RFC 3512
Configuring Networks and Devices with Simple Network Management Protocol (SNMP)
Pages: 83
Informational
Informational
Part 2 of 4 – Pages 9 to 41
3. Designing a MIB Module
Carefully considered MIB module designs are crucial to practical configuration with SNMP. As we have just seen, MIB objects designed for configuration can be very effective since they can be associated with integrated diagnostic, monitoring, and fault objects. MIB modules for configuration also scale when they expose their notion of template object types. Template objects can represent information at a higher level of abstraction than instance-level ones. This has the benefit of reducing the amount of instance-level data to move from management application to the agent on the managed element, when that instance-level data is brought about by applying a template object on the agent. Taken together, all of these objects can provide a robust configuration subsystem. The remainder of this section provides specific practices used in MIB module design with SMIv2 and SNMPv3.
3.1. MIB Module Design - General Issues
One of the first tasks in defining a MIB module is the creation of a model that reflects the scope and organization of the management information an agent will expose. MIB modules can be thought of as logical models providing one or more aspects/views of a subsystem. The objective for all MIB modules should be to serve one or more operational requirements such as accounting information collection, configuration of one or more parts of a system, or fault identification. However, it is important to include only those aspects of a subsystem that are proven to be operationally useful. In 1993, one of most widely deployed MIB modules supporting configuration was published, RFC 1493, which contained the BRIDGE- MIB. It defined the criteria used to develop the MIB module as follows: To be consistent with IAB directives and good engineering practice, an explicit attempt was made to keep this MIB as simple as possible. This was accomplished by applying the following criteria to objects proposed for inclusion: (1) Start with a small set of essential objects and add only as further objects are needed. (2) Require objects be essential for either fault or configuration management. (3) Consider evidence of current use and/or utility. (4) Limit the total (sic) of objects. (5) Exclude objects which are simply derivable from others in this or other MIBs. (6) Avoid causing critical sections to be heavily instrumented. The guideline that was followed is one counter per critical section per layer. Over the past eight years additional experience has shown a need to expand these criteria as follows: (7) Before designing a MIB module, identify goals and objectives for the MIB module. How much of the underlying system will be exposed depends on these goals.
(8) Minimizing the total number of objects is not an explicit goal,
but usability is. Be sure to consider deployment and usability
requirements.
(9) During configuration, consider supporting explicit error state,
capability and capacity objects.
(10) When evaluating rule (5) above, consider the impact on a
management application. If an object can help reduce a
management application's complexity, consider defining objects
that can be derived.
3.2. Naming MIB modules and Managed Objects
Naming of MIB modules and objects informally follows a set of best
practices. Originally, standards track MIB modules used RFC names.
As the MIB modules evolved, the practice changed to using more
descriptive names. Presently, Standards Track MIB modules define a
given area of technology such as ATM-MIB, and vendors then extend
such MIB modules by prefixing the company name to a given MIB module
as in ACME-ATM-MIB.
Object descriptors (the "human readable names" assigned to object
identifiers [2]) defined in standard MIB modules should be unique
across all MIB modules. Generally, a prefix is added to each managed
object that can help reference the MIB module it was defined in. For
example, the IF-MIB uses "if" prefix for descriptors of object types
such as ifTable, ifStackTable and so forth.
MIB module object type descriptors can include an abbreviation for
the function they perform. For example the objects that control
configuration in the example MIB module in Section 8 include "Cfg" as
part of the object descriptor, as in bldgHVACCfgDesiredTemp.
This is more fully realized when the object descriptors that include
the fault, configuration, accounting, performance and security [33]
abbreviations are combined with an organized OID assignment approach.
For example, a vendor could create a configuration branch in their
private enterprises area. In some cases this might be best done on a
per product basis. Whatever the approach used, "Cfg" might be
included in every object descriptor in the configuration branch.
This has two operational benefits. First, for those that do look at
instances of MIB objects, descriptors as seen through MIB browsers or
other command line tools assist in conveying the meaning of the
object type. Secondly, management applications can be pointed at
specific subtrees for fault or configuration, causing a more
efficient retrieval of data and a simpler management application with
potentially better performance.
3.3. Transaction Control And State Tracking
Transactions and keeping track of their state is an important consideration when performing any type of configuration activity regardless of the protocol. Here are a few areas to consider when designing transaction support into an SNMP-based configuration system.3.3.1. Conceptual Table Row Modification Practices
Any discussion of transaction control as it pertains to MIB module design often begins with how the creation or modification of object instances in a conceptual row in the MIB module is controlled. RowStatus [3] is a standard textual convention for the management of conceptual rows in a table. Specifically, the RowStatus textual convention that is used for the SYNTAX value of a single column in a table controls the creation, deletion, activation, and deactivation of conceptual rows of the table. When a table has been defined with a RowStatus object as one of its columns, changing an instance of the object to 'active' causes the row in which that object instance appears to become 'committed'. In a multi-table scenario where the configuration data must be spread over many columnar objects, a RowStatus object in one table can be used to cause the entire set of data to be put in operation or stored based on the definition of the objects. In some cases, very large amounts of data may need to be 'committed' all at once. In these cases, another approach is to configure all of the rows in all the tables required and have an "activate" object that has a set method that commits all the modified rows. The RowStatus textual convention specifies that, when used in a conceptual row, a description must define what can be modified. While the description of the conceptual row and its columnar object types is the correct place to derive this information on instance modifiability, it is often wrongly assumed in some implementations that: 1) objects either must all be presently set or none need be set to make a conceptual RowStatus object transition to active(1) 2) objects in a conceptual row cannot be modified once a RowStatus object is active(1). Restricting instance modifiability like this, so that after a RowStatus object is set to active(1) is in fact a reasonable limitation, since such a set of RowStatus may have agent system side-effects which depend on committed columnar
object instance values. However, where this restriction exists on
an object, it should be made clear in a DESCRIPTION clause such as
the following:
protocolDirDescr OBJECT-TYPE
SYNTAX DisplayString (SIZE (1..64))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A textual description of the protocol encapsulation.
A probe may choose to describe only a subset of the
entire encapsulation (e.g., only the highest layer).
This object is intended for human consumption only.
This object may not be modified if the associated
protocolDirStatus object is equal to active(1)."
::= { protocolDirEntry 4 }
Any such restrictions on columnar object instance modification while
a row's RowStatus object instance is set to active(1) should appear
in the DESCRIPTION clause of the RowStatus columnar OBJECT-TYPE as
well.
