The following diagram serves as reference. One communication path is illustrated between the end-point E1 and E2. The communication traffic enters intermediate system A, is physically transmitted over to System B.
Interface "P"
This interface handles the "peer-to-peer" relationship between systems. In a communication system, the high-level semantic for this interface follows:
forward( traffic unit )
Where traffic unit (TU) represents the unit to forward to the end-point(s). On a packet network layer, the usual traffic unit is the packet (of course :-) whilst on an optical network layer the traffic unit will be an octet.
Interface "C"
This interface handles the "client-server" relationship between network protocol layers in a system. In a communication system, the high-level semantic for this interface follows:
forward( traffic unit, destination address, QOS )
The parameter destination address can either be explicit or implicit: this depends on the network layer in question. The parameter QOS (Quality Of Service) too can either be explicit or implicit depending the network layer technology.
Peer-To-Peer relationship
The peer-to-peer relationship exists between separate (distinct) instantiations belonging to the same network protocol layer e.g. protocol sub-system N of system A and protocol sub-system N of system B. Physical proximity between sub-systems does not change the role of this interface.
Systems exchanging traffic through peer-to-peer relationships do so through a common address space. E.g. Internet Protocol.
Client-Server relationship
The client-server relationship exists between a pair of network protocol layers. This relationship is characterize by a fundamental premise: the server-side of the relationship does not expose client-specific state i.e. the server must be designed to be "client-agnostic".
The client-side "adapts" the traffic it must send by mapping the desired destination address to the address space semantics of the server-side. The same logic is applied to the QOS parameter.
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