Network Working Group                                        M. MacFaden
Request for Comments: 3512                     Riverstone Networks, Inc.
Category: Informational                                       D. Partain
                                                              J. Saperia
                                                    JDS Consulting, Inc.
                                                            W. Tackabury
                                              Gold Wire Technology, Inc.
                                                              April 2003

                 Configuring Networks and Devices with
               Simple Network Management Protocol (SNMP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document is written for readers interested in the Internet
   Standard Management Framework and its protocol, the Simple Network
   Management Protocol (SNMP).  In particular, it offers guidance in the
   effective use of SNMP for configuration management.  This information
   is relevant to vendors that build network elements, management
   application developers, and those that acquire and deploy this
   technology in their networks.

Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   3
      1.1. The Internet Standard Management Framework. . . . . . . .   3
      1.2. Configuration and the Internet Standard Management
           Frame-work. . . . . . . . . . . . . . . . . . . . . . . .   4
   2. Using SNMP as a Configuration Mechanism. . . . . . . . . . . .   5
      2.1. Transactions and SNMP . . . . . . . . . . . . . . . . . .   6
      2.2. Practical Requirements for Transactional Control. . . . .   6
      2.3. Practices in Configuration--Verification. . . . . . . . .   7
   3. Designing a MIB Module . . . . . . . . . . . . . . . . . . . .   9
      3.1. MIB Module Design - General Issues. . . . . . . . . . . .  10
      3.2. Naming MIB modules and Managed Objects. . . . . . . . . .  11
      3.3. Transaction Control And State Tracking. . . . . . . . . .  12

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           3.3.1. Conceptual Table Row Modification Practices. . . .  12
           3.3.2. Fate sharing with multiple tables. . . . . . . . .  13
           3.3.3. Transaction Control MIB Objects. . . . . . . . . .  14
           3.3.4. Creating And Activating New Table Rows . . . . . .  15
           3.3.5. Summary Objects and State Tracking . . . . . . . .  15
           3.3.6. Optimizing Configuration Data Transfer . . . . . .  18
      3.4. More Index Design Issues. . . . . . . . . . . . . . . . .  22
           3.4.1. Simple Integer Indexing. . . . . . . . . . . . . .  23
           3.4.2. Indexing with Network Addresses. . . . . . . . . .  23
      3.5. Conflicting Controls. . . . . . . . . . . . . . . . . . .  24
      3.6. Textual Convention Usage. . . . . . . . . . . . . . . . .  25
      3.7. Persistent Configuration. . . . . . . . . . . . . . . . .  26
      3.8. Configuration Sets and Activation . . . . . . . . . . . .  28
           3.8.1. Operational Activation Considerations. . . . . . .  28
           3.8.2. RowStatus and Deactivation . . . . . . . . . . . .  30
      3.9. SET Operation Latency . . . . . . . . . . . . . . . . . .  31
           3.9.1. Subsystem Latency, Persistence Latency,
                  and Activation Latency . . . . . . . . . . . . . .  33
      3.10. Notifications and Error Reporting. . . . . . . . . . . .  33
           3.10.1. Identifying Source of Configuration Changes . . .  34
           3.10.2. Limiting Unnecessary Transmission of
                   Notifications . . . . . . . . . . . . . . . . . .  34
           3.10.3. Control of Notification Subsystem . . . . . . . .  36
      3.11 Application Error Reporting . . . . . . . . . . . . . . .  36
      3.12 Designing MIB Modules for Multiple Managers . . . . . . .  37
      3.13 Other MIB Module Design Issues. . . . . . . . . . . . . .  39
           3.13.1. Octet String Aggregations . . . . . . . . . . . .  39
           3.13.2 Supporting multiple instances of a MIB Module. . .  40
           3.13.3 Use of Special Optional Clauses. . . . . . . . . .  41
   4. Implementing SNMP Configuration Agents . . . . . . . . . . . .  41
      4.1. Operational Consistency . . . . . . . . . . . . . . . . .  41
      4.2. Handling Multiple Managers. . . . . . . . . . . . . . . .  43
      4.3. Specifying Row Modifiability. . . . . . . . . . . . . . .  44
      4.4. Implementing Write-only Access Objects. . . . . . . . . .  44
   5. Designing Configuration Management Software. . . . . . . . . .  44
      5.1. Configuration Application Interactions
           with Managed Systems. . . . . . . . . . . . . . . . . . .  45
           5.1.1. SET Operations . . . . . . . . . . . . . . . . . .  46
           5.1.2. Configuration Transactions . . . . . . . . . . . .  46
           5.1.3. Tracking Configuration Changes . . . . . . . . . .  47
           5.1.4. Scalability of Data Retrieval. . . . . . . . . . .  48
   6. Deployment and Security Issues . . . . . . . . . . . . . . . .  48
      6.1. Basic assumptions about Configuration . . . . . . . . . .  48
      6.2. Secure Agent Considerations . . . . . . . . . . . . . . .  49
      6.3. Authentication Notifications. . . . . . . . . . . . . . .  49
      6.4. Sensitive Information Handling. . . . . . . . . . . . . .  50
   7. Policy-based Management. . . . . . . . . . . . . . . . . . . .  51
      7.1. What Is the Meaning of 'Policy-based' . . . . . . . . . .  51

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      7.2. Organization of Data in an SNMP-Based Policy System . . .  53
      7.3. Information Related to Policy-based Configuration . . . .  54
      7.4. Schedule and Time Issues. . . . . . . . . . . . . . . . .  56
      7.5. Conflict Detection, Resolution and Error Reporting. . . .  56
           7.5.1. Changes to Configuration Outside of the
                  Policy System. . . . . . . . . . . . . . . . . . .  57
      7.6. More about Notifications in a Policy System . . . . . . .  57
      7.7. Using Policy to Move Less Configuration Data. . . . . . .  57
   8. Example MIB Module With Template-based Data. . . . . . . . . .  58
      8.1. MIB Module Definition. . . . . . . .  . . . . . . . . . .  61
      8.2. Notes on MIB Module with Template-based Data. . . . . . .  73
      8.3. Examples of Usage of the MIB . . . . . . .. . . . . . . .  74
   9. Security Considerations . . . . . . . . . . .. . . . . . . . .  77
   10. Acknowledgments. . . . . . . . . . . . . . .  . . . . . . . .  78
   11. Normative References. . . . . . . . . . . . . . . . . . . . .  78
   12. Informative References. . . . . . . . . . . . . . . . . . . .  79
   13. Intellectual Property . . . . . . . . . . . . . . . . . . . .  81
   14. Editors' Addresses. . . . . . . . . . . . . . . . . . . . . .  82
   15. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  83

1.  Introduction

1.1.  The Internet Standard Management Framework

   The Internet Standard Management Framework has many components.  The
   purpose of this document is to describe effective ways of applying
   those components to the problems of configuration management.

   For reference purposes, the Internet Standard Management Framework
   presently consists of five major components:

   o  An overall architecture, described in RFC 3411 [1].

   o  Mechanisms for describing and naming objects and events for the
      purpose of management.  The first version of this Structure of
      Management Information (SMI) is called SMIv1 and described in STD
      16, RFC 1155 [15], STD 16, RFC 1212 [16] and RFC 1215 [17].  The
      second version, called SMIv2, is described in STD 58, RFC 2578
      [2], STD 58, RFC 2579 [3] and STD 58, RFC 2580 [4].

   o  Message protocols for transferring management information.  The
      first version of the SNMP message protocol is called SNMPv1 and
      described in STD 15, RFC 1157 [18].  A second version of the SNMP
      message protocol, which is not an Internet standards track
      protocol, is called SNMPv2c and described in RFC 1901 [19].  The
      third version of the message protocol is called SNMPv3 and
      described in RFC 3417 [5], RFC 3412 [6] and RFC 3414 [7].

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   o  Protocol operations for accessing management information.  The
      first set of protocol operations and associated PDU formats is
      described in STD 15, RFC 1157 [18].  A second set of protocol
      operations and associated PDU formats is described in RFC 3416

   o  A set of fundamental applications described in RFC 3413 [9] and
      the view-based access control mechanism described in RFC 3415

   A more detailed introduction to the current SNMP Management Framework
   can be found in RFC 3410 [12].

   Managed objects are accessed via a virtual information store, termed
   the Management Information Base or MIB.  Objects in the MIB are
   defined using the mechanisms defined in the SMI.

1.2.  Configuration and the Internet Standard Management Framework

   Data networks have grown significantly over the past decade.  This
   growth can be seen in terms of:

   Scale - Networks have more network elements, and the network
      elements are larger and place more demands on the systems managing
      them.  For example, consider a typical number and speed of
      interfaces in a modern core network element.  A managed
      metropolitan area network switch can have a port density much
      greater than the port density built into the expectations of the
      management systems that predated it.  There are also many more
      interrelationships within and between devices and device

   Functionality - network devices perform more functions.
      More protocols and network layers are required for the successful
      deployment of network services which depend on them.

   Rate of Change - the nature of modern network services
      causes updates, additions, and deletions of device configuration
      information more often than in the past.  No longer can it be
      assumed that a configuration will be specified once and then be
      updated rarely.  On the contrary, the trend has been towards much
      more frequent changes of configuration information.

   Correct configuration of network elements that make up data networks
   is a prerequisite to the successful deployment of the services on
   them.  The growth in size and complexity of modern networks increases
   the need for a standard configuration mechanism that is tightly
   integrated with performance and fault management systems.

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   The Internet Standard Management Framework has been used successfully
   to develop configuration management systems for a broad range of
   devices and networks.  A standard configuration mechanism that
   tightly integrates with performance and fault systems is needed not
   only to help reduce the complexity of management, but also to enable
   verification of configuration activities that create revenue-
   producing services.

   This document describes Current Practices that have been used when
   designing effective configuration management systems using the
   Internet Standard Management Framework (colloquially known as SNMP).
   It covers many basic practices as well as more complex agent and
   manager design issues that are raised by configuration management.
   We are not endeavoring to present a comprehensive how-to document for
   generalized SNMP agent, MIB module, or management application design
   and development.  We will, however, cover points of generalized SNMP
   software design and implementation practice, where the practice has
   been seen to benefit configuration management software.  So, for
   example, the requirement for management applications to be aware of
   agent limitations is discussed in the context of configuration
   operations, but many issues that a management application developer
   should consider with regard to manager-agent interactions are left
   for other documents and resources.

   Significant experience has been gained over the past ten years in
   configuring public and private data networks with SNMP.  During this
   time, networks have grown significantly as described above.  A
   response to this explosive growth has been the development of
   policy-based configuration management.  Policy-Based Configuration
   Management is a methodology wherein configuration information is
   derived from rules and network-wide objectives, and is distributed to
   potentially many network elements with the goal of achieving
   consistent network behavior throughout an administrative domain.

   This document presents lessons learned from these experiences and
   applies them to both conventional and policy-based configuration
   systems based on SNMP.

2.  Using SNMP as a Configuration Mechanism

   Configuration activity causes one or more state changes in an
   element.  While it often takes an arbitrary number of commands and
   amount of data to make up configuration change, it is critical that
   the configuration system treat the overall change operation
   atomically so that the number of states into which an element
   transitions is minimized.  The goal is for a change request either to
   be completely executed or not at all.  This is called transactional
   integrity.  Transactional integrity makes it possible to develop

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   reliable configuration systems that can invoke transactions and keep
   track of an element's overall state and work in the presence of error

2.1.  Transactions and SNMP

   Transactions can logically take place at very fine-grained levels
   such as an individual object instance or in very large aggregations
   that could include many object instances located in many tables on a
   managed device.  For this reason, reliance on transactional integrity
   only at the SNMP protocol level is insufficient.

2.2.  Practical Requirements for Transactional Control

   A well-designed and deployed configuration system should have the
   following features with regard to transactions and transactional

   1) Provide for flexible transaction control at many different levels
      of granularity.  At one extreme, an entire configuration may be
      delivered and installed on an element, or alternately one small
      attribute may be changed.

   2) The transaction control component should work at and understand a
      notion of the kind of multi-level "defaulting" as described in
      Section 7.1.  The key point here is that it may make most sense to
      configure systems at an abstract level rather than on an
      individual instance by instance basis as has been commonly
      practiced.  In some cases it is more effective to send a
      configuration command to a system that contains a set of
      'defaults' to be applied to instances that meet certain criteria.

   3) An effective configuration management system must allow
      flexibility in the definition of a successful transaction.  This
      cannot be done at the protocol level alone, but rather must be
      provided for throughout the application and the information that
      is being managed.  In the case of SNMP, the information would be
      in properly defined MIB modules.

   4) A configuration management system should provide time-indexed
      transaction control.  For effective rollback control, the
      configuration transactions and their successful or unsuccessful
      completion status must be reported by the managed elements and
      stored in a repository that supports such time indexing and can
      record the user that made the change, even if the change was not
      carried out by the system recording the change.

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   5) The managed system must support transactional security.  This
      means that depending on who is making the configuration request
      and where it is being made, it may be accepted or denied based on
      security policy that is in effect in the managed element.

   Effective transactional control is a responsibility shared between
   design, implementation, and operational practice.  Transaction
   control techniques for MIB module design are discussed in Section
   3.3.  Transaction control considerations for the agent implementation
   are discussed in Section 5.2.2.

2.3.  Practices in Configuration--Verification

   Verification of expected behavior subsequent to the commitment of
   change is an integral part of the configuration process.  To reduce
   the chance of making simple errors in configuration, many
   organizations employ the following change management procedure:

   pre-test - verify that the system is presently working properly

   change   - make configuration changes and wait for convergence
              (system or network stability)

   re-test  - verify once again that the system is working properly

   This procedure is commonly used to verify configuration changes to
   critical systems such as the domain name system (DNS).  DNS software
   kits provide diagnostic tools that allow automatic test
   procedures/scripts to be conducted.

   A planned configuration sequence can be aborted if the pre-
   configuration test result shows the state of the system as unstable.
   Debugging the unintended effects of two sets of changes in large
   systems is often more challenging than an analysis of the effects of
   a single set after test termination.

   Networks and devices under SNMP configuration readily support this
   change management procedure since the SNMP provides integrated
   monitoring, configuration and diagnostic capabilities.  The key is
   the sequencing of SNMP protocol operations to effect an integrated
   change procedure like the one described above.  This is usually a
   well-bounded affair for changes within a single network element or
   node.  However, there are times when configuration of a given element
   can impact other elements in a network.  Configuring network
   protocols such as IEEE 802.1D Spanning Tree or OSPF is especially
   challenging since the impact of a configuration change can directly
   affect stability (convergence) of the network the device is connected

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   An integrated view of configuration and monitoring provides an ideal
   platform from which to evaluate such changes.  For example, the MIB
   module governing IEEE 802.1D Spanning Tree (RFC 1493 [24]) provides
   the following object to monitor stability per logical bridge.

      dot1dStpTopChanges OBJECT-TYPE
          SYNTAX  Counter
          ACCESS  read-only
          STATUS  mandatory
             "The total number of topology changes detected by
             this bridge since the management entity was last
             reset or initialized."
             "IEEE 802.1D-1990: Section"
          ::= { dot1dStp 4 }

   Likewise, the OSPF MIB module provides a similar metric for stability
   per OSPF area.

      ospfSpfRuns OBJECT-TYPE
          SYNTAX   Counter32
          MAX-ACCESS   read-only
          STATUS   current
             "The number of times that the intra-area route
             table has been calculated using this area's
             link-state database.  This is typically done
             using Dijkstra's algorithm."
         ::= { ospfAreaEntry 4 }

   The above object types are good examples of a means of facilitating
   the principles described in Section 2.3.  That is, one needs to
   understand the behavior of a subsystem before configuration change,
   then be able to use the same means to retest and verify proper
   operation subsequent to configuration change.

   The operational effects of a given implementation often differ from
   one to another for any given standard configuration object.  The
   impact of a change to stability of systems such as OSPF should be
   documented in an agent-capabilities statement which is consistent
   with "Requirements for IP Version 4 Routers" [22], Section 1.3.4:

      A vendor needs to provide adequate documentation on all
      configuration parameters, their limits and effects.

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   Adherence to the above model is not fail-safe, especially when
   configuration errors are masked by long latencies or when
   configuration errors lead to oscillations in network stability.  For
   example, consider the situation of loading a new software version on
   a device, which leads to small, slow, cumulative memory leaks brought
   on by a certain traffic pattern that was not caught during vendor and
   customer test lab trials.

   In a network-based example, convergence in an autonomous system
   cannot be guaranteed when configuration changes are made since there
   are factors beyond the control of the operator, such as the state of
   other network elements.  Problems affecting this convergence may not
   be detected for a significant period of time after the configuration
   change.  Even for factors within the operator's control, there is
   often little verification done to prevent mis-configuration (as shown
   in the following example).

   Consider a change made to ospfIfHelloInterval and
   ospfIfRtrDeadInterval [24] timers in the OSPF routing protocol such
   that both are set to the same value.  Two routers may form an
   adjacency but then begin to cycle in and out of adjacency, and thus
   never reach a stable (converged) state.  Had the configuration
   process described at the beginning of this section been employed,
   this particular situation would have been discovered without
   impacting the production network.

   The important point to remember from this discussion is that
   configuration systems should be designed and implemented with
   verification tests in mind.

