SNMP: It stands for Simple Network Management Protocol.  It is an application layer protocol for network device management.

Using SNMP protocols management software can query, compile, store and display information about network devices.

There are multiple versions of the SNMP protocol, and many networked hardware devices implement some form of SNMP access. The most widely used version is SNMPv1, but it is in many ways insecure. SNMPv3 is recommended since it provides more advanced security features.

SNMP consist of devices containing SNMP agents and SNMP manager.

• SNMP agents do most of the work. They are responsible for gathering information, storing, querying and updating a database called the management information base (MIB) using the SNMP protocol. SNMP agents respond to most of the commands defined by the protocol. These include GetRequest, GetNextRequest, GetBulkRequest, SetRequest and InformRequest. An agent also sends Trap messages.

  The agent computer configures which managers should have access to its information. It reports information on devices it can connect to that are not configured for SNMP traffic. Due to this feature it gets the components online and SNMP accessible.

• SNMP manager queries agents for information and have data about all of the SNMP-enabled devices in their network and can issue requests to gather information and set certain properties. The manager can be any machine that sends query requests to SNMP agents with the correct credentials.
  The management component is a less complex than the client configuration as the management component simply requests data.
  The management component uses commands which include GetRequest, GetNextRequest, GetBulkRequest, SetRequest, InformRequest, and Response. It even responds to Trap and Response messages.

The Management Information Base (MIB) is a top-down hierarchical tree, predefined structure that stores information that can be queried. It is available to SNMP requests which originate from hosts that has been authenticated with the correct credentials by an SNMP manager. Object identifier (OID) is an entire address that refers to a specific node of the tree.

The SNMP Protocol Commands are as follows:

• Get: A Get message is sent by a manager to an agent to request the value of a specific OID. This request is answered with a Response message that is sent back to the manager with the data.
• GetNext: A GetNext message allows a manager to request the next sequential object in the MIB.
• Set: A Set message is sent by a manager to an agent in order to change the value held by a variable on the agent. It is used to control configuration information and is the only write operation defined by the protocol.
• GetBulk: It functions as if multiple GetNext requests were made from manager to agent. The reply back to the manager will contain as much data as possible as the packet allows.
• Response: Any requested information is sent by the agent back to the manager. It serves as a transport as well as an acknowledgement of receipt of the request. If the requested data cannot be returned then the response contains error fields that can be set with further information. A response message and Inform messages must be returned for any of the above requests.
• Trap: A trap message is sent by an agent to a manager. Traps are asynchronous notifications in that they are unsolicited by the manager receiving them. They are mainly used by agents to inform managers of events that are happening on their managed devices.
• Inform: To confirm the receipt of a trap, a manager sends an Inform message back to the agent. If the agent does not receive this message, it may continue to resend the trap message.