[AGENT++] usm counters - usmStatsNotInTimeWindows and usmStatsUnknownEngineIDs keeps incrementing by one after every snmpV3 get/set operation

[jc rani]

Hi

I ran agentpp example code cmd_ext_mib and performed snmp v3 get/set
request operation and got snmp response successfully. But found that during
every get/set operation, the counters -  usmStatsNotInTimeWindows and
usmStatsUnknownEngineIDs got incremented everytime.

For instance:

Initially the counter values are usmStatsUnknownEngineIDs - 0 ;
usmStatsUnknownEngineIDs - 0

After snmpwalk operation,
$ snmpwalk -v3 -u MD5DES -l authPriv -a MD5 -A MD5DESUserAuthPassword -x
DES -X MD5DESUserPrivPassword 20.0.0.162:12345 1.3.6.1.6.3.15.1.1
SNMP-USER-BASED-SM-MIB::usmStatsUnsupportedSecLevels.0 = Counter32: 0
*SNMP-USER-BASED-SM-MIB::usmStatsNotInTimeWindows.0 = Counter32: 1*
SNMP-USER-BASED-SM-MIB::usmStatsUnknownUserNames.0 = Counter32: 0
*SNMP-USER-BASED-SM-MIB::usmStatsUnknownEngineIDs.0 = Counter32: 1*
SNMP-USER-BASED-SM-MIB::usmStatsWrongDigests.0 = Counter32: 0
SNMP-USER-BASED-SM-MIB::usmStatsDecryptionErrors.0 = Counter32: 0

After another snmpwalk operation,
$ snmpwalk -v3 -u MD5DES -l authPriv -a MD5 -A MD5DESUserAuthPassword -x
DES -X MD5DESUserPrivPassword 20.0.0.162:12345 1.3.6.1.6.3.15.1.1
SNMP-USER-BASED-SM-MIB::usmStatsUnsupportedSecLevels.0 = Counter32: 0
*SNMP-USER-BASED-SM-MIB::usmStatsNotInTimeWindows.0 = Counter32: 2*
SNMP-USER-BASED-SM-MIB::usmStatsUnknownUserNames.0 = Counter32: 0
*SNMP-USER-BASED-SM-MIB::usmStatsUnknownEngineIDs.0 = Counter32: 2*
SNMP-USER-BASED-SM-MIB::usmStatsWrongDigests.0 = Counter32: 0
SNMP-USER-BASED-SM-MIB::usmStatsDecryptionErrors.0 = Counter32: 0

Performed a get operation on usmStatsUnknownEngineIDs
$ snmpget -v3 -u MD5DES -l authPriv -a MD5 -A MD5DESUserAuthPassword -x DES
-X MD5DESUserPrivPassword 20.0.0.162:12345 .1.3.6.1.6.3.15.1.1.4.0
*SNMP-USER-BASED-SM-MIB::usmStatsUnknownEngineIDs.0 = Counter32: 3*

Performed a get operation on usmStatsWrongDigests
$ snmpget -v3 -u MD5DES -l authPriv -a MD5 -A MD5DESUserAuthPassword -x DES
-X MD5DESUserPrivPassword 20.0.0.162:12345 .1.3.6.1.6.3.15.1.1.5.0
SNMP-USER-BASED-SM-MIB::usmStatsWrongDigests.0 = Counter32: 0

Now again performed a get operation on usmStatsUnknownEngineIDs
$ snmpget -v3 -u MD5DES -l authPriv -a MD5 -A MD5DESUserAuthPassword -x DES
-X MD5DESUserPrivPassword 20.0.0.162:12345 .1.3.6.1.6.3.15.1.1.4.0
*SNMP-USER-BASED-SM-MIB::usmStatsUnknownEngineIDs.0 = Counter32: 5*


So what I could infer is that after every get/set operation, the counters
-  usmStatsNotInTimeWindows and usmStatsUnknownEngineIDs gets incremented
by one.
Could you please explain why the counters gets incremented after every
get/set operation?

Thanks and Regards,
Rani