Layers are separate functional components.
Layers communicate sequentially with the layers above and below.
Each layer provides a well-defined service (API) to the layer above, using the services provided by layers below and its own private processing.
It provides great flexibility for each layer but sometimes we may want to break the layering for efficiency.
Reason for layering:
- Modularity
- Well defined service
- Reuse
- Separation of concerns
- Continuous improvement
- Peer-to-peer communications (for Internet)