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IETF RFC 2982
Distributed Management Expression MIB
Last modified on Tuesday, October 31st, 2000
 Permanent link to RFC 2982
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Network Working Group R. Kavasseri
Request for Comments: 2982 (Editor of this version)
Category: Standards Track B. Stewart
(Author of previous version)
Cisco Systems, Inc.
October 2000
Distributed Management Expression MIB
 Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2000). All Rights Reserved.
Abstract
This memo defines a portion of the Management Information Base (MIB)
for use with network management protocols in the Internet community.
In particular, it describes managed objects used for managing
expressions of MIB objects. The results of these expressions become
MIB objects usable like any other MIB object, such as for the test
condition for declaring an event.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Table of Contents
1 The SNMP Management Framework ............................... 2
2 Overview .................................................... 3
2.1 Usage ..................................................... 4
2.2 Persistence ............................................... 4
2.3 Operation ................................................. 4
2.3.1 Sampling ................................................ 5
2.3.2 Wildcards ............................................... 5
2.3.3 Evaluation .............................................. 5
2.3.4 Value Identification .................................... 6
2.4 Subsets ................................................... 6
2.4.1 No Wildcards ............................................ 6
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RFC 2982 Distributed Management Expression MIB October 2000
2.4.2 No Deltas ............................................... 7
2.5 Structure ................................................. 7
2.5.1 Resource ................................................ 7
2.5.2 Definition .............................................. 7
2.5.3 Value ................................................... 8
2.6 Examples .................................................. 8
2.6.1 Wildcarding ............................................. 8
2.6.2 Calculation and Conditional ............................. 10
3 Definitions ................................................. 12
4 Intellectual Property ....................................... 36
5 Acknowledgements ............................................ 37
6 References .................................................. 37
7 Security Considerations ..................................... 38
8 Author's Address ............................................ 40
9 Editor's Address ............................................ 40
10 Full Copyright Statement ................................... 41
1. The SNMP Management Framework
The SNMP Management Framework presently consists of five major
components:
o An overall architecture, described in RFC 2571 [RFC 2571].
o Mechanisms for describing and naming objects and events for the
purpose of management. The first version of this Structure of
Management Information (SMI) is called SMIv1 and described in
STD 16, RFC 1155 [RFC 1155], STD 16, RFC 1212 [RFC 1212] and RFC
1215 [RFC 1215]. The second version, called SMIv2, is described
in STD 58, RFC 2578 [RFC 2578], STD 58, RFC 2579 [RFC 2579] and
STD 58, RFC 2580 [RFC 2580].
o Message protocols for transferring management information. The
first version of the SNMP message protocol is called SNMPv1 and
described in STD 15, RFC 1157 [RFC 1157]. A second version of
the SNMP message protocol, which is not an Internet standards
track protocol, is called SNMPv2c and described in RFC 1901
[RFC 1901] and RFC 1906 [RFC 1906]. The third version of the
message protocol is called SNMPv3 and described in RFC 1906
[RFC 1906], RFC 2572 [RFC 2572] and RFC 2574 [RFC 2574].
o Protocol operations for accessing management information. The
first set of protocol operations and associated PDU formats is
described in STD 15, RFC 1157 [RFC 1157]. A second set of
protocol operations and associated PDU formats is described in
RFC 1905 [RFC 1905].
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RFC 2982 Distributed Management Expression MIB October 2000
o A set of fundamental applications described in RFC 2573
[RFC 2573] and the view-based access control mechanism described
in RFC 2575 [RFC 2575].
A more detailed introduction to the current SNMP Management Framework
can be found in RFC 2570 [RFC 2570].
Managed objects are accessed via a virtual information store, termed
the Management Information Base or MIB. Objects in the MIB are
defined using the mechanisms defined in the SMI.
This memo specifies a MIB module that is compliant to the SMIv2. A
MIB conforming to the SMIv1 can be produced through the appropriate
translations. The resulting translated MIB must be semantically
equivalent, except where objects or events are omitted because no
translation is possible (use of Counter64). Some machine readable
information in SMIv2 will be converted into textual descriptions in
SMIv1 during the translation process. However, this loss of machine
readable information is not considered to change the semantics of the
MIB.
2. Overview
Users of MIBs often desire MIB objects that MIB designers have not
provided. Furthermore, such needs vary from one management
philosophy to another. Rather than fill more and more MIBs with
standardized objects, the Expression MIB supports externally defined
expressions of existing MIB objects.
In the Expression MIB the results of an evaluated expression are MIB
objects that may be used like any other MIB objects. These custom-
defined objects are thus usable anywhere any other MIB object can be
used. For example, they can be used by a management application
directly or referenced from another MIB, such as the Event MIB
[MIBEventMIB]. They can even be used by the Expression MIB itself,
forming expressions of expressions.
The Expression MIB is instrumentation for a relatively powerful,
complex, high-level application, considerably different from simple
instrumentation for a communication driver or a protocol. The MIB is
appropriate in a relatively powerful, resource-rich managed system
and not necessarily in a severely limited environment.
Nevertheless, due to dependencies from the Event MIB [RFC 2981] and
the need to support as low-end a system as possible, the Expression
MIB can be somewhat stripped down for lower-power, lower-resource
implementations, as described in the Subsets section, below.
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Implementation of the Expression MIB in a managed system led to the
addition of objects that may not have been necessary in an
application environment with complete knowledge of compiled MIB
definitions. This is appropriate since implementation must be
possible within typical managed systems with some constraints on
system resources.
2.1. Usage
On managed systems that can afford the overhead, the Expression MIB
is a way to create new, customized MIB objects for monitoring.
Although these can save some network traffic and overhead on
management systems, that is often not a good tradeoff for objects
that are simply to be recorded or displayed.
An example of a use of the Expression MIB would be to provide custom
objects for the Event MIB [RFC 2981]. A complex expression can
evaluate to a rate of flow or a boolean and thus be subject to
testing as an event trigger, resulting in an SNMP notification.
Without these capabilities such monitoring would be limited to the
objects in predefined MIBs. The Expression MIB thus supports
powerful tools for the network manager faced with the monitoring of
large, complex systems that can support a significant level of self
management.
2.2. Persistence
Although like most MIBs this one has no explicit controls for the
persistence of the values set in configuring an expression, a robust,
polite implementation would certainly not force its managing
applications to reconfigure it whenever it resets.
Again, as with most MIBs, it is implementation specific how a system
provides and manages such persistence. To speculate, one could
imagine, for example, that persistence depended on the context in
which the expression was configured, or perhaps system-specific
characteristics of the expression's owner. Or perhaps everything in
a MIB such as this one, which is clearly aimed at persistent
configuration, is automatically part of a system's other persistent
configuration.
2.3. Operation
Most of the operation of the MIB is described or implied in the
object definitions but a few highlights bear mentioning here.
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2.3.1. Sampling
The MIB supports three types of object sampling for the MIB objects
that make up the expression: absolute, delta, and changed.
Absolute samples are simply the value of the MIB object at the time
it is sampled.
Absolute samples are not sufficient for expressions of counters, as
counters have meaning only as a delta (difference) from one sample to
the next. Thus objects may be sampled as deltas. Delta sampling
requires the application to maintain state for the value at the last
sample, and to do continuous sampling whether or not anyone is
looking at the results. It thus creates constant overhead.
