Exploring the Application Layer: DNS, FQDN, SMTP, Web-Based Email, and FTP Explained

The Application Layer
BY PROF. TRUPTI SISODE

Application Layer
 The Application Layer is the topmost layer of the OSI (Open Systems Interconnection) model
and the TCP/IP model. It serves as the interface between the user and the network by providing
services that directly interact with user applications.
 Key Functions of the Application Layer:
1. User Interaction: It provides the interface for user applications to interact with the network (e.g.,
web browsers, email clients).
2. Network Resource Access: Manages access to network resources, such as remote files or
databases.
3. Data Formatting and Representation: Ensures data is in the correct format for the application
(e.g., file formats like .jpeg or .html).
4. Communication Management: Handles communication services like email, file transfer, remote
login, and more.

Examples of Application Layer Protocols:
•HTTP/HTTPS: Used by web browsers to access websites.
•SMTP: Used for sending emails.
•FTP: Used for file transfers.
•DNS: Translates domain names into IP addresses.

Domain Name System
The Domain Name System (DNS) is a fundamental part of the internet that translates human-readable domain names
(like www.example.com) into IP addresses (like 192.0.2.1), which computers use to identify each other on the network.
1. Flat Namespace:
If you were using a flat namespace for employee names in the entire college, all employees would need to
have unique names across the entire organization. For example:
•mohan_IT
•mohan_CS
In this flat namespace:
•Single Scope: All names must be unique across the entire organization, leading to potentially confusing or
non-intuitive naming if multiple departments or contexts are involved.
2. Hierarchical namespace:
Example with College Departments:
Hierarchical Namespace:
1.College: Example College
•Department 1: IT Department
•Employee: mohan
•Department 2: CS Department
•Employee: mohan

Lables
 Conditions for lables:
 Labels are the individual components of a domain name.
 Alphanumeric: Labels can include letters (A-Z), numbers (0-9),
and hyphens (-).
•Length: Labels can be up to 63 characters long, and the total length
of a domain name cannot exceed 253 characters.
•Case Insensitive: Domain names are case-insensitive, meaning
Example.com is equivalent to example.com.

Domain name
Represented by a dot (.), it's the highest level of the hierarchy.
•Root Domain: .
•TLD: com, edu
•Second-Level Domain: mycollege
•Subdomains:
•compsc (subdomain of compsc.mycollege.com)
Pg(subdomain of pg.compsc.mycollege.com)

Domain name can be two types:
1.FQDN 2. PQDN
 1. Fully Qualified Domain Name (FQDN)
• Definition: An FQDN is a complete domain name that specifies the exact location of a resource within
the DNS hierarchy. It includes all levels of the domain name, from the specific hostname to the top-level
domain (TLD), and it is fully qualified with respect to the root domain.
Example: mail.google.com.
•Hostname: mail (indicates a specific service, like Google’s email service)
•Second-Level Domain: google (the company’s main domain)
•Top-Level Domain (TLD): com (indicates the domain type, commercial)
•Root: The trailing dot (.) represents the root of the DNS hierarchy, though it's usually implicit.
Explanation:
•Usage: To access Google’s email service, you would use the FQDN mail.google.com. This fully qualified
domain name specifies the exact location within the DNS hierarchy, ensuring that your request is directed to
Google's email servers.

2. PQDN
 2. Partially Qualified Domain Name (PQDN)
 Definition: A PQDN is a domain name that is not fully qualified. It may be missing one or more components of
the complete FQDN, which means it does not specify the exact location within the entire DNS hierarchy.
Example: mail.google
•Hostname: mail
•Second-Level Domain: google
Explanation:
•Usage: Internally within Google’s network or in contexts where the TLD is understood or assumed, mail.google
might be used. For instance, within Google’s internal systems or documentation, this partial name could be
sufficient, as the full domain context (.com) is understood or implied.
 We use Partially Qualified Domain Names (PQDNs) for convenience. They let users type shorter names, and
the system fills in the missing parts (like .com or .org) based on local network settings. This is especially useful
in internal networks to save time and effort.

