SNMPCONF Working Group M. MacFaden
Category: Informational Riverstone Networks, Inc
D. Partain
Ericsson
J. Saperia
JDS Consulting, Inc
W. Tackabury
Gold Wire Technology, Inc
Configuring Networks and Devices With SNMP
draft-ietf-snmpconf-bcp-10.txt
October 1, 2002
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document is written for readers interested in the Internet
Standard Management Framework and its protocol, the Simple Network
Management Protocol (SNMP). In particular, it offers guidance in the
effective use of SNMP for configuration management. This information
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is relevant to vendors that build network elements, management
application developers, and those that acquire and deploy this
technology in their networks.
1. Introduction
1.1. The Internet Standard Management Framework
The Internet Standard Management Framework has many components. The
purpose of this document is to describe effective ways of applying those
components to the problems of configuration management.
For reference purposes, the Internet Standard Management Framework
presently consists of five major components:
o An overall architecture, described in RFC 2571 [1].
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 [2], STD 16, RFC 1212 [3] and RFC
1215 [4]. The second version, called SMIv2, is described
in STD 58, RFC 2578 [5], STD 58, RFC 2579 [6] and
STD 58, RFC 2580 [7].
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 [8]. A second version of
the SNMP message protocol, which is not an Internet standards
track protocol, is called SNMPv2c and described in RFC 1901
[9] and RFC 1906 [10]. The third version of the
message protocol is called SNMPv3 and described in RFC 1906,
RFC 2572 [11] and RFC 2574 [12].
o Protocol operations for accessing management information. The
first set of protocol operations and associated PDU formats is
described in STD 15, RFC 1157 [8]. A second set of
protocol operations and associated PDU formats is described in
RFC 1905 [13].
o A set of fundamental applications described in RFC 2573
[14] and the view-based access control mechanism described
in RFC 2575 [15].
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A more detailed introduction to the current SNMP Management Framework
can be found in RFC 2570 [18].
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.
1.2. Configuration and the Internet Standard Management Framework
Data networks have grown significantly over the past decade. This growth
can be seen in terms of:
Scale - Networks have more network elements, and the network
elements are larger and place more demands on the systems
managing them. For example, consider a typical number and
speed of interfaces in a modern core network element. A
managed metropolitan area network switch can have a port
density much greater than the port density built into the
expectations of the management systems that predated it.
There are also many more interrelationships within and
between devices and device functions.
Functionality - network devices perform more functions.
More protocols and network layers are required for the
successful deployment of network services which depend
on them.
Rate of Change - the nature of modern network services
causes updates, additions, and deletions of device
configuration information more often than in the past.
No longer can it be assumed that a configuration will
be specified once and then be updated rarely. On the
contrary, the trend has been towards much more frequent
changes of configuration information.
Correct configuration of network elements that make up data networks is
a prerequisite to the successful deployment of the services on them. The
growth in size and complexity of modern networks increases the need for
a standard configuration mechanism that is tightly integrated with
performance and fault management systems.
The Internet Standard Management Framework has been used successfully to
develop configuration management systems for a broad range of devices
and networks. A standard configuration mechanism that tightly
integrates with performance and fault systems is needed not only to help
reduce the complexity of management, but also to enable verification of
configuration activities that create revenue-producing services.
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This document describes Best Current Practices that have been used when
designing effective configuration management systems using the Internet
Standard Management Framework (colloquially known as SNMP). It covers
many basic practices as well as more complex agent and manager design
issues that are raised by configuration management. We are not
endeavoring to present a comprehensive how-to document for generalized
SNMP agent, MIB module, or management application design and
development. We will, however, cover points of generalized SNMP
software design and implementation practice, where the practice has been
seen to benefit configuration management software. So, for example, the
requirement for management applications to be aware of agent limitations
is discussed in the context of configuration operations, but many issues
that a management application developer should consider with regard to
manager-agent interactions are left for other documents and resources.
Significant experience has been gained over the past ten years in
configuring public and private data networks with SNMP. During this
time, networks have grown significantly as described above. A response
to this explosive growth has been the development of policy-based
configuration management. Policy-Based Configuration Management is a
methodology configuration information is derived from rules and network-
wide objectives, and is distributed to potentially many network elements
with the goal of achieving consistent network behavior throughout an
administrative domain.
This document presents lessons learned from these experiences and
applies them to both conventional and policy-based configuration systems
based on SNMP.
2. Using SNMP as a Configuration Mechanism
Configuration activity causes one or more state changes in an element.
While it often takes an arbitrary number of commands and amount of data
to make up configuration change, it is critical that the configuration
system treat the overall change operation atomically so that the number
of states into which an element transitions is minimized. The goal is
for a change request either to be completely executed or not at all.
This is called transactional integrity. Transactional integrity makes it
possible to develop reliable configuration systems that can invoke
transactions and keep track of an element's overall state and work in
the presence of error states.
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2.1. Transactions and SNMP
Transactions can logically take place at very fine-grained levels such
as an individual object instance or in very large aggregations that
could include many object instances located in many tables on a managed
device. For this reason, reliance on transactional integrity only at
the SNMP protocol level is insufficient.
2.2. Practical Requirements for Transactional Control
A well-designed and deployed configuration system should have the
following features with regard to transactions and transactional
integrity.
1) Provide for flexible transaction control at many different levels
of granularity. At one extreme, an entire configuration may be
delivered and installed on an element, or alternately one small
attribute may be changed.
2) The transaction control component should work at and understand a
notion of the kind of multi-level "defaulting" as described in
Section 7.1. The key point here is that it may make most sense to
configure systems at an abstract level rather than on an individual
instance by instance basis as has been commonly practiced. In some
cases it is more effective to send a configuration command to a
system that contains a set of 'defaults' to be applied to instances
that meet certain criteria.
3) An effective configuration management system must allow
flexibility in the definition of a successful transaction. This
cannot be done at the protocol level alone, but rather must be
provided for throughout the application and the information that is
being managed. In the case of SNMP, the information would be in
properly defined MIB modules.
4) A configuration management system should provide time-indexed
transaction control. For effective rollback control, the
configuration transactions and their successful or unsuccessful
completion status must be reported by the managed elements and stored
in a repository that supports such time indexing and can record the
user that made the change, even if the change was not carried out by
the system recording the change.
5) The managed system must support transactional security. This means
that depending on who is making the configuration request and where
it is being made, it may be accepted or denied based on security
policy that is in effect in the managed element.
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Effective transactional control is a responsibility shared between
design, implementation, and operational practice. Transaction control
techniques for MIB module design are discussed in Section 3.3.
Transaction control considerations for the agent implementation are
discussed in Section 5.2.2.
2.3. Best Practices in Configuration--Verification
Verification of expected behavior subsequent to the commitment of change
is an integral part of the configuration process. To reduce the chance
of making simple errors in configuration, many organizations employ the
following change management procedure:
pre-test - verify that the system is presently working properly
change - make configuration changes and wait for convergence
(system or network stability)
re-test - verify once again that the system is working properly
This procedure is commonly used to verify configuration changes to
critical systems such as the domain name system (DNS). DNS software
kits provide diagnostic tools that allow automatic test
procedures/scripts to be conducted.
A planned configuration sequence can be aborted if the pre-configuration
test result shows the state of the system as unstable. Debugging the
unintended effects of two sets of changes in large systems is often more
challenging than an analysis of the effects of a single set after test
termination.
Networks and devices under SNMP configuration readily support this
change management procedure since the SNMP provides integrated
monitoring, configuration and diagnostic capabilities. The key is the
sequencing of SNMP protocol operations to effect an integrated change
procedure like the one described above. This is usually a well-bounded
affair for changes within a single network element or node. However,
there are times when configuration of a given element can impact other
elements in a network. Configuring network protocols such as IEEE 802.1D
Spanning Tree or OSPF is especially challenging since the impact of a
configuration change can directly affect stability (convergence) of the
network the device is connected to.
An integrated view of configuration and monitoring provides an ideal
platform from which to evaluate such changes. For example, the MIB
module governing IEEE 802.1D Spanning Tree (RFC 1493 [22]) provides the
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following object to monitor stability per logical bridge.
dot1dStpTopChanges OBJECT-TYPE
SYNTAX Counter
ACCESS read-only
STATUS mandatory
DESCRIPTION
"The total number of topology changes detected by
this bridge since the management entity was last
reset or initialized."
REFERENCE
"IEEE 802.1D-1990: Section 6.8.1.1.3"
::= { dot1dStp 4 }
Likewise, the OSPF MIB module provides a similar metric for stability
per OSPF area.
ospfSpfRuns OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times that the intra-area route
table has been calculated using this area's
link-state database. This is typically done
using Dijkstra's algorithm."
::= { ospfAreaEntry 4 }
The above object types are good examples of a means of facilitating the
principles described in Section 2.3. That is, one needs to understand
the behavior of a subsystem before configuration change, then be able to
use the same means to retest and verify proper operation subsequent to
configuration change.
The operational effects of a given implementation often differ from one
to another for any given standard configuration object. The impact of a
change to stability of systems such as OSPF should be documented in an
agent-capabilities statement which is consistent with "Requirements for
IP Version 4 Routers" [20], Section 1.3.4:
A vendor needs to provide adequate documentation on all
configuration parameters, their limits and effects.
Adherence to the above model is not fail-safe, especially when
configuration errors are masked by long latencies or when configuration
errors lead to oscillations in network stability. For example, consider
the situation of loading a new software version on a device, which leads
to small, slow, cumulative memory leaks brought on by a certain traffic
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pattern that was not caught during vendor and customer test lab trials.
In a network-based example, convergence in an autonomous system cannot
be guaranteed when configuration changes are made since there are
factors beyond the control of the operator, such as the state of other
network elements. Problems affecting this convergence may not be
detected for a significant period of time after the configuration
change. Even for factors within the operator's control, there is often
little verification done to prevent mis-configuration (as shown in the
following example).
Consider a change made to ospfIfHelloInterval and ospfIfRtrDeadInterval
[22] timers in the OSPF routing protocol such that both are set to the
same value. Two routers may form an adjacency but then begin to cycle in
and out of adjacency, and thus never reach a stable (converged) state.
Had the configuration process described at the beginning of this section
been employed, this particular situation would have been discovered
without impacting the production network.
The important point to remember from this discussion is that
configuration systems should be designed and implemented with
verification tests in mind.
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.
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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 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.
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(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 [5]) 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.
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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 [6] 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:
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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
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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
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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.
