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rfc:rfc7923

Internet Engineering Task Force (IETF) E. Voit Request for Comments: 7923 A. Clemm Category: Informational A. Gonzalez Prieto ISSN: 2070-1721 Cisco Systems

                                                             June 2016
          Requirements for Subscription to YANG Datastores

Abstract

 This document provides requirements for a service that allows client
 applications to subscribe to updates of a YANG datastore.  Based on
 criteria negotiated as part of a subscription, updates will be pushed
 to targeted recipients.  Such a capability eliminates the need for
 periodic polling of YANG datastores by applications and fills a
 functional gap in existing YANG transports (i.e., Network
 Configuration Protocol (NETCONF) and RESTCONF).  Such a service can
 be summarized as a "pub/sub" service for YANG datastore updates.
 Beyond a set of basic requirements for the service, various
 refinements are addressed.  These refinements include: periodicity of
 object updates, filtering out of objects underneath a requested a
 subtree, and delivery QoS guarantees.

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/rfc7923.

Voit, et al. Informational [Page 1] RFC 7923 YANG Subscription Requirements June 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
 2. Business Drivers ................................................3
    2.1. Pub/Sub in the Interface to the Routing System (I2RS) ......4
    2.2. Pub/Sub Variants on Network Elements .......................5
    2.3. Existing Generalized Pub/Sub Implementations ...............6
 3. Terminology .....................................................6
 4. Requirements ....................................................7
    4.1. Assumptions for Subscriber Behavior ........................7
    4.2. Subscription Service Requirements ..........................8
         4.2.1. General .............................................8
         4.2.2. Negotiation .........................................9
         4.2.3. Update Distribution ................................10
         4.2.4. Transport ..........................................11
         4.2.5. Security Requirements ..............................11
         4.2.6. Subscription QoS ...................................13
         4.2.7. Filtering ..........................................14
         4.2.8. Assurance and Monitoring ...........................15
 5. Security Considerations ........................................15
 6. References .....................................................16
    6.1. Normative References ......................................16
    6.2. Informative References ....................................16
 Acknowledgments ...................................................17
 Authors' Addresses ................................................18

Voit, et al. Informational [Page 2] RFC 7923 YANG Subscription Requirements June 2016

1. Introduction

 Applications interacting with YANG datastores require capabilities
 beyond the traditional client-server configuration of network
 elements.  One class of such applications are service-assurance
 applications, which must maintain a continuous view of operational
 data and state.  Another class of applications are security
 applications, which must continuously track changes made upon network
 elements to ensure compliance with corporate policy.
 Periodic fetching of data is not an adequate solution for
 applications requiring frequent or prompt updates of remote object
 state.  Applying polling-based solutions here imposes a load on
 networks, devices, and applications.  Additionally, polling solutions
 are brittle in the face of communication glitches, and have
 limitations in their ability to synchronize and calibrate retrieval
 intervals across a network.  These limitations can be addressed by
 including generic object subscription mechanisms within network
 elements, and allowing these mechanisms to be applied in the context
 of data that is conceptually contained in YANG datastores.
 This document aggregates requirements for such subscription from a
 variety of deployment scenarios.

2. Business Drivers

 For decades, information delivery of current network state has been
 accomplished either by fetching from operations interfaces, or via
 dedicated, customized networking protocols.  With the growth of
 centralized orchestration infrastructures, imperative policy
 distribution, and YANG's ascent as the dominant data modeling
 language for use in programmatic interfaces to network elements, this
 mixture of fetch plus custom networking protocols is no longer
 sufficient.  What is needed is a push mechanism that is able to
 deliver object changes as they happen.
 These push distribution mechanisms will not replace existing
 networking protocols.  Instead they will supplement these protocols,
 providing different response time, peering, scale, and security
 characteristics.
 Push solutions will not displace all existing operations
 infrastructure needs.  And SNMP and MIBs will remain widely deployed
 and the de facto choice for many monitoring solutions.  But some
 functions could be displaced.  Arguably the biggest shortcoming of
 SNMP for those applications concerns the need to rely on periodic
 polling, because it introduces an additional load on the network and
 devices, because it is brittle if polling cycles are missed, and

Voit, et al. Informational [Page 3] RFC 7923 YANG Subscription Requirements June 2016

 because it is hard to synchronize and calibrate across a network.  If
 applications can only use polling type interaction patterns with YANG
 datastores, similar issues can be expected.