1. Generic Domains
 Definition: Generic domains are top-level domains (TLDs) that are not associated with any
particular country. They are used for general purposes and are typically organized by category or
type.
 Examples:
• .com: Originally intended for commercial entities, but now used by all types of websites.
• .org: Generally used by non-profit organizations.
• .net: Originally intended for network-related entities, but now used widely.
• .edu: Reserved for educational institutions.
 Purpose: These domains help categorize websites based on their purpose or type rather than their
geographic location.
 Usage: Generic domains are used by businesses, organizations, and individuals around the world.
DNS in Internet

2. Country Code Domains
 Definition: Country code top-level domains (ccTLDs) are two-letter domains assigned to specific
countries or territories. They are used to represent geographic locations.
 Examples:
• .uk: United Kingdom
• .jp: Japan
• .in: India
• .au: Australia
 Purpose: These domains help identify websites that are associated with a particular country or
region. They often reflect local businesses, services, or organizations.
 Usage: Country code domains are used by entities within the respective countries or those targeting
a specific national audience.

3. Inverse Domains
Definition: Inverse domains, also known as reverse domains, are used for reverse DNS lookups. They map IP
addresses back to domain names. This is the opposite of the usual forward DNS lookup, which maps domain names
to IP addresses.
Examples:
•For IPAddress 192.0.2.1, the reverse lookup domain might be 1.2.0.192.in-addr.arpa.
Purpose: Inverse domains are used for tasks such as verifying the authenticity of an IP address and ensuring that IP
addresses resolve to their associated domain names.
Usage: Inverse domains are commonly used in network management and security to check if an IP address is
correctly associated with a domain name.
•.arpa: This is a special top-level domain (TLD) that stands for "Address and Routing Parameter Area." It is used
exclusively for technical infrastructure purposes in the DNS system.
•in-addr: This is short for "inverse address", and it indicates that the domain is being used for reverse lookups,
specifically for IPv4 addresses.

Resolution
The process of mapping name to an address and an address to a name is called Address
Mapping.
Resolution in the context of DNS (Domain Name System) refers to the process of converting a
human-readable domain name (like google.com) into its corresponding IP address (like
142.250.190.14), which is required for computers to locate and communicate with each
other on the internet.
This process involves querying DNS servers to find the IP address associated with the domain.
There are two types of resolution:
1.Recursive Resolution: The DNS server does all the work, querying other servers on behalf of
the client.
2.Iterative Resolution: The client is directed by each DNS server on where to ask next until it
finds the answer.

Recursive Name Address Resolution
Recursive DNS Resolution This process happens when your computer wants to visit a website, but
it doesn't know the website's IP address (like for google.com), so it asks for help from DNS servers.
Steps:
1.Your Computer (Client): You type google.com into your browser, and your computer asks the
local DNS server (let's say compcs.edu) for the IP address.
2.Local DNS Server: If compcs.edu doesn't know the IP, it asks the root DNS server for help.
3.Root DNS Server: The root DNS server doesn’t know google.com either, but it knows who can
help, so it sends compcs.edu to the .com DNS server.
4..com DNS Server: This server knows about websites that end with .com, so it points to the
Google DNS server.
5.Google DNS Server: Finally, the Google DNS server gives the exact IP address for google.com.
6.Back to Your Computer: The IP address is sent back through all the servers until it reaches your
computer. Now you can visit the website!

Recursive Name Address Resolution

Iterative Resolution
 Iterative DNS Resolution:
1. Client (Your Computer) asks compsc.edu for the IP.
2. compsc.edu doesn’t know, tells the client to ask the root server.
3. Root server sends the next step to try the .com DNS server.
4. .com DNS server directs to the Google DNS server.
5. Google DNS server gives the IP.
 In each step, the server responds quickly with where to go next, and the
client keeps asking until it gets the IP address.

