GetNextRequest SetRequest manager agent GetResponse Figure 14.1 SNMPv1 packet data units (PDUs). The get operation of the GetRequest PDU is used by the manager to retrieve the value of object instances from an agent. The agent uses the response operation of the GetResponse PDU to respond to the get operation with the object instances requested. If the agent cannot provide all the object instances in the requested list then it does not provide any value. The GetNext operation of the GetNextRequest PDU is used by the manager to retrieve the value of the next object instance in the MIB. This operation assumes that object values in the MIB can be accessed in tabular or sequen- tial form. The set operation of the SetRequest PDU is used by the manager to set the values of object instances in the MIB. The trap operation (Trap PDU) is used by agents to asynchronously notify the manager on the occurrence of an event. The PDUs GetRequest, GetNextRequest, SetRequest, and GetResponse have the format shown below: Request ID Error Status Error Index Variable Bindings Request ID is used as an identifier for correlating a response with the request. This way duplicate responses from the agent can be discarded at the manager. Error Status indicates the kind of error and can take values like genErr, bad- Value, tooBig, noSuchName, readOnly, and noError. Error Index gives the position of the object instance (variable) associated with the error. Variable binding is a list of object instance names and values. Let us check how error status works taking an example of GetRequest PDU sent to the agent. If the agent encounters errors while processing the GetRequest PDU from the manager then error status is "genError." Fundamentals of EMS, NMS and OSS/BSS When the agent receives a GetRequest PDU from the manager, the agent checks whether the object name is correct and available in MIB present in the network element being managed. When the object name is not correct or when the object name corresponds to an aggregate object type (not sup- po