Network Working Group                                  M. Bjorklund, Ed.
Internet-Draft                                            Tail-f Systems
Intended status: Standards Track                        J. Schoenwaelder
Expires: June 22, 2017                                 Jacobs University
                                                               P. Shafer
                                                               K. Watsen
                                                               R. Wilton
                                                       December 19, 2016

             A Revised Conceptual Model for YANG Datastores


   Datastores are a fundamental concept binding the YANG data modeling
   language to protocols transporting data defined in YANG data models,
   such as NETCONF or RESTCONF.  This document defines a revised
   conceptual model of datastores based on the experience gained with
   the initial simpler model and addressing requirements that were not
   well supported in the initial model.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on June 22, 2017.

Copyright Notice

   Copyright (c) 2016 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

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Original Model of Datastores  . . . . . . . . . . . . . . . .   4
   5.  Revised Model of Datastores . . . . . . . . . . . . . . . . .   6
     5.1.  The <intended> datastore  . . . . . . . . . . . . . . . .   8
     5.2.  The <applied> datastore . . . . . . . . . . . . . . . . .   8
       5.2.1.  Missing Resources . . . . . . . . . . . . . . . . . .   9
       5.2.2.  System-controlled Resources . . . . . . . . . . . . .   9
     5.3.  The <operational-state> datastore . . . . . . . . . . . .   9
   6.  Implications  . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Implications on NETCONF . . . . . . . . . . . . . . . . .   9
       6.1.1.  Migration Path  . . . . . . . . . . . . . . . . . . .  10
     6.2.  Implications on RESTCONF  . . . . . . . . . . . . . . . .  10
     6.3.  Implications on YANG  . . . . . . . . . . . . . . . . . .  11
     6.4.  Implications on Data Models . . . . . . . . . . . . . . .  11
   7.  Data Model Design Guidelines  . . . . . . . . . . . . . . . .  11
     7.1.  Auto-configured or Auto-negotiated Values . . . . . . . .  11
   8.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Example Data . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   This document provides a revised architectural framework for
   datastores as they are used by network management protocols such as
   NETCONF [RFC6241], RESTCONF [I-D.ietf-netconf-restconf] and the YANG
   [RFC7950] data modeling language.  Datastores are a fundamental
   concept binding management data models to network management
   protocols and agreement on a common architectural model of datastores
   ensures that data models can be written in a network management

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   protocol agnostic way.  This architectural framework identifies a set
   of conceptual datastores but it does not mandate that all network
   management protocols expose all these conceptual datastores.
   Furthermore, the architecture does not detail how data is encoded by
   network management protocols.

2.  Background

   NETCONF [RFC6241] provides the following definitions:

   o  datastore: A conceptual place to store and access information.  A
      datastore might be implemented, for example, using files, a
      database, flash memory locations, or combinations thereof.

   o  configuration datastore: The datastore holding the complete set of
      configuration data that is required to get a device from its
      initial default state into a desired operational state.

   YANG 1.1 [RFC7950] provides the following refinements when NETCONF is
   used with YANG (which is the usual case but note that NETCONF was
   defined before YANG did exist):

   o  datastore: When modeled with YANG, a datastore is realized as an
      instantiated data tree.

   o  configuration datastore: When modeled with YANG, a configuration
      datastore is realized as an instantiated data tree with
      configuration data.

   RFC 6244 defined operational state data as follows:

   o  Operational state data is a set of data that has been obtained by
      the system at runtime and influences the system's behavior 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.

   Section 4.3.3 of RFC 6244 discusses operational state and among other
   things mentions the option to consider operational state as being
   stored in another datastore.  Section 4.4 of this document then
   concludes that at the time of the writing, modeling state as a
   separate data tree is the recommended approach.

   Implementation experience and requests from operators
   [I-D.ietf-netmod-opstate-reqs], [I-D.openconfig-netmod-opstate]
   indicate that the datastore model initially designed for NETCONF and
   refined by YANG needs to be extended.  In particular, the notion of
   intended configuration and applied configuration has developed.

