SNMP Messages

Introduction

SNMP messages are used for two cases:

• A unidirectional request from any application to an SNMP application.

• A request from an SNMP application to any application, with a subsequent response.

• !SNMP-S-REQUEST

• !SNMP-S-RESPONSE

• !SNMP-C-REQUEST (unidirectional, no response)

Note that these messages begin with the special ! character, and so are treated as administration messages. They will be passed between applications using the dedicated administration message FIFO mechanism, and will always be processed before user-level messages are processed.

!SNMP-S-REQUEST

The !SNMP-S-REQUEST message is sent by SnmpApp to any application (including itself and the WatchdogApp) when receiving an external SNMP request addressed to that application.

The attributes of the !SNMP-S-REQUEST message are:

Each entry in the varbinds array contains the following fields:

Note that the type for each varbind entry will be one of the following, based on the internal implementation of RFC 3416:

• RFC 3416 SimpleSyntax:
  • integer-value
  • string-value
  • objectID-value
• RFC 3416 ApplicationSyntax:
  • ipAddress-value
  • counter-value
  • unsigned-integer-value
  • timeticks-value
  • big-counter-value
• RFC 3416 non-value indicators:
  • unSpecified
  • noSuchObject
  • noSuchInstance
  • endOfMibView

Note that the RFC 3416 application-wide syntax arbitrary-value is not supported.

An example request might be:

{
    pdu_type => 'get-request',
    varbinds => [
            {
                oid       => '1.3.6.1.2.1.1.3.0',
                oid_hex   => '2b06010201010300',
                oid_parts => [ '1', '3', '6', '1', '2', '1', '1', '3', '0' ],
                type      => 'integer-value',
                value     => 0
            }
        ]
    }
}

!SNMP-S-RESPONSE

The !SNMP-S-RESPONSE is returned by an application in response to an inbound !SNMP-S-REQUEST message.

The attributes of the !SNMP-S-RESPONSE message are:

The fields of each entry in the varbinds (or in an array varbind) are:

Neither oid_hex nor oid_parts are used in the response. The OID sent on the wire will be computed from the oid parameter.

Note that for strict compliance with RFC 3416, for get-request responses where no match to a local OID was found, the type for the variable binding should be noSuchInstance if a prefix match exists to a local OID and noSuchObject otherwise.

An example successful response might be:

{
    varbinds => [
        {
            oid   => '1.3.6.1.2.1.1.3.0',
            type  => 'integer-value',
            value => 0
        }
    ]
}

An example unsuccessful response might be:

{
    error_index  => 1,
    error_status => 2 # noSuchName
}

Refer to RFC 3416 for further details of how response messages should be constructed.

!SNMP-C-REQUEST

The !SNMP-C-REQUEST message is sent by any application to an SnmpApp and contains details for an SNMP v3 message to be sent to an external party.

Note that this message (and its counterpart sent externally) is unidirectional and does not have or expect a response. It is expected that this is used solely for sending SNMP v3 traps.

The attributes of the !SNMP-C-REQUEST message are: