1. A formal set of conventions governing the format and control of interaction among communicating functional units. Note: Protocols may govern portions of a network, types of service, or administrative procedures. For example, a data link protocol is the specification of methods whereby data communications over a data link are performed in terms of the particular transmission mode, control procedures, and recovery procedures. 2. In layered communications system architecture, a formal set of procedures that are adopted to facilitate functional interoperation within the layered hierarchy. 3. A set of rules and formats, semantic and syntactic, permitting information systems (IS's) to exchange information. 4. A set of semantic and syntactic rules that determine the behavior of entities in performing communication functions.