2.1. Pub/Sub in the Interface to the Routing System (I2RS)

 Various documents about the Interface to the Routing System (I2RS)
 highlight the need to provide pub/sub capabilities between network
 elements.  From [RFC7921], there are references throughout the
 document beginning in Section 6.2.  Some specific examples include:
 o  Section 7.6 of [RFC7921] provides high-level pub/sub
    (notification) guidance.
 o  Section 6.4.2 of [RFC7921] identifies "subscribing to an
    information stream of route changes" and "receiving notifications
    about peers coming up or going down".
 o  Section 6.3 of [RFC7921] notes that when Local Configuration
    preempts I2RS, external notification might be necessary.
 In addition, [USECASE] has relevant requirements.  A small subset
 includes:
 o  L-Data-REQ-12: The I2RS interface should support user
    subscriptions to data with the following parameters: push of data
    synchronously or asynchronously via registered subscriptions...
 o  L-DATA-REQ-07: The I2RS interface (protocol and instant messages
    (IMs)) should allow a subscriber to select portions of the data
    model.
 o  PI-REQ01: Monitor the available routes installed in the Routing
    Information Base (RIB) of each forwarding device, including near
    real-time notification of route installation and removal.
 o  BGP-REQ10: The I2RS client SHOULD be able to instruct the I2RS
    agent(s) to notify the I2RS client when the BGP processes on an
    associated routing system observe a route change to a specific set
    of IP Prefixes and associated prefixes.... The I2RS agent should
    be able to notify the client via the publish or subscribe
    mechanism.
 o  IGP-REQ-07: The I2RS interface (protocol and IMs) should support a
    mechanism where the I2RS Clients can subscribe to the I2RS Agent's
    notification of critical node IGP events.

Voit, et al. Informational [Page 4] RFC 7923 YANG Subscription Requirements June 2016

 o  MPLS-LDP-REQ-03: The I2RS Agent notifications should allow an I2RS
    client to subscribe to a stream of state changes regarding the LDP
    sessions or LDP Label Switched Paths (LSPs) from the I2RS Agent.
 o  L-Data-REQ-01: I2RS must be able to collect large data sets from
    the network with high frequency and resolution, and with minimal
    impact to the device's CPU and memory.
 Also, Section 7.4.3 of [RFC7922] includes this pub/sub requirement:
 o  I2RS agents MUST support publishing I2RS trace log information to
    that feed as described in [this document].  Subscribers would then
    receive a live stream of I2RS interactions in trace log format and
    could flexibly choose to do a number of things with the log
    messages.

2.2. Pub/Sub Variants on Network Elements

 This document is intended to cover requirements beyond I2RS.  Looking
 at history, there are many examples of switching and routing
 protocols that have done explicit or implicit pub/sub in the past.
 In addition, new policy notification mechanisms that operate on
 switches and routers are being specified now.  A small subset of
 current and past subscription mechanisms includes:
 o  Multicast topology establishment is accomplished before any
    content delivery is made to endpoints (IGMP, PIM, etc.).
 o  Secure Automation and Continuous Monitoring (SACM) allows
    subscription into devices, which may then push spontaneous changes
    in their configured hardware and software [SACMREQ].
 o  In MPLS VPNs [RFC6513], a Customer Edge router exchanges PIM
    control messages before Provider Edge (PE) Routing Adjacencies are
    passed [RFC6513].
 o  After OSPF establishes its adjacencies, Link State Advertisement
    will then commence [RFC2328].
 Worthy of note in the examples above is the wide variety of
 underlying transports.  A generalized pub/sub mechanism, therefore
 should be structured to support alternative transports.  Based on
 current I2RS requirements, NETCONF should be the initially supported
 transport due to the need for connection-oriented/unicast
 communication.  Eventual support for multicast and broadcast
 subscription update distribution will be needed as well.

