Unit VI: TRANSPORT LAYER & APPLICATION LAYER

Transport Layer

⭐Transport Layer Services

The transport layer is crucial in networking, as it provides end-to-end communication services between devices on a network. It sits between the application layer and the network layer, ensuring reliable data transfer and managing the connections between hosts. Here are the core services offered by the transport layer:

1) Segmentation and Reassembly

Segmentation:
- Large messages are divided into smaller, manageable segments by the transport layer.
- This segmentation allows data to fit within network constraints and makes it easier to transport across networks.
Reassembly:
- At the destination, the transport layer reassembles these segments back into the original message.
- Segments are reassembled in the correct order, based on their sequence numbers.

2) Connection Control

This service defines whether the communication will be connection-oriented or connectionless.
Connection-Oriented Communication (TCP):
- Establishes a connection between sender and receiver before data transmission.
- Provides reliable communication by ensuring the complete and accurate delivery of data.
- Uses mechanisms like a three-way handshake to establish connections (SYN, SYN-ACK, ACK).
Connectionless Communication (UDP):
- Data is sent without establishing a prior connection.
- Best for applications that need fast transmission and can tolerate occasional data loss (e.g., live video streaming).
- Does not perform error correction or flow control, resulting in faster transmission but less reliability.

3) Flow Control

Purpose: Prevents a fast sender from overwhelming a slower receiver.
Mechanism:
- Controls the rate at which data is sent.
- Adjusts based on the capacity of the receiving end to handle data.
TCP Flow Control (Sliding Window Protocol):
- Uses a "window" to manage the amount of data sent and acknowledged.
- The sender can only send a certain number of segments before waiting for an acknowledgment from the receiver.
- As acknowledgments are received, the window “slides” forward, allowing the sender to continue transmission.
Benefits:
- Prevents data loss due to overflow at the receiver’s buffer.
- Helps balance network traffic and avoid congestion.

4) Error Control

Purpose: Ensures data integrity by detecting and correcting errors in transmitted data.
Mechanism:
- Each segment includes a checksum, allowing the receiver to verify the integrity of received data.
- If an error is detected, the receiver can request retransmission of the corrupted data.
TCP Error Control:
- TCP uses a mechanism called Automatic Repeat reQuest (ARQ), where corrupted or lost packets are retransmitted.
- Types of ARQ:
  - Stop-and-Wait ARQ: Sends one segment at a time, waiting for acknowledgment before sending the next.
  - Go-Back-N ARQ: Sends multiple segments without waiting but retransmits all unacknowledged segments if an error is detected.
  - Selective Repeat ARQ: Only retransmits the specific segment where an error was detected.
UDP Error Control:
- UDP includes a basic checksum for error detection, but it does not provide retransmission or correction services.
- Less reliable than TCP, suitable for applications where speed is more critical than accuracy.

5) Multiplexing and Demultiplexing

Purpose: Enables multiple applications to share a single transport layer connection on both the sending and receiving ends.
Multiplexing:
- At the sender’s side, data from multiple applications is combined (or multiplexed) onto a single connection.
- Each segment includes information about the source application’s port number, identifying which application is sending the data.
Demultiplexing:
- At the receiver’s side, the transport layer reads the destination port number in each segment.
- Based on the port number, data is directed to the appropriate application.
Ports:
- Each application on a device uses a specific port number, which helps identify it.
- Standardized port numbers (e.g., HTTP - port 80, FTP - port 21) enable applications to communicate correctly.

6) Reliability

Purpose: Ensures data is delivered correctly and in the correct sequence.
TCP Reliability:
- TCP provides reliability by using mechanisms like acknowledgments, sequence numbers, and retransmission.
- Sequence Numbers:
  - Each segment is given a sequence number, allowing the receiver to reassemble data in the correct order.
  - Also helps detect and handle duplicate segments.
- Acknowledgments:
  - Receiver sends an acknowledgment (ACK) for each segment received.
  - If the sender doesn’t receive an ACK within a set time, it retransmits the segment.
- Retransmission:
  - Retransmission is triggered by timeouts or duplicate acknowledgments, ensuring the correct data is received.
UDP Reliability:
- UDP does not guarantee reliable delivery as it lacks acknowledgment and retransmission mechanisms.
- Suitable for real-time applications where occasional data loss is acceptable (e.g., online gaming, VoIP).

