SNMP Protocol Overview

The Simple Network Management Protocol (SNMP) is essentially a request-reply protocol running over UDP (ports 161 and 162), though TCP operation is possible. SNMP is an asymmetric protocol, operating between a management station (smart) and an agent (dumb). The agent is the device being managed - all its software has to do is implement a few simple packet types and a generic get-or-set function on its MIB variables. The management station presents the user interface. Simple management stations can be built with UNIX command-line utilities. More complex (and expensive) ones collect MIB data over time and use GUIs to draw network maps.

SNMP Version 1 is documented in RFC 1157. SNMP Version 2 is documented in several RFCs:

• RFC 1902 (MIB Structure)
• RFC 1903 (Textual Conventions)
• RFC 1904 (Conformance Statements)
• RFC 1905 (Protocol Operations)
• RFC 1906 (Transport Mappings)
• RFC 1907 (MIB)

SNMP's packet formats are described using Abstract Syntax Notation 1 (ASN.1), one of ISO's "Open" protocols. ASN.1 basically fills the role of XDR, but does so differently. ASN.1, like all OSI standard documents, is not freely available on-line.

An SNMP operation takes the form of a Protocol Data Unit (PDU), basically a fancy word for packet. Version 1 SNMP supports five possible PDUs:

• GetRequest / SetRequest supplies a list of objects and, possibly, values they are to be set to (SetRequest). In either case, the agent returns a GetResponse.

• GetResponse informs the management station of the results of a GetRequest or SetRequest by returning an error indication and a list of variable/value bindings.

• GetNextRequest is used to perform table transversal, and in other cases where the management station does not know the exact MIB name of the object it desires. GetNextRequest does not require an exact name to be specified; if no object exists of the specified name, the next object in the MIB is returned. Note that to support this, MIBs must be strictly ordered sets (and are).

• Trap is the only PDU sent by an agent on its own initiative. It is used to notify the management station of an unusual event that may demand further attention (like a link going down). In version 2, traps are named in MIB space. Newer MIBs specify management objects that control how traps are sent.