3.  Designing a MIB Module

   Carefully considered MIB module designs are crucial to practical
   configuration with SNMP.  As we have just seen, MIB objects designed
   for configuration can be very effective since they can be associated
   with integrated diagnostic, monitoring, and fault objects.  MIB
   modules for configuration also scale when they expose their notion of
   template object types.  Template objects can represent information at
   a higher level of abstraction than instance-level ones.  This has the
   benefit of reducing the amount of instance-level data to move from
   management application to the agent on the managed element, when that
   instance-level data is brought about by applying a template object on
   the agent.  Taken together, all of these objects can provide a robust
   configuration subsystem.

   The remainder of this section provides specific practices used in MIB
   module design with SMIv2 and SNMPv3.

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3.1.  MIB Module Design - General Issues

   One of the first tasks in defining a MIB module is the creation of a
   model that reflects the scope and organization of the management
   information an agent will expose.

   MIB modules can be thought of as logical models providing one or more
   aspects/views of a subsystem.  The objective for all MIB modules
   should be to serve one or more operational requirements such as
   accounting information collection, configuration of one or more parts
   of a system, or fault identification.  However, it is important to
   include only those aspects of a subsystem that are proven to be
   operationally useful.

   In 1993, one of most widely deployed MIB modules supporting
   configuration was published, RFC 1493, which contained the BRIDGE-
   MIB.  It defined the criteria used to develop the MIB module as

      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

   (3)  Consider evidence of current use and/or utility.

   (4)  Limit the total (sic) of objects.

   (5)  Exclude objects which are simply derivable from others in this
        or other MIBs.

   (6)  Avoid causing critical sections to be heavily instrumented.  The
        guideline that was followed is one counter per critical section
        per layer.

   Over the past eight years additional experience has shown a need to
   expand these criteria as follows:

   (7)  Before designing a MIB module, identify goals and objectives for
        the MIB module.  How much of the underlying system will be
        exposed depends on these goals.

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   (8)  Minimizing the total number of objects is not an explicit goal,
        but usability is.  Be sure to consider deployment and usability

   (9)  During configuration, consider supporting explicit error state,
        capability and capacity objects.

   (10) When evaluating rule (5) above, consider the impact on a
        management application.  If an object can help reduce a
        management application's complexity, consider defining objects
        that can be derived.

3.2.  Naming MIB modules and Managed Objects

   Naming of MIB modules and objects informally follows a set of best
   practices.  Originally, standards track MIB modules used RFC names.
   As the MIB modules evolved, the practice changed to using more
   descriptive names.  Presently, Standards Track MIB modules define a
   given area of technology such as ATM-MIB, and vendors then extend
   such MIB modules by prefixing the company name to a given MIB module
   as in ACME-ATM-MIB.

   Object descriptors (the "human readable names" assigned to object
   identifiers [2]) defined in standard MIB modules should be unique
   across all MIB modules.  Generally, a prefix is added to each managed
   object that can help reference the MIB module it was defined in.  For
   example, the IF-MIB uses "if" prefix for descriptors of object types
   such as ifTable, ifStackTable and so forth.

   MIB module object type descriptors can include an abbreviation for
   the function they perform.  For example the objects that control
   configuration in the example MIB module in Section 8 include "Cfg" as
   part of the object descriptor, as in bldgHVACCfgDesiredTemp.

   This is more fully realized when the object descriptors that include
   the fault, configuration, accounting, performance and security [33]
   abbreviations are combined with an organized OID assignment approach.
   For example, a vendor could create a configuration branch in their
   private enterprises area.  In some cases this might be best done on a
   per product basis.  Whatever the approach used, "Cfg" might be
   included in every object descriptor in the configuration branch.
   This has two operational benefits.  First, for those that do look at
   instances of MIB objects, descriptors as seen through MIB browsers or
   other command line tools assist in conveying the meaning of the
   object type.  Secondly, management applications can be pointed at
   specific subtrees for fault or configuration, causing a more
   efficient retrieval of data and a simpler management application with
   potentially better performance.

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3.3.  Transaction Control And State Tracking

   Transactions and keeping track of their state is an important
   consideration when performing any type of configuration activity
   regardless of the protocol.  Here are a few areas to consider when
   designing transaction support into an SNMP-based configuration

3.3.1.  Conceptual Table Row Modification Practices

   Any discussion of transaction control as it pertains to MIB module
   design often begins with how the creation or modification of object
   instances in a conceptual row in the MIB module is controlled.

   RowStatus [3] is a standard textual convention for the management of
   conceptual rows in a table.  Specifically, the RowStatus textual
   convention that is used for the SYNTAX value of a single column in a
   table controls the creation, deletion, activation, and deactivation
   of conceptual rows of the table.  When a table has been defined with
   a RowStatus object as one of its columns, changing an instance of the
   object to 'active' causes the row in which that object instance
   appears to become 'committed'.

   In a multi-table scenario where the configuration data must be spread
   over many columnar objects, a RowStatus object in one table can be
   used to cause the entire set of data to be put in operation or stored
   based on the definition of the objects.

   In some cases, very large amounts of data may need to be 'committed'
   all at once.  In these cases, another approach is to configure all of
   the rows in all the tables required and have an "activate" object
   that has a set method that commits all the modified rows.

   The RowStatus textual convention specifies that, when used in a
   conceptual row, a description must define what can be modified.
   While the description of the conceptual row and its columnar object
   types is the correct place to derive this information on instance
   modifiability, it is often wrongly assumed in some implementations

   1) objects either must all be presently set or none need be set to
      make a conceptual RowStatus object transition to active(1)

   2) objects in a conceptual row cannot be modified once a RowStatus
      object is active(1).  Restricting instance modifiability like
      this, so that after a RowStatus object is set to active(1) is in
      fact a reasonable limitation, since such a set of RowStatus may
      have agent system side-effects which depend on committed columnar

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      object instance values.  However, where this restriction exists on
      an object, it should be made clear in a DESCRIPTION clause such as
      the following:

      protocolDirDescr OBJECT-TYPE
        SYNTAX      DisplayString (SIZE (1..64))
        MAX-ACCESS  read-create
        STATUS      current
            "A textual description of the protocol encapsulation.
            A probe may choose to describe only a subset of the
            entire encapsulation (e.g., only the highest layer).

            This object is intended for human consumption only.

            This object may not be modified if the associated
            protocolDirStatus object is equal to active(1)."
        ::= { protocolDirEntry 4 }

   Any such restrictions on columnar object instance modification while
   a row's RowStatus object instance is set to active(1) should appear
   in the DESCRIPTION clause of the RowStatus columnar OBJECT-TYPE as

3.3.2.  Fate sharing with multiple tables

   An important principle associated with transaction control is fate
   sharing of rows in different tables.  Consider the case where a
   relationship has been specified between two conceptual tables of a
   MIB module (or tables in two different MIB modules).  In this
   context, fate sharing means that when a row of a table is deleted,
   the corresponding row in the other table is also deleted.  Fate
   sharing in a transaction control context can also be used with the
   activation of very large configuration changes.  If we have two
   tables that hold a set of configuration information, a row in one
   table might have to be put in the 'ready' state before the second can
   be put in the 'ready' state.  When that second table can be placed in
   the 'ready' state, then the entire transaction can be considered to
   have been 'committed'.

   Fate sharing of SNMP table data should be explicitly defined where
   possible using the SMI index qualifier AUGMENTS.  If the relationship
   between tables cannot be defined using SMIv2 macros, then the
   DESCRIPTION clause of the object types which particularly effect the
   cross-table relationship should define what should happen when rows
   in related tables are added or deleted.

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   Consider the relationship between the dot1dBasePortTable and the
   ifTable.  These tables have a sparse relationship.  If a given
   ifEntry supports 802.1D bridging then there is a dot1dBasePortEntry
   that has a pointer to it via dot1dBasePortIfIndex.

   Now, what should happen if an ifEntry that can bridge is deleted?
   Should the object dot1dBasePortIfIndex simply be set to 0 or should
   the dot1dBasePortEntry be deleted as well?  A number of acceptable
   design and practice techniques can provide the answer to these
   questions, so it is important for the MIB module designer to provide
   the guidance to guarantee consistency and interoperability.

   To this end, when two tables are related in such a way, ambiguities
   such as this should be avoided by having the DESCRIPTION clauses of
   the pertinent row object types define the fate sharing of entries in
   the respective tables.

3.3.3.  Transaction Control MIB Objects

   When a MIB module is defined that includes configuration object
   types, consider providing transaction control objects.  These objects
   can be used to cause a large transaction to be committed.  For
   example, we might have several tables that define the configuration
   of a portion of a system.  In order to avoid churn in the operational
   state of the system we might create a single scalar object that, when
   set to a particular value, will cause the activation of the rows in
   all the necessary tables.  Here are some examples of further usage
   for such object types:

   o  Control objects that are the 'write' or 'commit' objects.

      Such objects can cause all pending transactions (change MIB object
      values as a result of SET operations) to be committed to a
      permanent repository or operational memory, as defined by the
      semantics of the MIB objects.

   o  Control objects at different levels of configuration granularity.

      One of the decisions for a MIB module designer is what are the
      levels of granularity that make sense in practice.  For example,
      in the routing area, would changes be allowed on a per protocol
      basis such as BGP? If allowed at the BGP level, are sub-levels
      permitted such as per autonomous system? The design of these
      control objects will be impacted by the underlying software
      design.  RowStatus (see Section 3.3.1) also has important
      relevance as a general transaction control object.

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3.3.4.  Creating And Activating New Table Rows

   When designing read-create objects in a table, a MIB module designer
   should first consider the default state of each object in the table
   when a row is created.  Should an implementation of a standard MIB
   module vary in terms of the objects that need to be set in order to
   create an instance of a given row, an agent capabilities statement
   should be used to name the additional objects in that table using the

   It is useful when configuring new rows to use the notReady status to
   indicate row activation cannot proceed.

   When creating a row instance of a conceptual table, one should
   consider the state of instances of required columnar objects in the
   row.  The DESCRIPTION clause of such a required columnar object
   should specify it as such.

   During the period of time when a management application is attempting
   to create a row, there may be a period of time when not all of these
   required (and non-defaultable) columnar object instances have been
   set. Throughout this time, an agent should return a noSuchInstance
   error for a GET of any object instance of the row until such time
   that all of these required instance values are set.  The exception is
   the RowStatus object instance, for which a notReady(3) value should
   be returned during this period.

   One need only be concerned with the notReady value return for a
   RowStatus object when the row under creation does not yet have all of
   the required, non-defaultable instance values for the row.  One
   approach to simplifying in-row configuration transactions when
   designing MIB modules is to construct table rows that have no more
   instance data for columnar objects than will fit inside a single SET
   PDU.  In this case, the createAndWait() value for the RowStatus
   columnar object is not required.  It is possible to use createAndGo()
   in the same SET PDU, thus simplifying transactional management.

3.3.5.  Summary Objects and State Tracking

   Before beginning a new set of configuration transactions, a
   management application might want to checkpoint the state of the
   managed devices whose configuration it is about to change.  There are
   a number of techniques that a MIB module designer can provide to
   assist in the (re-)synchronization of the managed systems.  These
   objects can also be used to verify that the management application's
   notion of the managed system state is the same as that of the managed

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   These techniques include:

   1. Provide an object that reports the number of rows in a table

   2. Provide an object that flags when data in the table was last

   3. Send a notification message (InformRequests are preferable) to
      deliver configuration change.

   By providing an object containing the number of rows in a table,
   management applications can decide how best to retrieve a given
   table's data and may choose different retrieval strategies depending
   on table size.  Note that the availability of and application
   monitoring of such an object is not sufficient for determining the
   presence of table data change over a checkpointed duration since an
   equal number of row creates and deletes over that duration would
   reflect no change in the object instance value.  Additionally, table
   data change which does not change the number of rows in the table
   would not be reflected through simple monitoring of such an object

   Instead, the change in the value of any table object instance data
   can be tracked through an object that monitors table change state as
   a function of time.  An example is found in RFC 2790, Host Resources

   hrSWInstalledLastUpdateTime OBJECT-TYPE
       SYNTAX     TimeTicks
       MAX-ACCESS read-only
       STATUS     current
           "The value of sysUpTime when the hrSWInstalledTable
           was last completely updated.  Because caching of this
           data will be a popular implementation strategy,
           retrieval of this object allows a management station
           to obtain a guarantee that no data in this table is
           older than the indicated time."
       ::= { hrSWInstalled 2 }

   A similar convention found in many standards track MIB modules is the
   "LastChange" type object.

   For example, the ENTITY-MIB, RFC 2737 [34], provides the following

   entLastChangeTime OBJECT-TYPE
     SYNTAX      TimeStamp

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     MAX-ACCESS  read-only
     STATUS      current
             "The value of sysUpTime at the time a conceptual row is
             created, modified, or deleted in any of these tables:
                     - entPhysicalTable
                     - entLogicalTable
                     - entLPMappingTable
                     - entAliasMappingTable
                     - entPhysicalContainsTable"
     ::= { entityGeneral 1 }

   This convention is not formalized.  There tend to be small
   differences in what a table's LastChanged object reflects.  IF-MIB
   (RFC 2863 [20]) defines the following:

   ifTableLastChange  OBJECT-TYPE
       SYNTAX      TimeTicks
       MAX-ACCESS  read-only
       STATUS      current
               "The value of sysUpTime at the time of the last
               creation or deletion of an entry in the ifTable.  If
               the number of entries has been unchanged since the
               last re-initialization of the local network management
               subsystem, then this object contains a zero value."
       ::= { ifMIBObjects 5 }

   So, if an agent modifies a row with an SNMP SET on ifAdminStatus, the
   value of ifTableLastChange will not be updated.  It is important to
   be specific about what can cause an object to update so that
   management applications will be able to detect and more properly act
   on these changes.

   The final way to keep distributed configuration data consistent is to
   use an event-driven model, where configuration changes are
   communicated as they occur.  When the frequency of change to
   configuration is relatively low or polling a cache object is not
   desired, consider defining a notification that can be used to report
   all configuration change details.

   When doing so, the option is available to an SNMPv3 (or SNMPv2c)
   agent to deliver the notification using either a trap or an inform.
   The decision as to which PDU to deliver to the recipient is generally
   a matter of local configuration.  Vendors should recommend the use of
   informs over traps for NOTIFICATION-TYPE data since the agent can use
   the presence or absence of a response to help know whether it needs

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   to retransmit or not.  Overall, it is preferable to use an inform
   instead of a trap so that changes have a higher likelihood of
   confirmed end-to-end delivery.

   As a matter of MIB module design, when practical, the NOTIFICATION-
   TYPE should include in the PDU all of the modified columnar objects
   in a row of a table.  This makes it easier for the management
   application receiving the notification to keep track of what has
   changed in the row of a table and perform addition analysis on the
   state of the managed elements.

   However, the use of notifications to communicate the state of a
   rapidly changing object may not be ideal either.  This leads us back
   to the MIB module design question of what is the right level of
   granularity to expose.

   Finally, having to poll many "LastChange" objects does not scale
   well.  Consider providing a global LastChange type object to
   represent overall configuration in a given agent implementation.

3.3.6.  Optimizing Configuration Data Transfer

   Configuration management software should keep track of the current
   configuration of all devices under its control.  It should ensure
   that the result is a consistent view of the configuration of the
   network, which can help reduce inadvertent configuration errors.

   In devices that have very large amounts of configuration data, it can
   be costly to both the agent and the manager to have the manager
   periodically poll the entire contents of these configuration tables
   for synchronization purposes.  A benefit of good synchronization
   between the manager and the agent is that the manager can determine
   the smallest and most effective set of data to send to managed
   devices when configuration changes are required.  Depending on the
   table organization in the managed device and the agent
   implementation, this practice can reduce the burden on the managed
   device for activation of these configuration changes.

   In the previous section, we discussed the "LastChange" style of
   object.  When viewed against the requirements just described, the
   LastChange object is insufficient for large amounts of data.

   There are three design options that can be used to assist with the
   synchronization of the configuration data found in the managed device
   with the manager:

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   1) Design multiple indices to partition the data in a table logically
      or break a table into a set of tables to partition the data based
      on what an application will use the table for

   2) Use a time-based indexing technique

   3) Define a control MIB module that manages a separate data delivery
      protocol  Index Design

   Index design has a major impact on the amount of data that must be
   transferred between SNMP entities and can help to mitigate scaling
   issues with large tables.

   Many tables in standard MIB modules follow one of two indexing

   - Indexing based upon increasing Integer32 or Unsigned32 values of
      the kind one might find in an array.

   - Associative indexing, which refers to the technique of using
      potentially sparse indices based upon a "key" of the sort one
      would use for a hash table.

   When tables grow to a very large number of rows, using an associative
   indexing scheme offers the useful ability to efficiently retrieve
   only the rows of interest.

   For example, if an SNMP entity exposes a copy of the default-free
   Internet routing table as defined in the ipCidrRouteTable, it will
   presently contain around 100,000 rows.

