Computer networks unit v

RMK College of Engineering and Technology
CS 6551
Computer Networks
Department
of
Electronics and Communication Engineering

Unit 5
Application Layer
Prepared by
Jai Ganesh S
Asst.Professor - ECE

Syllabus
• Traditional applications
• Electronic Mail
• SMTP – Simple Mail Transfer Protocol
• POP 3
• IMAP
• MIME
• HTTP – Hyper Text Transfer Protocol
• Web Services
• DNS – Domain Name System
• SNMP – Simple Network Management Protocol

Introduction
• Applications – that are given to the END
USER.
• Every application is developed for a purpose
and have its own style of working

Traditional Applications
• Among all the applications our study focuses on 2 main traditional
applications. They are :

Reason for Traditional Applications
• We call these application as traditional application because they existed
since the early days of computer networks.

Before starting the Topic…
• There are 3 general points that have to be made clear.
1. Distinguish between the Application Program and Application Protocols
2. Each protocol reinvents the Simple Remote procedure call (RPC) mechanism (it is called
simple because it only specifies the procedure, format is taken care by companion
protocol)
3. Each protocol will have a companion protocol which defines the format of the contents.

Distinguish between the Application Program and Application
Protocols
• example

Each protocol reinvents the Simple Remote procedure call (RPC)
mechanism
• Remote Procedure Call (RPC) is a protocol that one program can use to request a
service from a program located in another computer in a network without having
to understand network details. (A procedure call is also sometimes known as a
function call or a subroutine call.)
• RPC uses the client/server model.

• Most of the application programs works on Query – Respond Principle.
• This RPC specifies, which mechanism to be followed to fetch information
from the source.

Each protocol will have a companion protocol
• This companion protocol specifies the format of the information to be
exchanged.

overview
• Application Program:
• Which is available to the end user
without knowing any knowledge
about the network.
• Application Protocol:
• Which specifies the RPC
mechanism to fetch information
available at remotely located
systems.
• Companion Protocol:
• Which specifies the format
of the data that are to be
exchanged.

E - Mail
• This is the one of the oldest applications.
• It is mandatory to distinguish between the User Interface – Transfer
Protocols – Companion Protocol

What happens when you press
Send Button
EMAIL CLIENT
MIME
EMAIL CLIENT

E - Mail
• Message Format – MIME
• Message Transfer – SMTP
• Mail Reader – POP / IMAP

Message Format
• Message format is defined by the companion protocols. Two majorly used
protocols are given below.
• RFC 822 - Standard for the format of ARPA Internet text messages
• MIME - Multipurpose Internet Mail Extensions.

RFC 822
• This defines 2 parts
• Header
• Body

• Few headers are obtained by the user.
• Ex:
• TO address
• CC address
• BCC address
• Sub Text
• Few headers are added automatically by the program
• Ex:
• From Address
• Mail Server
• Date and Time
Syntax:
<Header Type> : <Header Value>

MIME
• Multipurpose Internet Mail Extensions.
• This is an supplementary protocol that allows Non ASCII data to be sent through E Mail
• This enabled the email to carry any type of attachments like documents, pdf, image, etc.,

MIME Headers
• MIME mainly consists of 5 header:
• MIME - Version
• Content type
• Content transfer encoding
• Content ID
• Content Description

Data types and subtypes in MIME

Introduction
• Once the mails are composed they are transferred to the destination using
Mail Transfer Protocols.
• Some of the popular Mail Transfer Protocols are
• SMTP – Simple Mail Transfer Protocol
• POP – Post Office Protocol
• IMAP – Internet Message Access Protocol

SMTP
• The actual mail transfer is done through the Mail Transfer Agents.
• The protocol that defines the MTA Client and Server is called SMTP.
• SMTP is used 2 Times
• Between sender and mail server.
• Between 2 mail servers.

• SMTP simply defines how commands and responses must be sent back
and forth.

Message Reader / Message Access

Introduction
• SMPT is not involved in the third stage because SMTP is a Push Protocol.
• It pushes the message from the client to the server.
• The third stage need the Pull Protocol.
• The client must pull message from the server.
• Two of the popularly used message access agents are POP3 and IMAP

POP 3
• Post Office Protocol Version 3 (POP 3)
• Client POP 3 will be installed in the recipient’s computer and the server POP 3
is installed at the server.
• Message access will be initiated when the user wants to download the mail from
the server.

Modes of POP 3
• POP 3 have 2 modes
• Delete Mode
• In this mode, the mail is deleted from the mailbox after each retrieval.
• Keep Mode
• In this mode, the mail is kept is the personal computer for future access.

Exchange of Commands and Responses in POP 3

IMAP 4
• Internet Mail Access Protocol, Version 4
• IMAP 4 is similar to the POP 3 but with added features.
• The features are:
• A user can check the email header prior to downloading.
• A user can search the content of the email for a specific string
• A user can download partial email due to bandwidth constraints.
• User can create delete or rename mail boxes
• User can create hierarchy of mail boxes.

My Official Account – POP 3 Setting

My Official account - IMAP Settings

Video Presentation on E - Mail

Introduction
• The WWW is a repository of information
linked together from points all over the
world.
• The original goal of the web was to find a way to organize and retrieve information about
hyper linked documents.
• WWW uses HTTP Protocol to access the webpages on the internet. (Will be discussed
shortly)

Clients (Browsers)
1. Controller – Receives i/p from keyboard / mouse and uses the client program to access the documents.
2. Client protocol – This can be of any protocol discussed before like FTP, HTTP Etc.,
3. Interpreters – This can be any interpreter like java, Flash or HTML to display the appropriate content
CLIENT
(BROWSERS)
 There are variety of browsers available that interprets and displays
a web document.
 Every client consists of three parts

Server
• The webpage is stored at the server
• Each time the request is received the corresponding document
is sent to the client.
• The server can be more efficient by enabling “Multithreading”
(Answering Multiple requests)

URL – Uniform Resource Locator
• When a page needs to be accessed it must have a address.
• HTTP uses locators to access the webpages available at widely distributed areas.
• URL is a standard for specifying any kind of information on the internet.
• URL defines 4 Important things:

HTTP – Hyper Text Transfer Protocol

Introduction
• The Hyper Text Transfer Protocol (HTTP) is mainly used to access the data on the
world wide web.
• HTTP is the combination of FTP and SMTP.
• It is like FTP because it uses TCP Connection.
• It is like SMTP because data transferred between the server and client looks like
SMTP messages.

HTTP Transaction
• The client initiates the transaction by sending a request message and the
server replies through an response message.

Formats of Request and Response Messages

Status Code
• This field indicates the status of the request through codes.
• Codes in the range of
• 100  informational
• 200  Successful Request
• 300  Redirect the client to another URL
• 400  Error at client
• 500  Error at server

Status Codes and their Phrases

Header
• The format of the header is shown in the figure.
• The request header and Response header have different values.

Simple Network Management Protocol - SNMP

Computer networks unit v

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