Network Data Transmission and Protocol Data Units (PDUs)

Introduction to PDUs

• PDU (Protocol Data Unit): A unit of data specified in the protocol layer.
• Transmission Units: Data transferred across the network as a single unit.
• Ethernet Frames: Send data from one MAC address to another without caring about data content.

Data Encapsulation Across the OSI Model

• Layer 2 (Data Link): Uses a DLC frame header and trailer.
• Layer 3 (Network): Utilizes IP headers; doesn't care about internal data (e.g., UDP, TCP).
• Layer 4 (Transport): Involves TCP or UDP headers.
• Application Layers (5-7): Application data (e.g., web traffic).

Encapsulation and Decapsulation Process

• Sending Device:
  • Application data is encapsulated with TCP, IP, and frame headers.
  • Ethernet transmits data across the network.
• Receiving Device:
  • Decapsulates data by stripping frame, IP, and TCP headers.
  • Application data is then accessed.

Control Flags in Protocol Headers

• TCP Flags:

  • Examples: SYN, PSH (push), RST (reset), FIN (finish).
  • Control data flow by setting bits in headers.

• IP Flags:

  • Related to data fragmentation.
  • MTU (Maximum Transmission Unit): Determines packet size across a network.

MTU and Fragmentation

• Fragmentation:

  • Occurs when packets exceed MTU limits.
  • Involves segmenting data to fit the network's MTU.
  • Fragmentation can slow network throughput.

• MTU Considerations:

  • Know the MTU value throughout the network.
  • Adjust MTU settings for VPN tunneling or network changes.

Testing MTU with Ping Command

• Ping Utility: Tests MTU using ICMP packets.
• Windows Example:
  • Use ping -f for Don't Fragment bit.
  • Use -l to specify packet size.
  • Test against a server like Google's DNS 8.8.8.8.
• Response Analysis:
  • Successful ping confirms MTU compatibility.
  • Adjust packet size if fragmentation is needed.

Practical Application of MTU

• Design networks with consistent MTU values.
• Adjust MTU for different network paths and conditions.
• Use ping tests to determine optimal MTU settings.