   Associative indexing is used in the ipCidrRouteTable and allows one
   to retrieve, for example, all routes for a given IPv4 destination

   Yet, if the goal is to extract a copy of the table, the associative
   indexing reduces the throughput and potentially the performance of
   retrieval.  This is because each of the index objects are appended to
   the object identifiers for every object instance returned.

   ipCidrRouteEntry OBJECT-TYPE
      SYNTAX   IpCidrRouteEntry
      MAX-ACCESS not-accessible
      STATUS   current
       "A particular route to a particular destination,

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       under a particular policy."
      INDEX {

   A simple array-like index works efficiently since it minimizes the
   index size and complexity while increasing the number of rows that
   can be sent in a PDU.  If the indexing is not sparse, concurrency can
   be gained by sending multiple asynchronous non-overlapping collection
   requests as is explained in RFC 2819 [32], Page 41 (in the section
   pertaining to Host Group indexing).

      Should requirements dictate new methods of access, multiple
      indices can be defined such that both associative and simple
      indexing can coexist to access a single logical table.

   Two examples follow.

   First, consider the ifStackTable found in RFC 2863 [20] and the
   ifInvStackTable RFC 2864 [33].  They are logical equivalents with the
   order of the auxiliary (index) objects simply reversed.

   ifStackEntry  OBJECT-TYPE
        SYNTAX        IfStackEntry
        MAX-ACCESS    not-accessible
        STATUS        current
                "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
          "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 }

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   Second, table designs that can factor data into multiple tables with
   well-defined relationships can help reduce overall data transfer
   requirements.  The RMON-MIB, RFC 2819 [32], demonstrates a very
   useful technique of organizing tables into control and data
   components.  Control tables contain those objects that are configured
   and change infrequently, and the data tables contain information to
   be collected that can be large and may change quite frequently.

   As an example, the RMON hostControlTable provides a way to specify
   how to collect MAC addresses learned as a source or destination from
   a given port that provides transparent bridging of Ethernet packets.

   Configuration is accomplished using the hostControlTable.  It is
   indexed by a simple integer.  While this may seem to be array-like,
   it is common practice for command generators to encode the ifIndex
   into this simple integer to provide associative lookup capability.

   The RMON hostTable and hostTimeTable represent dependent tables that
   contain the results indexed by the hostControlTable entry.

   The hostTable is further indexed by the MAC address which provides
   the ability to reasonably search for a collection, such as the
   Organizationally Unique Identifier (OUI), the first three octets of
   the MAC address.

   The hostTimeTable is designed explicitly for fast transfer of bulk
   RMON data.  It demonstrates how to handle collecting large number of
   rows in the face of deletions and insertions by providing

   hostControlLastDeleteTime OBJECT-TYPE
   SYNTAX     TimeTicks
   MAX-ACCESS read-only
   STATUS     current
       "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 }  Time Based Indexing

   The TimeFilter as defined in RFC 2021 [44] and used in RMON2-MIB and
   Q-BRIDGE-MIB (RFC 2674 [26]) provides a way to obtain only those rows
   that have changed on or after some specified period of time has

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   One drawback to TimeFilter index tables is that a given row can
   appear at many points in time, which artificially inflates the size
   of the table when performing standard getNext or getBulk data
   retrieval.  Alternate Data Delivery Mechanisms

   If the amount of data to transfer is larger than current SNMP design
   restrictions permit, as in the case of OCTET STRINGS (64k minus
   overhead of IP/UDP header plus SNMP header plus varbind list plus
   varbind encoding), consider delivery of the data via an alternate
   method, such as FTP and use a MIB module to control that data
   delivery process.  In many cases, this problem can be avoided via
   effective MIB design.  In other words, object types requiring this
   kind of transfer size should be used judiciously, if at all.

   There are many enterprise MIB modules that provide control of the
   TFTP or FTP protocol.  Often the SNMP part defines what to send where
   and setting an object initiates the operation (for an example, refer
   to the CISCO-FTP-CLIENT-MIB, discussed in [38]).

   Various approaches exist for allowing a local agent process running
   within the managed node to take a template for an object instance
   (for example for a set of interfaces), and adapt and apply it to all
   of the actual instances within the node.  This is an architecture for
   one form of policy-based configuration (see [36], for example).  Such
   an architecture, which must be designed into the agent and some
   portions of the MIB module, affords the efficiency of specifying many
   copies of instance data only once, along with the execution
   efficiency of distributing the application of the instance data to
   the agent.

   Other work is currently underway to improve efficiency for bulk SNMP
   transfer operations [37].  The objective of these efforts is simply
   the conveyance of more information with less overhead.

3.4.  More Index Design Issues

   Section 3.3.5 described considerations for table row index design as
   it pertains to the synchronization of changes within sizable table
   rows. This section simply considers how to specify this syntactically
   and how to manage indices semantically.

   In many respects, the design issues associated with indices in a MIB
   module are similar to those in a database.  Care must be taken during
   the design phase to determine how often and what kind of information
   must be set or retrieved.  The next few points provide some guidance.

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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
   identifiers are unsigned long values, a question that arises is why
   not index from zero (0) instead of one(1)?

   RFC 2578 [2], Section 7.7, page 28 states the following: Instances
   identified by use of integer-valued objects should be numbered
   starting from one (i.e., not from zero).  The use of zero as a value
   for an integer-valued index object type should be avoided, except in
   special cases.  Consider the provisions afforded by the following
   textual convention from the Interfaces Group MIB module [33]:

   InterfaceIndexOrZero ::= TEXTUAL-CONVENTION
       DISPLAY-HINT "d"
       STATUS       current
           "This textual convention is an extension of the
           InterfaceIndex convention.  The latter defines a greater
           than zero value used to identify an interface or interface
           sub-layer in the managed system.  This extension permits the
           additional value of zero.  the value zero is object-specific
           and must therefore be defined as part of the description of
           any object which uses this syntax.  Examples of the usage of
           zero might include situations where interface was unknown,
           or when none or all interfaces need to be referenced."
       SYNTAX       Integer32 (0..2147483647)

3.4.2.  Indexing with Network Addresses

   There are many objects that use IPv4 addresses (SYNTAX IpAddress) as
   indexes.  One such table is the ipAddrTable from RFC 2011 [14] IP-
   MIB.  This limits the usefulness of the MIB module to IPv4.  To avoid
   such limitations, use the addressing textual conventions INET-
   ADDRESS-MIB [13] (or updates to that MIB module), which provides a
   generic way to represent addresses for Internet Protocols.  In using
   the InetAddress textual convention in this MIB, however, pay heed to
   the following advisory found in its description clause:

      When this textual convention is used as the syntax of an index
      object, there may be issues with the limit of 128 sub-identifiers
      specified in SMIv2, STD 58.  In this case, the OBJECT-TYPE
      declaration MUST include a 'SIZE' clause to limit the number of
      potential instance sub-identifiers.

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   One should consider the SMI limitation on the 128 sub-identifier
   specification when using certain kinds of network address index
   types.  The most likely practical liability encountered in practice
   has been with DNS names, which can in fact be in excess of 128 bytes.
   The problem can be, of course, compounded when multiple indices of
   this type are specified for a table.

3.5.  Conflicting Controls

   MIB module designers should avoid specifying read-write objects that
   overlap in function partly or completely.

   Consider the following situation where two read-write objects
   partially overlap when a dot1dBasePortEntry has a corresponding

   The BRIDGE-MIB defines the following managed object:

   dot1dStpPortEnable OBJECT-TYPE
                   disabled(2)            }
       ACCESS  read-write
       STATUS  mandatory
           "The enabled/disabled status of the port."
           "IEEE 802.1D-1990: Section"
       ::= { dot1dStpPortEntry 4 }

   The IF-MIB defines a similar managed object:

   ifAdminStatus OBJECT-TYPE
                   up(1),       -- ready to pass packets
                   testing(3)   -- in some test mode
       MAX-ACCESS  read-write
       STATUS      current
           "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

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           testing(3) states (or remains in the down(2) state)."
       ::= { ifEntry 7 }

   If ifAdminStatus is set to testing(3), the value to be returned for
   dot1dStpPortEnable is not defined.  Without clarification on how
   these two objects interact, management implementations will have to
   monitor both objects if bridging is detected and correlate behavior.

   The dot1dStpPortEnable object type could have been written with more
   information about the behavior of this object when values of
   ifAdminStatus which impact it change.  For example, text could be
   added that described proper return values for the dot1dStpPortEnable
   object instance for each of the possible values of ifAdminStatus.

   In those cases where overlap between objects is unavoidable, then as
   we have just described, care should be taken in the description of
   each of the objects to describe their possible interactions.  In the
   case of an object type defined after an incumbent object type, it is
   necessary to include in the DESCRIPTION of this later object type the
   details of these interactions.

3.6.  Textual Convention Usage

   Textual conventions should be used whenever possible to create a
   consistent semantic for an oft-recurring datatype.

   MIB modules often define a binary state object such as enable/disable
   or on/off.  Current practice is to use existing Textual Conventions
   and define the read-write object in terms of a TruthValue from
   SNMPv2-TC [3].  For example, the Q-BRIDGE-MIB [26] defines:

   dot1dTrafficClassesEnabled OBJECT-TYPE
       SYNTAX      TruthValue
       MAX-ACCESS  read-write
       STATUS      current
          "The value true(1) indicates that Traffic Classes are
          enabled on this bridge.  When false(2), the bridge
          operates with a single priority level for all traffic."
       DEFVAL      { true }
       ::= { dot1dExtBase 2 }

   Textual conventions that have a reasonable chance of being reused in
   other MIB modules ideally should also be defined in a separate MIB
   module to facilitate sharing of such object types.  For example, all
   ATM MIB modules draw on the ATM-TC-MIB [39] to reference and utilize
   common definitions for addressing, service class values, and the

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   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
       MAX-ACCESS  read-write
       STATUS      current
           "Current status of outdoor lighting."
       ::= { acmeOutDoorElectricalEntry 3 }

   This could be defined as follows using existing SNMPv2-TC TruthValue.

   acmePatioLightsOn OBJECT-TYPE
       SYNTAX  TruthValue
       MAX-ACCESS  read-write
       STATUS      current
           "Current status of outdoor lighting.  When set to true (1),
           this means that the lights are enabled and turned on.
           When set to false (2), the lights are turned off."
        ::= { acmeOutDoorElectricalEntry 3 }

3.7.  Persistent Configuration

   Many network devices have two levels of persistence with regard to
   configuration data.  In the first case, the configuration data sent
   to the device is persistent only until changed with a subsequent
   configuration operation, or the system is reinitialized.  The second
   level is where the data is made persistent as an inherent part of the
   acceptance of the configuration information.  Some configuration
   shares both these properties, that is, that on acceptance of new
   configuration data it is saved permanently and in memory.  Neither of
   these necessarily means that the data is used by the operational
   code.  Sometimes separate objects are required to activate this new
   configuration data for use by the operational code.

   However, many SNMP agents presently implement simple persistence
   models, which do not reflect all the relationships of the
   configuration data to the actual persistence model as described
   above.  Some SNMP set requests against MIB objects with MAX-ACCESS
   read-write are written automatically to a persistent store. In other
   cases, they are not.  In some of the latter cases, enterprise MIB

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   objects are required in order to get standard configuration stored,
   thus making it difficult for a generic application to have a
   consistent effect.

   There are standard conventions for saving configuration data.  The
   first method uses the Textual Convention known as StorageType [3]
   which explicitly defines a given row's persistence requirement.

   Examples include the RFC 3231 [25] definition for the schedTable row
   object schedStorageType of syntax StorageType, as well as similar row
   objects for virtually all of the tables of the SNMP View-based Access
   Control Model MIB [10].

   A second method for persistence simply uses the DESCRIPTION clause to
   define how instance data should persist.  RFC 2674 [26] explicitly
   defines Dot1qVlanStaticEntry data persistence as follows:

   dot1qVlanStaticTable OBJECT-TYPE
       SYNTAX      SEQUENCE OF Dot1qVlanStaticEntry
       MAX-ACCESS  not-accessible
       STATUS      current
        "A table containing static configuration information for
        each VLAN configured into the device by (local or
        network) management.  All entries are permanent and will
        be restored after the device is reset."
       ::= { dot1qVlan 3 }

   The current practice is a dual persistence model where one can make
   changes to run-time configuration as well as to a non-volatile
   configuration read at device initialization.  The DISMAN-SCHEDULE-MIB
   module [25] provides an example of this practice.  A row entry of its
   SchedTable specifies the parameters by which an agent MIB variable
   instance can be set to a specific value at some point in time and
   governed by other constraints and directives.  One of those is:

   schedStorageType OBJECT-TYPE
        SYNTAX      StorageType
        MAX-ACCESS  read-create
        STATUS      current
            "This object defines whether this scheduled action is kept
             in volatile storage and lost upon reboot or if this row is
             backed up by non-volatile or permanent storage.
             Conceptual rows having the value `permanent' must allow
             write access to the columnar objects schedDescr,
             schedInterval, schedContextName, schedVariable, schedValue,
             and schedAdminStatus.  If an implementation supports the

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             schedCalendarGroup, write access must be also allowed to
             the columnar objects schedWeekDay, schedMonth, schedDay,
             schedHour, schedMinute."
        DEFVAL { volatile }
        ::= { schedEntry 19 }

   It is important, however, to reiterate that the persistence is
   ultimately controlled by the capabilities and features (with respect
   to the storage model of management data) of the underlying system on
   which the MIB Module agent is being implemented.  This falls into
   very much the same kind of issue set as, for example, the situation
   where the size of data storage in the system for a Counter object
   type is not the same as that in the corresponding MIB Object Type.
   To generalize, the final word on the "when" and "how" of storage of
   persistent data is dictated by the system and the implementor of the
   agent on the system.

3.8.  Configuration Sets and Activation

   An essential notion for configuration of network elements with SNMP
   is awareness of the difference between the set of one or more
   configuration objects from the activation of those configuration
   changes in the actual subsystem.  That is, it often only makes sense
   to activate a group of objects as a single 'transaction'.

3.8.1.  Operational Activation Considerations

   A MIB module design must consider the implications of the preceding
   in the context of changes that will occur throughout a subsystem when
   changes are activated.  This is particularly true for configuration
   changes that are complex.  This complexity can be in terms of
   configuration data or the operational ramifications of the activation
   of the changes in the managed subsystem.  A practical technique to
   accommodate this kind of activation is the partitioning of contained
   configuration sets, as it pertains to their being activated as
   changes.  Any complex configuration should have a master on/off
   switch (MIB object type) as well as strategically placed on/off
   switches that partition the activation of configuration data in the
   managed subsystem.  These controls play a pivotal role during the
   configuration process as well as during subsequent diagnostics.
   Generally, a series of set operations should not cause an agent to
   activate each object, causing operational instability to be
   introduced with every changed object instance.  To avoid this
   liability, ideally a series of Set PDUs can install the configuration
   and a final set series of PDUs can activate the changes.

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   During diagnostic situations, certain on/off switches can be set to
   localize the perceived error instead of having to remove the

   An example of such an object from the OSPF Version 2 MIB [29] is the
   global ospfAdminStat:

   ospfAdminStat OBJECT-TYPE
       SYNTAX   Status
       MAX-ACCESS   read-write
       STATUS   current
          "The administrative status of  OSPF  in the
          router.  The value 'enabled' denotes that the
          OSPF Process is active on at least one interface;
          'disabled' disables it on all interfaces."
      ::= { ospfGeneralGroup 2 }

   Elsewhere in the OSPF MIB, the semantics of setting ospfAdminStat to
   enabled(2) are clearly spelled out.

   The Scheduling MIB [25] exposes such an object on each entry in the
   scheduled actions table, along with the corresponding stats object
   type (with read-only ACCESS) on the scheduled actions row instance.

   This reflects a recurring basic design pattern which brings about
   semantic clarity in the object type usage.  A table can expose one
   columnar object type which is strictly for administrative control.
   When read, an instance of this object type will reflect its last set
   or defaulted value.  A companion operational columnar object type,
   with MAX-ACCESS of read-only, provides the current state of
   activation or deactivation resulting from the last set of the
   administrative columnar instance.  It is fully expected that these
   administrative and operational columnar instances may reflect
   different values over some period of time of activation latency,
   which is why they are separate.  Further sections display some of the
   problems which can result from attempting to combine the operational
   and administrative row columns into a single object type.

   Note that all of this is independent of the RowStatus columnar
   object, and the notion of 'activation' as it pertains to RowStatus.
   A defined RowStatus object type should be strictly concerned with the
   management of the table row itself (with 'activation' indicating "the
   conceptual row is available for use by the managed device" [3], and
   not to be confused with any operational activation semantics).

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   In the following example, schedAdminStatus controls activation of the
   scheduled action, and schedOperStatus reports on its operational

   schedAdminStatus OBJECT-TYPE
       SYNTAX      INTEGER {
       MAX-ACCESS  read-create
       STATUS      current
           "The desired state of the schedule."
       DEFVAL { disabled }
       ::= { schedEntry 14 }

   schedOperStatus OBJECT-TYPE
       SYNTAX      INTEGER {
       MAX-ACCESS  read-only
       STATUS      current
           "The current operational state of this schedule.  The state
            enabled(1) indicates this entry is active and that the
            scheduler will invoke actions at appropriate times.  The
            disabled(2) state indicates that this entry is currently
            inactive and ignored by the scheduler.  The finished(3)
            state indicates that the schedule has ended.  Schedules
            in the finished(3) state are ignored by the scheduler.
            A one-shot schedule enters the finished(3) state when it
            deactivates itself."
       ::= { schedEntry 15 }

3.8.2.  RowStatus and Deactivation

   RowStatus objects should not be used to control
   activation/deactivation of a configuration.  While RowStatus looks
   ideally suited for such a purpose since a management application can
   set a row to active(1), then set it to notInService(2) to disable it
   then make it active(1) again, there is no guarantee that the agent
   won't discard the row while it is in the notInService(2) state.  RFC
   2579 [3], page 15 states:

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      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

   The DISMAN-SCHEDULE-MIB's managed object schedAdminStatus
   demonstrates how to separate row control from row activation.
   Setting the schedAdminStatus to disabled(2) does not cause the row to
   be aged out/removed from the table.

