SNMP requires two basic elements to function: an agent and a manager. A Management Information Base (MIB) is also required, which provides a small set of commands for the exchange of information between the agent and the manager, and lists the unique object identifier (OID) of each managed element in an SNMP network.
Agent – the devices (or software) that are managed. Provides the interface between the manager and the objects being managed or monitored. The agent typically stores and retrieves data as defined by the MIB and can asynchronously signal an event to the manager.
Manager – provides the interface between the system operator and the management system. The manager typically queries agents, gets responses from agents, sets variables in agents, and acknowledges asynchronous events from agents.
MIB – organized in a tree structure with individual variables, such as point status or description. A long numeric tag or object identifier (OID) is used to uniquely distinguish each variable in the MIB and in SNMP messages. The MIB provides five basic commands to communicate between the manager and the agent—Get, GetNext,GetResponse, Set, and Trap.
The MIB serves as a data dictionary or codebook that is used to assemble and interpret SNMP messages.
Both the agent and manager implement the full SNMP communications protocols—the User Datagram Protocol (UDP) and the Internet Protocol (IP).
The UDP is the IP transport layer protocol that supports
SNMP messages. Unlike TCP, UDP is a connectionless protocol. A UDP
host places messages on the network without first establishing a
connection with the recipient. UDP does not guarantee message delivery,
but it is a lightweight protocol that can transport a large number
of status messages without using too many network resources.
When the SNMP manager and agent communicate:
The Get and GetNext messages allow the manager to request information for a specific variable.
The agent, upon receiving a Get or GetNext message, issues a GetResponse message to the manager with either the information requested or an error indication as to why the request cannot be processed.
A Set message allows the manager to request a change be made to the value of a specific variable in the case of an alarm remote that operates a relay.
The agent responds with a GetResponse message indicating the change has been made or an error indication as to why the change cannot be made.
The Trap message allows the agent to spontaneously inform the manager of an ìimportant event.
Most of the messages (Get, GetNext, and Set) are issued only by the SNMP manager. Because the Trap message is the only command capable of being initiated by an agent, it is used by remote telemetry units (RTUs) to report alarms that notify the SNMP manager as soon as an alarm condition occurs instead of waiting for the SNMP manager to ask.
An SNMP trap is a change-of-state (COS) message issued by an SNMP agent that reports an event that can mean an alarm, a clear, or simply a status message.
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