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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 [32], provides the following
object:
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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 (RFC2863 [17])
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
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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:
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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.
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ipCidrRouteEntry OBJECT-TYPE
SYNTAX IpCidrRouteEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A particular route to a particular destina-
tion, 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 [30], 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 [17] and the
ifInvStackTable RFC 2864 [31]. 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 }
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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 [30], 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.
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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 [43] and used in RMON2-MIB and Q-
BRIDGE-MIB (RFC 2674 [24]) 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 [36]).
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 [34], 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
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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 [35]. 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 [5], 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 [31]:
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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 [44] IP-MIB.
This limits the usefulness of the MIB module to IPv4. To avoid such
limitations, use the addressing textual conventions INET-ADDRESS-MIB
[39] (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 need 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.
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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
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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. Best current practice is to use existing Textual Conventions and
define the read-write object in terms of a TruthValue from SNMPv2-TC
[6]. For example, the Q-BRIDGE-MIB [24] 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 [37] 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 }
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This could be defined as follows using existing SNMPv2-TC TruthValue.
acmePatioLightsOn OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"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 [6] which
explicitly defines a given row's persistence requirement.
Examples include the RFC 2591 [23] 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 [15].
A second method for persistence simply uses the DESCRIPTION clause to
define how instance data should persist. RFC 2674 [24] explicitly
defines Dot1qVlanStaticEntry data persistence as follows:
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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 }
Best 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 [23] 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.
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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 [27] 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
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enabled(2) are clearly spelled out.
The Scheduling MIB [23] 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" [6], 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 }
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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 [6], 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.
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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 }
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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 == spinCounterClockwise(3)
V
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 [17] provide such an example of the
separation of objects into desired and current state.
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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 [5], by the NOTIFICATION-TYPE macro. Notifications can
be sent in either SNMPv2c [9] 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
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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 snmpTargetParmsTable from RFC 2573 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
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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 [19].
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.
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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 2573 [14], 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 [38] 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:
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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 [6] 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.
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snmpSetSerialNo OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"An advisory lock used to allow several cooperating SNMPv2
entities, all acting in a manager role, to coordinate their
use of the SNMPv2 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 [14], 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 [30] has been used successfully for this purpose. More recently,
RFC 2571 [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 2591 [23], 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.
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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[5].
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 1907 SNMPv2-MIB [16].
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 [24]) is defined as follows:
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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
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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.
4. Implementing SNMP Configuration Agents
4.1. Operational Consistency
Successful deployment of SNMP configuration systems depends on
understanding the roles of MIB module design and agent design.
Both module and agent design need to be undertaken with an understanding
of how UDP/IP-based SNMP behaves. A best current practice in MIB design
is to consider the idempotency of settable objects. Idempotency
basically means being able to invoke the same set operation repeatedly
but resulting in only a single activation.
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Here is an example of the idempotency in action:
Manager Agent
-------- ------
Set1 (Object A, Value B) ---> receives set OK and responds
X<-------- Response PDU(OK) is dropped by network
Manager times out
and sends again
Set2 (Object A, Value B) ---> receives set OK (does nothing), responds
<-------- with a Response PDU(OK)
Manager receives OK
Had object A been defined in a stateful way, the set operation might
have caused the Set2 operation to fail as a result of interaction with
Set1. If the agent implementation is not aware of such a possible
situation on the second request, the agent may behave poorly by
performing the set request again rather than doing nothing.
The example above shows that all of the software that runs on a managed
element and in managed applications should be designed in concert when
possible. Particular emphasis should be placed at the logical boundaries
of the management system components in order to ensure correct
operation.
1. The first interface is between SNMP agents in managed
devices and the management applications themselves. The
MIB document is a contract between these two entities that
defines expected behavior - it is a type of API.
2. The second interface is between the agent and the instrumented
subsystem. In some cases, the instrumented subsystem will require
modification to allow for the dynamic nature of
SNMP-based configuration, control and monitoring
operations. Agent implementors must also be sensitive to
the operational code and device in order to minimize
the impact of management on the primary subsystems.
Additionally, while the SNMP protocol-level and MIB module-level
modeling of configuration operations may be idempotent and stateless
from one set operation to another, it may not be that way in the
underlying subsystem. It is possible that an agent may need to manage
this state in these subsystem architectures explicitly when it has
placed the underlying subsystem into an "intermediate" state at a point
in processing a series of SET PDUs. Alternatively, depending on the
underlying subsystem in question, the agent may be able to buffer all of
the configuration set operations prior to activating them in the
subsystem all at once (to accommodate the nature of the subsystem).
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As an example, it would be reasonable to define a MIB module to control
Virtual Private Network (VPN) forwarding, in which a management station
could set a set of ingress/egress IP addresses for the VPN gateway.
Perhaps the MIB module presumes that the level of transactionality is
the establishment of a single row in a table defining the address of the
ingress/egress gateway, along with some prefix information to assist in
routing at the VPN layer to that gateway. However, it would be
conceivable that in an underlying Layer 2 VPN subsystem instrumentation,
the requirement is that all existing gateways for a VPN be deleted
before a new one can be defined--that, in other words, in order to add a
new gateway, g(n), to a VPN, gateways g(1)..g(n-1) need to be removed,
and then all n gateways reestablished with the VPN forwarding service.
In this case, one could imagine an agent which has some sort of timer to
establish a bounded window for receipt of SETs for new VPN gateways, and
to activate them in this removal-then-reestablishment of existing and
new gateways at the end of this window.
4.2. Handling Multiple Managers
Devices are often modified by multiple management entities and with
different management techniques. It is sometimes the case that an
element is managed by different organizations such as when a device sits
between administrative domains.
There are a variety of approaches that management software can use to
ensure synchronization of information between the manager(s) and the
managed elements.
An agent should report configuration changes performed by different
entities. It should also distinguish configuration defined locally such
as a default or locally specified configuration made through an
alternate management interface such as a command line interface. When a
change has been made to the system via SNMP, CLI, or other method, a
managed element should send an notification to the manager(s) configured
as recipients of these applicable notifications. These management
applications should update their local configuration repositories and
then take whatever additional action is appropriate. This approach can
also be an early warning of undesired configuration changes.
Managers should also develop mechanisms to ensure that they are
synchronized with each other.
4.3. Specifying Row Modifiability
Once a RowStatus value is active(1) for a given row, the management
application should be able to determine what the semantics are for
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making additional changes to a row. The RMON MIB control table objects
spell out explicitly what managed objects in a row can and cannot be
changed once a given RowStatus goes active.
As described earlier, some operations take some time to complete. Some
systems also require that they remain in a particular state for some
period before moving to another. In some cases, a change to one value
may require re-initialization of the system. In all of these cases, the
DESCRIPTION clause should contain information about requirements of the
managed system and special restrictions that managers should observe.
4.4. Implementing Write-only Access Objects
The second version of the SNMP SMI dropped direct support for a write-
only object. It is therefore necessary to return something when reading
an object that you may have wished to have write-only semantics. Such
objects should have a DESCRIPTION clause that details what the return
values should be. However, regardless of the approach, the value
returned when reading the object instance should be meaningful in the
context of the object's semantics.
5. Designing Configuration Management Software
In this section, we describe practices that should be used when creating
and deploying management software that configures one or more systems
using SNMP. Functions all configuration management software should
provide, regardless of the method used to convey configuration
information to the managed systems are backup, fail-over, and
restoration. A management system should have the following features:
1. A method for restoring a previous configuration to one or
more devices. Ideally this restoration should be time indexed
so that a network can be restored to a configured state as of
a specific time and date.
2. A method for saving back up versions of the configuration
data in case of hardware or software failure.
3. A method of providing fail-over to a secondary (management)
system in case of a primary failure. This capability should be
deployed in such a way that it does not cause duplicate
polling of configuration.
These three capabilities are of course important for other types of
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management that are not the focus of this BCP.
5.1. Configuration Application Interactions with Managed Systems
>From the point of view of the design of the management application,
there are three basic requirements to evaluate relevant to SNMP protocol
operations and configuration:
o Set and configuration activation operations
o Notifications from the device
o Data retrieval and collection
Depending on the requirements of the specific services being configured,
many other requirements may, and probably will, also be present.
The design of the system should not assume that the objects in a device
that represent configuration data will remain unchanged over time.
As standard MIB modules evolve and vendors add private extensions, the
specific configuration parameters for a given operation are likely to
change over time. Even in the case of a configuration application that
is designed for a single vendor, the management application should allow
for variability in the MIB objects that will be used to configure the
device for a particular purpose. The best method to accomplish this is
by separating, as much as possible, the operational semantics of a
configuration operation from the actual data. One way that some
applications achieve this is by having the specific configuration
objects that are associated with a particular device be table driven
rather than hard coded. Ideally, management software should verify the
support in the devices it is expected to manage and report any
unexpected deviations to the operator. This approach is particularly
valuable when developing applications that are intended to support
equipment or software from multiple vendors.
5.1.1. SET Operations
Management software should be mindful of the environment in which SET
operations are being deployed. The intent here is to move configuration
information as efficiently as possible to the managed device. There are
many ways to achieve efficiency and some are specific to given devices.
One general case that all management software should employ is to reduce
the number of SET PDU exchanges between the managed device and the
management software to the smallest reasonable number. One approach to
this is to verify the largest number of variable bindings that can fit
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into a SET PDU for a managed device. In some cases, the number of
variable bindings to be sent in a particular PDU will be influenced by
the device, the specific MIB objects and other factors.
Maximizing the number of variable bindings in a SET PDU also has
benefits in the area of management application transaction initiation,
as we will discuss in the following section.
There are, though, agents that may have implementation limitations on
the number and order of varbinds they can handle in a single SET PDU.
In this case, sending fewer varbinds will be necessary.
As stated at the outset of this section, the management application
software designer must be sensitive to the design of the SNMP software
in the managed device. For example, the software in the managed device
may require that all that all related configuration information for an
operation be conveyed in a single PDU because it has no concept of a
transaction beyond a single SNMP PDU. Another example has to do with
the RowStatus textual convention. Some SNMP agents implement a subset
of the features available and as such the management application must
avoid using features that may not be supported in a specific table
implementation (such as createAndWait).
5.1.2. Configuration Transactions
There are several types of configuration transactions that can be
supported by SNMP-based configuration applications. They include
transactions on a scalar object, transactions in a single table (within
and across row instances), transactions across several tables in a
managed device and transactions across many devices. The manager's
ability to support these different transactions is partly dependent on
the design of the MIB objects used in the configuration operation.