3.3.2. Fate sharing with multiple tables
An important principle associated with transaction control is fate
sharing of rows in different tables. Consider the case where a
relationship has been specified between two conceptual tables of a
MIB module (or tables in two different MIB modules). In this
context, fate sharing means that when a row of a table is deleted,
the corresponding row in the other table is also deleted. Fate
sharing in a transaction control context can also be used with the
activation of very large configuration changes. If we have two
tables that hold a set of configuration information, a row in one
table might have to be put in the 'ready' state before the second can
be put in the 'ready' state. When that second table can be placed in
the 'ready' state, then the entire transaction can be considered to
have been 'committed'.
Fate sharing of SNMP table data should be explicitly defined where
possible using the SMI index qualifier AUGMENTS. If the relationship
between tables cannot be defined using SMIv2 macros, then the
DESCRIPTION clause of the object types which particularly effect the
cross-table relationship should define what should happen when rows
in related tables are added or deleted.
Consider the relationship between the dot1dBasePortTable and the ifTable. These tables have a sparse relationship. If a given ifEntry supports 802.1D bridging then there is a dot1dBasePortEntry that has a pointer to it via dot1dBasePortIfIndex. Now, what should happen if an ifEntry that can bridge is deleted? Should the object dot1dBasePortIfIndex simply be set to 0 or should the dot1dBasePortEntry be deleted as well? A number of acceptable design and practice techniques can provide the answer to these questions, so it is important for the MIB module designer to provide the guidance to guarantee consistency and interoperability. To this end, when two tables are related in such a way, ambiguities such as this should be avoided by having the DESCRIPTION clauses of the pertinent row object types define the fate sharing of entries in the respective tables.3.3.3. Transaction Control MIB Objects
When a MIB module is defined that includes configuration object types, consider providing transaction control objects. These objects can be used to cause a large transaction to be committed. For example, we might have several tables that define the configuration of a portion of a system. In order to avoid churn in the operational state of the system we might create a single scalar object that, when set to a particular value, will cause the activation of the rows in all the necessary tables. Here are some examples of further usage for such object types: o Control objects that are the 'write' or 'commit' objects. Such objects can cause all pending transactions (change MIB object values as a result of SET operations) to be committed to a permanent repository or operational memory, as defined by the semantics of the MIB objects. o Control objects at different levels of configuration granularity. One of the decisions for a MIB module designer is what are the levels of granularity that make sense in practice. For example, in the routing area, would changes be allowed on a per protocol basis such as BGP? If allowed at the BGP level, are sub-levels permitted such as per autonomous system? The design of these control objects will be impacted by the underlying software design. RowStatus (see Section 3.3.1) also has important relevance as a general transaction control object.
3.3.4. Creating And Activating New Table Rows
When designing read-create objects in a table, a MIB module designer should first consider the default state of each object in the table when a row is created. Should an implementation of a standard MIB module vary in terms of the objects that need to be set in order to create an instance of a given row, an agent capabilities statement should be used to name the additional objects in that table using the CREATION-REQUIRES clause. It is useful when configuring new rows to use the notReady status to indicate row activation cannot proceed. When creating a row instance of a conceptual table, one should consider the state of instances of required columnar objects in the row. The DESCRIPTION clause of such a required columnar object should specify it as such. During the period of time when a management application is attempting to create a row, there may be a period of time when not all of these required (and non-defaultable) columnar object instances have been set. Throughout this time, an agent should return a noSuchInstance error for a GET of any object instance of the row until such time that all of these required instance values are set. The exception is the RowStatus object instance, for which a notReady(3) value should be returned during this period. One need only be concerned with the notReady value return for a RowStatus object when the row under creation does not yet have all of the required, non-defaultable instance values for the row. One approach to simplifying in-row configuration transactions when designing MIB modules is to construct table rows that have no more instance data for columnar objects than will fit inside a single SET PDU. In this case, the createAndWait() value for the RowStatus columnar object is not required. It is possible to use createAndGo() in the same SET PDU, thus simplifying transactional management.3.3.5. Summary Objects and State Tracking
Before beginning a new set of configuration transactions, a management application might want to checkpoint the state of the managed devices whose configuration it is about to change. There are a number of techniques that a MIB module designer can provide to assist in the (re-)synchronization of the managed systems. These objects can also be used to verify that the management application's notion of the managed system state is the same as that of the managed device.
These techniques include:
1. Provide an object that reports the number of rows in a table
2. Provide an object that flags when data in the table was last
modified.
3. Send a notification message (InformRequests are preferable) to
deliver configuration change.
By providing an object containing the number of rows in a table,
management applications can decide how best to retrieve a given
table's data and may choose different retrieval strategies depending
on table size. Note that the availability of and application
monitoring of such an object is not sufficient for determining the
presence of table data change over a checkpointed duration since an
equal number of row creates and deletes over that duration would
reflect no change in the object instance value. Additionally, table
data change which does not change the number of rows in the table
would not be reflected through simple monitoring of such an object
instance.
Instead, the change in the value of any table object instance data
can be tracked through an object that monitors table change state as
a function of time. An example is found in RFC 2790, Host Resources
MIB:
hrSWInstalledLastUpdateTime OBJECT-TYPE
SYNTAX TimeTicks
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime when the hrSWInstalledTable
was last completely updated. Because caching of this
data will be a popular implementation strategy,
retrieval of this object allows a management station
to obtain a guarantee that no data in this table is
older than the indicated time."
::= { hrSWInstalled 2 }
A similar convention found in many standards track MIB modules is the
"LastChange" type object.
For example, the ENTITY-MIB, RFC 2737 [34], provides the following
object:
entLastChangeTime OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime at the time a conceptual row is
created, modified, or deleted in any of these tables:
- entPhysicalTable
- entLogicalTable
- entLPMappingTable
- entAliasMappingTable
- entPhysicalContainsTable"
::= { entityGeneral 1 }
This convention is not formalized. There tend to be small
differences in what a table's LastChanged object reflects. IF-MIB
(RFC 2863 [20]) defines the following:
ifTableLastChange OBJECT-TYPE
SYNTAX TimeTicks
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime at the time of the last
creation or deletion of an entry in the ifTable. If
the number of entries has been unchanged since the
last re-initialization of the local network management
subsystem, then this object contains a zero value."
::= { ifMIBObjects 5 }
So, if an agent modifies a row with an SNMP SET on ifAdminStatus, the
value of ifTableLastChange will not be updated. It is important to
be specific about what can cause an object to update so that
management applications will be able to detect and more properly act
on these changes.