   Finally, a reasonable agent implementation must consider how many
   rows will be allowed to be created in the notReady/notInService state
   such that resources are not exhausted by an errant application.

3.9.  SET Operation Latency

   Many standards track and enterprise MIB modules that contain read-
   write objects assume that an agent can complete a set operation as
   quickly as an agent can send back the status of the set operation to
   the application.

   Consider the subtle operational shortcomings in the following object.
   It both reports the current state and allows a SET operation to
   change to a possibly new state.

wheelRotationState  OBJECT-TYPE
      SYNTAX        INTEGER { unknown(0),
      MAX-ACCESS    read-write
      STATUS        current
      "The current state of a wheel."
  ::= { XXX 2 }

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With the object defined, the following example represents one possible

Time  Command Generator --------> <--- Command Responder
----- -----------------                -----------------
A  GetPDU(wheelRotationState.1.1)
|                          ResponsePDU(error-index 0,
|                                       error-code 0)
B                          wheelRotationState.1.1 == spinClockwise(2)
C  SetPDU(wheelRotationState.1.1 =
|                   spinCounterClockwise(3)
|                          ResponsePDU(error-index 0,
|                                       error-code 0)
D                          wheelRotationState.1.1
                                           == spinCounterClockwise(3)
E  GetPDU(wheelRotationState.1.1)
F                          ResponsePDU(error-index 0,
|                                       error-code 0)
V                          wheelRotationState.1.1 == spinClockwise(2)
   ....some time, perhaps seconds, later....
G       GetPDU(wheelRotationState.1.1)
H                         ResponsePDU(error-index 0,
|                                      error-code 0)
|                       wheelRotationState.1.1
V                                          == spinCounterClockwise(3)

   The response to the GET request at time E will often confuse
   management applications that assume the state of the object should be
   spinCounterClockwise(3).  In reality, the wheel is slowing down in
   order to come to the idle state then begin spinning counter

   This possibility of confusing and paradoxical interactions of
   administrative and operational state is inevitable when a single
   object type is used to control and report on both types of state.
   One common practice which we have already seen is to separate out the

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   desired (settable) state from current state.  The objects
   ifAdminStatus and ifOperStatus from RFC 2863 [20] provide such an
   example of the separation of objects into desired and current state.

3.9.1.  Subsystem Latency, Persistence Latency, and Activation Latency

   A second way latency can be introduced in SET operations is caused by
   delay in agent implementations that must interact with loosely
   coupled subsystems.  The time it takes the instrumented system to
   accept the new configuration information from the SNMP agent, process
   it and 'install' the updated configuration in the system or otherwise
   process the directives can often be longer than the SNMP response

   In these cases, it is desirable to provide a "current state" object
   type which can be polled by the management application to determine
   the state of control of the loosely coupled subsystem which was
   affected by its configuration update.

   More generally, some MIB objects may have high latencies associated
   with changes to their values.  This could be either a function of
   saving the changed value to a persistent storage type, and/or
   activating a subsystem that inherently has high latency as discussed
   above.  When defining such MIB objects, it might be wise to have the
   agent process set operations in the managed subsystem as soon as the
   Set PDU has been processed, and then update appropriate status
   objects when the save-to- persistent storage and (if applicable)
   activation has succeeded or is otherwise complete.  Another approach
   would be to cause a notification to be sent that indicates that the
   operation has been completed.

   When you describe an activation object, the DESCRIPTION clauses for
   these objects should give a hint about the likely latency for the
   completion of the operation.  Keep in mind that from a management
   software perspective (as presented in the example of schedAdminStatus
   in Section 3.8.1), the combined latency of saving-to-persistence and
   activation are not distinguishable when they are part of a single

3.10.  Notifications and Error Reporting

   For the purpose of this section, a 'notification' is as described in
   the SMIv2, RFC 2578 [2], by the NOTIFICATION-TYPE macro.
   Notifications can be sent in either SNMPv2c [19] or SNMPv3 TRAP or
   InformRequest PDUs.  Given the sensitivity of configuration
   information, it is recommended that configuration operations always
   be performed using SNMPv3 due to its enhanced security capabilities.
   InformRequest PDUs should be used in preference to TRAP PDUs since

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   the recipient of the InformRequest PDUs responds with a Response PDU.
   This acknowledgment can be used to avoid unnecessary retransmission
   of NOTIFICATION-TYPE information when retransmissions are in fact
   required.  The use of InformRequest PDUs (as opposed to TRAPs) is not
   at the control of the MIB module designer or agent implementor.  The
   determination as to whether or not a TRAP or InformRequest PDU is
   sent from an SNMPv2c or SNMPv3 agent is generally a function of the
   agent's local configuration (but can be controlled with MIB objects
   in SNMPv3).  To the extent notification timeout and retry values are
   determined by local configuration parameters, care should be taken to
   avoid unnecessary retransmission of InformRequest PDUs.

   Configuration change and error information conveyed in InformRequest
   PDUs can be an important part of an effective SNMP-based management
   system.  They also have the potential to be overused.  This section
   offers some guidance for effective definition of NOTIFICATION-TYPE
   information about configuration changes that can be carried in
   InformRequest PDUs.  Notifications can also play a key role for all
   kinds of error reporting from hardware failures to configuration and
   general policy errors.  These types of notifications should be
   designed as described in Section 3.11 (Application Error Reporting).

3.10.1.  Identifying Source of Configuration Changes

   A NOTIFICATION-TYPE designed to report configuration changes should
   report the identity of the management entity initiating the
   configuration change.  Specifically, if the entity is known to be a
   SNMP command generator, the transport address and SNMP parameters as
   found in table snmpTargetParamsTable from RFC 3413 SNMP-TARGET-MIB
   should be reported where  possible.  For reporting of configuration
   changes outside of the SNMP domain, the applicable change mechanism
   (for example, CLI vs. HTTP-based management client access) should be
   reported, along with whatever notion of "user ID" of the change
   initiator is applicable and available.

3.10.2.  Limiting Unnecessary Transmission of Notifications

   The design of event-driven synchronization models, essential to
   configuration management, can use notifications as an important
   enabling technique.  Proper usage of notifications allows the
   manager's view of the managed element's configuration to be in close
   synchronization with the actual state of the configuration of the
   managed element.

   When designing new NOTIFICATION-TYPEs, consider how to limit the
   number of notifications PDUs that will be sent with the notification
   information defined in the NOTIFICATION-TYPE in response to a
   configuration change or error event.

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   InformRequest PDUs, when compared to TRAP PDUs, have an inherent
   advantage when the concern is the reduction of unnecessary messages
   from the system generating the NOTIFICATION-TYPE data, when in fact
   retransmission of this data is required.  That is, an InformRequest
   PDU is acknowledged by the receiving entity with a Response PDU.  The
   receipt of this response allows the entity which generated the
   InformRequest PDU to verify (and record an audit entry, where such
   facilities exist on the agent system) that the message was received.
   As a matter of notification protocol, this receipt guarantee is not
   available when using TRAP PDUs, and if it is required, must be
   accomplished by the agent using some mechanism out of band to SNMP,
   and usually requiring the penalty of polling.

   Regardless of the specific PDUs used to convey them, one way to limit
   the unnecessary generation of notifications is to include in the
   NOTIFICATION-TYPE definition situations where it need not be sent.  A
   good example is the frDLCIStatusChange defined in FRAME-RELAY-DTE-
   MIB, RFC 2115 [21].

       OBJECTS  { frCircuitState }
       STATUS      current
          "This trap indicates that the indicated Virtual Circuit
          has changed state.  It has either been created or
          invalidated, or has toggled between the active and
          inactive states.  If, however, the reason for the state
          change is due to the DLCMI going down, per-DLCI traps
          should not be generated."
   ::= { frameRelayTraps 1 }

   There are a number of other techniques which can be used to reduce
   the unwanted generation of NOTIFICATION-TYPE information.  When
   defining notifications, the designer can specify a number of temporal
   limitations on the generation of specific instances of a
   NOTIFICATION-TYPE.  For example, a definition could specify that
   messages will not be sent more frequently than once every 60 seconds
   while the condition which led to the generation of the notification
   persists.  Alternately, a NOTIFICATION-TYPE DESCRIPTION clause could
   provide a fixed limit on the number of messages sent over the
   duration of the condition leading to sending the notification.

   If NOTIFICATION-TYPE transmission is "aggregated" in some way -
   bounded either temporally or by absolute system state change as
   described above - the optimal design technique is to have the data
   delivered with the notification reference the actual number of
   underlying managed element transitions which brought about the
   notification.  No matter which threshold is chosen to govern the

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   actual transmission of NOTIFICATION-TYPEs, the idea is to describe an
   aggregated event or related set of events in as few PDUs as possible.

3.10.3.  Control of Notification Subsystem

   There are standards track MIB modules that define objects that either
   augment or overlap control of notifications.  For instance, FRAME-
   RELAY-DTE-MIB RFC 2115 defines frTrapMaxRate and DOCS-CABLE-DEVICE-
   MIB defines a set of objects in docsDevEvent that provide for rate
   limiting and filtering of notifications.

   In the past, agents did not have a standard means to configure a
   notification generator.  With the availability of the SNMP-
   NOTIFICATION-MIB module in RFC 3413 [9], it is strongly recommended
   that the filtering functions of this MIB module be used.  This MIB
   facilitates the mapping of given NOTIFICATION-TYPEs and their
   intended recipients.

   If the mechanisms of the SNMP-NOTIFICATION-MIB are not suitable for
   this application, a explanation of why they are not suitable should
   be included in the DESCRIPTION clause of any replacement control

3.11.  Application Error Reporting

   MIB module designers should not rely on the SNMP protocol error
   reporting mechanisms alone to report application layer error state
   for objects that accept SET operations.

   Most MIB modules that exist today provide very little detail as to
   why a configuration request has failed.  Often the only information
   provided is via SNMP protocol errors which generally does not provide
   enough information about why an agent rejected a set request.
   Typically, there is an incumbent and sizable burden on the
   configuration application to determine if the configuration request
   failure is the result of a resource issue, a security issue, or an
   application error.

   Ideally, when a "badValue" error occurs for a given set request, an
   application can query the agent for more details on the error.  A
   badValue does not necessarily mean the command generator sent bad
   data.  An agent could be at fault.  Additional detailed diagnostic
   information may aid in diagnosing conditions in the integrated

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   Consider the requirement of conveying error information about a MIB
   expression 'object' set within the DISMAN-EXPRESSION-MIB [40] that
   occurs when the expression is evaluated.  Clearly, none of the
   available protocol errors are relevant when reporting an error
   condition that occurs when an expression is evaluated.  Instead, the
   DISMAN-EXPRESSION-MIB provides objects to report such errors (the
   expErrorTable).  Instead, the expErrorTable maintains information
   about errors that occur at evaluation time:

    expErrorEntry OBJECT-TYPE
        SYNTAX      ExpErrorEntry
        MAX-ACCESS  not-accessible
        STATUS      current
        "Information about errors in processing an expression.
        Entries appear in this table only when there is a matching
        expExpressionEntry and then only when there has been an
        error for that expression as reflected by the error codes
        defined for expErrorCode."
   INDEX       { expExpressionOwner, expExpressionName }

   More specifically, a MIB module can provide configuration
   applications with information about errors on the managed device by
   creating columnar object types in log tables that contain error
   information particular to errors that occur on row activation.

   Notifications with detailed failure information objects can also be
   used to signal configuration failures.  If this approach is used, the
   configuration of destinations for NOTIFICATION-TYPE data generated
   from configuration failures should be considered independently of the
   those for other NOTIFICATION-TYPEs which are generated for other
   operational reasons.  In other words, in many management
   environments, the network operators interested in NOTIFICATION-TYPEs
   generated from configuration failures may not completely overlap with
   the community of network operators interested in NOTIFICATION-TYPEs
   generated from, for example, network interface failures.

3.12.  Designing MIB Modules for Multiple Managers

   When designing a MIB module for configuration, there are several
   pertinent considerations to provide support for multiple managers.

   The first is to avoid any race conditions between two or more
   authorized management applications issuing SET protocol operations
   spanning over more than a single PDU.

   The standard textual convention document [3] defines TestAndIncr,
   often called a spinlock, which is used to avoid race conditions.

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   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
           "An advisory lock used to allow several cooperating
           command generator applications to coordinate their
           use of the SNMP set operation.

           This object is used for coarse-grain coordination.
           To achieve fine-grain coordination, one or more similar
           objects might be defined within each MIB group, as
   ::= { snmpSet 1 }

   Another prominent TestAndIncr example can be found in the SNMP-
   TARGET- MIB [9], snmpTargetSpinLock.

   Secondly, an agent should be able to report configuration as set by
   different entities as distinguishable from configuration defined
   external to the SNMP domain, such as application of a default or
   through an alternate management interface like a command line
   interface.  Section 3.10.1 describes considerations for this practice
   when designing NOTIFICATION-TYPEs.  The OwnerString textual
   convention from RMON-MIB RFC 2819 [32] has been used successfully for
   this purpose.  More recently, RFC 3411 [1] introduced the
   SnmpAdminString which has been designed as a UTF8 string.  This is
   more suitable for representing names in many languages.

   Experience has shown that usage of OwnerString to represent row
   ownership can be a useful diagnostic tool as well.  Specifically, the
   use of the string "monitor" to identify configuration set by an
   agent/local management has been prevalent and useful in applications.

   Thirdly, consider whether there is a need for multiple managers to
   configure the same set of tables.  If so, an "OwnerString" may be
   used as the first component of a table's index to allow VACM to be
   used to protect access to subsets of rows, at least at the level of
   securityName or groupName provided.  RFC 3231 [25], Section 6
   presents this technique in detail.  This technique does add
   complexity to the managed device and to the configuration management
   application since the manager will need to be aware of these
   additional columnar objects in configuration tables and act
   appropriately to set them.  Additionally, the agent must be

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   configured to provide the appropriate instance-level restrictions on
   the modifiability of the instances.

3.13.  Other MIB Module Design Issues

3.13.1.  Octet String Aggregations

   The OCTET STRING syntax can be used as an extremely flexible and
   useful datatype when defining managed objects that allow SET
   operation.  An octet string is capable of modeling many things and is
   limited in size to 65535 octets by SMIv2[2].

   Since OCTET STRINGS are very flexible, the need to make them useful
   to applications requires careful definition.  Otherwise, applications
   will at most simply be able to display and set them.

   Consider the following object from RFC 3418 SNMPv2-MIB [11].

   sysLocation OBJECT-TYPE
   SYNTAX      DisplayString (SIZE (0..255))
   MAX-ACCESS  read-write
   STATUS      current
           "The physical location of this node (e.g., `telephone
           closet, 3rd floor').  If the location is unknown, the value
           is the zero-length string."
   ::= { system 6 }

   Such informational object types have come to be colloquially known as
   "scratch pad objects".  While often useful, should an application be
   required to do more with this information than be able to read and
   set the value of this object, a more precise definition of the
   contents of the OCTET STRING is needed, since the actual format of an
   instance for such an object is unstructured.  Hence, alternatively,
   dividing the object type into several object type definitions can
   provide the required additional structural detail.

   When using OCTET STRINGS, avoid platform dependent data formats.
   Also avoid using OCTET STRINGS where a more precise SMI syntax such
   as SnmpAdminString or BITS would work.

   There are many MIB modules that attempt to optimize the amount of
   data sent/received in a SET/GET PDU by packing octet strings with
   aggregate data.  For example, the PortList syntax as defined in the
   Q-BRIDGE-MIB (RFC 2674 [26]) is defined as follows:

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    STATUS      current
        "Each octet within this value specifies a set of eight
        ports, with the first octet specifying ports 1 through
        8, the second octet specifying ports 9 through 16, etc.
        Within each octet, the most significant bit represents
        the lowest numbered port, and the least significant bit
        represents the highest numbered port.  Thus, each port
        of the bridge is represented by a single bit within the
        value of this object.  If that bit has a value of '1'
        then that port is included in the set of ports; the port
        is not included if its bit has a value of '0'."

   This compact representation saves on data transfer but has some
   limitations.  Such complex instance information is difficult to
   reference outside of the object or use as an index to a table.
   Additionally, with this approach, if a value within the aggregate
   requires change, the entire aggregated object instance must be

   Providing an SNMP table to represent aggregate data avoids the
   limitations of encoding data into OCTET STRINGs and is thus the
   better general practice.

   Finally, as previously mentioned in Section, one should
   consider the practical ramifications of instance transfer for object
   types of SYNTAX OCTET STRING where they have typical instance data
   requirements close to the upper boundary of SMIv2 OCTET STRING
   instance encoding.  Where such object types are truly necessary at
   all, SNMP/UDP may not be a very scalable means of transfer and
   alternatives should be explored.