To make use of any kind of transaction semantics effectively, SNMP
management software must be aware of the information in the MIB modules
that it is to configure so that it can effectively utilize RowStatus
objects for the control of transactions on one or more tables. Such
software must also be aware of control tables that the device supports
that are used to control the status of one or more other tables.
To the greatest extent possible, the management application should
provide the facility to support transactions across multiple devices.
This means that if a configuration operation is desired across multiple
devices, the manager can coordinate these configuration operations such
that they become active as close to simultaneously as possible.
Several practical means are present in the SNMP model that support
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management application level transactions. One was mentioned in the
preceding section, that transactions can be optimized by including the
maximum number of SET variable bindings possible in a single PDU sent to
the agent.
There is an important refinement to this. The set of read-create row
data objects for tables should be sent in a single PDU, and only placed
across multiple PDUs if absolutely necessary. The success of these set
operations should be verified through the response(s) to the Set PDU or
subsequent polling of the row data objects. The applicable RowStatus
object(s), may be set to active only after this verification. This is
the only tractable means of affording an opportunity for per-row
rollback, particularly when the configuration change is across table row
instances on multiple managed devices.
Finally, where a MIB module exposes the kind of helpful transaction
management object types that were discussed in Section 3.3.5, it is
clearly beneficial to the integrity of the management application's
capacity to handle transactions to make use of them.
5.1.3. Tracking Configuration Changes
As previously described in Section 3.3.5 (Summary Objects and State
Tracking), agents should provide the capability for notifications to be
sent to their configured management systems whenever a configuration
operation is completed or is detected to have failed. The management
application must be prepared to accept these notifications so that it
knows the current configured state of the devices under its control.
Upon receipt of the notification, the management application should use
getBulk or getNext to retrieve the configuration from the agent and
store the relevant contents in the management application database. The
GetBulkRequest-PDU is useful for this whenever supported by the managed
device, since it is more efficient than the GetNextRequest-PDU when
retrieving large amounts of data. For the purposes of backward
compatibility, the management station should also support and make use
of the GetNextRequest-PDU when the agent does not support the
GetBulkRequest-PDU.
Management systems should also provide configuration options with
defaults for users that tend to retrieve the smallest amount of data to
achieve the particular goal of the application, to avoid unnecessary
load on managed devices for the most common retrieval operations.
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5.1.4. Scalability of Data Retrieval
The techniques for efficient data retrieval described in the preceding
sections comprise only one aspect of what application developers should
consider in this regard when developing configuration applications.
Management applications should provide for distributed processing of the
configuration operations. This also extends to management functions that
are not the focus of this document. Techniques of distributed processing
can also be used to provide resilience in the case of network failures.
An SNMP-based configuration management system might be deployed in a
distributed fashion where three systems in different locations keep each
other synchronized. This synchronization can be accomplished without
additional polling of network devices through a variety of techniques.
In the case of a failure, a 'backup' system can take over the
configuration responsibilities from the failed manager without having to
re-synchronize with the managed elements since it will already be up to
date.
6. Deployment and Security Issues
Now that we have considered the design of SNMP MIB data for
configuration, agent implementation of its access, and management
application issues in configuration using SNMP, we turn to a variety of
operational considerations which transcend all three areas.
6.1. Basic assumptions about Configuration
The following basic assumptions are made about real world configuration
models.
1) Operations must understand and must be trained in the operation of
a given technology. No configuration system can prevent an untrained
operator from causing outages due to misconfiguration.
2) Systems undergoing configuration changes must be able to cope with
unexpected loss of communication at any time.
During configuration operations, network elements must take
appropriate measures to leave the configuration in a
consistent/recognizable state by either rolling back to a previously
valid state or changing to a well-defined or default state.
3) Configuration exists on a scale from relatively unchanging to a
high volume, high rate of change. The former is often referred to as
"set and forget" to indicate that the configuration changes quite
infrequently. The latter, "near real-time change control" implies a
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high frequency of configuration change. Design of configuration
management must take into account the rate and volume of change
expected in a given configuration subsystem.
6.2. Secure Agent Considerations
Vendors should not ship a device with a community string 'public' or
'private', and agents should not define default community strings except
when needed to bootstrap devices that do not have secondary management
interfaces. Defaults lead to security issues that have been recognized
and exploited. When using SNMPv1, supporting read-only community strings
is a common practice.
Version 3 of the SNMP represents the current standard for the Internet
Management Framework and is recommended for all network management
applications. In particular, SNMPv3 provides authorization,
authentication, and confidentiality protection and is essential to
meeting the security considerations for all management of devices that
support SNMP-based configuration.
6.3. Authentication Notifications
The default state of RFC 1215 [4] Authentication notifications should be
off. One does not want to risk accidentally sending out authentication
failure information, which by itself could constitute a security
liability. Enabling authentication Notifications should be done in the
context of a management security scheme which considers the proper
recipients of this information.
There are other liabilities where authentication notifications are
generated without proper security infrastructure. When notifications are
sent in SNMPv1 trap PDUs, unsolicited packets to a device can causes one
or more trap PDUs to be created and sent to management stations. If
these traps flow on shared access media and links, the community string
from the trap may be gleaned and exploited to gain access to the device.
At the very least, this risk should be mitigated by having the
authentication trap PDU be conveyed with a community string which is
only used for authentication traps from the agent, and would be useless
for access inbound to the agent to get at other management data.
A further liability of authentication traps can be seen when they are
being generated in the face of a Denial Of Service (DOS) attack, in the
form of a flood of PDUs with invalid community strings, on the agent
system. If it is bad enough that the system is having to respond to and
recover from the invalid agent data accesses, but the problem will be
compounded if a separate Authentication notification PDU is sent to each
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recipient on the management network.
6.4. Sensitive Information Handling
Some MIB modules contain objects that may contain data for keys,
passwords and other such sensitive information and hence must be
protected from unauthorized access. MIB documents that are created in
the IETF must have a 'Security Considerations' section, which details
how sensitive information should be protected. Similarly, MIB module
designers who create MIB documents for private MIB objects should
include similar information so that users of the products containing
these objects can take appropriate precautions.
Even if a device does support DES, it should be noted that configuration
of keys for other protocols via SNMP Sets protected by DES should not be
allowed if the other keys are longer than the 56 bit DES keys protecting
the SNMP transmission.
The DESCRIPTION clause for these object types and their Security
Considerations sections in the documents which define them should make
it clear how and why these specific objects are sensitive and that a
user should only make them accessible for encrypted SNMP access. Vendors
should also document sensitive objects in a similar fashion.
Confidentiality is not a mandatory portion of the SNMPv3 management
framework [11].
Prior to SNMPv3, providing customized views of MIB module data was
difficult. This led to objects being defined such as the following from
[40].
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docsDevNmAccessEntry OBJECT-TYPE
SYNTAX DocsDevNmAccessEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"An entry describing access to SNMP objects by a
particular network management station. An entry in
this table is not readable unless the management station
has read-write permission (either implicit if the table
is empty, or explicit through an entry in this table.
Entries are ordered by docsDevNmAccessIndex. The first
matching entry (e.g. matching IP address and community
string) is used to derive access."
INDEX { docsDevNmAccessIndex }
::= { docsDevNmAccessTable 1 }
New MIB modules should capitalize on existing security capabilities of
SNMPv3 Framework. One way they can do this is by indicating the level
of security appropriate to different object types. For example, objects
that change the configuration of the system might be protected by using
the authentication mechanisms in SNMPv3. Specifically, it is useful to
design MIB module object grouping with considerations for VACM views
definition, such that users can define and properly scope what tables
are visible to a given user and view.
7. Policy-based Management
In some designs and implementations, a common practice used to move
large amounts of data involves using SNMP as a control channel in
combination with other protocols defined for transporting bulk data.
This approach is sub-optimal since it raises a number of security and
other concerns. Transferring large amounts of configuration data via
SNMP can be efficiently performed with several of the techniques
described earlier in this document. This policy section shows how even
greater efficiency can be achieved using a set of relatively new design
mechanisms. This section gives background and defines terms that are
relevant to this field and describes some deployment approaches.
7.1. What Is the Meaning of 'Policy-based'?
In the past few years of output from standards organizations and
networking vendor marketing departments, the term 'policy' has been
heavily used, touted, and contorted in meaning. The result is that the
true meaning of 'policy' is unclear without greater qualification where
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it is used.
[41] gives the term 'policy' two explicit definitions:
- A definite goal, course or method of action to guide and
determine present and future decisions. "Policies" are
implemented or executed within a particular context
(such as policies defined within a business unit).
- Policies as a set of rules to administer, manage, and
control access to network resources.
Note that these two views are not contradictory since
individual rules may be defined in support of business
goals.
As it pertains to our discussion of the term 'policy-based
configuration', the meaning is significantly more specific. In this
context, we refer to a way of integrating data and the management
actions which use it in such a way that:
- there is the ability to specify "default" configuration data for a
number of instances of managed elements, where those instances can
be correlated in some data driven or algorithmic way. The engine
to do this correlation and activate instances from defaults may
reside in the agent or externally. Where the representation of
these defaults are in the MIB design itself, the object types
supporting this notion are referred to as "template objects".
- the activation of instance data derived from template object
types results from minimal activation directives from the
management application, once the instances of the template
object types have been established.
- somewhat independently, the architecture of the overall
management agent may accommodate the definition and evaluation
of management and configuration policies. The side-effects
of the evaluation of these policies typically include the
activation of certain configuration directives. Where
management data design exposes template object types, the
policy-driven activation can (and ideally, should) include the
application of template object instances to the analogous
managed element instance-level values.
As it pertains to template object data, the underlying notions implied
here have been prevalent for some time in non-SNMP management regimes.
A common feature of many command line interfaces for configuring routers
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is the specification of one or more access control lists. These
typically provide a set of IP prefixes, BGP autonomous system numbers,
or other such identifying constructs (see, for example, [42]). Once
these access control lists are assembled, their application to various
interfaces, routing processes, and the like are specified typically in
the configuration of what the access control list is applied to.
Consistent with the prior properties to define our use of policy-based
configuration, a) the access list is defined independent from its point
of application, and b) its application is independent of the access list
definition. For example, changing the application of an access list
from one interface to the other does not require a change in the access
list itself. The first point just mentioned suggests what is necessary
for template-based data organization. The second suggests its
application in a policy-based manner.
