SNMPCONF Working Group M. MacFaden
Category: Best Current Practice Riverstone Networks, Inc
J. Saperia
JDS Consulting, Inc
W. Tackabury
Gold Wire Technology, Inc
Configuring Networks and Devices With SNMP
draft-ietf-snmpconf-bcp-06.txt
September 4, 2001
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document is for a variety of readers interested in the Internet
Standard Management Framework, the Simple Network Management Protocol
(SNMP). In particular, it offers guidance in the effective use of
SNMP in configuration management. This information is relevant to
vendors that build network elements, management application develop-
ers, and those that acquire and deploy this technology in their
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networks.
1. INTRODUCTION
Data networks have grown significantly over the past decade. This growth
can be seen in terms of:
Scale - Data networks have grown in many dimensions:
they have more network elements, and the network elements are
larger. For example the number and speed of interfaces. There are
also many more interrelationships
within and between devices.
Functionality - network devices perform more functions.
More protocols and network layers are
required for the successful deployment of a wide array
of network services.
Time - changes to devices occur more often than in the
past. The need for dynamic configuration has grown faster than
the traditional set-and-forget style configuration.
Correct configuration of network elements that make up data networks is
a prerequisite to the successful deployment of services of them. The
growth in size and complexity of modern networks increases the need for
a standard configuration mechanism that is tightly integrated with per-
formance and fault management systems.
The Internet Standard Management Framework, SNMP, is used successfully
to develop configuration management systems for a broad range of devices
and networks. A standard configuration mechanism that tightly inte-
grates with performance and fault systems is needed not only to help
reduce the complexity of management, but to enable verification of con-
figuration activities that create bill-able services.
This document describes Best Current Practices that have been used when
designing effective configuration management systems using the Internet
Standard Management Framework (or, more colloquially, SNMP). It covers
many basic practices as well as more complex agent and manager design
issues that are raised in configuration management.
Significant experience has been gained over the past ten years in con-
figuring public and private data networks with SNMP. Policy Based Con-
figuration Management, is a methodology where configuration information
is distributed to potentially many network elements with the goal of
achieving consistent network behavior throughout an administrative
domain.
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This document presents lessons learned from these experiences and
applies them to both conventional and policy based configuration systems
based on SNMP.
1.1. Document Organization
This document is divided into eight sections:
Section 1 - Introduction and document organization
Section 2 - Using SNMP as a configuration mechanism
Section 3 - Designing a MIB Module
Section 4 - Implementing SNMP configuration agents
Section 5 - Designing configuration management
software
Section 6 - Deployment and Security Issues
Section 7 - Policy Management
Section 8 - Example MIB Module for configuration
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 the number of
states into which an element transitions is minimized. The goal is that
a change request is either completely executed or not at all. This is
called transactional integrity. Transactional integrity makes it possi-
ble to develop reliable configuration systems that can invoke transac-
tions and keep track of an elements overall state and work in the pres-
ence of error states.
2.1. Transactions and SNMP
Transactions can logically take place at very fine-grained levels such
as an individual object or in very large aggregations such as an entire
configuration file. For this reason, reliance on transactional integrity
only at the protocol level is insufficient.
MIB Module design plays a significant role in how well SNMP transaction
integrity will work. For example, The Structure of Management Informa-
tion Version 2 (SMIv2), RFC 2579, defines textual conventions that help
support configuration. The RowStatus object which defines a standard
object for the management of conceptual rows in a table is one example.
A RowStatus object can be used in many ways to help with transaction
control. The case where a single row activation is equivalent to a level
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of a transaction is an example. When a RowStatus object is moved to the
active state, the entire row is 'committed'.
In a multi-table scenario where the amount of 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.
A well designed SNMP-based management system addresses all the issues
described above effectively.
2.2. Practical Requirements for Transactional Control
A well designed and deployed configuration system should have the fol-
lowing 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 one small attribute may be changed. Very
granular changes should invoke behavior according to the change being
made. This is often termed "no surprises."
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 trans-
action control. For effective rollback control, the configuration trans-
actions and their successful or unsuccessful completion status must be
reported by the managed elements and stored in a repository that sup-
ports such time indexing and can record the user that made the change,
even if the change was not carried out the system recording the change.
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5) The managed system must support transactional security. This means
that depending on where and who is making the configuration request, it
may be accepted or denied based on security policy that is in effect in
the managed element.
2.3. Best Practices in Configuration
Debugging 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/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 crit-
ical systems such as the domain name system (DNS). DNS software kits
provide diagnostic tools that facilitate automatic test proce-
dures/scripts to be defined. Strict adherence to this procedure ensures
service remains intact since any failure of the test detected after the
change can be rolled back to the prior state.
Likewise, a planned configuration sequence can be aborted if pre-config-
uration test results show the state of the system as unstable. Debugging
two sets of changes in large systems is often more challenging than one.
Networks and devices under SNMP configuration readily support this
change management procedure since the SNMP provides integrated monitor-
ing, configuration and diagnostic capabilities. For example, the Ether-
Like-MIB, RFC 2665, defines diagnostic tests as follows:
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dot3TestLoopBack OBJECT-IDENTITY
STATUS current
DESCRIPTION "This test configures the MAC chip and executes
an internal loopback test of memory, data paths,
and the MAC chip logic. This loopback test can
only be executed if the interface is offline.
Once the test has completed, the MAC chip should
be reinitialized for network operation, but it
should remain offline.
If an error occurs during a test, the
appropriate test result object will be set
to indicate a failure. The two OBJECT
IDENTIFIER values dot3ErrorInitError and
dot3ErrorLoopbackError may be used to provided
more information as values for an appropriate
test result code object."
::= { dot3Tests 2 }
There are times when configuration of a given element can impact other
network 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 (conver-
gence) 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 IEEE 802.1D Spanning
Tree, RFC 1493 provides the 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.
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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 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.
The above model is not fail safe, especially when configuration errors
are masked by long latencies or when configuration errors lead to oscil-
lations in network stability. For example, consider the situation where
loading a new software version on a device leads to a small, slow memory
leak brought on by a certain traffic 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 fac-
tors beyond control of the operator such as the state of other network
elements. Even for factors within the operator's control, there is often
little verification done to prevent mis-configuration such as 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 defined above been employed, this particu-
lar situation would have been discovered without impact on the produc-
tion network.
The important point to remember from this discussion is that configura-
tion systems should be designed and implemented with verification tests
in mind.
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3. DESIGNING A MIB MODULE
Well-thought out MIB module designs are crucial for practical configura-
tion with SNMP. MIB Modules for configuration can be very effective
since they can be integrated with diagnostic, monitoring and fault
objects. MIB Modules for configuration also scale when integrated policy
objects. Policy objects represent information at an or higher level of
abstraction, than instance level ones. 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 infor-
mation an agent will expose.
MIB modules can be thought of as logical models providing one or more
aspects/views of a subsystem. The objective for all MIB modules should
be to serve one or more operational requirements such as accounting
information collection, configuration of one or more parts of a system,
or fault identification.
Include only those aspects of a subsystem that are proven to be opera-
tionally useful.
In 1993, one of most widely deployed MIB modules supporting configura-
tion was published, RFC 1493 containing 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
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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:
1. Before MIB Module design, identify goals and objectives
for the MIB module. How much of the underlying system
will be exposed depends on goals set.
2. Minimizing the total number of objects is not explicit goal,
but usability is. Be sure to consider
deployment and usability requirements.
3. During configuration, consider supporting explicit
error state, capability and capacity objects.
4. 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 generally follows a set of best prac-
tices. 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 iden-
tifiers) defined in standard MIB modules should be unique across all MIB
modules. However, MIB Modules do not define a naming scope. 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
objects such as ifTable, ifStackTable and so forth.
The names for MIB objects 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 name as in:
bldgHVACCfgDesiredTemp.
The power of this approach is more fully realized when the names that
include the fault, configuration, accounting, performance and security
[34] 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
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a per product basis. Which ever approach is used, Cfg might be included
in every object in the configuration branch. This has two operational
benefits. First for those that do look at MIB Object names through MIB
Browsers or other simple command line tools, the name can more com-
pletely convey the meaning of that object. Secondly, management applica-
tions can be pointed at specific sub trees 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 the state of them is an important con-
sideration when performing any type of configuration activity regardless
of the protocol. SNMP is no example. Here are a few areas to consider
when designing transaction support into an SNMP-based configuration sys-
tem.
