Data Encapsulation and Decapsulation

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A network protocol is a set of rules for exchanging data in a structured format.

• 2 principle functions:
  • Addressing
    • describes where data messages should go
  • Encapsulation
    • describes how data messages should be packaged for transmission
    • e.g., like an envelope for a letter
    • each layer requires its own encapsulation
    • each layer adds fields in a header to whatever payload data it receives from an application or protocol
• a network uses many different protocols
• not all protocols conform exactly to OSI model
• at each layer, for two nodes to communicate they must be running the same protocol
  • communication between nodes at the same layer is same layer interaction
  • for communication on each node, each layer provides services for the layer above and uses the services of the layer below
    • referred to as adjacent layer interaction

Encapsulation

• when a message is sent from one node to another:
  • messages travels down the stack of layers on sending node
  • reaches receiving node via the transmission media
  • passes up the stack on the receiving node
  • at each layer except physical layer, the sending node adds a header to the data payload
    • forms a chunk of data called a protocol data unit (PDU)
  • this process is encapsulation

Example

• Data generated by application on sending node (e.g. HTTP) with its own application header
• at transport layer, a TCP header is added to the application data
• at network layer, the TCP segment is wrapped in an IP header
• at data link layer, the IP packet is encapsulated in an Ethernet frame
• at the physical layer, the stream of bits making up the frame is transmitted over the network as a modulated electrical signal

Decapsulation

• Decapsulation is the reverse process performed by the receiving node:
  • receives stream of bits at the physical layer
  • decodes an Ethernet frame
  • extracts the IP packet from the frame
  • resolves the information in IP header
  • resolves the information in the TCP header
  • resolves the information in the Application header
  • extracts the HTTP data for processing by a software application