Protocol definition: a protocol is a set of rules governing the time sequence of events that must take place at which relationship level?

Introduction / Context:
In layered network architectures, adjacent layers interact via service interfaces, whereas like-numbered layers on different hosts communicate using protocols. Understanding this separation clarifies the difference between service primitives and protocol data units, and avoids mislabeling device-to-device behaviors as protocol-layer interactions.

Given Data / Assumptions:

• Protocols specify formats, timings, and semantics between entities at the same layer.
• Interfaces describe how one layer offers services to the layer above.
• The question asks where protocol rules apply.

Concept / Approach:
Protocols govern communication between peers—for example, TCP on one host with TCP on another. Across an interface, the relationship is service provision/consumption, not protocol peer exchange. “Between modems” names a device pair, not a layer relationship. “Between an interface” is grammatically incorrect and conceptually imprecise.

Step-by-Step Solution:

Verification / Alternative check:
OSI terminology distinguishes Service Access Points (interfaces) from protocols (peer-to-peer rules). This confirms the selection.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing service interfaces with protocols; conflating device roles with layer relationships.

Final Answer:
between peers.