Data Communication & Computer Networks

(Course Code: 3143201), Sem.- IV

Unit- 6
Application Layer Protocols

Dr. Mani Shekhar Gupta,

Department of Information & Communication Technology
Adani University
Gujarat, India

May 10, 2023

 Internet Applications
⚫ Internet: The Internet is a global web
of computers connected to each
router workstation
other by wires, (mostly phone lines).
It is hierarchical “network of server
networks” mobile
⚫ Major Components: Hosts, Routers, local ISP
Communication links
⚫ Protocols: for sending, receiving of
messages e.g., Telnet, HTTP, FTP,
regional ISP
DNS, PPP
⚫ Internet standards
⚫ RFC: Request for comments
⚫ IETF: Internet Engineering Task
Force
company
network

2
 Internet Applications
WHAT IS AN INTERNET APPLICATION?

Internet application is an interactive, compiled

application that can be accessed through a corporate.

Internet applications can perform complex business

processes on either the client or the server.

In a server-based Internet application, the application

uses the Internet protocol to receive requests from a
client, typically a Web browser, process associated
code, and return data to the browser.
Some Internet applications
⚫ E-mail ⚫ Internet telephone
⚫ Web ⚫ Real-time video
⚫ Instant messaging conference
⚫ Remote login ⚫ Massive parallel
⚫ P2P file sharing computing
⚫ Multi-user network games
⚫ Streaming stored video
clips
INTERNET TERMS
⚫ WWW
⚫ Browser
⚫ Search Engine
⚫ URL
⚫ Domain
⚫ Html
⚫ WWW: The World Wide Web "The World Wide Web is the universe
of network-accessible information, an embodiment of human
knowledge."

⚫ BROWSERS: Software program that allows a person to view WWW

documents. Examples of browsers are Netscape, Microsoft Internet
Explorer, Mosaic, Mac web, and Net cruiser.

⚫ SEARCH ENGINES: A web site that will help you search the
Internet for key words, subjects. Search engines are programs that
search documents for specified keywords and returns a list of the
documents where the keywords were found. A search engine is really
a general class of programs, however, the term is often used to
specifically describe systems like Google, Bing and Yahoo! Search
that enable users to search for documents on the WWW.
⚫ URL UNIFORM RESOURCE LOCATOR: The unique address of
any web page. It tells your computer where the information is stored
so it can be viewed.

⚫ DOMAIN: A way to indicate what type of site you may be viewing.

Some common domains are .com-commercial, .org- non-profit and
research organizations, .gov- government agency, .edu-education.
More are constantly being added so theses should only be used as
guidelines to help you know what type of site you are on.

⚫ HTML: Standard hyper text markup language used to create web

pages.
 APPLICATIONS OF INTERNET

⚫ Communication
⚫ Job Searches
⚫ Finding books & Study materials
⚫ Health & Medicine
⚫ Travel
⚫ Entertainment
⚫ Shopping
⚫ Stock market updates
⚫ Research
⚫ Business
Electronic Mail

Sending/Receiving Mail
Addresses
User Agent
MIME
Mail Transfer Agent
Mail Access Protocols
 Electronic Mail

» Electronic mail or email is one of the most popular

network service.
» Electronic mail is used for sending a single
messages that includes:
» Text,
» Voice
» Video
» Image
» To one or more recipients.

» Simple Mail Transfer Protocol (SMTP) is standard

mechanism for electronic mail in Internet.
 Electronic Mail → Sending/Receiving Mail → Sending Mail:

» To send mail, the user creates mail that looks very similar to postal mail.
» It has an envelope and a message as shown in Fig. below:

Figure: Format of an email

 Electronic Mail → Sending/Receiving Mail:

Sending Mail
Envelop:
» The envelop usually contains the sender address, the receiver address
and other information.

Message:
» The message contains the headers and the body part.
» The headers of the message defines the sender, the receiver, the subject
of the message and other information.
» The body of the message contains the actual information to be read by
the recipient.
Receiving Mail
» The email system periodically checks the mailboxes.
» If a user has mail, it inform the user with a notice.
» If the user is ready to read the mail, a list displayed in which each line contains
a summary of the information about a particular message in the mailbox.
» The summary usually includes:
» The sender mail address,
» The subject,
» And the time the mail was sent or received.
» The user can select any of the message and display its contents on the
screen.
 Electronic Mail → Addresses:

» To deliver mail, a mail handling system must use an addressing

system with unique addresses.

» The addressing system used by SMTP consists of two parts:

1. A local part and
2. A domain name.
» Separated by an @ sign, as shown in Fig. below:

Local Part:
» The local part defines the name of a special file, called the user mailbox,
where all the mail received for a user, is stored for retrieval by the user agent.

Domain Name:
» The second part of the address is the domain name.
» An organization usually selects one or more hosts to receive and send email;
they are sometimes called mail exchanges.
» The domain name assigned to each mail exchanger either comes from the
DNS database or is logical name (e.g., the name of the organization).
 Electronic Mail → User Agent (UA):

» User Agent (UA) is the first component of Electronic Mail system.

