SNMPv1: Simple Network Management Protocol version 1

Simple Network Management Protocol (SNMP ) is the protocol developed to manage nodes (servers, workstations, routers, switches and hubs etc.) on an IP network. SNMP is an application protocol, which is encapsulated in UDP . The general SNMP information can be found in the SNMP document . 

Currently, there are three versions of SNMP defined: SNMP v1, SNMP v2 and SNMP v3 . In this document, we focus on  SNMPv1. SNMPv1 is a simple request/response protocol. In the SNMPv1 framework, the network-management system installed a request, and managed devices return responses.

The message format for all versions of SNMP is similar except PDU. The PDU (Protocol Data Unit) for SNMPv1 have five different PDU types: GetRequest, GetNextRequest, GetResponse, SetRequest, and Trap. A general description of each of these is given in the next section.

The format for GetRequest, GetNext Request, GetResponse and SetRequest PDUs is shown here.

• PDU type- Specifies the type of PDU transmitted:  0 GetRequest, 1 GetNextRequest, 2 GetResponse and 3 SetRequest.
• Request ID- Associates SNMP requests with responses.
• Error status- Indicates one of a number of errors and error types. Only the response operation sets this field. Other operations set this field to zero.
• Error index- Associates an error with a particular object instance. Only the response operation sets this field. Other operations set this field to zero.
• Variable bindings- Serves as the data field of the SNMPv1 PDU. Each variable binding associates a particular object instance with its current value (with the exception of Get and GetNext requests, for which the value is ignored).

The format of the Trap PDU is shown below:

• PDU type --Specifies the type of PDU (4=Trap).
• Enterprise -- Identifies the management enterprise under whose registration authority the trap was defined.
• Agent address- - IP address of the agent, used for further identification.
• Generic trap type -- Field describing the event being reported. The following seven values are defined:
• Specific trap type -- Used to identify a non-generic trap when the Generic Trap Type is enterprise specific.
• Timestamp -- Value of the sysUpTime object, representing the amount of time elapsed between the last (re-)initialization and the generation of that Trap.