The OSI Reference Model

Key concepts:

•Layers

•Communications between two adjacent layers

•Encapsulation

•Multiplexing and demultiplexing

•Tunneling

Source: http://bzupages.net/f8/osi-model-7-a-110/

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Network Security 3

Source: http://www.petri.co.il/osi_concepts.htm

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• Source: http://www.novell.com/info/primer/prim05.html

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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.

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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

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• Source: http://en.wikipedia.org/wiki/TCP_IP

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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.

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• Source: http://en.wikipedia.org/wiki/TCP_IP

• 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 (http://en.wikipedia.org/wiki/Tunneling_protocol)

– Example: HTTP can be used to tunnel a protocol that would otherwise be blocked by the firewall.

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Encapsulation vs Tunneling