⭐TCP - Header Format and Handshaking Operation

TCP (Transmission Control Protocol) is a reliable, connection-oriented protocol in the transport layer. It provides accurate, in-order data transfer between devices by using mechanisms such as error control, flow control, and connection establishment. Here's an in-depth look at the TCP header format and the TCP three-way handshake used for connection setup.

1) TCP Header Format

Each TCP segment includes a header containing various fields that control data transfer and reliability. The main fields in the TCP header are:

Source Port (16 bits):
- Identifies the sending application on the sender’s side.
- Helps the receiver know which application or process sent the segment.
Destination Port (16 bits):
- Identifies the receiving application on the receiver’s side.
- Used to deliver the segment to the correct application or process.
Sequence Number (32 bits):
- Specifies the order of bytes in a segment to help reassemble data at the receiver.
- Helps ensure that data is received in the correct sequence.
- The sequence number of the first byte in the segment is indicated here.
Acknowledgment Number (32 bits):
- Used to acknowledge receipt of data from the sender.
- This field contains the next expected byte from the sender, confirming all prior bytes have been received correctly.
Data Offset (4 bits):
- Indicates the length of the TCP header in 32-bit words.
- Helps the receiver locate the start of the actual data in the segment.
Reserved (3 bits):
- Reserved for future use, always set to zero.
- Ensures compatibility with future versions or additional TCP features.
Flags (9 bits):
- Contains control flags, each indicating a specific purpose. The most common flags are:
  - URG (Urgent): Indicates that certain data is marked as urgent.
  - ACK (Acknowledgment): Shows that the acknowledgment number field is valid.
  - PSH (Push): Instructs the receiver to deliver data immediately.
  - RST (Reset): Resets the connection if there’s an error or unexpected condition.
  - SYN (Synchronize): Initiates a connection and synchronizes sequence numbers.
  - FIN (Finish): Indicates the end of data from the sender, requesting to close the connection.
Window Size (16 bits):
- Indicates the amount of data (in bytes) the sender is willing to accept.
- Used for flow control to manage data transfer rates and prevent buffer overflow at the receiver.
Checksum (16 bits):
- Provides error-checking for both the TCP header and data.
- The sender calculates the checksum, and the receiver verifies it to ensure data integrity.
Urgent Pointer (16 bits):
- Used when the URG flag is set.
- Points to the sequence number of urgent data in the segment, instructing the receiver to prioritize this data.
Options (Variable length):
- Allows additional features to be included, such as maximum segment size (MSS) or window scaling.
- Common options include setting up parameters for the connection.
Padding:
- Extra bits added to ensure the TCP header is a multiple of 32 bits.
- Ensures proper alignment and compatibility with the network protocols.

2) TCP Three-Way Handshake Operation

The three-way handshake is a process that establishes a reliable connection between a client and server before data transmission. It ensures both parties are synchronized and ready to communicate.

Steps of the Three-Way Handshake:

Step 1: SYN (Synchronize)
- The client initiates the connection by sending a TCP segment with the SYN flag set.
- This segment includes an initial sequence number (ISN) chosen by the client.
- The SYN segment indicates that the client wants to establish a connection with the server.
Step 2: SYN-ACK (Synchronize-Acknowledgment)
- The server responds to the client’s SYN request with a SYN-ACK segment.
- The server sets both the SYN and ACK flags.
- The SYN-ACK segment contains the server’s initial sequence number (ISN) and an acknowledgment number, which is the client’s ISN plus one.
- This acknowledgment confirms receipt of the client’s SYN and establishes the server’s own sequence number for the connection.
Step 3: ACK (Acknowledgment)
- The client responds with an ACK segment, confirming the server’s SYN-ACK.
- This segment has the ACK flag set, and the acknowledgment number matches the server’s ISN plus one.
- The connection is now established, and data can be transmitted between the client and server.

Key Points of the Three-Way Handshake:

Reliable Connection: This process ensures that both the client and server agree on initial sequence numbers, which helps keep data in the correct order.
Error Handling: If one of the steps fails, the connection setup process will timeout or reset, and the handshake may restart.
State Transition: Both the client and server go through specific states (e.g., SYN-SENT, SYN-RECEIVED) during the handshake, which helps in managing the connection’s lifecycle.