Caching:
 Caching in resolution typically refers to the process of storing the results of previous queries or
computations to avoid redundant calculations and speed up future resolutions. This concept is widely
used in various fields, including computer science, web development, and networking.
DNS Name Caching
•DNS Resolver Cache: When a DNS resolver translates a domain name (like example.com) into its
corresponding IP address, it stores this mapping in a cache. If the same domain name is queried again
within a certain period (known as the Time-To-Live or TTL), the resolver returns the cached IP address
instead of querying the authoritative DNS servers again. This speeds up the resolution process and
reduces DNS traffic.

Email:
 An email (electronic mail) is a method of exchanging digital messages over the
internet. It allows users to send and receive text, images, files, and other types of
media through their email accounts. Email systems consist of two main components:
1. User Agents (UA): These are the applications or interfaces that help users compose,
send, receive, and manage emails (e.g., Gmail, Outlook).
2. Mail Transfer Agents (MTA): These are servers that route and deliver emails
between the sender and receiver.

Email Architecture Scenario 1:
1. Two Users on the Same Computer: Alice and Bob have their own
mailboxes on the same computer.
2. Sending a Message: Alice writes a message using her email app (User
Agent) and sends it to Bob’s mailbox.
3. Message Delivery: The message goes into Bob’s mailbox on the computer.
4. Reading the Message: Bob checks his mailbox whenever he wants to read
Alice’s message.
 In short, Alice and Bob use their mailboxes on the same computer to send
and receive messages.

Sender and Receiver connected to the same
system Scenario 1

1. Different Systems: Alice and Bob are on different computers or servers.
2. User Agents (UA):
1. Alice’s UA: Helps Alice write and send her email.
2. Bob’s UA: Helps Bob receive and read his email.
3. Message Transfer Agents (MTA):
1. Alice’s MTA: Sends Alice’s email from her computer to the internet.
2. Bob’s MTA: Receives the email from the internet and delivers it to Bob’s mailbox.
4. Sending and Receiving:
1. Alice writes an email and sends it through her UA.
2. The email travels over the internet, handled by Alice’s MTA.
3. Bob’s MTA receives the email and puts it in Bob’s mailbox.
4. Bob reads the email using his UA.
 In summary, Alice and Bob use their own email apps and servers to send and receive messages across different systems.

Scenario 2: Sender and Receiver connected
to different systems

1.Alice (Remote User): Uses her email app (UA) to send a message over the internet to her
email server using an MTA client.
2.Email Transfer: The message goes from Alice’s email server to Bob’s email server over the
internet.
3.Bob (Local User): Uses his email app (UA) to retrieve and read the message from his server.
In summary: Alice sends an email from her remote location, and Bob, who is directly connected,
receives and reads it.
•Without Internet(local): You can access previously downloaded emails and compose new
messages.
•With Internet(remote): You need it to initially connect, sync, and send new messages to the
email server.

Scenario 3: One user is a remote user

 This is the most common scenario. In this scenario, A connects to her mail server using a
LAN or WAN link. Receiver B also connects to his mail server using a LAN or WAN
connection i.e. both A and B are not directly connected to the mail server.
 i. A uses a UA and MTA to send the mail to her mail server.
 ii. The MTA client at the mail server sends it to B's mail server.
 iii. B uses a Mail Access Agent (MAA) to retrieve the message from server.
 iv. The MAA client pulls the messages from the mail server and pushes them into a
special MAA
 v. B uses a MAA client to retrieve messages from the MAA server.

Scenario 4: Both users are remote users

User Agent
 Email User Agent: An application that enables users to
manage their email accounts, including sending,
receiving, and organizing emails
 Web User Agent: A software application like a web
browser that allows users to access and interact with
websites.