Changed sampling is a simple fallout of delta sampling where rather
than a difference the result is a boolean indicating whether or not
the object changed value since the last sample.
2.3.2. Wildcards
Wildcards allow the application of a single expression to multiple
instances of the same MIB object. The definer of the expression
indicates this choice and provides a partial object identifier, with
some or all of the instance portion left off. The application then
does the equivalent of GetNext to obtain the object values, thus
discovering the instances.
All wildcarded objects in an expression must have the same semantics
for the missing portion of their object identifiers. Otherwise, any
successful evaluation of the wildcarded expression would be the
result of the accidental matching of the wildcarded portion of the
object identifiers in the expression. Such an evaluation will likely
produce results which are not meaningful.
The expression can be evaluated only for those instances where all
the objects in the expression are available with the same value for
the wildcarded portion of the instance.
2.3.3. Evaluation
There are two important aspects of evaluation that may not be
obvious: what objects and when.
What objects get used in the evaluation depends on the type of
request and whether or not the expression contains wildcarded
objects. If the request was a Get, that locks down the instances to
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be used. If the request was a GetNext or GetBulk, the application
must work its way up to the next full set of objects for the
expression.
Evaluation of expressions happens at two possible times, depending on
the sampling method (delta or absolute) used to evaluate the
expression.
If there are no delta or change values in an expression, the
evaluation occurs on demand, i.e. when a requester attempts to read
the value of the expression. In this case all requesters get a
freshly calculated value.
For expressions with delta or change values, evaluation goes on
continuously, every sample period. In this case requesters get the
value as of the last sample period. For any given sample period of a
given expression, only those instances exist that provided a full set
of object values. It may be possible that a delta expression which
was evaluated successfully for one sample period may not be
successfully evaluated in the next sample period. This may, for
example, be due to missing instances for some or all of the objects
in the expression. In such cases, the value from the previous sample
period (with the successful evaluation) must not be carried forward
to the next sample period (with the failed evaluation).
2.3.4. Value Identification
Values resulting from expression evaluation are identified with a
combination of the object identifier (OID) for the data type from
expValueTable (such as expValueCounter32Val), the expression owner,
the expression name, and an OID fragment.
The OID fragment is not an entire OID beginning with iso.dod.org
(1.3.6). Rather it begins with 0.0. The remainder is either another
0 when there is no wildcarding or the instance that satisfied the
wildcard if there is wildcarding.
2.4. Subsets
To pare down the Expression MIBs complexity and use of resources an
implementor can leave out various parts.
2.4.1. No Wildcards
Leaving out wildcarding significantly reduces the complexity of
retrieving values to evaluate expressions and the processing required
to do so. Such an implementation would allow expressions made up of
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RFC 2982 Distributed Management Expression MIB October 2000
individual MIB objects but would not be suitable for expressions
applied across large tables as each instance in the table would
require a separate expression definition.
Furthermore it would not be suitable for tables with arbitrary,
dynamic instances, as expressions definitions could not predict what
instance values to use.
An implementation without wildcards might be useful for a self-
managing system with small tables or few dynamic instances, or one
that can do calculations only for a few key objects.
2.4.2. No Deltas
Leaving out delta processing significantly reduces state that must be
kept and the burden of ongoing processing even when no one is looking
at the results. Unfortunately it also makes expressions on counters
unusable, as counters have meaning only as deltas.
An implementation without deltas might be useful for a severely
limited, self-managing system that has no need for expressions or
events on counters. Although conceivable, such systems would be
rare.
2.5. Structure
The MIB has the following sections:
o Resource -- management of the MIB's use of system resources.
o Definition -- definition of expressions.
o Value -- values of evaluated expressions.
2.5.1. Resource
The resource section has objects to manage resource usage by
wildcarded delta expressions, a potential major consumer of CPU and
memory.
2.5.2. Definition
The definition section contains the tables that define expressions.
The expression table, indexed by expression owner and expression
name, contains those parameters that apply to the entire expression,
such as the expression itself, the data type of the result, and the
sampling interval if it contains delta or change values.
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The object table, indexed by expression owner, expression name and
object index within each expression, contains the parameters that
apply to the individual objects that go into the expression,
including the object identifier, sample type, discontinuity
indicator, and such.
2.5.3. Value
The value section contains the values of evaluated expressions.
The value table, indexed by expression owner, expression name and
instance fragment contains a "discriminated union" of evaluated
expression results. For a given expression only one of the columns
is instantiated, depending on the result data type for the
expression. The instance fragment is a constant or the final section
of the object identifier that filled in a wildcard.
2.6. Examples
The examples refer to tables and objects defined below in the MIB
itself. They may well make more sense after reading those
definitions.
2.6.1. Wildcarding
An expression may use wildcarded MIB objects that result in multiple
values for the expression. To specify a wildcarded MIB object a
management application leaves off part or all of the instance portion
of the object identifier, and sets expObjectWildcard to true(1) for
that object. For our example we'll use a counter of total blessings
from a table of people. Another table, indexed by town and person
has blessings just from that town.
So the index clauses are:
personEntry OBJECT-TYPE
...
INDEX { personIndex }
And:
townPersonEntry OBJECT-TYPE
...
INDEX { townIndex, personIndex }
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In our friendly application we may have entered our expression as:
100 * townPersonBlessings.976.* / personBlessings.*
What goes in expExpression is:
100*$1/$2
For example purposes we'll use some slightly far-fetched OIDs. The
People MIB is 1.3.6.1.99.7 and the Town MIB is 1.3.6.1.99.11, so for
our two counters the OIDs are:
personBlessings 1.3.6.1.99.7.1.3.1.4
townPersonBlessings 1.3.6.1.99.11.1.2.1.9
The rule for wildcards is that all the wildcarded parts have to match
exactly. In this case that means we have to hardwire the town and
only the personIndex can be wildcarded. So our values for
expObjectID are:
1.3.6.1.99.7.1.3.1.4
1.3.6.1.99.11.1.2.1.9.976
We're hardwired to townIndex 976 and personIndex is allowed to vary.
The value of expExpressionPrefix can be either of those two counter
OIDs (including the instance fragment in the second case), since
either of them takes you to a MIB definition where you can look at
the INDEX clause and figure out what's been left off. What's been
left off doesn't have to work out to be the same object, but it does
have to work out to be the same values (semantics) for the result to
make sense. Note that the managed system can not typically check
such semantics and if given nonsense will return nonsense.
If we have people numbered 6, 19, and 42 in town number 976, the
successive values of expValueInstance will be:
0.0.6
0.0.19
0.0.42
So there will be three values in expValueTable, with those OIDs as
the expValueInstance part of their indexing.
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RFC 2982 Distributed Management Expression MIB October 2000
2.6.2. Calculation and Conditional
The following formula for line utilization of a half-duplex link is
adapted from [PracPersp].
utilization = (ifInOctets + ifOutOctets) * 800 / seconds / ifSpeed
The expression results in the percentage line utilization per second.
The total octets are multiplied by 8 to get bits and 100 to scale up
the percentage as an integer.
The following Expression MIB object values implement this as an
expression for all ifIndexes that directly represent actual hardware.
Since the octet counters are Counter32 values, they must be delta
sampled to be meaningful. The sample period is 6 seconds but for
accuracy and independence is calculated as a delta of sysUpTime.
