Internet Engineering Task Force (IETF)                          A. AtlasRequest for Comments: 7921                              Juniper NetworksCategory: Informational                                       J. HalpernISSN: 2070-1721                                                 Ericsson                                                                S. Hares                                                                  Huawei                                                                 D. Ward                                                           Cisco Systems                                                               T. Nadeau                                                                 Brocade                                                               June 2016        An Architecture for the Interface to the Routing SystemAbstract   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 Section 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained at   http://www.rfc-editor.org/info/rfc7921.Atlas, et al.                 Informational                     [Page 1]RFC 7921                    I2RS Architecture                  June 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Atlas, et al.                 Informational                     [Page 2]RFC 7921                    I2RS Architecture                  June 2016Table of Contents   1. Introduction ....................................................4      1.1. Drivers for the I2RS Architecture ..........................5      1.2. Architectural Overview .....................................6   2. Terminology ....................................................11   3. Key Architectural Properties ...................................13      3.1. Simplicity ................................................13      3.2. Extensibility .............................................14      3.3. Model-Driven Programmatic Interfaces ......................14   4. Security Considerations ........................................15      4.1. Identity and Authentication ...............................17      4.2. Authorization .............................................18      4.3. Client Redundancy .........................................19      4.4. I2RS in Personal Devices ..................................19   5. Network Applications and I2RS Client ...........................19      5.1. Example Network Application: Topology Manager .............20   6. I2RS Agent Role and Functionality ..............................20      6.1. Relationship to Its Routing Element .......................20      6.2. I2RS State Storage ........................................21           6.2.1. I2RS Agent Failure .................................21           6.2.2. Starting and Ending ................................22           6.2.3. Reversion ..........................................23      6.3. Interactions with Local Configuration .....................23           6.3.1. Examples of Local Configuration vs. I2RS                  Ephemeral Configuration ............................24      6.4. Routing Components and Associated I2RS Services ...........26           6.4.1. Routing and Label Information Bases ................28           6.4.2. IGPs, BGP, and Multicast Protocols .................28           6.4.3. MPLS ...............................................29           6.4.4. Policy and QoS Mechanisms ..........................29           6.4.5. Information Modeling, Device Variation, and                  Information Relationships ..........................29                  6.4.5.1. Managing Variation: Object                           Classes/Types and Inheritance .............29                  6.4.5.2. Managing Variation: Optionality ...........30                  6.4.5.3. Managing Variation: Templating ............31                  6.4.5.4. Object Relationships ......................31                           6.4.5.4.1. Initialization .................31                           6.4.5.4.2. Correlation Identification .....32                           6.4.5.4.3. Object References ..............32                           6.4.5.4.4. Active References ..............32   7. I2RS Client Agent Interface ....................................32      7.1. One Control and Data Exchange Protocol ....................32      7.2. Communication Channels ....................................33      7.3. Capability Negotiation ....................................33      7.4. Scope Policy Specifications ...............................34      7.5. Connectivity ..............................................34Atlas, et al.                 Informational                     [Page 3]RFC 7921                    I2RS Architecture                  June 2016      7.6. Notifications .............................................35      7.7. Information Collection ....................................35      7.8. Multi-headed Control ......................................36      7.9. Transactions ..............................................36   8. Operational and Manageability Considerations ...................37   9. References .....................................................38      9.1. Normative References ......................................38      9.2. Informative References ....................................38   Acknowledgements ..................................................39   Authors' Addresses ................................................401.  Introduction   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.Atlas, et al.                 Informational                     [Page 4]RFC 7921                    I2RS Architecture                  June 2016   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.  Section 4 provides an   overview of the security considerations for the I2RS architecture.   The detailed requirements for I2RS protocol security are contained in   [I2RS-PROT-SEC], and the detailed security requirements for   environment in which the I2RS protocol exists are contained in   [I2RS-ENV-SEC].1.1.  Drivers for the I2RS Architecture   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 Section 5 of [RFC7920].   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/orAtlas, et al.                 Informational                     [Page 5]RFC 7921                    I2RS Architecture                  June 2016   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.1.2.  Architectural Overview   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 [RFC7920], 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.Atlas, et al.                 Informational                     [Page 6]RFC 7921                    I2RS Architecture                  June 2016   The I2RS agent provides read and write access to selected data on the   routing element that are organized into I2RS services.  Section 4   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 7.6 and 7.7 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.Atlas, et al.                 Informational                     [Page 7]RFC 7921                    I2RS Architecture                  June 2016        ******************   *****************  *****************        *  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 AgentsAtlas, et al.                 Informational                     [Page 8]RFC 7921                    I2RS Architecture                  June 2016   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.Atlas, et al.                 