Network Working Group                                          P. Shafer
Internet-Draft                                          Juniper Networks
Intended status: Informational                              May                         February 27, 2009 2010
Expires: November 28, 2009 August 31, 2010

    An NETCONF- and NETMOD-based Architecture for Network Management


   NETCONF gives access to native capabilities of the devices within a
   network, defining methods for manipulating configuration databases,
   retrieving operational data, and invoking specific operations.  YANG
   provides the means to define the content carried via NETCONF, both
   data and operations.  Using both technologies, standard modules can
   be defined to give interoperability and commonality to devices, while
   still allowing devices to express their unique capabilities.

   This document describes how NETCONF and YANG help build network
   management applications that meet the needs of network operators.

Status of this Memo

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Copyright Notice

   Copyright (c) 2009 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document ( document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.


   NETCONF gives access to native capabilities  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the devices within a
   network, defining methods for manipulating configuration databases,
   retrieving operational data, Trust Legal Provisions and invoking specific operations.  YANG
   provides are provided without warranty as
   described in the means Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to define allow
   modifications of such material outside the content carried via NETCONF, both
   data and operations.  Using both technologies, standard modules can IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be defined to give interoperability modified
   outside the IETF Standards Process, and commonality derivative works of it may
   not be created outside the IETF Standards Process, except to devices, while
   still allowing devices format
   it for publication as an RFC or to express their unique capabilities.

   This document describes how NETCONF and YANG help build network
   management applications that meet the needs of network operators. translate it into languages other
   than English.

Table of Contents

   1.  Key Words  . . . . . . . . . . . . . . . . . . . . . . . . . .  3  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4  5
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4  5
     2.2.  NETCONF  . . . . . . . . . . . . . . . . . . . . . . . . .  4  5
     2.3.  YANG . . . . . . . . . . . . . . . . . . . . . . . . . . .  6  7
       2.3.1.  Extensibility Model  . . . . . . . . . . . . . . . . .  8  9
   3.  An Architecture for NETMOD . . . . . . . . . . . . . . . . . . 10 11
   4.  YANG and Related Technologies  . . . . . . . . . . . . . . . . 13 14
     4.1.  YIN  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 14
     4.2.  DSDL (Relax NG)  . . . . . . . . . . . . . . . . . . . . . 13 14
     4.3.  YANG Types . . . . . . . . . . . . . . . . . . . . . . . . 14 15
   5.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . . 15 16
     5.1.  Device Developer . . . . . . . . . . . . . . . . . . . . . 15 16
     5.2.  Generic Content Support  . . . . . . . . . . . . . . . . . 15 16
     5.3.  XML "over the wire" Definitions  . . . . . . . . . . . . . 15 16
     5.4.  Application Developer  . . . . . . . . . . . . . . . . . . 15 16
       5.4.1.  Hard Coded . . . . . . . . . . . . . . . . . . . . . . 15 16
       5.4.2.  Bottom Up  . . . . . . . . . . . . . . . . . . . . . . 16 17
       5.4.3.  Top Down . . . . . . . . . . . . . . . . . . . . . . . 16 17
   6.  Modeling Considerations  . . . . . . . . . . . . . . . . . . . 17 18
   7.  Data Distinctions  . . . . . . . . . . . . . . . . . . . . . . 19
     7.1.  Background . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . 19
       7.2.1.  Example 1: IP Routing Table  . . . . . . . . . . . . . 20
       7.2.2.  Example 2: Interfaces  . . . . . . . . . . . . . . . . 20
       7.2.3.  Example 3: Account Information . . . . . . . . . . . . 20
     7.3.  Implications . . . . . . . . . . . . . . . . . . . . . . . 21
       7.3.1.  Data Models  . . . . . . . . . . . . . . . . . . . . . 21
       7.3.2.  Additional Operations to Retrieve Operational State  . 21
       7.3.3.  Introduction of an Operational State Datastore . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8. 22
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 19 23
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20 24

1.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, [RFC2119].

2.  Introduction

   Networks are increasing in complexity and capacity, as well as the
   density of the services deployed upon them.  Uptime, reliability, and
   predictable latency requirements drive the need for automation.