Email User Agent Services
•Composing: Create and send new emails.
•Reading: View incoming emails.
•Replying: Respond to received emails.
•Forwarding: Share received emails with others.
•Handling Mailbox: Organize, search, and manage emails in your
mailbox.

Typer of User Agents
1. Command-Driven User Agents
•Definition: User Agents that operate primarily through text-based commands, typically in a
command-line interface (CLI). Users interact with these UAs by typing commands and
receiving text-based responses.
•Characteristics:
•Text-Based: Interaction is through textual input and output.
•Keyboard-Centric: Commands are entered via the keyboard.
•Efficient for Power Users: Often preferred by advanced users who are comfortable with
command-line environments.
•Examples:
•Email Clients:
•Mutt: A text-based email client for Unix-like systems.
•Pine (now Alpine): Another text-based email client.

2. GUI-Based User Agents
•Definition: User Agents that operate through a graphical user interface (GUI), providing visual
elements like windows, buttons, and icons. Users interact with these UAs using graphical elements
rather than text-based commands.
•Characteristics:
•Graphical Interface: Interaction is through visual elements like menus, buttons, and icons.
•User-Friendly: Designed to be more intuitive and accessible, especially for users less familiar with
command-line interfaces.
•Rich Functionality: Often includes additional features like drag-and-drop, visual feedback, and
multimedia support.
•Examples:
•Email Clients:
•Microsoft Outlook: A popular email client with a rich graphical interface.
•Apple Mail: The default email client for macOS with a GUI.

Format of An Email
Envelope
•Definition: Metadata used for routing and delivering the email.
•Components:
•Envelope Sender: Email address of the sender.
•Envelope Recipient: Email address of the recipient.
•Purpose: Helps mail servers deliver the email to the correct address.
Message
•Definition: The actual content of the email.
•Components:
•Header: Includes To, From, Subject, and Date.
•Body: The main content or text of the email.
•Attachments: Files included with the email.
•Purpose: Contains the information or files the sender wants to communicate.

email address
An email address is divided into two main parts:
1. Local Part
•Definition: The part before the "@" symbol in an email address.
•Purpose: Identifies the specific mailbox or user on the mail server.
•Example: In john.doe@example.com, the local part is john.doe.
2. Domain Name
•Definition: The part after the "@" symbol in an email address.
•Purpose: Specifies the mail server that handles the email. It includes the domain of the
organization or service provider.
•Example: In john.doe@example.com, the domain name is example.com.

MIME(Multipurpose Internet Mail Extensions)
 MIME is a kind of add-on or a supplementary protocol that allows non-ASCII data to be sent
through SMTP. It allows the users to exchange different kinds of data files on the Internet: audio,
video, images, application programs as well.
 Why do we need MIME?
Limitations of Simple Mail Transfer Protocol (SMTP):
1. SMTP has a very simple structure
2. SMTP simplicity however comes with a price as it only sends messages in NVT 7-bit ASCII format.
3. SMTP cannot be used for languages that do not support 7-bit ASCII format such as French, German,
Russian, Chinese and Japanese, etc. so it cannot be transmitted using SMTP. So, in order to make
SMTP more broad, we use MIME.
4. SMTP cannot be used to send binary files or video or audio data.

MIME Header:

It is added to the original e-mail header section to define transformation. There are five
headers that we add to the original header:
1. MIME-Version – Defines the version of the MIME protocol. It must have the
parameter Value 1.0, which indicates that message is formatted using MIME.
2. Content-Type – Type of data used in the body of the message. They are of different types
like text data (plain, HTML), audio content, or video content.
3. Content-Type Encoding – It defines the method used for encoding the message. Like 7-bit
encoding, 8-bit encoding, etc.
4. Content Id – It is used for uniquely identifying the message.
5. Content description – It defines whether the body is actually an image, video, or audio.