3.13.2.  Supporting multiple instances of a MIB Module

   When defining new MIB modules, one should consider if there could
   ever be multiple instances of this MIB module in a single SNMP

   MIB modules exist that assume a one to many relationship, such as
   MIBs for routing protocols which can accommodate multiple "processes"
   of the underlying protocol and its administrative framework.
   However, the majority of MIB modules assume a one-to-one relationship
   between the objects found in the MIB module and how many instances
   will exist on a given SNMP agent.  The OSPF-MIB, IP-MIB, BRIDGE-MIB
   are all examples that are defined for a single instance of the

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   It is clear that single instancing of these MIB modules limits
   implementations that might support multiple instances of OSPF, IP
   stacks or logical bridges.

   In such cases, the ENTITY-MIB [RFC2737] can provide a means for
   supporting the one-to-many relationship through naming scopes using
   the entLogicalTable.  Keep in mind, however, that there are some
   drawbacks to this approach.

   1) One cannot issue a PDU request that spans naming scopes.  For
      example, given two instances of BRIDGE-MIB active in a single
      agent, one PDU cannot contain a request for dot1dBaseNumPorts from
      both the first and second instances.

   2) Reliance on this technique creates a dependency on the Entity MIB
      for an application to be able to access multiple instances of

   Alternately, completely independently of the Entity MIB, multiple MIB
   module instances can be scoped by different SNMP contexts.  This
   does, however, require the coordination of this technique with the
   administrative establishment of contexts in the configured agent

3.13.3.  Use of Special Optional Clauses

   When defining integer-based objects for read-create, read-write and
   read-only semantics, using the UNITS clause is recommended in
   addition to specification in the DESCRIPTION clause of any particular
   details of how UNITs are to be interpreted.

   The REFERENCE clause is also recommended as a way to help an
   implementer track down related information on a given object.  By
   adding a REFERENCE clause to the specific underlying technology
   document, multiple separate implementations will be more likely to

4.  Implementing SNMP Configuration Agents

4.1.  Operational Consistency

   Successful deployment of SNMP configuration systems depends on
   understanding the roles of MIB module design and agent design.

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   Both module and agent design need to be undertaken with an
   understanding of how UDP/IP-based SNMP behaves.  A current practice
   in MIB design is to consider the idempotency of settable objects.
   Idempotency basically means being able to invoke the same set
   operation repeatedly but resulting in only a single activation.

   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
Manager times out
and sends again
Set2 (Object A, Value B) --->       receives set OK (does nothing),
                          <-------- with a Response PDU(OK)
Manager receives OK

   Had object A been defined in a stateful way, the set operation might
   have caused the Set2 operation to fail as a result of interaction
   with Set1.  If the agent implementation is not aware of such a
   possible situation on the second request, the agent may behave poorly
   by performing the set request again rather than doing nothing.

   The example above shows that all of the software that runs on a
   managed element and in managed applications should be designed in
   concert when possible.  Particular emphasis should be placed at the
   logical boundaries of the management system components in order to
   ensure correct operation.

   1. The first interface is between SNMP agents in managed devices and
      the management applications themselves.  The MIB document is a
      contract between these two entities that defines expected behavior
      - it is a type of API.

   2. The second interface is between the agent and the instrumented
      subsystem.  In some cases, the instrumented subsystem will require
      modification to allow for the dynamic nature of SNMP-based
      configuration, control and monitoring operations.  Agent
      implementors must also be sensitive to the operational code and
      device in order to minimize the impact of management on the
      primary subsystems.

   Additionally, while the SNMP protocol-level and MIB module-level
   modeling of configuration operations may be idempotent and stateless
   from one set operation to another, it may not be that way in the

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   underlying subsystem.  It is possible that an agent may need to
   manage this state in these subsystem architectures explicitly when it
   has placed the underlying subsystem into an "intermediate" state at a
   point in processing a series of SET PDUs.  Alternatively, depending
   on the underlying subsystem in question, the agent may be able to
   buffer all of the configuration set operations prior to activating
   them in the subsystem all at once (to accommodate the nature of the

   As an example, it would be reasonable to define a MIB module to
   control Virtual Private Network (VPN) forwarding, in which a
   management station could set a set of ingress/egress IP addresses for
   the VPN gateway.  Perhaps the MIB module presumes that the level of
   transactionality is the establishment of a single row in a table
   defining the address of the ingress/egress gateway, along with some
   prefix information to assist in routing at the VPN layer to that
   gateway.  However, it would be conceivable that in an underlying
   Layer 2 VPN subsystem instrumentation, the requirement is that all
   existing gateways for a VPN be deleted before a new one can be
   defined--that, in other words, in order to add a new gateway, g(n),
   to a VPN, gateways g(1)..g(n-1) need to be removed, and then all n
   gateways reestablished with the VPN forwarding service.  In this
   case, one could imagine an agent which has some sort of timer to
   establish a bounded window for receipt of SETs for new VPN gateways,
   and to activate them in this removal-then-reestablishment of existing
   and new gateways at the end of this window.

4.2.  Handling Multiple Managers

   Devices are often modified by multiple management entities and with
   different management techniques.  It is sometimes the case that an
   element is managed by different organizations such as when a device
   sits between administrative domains.

   There are a variety of approaches that management software can use to
   ensure synchronization of information between the manager(s) and the
   managed elements.

   An agent should report configuration changes performed by different
   entities.  It should also distinguish configuration defined locally
   such as a default or locally specified configuration made through an
   alternate management interface such as a command line interface.
   When a change has been made to the system via SNMP, CLI, or other
   method, a managed element should send an notification to the
   manager(s) configured as recipients of these applicable
   notifications.  These management applications should update their

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   local configuration repositories and then take whatever additional
   action is appropriate.  This approach can also be an early warning of
   undesired configuration changes.

   Managers should also develop mechanisms to ensure that they are
   synchronized with each other.

4.3.  Specifying Row Modifiability

   Once a RowStatus value is active(1) for a given row, the management
   application should be able to determine what the semantics are for
   making additional changes to a row.  The RMON MIB control table
   objects spell out explicitly what managed objects in a row can and
   cannot be changed once a given RowStatus goes active.

   As described earlier, some operations take some time to complete.
   Some systems also require that they remain in a particular state for
   some period before moving to another.  In some cases, a change to one
   value may require re-initialization of the system.  In all of these
   cases, the DESCRIPTION clause should contain information about
   requirements of the managed system and special restrictions that
   managers should observe.

4.4.  Implementing Write-only Access Objects

   The second version of the SNMP SMI dropped direct support for a
   write-only object. It is therefore necessary to return something when
   reading an object that you may have wished to have write-only
   semantics.  Such objects should have a DESCRIPTION clause that
   details what the return values should be.  However, regardless of the
   approach, the value returned when reading the object instance should
   be meaningful in the context of the object's semantics.

5.  Designing Configuration Management Software

   In this section, we describe practices that should be used when
   creating and deploying management software that configures one or
   more systems using SNMP.  Functions all configuration management
   software should provide, regardless of the method used to convey
   configuration information to the managed systems are backup, fail-
   over, and restoration.  A management system should have the following

   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.

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   2. A method for saving back up versions of the configuration data in
      case of hardware or software failure.

   3. A method of providing fail-over to a secondary (management) system
      in case of a primary failure.  This capability should be deployed
      in such a way that it does not cause duplicate polling of

   These three capabilities are of course important for other types of
   management that are not the focus of this BCP.

5.1.  Configuration Application Interactions with Managed Systems

   From the point of view of the design of the management application,
   there are three basic requirements to evaluate relevant to SNMP
   protocol operations and configuration:

      o Set and configuration activation operations

      o Notifications from the device

      o Data retrieval and collection

   Depending on the requirements of the specific services being
   configured, many other requirements may, and probably will, also be

   The design of the system should not assume that the objects in a
   device that represent configuration data will remain unchanged over

   As standard MIB modules evolve and vendors add private extensions,
   the specific configuration parameters for a given operation are
   likely to change over time.  Even in the case of a configuration
   application that is designed for a single vendor, the management
   application should allow for variability in the MIB objects that will
   be used to configure the device for a particular purpose.  The best
   method to accomplish this is by separating, as much as possible, the
   operational semantics of a configuration operation from the actual
   data. One way that some applications achieve this is by having the
   specific configuration objects that are associated with a particular
   device be table driven rather than hard coded.  Ideally, management
   software should verify the support in the devices it is expected to
   manage and report any unexpected deviations to the operator.  This
   approach is particularly valuable when developing applications that
   are intended to support equipment or software from multiple vendors.

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5.1.1.  SET Operations

   Management software should be mindful of the environment in which SET
   operations are being deployed.  The intent here is to move
   configuration information as efficiently as possible to the managed
   device.  There are many ways to achieve efficiency and some are
   specific to given devices.  One general case that all management
   software should employ is to reduce the number of SET PDU exchanges
   between the managed device and the management software to the
   smallest reasonable number.  One approach to this is to verify the
   largest number of variable bindings that can fit into a SET PDU for a
   managed device.  In some cases, the number of variable bindings to be
   sent in a particular PDU will be influenced by the device, the
   specific MIB objects and other factors.

   Maximizing the number of variable bindings in a SET PDU also has
   benefits in the area of management application transaction
   initiation, as we will discuss in the following section.

   There are, though, agents that may have implementation limitations on
   the number and order of varbinds they can handle in a single SET PDU.
   In this case, sending fewer varbinds will be necessary.

   As stated at the outset of this section, the management application
   software designer must be sensitive to the design of the SNMP
   software in the managed device.  For example, the software in the
   managed device may require that all that all related configuration
   information for an operation be conveyed in a single PDU because it
   has no concept of a transaction beyond a single SNMP PDU.  Another
   example has to do with the RowStatus textual convention.  Some SNMP
   agents implement a subset of the features available and as such the
   management application must avoid using features that may not be
   supported in a specific table implementation (such as createAndWait).

5.1.2.  Configuration Transactions

   There are several types of configuration transactions that can be
   supported by SNMP-based configuration applications.  They include
   transactions on a scalar object, transactions in a single table
   (within and across row instances), transactions across several tables
   in a managed device and transactions across many devices.  The
   manager's ability to support these different transactions is partly
   dependent on the design of the MIB objects used in the configuration

   To make use of any kind of transaction semantics effectively, SNMP
   management software must be aware of the information in the MIB
   modules that it is to configure so that it can effectively utilize

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   RowStatus objects for the control of transactions on one or more
   tables.  Such software must also be aware of control tables that the
   device supports that are used to control the status of one or more
   other tables.

   To the greatest extent possible, the management application should
   provide the facility to support transactions across multiple devices.
   This means that if a configuration operation is desired across
   multiple devices, the manager can coordinate these configuration
   operations such that they become active as close to simultaneously as

   Several practical means are present in the SNMP model that support
   management application level transactions.  One was mentioned in the
   preceding section, that transactions can be optimized by including
   the maximum number of SET variable bindings possible in a single PDU
   sent to the agent.

   There is an important refinement to this.  The set of read-create row
   data objects for tables should be sent in a single PDU, and only
   placed across multiple PDUs if absolutely necessary.  The success of
   these set operations should be verified through the response(s) to
   the Set PDU or subsequent polling of the row data objects.  The
   applicable RowStatus object(s), may be set to active only after this
   verification.  This is the only tractable means of affording an
   opportunity for per-row rollback, particularly when the configuration
   change is across table row instances on multiple managed devices.

   Finally, where a MIB module exposes the kind of helpful transaction
   management object types that were discussed in Section 3.3.5, it is
   clearly beneficial to the integrity of the management application's
   capacity to handle transactions to make use of them.

5.1.3.  Tracking Configuration Changes

   As previously described in Section 3.3.5 (Summary Objects and State
   Tracking), agents should provide the capability for notifications to
   be sent to their configured management systems whenever a
   configuration operation is completed or is detected to have failed.
   The management application must be prepared to accept these
   notifications so that it knows the current configured state of the
   devices under its control.  Upon receipt of the notification, the
   management application should use getBulk or getNext to retrieve the
   configuration from the agent and store the relevant contents in the
   management application database.  The GetBulkRequest-PDU is useful
   for this whenever supported by the managed device, since it is more
   efficient than the GetNextRequest-PDU when retrieving large amounts

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   of data.  For the purposes of backward compatibility, the management
   station should also support and make use of the GetNextRequest-PDU
   when the agent does not support the GetBulkRequest-PDU.

   Management systems should also provide configuration options with
   defaults for users that tend to retrieve the smallest amount of data
   to achieve the particular goal of the application, to avoid
   unnecessary load on managed devices for the most common retrieval

5.1.4.  Scalability of Data Retrieval

   The techniques for efficient data retrieval described in the
   preceding sections comprise only one aspect of what application
   developers should consider in this regard when developing
   configuration applications.  Management applications should provide
   for distributed processing of the configuration operations.  This
   also extends to management functions that are not the focus of this
   document.  Techniques of distributed processing can also be used to
   provide resilience in the case of network failures.  An SNMP-based
   configuration management system might be deployed in a distributed
   fashion where three systems in different locations keep each other
   synchronized.  This synchronization can be accomplished without
   additional polling of network devices through a variety of
   techniques.  In the case of a failure, a 'backup' system can take
   over the configuration responsibilities from the failed manager
   without having to re-synchronize with the managed elements since it
   will already be up to date.

6.  Deployment and Security Issues

   Now that we have considered the design of SNMP MIB data for
   configuration, agent implementation of its access, and management
   application issues in configuration using SNMP, we turn to a variety
   of operational considerations which transcend all three areas.

6.1.  Basic assumptions about Configuration

   The following basic assumptions are made about real world
   configuration models.

   1) Operations must understand and must be trained in the operation of
      a given technology.  No configuration system can prevent an
      untrained operator from causing outages due to misconfiguration.

   2) Systems undergoing configuration changes must be able to cope with
      unexpected loss of communication at any time.

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      During configuration operations, network elements must take
      appropriate measures to leave the configuration in a
      consistent/recognizable state by either rolling back to a
      previously valid state or changing to a well-defined or default

   3) Configuration exists on a scale from relatively unchanging to a
      high volume, high rate of change.  The former is often referred to
      as "set and forget" to indicate that the configuration changes
      quite infrequently.  The latter, "near real-time change control"
      implies a high frequency of configuration change.  Design of
      configuration management must take into account the rate and
      volume of change expected in a given configuration subsystem.

6.2.  Secure Agent Considerations

   Vendors should not ship a device with a community string 'public' or
   'private', and agents should not define default community strings
   except when needed to bootstrap devices that do not have secondary
   management interfaces.  Defaults lead to security issues that have
   been recognized and exploited.  When using SNMPv1, supporting read-
   only community strings is a common practice.

   Version 3 of the SNMP represents the current standard for the
   Internet Management Framework and is recommended for all network
   management applications.  In particular, SNMPv3 provides
   authorization, authentication, and confidentiality protection and is
   essential to meeting the security considerations for all management
   of devices that support SNMP-based configuration.

6.3.  Authentication Notifications

   The default state of RFC 1215 [17] Authentication notifications
   should be off.  One does not want to risk accidentally sending out
   authentication failure information, which by itself could constitute
   a security liability.  Enabling authentication Notifications should
   be done in the context of a management security scheme which
   considers the proper recipients of this information.

   There are other liabilities where authentication notifications are
   generated without proper security infrastructure.  When notifications
   are sent in SNMPv1 trap PDUs, unsolicited packets to a device can
   causes one or more trap PDUs to be created and sent to management
   stations.  If these traps flow on shared access media and links, the
   community string from the trap may be gleaned and exploited to gain
   access to the device.  At the very least, this risk should be
   mitigated by having the authentication trap PDU be conveyed with a

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   community string which is only used for authentication traps from the
   agent, and would be useless for access inbound to the agent to get at
   other management data.

   A further liability of authentication traps can be seen when they are
   being generated in the face of a Denial Of Service (DOS) attack, in
   the form of a flood of PDUs with invalid community strings, on the
   agent system.  If it is bad enough that the system is having to
   respond to and recover from the invalid agent data accesses, but the
   problem will be compounded if a separate Authentication notification
   PDU is sent to each recipient on the management network.

6.4.  Sensitive Information Handling

   Some MIB modules contain objects that may contain data for keys,
   passwords and other such sensitive information and hence must be
   protected from unauthorized access.  MIB documents that are created
   in the IETF must have a 'Security Considerations' section, which
   details how sensitive information should be protected.  Similarly,
   MIB module designers who create MIB documents for private MIB objects
   should include similar information so that users of the products
   containing these objects can take appropriate precautions.

   Even if a device does support DES, it should be noted that
   configuration of keys for other protocols via SNMP Sets protected by
   DES should not be allowed if the other keys are longer than the 56
   bit DES keys protecting the SNMP transmission.

   The DESCRIPTION clause for these object types and their Security
   Considerations sections in the documents which define them should
   make it clear how and why these specific objects are sensitive and
   that a user should only make them accessible for encrypted SNMP
   access.  Vendors should also document sensitive objects in a similar

   Confidentiality is not a mandatory portion of the SNMPv3 management
   framework [6].

   Prior to SNMPv3, providing customized views of MIB module data was
   difficult.  This led to objects being defined such as the following
   from [41].

