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Network Working Group                                          A. Shaikh
Internet-Draft                                                    Google
Intended status: Informational                                 R. Shakir
Expires: September 10, 2015                                           BT
                                                              K. D'Souza
                                                                    AT&T
                                                                 L. Fang
                                                               Microsoft
                                                           March 9, 2015


         Operational Structure and Organization of YANG Models
               draft-openconfig-netmod-model-structure-00

Abstract

   This document presents an approach for organizing YANG models in a
   comprehensive structure that defines how individual models may be
   composed to configure and operate network infrastructure and
   services.  The structure is itself represented as a YANG model rooted
   at a device, with all of the related component models logically
   organized in a way that is operationally intuitive.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 10, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The large number of configuration models recently published cover
   much of networking protocols and technology and, in theory, enable a
   programmatic, model-driven approach for configuring network devices.
   These models have been largely developed individually and in
   isolation, however, making it challenging to use them together to
   fully configure a device, or manage a set of devices comprising a
   service.  For example, standard models for interface management
   [RFC7223] and system management [RFC7317] are available but there is
   no guidance for how they should be used together, or combined with
   other models for routing protocols, ACLs, etc. to form a complete
   model.  Recently, some frameworks (e.g., [RTG-CFG]  and [RTG-POLICY])
   that tie models together have been developed, but they are
   incomplete, covering only a subset of related models.

1.1.  Goals and approach

   In this document, we describe a structure for organizing YANG
   [RFC6020] models that is broadly applicable to physical and virtual
   devices.  Individual models are composed such that the data they
   define can be accessed in a predictable and operationally intuitive
   way that is common across implementations.  This organization enables
   several important capabilities:

   o  a common schema to access data related to all aspects of a device

   o  an extensible structure that makes it clear where additional
      models or data should be fit (e.g., using YANG augmentation or
      imports)

   o  a place for including metadata that provides useful information
      about the corresponding individual models, such as which
      organization provides them, which vendors support them, or which
      version of the model is deployed

   o  a common infrastructure model layer on which higher layer service
      models can be built, for example by specifying which models are
      needed to provide the service





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   o  an ability to express an instance of the structure consisting of
      models that have been validated to work together (i.e., with
      information about sources of the models, their versions, etc.), so
      that operators can easily identify a set of models that is known
      to be mutually consistent

   Our approach is to organize the models describing various aspects of
   network infrastructure, including devices and their subsystems, and
   relevant protocols operating at the link and network layers.  The
   proposal does not consider a common model for higher level network
   services, nor does it specify details of how hardware-related data
   should be organized.  Both of these are challenging to standardize --
   services are subject to operational and business considerations that
   vary across network operators, and hardware models are necessarily
   dependent on specific platform features and architecture -- and are
   thus out of scope of this document.  We instead consider the set of
   models that are commonly used by network operators, and suggest a
   corresponding organization.

   As with other models developed from an operator perspective, the
   intent is not to be exhaustive by including all possible models in
   the overall structure, whether currently available or not.  We focus
   on components that are deemed most useful for network operators
   across a variety of use cases.  We recognize, however, that
   additional models will be needed in some cases, and this structure is
   useful for describing how new models can be fit into the overall
   structure.

2.  Model overview

   The model organization can itself be thought of as a "meta- model",
   in that it describes the relationships between individual models.  We
   choose to represent it also as simple YANG model consisting of lists
   and containers to serve as anchor points for the corresponding
   individual models.

   As shown below, our model is rooted at a "device", which represents a
   network router, switch, or similar device.  The model is applicable
   to both physical, hardware-based devices, as well as software-based
   devices such as virtual network functions (VNFs).  It does not follow
   the hierarchy of any particular implementation, and hence is vendor-
   neutral.  Nevertheless, the structure should be familiar to network
   operators and also readily mapped to vendor implementations.