The expObjectTable entry for ifInOctets has an expObjectConditional
that checks for being a hardware interface. Only one object in the
expression needs that check associated, since it applies to the whole
expression. Since ifConnectorPresent is a TruthValue with values of
1 or 2 rather than 0 and non-zero, it must also be in an expression
rather than used directly for the conditional.
The interface-specific discontinuity indicator is supplied only for
ifInOctets since invalidating that sample will invalidate an attempt
at evaluation, effectively invalidating ifOutOctets as well
(correctly, because it has the same indicator).
For notational clarity, in the rest of this document, a string in
quotes as part of the object instance indicates the value that would
actually be one subidentifier per byte. The objects all belong to
owner "me".
Also for clarity OIDs are expressed as the object descriptor and
instance. In fact they must be supplied numerically, with all
subidentifiers in place before the part for the particular object and
instance.
What the user would set in expExpressionTable:
expExpression.2."me".4."hard" = "$1==1"
expExpressionValueType.2."me".4."hard" = unsigned32
expExpressionRowStatus.2."me"4."hard" = 'active'
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RFC 2982 Distributed Management Expression MIB October 2000
expExpression.2."me".4."util" = "($1+$2)*800/$4/$3"
expExpressionValueType.2."me".4."util" = integer32
expExpressionDeltaInterval.2."me".4."util" = 6
expExpressionRowStatus.2."me"4."util" = 'active'
What the user would set in expObjectTable:
expObjectID.2."me".4."hard".1 = ifConnectorPresent
expObjectWildcard.2."me".4."hard".1 = 'true'
expObjectSampleType.2."me".4."hard".1 = 'absoluteValue'
expObjectRowStatus.2."me".4."hard".1 = 'active'
expObjectID.2."me".4."util".1 = ifInOctets
expObjectWildcard.2."me".4."util".1 = 'true'
expObjectSampleType.2."me".4."util".1 = 'deltaValue'
expObjectConditional.2."me".4."util".1 =
expValueUnsigned32Val.4."hard".0.0
expObjectConditionalWildcard.2."me".4."util".1 = 'true'
expObjectDiscontinuityID.2."me".4."util".1 =
ifCounterDiscontinuityTime
expObjectDiscontinuityIDWildcard.2."me".4."util".1 = 'true'
expObjectRowStatus.2."me".4."util".1 = 'active'
expObjectID.2."me".4."util".2 = ifOutOctets
expObjectWildcard.2."me".4."util".2 = 'true'
expObjectSampleType.2."me".4."util".2 = 'deltaValue'
expObjectRowStatus.2."me".4."util".2 = 'active'
expObjectID.2."me".4."util".3 = ifSpeed
expObjectWildcard.2."me".4."util".3 = 'true'
expObjectSampleType.2."me".4."util".3 = 'absoluteValue'
expObjectRowStatus.2."me".4."util".3 = 'active'
expObjectID.2."me".4."util".4 = sysUpTime.0
expObjectWildcard.2."me".4."util".4 = 'false'
expObjectSampleType.2."me".4."util".4 = 'deltaValue'
expObjectRowStatus.2."me".4."util".4 = 'active'
These settings will result in populating one column of expValueTable:
expValueInteger32Val.2."me".4."util".0.0.?
The subidentifier represented by "?" above represents one
subidentifier that takes on a value of ifIndex and identifies a row
for each ifIndex value where ifConnectorPresent is 'true' and the
interface was present for two samples to provide a delta.
Kavasseri & Stewart Standards Track PAGE 11
RFC 2982 Distributed Management Expression MIB October 2000
This value could in turn be used as an event threshold [RFC 2981] to
watch for overutilization of all hardware network connections.
3. Definitions
DISMAN-EXPRESSION-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
Integer32, Gauge32, Unsigned32,
Counter32, Counter64, IpAddress,
TimeTicks, mib-2, zeroDotZero FROM SNMPv2-SMI
RowStatus, TruthValue, TimeStamp FROM SNMPv2-TC
sysUpTime FROM SNMPv2-MIB
SnmpAdminString FROM SNMP-FRAMEWORK-MIB
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF;
dismanExpressionMIB MODULE-IDENTITY
LAST-UPDATED "200010160000Z" -- 16 October 2000
ORGANIZATION "IETF Distributed Management Working Group"
CONTACT-INFO "Ramanathan Kavasseri
Cisco Systems, Inc.
170 West Tasman Drive,
San Jose CA 95134-1706.
Phone: +1 408 527 2446
Email: ramk@cisco.com"
DESCRIPTION
"The MIB module for defining expressions of MIB objects for
management purposes."
-- Revision History
REVISION "200010160000Z" -- 16 October 2000
DESCRIPTION "This is the initial version of this MIB.
Published as RFC 2982"
::= { mib-2 90 }
dismanExpressionMIBObjects OBJECT IDENTIFIER ::=
{ dismanExpressionMIB 1 }
expResource OBJECT IDENTIFIER ::= { dismanExpressionMIBObjects 1 }
expDefine OBJECT IDENTIFIER ::= { dismanExpressionMIBObjects 2 }
expValue OBJECT IDENTIFIER ::= { dismanExpressionMIBObjects 3 }
--
-- Resource Control
--
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RFC 2982 Distributed Management Expression MIB October 2000
expResourceDeltaMinimum OBJECT-TYPE
SYNTAX Integer32 (-1 | 1..600)
UNITS "seconds"
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"The minimum expExpressionDeltaInterval this system will
accept. A system may use the larger values of this minimum to
lessen the impact of constantly computing deltas. For larger
delta sampling intervals the system samples less often and
suffers less overhead. This object provides a way to enforce
such lower overhead for all expressions created after it is
set.
The value -1 indicates that expResourceDeltaMinimum is
irrelevant as the system will not accept 'deltaValue' as a
value for expObjectSampleType.
Unless explicitly resource limited, a system's value for
this object should be 1, allowing as small as a 1 second
interval for ongoing delta sampling.
Changing this value will not invalidate an existing setting
of expObjectSampleType."
::= { expResource 1 }
expResourceDeltaWildcardInstanceMaximum OBJECT-TYPE
SYNTAX Unsigned32
UNITS "instances"
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"For every instance of a deltaValue object, one dynamic instance
entry is needed for holding the instance value from the previous
sample, i.e. to maintain state.
This object limits maximum number of dynamic instance entries
this system will support for wildcarded delta objects in
expressions. For a given delta expression, the number of
dynamic instances is the number of values that meet all criteria
to exist times the number of delta values in the expression.
A value of 0 indicates no preset limit, that is, the limit
is dynamic based on system operation and resources.
Unless explicitly resource limited, a system's value for
this object should be 0.
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Changing this value will not eliminate or inhibit existing delta
wildcard instance objects but will prevent the creation of more
such objects.
An attempt to allocate beyond the limit results in expErrorCode
being tooManyWildcardValues for that evaluation attempt."
::= { expResource 2 }
expResourceDeltaWildcardInstances OBJECT-TYPE
SYNTAX Gauge32
UNITS "instances"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of currently active instance entries as
defined for expResourceDeltaWildcardInstanceMaximum."
::= { expResource 3 }
expResourceDeltaWildcardInstancesHigh OBJECT-TYPE
SYNTAX Gauge32
UNITS "instances"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The highest value of expResourceDeltaWildcardInstances
that has occurred since initialization of the managed
system."