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   docsDevNmAccessEntry OBJECT-TYPE
       SYNTAX      DocsDevNmAccessEntry
       MAX-ACCESS  not-accessible
       STATUS      current
           "An entry describing access to SNMP objects by a
            particular network management station.  An entry in
            this table is not readable unless the management station
            has read-write permission (either implicit if the table
            is empty, or explicit through an entry in this table.
            Entries are ordered by docsDevNmAccessIndex.  The first
            matching entry (e.g., matching IP address and community
            string) is used to derive access."
       INDEX { docsDevNmAccessIndex  }
       ::= {  docsDevNmAccessTable 1 }

   New MIB modules should capitalize on existing security capabilities
   of SNMPv3 Framework.  One way they can do this is by indicating the
   level of security appropriate to different object types.  For
   example, objects that change the configuration of the system might be
   protected by using the authentication mechanisms in SNMPv3.
   Specifically, it is useful to design MIB module object grouping with
   considerations for VACM views definition, such that users can define
   and properly scope what tables are visible to a given user and view.

7.  Policy-based Management

   In some designs and implementations, a common practice used to move
   large amounts of data involves using SNMP as a control channel in
   combination with other protocols defined for transporting bulk data.
   This approach is sub-optimal since it raises a number of security and
   other concerns.  Transferring large amounts of configuration data via
   SNMP can be efficiently performed with several of the techniques
   described earlier in this document.  This policy section shows how
   even greater efficiency can be achieved using a set of relatively new
   design mechanisms.  This section gives background and defines terms
   that are relevant to this field and describes some deployment

7.1.  What Is the Meaning of 'Policy-based'?

   In the past few years of output from standards organizations and
   networking vendor marketing departments, the term 'policy' has been
   heavily used, touted, and contorted in meaning.  The result is that
   the true meaning of 'policy' is unclear without greater qualification
   where it is used.

   [42] gives the term 'policy' two explicit definitions:

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   -  A definite goal, course or method of action to guide and determine
      present and future decisions.  "Policies" are implemented or
      executed within a particular context (such as policies defined
      within a business unit).

   -  Policies as a set of rules to administer, manage, and control
      access to network resources.

      Note that these two views are not contradictory since individual
      rules may be defined in support of business goals.

   As it pertains to our discussion of the term 'policy-based
   configuration', the meaning is significantly more specific.  In this
   context, we refer to a way of integrating data and the management
   actions which use it in such a way that:

   -  there is the ability to specify "default" configuration data for a
      number of instances of managed elements, where those instances can
      be correlated in some data driven or algorithmic way.  The engine
      to do this correlation and activate instances from defaults may
      reside in the agent or externally.  Where the representation of
      these defaults are in the MIB design itself, the object types
      supporting this notion are referred to as "template objects".

   -  the activation of instance data derived from template object types
      results from minimal activation directives from the management
      application, once the instances of the template object types have
      been established.

   -  somewhat independently, the architecture of the overall management
      agent may accommodate the definition and evaluation of management
      and configuration policies.  The side-effects of the evaluation of
      these policies typically include the activation of certain
      configuration directives.  Where management data design exposes
      template object types, the policy-driven activation can (and
      ideally, should) include the application of template object
      instances to the analogous managed element instance-level values.

   As it pertains to template object data, the underlying notions
   implied here have been prevalent for some time in non-SNMP management
   regimes.  A common feature of many command line interfaces for
   configuring routers is the specification of one or more access
   control lists.  These typically provide a set of IP prefixes, BGP
   autonomous system numbers, or other such identifying constructs (see,
   for example, [42]).  Once these access control lists are assembled,
   their application to various interfaces, routing processes, and the
   like are specified typically in the configuration of what the access
   control list is applied to.  Consistent with the prior properties to

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   define our use of policy-based configuration, a) the access list is
   defined independent from its point of application, and b) its
   application is independent of the access list definition.  For
   example, changing the application of an access list from one
   interface to the other does not require a change in the access list
   itself.  The first point just mentioned suggests what is necessary
   for template-based data organization.  The second suggests its
   application in a policy-based manner.

   Let us now examine the motivation for such a system or subsystem
   (perhaps bounded at the level of a 'template-enabled' MIB module,
   given the above definition).  Let us explore the importance of
   policy-based techniques to configuration specifically.

7.2.  Organization of Data in an SNMP-Based Policy System

   The number of configurable parameters and 'instances' such as
   interfaces has increased as equipment has become larger and more

   At the same time, there is a need to configure many of these systems
   to operate in a coordinated fashion.  This enables the delivery of
   new specialized services that require this coordinated configuration.
   Examples include delivery of virtual private networks and connections
   that guarantee specific service levels.

   The growth in size and complexity of configuration information has
   significant implications for its organization as well as its
   efficient transfer to the management agent.  As an example, an agent
   that implements the Bridge MIB [24] could be used to represent a
   large VLAN with some 65,000 port entries.  Configuring such a VLAN
   would require the establishment of dot1dStpPortTable and
   dot1DStaticTable entries for each such virtual port.  Each table
   entry would contain several parameters.  A more efficient approach is
   to provide default values for the creation of new entries that are
   appropriate to the VLAN environment in our example.  The local
   management infrastructure should then iterate across the system
   setting the default values to the selected ports as groups.

   To date, this kind of large-scale configuration has been accomplished
   with file transfer, by setting individual MIB objects, or with many
   CLI commands.  In each of these approaches the details for each
   instance are contained in the file, CLI commands or MIB objects.
   That is, they contain not only the value, and type of object, but
   also the exact instance of the object to which to apply the value.
   It is this property that tends to make configuration operations
   explode as the number of instances (such as interfaces) grows.  This
   per-instance approach can work for a few machines configured by

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   experts, but there is a need for a more scalable solution.
   Template-based data organization and policy-based management
   abstracts the details above the instance level, which means that
   fewer SET requests are sent to a managed device.

   Realization of such a policy-driven system requires agents that can
   take defaults and apply them to instances based on a rule that
   defines under what conditions the defaults (policy) are to be
   applied.  A policy-driven configuration system which is to be
   scalable needs to expose a means of layering its application of
   defaults at discrete ranges of granularity.  The spectrum of that
   granularity might have a starting hierarchy point to apply defaults
   at the breadth of a network service.

   Ultimately, such a layering ends up with features to support
   instance-level object instance data within the running agent.

   An example of this kind of layering is implicit in the principle of
   operations of a SNMPCONF Policy-Based Management MIB [36] (PM-MIB)
   implementation.  However, other entity management systems have been
   employing these kinds of techniques end-to-end for some time, in some
   cases using SNMP, in some cases using other encodings and transfer
   technologies.  What the PM-MIB seeks to establish, in an environment
   ideal for its deployment, is an adaptation between MIB module data
   which was not designed using template object types, and the ability
   to allow the PM-MIB agent engine to apply instances of that data as
   though it were template-based.

7.3.  Information Related to Policy-based Configuration

   In order for effective policy management to take place, a range of
   information about the network elements is needed to avoid making poor
   policy decisions.  Even in those cases where policy-based
   configuration is not in use, much of the information described in
   this section can be useful input to the decision-making process about
   what type of configuration operations to do.

   For this discussion it is important to make distinctions between
   distribution of policy to a system, activation of a policy in a
   system, and changes/failures that take place during the time the
   policy is expected to be active.  For example, if an interface is
   down that is included in a policy that is distributed, there may not
   be an error since the policy may not be scheduled for activation
   until a later time.

   On the other hand, if a policy is distributed and applied to an
   interface that should be operational and it is not, clearly this is a
   problem, although it is not an error in the configuration policy

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   itself.  With this as background, here are some areas to consider
   that are important to making good policy configuration decisions and
   establishing when a policy has 'failed'.

   o  The operational state of network elements that are to be

      Care should be taken to determine if the sub-components to be
      configured are available for use.  In some cases the elements may
      not be available.  The policy configuration software should
      determine if this is a prerequisite to policy installation or if
      the condition is even acceptable.  This decision is separate from
      the one to be made about policy activation.  Installation is when
      the policy is sent from the policy manager to the managed device
      and activation is turning on the policy.  In those cases where
      policy is distributed when the sub-component such as an interface
      or disk is not available, the managed system should send a
      notification to the designated management station when the policy
      is to become active or if the resource is still not available.

   o  The capabilities of the devices in the network.

      A capability can be almost any unit of work a network element can
      perform.  These include routing protocols supported, Web server
      and OS versions, queuing mechanisms supported on each interface
      that can be used to support different qualities of service, and
      many others.  This information can be obtained from the
      capabilities table of the Policy MIB module [36].

      Historically, management applications have had to obtain this type
      of information by issuing get requests for objects they might want
      to use.  This approach is far less efficient since it requires
      many get requests and is more error prone since some instances
      will not exist until configured.  The new capabilities table is an
      improvement on the current technique.

   o  The capacity of the devices to perform the desired work.

      Capability is an ability to perform the desired work while a
      capacity is a measure of how much of that capability the system
      has.  The policy configuration application should, wherever
      possible, evaluate the capacity of the network element to perform
      the work identified by the policy.  In some systems it will not be
      possible to obtain the capacity of the managed elements to perform
      the desired work directly, even though it may be possible to
      monitor the amount of work the element performs.  In these cases,
      the management application may benefit from pre-configured

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      information about the capacity of different network elements so
      that evaluations of the resources available can be done before
      distributing new policies.

      Utilization refers to how much capacity for a particular
      capability has been consumed.  For devices that have been under
      policy configuration control for any period of time, a certain
      percentage of the available capacity of the managed elements will
      be used.  Policies should not be distributed to systems that do
      not have the resources to carry out the policy in a reasonable
      period of time.

7.4.  Schedule and Time Issues

   This section applies equally to systems that are not policy-based as
   well as policy-based systems, since configuration operations often
   need to be synchronized across time zones.  Wherever possible, the
   network elements should support time information using the standard
   DateAndTime TC that includes local time zone information.  Policy-
   based management often requires more complex time expressions than
   can be conveyed with the DateAndTime TC.  See the Policy-Based
   Management MIB document [36] for more information.  Some deployed
   systems do not store complex notions of local time and thus may not
   be able to process policy directives properly that contain time zone
   relevant data.  For this reason, policy management applications
   should have the ability to ascertain the time keeping abilities of
   the managed system and make adjustments to the policy for those
   systems that are time-zone challenged.

7.5.  Conflict Detection, Resolution and Error Reporting

   Policies sent to a device may contain conflicting instructions.
   Detection of such commands can occur at the device or management
   level and may be resolved using any number of mechanisms (examples
   are, last configuration set wins, or abort change).  These unintended
   conflicts should be reported.  Conflicts can occur at different
   levels in a chain of commands.  Each 'layer' in policy management
   system should be able to check for some errors and report them.  This
   is conceptually identical to programs raising an exception and
   passing that information on to software that can do something
   meaningful with it.

   At the instance level, conflict detection has been performed in a
   limited way for some time in software that realizes MIB objects at
   this level of resolution.  This detection is independent of policy.
   The types of 'conflicts' usually checked for are resource
   availability and validity of the set operations.  In a policy enabled

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   system, there are no additional requirements for this software
   assuming that good error detection and reporting appropriate to this
   level have already been implemented.

7.5.1.  Changes to Configuration Outside of the Policy System

   It is essential to consider changes to configuration that are
   initiated outside of the policy system.  A goal of SNMP-based policy
   management is to coexist with other kinds of management software that
   have historically been instance based management.  The best example
   is the command line interface.

   A notification should be sent whenever an out-of-policy control
   change is made to an element that is under the control of policy.
   This notification should include the policy that was affected, the
   instance of the element that was changed and the object and value
   that it was changed to.

   Even for those systems that have no concept of policy control, the
   ideas presented above make sense.  That is, if SNMP co-exists with
   other access methods such as a CLI, it is essential that the
   management station remain synchronized with changes that might have
   been made to the managed device using other methods.  As a result,
   the approach of sending a notification when another access method
   makes a change is a good one.  Of course this should be configurable
   by the user.

7.6.  More about Notifications in a Policy System

   Notifications can be useful in determining a failure of a policy as a
   result of an error in the policy or element(s) under policy control.
   As with all notifications, they should be defined and controlled in
   such a way that they do not create a problem by sending more than are
   helpful over a specific period of time.  For example, if a policy is
   controlling 1,000 interfaces and fails, one notification rather than
   1,000 may be the better approach.  In addition, such notifications
   should be defined to include as much information as possible to aid
   in problem resolution.

7.7.  Using Policy to Move Less Configuration Data

   One of the advantages of policy-based configuration with SNMP is that
   many configuration operations can be conveyed with a small amount of
   data.  Changing a single configuration parameter for each of 100
   interfaces on a system might require 100 CLI commands or 100 SNMP
   variable bindings using conventional techniques.

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   Using policy-based configuration with SNMP, a single SET PDU can be
   sent with the policy information necessary to apply a configuration
   change to 100 similar interfaces.  This efficiency gain is the result
   of eliminating the need to send the value for each instance to be
   configured.  The 'default' for each of the instances included in the
   policy is sent, and the rule for selection of the instances that the
   default is to be applied to can also be carried (see the Policy MIB
   module [36]).

   To extend the example above, assume that there are 10 parameters that
   need to change.  Using conventional techniques, there would now be
   1,000 variable bindings, one for each instance of each new value for
   each interface.  Using policy-based configuration with SNMP, it is
   still likely that all the information can be conveyed in one SET PDU.
   The only difference in this case is that there are ten parameters
   sent that will be the 'template' used to create instances on the
   managed interfaces.

   This efficiency gain not only applies to SET operations, but also to
   those management operations that require configuration information.
   Since the policy is also held in the storage for cross-instance
   defaults (for example, the pmPolicyTable in [36]), an entire data set
   that potentially controls hundreds of rows of information can be
   retrieved in a single GET request.

   A policy-friendly data organization such as this is consistent and
   integrates well with MIB module objects which support "summary"
   activation and activation reporting, of the kind discussed in Section

8.  Example MIB Module With Template-based Data

   This section defines a MIB module that controls the heating and air
   conditioning system for a large building.  It contains both
   configuration and counter objects that allow operators to see how
   much cooling or heating a particular configuration has consumed.
   Objects that represent the configuration information at a "default"
   level (as referenced above) are also included.

   These tables, in combination with the application of the tables' row
   instance data as templated 'defaults', will allow operators to
   configure and monitor many rooms at once, change the configuration
   parameters based on time of day, and make a number of other
   sophisticated decisions based on the 'policy' implied by these
   defaults and their application.  For this reason, these configuration
   controls have their instances specified from template object types.

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   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


A single index reference to a table that 'glues' configuration
information defaults with descriptive information:


A set of objects that show status and units of
work (bldgHVACCoolOrHeatMins) and standard per-row
SnmpAdminString, StorageType, and RowStatus columnar


Step 2: A configuration description table.  The purpose of this
        table is to provide a unique string identifier for
        templates.  These may be driven by policies in a
        network.  If it were necessary to configure devices
        to deliver a particular quality of service, the
        index string of this table could be the name and the
        description part, it could be a brief description of the
        underlying motivation such as: "provides extra heat to
        corner offices to counteract excessive exterior wind

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        chill".  Standard owner and status objects may also
        be helpful and are included here.  The row columnar
        objects are:


Notice that to this point we have provided no
configuration information.  That will be in the next
table.  Some readers may wonder why this table is not
combined with the configuration template table described
in the next step.  In fact, they can be.  The reason for
having a separate table is that as systems become more
complex, there may be more than one configuration table
that points to these descriptions.  Another reason for
two tables is that this in not reproduced for every
template and instance, which can save some additional
data movement.  Every designer will have to evaluate the
tradeoffs between number of objects and data movement
efficiency just as with other MIB modules.

Step 3: The bldgHVACCfgTemplateTable contains the specific
        configuration parameters that are pointed to by the
        bldgHVACConfigPtr object.  Note that many rows in the
        bldgHVACTable can point to an entry in this table.  It
        is also possible for entries to be used by 1 or 0 rows
        of the bldgHVACTable.  It is the property of allowing
        multiple rows (instances) in the bldgHVACTable to
        point to a row in this table that can produce such
        efficiency gains from policy-based management with
        SNMP.  Also notice that the configuration data is tied
        directly to the counter data so that people can see
        how configurations impact behavior.

The objects in this table are all that are necessary
for configuration and connection to the other tables as
well as the usual SnmpAdminString, StorageType, and
RowStatus objects:

A simple index to the table:


The configuration objects:

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Administrative objects for SnmpAdminString and


8.1.  MIB Module Definition

    MODULE-IDENTITY, Counter32,
    Gauge32, OBJECT-TYPE, Unsigned32, experimental
        FROM SNMPv2-SMI
    TimeStamp, RowStatus, StorageType
        FROM SNMPv2-TC

    LAST-UPDATED "200303270000Z"
    ORGANIZATION "SNMPCONF working group
        "Jon Saperia
        Postal:     JDS Consulting
                    174 Chapman Street
                    Watertown, MA 02472
        Phone:      +1 617 744 1079

        Wayne Tackabury
        Postal:     Gold Wire Technology
                    411 Waverley Oaks Rd.
                    Waltham, MA 02452
        Phone:      +1 781 398 8800

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        Michael MacFaden
        Postal:     Riverstone Networks
                    5200 Great America Pkwy.
                    Santa Clara, CA 95054
        Phone:      +1 408 878 6500

        David Partain
        Postal:     Ericsson AB
                    P.O. Box 1248
                    SE-581 12  Linkoping
        "This example MIB module defines a set of management objects
        for heating ventilation and air conditioning systems.  It
        also includes objects that can be used to create policies
        that are applied to rooms.  This eliminates the need to send
        per-instance configuration commands to the system.