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   +--rw device
         +--rw info
         |  +--rw device-type?
         |     ...
         +--rw hardware
         +--rw system
         |     ...
         +--rw interfaces
         |     ...
         +--rw acl
         +--rw qos
         +--rw logical-routers
               ...

   The key subsystems are represented at the top level of the device,
   including, system-wide configuration, interfaces, and routing
   instances.  The info section can be used for basic device information
   such as its type (e.g., physical or virtual), vendor, and model.  For
   physical devices, the hardware container is intended to be a
   placeholder for platform-specific configuration and operational state
   data.  For example, a common structure for the hardware model might
   include chassis, linecards, and ports, but we leave this unspecified.

2.1.  System model components

   The system container includes a number of subsystems that are
   typically configured globally for the device.  Some of these, such as
   DHCP, Ethernet CFM, or sampling configuration also may have data that
   is associated with an interface.  For simplicity, these relationships
   are not represented in this structural model.  The currently defined
   subsystems are shown below:

      +--rw device
         +--rw system
            +--rw dns
            +--rw ntp
            +--rw dhcp
            +--rw syslog
            +--rw ssh
            +--rw stat-coll
            +--rw oam
            |  +--rw snmp
            |  +--rw cfm
            |  +--rw twamp
            +--rw aaa
            |  +--rw tacacs
            |  +--rw radius
            +--rw users



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2.2.  Interface model components

   Interfaces are a crucial part of any network device's configuration
   and operational state.  They generally include a combination of raw
   physical interfaces, link-layer interfaces, addressing configuration,
   and logical interfaces that may not be tied to any physical
   interface.  Several system services, and layer 2 and layer 3
   protocols may also associate configuration or operational state data
   with different types of interfaces (these relationships are not shown
   for simplicity).  The interfaces container includes a number of
   commonly used components as examples:

      +--rw device
         +--rw interfaces
            +--rw ethernet
            |  +--rw aggregates
            |  +--rw vlans
            |  +--rw lfm
            +--rw sonet-sdh
            +--rw addressing
            |  +--rw ipv4
            |  |  +--rw vrrp
            |  +--rw ipv6
            |     +--rw vrrp
            +--rw tunnels

2.3.  Logical routing instances

   Logical routers represent the capability on some devices to partition
   resources into independent logical routers.  In physical devices,
   some hardware features are shared across partitions, but routing
   protocol instances, routing tables, and configuration are managed
   separately.  In virtual routers or VNFs, this may correspond to
   establishing multiple logical instances using a single software
   installation.  The model supports configuration of multiple routing
   instances on a single device by creating a list of logical routers,
   each with their own configuration and operational state related to
   routing and switching protocols, as shown below:

      +--rw device
         +--rw logical-routers
            +--rw logical-router* [router-id]
               +--rw router-id            uint8
               +--rw router-name?         string
               +--rw layer-2-protocols
               |     ...
               +--rw layer-3-protocols
                     ...



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2.4.  VRFs and global routing configuration

   Virtual routing and forwarding instances (VRFs) are commonly used to
   isolate routing domains, for example to create virtual private
   networks, each with their own active protocols and routing policies.
   Devices also have a global instance of each routing protocol that may
   also exchange routes with VRFs through routing policies.  The model
   describes protocols and policies for both VRF routing instances and
   the global instance.  The routing policy framework is expected to
   follow [RTG-POLICY], which enables import / export policies to be
   expressed with respect to a VRF, or the global routing instance.

   +--rw device
         +--rw logical-routers
            +--rw logical-router* [router-id]
               +--rw router-id
               +--rw router-name?
               +--rw layer-3-protocols
                  +--rw global
                  |     ...
                  +--rw vrf* [vrf-name]
                  |     ...
                  +--rw routing-policy
                        ...

3.  Populating the structural model

   The structural model in this document describes how individual YANG
   models may be used together to represent the configuration and
   operational state for all parts of a physical or virtual device.  It
   does not, however, document the actual model in its entirety.  In
   this section, we outline an option for creating the full model and
   also describe how it may be used.