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   Furthermore, separating operational state data from configuration
   data in a separate branch in the data model has been found
   operationally complicated.  The relationship between the branches is
   not machine readable and filter expressions operating on
   configuration data and on related operational state data are

3.  Terminology

   This document defines the following terms:

   o  configuration data: Data that determines how a device behaves.
      Configuration data can originate from different sources.  In YANG
      1.1, configuration data is the "config true" nodes.

   o  static configuration data: Configuration data that is eventually
      persistent and used to get a device from its initial default state
      into its desired operational state.

   o  dynamic configuration data: Configuration data that is obtained
      dynamically during the operation of a device through interaction
      with other systems and not persistent.

   o  system configuration data: Configuration data that is supplied by
      the device itself.

   o  data-model-defined configuration data: Configuration data that is
      not explicitly provided but for which a value defined in the data
      model is used.  In YANG 1.1, such data can be defined with the
      "default" statement or in "description" statements.

4.  Original Model of Datastores

   The following drawing shows the original model of datastores as it is
   currently used by NETCONF [RFC6241]:

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     +-------------+                 +-----------+
     | <candidate> |                 | <startup> |
     |  (ct, rw)   |<---+       +--->| (ct, rw)  |
     +-------------+    |       |    +-----------+
            |           |       |           |
            |         +-----------+         |
            +-------->| <running> |<--------+
                      | (ct, rw)  |
                     operational state  <--- control plane
                         (cf, ro)

     ct = config true; cf = config false
     rw = read-write; ro = read-only
     boxes denote datastores

   Note that read-only (ro) and read-write (rw) is to be understood at a
   conceptual level.  In NETCONF, for example, support for the
   <candidate> and <startup> datastores is optional and the <running>
   datastore does not have to be writable.  Furthermore, the <startup>
   datastore can only be modified by copying <running> to <startup> in
   the standardized NETCONF datastore editing model.  The RESTCONF
   protocol does not expose these differences and instead provides only
   a writable unified datastore, which hides whether edits are done
   through a <candidate> datastore or by directly modifying the
   <running> datastore or via some other implementation specific
   mechanism.  RESTCONF also hides how configuration is made persistent.
   Note that implementations may also have additional datastores that
   can propagate changes to the <running> datastore.  NETCONF explicitly
   mentions so called named datastores.

   Some observations:

   o  Operational state has not been defined as a datastore although
      there were proposals in the past to introduce an operational state

   o  The NETCONF <get/> operation returns the content of the <running>
      configuration datastore together with the operational state.  It
      is therefore necessary that config false data is in a different
      branch than the config true data if the operational state data can
      have a different lifetime compared to configuration data or if
      configuration data is not immediately or successfully applied.

   o  Several implementations have proprietary mechanisms that allow
      clients to store inactive data in the <running> datastore; this

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      inactive data is only exposed to clients that indicate that they
      support the concept of inactive data; clients not indicating
      support for inactive data receive the content of the <running>
      datastore with the inactive data removed.  Inactive data is
      conceptually removed during validation.

   o  Some implementations have proprietary mechanisms that allow
      clients to define configuration templates in <running>.  These
      templates are expanded automatically by the system, and the
      resulting configuration is applied internally.

   o  Some operators have reported that it is essential for them to be
      able to retrieve the configuration that has actually been
      successfully applied, which may be a subset or a superset of the
      <running> configuration.

5.  Revised Model of Datastores

   Below is a new conceptual model of datastores extending the original
   model in order reflect the experience gained with the original model.

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     +-------------+                 +-----------+
     | <candidate> |                 | <startup> |
     |  (ct, rw)   |<---+       +--->| (ct, rw)  |
     +-------------+    |       |    +-----------+
            |           |       |           |
            |         +-----------+         |
            +-------->| <running> |<--------+
                      | (ct, rw)  |
                            |        // e.g., removal of 'inactive'
                            |        // nodes, expansion of templates
                      | <intended> | // subject to validation
                      | (ct, ro)   |
                            |        // e.g., missing resources or
                            |        // delays
                      | <applied> |<---+--- dynamic configuration
                      | (ct, ro)  |    |      protocols
                      +-----------+    +--- control-plane datastores
                            |          +--- auto-discovery
                            |    +-----+--- control-plane protocols
                            |    |     +--- control-plane datastores
                            v    v
                  | <operational-state> |
                  | (ct + cf, ro)       |

     ct = config true; cf = config false
     rw = read-write; ro = read-only
     boxes denote datastores

   The model foresees control-plane datastores that are by definition
   not part of the persistent configuration of a device.  In some
   contexts, these have been termed ephemeral datastores since the
   information is ephemeral, i.e., lost upon reboot.  The control-plane
   datastores interact with the rest of the system through the <applied>
   or <operational-state> datastores, depending on the type of data they
   contain.  Note that the ephemeral datastore discussed in I2RS
   documents maps to a control-plane datastore in the revised datastore
   model described here.

