A service is a set of primitives or we call it as operations where a user can invoke to access the service.
The selective primitives tell the service to perform an action taken by peer nodes.
The initiating node wants to establish a connection send CONNECT.
Request − A node wants to do some work or a packet is being sent.
The receiver accepts the request and CONNECT.
Indication − A node is saying that it wants to set up a connection to it.
The CONNECT.
Response is primitive to tell whether it wants to accept or reject the proposed connection.
The node issuing the initial CONNECT. request finds out what happened via a CONNECT. Confirm primitive.
Types of Service Primitives
Generally service primitives are divided into four classes −
Request − A service node wants some service from its adjacent layer to pass the parameters to mention the requested service.
Indication − Another Service node or receiver node gets an indication that a procedure has been invoked by the adjacent service node.
Response − A receiver service node acknowledges or completes some procedure.
Confirm − service nodes acknowledge the permission to get connected or not.
The different types of service primitives are explained below −
| Primitive | Explanation |
|---|---|
| Indication | Information given by an node of an event Example − The receiver just received a connection request. |
| Request | Service is requested by the node. Example − Requesting for a connection to a remote computer. |
| Response | Node is responding to an event. Example − the receiver sending the permission to connect. |
| Confirm | A node acknowledges the response to its request. Example − receiver acknowledge after receiving the data from sender |
Parameters of Service Primitives
The parameters of primitives are: CONNECT, DATA, DISCONNECT which are used to establish a connection, to send data, and Disconnect a connection.
Generally services can be either confirmed or unconfirmed.
Confirmed service − the connection oriented services are called confirmed services.
Unconfirmed service − the connectionless services are called unconfirmed services.
Connection Oriented Service
The service primitives for the connection oriented services are as follows −
CONNECT. request: Request a connection to be established.
CONNECT. indication: Signal the called party (phone ringing).
CONNECT. response: Accept / reject calls.
CONNECT. confirm: Tell the caller whether the call was accepted.
DATA. request: Request that data be sent.
DATA. indication: Signal the arrival of data.
DISCONNECT. request: Request that a connection be released.
DISCONNECT. indication: Signal the peer about the request.
Example: Modern Telephone and Internet Services
Connection less Service
The service primitives for the connection less services are as follows −
SENDING. data − Sender just sending data (traditional postal service).
REPORT. confirm − availability of reports the data reached at destination (registered post)