Model of SMTP System :
In the SMTP model user deals with the user agent (UA), for example, Microsoft Outlook, Netscape,
Mozilla, etc. To exchange the mail using TCP, MTA is used. The user sending the mail doesn’t have to
deal with MTA as it is the responsibility of the system admin to set up a local MTA. The MTA
maintains a small queue of mail so that it can schedule repeat delivery of mail in case the receiver is
not available. The MTA delivers the mail to the mailboxes and the information can later be
downloaded by the user agents.
SMTP :

The following table shows the SMTP commands. The first 5 commands are mandatory commands
and must be supported by every SMTP software.
SMTP commands :

The response is sent back as a 3 digit number which specifies the result of the command.
Example: 450 means mailbox not available. 220 means service ready, 421 means service
not ready etc.
Typical SMTP procedure is given below
SMTP Procedure :

Message Access Agents (MAAs) are protocols used
to retrieve and manage emails from a mail server.
The two most common MAAs are POP (Post Office
Protocol) and IMAP (Internet Message Access
Protocol).
(MAA)Message Access Agent: POP and IMAP

1. POP (Post Office Protocol)
2. IMAP (Internet Message Access Protocol)
•POP: Downloads emails to a local device, removing them from the server. Best for users accessing
email from one device.
•Email Storage: Downloads and removes from server.
•Best For: Single-device access.
•IMAP: Keeps emails on the server and synchronizes across multiple devices. Best for users needing
access from various devices.
•Email Storage: Keeps emails on the server.
•Best For: Multi-device access and synchronization.

Web-Based Email
• Definition: Accessible via web browsers, offering features like cloud storage, real-time synchronization,
and additional tools integrated into the email service. Ideal for accessing email from any device without
needing separate software.
• Examples: Gmail, Yahoo Mail, Outlook.com.
 Key Features:
• Accessible Anywhere: Use any device with an internet connection.
• User-Friendly Interface: Manage emails with a graphical interface.
• Cloud Storage: Emails and attachments are stored online.
• Real-Time Sync: Emails and settings update across devices.
 Example:
• Gmail: Check inbox, compose emails, use folders, and search within a web browser.

FTP
 FTP (File Transfer Protocol) is a standard network protocol used for
transferring files between a client and a server over a TCP-based network, such
as the Internet.
 Connections in FTP:
 FTP uses two separate connections:
• Control Connection (Port 21): This is responsible for session management and
command exchange between the control processes of the client and the server. It
is established over the TCP/IP network.
• Data Connection (Port 20 or other): This is responsible for the actual transfer
of data (files or directories). It is opened between the data transfer processes of
the client and server.

FTP Model:
 FTP Client Components:
•User Interface: This allows the user to interact with the FTP client, giving
commands like uploading or downloading files.
•Client Control Process: This is responsible for managing the control
connection (typically on port 21), which is used to send commands (like login or
navigation) to the FTP server.
•Client Data Transfer Process: This component handles the actual transfer of
files (via the data connection) between the client and the server. It works once
the control process establishes the connection.

 FTP Server Components:
The FTP server has two main components:

Server Control Process: This corresponds to the client control process and is responsible
for managing the control connection, listening for and responding to the commands from
the client.
Server Data Transfer Process: Similar to the client data transfer process, this is
responsible for handling the transfer of files through the data connection, either uploading
or downloading data between the client and the server

Communication over Control Connection:
 The control connection is primarily responsible for managing the session between the FTP client and server.
This connection remains open throughout the FTP session, even if no files are being transferred.
 The control connection doesn't send actual files because it's specifically designed for sending commands and
managing the FTP session.
•Port: Uses port 21 for communication.
•Purpose: Manages the session and exchanges commands (like login, directory navigation) between the client
and server.
•Process:
•The client sends commands (e.g., USER to log in).
•The server responds with status codes (e.g., 200 OK).
•Commands: Includes commands like USER, PASS, QUIT, LIST, etc.
•Always Open: The control connection stays open during the entire session but doesn't transfer files, only
instructions.
This connection handles session management and control messages, not the actual file data.