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5.1.  The <intended> datastore

   The <intended> datastore is a read-only datastore that consists of
   config true nodes.  It is tightly coupled to <running>.  When data is
   written to <running>, the data that is to be validated is also
   conceptually written to <intended>.  Validation is performed on the
   contents of <intended>.

   On a traditional NETCONF implementation, <running> and <intended> are
   always the same.

   Currently there are no standard mechanisms defined that affect
   <intended> so that it would have different contents than <running>,
   but this architecture allows for such mechanisms to be defined.

   One example of such a mechanism is support for marking nodes as
   inactive in <running>.  Inactive nodes are not copied to <intended>,
   and are thus not taken into account when validating the

   Another example is support for templates.  Templates are expanded
   when copied into <intended>, and the result is validated.

5.2.  The <applied> datastore

   The <applied> datastore is a read-only datastore that consists of
   config true nodes.  It contains the currently active configuration on
   the device.  This data can come from several sources; from
   <intended>, from dynamic configuration protocols (e.g., DHCP), or
   from control-plane datastores.

   As data flows into the <applied> and <operational-state> datastores,
   it is conceptually marked with a metadata annotation ([RFC7952]) that
   indicates its origin.  The "origin" metadata annotation is defined in
   Section 8.  The values are YANG identities.  The following identities
   are defined:

     +-- origin
         +-- static
         +-- dynamic
         +-- data-model
         +-- system

   These identities can be further refined, e.g., there might be an
   identity "dhcp" derived from "dynamic".

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   The <applied> datastore contains the subset of the instances in the
   <operational-state> datastore where the "origin" values are derived
   from or equal to "static" or "dynamic".

5.2.1.  Missing Resources

   Sometimes some parts of <intended> configuration refer to resources
   that are not present and hence parts of the <intended> configuration
   cannot be applied.  A typical example is an interface configuration
   that refers to an interface that is not currently present.  In such a
   situation, the interface configuration remains in <intended> but the
   interface configuration will not appear in <applied>.

5.2.2.  System-controlled Resources

   Sometimes resources are controlled by the device and such system
   controlled resources appear in (and disappear from) the
   <operational-state> dynamically.  If a system controlled resource has
   matching configuration in <intended> when it appears, the system will
   try to apply the configuration, which causes the configuration to
   appear in <applied> eventually (if application of the configuration
   was successful).

5.3.  The <operational-state> datastore

   The <operational-state> datastore is a read-only datastore that
   consists of config true and config false nodes.  In the original
   NETCONF model the operational state only had config false nodes.  The
   reason for incorporating config true nodes here is to be able to
   expose all operational settings without having to replicate
   definitions in the data models.

   The <operational-state> datastore contains all configura data
   actually used by the system, i.e., all applied configuration, system
   configuration and data-model-defined configuration.  This data is
   marked with the "origin" metadata annotation.  In addition, the
   <operational-state> datastore also contains state data.

   In the <operational-state> datastore, semantic constraints defined in
   the data model are not applied.  See Appendix B.

6.  Implications

6.1.  Implications on NETCONF

   o  A mechanism is needed to announce support for <intended>,
      <applied>, and <operational-state>.

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   o  Support for <intended>, <applied>, and <operational-state> should
      be optional to implement.

   o  For systems supporting <intended> or <applied> configuration
      datastores, the <get-config/> operation may be used to retrieve
      data stored in these new datastores.

   o  A new operation should be added to retrieve the operational state
      data store (e.g., <get-state/>).  An alternative is to define a
      new operation to retrieve data from any datastore (e.g.,
      <get-data> with the name of the datastore as a parameter).  In
      principle <get-config/> could work but it would be a confusing

   o  The <get/> operation will be deprecated since it returns data from
      two datastores that may overlap in the revised datastore model.

6.1.1.  Migration Path

   A common approach in current data models is to have two separate
   trees "/foo" and "/foo-state", where the former contains config true
   nodes, and the latter config false nodes.  A data model that is
   designed for the revised architectural framework presented in this
   document will have a single tree "/foo" with a combination of config
   true and config false nodes.

   A server that implements the <operational-state> datastore can
   implement a module of the old design.  In this case, some instances
   are probably reported both in the "/foo" tree and in the "/foo-state"

   A server that does not implement the <operational-state> datastore
   can implement a module of the new design, but with limited
   functionality.  Specifically, it may not be possible to retrieve all
   operationally used instances (e.g., dynamically configured or system-
   controlled).  The same limitation applies to a client that does not
   implement the <operational-state> datastore, but talks to a server
   that implements it.

