Protocol Layers
Each layer implements a service via its own internal-laye actions and relies on services provided by layers below. Layering explicitly structure allows identification and relationship of system’s pieces; as well as modularization eases maintenance.
- Application Layer
- Support network applications
- Transport Layer
- process-to-process data transfer
- Network Layer
- routing of datagrams from source to destination
- Link Layer
- data transfer between neightboring network elements
- Physical Layer
- bits on the wire
- Application exchanges messages \(M\) to implement some application service using services provided by transport layer.
- Transport layer protocol encapsulates application-layer message \(M\) with transport-layer header \(H_t\) to create a transport-layer segment.
- Network-layer protocol encapsulates transport-layer segments \([H_t | M]\) with network-layer header \(H_n\) to create a network-layer datagram.
- Link-layer protocol encapsulates network datagram \([H_n | H_t | M]\) with link-layer header \(H_\ell\) to create a link-layer frame