3.3.1. Fate sharing with multiple tables
Fate sharing of SNMP tables should be explicitly defined where possible
using SMI macros such as AUGMENTS. If the relationship between tables
cannot be defined using SMIv2 macros, then the DESCRIPTION clause should
define what should happen when rows in related tables are added or
deleted.
Consider the relationship between the dot1dBasePortTable and the
ifTable, which have a sparse relationship. If a given ifEntry supports
802.1D bridging then there is a dot1dBasePortEntry that has a pointer to
it via dot1dBasePortIfIndex.
Now what should happen if an ifEntry that can bridge is deleted? Should
the object dot1dBasePortIfIndex simply be set to 0 or should the
dot1dBasePortEntry be deleted as well?
When two tables are related, the DESCRIPTION clauses should define the
fate sharing of entries in the respective tables.
3.3.2. Transaction Control MIB Objects
When a standard MIB module is defined that includes configuration opera-
tions, consider providing transaction control objects in places where
they can help reduce churn in the underlying systems being configured.
Here are some examples:
Control objects that are the 'write' or 'commit' objects.
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Such objects will cause all pending transactions (change MIB object val-
ues as a result of SET operations) to be committed to a permanent repos-
itory or operational memory as defined by the semantics of the MIB
objects.
Control objects at different levels of configuration granularity.
One of the decisions for a MIB module designer is what levels of granu-
larity that make sense. 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 soft-
ware design. RowStatus also has important relevance as a general trans-
action control object.
3.3.3. Usage of Row notReady Status
It is useful when configuring new rows to use the notReady status to
indicate row activation cannot proceed.
When designing read-create objects in a table containing a RowStatus
object, a MIB module designer should first consider the default state of
each object in the table when a row is created via one simple createAnd-
Wait(5) PDU. If no default state is applicable but the object must be
set to some value, the DESCRIPTION clause should specify this object as
mandatory. In addition, an SNMP get of such an object then should return
a noSuchInstance error if the object has not yet been set. A read of the
RowStatus columnar object should return notReady(3) until all such
mandatory and non-defaultable objects have been set with acceptable val-
ues.
Should a given implementation vary from a standard MIB module 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. Not
implementing the above may result in a management application being mis-
led that a transition to active(1) state will succeed without further
action by only polling the RowStatus object and receiving the notInSer-
vice(2) value from an agent.
3.3.4. Summary Objects and State Tracking
Prior to making a configuration change, management software generally
will synchronize with agents and backup existing configuration.
The three factors that influence this process are:
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1. Amount of configuration data to transfer
2. Frequency of change to the data
3. Accessibility of the data
To make this process simple and efficient, consider using the following
techniques in a MIB module.
1. Provide an object that reports the number of rows in a table
2. Provide an object that flags when data in the table
was last modified.
3. Send a notification (inform) to deliver configuration change.
By providing an object containing the number of rows in a table, manage-
ment applications can decide how best to retrieve a given table's data
and may choose different access strategies depending on table size.
Once a table has been synchronized, keeping it synchronized requires an
object to monitor table state. 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 RFC2737 provides the following object:
entLastChangeTime OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime at the time a conceptual row is
created, modified, or deleted in any of these tables:
- entPhysicalTable
- entLogicalTable
- entLPMappingTable
- entAliasMappingTable
- entPhysicalContainsTable"
::= { entityGeneral 1 }
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This convention is not formalized, and there tend to be small differ-
ences in what a table's LastChanged object reflects. IF-MIB, RFC2863,
[31] 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 performs a row modification via an SNMP SET on ifAdmin-
Status, the value of ifTableLastChange will not be updated. Be specific
in what causes your object to update so caching will work.
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 rela-
tively low or polling a cache object is not desired, consider defining a
notification that will report all configuration change details.
Use an inform instead of a trap notification PDU so that changes are
reliably communicated.
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 reason-
ably. Consider providing a global LastChange type object to represent
overall configuration in a given agent implementation.
3.3.5. Advanced Synchronization Considerations
For very large tables or for tables whose data changes frequently, a row
count and LastChange style object as described in the previous section
for caching a whole table's contents in a management application may not
work.
There are three design choices to consider:
1) Design multiple indices to partition
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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
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.
Most tables in standard MIB modules follow one of two indexing models:
associative indexing or positional/array-like indexing that starts from
one (1) or greater value.
When tables get to a very large number of rows, using an associative
indexing scheme offers a useful ability to efficiently retrieve only the
rows of interest.
For example, if an SNMP entity exposes a copy of the default-free Inter-
net routing table as defined in the ipCidrRouteTable, it will presently
contain around 100,000 rows.
Associative indexing is used in ipCidrRouteTable allows one to retrieve
say 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 index-
ing 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 returned.
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
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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 Page 41 section on 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 [31] and the ifInvS-
tackTable RFC 2864 [32]. They are logical equivalents with the order of
the auxiliary (index) objects simply reversed.
ifStackEntry OBJECT-TYPE
SYNTAX IfStackEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information on a particular relationship between
two sub-layers, specifying that one sub-layer runs
on 'top' of the other sub-layer. Each sub-layer
corresponds to a conceptual row in the ifTable."
INDEX { ifStackHigherLayer, ifStackLowerLayer }
::= { ifStackTable 1 }
ifInvStackEntry OBJECT-TYPE
SYNTAX IfInvStackEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Information on a particular relationship between two
sub-layers, specifying that one sub-layer runs underneath
the other sub-layer. Each sub-layer corresponds to a
conceptual row in the ifTable."
INDEX { ifStackLowerLayer, ifStackHigherLayer }
::= { ifInvStackTable 1 }
Second, table designs that can factor data into multiple tables with
well-defined relationships can help reduce overall data transfer
requirements. The RMON-MIB, RFC 2819, demonstrates a very useful tech-
nique 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
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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
use for command generators to encode the ifIndex into this simple inte-
ger to provide associative lookup capability.
The RMON hostTable and hostTimeTable represent dependent tables that
contains the results indexed by the hostControlTable entry.
The hostTable table is further indexed by the MAC address which provides
the ability to reasonably search a collection such as the Organization-
ally Unique Identifier (OUI) which is 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 hostControlLastDelete-
Time.
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 }
2) Time Based Indexing
The TimeFilter as defined in RFC 2021 and used in RMON2-MIB and Q-
BRIDGE-MIB (RFC2674) provide 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 a standard getNext or getBulk data retrieval.
3) Divide the Work
If the amount of data to transfer is larger than current SNMP design
restrictions permit as in the case of OCTET STRINGS (64k less overhead
of IP/UDP header plus SNMP header plus varbind list plus varbind encod-
ing), consider delivery of the data via an alternate method such as FTP
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and use a MIB module to control that data delivery process. In many
cases this problem can be avoided via effective MIB design.
There exist many enterprise MIB modules that provide control over the
TFTP or FTP protocol. Often the SNMP part defines what to send where and
then performs a set of an object to make the operation occur. This style
can be found in the REMOPS-MIB module as well.
The SNMPCONF work directly address the amount of configuration and other
data can be carried via SNMP. Other work is currently underway to
improve efficiency for all SNMP operations. This means much more infor-
mation will be conveyed with less overhead.
3.3.6. Conceptual Table Modification Practices
The RowStatus textual convention specifies that when used, in a concep-
tual row, a description must define what can be modified. In fact, it
is often wrongly assumed in implementations that objects:
1) either must all be presently set or none need be to make
a conceptual RowStatus object transition to active(1)
2) that objects in a conceptual row cannot be modified once
a RowStatus object is active(1).
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 }
3.4. Index Design Issues
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.
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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 iden-
tifiers are unsigned long values, a question that arises is why not
index from zero (0) instead of one(1)?
RFC 2578, Section 7.7, page 28 states the following: Instances identi-
fied 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 should be avoided, except in special cases.
