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

2. Source: http://bzupages.net/f8/osi-model-7-a-110/http://bzupages.net/f8/osi-model-7-a-110/ T. A. YangNetwork Security2

3. 3 Source: http://www.petri.co.il/osi_concepts.htmhttp://www.petri.co.il/osi_concepts.htm T. A. Yang

4. Source: http://www.novell. com/info/primer/p rim05.html http://www.novell. com/info/primer/p rim05.html T. A. YangNetwork Security4

5. 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. T. A. YangNetwork Security5

6. 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. T. A. YangNetwork Security6

7. Source: http://en.wikipedia.org/wiki/TCP_IPhttp://en.wikipedia.org/wiki/TCP_IP T. A. YangNetwork Security7

8. 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. T. A. YangNetwork Security8 Source: http://en.wikipedia.or g/wiki/TCP_IP http://en.wikipedia.or g/wiki/TCP_IP

9. 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)http://en.wikipedia.org/wiki/Tunneling_protocol –Example: HTTP can be used to tunnel a protocol that would otherwise be blocked by the firewall. T. A. YangNetwork Security9 Encapsulation vs Tunneling