        Copyright (C) The Internet Society (2003).  This version of
        this MIB module is part of RFC 3512; see the RFC itself for
        full legal notices."

    REVISION "200303270000Z"
        "Initial version of BLDG-HVAC-MIB as published in RFC 3512."
    ::= { experimental 122 }

bldgHVACObjects         OBJECT IDENTIFIER ::= { bldgHVACMIB 1 }
bldgConformance         OBJECT IDENTIFIER ::= { bldgHVACMIB 2 }

-- Textual Conventions

BldgHvacOperation  ::= TEXTUAL-CONVENTION
    STATUS             current
        "Operations supported by a heating and cooling system.
        A reference to underlying general systems would go here."
-- HVAC Objects Group

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    MAX-ACCESS  not-accessible
    STATUS      current
        "This table is the representation and data control
        for building HVAC by each individual office.
        The table has rows for, and is indexed by a specific
        floor and office number.  Each such row includes
        HVAC statistical and current status information for
        the associated office.  The row also contains a
        bldgHVACCfgTemplate columnar object that relates the
        bldgHVACTable row to a row in the bldgHVACCfgTemplateTable.
        If this value is nonzero, then the instance in the row
        that has a value for how the HVAC has been configured
        in the associated template (bldgHVACCfgTeplateTable row).
        Hence, the bldgHVACCfgTeplateTable row contains the
        specific configuration values for the offices as described
        in this table."
    ::= { bldgHVACObjects 1 }

    SYNTAX       BldgHVACEntry
    MAX-ACCESS   not-accessible
    STATUS       current
        "A row in the bldgHVACTable.  Each row represents a particular
        office in the building, qualified by its floor and office
        number.  A given row instance can be created or deleted by
        set operations  upon its bldgHVACStatus columnar
        object instance."
    INDEX { bldgHVACFloor, bldgHVACOffice }
        ::= { bldgHVACTable 1 }

BldgHVACEntry ::= SEQUENCE {
        bldgHVACFloor             Unsigned32,
        bldgHVACOffice            Unsigned32,
        bldgHVACCfgTemplate       Unsigned32,
        bldgHVACFanSpeed          Gauge32,
        bldgHVACCurrentTemp       Gauge32,
        bldgHVACCoolOrHeatMins    Counter32,
        bldgHVACDiscontinuityTime TimeStamp,
        bldgHVACOwner             SnmpAdminString,
        bldgHVACStorageType       StorageType,
        bldgHVACStatus            RowStatus

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    SYNTAX      Unsigned32 (1..1000)
    MAX-ACCESS  not-accessible
    STATUS      current
        "This portion of the index indicates the floor of the
         building.  The ground floor is considered the
         first floor.  For the purposes of this example,
         floors under the ground floor cannot be
         controlled using this MIB module."
    ::= { bldgHVACEntry 1 }

    SYNTAX      Unsigned32 (1..2147483647)
    MAX-ACCESS  not-accessible
    STATUS      current
        "This second component of the index specifies the
        office number."
    ::= { bldgHVACEntry 2 }

bldgHVACCfgTemplate  OBJECT-TYPE
    SYNTAX          Unsigned32
    MAX-ACCESS      read-create
    STATUS          current
        "The index (bldgHVACCfgTemplateIndex instance)
        of an entry in the 'bldgHVACCfgTemplateTable'.
        The bldgHVACCfgTable row instance referenced
        is a pre-made configuration 'template'
        that represents the configuration described
        by the bldgHVACCfgTemplateInfoDescr object.  Note
        that not all configurations will be under a
        defined template.  As a result, a row in this
        bldgHVACTable may point to an entry in the
        bldgHVACCfgTemplateTable that does not in turn
        have a reference (bldgHVACCfgTemplateInfo) to an
        entry in the bldgHVACCfgTemplateInfoTable.  The
        benefit of this approach is that all
        configuration information is available in one
        table whether all elements in the system are
        derived from configured templates or not.

        Where the instance value for this colunmar object
        is zero, this row represents data for an office
        whose HVAC status can be monitored using the
        read-only columnar object instances of this
        row, but is not under the configuration control

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        of the agent."
    ::= { bldgHVACEntry 3 }

    SYNTAX            Gauge32
    UNITS             "revolutions per minute"
    MAX-ACCESS        read-only
    STATUS            current
        "Shows the revolutions per minute of the fan.  Fan speed
        will vary based on the difference between
        bldgHVACCfgTemplateDesiredTemp and bldgHVACCurrentTemp.  The
        speed is measured in revolutions of the fan blade per minute."
    ::= { bldgHVACEntry 4 }

bldgHVACCurrentTemp  OBJECT-TYPE
    SYNTAX            Gauge32
    UNITS             "degrees in celsius"
    MAX-ACCESS        read-only
    STATUS            current
        "The current measured temperature in the office.  Should
        the current temperature be measured at a value of less
        than zero degrees celsius, a read of the instance
        for this object will return a value of zero."
    ::= { bldgHVACEntry 5 }

    SYNTAX            Counter32
    UNITS             "minutes"
    MAX-ACCESS        read-only
    STATUS            current
        "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 parameters that impact
        configuration are changed, the subsystem must be
        re-initialized.  Discontinuities in the value of this counter
        can occur at re-initialization of the management system,
        and at other times as indicated by the value of
    ::= { bldgHVACEntry 6 }

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bldgHVACDiscontinuityTime OBJECT-TYPE
    SYNTAX      TimeStamp
    MAX-ACCESS  read-only
    STATUS      current
        "The value of sysUpTime on the most recent occasion at which
        any heating or cooling operation for the office designated
        by this row instance experienced a discontinuity.  If
        no such discontinuities have occurred since the last re-
        initialization of the this row, then this object contains a
        zero value."
    ::= { bldgHVACEntry 7 }

    SYNTAX            SnmpAdminString
    MAX-ACCESS        read-create
    STATUS            current
        "The identity of the operator/system that
        last modified this entry.  When a new entry
        is created, a valid SnmpAdminString must
        be supplied.  If, on the other hand, this
        entry is populated by the agent 'discovering'
        unconfigured rooms, the empty string is a valid
        value for this object."
    ::= { bldgHVACEntry 8 }

bldgHVACStorageType  OBJECT-TYPE
    SYNTAX            StorageType
    MAX-ACCESS        read-create
    STATUS            current
        "The persistence of this row of the table in system storage,
        as it pertains to permanence across system resets.  A columnar
        instance of this object with value 'permanent' need not allow
        write-access to any of the columnar object instances in the
        containing row."
    ::= { bldgHVACEntry 9  }

    SYNTAX            RowStatus
    MAX-ACCESS        read-create
    STATUS            current
        "Controls and reflects the creation and activation status of
        a row in this table.

        No attempt to modify a row columnar object instance value in

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        the bldgHVACTable should be issued while the value of
        bldgHVACStatus is active(1).  Should an agent receive a SET
        PDU attempting such a modification in this state, an
        inconsistentValue error should be returned as a result of
        the SET attempt."
    ::= { bldgHVACEntry 10 }
-- HVAC Configuration Template Table

bldgHVACCfgTemplateInfoTable  OBJECT-TYPE
    SYNTAX      SEQUENCE OF BldgHVACCfgTemplateInfoEntry
    MAX-ACCESS  not-accessible
    STATUS      current
        "This table provides unique string identification for
        HVAC templates in a network.  If it were necessary to
        configure rooms to deliver a particular quality of climate
        control with regard to cooling or heating, the index string
        of a row in this table could be the template name.
        The bldgHVACCfgCfgTemplateInfoDescription
        contains a brief description of the template service objective
        such as: provides summer cooling settings for executive
        offices.  The bldgHVACCfgTemplateInfo in the
        bldgHVACCfgTemplateTable will contain the pointer to the
        relevant row in this table if it is intended that items
        that point to a row in the bldgHVACCfgTemplateInfoTable be
        identifiable as being under template control though this

    ::= { bldgHVACObjects 2 }

bldgHVACCfgTemplateInfoEntry  OBJECT-TYPE
    SYNTAX       BldgHVACCfgTemplateInfoEntry
    MAX-ACCESS   not-accessible
    STATUS       current
        "Each row represents a particular template and
        description.  A given row instance can be created or
        deleted by set operations upon its
        bldgHVACCfgTemplateInfoStatus columnar object
    INDEX { bldgHVACCfgTemplateInfoIndex }
    ::= { bldgHVACCfgTemplateInfoTable 1 }

BldgHVACCfgTemplateInfoEntry ::= SEQUENCE {
        bldgHVACCfgTemplateInfoIndex          Unsigned32,
        bldgHVACCfgTemplateInfoID             SnmpAdminString,

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        bldgHVACCfgTemplateInfoDescr          SnmpAdminString,
        bldgHVACCfgTemplateInfoOwner          SnmpAdminString,
        bldgHVACCfgTemplateInfoStatus         RowStatus,
        bldgHVACCfgTemplateInfoStorType       StorageType

bldgHVACCfgTemplateInfoIndex   OBJECT-TYPE
       SYNTAX       Unsigned32 (1..2147483647)
       MAX-ACCESS   not-accessible
       STATUS       current
           "The unique index to a row in this table."
        ::= { bldgHVACCfgTemplateInfoEntry 1 }

bldgHVACCfgTemplateInfoID  OBJECT-TYPE
    SYNTAX       SnmpAdminString
    MAX-ACCESS   read-create
    STATUS       current
        "Textual identifier for this table row, and, consequently
        the template.  This should be a unique name within
        an administrative domain for a particular template so that
        all systems in a network that are under the same template
        can have the same 'handle' (e.g., 'Executive Offices',
        'Lobby Areas')."
    ::= { bldgHVACCfgTemplateInfoEntry 2 }

bldgHVACCfgTemplateInfoDescr   OBJECT-TYPE
    SYNTAX       SnmpAdminString
    MAX-ACCESS   read-create
    STATUS       current
        "A general description of the template.  One example might
        be - Controls the cooling for offices on higher floors
        during the summer."
    ::= { bldgHVACCfgTemplateInfoEntry 3 }

bldgHVACCfgTemplateInfoOwner  OBJECT-TYPE
    SYNTAX            SnmpAdminString
    MAX-ACCESS        read-create
    STATUS            current
        "The identity of the operator/system that last modified
        this entry."
    ::= { bldgHVACCfgTemplateInfoEntry 4 }

bldgHVACCfgTemplateInfoStatus  OBJECT-TYPE

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    SYNTAX            RowStatus
    MAX-ACCESS        read-create
    STATUS            current
        "The activation status of this row.

        No attempt to modify a row columnar object instance value in
        the bldgHVACCfgTemplateInfo Table should be issued while the
        value of bldgHVACCfgTemplateInfoStatus is active(1).
        Should an agent receive a SET PDU attempting such a modification
        in this state, an inconsistentValue error should be returned as
        a result of the SET attempt."
    ::= { bldgHVACCfgTemplateInfoEntry 5 }

bldgHVACCfgTemplateInfoStorType   OBJECT-TYPE
    SYNTAX            StorageType
    MAX-ACCESS        read-create
    STATUS            current
        "The persistence of this row of the table in system storage,
         as it pertains to permanence across system resets.  A columnar
        instance of this object with value 'permanent' need not allow
        write-access to any of the columnar object instances in the
        containing row."
    ::= { bldgHVACCfgTemplateInfoEntry 6  }

-- HVAC Configuration Template Table
bldgHVACCfgTemplateTable    OBJECT-TYPE
    SYNTAX      SEQUENCE OF BldgHVACCfgTemplateEntry
    MAX-ACCESS  not-accessible
    STATUS      current
        "This table contains the templates, which
        can be used to set defaults that will
        be applied to specific offices.  The application
        of those values is accomplished by having a row
        instance of the bldgHVACTable reference a row of
        this table (by the value of the former's
        bldgHVACCfgTemplate columnar instance).  Identifying
        information concerning a row instance of this table
        can be found in the columnar data of the row instance
        of the bldgHVACCfgTemplateInfoTable entry referenced
        by the bldgHVACCfgTemplateInfo columnar object of
        this table."
    ::= { bldgHVACObjects 3 }

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bldgHVACCfgTemplateEntry  OBJECT-TYPE
    SYNTAX       BldgHVACCfgTemplateEntry
    MAX-ACCESS   not-accessible
    STATUS       current
        "Each row represents a single set of template parameters
        that can be applied to selected instances - in this case
        offices.  These policies will be turned on and off by the
        policy module through its scheduling facilities.

        A given row instance can be created or
        deleted by set operations upon its
        bldgHVACCfgTemplateStatus columnar object instance."
    INDEX { bldgHVACCfgTemplateIndex }
    ::= { bldgHVACCfgTemplateTable 1 }

BldgHVACCfgTemplateEntry ::= SEQUENCE {
        bldgHVACCfgTemplateIndex           Unsigned32,
        bldgHVACCfgTemplateDesiredTemp     Gauge32,
        bldgHVACCfgTemplateCoolOrHeat      BldgHvacOperation,
        bldgHVACCfgTemplateInfo            Unsigned32,
        bldgHVACCfgTemplateOwner           SnmpAdminString,
        bldgHVACCfgTemplateStorage         StorageType,
        bldgHVACCfgTemplateStatus          RowStatus

bldgHVACCfgTemplateIndex    OBJECT-TYPE
    SYNTAX      Unsigned32 (1..2147483647)
    MAX-ACCESS  not-accessible
    STATUS      current
        "A unique value for each defined template in this
        table.  This value can be referenced as a row index
        by any MIB module that needs access to this information.
        The bldgHVACCfgTemplate will point to entries in this
    ::= { bldgHVACCfgTemplateEntry 1 }

bldgHVACCfgTemplateDesiredTemp OBJECT-TYPE
    SYNTAX            Gauge32
    UNITS             "degrees in celsius"
    MAX-ACCESS        read-create
    STATUS            current
        "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

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        change and then reactivating the row."
    ::= { bldgHVACCfgTemplateEntry 2 }

bldgHVACCfgTemplateCoolOrHeat  OBJECT-TYPE
    SYNTAX             BldgHvacOperation
    MAX-ACCESS         read-create
    STATUS             current
        "This controls the heating and cooling mechanism and is
        set-able by building maintenance.  It is permitted to
        change this value by first moving the row to an inactive
        state, making the change and then reactivating the row."
    ::= { bldgHVACCfgTemplateEntry 3 }

bldgHVACCfgTemplateInfo OBJECT-TYPE
    SYNTAX             Unsigned32
    MAX-ACCESS         read-create
    STATUS             current
        "This object points to a row in the
        bldgHVACCfgTemplateInfoTable.  This controls the
        heating and cooling mechanism and is set-able by
        building maintenance.  It is permissible to change
        this value by first moving the row to an inactive
        state, making the change and then reactivating
        the row.  A value of zero means that this entry
        is not associated with a named template found
        in the bldgHVACCfgTemplateInfoTable."
    ::= { bldgHVACCfgTemplateEntry 4 }

bldgHVACCfgTemplateOwner  OBJECT-TYPE
    SYNTAX            SnmpAdminString
    MAX-ACCESS        read-create
    STATUS            current
        "The identity of the administrative entity
        that created this row of the table."
    ::= { bldgHVACCfgTemplateEntry 5 }

bldgHVACCfgTemplateStorage  OBJECT-TYPE
    SYNTAX            StorageType
    MAX-ACCESS        read-create
    STATUS            current
        "The persistence of this row of the table across
         system resets.  A columnar instance of this object with
         value 'permanent' need not allow write-access to any
         of the columnar object instances in the containing row."

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    ::= { bldgHVACCfgTemplateEntry 6 }

bldgHVACCfgTemplateStatus  OBJECT-TYPE
    SYNTAX            RowStatus
    MAX-ACCESS        read-create
    STATUS            current
        "The activation status of this row of the table.

        No attempt to modify a row columnar object instance value in
        the bldgHVACCfgTemplateTable should be issued while the
        value of bldgHVACCfgTemplateStatus is active(1).
        Should an agent receive a SET PDU attempting such a modification
        in this state, an inconsistentValue error should be returned as
        a result of the SET attempt."
    ::= { bldgHVACCfgTemplateEntry 7 }

-- Conformance Information

bldgCompliances  OBJECT IDENTIFIER ::= { bldgConformance 1 }
bldgGroups       OBJECT IDENTIFIER ::= { bldgConformance 2 }

-- Compliance Statements

    STATUS current
        "The requirements for conformance to the BLDG-HVAC-MIB.  The
         bldgHVACObjects group must be implemented to conform to the
    MODULE -- this module

    GROUP bldgHVACObjectsGroup
        "The bldgHVACObjects is mandatory for all systems that
        support HVAC systems."
    ::= { bldgCompliances 1 }

        bldgHVACFanSpeed, bldgHVACCurrentTemp,
        bldgHVACCoolOrHeatMins, bldgHVACDiscontinuityTime,
        bldgHVACOwner, bldgHVACStatus,
        bldgHVACStorageType, bldgHVACCfgTemplateInfoID,
        bldgHVACCfgTemplateInfoDescr, bldgHVACCfgTemplateInfoOwner,

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    STATUS current
        "The bldgHVACObjects Group."
    ::= { bldgGroups 1 }


8.2.  Notes on MIB Module with Template-based Data

   The primary purpose of the example "HVAC" MIB module is to show how
   to construct a single module that includes configuration, template,
   counter and state information in a single module.  If this were a
   'real' module we would also have included definitions for
   notifications for the configuration change operations as previously
   described.  We also would have included notifications for faults and
   other counter threshold events.

