Meaning – The term protocol boundary, refers to the signals and rules governing interactions between two components within a node.
Communicating systems use well-defined formats for exchanging various messages. Each message has an exact meaning intended to elicit a response from a range of possible responses pre-determined for that particular situation. The specified behavior is typically independent of how it is to be implemented. Communication protocols have to be agreed upon by the parties involved.
A programming language describes the same for computations, so there is a close analogy between protocols and programming languages: protocols are to communicate what programming languages are to computations. An alternate formulation states that protocols are to communicate what algorithms are to computation.
Example of usage – “Getting the data across a network is only part of the problem for a protocol. The data received has to be evaluated in the context of the progress of the conversation, so a protocol must include rules describing the context.”