Let us now examine the motivation for such a system or subsystem
(perhaps bounded at the level of a 'template-enabled' MIB module, given
the above definition). Let us explore the importance of policy-based
techniques to configuration specifically.
7.2. Organization of Data in an SNMP-Based Policy System
The number of configurable parameters and 'instances' such as interfaces
has increased as equipment has become larger and more complex.
At the same time, there is a need to configure many of these systems to
operate in a coordinated fashion. This enables the delivery of new
specialized services that require this coordinated configuration.
Examples include delivery of virtual private networks and connections
that guarantee specific service levels.
The growth in size and complexity of configuration information has
significant implications for its organization as well as its efficient
transfer to the management agent. As an example, an agent that
implements the Bridge MIB [22] could be used to represent a large VLAN
with some 65,000 port entries. Configuring such a VLAN would require
the establishment of dot1dStpPortTable and dot1DStaticTable entries for
each such virtual port. Each table entry would contain several
parameters. A more efficient approach is to provide default values for
the creation of new entries that are appropriate to the VLAN environment
in our example. The local management infrastructure should then iterate
across the system setting the default values to the selected ports as
groups.
To date, this kind of large-scale configuration has been accomplished
with file transfer, by setting individual MIB objects, or with many CLI
commands. In each of these approaches the details for each instance are
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contained in the file, CLI commands or MIB objects. That is, they
contain not only the value, and type of object, but also the exact
instance of the object to which to apply the value. It is this property
that tends to make configuration operations explode as the number of
instances (such as interfaces) grows. This per-instance approach can
work for a few machines configured by experts, but there is a need for a
more scalable solution. Template-based data organization and policy-
based management abstracts the details above the instance level, which
means that fewer SET requests are sent to a managed device.
Realization of such a policy-driven system requires agents that can take
defaults and apply them to instances based on a rule that defines under
what conditions the defaults (policy) are to be applied. A policy-
driven configuration system which is to be scalable needs to expose a
means of layering its application of defaults at discrete ranges of
granularity. The spectrum of that granularity might have a starting
hierarchy point to apply defaults at the breadth of a network service.
Ultimately, such a layering ends up with features to support instance-
level object instance data within the running agent.
An example of this kind of layering is implicit in the principle of
operations of a SNMPCONF Policy-Based Management MIB [34] (PM-MIB)
implementation. However, other entity management systems have been
employing these kinds of techniques end-to-end for some time, in some
cases using SNMP, in some cases using other encodings and transfer
technologies. What the PM-MIB seeks to establish, in an environment
ideal for its deployment, is an adaptation between MIB module data which
was not designed using template object types, and the ability to allow
the PM-MIB agent engine to apply instances of that data as though it
were template-based.
7.3. Information Related to Policy-based Configuration
In order for effective policy management to take place, a range of
information about the network elements is needed to avoid making poor
policy decisions. Even in those cases where policy-based configuration
is not in use, much of the information described in this section can be
useful input to the decision-making process about what type of
configuration operations to do.
For this discussion it is important to make distinctions between
distribution of policy to a system, activation of a policy in a system,
and changes/failures that take place during the time the policy is
expected to be active. For example, if an interface is down that is
included in a policy that is distributed, there may not be an error
since the policy may not be scheduled for activation until a later time.
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On the other hand, if a policy is distributed and applied to an
interface that should be operational and it is not, clearly this is a
problem, although it is not an error in the configuration policy itself.
With this as background, here are some areas to consider that are
important to making good policy configuration decisions and establishing
when a policy has 'failed'.
o The operational state of network elements that are to be
configured.
Care should be taken to determine if the sub-components to be
configured are available for use. In some cases the elements may
not be available. The policy configuration software should
determine if this is a prerequisite to policy installation or if
the condition is even acceptable. This decision is separate from
the one to be made about policy activation. Installation is when
the policy is sent from the policy manager to the managed device
and activation is turning on the policy. In those cases where
policy is distributed when the sub-component such as an interface
or disk is not available, the managed system should send a
notification to the designated management station when the policy
is to become active or if the resource is still not available.
o The capabilities of the devices in the network.
A capability can be almost any unit of work a network element can
perform. These include routing protocols supported, Web server and
OS versions, queuing mechanisms supported on each interface that
can be used to support different qualities of service, and many
others. This information can be obtained from the capabilities
table of the Policy MIB module [34].
Historically, management applications have had to obtain this type
of information by issuing get requests for objects they might want
to use. This approach is far less efficient since it requires many
get requests and is more error prone since some instances will not
exist until configured. The new capabilities table is an
improvement on the current technique.
o The capacity of the devices to perform the desired work.
Capability is an ability to perform the desired work while a
capacity is a measure of how much of that capability the system
has. The policy configuration application should, wherever
possible, evaluate the capacity of the network element to perform
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the work identified by the policy. In some systems it will not be
possible to obtain the capacity of the managed elements to perform
the desired work directly, even though it may be possible to
monitor the amount of work the element performs. In these cases,
the management application may benefit from pre-configured
information about the capacity of different network elements so
that evaluations of the resources available can be done before
distributing new policies.
Utilization refers to how much capacity for a particular capability
has been consumed. For devices that have been under policy
configuration control for any period of time, a certain percentage
of the available capacity of the managed elements will be used.
Policies should not be distributed to systems that do not have the
resources to carry out the policy in a reasonable period of time.
7.4. Schedule and Time Issues
This section applies equally to systems that are not policy-based as
well as policy-based systems, since configuration operations often need
to be synchronized across time zones. Wherever possible, the network
elements should support time information using the standard DateAndTime
TC that includes local time zone information. Policy-based management
often requires more complex time expressions than can be conveyed with
the DateAndTime TC. See the Policy-Based Management MIB document [34]
for more information. Some deployed systems do not store complex
notions of local time and thus may not be able to process policy
directives properly that contain time zone relevant data. For this
reason, policy management applications should have the ability to
ascertain the time keeping abilities of the managed system and make
adjustments to the policy for those systems that are time-zone
challenged.
7.5. Conflict Detection, Resolution and Error Reporting
Policies sent to a device may contain conflicting instructions.
Detection of such commands can occur at the device or management level
and may be resolved using any number of mechanisms (examples are, last
configuration set wins, or abort change). These unintended conflicts
should be reported. Conflicts can occur at different levels in a chain
of commands. Each 'layer' in policy management system should be able to
check for some errors and report them. This is conceptually identical
to programs raising an exception and passing that information on to
software that can do something meaningful with it.
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At the instance level, conflict detection has been performed in a
limited way for some time in software that realizes MIB objects at this
level of resolution. This detection is independent of policy. The types
of 'conflicts' usually checked for are resource availability and
validity of the set operations. In a policy enabled system, there are no
additional requirements for this software assuming that good error
detection and reporting appropriate to this level have already been
implemented.
7.5.1. Changes to Configuration Outside of the Policy System
It is essential to consider changes to configuration that are initiated
outside of the policy system. A goal of SNMP-based policy management is
to coexist with other kinds of management software that have
historically been instance based management. The best example is the
command line interface.
A notification should be sent whenever an out-of-policy control change
is made to an element that is under the control of policy. This
notification should include the policy that was affected, the instance
of the element that was changed and the object and value that it was
changed to.
Even for those systems that have no concept of policy control, the ideas
presented above make sense. That is, if SNMP co-exists with other access
methods such as a CLI, it is essential that the management station
remain synchronized with changes that might have been made to the
managed device using other methods. As a result, the approach of sending
a notification when another access method makes a change is a good one.
Of course this should be configurable by the user.
7.6. More about Notifications in a Policy System
Notifications can be useful in determining a failure of a policy as a
result of an error in the policy or element(s) under policy control. As
with all notifications, they should be defined and controlled in such a
way that they do not create a problem by sending more than are helpful
over a specific period of time. For example, if a policy is controlling
1,000 interfaces and fails, one notification rather than 1,000 may be
the better approach. In addition, such notifications should be defined
to include as much information as possible to aid in problem resolution.
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7.7. Using Policy to Move Less Configuration Data
One of the advantages of policy-based configuration with SNMP is that
many configuration operations can be conveyed with a small amount of
data. Changing a single configuration parameter for each of 100
interfaces on a system might require 100 CLI commands or 100 SNMP
variable bindings using conventional techniques.
Using policy-based configuration with SNMP, a single SET PDU can be sent
with the policy information necessary to apply a configuration change to
100 similar interfaces. This efficiency gain is the result of
eliminating the need to send the value for each instance to be
configured. The 'default' for each of the instances included in the
policy is sent, and the rule for selection of the instances that the
default is to be applied to can also be carried (see the Policy MIB
module [34]).
To extend the example above, assume that there are 10 parameters that
need to change. Using conventional techniques, there would now be 1,000
variable bindings, one for each instance of each new value for each
interface. Using policy-based configuration with SNMP, it is still
likely that all the information can be conveyed in one SET PDU. The only
difference in this case is that there are ten parameters sent that will
be the 'template' used to create instances on the managed interfaces.
This efficiency gain not only applies to SET operations, but also to
those management operations that require configuration information.
Since the policy is also held in the storage for cross-instance defaults
(for example, the pmPolicyTable in [34]), an entire data set that
potentially controls hundreds of rows of information can be retrieved in
a single GET request.
A policy-friendly data organization such as this is consistent and
integrates well with MIB module objects which support "summary"
activation and activation reporting, of the kind discussed in Section
3.3.5.
8. Example MIB Module With Template-based Data
This section defines a MIB module that controls the heating and air
conditioning system for a large building. It contains both configuration
and counter objects that allow operators to see how much cooling or
heating a particular configuration has consumed. Objects that represent
the configuration information at a "default" level (as referenced above)
are also included.
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These tables, in combination with the application of the tables' row
instance data as templated 'defaults', will allow operators to configure
and monitor many rooms at once, change the configuration parameters
based on time of day, and make a number of other sophisticated decisions
based on the 'policy' implied by these defaults and their application.
For this reason, these configuration controls have their instances
specified from template object types.
In our simplified Heating Ventilation and Air Conditioning (HVAC) model
we will create three tables based on a simple analysis. More complicated
systems will need more tables, but the principles will be the same.