3.1.  Constructing the device model

   One of the challenges in assembling existing YANG models is that they
   are generally written with the assumption that each model is at the
   root of the configuration or state tree.  Combining models then
   results in a multi-rooted tree that does not follow any logical
   construction and makes it difficult to work with operationally.  In
   some cases, models explicitly reference other models (e.g., via
   augmentation) to define a relationship, but this is the case for only
   a few existing models.

   Some examples include the interfaces [RFC7223] and IP management
   [RFC7277] models, and proposed IS-IS [RTG-ISIS], OSPF [RTG-OSPF] and
   routing configuration [RTG-CFG] models.



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3.2.  Pull approach for model composition

   To enable model composition, one possible approach is to avoid using
   root-level containers in individual component models.  Instead, the
   top level container (and all other data definitions) can be enclosed
   in a YANG 'grouping' statement so that when the model is imported by
   another model, its location in the configuration tree can be
   controlled by the importing YANG module with the 'uses' statement.
   One advantage of this approach is that the importing module has the
   flexibility to readily use the data definitions where the author
   deems appropriate.

   One obvious drawback is that individual models no longer contain any
   of their own data definitions and must be used by a higher-level
   model for their data nodes to become active.  Some judgment as to
   which models are more suited for inclusion in higher level models is
   also necessary to decide when the corresponding YANG module should
   contain only groupings.  Another potential drawback is that this
   approach does not define a common structure for models to fit
   together, limiting interoperability due to implementations using
   different structures.  To address this, a top-level standard model
   structure could be defined and updated to import new models into the
   hierarchy as they are defined.

3.3.  "Push" approach for model composition

   An alternative approach is to develop a top level model which defines
   the overall structure of the models, similar to the structure
   described in Section 2.  Individual models may augment the top level
   model with their data nodes in the appropriate locations.  The
   drawback is the need for a pre-defined top level model structure.  On
   the other hand, when this top level model is standardized, it can
   become the basis for a vendor-neutral way to manage devices, assuming
   that the component models are supported by a given implementation.

   One question in both approaches is what the root of the top- level
   model should be.  In this document we selected to base the mode at a
   device because this layer should be common across many use cases and
   implementations.  Starting at a higher layer (e.g., services) makes
   defining and agreeing on a common organization more challenging as
   discussed in Section 1.1.

   Ideally, one could consider a hybrid construction mechanism that
   supports both styles of model composition.  For example, a YANG
   compiler directive could be used to indicate whether an individual
   model should assume it is at the root, or whether it is meant for
   inclusion in other higher-level models.




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4.  Additional use cases

   The goal of this document is to motivate the need for an overall
   structure for YANG data models that allows all of the data to be
   accessed in a common, logical way.  With such a structure defined
   itself as a simple YANG model, it is possible to consider additional
   use cases.

4.1.  Model catalog

   YANG data models are being developed in a number of organizations,
   including standards bodies such as IETF, ONF, and IEEE, as well as
   open source projects and ad-hoc working groups.  In addition to
   understanding how these models can work together, another challenge
   for users is the complexity of tracking which organization created a
   given model, and the capabilities and coverage each model provides.
   This becomes even more difficult when multiple overlapping models are
   available for a particular component.

   Such a catalog could also be locally defined by an operator to
   describe the models needed to instantiate and manage different
   services.

   The idea of a model catalog is similar to service catalogs in
   traditional IT environments.  Service catalogs serve as a software-
   based registries of available services with information needed to
   discover and invoke available services.

