Network Protocol Stack Overview

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.