6.2.  Implications on RESTCONF

   o  The {+restconf}/data resource represents the combined
      configuration and state data resources that can be accessed by a
      client.  This is effectively bundling <running> together with
      <operational-state>, much like the <get/> operation of NETCONF.
      This design should be deprecated.

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   o  A new query parameter is needed to indicate that data from
      <operational-state> is requested.

6.3.  Implications on YANG

   o  Some clarifications may be needed if this revised model is
      adopted.  YANG currently describes validation in terms of the
      <running> configuration datastore while it really happens on the
      <intended> configuration datastore.

6.4.  Implications on Data Models

   o  Since the NETCONF <get/> operation returns the content of the
      <running> configuration datastore and the operational state
      together in one tree, data models were often forced to branch at
      the top-level into a config true branch and a structurally similar
      config false branch that replicated some of the config true nodes
      and added state nodes.  With the revised datastore model this is
      not needed anymore since the different datastores handle the
      different lifetimes of data objects.  Introducing this model
      together with the deprecation of the <get/> operation makes it
      possible to write simpler models.

   o  There may be some differences in the value set of some nodes that
      are used for both configuration and state.  At this point of time,
      these are considered to be rare cases that can be dealt with using
      different nodes for the configured and state values.

   o  It is important to design data models with clear semantics that
      work equally well for instantiation in a configuration datastore
      and instantiation in the <operational-state> datastore.

7.  Data Model Design Guidelines

7.1.  Auto-configured or Auto-negotiated Values

   Sometimes configuration leafs support special values that instruct
   the system to automatically configure a value.  An example is an MTU
   that is configured to 'auto' to let the system determine a suitable
   MTU value.  Another example is Ethernet auto-negotiation of link
   speed.  In such a situation, it is recommended to model this as two
   separate leafs, one config true leaf for the input to the auto-
   negotiation process, and one config false leaf for the output from
   the process.

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8.  Data Model

   <CODE BEGINS> file "ietf-yang-architecture@2016-10-13.yang"

   module ietf-yang-architecture {
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-architecture";
     prefix arch;

     import ietf-yang-metadata {
       prefix md;

       "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

       "WG Web:   <>

        WG List:  <>

        Editor:   Martin Bjorklund

       "This YANG module defines an 'origin' metadata annotation,
        and a set of identities for the origin value.  The 'origin'
        metadata annotation is used to mark data in the applied
        and operational state datastores with information on where
        the data originated.

        Copyright (c) 2016 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Simplified BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents

        This version of this YANG module is part of RFC XXXX
        (; see the RFC itself
        for full legal notices.";

     revision 2016-10-13 {
         "Initial revision.";

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         "RFC XXXX: A Revised Conceptual Model for YANG Datastores";

      * Identities

     identity origin {
         "Abstract base identitiy for the origin annotation.";

     identity static {
       base origin;
         "Denotes data from static configuration (e.g., <intended>).";

     identity dynamic {
       base origin;
         "Denotes data from dynamic configuration protocols
          or dynamic datastores (e.g., DHCP).";

     identity system {
       base origin;
         "Denotes data created by the system independently of what
          has been configured.";

     identity data-model {
       base origin;
         "Denotes data that does not have an explicitly configured
          value, but has a default value in use.  Covers both simple
          defaults and complex defaults.";

      * Metadata annotations

     md:annotation origin {
       type identityref {
         base origin;

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9.  IANA Considerations


10.  Security Considerations

   This document discusses a conceptual model of datastores for network
   management using NETCONF/RESTCONF and YANG.  It has no security
   impact on the Internet.

11.  Acknowledgments

   This document grew out of many discussions that took place since
   2010.  Several Internet-Drafts ([I-D.bjorklund-netmod-operational],
   [I-D.wilton-netmod-opstate-yang], [I-D.ietf-netmod-opstate-reqs],
   [I-D.kwatsen-netmod-opstate], [I-D.openconfig-netmod-opstate]) and
   [RFC6244] touched on some of the problems of the original datastore
   model.  The following people were authors to these Internet-Drafts or
   otherwise actively involved in the discussions that led to this

   o  Lou Berger, LabN Consulting, L.L.C., <>

   o  Andy Bierman, YumaWorks, <>

   o  Marcus Hines, Google, <>

   o  Christian Hopps, Deutsche Telekom, <>

   o  Acee Lindem, Cisco Systems, <>

   o  Ladislav Lhotka, CZ.NIC, <>

   o  Thomas Nadeau, Brocade Networks, <>

   o  Anees Shaikh, Google, <>

   o  Rob Shakir, Google, <>

   Juergen Schoenwaelder was partly funded by Flamingo, a Network of
   Excellence project (ICT-318488) supported by the European Commission
   under its Seventh Framework Programme.