Indexing tables starting from one(1) simplifies implementation as well
as allows for protocol expansion as to what .0 means for non-scalar
objects. Agent implementations that contain tables that have simple
integer indexes starting from zero(0) require the agent to disambiguate
a varbind's instance information in more detail than it otherwise might
have. For example, a getNext ifInOctets from getNext ifInOctets.0
becomes necessary.
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 IP-MIB. This
limits the usefulness of the MIB module to IPv4. To avoid such limita-
tions, use the INET-ADDRESS-MIB, which provides a generic way to repre-
sent addresses for Internet Protocols.
3.5. Conflicting Controls
MIB module designers should avoid specifying read-write objects that
overlap in function partly or completely.
Consider the following situation where two read-write objects partially
overlap when a dot1dBasePortEntry has a corresponding ifEntry.
The BRIDGE-MIB defines the following managed object:
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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.
When this situation has been corrected, one of the two objects should
have the STATUS set to deprecated.
3.6. Textual Convention Usage
Textual conventions should be used whenever possible to create a consis-
tent semantic for an oft-recurring datatype.
Often, MIB modules define a binary state object such as enable/disable
or on/off. Best current practice is to use existing Textual Conventions
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and define the read-write object in terms of a TruthValue from SNMPv2-TC
[RFC2579]. For example, the Q-BRIDGE-MIB [RFC2674] 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 mod-
ule to facilitate sharing of such objects. For example, all ATM MIB mod-
ules draw on the ATM-TC-MIB to define common definitions.
To simplify management, it is recommended that existing SNMPv2-TC based
definitions be used when possible. For example, consider the following
object definition:
acmePatioLights OBJECT-TYPE
SYNTAX INTEGER {
on(1),
off(2),
}
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"Current status of outdoor lighting."
::= { acmeOutDoorElectricalEntry 3 }
This could be defined as follows using existing SNMPv2-TC TruthValue.
AcmePatioLightsEnabledOn OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"Current status of outdoor lighting."
::= { acmeOutDoorElectricalEntry 3 }
3.7. Persistent Configuration
Many SNMP agents presently implement simple persistence models. SNMP
set requests against MIB objects with MAX-ACCESS read-write are typi-
cally written automatically to a persistent store. In other cases,
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enterprise MIB objects are required in order to get standard configura-
tion 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] defines explic-
itly Dot1qVlanStaticEntry data persistence as follows:
dot1qVlanStaticTable OBJECT-TYPE
SYNTAX SEQUENCE OF Dot1qVlanStaticEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A table containing static configuration information for
each VLAN configured into the device by (local or
network) management. All entries are permanent and will
be restored after the device is reset."
::= { dot1qVlan 3 }
Best current practice is a dual persistence model where one can make
changes to run time configuration as well as to a non-volatile configu-
ration read at device initialization. The DISMAN-SCHEDULE-MIB module
provides an example of this practice.
3.8. Configuration Sets and Activation
An essential notion for configuration of network elements is awareness
of the difference between the set of one or more configuration objects
from the activation of those configuration changes in the actual subsys-
tem.
The document "Requirements for IP Version 4 Routers" [20], section 1.3.4
states:
A vendor needs to provide adequate documentation on all
configuration parameters, their limits and effects.
Any complex configuration should have a master on/off switch as well as
strategically placed on/off switches to control the sectional employment
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of configuration data. These controls play a pivotal role during the
configuration process as well as during subsequent diagnostics. Gener-
ally a series of set operations should not cause an agent to act on each
object causing convergence/stability possibly to be lost on each and
every set. Ideally a series of Set PDUs would install the configuration
and a final set series of PDUs would activate the changes.
During diagnostic situations, certain on/off switches can be set to
localize the perceived error instead of having to remove the configura-
tion.
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 inter-
face; 'disabled' disables it on all inter-
faces."
::= { ospfGeneralGroup 2 }
Elsewhere in the OSPF MIB, the semantics of setting ospfAdminStat to
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 read status object
on the entry 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 }
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, page 15 which 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 schedAd-
minStatus to disabled(2) does not cause the row to be aged out/removed
from the table.
Lastly, a reasonable agent implementation must consider how many rows
will be allowed to be created in the notReady/notInService state such
that resources are not exhausted by an errant application.
3.9. SET operation Latency
Many standards track and enterprise MIB modules that contain read-write
objects assume that an agent can complete a set operation as quickly as
an agent can send back the status of the set operation to the
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application.
Consider the following object that both reports the current state as
well as allows a SET operation to change to a possibly new state.
wheelRotationState OBJECT-TYPE
SYNTAX INTEGER { unknown(0),
idle(1),
spinClockwise(2),
spinCounterClockwise(3)
}
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"The current state of a wheel."
::= { XXX 2 }
With the object defined, the following example represents
one possible transaction.
Time Command Generator --------> <--- Command Responder
----- ----------------- -----------------
|
A GetPDU(wheelRotationState.1.1)
|
| ResponsePDU(error-index 0,
| error-code 0)
|
B wheelRotationState.1.1 == spinClockwise(2)
|
C SetPDU(wheelRotationState.1.1 =
| spinCounterClockwise(3)
|
| ResponsePDU(error-index 0,
| error-code 0)
|
D wheelRotationState.1.1 == spinCounterClockwise(3)
|
E GetPDU(wheelRotationState.1.1)
|
F ResponsePDU(error-index 0,
| error-code 0)
|
V wheelRotationState.1.1 == spinClockwise(2)
....some time, perhaps seconds, later....
|
G GetPDU(wheelRotationState.1.1)
|
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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 spinCounter-
Clockwise(3). In reality, the wheel is slowing down in order to come to
the idle state then begin spinning counter clockwise.
One common practice is to separate out desired (settable) state from
current state. The objects ifAdminStatus and ifOperStatus from RFC 2863
provide such an example of the separation of objects into desired and
current state.
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 commands in the system can often be longer than the SNMP response
timeout.
3.10. Notifications and Error Reporting
Notifications can be an important part of an effective SNMP-based man-
agement system. They can often be over-used. This section offers some
guidance for effective notification generation. 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
must be designed in as shown in the section on Application Error Report-
ing.
3.10.1. Designing Notifications
Notifications can play an important role in configuration. With SNMPv2c
and SNMPv3, informs allow the design of reliable, event-driven synchro-
nization models that can aid configuration.
When designing new notifications, consider how to limit the number of
notifications (traps or informs) that can be sent in response to an
event. RMON I[30] defines a generic trap capability in the alarmTable
and provides one of many ways to prevent the quantity of notifications
from overwhelming a management system.
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Another way to limit the volume of a particular notification is to
define 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 }
3.10.2. 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 fil-
tering of notifications.
In the past, agents did not have a standard means to configure a notifi-
cation generator. With the availability of the SNMP-NOTIFICATION-MIB
Module in RFC 2573, it is strongly recommended that the filtering func-
tions of this MIB module be used. If not, then a justification of why it
is not necessary or suitable should be made clear in the MIB module in
the description clause of any replacement control objects.
3.10.3. Application Error Reporting
MIB Module designers should not rely on the SNMP protocol error report-
ing 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 is generally not enough information as to
why a agent rejected a set request. The burden on the configuration
application to determine if there is a resource issue, a security issue,
or application error issue is large.
Ideally when a "badValue" error occurs for a given set request, an
application can query the agent for more detail on the error to report
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on. A badValue does not necessarily mean the command generator sent bad
data. An agent could be at fault. Additional detailed diagnostic infor-
mation may aid in debugging the integrated system.
As an example of tracking errors, consider the hrPrinterTable from the
HOST-RESOURCES-MIB, RFC 2790:
hrPrinterDetectedErrorState OBJECT-TYPE
SYNTAX OCTET STRING
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"This object represents any error conditions detected
by the printer. The error conditions are encoded as
bits in an octet string, with the following
definitions:
Condition Bit #
lowPaper 0
noPaper 1
lowToner 2
noToner 3
doorOpen 4
jammed 5
offline 6
serviceRequested 7
inputTrayMissing 8
outputTrayMissing 9
markerSupplyMissing 10
outputNearFull 11
outputFull 12
inputTrayEmpty 13
overduePreventMaint 14
Bits are numbered starting with the most significant
bit of the first byte being bit 0, the least
significant bit of the first byte being bit 7, the
most significant bit of the second byte being bit 8,
and so on. A one bit encodes that the condition was
detected, while a zero bit encodes that the condition
was not detected.