Voit, et al. Informational [Page 5] RFC 7923 YANG Subscription Requirements June 2016

2.3. Existing Generalized Pub/Sub Implementations

 TIBCO, RSS, Common Object Request Broker Architecture (CORBA), and
 other technologies all show precursor pub/sub technologies.  However,
 there are new needs (described in Section 4 below) that these
 technologies do not serve.  We need a new pub/sub technology.
 There are at least two widely deployed generalized pub/sub
 implementations that come close to current needs: Extensible
 Messaging and Presence Protocol (XMPP) [XEP-0060] and Data
 Distribution Service (DDS) [OMG-DDS].  Both serve as proof-points
 that a highly scalable distributed datastore implementation
 connecting millions of edge devices is possible.
 Because of these proof-points, we can be comfortable that the
 underlying technologies can enable reusable generalized YANG object
 distribution.  Analysis will need to fully dimension the speed and
 scale of such object distribution for various subtree sizes and
 transport types.

3. Terminology

 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].  Although
 this document is not a protocol specification, the use of this
 language clarifies the instructions to protocol designers producing
 solutions that satisfy the requirements set out in this document.
 A Subscriber makes requests for set(s) of YANG object data.
 A Publisher is responsible for distributing subscribed YANG object
 data per the terms of a subscription.  In general, a Publisher is the
 owner of the YANG datastore that is subjected to the subscription.
 A Receiver is the target to which a Publisher pushes updates.  In
 general, the Receiver and Subscriber will be the same entity.  A
 Subscription Service provides subscriptions to Subscribers of YANG
 data.
 A Subscription Service interacts with the Publisher of the YANG data
 as needed to provide the data per the terms of the subscription.
 A subscription request for one or more YANG subtrees (including
 single leafs) is made by the Subscriber of a Publisher and is
 targeted to a Receiver.  A subscription may include constraints that
 dictate how often or under what conditions YANG information updates
 might be sent.

Voit, et al. Informational [Page 6] RFC 7923 YANG Subscription Requirements June 2016

 A subscription is a contract between a Subscription Service and a
 Subscriber that stipulates the data to be pushed and the associated
 terms.
 A datastore is defined in [RFC6241].
 An Update provides object changes that have occurred within
 subscribed YANG subtree(s).  An Update must include the current
 status of (data) node instances for which filtering has indicated
 they have different status than previously provided.  An Update may
 include a bundled set of ordered/sequential changes for a given
 object that have been made since the last update.
 A Filter contains evaluation criteria, which are evaluated against
 YANG object(s) within a subscription.  There are two types of
 Filters: Subtree Filters, which identify selected objects/nodes
 published under a target data node, and object element and attribute
 Filters where an object should only be published if it has properties
 meeting specified Filter criteria.

4. Requirements

 Many of the requirements within this section have been adapted from
 the XMPP [XEP-0060] and DDS [OMG-DDS] requirements specifications.

4.1. Assumptions for Subscriber Behavior

 This document provides requirements for the Subscription Service.  It
 does not define all the requirements for the Subscriber/Receiver.
 However in order to frame the desired behavior of the Subscription
 Service, it is important to specify key input constraints.
 A Subscriber SHOULD avoid attempting to establish multiple
 subscriptions pertaining to the same information, i.e., referring to
 the same datastore YANG subtrees.
 A Subscriber MAY provide subscription QoS criteria to the
 Subscription Service; if the Subscription Service is unable to meet
 those criteria, the subscription SHOULD NOT be established.
 When a Subscriber and Receiver are the same entity and the transport
 session is lost/terminated, the Subscriber MUST re-establish any
 subscriptions it previously created via signaling over the transport
 session.  That is, there is no requirement for the life span of such
 signaled subscriptions to extend beyond the life span of the
 transport session.

