Encapsulation and Decapsulation

Encapsulation

Encapsulation is the process where each protocol layer “wraps” the data it receives with its own header (and sometimes a trailer) before handing it to the layer below. This modular approach lets each layer focus solely on its responsibility—port addressing, routing, framing, error checks, etc. without worrying about the rest of the stack.

Protocol Data Units (PDUs)

Layer	PDU Name	What’s Added
Application	Data	Application‑specific formatting (e.g., HTTP headers)
Transport	Segment (TCP)Datagram (UDP)	Source/destination ports,sequence numbers, flags
Network	Packet	Source/destination IP addresses,TTL, protocol field
Data Link	Frame	Source/destination MACs,frame check sequence (FCS) trailer

Step‑by‑Step Encapsulation

Application Layer
- Your application (web browser, chat client, etc.) generates the raw payload and passes it down.
Transport Layer
- TCP adds a header with ports, sequence/ACK numbers, flags, window size, checksum → TCP segment.
- UDP adds ports, length, checksum → UDP datagram.
Network Layer
- The IP layer prepends source and destination IP addresses, TTL, fragmentation info → IP packet.
Data Link Layer
- Ethernet/Wi‑Fi wraps the packet with source/destination MAC addresses and a trailer with FCS for error detection → Frame.
Physical Layer
- The frame is converted to electrical, optical, or radio signals and transmitted over the medium.

Note: Frames often include both a header (addresses, type) and a trailer (error‑check sequence).

Decapsulation

On the receiving side, each layer strips off its header/trailer and passes the remaining PDU up to the layer above:

Frame → Packet: Data Link checks FCS, removes header/trailer
Packet → Segment/Datagram: Network verifies IP checksum, removes IP header