   The problems with network management are not simple.  They are
   complex and intricate.  But these problems must be solved for
   networks to meet the stability needs of existing services while
   incorporating new services in a world where the growth of the
   networks is exhausting the supply of qualified networking engineers.
   We need to move from a CLI world into a world of automation, but that
   automation must be robust and trustworthy.

   This document presents an architecture based on NETCONF ([RFC4741])
   and [YANG].  NETCONF and YANG address the problems of network
   management with flexibility and expressiveness.  NETCONF allows any
   manner of configuration and operational data to be carried with few
   rules governing the data.  YANG allows data models to be defined that
   are flexible and extensible in ways that allow the data to be
   cohesive and structured, but not rigid.

   This approach allows the device to express its native capabilities in
   a way that is flexible and extensible.  Evolution of devices and data
   models are permitted and managed.

2.1.  Terminology

   The document mirrors terminology from NETCONF.  NETCONF uses a simple
   RPC-based mechanism to facilitate communication between a client and
   a server.  The client can be a script or application typically
   running as part of a network management system.  The server is
   typically a network device.  The terms "device" and "server" are used
   interchangeably in this document, as are "client" and "application".


   NETCONF defines an XML-based RPC mechanism that leverages the
   simplicity and availability of high-quality XML parsers.  XML gives a
   rich, flexible, hierarchical, standard representation of data that
   matches the needs of networking devices.  NETCONF carries
   configuration data and operations encoded in XML using an RPC
   mechanism over a connection-oriented transport.

   XML's hierarchical data representation allows complex networking data
   to be rendered in a natural way.  For example, the following
   configuration places interfaces in OSPF areas.  The <ospf> element
   contains a list of <area> elements, each of which contain a list of
   <interface> elements.  The <name> element identifies the specific
   area or interface.  Additional configuration for each area or
   interface appears directly inside the appropriate element.

         <ospf xmlns="">


               <!-- The priority for this interface -->





   NETCONF includes mechanisms for controlling configuration datastores,
   fetching state data, receiving notifications, and allows for
   additional RPC methods.  Configuration operations include the ability
   to lock datastores to isolate one application from the actions of
   others, the ability to save and restore configuration data sets, and
   the ability to discover (via the <hello> message) the capabilities of
   the device.

2.3.  YANG

   YANG is the data model language for NETCONF that allows the
   description of hierarchies of data model nodes ("nodes") and the
   constraints that exist amongst them.  YANG defines data models and
   how to manipulate those models via NETCONF protocol operations.

   Each YANG module defines a data model, uniquely identified by a
   namespace URI.  These data models are extensible in a manner that
   allows tight integration of standard data models and proprietary data
   models.  Models are built from organizational containers, lists of
   data instances and leaf data values.

       module ietf-ospf {
           prefix ospf;

           import network-types {  // Access another module's def'ns
               prefix nett;

           container ospf {   // Declare the top-level tag
               list area {    // Declare a list of "area" nodes
                   key name;  // The key "name" identifies list members
                   leaf name {
                       type nett:area-id;
                   list interface {
                       key name;
                       leaf name {
                           type nett:interface-name;
                       leaf priority {
                           description "Designated router priority";
                           type uint {       // The type and range are
                               range 0..255; //  constraints on valid
                           }                 //  values for "priority".
                       leaf metric {
                           type uint {
                               range 1..65535;
                       leaf dead-interval {
                           units seconds;
                           type uint {
                               range 1..65535;

   A YANG module defines a data model in terms of the data, its
   hierarchical organization, and the constraints on that data.  YANG
   defines how this data is represented in XML and how that data is used
   in NETCONF operations.

2.3.1.  Extensibility Model

   XML includes the concept of namespaces, allowing XML elements from
   different sources to be combined in the same hierarchy without
   risking collision.  YANG modules define content for specific
   namespaces, but one module may augment the definition of another
   module, introducing elements from that module's namespace into the
   first module's hierarchy.

   Since one module can augment another module's definition, hierarchies
   of definitions are allowed to grow, as definitions from multiple
   sources are added to the base hierarchy.  These augmentations are
   qualified using the namespace of the source module, helping to avoid
   issues with name conflicts as the modules change over time.

