<pre>Internet Engineering Task Force (IETF)                          A. Atlas
Request for Comments: 7921                              Juniper Networks
Category: Informational                                       J. Halpern
ISSN: 2070-1721                                                 Ericsson
                                                                S. Hares
                                                                  Huawei
                                                                 D. Ward
                                                           Cisco Systems
                                                               T. Nadeau
                                                                 Brocade
                                                               June 2016


        <span class="h1">An Architecture for the Interface to the Routing System</span>

Abstract

   This document describes the IETF architecture for a standard,
   programmatic interface for state transfer in and out of the Internet
   routing system.  It describes the high-level architecture, the
   building blocks of this high-level architecture, and their
   interfaces, with particular focus on those to be standardized as part
   of the Interface to the Routing System (I2RS).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc7921">http://www.rfc-editor.org/info/rfc7921</a>.












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Copyright Notice

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

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.





































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Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-4">4</a>
      <a href="#section-1.1">1.1</a>. Drivers for the I2RS Architecture ..........................<a href="#page-5">5</a>
      <a href="#section-1.2">1.2</a>. Architectural Overview .....................................<a href="#page-6">6</a>
   <a href="#section-2">2</a>. Terminology ....................................................<a href="#page-11">11</a>
   <a href="#section-3">3</a>. Key Architectural Properties ...................................<a href="#page-13">13</a>
      <a href="#section-3.1">3.1</a>. Simplicity ................................................<a href="#page-13">13</a>
      <a href="#section-3.2">3.2</a>. Extensibility .............................................<a href="#page-14">14</a>
      <a href="#section-3.3">3.3</a>. Model-Driven Programmatic Interfaces ......................<a href="#page-14">14</a>
   <a href="#section-4">4</a>. Security Considerations ........................................<a href="#page-15">15</a>
      <a href="#section-4.1">4.1</a>. Identity and Authentication ...............................<a href="#page-17">17</a>
      <a href="#section-4.2">4.2</a>. Authorization .............................................<a href="#page-18">18</a>
      <a href="#section-4.3">4.3</a>. Client Redundancy .........................................<a href="#page-19">19</a>
      <a href="#section-4.4">4.4</a>. I2RS in Personal Devices ..................................<a href="#page-19">19</a>
   <a href="#section-5">5</a>. Network Applications and I2RS Client ...........................<a href="#page-19">19</a>
      <a href="#section-5.1">5.1</a>. Example Network Application: Topology Manager .............<a href="#page-20">20</a>
   <a href="#section-6">6</a>. I2RS Agent Role and Functionality ..............................<a href="#page-20">20</a>
      <a href="#section-6.1">6.1</a>. Relationship to Its Routing Element .......................<a href="#page-20">20</a>
      <a href="#section-6.2">6.2</a>. I2RS State Storage ........................................<a href="#page-21">21</a>
           <a href="#section-6.2.1">6.2.1</a>. I2RS Agent Failure .................................<a href="#page-21">21</a>
           <a href="#section-6.2.2">6.2.2</a>. Starting and Ending ................................<a href="#page-22">22</a>
           <a href="#section-6.2.3">6.2.3</a>. Reversion ..........................................<a href="#page-23">23</a>
      <a href="#section-6.3">6.3</a>. Interactions with Local Configuration .....................<a href="#page-23">23</a>
           6.3.1. Examples of Local Configuration vs. I2RS
                  Ephemeral Configuration ............................<a href="#page-24">24</a>
      <a href="#section-6.4">6.4</a>. Routing Components and Associated I2RS Services ...........<a href="#page-26">26</a>
           <a href="#section-6.4.1">6.4.1</a>. Routing and Label Information Bases ................<a href="#page-28">28</a>
           <a href="#section-6.4.2">6.4.2</a>. IGPs, BGP, and Multicast Protocols .................<a href="#page-28">28</a>
           <a href="#section-6.4.3">6.4.3</a>. MPLS ...............................................<a href="#page-29">29</a>
           <a href="#section-6.4.4">6.4.4</a>. Policy and QoS Mechanisms ..........................<a href="#page-29">29</a>
           6.4.5. Information Modeling, Device Variation, and
                  Information Relationships ..........................<a href="#page-29">29</a>
                  6.4.5.1. Managing Variation: Object
                           Classes/Types and Inheritance .............<a href="#page-29">29</a>
                  <a href="#section-6.4.5.2">6.4.5.2</a>. Managing Variation: Optionality ...........<a href="#page-30">30</a>
                  <a href="#section-6.4.5.3">6.4.5.3</a>. Managing Variation: Templating ............<a href="#page-31">31</a>
                  <a href="#section-6.4.5.4">6.4.5.4</a>. Object Relationships ......................<a href="#page-31">31</a>
                           <a href="#section-6.4.5.4.1">6.4.5.4.1</a>. Initialization .................<a href="#page-31">31</a>
                           <a href="#section-6.4.5.4.2">6.4.5.4.2</a>. Correlation Identification .....<a href="#page-32">32</a>
                           <a href="#section-6.4.5.4.3">6.4.5.4.3</a>. Object References ..............<a href="#page-32">32</a>
                           <a href="#section-6.4.5.4.4">6.4.5.4.4</a>. Active References ..............<a href="#page-32">32</a>
   <a href="#section-7">7</a>. I2RS Client Agent Interface ....................................<a href="#page-32">32</a>
      <a href="#section-7.1">7.1</a>. One Control and Data Exchange Protocol ....................<a href="#page-32">32</a>
      <a href="#section-7.2">7.2</a>. Communication Channels ....................................<a href="#page-33">33</a>
      <a href="#section-7.3">7.3</a>. Capability Negotiation ....................................<a href="#page-33">33</a>
      <a href="#section-7.4">7.4</a>. Scope Policy Specifications ...............................<a href="#page-34">34</a>
      <a href="#section-7.5">7.5</a>. Connectivity ..............................................<a href="#page-34">34</a>



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      <a href="#section-7.6">7.6</a>. Notifications .............................................<a href="#page-35">35</a>
      <a href="#section-7.7">7.7</a>. Information Collection ....................................<a href="#page-35">35</a>
      <a href="#section-7.8">7.8</a>. Multi-headed Control ......................................<a href="#page-36">36</a>
      <a href="#section-7.9">7.9</a>. Transactions ..............................................<a href="#page-36">36</a>
   <a href="#section-8">8</a>. Operational and Manageability Considerations ...................<a href="#page-37">37</a>
   <a href="#section-9">9</a>. References .....................................................<a href="#page-38">38</a>
      <a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-38">38</a>
      <a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-38">38</a>
   Acknowledgements ..................................................<a href="#page-39">39</a>
   Authors' Addresses ................................................<a href="#page-40">40</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   Routers that form the Internet routing infrastructure maintain state
   at various layers of detail and function.  For example, a typical
   router maintains a Routing Information Base (RIB) and implements
   routing protocols such as OSPF, IS-IS, and BGP to exchange
   reachability information, topology information, protocol state, and
   other information about the state of the network with other routers.

   Routers convert all of this information into forwarding entries,
   which are then used to forward packets and flows between network
   elements.  The forwarding plane and the specified forwarding entries
   then contain active state information that describes the expected and
   observed operational behavior of the router and that is also needed
   by the network applications.  Network-oriented applications require
   easy access to this information to learn the network topology, to
   verify that programmed state is installed in the forwarding plane, to
   measure the behavior of various flows, routes or forwarding entries,
   as well as to understand the configured and active states of the
   router.  Network-oriented applications also require easy access to an
   interface, which will allow them to program and control state related
   to forwarding.

   This document sets out an architecture for a common, standards-based
   interface to this information.  This Interface to the Routing System
   (I2RS) facilitates control and observation of the routing-related
   state (for example, a Routing Element RIB manager's state), as well
   as enabling network-oriented applications to be built on top of
   today's routed networks.  The I2RS is a programmatic asynchronous
   interface for transferring state into and out of the Internet routing
   system.  This I2RS architecture recognizes that the routing system
   and a router's Operating System (OS) provide useful mechanisms that
   applications could harness to accomplish application-level goals.
   These network-oriented applications can leverage the I2RS
   programmatic interface to create new ways to combine retrieving
   Internet routing data, analyzing this data, and setting state within
   routers.



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   Fundamental to I2RS are clear data models that define the semantics
   of the information that can be written and read.  I2RS provides a way
   for applications to customize network behavior while leveraging the
   existing routing system as desired.  I2RS provides a framework for
   applications (including controller applications) to register and to
   request the appropriate information for each particular application.

   Although the I2RS architecture is general enough to support
   information and data models for a variety of data, and aspects of the
   I2RS solution may be useful in domains other than routing, I2RS and
   this document are specifically focused on an interface for routing
   data.

   Security is a concern for any new I2RS.  <a href="#section-4">Section 4</a> provides an
   overview of the security considerations for the I2RS architecture.
   The detailed requirements for I2RS protocol security are contained in
   [<a href="#ref-I2RS-PROT-SEC">I2RS-PROT-SEC</a>], and the detailed security requirements for
   environment in which the I2RS protocol exists are contained in
   [<a href="#ref-I2RS-ENV-SEC">I2RS-ENV-SEC</a>].

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Drivers for the I2RS Architecture</span>

   There are four key drivers that shape the I2RS architecture.  First
   is the need for an interface that is programmatic and asynchronous
   and that offers fast, interactive access for atomic operations.
   Second is the access to structured information and state that is
   frequently not directly configurable or modeled in existing
   implementations or configuration protocols.  Third is the ability to
   subscribe to structured, filterable event notifications from the
   router.  Fourth, the operation of I2RS is to be data-model-driven to
   facilitate extensibility and provide standard data models to be used
   by network applications.

   I2RS is described as an asynchronous programmatic interface, the key
   properties of which are described in <a href="./rfc7920#section-5">Section&nbsp;5 of [RFC7920]</a>.

   The I2RS architecture facilitates obtaining information from the
   router.  The I2RS architecture provides the ability to not only read
   specific information, but also to subscribe to targeted information
   streams, filtered events, and thresholded events.