» A User Agent is sometimes called a mail reader, but this terminology is
confusing, so we prefer to use the term User Agent.

» Services Provided by a User Agent:

» A User Agent is a software package (program) that
» Composes,
» Reads,
» Replies to and
» Forward messages.
» It also handles mail boxes.
» Fig. below shows the services of a typical user agent.

Figure: User agent

 Electronic Mail → Mail Transfer Agent (MTA) :

» The actual mail transfers done through Mail Transfer Agents (MTAs).
» To send mail, a system must have a client MTA and
» To receive a mail, a system, must have a server MTA.

» In Internet, message transfer is done through protocol (and software) named

Simple Mail Transfer Protocol (SMTP).
» To send a message, we need a client SMTP and a server SMTP.

» Fig. below Alice is shown sending email to Bob with the SMTP clients and
server needed.

Figure MTA client and server

 Electronic Mail → Mail Access Protocols:

» The first and second stages of mail delivery use SMTP.

» However, SMTP is not involved in the third stage.
» Because SMTP is a push protocol: it pushes messages from sender
to receiver even if the receiver does not want it.
» The operation of SMTP starts with sender, not the receiver.

» On the other hand, the third stage needs a pull protocol: the
operation must start with the recipient.
» The mail must stay in the mail server mailbox until the recipient
retrieves it.

» The third stage uses a Mail Access Protocol.

» Currently two mail access protocols are available:

1. Post Office Protocol, version 3. (POP3) and
2. Internet Mail Access Protocol, version 4 (IMAP4).
Different scenarios in electronic mail

First scenario in electronic mail

When the sender and the receiver of an e-mail are on the same system, we
need only two user agents.
Second scenario in electronic mail

When the sender and the receiver of an e-mail are on different systems, we
need two UAs and a pair of MTAs (client and server).
Third scenario in electronic mail

When the sender is connected to the mail server via a LAN or a WAN, we
need two UAs and two pairs of MTAs (client and server).
Fourth scenario in electronic mail

When both sender and receiver are connected to the mail server via a LAN or
a WAN, we need two UAs, two pairs of MTAs and a pair of MAAs.
This is the most common situation today.
 MIME: Multipurpose Internet Mail Extensions

Multipurpose Internet Mail Extensions:

» SMTP is a simple mail transfer protocol.

» Its simplicity, however comes with a price.
» SMTP has some limitations e.g.
» SMTP can send messages only in 7-bit ASCII format.
» SMTP cannot be used for languages that are not supported
by 7-bit ASCII characters (such as French, German, Russian
Chinese and Japanese).
» SMTP cannot be used to send binary files (files that are
stored as stream of 0s and 1s without using any character
code), video and audio.

» MIME is a supplementary protocol that allows non-ASCII data to

be sent through SMTP.
» MIME is not a mail protocol and cannot replace SMTP.
» MIME is only an extension to SMTP.
» MIME transforms non-ASCII data at the sender site ASCII data and
delivers to the client SMTP to be sent through the Internet.
» The server SMTP at the receiving side receives the ASCII data and
delivers them to MIME to be transformed to the original data.

» We can think of MIME as a set of software functions that transforms

non-ASCII data to ASCII data and vice versa, as shown in Fig. below:

Figure: MIME
» MIME defines five headers that can be added to the original SMTP
header section to define the transformation parameters:
1. MIME-Version.
2. Content-Type.
3. Content-Transfer-Encoding.
4. Content-Id.
5. Content-Description.
» Fig. below shows the original header and the extended header.

Figure: MIME header

File Transfer Protocol

Connections
Communication
File Transfer
User Interface
 File Transfer Protocol :

» File Transfer Protocol (FTP) is the standard mechanism provided by the Internet for
copying a file from one host to another.
» Transferring files from one computer to another is one of the most common tasks
expected from a networking or inter-networking environment.
» Although transferring files from one system to another seems simple and straight
forward, but there are some problems:
» What if the two systems are using different file name conventions?
» What if two systems have different ways to represent text and data?
» What if two systems have different directory structure?
» All these and other problems are solved by FTP in a very simple manner.

» FTP differs from other client-server applications in that it establishes two connections
between the client and the server.
1. One connection is used for data transfer,
2. The other is for control information (commands and responses).
» Separation of commands and data transfer makes FTP more efficient.
» The control connection uses very simple rules of communication.
» We need to transfer only one line of command or a line of response at a time.
» The data connection, needs more complex rules due to variety of data types
transferred.
» FTP uses two well-known TCP ports:
1. Port 20: used for data connection,
2. Port 21: used for control connection.
 File Transfer Protocol (Structure) :

» Fig. below shows the basic model of FTP.

» The client has three components:
1. User Interface,
2. Client Control Process and
3. Client data transfer process.
» The server has two components:
1. Server control process and
2. Server data transfer process.
» The control connection is made between the control processes.
» The data connection is made between the data transfer processes.

