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Network Working Group                                                     J. Bi
Internet Draft                                                   Tsinghua Univ.
Intended status: Standard Track                                          Q. Sun
Expires: June 2016                                                       C. Xie
                                                                  China Telecom
                                                                         Y. Zha
                                                            Huawei Technologies


                                                              December 16, 2015


                          Declarative Policy Model
                       draft-bi-declarative-policy-03


Abstract

   This document describes a declarative policy model to describe the user's
   declarative on network policy. The declarative policy model is a specific data
   model specifies the desired state of the network system. It helps the service
   management to model the policy (a set of states described by constraints) that
   defines the final results of a VPN service without specifying how it is
   monitored and managed during its lifecycle. One application for Distributed
   Data Center (DDC) scenarios with policy enforcement is provided with details of
   how to convert high level declarative policy into lower level configurations.

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), its areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This Internet-Draft will expire on June 16, 2015.



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Copyright Notice

   Copyright (c) 2014 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 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.

Table of Contents

   1. Introduction .......................................................... 2
   2. Conventions used in this document ..................................... 3
   3. Policy Based Service Management Framework.............................. 3
   4. Declarative Policy Configuration Modules .............................. 4
      4.1. Declarative Policy Framework ..................................... 4
      4.2. Declarative Policy Model ......................................... 5
   5. Declarative Policy Applications in DDC services ...................... 14
      5.1. Policy Based Traffic Steering Case study ........................ 14
      5.2. Declarative Policy Enforcement .................................. 19
   6. Security Considerations .............................................. 20
   7. IANA Considerations .................................................. 20
   8. Acknowledgments ...................................................... 21
   9. References............................................................ 21
      9.1. Normative References ............................................ 21
      9.2. Informative References .......................................... 21

1. Introduction

   In order to support emerging DDC service with VPN connection as well as new
   services, it brings new requirements on Policy based service management for
   service providers. Instead of manual configuration of the network
   infrastructures, policy based service management is achieved by using policies
   that can be created by the operators once and the service management refers to
   these policies to infer how a given service needs to be provisioned considering
   the current state of the network.

   With the general trend of NFV/SDN, network management and control is moving
   from low level operation to high level decision making. Human to machine
   communication is changing from imperative to declarative. In the other hand,
   human declarativeion will be more expressed by declarative policy.




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   In SUPA framework, network policy consists of different layers with generic
   policy on top while declarative policy and ECA (Event-Condition-Action) policy.
   ECA policy is a predefined rule or a set of rules that the service management
   use to map the service to the lower level network infrastructures as shown in
   draft [draft-chen-supa-eca-data-model]. Oh the other hand, declarative policy
   is higher level abstraction which only specifies the needs or the final state
   of the system without knowing the low level network infrastructures. In this
   way, the declarative policy defines the final state and the constraint for the
   network behaviors.

   Meanwhile, DDC service which is mainly relied on VPN [RFC4110] needs policy
   based management and controlling capability from the service management systems
   to facilitate the service deployment both inter data centers and within data
   center.

   This document introduces YANG [RFC6020] [RFC6021] data models for declarative
   policy configuration.  Such models can facilitate the standardization for the
   interface of SUPA, as they are compatible to a variety of protocols such as
   NETCONF [RFC6241] and [RESTCONF]. Please note that in the context of SUPA, the
   term "application" refers to an operational and management applications
   employed, and possibly implemented, by an operator. The policy model is based
   on the first example DDC services.

   Also certain foundational concepts of the model are declarative left open to
   enable future extension. With respect to the scope, defining an information
   model on the top level and a corresponding data model based on yang to express
   the declarative policy for traffic optimization in specific DDC service use
   case is initial goal of this document. In addition to the generic policy model,
   here the traffic optimization policy in DDC use case provides a concrete
   example for a specific network service, as what constitutes an declarative
   policy could be enforced and executed depending on the context where it is used,
   e.g. there could be tenant specific policies, site specific, network domain
   specific etc.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in [RFC2119]. In this document, these words will
   appear with that interpretation   only when in ALL CAPS. Lower case uses of
   these words are not to be interpreted as carrying [RFC2119] significance.

3. Policy Based Service Management Framework

   The usage of policy rules to manage the behavior of one or more managed
   entities helps to simplify the service deployment and management. Policy is
   about governance, and can be expressed differently: e.g. ''When this threshold


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   is violated, change the route path of the flow'' is a lower level policy and
   can be executed with direct operations. ''The links between DCs require LOAD
   BALANCE'' is a higher level policy which cannot be directly executed. Different
   users and operators may have different requirements on policies and level of
   policy abstractions depends on their operational capability and SLA
   requirements.