   Such an interface also facilitates the injection of ephemeral state
   into the routing system.  Ephemeral state on a router is the state
   that does not survive the reboot of a routing device or the reboot of
   the software handling the I2RS software on a routing device.  A non-
   routing protocol or application could inject state into a routing
   element via the state-insertion functionality of I2RS and that state
   could then be distributed in a routing or signaling protocol and/or



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   be used locally (e.g., to program the co-located forwarding plane).
   I2RS will only permit modification of state that would be possible to
   modify via Local Configuration; no direct manipulation of protocol-
   internal, dynamically determined data is envisioned.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Architectural Overview</span>

   Figure 1 shows the basic architecture for I2RS between applications
   using I2RS, their associated I2RS clients, and I2RS agents.
   Applications access I2RS services through I2RS clients.  A single
   I2RS client can provide access to one or more applications.  This
   figure also shows the types of data models associated with the
   routing system (dynamic configuration, static configuration, Local
   Configuration, and routing and signaling configuration) that the I2RS
   agent data models may access or augment.

   Figure 1 is similar to Figure 1 in [<a href="./rfc7920" title="&quot;Problem Statement for the Interface to the Routing System&quot;">RFC7920</a>], but the figure in this
   document shows additional detail on how the applications utilize I2RS
   clients to interact with I2RS agents.  It also shows a logical view
   of the data models associated with the routing system rather than a
   functional view (RIB, Forwarding Information Base (FIB), topology,
   policy, routing/signaling protocols, etc.)

   In Figure 1, Clients A and B each provide access to a single
   application (Applications A and B, respectively), while Client P
   provides access to multiple applications.

   Applications can access I2RS services through local or remote
   clients.  A local client operates on the same physical box as the
   routing system.  In contrast, a remote client operates across the
   network.  In the figure, Applications A and B access I2RS services
   through local clients, while Applications C, D, and E access I2RS
   services through a remote client.  The details of how applications
   communicate with a remote client is out of scope for I2RS.

   An I2RS client can access one or more I2RS agents.  In Figure 1,
   Clients B and P access I2RS agents 1 and 2.  Likewise, an I2RS agent
   can provide service to one or more clients.  In this figure, I2RS
   agent 1 provides services to Clients A, B, and P while Agent 2
   provides services to only Clients B and P.

   I2RS agents and clients communicate with one another using an
   asynchronous protocol.  Therefore, a single client can post multiple
   simultaneous requests, either to a single agent or to multiple
   agents.  Furthermore, an agent can process multiple requests, either
   from a single client or from multiple clients, simultaneously.





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   The I2RS agent provides read and write access to selected data on the
   routing element that are organized into I2RS services.  <a href="#section-4">Section 4</a>
   describes how access is mediated by authentication and access control
   mechanisms.  Figure 1 shows I2RS agents being able to write ephemeral
   static state (e.g., RIB entries) and to read from dynamic static
   (e.g., MPLS Label Switched Path Identifier (LSP-ID) or number of
   active BGP peers).

   In addition to read and write access, the I2RS agent allows clients
   to subscribe to different types of notifications about events
   affecting different object instances.  One example of a notification
   of such an event (which is unrelated to an object creation,
   modification or deletion) is when a next hop in the RIB is resolved
   in a way that allows it to be used by a RIB manager for installation
   in the forwarding plane as part of a particular route.  Please see
   Sections <a href="#section-7.6">7.6</a> and <a href="#section-7.7">7.7</a> for details.

   The scope of I2RS is to define the interactions between the I2RS
   agent and the I2RS client and the associated proper behavior of the
   I2RS agent and I2RS client.































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        ******************   *****************  *****************
        *  Application C *   * Application D *  * Application E *
        ******************   *****************  *****************
                 ^                  ^                   ^
                 |--------------|   |    |--------------|
                                |   |    |
                                v   v    v
                              ***************
                              *  Client P   *
                              ***************
                                   ^     ^
                                   |     |-------------------------|
         ***********************   |      ***********************  |
         *    Application A    *   |      *    Application B    *  |
         *                     *   |      *                     *  |
         *  +----------------+ *   |      *  +----------------+ *  |
         *  |   Client A     | *   |      *  |   Client B     | *  |
         *  +----------------+ *   |      *  +----------------+ *  |
         ******* ^ *************   |      ***** ^ ****** ^ ******  |
                 |                 |            |        |         |
                 |   |-------------|            |        |   |-----|
                 |   |   -----------------------|        |   |
                 |   |   |                               |   |
    ************ v * v * v *********   ***************** v * v ********
    *  +---------------------+     *   *  +---------------------+     *
    *  |     Agent 1         |     *   *  |    Agent 2          |     *
    *  +---------------------+     *   *  +---------------------+     *
    *     ^        ^  ^   ^        *   *     ^        ^  ^   ^        *
    *     |        |  |   |        *   *     |        |  |   |        *
    *     v        |  |   v        *   *     v        |  |   v        *
    * +---------+  |  | +--------+ *   * +---------+  |  | +--------+ *
    * | Routing |  |  | | Local  | *   * | Routing |  |  | | Local  | *
    * |   and   |  |  | | Config | *   * |   and   |  |  | | Config | *
    * |Signaling|  |  | +--------+ *   * |Signaling|  |  | +--------+ *
    * +---------+  |  |         ^  *   * +---------+  |  |         ^  *
    *    ^         |  |         |  *   *    ^         |  |         |  *
    *    |    |----|  |         |  *   *    |    |----|  |         |  *
    *    v    |       v         v  *   *    v    |       v         v  *
    *  +----------+ +------------+ *   *  +----------+ +------------+ *
    *  |  Dynamic | |   Static   | *   *  |  Dynamic | |   Static   | *
    *  |  System  | |   System   | *   *  |  System  | |   System   | *
    *  |  State   | |   State    | *   *  |  State   | |   State    | *
    *  +----------+ +------------+ *   *  +----------+ +------------+ *
    *                              *   *                              *
    *  Routing Element 1           *   *  Routing Element 2           *
    ********************************   ********************************

             Figure 1: Architecture of I2RS Clients and Agents



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   Routing Element:  A Routing Element implements some subset of the
      routing system.  It does not need to have a forwarding plane
      associated with it.  Examples of Routing Elements can include:

      *  A router with a forwarding plane and RIB Manager that runs
         IS-IS, OSPF, BGP, PIM, etc.,

      *  A BGP speaker acting as a Route Reflector,

      *  A Label Switching Router (LSR) that implements RSVP-TE,
         OSPF-TE, and the Path Computation Element (PCE) Communication
         Protocol (PCEP) and has a forwarding plane and associated RIB
         Manager, and

      *  A server that runs IS-IS, OSPF, and BGP and uses Forwarding and
         Control Element Separation (ForCES) to control a remote
         forwarding plane.

      A Routing Element may be locally managed, whether via command-line
      interface (CLI), SNMP, or the Network Configuration Protocol
      (NETCONF).

   Routing and Signaling:  This block represents that portion of the
      Routing Element that implements part of the Internet routing
      system.  It includes not merely standardized protocols (i.e.,
      IS-IS, OSPF, BGP, PIM, RSVP-TE, LDP, etc.), but also the RIB
      Manager layer.

   Local Configuration:  The black box behavior for interactions between
      the ephemeral state that I2RS installs into the routing element;
      Local Configuration is defined by this document and the behaviors
      specified by the I2RS protocol.

   Dynamic System State:  An I2RS agent needs access to state on a
      routing element beyond what is contained in the routing subsystem.
      Such state may include various counters, statistics, flow data,
      and local events.  This is the subset of operational state that is
      needed by network applications based on I2RS that is not contained
      in the routing and signaling information.  How this information is
      provided to the I2RS agent is out of scope, but the standardized
      information and data models for what is exposed are part of I2RS.

   Static System State:  An I2RS agent needs access to static state on a
      routing element beyond what is contained in the routing subsystem.
      An example of such state is specifying queueing behavior for an
      interface or traffic.  How the I2RS agent modifies or obtains this
      information is out of scope, but the standardized information and
      data models for what is exposed are part of I2RS.



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   I2RS agent:  See the definition in <a href="#section-2">Section 2</a>.

   Application:  A network application that needs to observe the network
      or manipulate the network to achieve its service requirements.

   I2RS client:  See the definition in <a href="#section-2">Section 2</a>.

   As can be seen in Figure 1, an I2RS client can communicate with
   multiple I2RS agents.  Similarly, an I2RS agent may communicate with
   multiple I2RS clients -- whether to respond to their requests, to
   send notifications, etc.  Timely notifications are critical so that
   several simultaneously operating applications have up-to-date
   information on the state of the network.

   As can also be seen in Figure 1, an I2RS agent may communicate with
   multiple clients.  Each client may send the agent a variety of write
   operations.  In order to keep the protocol simple, two clients should
   not attempt to write (modify) the same piece of information on an
   I2RS agent.  This is considered an error.  However, such collisions
   may happen and <a href="#section-7.8">Section 7.8</a> ("Multi-headed Control") describes how the
   I2RS agent resolves collision by first utilizing priority to resolve
   collisions and second by servicing the requests in a first-in, first-
   served basis.  The I2RS architecture includes this definition of
   behavior for this case simply for predictability, not because this is
   an intended result.  This predictability will simplify error handling
   and suppress oscillations.  If additional error cases beyond this
   simple treatment are required, these error cases should be resolved
   by the network applications and management systems.

   In contrast, although multiple I2RS clients may need to supply data
   into the same list (e.g., a prefix or filter list), this is not
   considered an error and must be correctly handled.  The nuances so
   that writers do not normally collide should be handled in the
   information models.

   The architectural goal for I2RS is that such errors should produce
   predictable behaviors and be reportable to interested clients.  The
   details of the associated policy is discussed in <a href="#section-7.8">Section 7.8</a>.  The
   same policy mechanism (simple priority per I2RS client) applies to
   interactions between the I2RS agent and the CLI/SNMP/NETCONF as
   described in <a href="#section-6.3">Section 6.3</a>.

   In addition, it must be noted that there may be indirect interactions
   between write operations.  A basic example of this is when two
   different but overlapping prefixes are written with different
   forwarding behavior.  Detection and avoidance of such interactions is
   outside the scope of the I2RS work and is left to agent design and
   implementation.



