Networking is complex with many functions (addressing, routing, reliability, applications). Without organization, protocols would be monolithic and impossible to modify independently.
A conceptual framework that divides network functionality into layers, where each layer provides services to the layer above and uses services from the layer below.
- Each layer has a specific responsibility (e.g., physical, link, network, transport, application)
- Data is encapsulated with headers as it moves down layers
- Data is decapsulated as it moves up layers
- Peer entities at the same layer communicate using protocols
- Adjacent layers interact through service interfaces (SAPs)
- Modular: each layer can be modified independently
- Encapsulation: each layer adds its header
- Service abstraction: upper layers don’t need to know lower layer details
- Two main models: OSI (7 layers) and TCP/IP (4-5 layers)
- Built from: Encapsulation — wrapping data with headers
- Built from: Service — what each layer provides
- Built from: Protocol — how peers communicate at each layer
- Related: IP Model — practical 4-layer model
- Related: OSI Model — conceptual 7-layer model
- Strict layering can reduce efficiency (extra headers, processing)
- Some modern protocols blur layer boundaries (e.g., MPLS)
- TCP/IP model is descriptive (how internet works), OSI is prescriptive (how it should work)