Informational                     [Page 9]RFC 7921                    I2RS Architecture                  June 2016   I2RS agent:  See the definition in Section 2.   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 Section 2.   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 Section 7.8 ("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 Section 7.8.  The   same policy mechanism (simple priority per I2RS client) applies to   interactions between the I2RS agent and the CLI/SNMP/NETCONF as   described in Section 6.3.   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.Atlas, et al.                 Informational                    [Page 10]RFC 7921                    I2RS Architecture                  June 20162.  Terminology   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.Atlas, et al.                 Informational                    [Page 11]RFC 7921                    I2RS Architecture                  June 2016   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.Atlas, et al.                 Informational                    [Page 12]RFC 7921                    I2RS Architecture                  June 2016   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 [RFC6536] 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 [RFC2119].3.  Key Architectural Properties   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 [RFC7920] and because they may vary based on the   particular use cases.3.1.  Simplicity   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.Atlas, et al.                 Informational                    [Page 13]RFC 7921                    I2RS Architecture                  June 2016   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?"3.2.  Extensibility   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.3.3.  Model-Driven Programmatic Interfaces   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 andAtlas, et al.                 Informational                    [Page 14]RFC 7921                    I2RS Architecture                  June 2016   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.4.  Security Considerations   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 [RFC6241] and RESTCONF   [RESTCONF].  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 forAtlas, et al.                 Informational                    [Page 15]RFC 7921                    I2RS Architecture                  June 2016   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 [I2RS-PROT-SEC], and the global I2RS security   environment requirements are contained [I2RS-ENV-SEC].   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   Section 4.1) 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 Section 4.2).  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 forAtlas, et al.                 Informational                    [Page 16]RFC 7921                    I2RS Architecture                  June 2016   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 Section 7.2, 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 Section 7.2).  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.4.1.  Identity and Authentication   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.Atlas, et al.                 Informational                    [Page 17]RFC 7921                    I2RS Architecture                  June 20164.2.  Authorization   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) [RFC6536]; it   allows arbitrary access to be specified for a data node instance   identifier while defining meaningful manipulable defaults.  The   identity within NACM [RFC6536] 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.Atlas, et al.                 Informational                    [Page 18]RFC 7921                    I2RS Architecture                  June 20164.3.  Client Redundancy   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.4.4.  I2RS in Personal Devices   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.5.  Network Applications and I2RS Client   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 Section 4.2 do   apply.  As discussed in Section 4.1, 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.Atlas, et al.                 Informational                    [Page 19]RFC 7921                    I2RS Architecture                  June 2016   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.5.1.  Example Network Application: Topology Manager   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 [RFC7752], 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.6.  I2RS Agent Role and Functionality   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.6.1.  Relationship to Its Routing Element   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 agentAtlas, et al.                 Informational                    [Page 20]RFC 7921                    I2RS Architecture                  June 2016   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.6.2.  I2RS State Storage   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.6.2.1.  I2RS Agent Failure   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.Atlas, et al.                 Informational                    [Page 21]RFC 7921                    I2RS Architecture                  June 2016   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.6.2.2.  Starting and Ending   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.Atlas, et al.                 Informational                    [Page 22]RFC 7921                    I2RS Architecture                  June 20166.2.3.  Reversion   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 1.2 and 7.8)   An I2RS client may register for notifications, subject to its   notification scope, regarding state modification or removal by a   particular I2RS client.6.3.  Interactions with Local Configuration   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.Atlas, et al.                 Informational                    [Page 23]RFC 7921                    I2RS Architecture                  June 2016   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.6.3.1.  Examples of Local Configuration vs. I2RS Ephemeral Configuration   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.Atlas, et al.                 Informational                    [Page 24]RFC 7921                    I2RS Architecture                  June 2016   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.Atlas, et al.                 Informational                    [Page 25]RFC 7921                    I2RS Architecture                  June 2016   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.6.4.  Routing Components and Associated I2RS Services   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.Atlas, et al.                 Informational                    [Page 26]RFC 7921                    I2RS Architecture                  June 2016   The initial services included in the I2RS architecture are as   follows.    ***************************     **************    *****************    *      I2RS Protocol      *     *            *    *    Dynamic    *    *                         *     * Interfaces *    *    Data &     *    *  +--------+  +-------+  *     *            *    *  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 & 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).   Section 6.4.5 discusses information modeling constructs and the range   of relationship types that are applicable.Atlas, et al.                 Informational                    [Page 27]RFC 7921                    I2RS Architecture                  June 20166.4.1.  