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<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Terminology</span>

   The following terminology is used in this document.

   agent or I2RS agent:   An I2RS agent provides the supported I2RS
      services from the local system's routing subsystems by interacting
      with the routing element to provide specified behavior.  The I2RS
      agent understands the I2RS protocol and can be contacted by I2RS
      clients.

   client or I2RS client:   A client implements the I2RS protocol, uses
      it to communicate with I2RS agents, and uses the I2RS services to
      accomplish a task.  It interacts with other elements of the
      policy, provisioning, and configuration system by means outside of
      the scope of the I2RS effort.  It interacts with the I2RS agents
      to collect information from the routing and forwarding system.
      Based on the information and the policy-oriented interactions, the
      I2RS client may also interact with I2RS agents to modify the state
      of their associated routing systems to achieve operational goals.
      An I2RS client can be seen as the part of an application that uses
      and supports I2RS and could be a software library.

   service or I2RS service:   For the purposes of I2RS, a service refers
      to a set of related state access functions together with the
      policies that control their usage.  The expectation is that a
      service will be represented by a data model.  For instance, 'RIB
      service' could be an example of a service that gives access to
      state held in a device's RIB.

   read scope:   The read scope of an I2RS client within an I2RS agent
      is the set of information that the I2RS client is authorized to
      read within the I2RS agent.  The read scope specifies the access
      restrictions to both see the existence of data and read the value
      of that data.

   notification scope:   The notification scope is the set of events and
      associated information that the I2RS client can request be pushed
      by the I2RS agent.  I2RS clients have the ability to register for
      specific events and information streams, but must be constrained
      by the access restrictions associated with their notification
      scope.

   write scope:   The write scope is the set of field values that the
      I2RS client is authorized to write (i.e., add, modify or delete).
      This access can restrict what data can be modified or created, and
      what specific value sets and ranges can be installed.





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   scope:   When unspecified as either read scope, write scope, or
      notification scope, the term "scope" applies to the read scope,
      write scope, and notification scope.

   resources:   A resource is an I2RS-specific use of memory, storage,
      or execution that a client may consume due to its I2RS operations.
      The amount of each such resource that a client may consume in the
      context of a particular agent may be constrained based upon the
      client's security role.  An example of such a resource could
      include the number of notifications registered for.  These are not
      protocol-specific resources or network-specific resources.

   role or security role:   A security role specifies the scope,
      resources, priorities, etc., that a client or agent has.  If an
      identity has multiple roles in the security system, the identity
      is permitted to perform any operations any of those roles permit.
      Multiple identities may use the same security role.

   identity:   A client is associated with exactly one specific
      identity.  State can be attributed to a particular identity.  It
      is possible for multiple communication channels to use the same
      identity; in that case, the assumption is that the associated
      client is coordinating such communication.

   identity and scope:   A single identity can be associated with
      multiple roles.  Each role has its own scope, and an identity
      associated with multiple roles can use the combined scope of all
      its roles.  More formally, each identity has:

      *  a read scope that is the logical OR of the read scopes
         associated with its roles,

      *  a write scope that is the logical OR of the write scopes
         associated with its roles, and

      *  a notification scope that is the logical OR of the notification
         scopes associated with its roles.

   secondary identity:   An I2RS client may supply a secondary opaque
      identifier for a secondary identity that is not interpreted by the
      I2RS agent.  An example of the use of the secondary opaque
      identifier is when the I2RS client is a go-between for multiple
      applications and it is necessary to track which application has
      requested a particular operation.







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   ephemeral data:   Ephemeral data is data that does not persist across
      a reboot (software or hardware) or a power on/off condition.
      Ephemeral data can be configured data or data recorded from
      operations of the router.  Ephemeral configuration data also has
      the property that a system cannot roll back to a previous
      ephemeral configuration state.

   group:   The NETCONF Access Control Model [<a href="./rfc6536" title="&quot;Network Configuration Protocol (NETCONF) Access Control Model&quot;">RFC6536</a>] uses the term
      "group" in terms of an administrative group that supports the
      well-established distinction between a root account and other
      types of less-privileged conceptual user accounts.  "Group" still
      refers to a single identity (e.g., root) that is shared by a group
      of users.

   routing system/subsystem:   A routing system or subsystem is a set of
      software and/or hardware that determines where packets are
      forwarded.  The I2RS agent is a component of a routing system.
      The term "packets" may be qualified to be layer 1 frames, layer 2
      frames, or layer 3 packets.  The phrase "Internet routing system"
      implies the packets that have IP as layer 3.  A routing
      "subsystem" indicates that the routing software/hardware is only
      the subsystem of another larger system.

   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 [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Key Architectural Properties</span>

   Several key architectural properties for the I2RS protocol are
   elucidated below (simplicity, extensibility, and model-driven
   programmatic interfaces).  However, some architectural properties
   such as performance and scaling are not described below because they
   are discussed in [<a href="./rfc7920" title="&quot;Problem Statement for the Interface to the Routing System&quot;">RFC7920</a>] and because they may vary based on the
   particular use cases.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Simplicity</span>

   There have been many efforts over the years to improve access to the
   information available to the routing and forwarding system.  Making
   such information visible and usable to network management and
   applications has many well-understood benefits.  There are two
   related challenges in doing so.  First, the quantity and diversity of
   information potentially available is very large.  Second, the
   variation both in the structure of the data and in the kinds of
   operations required tends to introduce protocol complexity.





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   While the types of operations contemplated here are complex in their
   nature, it is critical that I2RS be easily deployable and robust.
   Adding complexity beyond what is needed to satisfy well known and
   understood requirements would hinder the ease of implementation, the
   robustness of the protocol, and the deployability of the protocol.
   Overly complex data models tend to ossify information sets by
   attempting to describe and close off every possible option,
   complicating extensibility.

   Thus, one of the key aims for I2RS is to keep the protocol and
   modeling architecture simple.  So for each architectural component or
   aspect, we ask ourselves, "Do we need this complexity, or is the
   behavior merely nice to have?"  If we need the complexity, we should
   ask ourselves, "Is this the simplest way to provide this complexity
   in the I2RS external interface?"

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Extensibility</span>

   Extensibility of the protocol and data model is very important.  In
   particular, given the necessary scope limitations of the initial
   work, it is critical that the initial design include strong support
   for extensibility.

   The scope of I2RS work is being designed in phases to provide
   deliverable and deployable results at every phase.  Each phase will
   have a specific set of requirements, and the I2RS protocol and data
   models will progress toward these requirements.  Therefore, it is
   clearly desirable for the I2RS data models to be easily and highly
   extensible to represent additional aspects of the network elements or
   network systems.  It should be easy to integrate data models from
   I2RS with other data.  This reinforces the criticality of designing
   the data models to be highly extensible, preferably in a regular and
   simple fashion.

   The I2RS Working Group is defining operations for the I2RS protocol.
   It would be optimistic to assume that more and different ones may not
   be needed when the scope of I2RS increases.  Thus, it is important to
   consider extensibility not only of the underlying services' data
   models, but also of the primitives and protocol operations.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Model-Driven Programmatic Interfaces</span>

   A critical component of I2RS is the standard information and data
   models with their associated semantics.  While many components of the
   routing system are standardized, associated data models for them are
   not yet available.  Instead, each router uses different information,
   different mechanisms, and different CLI, which makes a standard
   interface for use by applications extremely cumbersome to develop and



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   maintain.  Well-known data modeling languages exist and may be used
   for defining the data models for I2RS.

   There are several key benefits for I2RS in using model-driven
   architecture and protocol(s).  First, it allows for data-model-
   focused processing of management data that provides modular
   implementation in I2RS clients and I2RS agents.  The I2RS client only
   needs to implement the models the I2RS client is able to access.  The
   I2RS agent only needs to implement the data models the I2RS agent
   supports.

   Second, tools can automate checking and manipulating data; this is
   particularly valuable for both extensibility and for the ability to
   easily manipulate and check proprietary data models.

   The different services provided by I2RS can correspond to separate
   data models.  An I2RS agent may indicate which data models are
   supported.

   The purpose of the data model is to provide a definition of the
   information regarding the routing system that can be used in
   operational networks.  If routing information is being modeled for
   the first time, a logical information model may be standardized prior
   to creating the data model.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Security Considerations</span>

   This I2RS architecture describes interfaces that clearly require
   serious consideration of security.  As an architecture, I2RS has been
   designed to reuse existing protocols that carry network management
   information.  Two of the existing protocols that are being reused for
   the I2RS protocol version 1 are NETCONF [<a href="./rfc6241" title="&quot;Network Configuration Protocol (NETCONF)&quot;">RFC6241</a>] and RESTCONF
   [<a href="#ref-RESTCONF" title="&quot;RESTCONF Protocol&quot;">RESTCONF</a>].  Additional protocols may be reused in future versions of
   the I2RS protocol.

   The I2RS protocol design process will be to specify additional
   requirements (including security) for the existing protocols in order
   in order to support the I2RS architecture.  After an existing
   protocol (e.g., NETCONF or RESTCONF) has been altered to fit the I2RS
   requirements, then it will be reviewed to determine if it meets these
   requirements.  During this review of changes to existing protocols to
   serve the I2RS architecture, an in-depth security review of the
   revised protocol should be done.

   Due to the reuse strategy of the I2RS architecture, this security
   section describes the assumed security environment for I2RS with
   additional details on a) identity and authentication, b)
   authorization, and c) client redundancy.  Each protocol proposed for



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   inclusion as an I2RS protocol will need to be evaluated for the
   security constraints of the protocol.  The detailed requirements for
   the I2RS protocol and the I2RS security environment will be defined
   within these global security environments.

   The I2RS protocol security requirements for I2RS protocol version 1
   are contained in [<a href="#ref-I2RS-PROT-SEC">I2RS-PROT-SEC</a>], and the global I2RS security
   environment requirements are contained [<a href="#ref-I2RS-ENV-SEC">I2RS-ENV-SEC</a>].

   First, here is a brief description of the assumed security
   environment for I2RS.  The I2RS agent associated with a Routing
   Element is a trusted part of that Routing Element.  For example, it
   may be part of a vendor-distributed signed software image for the
   entire Routing Element, or it may be a trusted signed application
   that an operator has installed.  The I2RS agent is assumed to have a
   separate authentication and authorization channel by which it can
   validate both the identity and permissions associated with an I2RS
   client.  To support numerous and speedy interactions between the I2RS
   agent and I2RS client, it is assumed that the I2RS agent can also
   cache that particular I2RS clients are trusted and their associated
   authorized scope.  This implies that the permission information may
   be old either in a pull model until the I2RS agent re-requests it or
   in a push model until the authentication and authorization channel
   can notify the I2RS agent of changes.

