Discuss the service primitives in a client server model?

Answer 1

A service is formally specified by a set of primitives or operations available to the user to access the service. These primitives tell the service to perform some action or report an action taken by the peer entity. The primitives for the connection oriented service are given in following table. Primitives Meaning Listen Block waiting for an incoming connection Connect Establish a connection with waiting peer Receive Block waiting for an incoming message Send Send a message to the peer Disconnect Terminate a connection Table: - Service primitives for a connection oriented service Communication in a simple client server model using the above service primitives is illustrated in figure [A]. First the server executes LISTEN to indicate that it is ready to accept incoming connections. The client executes CONNECT (1) to establish the connection with the server. The server now unblocks the listener and sends back an acknowledgement (2). Thus the connection is established. Figure [A]:- Simple client server model on a connection oriented network
The next step for a server is to execute RECEIVE (3) to prepare to accept the first request. The arrival of the request packet unblocks the server so that it can process the request. After it has done the work it uses SEND (4) to answer to the client. After all the data transfer is done it can use DISCONNECT (5) for suspending the client. When the server gets this packet, it also issues a DISCONNECT (6) and when it reaches the client, the client process is releases and the connection is broken. In the process packets may get lost, timings may be wrong; many other complex issues may arise. The Relationship of Services to Protocols Figure [B]: Relationship between the service and protocols A service is a set of primitives that a layer provides to the layer above it. The service defines what operation the layer is prepared to perform on behalf of its users. It says nothing about the implementation of these operations. A protocol is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer. Figure [B] illustrates the relationship of services to protocols. Entities use protocols to implement their service primitives. Protocols relate to the packets sent between entities.