Voit, et al. Informational [Page 7] RFC 7923 YANG Subscription Requirements June 2016

 A Subscriber MUST be able to infer when a Subscription Service is no
 longer active and when no more updates are being sent.
 A Subscriber MAY check with a Subscription Service to validate the
 existence and monitored subtrees of a subscription.
 A Subscriber MUST be able to periodically lease and extend the lease
 of a subscription from a Subscription Service.

4.2. Subscription Service Requirements

4.2.1. General

 A Subscription Service MUST support the ability to create, renew,
 time out, and terminate a subscription.
 A Subscription Service MUST be able to support and independently
 track multiple subscription requests by the same Subscriber.
 A Subscription Service MUST be able to support an add/change/delete
 of subscriptions to multiple YANG subtrees as part of the same
 subscription request.
 A Subscription Service MUST support subscriptions against operational
 datastores, configuration datastores, or both.
 A Subscription Service MUST be able support filtering so that the
 subscribed updates under a target node might publish only operational
 data, only configuration data, or both.
 A subscription MAY include Filters as defined within a subscription
 request, therefore the Subscription Service MUST publish only data
 nodes that meet the Filter criteria within a subscription.
 A Subscription Service MUST support the ability to subscribe to
 periodic updates.  The subscription period MUST be configurable as
 part of the subscription request.
 A Subscription Service SHOULD support the ability to subscribe to
 updates on-change, i.e., whenever values of subscribed data objects
 change.
 For on-change updates, the Subscription Service MUST support a
 dampening period that needs to be passed before the first or
 subsequent on-change updates are sent.  The dampening period SHOULD
 be configurable as part of the subscription request.

Voit, et al. Informational [Page 8] RFC 7923 YANG Subscription Requirements June 2016

 A Subscription Service MUST allow subscriptions to be monitored.
 Specifically, a Subscription Service MUST at a minimum maintain
 information about which subscriptions are being serviced, the terms
 of those subscriptions (e.g., what data is being subscribed,
 associated Filters, update policy -- on change, periodic), and the
 overall status of the subscription -- e.g., active or suspended.
 A Subscription Service MUST support the termination of a subscription
 when requested by the Subscriber.
 A Subscription Service SHOULD support the ability to suspend and to
 resume a subscription on request of a client.
 A Subscription Service MAY at its discretion revoke or suspend an
 existing subscription.  Reasons may include transitory resource
 limitation, credential expiry, failure to reconfirm a subscription,
 loss of connectivity with the Receiver, operator command-line
 interface (CLI), and/or others.  When this occurs, the Subscription
 Service MUST notify the Subscriber and update the subscription
 status.
 A Subscription Service MAY offer the ability to modify a subscription
 Filter.  If such an ability is offered, the service MUST provide
 subscribers with an indication telling at what point the modified
 subscription goes into effect.

4.2.2. Negotiation

 A Subscription Service MUST be able to negotiate the following terms
 of a subscription:
 o  The policy, i.e., whether updates are on-change or periodic
 o  The interval, for periodic publication policy
 o  The on-change policy dampening period (if the on-change policy is
    supported)
 o  Any Filters associated with a subtree subscription
 A Subscription Service SHOULD be able to negotiate QoS criteria for a
 subscription.  Examples of subscription QoS criteria may include
 reliability of the Subscription Service, reaction time between a
 monitored YANG subtree/object change and a corresponding notification
 push, and the Subscription Service's ability to support certain
 levels of object liveliness.