The final way to keep distributed configuration data consistent is to
use an event-driven model, where configuration changes are
communicated as they occur. When the frequency of change to
configuration is relatively low or polling a cache object is not
desired, consider defining a notification that can be used to report
all configuration change details.
When doing so, the option is available to an SNMPv3 (or SNMPv2c)
agent to deliver the notification using either a trap or an inform.
The decision as to which PDU to deliver to the recipient is generally
a matter of local configuration. Vendors should recommend the use of
informs over traps for NOTIFICATION-TYPE data since the agent can use
the presence or absence of a response to help know whether it needs
to retransmit or not. Overall, it is preferable to use an inform instead of a trap so that changes have a higher likelihood of confirmed end-to-end delivery. As a matter of MIB module design, when practical, the NOTIFICATION- TYPE should include in the PDU all of the modified columnar objects in a row of a table. This makes it easier for the management application receiving the notification to keep track of what has changed in the row of a table and perform addition analysis on the state of the managed elements. However, the use of notifications to communicate the state of a rapidly changing object may not be ideal either. This leads us back to the MIB module design question of what is the right level of granularity to expose. Finally, having to poll many "LastChange" objects does not scale well. Consider providing a global LastChange type object to represent overall configuration in a given agent implementation.3.3.6. Optimizing Configuration Data Transfer
Configuration management software should keep track of the current configuration of all devices under its control. It should ensure that the result is a consistent view of the configuration of the network, which can help reduce inadvertent configuration errors. In devices that have very large amounts of configuration data, it can be costly to both the agent and the manager to have the manager periodically poll the entire contents of these configuration tables for synchronization purposes. A benefit of good synchronization between the manager and the agent is that the manager can determine the smallest and most effective set of data to send to managed devices when configuration changes are required. Depending on the table organization in the managed device and the agent implementation, this practice can reduce the burden on the managed device for activation of these configuration changes. In the previous section, we discussed the "LastChange" style of object. When viewed against the requirements just described, the LastChange object is insufficient for large amounts of data. There are three design options that can be used to assist with the synchronization of the configuration data found in the managed device with the manager:
1) Design multiple indices to partition the data in a table logically
or break a table into a set of tables to partition the data based
on what an application will use the table for
2) Use a time-based indexing technique
3) Define a control MIB module that manages a separate data delivery
protocol
3.3.6.1. Index Design
Index design has a major impact on the amount of data that must be
transferred between SNMP entities and can help to mitigate scaling
issues with large tables.
Many tables in standard MIB modules follow one of two indexing
models:
- Indexing based upon increasing Integer32 or Unsigned32 values of
the kind one might find in an array.
- Associative indexing, which refers to the technique of using
potentially sparse indices based upon a "key" of the sort one
would use for a hash table.
When tables grow to a very large number of rows, using an associative
indexing scheme offers the useful ability to efficiently retrieve
only the rows of interest.
For example, if an SNMP entity exposes a copy of the default-free
Internet routing table as defined in the ipCidrRouteTable, it will
presently contain around 100,000 rows.
Associative indexing is used in the ipCidrRouteTable and allows one
to retrieve, for example, all routes for a given IPv4 destination
192.0.2/24.
Yet, if the goal is to extract a copy of the table, the associative
indexing reduces the throughput and potentially the performance of
retrieval. This is because each of the index objects are appended to
the object identifiers for every object instance returned.
ipCidrRouteEntry OBJECT-TYPE
SYNTAX IpCidrRouteEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A particular route to a particular destination,
under a particular policy."
INDEX {
ipCidrRouteDest,
ipCidrRouteMask,
ipCidrRouteTos,
ipCidrRouteNextHop
}
A simple array-like index works efficiently since it minimizes the
index size and complexity while increasing the number of rows that
can be sent in a PDU. If the indexing is not sparse, concurrency can
be gained by sending multiple asynchronous non-overlapping collection
requests as is explained in RFC 2819 [32], Page 41 (in the section
pertaining to Host Group indexing).
Should requirements dictate new methods of access, multiple
indices can be defined such that both associative and simple
indexing can coexist to access a single logical table.
Two examples follow.
First, consider the ifStackTable found in RFC 2863 [20] and the
ifInvStackTable RFC 2864 [33]. They are logical equivalents with the
order of the auxiliary (index) objects simply reversed.
ifStackEntry OBJECT-TYPE
SYNTAX IfStackEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information on a particular relationship between
two sub-layers, specifying that one sub-layer runs
on 'top' of the other sub-layer. Each sub-layer
corresponds to a conceptual row in the ifTable."
INDEX { ifStackHigherLayer, ifStackLowerLayer }
::= { ifStackTable 1 }
ifInvStackEntry OBJECT-TYPE
SYNTAX IfInvStackEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information on a particular relationship between two
sub-layers, specifying that one sub-layer runs underneath
the other sub-layer. Each sub-layer corresponds to a
conceptual row in the ifTable."
INDEX { ifStackLowerLayer, ifStackHigherLayer }
::= { ifInvStackTable 1 }
Second, table designs that can factor data into multiple tables with well-defined relationships can help reduce overall data transfer requirements. The RMON-MIB, RFC 2819 [32], demonstrates a very useful technique of organizing tables into control and data components. Control tables contain those objects that are configured and change infrequently, and the data tables contain information to be collected that can be large and may change quite frequently. As an example, the RMON hostControlTable provides a way to specify how to collect MAC addresses learned as a source or destination from a given port that provides transparent bridging of Ethernet packets. Configuration is accomplished using the hostControlTable. It is indexed by a simple integer. While this may seem to be array-like, it is common practice for command generators to encode the ifIndex into this simple integer to provide associative lookup capability. The RMON hostTable and hostTimeTable represent dependent tables that contain the results indexed by the hostControlTable entry. The hostTable is further indexed by the MAC address which provides the ability to reasonably search for a collection, such as the Organizationally Unique Identifier (OUI), the first three octets of the MAC address. The hostTimeTable is designed explicitly for fast transfer of bulk RMON data. It demonstrates how to handle collecting large number of rows in the face of deletions and insertions by providing hostControlLastDeleteTime. hostControlLastDeleteTime OBJECT-TYPE SYNTAX TimeTicks MAX-ACCESS read-only STATUS current DESCRIPTION "The value of sysUpTime when the last entry was deleted from the portion of the hostTable associated with this hostControlEntry. If no deletions have occurred, this value shall be zero." ::= { hostControlEntry 4 }3.3.6.2. Time Based Indexing
The TimeFilter as defined in RFC 2021 [44] and used in RMON2-MIB and Q-BRIDGE-MIB (RFC 2674 [26]) provides a way to obtain only those rows that have changed on or after some specified period of time has passed.