::= { expResource 4 }
expResourceDeltaWildcardInstanceResourceLacks OBJECT-TYPE
SYNTAX Counter32
UNITS "instances"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times this system could not evaluate an
expression because that would have created a value instance in
excess of expResourceDeltaWildcardInstanceMaximum."
::= { expResource 5 }
--
-- Definition
--
-- Expression Definition Table
--
expExpressionTable OBJECT-TYPE
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SYNTAX SEQUENCE OF ExpExpressionEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A table of expression definitions."
::= { expDefine 1 }
expExpressionEntry OBJECT-TYPE
SYNTAX ExpExpressionEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information about a single expression. New expressions
can be created using expExpressionRowStatus.
To create an expression first create the named entry in this
table. Then use expExpressionName to populate expObjectTable.
For expression evaluation to succeed all related entries in
expExpressionTable and expObjectTable must be 'active'. If
these conditions are not met the corresponding values in
expValue simply are not instantiated.
Deleting an entry deletes all related entries in expObjectTable
and expErrorTable.
Because of the relationships among the multiple tables for an
expression (expExpressionTable, expObjectTable, and
expValueTable) and the SNMP rules for independence in setting
object values, it is necessary to do final error checking when
an expression is evaluated, that is, when one of its instances
in expValueTable is read or a delta interval expires. Earlier
checking need not be done and an implementation may not impose
any ordering on the creation of objects related to an
expression.
To maintain security of MIB information, when creating a new row in
this table, the managed system must record the security credentials
of the requester. These security credentials are the parameters
necessary as inputs to isAccessAllowed from the Architecture for
Describing SNMP Management Frameworks. When obtaining the objects
that make up the expression, the system must (conceptually) use
isAccessAllowed to ensure that it does not violate security.
The evaluation of the expression takes place under the
security credentials of the creator of its expExpressionEntry.
Values of read-write objects in this table may be changed
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RFC 2982 Distributed Management Expression MIB October 2000
at any time."
INDEX { expExpressionOwner, expExpressionName }
::= { expExpressionTable 1 }
ExpExpressionEntry ::= SEQUENCE {
expExpressionOwner SnmpAdminString,
expExpressionName SnmpAdminString,
expExpression OCTET STRING,
expExpressionValueType INTEGER,
expExpressionComment SnmpAdminString,
expExpressionDeltaInterval Integer32,
expExpressionPrefix OBJECT IDENTIFIER,
expExpressionErrors Counter32,
expExpressionEntryStatus RowStatus
}
expExpressionOwner OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(0..32))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"The owner of this entry. The exact semantics of this
string are subject to the security policy defined by the
security administrator."
::= { expExpressionEntry 1 }
expExpressionName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE (1..32))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"The name of the expression. This is locally unique, within
the scope of an expExpressionOwner."
::= { expExpressionEntry 2 }
expExpression OBJECT-TYPE
SYNTAX OCTET STRING (SIZE (1..1024))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The expression to be evaluated. This object is the same
as a DisplayString (RFC 1903) except for its maximum length.
Except for the variable names the expression is in ANSI C
syntax. Only the subset of ANSI C operators and functions
listed here is allowed.
Variables are expressed as a dollar sign ('$') and an
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integer that corresponds to an expObjectIndex. An
example of a valid expression is:
($1-$5)*100
Expressions must not be recursive, that is although an expression
may use the results of another expression, it must not contain
any variable that is directly or indirectly a result of its own
evaluation. The managed system must check for recursive
expressions.
The only allowed operators are:
( )
- (unary)
+ - * / %
& | ^ << >> ~
! && || == != > >= < <=
Note the parentheses are included for parenthesizing the
expression, not for casting data types.
The only constant types defined are:
int (32-bit signed)
long (64-bit signed)
unsigned int
unsigned long
hexadecimal
character
string
oid
The default type for a positive integer is int unless it is too
large in which case it is long.
All but oid are as defined for ANSI C. Note that a
hexadecimal constant may end up as a scalar or an array of
8-bit integers. A string constant is enclosed in double
quotes and may contain back-slashed individual characters
as in ANSI C.
An oid constant comprises 32-bit, unsigned integers and at
least one period, for example:
0.
.0
1.3.6.1
Kavasseri & Stewart Standards Track PAGE 17
RFC 2982 Distributed Management Expression MIB October 2000
No additional leading or trailing subidentifiers are automatically
added to an OID constant. The constant is taken as expressed.
Integer-typed objects are treated as 32- or 64-bit, signed
or unsigned integers, as appropriate. The results of
mixing them are as for ANSI C, including the type of the
result. Note that a 32-bit value is thus promoted to 64 bits
only in an operation with a 64-bit value. There is no
provision for larger values to handle overflow.
Relative to SNMP data types, a resulting value becomes
unsigned when calculating it uses any unsigned value,
including a counter. To force the final value to be of
data type counter the expression must explicitly use the
counter32() or counter64() function (defined below).
OCTET STRINGS and OBJECT IDENTIFIERs are treated as
one-dimensioned arrays of unsigned 8-bit integers and
unsigned 32-bit integers, respectively.
IpAddresses are treated as 32-bit, unsigned integers in
network byte order, that is, the hex version of 255.0.0.0 is
0xff000000.
Conditional expressions result in a 32-bit, unsigned integer
of value 0 for false or 1 for true. When an arbitrary value
is used as a boolean 0 is false and non-zero is true.
Rules for the resulting data type from an operation, based on
the operator:
For << and >> the result is the same as the left hand operand.
For &&, ||, ==, !=, <, <=, >, and >= the result is always
Unsigned32.
For unary - the result is always Integer32.
For +, -, *, /, %, &, |, and ^ the result is promoted according
to the following rules, in order from most to least preferred:
If left hand and right hand operands are the same type,
use that.
If either side is Counter64, use that.
If either side is IpAddress, use that.
Kavasseri & Stewart Standards Track PAGE 18
RFC 2982 Distributed Management Expression MIB October 2000
If either side is TimeTicks, use that.
If either side is Counter32, use that.
Otherwise use Unsigned32.
The following rules say what operators apply with what data
types. Any combination not explicitly defined does not work.
For all operators any of the following can be the left hand or
right hand operand: Integer32, Counter32, Unsigned32, Counter64.
The operators +, -, *, /, %, <, <=, >, and >= work with
TimeTicks.
The operators &, |, and ^ work with IpAddress.
The operators << and >> work with IpAddress but only as the
left hand operand.
The + operator performs a concatenation of two OCTET STRINGs or
two OBJECT IDENTIFIERs.
The operators &, | perform bitwise operations on OCTET STRINGs.
If the OCTET STRING happens to be a DisplayString the results
may be meaningless, but the agent system does not check this as
some such systems do not have this information.
The operators << and >> perform bitwise operations on OCTET
STRINGs appearing as the left hand operand.