   Implementation and Instance Extensions:

   Just as with networking technologies, vendors may wish to add
   extensions that can distinguish their products from the competition.
   If an HVAC vendor also wanted to support humidity control, they could
   add that facility to their equipment and use AUGMENTS for the
   bldgHVACTemplateTable with two objects, one that indicates the
   desired humidity and the other, the actual.  The
   bldgHVACTemplateTable could also be extended using this same approach
   so that HVAC policies could easily be extended to support this

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8.3.  Examples of Usage of the MIB

   The following two examples use two templates to configure the
   temperature in executive offices and in conference rooms.  The
   "conference rooms" template is applied to all conference rooms (which
   happen to be office 104 on each floor), and the "executive offices"
   template is applied to executive offices.

   If offices 24, 25, and 26 on the third floor are executive offices,
   the values in the bldgHVACTable might be:

   bldgHVACCfgTemplate.3.24 = 2
   bldgHVACFanSpeed.3.24 = 2989
   bldgHVACCurrentTemp.3.24 = 24
   bldgHVACCoolOrHeatMins.3.24 = 123
   bldgHVACDiscontinuityTime.3.24 = sysUpTime + 12h + 21m
   bldgHVACOwner.3.24 = "policy engine"
   bldgHVACStorageType.3.24 = nonVolatile(3)
   bldgHVACStatus.3.24 = active(1)

   bldgHVACCfgTemplate.3.25 = 2
   bldgHVACFanSpeed.3.25 = 0
   bldgHVACCurrentTemp.3.25 = 22
   bldgHVACCoolOrHeatMins.3.25 = 298
   bldgHVACDiscontinuityTime.3.25 = sysUpTime + 4h + 2m
   bldgHVACOwner.3.25 = "policy engine"
   bldgHVACStorageType.3.25 = nonVolatile(3)
   bldgHVACStatus.3.25 = active(1)

   bldgHVACCfgTemplate.3.26 = 2
   bldgHVACFanSpeed.3.26 = 0
   bldgHVACCurrentTemp.3.26 = 22
   bldgHVACCoolOrHeatMins.3.26 = 982
   bldgHVACOwner.3.26 = "policy engine"
   bldgHVACStorageType.3.26 = nonVolatile(3)
   bldgHVACStatus.3.26 = active(1)

   The second entry in the bldgHVACCfgTemplateTable, to which all of the
   above point, might have the following configuration:

   bldgHVACCfgTemplateDesiredTemp.2 = 22
   bldgHVACCfgTemplateCoolOrHeat.2 = cool(2)
   bldgHVACCfgTemplateInfo.2 = 2
   bldgHVACCfgTemplateOwner.2 = "Senior Executive assistant"
   bldgHVACCfgTemplateStorage.2 = nonVolatile(3)
   bldgHVACCfgTemplateStatus.2 = active(1)

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   and the associated template information ("executive offices") might

   bldgHVACCfgTemplateInfoID.2 = "executive offices"
   bldgHVACCfgTemplateInfoDescr.2 = "Controls temperature for executive
   bldgHVACCfgTemplateInfoOwner.2 = "Senior Executive assistant"
   bldgHVACCfgTemplateInfoStorType.2 = nonVolatile(3)
   bldgHVACCfgTemplateInfoStatus.2 = active(1)

   The policy engine can now associate instances of executive offices
   with the template called "executive offices" and apply the values in
   the second entry of the bldgHVACCfgTemplateTable to each of the
   instances of the executive offices.  This will then attempt to set
   the temperature in executive offices to 22 degrees celsius.

   It is also possible that there may be an office configured for a
   particular temperature, but without using a template.  For example,
   office 28 on the third floor might look like this:

   bldgHVACCfgTemplate.3.28 = 3
   bldgHVACFanSpeed.3.28 = 50
   bldgHVACCurrentTemp.3.28 = 26
   bldgHVACCoolOrHeatMins.3.28 = 0
   bldgHVACDiscontinuityTime.3.28 = 0
   bldgHVACOwner.3.28 = "Executive with poor circulation"
   bldgHVACStorageType.3.28 = nonVolatile(3)
   bldgHVACStatus.3.28 = active(1)

   The entry in the bldgHVACCfgTemplateTable (to which
   bldgHVACCfgTemplate.3.28 points) might instead look like:

   bldgHVACCfgTemplateDesiredTemp.3 = 28
   bldgHVACCfgTemplateCoolOrHeat.3 = cool(2)
   bldgHVACCfgTemplateInfo.3 = 0.0
   bldgHVACCfgTemplateOwner.3 = "Executive with poor circulation"
   bldgHVACCfgTemplateStorage.3 = nonVolatile(3)
   bldgHVACCfgTemplateStatus.3 = active(1)

   Note that this entry does not point to a template.

   If the executive's circulation improves so that the temperature
   should be aligned with other executive offices, this is accomplished
   by changing the value of bldgHVACCfgTemplate.3.28 from
   bldgHVACCfgTemplateInfoID.3 to bldgHVACCfgTemplateInfoID.2 (shown

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   Finally, there might be offices for which there is no configured
   temperature but management applications can read the current
   temperature, fan speed, and cooling or heating minutes from the
   bldgHVACTable.  In that case, the value of bldgHVACCfgTemplate will
   be a zero index ("null"), as will the value of bldgHVACOwner.

   bldgHVACCfgTemplate.4.2 = 0
   bldgHVACFanSpeed.3.28 = 50
   bldgHVACCurrentTemp.3.28 = 26
   bldgHVACCoolOrHeatMins.3.28 = 0
   bldgHVACDiscontinuityTime.3.28 = 0
   bldgHVACOwner.3.28 = ""
   bldgHVACStorageType.3.28 = nonVolatile(3)
   bldgHVACStatus.3.28 = active(1)

   As a second example, the conference rooms on several floors are
   configured using the "conference rooms" template.  When the values in
   the bldgHVACTable pertaining to conference rooms are read, it might
   look like:

   bldgHVACCfgTemplate.12.104 = bldgHVACCfgTemplateDesiredTemp.1
   bldgHVACFanSpeed.12.104 = 1423
   bldgHVACCurrentTemp.12.104 = 21
   bldgHVACCoolOrHeatMins.12.104 = 2193
   bldgHVACDiscontinuityTime.12.104 = sysUpTime + 36h + 15m
   bldgHVACOwner.12.104 =  = "Bob the Conference Guy"
   bldgHVACStorageType.12.104 = nonVolatile(3)
   bldgHVACStatus.12.104 = active(1)

   bldgHVACCfgTemplate.14.104 = bldgHVACCfgTemplateDesiredTemp.1
   bldgHVACFanSpeed.14.104 = 1203
   bldgHVACCurrentTemp.14.104 = 20
   bldgHVACCoolOrHeatMins.14.104 = 293
   bldgHVACDiscontinuityTime.14.104 = sysUpTime + 5h + 54m
   bldgHVACOwner.14.104 =  = "Bob the Conference Guy"
   bldgHVACStorageType.14.104 = nonVolatile(3)
   bldgHVACStatus.14.104 = active(1)

   bldgHVACCfgTemplate.15.104 = bldgHVACCfgTemplateDesiredTemp.1
   bldgHVACFanSpeed.15.104 = 12
   bldgHVACCurrentTemp.15.104 = 19
   bldgHVACCoolOrHeatMins.15.104 = 1123
   bldgHVACDiscontinuityTime.15.103 = sysUpTime + 2d + 2h + 7m
   bldgHVACOwner.15.104 =  = "Bob the Conference Guy"
   bldgHVACStorageType.15.104 = nonVolatile(3)
   bldgHVACStatus.15.104 = active(1)

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   The desired temperature and whether to heat or cool is configured in
   the first entry of the bldgHVACCfgTemplateTable, which tries to set
   the temperature to 19 degrees celsius in conference rooms:

   bldgHVACCfgTemplateDesiredTemp.1 = 19
   bldgHVACCfgTemplateCoolOrHeat.1 = cool(2)
   bldgHVACCfgTemplateInfo.1 = bldgHVACCfgTemplateInfoID.1
   bldgHVACCfgTemplateOwner.1 = "Bob the Conference Guy"
   bldgHVACCfgTemplateStorage.1 = nonVolatile(3)
   bldgHVACCfgTemplateStatus.1 = active(1)

   The associated template information would then have:

   bldgHVACCfgTemplateInfoID.1 = "conference rooms"
   bldgHVACCfgTemplateInfoDescr.1 = "Controls temperature in conference
   rooms" bldgHVACCfgTemplateInfoOwner.1 = "Bob the Conference Guy"
   bldgHVACCfgTemplateInfoStorType.1 = nonVolatile(3)
   bldgHVACCfgTemplateInfoStatus.1 = active(1)

   The policy system can then apply this template (cool to 19 degrees
   Celsius) to its notion of all of the conference rooms in the

9.  Security Considerations

   This document discusses practices and methods for using the SNMP for
   management and distribution of configuration information for network
   elements.  Any effective use of the SNMP in this application must
   concern itself with issues of authentication of the management
   entities initiating configuration change and management, in addition
   to the integrity of the configuration data itself.  Other more subtle
   considerations also exist.

   To that end, the section of this document entitled "Deployment and
   Security Issues" covers these security considerations to the extent
   they affect the current practices described throughout this document.
   In particular, in the subsection entitled "Secure Agent
   Considerations", there is a recommendation for the usage of Version 3
   of the SNMP, and its essential presumption as a foundation for other
   practices described throughout.  With the exception of a small number
   of cases where a mention is made to the contrary to illustrate
   techniques for coexistence with application entities dependent upon
   earlier versions of the SNMP, that recommendation of usage of Version
   3 of the SNMP is reiterated here.

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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 T. Perkins
      Randy Presuhn
      Dan Romascanu
      Shawn Routhier
      Steve Waldbusser
      Bert Wijnen

11.  Normative References

   [1]  Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for
        Describing Simple Network Management Protocol (SNMP) Management
        Frameworks", STD 62, RFC 3411, December 2002.

   [2]  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Structure of
        Management Information Version 2 (SMIv2)", STD 58, RFC 2578,
        April 1999.

   [3]  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Textual
        Conventions for SMIv2", STD 58, RFC 2579, April 1999.

   [4]  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Conformance
        Statements for SMIv2", STD 58, RFC 2580, April 1999.

   [5]  Presuhn, R. (Ed.), "Transport Mappings for the Simple Network
        Management Protocol (SNMPv2)", STD 62, RFC 3417, December 2002.

   [6]  Case, J., Harrington D., Presuhn R. and B. Wijnen, "Message
        Processing and Dispatching for the Simple Network Management
        Protocol (SNMP)", STD 62, RFC 3412, December 2002.

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RFC 3512       Configuring Networks and Devices with SNMP     April 2003

   [7]  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
        for version 3 of the Simple Network Management Protocol
        (SNMPv3)", STD 62, RFC 3414, December 2002.

   [8]  Presuhn, R. (Ed.), "Version 2 of the Protocol Operations for the
        Simple Network Management Protocol (SNMP)", STD 62, RFC 3416,
        December 2002.

   [9]  Levi, D., Meyer, P., and B. Stewart, "Simple Network Management
        Protocol Applications", STD 62, RFC 3413, December 2002.

   [10] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-based Access
        Control Model (VACM) for the Simple Network Management Protocol
        (SNMP)", STD 62, RFC 3415, December 2002.

   [11] Presuhn, R. (Ed.), "Management Information Base for the Simple
        Network Management Protocol (SNMPv2)", STD 62, RFC 3418,
        December 2002.

   [12] Case, J., Mundy, R., Partain, D. and B. Stewart, "Introduction
        and Applicability Statements for Internet-Standard Management
        Framework", RFC 3410, December 2002.

   [13] Daniele, M., Haberman, B., Routhier, S. and J. Schoenwaelder,
        "Textual Conventions for Internet Network Addresses", RFC 3291,
        May 2002.

   [14] McCloghrie, K. (Ed.), "SNMPv2 Management Information Base for
        the Internet Protocol using SMIv2", RFC 2011, November 1996.

12.  Informative References

   [15] Rose, M. and K. McCloghrie, "Structure and Identification of
        Management Information for TCP/IP-based Internets", STD 16, RFC
        1155, May 1990.

   [16] Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD 16,
        RFC 1212, March 1991.

   [17] Rose, M., "A Convention for Defining Traps for use with the
        SNMP", RFC 1215, March 1991.

   [18] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple
        Network Management Protocol", STD 15, RFC 1157, May 1990.

   [19] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser,
        "Introduction to Community-based SNMPv2", RFC 1901, January

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RFC 3512       Configuring Networks and Devices with SNMP     April 2003

   [20] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
        RFC 2863, June 2000.

   [21] Brown, C. and F. Baker, "Management Information Base for Frame
        Relay DTEs Using SMIv2", RFC 2115, September 1997.

   [22] Baker, F. (Ed.), "Requirements for IP Version 4 Routers", RFC
        1812, June 1995.

   [23] Hawkinson, J. and T. Bates, "Guidelines for Creation, Selection,
        and Registration of an Autonomous System (AS)", BCP 6, RFC 1930,
        March 1996.

   [24] Decker, E., Langille, P., Rijsinghani, A. and K. McCloghrie,
        "Definitions of Managed Objects for Bridges", RFC 1493, July

   [25] Levi, D. and J. Schoenwaelder "Definitions of Managed Objects
        for Scheduling Management Operations", RFC 3231, January 2002.

   [26] Bell, E., Smith, A., Langille, P., Rijsinghani, A. and K.
        McCloghrie, "Definitions of Managed Objects for Bridges with
        Traffic Classes, Multicast Filtering and Virtual LAN
        Extensions", RFC 2674, August 1999.

   [27] Baker, F., "IP Forwarding Table MIB", RFC 2096, January 1997.

   [28] St. Johns, M. (Ed.), "Radio Frequency (RF) Interface Management
        Information Base for MCNS/DOCSIS compliant RF interfaces", RFC
        2670, August 1999.

   [29] Baker, F. and R. Coltun, "OSPF Version 2 Management Information
        Base", RFC 1850, November 1995.

   [30] Blake, S., Black, D., Carlson M., Davies, E., Wang, Z. and W.
        Weiss, "An Architecture for Differentiated Services ", RFC 2475,
        December 1998.

   [31] Willis, S., Burruss, J. and J. Chu (Ed.), "Definitions of
        Managed Objects for the Fourth Version of the Border Gateway
        Protocol (BGP-4) using SMIv2", RFC 1657, July 1994.

   [32] Waldbusser, S., "Remote Network Monitoring Management
        Information Base", RFC 2819, May 2000.

   [33] McCloghrie, K. and G. Hanson, "The Inverted Stack Table
        Extension to the Interfaces Group MIB", RFC 2864, June 2000.

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RFC 3512       Configuring Networks and Devices with SNMP     April 2003

   [34] McCloghrie, K. and A. Bierman, "Entity MIB (Version 2)", RFC
        2737, December 1999.

   [35] ITU-T,, Recommendation M.3010., PRINCIPLES FOR A

   [36] Waldbusser, S., Saperia, J., and Hongal, T., "Policy Based
        Management MIB", Work-in-progress.

   [37] Heintz, L., "SNMP Row Operations Extensions", Work-in-progress.

   [38] Zeltserman, D., "A Practical Guide to Snmpv3 and Network
        Management", Prentice Hall, 1999.

   [39] Noto, M., Spiegel, E. and K. Tesink, "Definitions of Textual
        Conventions and OBJECT-IDENTITIES for ATM Management", RFC 2514,
        February 1999.

   [40] Kassaveri, R., Editor, "Distributed Management Expression MIB",
        RFC 2982, October 2000.

   [41] St. Johns, M., "DOCSIS Cable Device MIB Cable Device Management
        Information Base for DOCSIS compliant Cable Modems and Cable
        Modem Termination Systems", RFC 2669, August 1999.

   [42] Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M.,
        Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J. and S.
        Waldbusser, "Terminology for Policy-Based Management", RFC 3198,
        November 2001.


   [44] Waldbusser, S., "Remote Network Monitoring Management
        Information Base Version 2 using SMIv2", RFC 2021, January 1997.

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

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RFC 3512       Configuring Networks and Devices with SNMP     April 2003

   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

14. Editors' Addresses

   Michael R. MacFaden
   Riverstone Networks, Inc
   5200 Great America Parkway
   Santa Clara, CA 95054


   David Partain
   Ericsson AB
   P.O. Box 1248
   SE-581 12  Linkoping


   Jon Saperia
   JDS Consulting
   174 Chapman Street
   Watertown, MA 02472


   Wayne F. Tackabury
   Gold Wire Technology
   411 Waverley Oaks Rd.
   Waltham, MA  02452


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15.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   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


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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