FTP commands used over the control connection:
•ABOR (Abort):
•Cancels an ongoing file transfer or any other FTP command that is currently being executed.
•Example: ABOR
•Usage: If a file transfer is taking too long or an error occurs, you can use ABOR to stop it.
•CWD (Change Working Directory):
•Changes the current directory on the FTP server to a specified directory.
•Example: CWD /directory_name
•Usage: Helps navigate between folders on the server.
•DELE (Delete File):
•Deletes a specified file from the server.
•Example: DELE filename.txt
•Usage: Removes unwanted or unnecessary files from the server.
•NLST (Name List):
•Lists the names of files in the current directory (similar to LIST but without detailed information like file sizes
or dates).
•Example: NLST
•Usage: Useful when you just need a simple list of filenames without extra details.

Communication over Data Connection (FTP)
 Definition:
The data connection in FTP is a temporary connection used for transferring files and
directory listings between the client and server. It is separate from the control connection
and is opened as needed for each file transfer, then closed afterward.
The flow emphasizes that the data connection handles the actual transfer of files based on client-defined
parameters.

File Type:
•ASCII (Text):
•For transferring plain text files. Converts end-of-line characters between systems
(e.g., Windows to UNIX).
•Example: Text files, HTML, scripts.
•EBCDIC:
•Used for text files encoded in EBCDIC (Extended Binary Coded Decimal
Interchange Code), which is mainly used in older IBM mainframes.
•Binary (Image):
•For transferring files in binary format (no conversion). Best for non-text files like
images, videos, or executables.
•
Example: .jpg, .zip, .exe.

Data Structure:
•File Structure:
•The default structure where files are treated as continuous sequences
of data (bytes).
•Record Structure:
•Data is transferred as distinct records. Typically used in systems where
data is processed in record formats (like mainframes).
•Page Structure:
•Files are divided into pages, and each page contains a header. This is
rarely used in modern implementations.

Transmission Modes:
•Stream Mode:
•Data is sent as a continuous stream, and the end of the file is
indicated by closing the connection. It is the most common and
simplest mode.
•Block Mode:
•Data is sent in blocks, each with a header containing control
information. Useful for transferring large files in segments.
•Compressed Mode:
•Data is compressed using simple algorithms (like run-length
encoding) before being sent, reducing the size of the transfer.

Anonymous FTP
 Anonymous FTP allows users to access files on a server without a personal account. Users log in
with "anonymous" as the username and typically provide an email as the password. It's commonly
used for distributing public files like software and datasets.
 Key Points:
• Public Access: Anyone can download files.
• Read-Only: Users can’t upload files.
• Security Risks: Sensitive info shouldn’t be shared.
 Anonymous" means you don't need to provide any personal details to log in. You can access data or
files without creating an account or entering specific identifying information. In anonymous FTP, for
example, you simply log in with "anonymous" and often provide an email address as a password
(which is usually not verified), allowing you to access public files easily.

www
 The World Wide Web (WWW) is a system of interlinked hypertext documents and multimedia content that is accessed
via the internet. It allows users to view and interact with information in various forms, including text, images, videos, and
more.
 Components:
•Hypertext:
•Uses hyperlinks to connect different pieces of content, allowing users to navigate from one page to another easily.
•Web Pages:
•Consists of web pages written primarily in HTML (Hypertext Markup Language) that can be accessed using web browsers.
•URLs:
•Each resource on the web is identified by a Uniform Resource Locator (URL), which specifies its location.
•Protocols:
•Relies on protocols like HTTP (Hypertext Transfer Protocol) and HTTPS (HTTP Secure) for data transfer between clients
(browsers) and servers.
•Multimedia:
•Supports various types of content, including text, images, audio, and video, enriching the user experience.