   The document focused on developing a model that abstracts network resources and
   services and a methodology by which the management and monitoring of network
   services can be done using standardized policy rules. Figure 1 illustrates the
   network policy model which contains generic model that has ECA model and
   declarative policy model.

             +--------------------------------------------+
             |             Generic Policy                 |
             |                                            |
             |    +------------+     +-----------------+  |
             |    | ECA Policy |     | Declarative     |  |
             |    | Data Model |     |Policy Data Model|  |
             |    +------------+     +-----------------+  |
             +--------------------------------------------+
                 Figure 1: Overview of policy model structure

4. Declarative Policy Configuration Modules

   In this section, an declarative policy model is defined with some high level
   descriptions on the structure. The policy model and policy configuration are
   based on a set of specific network services and the framework of SUPA [SUPA-
   framework]. Meanwhile, the policy model should be working on the orchestration
   level which is above network element and below OSS level based on the YANG
   model classification in [draft-bogdanovic-netmod-yang-model classification-02]

4.1. Declarative Policy Framework

   Unlike the ECA policy model that specify the corresponding action, declarative
   policy only express what should be done, not how to do it. It specifies
   criteria for choosing a set of states, any of which is acceptable. Moreover,
   rationality is generated by policy engine by enforcing the policy into real
   actions and make sure the goal has been fulfilled. In order to express what is
   needed, the declarative policy is defined with description of state and
   behavior constraint.

   Desired state: The description of the final state of the system, in another
   word, the goal or the declarative of the policy management. In SUPA scope, it
   consists of constraints. Here the constraint is defined with First Order Logic
   (FOL) which has 'predicate' expression to describe the relation or attributes,



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   and 'quantifier' to describe the quantity of the objects in the domain to meet
   the predicate expression.

   Behavior constraint: a set of constraints to limit the possible operations or
   states in processing the policy goal to achieve the final state.

   Note that state and behavior can be described as constraint or a set of
   constraints.

            +--------------------------+
            |     PolicyRuleMetaData   |
            +--------------------------+
                        |
               +--------+------+
               |   PolicyRule  |
               +-----------+---+
                           |
                     ----------------------
                     |                    |
           +--------------+     +----------------+
           |  ECA Policy  |     |  Declarative   |
           |    Model     |     | Policy Model   |
           +--------------+     ++---+-----------+
                                 |   |
           -----------------------   |
           |                         |
   +-----------------+    +---------------------+
   |  Desired State  |    | Behavior Constraint |
   +-----------------+    +---------------------+

   Figure 2: Overview of information declarative policy model

4.2. Declarative Policy Model

   The hierarchy of the declarative base policy model is shown as figure 3 and the
   entire data model is defined as follows.












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   module: ietf-declarative-policy
      +--rw policy
         +--rw policy-name?              string
         +--rw policy-priority?          uint8
         +--rw policy-validity-period
         |  +--rw start?         yang:date-and-time
         |  +--rw end?           yang:date-and-time
         |  +--rw duration?      uint32
         |  +--rw periodicity?   enumeration
         +--rw policy-target
         |  +--rw profileType?          string
         |  +--rw domain-name?          string
         |  +--rw admin-subnet?         string
         |  +--rw business-type-name?   string
         |  +--rw instance
         +--rw policy-atomic
            +--rw declarative-policy
               +--rw policy-rule-deploy-status?   enumeration
               +--rw policy-rule-exec-status?     enumeration
               +--rw declarative-policy-rule
                  +--rw desired-state
                  |  +--rw constraint-on-state
                  |  |  +--rw quantified-variable
                  |  |  |  +--rw quantifier?   enumeration
                  |  |  |  +--rw element?      string
                  |  |  |  +--rw set?          string
                  |  |  +--rw predicate
                  |  |     +--rw object?           string
                  |  |     +--rw logic-operator?   enumeration
                  |  |     +--rw value?            string
                  |  +--rw constraint-priority?   uint8


         Figure 3. Hierarchy of the declarative policy data model



 <CODE BEGINS> file "ietf-declarative-policy@2015-11-18.yang"

   module ietf-declarative-policy {
     namespace "urn:ietf:params:xml:ns:yang:ietf-declarative-policy";
     // replace with IANA namespace when assigned
     prefix policy;

     import ietf-yang-types {
       prefix yang;
     }


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     organization "IETF";
     contact
       "Editor: Yiyong Zha";

     description
       "This YANG module defines a component that describing
        the declarative policy data model to describe the states and
        constraints.