   Mutual authentication between the I2RS client and I2RS agent is
   required.  An I2RS client must be able to trust that the I2RS agent
   is attached to the relevant Routing Element so that write/modify
   operations are correctly applied and so that information received
   from the I2RS agent can be trusted by the I2RS client.

   An I2RS client is not automatically trustworthy.  Each I2RS client is
   associated with an identity with a set of scope limitations.
   Applications using an I2RS client should be aware that the scope
   limitations of an I2RS client are based on its identity (see
   <a href="#section-4.1">Section 4.1</a>) and the assigned role that the identity has.  A role
   sets specific authorization limits on the actions that an I2RS client
   can successfully request of an I2RS agent (see <a href="#section-4.2">Section 4.2</a>).  For
   example, one I2RS client may only be able to read a static route
   table, but another client may be able add an ephemeral route to the
   static route table.

   If the I2RS client is acting as a broker for multiple applications,
   then managing the security, authentication, and authorization for
   that communication is out of scope; nothing prevents the broker from
   using the I2RS protocol and a separate authentication and
   authorization channel from being used.  Regardless of the mechanism,
   an I2RS client that is acting as a broker is responsible for



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   determining that applications using it are trusted and permitted to
   make the particular requests.

   Different levels of integrity, confidentiality, and replay protection
   are relevant for different aspects of I2RS.  The primary
   communication channel that is used for client authentication and then
   used by the client to write data requires integrity, confidentiality
   and replay protection.  Appropriate selection of a default required
   transport protocol is the preferred way of meeting these
   requirements.

   Other communications via I2RS may not require integrity,
   confidentiality, and replay protection.  For instance, if an I2RS
   client subscribes to an information stream of prefix announcements
   from OSPF, those may require integrity but probably not
   confidentiality or replay protection.  Similarly, an information
   stream of interface statistics may not even require guaranteed
   delivery.  In <a href="#section-7.2">Section 7.2</a>, additional logins regarding multiple
   communication channels and their use is provided.  From the security
   perspective, it is critical to realize that an I2RS agent may open a
   new communication channel based upon information provided by an I2RS
   client (as described in <a href="#section-7.2">Section 7.2</a>).  For example, an I2RS client
   may request notifications of certain events, and the agent will open
   a communication channel to report such events.  Therefore, to avoid
   an indirect attack, such a request must be done in the context of an
   authenticated and authorized client whose communications cannot have
   been altered.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Identity and Authentication</span>

   As discussed above, all control exchanges between the I2RS client and
   agent should be authenticated and integrity-protected (such that the
   contents cannot be changed without detection).  Further, manipulation
   of the system must be accurately attributable.  In an ideal
   architecture, even information collection and notification should be
   protected; this may be subject to engineering trade-offs during the
   design.

   I2RS clients may be operating on behalf of other applications.  While
   those applications' identities are not needed for authentication or
   authorization, each application should have a unique opaque
   identifier that can be provided by the I2RS client to the I2RS agent
   for purposes of tracking attribution of operations to an application
   identifier (and from that to the application's identity).  This
   tracking of operations to an application supports I2RS functionality
   for tracing actions (to aid troubleshooting in routers) and logging
   of network changes.




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<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Authorization</span>

   All operations using I2RS, both observation and manipulation, should
   be subject to appropriate authorization controls.  Such authorization
   is based on the identity and assigned role of the I2RS client
   performing the operations and the I2RS agent in the network element.
   Multiple identities may use the same role(s).  As noted in the
   definitions of "identity" and "role" above, if multiple roles are
   associated with an identity then the identity is authorized to
   perform any operation authorized by any of its roles.

   I2RS agents, in performing information collection and manipulation,
   will be acting on behalf of the I2RS clients.  As such, each
   operation authorization will be based on the lower of the two
   permissions of the agent itself and of the authenticated client.  The
   mechanism by which this authorization is applied within the device is
   outside of the scope of I2RS.

   The appropriate or necessary level of granularity for scope can
   depend upon the particular I2RS service and the implementation's
   granularity.  An approach to a similar access control problem is
   defined in the NETCONF Access Control Model (NACM) [<a href="./rfc6536" title="&quot;Network Configuration Protocol (NETCONF) Access Control Model&quot;">RFC6536</a>]; it
   allows arbitrary access to be specified for a data node instance
   identifier while defining meaningful manipulable defaults.  The
   identity within NACM [<a href="./rfc6536" title="&quot;Network Configuration Protocol (NETCONF) Access Control Model&quot;">RFC6536</a>] can be specified as either a user name
   or a group user name (e.g., Root), and this name is linked a scope
   policy that is contained in a set of access control rules.
   Similarly, it is expected the I2RS identity links to one role that
   has a scope policy specified by a set of access control rules.  This
   scope policy can be provided via Local Configuration, exposed as an
   I2RS service for manipulation by authorized clients, or via some
   other method (e.g., Authentication, Authorization, and Accounting
   (AAA) service)

   While the I2RS agent allows access based on the I2RS client's scope
   policy, this does not mean the access is required to arrive on a
   particular transport connection or from a particular I2RS client by
   the I2RS architecture.  The operator-applied scope policy may or may
   not restrict the transport connection or the identities that can
   access a local I2RS agent.

   When an I2RS client is authenticated, its identity is provided to the
   I2RS agent, and this identity links to a role that links to the scope
   policy.  Multiple identities may belong to the same role; for
   example, such a role might be an Internal-Routes-Monitor that allows
   reading of the portion of the I2RS RIB associated with IP prefixes
   used for internal device addresses in the AS.




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<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Client Redundancy</span>

   I2RS must support client redundancy.  At the simplest, this can be
   handled by having a primary and a backup network application that
   both use the same client identity and can successfully authenticate
   as such.  Since I2RS does not require a continuous transport
   connection and supports multiple transport sessions, this can provide
   some basic redundancy.  However, it does not address the need for
   troubleshooting and logging of network changes to be informed about
   which network application is actually active.  At a minimum, basic
   transport information about each connection and time can be logged
   with the identity.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  I2RS in Personal Devices</span>

   If an I2RS agent or I2RS client is tightly correlated with a person
   (such as if an I2RS agent is running on someone's phone to control
   tethering), then this usage can raise privacy issues, over and above
   the security issues that normally need to be handled in I2RS.  One
   example of an I2RS interaction that could raise privacy issues is if
   the I2RS interaction enabled easier location tracking of a person's
   phone.  The I2RS protocol and data models should consider if privacy
   issues can arise when clients or agents are used for such use cases.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Network Applications and I2RS Client</span>

   I2RS is expected to be used by network-oriented applications in
   different architectures.  While the interface between a network-
   oriented application and the I2RS client is outside the scope of
   I2RS, considering the different architectures is important to
   sufficiently specify I2RS.

   In the simplest architecture of direct access, a network-oriented
   application has an I2RS client as a library or driver for
   communication with routing elements.

   In the broker architecture, multiple network-oriented applications
   communicate in an unspecified fashion to a broker application that
   contains an I2RS client.  That broker application requires additional
   functionality for authentication and authorization of the network-
   oriented applications; such functionality is out of scope for I2RS,
   but similar considerations to those described in <a href="#section-4.2">Section 4.2</a> do
   apply.  As discussed in <a href="#section-4.1">Section 4.1</a>, the broker I2RS client should
   determine distinct opaque identifiers for each network-oriented
   application that is using it.  The broker I2RS client can pass along
   the appropriate value as a secondary identifier, which can be used
   for tracking attribution of operations.




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   In a third architecture, a routing element or network-oriented
   application that uses an I2RS client to access services on a
   different routing element may also contain an I2RS agent to provide
   services to other network-oriented applications.  However, where the
   needed information and data models for those services differs from
   that of a conventional routing element, those models are, at least
   initially, out of scope for I2RS.  The following section describes an
   example of such a network application.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Example Network Application: Topology Manager</span>

   A Topology Manager includes an I2RS client that uses the I2RS data
   models and protocol to collect information about the state of the
   network by communicating directly with one or more I2RS agents.  From
   these I2RS agents, the Topology Manager collects routing
   configuration and operational data, such as interface and Label
   Switched Path (LSP) information.  In addition, the Topology Manager
   may collect link-state data in several ways -- via I2RS models, by
   peering with BGP-LS [<a href="./rfc7752" title="&quot;North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP&quot;">RFC7752</a>], or by listening into the IGP.

   The set of functionality and collected information that is the
   Topology Manager may be embedded as a component of a larger
   application, such as a path computation application.  As a stand-
   alone application, the Topology Manager could be useful to other
   network applications by providing a coherent picture of the network
   state accessible via another interface.  That interface might use the
   same I2RS protocol and could provide a topology service using
   extensions to the I2RS data models.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  I2RS Agent Role and Functionality</span>

   The I2RS agent is part of a routing element.  As such, it has
   relationships with that routing element as a whole and with various
   components of that routing element.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Relationship to Its Routing Element</span>

   A Routing Element may be implemented with a wide variety of different
   architectures: an integrated router, a split architecture,
   distributed architecture, etc.  The architecture does not need to
   affect the general I2RS agent behavior.

   For scalability and generality, the I2RS agent may be responsible for
   collecting and delivering large amounts of data from various parts of
   the routing element.  Those parts may or may not actually be part of
   a single physical device.  Thus, for scalability and robustness, it
   is important that the architecture allow for a distributed set of
   reporting components providing collected data from the I2RS agent



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   back to the relevant I2RS clients.  There may be multiple I2RS agents
   within the same router.  In such a case, they must have non-
   overlapping sets of information that they manipulate.