This object is useful for alerting an operator to
specific warning or error conditions that may occur,
especially those requiring human intervention."
::= { hrPrinterEntry 2 }
Notifications can also be used to signal configuration failures with
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detailed failure information objects.
3.11. Other MIB Module Design Issues
3.11.1. Octet String Aggregations
The OCTET STRING syntax is 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.
For example:
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 }
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 defini-
tion of the contents of the OCTET STRING is needed.
When using OCTET STRINGS, avoid platform dependent data formats. Also
avoid using OCTET STRINGS where a more precise SMI syntax such as Snm-
pAdminString 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) is defined as follows:
PortList ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Each octet within this value specifies a set of eight
ports, with the first octet specifying ports 1 through
8, the second octet specifying ports 9 through 16, etc.
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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 limita-
tions. Such complex instance information is difficult to reference out-
side of the object or use as an index a table.
Providing an SNMP Table to represent aggregate data does avoids the lim-
itations of encoding data into OCTET STRINGS and is thus the better gen-
eral practice.
3.11.2. 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.
There exist MIB modules that assume a one to many relationship, such as
the SYSAPPL-MIB [RFC2287]. 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 implemen-
tations that might support multiple instances of OSPF, IP Stack or logi-
cal bridges.
In such cases, the ENTITY-MIB [RFC2737] can provide a means for sup-
porting the one-to-many relationship through naming scopes using the
entLogicalTable. There are some drawbacks to this approach.
1) One cannot issue PDU request that span naming scopes
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) It creates a dependency on the Entity MIB for
an application to be able to access multiple
instance information.
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3.11.3. Use of special optional clauses
When defining integer based objects for both read-write and read-only
semantics, using the UNITS clause is recommended in addition to specifi-
cation in the DESCRIPTION clause of any particular details.
The REFERENCE clause is also recommended to help an implementer track
down related information on a given object.
4. IMPLEMENTING SNMP CONFIGURATION AGENTS
4.1. Operational Consistency
Successful deployment of SNMP configuration systems depends on under-
standing the roles of MIB module design and agent design.
Both components must have been designed 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 repeatedly with the effect of it
resulting in an activation only once.
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 stateful way, the set operation may cause
Set2 operation to fail owing to interactions with the operation from
Set1. If the implementation of the agent on the second request is not
aware of such a situation, the agent may behave poorly by actually re-
implementing the set request instead of doing nothing.
The example above shows that all the software that runs on a managed
element and in managed applications should be designed and thought out
together where 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 the SNMP agents in the managed
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devices and the management systems themselves. The MIB document is a
contract between these two entities that defines expected behaviors -
it is a type of API.
2. 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 approaches that minimize
the management impact on those operational elements.
4.2. Handling Multiple Managers
Devices are often modified by multiple management entities and with dif-
ferent 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 set by different entities.
It should also distinguish configuration defined locally such as a
default or locally specified configuration made through an alternate
management interface like command line interface. When a change has been
made to the system via SNMP, CLI, or other method, a managed element
should send an inform to the manager(s) to which it has been assigned.
The managers 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 synchro-
nized with each other.
4.3. Designing MIB Modules for Multiple Managers
When designing a MIB Module for configuration, consider the following to
provide support for multiple managers.
The first consideration 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 [RFC2579] defines TestAndIncr,
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often called a spinlock, which is used to avoid race conditions.
A MIB module designer may explicitly define a synchronization object of
syntax TestAndIncr or may choose to rely on snmpSetSerialNo (a global
spinlock object) as defined in SNMPv2-MIB.
snmpSetSerialNo OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"An advisory lock used to allow several cooperating 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 example can be found in the SNMP-TARGET-MIB snmpTargetSpinLock.
Second, an agent should be able to report configuration as set by dif-
ferent entities as well as distinguish configuration defined locally
such as a default or locally specified configuration made through an
alternate management interfaces like a command line interface. The Own-
erString textual convention from RMON-MIB RFC 2819 [30] has been used
successfully for this purpose. More recently, RFC 2271 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 owner-
ship 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 useful in applications.
Third, consider whether there is a need for multiple managers to config-
ure 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 per manager. RFC 2591 section 6 presents this
technique in detail.
4.4. 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 mak-
ing additional changes to a row. RMON I MIB control table objects spell
out explicitly what managed objects in a row can and cannot be changed
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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 observed.
4.5. Order of varbinds in a SET PDU
When a given SET PDU contains multiple varbinds, agents conforming to
the SNMP should be able to process the varbinds in any given order.
In practice, it is often preferable that management applications send
the varbinds in the order they are defined in the MIB module.
4.6. 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 on a read of
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.
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
with SNMP. Functions all configuration management software should pro-
vide, regardless of the method used to convey configuration information
to the managed systems are: backup, fail-over, and restoration. A man-
agement 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
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polling of configuration.
These three capabilities are of course important for other types of man-
agement that are not the focus of this BCP.
5.1. Designing Configuration Management Software
This section focuses on general issues related to the development of
SNMP based applications (command generators) that configure one or more
network elements. Special consideration is given to what has come to be
known as policy-based management with SNMP. Effective software for the
configuration of one or many network elements requires thoughtful design
before starting implementation. This is true regardless of the technol-
ogy used to represent and transfer the configuration information. Two
general principles in management application design are:
- Ensure minimum movement of data
- Transaction control should be synchronized with remote system
5.2. Protocol Operations
There are three basic areas to evaluate relevant to SNMP protocol opera-
tions and configuration:
o Set and configuration activation operations
o Notifications from the device
o Data retrieval and collection
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 con-
figuration operation from the actual data. One way that some applica-
tions 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
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devices it is intended 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.
5.3. 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 to the managed device as possible. 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 man-
agement software to the smallest reasonable number. One approach to this
is to verify the largest number of variable bindings that can fit 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 SET variable bindings within a PDU has beneficial implica-
tions in the area of management application transaction initiation, as
well, as we will discuss in the following section.
Yet there are agents that may have implementation limitations on the
number and order of varbinds it can handle in a single SET PDU. In this
case, sending fewer varbinds will be necessary.
5.4. Configuration Transactions
There are several types of configuration transactions that can be sup-
ported by SNMP-based configuration applications. They include transac-
tions on a scalar object, a single table, 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 within the scope of the con-
figuration operation.
To make use of any kind of transaction semantics effectively, SNMP man-
agement 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 pro-
vide the facility to support transactions across multiple devices. This
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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 man-
agement application level transactions. One was mentioned in the pre-
ceding section. 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 or set of PDUs if
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 to set to
active only after this verification. This is the only effective means
of affording an opportunity for per-row rollback, particularly when the
configuration change is across table row instances on multiple managed
devices.
5.5. Notifications
As described throughout the section on Agent Software Development,
agents should provide the capability for notifications to be sent to
their configured management systems whenever a configuration operation
is attempted or completed. The management application MUST be prepared
to accept these notifications so that it knows the current configured
state of the devices it has been deployed to control. Some configura-
tion management applications may consume data from the managed devices
that reflect configuration, operational and utilization state informa-
tion. The GetBulkRequest-PDU is useful here whenever supported by the
managed device. For the purposes of backward compatibility, the manage-
ment station should also support and make use of the GetNextRequest-PDU
in these cases.
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.
5.6. Scale of the Management Software
Efficient data retrieval described above is only part of the dimension
of scale that application developers should consider when developing
configuration applications. Management applications should provide for
distributed processing of the configuration operations. This also
extends to other management functions not the focus of this document.
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This capability can also be used to provide resilience in the case of
network failures as well. An SNMP-based configuration management system
might be deployed in a distributed fashion where three systems in dif-
ferent locations keep each other synchronized. This synchronization can
be accomplished without additional polling of network devices through a
variety of techniques between each of the three managers. In the case of
a failure, a 'backup' system can take over the configuration responsi-
bilities of 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
Network devices are configured using many mechanisms, however two meth-
ods remain the most common: SNMP and Command Line Interface (CLI).