        Terms and Acronyms
        ";
       revision 2015-10-10 {
       reference "ietf-declarative-policy@2015-10-10";
     }
     revision 2015-09-10 {
       reference "ietf-declarative-policy@2015-9-10";
     }
     container policy{
       description
         "This defines a policy data model with declarative policy
         constraint";
       leaf policy-name {
         type string;
         description
           "The name of the policy";
       }
       leaf policy-priority {
         type uint8;
         description
           "The priority of the defined policy";
       }
       container policy-validity-period {
         description
           "The valid time of the policy. E.g., the policy
           will be valid 9am-9am daily";
          leaf start {
           type yang:date-and-time;
           description
             "When the policy will be start to work.";
         }
         leaf end {
           type yang:date-and-time;
           description
             "When the policy will be terminated.";
         }
         leaf duration {


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           type uint32;
           description
             "How long the policy will be valid.";
         }
         leaf periodicity {
           type enumeration {
             enum daily {
               value 0;
               description
                 "The policy will be repeated daily.";
             }
             enum monthly {
               value 1;
               description
                 "The policy will be repeated monthly.";
             }
           }
           description
             "How the policy will be repeated.";
         }
       }
       container policy-target {
         description
           "SUPAPolicyTarget is an abstract class that
           defines a set of managed objects that may be
           affected by the actions of a SUPAPolicyStatement.";
         leaf profileType {
           type string;
           description
             "Which profile the policy will be worked on.";
         }
         leaf domain-name {
           type string;
           description
             "Which domain the policy will be worked on.";
         }
         leaf admin-subnet {
           type string;
           description
             "Which subnet the policy will be worked on.";
         }
         leaf business-type-name {
           type string;
           description
             "Which business the policy will be worked on.";
         }
         container instance  {


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           description
             "Which instance the policy will be worked on? E.g.,
             a VPN, a flow or a link";
           }
       }
       container policy-atomic {
         description
           "Define an atomic declarative policy.";
         container declarative-policy {
           description
             "supa declarative policy only describes the desired
             state and constraints of the policy and what is
             wanted without
             telling how to do.";
           leaf policy-rule-deploy-status {
             type enumeration {
               enum undefined{
                 value 0;
                 description "undefined";
               }
               enum enabled{
                 value 1;
                 description "deployed and enabled";
               }
               enum in-test{
                 value 2;
                 description "deployed and in test";
               }
               enum not-enabled{
                 value 3;
                 description "deployed but not enabled";
               }
               enum ready{
                 value 4;
                 description "ready to be deployed";
               }
               enum not-deployed{
                 value 5;
                 description "not deployed";
               }
             }
             description
               "Describes the deploy status of the policy.";
           }
           leaf policy-rule-exec-status {
             type enumeration {
               enum undefined{


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                 value 0;
                 description "undefined";
               }
               enum OPmode-executed-succeeded{
                 value 1;
                 description
                   "executed and SUCEEDED (operational mode)";
               }
               enum OPmode-executed-failed{
                 value 2;
                 description
                   "executed and FAILED (operational mode)";
               }
               enum OPmode-executing{
                 value 3;
                 description
                   "currently executing (operational mode)";
               }
               enum TEmode-executed-succeeded{
                 value 4;
                 description
                   "executed and SUCEEDED (test mode)";
               }
               enum TEmode-executed-failed{
                 value 5;
                 description
                   "executed and FAILED (test mode)";
               }
               enum TEmode-executing{
                 value 6;
                 description
                   "currently executing (test mode)";
               }
             }
             description
               "Describes the execution status of the policy.";
           }

           container declarative-policy-rule {
             description
               "The declarative policy rule is defined as
               constraints on states and actions.";
             grouping FOL-statement {
               description
                 "SUPA declarative policy is based on constraint.
                 The constraint is defined as first order logic
                 statement which consists of quantified variables


