Let's continue to discuss the SNMP. Now, I am using my old ASUS laptop in home, I can not found "del" and "home" key again.

Device Activity
Inactive : No monitoring is be done,if you received a message from this device,you would ignore it.
Reactive: No monitoring is being done,you react the problem if it occurs.
Interactive: You monitor the network elements,and interactive with it to eliminate the side-effect alarm and isolate the root cause.
Proactive:You monitor the network elements,and run the predefined automatic restoral process when the special problem occures,it is possible to minimize downtime.

Some tools aid to analysis network elements' problem: Trend analysis, Response time reporting ,Alarm correlation and Trouble resolution.

In general,the ability to monitor and proactive a network element begins with trend analysis and reporting.The goal of trend analysis is to identify when the network element reach the maximum capacity.
We must plan for both scheduled and emergency change in network elements.

SNMP uses the UDP (which is defined in RFC 768) as the transport protocol for passing data between managers and agents. which means no end-to-end connections is made between NMS and agent when datagram are sent back and forth. This feature make it unreliable since there is no acknowledgement of lost datagrams at the protocol level. It's up to the SNMP application to determine if datagrams are lost and retransmit them if it so desire.
The NMS sends a UDP request to a agent and wait for a response, the length of time that NMS waits depends on how it's configured. If the timeout is reached and the NMS have not heard back from the agent, It assumes the datagram was lost and retransmits the request again. The number of times that the NMS retransmits packages is also configurable.