⭐UDP - Header Format

UDP (User Datagram Protocol) is a simple, connectionless protocol in the transport layer that provides fast, efficient communication without the reliability mechanisms of TCP. It is best suited for applications where speed is critical and occasional data loss is acceptable, such as video streaming, online gaming, and voice-over-IP (VoIP). UDP’s header format is straightforward, containing minimal fields to reduce processing overhead.

The UDP header is 8 bytes (64 bits) long, consisting of four fields, each 16 bits. Here’s a detailed breakdown of each field in the UDP header:

Source Port (16 bits)
- Identifies the port number of the sending application.
- Allows the receiver to know which application or process on the sender’s side sent the packet.
- Optional: Can be set to zero if the sender does not require a reply.
Destination Port (16 bits)
- Identifies the port number of the receiving application.
- Ensures the UDP datagram is delivered to the correct application or service on the receiving device.
- Commonly used port numbers include 53 for DNS, 67 for DHCP, and 123 for NTP (Network Time Protocol).
Length (16 bits)
- Specifies the total length of the UDP datagram, including both the header and the data.
- Minimum length is 8 bytes (header only), but it can vary based on the size of the data.
- Helps the receiver determine where the UDP datagram ends.
Checksum (16 bits)
- Used for error-checking the header and data to ensure data integrity.
- The checksum is calculated by the sender and verified by the receiver.
- Optional in IPv4 but mandatory in IPv6. If set to zero in IPv4, it indicates no checksum is calculated.

Key Characteristics of UDP

Connectionless:
- UDP does not establish a connection before data transfer.
- Each datagram is sent independently, with no acknowledgment or retransmission in case of data loss.
No Flow Control or Congestion Control:
- UDP does not manage the rate of data transmission or respond to network congestion.
- This results in faster data transmission but can cause data to be lost in congested networks.
Minimal Header Overhead:
- With only 8 bytes, the UDP header is lightweight, reducing the data transmission time.
- Ideal for applications needing low latency and high-speed transmission.
Best-Effort Delivery:
- UDP provides a "best-effort" service, meaning it delivers data without guarantees for order or delivery.
- Applications that can tolerate occasional data loss (e.g., live streaming) often use UDP.

Summary of UDP Header Fields

Field	Size	Purpose
Source Port	16 bits	Identifies the sending application’s port.
Destination Port	16 bits	Identifies the receiving application’s port.
Length	16 bits	Specifies total length of the datagram.
Checksum	16 bits	Provides error-checking for data integrity.

Application Layer

⭐Domain Name System (DNS)

The Domain Name System (DNS) is a system that translates human-readable domain names (like www.example.com) into IP addresses (like 192.168.1.1), which computers use to locate and communicate with each other on the internet. DNS acts as the "phonebook" of the internet, allowing users to access websites and services without needing to remember IP addresses.

1. Purpose of DNS

DNS makes it easier for users to access websites by using domain names instead of complex IP addresses.
Translates domain names to IP addresses, which are required to route data to the correct destination.

2. Components of DNS

DNS Resolver:
- A client-side service (often provided by ISPs or local networks) that receives DNS queries from user applications.
- Queries DNS servers on behalf of the client to find the corresponding IP address for a given domain.
DNS Server:
- Root DNS Servers:
  - Top-level servers that manage the DNS namespace hierarchy and direct queries to the appropriate Top-Level Domain (TLD) servers.
- TLD (Top-Level Domain) Servers:
  - Responsible for handling requests for specific domain extensions, like .com, .org, .net.
  - Direct queries to the relevant authoritative DNS servers for the specific domain.
- Authoritative DNS Servers:
  - Hold the actual DNS records for specific domains (e.g., example.com) and return the IP address for the domain requested.

3. DNS Hierarchical Structure

Root Level:
- The highest level in the DNS hierarchy; represented by a dot (".").
- Directs requests to appropriate TLD servers.
Top-Level Domains (TLDs):
- Include common domain extensions like .com, .org, .edu, as well as country-specific TLDs like .uk and .jp.
Second-Level Domains:
- Unique names registered under TLDs (e.g., example in example.com).
- Managed by individuals or organizations who own the domain.
Subdomains:
- Additional subdivisions within a domain, often used to organize content (e.g., blog.example.com).

