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

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)