Data Transfer and Protocol Data Units (PDUs)

Introduction to PDUs

• A Protocol Data Unit (PDU) is essential for moving data across a network.
• Sometimes referred to as transmission units, PDUs encapsulate a piece of data for transfer.

Layers of Data Transfer

Ethernet (Layer 2)

• Data is sent within an Ethernet frame from one MAC address to another.
• Ethernet does not interpret the data inside the frame; it simply encapsulates it.

IP Layer (Layer 3)

• IP layer sends data between IP addresses.
• It encapsulates UDP, TCP, or other data without concern for the content.

Transport Layer (Layer 4)

• TCP or UDP headers are added to the data from the application layer before sending it.
• Each layer adds its own header, encapsulating the data further.

Application Layers (Layer 5-7)

• Application data can include web content, emails, etc.
• Data is encapsulated in the TCP or UDP header for transfer.

Encapsulation Process

• Sending Device:
  • Encapsulates application data with TCP header, IP header, and Ethernet frame header.
• Receiving Device:
  • Decapsulates data by removing headers to access the original application data.

Control Information and Flags in Headers

TCP Header Flags

• TCP flags indicate how data should be processed:
  • SYN: Synchronizes sequence numbers.
  • PSH: Pushes data to the application without buffering.
  • RST: Resets the connection.
  • FIN: Marks the last packet.

IP Header Flags

• IP flags mainly deal with data fragmentation, crucial for navigating networks with varying MTUs (Maximum Transmission Units).

Maximum Transmission Unit (MTU)

• MTU defines the largest packet size that can be sent without fragmentation.
• Fragmentation can slow down network traffic and increase overhead.
• MTU values can vary across network hops.

Fragmentation Example

• If the MTU is set to 16 bytes, sending 44 bytes requires fragmentation into 3 packets.
• Design Consideration: MTUs are typically set during network design and rarely change.

Testing MTU with Ping Command

• Use the ping command to determine MTU values:
  • Windows Command: ping -f -l <size> <destination>
  • For example, ping -f -l 1472 8.8.8.8 tests MTU against Google DNS.
• Adjust the size until the packet can be sent without fragmentation.

Conclusion

• Understanding PDUs, MTUs, and encapsulation/decapsulation is essential for efficient data transfer in networks.