Effective use of these mechanisms involves an operational methodology
for deploying changes to networks in a cautious and incremental manner
with well-documented procedures. The collective intent of these proce-
dures is to guarantee that a configuration change to the network has the
intended effect on the affected network elements. Here is one such pro-
cedural model in detail:
Network Scope Input Procedural Step Output
------------------ ----- --------------- ------
Lab isolated from (1) Stage configuration Verify reliability
main network change with test of config change
device set and
(SNMP action: interoperability
determine distribution with prior system
of CLI and SNMP versions and
set actions for backwards
configuration change) compatibility
with prior
configuration
(2) Plan noncritical/ Fall back strategy
off hour window in case of change
(SNMP action: none) failure
Segmented edge of (1),(2) (3) Apply change to CLI Verified device
main network of network segment accept of
network elements configuration
(SNMP action: none) change
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(1),(3) (4) Observation of Verified device
devices/networks over and network
time after change stability, proper
(SNMP action: effect of changes
monitor status of
SNMP read-only objects
which are companions
to SNMP set objects
for configuration
change)
(4) (5) Save changes to Verify integrity
persistent storage of changes after
of devices device restart
(SNMP action: SET
on controls to move
running config to
restart/persistent)
Expanded network (1),(2),(5) (6) Deploy changes, Verification of
segments, iterative using CLI and SNMP. safety from any
to complete network Keep prior unanticipated
configuration on some SNMP defaulting
devices to allow behavior in mixed
fallback in case environment
of failure
(SNMP action: SET
on SNMP objects
determined in (1))
(4),(6) (7) Continue to Checkpointed
observe devices and verification
network for of anticipated
behavioral/service behavior
anomalies
(SNMP action:
same as (4) across
devices of expanded
network)
(7) (8) Expand deployment Checkpointed
and iteration to (6) verification
as called for of anticipated
(SNMP action: behavior
same as (6) across
expanded network)
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Network revision (8) Update archived Change deployment
control system configurations, complete.
change log.
(SNMP action: none)
Procedures such as those above bring about a form of operational trans-
actionality, which works alongside (and employs) SNMP transactionality.
The key goals throughout are
o Transactionality of configuration change deployment
o Persistence of configuration change which can be verified for
resulting stability, convergence, and realization of intended
effect of change.
Considerations for bringing about this verification are discussed
in the following section.
6.1. Basic assumptions about Configuration
The following basic assumptions are made about real world configuration
models.
One, 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.
Two, systems undergoing configuration changes must be able to cope with
unexpected loss of communication at any time.
Network elements in conjunction with the configuration mechanism must
take appropriate measures to leave the configuration in a consis-
tent/recognizable state by either rolling back to a previously valid
state or changing to a well-defined or default state.
Three, configuration exists on a scale from relatively unchanging to a
high volume, high rate of configuration change. The former is often
referred to as "set and forget" and the later "near real-time change
control." 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
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where to bootstrap devices that do not have secondary management inter-
faces. Defaults lead to security issues, that have been recognized and
exploited. When using SNMPv1, supporting read-only community strings is
a common practice.
SNMPv3 provides authentication and privacy protection and is recommended
for all devices that support SNMP-based configuration.
6.3. Authentication Traps
The default state of RFC 1215 [4] Authentication traps should be off.
In the "Notification" section of this document's discussion on MIB agent
design, issues and recommendations on throttling traps were raised.
Where notifications are sent in SNMPv1 trap PDUs, unsolicited packets to
a device can causes one ore 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.
6.4. Sensitive Information Handling
Some MIB modules contain objects that may contain data for keys, pass-
words and other such sensitive information and hence must be protected
from unauthorized access.
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 and their defining MIB RFC Security
Considerations section 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.
As noted in the section on Designing Configuration Objects, when writing
standards track MIB modules, one must implement those objects that are
part of the various standards-track specifications. Confidentiality is
not a must implement portion of the SNMPv3 management framework [11].
Prior to SNMPv3, providing customized views of MIB module data was dif-
ficult. This led to objects being defined such as the following.
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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.
Organize your objects into groups such that VACM views can be defined to
properly scope what tables are visible to a given user and view. See the
prior section "Naming MIB modules and Managed Objects "
7. POLICY BASED MANAGEMENT
A common practice used to move large amounts of data that some vendors
employ 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 effi-
ciently performed with several of the techniques described earlier in
this document. This policy section shows how even greater efficiency can
be achieved using the new SNMPCONF technology. This section gives back-
ground and defines terms that are relevant to this field and describes
some deployment approaches.
7.1. 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 spe-
cific service levels.
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The growth in size and complexity of configuration information has sig-
nificant implications for its organization as well as its efficient
transfer to the management agent. As an example, an agent that imple-
ments the Bridge MIB [22] could be used to represent a large VLAN with
some 65,000 port entries. To configure such a VLAN, it 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 sys-
tem setting the default values to the selected ports.
To date, that configuration has been accomplished to now is with file
transfer, by setting individual MIB objects, or with many CLI commands.
In each of these approaches the details for each instance are 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 con-
figuration operations explode as the number of instances such as inter-
faces grows. This per-instance approach can work for a few machines
configured by experts, but there is a need for a more scalable solution.
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) is 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, as a common example, have a
starting hierarchy point to apply defaults at the breadth of a network
service. It could then allow refinement of these defaults across the
domains of network control to realize the network service, the specific
network protocol and algorithmic mechanisms encompassed within those
domains, and so forth.
Ultimately, such a layering needs to extend itself to attribute granu-
larity at the lowest level, that of the managed element instance itself.
At this level, the notion of "defaulting" the setting of a configuration
controls are either applied at the lowest level (where "level" is at
some point in the hierarchy described above), or explicitly specified
for the instance.
An example of this kind of layering is implicit in the principle of
operations of a SNMPConf Policy Based Management MIB [35] implementa-
tion. Appendix A has an example of the interaction between such a
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Policy Agent and the rest of the system for OSPF.
7.2. 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
a useful input to the decision-making process about what type of config-
uration operations to do.
For this discussion it is important to make distinctions between distri-
bution of policy to a system, activation of a policy in a system, and
changes/failures that take place during the time it is expected to be
active. For example, if an interface is down that is included in a pol-
icy that is distributed, there may not be an error since the policy may
not be scheduled for activation until a later time. On the other hand if
a policy is distributed and applied to an interface that should be oper-
ational and it is not, clearly this is a problem. With this as back-
ground, 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 config-
ured.
Care should be taken to determine if the sub-components to be con-
figured 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 dis-
tributed 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
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others. this information can be obtained from the capabilities
table of the Policy MIB module.
o The capacity of the devices to perform the desired work.
Capability is an ability to perform the desired work while a capac-
ity is a measure of how much of that capability the system has. The
policy configuration application should, wherever possible, evalu-
ate the capacity of the network element to perform the work identi-
fied by the policy. In some systems it will not be possible to
directly obtain the capacity of the managed elements to perform the
desired work, even though it may be possible to monitor the amount
of work the element performs. In these cases, the management appli-
cation may benefit from pre-configured information about the capac-
ity of different network elements so that evaluations of the
resources available can be made before distributing new policies.
Utilization refers to how much capacity for a particular capability
has been consumed. For devices that have been under policy configu-
ration 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.
7.3. 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 for
more information. Some deployed systems do not store complex notions of
local time and thus may not be able to properly process policy direc-
tives that contain time zone relevant data. For this reason, policy man-
agement applications should have the ability to ascertain the time keep-
ing abilities of the managed system and make adjustments to the policy
for those systems that are time-zone challenged.
7.4. Conflict Detection, Resolution and Error Reporting
Policies sent to a device may contain conflicting instructions. Detec-
tion of such commands can occur at the device or management level and
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may be resolved using any number of mechanisms (examples are, last con-
figuration 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 and exception and passing that information on to
software that can do something meaningful with it.
At the instance level, conflict detection has been performed in a lim-
ited 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 evaluated are for 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.
For software that realizes MIB module objects at the protocol/data han-
dling mechanism and/or implementation thereof, failures should be
reported such that the specific policy that has been impacted can be
related with the specific element that failed. Beyond this basic report-
ing which is does not perform any policy conflict detection, there need
be no requirements. See the Policy MIB Module document [35] for addi-
tional information on and example handling of policy precedence and con-
flict detection.