One drawback to TimeFilter index tables is that a given row can appear at many points in time, which artificially inflates the size of the table when performing standard getNext or getBulk data retrieval.3.3.6.3. Alternate Data Delivery Mechanisms
If the amount of data to transfer is larger than current SNMP design restrictions permit, as in the case of OCTET STRINGS (64k minus overhead of IP/UDP header plus SNMP header plus varbind list plus varbind encoding), consider delivery of the data via an alternate method, such as FTP and use a MIB module to control that data delivery process. In many cases, this problem can be avoided via effective MIB design. In other words, object types requiring this kind of transfer size should be used judiciously, if at all. There are many enterprise MIB modules that provide control of the TFTP or FTP protocol. Often the SNMP part defines what to send where and setting an object initiates the operation (for an example, refer to the CISCO-FTP-CLIENT-MIB, discussed in [38]). Various approaches exist for allowing a local agent process running within the managed node to take a template for an object instance (for example for a set of interfaces), and adapt and apply it to all of the actual instances within the node. This is an architecture for one form of policy-based configuration (see [36], for example). Such an architecture, which must be designed into the agent and some portions of the MIB module, affords the efficiency of specifying many copies of instance data only once, along with the execution efficiency of distributing the application of the instance data to the agent. Other work is currently underway to improve efficiency for bulk SNMP transfer operations [37]. The objective of these efforts is simply the conveyance of more information with less overhead.3.4. More Index Design Issues
Section 3.3.5 described considerations for table row index design as it pertains to the synchronization of changes within sizable table rows. This section simply considers how to specify this syntactically and how to manage indices semantically. In many respects, the design issues associated with indices in a MIB module are similar to those in a database. Care must be taken during the design phase to determine how often and what kind of information must be set or retrieved. The next few points provide some guidance.
3.4.1. Simple Integer Indexing
When indexing tables using simple Integer32 or Unsigned32, start with one (1) and specify the maximum range of the value. Since object identifiers are unsigned long values, a question that arises is why not index from zero (0) instead of one(1)? RFC 2578 [2], Section 7.7, page 28 states the following: Instances identified by use of integer-valued objects should be numbered starting from one (i.e., not from zero). The use of zero as a value for an integer-valued index object type should be avoided, except in special cases. Consider the provisions afforded by the following textual convention from the Interfaces Group MIB module [33]: InterfaceIndexOrZero ::= TEXTUAL-CONVENTION DISPLAY-HINT "d" STATUS current DESCRIPTION "This textual convention is an extension of the InterfaceIndex convention. The latter defines a greater than zero value used to identify an interface or interface sub-layer in the managed system. This extension permits the additional value of zero. the value zero is object-specific and must therefore be defined as part of the description of any object which uses this syntax. Examples of the usage of zero might include situations where interface was unknown, or when none or all interfaces need to be referenced." SYNTAX Integer32 (0..2147483647)3.4.2. Indexing with Network Addresses
There are many objects that use IPv4 addresses (SYNTAX IpAddress) as indexes. One such table is the ipAddrTable from RFC 2011 [14] IP- MIB. This limits the usefulness of the MIB module to IPv4. To avoid such limitations, use the addressing textual conventions INET- ADDRESS-MIB [13] (or updates to that MIB module), which provides a generic way to represent addresses for Internet Protocols. In using the InetAddress textual convention in this MIB, however, pay heed to the following advisory found in its description clause: When this textual convention is used as the syntax of an index object, there may be issues with the limit of 128 sub-identifiers specified in SMIv2, STD 58. In this case, the OBJECT-TYPE declaration MUST include a 'SIZE' clause to limit the number of potential instance sub-identifiers.
One should consider the SMI limitation on the 128 sub-identifier specification when using certain kinds of network address index types. The most likely practical liability encountered in practice has been with DNS names, which can in fact be in excess of 128 bytes. The problem can be, of course, compounded when multiple indices of this type are specified for a table.3.5. Conflicting Controls
MIB module designers should avoid specifying read-write objects that overlap in function partly or completely. Consider the following situation where two read-write objects partially overlap when a dot1dBasePortEntry has a corresponding ifEntry. The BRIDGE-MIB defines the following managed object: dot1dStpPortEnable OBJECT-TYPE SYNTAX INTEGER { enabled(1), disabled(2) } ACCESS read-write STATUS mandatory DESCRIPTION "The enabled/disabled status of the port." REFERENCE "IEEE 802.1D-1990: Section 4.5.5.2" ::= { dot1dStpPortEntry 4 } The IF-MIB defines a similar managed object: ifAdminStatus OBJECT-TYPE SYNTAX INTEGER { up(1), -- ready to pass packets down(2), testing(3) -- in some test mode } MAX-ACCESS read-write STATUS current DESCRIPTION "The desired state of the interface. The testing(3) state indicates that no operational packets can be passed. When a managed system initializes, all interfaces start with ifAdminStatus in the down(2) state. As a result of either explicit management action or per configuration information retained by the managed system, ifAdminStatus is then changed to either the up(1) or
testing(3) states (or remains in the down(2) state)."
::= { ifEntry 7 }
If ifAdminStatus is set to testing(3), the value to be returned for
dot1dStpPortEnable is not defined. Without clarification on how
these two objects interact, management implementations will have to
monitor both objects if bridging is detected and correlate behavior.
The dot1dStpPortEnable object type could have been written with more
information about the behavior of this object when values of
ifAdminStatus which impact it change. For example, text could be
added that described proper return values for the dot1dStpPortEnable
object instance for each of the possible values of ifAdminStatus.
In those cases where overlap between objects is unavoidable, then as
we have just described, care should be taken in the description of
each of the objects to describe their possible interactions. In the
case of an object type defined after an incumbent object type, it is
necessary to include in the DESCRIPTION of this later object type the
details of these interactions.
3.6. Textual Convention Usage
Textual conventions should be used whenever possible to create a
consistent semantic for an oft-recurring datatype.
MIB modules often define a binary state object such as enable/disable
or on/off. Current practice is to use existing Textual Conventions
and define the read-write object in terms of a TruthValue from
SNMPv2-TC [3]. For example, the Q-BRIDGE-MIB [26] defines:
dot1dTrafficClassesEnabled OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"The value true(1) indicates that Traffic Classes are
enabled on this bridge. When false(2), the bridge
operates with a single priority level for all traffic."