   For example, if the above OSPF configuration were the standard, a
   vendor module may augment this with vendor-specific extensions.

       module vendorx-ospf {
           prefix vendorx;

           import ietf-ospf {
               prefix ospf;

           augment /ospf:ospf/ospf:area/ospf:interfaces {
               leaf no-neighbor-down-notification {
                   type empty;
                   description "Don't inform other protocols about"
                             + " neighbor down events";

   The <no-neighbor-down-notification> element is then placed in the
   vendorx namespace:

       <protocols xmlns=""
         <ospf xmlns="">




   Augmentations are seamlessly integrated with base modules, allowing
   them to be fetched, archived, loaded, and deleted within their
   natural hierarchy.  If a client application asks for the
   configuration for a specific OSPF area, it will receive the sub-
   hierarchy for that area, complete with any augmentations.

3.  An Architecture for NETMOD

   In the NETMOD architecture, each device vendor implements a set of
   data models in their devices.  These models are either standard data
   models, defined in YANG modules published by a standards body, or
   proprietary data models, defined in YANG modules published by

   Standard data models define content that is independent of the
   vendor, allowing client applications to request specific behavior
   without concern for the vendor, product line, or installed software
   revision.  The translation between the standard model and the device
   specific behavior is performed by the device, freeing the application
   from such concerns.

   Proprietary data models allow the vendor to accurately describe the
   content and behavior of their devices in explicit detail.
   Applications may take advantage of these specifics to give their
   users complete control over the device.

   When a NETCONF session begins, the namespaces for all supported
   modules are announced as capabilities via the device's <hello>
   message.  The device should also support the schema discovery
   mechanism [ref], enabling applications to discover the location from
   which the modules may be downloaded.

   The schema discovery for standard YANG modules should list a common,
   standard location for these modules, presumably one set by the
   organization that defined the standard.

   When an application connects with a device, it receives the list of
   capabilities supported by that device.  The application may compare
   the set of capabilities announced by the device with the set of
   modules the application is aware of.  Any new modules or new
   revisions of known modules may be downloaded as needed from the
   locations given via the schema discovery mechanism.

   Once the application has access to the YANG modules, it may
   manipulate the device as a "YANG data browser", capable of parsing
   the elements sent from the device with an understanding of the
   organization of the data.  The module describes the syntax of the
   data and constraints on that data, allowing the application to create
   data that abides by those constraints.

   To have a real understanding of a module's content, the application
   may need to incorporate logic specific to that module.  Semantic
   information contained in description statements is not machine
   readable, but module-specific custom work can be done to tailor the
   user interface to the particular semantic needs of a module.

   For example, a module could define the "location" of a device using
   longitude and latitude, and the application can use the "browser"
   style to display this data using input fields in a web form.  Custom
   logic would be needed to take the value of these fields and place the
   device on a map of the world, and additional logic would be needed to
   update the data values when the user drags the device from Dallas to

   If an application is meant to manage a specific problem, it may model
   the data internally in whatever form is most convenient to its
   organizational needs.  When the application interacts with a device,
   it may choose one of two paths.  If the device implements a standard
   module, the application may generate content for that standard by
   translating its internal form into the standard one.

   If the device doesn't implement such a standard or no such standard
   exists, the application may use a transformation that is particular
   to that device's vendor, product model, hardware, or software.
   Depending on the application, this transformation may be provided by
   the application vendor, the device vendor, a third-party, or the

   For a popular application, the device vendor may wish to provide this
   transformation to increase market acceptance of their devices.  For
   popular devices, the application may provide this transformation as a
   means of making the application useful in the maximum number of
   provider networks.  For problem domains where the mapping from the
   application to the device is not straight-forward or requires
   tailoring to the specific provider or environment, the provider may
   wish to control this transformation.  Additionally, other parties may
   make such transformations available via open source.

   Note that both cases may appear within a single application on an "as
   needed" basis.  If the device announces the capability for the
   standard YANG module, the application may transmit to the device via
   NETCONF the content in the standard modules format.  If the device
   does not announce the appropriate capability, the application may
   find a transformation that matches the device, perform the
   transformation on the standard data to produce device native
   configuration, and transmit via NETCONF that device configuration to
   the device.