The only functions defined are:
counter32
counter64
arraySection
stringBegins
stringEnds
stringContains
oidBegins
oidEnds
oidContains
average
maximum
minimum
sum
exists
Kavasseri & Stewart Standards Track PAGE 19
RFC 2982 Distributed Management Expression MIB October 2000
The following function definitions indicate their parameters by
naming the data type of the parameter in the parameter's position
in the parameter list. The parameter must be of the type indicated
and generally may be a constant, a MIB object, a function, or an
expression.
counter32(integer) - wrapped around an integer value counter32
forces Counter32 as a data type.
counter64(integer) - similar to counter32 except that the
resulting data type is 'counter64'.
arraySection(array, integer, integer) - selects a piece of an
array (i.e. part of an OCTET STRING or OBJECT IDENTIFIER). The
integer arguments are in the range 0 to 4,294,967,295. The
first is an initial array index (one-dimensioned) and the second
is an ending array index. A value of 0 indicates first or last
element, respectively. If the first element is larger than the
array length the result is 0 length. If the second integer is
less than or equal to the first, the result is 0 length. If the
second is larger than the array length it indicates last
element.
stringBegins/Ends/Contains(octetString, octetString) - looks for
the second string (which can be a string constant) in the first
and returns the one-dimensioned arrayindex where the match began.
A return value of 0 indicates no match (i.e. boolean false).
oidBegins/Ends/Contains(oid, oid) - looks for the second OID
(which can be an OID constant) in the first and returns the
the one-dimensioned index where the match began. A return value
of 0 indicates no match (i.e. boolean false).
average/maximum/minimum(integer) - calculates the average,
minimum, or maximum value of the integer valued object over
multiple sample times. If the object disappears for any
sample period, the accumulation and the resulting value object
cease to exist until the object reappears at which point the
calculation starts over.
sum(integerObject*) - sums all available values of the
wildcarded integer object, resulting in an integer scalar. Must
be used with caution as it wraps on overflow with no
notification.
exists(anyTypeObject) - verifies the object instance exists. A
return value of 0 indicates NoSuchInstance (i.e. boolean
false)."
Kavasseri & Stewart Standards Track PAGE 20
RFC 2982 Distributed Management Expression MIB October 2000
::= { expExpressionEntry 3 }
expExpressionValueType OBJECT-TYPE
SYNTAX INTEGER { counter32(1), unsigned32(2), timeTicks(3),
integer32(4), ipAddress(5), octetString(6),
objectId(7), counter64(8) }
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The type of the expression value. One and only one of the
value objects in expValueTable will be instantiated to match
this type.
If the result of the expression can not be made into this type,
an invalidOperandType error will occur."
DEFVAL { counter32 }
::= { expExpressionEntry 4 }
expExpressionComment OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A comment to explain the use or meaning of the expression."
DEFVAL { ''H }
::= { expExpressionEntry 5 }
expExpressionDeltaInterval OBJECT-TYPE
SYNTAX Integer32 (0..86400)
UNITS "seconds"
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Sampling interval for objects in this expression with
expObjectSampleType 'deltaValue'.
This object has no effect if the the expression has no
deltaValue objects.
A value of 0 indicates no automated sampling. In this case
the delta is the difference from the last time the expression
was evaluated. Note that this is subject to unpredictable
delta times in the face of retries or multiple managers.
A value greater than zero is the number of seconds between
automated samples.
Until the delta interval has expired once the delta for the
Kavasseri & Stewart Standards Track PAGE 21
RFC 2982 Distributed Management Expression MIB October 2000
object is effectively not instantiated and evaluating
the expression has results as if the object itself were not
instantiated.
Note that delta values potentially consume large amounts of
system CPU and memory. Delta state and processing must
continue constantly even if the expression is not being used.
That is, the expression is being evaluated every delta interval,
even if no application is reading those values. For wildcarded
objects this can be substantial overhead.
Note that delta intervals, external expression value sampling
intervals and delta intervals for expressions within other
expressions can have unusual interactions as they are impossible
to synchronize accurately. In general one interval embedded
below another must be enough shorter that the higher sample
sees relatively smooth, predictable behavior. So, for example,
to avoid the higher level getting the same sample twice, the
lower level should sample at least twice as fast as the higher
level does."
DEFVAL { 0 }
::= { expExpressionEntry 6 }
expExpressionPrefix OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"An object prefix to assist an application in determining
the instance indexing to use in expValueTable, relieving the
application of the need to scan the expObjectTable to
determine such a prefix.
See expObjectTable for information on wildcarded objects.
If the expValueInstance portion of the value OID may
be treated as a scalar (that is, normally, 0) the value of
expExpressionPrefix is zero length, that is, no OID at all.
Note that zero length implies a null OID, not the OID 0.0.
Otherwise, the value of expExpressionPrefix is the expObjectID
value of any one of the wildcarded objects for the expression.
This is sufficient, as the remainder, that is, the instance
fragment relevant to instancing the values, must be the same for
all wildcarded objects in the expression."
::= { expExpressionEntry 7 }
expExpressionErrors OBJECT-TYPE
Kavasseri & Stewart Standards Track PAGE 22
RFC 2982 Distributed Management Expression MIB October 2000
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of errors encountered while evaluating this
expression.
Note that an object in the expression not being accessible,
is not considered an error. An example of an inaccessible
object is when the object is excluded from the view of the
user whose security credentials are used in the expression
evaluation. In such cases, it is a legitimate condition
that causes the corresponding expression value not to be
instantiated."
::= { expExpressionEntry 8 }
expExpressionEntryStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The control that allows creation and deletion of entries."
::= { expExpressionEntry 9 }
--
-- Expression Error Table
--
expErrorTable OBJECT-TYPE
SYNTAX SEQUENCE OF ExpErrorEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A table of expression errors."
::= { expDefine 2 }
expErrorEntry OBJECT-TYPE
SYNTAX ExpErrorEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information about errors in processing an expression.
Entries appear in this table only when there is a matching
expExpressionEntry and then only when there has been an
error for that expression as reflected by the error codes
defined for expErrorCode."
INDEX { expExpressionOwner, expExpressionName }
Kavasseri & Stewart Standards Track PAGE 23
RFC 2982 Distributed Management Expression MIB October 2000
::= { expErrorTable 1 }
ExpErrorEntry ::= SEQUENCE {
expErrorTime TimeStamp,
expErrorIndex Integer32,
expErrorCode INTEGER,
expErrorInstance OBJECT IDENTIFIER
}
expErrorTime OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime the last time an error caused a
failure to evaluate this expression."
::= { expErrorEntry 1 }
expErrorIndex OBJECT-TYPE
SYNTAX Integer32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The one-dimensioned character array index into
expExpression for where the error occurred. The value
zero indicates irrelevance."
::= { expErrorEntry 2 }
expErrorCode OBJECT-TYPE
SYNTAX INTEGER {
invalidSyntax(1),
undefinedObjectIndex(2),
unrecognizedOperator(3),
unrecognizedFunction(4),
invalidOperandType(5),
unmatchedParenthesis(6),
tooManyWildcardValues(7),
recursion(8),
deltaTooShort(9),
resourceUnavailable(10),
divideByZero(11)
}
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The error that occurred. In the following explanations the
expected timing of the error is in parentheses. 'S' means
the error occurs on a Set request. 'E' means the error
Kavasseri & Stewart Standards Track PAGE 24
RFC 2982 Distributed Management Expression MIB October 2000
occurs on the attempt to evaluate the expression either due to
Get from expValueTable or in ongoing delta processing.
invalidSyntax the value sent for expExpression is not
valid Expression MIB expression syntax
(S)
undefinedObjectIndex an object reference ($n) in
expExpression does not have a matching
instance in expObjectTable (E)
unrecognizedOperator the value sent for expExpression held an
unrecognized operator (S)
unrecognizedFunction the value sent for expExpression held an
unrecognized function name (S)
invalidOperandType an operand in expExpression is not the
right type for the associated operator
or result (SE)
unmatchedParenthesis the value sent for expExpression is not
correctly parenthesized (S)
tooManyWildcardValues evaluating the expression exceeded the
limit set by
expResourceDeltaWildcardInstanceMaximum
(E)
recursion through some chain of embedded
expressions the expression invokes itself
(E)
deltaTooShort the delta for the next evaluation passed
before the system could evaluate the
present sample (E)
resourceUnavailable some resource, typically dynamic memory,
was unavailable (SE)
divideByZero an attempt to divide by zero occurred
(E)
For the errors that occur when the attempt is made to set
expExpression Set request fails with the SNMP error code
'wrongValue'. Such failures refer to the most recent failure to
Set expExpression, not to the present value of expExpression
which must be either unset or syntactically correct.