Architecture of WWW
following figure shows the architecture of WWW. A client needs to fetch some information from server A. It
sends a request from its browser. The request content fetch some information document and the address of
the site in a URL. The server A sends the document to the client. In the document, the client finds some
reference to another document at server B. It sends a request to server B for the document.

Client browser
 A client browser can be viewed as having three primary roles:
 1. Controller:
• The browser acts as a controller by managing the interaction between the user and the web server. It sends
requests for web resources (like HTML pages, images, etc.) and handles responses from the server,
orchestrating the entire process of loading a webpage.
 2. Client Protocol:
• The browser uses protocols like HTTP or HTTPS to communicate with web servers. It formats requests
and interprets responses according to these protocols, ensuring that data is correctly sent and received.
 3. Interpreter:
• The browser serves as an interpreter for web content. It interprets HTML to render web pages visually,
processes CSS for styling, and executes JavaScript to enable interactive features on the page. This allows
users to view and interact with web content as intended by the developers.

Server
 A server is a computer or software that provides resources, data, services, or programs to other computers, known
as clients, over a network. In the context of the World Wide Web, a web server stores, processes, and delivers web
pages to clients (browsers) via the internet using protocols like HTTP or HTTPS.
 URL :A Uniform Resource Locator (URL) is a specific type of Uniform Resource Identifier (URI) that provides
the address of a resource on the web and the protocol used to access it.
 Components of a URL:
1.Protocol
•The method used to access the resource. Common protocols include:
•HTTP: Hypertext Transfer Protocol
•HTTPS: HTTP Secure (encrypted)
•FTP: File Transfer Protocol
2. Host Computer
•The domain name or IP address of the server hosting the resource. It identifies where the resource is located.
•Example: www.example.com or 192.168.1.1

3.Port Number
•An optional component that specifies the port on the server used for the connection. If not specified, default
ports are used:
•80 for HTTP
•443 for HTTPS
•21 for FTP
•Example: :8080 (if the port is not default)
4. Path
•The specific location on the server where the resource is stored. It typically starts with a slash (/).
•Example: /folder/page
5. Filename
•The name of the specific file being requested, often including its extension.
•Example: page.html
Full URL Example:
https://www.example.com:8080/folder/page.html

Cookies:
 Cookies are small files stored on your device by websites to remember your
preferences, login info, or browsing activity. They help improve your web experience
but can also track online behavior. There are session cookies (temporary), persistent
cookies (long-term), and third-party cookies (used by advertisers for tracking).
•Session Cookies: Temporary, deleted when you close the browser.
•Persistent Cookies: Long-term, stored even after closing the browser to remember
preferences.
•First-party Cookies: Set by the website you're visiting first time, for a better user
experience.
•Third-party Cookies: Set by external sites (e.g., advertisers) to track your activity
across websites.

Web Documents:
 Web Documents are the content displayed on web pages, and they can be classified into two main types:
1. Static Web Documents:
1. These are fixed and do not change unless manually updated by a developer.
2. They are created using basic HTML and display the same content to all users.
3. Example: A simple HTML webpage with text and images that remains the same for everyone.
2. Dynamic Web Documents:
1. These are interactive and can change based on user input, time, or other conditions.
2. They are often generated using server-side scripts (e.g., PHP, Python) or client-side scripts (e.g., JavaScript).
3. Example: A personalized webpage like a social media feed that updates content dynamically for each user.
 Active documents are web pages that can interact with users and update in real-time, like online games, live
chats, or interactive maps. They use programs like JavaScript to make the page responsive and dynamic.