   In both cases, the key is the ability to discover the capabilities of
   the specific device, download the YANG modules that support those
   capabilities, gain an understanding of those data models and their
   constraints, generate appropriate content, and transmit that content
   to the device.

4.  YANG and Related Technologies

   The YANG data modeling language is the central piece of a group of
   related technologies.  The YANG language itself, described in [ref],
   defines the syntax of the language and its statements, the meaning of
   those statements, and how to combine them to build the hierarchy of
   nodes that describe a data model.

   That document also defines the "on the wire" XML content for NETCONF
   operations on data models defined in YANG modules.  This includes the
   basic mapping between YANG data tree nodes and XML elements, as well
   as mechanisms used in <edit-config> content to manipulate that data,
   such as arranging the order of nodes within a list.

   YANG uses a syntax that is regular and easily described, primarily
   designed for human readability.  YANG's syntax is friendly to email,
   diff, patch, and the constraints of RFC formatting.

4.1.  YIN

   In some environments, incorporating a YANG parser may not be an
   acceptable option.  For those scenarios, an XML grammar for YANG is
   defined in YIN (YANG Independent Notation) [ref].  YIN allows the use
   of XML parsers which are readily available in both open source and
   commercial versions.  Conversion between YANG and YIN is direct,
   loss-less and reversible.  YANG statements are converted to XML
   elements, preserving the structure and content of YANG, but enabling
   the use of off-the-shelf XML parsers rather than requiring the
   integration of a YANG parser.  YIN maintains complete semantic
   equivalence with YANG.

4.2.  DSDL (Relax NG)

   Since NETCONF content is encoded in XML, it is natural to use XML
   schema languages for their validation.  To facilitate this, YANG
   offers a standardized mapping of YANG modules into Document Schema
   Description Languages (DSDL) [DSDL].

   DSDL is considered to be the best choice for the given purpose
   because it addresses not only grammar and datatypes of XML documents
   but also semantic constraints and rules for modifying information set
   of the document.

   In addition, DSDL offers formal means for coordinating multiple
   independent schemas and specifying how to apply the schemas to the
   various parts of the document.  This is useful since YANG content is
   typically composed of multiple vocabularies.

4.3.  YANG Types

   YANG supports a number of builtin types, and allows additional types
   to be derived from those types in an extensible manner.  New types
   can add additional restrictions to allowable data values.

   A standard type library for use by YANG is available [ref].  These
   YANG modules define commonly used data types for IETF-related

5.  Applicability

   The data model in a YANG module yields value in five specific areas.

5.1.  Device Developer

   The YANG model tells the device developer what data is being modeled.
   The developer reads the YANG models, absorbs the zen of the model,
   and writes code that supports the model.  The model describes the
   data hierarchy and associated constraints, and the description and
   reference material helps the developer understand how to transform
   the models view into the device native implementation.

5.2.  Generic Content Support

   The YANG model can be compiled into a YANG-based engine for either
   the client or server side.  Incoming data can be validated, as can
   outgoing data.  The complete configuration datastore may be validated
   in accordance with the constraints described in the data model.

   Serializers and deserializers for generating and receiving NETCONF
   content can be driven by the meta-data in the model.  As data is
   received, the meta-data is consulted to ensure the validity of
   incoming XML elements.

5.3.  XML "over the wire" Definitions

   The YANG module dictates the XML encoding sent "over the wire",
   though actual transmission should be encrypted so as not to appear as
   readable text on the physical media.  The rules that define the
   encoding are fixed, so the YANG module can be used to ascertain
   whether a specific NETCONF payload is obeying the rules.

5.4.  Application Developer

   The YANG module tells the application developer what data can be
   modeled.  Developers can inspect the modules and take one of three
   distinct views.  In this section, we will consider them and the
   impact of YANG on their design.  In the real world, most applications
   are a mixture of these approaches.