DEFVAL { true }
::= { dot1dExtBase 2 }
Textual conventions that have a reasonable chance of being reused in
other MIB modules ideally should also be defined in a separate MIB
module to facilitate sharing of such object types. For example, all
ATM MIB modules draw on the ATM-TC-MIB [39] to reference and utilize
common definitions for addressing, service class values, and the
like.
To simplify management, it is recommended that existing SNMPv2-TC
based definitions be used when possible. For example, consider the
following object definition:
acmePatioLights OBJECT-TYPE
SYNTAX INTEGER {
on(1),
off(2),
}
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"Current status of outdoor lighting."
::= { acmeOutDoorElectricalEntry 3 }
This could be defined as follows using existing SNMPv2-TC TruthValue.
acmePatioLightsOn OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
DESCRI2096PTION
"Current status of outdoor lighting. When set to true (1),
this means that the lights are enabled and turned on.
When set to false (2), the lights are turned off."
::= { acmeOutDoorElectricalEntry 3 }
3.7. Persistent Configuration
Many network devices have two levels of persistence with regard to
configuration data. In the first case, the configuration data sent
to the device is persistent only until changed with a subsequent
configuration operation, or the system is reinitialized. The second
level is where the data is made persistent as an inherent part of the
acceptance of the configuration information. Some configuration
shares both these properties, that is, that on acceptance of new
configuration data it is saved permanently and in memory. Neither of
these necessarily means that the data is used by the operational
code. Sometimes separate objects are required to activate this new
configuration data for use by the operational code.
However, many SNMP agents presently implement simple persistence
models, which do not reflect all the relationships of the
configuration data to the actual persistence model as described
above. Some SNMP set requests against MIB objects with MAX-ACCESS
read-write are written automatically to a persistent store. In other
cases, they are not. In some of the latter cases, enterprise MIB
objects are required in order to get standard configuration stored, thus making it difficult for a generic application to have a consistent effect. There are standard conventions for saving configuration data. The first method uses the Textual Convention known as StorageType [3] which explicitly defines a given row's persistence requirement. Examples include the RFC 3231 [25] definition for the schedTable row object schedStorageType of syntax StorageType, as well as similar row objects for virtually all of the tables of the SNMP View-based Access Control Model MIB [10]. A second method for persistence simply uses the DESCRIPTION clause to define how instance data should persist. RFC 2674 [26] explicitly defines Dot1qVlanStaticEntry data persistence as follows: dot1qVlanStaticTable OBJECT-TYPE SYNTAX SEQUENCE OF Dot1qVlanStaticEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "A table containing static configuration information for each VLAN configured into the device by (local or network) management. All entries are permanent and will be restored after the device is reset." ::= { dot1qVlan 3 } The current practice is a dual persistence model where one can make changes to run-time configuration as well as to a non-volatile configuration read at device initialization. The DISMAN-SCHEDULE-MIB module [25] provides an example of this practice. A row entry of its SchedTable specifies the parameters by which an agent MIB variable instance can be set to a specific value at some point in time and governed by other constraints and directives. One of those is: schedStorageType OBJECT-TYPE SYNTAX StorageType MAX-ACCESS read-create STATUS current DESCRIPTION "This object defines whether this scheduled action is kept in volatile storage and lost upon reboot or if this row is backed up by non-volatile or permanent storage. Conceptual rows having the value `permanent' must allow write access to the columnar objects schedDescr, schedInterval, schedContextName, schedVariable, schedValue, and schedAdminStatus. If an implementation supports the
schedCalendarGroup, write access must be also allowed to
the columnar objects schedWeekDay, schedMonth, schedDay,
schedHour, schedMinute."
DEFVAL { volatile }
::= { schedEntry 19 }
It is important, however, to reiterate that the persistence is
ultimately controlled by the capabilities and features (with respect
to the storage model of management data) of the underlying system on
which the MIB Module agent is being implemented. This falls into
very much the same kind of issue set as, for example, the situation
where the size of data storage in the system for a Counter object
type is not the same as that in the corresponding MIB Object Type.
To generalize, the final word on the "when" and "how" of storage of
persistent data is dictated by the system and the implementor of the
agent on the system.
3.8. Configuration Sets and Activation
An essential notion for configuration of network elements with SNMP
is awareness of the difference between the set of one or more
configuration objects from the activation of those configuration
changes in the actual subsystem. That is, it often only makes sense
to activate a group of objects as a single 'transaction'.
3.8.1. Operational Activation Considerations
A MIB module design must consider the implications of the preceding
in the context of changes that will occur throughout a subsystem when
changes are activated. This is particularly true for configuration
changes that are complex. This complexity can be in terms of
configuration data or the operational ramifications of the activation
of the changes in the managed subsystem. A practical technique to
accommodate this kind of activation is the partitioning of contained
configuration sets, as it pertains to their being activated as
changes. Any complex configuration should have a master on/off
switch (MIB object type) as well as strategically placed on/off
switches that partition the activation of configuration data in the
managed subsystem. These controls play a pivotal role during the
configuration process as well as during subsequent diagnostics.
Generally, a series of set operations should not cause an agent to
activate each object, causing operational instability to be
introduced with every changed object instance. To avoid this
liability, ideally a series of Set PDUs can install the configuration
and a final set series of PDUs can activate the changes.
During diagnostic situations, certain on/off switches can be set to localize the perceived error instead of having to remove the configuration. An example of such an object from the OSPF Version 2 MIB [29] is the global ospfAdminStat: ospfAdminStat OBJECT-TYPE SYNTAX Status MAX-ACCESS read-write STATUS current DESCRIPTION "The administrative status of OSPF in the router. The value 'enabled' denotes that the OSPF Process is active on at least one interface; 'disabled' disables it on all interfaces." ::= { ospfGeneralGroup 2 } Elsewhere in the OSPF MIB, the semantics of setting ospfAdminStat to enabled(2) are clearly spelled out. The Scheduling MIB [25] exposes such an object on each entry in the scheduled actions table, along with the corresponding stats object type (with read-only ACCESS) on the scheduled actions row instance. This reflects a recurring basic design pattern which brings about semantic clarity in the object type usage. A table can expose one columnar object type which is strictly for administrative control. When read, an instance of this object type will reflect its last set or defaulted value. A companion operational columnar object type, with MAX-ACCESS of read-only, provides the current state of activation or deactivation resulting from the last set of the administrative columnar instance. It is fully expected that these administrative and operational columnar instances may reflect different values over some period of time of activation latency, which is why they are separate. Further sections display some of the problems which can result from attempting to combine the operational and administrative row columns into a single object type. Note that all of this is independent of the RowStatus columnar object, and the notion of 'activation' as it pertains to RowStatus. A defined RowStatus object type should be strictly concerned with the management of the table row itself (with 'activation' indicating "the conceptual row is available for use by the managed device" [3], and not to be confused with any operational activation semantics).