   To facilitate operations, deployment, and troubleshooting, it is
   important that traceability of the requests received by I2RS agent's
   and actions taken be supported via a common data model.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  I2RS State Storage</span>

   State modification requests are sent to the I2RS agent in a routing
   element by I2RS clients.  The I2RS agent is responsible for applying
   these changes to the system, subject to the authorization discussed
   above.  The I2RS agent will retain knowledge of the changes it has
   applied, and the client on whose behalf it applied the changes.  The
   I2RS agent will also store active subscriptions.  These sets of data
   form the I2RS datastore.  This data is retained by the agent until
   the state is removed by the client, it is overridden by some other
   operation such as CLI, or the device reboots.  Meaningful logging of
   the application and removal of changes are recommended.  I2RS-applied
   changes to the routing element state will not be retained across
   routing element reboot.  The I2RS datastore is not preserved across
   routing element reboots; thus, the I2RS agent will not attempt to
   reapply such changes after a reboot.

<span class="h4"><a class="selflink" id="section-6.2.1" href="#section-6.2.1">6.2.1</a>.  I2RS Agent Failure</span>

   It is expected that an I2RS agent may fail independently of the
   associated routing element.  This could happen because I2RS is
   disabled on the routing element or because the I2RS agent, which may
   be a separate process or even running on a separate processor,
   experiences an unexpected failure.  Just as routing state learned
   from a failed source is removed, the ephemeral I2RS state will
   usually be removed shortly after the failure is detected or as part
   of a graceful shutdown process.  To handle these two types of
   failures, the I2RS agent MUST support two different notifications: a
   notification for the I2RS agent terminating gracefully, and a
   notification for the I2RS agent starting up after an unexpected
   failure.  The two notifications are described below followed by a
   description of their use in unexpected failures and graceful
   shutdowns.










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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


   NOTIFICATION_I2RS_AGENT_TERMINATING:   This notification reports that
      the associated I2RS agent is shutting down gracefully and that
      I2RS ephemeral state will be removed.  It can optionally include a
      timestamp indicating when the I2RS agent will shut down.  Use of
      this timestamp assumes that time synchronization has been done,
      and the timestamp should not have granularity finer than one
      second because better accuracy of shutdown time is not guaranteed.

   NOTIFICATION_I2RS_AGENT_STARTING:   This notification signals to the
      I2RS client(s) that the associated I2RS agent has started.  It
      includes an agent-boot-count that indicates how many times the
      I2RS agent has restarted since the associated routing element
      restarted.  The agent-boot-count allows an I2RS client to
      determine if the I2RS agent has restarted.  (Note: This
      notification will be sent by the I2RS agent to I2RS clients that
      are known by the I2RS agent after a reboot.  How the I2RS agent
      retains the knowledge of these I2RS clients is out of scope of
      this architecture.)

   There are two different failure types that are possible, and each has
   different behavior.

   Unexpected failure:   In this case, the I2RS agent has unexpectedly
      crashed and thus cannot notify its clients of anything.  Since
      I2RS does not require a persistent connection between the I2RS
      client and I2RS agent, it is necessary to have a mechanism for the
      I2RS agent to notify I2RS clients that had subscriptions or
      written ephemeral state; such I2RS clients should be cached by the
      I2RS agent's system in persistent storage.  When the I2RS agent
      starts, it should send a NOTIFICATION_I2RS_AGENT_STARTING to each
      cached I2RS client.

   Graceful shutdowns:   In this case, the I2RS agent can do specific
      limited work as part of the process of being disabled.  The I2RS
      agent must send a NOTIFICATION_I2RS_AGENT_TERMINATING to all its
      cached I2RS clients.  If the I2RS agent restarts after a graceful
      termination, it will send a NOTIFICATION_I2RS_AGENT_STARTING to
      each cached I2RS client.

<span class="h4"><a class="selflink" id="section-6.2.2" href="#section-6.2.2">6.2.2</a>.  Starting and Ending</span>

   When an I2RS client applies changes via the I2RS protocol, those
   changes are applied and left until removed or the routing element
   reboots.  The network application may make decisions about what to
   request via I2RS based upon a variety of conditions that imply
   different start times and stop times.  That complexity is managed by
   the network application and is not handled by I2RS.




<span class="grey">Atlas, et al.                 Informational                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


<span class="h4"><a class="selflink" id="section-6.2.3" href="#section-6.2.3">6.2.3</a>.  Reversion</span>

   An I2RS agent may decide that some state should no longer be applied.
   An I2RS client may instruct an agent to remove state it has applied.
   In all such cases, the state will revert to what it would have been
   without the I2RS client-agent interaction; that state is generally
   whatever was specified via the CLI, NETCONF, SNMP, etc., I2RS agents
   will not store multiple alternative states, nor try to determine
   which one among such a plurality it should fall back to.  Thus, the
   model followed is not like the RIB, where multiple routes are stored
   at different preferences.  (For I2RS state in the presence of two
   I2RS clients, please see Sections <a href="#section-1.2">1.2</a> and <a href="#section-7.8">7.8</a>)

   An I2RS client may register for notifications, subject to its
   notification scope, regarding state modification or removal by a
   particular I2RS client.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Interactions with Local Configuration</span>

   Changes may originate from either Local Configuration or from I2RS.
   The modifications and data stored by I2RS are separate from the local
   device configuration, but conflicts between the two must be resolved
   in a deterministic manner that respects operator-applied policy.  The
   deterministic manner is the result of general I2RS rules, system
   rules, knobs adjusted by operator-applied policy, and the rules
   associated with the YANG data model (often in "MUST" and "WHEN"
   clauses for dependencies).

   The operator-applied policy knobs can determine whether the Local
   Configuration overrides a particular I2RS client's request or vice
   versa.  Normally, most devices will have an operator-applied policy
   that will prioritize the I2RS client's ephemeral configuration
   changes so that ephemeral data overrides the Local Configuration.

   These operator-applied policy knobs can be implemented in many ways.
   One way is for the routing element to configure a priority on the
   Local Configuration and a priority on the I2RS client's write of the
   ephemeral configuration.  The I2RS mechanism would compare the I2RS
   client's priority to write with that priority assigned to the Local
   Configuration in order to determine whether Local Configuration or
   I2RS client's write of ephemeral data wins.

   To make sure the I2RS client's requests are what the operator
   desires, the I2RS data modules have a general rule that, by default,
   the Local Configuration always wins over the I2RS ephemeral
   configuration.





<span class="grey">Atlas, et al.                 Informational                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


   The reason for this general rule is if there is no operator-applied
   policy to turn on I2RS ephemeral overwrites of Local Configuration,
   then the I2RS overwrites should not occur.  This general rule allows
   the I2RS agents to be installed in routing systems and the
   communication tested between I2RS clients and I2RS agents without the
   I2RS agent overwriting configuration state.  For more details, see
   the examples below.

   In the case when the I2RS ephemeral state always wins for a data
   model, if there is an I2RS ephemeral state value, it is installed
   instead of the Local Configuration state value.  The Local
   Configuration information is stored so that if/when an I2RS client
   removes I2RS ephemeral state, the Local Configuration state can be
   restored.

   When the Local Configuration always wins, some communication between
   that subsystem and the I2RS agent is still necessary.  As an I2RS
   agent connects to the routing subsystem, the I2RS agent must also
   communicate with the Local Configuration to exchange model
   information so the I2RS agent knows the details of each specific
   device configuration change that the I2RS agent is permitted to
   modify.  In addition, when the system determines that a client's I2RS
   state is preempted, the I2RS agent must notify the affected I2RS
   clients; how the system determines this is implementation dependent.

   It is critical that policy based upon the source is used because the
   resolution cannot be time based.  Simply allowing the most recent
   state to prevail could cause race conditions where the final state is
   not repeatably deterministic.

<span class="h4"><a class="selflink" id="section-6.3.1" href="#section-6.3.1">6.3.1</a>.  Examples of Local Configuration vs. I2RS Ephemeral Configuration</span>

   A set of examples is useful in order to illustrated these
   architecture principles.  Assume there are three routers: Router A,
   Router B, and Router C.  There are two operator-applied policy knobs
   that these three routers must have regarding ephemeral state.

   o  Policy Knob 1: Ephemeral configuration overwrites Local
      Configuration.

   o  Policy Knob 2: Update of Local Configuration value supersedes and
      overwrites the ephemeral configuration.









<span class="grey">Atlas, et al.                 Informational                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


   For Policy Knob 1, the routers with an I2RS agent receiving a write
   for an ephemeral entry in a data model must consider the following:

   1.  Does the operator policy allow the ephemeral configuration
       changes to have priority over existing Local Configuration?

   2.  Does the YANG data model have any rules associated with the
       ephemeral configuration (such as the "MUST" or "WHEN" rule)?

   For this example, there is no "MUST" or "WHEN" rule in the data being
   written.

   The policy settings are:

               Policy Knob 1           Policy Knob 2
               ===================     ==================
   Router A    ephemeral has           ephemeral has
               priority                priority

   Router B    Local Configuration     Local Configuration
               has priority            has priority

   Router C    ephemeral has           Local Configuration
               priority                has priority

   Router A has the normal operator policy in Policy Knob 1 and Policy
   Knob 2 that prioritizes ephemeral configuration over Local
   Configuration in the I2RS agent.  An I2RS client sends a write to an
   ephemeral configuration value via an I2RS agent in Router A.  The
   I2RS agent overwrites the configuration value in the intended
   configuration, and the I2RS agent returns an acknowledgement of the
   write.  If the Local Configuration value changes, Router A stays with
   the ephemeral configuration written by the I2RS client.

   Router B's operator has no desire to allow ephemeral writes to
   overwrite Local Configuration even though it has installed an I2RS
   agent.  Router B's policy prioritizes the Local Configuration over
   the ephemeral write.  When the I2RS agent on Router B receives a
   write from an I2RS client, the I2RS agent will check the operator
   Policy Knob 1 and return a response to the I2RS client indicating the
   operator policy did not allow the overwriting of the Local
   Configuration.

   The Router B case demonstrates why the I2RS architecture sets the
   default to the Local Configuration wins.  Since I2RS functionality is
   new, the operator must enable it.  Otherwise, the I2RS ephemeral
   functionality is off.  Router B's operators can install the I2RS code
   and test responses without engaging the I2RS overwrite function.