Step 1: As with any other MIB module design, the first step
is to determine what objects are necessary for
configuration and control operations. The first table
to be created is a fairly traditional monitoring
table. It includes indices so that we will know what
rooms the counters and status objects are for. It
includes an object that is a RowPointer to a table
that contains configuration information. The objects
for the bldgHVACTable, our first table in the HVAC
MIB module are:
Index objects that identify what floor and office we are
managing:
bldgHVACFloor
bldgHVACOffice
A single index reference to a table that 'glues' configuration
information defaults with descriptive information:
bldgHVACCfgTemplate
A set of objects that show status and units of
work (bldgHVACCoolOrHeatMins) and standard per-row
SnmpAdminString, StorageType, and RowStatus columnar
objects:
bldgHVACFanSpeed
bldgHVACCurrentTemp
bldgHVACCoolOrHeatMins
bldgHVACDiscontinuityTime
bldgHVACOwner
bldgHVACStatus
Step 2: A configuration description table. The purpose of this
table is to provide a unique string identifier for
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templates. These may be driven by policies in a
network. If it were necessary to configure devices
to deliver a particular quality of service, the
index string of this table could be the name and the
description part, it could be a brief description of the
underlying motivation such as: "provides extra heat to
corner offices to counteract excessive exterior wind
chill". Standard owner and status objects may also
be helpful and are included here. The row columnar
objects are:
bldgHVACCfgTemplateInfoIndex
bldgHVACCfgTemplateInfoID
bldgHVACCfgTemplateInfoDescr
bldgHVACCfgTemplateInfoOwner
bldgHVACCfgTemplateInfoStatus
Notice that to this point we have provided no
configuration information. That will be in the next
table. Some readers may wonder why this table is not
combined with the configuration template table described
in the next step. In fact, they can be. The reason for
having a separate table is that as systems become more
complex, there may be more than one configuration table
that points to these descriptions. Another reason for
two tables is that this in not reproduced for every
template and instance, which can save some additional
data movement. Every designer will have to evaluate the
tradeoffs between number of objects and data movement
efficiency just as with other MIB modules.
Step 3: The bldgHVACCfgTemplateTable contains the specific
configuration parameters that are pointed to by the
bldgHVACConfigPtr object. Note that many rows in the
bldgHVACTable can point to an entry in this table. It
is also possible for entries to be used by 1 or 0 rows
of the bldgHVACTable. It is the property of allowing
multiple rows (instances) in the bldgHVACTable to
point to a row in this table that can produce such
efficiency gains from policy-based management with
SNMP. Also notice that the configuration data is tied
directly to the counter data so that people can see
how configurations impact behavior.
The objects in this table are all that are necessary
for configuration and connection to the other tables as
well as the usual SnmpAdminString, StorageType, and
RowStatus objects:
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A simple index to the table:
bldgHVACCfgTemplateIndex
The configuration objects:
bldgHVACCfgTemplateDesiredTemp
bldgHVACCfgTemplateCoolOrHeat
Administrative objects for SnmpAdminString and
RowStatus:
bldgHVACCfgTemplateInfo
bldgHVACCfgTemplateOwner
bldgHVACCfgTemplateStorage
bldgHVACCfgTemplateStatus
8.1. MIB Module Definition
BLDG-HVAC-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, Counter32,
Gauge32, OBJECT-TYPE, Unsigned32, experimental
FROM SNMPv2-SMI
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
TEXTUAL-CONVENTION,
TimeStamp, RowStatus, StorageType
FROM SNMPv2-TC
SnmpAdminString
FROM SNMP-FRAMEWORK-MIB;
bldgHVACMIB MODULE-IDENTITY
LAST-UPDATED "200208010000Z"
ORGANIZATION "SNMPCONF working group
E-mail: snmpconf@snmp.com"
CONTACT-INFO
"Jon Saperia
Postal: JDS Consulting
174 Chapman Street
Watertown, MA 02472
U.S.A.
Phone: +1 617 744 1079
E-mail: saperia@jdscons.com
Wayne Tackabury
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Postal: Gold Wire Technology
411 Waverley Oaks Rd.
Waltham, MA 02452
U.S.A.
Phone: +1 781 398 8800
E-mail: wayne@goldwiretech.com
Michael MacFaden
Postal: Riverstone Networks
5200 Great America Pkwy.
Santa Clara, CA 95054
U.S.A.
Phone: +1 408 878 6500
E-mail: mrm@riverstonenet.com
David Partain
Postal: Ericsson AB
P.O. Box 1248
SE-581 12 Linkoping
Sweden
E-mail: David.Partain@ericsson.com"
DESCRIPTION
"This example MIB module defines a set of
management objects for heating ventilation and
air conditioning systems. It also includes
objects that can be used to create policies
that are applied to rooms. This eliminates
the need to send per-instance configuration
commands to the system.
Note that for this MIB module to compile
successfully, an assignment from IANA would be
needed to replace the XXX below."
REVISION "200208010000Z"
DESCRIPTION
"Current version of BLDG-HVAC-MIB, updated
to address comments from working group membership."
::= { experimental XXX }
bldgHVACObjects OBJECT IDENTIFIER ::= { bldgHVACMIB 1 }
bldgConformance OBJECT IDENTIFIER ::= { bldgHVACMIB 2 }
--
-- Textual Conventions
--
BldgHvacOperation ::= TEXTUAL-CONVENTION
STATUS current
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DESCRIPTION
"Operations supported by a heating and cooling system.
A reference to underlying general systems would go here."
SYNTAX INTEGER {
heat(1),
cool(2)
}
--
-- HVAC Objects Group
--
bldgHVACTable OBJECT-TYPE
SYNTAX SEQUENCE OF BldgHVACEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is the representation and data control
for building HVAC by each individual office.
The table has rows for, and is indexed by a specific
floor and office number. Each such row includes
HVAC statistical and current status information for
the associated office. The row also contains a
bldgHVACCfgTemplate columnar object that relates the
bldgHVACTable row to a row in the bldgHVACCfgTemplateTable.
If this value is nonzero, then the instance in the row
that has a value for how the HVAC has been configured
in the associated template (bldgHVACCfgTeplateTable row).
Hence, the bldgHVACCfgTeplateTable row contains the
specific configuration values for the offices as described
in this table."
::= { bldgHVACObjects 1 }
bldgHVACEntry OBJECT-TYPE
SYNTAX BldgHVACEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A row in the bldgHVACTable. Each row represents a particular
office in the building, qualified by its floor and office
number. A given row instance can be created or deleted by
set operations upon its bldgHVACStatus columnar
object instance."
INDEX { bldgHVACFloor, bldgHVACOffice }
::= { bldgHVACTable 1 }
BldgHVACEntry ::= SEQUENCE {
bldgHVACFloor Unsigned32,
bldgHVACOffice Unsigned32,
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bldgHVACCfgTemplate Unsigned32,
bldgHVACFanSpeed Gauge32,
bldgHVACCurrentTemp Gauge32,
bldgHVACCoolOrHeatMins Counter32,
bldgHVACDiscontinuityTime TimeStamp,
bldgHVACOwner SnmpAdminString,
bldgHVACStorageType StorageType,
bldgHVACStatus RowStatus
}
bldgHVACFloor OBJECT-TYPE
SYNTAX Unsigned32 (1..1000)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This portion of the index indicates the floor of the
building. The ground floor is considered the
first floor. For the purposes of this example,
floors under the ground floor cannot be
controlled using this MIB module."
::= { bldgHVACEntry 1 }
bldgHVACOffice OBJECT-TYPE
SYNTAX Unsigned32 (1..2147483647)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This second component of the index specifies the
office number."
::= { bldgHVACEntry 2 }
bldgHVACCfgTemplate OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The index (bldgHVACCfgTemplateIndex instance)
of an entry in the 'bldgHVACCfgTemplateTable'.
The bldgHVACCfgTable row instance referenced
is a pre-made configuration 'template'
that represents the configuration described
by the bldgHVACCfgTemplateInfoDescr object. Note
that not all configurations will be under a
defined template. As a result, a row in this
bldgHVACTable may point to an entry in the
bldgHVACCfgTemplateTable that does not in turn
have a reference (bldgHVACCfgTemplateInfo) to an
entry in the bldgHVACCfgTemplateInfoTable. The
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benefit of this approach is that all
configuration information is available in one
table whether all elements in the system are
derived from configured templates or not.
Where the instance value for this colunmar object
is zero, this row represents data for an office
whose HVAC status can be monitored using the
read-only columnar object instances of this
row, but is not under the configuration control
of the agent."
::= { bldgHVACEntry 3 }
bldgHVACFanSpeed OBJECT-TYPE
SYNTAX Gauge32
UNITS "revolutions per minute"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"Shows the revolutions per minute of the fan. Fan speed
will vary based on the difference between
bldgHVACCfgTemplateDesiredTemp and bldgHVACCurrentTemp. The
speed is measured in revolutions of the fan blade per minute."
::= { bldgHVACEntry 4 }
bldgHVACCurrentTemp OBJECT-TYPE
SYNTAX Gauge32
UNITS "degrees in celsius"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The current measured temperature in the office. Should
the current temperature be measured at a value of less
than zero degrees celsius, a read of the instance
for this object will return a value of zero."
::= { bldgHVACEntry 5 }
bldgHVACCoolOrHeatMins OBJECT-TYPE
SYNTAX Counter32
UNITS "minutes"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The total number of heating or cooling minutes that have
been consumed since the row was activated. Notice that
whether the minutes represent heating or cooling is a
function of the configuration of this row. If the system
is re-initialized from a cooling to heating function or
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vice versa, then the counter would start over again. This
effect is similar to a reconfiguration of some network
interface cards. When parameters that impact
configuration are changed, the subsystem must be
re-initialized. Discontinuities in the value of this counter
can occur at re-initialization of the management system,
and at other times as indicated by the value of
bldgHVACDiscontinuityTime."
::= { bldgHVACEntry 6 }
bldgHVACDiscontinuityTime OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime on the most recent occasion at which
any heating or cooling operation for the office designated
by this row instance experienced a discontinuity. If
no such discontinuities have occurred since the last re-
initialization of the this row, then this object contains a
zero value."
::= { bldgHVACEntry 7 }
bldgHVACOwner OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The identity of the operator/system that
last modified this entry. When a new entry
is created, a valid SnmpAdminString must
be supplied. If, on the other hand, this
entry is populated by the agent 'discovering'
unconfigured rooms, the empty string is a valid
value for this object."
::= { bldgHVACEntry 8 }
bldgHVACStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The persistence of this row of the table in system storage,
as it pertains to permanence across system resets."