Changes to configuration outside of the policy system:
A goal of SNMP-based policy management is to coexist with other forms of
management software that has historically been instance based manage-
ment. The best example is command line interface. Here are some guide-
lines for handling these changes.
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 notifi-
cation should include the policy that was changed, the instance of the
element that was changed and the object and value that it was changed
to.
An element under the control of policy that has been changed remains a
member of the policy group until the attributes in the Role table that
caused it to match the policy in the first place are modified. An ele-
ment that has been modified by a an out of policy mechanism, while
remaining a member of the policy does not get overridden by a policy
until its value is made the same as the extant policy with the highest
precedence for this element, and by implication then returned to policy
control. A notification should be sent when this action is taken.
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7.5. 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 add to 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 it 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.
7.6. 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 inter-
faces 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 eliminat-
ing 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 can also be carried (see the Policy MIB Module).
To extend the example above, let's say 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 pmPolicyTable, and entire policy
that potentially controls hundreds of rows of information can be
retrieved in a single GET request.
8. Example MIB Module For Policy-Based Management
In this section, we define a MIB Module that controls the heating and
air conditioning for a large building. It contains both configuration
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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 "mechanism" level (as referenced
above) are also included.
These tables in combination with the policy MIB Module will allow opera-
tors to configure and monitor many rooms all at once, change the config-
uration parameters based on time of day, and make a number of other
sophisticated decisions based on policy.
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 pointer to a table that 'glues' configuration
information defaults with descriptive information:
bldgHVACCfgTemplatePtr
A set of objects that show status and units of work
(bldgHVACCoolOrHeatMins) and standard SnmpAdminString and RowStatus
objects:
bldgHVACFanSpeed
bldgHVACCurrentTemp
bldgHVACCoolOrHeatMins
bldgHVACOwner
bldgHVACStatus
Step 2: A configuration description table. The purpose of this table is
to provide a unique string identifier for 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, could be a brief
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description of the policy such as: "provides high quality packet
forwarding for Voice over IP customers". The details are:
bldgHVACCfgPolicyIndex
bldgHVACCfgPolicyDescription
bldgHVACCfgPolicyOwner
bldgHVACCfgPolicyStatus
Standard owner and status objects may also be helpful and so are
included here.
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 policy and
instance which can save some additional data movement. Every
designer will have to evaluate the tradeoffs of number of
objects and data movement efficiency just as with other MIB
modules.
Step 3: This is the bldgHVACCfgTemplateTable. It contains the specific
configuration parameters that are pointed to by the
bldgHVACConfigPtr object. Note that many 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 in Policy 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 of those necessary for
configuration and connection to the other tables as well as the
usual SnmpAdminString and RowStatus objects:
A simple index to the table:
bldgHVACCfgTemplateIndex
The configuration objects:
bldgHVACCfgTemplateDesiredTemp
bldgHVACCfgTemplateCoolOrHeat
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Administrative objects for SnmpAdminString and RowStatus:
bldgHVACCfgTemplateDescrPtr
bldgHVACCfgTemplateOwner
bldgHVACCfgTemplateStorage
bldgHVACCfgTemplateStatus
BLDG-HVAC-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, Counter32,
Gauge32, OBJECT-TYPE, Integer32, Unsigned32, experimental
FROM SNMPv2-SMI
MODULE-COMPLIANCE, OBJECT-GROUP
FROM SNMPv2-CONF
TEXTUAL-CONVENTION,
RowStatus, RowPointer, StorageType, DisplayString
FROM SNMPv2-TC
SnmpAdminString
FROM SNMP-FRAMEWORK-MIB;
bldgHVACMIB MODULE-IDENTITY
LAST-UPDATED "200007090000Z"
ORGANIZATION "SNMPCONF working group"
CONTACT-INFO
" Someone
Some place
phone number
email address, web site"
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 which
eliminates the need to send per instance configuration
commands to the system. Note that before this MIB Module
will successfully compile an assignment from IANA would
be needed to replace the XXX below."
REVISION "200105101230Z"
DESCRIPTION
"Initial version of BLDG-HVAC-MIB."
::= { experimental 9999 }
bldgHVACObjects OBJECT IDENTIFIER ::= { bldgHVACMIB 1 }
bldgConformance OBJECT IDENTIFIER ::= { bldgHVACMIB 2 }
--
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-- Textual Conventions
--
HvacOperation ::= TEXTUAL-CONVENTION
STATUS current
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 simple table is an example of objects that are at the
instance level of specificity. That is, they are indexed by a
specific floor and office number. These objects include
general counter and status information. This table includes
the bldgHVACCfgTemplatePtr that points to a row in the
bldgHVACCfgTemplateTable. If this value is not null, then the
instance in the row that has a value for this object is
being configured using templates and this is where the
configuration information for the HVAC operation will be
installed."
::= { bldgHVACObjects 1 }
bldgHVACEntry OBJECT-TYPE
SYNTAX BldgHVACEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Each row represents a particular office, a pointer to the
desired HVAC configuration settings and usage information for
that location since this setting was initialized."
INDEX { bldgHVACFloor, bldgHVACOffice }
::= { bldgHVACTable 1 }
BldgHVACEntry ::= SEQUENCE {
bldgHVACFloor Integer32,
bldgHVACOffice Integer32,
bldgHVACCfgTemplatePtr RowPointer,
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bldgHVACFanSpeed Gauge32,
bldgHVACCurrentTemp Gauge32,
bldgHVACCoolOrHeatMins Counter32,
bldgHVACOwner SnmpAdminString,
bldgHVACStatus RowStatus
}
bldgHVACFloor OBJECT-TYPE
SYNTAX Integer32 (1..2147483647)
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"This portion of the index indicates the floor of the
building."
::= { bldgHVACEntry 1 }
bldgHVACOffice OBJECT-TYPE
SYNTAX Integer32 (1..2147483647)
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"This second component of the index specifies the
office number."
::= { bldgHVACEntry 2 }
bldgHVACCfgTemplatePtr OBJECT-TYPE
SYNTAX RowPointer
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"The pointer to an entry in the
'bldgHVACCfgTemplateTable'. The entry pointed to is a
pre-made configuration that represents the configuration
described by the bldgHVACCfgDescription object. Note that
not all configurations will be under a defined policy. As
a result, a row in this bldgHVACTable may point to an
entry in the bldgHVACCfgTemplateTable that does not in
turn have a pointer to an entry in the
bldgHVACCfgPolicyTable. The benefit of this approach is
that all configuration information is available in one
table whether all elements in the system are under policy
control or not."
::= { bldgHVACEntry 3 }
bldgHVACFanSpeed OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
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DESCRIPTION
"Shows the revolutions per minute of the fan. Fan speed
will vary based on the difference between
bldgHVACDesiredTemp and bldgHVACurrentTemp."
::= { bldgHVACEntry 4 }
bldgHVACCurrentTemp OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The current measured temperature in the office."
::= { bldgHVACEntry 5 }
bldgHVACCoolOrHeatMins OBJECT-TYPE
SYNTAX Counter32
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
vice versa, then the counter would start over again. This
effect is similar to a reconfiguration of some network
interface cards. When some parameters that impact
configuration are changed, the subsystem must be
re-initialized."
::= { bldgHVACEntry 6 }
bldgHVACOwner OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The identity of the operator/system that last modified
this entry."
::= { bldgHVACEntry 7 }
bldgHVACStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this row."
::= { bldgHVACEntry 8 }
--
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-- HVAC Configuration Policy Table
--
bldgHVACCfgPolicyTable OBJECT-TYPE
SYNTAX SEQUENCE OF BldgHVACCfgPolicyEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table provides a unique string identifier for HVAC
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 policy name. The
bldgHVACCfgCfgPolicyDescription contains a brief
description of the policy such as: provides high quality
packet forwarding for Voice over IP customers. The
bldgHVACCfgDescrPolicyPtr 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 bldgHVACCfgTemplateTable be identifiable as being
under policy control though this mechanism."
::= { bldgHVACObjects 2 }
bldgHVACCfgPolicyEntry OBJECT-TYPE
SYNTAX BldgHVACCfgPolicyEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Each row represents a particular policy and description."