<span class="grey">Atlas, et al.                 Informational                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


   Router C's operator sets Policy Knob 1 for the I2RS clients to
   overwrite existing Local Configuration and Policy Knob 2 for the
   Local Configuration changes to update ephemeral state.  To understand
   why an operator might set the policy knobs this way, consider that
   Router C is under the control of an operator that has a back-end
   system that re-writes the Local Configuration of all systems at 11
   p.m. each night.  Any ephemeral change to the network is only
   supposed to last until 11 p.m. when the next Local Configuration
   changes are rolled out from the back-end system.  The I2RS client
   writes the ephemeral state during the day, and the I2RS agent on
   Router C updates the value.  At 11 p.m., the back-end configuration
   system updates the Local Configuration via NETCONF, and the I2RS
   agent is notified that the Local Configuration updated this value.
   The I2RS agent notifies the I2RS client that the value has been
   overwritten by the Local Configuration.  The I2RS client in this use
   case is a part of an application that tracks any ephemeral state
   changes to make sure all ephemeral changes are included in the next
   configuration run.

<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  Routing Components and Associated I2RS Services</span>

   For simplicity, each logical protocol or set of functionality that
   can be compactly described in a separable information and data model
   is considered as a separate I2RS service.  A routing element need not
   implement all routing components described nor provide the associated
   I2RS services.  I2RS services should include a capability model so
   that peers can determine which parts of the service are supported.
   Each I2RS service requires an information model that describes at
   least the following: data that can be read, data that can be written,
   notifications that can be subscribed to, and the capability model
   mentioned above.




















<span class="grey">Atlas, et al.                 Informational                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


   The initial services included in the I2RS architecture are as
   follows.

    ***************************     **************    *****************
    *      I2RS Protocol      *     *            *    *    Dynamic    *
    *                         *     * Interfaces *    *    Data &amp;     *
    *  +--------+  +-------+  *     *            *    *  Statistics   *
    *  | Client |  | Agent |  *     **************    *****************
    *  +--------+  +-------+  *
    *                         *        **************    *************
    ***************************        *            *    *           *
                                       *  Policy    *    * Base QoS  *
    ********************    ********   *  Templates *    * Templates *
    *       +--------+ *    *      *   *            *    *************
    *  BGP  | BGP-LS | *    * PIM  *   **************
    *       +--------+ *    *      *
    ********************    ********       ****************************
                                           * MPLS +---------+ +-----+ *
    **********************************     *      | RSVP-TE | | LDP | *
    *    IGPs      +------+ +------+ *     *      +---------+ +-----+ *
    *  +--------+  | OSPF | |IS-IS | *     * +--------+               *
    *  | Common |  +------+ +------+ *     * | Common |               *
    *  +--------+                    *     * +--------+               *
    **********************************     ****************************

    **************************************************************
    * RIB Manager                                                *
    *  +-------------------+  +---------------+   +------------+ *
    *  | Unicast/multicast |  | Policy-Based  |   | RIB Policy | *
    *  | RIBs &amp; LIBs       |  | Routing       |   | Controls   | *
    *  | route instances   |  | (ACLs, etc)   |   +------------+ *
    *  +-------------------+  +---------------+                  *
    **************************************************************

                    Figure 2: Anticipated I2RS Services

   There are relationships between different I2RS services -- whether
   those be the need for the RIB to refer to specific interfaces, the
   desire to refer to common complex types (e.g., links, nodes, IP
   addresses), or the ability to refer to implementation-specific
   functionality (e.g., pre-defined templates to be applied to
   interfaces or for QoS behaviors that traffic is directed into).
   <a href="#section-6.4.5">Section 6.4.5</a> discusses information modeling constructs and the range
   of relationship types that are applicable.







<span class="grey">Atlas, et al.                 Informational                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


<span class="h4"><a class="selflink" id="section-6.4.1" href="#section-6.4.1">6.4.1</a>.  Routing and Label Information Bases</span>

   Routing elements may maintain one or more information bases.
   Examples include Routing Information Bases such as IPv4/IPv6 Unicast
   or IPv4/IPv6 Multicast.  Another such example includes the MPLS Label
   Information Bases, per platform, per interface, or per context.  This
   functionality, exposed via an I2RS service, must interact smoothly
   with the same mechanisms that the routing element already uses to
   handle RIB input from multiple sources.  Conceptually, this can be
   handled by having the I2RS agent communicate with a RIB Manager as a
   separate routing source.

   The point-to-multipoint state added to the RIB does not need to match
   to well-known multicast protocol installed state.  The I2RS agent can
   create arbitrary replication state in the RIB, subject to the
   advertised capabilities of the routing element.

<span class="h4"><a class="selflink" id="section-6.4.2" href="#section-6.4.2">6.4.2</a>.  IGPs, BGP, and Multicast Protocols</span>

   A separate I2RS service can expose each routing protocol on the
   device.  Such I2RS services may include a number of different kinds
   of operations:

   o  reading the various internal RIB(s) of the routing protocol is
      often helpful for understanding the state of the network.
      Directly writing to these protocol-specific RIBs or databases is
      out of scope for I2RS.

   o  reading the various pieces of policy information the particular
      protocol instance is using to drive its operations.

   o  writing policy information such as interface attributes that are
      specific to the routing protocol or BGP policy that may indirectly
      manipulate attributes of routes carried in BGP.

   o  writing routes or prefixes to be advertised via the protocol.

   o  joining/removing interfaces from the multicast trees.

   o  subscribing to an information stream of route changes.

   o  receiving notifications about peers coming up or going down.

   For example, the interaction with OSPF might include modifying the
   local routing element's link metrics, announcing a locally attached
   prefix, or reading some of the OSPF link-state database.  However,
   direct modification of the link-state database must not be allowed in
   order to preserve network state consistency.



<span class="grey">Atlas, et al.                 Informational                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


<span class="h4"><a class="selflink" id="section-6.4.3" href="#section-6.4.3">6.4.3</a>.  MPLS</span>

   I2RS services will be needed to expose the protocols that create
   transport LSPs (e.g., LDP and RSVP-TE) as well as protocols (e.g.,
   BGP, LDP) that provide MPLS-based services (e.g., pseudowires,
   L3VPNs, L2VPNs, etc).  This should include all local information
   about LSPs originating in, transiting, or terminating in this Routing
   Element.

<span class="h4"><a class="selflink" id="section-6.4.4" href="#section-6.4.4">6.4.4</a>.  Policy and QoS Mechanisms</span>

   Many network elements have separate policy and QoS mechanisms,
   including knobs that affect local path computation and queue control
   capabilities.  These capabilities vary widely across implementations,
   and I2RS cannot model the full range of information collection or
   manipulation of these attributes.  A core set does need to be
   included in the I2RS information models and supported in the expected
   interfaces between the I2RS agent and the network element, in order
   to provide basic capabilities and the hooks for future extensibility.

   By taking advantage of extensibility and subclassing, information
   models can specify use of a basic model that can be replaced by a
   more detailed model.

<span class="h4"><a class="selflink" id="section-6.4.5" href="#section-6.4.5">6.4.5</a>.  Information Modeling, Device Variation, and Information</span>
<span class="h4">        Relationships</span>

   I2RS depends heavily on information models of the relevant aspects of
   the Routing Elements to be manipulated.  These models drive the data
   models and protocol operations for I2RS.  It is important that these
   information models deal well with a wide variety of actual
   implementations of Routing Elements, as seen between different
   products and different vendors.  There are three ways that I2RS
   information models can address these variations: class or type
   inheritance, optional features, and templating.

<span class="h5"><a class="selflink" id="section-6.4.5.1" href="#section-6.4.5.1">6.4.5.1</a>.  Managing Variation: Object Classes/Types and Inheritance</span>

   Information modeled by I2RS from a Routing Element can be described
   in terms of classes or types or object.  Different valid inheritance
   definitions can apply.  What is appropriate for I2RS to use is not
   determined in this architecture; for simplicity, "class" and
   "subclass" will be used as the example terminology.  This I2RS
   architecture does require the ability to address variation in Routing
   Elements by allowing information models to define parent or base
   classes and subclasses.





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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


   The base or parent class defines the common aspects that all Routing
   Elements are expected to support.  Individual subclasses can
   represent variations and additional capabilities.  When applicable,
   there may be several levels of refinement.  The I2RS protocol can
   then provide mechanisms to allow an I2RS client to determine which
   classes a given I2RS agent has available.  I2RS clients that only
   want basic capabilities can operate purely in terms of base or parent
   classes, while a client needing more details or features can work
   with the supported subclass(es).

   As part of I2RS information modeling, clear rules should be specified
   for how the parent class and subclass can relate; for example, what
   changes can a subclass make to its parent?  The description of such
   rules should be done so that it can apply across data modeling tools
   until the I2RS data modeling language is selected.

<span class="h5"><a class="selflink" id="section-6.4.5.2" href="#section-6.4.5.2">6.4.5.2</a>.  Managing Variation: Optionality</span>

   I2RS information models must be clear about what aspects are
   optional.  For instance, must an instance of a class always contain a
   particular data field X?  If so, must the client provide a value for
   X when creating the object or is there a well-defined default value?
   From the Routing Element perspective, in the above example, each
   information model should provide information regarding the following
   questions:

   o  Is X required for the data field to be accepted and applied?

   o  If X is optional, then how does "X" as an optional portion of the
      data field interact with the required aspects of the data field?

   o  Does the data field have defaults for the mandatory portion of the
      field and the optional portions of the field?

   o  Is X required to be within a particular set of values (e.g.,
      range, length of strings)?

   The information model needs to be clear about what read or write
   values are set by the client and what responses or actions are
   required by the agent.  It is important to indicate what is required
   or optional in client values and agent responses/actions.










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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc7921">RFC 7921</a>                    I2RS Architecture                  June 2016</span>


<span class="h5"><a class="selflink" id="section-6.4.5.3" href="#section-6.4.5.3">6.4.5.3</a>.  Managing Variation: Templating</span>

   A template is a collection of information to address a problem; it
   cuts across the notions of class and object instances.  A template
   provides a set of defined values for a set of information fields and
   can specify a set of values that must be provided to complete the
   template.  Further, a flexible template scheme may allow some of the
   defined values to be overwritten.

   For instance, assigning traffic to a particular service class might
   be done by specifying a template queueing with a parameter to
   indicate Gold, Silver, or Best Effort.  The details of how that is
   carried out are not modeled.  This does assume that the necessary
   templates are made available on the Routing Element via some
   mechanism other than I2RS.  The idea is that by providing suitable
   templates for tasks that need to be accomplished, with templates
   implemented differently for different kinds of Routing Elements, the
   client can easily interact with the Routing Element without concern
   for the variations that are handled by values included in the
   template.

