Textual Conventions for Additional High Capacity Data Types
Extracted elements (14)
Both CounterBasedGauge64 and ZeroBasedCounter64 use Counter64 as their base type rather than a dedicated Gauge64 or Unsigned64 because no such base type exists in SMIv2. This ensures ASN.1 encoding is identical to Counter64, identified by [APPLICATION 6], preserving backward compatibility.
Sub-ranges cannot be specified on TC-derived types and no DEFVAL clause can be used, because these constraints are not allowed on Counter64 objects in SMIv2. Similarly, these types cannot be used in an INDEX clause due to the lack of an INDEX mapping for Counter64.
These textual conventions are explicitly described as a short-term solution intended to be deprecated once a proper long-term solution (a native Gauge64 or Unsigned64 base type) is defined and deployed in SMIv2. MIB objects using these TCs may also need to be deprecated.
ZeroBasedCounter64 is modeled after the ZeroBasedCounter32 TC from the RMON-2 MIB (RFC 2021), providing a 64-bit analogue for use in tables with constantly changing INDEX spaces or where the TimeFilter mechanism is in use.
CounterBasedGauge64 is not strictly supported in SMIv2 because the 'always increasing' and 'counter wrap' semantics of Counter64 are not preserved. Management applications relying solely on the Counter64 ASN.1 tag may mistakenly treat CounterBasedGauge64 objects as Counter64 counters.
The textual conventions defined here cannot be translated to SMIv1 because the Counter64 type does not exist in SMIv1. These TCs are only usable in SMIv2-compliant MIB modules.
ZeroBasedCounter64 departs from Counter64 semantics in that objects must start at zero rather than an arbitrary initial value. The document notes this is not likely to adversely affect management applications expecting Counter64 semantics.
Management stations using ZeroBasedCounter64 must be aware of the minimum time to wrap the counter and the actual polling interval, and must discard data if the actual time between polls is too long or no minimum wrap time is defined.
Objects defined with ZeroBasedCounter64 are required to start at the value zero on creation, and thereafter count appropriate events, wrapping back to zero when the value 2^64 is reached.
The MAX-ACCESS of objects defined with these TC-derived types must be read-only, because the underlying Counter64 base type has a MAX-ACCESS of read-only.
CounterBasedGauge64 is a textual convention defining a 64-bit gauge using Counter64 syntax. It represents a non-negative integer that may increase or decrease between 0 and 2^64-1, clamping at its maximum or minimum when the modeled information exceeds those bounds.
The HCNUM-TC MIB module is assigned OID mib-2 78 (i.e., 1.3.6.1.2.1.78). It imports Counter64 from SNMPv2-SMI and TEXTUAL-CONVENTION from SNMPv2-TC.
ZeroBasedCounter64 is a textual convention defining a 64-bit counter that is initialized to zero on creation and wraps back to zero upon reaching 2^64. It is intended for use in tables instantiated by management application action.
This module defines only textual conventions and no management objects. Meaningful security considerations must be written in the MIB modules that use these TCs to define actual management objects.