::= { bldgHVACEntry 9 }
bldgHVACStatus OBJECT-TYPE
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SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Controls and reflects the creation and activation status of
a row in this table.
No attempt to modify a row columnar object instance value in
the bldgHVACTable should be issued while the value of
bldgHVACStatus is active(1). Should an agent receive a SET
PDU attempting such a modification in this state, an
inconsistentValue error should be returned as a result of
the SET attempt."
::= { bldgHVACEntry 10 }
--
-- HVAC Configuration Template Table
--
bldgHVACCfgTemplateInfoTable OBJECT-TYPE
SYNTAX SEQUENCE OF BldgHVACCfgTemplateInfoEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table provides unique string identification for
HVAC templates in a network. If it were necessary to
configure rooms to deliver a particular quality of climate
control with regard to cooling or heating, the index string
of a row in this table could be the template name.
The bldgHVACCfgCfgTemplateInfoDescription
contains a brief description of the template service objective
such as: provides summer cooling settings for executive
offices. The bldgHVACCfgTemplateInfo in the
bldgHVACCfgTemplateTable will contain the pointer to the
relevant row in this table if it is intended that items
that point to a row in the bldgHVACCfgTemplateInfoTable be
identifiable as being under template control though this
mechanism."
::= { bldgHVACObjects 2 }
bldgHVACCfgTemplateInfoEntry OBJECT-TYPE
SYNTAX BldgHVACCfgTemplateInfoEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Each row represents a particular template and
description. A given row instance can be created or
deleted by set operations upon its
bldgHVACCfgTemplateInfoStatus columnar object
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instance."
INDEX { bldgHVACCfgTemplateInfoIndex }
::= { bldgHVACCfgTemplateInfoTable 1 }
BldgHVACCfgTemplateInfoEntry ::= SEQUENCE {
bldgHVACCfgTemplateInfoIndex Unsigned32,
bldgHVACCfgTemplateInfoID SnmpAdminString,
bldgHVACCfgTemplateInfoDescr SnmpAdminString,
bldgHVACCfgTemplateInfoOwner SnmpAdminString,
bldgHVACCfgTemplateInfoStatus RowStatus,
bldgHVACCfgTemplateInfoStorType StorageType
}
bldgHVACCfgTemplateInfoIndex OBJECT-TYPE
SYNTAX Unsigned32 (1..2147483647)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"The unique index to a row in this table."
::= { bldgHVACCfgTemplateInfoEntry 1 }
bldgHVACCfgTemplateInfoID OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Textual identifier for this table row, and, consequently
the template. This should be a unique name within
an administrative domain for a particular template so that
all systems in a network that are under the same template
can have the same 'handle' (e.g., 'Executive Offices',
'Lobby Areas')."
::= { bldgHVACCfgTemplateInfoEntry 2 }
bldgHVACCfgTemplateInfoDescr OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A general description of the template. One example might
be - Controls the cooling for offices on higher floors
during the summer."
::= { bldgHVACCfgTemplateInfoEntry 3 }
bldgHVACCfgTemplateInfoOwner OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
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STATUS current
DESCRIPTION
"The identity of the operator/system that last modified
this entry."
::= { bldgHVACCfgTemplateInfoEntry 4 }
bldgHVACCfgTemplateInfoStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The activation status of this row.
No attempt to modify a row columnar object instance value in
the bldgHVACCfgTemplateInfo Table should be issued while the
value of bldgHVACCfgTemplateInfoStatus is active(1).
Should an agent receive a SET PDU attempting such a modification
in this state, an inconsistentValue error should be returned as
a result of the SET attempt."
::= { bldgHVACCfgTemplateInfoEntry 5 }
bldgHVACCfgTemplateInfoStorType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The persistence of this row of the table in system storage,
as it pertains to permanence across system resets."
::= { bldgHVACCfgTemplateInfoEntry 6 }
--
-- HVAC Configuration Template Table
--
bldgHVACCfgTemplateTable OBJECT-TYPE
SYNTAX SEQUENCE OF BldgHVACCfgTemplateEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table contains the templates, which
can be used to set defaults that will
be applied to specific offices. The application
of those values is accomplished by having a row
instance of the bldgHVACTable reference a row of
this table (by the value of the former's
bldgHVACCfgTemplate columnar instance). Identifying
information concerning a row instance of this table
can be found in the columnar data of the row instance
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iof the bldgHVACCfgTemplateInfoTable entry referenced
by the bldgHVACCfgTemplateInfo columnar object of
this table."
::= { bldgHVACObjects 3 }
bldgHVACCfgTemplateEntry OBJECT-TYPE
SYNTAX BldgHVACCfgTemplateEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Each row represents a single set of template parameters
that can be applied to selected instances - in this case
offices. These policies will be turned on and off by the
policy module through its scheduling facilities.
A given row instance can be created or
deleted by set operations upon its
bldgHVACCfgTemplateStatus columnar object instance."
INDEX { bldgHVACCfgTemplateIndex }
::= { bldgHVACCfgTemplateTable 1 }
BldgHVACCfgTemplateEntry ::= SEQUENCE {
bldgHVACCfgTemplateIndex Unsigned32,
bldgHVACCfgTemplateDesiredTemp Gauge32,
bldgHVACCfgTemplateCoolOrHeat BldgHvacOperation,
bldgHVACCfgTemplateInfo Unsigned32,
bldgHVACCfgTemplateOwner SnmpAdminString,
bldgHVACCfgTemplateStorage StorageType,
bldgHVACCfgTemplateStatus RowStatus
}
bldgHVACCfgTemplateIndex OBJECT-TYPE
SYNTAX Unsigned32 (1..2147483647)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A unique value for each defined template in this
table. This value can be referenced as a row index
by any MIB module that needs access to this information.
The bldgHVACCfgTemplate will point to entries in this
table."
::= { bldgHVACCfgTemplateEntry 1 }
bldgHVACCfgTemplateDesiredTemp OBJECT-TYPE
SYNTAX Gauge32
UNITS "degrees in celsius"
MAX-ACCESS read-create
STATUS current
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DESCRIPTION
"This is the desired temperature setting. It might be
changed at different times of the day or based on
seasonal conditions. It is permitted to change this value
by first moving the row to an inactive state, making the
change and then reactivating the row."
::= { bldgHVACCfgTemplateEntry 2 }
bldgHVACCfgTemplateCoolOrHeat OBJECT-TYPE
SYNTAX BldgHvacOperation
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"This controls the heating and cooling mechanism and is
set-able by building maintenance. It is permitted to
change this value by first moving the row to an inactive
state, making the change and then reactivating the row."
::= { bldgHVACCfgTemplateEntry 3 }
bldgHVACCfgTemplateInfo OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"This object points to a row in the
bldgHVACCfgTemplateInfoTable. This controls the
heating and cooling mechanism and is set-able by
building maintenance. It is permissible to change
this value by first moving the row to an inactive
state, making the change and then reactivating
the row. A value of zero means that this entry
is not associated with a named template found
in the bldgHVACCfgTemplateInfoTable."
::= { bldgHVACCfgTemplateEntry 4 }
bldgHVACCfgTemplateOwner OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The identity of the administrative entity
that created this row of the table."
::= { bldgHVACCfgTemplateEntry 5 }
bldgHVACCfgTemplateStorage OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
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DESCRIPTION
"The persistence of this row of the table across
system resets."
::= { bldgHVACCfgTemplateEntry 6 }
bldgHVACCfgTemplateStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The activation status of this row of the table.
No attempt to modify a row columnar object instance value in
the bldgHVACCfgTemplateTable should be issued while the
value of bldgHVACCfgTemplateStatus is active(1).
Should an agent receive a SET PDU attempting such a modification
in this state, an inconsistentValue error should be returned as
a result of the SET attempt."
::= { bldgHVACCfgTemplateEntry 7 }
--
-- Conformance Information
--
bldgCompliances OBJECT IDENTIFIER ::= { bldgConformance 1 }
bldgGroups OBJECT IDENTIFIER ::= { bldgConformance 2 }
-- Compliance Statements
bldgCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The requirements for conformance to the BLDG-HVAC-MIB. The
bldgHVACObjects group must be implemented to conform to the
BLDG-HVAC-MIB."
MODULE -- this module
GROUP bldgHVACObjectsGroup
DESCRIPTION
"The bldgHVACObjects is mandatory for all systems that
support HVAC systems."
::= { bldgCompliances 1 }
bldgHVACObjectsGroup OBJECT-GROUP
OBJECTS {
bldgHVACCfgTemplate,
bldgHVACFanSpeed, bldgHVACCurrentTemp,
bldgHVACCoolOrHeatMins, bldgHVACDiscontinuityTime,
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bldgHVACOwner, bldgHVACStatus,
bldgHVACStorageType, bldgHVACCfgTemplateInfoID,
bldgHVACCfgTemplateInfoDescr, bldgHVACCfgTemplateInfoOwner,
bldgHVACCfgTemplateInfoStatus,
bldgHVACCfgTemplateInfoStorType,
bldgHVACCfgTemplateDesiredTemp,
bldgHVACCfgTemplateCoolOrHeat,
bldgHVACCfgTemplateInfo,
bldgHVACCfgTemplateOwner,bldgHVACCfgTemplateStorage,
bldgHVACCfgTemplateStatus
}
STATUS current
DESCRIPTION
"The bldgHVACObjects Group."
::= { bldgGroups 1 }
END
8.2. Notes on MIB Module with Template-based Data
The primary purpose of the example "HVAC" MIB module is to show how to
construct a single module that includes configuration, template, counter
and state information in a single module. If this were a 'real' module
we would also have included definitions for notifications for the
configuration change operations as previously described. We also would
have included notifications for faults and other counter threshold
events.
Implementation and Instance Extensions:
Just as with networking technologies, vendors may wish to add extensions
that can distinguish their products from the competition. If an HVAC
vendor also wanted to support humidity control, they could add that
facility to their equipment and use AUGMENTS for the
bldgHVACTemplateTable with two objects, one that indicates the desired
humidity and the other, the actual. The bldgHVACTemplateTable could also
be extended using this same approach so that HVAC policies could easily
be extended to support this vendor.
8.3. Examples of Usage of the MIB
The following two examples use two templates to configure the
temperature in executive offices and in conference rooms. The
"conference rooms" template is applied to all conference rooms (which
happen to be office 104 on each floor), and the "executive offices"
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template is applied to executive offices.
