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

Key Concepts

• Protocol Data Unit (PDU):

  • Also known as transmission units.
  • Represents a segment of data transmitted as a single unit across a network.

• Ethernet and Frame Encapsulation:

  • Ethernet frames transfer data from one MAC address to another.
  • Ethernet encapsulates data without concern for the contents.

• IP Layer (Layer 3):

  • Transmits data from one IP address to another.
  • Includes UDP/TCP data within IP packets.
  • IP layer doesn't care about the data type inside.

• TCP/UDP Segmentation:

  • TCP segments or UDP datagrams are part of IP packets.
  • Headers define how data should be processed.

OSI Model and Data Encapsulation

• Layers 5, 6, 7 (Application Layers):

  • Application data (e.g., web browsing) sent across the network.

• Layer 4 (Transport Layer):

  • Uses TCP/UDP headers in front of application data.

• Layer 3 (Network Layer):

  • IP headers are added to transport data between IP addresses.

• Layer 2 (Data Link Layer):

  • DLC header and sometimes a trailer encapsulate everything.
  • Ensures communication across networks.

Encapsulation and Decapsulation Process

• Sending Device:

  • Begins with application data.
  • Encapsulates with TCP, IP, and Ethernet headers.

• Receiving Device:

  • Decapsulates by removing headers to access application data.

Header Information and Control Flags

• TCP Flags:

  • Control flags (SYN, PSH, RST, FIN) dictate data flow.

• IP Flags:

  • Primarily manage data fragmentation.
  • Maximum Transmission Unit (MTU) defines max packet size.

Data Fragmentation

• Importance of MTU:

  • Determines the max size of data sent without fragmentation.
  • Sending larger data requires fragmentation, affecting performance.

• Fragmentation Process:

  • If MTU is smaller than the data, the data must be fragmented.
  • Networks might require different MTUs.

• Testing MTU with Ping:

  • Use ping with -f (Don't Fragment) and -l (length) to test.
  • Windows uses ping -f -l 1472 8.8.8.8 to test against Google DNS.

• Adjusting MTU:

  • Reduce size until data transmits successfully.
  • VPNs may require specific MTU settings.

Conclusion

• Understanding PDUs, encapsulation, and MTU is crucial for network efficiency.
• Proper header and flag use ensures effective data transmission.