Errors that occur during evaluation for a Get* operation return
the SNMP error code 'genErr' except for 'tooManyWildcardValues'
and 'resourceUnavailable' which return the SNMP error code
'resourceUnavailable'."
::= { expErrorEntry 3 }
expErrorInstance OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-only
Kavasseri & Stewart Standards Track PAGE 25
RFC 2982 Distributed Management Expression MIB October 2000
STATUS current
DESCRIPTION
"The expValueInstance being evaluated when the error
occurred. A zero-length indicates irrelevance."
::= { expErrorEntry 4 }
--
-- Object Table
--
expObjectTable OBJECT-TYPE
SYNTAX SEQUENCE OF ExpObjectEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A table of object definitions for each expExpression.
Wildcarding instance IDs:
It is legal to omit all or part of the instance portion for
some or all of the objects in an expression. (See the
DESCRIPTION of expObjectID for details. However, note that
if more than one object in the same expression is wildcarded
in this way, they all must be objects where that portion of
the instance is the same. In other words, all objects may be
in the same SEQUENCE or in different SEQUENCEs but with the
same semantic index value (e.g., a value of ifIndex)
for the wildcarded portion."
::= { expDefine 3 }
expObjectEntry OBJECT-TYPE
SYNTAX ExpObjectEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information about an object. An application uses
expObjectEntryStatus to create entries in this table while
in the process of defining an expression.
Values of read-create objects in this table may be
changed at any time."
INDEX { expExpressionOwner, expExpressionName, expObjectIndex }
::= { expObjectTable 1 }
ExpObjectEntry ::= SEQUENCE {
expObjectIndex Unsigned32,
expObjectID OBJECT IDENTIFIER,
expObjectIDWildcard TruthValue,
Kavasseri & Stewart Standards Track PAGE 26
RFC 2982 Distributed Management Expression MIB October 2000
expObjectSampleType INTEGER,
expObjectDeltaDiscontinuityID OBJECT IDENTIFIER,
expObjectDiscontinuityIDWildcard TruthValue,
expObjectDiscontinuityIDType INTEGER,
expObjectConditional OBJECT IDENTIFIER,
expObjectConditionalWildcard TruthValue,
expObjectEntryStatus RowStatus
}
expObjectIndex OBJECT-TYPE
SYNTAX Unsigned32 (1..4294967295)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Within an expression, a unique, numeric identification for an
object. Prefixed with a dollar sign ('$') this is used to
reference the object in the corresponding expExpression."
::= { expObjectEntry 1 }
expObjectID OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The OBJECT IDENTIFIER (OID) of this object. The OID may be
fully qualified, meaning it includes a complete instance
identifier part (e.g., ifInOctets.1 or sysUpTime.0), or it
may not be fully qualified, meaning it may lack all or part
of the instance identifier. If the expObjectID is not fully
qualified, then expObjectWildcard must be set to true(1).
The value of the expression will be multiple
values, as if done for a GetNext sweep of the object.
An object here may itself be the result of an expression but
recursion is not allowed.
NOTE: The simplest implementations of this MIB may not allow
wildcards."
::= { expObjectEntry 2 }
expObjectIDWildcard OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A true value indicates the expObjecID of this row is a wildcard
object. False indicates that expObjectID is fully instanced.
If all expObjectWildcard values for a given expression are FALSE,
Kavasseri & Stewart Standards Track PAGE 27
RFC 2982 Distributed Management Expression MIB October 2000
expExpressionPrefix will reflect a scalar object (i.e. will
be 0.0).
NOTE: The simplest implementations of this MIB may not allow
wildcards."
DEFVAL { false }
::= { expObjectEntry 3 }
expObjectSampleType OBJECT-TYPE
SYNTAX INTEGER { absoluteValue(1), deltaValue(2),
changedValue(3) }
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The method of sampling the selected variable.
An 'absoluteValue' is simply the present value of the object.
A 'deltaValue' is the present value minus the previous value,
which was sampled expExpressionDeltaInterval seconds ago.
This is intended primarily for use with SNMP counters, which are
meaningless as an 'absoluteValue', but may be used with any
integer-based value.
A 'changedValue' is a boolean for whether the present value is
different from the previous value. It is applicable to any data
type and results in an Unsigned32 with value 1 if the object's
value is changed and 0 if not. In all other respects it is as a
'deltaValue' and all statements and operation regarding delta
values apply to changed values.
When an expression contains both delta and absolute values
the absolute values are obtained at the end of the delta
period."
DEFVAL { absoluteValue }
::= { expObjectEntry 4 }
sysUpTimeInstance OBJECT IDENTIFIER ::= { sysUpTime 0 }
expObjectDeltaDiscontinuityID OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The OBJECT IDENTIFIER (OID) of a TimeTicks, TimeStamp, or
DateAndTime object that indicates a discontinuity in the value
at expObjectID.
Kavasseri & Stewart Standards Track PAGE 28
RFC 2982 Distributed Management Expression MIB October 2000
This object is instantiated only if expObjectSampleType is
'deltaValue' or 'changedValue'.
The OID may be for a leaf object (e.g. sysUpTime.0) or may
be wildcarded to match expObjectID.
This object supports normal checking for a discontinuity in a
counter. Note that if this object does not point to sysUpTime
discontinuity checking must still check sysUpTime for an overall
discontinuity.
If the object identified is not accessible no discontinuity
check will be made."
DEFVAL { sysUpTimeInstance }
::= { expObjectEntry 5 }
expObjectDiscontinuityIDWildcard OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A true value indicates the expObjectDeltaDiscontinuityID of
this row is a wildcard object. False indicates that
expObjectDeltaDiscontinuityID is fully instanced.
This object is instantiated only if expObjectSampleType is
'deltaValue' or 'changedValue'.
NOTE: The simplest implementations of this MIB may not allow
wildcards."
DEFVAL { false }
::= { expObjectEntry 6 }
expObjectDiscontinuityIDType OBJECT-TYPE
SYNTAX INTEGER { timeTicks(1), timeStamp(2), dateAndTime(3) }
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The value 'timeTicks' indicates the expObjectDeltaDiscontinuityID
of this row is of syntax TimeTicks. The value 'timeStamp' indicates
syntax TimeStamp. The value 'dateAndTime indicates syntax
DateAndTime.
This object is instantiated only if expObjectSampleType is
'deltaValue' or 'changedValue'."