INDEX { bldgHVACCfgPolicyIndex }
::= { bldgHVACCfgPolicyTable 1 }
BldgHVACCfgPolicyEntry ::= SEQUENCE {
bldgHVACCfgPolicyIndex DisplayString,
bldgHVACCfgPolicyDescription DisplayString,
bldgHVACCfgPolicyOwner SnmpAdminString,
bldgHVACCfgPolicyStatus RowStatus
}
bldgHVACCfgPolicyIndex OBJECT-TYPE
SYNTAX Integer32 (1..2147483647)
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The index to this table. This should be a unique name
within an administrative domain for a particular policy
so that all systems in a network that are under the same
policy can have the same 'handle'."
::= { bldgHVACCfgPolicyEntry 1 }
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bldgHVACCfgPolicyDescription OBJECT-TYPE
SYNTAX Integer32
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A general description of the policy. One example might be -
Controls the cooling policy for offices on higher floors
during the summer."
::= { bldgHVACCfgPolicyEntry 2 }
bldgHVACCfgPolicyOwner OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The identity of the operator/system that last modified
this entry."
::= { bldgHVACCfgPolicyEntry 3 }
bldgHVACCfgPolicyStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this row."
::= { bldgHVACCfgPolicyEntry 4 }
--
-- HVAC Configuration Template Table
--
bldgHVACCfgTemplateTable OBJECT-TYPE
SYNTAX SEQUENCE OF BldgHVACCfgTemplateEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table can be used to set policy defaults that will be
placed on the specific offices that match the
pmPolicyFilter. See the policy MIB Module for more
information."
::= { bldgHVACObjects 3 }
bldgHVACCfgTemplateEntry OBJECT-TYPE
SYNTAX BldgHVACCfgTemplateEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Each row represents a single set of policy parameters that
Various Authors [Page 54]
RFC DRAFT Expires March 2002 May 2001
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."
INDEX { bldgHVACCfgTemplateIndex }
::= { bldgHVACCfgTemplateTable 1 }
BldgHVACCfgTemplateEntry ::= SEQUENCE {
bldgHVACCfgTemplateIndex Unsigned32,
bldgHVACCfgTemplateDesiredTemp Gauge32,
bldgHVACCfgTemplateCoolOrHeat HvacOperation,
bldgHVACCfgTemplateDescrPtr RowPointer,
bldgHVACCfgTemplateOwner SnmpAdminString,
bldgHVACCfgTemplateStorage StorageType,
bldgHVACCfgTemplateStatus RowStatus
}
bldgHVACCfgTemplateIndex OBJECT-TYPE
SYNTAX Unsigned32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A unique value for each defined template in this
table. This value can be pointed to from any MIB Module
that needs access to this information. The
bldgHVACCfgTemplatePtr will point to entries in this
table."
::= { bldgHVACCfgTemplateEntry 1 }
bldgHVACCfgTemplateDesiredTemp OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-create
STATUS current
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 HvacOperation
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
Various Authors [Page 55]
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state, making the change and then reactivating the row."
::= { bldgHVACCfgTemplateEntry 3 }
bldgHVACCfgTemplateDescrPtr OBJECT-TYPE
SYNTAX RowPointer
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 4 }
bldgHVACCfgTemplateOwner OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The identity that created this row of the table."
::= { bldgHVACCfgTemplateEntry 5 }
bldgHVACCfgTemplateStorage OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
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 status of this row of the table."
::= { bldgHVACCfgTemplateEntry 7 }
--
-- Conformance Information
--
bldgCompliances OBJECT IDENTIFIER ::= { bldgConformance 1 }
bldgGroups OBJECT IDENTIFIER ::= { bldgConformance 2 }
-- Compliance Statements
Various Authors [Page 56]
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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 bldgHVACObjects
DESCRIPTION
"The bldgHVACObjects is mandatory for all systems that
support HVAC systems."
::= { bldgCompliances 1 }
bldgHVACObjectsGroup OBJECT-GROUP
OBJECTS {
bldgHVACFloor, bldgHVACOffice, bldgHVACCfgTemplatePtr,
bldgHVACFanSpeed, bldgHVACCurrentTemp,
bldgHVACCoolOrHeatMins, bldgHVACOwner, bldgHVACStatus,
bldgHVACCfgPolicyIndex, bldgHVACCfgPolicyDescription,
bldgHVACCfgPolicyOwner, bldgHVACCfgPolicyStatus,
bldgHVACCfgTemplateIndex, bldgHVACCfgTemplateDesiredTemp,
bldgHVACCfgTemplateCoolOrHeat,
bldgHVACCfgTemplateDescrPtr,
bldgHVACCfgTemplateOwner,bldgHVACCfgTemplateStorage,
bldgHVACCfgTemplateStatus
}
STATUS current
DESCRIPTION
"The bldgHVACObjects Group."
::= { bldgGroups 1 }
END
8.1. NOTES ON MIB MODULE FOR POLICY-BASED MANAGEMENT
The primary purpose to the sample MIB module is to show how to construct
a single module that includes configuration, policy, 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:
Various Authors [Page 57]
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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 bldgHVACPolicyTable
with two objects, one that indicates the desired humidity and the other,
the actual. The bldgHVACPolicyTable could also be extended using this
same approach so that HVAC policies could easily be extended to support
this vendor.
Fate Sharing
A property of two Internet protocol resources or management data control
objects wherein there is a dependency between their individual state
transitions. If the relationship between them becomes inoperable or
invalidated, there will be a predictable behavior for the removal of the
relationship (and possibly, one or both of its parties in their
entirety).
Persistence
A property of management data that defines its permanence. Specifi-
cally, for the purposes of this document, persistence defines whether a
change in configuration on a managed element will have that change
reflected across power cycles (and associated operational configuration
data store re-initialization) of the managed element.
Time-indexed
For this document, this refers to data (and in particular, data describ-
ing events of configuration set or notification), which is tagged with
the agent or management station time of event occurrence. This is often
used in SNMP management systems to correlate events over an elapsed
indexed time sequence with each other for purposes of transactional
grouping.
TimeStamp
A textual convention containing a managed element sysUpTime value, gen-
erally to reflect the element-relative time of the occurrence of a given
associated event.
Transaction
A finite group of changes that when taken as a whole can be applied or
rolled back in one operation. For example, a single SNMP SET PDU repre-
sents a transaction for which the state before the set is restored when
any individual element in the variable-bindings list fail to be applied
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RFC DRAFT Expires March 2002 May 2001
thus returning the device to exactly the same state before the SET was
executed.
9. APPENDIX A - Policy Example
OSPF, is a common routing protocol. RFC 1850 contains the OSPF Version 2
MIB Module. In that module are a number of configuration parameters, two
have been selected for policy control in this example; the Hello and
Dead interval timers.
This example depicts how the policyScript can be used to maintain common
Hello and Dead interval Across different subnets in an OSPF routing
domain.
The 'ospfIfHelloInterval' is the length of time, in seconds, between the
Hello packets that the router sends on the interface. This value must be
same for all routers attached to a common network. The 'ospfIfRtrDead-
Interval' is the number of seconds that a router's Hello packets have
not been seen before its neighbors declare the router down. This value
must be the same for all routers attached to a common network.
A policy to maintain common Hello and Dead interval across different
sub-net can be expressed as:
If network is 10.50.5.0/24
set ospfIfHelloInterval = 15 and ospfIfRtrDeadInterval = 60
else /* default value */
set ospfIfHelloInterval = 10 and ospfIfRtrDeadInterval = 40
The picture below depicts realization of this policy using policyScript.
