Slide 15.5: A web architecture
Slide 15.7: The TCP/IP model
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The Open Systems Interconnect (OSI) Model

OSI is a model of network architecture and a suite of protocols (a protocol stack) to implement it, developed by International Organization for Standardization (ISO) in 1978 as a framework for international standards in heterogeneous computer network architecture. The OSI architecture is split between seven layers, from lowest to highest:
  1. Physical layer,
  2. Data Link layer,
  3. Network layer,
  4. Transport layer,
  5. Session layer,
  6. Presentation layer, and
  7. Application layer.

Each layer uses the layer immediately below it and provides a service to the layer above. In some implementations a layer may itself be composed of sub-layers. Each level “talks to” the equivalent level on the other node. Transmission errors call for retransmission of affected packet or packets.