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                 and predicate expression.";
               container quantified-variable {
                 description
                   "The quantified-variable defines existence
                   of the element that is being described. It is
                   the major difference of FOL from propositional
                   logic.";
                 leaf quantifier {
                   type enumeration {
                     enum none {
                       value 0;
                       description
                         "None: no one in the set meets
                         certain requirement.";
                     }
                     enum all{
                       value 1;
                       description
                         "All: all elements in the set meets
                         certain requirement.";
                     }
                     enum some{
                       value 2;
                       description
                         "At least one: there is at least one
                         element in the set meets certain
                         requirement.";
                     }
                     enum lone{
                       value 3;
                       description
                         "Also called At most one: there is at
                         most one element in the set meets
                         certain requirement.";
                     }
                     enum singleton{
                       value 4;
                       description
                         "Singleton: there is only one element
                         in the set meets certain requirement.";
                     }
                   }
                   description
                     "Quantifier defines the quantity of
                     specimens in the domain of discourse that
                     satisfy an predicate expression.";
                 }


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                 leaf element {
                   type string;
                   description
                     "The objects of constraints, which is the
                     elements of the predicate";
                 }
                 leaf set {
                   type string;
                   description
                     "The set of which the element belongs to. It
                     is usually pre-defined as policy target or
                     certain problem domain such as a subnet. ";
                 }
               }
               container predicate {
                 description
                   "Predicate expression is no constraint that
                   holds for all the predefined quantified
                   variables. ";
                 leaf object {
                   type string;
                   description
                     "The object of constraints, which is the
                     subject of the predicate. It can be but
                     not must be the same of element in quantified
                     variables.";
                 }
                 leaf logic-operator {
                   type enumeration{
                     enum unknown {
                       value 0;
                       description "Unknown";
                     }
                     enum match {
                       value 1;
                       description "Match";
                     }
                     enum greater-than {
                       value 2;
                       description "Greater than";
                     }
                     enum greater-than-or-equal-to {
                       value 3;
                       description "Greater than or equal to";
                     }
                     enum less-than {
                       value 4;


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                       description "Less than";
                     }
                     enum less-than-or-equal-to {
                       value 5;
                       description "Less than or equal to";
                     }
                     enum equal-to {
                       value 6;
                       description "Equal to";
                     }
                     enum not-equal-to {
                       value 7;
                       description "Not equal to";
                     }
                     enum in {
                       value 8;
                       description "IN";
                     }
                     enum not-in {
                       value 9;
                       description "NOT IN";
                     }
                   }
                   description
                     "A predicate is commonly understood to be a
                     Boolean-valued expression.";
                 }
                 leaf value {
                   type string;
                     description
                       "The value in the predicate expression to
                       describe the relationship of the objects.";
                 }
               }
             }
             container desired-state {
               description
                 "Describes the final state based on user's
                 goal. Just defines one of multiple states
                 without telling how";
               container constraint-on-state {
                 uses FOL-statement;
                 description
                   "Use first order logic to describe the
                   constraint. FOL statement includes
                   quantifier and predicate to describe the
                   state.";


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               }
             leaf constraint-priority {
                 type uint8;
                 description
                   "0-7 denotes the priority of this state
                   constraint";
               }
             }
           }
         }
       }
     }
   }

   <CODE ENDS>

5. Declarative Policy Applications in DDC services

5.1. Policy Based Traffic Steering Case study

Traffic Steering use case description:

  In one set of links, keep all link utilization below 70%.

  If some flows need to move to other link, keep Gold user flows untouched.



  After analyze above case, we structure the description as following:

  Related objects:  links   flow (user type)

  Goal            all link utilization < 70%

  Constraint:    keep Gold user flows untouched



  The service model of this use case:









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                 +--------------+              +--------------+
                 | Link         +--------------+  Flow        |
                 +--------------+ m          n +--------------+
                 | uti: float   |              | bw:int       |
                 | phybw: int   |              | srcip:IPAddr |
                 |              |              | destip:IPAddr|
                 |              |              | userlevel:   |
                 |              |              | enum(Gold,   |
                 +--------------+              |   Normal)    |
                                               |              |
                                               +--------------+
           Figure 4. Service model of traffic steering policy use case

  Link attribute

  Uti: link bandwidth utility

  Phybw: physical bandwidth of the link

  Flow attribute

  bw: the bandwidth of the flow

  srcip,destip: the source and dest ip address of the flow

  userlevel: the user's service level of the flow, it can be gold or normal.




















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       Please view in a fixed-width font such as Courier.