1.Request Message:
Request Line:
1. Contains the request type (method) (e.g., GET, POST).
2. Contains the URL (web address of the resource being requested).
3. Contains the version of HTTP being used (e.g., HTTP/1.1).
Response Message:
Status Line:
4. Contains the HTTP version, status code (e.g., 200 for success), and status phrase (e.g., OK
or Not Found).
Status Line
• HTTP Version: The version of HTTP being used (e.g., HTTP/1.1).
• Status Code: A three-digit code indicating the outcome of the request (e.g., 200, 404, 500).
• Reason Phrase: A short description of the status code (e.g., OK, Not Found, Internal Server Error).
Here are the purposes of the common HTTP request methods:
1.GET: Retrieve data from a server.
2.POST: Submit data to the server for processing.
3.PUT: Update or create a resource on the server.
4.DELETE: Remove a specified resource from the server.
5.HEAD: Retrieve only the headers of a resource.
6.OPTIONS: Describe the communication options for the target resource.
7.PATCH: Apply partial modifications to a resource.

2. Header:
• Here’s a brief overview of HTTP header types:
• 1. General Headers
• Purpose: Apply to both requests and responses.
• Examples: Date, Connection.
• 2. Request Headers
• Purpose: Provide information about the client and the request.
• Examples: Host, User-Agent, Accept, Authorization.
• 3. Response Headers
• Purpose: Provide information about the server and the response.
• Examples: Content-Type, Content-Length, Cache-Control, Location.
• 4. Entity Headers:
• Purpose: Describe the body of the resource.
• Examples: Content-Encoding, Content-Language, Last-Modified, ETag.
• 5. Custom Headers:
• Purpose: User-defined headers for specific applications.
• Examples: X-Requested-With, X-Frame-Options.

•Empty Line
•Indicates the end of the headers.
•Body (optional)
•Contains data sent to the server, primarily in POST requests.
•Format can vary (e.g., JSON, form data).
Example:
Client sends a GET request for image
named pic and tells server that it can
accept images in jpg format. The
response contains the status line and
three lines of header.

Persistent Connection
•What it is: One TCP connection used for multiple requests/responses.
•How it works: Keeps the connection open after sending a response,
allowing more requests.
•Example: Default in HTTP/1.1 with Connection: keep-alive.
•Benefits:
•Reduces loading time for multiple resources.
•More efficient use of network resources.

Non-Persistent Connection
•What it is: A new TCP connection for each request/response.
•How it works: Closes the connection after sending the response.
•Example: Default in HTTP/1.0.
•Drawbacks:
•Higher latency due to connection setup time.
•Less efficient; more connections mean more overhead.
•Persistent: Better for performance and resource use.
•Non-Persistent: Simpler but slower due to frequent connections.

proxy server
 A proxy server is like a middleman between your device (client) and the internet. It
forwards your requests to other servers and sends their responses back to you.
 Example
• Scenario: You want to watch a video that’s only available in another country.
• Using a Proxy:
• You connect to a proxy server located in that country.
• You request the video.
• The proxy server accesses the video from the target server and sends it back to you, making it
appear as if you are in that country.

Gateways
 A gateway is a device or a system that acts as a bridge between two different networks
or systems that use different communication protocols. It allows data to flow between
networks that otherwise wouldn't be able to communicate directly with each other.
 It translates communication from one protocol to another, enabling data exchange
between systems that use different protocols.
•Your phone or laptop = small local network using Wi-Fi.
•Internet = huge, global network using many different technologies.
•Gateway (Wi-Fi router) = the device that helps these two connect and "talk" to
each other, just like a translator in a conversation!

Types of Gateways
 Transport Gateway
• Purpose: Connects different transport protocols (e.g., TCP/IP, UDP).
• Functionality:
• Converts data packets from one transport protocol to another.
• Manages and routes traffic between networks using different transport layers.
• Example: A gateway that connects a local area network (LAN) using TCP/IP to another network using a different transport
protocol.
 2. Application Gateway
• Purpose: Connects different application-layer protocols (e.g., HTTP, FTP).
• Functionality:
• Acts as a bridge for applications, translating requests and responses between different application protocols.
• Provides additional functionalities like security, load balancing, and monitoring.
• Example: A web application firewall that filters and forwards HTTP requests to a web server, translating between HTTP and
HTTPS.

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