Routing and Label Information Bases   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.6.4.2.  IGPs, BGP, and Multicast Protocols   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.Atlas, et al.                 Informational                    [Page 28]RFC 7921                    I2RS Architecture                  June 20166.4.3.  MPLS   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.6.4.4.  Policy and QoS Mechanisms   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.6.4.5.  Information Modeling, Device Variation, and Information        Relationships   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.6.4.5.1.  Managing Variation: Object Classes/Types and Inheritance   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.Atlas, et al.                 Informational                    [Page 29]RFC 7921                    I2RS Architecture                  June 2016   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.6.4.5.2.  Managing Variation: Optionality   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.Atlas, et al.                 Informational                    [Page 30]RFC 7921                    I2RS Architecture                  June 20166.4.5.3.  Managing Variation: Templating   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.6.4.5.4.  Object Relationships   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.6.4.5.4.1.  Initialization   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.Atlas, et al.                 Informational                    [Page 31]RFC 7921                    I2RS Architecture                  June 20166.4.5.4.2.  Correlation Identification   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.6.4.5.4.3.  Object References   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.6.4.5.4.4.  Active References   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.7.  I2RS Client Agent Interface7.1.  One Control and Data Exchange Protocol   This I2RS architecture assumes a data-model-driven protocol where the   data models are defined in YANG 1.1 [YANG1.1] and associated YANG   based model documents [RFC6991], [RFC7223], [RFC7224], [RFC7277],   [RFC7317].  Two of the protocols to be expanded to support the I2RS   protocol are NETCONF [RFC6241] and RESTCONF [RESTCONF].  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 ofAtlas, et al.                 Informational                    [Page 32]RFC 7921                    I2RS Architecture                  June 2016   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.7.2.  Communication Channels   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.7.3.  Capability Negotiation   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 Section 6.4), 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 andAtlas, et al.                 Informational                    [Page 33]RFC 7921                    I2RS Architecture                  June 2016   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.7.4.  Scope Policy Specifications   As Sections 4.1 and 4.2 describe, each I2RS client will have a unique   identity and may have a secondary identity (see Section 2) to aid in   troubleshooting.  As Section 4 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.7.5.  Connectivity   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'sAtlas, et al.                 Informational                    [Page 34]RFC 7921                    I2RS Architecture                  June 2016   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.7.6.  Notifications   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.7.7.  Information Collection   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.Atlas, et al.                 Informational                    [Page 35]RFC 7921                    I2RS Architecture                  June 20167.8.  Multi-headed Control   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.7.9.  Transactions   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.Atlas, et al.                 Informational                    [Page 36]RFC 7921                    I2RS Architecture                  June 2016   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.8.  Operational and Manageability Considerations   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.Atlas, et al.                 Informational                    [Page 37]RFC 7921                    I2RS Architecture                  June 2016   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.9.  References9.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC7920]  Atlas, A., Ed., Nadeau, T., Ed., and D. Ward, "Problem              Statement for the Interface to the Routing System",              RFC 7920, DOI 10.17487/RFC7920, June 2016,              <http://www.rfc-editor.org/info/rfc7920>.9.2.  Informative References   [I2RS-ENV-SEC]              Migault, D., Ed., Halpern, J., and S. Hares, "I2RS              Environment Security Requirements", Work in Progress,              draft-ietf-i2rs-security-environment-reqs-01, April 2016.   [I2RS-PROT-SEC]              Hares, S., Migault, D., and J. Halpern, "I2RS Security              Related Requirements", Work in Progress, draft-ietf-i2rs-              protocol-security-requirements-06, May 2016.   [RESTCONF] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF              Protocol", Work in Progress, draft-ietf-netconf-              restconf-14, June 2016.   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,              and A. Bierman, Ed., "Network Configuration Protocol              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,              <http://www.rfc-editor.org/info/rfc6241>.   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration              Protocol (NETCONF) Access Control Model", RFC 6536,              DOI 10.17487/RFC6536, March 2012,              <http://www.rfc-editor.org/info/rfc6536>.Atlas, et al.                 Informational                    [Page 38]RFC 7921                    I2RS Architecture                  June 2016   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",              RFC 6991, DOI 10.17487/RFC6991, July 2013,              <http://www.rfc-editor.org/info/rfc6991>.   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,              <http://www.rfc-editor.org/info/rfc7223>.   [RFC7224]  Bjorklund, M., "IANA Interface Type YANG Module",              RFC 7224, DOI 10.17487/RFC7224, May 2014,              <http://www.rfc-editor.org/info/rfc7224>.   [RFC7277]  Bjorklund, M., "A YANG Data Model for IP Management",              RFC 7277, DOI 10.17487/RFC7277, June 2014,              <http://www.rfc-editor.org/info/rfc7277>.   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for              System Management", RFC 7317, DOI 10.17487/RFC7317, August              2014, <http://www.rfc-editor.org/info/rfc7317>.   [RFC7752]  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", RFC 7752,              DOI 10.17487/RFC7752, March 2016,              <http://www.rfc-editor.org/info/rfc7752>.   [YANG1.1]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",              Work in Progress, draft-ietf-netmod-rfc6020bis-14, 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.Atlas, et al.                 Informational                    [Page 39]RFC 7921                    I2RS Architecture                  June 2016Authors' 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.comAtlas, et al.                 Informational                    [Page 40]