In the following example, schedAdminStatus controls activation of the
scheduled action, and schedOperStatus reports on its operational
status:
schedAdminStatus OBJECT-TYPE
SYNTAX INTEGER {
enabled(1),
disabled(2)
}
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The desired state of the schedule."
DEFVAL { disabled }
::= { schedEntry 14 }
schedOperStatus OBJECT-TYPE
SYNTAX INTEGER {
enabled(1),
disabled(2),
finished(3)
}
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The current operational state of this schedule. The state
enabled(1) indicates this entry is active and that the
scheduler will invoke actions at appropriate times. The
disabled(2) state indicates that this entry is currently
inactive and ignored by the scheduler. The finished(3)
state indicates that the schedule has ended. Schedules
in the finished(3) state are ignored by the scheduler.
A one-shot schedule enters the finished(3) state when it
deactivates itself."
::= { schedEntry 15 }
3.8.2. RowStatus and Deactivation
RowStatus objects should not be used to control
activation/deactivation of a configuration. While RowStatus looks
ideally suited for such a purpose since a management application can
set a row to active(1), then set it to notInService(2) to disable it
then make it active(1) again, there is no guarantee that the agent
won't discard the row while it is in the notInService(2) state. RFC
2579 [3], page 15 states:
The agent must detect conceptual rows that have been in either
state for an abnormally long period of time and remove them. It
is the responsibility of the DESCRIPTION clause of the status
column to indicate what an abnormally long period of time would
be.
The DISMAN-SCHEDULE-MIB's managed object schedAdminStatus
demonstrates how to separate row control from row activation.
Setting the schedAdminStatus to disabled(2) does not cause the row to
be aged out/removed from the table.
Finally, a reasonable agent implementation must consider how many
rows will be allowed to be created in the notReady/notInService state
such that resources are not exhausted by an errant application.
3.9. SET Operation Latency
Many standards track and enterprise MIB modules that contain read-
write objects assume that an agent can complete a set operation as
quickly as an agent can send back the status of the set operation to
the application.
Consider the subtle operational shortcomings in the following object.
It both reports the current state and allows a SET operation to
change to a possibly new state.
wheelRotationState OBJECT-TYPE
SYNTAX INTEGER { unknown(0),
idle(1),
spinClockwise(2),
spinCounterClockwise(3)
}
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"The current state of a wheel."
::= { XXX 2 }
With the object defined, the following example represents one possible
transaction.
Time Command Generator --------> <--- Command Responder
----- ----------------- -----------------
|
A GetPDU(wheelRotationState.1.1)
|
| ResponsePDU(error-index 0,
| error-code 0)
|
B wheelRotationState.1.1 == spinClockwise(2)
|
C SetPDU(wheelRotationState.1.1 =
| spinCounterClockwise(3)
|
| ResponsePDU(error-index 0,
| error-code 0)
|
D wheelRotationState.1.1
== spinCounterClockwise(3)
|
E GetPDU(wheelRotationState.1.1)
|
F ResponsePDU(error-index 0,
| error-code 0)
|
V wheelRotationState.1.1 == spinClockwise(2)
....some time, perhaps seconds, later....
|
G GetPDU(wheelRotationState.1.1)
|
H ResponsePDU(error-index 0,
| error-code 0)
| wheelRotationState.1.1
V == spinCounterClockwise(3)
The response to the GET request at time E will often confuse
management applications that assume the state of the object should be
spinCounterClockwise(3). In reality, the wheel is slowing down in
order to come to the idle state then begin spinning counter
clockwise.
This possibility of confusing and paradoxical interactions of
administrative and operational state is inevitable when a single
object type is used to control and report on both types of state.
One common practice which we have already seen is to separate out the
desired (settable) state from current state. The objects ifAdminStatus and ifOperStatus from RFC 2863 [20] provide such an example of the separation of objects into desired and current state.3.9.1. Subsystem Latency, Persistence Latency, and Activation Latency
A second way latency can be introduced in SET operations is caused by delay in agent implementations that must interact with loosely coupled subsystems. The time it takes the instrumented system to accept the new configuration information from the SNMP agent, process it and 'install' the updated configuration in the system or otherwise process the directives can often be longer than the SNMP response timeout. In these cases, it is desirable to provide a "current state" object type which can be polled by the management application to determine the state of control of the loosely coupled subsystem which was affected by its configuration update. More generally, some MIB objects may have high latencies associated with changes to their values. This could be either a function of saving the changed value to a persistent storage type, and/or activating a subsystem that inherently has high latency as discussed above. When defining such MIB objects, it might be wise to have the agent process set operations in the managed subsystem as soon as the Set PDU has been processed, and then update appropriate status objects when the save-to- persistent storage and (if applicable) activation has succeeded or is otherwise complete. Another approach would be to cause a notification to be sent that indicates that the operation has been completed. When you describe an activation object, the DESCRIPTION clauses for these objects should give a hint about the likely latency for the completion of the operation. Keep in mind that from a management software perspective (as presented in the example of schedAdminStatus in Section 3.8.1), the combined latency of saving-to-persistence and activation are not distinguishable when they are part of a single operation.3.10. Notifications and Error Reporting
For the purpose of this section, a 'notification' is as described in the SMIv2, RFC 2578 [2], by the NOTIFICATION-TYPE macro. Notifications can be sent in either SNMPv2c [19] or SNMPv3 TRAP or InformRequest PDUs. Given the sensitivity of configuration information, it is recommended that configuration operations always be performed using SNMPv3 due to its enhanced security capabilities. InformRequest PDUs should be used in preference to TRAP PDUs since
the recipient of the InformRequest PDUs responds with a Response PDU. This acknowledgment can be used to avoid unnecessary retransmission of NOTIFICATION-TYPE information when retransmissions are in fact required. The use of InformRequest PDUs (as opposed to TRAPs) is not at the control of the MIB module designer or agent implementor. The determination as to whether or not a TRAP or InformRequest PDU is sent from an SNMPv2c or SNMPv3 agent is generally a function of the agent's local configuration (but can be controlled with MIB objects in SNMPv3). To the extent notification timeout and retry values are determined by local configuration parameters, care should be taken to avoid unnecessary retransmission of InformRequest PDUs. Configuration change and error information conveyed in InformRequest PDUs can be an important part of an effective SNMP-based management system. They also have the potential to be overused. This section offers some guidance for effective definition of NOTIFICATION-TYPE information about configuration changes that can be carried in InformRequest PDUs. Notifications can also play a key role for all kinds of error reporting from hardware failures to configuration and general policy errors. These types of notifications should be designed as described in Section 3.11 (Application Error Reporting).3.10.1. Identifying Source of Configuration Changes
A NOTIFICATION-TYPE designed to report configuration changes should report the identity of the management entity initiating the configuration change. Specifically, if the entity is known to be a SNMP command generator, the transport address and SNMP parameters as found in table snmpTargetParamsTable from RFC 3413 SNMP-TARGET-MIB should be reported where possible. For reporting of configuration changes outside of the SNMP domain, the applicable change mechanism (for example, CLI vs. HTTP-based management client access) should be reported, along with whatever notion of "user ID" of the change initiator is applicable and available.3.10.2. Limiting Unnecessary Transmission of Notifications
The design of event-driven synchronization models, essential to configuration management, can use notifications as an important enabling technique. Proper usage of notifications allows the manager's view of the managed element's configuration to be in close synchronization with the actual state of the configuration of the managed element. When designing new NOTIFICATION-TYPEs, consider how to limit the number of notifications PDUs that will be sent with the notification information defined in the NOTIFICATION-TYPE in response to a configuration change or error event.