   The current model structure described in Section 2 focuses on
   describing relationships between the models, however there are
   several examples of additional metadata that could be captured for
   each component model in the overall structural model:

   o  origin and responsible party for maintenance of the model with
      contact information.  In IETF standard models, the YANG
      'organization' and 'contact' statement contents are a good
      example, but this is not necessarily the case for models from
      other sources.

   o  license under which the model is distributed, e.g., open source or
      as part of a commercial license

   o  classification of the model, including its category / subcategory,
      whether the model is intended to be used standalone, etc.

   o  model dependencies, e.g., a list of other modules that are
      required




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   o  namespace information, including base namespace, prefixes, etc. to
      enable importing the model

   o  pointer to the YANG code, if it is freely downloadable

   o  implementation information, for example, a list of available
      implementations that support the model from vendors, open source
      projects, etc.

   o  authentication information to allow users to verify that the model
      they download does in fact originate from the stated organization

   For such an approach to be useful, we also require a registration
   system where model developers can register information about their
   models, and update it as needed.  The IANA XML Registry" [RFC3688]
   provides a basic registry for YANG models, but the information is
   somewhat limited and is currently targeted at IETF-standardized
   models only.  Further details on the proposal for such a registry may
   be forthcoming in further revisions to this document.

4.2.  Service-layer composition

   The proposed structural model covers a wide variety of components and
   protocols, and clearly not all of them are needed for all services.
   Another envisioned use case for the structural model is the ability
   to reference the set of models that are needed for specific use cases
   or services.  The intent is that the set would be based on best
   operational practices as defined by users or operators who run such
   services.

   One approach for this would be to define a 'service overlay' model,
   for example for Layer 3 VPN services, that defines the set of
   required configuration and state models, such as VRFs, interfaces,
   BGP, policy, ACLs, and QoS.  Similar overlay models can be defined
   for other services or use cases, for example, basic Internet
   operations such as adding new peers or customers, or setting up Layer
   2 VPNs.  Note these overlay models may be complementary to actual
   configuration models for such services, which may focus on providing
   an abstracted set of configuration or operational state variables,
   which would then be mapped onto device level variables.  We leave
   discussion of such mapping mechanisms to future revisions.

5.  Security Considerations

   The model structure described in this document does not define actual
   configuration and state data, hence it is not directly responsible
   for security risks.




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   However, each of the component models that provide the corresponding
   configuration and state data should be considered sensitive from a
   security standpoint since they generally manipulate aspects of
   network configurations.  Each component model should be carefully
   evaluated to determine its security risks, along with mitigations to
   reduce such risks.

6.  IANA Considerations

   This YANG model currently uses a temporary ad-hoc namespace.  If it
   is placed or redirected for the standards track, an appropriate
   namespace URI will be registered in the IETF XML Registry" [RFC3688].
   The YANG structure modules will be registered in the "YANG Module
   Names" registry [RFC6020].

7.  YANG module

   The model structure is described by the YANG module below.

7.1.  Model structure

   <CODE BEGINS> file model-structure.yang
   module model-structure {

     yang-version "1";

     // namespace
     namespace "http://openconfig.net/yang/structure";

     prefix "struct";

     // import some basic types


     // meta
     organization "OpenConfig working group";

     contact
       "OpenConfig working group
       netopenconfig@googlegroups.com";

     description
       "This module describes a model structure for YANG
       configuration and operational state data models. Its intent is to
       describe how individual device protocol and feature models fit
       together and interact.";

     revision "2015-03-06" {



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       description
         "Initial revision";
       reference "TBD";
     }

     // extension statements

     // feature statements

     // identity statements

     // typedef statements

     // grouping statements

     grouping info {
       description
         "base system information";

       container info {
         description
           "This container is for base system information, including
           device type (e.g., physcal or virtual), model, serial no.,
           location, etc.";

         leaf device-type {
           //TODO: consider changing to an identity if finer grained
           // device type classification is envisioned
           type enumeration {
             enum PHYSICAL {
               description "physical or hardware device";
             }
             enum VIRTUAL {
               description "virtual or software device";
             }
           }
           description
             "Type of the device, e.g., physical or virtual.  This node
             may be used to activate other containers in the model";
         }

