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The OSI Reference Model. Key concepts: Layers Communications between two adjacent layers Encapsulation Multiplexing and demultiplexing Tunneling. Source: Source:

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the osi reference model

The OSI Reference Model

Key concepts:


Communications between two adjacent layers


Multiplexing and demultiplexing


multiplexing and demultiplexing
Multiplexing and Demultiplexing
  • Multiplexing: Multiple types of data streams are delivered on a shared channel.
  • Can occur at any layer
    • Example: Application packets of different types (e.g., HTTP, SMTP, etc.) are encapsulated into TCP packets at the transport layer.

- Another example: TCP and UDP packets are multiplexed into IP packets.

Network Security

multiplexing and demultiplexing1
Multiplexing and Demultiplexing
  • Demultiplexing: The reverse operation of multiplexing. Different types of data streams delivered on a shared channel are split into the original types of packets.
  • Two operations:
    • Given a packet, check the type of the payload.
    • Remove the header and send the payload to the appropriate protocol one level above.

Network Security

encapsulation vs tunneling
Encapsulation vs Tunneling
  • Encapsulation: A packet is encapsulated into the payload of a lower layer packet; a new header is added at the lower layer.
  • Source:

Network Security

encapsulation vs tunneling1
Encapsulation vs Tunneling
  • Tunneling: A packet (the payload protocol) is encapsulated into another protocol at the same or higher layer (the delivery protocol).
    • Example: A IP packet is encapsulated into a IPsec packet.
  • Tunneling can be used to circumvent a firewall policy (
    • Example: HTTP can be used to tunnel a protocol that would otherwise be blocked by the firewall.

Network Security