End systems connect to internet via ISPs
Access ISPs interconnected
- Any 2 hosts can send packets to each other
Resulting network of networks is compllex
Complex because structure is driven by economy / national policies
O(N^2) if we connect every ISP to other access ISPs
Infra which provides services to applications
Provides programming interface to apps
hooks which allow sending & receiving app programs to connect to internet.
Provides service options, analogous to postal service.
Each layer implements a service by performing actions within that layer, rely on services provided by the layer below it.