       }
     }

     grouping hardware {
       description
         "hardware / vendor -specific data relevant to the platform";




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       container hardware {
         description
           "This container is an anchor point for platform-specific
           configuration and operational state data.  It may be further
           organized into chassis, linecards, ports, etc.  It is
           expected that vendor or platform-specific augmentations
           would be used to populate this part of the device model";
       }
     }

     grouping l2-protocol-members {
       description "containers for each layer 2 protocol model";

       container vsi {
         description "virtual switch instance (or virtual forwarding
         instance) for use in PWE3 / VPLS services";

       }

       container ipv6-ndp {
         description "IPv6 neighbor discovery";
         reference "RFC 4861 - Neighbor Discovery for IP version 6
         (IPv6)";
       }

       container arp {
         description "Address resolution protocol";
         reference "STD 37 - An Ethernet Address Resolution Protocol";
       }

       container rstp {
         description "rapid spanning tree protocol";
         reference "IEEE 802.1D-2004";
       }

       container lldp {
         description "link layer discovery protocol";
         reference "IEEE 802.1AB";
       }

       container ptp {
         description
           "precision time protocol for time synchronization services.
           PTP also typically requires per-interface configuration";
         reference "IEEE 1588-2008";
       }
     }




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     grouping l2-protocols {
       description "Layer 2 protocol models";

       container layer-2-protocols {
         description "layer 2 protocols and features";

         uses l2-protocol-members;

       }

     }

     grouping igp-protocol-members {
       description "containers for IGPs";

       container is-is {
         description "IS-IS IGP routing protocol";
         reference "RFC 1195 - Use of OSI IS-IS for Routing in TCP/IP
         and Dual Environments";
       }

       container ospf {
         description "OSPF IGP routing protocols";

         container ospf2 {
           description "OSPF v2";
           reference "RFC 2328 - OSPF Version 2";
         }

         container ospf3 {
           description "OSPF v3";
           reference "RFC 5340 - OSPF for IPv6";
         }
       }

       container igp-common {
         description "Common parameters for IGP protocols";
       }
     }

     grouping l3-protocol-members-vrf {
       description "containers for layer 3 protocol that are supported
       in a VRF instance";

       container bgp {
         description "BGP 4";
         reference "RFC 4271 - A Border Gateway Protocol 4 (BGP-4)";
       }



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       container igp {
         description "interior gateway protocols";

         uses igp-protocol-members;
       }

       container bfd {
         description "bidirectional forwarding detection";
         reference "RFC 5880 - Bidirectional Forwarding Detection
         (BFD)";
       }

       container pim {
         description "protocol independent multicast";
         reference "RFC 4601 - Protocol Independent Multicast -
         Sparse Mode (PIM-SM): Protocol Specification (Revised)";
       }

       container igmp {
         description "Internet group management protocol";
         reference "RFC 3376 - Internet Group Management Protocol,
         Version 3";
       }

       container static-routes {
         description "static route that are manually created";
       }

     }

     grouping l3-protocols-misc {
       description "containers for other features operating at the
       network layer";

     }

     grouping l3-protocols-mpls {
       description "models related to MPLS and TE";

       container mpls-te {
         description "MPLS and traffic engineering";

         container global {
           description "global MPLS configuration";
         }

         container signaling {
           description "MPLS signaling protocols";



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           container rsvp {
             description "RSVP signaling";
             reference "RFC 3209 - RSVP-TE: Extensions to RSVP for LSP
             Tunnels";
           }

           container segment-routing {
             description "SR signaling";
             reference "Segment Routing Architecture -
             draft-filsfils-spring-segment-routing-04";
           }

           container ldp {
             description "label distribution protocol";
             reference "RFC 5036 - LDP Specification";
           }
         }

         container label-switched-paths {
           description "models for different types of LSPs";

           container constrained-path {
             description "traffic-engineered, or constrained path LSPs";
           }

           container igp-congruent {
             description "LSPs that follow the IGP-computed path";
           }

           container static {
             description "statically configured LSPs";
           }
         }
       }
     }

     grouping l3-protocol-members {
       description "containers for all layer 3 protocols";

       uses l3-protocol-members-vrf;
       uses l3-protocols-misc;
       uses l3-protocols-mpls;

