SNMP (Simple Network Management Protocol)

Network management via SNMP is based on an agent manager model. The central management station is the system from which the administrator monitors and controls the various network participants. For this purpose, management software is installed that enables SNMP data retrieval and the initiation of certain actions. The agents, which are also applications, are the counterpart on the side of the individual network components. You enter the relevant data on the target host and pass it on to the management station, but you can also make settings yourself and trigger certain actions. These kinds of agent applications are already in use by default in most popular Windows and Linux systems, for example in the form of the snmpd daemon (Linux only).

The SNMP protocol specifies seven possible message types for communication between manager and agent:

• GET request: GET requests are the default messages for retrieving a specific record on the intended network device.
• GETNEXT request: This message format is required if subsequent data records need to be queried, e.g. for tables.
• GETBULK request: If a defined number of data records are to be retrieved with a single request, the manager application can send a GETBULK request (from SNMPv2). Such a request is comparable to several successive GETNEXT requests.
• SET request: SET requests allow the manager to change one or more data records of the intended network device or to trigger certain actions. An example situation in which several adjustments are necessary is configuring an IP address, which also requires the specification of a network mask at the same time.
• GET response: If the manager has requested one or more data records or initiated changes or actions, the agent responds with GET responses. These response packages contain either the requested data, a confirmation of the adjustments, or an error message if the requests could not be answered correctly.
• SNMP trap: SNMP traps are agent messages sent without prompts from the manager station. This might happen if something unexpected occurs. The SNMP traps can communicate the event in two ways. The first option is to add a unique identification number, the meaning of which the manager can look up in the information database (MIB). If option number two is selected, the SNMP traps not only inform about the event, but also contain the corresponding data without displaying a specific identification number.
• INFORM request: INFORM requests basically fulfill the same function as SNMP traps. In contrast to these, however, the INFORM packages are characterized by the fact that their receipt is acknowledged by the manager. As a result, the agent can resend the message if it has not reached the manager in the first attempt.

As already mentioned, the Simple Network Management Protocol prescribes the use of the connectionless transport protocol UDP for the transmission of the listed message packets. SNMP uses UDP port 161 for the various GET queries to the agents (including replies), while the automatically sent SNMP traps are sent via UDP port 162.