DEFVAL { timeTicks }
::= { expObjectEntry 7 }
Kavasseri & Stewart Standards Track PAGE 29
RFC 2982 Distributed Management Expression MIB October 2000
expObjectConditional OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The OBJECT IDENTIFIER (OID) of an object that overrides
whether the instance of expObjectID is to be considered
usable. If the value of the object at expObjectConditional
is 0 or not instantiated, the object at expObjectID is
treated as if it is not instantiated. In other words,
expObjectConditional is a filter that controls whether or
not to use the value at expObjectID.
The OID may be for a leaf object (e.g. sysObjectID.0) or may be
wildcarded to match expObjectID. If expObject is wildcarded and
expObjectID in the same row is not, the wild portion of
expObjectConditional must match the wildcarding of the rest of
the expression. If no object in the expression is wildcarded
but expObjectConditional is, use the lexically first instance
(if any) of expObjectConditional.
If the value of expObjectConditional is 0.0 operation is
as if the value pointed to by expObjectConditional is a
non-zero (true) value.
Note that expObjectConditional can not trivially use an object
of syntax TruthValue, since the underlying value is not 0 or 1."
DEFVAL { zeroDotZero }
::= { expObjectEntry 8 }
expObjectConditionalWildcard OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A true value indicates the expObjectConditional of this row is
a wildcard object. False indicates that expObjectConditional is
fully instanced.
NOTE: The simplest implementations of this MIB may not allow
wildcards."
DEFVAL { false }
::= { expObjectEntry 9 }
expObjectEntryStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
Kavasseri & Stewart Standards Track PAGE 30
RFC 2982 Distributed Management Expression MIB October 2000
STATUS current
DESCRIPTION
"The control that allows creation/deletion of entries.
Objects in this table may be changed while
expObjectEntryStatus is in any state."
::= { expObjectEntry 10 }
--
-- Expression Value Table
--
expValueTable OBJECT-TYPE
SYNTAX SEQUENCE OF ExpValueEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A table of values from evaluated expressions."
::= { expValue 1 }
expValueEntry OBJECT-TYPE
SYNTAX ExpValueEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A single value from an evaluated expression. For a given
instance, only one 'Val' object in the conceptual row will be
instantiated, that is, the one with the appropriate type for
the value. For values that contain no objects of
expObjectSampleType 'deltaValue' or 'changedValue', reading a
value from the table causes the evaluation of the expression
for that value. For those that contain a 'deltaValue' or
'changedValue' the value read is as of the last sampling
interval.
If in the attempt to evaluate the expression one or more
of the necessary objects is not available, the corresponding
entry in this table is effectively not instantiated.
To maintain security of MIB information, when creating a new
row in this table, the managed system must record the security
credentials of the requester. These security credentials are
the parameters necessary as inputs to isAccessAllowed from
[RFC 2571]. When obtaining the objects that make up the
expression, the system must (conceptually) use isAccessAllowed to
ensure that it does not violate security.
The evaluation of that expression takes place under the
Kavasseri & Stewart Standards Track PAGE 31
RFC 2982 Distributed Management Expression MIB October 2000
security credentials of the creator of its expExpressionEntry.
To maintain security of MIB information, expression evaluation must
take place using security credentials for the implied Gets of the
objects in the expression as inputs (conceptually) to
isAccessAllowed from the Architecture for Describing SNMP
Management Frameworks. These are the security credentials of the
creator of the corresponding expExpressionEntry."
INDEX { expExpressionOwner, expExpressionName,
IMPLIED expValueInstance }
::= { expValueTable 1 }
ExpValueEntry ::= SEQUENCE {
expValueInstance OBJECT IDENTIFIER,
expValueCounter32Val Counter32,
expValueUnsigned32Val Unsigned32,
expValueTimeTicksVal TimeTicks,
expValueInteger32Val Integer32,
expValueIpAddressVal IpAddress,
expValueOctetStringVal OCTET STRING,
expValueOidVal OBJECT IDENTIFIER,
expValueCounter64Val Counter64
}
expValueInstance OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"The final instance portion of a value's OID according to
the wildcarding in instances of expObjectID for the
expression. The prefix of this OID fragment is 0.0,
leading to the following behavior.
If there is no wildcarding, the value is 0.0.0. In other
words, there is one value which standing alone would have
been a scalar with a 0 at the end of its OID.
If there is wildcarding, the value is 0.0 followed by
a value that the wildcard can take, thus defining one value
instance for each real, possible value of the wildcard.
So, for example, if the wildcard worked out to be an ifIndex,
there is an expValueInstance for each applicable ifIndex."
::= { expValueEntry 1 }
expValueCounter32Val OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
Kavasseri & Stewart Standards Track PAGE 32
RFC 2982 Distributed Management Expression MIB October 2000
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'counter32'."
::= { expValueEntry 2 }
expValueUnsigned32Val OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'unsigned32'."
::= { expValueEntry 3 }
expValueTimeTicksVal OBJECT-TYPE
SYNTAX TimeTicks
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'timeTicks'."
::= { expValueEntry 4 }
expValueInteger32Val OBJECT-TYPE
SYNTAX Integer32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'integer32'."
::= { expValueEntry 5 }
expValueIpAddressVal OBJECT-TYPE
SYNTAX IpAddress
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'ipAddress'."
::= { expValueEntry 6 }
expValueOctetStringVal OBJECT-TYPE
SYNTAX OCTET STRING (SIZE (0..65536))
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'octetString'."
::= { expValueEntry 7 }
expValueOidVal OBJECT-TYPE
SYNTAX OBJECT IDENTIFIER
MAX-ACCESS read-only
Kavasseri & Stewart Standards Track PAGE 33
RFC 2982 Distributed Management Expression MIB October 2000
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'objectId'."
::= { expValueEntry 8 }
expValueCounter64Val OBJECT-TYPE
SYNTAX Counter64
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value when expExpressionValueType is 'counter64'."
::= { expValueEntry 9 }
--
-- Conformance
--
dismanExpressionMIBConformance OBJECT IDENTIFIER ::=
{ dismanExpressionMIB 3 }
dismanExpressionMIBCompliances OBJECT IDENTIFIER ::=
{ dismanExpressionMIBConformance 1 }
dismanExpressionMIBGroups OBJECT IDENTIFIER ::=
{ dismanExpressionMIBConformance 2 }
-- Compliance
dismanExpressionMIBCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for entities which implement
the Expression MIB."
MODULE -- this module
MANDATORY-GROUPS {
dismanExpressionResourceGroup,
dismanExpressionDefinitionGroup,
dismanExpressionValueGroup
}
OBJECT expResourceDeltaMinimum
SYNTAX Integer32 (-1 | 60..600)
DESCRIPTION
"Implementation need not allow deltas or it may
implement them and restrict them to higher values."
OBJECT expObjectSampleType
WRITE-SYNTAX INTEGER { absoluteValue(1) }
DESCRIPTION
"Implementation may disallow deltas calculation or
Kavasseri & Stewart Standards Track PAGE 34
RFC 2982 Distributed Management Expression MIB October 2000
change detection."
OBJECT expObjectIDWildcard
WRITE-SYNTAX INTEGER { false(2) }
DESCRIPTION
"Implementation may allow wildcards."
OBJECT expObjectDiscontinuityIDWildcard
WRITE-SYNTAX INTEGER { false(2) }
DESCRIPTION
"Implementation need not allow wildcards."
OBJECT expObjectConditionalWildcard
WRITE-SYNTAX INTEGER { false(2) }
DESCRIPTION
"Implementation need not allow deltas wildcards."