     }

     grouping l3-routing-policy {
       description "containers for routing policy models";




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       container common {
         description "generic routing policy framework and
         configuration parameters";
       }

       container bgp-policy {
         description "BGP-specific routing policy parameters";
       }

       container igp-policy {
         description "IGP routing policy knobs -- may include
         policy parameters for specific IGPs";
       }

       container vrf-policy {
         description "import/export policies for VRFs";
       }
     }

     grouping l3-protocols {
       description "Layer 3 protocol models";

       container layer-3-protocols {
         description "layer 3 protocols and features";

         container global {
           description "router-wide instance of each routing protocol";

           uses l3-protocol-members;

         }

         list vrf {
           key vrf-name;
           description "list of VRF instances";

           leaf vrf-name {
             type string;
             description "name or id of the routing instance / VRF";
           }

           uses l3-protocol-members-vrf;
         }

         container routing-policy {
           description "models related to routing policy across
           protocols and VRFs";




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           uses l3-routing-policy;

         }
       }
     }

     grouping interface-ip-common {
       description
         "interface-specific configuration for IP interfaces, IPv4 and
         IPv6";

       container vrrp {
         description "virtual router redundancy protocol";
         reference "RFC 5798 - Virtual Router Redundancy Protocol
         (VRRP) Version 3 for IPv4 and IPv6";
       }
     }

     grouping interface-addr-families {
       description
         "containers for addr family-specific data attached
         to interfaces";

       container ipv4 {
         description "IPv4 interfaces";

         uses interface-ip-common;
       }

       container ipv6 {
         description "IPv6 interfaces";

         uses interface-ip-common;
       }
     }

     grouping interfaces {
       description "interface-related models";

       container interfaces {
         description "various interface models";

         container ethernet {
           description "Ethernet interface config, e.g., 10, 40,
           100GBE";

           container aggregates {
             description "LAGs, LACP, etc. for Ethernet interfaces";



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             reference "IEEE 802.1ad, 802.1AX";
           }

           container vlans {
             description "VLANs, 802.1q, q-in-q, etc.";
             reference "IEEE 802.1Q";
           }

           container lfm {
             description
               "Link-layer fault management for Ethernet interfaces";
             reference "IEEE 802.3ah";
           }
         }

         container sonet-sdh {
           description "SONET/SDH interfaces";
           reference
             "SDH: ITU standards G.707, G.783, G.784, and G.803
             SONET: ANSI standard T1.105";

         }

         container addressing {
           description "addressing and other interface-specific data,
           e.g., data plane protocols";

           uses interface-addr-families;
         }

         container tunnels {
           description
             "logical tunnel interfaces incl. GRE, VxLAN, L2TP etc.";
         }

       }
     }

     grouping oam {
       description "containers for features related to operations,
       administration, and management";

       container oam {
         description "commonly use OAM functions on devices";

         container snmp {
           description "SNMP server information, e.g., allowed clients";
         }



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         container cfm {
           description
             "Ethernet connectivity fault management.  Also includes
             options that are associated with specific interfaces, such
             as maintenance endpoint domains.";
           reference "IEEE 802.1ag";
         }

         container twamp {
           description
             "Two-way active measurement protocol for measuring
             round-trip IP layer performance.";
           reference "RFC 5357 A Two-Way Active Measurement Protocol
           (TWAMP)";
         }
       }
     }

     grouping system-services {
       description "containers for system service models";

       container dns {
         description "domain name service and resolver configurration";
       }

       container ntp {
         description "network time protocol configuration";
       }

       container dhcp {
         description "dhcp and relay services";
       }

       container syslog {
         description "syslog configuration";
       }

       container ssh {
         description "ssh server configuration";
       }

       container stat-coll {
         description
           "mechanisms for data collection from devices, including
           packet and flow-level sampling";
       }

       uses oam;