                        +--------------+     +--------------+
                        | Policy       |-----|Rule(abstract)|
                        +--------------+     +--------------+
                                                    A
                                            +-------+
                                            |
                                    +-------+--------------+
                                    | Declarative- rule    |
                                    +----------------------+
        A                               V
    ----+  subclass                     +
                                        |
        V                       +----------------------+
    ----+  composition          | desired-state        |
                                +----------------------+

            Figure 5. policy model of traffic steering policy use case


   The desired-state may contain one or multiple constraint, which describe the
   desired states. In this case, it has a constraint like following string:

                        no link:Link  link.uti > 0.7

   above description can be read by the constraint solver combining the service
   model defined above. It defines a variable "link" and announce it as Link type,
   and says that there is no link in question, whose uti greater than 0.7(the
   threshold)

   There is another constraint that: "If some flows need to move to other link,
   keep Gold user flows untouched." This is another kind of constraint that
   confines the result, also can be constrained by desired-state.

   Before giving out the constraint programming script, we introduce a 'guide'
   model first which is internally build for the traffic steering problem domain.
   The guide model is consumed by a multi-constraint solver. 'Guide' model is the
   bridge to connect goal to actions.  One guide model may cover a problem domain
   and serve many declarative policies.

   The guide model can be seen in figure 6






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                           +------------------+
                           |    Guide         |
                           +------------------+
                             V  V     V    V
                             |  |     |    +---------------------+
           +-----------------+  |     +-------+                  |
           |                    |             |        +------------+
     +----------+     +-------------+  +-----------+   |  Action    |
     | Select   |     |  Constraint |  |  Fact     |   |  Container |
     +----------+     +-------------+  +-----------+   +------------+

   Figure 6. Build in guide model for traffic steering problem domain


   The Guide model contains 'Select', 'Constraint', 'Fact', 'Action container'.
   The objective of guide model is to guide the constraint solver output actions
   which can map to low layer functions.

   Guide expresses the sentence: 'Select' objects which meet the 'Constraint',
   following the 'Fact' and output actions formatted in 'Action Container'.

   'Select' may coexist with other guidance type to let the constraint solver know
   the problem solving direction. Most cases of multiple constraint solving can
   belong to select. For example, select host to run VM, select physical network
   resource for virtual network path.
   The Select string in this case:
   flowset: set Flow
   linkB: set Link
   flow:Flow in flowset
   tolink:Link in linkB


   'Constraint' contains all common constraints under this problem domain. It can
   be extended to express customer's constraints which in goal/declarative policy
   model. Constraint in guide also hides some complexity from customer.

   The Constraint string in this case:
   any link1:Link link1.uti>threshold
   flowset in link1.flows
   link1 not in linkB


   'Fact' is to input some basic behavior of the problem domain to the solver and
   let the solver do the work and at the same time keep service agnostic.
   The Fact string in this case:
   link1.uti -= flow.bw/link1.phybw
   tolink.uti+= flow.bw/link1.phybw


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   'Action container' is regulating the output action which solver must output. It
   connects the goal/declarative policy engine to the lower action system.
   The Action container string in this case:
   move(flow:Flow,tolink:Link)

   This action container assume that there is a low layer function corresponds to
   the abstract move action container and the function can perform the work that
   move the flow to the target link tolink.


   Come back to the goal-rule model, there is behavior constraint which extends
   the Constraint in guide model. The goal in goal-rule also consists of
   constraints; the behavior constraint is a little different that it confines the
   intermediate behavior which modeled in the problem domains guide model.

   The constraint in this case may like follow:
   threshold = 0.7
   no flow.userlevel = Gold




























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5.2. Declarative Policy Enforcement

   Based on the service model and policy model traffic steering use case
   introduced in above section, this section introduce an example of policy
   framework and briefly illustrate how to enforce the declarative policy.

                   +---------------------------------------+
                   |      SUPA policy service API          |
                   |                                       |
                   +--------|--------------------|---------+
                            |                    |
                    /-------v--------\   /-------v--------\
                    | Service model  |   |   Policy       |
                    |                |   |   repository   |
                    \-------|--------/   \-------|--------/
                            +------------+       |
                   /----------\      +---v-------v---------+
                   | Context  |----->|   Policy Engine     |
                   | data     |      |                     |
                   \----------/      | +-----+   +-------+ |
                                     | | ECA |   |Decla  | |
                   /----------\      | |     |   |rative | |
                   | Event    |----->| +-----+   +-------+ |
                   | data     |      |                     |
                   \-----A----/      +-----------|---------+
                         |                       |
                   +-------------+ +-------------v---------+
                   |  Collector  | |   Action Scheduler    |
                   +-----A-------+ +-------------|---------+
                        |event                  v action

              Figure 7. Declarative policy framework as an example

   Figure 7 shows the example framework. In the framework, the policy engine takes
   the key role who translates the declarative policy to lower layer actions. The
   policy engine is problem domain agnostic. It depends on service model and
   policy model to operate on problem domains.

