[Day 4] 1. Transport Layer

• The Transport Layer in the OSI model ensures reliability of data transmission and is composed of mechanisms like flow control and error control. It manages data segments by using protocols such as:
  • TCP (Transmission Control Protocol): Reliable but slower, ensures data is delivered correctly.
  • UDP (User Datagram Protocol): Fast but unreliable, does not guarantee delivery or order.

Port Numbers

• Ports range from 1 to 65535.
• Well-known ports (1–1024) are reserved for system-level services (e.g., HTTP, HTTPS).
• Ports from 4096 to 65535 can be used dynamically by applications for various processes.
• When accessing a website like
  Copy

  https://google.com

  , the server sends data to your device on a specific port number, which is included in the segment (data unit at the Transport Layer). This port number helps direct the data to the appropriate application on the receiving system.

Port Security and Firewalls

• An attacker may attempt to exploit open ports by scanning them.
• Windows Firewall and corporate firewalls like Honeywell Firewall block unused or suspicious ports to prevent unauthorized access.
• In secure environments, only the ports required for specific applications are opened, and others are blocked by default.

Session IDs and Multiple Instances

• A Session ID is used to manage multiple instances of the same application. While different applications are identified by their port numbers, different instances of the same application use session IDs to ensure proper data routing.

Data Link Layer (Layer 2)

• Manages the MAC addresses and Logical Link Control (LLC). The Data Link Layer interfaces with both the Physical Layer and the Network Layer.

Network Layer (Layer 3)

• Responsible for identifying the protocol being used (e.g., TCP or UDP). It assigns the protocol number and routes packets to the destination.

Encapsulation Process

1. Frame: Contains data and type information.
2. Packet: Contains network protocol information (IP address).
3. Segment: Contains transport layer information (port number).
4. Process: The data reaches the application that requested it.