includes the web page at site
B.
The reference generally contains the URL of site B. And the client is interested to take a look
at this document too. Then after the client sends the request to the new site and then the
new page is retrieved.
Now we will cover the components of WWW in detail.

1.Client/Browser
The Client/Web browser is basically a program that is used to communicate with the
webserver on the Internet.
 Each browser mainly comprises of three components and these are:
o Controller
o Interpreter
o Client Protocols
 The Controller mainly receives the input from the input device, after that it uses the
client programs in order to access the documents.
  After accessing the document, the controller makes use of an interpreter in order to
display the document on the screen.
 An interpreter can be Java, HTML, javascript mainly depending upon the type of the
document.
 The Client protocol can be FTP, HTTP, TELNET.

2.Server
The Computer that is mainly available for the network resources and in order to provide
services to the other computer upon request is generally known as the server.
 The Web pages are mainly stored on the server.
 Whenever the request of the client arrives then the corresponding document is sent
to the client.
 The connection between the client and the server is TCP.
 It can become more efficient through multithreading or multiprocessing. Because in
this case, the server can answer more than one request at a time.

3.URL
URL is an abbreviation of the Uniform resource locator.
 It is basically a standard used for specifying any kind of information on the Internet.
 In order to access any page the client generally needs an address.
 To facilitate the access of the documents throughout the world HTTP generally makes
use of Locators.
URL mainly defines the four things:
  Protocol
It is a client/server program that is mainly used to retrieve the document. A
commonly used protocol is HTTP.
 Host Computer
It is the computer on which the information is located. It is not mandatory because it
is the name given to any computer that hosts the web page.
 Port
The URL can optionally contain the port number of the server. If the port number is
included then it is generally inserted in between the host and path and is generally
separated from the host by the colon.
 Path
It indicates the pathname of the file where the information is located.

4.HTML
HTML is an abbreviation of Hypertext Markup Language.
 It is generally used for creating web pages.
 It is mainly used to define the contents, structure, and organization of the web page.

5.XML
XML is an abbreviation of Extensible Markup Language. It mainly helps in order to define
the common syntax in the semantic web.
Features of WWW
Given below are some of the features provided by the World Wide Web:
 Provides a system for Hypertext information
 Open standards and Open source
 Distributed.
 Mainly makes the use of Web Browser in order to provide a single interface for many
services.
 Dynamic
 Interactive
 Cross-Platform
Advantages of WWW
Given below are the benefits offered by WWW:
 It mainly provides all the information for Free.
 Provides rapid Interactive way of Communication.
 It is accessible from anywhere.
 It has become the Global source of media.
 It mainly facilitates the exchange of a huge volume of data.

Disadvantages of WWW
There are some drawbacks of the WWW and these are as follows;
 It is difficult to prioritize and filter some information.
 There is no guarantee of finding what one person is looking for.
 There occurs some danger in case of overload of Information.
 There is no quality control over the available data.
 There is no regulation.
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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

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.
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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.
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