5.4.1.  Hard Coded

   An application can be coded against the specific, well-known contents
   of YANG modules, implementing their organization, rules, and logic
   directly with explicit knowledge.  For example, a script could be
   written to change the domain name of a set of devices using a
   standard YANG module that includes such a leaf node.  This script
   takes the new domain name as an argument and insert it into a string
   containing the rest of the XML encoding as required by the YANG
   module.  This content is then sent via NETCONF to the devices.

   This type of application is useful for small, fixed problems where
   the cost and complexity of flexibility is overwhelmed by the ease of
   hard coding direct knowledge into the application.

5.4.2.  Bottom Up

   An application may take a generic, bottom up approach to
   configuration, concentrating on the device's data directly and
   treating that data without specific understanding.

   YANG modules may be used to drive the operation of the YANG
   equivalent of a "MIB Browser".  Such an application manipulates the
   device's configuration data based on the data organization contained
   in the YANG module.  For example, a GUI may present a straight-
   forward visualization where elements of the YANG hierarchy are
   depicted in a hierarchy of folders or GUI panels.  Clicking on a line
   expands to the contents of the matching content.

   This type of GUI can easily be built by generating XSLT stylesheets
   from the YANG data models.  An XSLT engine can then be used to turn
   configuration data into a set of web pages.

   The YANG modules allows the application to enforce a set of
   constraints without understanding the semantics of the YANG module.

5.4.3.  Top Down

   In contrast to the bottom-up approach, the top-down approach allows
   the application to take a view of the configuration data which is
   distinct from the standard and/or proprietary YANG modules.  The
   application is free to construct its own model for data organization
   and to present this model to the user.  When the application needs to
   transmit data to a device, the application transforms its data from
   the problem-oriented view of the world into the data needed for that
   particular device.  This transformation is under the control and
   maintenance of the application, allowing the transformation to be
   changed and updated without affecting the device.

   For example, an application could be written that models VPNs in a
   network-oriented view.  The application would need to transform these
   high-level VPN definitions into the configuration data that would be
   handed to any particular device within a VPN.

6.  Modeling Considerations

   In developing good data models, there are many conflicting interests
   the data modeler must keep in mind.  Modelers need to be aware of
   four types of behavior in modeled device:

   o  [strict compliance] behavior that follow the model completely

   o  [modeled deviations] behavior that follows within deviations
      allowed by the model

   o  [allowable deviations] behavior that falls outside the model, but
      can still be handled

   o  [unacceptable deviations] behavior that is not at all consistent
      with the model

   Once the model is published, an implementer may decide to make a
   particular data model node configurable, where the standard model
   describes it a state data.  The implementation reports the value
   normally and may have an "out of band" mechanism for reporting that
   this device behaves in a different manner than the standard.
   Applications capable of discovering such behavior can make
   allowances, but applications that do not discover such behavior can
   continue treating the implementation as if it were compliant.

   Rarely, implementations may make decisions that prevent compliance
   with the standard.  Such occasions are regrettable, but they remain a
   part of reality, and modelers and application writers ignore them at
   their own risk.  An implementation that emits an integer leaf as
   "cow" would be difficult to manage, but applications must expect to
   encounter such misbehaving devices in the field.

   Despite this, both client and server should view the YANG module as a
   contract, with both sides agreeing to abide by the terms.  The
   modeler should be explicit about the terms of such a contract, and
   both client and server implementations should strive to faithfully
   and accurately implement the data model described in the YANG module.

7.  Security Considerations  Data Distinctions

   The distinction between configuration data, operational state data,
   and statistics is important to understand for data model writers and
   people who plan to extend the NETCONF protocol.  This document defines section first
   discusses some background and then provides a language with which to write definition and read
   descriptions of management information.  The language itself has no
   security impact on some

7.1.  Background

   During the Internet.

8. IAB Network Management Workshop documented in RFC 3535,
   operators did formulate the following two requirements:

    2.  It is necessary to make a clear distinction between
        configuration data, data that describes operational state
        and statistics.  Some devices make it very hard to determine
        which parameters were administratively configured and which
        were obtained via other mechanisms such as routing

    3.  It is required to be able to fetch separately configuration
        data, operational state data, and statistics from devices,
        and to be able to compare these between devices.