4. DNS Records

DNS records define specific information about a domain and its services. Some common DNS record types include:

A Record (Address Record):
- Maps a domain name to an IPv4 address.
AAAA Record (IPv6 Address Record):
- Maps a domain name to an IPv6 address.
CNAME Record (Canonical Name Record):
- Creates an alias for a domain name, allowing multiple domain names to point to the same IP address.
- Useful for linking subdomains to the main domain without needing separate IP addresses.
MX Record (Mail Exchange Record):
- Specifies the mail server responsible for receiving email on behalf of the domain.
- Essential for routing emails to the correct mail server.
NS Record (Name Server Record):
- Indicates the authoritative DNS servers for a domain.
PTR Record (Pointer Record):
- Maps an IP address back to a domain name, commonly used in reverse DNS lookups.
TXT Record (Text Record):
- Holds text information for various uses, often for verification or security purposes (e.g., SPF for email authentication).

5. DNS Resolution Process

The DNS resolution process involves several steps that convert a domain name into an IP address:

Step 1: User Request:
- A user enters a domain name (e.g., www.example.com) in a browser.
- The user’s computer (DNS resolver) checks its local cache for the IP address.
Step 2: Query to DNS Resolver:
- If not cached locally, the request is sent to a DNS resolver, typically provided by the ISP.
Step 3: Query to Root DNS Server:
- If the resolver doesn’t know the IP, it sends the query to a root DNS server.
- The root server responds with the address of the relevant TLD server.
Step 4: Query to TLD Server:
- The resolver queries the TLD server (e.g., the .com server for example.com).
- The TLD server responds with the address of the authoritative DNS server for the specific domain.
Step 5: Query to Authoritative DNS Server:
- The resolver contacts the authoritative DNS server for the domain (e.g., example.com).
- The authoritative server responds with the IP address of the requested domain.
Step 6: Return IP to Client:
- The resolver returns the IP address to the user’s computer.
- The browser uses the IP to connect to the web server hosting the domain.

6. Types of DNS Queries

Recursive Query:
- The DNS resolver queries other servers on behalf of the client until it finds the answer or reaches a timeout.
- Used when a full DNS resolution is required.
Iterative Query:
- The DNS resolver provides a referral to other DNS servers if it doesn’t know the answer.
- The client then queries each server in sequence until the IP address is found.
Non-Recursive Query:
- Used when the DNS resolver already has the IP address cached, returning it immediately to the client without further queries.

7. DNS Caching

Purpose:
- To reduce the load on DNS servers and speed up the resolution process.
- Allows frequently accessed domain names to be resolved faster.
How It Works:
- DNS resolvers, browsers, and operating systems cache DNS records for a specified time (known as the Time-to-Live, or TTL).
- Cached records allow repeated queries to be resolved quickly without re-contacting the DNS servers.

8. DNS Security

DNS Security Extensions (DNSSEC):
- DNSSEC adds security to DNS by verifying the authenticity of DNS responses.
- Prevents attacks like DNS spoofing by using digital signatures to ensure data integrity.
DNS Spoofing:
- A type of attack where a malicious actor provides false DNS responses to redirect users to fraudulent websites.
- DNSSEC helps mitigate this risk by verifying response authenticity.

⭐E-Mail (Electronic Mail)

E-mail, or electronic mail, is a widely used method for exchanging digital messages over the internet. E-mail allows users to send and receive text, images, files, and links to other users around the world. It’s a fast, reliable, and essential communication tool for personal, educational, and professional use.

1. Basic Components of E-Mail

Email Address:
- Unique identifier for each user, composed of a username, the "@" symbol, and the domain name (e.g., username@example.com).
- The username identifies the individual user, and the domain name identifies the mail server.
Inbox:
- The folder where received emails are stored.
- Users can view, read, and manage incoming messages here.
Subject:
- A brief description or title of the email content, giving the recipient an idea of what the message is about.
Body:
- The main content area of the email, where the actual message is written.
- May include text, images, and even attachments.
Attachments:
- Files that are sent along with the email, such as documents, images, or videos.
- Attachments are typically downloaded by the recipient for viewing.