   If implementation variation can be exposed in other ways, templates
   may not be needed.  However, templates themselves could be objects
   referenced in the protocol messages, with Routing Elements being
   configured with the proper templates to complete the operation.  This
   is a topic for further discussion.

<span class="h5"><a class="selflink" id="section-6.4.5.4" href="#section-6.4.5.4">6.4.5.4</a>.  Object Relationships</span>

   Objects (in a Routing Element or otherwise) do not exist in
   isolation.  They are related to each other.  One of the important
   things a class definition does is represent the relationships between
   instances of different classes.  These relationships can be very
   simple or quite complicated.  The following sections list the
   information relationships that the information models need to
   support.

<span class="h6"><a class="selflink" id="section-6.4.5.4.1" href="#section-6.4.5.4.1">6.4.5.4.1</a>.  Initialization</span>

   The simplest relationship is that one object instance is initialized
   by copying another.  For example, one may have an object instance
   that represents the default setup for a tunnel, and all new tunnels
   have fields copied from there if they are not set as part of
   establishment.  This is closely related to the templates discussed
   above, but not identical.  Since the relationship is only momentary,
   it is often not formally represented in modeling but only captured in
   the semantic description of the default object.




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<span class="h6"><a class="selflink" id="section-6.4.5.4.2" href="#section-6.4.5.4.2">6.4.5.4.2</a>.  Correlation Identification</span>

   Often, it suffices to indicate in one object that it is related to a
   second object, without having a strong binding between the two.  So
   an identifier is used to represent the relationship.  This can be
   used to allow for late binding or a weak binding that does not even
   need to exist.  A policy name in an object might indicate that if a
   policy by that name exists, it is to be applied under some
   circumstance.  In modeling, this is often represented by the type of
   the value.

<span class="h6"><a class="selflink" id="section-6.4.5.4.3" href="#section-6.4.5.4.3">6.4.5.4.3</a>.  Object References</span>

   Sometimes the relationship between objects is stronger.  A valid ARP
   entry has to point to the active interface over which it was derived.
   This is the classic meaning of an object reference in programming.
   It can be used for relationships like containment or dependence.
   This is usually represented by an explicit modeling link.

<span class="h6"><a class="selflink" id="section-6.4.5.4.4" href="#section-6.4.5.4.4">6.4.5.4.4</a>.  Active References</span>

   There is an even stronger form of coupling between objects if changes
   in one of the two objects are always to be reflected in the state of
   the other.  For example, if a tunnel has an MTU (maximum transmit
   unit), and link MTU changes need to immediately propagate to the
   tunnel MTU, then the tunnel is actively coupled to the link
   interface.  This kind of active state coupling implies some sort of
   internal bookkeeping to ensure consistency, often conceptualized as a
   subscription model across objects.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  I2RS Client Agent Interface</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  One Control and Data Exchange Protocol</span>

   This I2RS architecture assumes a data-model-driven protocol where the
   data models are defined in YANG 1.1 [<a href="#ref-YANG1.1" title="&quot;The YANG 1.1 Data Modeling Language&quot;">YANG1.1</a>] and associated YANG
   based model documents [<a href="./rfc6991" title="&quot;Common YANG Data Types&quot;">RFC6991</a>], [<a href="./rfc7223" title="&quot;A YANG Data Model for Interface Management&quot;">RFC7223</a>], [<a href="./rfc7224" title="&quot;IANA Interface Type YANG Module&quot;">RFC7224</a>], [<a href="./rfc7277" title="&quot;A YANG Data Model for IP Management&quot;">RFC7277</a>],
   [<a href="./rfc7317" title="&quot;A YANG Data Model for System Management&quot;">RFC7317</a>].  Two of the protocols to be expanded to support the I2RS
   protocol are NETCONF [<a href="./rfc6241" title="&quot;Network Configuration Protocol (NETCONF)&quot;">RFC6241</a>] and RESTCONF [<a href="#ref-RESTCONF" title="&quot;RESTCONF Protocol&quot;">RESTCONF</a>].  This helps
   meet the goal of simplicity and thereby enhances deployability.  The
   I2RS protocol may need to use several underlying transports (TCP,
   SCTP (Stream Control Transport Protocol), DCCP (Datagram Congestion
   Control Protocol)), with suitable authentication and integrity-
   protection mechanisms.  These different transports can support
   different types of communication (e.g., control, reading,
   notifications, and information collection) and different sets of





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   data.  Whatever transport is used for the data exchange, it must also
   support suitable congestion-control mechanisms.  The transports
   chosen should be operator and implementor friendly to ease adoption.

   Each version of the I2RS protocol will specify the following: a)
   which transports may be used by the I2RS protocol, b) which
   transports are mandatory to implement, and c) which transports are
   optional to implement.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Communication Channels</span>

   Multiple communication channels and multiple types of communication
   channels are required.  There may be a range of requirements (e.g.,
   confidentiality, reliability), and to support the scaling, there may
   need to be channels originating from multiple subcomponents of a
   routing element and/or to multiple parts of an I2RS client.  All such
   communication channels will use the same higher-layer I2RS protocol
   (which combines secure transport and I2RS contextual information).
   The use of additional channels for communication will be coordinated
   between the I2RS client and the I2RS agent using this protocol.

   I2RS protocol communication may be delivered in-band via the routing
   system's data plane.  I2RS protocol communication might be delivered
   out-of-band via a management interface.  Depending on what operations
   are requested, it is possible for the I2RS protocol communication to
   cause the in-band communication channels to stop working; this could
   cause the I2RS agent to become unreachable across that communication
   channel.

<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  Capability Negotiation</span>

   The support for different protocol capabilities and I2RS services
   will vary across I2RS clients and Routing Elements supporting I2RS
   agents.  Since each I2RS service is required to include a capability
   model (see <a href="#section-6.4">Section 6.4</a>), negotiation at the protocol level can be
   restricted to protocol specifics and which I2RS services are
   supported.

   Capability negotiation (such as which transports are supported beyond
   the minimum required to implement) will clearly be necessary.  It is
   important that such negotiations be kept simple and robust, as such
   mechanisms are often a source of difficulty in implementation and
   deployment.

   The protocol capability negotiation can be segmented into the basic
   version negotiation (required to ensure basic communication), and the
   more complex capability exchange that can take place within the base
   protocol mechanisms.  In particular, the more complex protocol and



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   mechanism negotiation can be addressed by defining information models
   for both the I2RS agent and the I2RS client.  These information
   models can describe the various capability options.  This can then
   represent and be used to communicate important information about the
   agent and the capabilities thereof.

<span class="h3"><a class="selflink" id="section-7.4" href="#section-7.4">7.4</a>.  Scope Policy Specifications</span>

   As Sections <a href="#section-4.1">4.1</a> and <a href="#section-4.2">4.2</a> describe, each I2RS client will have a unique
   identity and may have a secondary identity (see <a href="#section-2">Section 2</a>) to aid in
   troubleshooting.  As <a href="#section-4">Section 4</a> indicates, all authentication and
   authorization mechanisms are based on the primary identity, which
   links to a role with scope policy for reading data, for writing data,
   and for limiting the resources that can be consumed.  The
   specifications for data scope policy (for read, write, or resources
   consumption) need to specify the data being controlled by the policy,
   and acceptable ranges of values for the data.

<span class="h3"><a class="selflink" id="section-7.5" href="#section-7.5">7.5</a>.  Connectivity</span>

   An I2RS client may or may not maintain an active communication
   channel with an I2RS agent.  Therefore, an I2RS agent may need to
   open a communication channel to the client to communicate previously
   requested information.  The lack of an active communication channel
   does not imply that the associated I2RS client is non-functional.
   When communication is required, the I2RS agent or I2RS client can
   open a new communication channel.

   State held by an I2RS agent that is owned by an I2RS client should
   not be removed or cleaned up when a client is no longer
   communicating, even if the agent cannot successfully open a new
   communication channel to the client.

   For many applications, it may be desirable to clean up state if a
   network application dies before removing the state it has created.
   Typically, this is dealt with in terms of network application
   redundancy.  If stronger mechanisms are desired, mechanisms outside
   of I2RS may allow a supervisory network application to monitor I2RS
   clients and, based on policy known to the supervisor, clean up state
   if applications die.  More complex mechanisms instantiated in the
   I2RS agent would add complications to the I2RS protocol and are thus
   left for future work.

   Some examples of such a mechanism include the following.  In one
   option, the client could request state cleanup if a particular
   transport session is terminated.  The second is to allow state
   expiration, expressed as a policy associated with the I2RS client's




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   role.  The state expiration could occur after there has been no
   successful communication channel to or from the I2RS client for the
   policy-specified duration.

<span class="h3"><a class="selflink" id="section-7.6" href="#section-7.6">7.6</a>.  Notifications</span>

   As with any policy system interacting with the network, the I2RS
   client needs to be able to receive notifications of changes in
   network state.  Notifications here refer to changes that are
   unanticipated, represent events outside the control of the systems
   (such as interface failures on controlled devices), or are
   sufficiently sparse as to be anomalous in some fashion.  A
   notification may also be due to a regular event.

   Such events may be of interest to multiple I2RS clients controlling
   data handled by an I2RS agent and to multiple other I2RS clients that
   are collecting information without exerting control.  The
   architecture therefore requires that it be practical for I2RS clients
   to register for a range of notifications and for the I2RS agents to
   send notifications to a number of clients.  The I2RS client should be
   able to filter the specific notifications that will be received; the
   specific types of events and filtering operations can vary by
   information model and need to be specified as part of the information
   model.

   The I2RS information model needs to include representation of these
   events.  As discussed earlier, the capability information in the
   model will allow I2RS clients to understand which events a given I2RS
   agent is capable of generating.

   For performance and scaling by the I2RS client and general
   information confidentiality, an I2RS client needs to be able to
   register for just the events it is interested in.  It is also
   possible that I2RS might provide a stream of notifications via a
   publish/subscribe mechanism that is not amenable to having the I2RS
   agent do the filtering.

