The principle of SNMP is simple. It works by polling and answering, and adopts centralized or centralized distributed control mode to control and manage the whole network. The whole network management system includes four parts: SNMP manager, SNMP agent, management information base and management protocol.

SNMP is located in the application layer, and uses two ports of UDP (16 1 and 162) to exchange management information between administrators and agents. UDP port 16 1 is used for data transceiver, and UDP port 162 is used for proxy alarm (i.e. sending Trap messages).

Snmpv1(RFC1157) adopts centralized management mode, and only one SNMP manager polls and manages multiple agents in the whole network. Five types of PDUs are exchanged between administrators/agents:

The GetRequest 0 administrator queries the agent for the value of the specified variable.

GetNextRequest 1 The administrator goes to the agent to query the value of the next variable.

GetResponse 2 proxy to the administrator, sending back the execution result (correct/wrong).

The SetRequest 3 administrator is set as a proxy and sets the values of variables maintained by the proxy.

Trap 4 agent administrator, actively transmitting alarm information.