If offices 24, 25, and 26 on the third floor are executive offices, the
values in the bldgHVACTable might be:
bldgHVACCfgTemplate.3.24 = 2
bldgHVACFanSpeed.3.24 = 2989
bldgHVACCurrentTemp.3.24 = 24
bldgHVACCoolOrHeatMins.3.24 = 123
bldgHVACDiscontinuityTime.3.24 = sysUpTime + 12h + 21m
bldgHVACOwner.3.24 = "policy engine"
bldgHVACStorageType.3.24 = nonVolatile(3)
bldgHVACStatus.3.24 = active(1)
bldgHVACCfgTemplate.3.25 = 2
bldgHVACFanSpeed.3.25 = 0
bldgHVACCurrentTemp.3.25 = 22
bldgHVACCoolOrHeatMins.3.25 = 298
bldgHVACDiscontinuityTime.3.25 = sysUpTime + 4h + 2m
bldgHVACOwner.3.25 = "policy engine"
bldgHVACStorageType.3.25 = nonVolatile(3)
bldgHVACStatus.3.25 = active(1)
bldgHVACCfgTemplate.3.26 = 2
bldgHVACFanSpeed.3.26 = 0
bldgHVACCurrentTemp.3.26 = 22
bldgHVACCoolOrHeatMins.3.26 = 982
bldgHVACOwner.3.26 = "policy engine"
bldgHVACStorageType.3.26 = nonVolatile(3)
bldgHVACStatus.3.26 = active(1)
The second entry in the bldgHVACCfgTemplateTable, to which all of the
above point, might have the following configuration:
bldgHVACCfgTemplateDesiredTemp.2 = 22
bldgHVACCfgTemplateCoolOrHeat.2 = cool(2)
bldgHVACCfgTemplateInfo.2 = 2
bldgHVACCfgTemplateOwner.2 = "Senior Executive assistant"
bldgHVACCfgTemplateStorage.2 = nonVolatile(3)
bldgHVACCfgTemplateStatus.2 = active(1)
and the associated template information ("executive offices") might be:
bldgHVACCfgTemplateInfoID.2 = "executive offices"
bldgHVACCfgTemplateInfoDescr.2 = "Controls temperature for executive offices"
bldgHVACCfgTemplateInfoOwner.2 = "Senior Executive assistant"
bldgHVACCfgTemplateInfoStorType.2 = nonVolatile(3)
bldgHVACCfgTemplateInfoStatus.2 = active(1)
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fi
The policy engine can now associate instances of executive
offices with the template called "executive offices" and apply
the values in the second entry of the bldgHVACCfgTemplateTable to
each of the instances of the executive offices. This will then
attempt to set the temperature in executive offices to 22 degrees
celsius.
It is also possible that there may be an office configured
for a particular temperature, but without using a template.
For example, office 28 on the third floor might look like this:
bldgHVACCfgTemplate.3.28 = 3
bldgHVACFanSpeed.3.28 = 50
bldgHVACCurrentTemp.3.28 = 26
bldgHVACCoolOrHeatMins.3.28 = 0
bldgHVACDiscontinuityTime.3.28 = 0
bldgHVACOwner.3.28 = "Executive with poor circulation"
bldgHVACStorageType.3.28 = nonVolatile(3)
bldgHVACStatus.3.28 = active(1)
The entry in the bldgHVACCfgTemplateTable (to which
bldgHVACCfgTemplate.3.28 points) might instead look like:
bldgHVACCfgTemplateDesiredTemp.3 = 28
bldgHVACCfgTemplateCoolOrHeat.3 = cool(2)
bldgHVACCfgTemplateInfo.3 = 0.0
bldgHVACCfgTemplateOwner.3 = "Executive with poor circulation"
bldgHVACCfgTemplateStorage.3 = nonVolatile(3)
bldgHVACCfgTemplateStatus.3 = active(1)
Note that this entry does not point to a template.
If the executive's circulation improves so that the temperature should
be aligned with other executive offices, this is accomplished by
changing the value of bldgHVACCfgTemplate.3.28 from
bldgHVACCfgTemplateInfoID.3 to bldgHVACCfgTemplateInfoID.2 (shown
above).
Finally, there might be offices for which there is no configured
temperature but management applications can read the current
temperature, fan speed, and cooling or heating minutes from the
bldgHVACTable. In that case, the value of bldgHVACCfgTemplate will be a
zero index ("null"), as will the value of bldgHVACOwner.
bldgHVACCfgTemplate.4.2 = 0
bldgHVACFanSpeed.3.28 = 50
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bldgHVACCurrentTemp.3.28 = 26
bldgHVACCoolOrHeatMins.3.28 = 0
bldgHVACDiscontinuityTime.3.28 = 0
bldgHVACOwner.3.28 = ""
bldgHVACStorageType.3.28 = nonVolatile(3)
bldgHVACStatus.3.28 = active(1)
As a second example, the conference rooms on several floors are
configured using the "conference rooms" template. When the values in
the bldgHVACTable pertaining to conference rooms are read, it might look
like:
bldgHVACCfgTemplate.12.104 = bldgHVACCfgTemplateDesiredTemp.1
bldgHVACFanSpeed.12.104 = 1423
bldgHVACCurrentTemp.12.104 = 21
bldgHVACCoolOrHeatMins.12.104 = 2193
bldgHVACDiscontinuityTime.12.104 = sysUpTime + 36h + 15m
bldgHVACOwner.12.104 = = "Bob the Conference Guy"
bldgHVACStorageType.12.104 = nonVolatile(3)
bldgHVACStatus.12.104 = active(1)
bldgHVACCfgTemplate.14.104 = bldgHVACCfgTemplateDesiredTemp.1
bldgHVACFanSpeed.14.104 = 1203
bldgHVACCurrentTemp.14.104 = 20
bldgHVACCoolOrHeatMins.14.104 = 293
bldgHVACDiscontinuityTime.14.104 = sysUpTime + 5h + 54m
bldgHVACOwner.14.104 = = "Bob the Conference Guy"
bldgHVACStorageType.14.104 = nonVolatile(3)
bldgHVACStatus.14.104 = active(1)
bldgHVACCfgTemplate.15.104 = bldgHVACCfgTemplateDesiredTemp.1
bldgHVACFanSpeed.15.104 = 12
bldgHVACCurrentTemp.15.104 = 19
bldgHVACCoolOrHeatMins.15.104 = 1123
bldgHVACDiscontinuityTime.15.103 = sysUpTime + 2d + 2h + 7m
bldgHVACOwner.15.104 = = "Bob the Conference Guy"
bldgHVACStorageType.15.104 = nonVolatile(3)
bldgHVACStatus.15.104 = active(1)
The desired temperature and whether to heat or cool is configured in the
first entry of the bldgHVACCfgTemplateTable, which tries to set the
temperature to 19 degrees celsius in conference rooms:
bldgHVACCfgTemplateDesiredTemp.1 = 19
bldgHVACCfgTemplateCoolOrHeat.1 = cool(2)
bldgHVACCfgTemplateInfo.1 = bldgHVACCfgTemplateInfoID.1
bldgHVACCfgTemplateOwner.1 = "Bob the Conference Guy"
bldgHVACCfgTemplateStorage.1 = nonVolatile(3)
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bldgHVACCfgTemplateStatus.1 = active(1)
The associated template information would then have:
bldgHVACCfgTemplateInfoID.1 = "conference rooms"
bldgHVACCfgTemplateInfoDescr.1 = "Controls temperature in conference
rooms" bldgHVACCfgTemplateInfoOwner.1 = "Bob the Conference Guy"
bldgHVACCfgTemplateInfoStorType.1 = nonVolatile(3)
bldgHVACCfgTemplateInfoStatus.1 = active(1)
The policy system can then apply this template (cool to 19 degrees
Celsius) to its notion of all of the conference rooms in the building.
9. Acknowledgments
This document was produced by the SNMPCONF Working Group. In particular,
the editors wish to thank:
Christopher Anderson
Andy Bierman
Greg Bruell
Dr Jeffrey Case
Chris Elliott
Joel Halpern
Pablo Halpern
Wes Hardaker
David Harrington
Harrie Hazewinkel
Thippanna Hongal
Bob Moore
David T. Perkins
Randy Preshun
Dan Romanescu
Shawn Routhier
Steve Waldbusser
Bert Wijnen
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10. References
[1] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for
Describing SNMP Management Frameworks", RFC 2571, April 1999.
[2] Rose, M., and K. McCloghrie, "Structure and Identification of Man-
agement Information for TCP/IP-based Internets", STD 16, RFC 1155,
May 1990.
[3] Rose, M., and K. McCloghrie, "Concise MIB Definitions", STD 16, RFC
1212, March 1991.
[4] Rose, M., "A Convention for Defining Traps for use with the SNMP",
RFC 1215, March 1991.
[5] 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.
[6] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M.,
and S. Waldbusser, "Textual Conventions for SMIv2", STD 58, RFC
2579, April 1999.
[7] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose, M.,
and S. Waldbusser, "Conformance Statements for SMIv2", STD 58, RFC
2580, April 1999.
[8] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network
Management Protocol", STD 15, RFC 1157, May 1990.
[9] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Introduc-
tion to Community-based SNMPv2", RFC 1901, January 1996.
[10] 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.
[11] Case, J., Harrington D., Presuhn R., and B. Wijnen, "Message Pro-
cessing and Dispatching for the Simple Network Management Protocol
(SNMP)", RFC 2572, April 1999.
[12] Blumenthal, U., and B. Wijnen, "User-based Security Model (USM) for
version 3 of the Simple Network Management Protocol (SNMPv3)", RFC
2574, April 1999.
[13] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Protocol
Operations for Version 2 of the Simple Network Management Protocol
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(SNMPv2)", RFC 1905, January 1996.
[14] Levi, D., Meyer, P., and B. Stewart, "SNMPv3 Applications", RFC
2573, April 1999.
[15] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based Access Con-
trol Model (VACM) for the Simple Network Management Protocol
(SNMP)", RFC 2575, April 1999.
[16] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, q(Management
Information Base for Version 2 of the Simple Network Management
Protocol (SNMPv2) q, RFC 1907, January 1996.
[17] McCloghrie, K., and Kastenholz, F., "The Interfaces Group MIB", RFC
2863, June 2000.
[18] Case, J., Mundy, R., Partain, D., and B. Stewart, "Introduction to
Version 3 of the Internet-standard Network Management Framework",
RFC 2570, April 1999.
