Lecture Notes on Network Data Transmission and Protocol Data Units (PDU)

Introduction to PDUs

• Protocol Data Unit (PDU): Fundamental unit of data for network transmission.
• Also referred to as transmission units.
• Example: Ethernet Frame used to send data from one MAC address to another without concern for the content.

OSI Model Layers

1. Application Layers (OSI Layers 5-7)

   • Handles data from applications (e.g., web browsers).
   • Application data exists in these layers.

2. Transport Layer (OSI Layer 4)

   • Uses protocols like TCP/UDP.
   • Encapsulates application data with TCP/UDP headers.
   • Introduces terms like TCP segments and UDP datagrams.

3. Network Layer (OSI Layer 3)

   • IP addresses are used to send data packets.
   • Uses IP headers for routing packets across networks.

4. Data Link Layer (OSI Layer 2)

   • Adds DLC frame headers and trailers.
   • Encapsulates the entire packet with necessary addressing (e.g., MAC addresses).

Encapsulation and Decapsulation Process

• Encapsulation: Adding headers/trailers to data at each layer for transmission.
• Decapsulation: Removing headers/trailers to retrieve original data at the destination.
• Involves transitioning data from source to destination by encapsulating through layers.

TCP/IP Headers and Flags

• Control Flags: Bits in headers that control data processing.
  • SYN: Synchronization of sequence numbers.
  • ACK: Acknowledgment data.
  • PSH: Push data to application without buffering.
  • RST: Reset connection.
  • FIN: Indicates last packet from sender.
• IP Headers also contain flags, mostly for data fragmentation control.

Maximum Transmission Unit (MTU)

• MTU: Maximum size of a packet that can be sent without fragmentation.
• Fragmentation occurs when packets exceed MTU; slows down network.
• MTU Discovery: Systems try to automatically determine MTU size.
• ICMP Filtering: Can hinder automatic MTU discovery.

Fragmentation and Its Impacts

• Fragmentation required if packet size exceeds network's MTU.
• Example: Sending 44 bytes of data over a network with 16 bytes MTU requires splitting into multiple frames.
• MTUs are usually stable once set during network creation, unless using tunneling (e.g., VPNs).

Practical MTU Testing

• Utilize ping command with options to test MTU:

  • -F: Don't fragment.
  • -L: Set length of data.

• Example: Testing MTU with Google's DNS (8.8.8.8)

  • Ideal MTU for Ethernet: 1,472 bytes.
  • Use ping -F -L 1472 8.8.8.8 to confirm MTU compatibility.

Conclusion

• Understanding PDUs, encapsulation, and MTU are key for efficient networking.
• Proper configuration and testing ensure optimal network performance and data flow.