Devices from different manufacturers need to communicate, but they have different hardware, software, and data formats. Without agreed-upon rules, devices cannot understand each other’s messages.
A set of rules, formats, and procedures that define how devices communicate — specifying message formats, timing, sequencing, and error handling.
- Defines message formats (headers, payload structure)
- Specifies message semantics (what each field means)
- Establishes rules for when messages are sent and how to respond
- Includes error detection and handling procedures
- Operates between peer entities at the same layer across different systems
- Horizontal relationship: governs peer-to-peer communication across systems
- Defines the “how” of communication at a layer
- Enables interoperability between different implementations
- Part of a protocol suite (e.g., TCP/IP suite)
- Contrasts with: Service — protocol is “how”, service is “what”
- Built from: Layered Model — protocols exist at each layer
- Related: TCP — transport layer protocol example
- Related: IP Protocol — network layer protocol example
- Related: Protocol Suite — collection of related protocols
- Protocol specification vs implementation: specs can be ambiguous leading to interop issues
- Protocol ossification: widely deployed protocols become hard to change (e.g., TCP)
- Versioning: protocols need backward compatibility as they evolve