[19] Brown, C., and F. Baker, "Management Information Base for Frame
Relay DTEs Using SMIv2", RFC 2115, September 1997.
[20] Baker, F, "Requirements for IP Version 4 Routers", RFC 1812, June
1995.
[21] Hawkinson, J., and T. Bates, "Guidelines for Creation, Selection,
and Registration of an Autonomous System (AS)", RFC 1930, March
1996.
[22] Decker, E., Langille, P., Rijsinghani, A., and K. McCloghrie, "Def-
initions of Managed Objects for Bridges", RFC 1493, July 1993.
[23] Levi, D., and J. Schoenwaelder "Definitions of Managed Objects for
Scheduling Management Operations", RFC 2591, May 1999.
[24] Bell, E., Smith, A., Langille, P., Rijsinghani, A., and K.
McCloghrie, "Definitions of Managed Objects for Bridges with Traf-
fic Classes, Multicast Filtering and Virtual LAN Extensions, RFC
2674, August 1999.
[25] Baker, F., "IP Forwarding Table MIB", RFC 2096, January 1997.
[26] St. Johns, M., "Radio Frequency (RF) Interface Management Informa-
tion Base for MCNS/DOCSIS compliant RF interfaces", RFC 2670,
August 1999.
[27] Baker, F., and R. Coltun, "OSPF Version 2 Management Information
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RFC DRAFT Expires April 2003 October 2002
Base", RFC 1850, November 1995.
[28] Blake, S. Black, D., Carlson M., Davies E. Wang Z., Weiss W., "An
Architecture for Differentiated Services ", RFC 2475, December
1998.
[29] Willis, S. and J. Chu., "Definitions of Managed Objects for the
Fourth Version of the Border Gateway Protocol (BGP-4) using SMIv2",
RFC 1657, July 1994.
[30] Waldbusser, S."Remote Network Monitoring Management Information
Base", RFC 2819, May 2000.
[31] McCloghrie, K., and Hanson, G., "The Inverted Stack Table Extension
to the Interfaces Group MIB", RFC 2863, June 2000.
[32] McCloghrie, K. and Bierman, A., "Entity MIB (Version 2)", RFC 2737,
December, 1999.
[33] ITU-T,, Recommendation M.3010., PRINCIPLES FOR A TELECOMMUNICATIONS
MANAGEMENT NETWORK. February, 2000.
[34] Waldbusser, S., Saperia, J., and Hongal, T., "Policy Based Manage-
ment MIB", Work-in-progress.
[35] Heintz, L., "SNMP Row Operations Extensions", Work-in-progress.
[36] Zeltserman, D., "A Practical Guide to Snmpv3 and Network Manage-
ment", Prentice Hall, 1999.
[37] Noto, M., Spiegel, E., and Tesink, K., "Definitions of Textual Con-
ventions and OBJECT-IDENTITIES for ATM Management", RFC 2514,
February, 1999.
[38] Kassaveri, R., and Stewart, B., "Distributed Management Expression
MIB", RFC 2982, October, 2000.
[39] Daniele, M., Haberman, B., Routhier, S., and Schoenwaelder, J.,
"Textual Conventions for Internet Network Addresses", RFC 2851,
June 2000.
[40] St. Johns, M., "DOCSIS Cable Device MIB Cable Device Management
Information Base for DOCSIS compliant Cable Modems and Cable Modem
Termination Systems", RFC 2669, August, 1999.
[41] Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M.,
Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J., and Wald-
busser, S., "Terminology for Policy-Based Management", work-in-
Various Authors [Page 80]
RFC DRAFT Expires April 2003 October 2002
progress, July, 2001.
[42] http://wwww.cisco.com/univercd/cc/td/product/software/ios113ed/
11ed_cr/secur_c/scprt/scacls.pdf.
[43] Waldbusser, S., "Remote Network Monitoring Management Information
Base Version 2 using SMIv2", RFC 2021, January 1997.
[44] McCloghrie, k., "SNMPv2 Management Information Base for the Inter-
net Protocol using SMIv2", RFC 2011, November 1996.
11. Editors' Addresses
Michael R. MacFaden
Riverstone Networks, Inc
5200 Great America Parkway
Santa Clara, CA 95054
email - mrm@riverstonenet.com
David Partain
Ericsson AB
P.O. Box 1248
SE-581 12 Linkoping
Sweden
email: David.Partain@ericsson.com
Jon Saperia
JDS Consulting
174 Chapman Street
Watertown, MA 02472
email - saperia@jdscons.com
Wayne F. Tackabury
Gold Wire Technology
411 Waverley Oaks Rd.
Waltham, MA 02452
email - wayne@goldwiretech.com
12. 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
Various Authors [Page 81]
RFC DRAFT Expires April 2003 October 2002
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.
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.
13. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. The Internet Standard Management Framework . . . . . . . . . . 2
1.2. Configuration and the Internet Standard Management Frame-
work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Using SNMP as a Configuration Mechanism . . . . . . . . . . . . . 4
2.1. Transactions and SNMP . . . . . . . . . . . . . . . . . . . . . 5
2.2. Practical Requirements for Transactional Control . . . . . . . 5
2.3. Best Practices in Configuration--Verification . . . . . . . . . 6
3. Designing a MIB Module . . . . . . . . . . . . . . . . . . . . . 8
3.1. MIB Module Design - General Issues . . . . . . . . . . . . . . 8
3.2. Naming MIB modules and Managed Objects . . . . . . . . . . . . 10
3.3. Transaction Control And State Tracking . . . . . . . . . . . . 10
3.3.1. Conceptual Table Row Modification Practices . . . . . . . . . 11
3.3.2. Fate sharing with multiple tables . . . . . . . . . . . . . . 12
3.3.3. Transaction Control MIB Objects . . . . . . . . . . . . . . . 13
3.3.4. Creating And Activating New Table Rows . . . . . . . . . . . 13
3.3.5. Summary Objects and State Tracking . . . . . . . . . . . . . 14
3.3.6. Optimizing Configuration Data Transfer . . . . . . . . . . . 17
3.3.6.1. Index Design . . . . . . . . . . . . . . . . . . . . . . . 18
3.3.6.2. Time Based Indexing . . . . . . . . . . . . . . . . . . . . 21
3.3.6.3. Alternate Data Delivery Mechanisms . . . . . . . . . . . . 21
3.4. More Index Design Issues . . . . . . . . . . . . . . . . . . . 22
3.4.1. Simple Integer Indexing . . . . . . . . . . . . . . . . . . . 22
3.4.2. Indexing with Network Addresses . . . . . . . . . . . . . . . 23
3.5. Conflicting Controls . . . . . . . . . . . . . . . . . . . . . 23
3.6. Textual Convention Usage . . . . . . . . . . . . . . . . . . . 25
3.7. Persistent Configuration . . . . . . . . . . . . . . . . . . . 26
3.8. Configuration Sets and Activation . . . . . . . . . . . . . . . 28
3.8.1. Operational Activation Considerations . . . . . . . . . . . . 28
3.8.2. RowStatus and Deactivation . . . . . . . . . . . . . . . . . 30
3.9. SET Operation Latency . . . . . . . . . . . . . . . . . . . . . 31
3.9.1. Subsystem Latency, Persistence Latency, and Activation
Latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.10. Notifications and Error Reporting . . . . . . . . . . . . . . 33
3.10.1. Identifying Source of Configuration Changes . . . . . . . . 34
3.10.2. Limiting Unnecessary Transmission of Notifications . . . . . 34
3.10.3. Control of Notification Subsystem . . . . . . . . . . . . . 36
3.11. Application Error Reporting . . . . . . . . . . . . . . . . . 36
3.12. Designing MIB Modules for Multiple Managers . . . . . . . . . 37
3.13. Other MIB Module Design Issues . . . . . . . . . . . . . . . . 39
3.13.1. Octet String Aggregations . . . . . . . . . . . . . . . . . 39
3.13.2. Supporting multiple instances of a MIB Module . . . . . . . 40
3.13.3. Use of Special Optional Clauses . . . . . . . . . . . . . . 41
4. Implementing SNMP Configuration Agents . . . . . . . . . . . . . 41
4.1. Operational Consistency . . . . . . . . . . . . . . . . . . . . 41
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4.2. Handling Multiple Managers . . . . . . . . . . . . . . . . . . 43
4.3. Specifying Row Modifiability . . . . . . . . . . . . . . . . . 43
4.4. Implementing Write-only Access Objects . . . . . . . . . . . . 44
5. Designing Configuration Management Software . . . . . . . . . . . 44
5.1. Configuration Application Interactions with Managed Systems
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.1. SET Operations . . . . . . . . . . . . . . . . . . . . . . . 45
5.1.2. Configuration Transactions . . . . . . . . . . . . . . . . . 46
5.1.3. Tracking Configuration Changes . . . . . . . . . . . . . . . 47
5.1.4. Scalability of Data Retrieval . . . . . . . . . . . . . . . . 48
6. Deployment and Security Issues . . . . . . . . . . . . . . . . . 48
6.1. Basic assumptions about Configuration . . . . . . . . . . . . . 48
6.2. Secure Agent Considerations . . . . . . . . . . . . . . . . . . 49
6.3. Authentication Notifications . . . . . . . . . . . . . . . . . 49
6.4. Sensitive Information Handling . . . . . . . . . . . . . . . . 50
7. Policy-based Management . . . . . . . . . . . . . . . . . . . . . 51
7.1. What Is the Meaning of 'Policy-based'? . . . . . . . . . . . . 51
7.2. Organization of Data in an SNMP-Based Policy System . . . . . . 53
7.3. Information Related to Policy-based Configuration . . . . . . . 54
7.4. Schedule and Time Issues . . . . . . . . . . . . . . . . . . . 56
7.5. Conflict Detection, Resolution and Error Reporting . . . . . . 56
7.5.1. Changes to Configuration Outside of the Policy System . . . . 57
7.6. More about Notifications in a Policy System . . . . . . . . . . 57
7.7. Using Policy to Move Less Configuration Data . . . . . . . . . 58
8. Example MIB Module With Template-based Data . . . . . . . . . . . 58
8.1. MIB Module Definition . . . . . . . . . . . . . . . . . . . . . 61
8.2. Notes on MIB Module with Template-based Data . . . . . . . . . 73
8.3. Examples of Usage of the MIB . . . . . . . . . . . . . . . . . 73
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 77
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
11. Editors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 81
12. Intellectual Property . . . . . . . . . . . . . . . . . . . . . 81
13. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 82
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