PDU

Protocol Data Units (PDUs)

  • PDUs are data containers at each layer of the OSI model that encapsulate information from higher layers
  • Each layer adds its own header (and sometimes trailer) to create a new PDU for the layer below
  • Critical concept: Data flows DOWN the stack on sending device, UP the stack on receiving device
  • Understanding PDU names and encapsulation process is fundamental to troubleshooting network issues

PDU Names by OSI Layer

OSI Layer Layer Name PDU Name Header Added Example Protocols
7-5 Application/Presentation/Session Data Application headers HTTP, FTP, SMTP
4 Transport Segment (TCP) / Datagram (UDP) TCP/UDP header TCP, UDP
3 Network Packet IP header IPv4, IPv6, ICMP
2 Data Link Frame Ethernet header + trailer Ethernet, Wi-Fi
1 Physical Bits Electrical/optical signals Cables, radio waves

Encapsulation Process (Sending Data)

  • Step 1: Application creates data (Layer 7-5)
  • Step 2: Transport layer adds TCP/UDP header → creates segment/datagram (Layer 4)
  • Step 3: Network layer adds IP header → creates packet (Layer 3)
  • Step 4: Data Link adds Ethernet header/trailer → creates frame (Layer 2)
  • Step 5: Physical layer converts to bits for transmission (Layer 1)

De-encapsulation Process (Receiving Data)

  • Step 1: Physical layer receives bits and passes to Data Link (Layer 1→2)
  • Step 2: Data Link removes Ethernet header/trailer from frame → extracts packet (Layer 2→3)
  • Step 3: Network layer removes IP header from packet → extracts segment (Layer 3→4)
  • Step 4: Transport layer removes TCP/UDP header from segment → extracts data (Layer 4→7)
  • Step 5: Application processes the data (Layer 7)

Key Header Information by Layer

Transport Layer Headers (Layer 4):

  • Source/destination port numbers (identifies applications)
  • Sequence numbers (TCP only - for reliable delivery)
  • Window size (flow control)
  • Flags (SYN, ACK, FIN for TCP connection management)

Network Layer Headers (Layer 3):

  • Source/destination IP addresses (logical addressing)
  • TTL/Hop Limit (prevents routing loops)
  • Protocol field (identifies next layer protocol: TCP=6, UDP=17)
  • Fragmentation information

Data Link Layer Headers (Layer 2):

  • Source/destination MAC addresses (physical addressing)
  • EtherType field (identifies Layer 3 protocol: IPv4=0x0800, IPv6=0x86DD)
  • Frame Check Sequence (error detection)

Vocabulary

Encapsulation: Process of adding headers/trailers as data moves down OSI layers

De-encapsulation: Process of removing headers/trailers as data moves up OSI layers

Header: Control information added by each layer containing addressing and protocol information

Trailer: Additional control information added after data (primarily used at Data Link layer for error checking)

Maximum Transmission Unit (MTU): Largest frame size that can be transmitted (Ethernet standard = 1500 bytes)

Real-World Examples

Web Browsing Scenario:

  • Browser creates HTTP request (data)
  • TCP adds port 80 destination (segment)
  • IP adds web server address (packet)
  • Ethernet adds switch MAC address (frame)
  • Transmitted as electrical signals (bits)

Email Transmission:

  • Email client creates SMTP message (data)
  • TCP adds port 25 destination (segment)
  • IP routes through multiple networks (packet)
  • Each hop re-frames with new MAC addresses (frame)

Notes

  • Remember: PDU names are layer-specific - using correct terminology is crucial for CCNA exam
  • Segment vs Datagram: TCP creates segments (connection-oriented), UDP creates datagrams (connectionless)
  • Frame size limitations: If packet exceeds MTU, fragmentation occurs at Layer 3 (creates multiple frames)
  • Key troubleshooting insight: Problems at each layer affect different PDU characteristics (corrupted frames = Layer 2, routing issues = Layer 3)
  • Wireshark analysis tip: You can see all headers in captured frames - practice identifying each layer’s contribution
  • Don’t confuse encapsulation (adding headers) with tunneling (wrapping entire packets in new packets)