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12.  References

12.1.  Normative References

              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", draft-ietf-netconf-restconf-18 (work in
              progress), October 2016.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC7952]  Lhotka, L., "Defining and Using Metadata with YANG",
              RFC 7952, DOI 10.17487/RFC7952, August 2016,

12.2.  Informative References

              Bjorklund, M. and L. Lhotka, "Operational Data in NETCONF
              and YANG", draft-bjorklund-netmod-operational-00 (work in
              progress), October 2012.

              Watsen, K. and T. Nadeau, "Terminology and Requirements
              for Enhanced Handling of Operational State", draft-ietf-
              netmod-opstate-reqs-04 (work in progress), January 2016.

              Watsen, K., Bierman, A., Bjorklund, M., and J.
              Schoenwaelder, "Operational State Enhancements for YANG,
              NETCONF, and RESTCONF", draft-kwatsen-netmod-opstate-02
              (work in progress), February 2016.

              Shakir, R., Shaikh, A., and M. Hines, "Consistent Modeling
              of Operational State Data in YANG", draft-openconfig-
              netmod-opstate-01 (work in progress), July 2015.

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              Wilton, R., ""With-config-state" Capability for NETCONF/
              RESTCONF", draft-wilton-netmod-opstate-yang-02 (work in
              progress), December 2015.

   [RFC6244]  Shafer, P., "An Architecture for Network Management Using
              NETCONF and YANG", RFC 6244, DOI 10.17487/RFC6244, June
              2011, <>.

Appendix A.  Example Data

   In this example, the following fictional module is used:

   module example-system {
     yang-version 1.1;
     namespace urn:example:system;
     prefix sys;

     import ietf-inet-types {
       prefix inet;

     container system {
       leaf hostname {
         type string;

       list interface {
         key name;

         leaf name {
           type string;

         container auto-negotiation {
           leaf enabled {
             type boolean;
             default true;
           leaf speed {
             type uint32;
             units mbps;
               "The advertised speed, in mbps.";

         leaf speed {

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           type uint32;
           units mbps;
           config false;
             "The speed of the interface, in mbps.";

         list address {
           key ip;

           leaf ip {
             type inet:ip-address;
           leaf prefix-length {
             type uint8;

   The operator has configured the host name and two interfaces, so the
   contents of <intended> is:

   <system xmlns="urn:example:system">





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   The system has detected that the hardware for one of the configured
   interfaces ("eth1") is not yet present, so the configuration for that
   interface is not applied.  Further, the system has received a host
   name and an additional IP address for "eth0" over DHCP.  This is
   reflected in <applied>:


     <hostname arch:origin="arch:dynamic">bar</hostname>

     <interface arch:origin="arch:static">
       <address arch:origin="arch:dynamic">


   In <operational-state>, all data from <applied> is present, in
   addition to a default value, a loopback interface automatically added
   by the system, and the result of the "speed" auto-negotiation:

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     <hostname arch:origin="arch:dynamic">bar</hostname>

     <interface arch:origin="arch:static">
         <enabled arch:origin="arch:data-model">true</enabled>
       <address arch:origin="arch:dynamic">

     <interface arch:origin="arch:system">


Appendix B.  Open Issues

   1.  Do we need another DS <active> inbetween <running> and
       <intended>?  This DS would allow a client to see all active
       nodes, including unexpanded templates.

   2.  How do we handle semantical constraints in <operational-state>?
       Are they just ignored?  Do we need a new YANG statement to define
       if a "must" constraints applies to the <operational-state>?

   3.  Should it be possible to ask for <applied> in RESTCONF?

   4.  Better name for "static configuration"?

   5.  Better name for "intended"?

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Authors' Addresses

   Martin Bjorklund (editor)
   Tail-f Systems


   Juergen Schoenwaelder
   Jacobs University


   Phil Shafer


   Kent Watsen


   Rob Wilton


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