InformRequest PDUs, when compared to TRAP PDUs, have an inherent advantage when the concern is the reduction of unnecessary messages from the system generating the NOTIFICATION-TYPE data, when in fact retransmission of this data is required. That is, an InformRequest PDU is acknowledged by the receiving entity with a Response PDU. The receipt of this response allows the entity which generated the InformRequest PDU to verify (and record an audit entry, where such facilities exist on the agent system) that the message was received. As a matter of notification protocol, this receipt guarantee is not available when using TRAP PDUs, and if it is required, must be accomplished by the agent using some mechanism out of band to SNMP, and usually requiring the penalty of polling. Regardless of the specific PDUs used to convey them, one way to limit the unnecessary generation of notifications is to include in the NOTIFICATION-TYPE definition situations where it need not be sent. A good example is the frDLCIStatusChange defined in FRAME-RELAY-DTE- MIB, RFC 2115 [21]. frDLCIStatusChange NOTIFICATION-TYPE OBJECTS { frCircuitState } STATUS current DESCRIPTION "This trap indicates that the indicated Virtual Circuit has changed state. It has either been created or invalidated, or has toggled between the active and inactive states. If, however, the reason for the state change is due to the DLCMI going down, per-DLCI traps should not be generated." ::= { frameRelayTraps 1 } There are a number of other techniques which can be used to reduce the unwanted generation of NOTIFICATION-TYPE information. When defining notifications, the designer can specify a number of temporal limitations on the generation of specific instances of a NOTIFICATION-TYPE. For example, a definition could specify that messages will not be sent more frequently than once every 60 seconds while the condition which led to the generation of the notification persists. Alternately, a NOTIFICATION-TYPE DESCRIPTION clause could provide a fixed limit on the number of messages sent over the duration of the condition leading to sending the notification. If NOTIFICATION-TYPE transmission is "aggregated" in some way - bounded either temporally or by absolute system state change as described above - the optimal design technique is to have the data delivered with the notification reference the actual number of underlying managed element transitions which brought about the notification. No matter which threshold is chosen to govern the
actual transmission of NOTIFICATION-TYPEs, the idea is to describe an aggregated event or related set of events in as few PDUs as possible.3.10.3. Control of Notification Subsystem
There are standards track MIB modules that define objects that either augment or overlap control of notifications. For instance, FRAME- RELAY-DTE-MIB RFC 2115 defines frTrapMaxRate and DOCS-CABLE-DEVICE- MIB defines a set of objects in docsDevEvent that provide for rate limiting and filtering of notifications. In the past, agents did not have a standard means to configure a notification generator. With the availability of the SNMP- NOTIFICATION-MIB module in RFC 3413 [9], it is strongly recommended that the filtering functions of this MIB module be used. This MIB facilitates the mapping of given NOTIFICATION-TYPEs and their intended recipients. If the mechanisms of the SNMP-NOTIFICATION-MIB are not suitable for this application, a explanation of why they are not suitable should be included in the DESCRIPTION clause of any replacement control objects.3.11. Application Error Reporting
MIB module designers should not rely on the SNMP protocol error reporting mechanisms alone to report application layer error state for objects that accept SET operations. Most MIB modules that exist today provide very little detail as to why a configuration request has failed. Often the only information provided is via SNMP protocol errors which generally does not provide enough information about why an agent rejected a set request. Typically, there is an incumbent and sizable burden on the configuration application to determine if the configuration request failure is the result of a resource issue, a security issue, or an application error. Ideally, when a "badValue" error occurs for a given set request, an application can query the agent for more details on the error. A badValue does not necessarily mean the command generator sent bad data. An agent could be at fault. Additional detailed diagnostic information may aid in diagnosing conditions in the integrated system.
Consider the requirement of conveying error information about a MIB expression 'object' set within the DISMAN-EXPRESSION-MIB [40] that occurs when the expression is evaluated. Clearly, none of the available protocol errors are relevant when reporting an error condition that occurs when an expression is evaluated. Instead, the DISMAN-EXPRESSION-MIB provides objects to report such errors (the expErrorTable). Instead, the expErrorTable maintains information about errors that occur at evaluation time: 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 } More specifically, a MIB module can provide configuration applications with information about errors on the managed device by creating columnar object types in log tables that contain error information particular to errors that occur on row activation. Notifications with detailed failure information objects can also be used to signal configuration failures. If this approach is used, the configuration of destinations for NOTIFICATION-TYPE data generated from configuration failures should be considered independently of the those for other NOTIFICATION-TYPEs which are generated for other operational reasons. In other words, in many management environments, the network operators interested in NOTIFICATION-TYPEs generated from configuration failures may not completely overlap with the community of network operators interested in NOTIFICATION-TYPEs generated from, for example, network interface failures.3.12. Designing MIB Modules for Multiple Managers
When designing a MIB module for configuration, there are several pertinent considerations to provide support for multiple managers. The first is to avoid any race conditions between two or more authorized management applications issuing SET protocol operations spanning over more than a single PDU. The standard textual convention document [3] defines TestAndIncr, often called a spinlock, which is used to avoid race conditions.
