4. Discovery

The User-based Security Model requires that a discovery process obtains sufficient information about other SNMP engines in order to communicate with them. Discovery requires an non-authoritative SNMP engine to learn the authoritative SNMP engine's snmpEngineID value before communication may proceed. This may be accomplished by generating a Request message with a securityLevel of noAuthNoPriv, a msgUserName of zero-length, a msgAuthoritativeEngineID value of zero length, and the varBindList left empty. The response to this message will be a Report message containing the snmpEngineID of the authoritative SNMP engine as the value of the msgAuthoritativeEngineID field within the msgSecurityParameters field. It also contains a Report PDU with the usmStatsUnknownEngineIDs counter in the varBindList.

If authenticated communication is required, then the discovery process should also establish time synchronization with the authoritative SNMP engine. This may be accomplished by sending an authenticated Request message with the value of msgAuthoritativeEngineID set to the newly learned snmpEngineID and with the values of msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime set to zero. For an authenticated Request message, a valid userName must be used in the msgUserName field. The response to this authenticated message will be a Report message containing the up to date values of the authoritative SNMP engine's snmpEngineBoots and snmpEngineTime as the value of the msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields respectively. It also contains the usmStatsNotInTimeWindows counter in the varBindList of the Report PDU. The time synchronization then happens automatically as part of the procedures in section 3.2 step 7b. See also section 2.3.