2. E-Mail Protocols

Several protocols are used to send, receive, and store email messages:

SMTP (Simple Mail Transfer Protocol):
- Used for sending emails from a client to a mail server and between mail servers.
- Operates on port 25 or port 587 (for encrypted connections).
- SMTP is a "push" protocol, meaning it is used to send messages from a sender's mail server to a receiver’s mail server.
POP3 (Post Office Protocol version 3):
- Used for retrieving emails from the mail server to a client.
- Operates on port 110 or port 995 (for encrypted connections).
- POP3 downloads emails to the client’s device and, by default, removes them from the server (unless set otherwise), meaning emails are accessible offline.
IMAP (Internet Message Access Protocol):
- Also used for retrieving emails from the server to the client.
- Operates on port 143 or port 993 (for encrypted connections).
- IMAP allows emails to stay on the server and syncs changes across devices, allowing users to access their email from multiple devices and locations.

3. Structure of an E-Mail Message

Header:
- Contains metadata about the email, including:
  - From: Indicates the sender’s email address.
  - To: Lists the primary recipient(s)’ email addresses.
  - CC (Carbon Copy): Allows the sender to send a copy of the email to additional recipients who are not the main addressees.
  - BCC (Blind Carbon Copy): Sends a copy of the email to additional recipients without disclosing their email addresses to other recipients.
  - Date: Shows when the email was sent.
  - Subject: Provides a short description of the email’s content.
  - Message-ID: A unique identifier for each email message.
Body:
- Contains the actual message text and any HTML or formatting applied to the text.
- May include embedded links, images, and other multimedia elements.
Attachments:
- Optional files that can be included with the email, such as documents, images, videos, or compressed files.

4. E-Mail Services

E-mail services can be provided by:

Web-Based E-Mail:
- Services accessible through a web browser (e.g., Gmail, Yahoo Mail, Outlook.com).
- Users log in online and can access their inbox, compose, send, and manage emails from any device with internet access.
Client-Based E-Mail:
- Software applications installed on a device, like Microsoft Outlook, Mozilla Thunderbird, or Apple Mail.
- Uses protocols (like POP3 or IMAP) to download and sync emails from the server.
Corporate E-Mail Servers:
- E-mail systems used by businesses to provide employees with an email address and manage communication within the company (e.g., Microsoft Exchange Server, Google Workspace).

5. E-Mail Working Process

Step 1: Composing and Sending:
- A user writes an email and clicks "Send."
- The email client forwards the email to the sender’s SMTP server.
Step 2: SMTP Server Processing:
- The SMTP server checks the recipient’s domain and forwards the email to the recipient’s mail server if it’s on a different domain.
- If the sender and recipient are on the same domain, the email is delivered directly to the recipient’s inbox.
Step 3: Receiving:
- The recipient’s mail server receives the email and stores it.
- The recipient retrieves the email via POP3 or IMAP, depending on their email client setup.
Step 4: Viewing and Storing:
- The recipient views the email, and if using IMAP, the email remains on the server.
- If using POP3, the email may be downloaded and removed from the server.

6. Spam and Filtering

Spam:
- Unwanted or unsolicited emails often containing advertisements, phishing attempts, or malware.
- Most email services have filters to detect and move spam emails to a separate "Spam" folder.
Filtering:
- Allows users to categorize, sort, and manage emails automatically.
- Filters can move emails to specific folders, mark them as important, or block certain senders.

7. Email Security Mechanisms

Authentication:
- Ensures the sender is who they claim to be.
- Common authentication methods include SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance).
Encryption:
- Protects email content from unauthorized access.
- TLS (Transport Layer Security) encrypts emails between servers.
- End-to-End Encryption (like PGP or S/MIME) encrypts email content directly between the sender and recipient, making it readable only to them.
Digital Signatures:
- Used to verify the authenticity and integrity of the email.
- Digital signatures ensure that the email has not been altered during transit.

8. Advantages and Disadvantages of E-Mail

Advantages:
- Fast and cost-effective communication across long distances.
- Ability to attach and send files, links, and multimedia.
- Accessible on multiple devices and platforms.
Disadvantages:
- Susceptibility to spam, phishing, and other types of cyber-attacks.
- Emails can be intercepted without encryption, compromising privacy.
- Overuse can lead to information overload and reduced productivity.

⭐FTP (File Transfer Protocol)

FTP (File Transfer Protocol) is a standard protocol used to transfer files between a client and a server on a computer network. FTP allows users to upload, download, delete, rename, move, and copy files on a remote server over the internet or within a local network.

1. Purpose of FTP

FTP facilitates the transfer of files between two systems, typically a client and a remote server.
It is widely used for sharing files, uploading website content to a server, and downloading files from servers.

2. How FTP Works

FTP operates on a client-server model, where an FTP client connects to an FTP server to request file transfers.
The client initiates the connection, and the server responds by providing access to files or folders based on permissions.
FTP uses two main connections:
- Control Connection: Used to send commands from the client to the server and receive responses.
- Data Connection: Used to transfer files between the client and the server.

3. FTP Ports

FTP typically uses two network ports to establish a connection:
- Port 21 for the control connection.
- Port 20 for the data connection, although this can vary depending on the transfer mode.

4. Types of FTP Connections (Modes)

Active Mode:
- In active mode, the client opens a port and waits for the server to initiate the data connection.
- The server connects to the client’s open port to transfer data.
- Sometimes blocked by firewalls due to security concerns with the incoming server connection.
Passive Mode:
- In passive mode, the server opens a port and waits for the client to connect.
- The client initiates both control and data connections, making passive mode more firewall-friendly.
- Used commonly when clients are behind a firewall or NAT (Network Address Translation).

5. FTP Authentication

Anonymous FTP:
- Allows users to connect to the server without a unique username or password.
- Commonly used by public servers to share files that don’t require restricted access.
Authenticated FTP:
- Requires users to log in with a username and password.
- Ensures that only authorized users can access or manage files on the server.

6. FTP Commands

FTP has several basic commands for managing files and directories on a remote server. Some common FTP commands include:

USER and PASS: Used for user login authentication.
LIST: Lists the files and directories in the current directory on the server.
RETR: Retrieves (downloads) a file from the server to the client.
STOR: Uploads a file from the client to the server.
DELE: Deletes a file on the server.
MKD: Creates a new directory on the server.
RMD: Removes a directory on the server.
PWD: Displays the current working directory on the server.
CWD: Changes the current directory on the server.

7. FTP Transfer Modes

ASCII Mode:
- Transfers text files and converts line endings to match the target system.
- Suitable for plain text files (e.g., .txt, .html) and scripts.
Binary Mode:
- Transfers files byte-by-byte with no conversion, preserving the original file format.
- Suitable for non-text files (e.g., images, videos, software).

8. Types of FTP

Standard FTP:
- Unencrypted and transfers data, including usernames and passwords, in plain text.
- Not secure and prone to interception by attackers.
FTPS (FTP Secure):
- An extension of FTP that adds SSL/TLS encryption.
- Encrypts the control and data connections, making it more secure than standard FTP.
SFTP (SSH File Transfer Protocol):
- Not technically part of FTP but often used as a secure alternative.
- Works over SSH (Secure Shell) to provide encryption and secure data transfer.
- Typically runs on port 22, the standard port for SSH.

9. FTP Client Software

To use FTP, clients need special software known as an FTP client.
Examples of FTP clients include:
- FileZilla: A popular open-source FTP client.
- WinSCP: An FTP and SFTP client for Windows.
- Cyberduck: A cross-platform client for FTP and SFTP.
FTP clients have graphical user interfaces (GUIs) or can be accessed via command-line interfaces.

10. Advantages and Disadvantages of FTP

Advantages:
- Enables large file transfers over networks.
- Provides various access levels and permissions for different users.
- Simple and efficient for file transfer tasks, especially over local networks.
Disadvantages:
- Standard FTP is unencrypted, making it insecure for sensitive data.
- Requires additional configuration to work with firewalls (especially in active mode).
- Vulnerable to attacks like eavesdropping and interception if not used with secure protocols (FTPS or SFTP).

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Unit VI: TRANSPORT LAYER & APPLICATION LAYER | CSE306: COMPUTER NETWORKS | B.Tech CSE