::= { dismanExpressionMIBCompliances 1 }
-- Units of Conformance
dismanExpressionResourceGroup OBJECT-GROUP
OBJECTS {
expResourceDeltaMinimum,
expResourceDeltaWildcardInstanceMaximum,
expResourceDeltaWildcardInstances,
expResourceDeltaWildcardInstancesHigh,
expResourceDeltaWildcardInstanceResourceLacks
}
STATUS current
DESCRIPTION
"Expression definition resource management."
::= { dismanExpressionMIBGroups 1 }
dismanExpressionDefinitionGroup OBJECT-GROUP
OBJECTS {
expExpression,
expExpressionValueType,
expExpressionComment,
expExpressionDeltaInterval,
expExpressionPrefix,
expExpressionErrors,
expExpressionEntryStatus,
expErrorTime,
expErrorIndex,
expErrorCode,
expErrorInstance,
Kavasseri & Stewart Standards Track PAGE 35
RFC 2982 Distributed Management Expression MIB October 2000
expObjectID,
expObjectIDWildcard,
expObjectSampleType,
expObjectDeltaDiscontinuityID,
expObjectDiscontinuityIDWildcard,
expObjectDiscontinuityIDType,
expObjectConditional,
expObjectConditionalWildcard,
expObjectEntryStatus
}
STATUS current
DESCRIPTION
"Expression definition."
::= { dismanExpressionMIBGroups 2 }
dismanExpressionValueGroup OBJECT-GROUP
OBJECTS {
expValueCounter32Val,
expValueUnsigned32Val,
expValueTimeTicksVal,
expValueInteger32Val,
expValueIpAddressVal,
expValueOctetStringVal,
expValueOidVal,
expValueCounter64Val
}
STATUS current
DESCRIPTION
"Expression value."
::= { dismanExpressionMIBGroups 3 }
END
4. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards- related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
Kavasseri & Stewart Standards Track PAGE 36
RFC 2982 Distributed Management Expression MIB October 2000
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
5. Acknowledgements
This MIB contains considerable contributions from the Distributed
Management Design Team (Andy Bierman, Maria Greene, Bob Stewart, and
Steve Waldbusser), and colleagues at Cisco who did the first
implementation.
6. References
[RFC 2571] Harrington, D., Presuhn, R. and B. Wijnen, "An
Architecture Describing SNMP Management Frameworks", RFC
2571, April 1999.
[RFC 1155] Rose, M. and K. McCloghrie, "Structure and Identification
of Management Information for TCP/IP-based Internets",
STD 16, RFC 1155, May 1990.
[RFC 1212] Rose, M. and K. McCloghrie, "Concise MIB Definitions",
STD 16, RFC 1212, March 1991.
[RFC 1215] Rose, M., "A Convention for Defining Traps for use with
the SNMP", RFC 1215, March 1991.
[RFC 2578] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J.,
Rose, M. and S. Waldbusser, "Structure of Management
Information Version 2 (SMIv2)", STD 58, RFC 2578, April
1999.
[RFC 2579] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J.,
Rose, M. and S. Waldbusser, "Textual Conventions for
SMIv2", STD 58, RFC 2579, April 1999.
[RFC 2580] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J.,
Rose, M. and S. Waldbusser, "Conformance Statements for
SMIv2", STD 58, RFC 2580, April 1999.
[RFC 1157] Case, J., Fedor, M., Schoffstall, M. and J. Davin,
"Simple Network Management Protocol", STD 15, RFC 1157,
May 1990.
Kavasseri & Stewart Standards Track PAGE 37
RFC 2982 Distributed Management Expression MIB October 2000
[RFC 1901] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Introduction to Community-based SNMPv2", RFC 1901,
January 1996.
[RFC 1906] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Transport Mappings for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1906, January 1996.
[RFC 2572] Case, J., Harrington D., Presuhn R. and B. Wijnen,
"Message Processing and Dispatching for the Simple
Network Management Protocol (SNMP)", RFC 2572, April
1999.
[RFC 2574] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", RFC 2574, April 1999.
[RFC 1905] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Protocol Operations for Version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1905, January 1996.
[RFC 2573] Levi, D., Meyer, P. and B. Stewart, "SNMPv3
Applications", RFC 2573, April 1999.
[RFC 2575] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", RFC 2575, April 1999.
[RFC 2570] Case, J., Mundy, R., Partain, D. and B. Stewart,
"Introduction to Version 3 of the Internet-standard
Network Management Framework", RFC 2570, April 1999.
[RFC 1903] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser,
"Coexistence between Version 1 and version 2 of the
Internet-standard Network Management Framework", RFC
1903, January 1996.
[RFC 2981] Stewart, B., "Event MIB", RFC 2981, October 2000.
[PracPersp] Leinwand, A. and K. Fang, "Network Management: A
Practical Perspective", Addison-Wesley Publishing
Company, Inc., 1993.
7. Security Considerations
Expression MIB security involves two perspectives: protection of
expressions from tampering or unauthorized use of resources, and
protection of the objects used to calculate the expressions.
Kavasseri & Stewart Standards Track PAGE 38
RFC 2982 Distributed Management Expression MIB October 2000
Security of expression definitions and results depends on the
expression owner (expExpressionOwner). With view-based access
control [RFC 2575] a network manager can control who has what level of
access to what expressions.
Access control for the objects within the expression depends on the
security credentials of the expression creator. These are the
security credentials used to get the objects necessary to evaluate
the expression. They are the security credentials that were used to
set the expExpressionRowStatus object for that expression to
'active', as recorded by the managed system.
This means that the results of an expression could potentially be
made available to someone who does not have access to the raw data
that went into them. This could be either legitimate or a security
violation, depending on the specific situation and security policy.
To facilitate the provisioning of access control by a security
administrator for this MIB itself using the View-Based Access Control
Model (VACM) defined in RFC 2575 [RFC 2575] for tables in which
multiple users may need to independently create or modify entries,
the initial index is used as an "owner index". Such an initial index
has a syntax of SnmpAdminString, and can thus be trivially mapped to
a securityName or groupName as defined in VACM, in accordance with a
security policy.
All entries in related tables belonging to a particular user will
have the same value for this initial index. For a given user's
entries in a particular table, the object identifiers for the
information in these entries will have the same subidentifiers
(except for the "column" subidentifier) up to the end of the encoded
owner index. To configure VACM to permit access to this portion of
the table, one would create vacmViewTreeFamilyTable entries with the
value of vacmViewTreeFamilySubtree including the owner index portion,
and vacmViewTreeFamilyMask "wildcarding" the column subidentifier.
More elaborate configurations are possible.
Kavasseri & Stewart Standards Track PAGE 39
RFC 2982 Distributed Management Expression MIB October 2000
8. Author's Address
Bob Stewart
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
U.S.A.
9. Editor's Address
Ramanathan Kavasseri
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
U.S.A.
Phone: +1 408 527 2446
EMail: ramk@cisco.com
Kavasseri & Stewart Standards Track PAGE 40
RFC 2982 Distributed Management Expression MIB October 2000
10. Full Copyright Statement
Copyright © The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Kavasseri & Stewart Standards Track PAGE 41
Distributed Management Expression MIB
RFC TOTAL SIZE: 81371 bytes
PUBLICATION DATE: Tuesday, October 31st, 2000
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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