   The NETCONF protocol defined in RFC 4741 distinguishes two types of
   data, namely configuration data and state data:

      Configuration data is the set of writable data that is
      required to transform a system from its initial default state
      into its current state.

      State data is the additional data on a system that is not
      configuration data such as read-only status information and
      collected statistics.

   NETCONF does not follow the distinction formulated by the operators
   between configuration data, operational state data, and statistical
   data, since it considers state data to include both statistics and
   operational state data.

7.2.  Definitions

   Below is a definition for configuration data, operational state data,
   and statistical data.  The definition borrows from previous work.

   o  Configuration data is the set of writable data that is required to
      transform a system from its initial default state into its current
      state.  [RFC 4741]

   o  Operational state data is a set of data that has been obtained by
      the system at runtime and influences the system's behaviour
      similar to configuration data.  In contrast to configuration data,
      operational state is transient and modified by interactions with
      internal components or other systems via specialized protocols.

   o  Statistical data is the set of read-only data created by a system
      itself.  It describes the performance of the system and its

   The following examples help to clarify the difference between
   configuration data, operational state data and statistical data.

7.2.1.  Example 1: IP Routing Table

   IP routing tables can contain entries that are statically configured
   (configuration data) as well as entries obtained from routing
   protocols such as OSPF (operational state data).  In addition, a
   routing engine might collect statistics like how often a particular
   routing table entry has been used.

7.2.2.  Example 2: Interfaces

   Network interfaces usually comes with a large number of attributes
   that are specific to the interface type and in some cases specific to
   the cable plugged into an interface.  Examples are the maximum
   transmission unit of an interface or the speed detected by an
   Ethernet interface.

   In many deployments, systems use the interface attributes detected
   when an interface is initialized.  As such, these attributes
   constitute operational state.  However, there are usually provisions
   to overwrite the discovered attributes with static configuration
   data, like for example configuring the interface MTU to use a
   specific value or forcing an Ethernet interface to run at a given

   The system will record statistics (counters) measuring the number of
   packets, bytes, and errors received and transmitted on each

7.2.3.  Example 3: Account Information

   Systems usually maintain static configuration information about the
   accounts on the system.  In addition, systems can obtain information
   about accounts from other sources (e.g.  LDAP, NIS) dynamically,
   leading to operational state data.  Information about account usage
   are examples of statistic data.

   Note that configuration data supplied to a system in order to create
   a new account might be supplemented with additional configuration
   information determined by the system when the account is being
   created (such as a unique account id).  Even though the system might
   create such information, it usually becomes part of the static
   configuration of the system since this data is not transient.

7.3.  Implications

   The primary focus of YANG is configuration data.  There is no single
   mechanism defined for the separation of operational state data and
   statistics since NETCONF treats them both as state data.  This
   section describes several different options for addressing this

7.3.1.  Data Models

   The first option is to have data models that provide explicitly
   differentiate between configuration data and operational state data.
   This leads to duplication of data structures and might not scale well
   from a modeling perspective.

   For example, the configured duplex value and the operational duplex
   value would be distinct leafs in the data model.

7.3.2.  Additional Operations to Retrieve Operational State

   The NETCONF protocol can be extended with new protocol operations
   that specifically allow the retrieval of all operational state, e.g.
   by introducing a <get-ops> operation (and perhaps also a <get-stats>

7.3.3.  Introduction of an Operational State Datastore

   Another option could be to introduce a new "configuration" data store
   that represents the operational state.  A <get-config> operation on
   the <operational> data store would then return the operational state
   determining the behaviour of the box instead of its static and
   explicit configuration state.

8.  Security Considerations

   This document defines a language with which to write and read
   descriptions of management information.  The language itself has no
   security impact on the Internet.

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4741]  Enns, R., "NETCONF Configuration Protocol", RFC 4741,
              December 2006.

   [YANG]     Bjorklund, M., Ed., "YANG - A data modeling language for
              NETCONF", draft-ietf-netmod-yang-05 draft-ietf-netmod-yang-11 (work in progress).

Author's Address

   Phil Shafer
   Juniper Networks