Voit, et al. Informational [Page 9] RFC 7923 YANG Subscription Requirements June 2016

 In cases where a subscription request cannot be fulfilled due to
 insufficient platform resources, the Subscription Service SHOULD
 include within its decline hints on criteria that would have been
 acceptable when the subscription request was made.  For example, if
 periodic updates were requested with update intervals that were too
 short for the specified data set, an alternative acceptable interval
 period might be returned from the Publisher.  If on-change updates
 were requested with too aggressive a dampening period, then an
 acceptable dampening period may be returned, or alternatively an
 indication that only periodic updates are supported for the requested
 object(s).

4.2.3. Update Distribution

 For on-change updates, the Subscription Service MUST only send deltas
 to the object data for which a change occurred.  (Otherwise the
 subscriber might not know what has actually undergone change.)  The
 updates for each object MUST include an indication of whether it was
 removed, added, or changed.
 When a Subscription Service is not able to send updates per its
 subscription contract, the subscription MUST notify subscribers and
 put the subscription into a state indicating that the subscription
 was suspended by the service.  When able to resume service,
 subscribers need to be notified as well.  If unable to resume
 service, the Subscription Service MAY terminate the subscription and
 notify Subscribers accordingly.
 When a subscription with on-change updates is suspended and then
 resumed, the first update SHOULD include updates of any changes that
 occurred while the subscription was suspended, with the current
 value.  The Subscription Service MUST provide a clear indication when
 this capability is not supported (because in this case, a client
 application may have to synchronize state separately).
 Multiple objects being pushed to a Subscriber, perhaps from different
 subscriptions, SHOULD be bundled together into a single Update.
 The sending of an Update MUST NOT be delayed beyond the Push Latency
 of any enclosed object changes.
 The sending of an Update MUST NOT be delayed beyond the dampening
 period of any enclosed object changes.
 The sending of an Update MUST NOT occur before the dampening period
 expires for any enclosed object changes.

Voit, et al. Informational [Page 10] RFC 7923 YANG Subscription Requirements June 2016

 A Subscription Service MAY, as an option, support a replay capability
 so that a set of updates generated during a previous time internal
 can be sent to a Receiver.

4.2.4. Transport

 It is possible for updates coming from a Subscription Service to be
 pushed over different types of transports such as NETCONF, RESTCONF,
 and HTTP.  Beyond existing transports, this Subscription Service will
 be applicable for emerging protocols such as those being defined in
 [USECASE].  The need for such transport flexibility drives the
 following requirements:
 o  A Subscription Service SHOULD support different transports.
 o  A Subscription Service SHOULD support different encodings of a
    payload.
 o  It MUST be possible for Receivers to associate the update with a
    specific subscription.
 o  In the case of connection-oriented transport, when a transport
    connection drops, the associated subscription SHOULD be
    terminated.  It is up the Subscriber to request a new
    subscription.

4.2.5. Security Requirements

 Some uses of this Subscription Service will push privacy-sensitive
 updates and metadata.  For privacy-sensitive deployments,
 subscription information MUST be bound within secure, encrypted
 transport-layer mechanisms.  For example, if NETCONF is used as
 transport, then [RFC7589] would be a valid option to secure the
 transported information.  The Subscription Service can also be used
 with emerging privacy-sensitive deployment contexts as well.  As an
 example, deployments based on [USECASE] would apply these
 requirements in conjunction with those documented within
 [I2RS-ENV-SEC] and [I2RS-PROT-SEC] to secure ephemeral state
 information being pushed from a network element.
 As part of the subscription establishment, mutual authentication MUST
 be used between the Subscriber and the Subscription Service.
 Subscribers MUST NOT be able to pose as the original Subscription
 Service.