<span class="h3"><a class="selflink" id="section-7.7" href="#section-7.7">7.7</a>.  Information Collection</span>

   One of the other important aspects of I2RS is that it is intended to
   simplify collecting information about the state of network elements.
   This includes both getting a snapshot of a large amount of data about
   the current state of the network element and subscribing to a feed of
   the ongoing changes to the set of data or a subset thereof.  This is
   considered architecturally separate from notifications due to the
   differences in information rate and total volume.





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<span class="h3"><a class="selflink" id="section-7.8" href="#section-7.8">7.8</a>.  Multi-headed Control</span>

   As described earlier, an I2RS agent interacts with multiple I2RS
   clients who are actively controlling the network element.  From an
   architecture and design perspective, the assumption is that by means
   outside of this system, the data to be manipulated within the network
   element is appropriately partitioned so that any given piece of
   information is only being manipulated by a single I2RS client.

   Nonetheless, unexpected interactions happen, and two (or more) I2RS
   clients may attempt to manipulate the same piece of data.  This is
   considered an error case.  This architecture does not attempt to
   determine what the right state of data should be when such a
   collision happens.  Rather, the architecture mandates that there be
   decidable means by which I2RS agents handle the collisions.  The
   mechanism for ensuring predictability is to have a simple priority
   associated with each I2RS client, and the highest priority change
   remains in effect.  In the case of priority ties, the first I2RS
   client whose attribution is associated with the data will keep
   control.

   In order for this approach to multi-headed control to be useful for
   I2RS clients, it is necessary that an I2RS client can register to
   receive notifications about changes made to writeable data, whose
   state is of specific interest to that I2RS client.  This is included
   in the I2RS event mechanisms.  This also needs to apply to changes
   made by CLI/NETCONF/SNMP within the write scope of the I2RS agent, as
   the same priority mechanism (even if it is "CLI always wins") applies
   there.  The I2RS client may then respond to the situation as it sees
   fit.

<span class="h3"><a class="selflink" id="section-7.9" href="#section-7.9">7.9</a>.  Transactions</span>

   In the interest of simplicity, the I2RS architecture does not include
   multi-message atomicity and rollback mechanisms.  Rather, it includes
   a small range of error handling for a set of operations included in a
   single message.  An I2RS client may indicate one of the following
   three methods of error handling for a given message with multiple
   operations that it sends to an I2RS agent:

   Perform all or none:  This traditional SNMP semantic indicates that
      the I2RS agent will keep enough state when handling a single
      message to roll back the operations within that message.  Either
      all the operations will succeed, or none of them will be applied,
      and an error message will report the single failure that caused
      them not to be applied.  This is useful when there are, for
      example, mutual dependencies across operations in the message.




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   Perform until error:  In this case, the operations in the message are
      applied in the specified order.  When an error occurs, no further
      operations are applied, and an error is returned indicating the
      failure.  This is useful if there are dependencies among the
      operations and they can be topologically sorted.

   Perform all storing errors:  In this case, the I2RS agent will
      attempt to perform all the operations in the message and will
      return error indications for each one that fails.  This is useful
      when there is no dependency across the operation or when the I2RS
      client would prefer to sort out the effect of errors on its own.

   In the interest of robustness and clarity of protocol state, the
   protocol will include an explicit reply to modification or write
   operations even when they fully succeed.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Operational and Manageability Considerations</span>

   In order to facilitate troubleshooting of routing elements
   implementing I2RS agents, the routing elements should provide for a
   mechanism to show actively provisioned I2RS state and other I2RS
   agent internal information.  Note that this information may contain
   highly sensitive material subject to the security considerations of
   any data models implemented by that agent and thus must be protected
   according to those considerations.  Preferably, this mechanism should
   use a different privileged means other than simply connecting as an
   I2RS client to learn the data.  Using a different mechanism should
   improve traceability and failure management.

   Manageability plays a key aspect in I2RS.  Some initial examples
   include:

   Resource Limitations:   Using I2RS, applications can consume
      resources, whether those be operations in a time frame, entries in
      the RIB, stored operations to be triggered, etc.  The ability to
      set resource limits based upon authorization is important.

   Configuration Interactions:   The interaction of state installed via
      I2RS and via a router's configuration needs to be clearly defined.
      As described in this architecture, a simple priority that is
      configured is used to provide sufficient policy flexibility.

   Traceability of Interactions:   The ability to trace the interactions
      of the requests received by the I2RS agent's and actions taken by
      the I2RS agents is needed so that operations can monitor I2RS
      agents during deployment, and troubleshoot software or network
      problems.




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   Notification Subscription Service:  The ability for an I2RS client to
      subscribe to a notification stream pushed from the I2RS agent
      (rather than having I2RS client poll the I2RS agent) provides a
      more scalable notification handling for the I2RS agent-client
      interactions.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC7920">RFC7920</a>]  Atlas, A., Ed., Nadeau, T., Ed., and D. Ward, "Problem
              Statement for the Interface to the Routing System",
              <a href="./rfc7920">RFC 7920</a>, DOI 10.17487/RFC7920, June 2016,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7920">http://www.rfc-editor.org/info/rfc7920</a>&gt;.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Informative References</span>

   [<a id="ref-I2RS-ENV-SEC">I2RS-ENV-SEC</a>]
              Migault, D., Ed., Halpern, J., and S. Hares, "I2RS
              Environment Security Requirements", Work in Progress,
              <a href="./draft-ietf-i2rs-security-environment-reqs-01">draft-ietf-i2rs-security-environment-reqs-01</a>, April 2016.

   [<a id="ref-I2RS-PROT-SEC">I2RS-PROT-SEC</a>]
              Hares, S., Migault, D., and J. Halpern, "I2RS Security
              Related Requirements", Work in Progress, <a href="./draft-ietf-i2rs-protocol-security-requirements-06">draft-ietf-i2rs-</a>
              <a href="./draft-ietf-i2rs-protocol-security-requirements-06">protocol-security-requirements-06</a>, May 2016.

   [<a id="ref-RESTCONF">RESTCONF</a>] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", Work in Progress, <a href="./draft-ietf-netconf-restconf-14">draft-ietf-netconf-</a>
              <a href="./draft-ietf-netconf-restconf-14">restconf-14</a>, June 2016.

   [<a id="ref-RFC6241">RFC6241</a>]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", <a href="./rfc6241">RFC 6241</a>, DOI 10.17487/RFC6241, June 2011,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6241">http://www.rfc-editor.org/info/rfc6241</a>&gt;.

   [<a id="ref-RFC6536">RFC6536</a>]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", <a href="./rfc6536">RFC 6536</a>,
              DOI 10.17487/RFC6536, March 2012,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6536">http://www.rfc-editor.org/info/rfc6536</a>&gt;.






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   [<a id="ref-RFC6991">RFC6991</a>]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              <a href="./rfc6991">RFC 6991</a>, DOI 10.17487/RFC6991, July 2013,
              &lt;<a href="http://www.rfc-editor.org/info/rfc6991">http://www.rfc-editor.org/info/rfc6991</a>&gt;.

   [<a id="ref-RFC7223">RFC7223</a>]  Bjorklund, M., "A YANG Data Model for Interface
              Management", <a href="./rfc7223">RFC 7223</a>, DOI 10.17487/RFC7223, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7223">http://www.rfc-editor.org/info/rfc7223</a>&gt;.

   [<a id="ref-RFC7224">RFC7224</a>]  Bjorklund, M., "IANA Interface Type YANG Module",
              <a href="./rfc7224">RFC 7224</a>, DOI 10.17487/RFC7224, May 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7224">http://www.rfc-editor.org/info/rfc7224</a>&gt;.

   [<a id="ref-RFC7277">RFC7277</a>]  Bjorklund, M., "A YANG Data Model for IP Management",
              <a href="./rfc7277">RFC 7277</a>, DOI 10.17487/RFC7277, June 2014,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7277">http://www.rfc-editor.org/info/rfc7277</a>&gt;.

   [<a id="ref-RFC7317">RFC7317</a>]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", <a href="./rfc7317">RFC 7317</a>, DOI 10.17487/RFC7317, August
              2014, &lt;<a href="http://www.rfc-editor.org/info/rfc7317">http://www.rfc-editor.org/info/rfc7317</a>&gt;.

   [<a id="ref-RFC7752">RFC7752</a>]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", <a href="./rfc7752">RFC 7752</a>,
              DOI 10.17487/RFC7752, March 2016,
              &lt;<a href="http://www.rfc-editor.org/info/rfc7752">http://www.rfc-editor.org/info/rfc7752</a>&gt;.

   [<a id="ref-YANG1.1">YANG1.1</a>]  Bjorklund, M., Ed., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22The+YANG+1.1+Data+Modeling+Language%22'>"The YANG 1.1 Data Modeling Language"</a>,
              Work in Progress, <a href="./draft-ietf-netmod-rfc6020bis-14">draft-ietf-netmod-rfc6020bis-14</a>, June
              2016.

Acknowledgements

   Significant portions of this draft came from "Interface to the
   Routing System Framework" (February 2013) and "A Policy Framework for
   the Interface to the Routing System" (February 2013).

   The authors would like to thank Nitin Bahadur, Shane Amante, Ed
   Crabbe, Ken Gray, Carlos Pignataro, Wes George, Ron Bonica, Joe
   Clarke, Juergen Schoenwalder, Jeff Haas, Jamal Hadi Salim, Scott
   Brim, Thomas Narten, Dean Bogdanovic, Tom Petch, Robert Raszuk,
   Sriganesh Kini, John Mattsson, Nancy Cam-Winget, DaCheng Zhang, Qin
   Wu, Ahmed Abro, Salman Asadullah, Eric Yu, Deborah Brungard, Russ
   Housley, Russ White, Charlie Kaufman, Benoit Claise, Spencer Dawkins,
   and Stephen Farrell for their suggestions and review.







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Authors' Addresses

   Alia Atlas
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States

   Email: akatlas@juniper.net


   Joel Halpern
   Ericsson

   Email: Joel.Halpern@ericsson.com


   Susan Hares
   Huawei
   7453 Hickory Hill
   Saline, MI  48176
   United States

   Phone: +1 734-604-0332
   Email: shares@ndzh.com


   Dave Ward
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   United States

   Email: wardd@cisco.com


   Thomas D. Nadeau
   Brocade

   Email: tnadeau@lucidvision.com











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