A MIB module designer may explicitly define a synchronization object
of syntax TestAndIncr or may choose to rely on snmpSetSerialNo (a
global spinlock object) as defined in SNMPv2-MIB.
snmpSetSerialNo OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"An advisory lock used to allow several cooperating
command generator applications to coordinate their
use of the SNMP set operation.
This object is used for coarse-grain coordination.
To achieve fine-grain coordination, one or more similar
objects might be defined within each MIB group, as
appropriate."
::= { snmpSet 1 }
Another prominent TestAndIncr example can be found in the SNMP-
TARGET- MIB [9], snmpTargetSpinLock.
Secondly, an agent should be able to report configuration as set by
different entities as distinguishable from configuration defined
external to the SNMP domain, such as application of a default or
through an alternate management interface like a command line
interface. Section 3.10.1 describes considerations for this practice
when designing NOTIFICATION-TYPEs. The OwnerString textual
convention from RMON-MIB RFC 2819 [32] has been used successfully for
this purpose. More recently, RFC 3411 [1] introduced the
SnmpAdminString which has been designed as a UTF8 string. This is
more suitable for representing names in many languages.
Experience has shown that usage of OwnerString to represent row
ownership can be a useful diagnostic tool as well. Specifically, the
use of the string "monitor" to identify configuration set by an
agent/local management has been prevalent and useful in applications.
Thirdly, consider whether there is a need for multiple managers to
configure the same set of tables. If so, an "OwnerString" may be
used as the first component of a table's index to allow VACM to be
used to protect access to subsets of rows, at least at the level of
securityName or groupName provided. RFC 3231 [25], Section 6
presents this technique in detail. This technique does add
complexity to the managed device and to the configuration management
application since the manager will need to be aware of these
additional columnar objects in configuration tables and act
appropriately to set them. Additionally, the agent must be
configured to provide the appropriate instance-level restrictions on the modifiability of the instances.3.13. Other MIB Module Design Issues
3.13.1. Octet String Aggregations
The OCTET STRING syntax can be used as an extremely flexible and useful datatype when defining managed objects that allow SET operation. An octet string is capable of modeling many things and is limited in size to 65535 octets by SMIv2[2]. Since OCTET STRINGS are very flexible, the need to make them useful to applications requires careful definition. Otherwise, applications will at most simply be able to display and set them. Consider the following object from RFC 3418 SNMPv2-MIB [11]. sysLocation OBJECT-TYPE SYNTAX DisplayString (SIZE (0..255)) MAX-ACCESS read-write STATUS current DESCRIPTION "The physical location of this node (e.g., `telephone closet, 3rd floor'). If the location is unknown, the value is the zero-length string." ::= { system 6 } Such informational object types have come to be colloquially known as "scratch pad objects". While often useful, should an application be required to do more with this information than be able to read and set the value of this object, a more precise definition of the contents of the OCTET STRING is needed, since the actual format of an instance for such an object is unstructured. Hence, alternatively, dividing the object type into several object type definitions can provide the required additional structural detail. When using OCTET STRINGS, avoid platform dependent data formats. Also avoid using OCTET STRINGS where a more precise SMI syntax such as SnmpAdminString or BITS would work. There are many MIB modules that attempt to optimize the amount of data sent/received in a SET/GET PDU by packing octet strings with aggregate data. For example, the PortList syntax as defined in the Q-BRIDGE-MIB (RFC 2674 [26]) is defined as follows:
PortList ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Each octet within this value specifies a set of eight
ports, with the first octet specifying ports 1 through
8, the second octet specifying ports 9 through 16, etc.
Within each octet, the most significant bit represents
the lowest numbered port, and the least significant bit
represents the highest numbered port. Thus, each port
of the bridge is represented by a single bit within the
value of this object. If that bit has a value of '1'
then that port is included in the set of ports; the port
is not included if its bit has a value of '0'."
SYNTAX OCTET STRING
This compact representation saves on data transfer but has some
limitations. Such complex instance information is difficult to
reference outside of the object or use as an index to a table.
Additionally, with this approach, if a value within the aggregate
requires change, the entire aggregated object instance must be
written.
Providing an SNMP table to represent aggregate data avoids the
limitations of encoding data into OCTET STRINGs and is thus the
better general practice.
Finally, as previously mentioned in Section 3.3.6.3, one should
consider the practical ramifications of instance transfer for object
types of SYNTAX OCTET STRING where they have typical instance data
requirements close to the upper boundary of SMIv2 OCTET STRING
instance encoding. Where such object types are truly necessary at
all, SNMP/UDP may not be a very scalable means of transfer and
alternatives should be explored.
3.13.2. Supporting multiple instances of a MIB Module
When defining new MIB modules, one should consider if there could
ever be multiple instances of this MIB module in a single SNMP
entity.
MIB modules exist that assume a one to many relationship, such as
MIBs for routing protocols which can accommodate multiple "processes"
of the underlying protocol and its administrative framework.
However, the majority of MIB modules assume a one-to-one relationship
between the objects found in the MIB module and how many instances
will exist on a given SNMP agent. The OSPF-MIB, IP-MIB, BRIDGE-MIB
are all examples that are defined for a single instance of the
technology.
It is clear that single instancing of these MIB modules limits implementations that might support multiple instances of OSPF, IP stacks or logical bridges. In such cases, the ENTITY-MIB [RFC2737] can provide a means for supporting the one-to-many relationship through naming scopes using the entLogicalTable. Keep in mind, however, that there are some drawbacks to this approach. 1) One cannot issue a PDU request that spans naming scopes. For example, given two instances of BRIDGE-MIB active in a single agent, one PDU cannot contain a request for dot1dBaseNumPorts from both the first and second instances. 2) Reliance on this technique creates a dependency on the Entity MIB for an application to be able to access multiple instances of information. Alternately, completely independently of the Entity MIB, multiple MIB module instances can be scoped by different SNMP contexts. This does, however, require the coordination of this technique with the administrative establishment of contexts in the configured agent system.3.13.3. Use of Special Optional Clauses
When defining integer-based objects for read-create, read-write and read-only semantics, using the UNITS clause is recommended in addition to specification in the DESCRIPTION clause of any particular details of how UNITs are to be interpreted. The REFERENCE clause is also recommended as a way to help an implementer track down related information on a given object. By adding a REFERENCE clause to the specific underlying technology document, multiple separate implementations will be more likely to interoperate.
(page 41 continued on part 3)