Remote Monitoring MIB Extensions for Interface Parameters Monitoring
Extracted elements (23)
A more generic usrTopN approach (sorting any table with any index type) was considered but abandoned because it is difficult to define a normalization factor for a generic approach; requiring the application to supply a scaling factor for every possible instance was deemed undesirable by the working group.
The MIB restricts sortable objects to those in tables indexed solely by an InterfaceIndex object. This was a deliberate scope limitation that simplifies the agent's search algorithm and result table construction, at the cost of not supporting the more generic 'usrTopN' function.
interfaceTopNObjectSampleType bandwidthPercentage(3) is designed for octet counters; its use with packet or error counters is explicitly not recommended because the bandwidth normalization formula is not meaningful for non-octet metrics.
At row creation the agent SHOULD set interfaceTopNNormalizationFactor to the effective speed of the interface, providing a sensible default for normalization.
If interfaceTopNRowStatus is set to a value other than active(1), all associated entries in interfaceTopNTable SHALL be deleted by the agent.
Implementers are RECOMMENDED to use SNMPv3 with the User-based Security Model (USM) and View-based Access Control Model (VACM) to protect access to this MIB, as SNMPv1 alone provides no access control.
interfaceTopNObjectVariable, interfaceTopNObjectSampleType, interfaceTopNNormalizationReq, and related control objects MUST NOT be modified while the associated interfaceTopNRowStatus is active(1).
The agent MUST NOT lower interfaceTopNGrantedSize except as a result of a SET to the associated interfaceTopNRequestedSize object.
The agent MUST return a badValue error on a SET of interfaceTopNObjectVariable if the supplied variable is not available in the selected MIB view or does not conform to the stated conditions (e.g., not indexed solely by InterfaceIndex).
The probe MUST only grant write access to interfaceTopNObjectVariable in MIB views that have read access to all objects on the probe, because no SNMP access-control mechanism can otherwise restrict the counter data retrievable via the TopN mechanism.
The value of interfaceTopNNormalizationReq MUST be false if the effective speed of the interface sub-layer as determined from ifSpeed is zero, conforming to the ifSpeed definition in RFC 2863 for sub-layers with no concept of bandwidth.
interfaceTopNCaps is a read-only BITS object that declares which of the 76 defined interface statistics (ifInOctets through dot1dTpPortInDiscards, covering IF-MIB, EtherLike-MIB, TOKENRING-MIB, RMON-MIB, and BRIDGE-MIB counters) the agent is capable of using as a sorting criterion for TopN reports.
interfaceTopNControlTable is an RMON-style control table where a management station creates rows to configure TopN reports: selecting the variable, sample type, normalization, duration, and requested report size. Entries in interfaceTopNTable are static after the report completes and are associated with a control entry.
interfaceTopNDataSourceIndex identifies the interface that each ranked entry represents. For IF-MIB, EtherLike-MIB, and TOKENRING-MIB variables it equals ifIndex; for RMON-MIB variables it is the interface pointed to by etherStatsDataSource; for BRIDGE-MIB variables it is the interface pointed to by dot1dBasePortIfIndex.
interfaceTopNNormalizationReq (TruthValue) and interfaceTopNNormalizationFactor (Integer32) together allow values to be scaled by (normalizationFactor / ifSpeed) before ranking, enabling meaningful comparison of counters across interfaces running at different speeds (e.g., 10/100/1000 Mbps).
interfaceTopNObjectSampleType selects how the chosen variable is measured: absoluteValue(1) copies the counter directly, deltaValue(2) computes the change over the interval, and bandwidthPercentage(3) records octets-sent divided by total-octets-at-100%-bandwidth multiplied by 1000 to yield tenths-of-a-percent (0..1000).
interfaceTopNObjectVariable is an INTEGER (0..75) specifying which MIB counter to rank interfaces by. Values 0-22 map to IF-MIB objects, 23-37 to EtherLike-MIB, 38-55 to TOKENRING-MIB, 56-72 to RMON-MIB, and 73-75 to BRIDGE-MIB.
interfaceTopNRequestedSize (read-create, default 10) and interfaceTopNGrantedSize (read-only) implement a size-negotiation mechanism: the management station requests N entries and the agent grants as many as resources allow.
interfaceTopNTimeRemaining triggers a new collection when set to a non-zero value by the management station; existing entries become inaccessible during collection. When it decrements to zero the report becomes accessible. Setting it to zero while a report is running aborts the collection without creating entries.
interfaceTopNValue (Gauge32) holds the ranked value for 32-bit counters and for bandwidthPercentage results; interfaceTopNValue64 (CounterBasedGauge64) holds the value when the selected variable is a 64-bit counter. The inapplicable object is set to zero. Entries are sorted in decreasing order of value; zero-value interfaces are excluded.
The INTERFACETOPN-MIB module is registered under the RMON MIB OID tree as rmon 27 (interfaceTopNMIB ::= { rmon 27 }), extending the RMON-MIB (RFC 2819) and SMON MIB (RFC 2613) namespaces.
interfaceTopNDataSourceIndex and interfaceTopNValue may contain sensitive information about interface utilization; GET access to these objects should be controlled, and their values should possibly be encrypted when transmitted via SNMP.
Read-write and read-create objects in this MIB (including the control table entries) can negatively affect network operations if SET operations are allowed in a non-secure environment without proper protection. SNMPv1 provides no access control even on a network secured by IPsec.