The SNMP agents can also initiate communication with the managers by sending so called ``Traps'' . This can for example be used to notify the network operator upon special events like failures of links etc.
A connectionless transport protocol, UDP, is employed to convey SNMP messages between the agents and managers. Even though with this protocol delivery is not guaranteed, the reality is that a vast majority of the messages are successfully delivered. In addition most manager programs keep track of requests that they have sent. If a response is not received within a time-out interval, the manager program retransmits the request. However, using an unreliable protocol to carry SNMP has one disadvantage when transmitting the unsolicited trap messages from an agent to a manager. The agent has no way of determining if a trap was actually received by the intended manager (the manager may not even be running!).