Multilayer protocol
A protocol whose functions span several OSI layers, or a suite that nests protocols inside one another; the flexibility also allows covert channels and filter bypass.
A multilayer protocol is one whose work is not confined to a single layer of the OSI model. The TCP/IP suite is the broad example: its protocols are stacked, and each layer wraps the one above through encapsulation. DNP3, used in industrial control and utility networks, is the example study sources commonly give of a single protocol spanning several layers, since it handles functions that the OSI model would divide between the application, transport and data link layers. The ISC2 exam outline lists the implications of multilayer protocols as a Domain 4 topic.
Those implications cut both ways. Layering makes protocols flexible: new protocols can be carried inside existing ones, and encryption can be added at different layers. The same flexibility helps an attacker. Data can be hidden inside a protocol that a filter trusts, creating a covert channel. A control that examines only the outer header can be bypassed by what is nested inside, and encryption at one layer can prevent inspection at another. A boundary enforced by a single field can be stepped over, as double tagging does in VLAN hopping.
Exam relevance: a scenario is likely to describe traffic hidden inside an allowed protocol, or a filter defeated by nesting. Candidates are expected to connect this to multilayer protocols and to know DNP3 as the example study sources commonly give.