Figure FTP
 File Transfer Protocol (Working) :

» The control connection is maintained during the entire interactive FTP

session.
» The data connection is opened and then closed after each file is transferred.
» It opens each time commands that involve transferring files are used,
» And it closes when the file is transferred.

» We can say that, when FTP session starts,

» The control connection opens,
» While the control connection is open, the data connection can be
opened and closed multiple times if several files are transferred.

Figure FTP
 FTP → File Transfer :

» File transfer occurs over the data connection under the control of the
commands sent over the control connection.
» However, remember that file transfer in FTP means one of three
things as shown in Fig. below:
1. A file is to be copied from the server to the client.
» This is called file retrieving a file.
2. A file is to copied from the client to the server.
» This is called storing a file.
3. A list of directory of file names is to be sent from the server to the
client.
» FTP treats a list of directory or file name as a file.
» It is sent over the data connection.

Figure File transfer

Simple Network Management
Protocol (SNMP)
What is it?
A Protocol that Facilitates the exchange of management information
between network devices.

Why was it developed?

To control and monitor status of network devices

How is it beneficial?
Enables network administrators to:
❑ Manage network performance
❑ Find and solve network problems
❑ Plan for network growth
 SNMP Functions and Design Issues
⚫ Configuration management
⚫ Performance management
⚫ System tuning and capacity planning
⚫ Fault management
⚫ Security
⚫ Accounting management
⚫ Plan, organize, direct, control, staff the voice/data network operation
⚫ Manage operations and provide support
⚫ Ensure reliability
⚫ Manage LANs and connections to backbone
⚫ Technological developments
⚫ Manage the communications budget
⚫ Evaluate and acquire communications hardware, software, and services
SNMP Basic Components

⚫ Network Management Station

⚫ Collects and stores management information, and makes this
information available to Network Management System using SNMP
⚫ It could be a work station or PC
⚫ Network Management System
⚫ Executes applications that monitor and control managed devices
⚫ Agent
⚫ A network-management software module that resides in a managed
device
⚫ Management Information Base (MIB)
⚫ Used by both the manager and the agent to store and exchange
management information
 NMS

User Interface
Network
Management
Station Network
Management
Management Architecture
Application

SNMP
AGENT AGENT AGENT

MIB MIB MIB

Managed
Devices
Router
Host Printe
 How SNMP Works

Get-request
Get-next-request
Set-request

Get-response
Network MIB
Management Station SNMP Agent
Traps

Get-request: Request value of variable.

Get-next-request: Request the value of next variable (like a table).
Set-request: update one or more variables (for reconfiguration).
Get-response: agents sends a reply to manager.
Traps: agent notifies manager about an abnormal situation.
With a Network Management System, you can monitor a large network
from a central console.
CMIP (common management information
protocol)
⚫ Cross-layer functions:
⚫ Management functions- Functions that permit to configure, monitor,
terminate the communications of two or more entities: there is a
specific application layer protocol, common management information
protocol (CMIP) and its corresponding service, common management
information service (CMIS), they need to interact with every layer in
order to deal with their instances.

⚫ CMIP is the network management protocol specified by the ISO/OSI

Network management model and is further defined by the ITU-T.

⚫ CMIP models management information in terms of managed objects

and allows both modification and performing actions on managed
objects.
⚫ Managed objects are described using GDMO (Guidelines for the
Definition of Managed Objects), and can be identified by a
distinguished name, from the X.500 directory.
⚫ CMIP play a similar role to that of SNMP in the protocol stack.
⚫ Protocol data unit and maps it to a remote operations service element
(ROSE).
⚫ Communication between distributed applications in the ISO model.

⚫ CMIP provides more detailed representation and the ability to create

objects and inherit from other objects.
⚫ CMIP also provides good security (support authorization, access control,
and security logs) and flexible reporting of unusual network conditions.
⚫ CMIP is implemented in association with the Association Control Service
Element (ACSE) and ROSE protocols. ACSE is used to manage
associations between management applications (i.e. manage connections
between CMIP agents). ROSE is employed for all data exchange
interactions.
⚫ There is also a form of CMIS that is developed to operate directly on top of
the LLC sublayer. It is called the LAN/MAN Management Protocol
(LMMP), formerly it was the Common Management Information Services
and Protocol over IEEE 802 Logical Link Control (CMOL).
HTTP

The Hypertext Transfer Protocol (HTTP) is a protocol

used mainly to access data on the World Wide Web.
Client-Server model
URL

http://
HTTP transaction

HTTP uses the services of TCP on well-

known port.
Request and response messages (all in Plain Text)
Methods
Status codes
Status codes (continued)
Header format
Persistent vs Nonpersistent
Connection
⚫ In a nonpersistent connection, one TCP connection
is made for each request/response.
⚫ In a persistent connection, the server leaves the
connection open for more requests after sending a
response.
⚫ The server can close the connection at the request of a
client or if a time-out has been reached.