In layered architecture
Protocol is a set of rules to be followed by peers to communicate.
Peer entities are entities at the same layer.
Two entities at same layer communicate using PDU(Protocol Data Unit) for that layer.
A layer in layered architecture offers a service to the layer above it and uses the services from layer below it.SAP(Service Access Point) is interface between two layers and the data unit is called SDU (Service Data unit).
Protocol may be connectionless or connection oriented.In connection oriented protcol some sort of handsaking or syncronization is required between communicating partries prior to any communication.An example of connection oriented protocol is our telephone system.In connection less protcol no handsaking is required for communication.An example of connection less protocol is Postal System.