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     }

     grouping system-aaa {
       description "AAA-related services";

       container aaa {
         description "authentication, authorization, and accounting";

         container tacacs {
           description "TACACS+ configuration";
         }

         container radius {
           description "RADIUS";
           reference "RFC 2865 - Remote Authentication Dial In User
           Service (RADIUS)";
         }
       }
     }

     grouping system {
       description "system-wide services";

       container system {
         description "system services";

         uses system-services;
         uses system-aaa;

         container users {
           description "local user configuration";
         }
       }
     }

     grouping acl {
       description "forwarding rules";

       container acl {
         description "ACLs and packet forwarding rules";
       }
     }

     grouping qos {
       description "QoS features";

       container qos {
         description "QoS, including policing, shaping, etc.";



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       }
     }

     // data definition statements

     container device {
       description "top-level anchor point for models.  Device is a
       generic L2/L3 network element";

       uses info;
       uses hardware;
       uses system;
       uses interfaces;
       uses acl;
       uses qos;

       container logical-routers {
         description "devices may support multiple logical router
         instances";

         list logical-router {

           key router-id;
           description "list of logical router instances";

           leaf router-id {
             type uint8; // expect a small number of logical routers
             description "identifier of the logical router instance";
           }

           leaf router-name {
             type string; // expect a small number of logical routers
             description "identifier of the logical router instance";
           }

           uses l2-protocols;
           uses l3-protocols;

         }
       }

     }

     // augment statements

     // rpc statements

     // notification statements



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   }
   <CODE ENDS>


8.  References

8.1.  Normative references

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2014.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, May 2014.

   [RFC7277]  Bjorklund, M., "A YANG Data Model for IP Management", RFC
              7277, June 2014.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, August 2014.

   [RFC3688]  Mealling, M., "The IETF XML Registry", RFC 3688, January
              2004.

8.2.  Informative references

   [RTG-CFG]  Lhotka, L., "A YANG Data Model for Routing Management",
              draft-ietf-netmod-routing-cfg-16 (work in progress),
              October 2014.

   [RTG-POLICY]
              Shaikh, A., Shakir, R., D'Souza, K., and C. Chase,
              "Routing Policy Configuration Model for Service Provider
              Networks", draft-shaikh-rtgwg-policy-model-00 (work in
              progress), January 2015.

   [RTG-OSPF]
              Yeung, D., Qu, Y., Zhang, J., and D. Bogdanovic, "Yang
              Data Model for OSPF Protocol", draft-yeung-netmod-ospf-02
              (work in progress), October 2014.

   [RTG-ISIS]
              Litkowski, S., Yeung, D., Lindem, A., Zhang, J., and L.
              Lhotka, "YANG Data Model for ISIS protocol", draft-ietf-
              isis-yang-isis-cfg-01 (work in progress), October 2014.






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Appendix A.  Acknowledgements

   The authors are grateful for valuable contributions to this document
   and the associated models from: Deepak Bansal, Paul Borman, Chris
   Chase, Josh George, Marcus Hines, and Jim Uttaro.

Authors' Addresses

   Anees Shaikh
   Google
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043
   US

   Email: aashaikh@google.com


   Rob Shakir
   BT
   pp. C3L, BT Centre
   81, Newgate Street
   London  EC1A 7AJ
   UK

   Email: rob.shakir@bt.com
   URI:   http://www.bt.com/


   Kevin D'Souza
   AT&T
   200 S. Laurel Ave
   Middletown, NJ
   US

   Email: kd6913@att.com


   Luyuan Fang
   Microsoft
   205 108th Ave. NE, Suite 400
   Bellevue, WA
   US

   Email: lufang@microsoft.com







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