+--------------------------------+
|pmElementTypeRegTable |
|index=1 //ospfIfEntry |
|oid=1.3.6.1.2.1.14.7.1 |
|Name = ospfIfEntry |
+--------------------------------+
+--------------+
|pmPolicyTable |
|Filter=1 -----+----+
|Action=2 -----+----|-----------------------+
+--------------+ | |
| |
+------------V-----+ +--------V----------+
|pmPolicyCodeTable | |pmPolicyCodeTable |
|Index =1 | |Index=2 |
Various Authors [Page 59]
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|Segment=1 | |Segment=1 |
+--------+---------+ +-----------------+-+
| |
| |
+---------------V------------------------------+ |
| | |
| if(insubtree(elementName(),"ospfIfEntry")&&| |
| (getvar("ospfIfStatus")==1) && | |
| (inSubNet(getvar("ospfIfIpAddress.$0"), | |
| "10.50.5.0/24")= 0)) | |
| return 0; | |
| else | |
| return 1; | |
+----------------------------------------------+ |
|
+----------------------------------------------------V---------+
| var pdu; |
| int HELLO_INTERVAL1=15; /* 15 seconds */ |
| int RTRDEAD_INTERVAL1=60; /* 60 seconds */ |
| var = newPDU(); |
| writeVar(pdu,0, " ospfIfHelloInterval",iv(0), |
| HELLO_INTERVAL1); |
| writeVar(pdu,0, " ospfIfRtrDeadInterval",iv(0), |
| RTRDEAD_INTERVAL1);|
| snmpsend(pdu,2,OP_SET); |
+--------------------------------------------------------------+
10. 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 Partain
Various Authors [Page 60]
RFC DRAFT Expires March 2002 May 2001
Randy Preshun
Dan Romanescu
Shawn Routhier
Steve Waldbusser
11. 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.
Various Authors [Page 61]
RFC DRAFT Expires March 2002 May 2001
[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
(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 F. Kastenholz, "The Interfaces Group MIB using
SMIv2", RFC 2233, Cisco Systems, FTP Software, November 1997.
[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.
Various Authors [Page 62]
RFC DRAFT Expires March 2002 May 2001
[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
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 Kastenholz, F., "The Interfaces Group MIB", RFC
2863, June 2000.
[32] McCloghrie, K., and Hanson, G., "The Inverted Stack Table Extension
to the Interfaces Group MIB", RFC 2864, June 2000.
[33] McCloghrie, K. and Bierman, A., "Entity MIB (Version 2)", RFC 2737,
December, 1999.
[34] ITU-T,, Recommendation M.3010., PRINCIPLES FOR A TELECOMMUNICATIONS
MANAGEMENT NETWORK. February, 2000.
[35] Waldbusser, S., Saperia, J., and Hongal, T., "Policy Based Manage-
ment MIB", Work-in-progress.
12. EDITORS' ADDRESSES
Michael R. MacFaden
Riverstone Networks, Inc
5200 Great America Parkway
Santa Clara, CA 95054
email - mrm@riverstonenet.com
Jon Saperia
JDS Consulting
174 Chapman Street
Various Authors [Page 63]
RFC DRAFT Expires March 2002 May 2001
Watertown, MA 02472
email - saperia@jdscons.com
Wayne F. Tackabury
Gold Wire Technology
411 Waverley Oaks Rd.
Waltham, MA 02452
email - wayne@goldwiretech.com
13. INTELLECTUAL PROPERTY
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on
the IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such pro-
prietary 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 Execu-
tive Director.
14. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain
it or assist in its implementation may be prepared, copied, pub-
lished and distributed, in whole or in part, without restriction of
any kind, provided that the above copyright notice and this
Various Authors [Page 64]
RFC DRAFT Expires March 2002 May 2001
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 proce-
dures 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 ENGI-
NEERING 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 WAR-
RANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Table of Contents
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Document Organization . . . . . . . . . . . . . . . . . . 3
2. USING SNMP AS A CONFIGURATION MECHANISM . . . . . . . . . . 3
2.1. Transactions and SNMP . . . . . . . . . . . . . . . . . . 3
2.2. Practical Requirements for Transactional Control . . . . . 4
2.3. Best Practices in Configuration . . . . . . . . . . . . . 5
3. DESIGNING A MIB MODULE . . . . . . . . . . . . . . . . . . . 8
3.1. MIB Module Design - General Issues . . . . . . . . . . . . 8
3.2. Naming MIB modules and Managed Objects . . . . . . . . . . 9
3.3. Transaction Control And State Tracking . . . . . . . . . . 10
3.3.1. Fate sharing with multiple tables . . . . . . . . . . . 10
3.3.2. Transaction Control MIB Objects . . . . . . . . . . . . 10
3.3.3. Usage of Row notReady Status . . . . . . . . . . . . . . 11
3.3.4. Summary Objects and State Tracking . . . . . . . . . . . 11
3.3.5. Advanced Synchronization Considerations . . . . . . . . 13
3.3.6. Conceptual Table Modification Practices . . . . . . . . 17
3.4. Index Design Issues . . . . . . . . . . . . . . . . . . . 17
3.4.1. Simple Integer Indexing . . . . . . . . . . . . . . . . 18
3.4.2. Indexing with Network Addresses . . . . . . . . . . . . 18
3.5. Conflicting Controls . . . . . . . . . . . . . . . . . . . 18
3.6. Textual Convention Usage . . . . . . . . . . . . . . . . . 19
3.7. Persistent Configuration . . . . . . . . . . . . . . . . . 20
Various Authors [Page 65]
RFC DRAFT Expires March 2002 May 2001
3.8. Configuration Sets and Activation . . . . . . . . . . . . 21
3.9. SET operation Latency . . . . . . . . . . . . . . . . . . 23
3.10. Notifications and Error Reporting . . . . . . . . . . . . 25
3.10.1. Designing Notifications . . . . . . . . . . . . . . . . 25
3.10.2. Control of Notification Subsystem . . . . . . . . . . . 26
3.10.3. Application Error Reporting . . . . . . . . . . . . . . 26
3.11. Other MIB Module Design Issues . . . . . . . . . . . . . 28
3.11.1. Octet String Aggregations . . . . . . . . . . . . . . . 28
3.11.2. Supporting multiple instances of a MIB Module . . . . . 29
3.11.3. Use of special optional clauses . . . . . . . . . . . . 30
4. IMPLEMENTING SNMP CONFIGURATION AGENTS . . . . . . . . . . . 30
4.1. Operational Consistency . . . . . . . . . . . . . . . . . 30
4.2. Handling Multiple Managers . . . . . . . . . . . . . . . . 31
4.3. Designing MIB Modules for Multiple Managers . . . . . . . 31
4.4. Specifying Row Modifiability . . . . . . . . . . . . . . . 32
4.5. Order of varbinds in a SET PDU . . . . . . . . . . . . . . 33
4.6. Implementing write-only access objects . . . . . . . . . . 33
5. DESIGNING CONFIGURATION MANAGEMENT SOFTWARE . . . . . . . . 33
5.1. Designing Configuration Management Software . . . . . . . 34
5.2. Protocol Operations . . . . . . . . . . . . . . . . . . . 34
5.3. SET Operations . . . . . . . . . . . . . . . . . . . . . . 35
5.4. Configuration Transactions . . . . . . . . . . . . . . . . 35
5.5. Notifications . . . . . . . . . . . . . . . . . . . . . . 36
5.6. Scale of the Management Software . . . . . . . . . . . . . 36
6. DEPLOYMENT AND SECURITY ISSUES . . . . . . . . . . . . . . . 37
6.1. Basic assumptions about Configuration . . . . . . . . . . 39
6.2. Secure Agent Considerations . . . . . . . . . . . . . . . 39
6.3. Authentication Traps . . . . . . . . . . . . . . . . . . . 40
6.4. Sensitive Information Handling . . . . . . . . . . . . . . 40
7. POLICY BASED MANAGEMENT . . . . . . . . . . . . . . . . . . 41
7.1. Organization of Data in an SNMP-Based Policy System . . . 41
7.2. Information Related to Policy Based Configuration . . . . 43
7.3. Schedule and Time Issues . . . . . . . . . . . . . . . . . 44
7.4. Conflict Detection, Resolution and Error Reporting . . . . 44
7.5. Notifications in a Policy System . . . . . . . . . . . . . 46
7.6. Using Policy to Move Less Configuration Data . . . . . . . 46
8. Example MIB Module For Policy-Based Management . . . . . . . 46
8.1. NOTES ON MIB MODULE FOR POLICY-BASED MANAGEMENT . . . . . 57
9. APPENDIX A - Policy Example . . . . . . . . . . . . . . . . 59
10. ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . 60
11. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . 61
Various Authors [Page 66]
