Service is a set of primitives (operations) that a layer provides to the layer prepared to perform on above it. The service defines what operations the layer is prepared behalf of its users, but it says nothing at all about how these operations are implemented. A service relates to an interface between two layers, with the lower layer being the service provider and the upper layer being the service user.
A protocol is a set of rules governing the format and meaning of frames, packets, or messages that are exchanged by the peer entities within a layer. Entities use protocols in order to implement their service definitions. They are free to change their protocols at will, provided they do not change the service visible to their users. In this way, the service and the protocol are completely decoupled.
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