   Following give some brief illustration around the traffic steering use case
   that how the policy engine do declarative policy enforcement.

   First, to inject the capability of operating on traffic steering policy to the
   policy system, the guide model (see section 5.1) and the service mode (see
   section 5.1) is input to the system.





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   Then, under some concrete traffic steering scenario, one user can express the
   declarative by transfer the desired state and constraint to the system. After
   verifying the policy language script against the service model and policy
   syntax, the policy framework will save the user's declarative policy in policy
   repository.

   When the policy is activated, the policy engine may get data from context data
   store, in this case, the data include the link, flow and their relationship
   information. The policy engine is guided by the guide model and user's
   declarative policy model, evaluates whether some selected data is compatible
   with the constraints and desired states.

   After finding out data, the policy engine will fill the <flow,link> tuples
   which is the result of the 'select' to the action container, in this case is
   move(flow:Flow,tolink:Link)

   Finally the policy engine output a list of actions such like
   move(flow1,link3)
   move(flow2,link5)
   move(flow4,link2)
   ...

   The flow1, flow2, flow4, link3, link5, link2 all comes from context data store
   as shown in figure 4. The policy engine even does not know what 'move' is, but
   the system can map the abstract move action to a con12444crete function at
   lower layer to perform the movement.

   The collecting context data, making decision and output action circle may
   perform one or multiple times to change the traffic steering system to a new
   steady state and meet the user's declarative/goal.

6. Security Considerations

   TBD



7. IANA Considerations

   This document has no actions for IANA.









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8. Acknowledgments

   This document has benefited from reviews, suggestions, comments and proposed
   text provided by the following members, listed in alphabetical order: Felix Lu,
   Juergen Schoenwaelder, John Strassner, and Min Zha.

9. References

9.1. Normative References

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

   [RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
             Network Configuration Protocol (NETCONF)", RFC 6020,
             October 2010.

   [RFC6021] Schoenwaelder, J., "Common YANG Data Types", RFC 6021,
             October 2010.

   [RFC3272] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and X.
             Xiao, "Overview and Principles of Internet Traffic
             Engineering", RFC 3272, May 2002.

9.2. Informative References

   [SUPA-framework] C. Zhou, L. M. Contreras, Q. Sun, and P. Yegani, " The
   Framework of Shared Unified Policy Automation (SUPA) ", IETF Internet draft,
   draft-zhou-supa-framework, January 2015.

   [SUPA-problem-statement] G. Karagiannis, Q. Sun, Luis M. Contreras, P. Yegani,
   and JF Tremblay, "Problem Statement for Shared Unified Policy Automation
   (SUPA)", IETF Internet draft, draft-karagiannis-supa-problem-statement, January
   2015.

   [SUPA-DDC] Y. Cheng,and JF. Tremblay, ''Use Cases for Distributed Data Center
   Applications in SUPA'', IETF Internet draft, draft-cheng-supa-ddc-use-cases,
   January 2015.

   [RESTCONF] Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando, "RESTCONF
   Protocol", draft-ietf-netconf-restconf (work in progress), July 2014.

   [POLICY MODEL] Z. Wang, L. Dunbar, Q. Wu, ''Network Policy YANG Data Model''
   draft-wang-netmod-yang-policy-dm, January 2015.


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Authors' Addresses

   Jun Bi
   Tsinghua University
   Network Research Center, Tsinghua University
   Beijing  100084
   P.R. China

   Email: junbi@tsinghua.edu.cn

   Qiong Sun
   China Telecom
   No.118 Xizhimennei street, Xicheng District
   Beijing  100035
   P.R. China

   Email: sunqiong@ctbri.com.cn

   Chongfeng Xie
   China Telecom
   No.118 Xizhimennei street, Xicheng District
   Beijing  100035
   P.R. China

   Email: xiechf@ctbri.com.cn

   Yiyong Zha
   Huawei Technologies
   Section F, Huawei Industrial Base, Longgang District
   Shenzhen 518129
   P.R. China

   Email: zhayiyong@huawei.com















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