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Notes of Computer Networks and Security [CT 702]

Application Layer

Application Layer

- Application layer is the top most layer in OSI model which is responsible for interacting with user and user applications.
- It is the layer where actual communication is initiated and reflected.
- Irrespective of software used, it is the protocol which is considered at application layer used by that software.

Web - HTTP

Web Server

- Web server is a computer system that processes requests via HTTP protocol, used to distribute information on WWW.
- It stores, processes and delivers web pages to clients.
- The communication between client and server takes place using HTTP.

Hyper Text Transfer Protocol (HTTP)

- HTTP is an application protocol for distributed, collaborative and hypermedia information system used for data communication in WWW.
- It acts as a request-response protocol in client-server computing model.
- A request message consists of request line, request header field, empty line and optional message body.
- A response message consists of status line, response header field, empty line and optional message body.

Example Communication over HTTP


Consider that a http client be web browser which requests


Client request:
- A client sends a request in request message as:
GET / HTTP/1.1 (Request line)
Host: (Request header field)
(Empty line)

Server response:
HTTP/1.1 200 OK
Date: Mon, 23 May 2016 22:38:45 GMT
Content-Type: text/html; charset = UTF-8
Content-Encoding: UTF-8
Content-Length: 138
Last Modified: Wed, 11 April 2015 11:22:32 GMT
Server: Apache/
Etag: “3f80f-1b6-3e1cb03b”
Accept-Ranges: bytes
Connection: close

< html >
< head >
< title > Egnite Notes < / title >
< / head >
< body >
Welcome to Egnite Notes.
< / body >
< / html >


File Transfer - FTP, PuTTY, WinSCP

File Transfer Server

- A file server is a computer responsible for control storage and management of data files.
- It allows users to share information over a network.

File Transfer Protocol (FTP)

- FTP is a standard network protocol used for transfer of files from a server to a client using client-server architecture on a network.

Communication in FTP

- FTP may run in active or passive mode, which determines how data connection is established.
- In active mode, client starts listening for incoming data connections from server on port M. It sends FTP command PORT M to inform server on which port it is listening. The server then initiates data channel to the client from its port 20.
- In passive mode, the client uses control connection to send PASV command to the server and recieves server IP address and server port no from the server. The client then uses to open a data connection from an arbitary client port to server IP address and server port no received.


- PuTTY is a free and open source file transfer application.
- It supports network protocols like SCP, SSH, Telnet, rlogin.
- It allows local, remote or dynamic port forwarding with SSH.
- It provides user control over SSH encryption key and protocol version.
- It comes bundled with command line SCP client (pscp) and SFTP client (psftp).

WinSCP (Windows Secure Copy)

- It is a free and open source SFTP, FTP, WebDAV and SCP client for Microsoft Windows.
- It allows secure file transfer between a local and a remote computer.
- It is based on implementation of SSH protocol from PuTTY and FTP protocol from FileZilla.


Electronic Mail

- Mail server is the computer system that is responsible to forward mails towards its intended recipient.
- Every email that is sent passes through a series of mail servers before reaching recipient.
- Without the series of mail servers, it would be possible to send emails within same domain only.

Process of sending email

- The email client connects to the domain’s SMTP server.
- The email client communicate with SMTP and provides your email address, recipient email address and message body.
- The SMTP server processes the recipient’s email address. If domain of sender and receiver is same, the message is routed directly over to domain’s POP3 or IMAP server. Otherwise, SMTP server communicates with other domain’s server.
- SMTP server communicates with DNS to find recipient’s server and the DNS provides IP address.
- The SMTP server can connect to recipient SMTP server by routing messages along a series of unrelated SMTP servers.
- The recipient SMTP server scans incoming message and forwards to domain’s POP3 or IMAP server if it recognizes the sender’s domain and username.

Simple Mail Transfer Protocol (SMTP)

- It is an email transmit text based protocol which moves the email on and across networks using a process called ‘store and forward’.
- It works with Main Transfer Agent to send communication to the right computer.
- It provides a set of codes that simply communicate email messages between email servers.
- When you send out a message, it is turned into strings of text separated by code words that identify the purpose of each section.
- It provides those codes to servers.
- Email server software helps to understand their meaning.
- As message travels towards destination, it passes through a no of computers.
- Each computer stores it before moving on to next computer in the path.
- It is able to transfer text only.

Support for images in SMTP

- Multipurpose Internet Mail Extensions can be used to support image messages in SMTP.
- MIME encodes the non-text content into plain text, which can be transmitted via SMTP.
- It consists of MIME header.
- MIME header includes:
a) MIME-Version = indicates MIME formatted messages
b) Content-Type = indicates media type of message content. For image; image/png
c) Content-Disposition = specify presentation styles of mail messages.
d) Content-Transfer-Encoding = indicates whether or not binary to text encoding scheme is used.
- The non ASCII data uses MIME encoded word syntax.
- The syntax uses string of ASCII characters indicating both original character encoding (charset) and content-transfer-encoding used to map bytes of charset into ASCII characters.

Post Office Protocol 3 (POP3)

- It is the standard client/server protocol for receiving emails.
- Email is received and held for the user by the Internet server.
- Periodically, the user check their mail box on the server and download any mail.
- As soon as the user downloaded the mail, POP3 deletes the mail on the server.
- It is a kind of ‘store and forward’ service.

Internet Message Access Protocol (IMAP)

- it is the standard protocol for receiving emails in which the stored messages on the mail server can be viewed and manipulated by the end users as though they are stored locally.
- Users can organize messages into folders on the server.
- It is a kind of remote file server.
- It also supports multiple logins.

DNS Concept

Domain Name Server

- DNS is a protocol that helps and supports protocols used by users and works on client/server model.
- It uses UDP protocol for transport layer communication,
- It uses hierarchical domain based naming scheme.
- A DNS server is configured with fully qualified domain names (FQDN) and email addresses mapped with their respective IP addresses.

Working of DNS

- When you type into your browser, the browser sends a query over Internet to find website for the requested URL. The query is forwarded firstly to recursive resolver.
- The recursive resolver takes to root server. It provides DNS information about top level domain such as .com
- The TLD DNS server stores address info for second level domains. When query reaches to TLD server, it provides IP address of required DNS.
- The query is then sent to DNS which knows the IP address of full requested domain and is returned to recursive resolver.
- The recursive resolver provides the IP address to the browser.
- Finally, the browser sends a request to IP address of website to retrieve web contents.

Socket Programming

- It is the process of network programming that provides API and interfaces to handle sockets for communicating using standard mechanisms built into network hardware and OS.

Proxy Caching

- It is the feature of proxy servers that stores content on the proxy server itself, allowing web services to share those resources to more users.
- It allows a server to act as an intermediary between a user and a provider of web content.



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