Voit, et al. Informational [Page 11] RFC 7923 YANG Subscription Requirements June 2016

 Versioning of any subscription protocols MUST be supported so that
 the capabilities and behaviors expected of specific technology
 implementations can be exposed.
 A subscription could be used to attempt to retrieve information to
 which a client has no authorized access.  Therefore, it is important
 that data being pushed based on subscriptions is authorized in the
 same way that regular data retrieval operations are authorized.  Data
 being pushed to a client MUST be filtered accordingly, just like if
 the data were being retrieved on demand.  For Unicast transports, the
 NETCONF Authorization Control Model applies.
 Additions or changes within a subscribed subtree structure MUST be
 validated against authorization methods before subscription updates,
 including new subtree information, are pushed.
 A loss of authenticated access to the target subtree or node SHOULD
 be communicated to the Subscriber.
 For any encrypted information exchanges, commensurate strength
 security mechanisms MUST be available and SHOULD be used.  This
 includes all stages of the subscription and update push process.
 Subscription requests, including requests to create, terminate,
 suspend, and resume subscriptions MUST be properly authorized.
 When the Subscriber and Receiver are different, the Receiver MUST be
 able to terminate any subscription to it where objects are being
 delivered over a Unicast transport.
 A Subscription Service SHOULD decline a subscription request if it is
 likely to deplete its resources.  It is preferable to decline a
 subscription when originally requested, rather than having to
 terminate it prematurely later.
 When the Subscriber and Receiver are different, and when the
 underlying transport connection passes credentials as part of
 transport establishment, then potentially pushed objects MUST be
 excluded from a push update if that object doesn't have read access
 visibility for that Receiver.

Voit, et al. Informational [Page 12] RFC 7923 YANG Subscription Requirements June 2016

4.2.6. Subscription QoS

 A Subscription Service SHOULD be able to negotiate the following
 subscription QoS parameters with a Subscriber: Dampening,
 Reliability, Deadline, and Bundling.
 A Subscription Service SHOULD be able to interpret subscription QoS
 parameters, and only establish a subscription if it is possible to
 meet the QoS needs of the provided QoS parameters.

4.2.6.1. Liveliness

 A Subscription Service MUST be able to respond to requests to verify
 the Liveliness of a subscription.
 A Subscription Service MUST be able to report the currently monitored
 Nodes of a subscription.

4.2.6.2. Dampening

 A Subscription Service MUST be able to negotiate the minimum time
 separation since the previous update before transmitting a subsequent
 update for subscription.  (Note: this is intended to confine the
 visibility of volatility into something digestible by the receiver.)

4.2.6.3. Reliability

 A Subscription Service MAY send Updates over Best Effort and Reliable
 transports.

4.2.6.4. Coherence

 For a particular subscription, every update to a subscribed object
 MUST be sent to the Receiver in sequential order.

4.2.6.5. Presentation

 The Subscription Service MAY have the ability to bundle a set of
 discrete object notifications into a single publishable update for a
 subscription.  A bundle MAY include information on different Data
 Nodes and/or multiple updates about a single Data Node.
 For any bundled updates, the Subscription Service MUST provide
 information for a Receiver to reconstruct the order and timing of
 updates.

Voit, et al. Informational [Page 13] RFC 7923 YANG Subscription Requirements June 2016

4.2.6.6. Deadline

 The Subscription Service MUST be able to push updates at a regular
 cadence that corresponds with the Subscriber's specified start and
 end timestamps.  (Note: the regular cadence can drive one update, a
 discrete quantity of updates, or an unbounded set of periodic
 updates.)

4.2.6.7. Push Latency

 The Subscription Service SHOULD be able to delay Updates on object
 push for a configurable period per Subscriber.
 It MUST be possible for an administrative entity to determine the
 Push latency between object change in a monitored subtree and the
 Subscription Service Push of the update transmission.

4.2.6.8. Relative Priority

 The Subscription Service SHOULD support the relative prioritization
 of subscriptions so that the dequeuing and discarding of push updates
 can consider this if there is insufficient bandwidth between the
 Publisher and the Receiver.

4.2.7. Filtering

 If no filtering criteria are provided, or if filtering criteria are
 met, updates for a subscribed object MUST be pushed, subject to the
 QoS limits established for the subscription.
 It MUST be possible for the Subscription Service to receive Filter(s)
 from a Subscriber and apply them to the corresponding object(s)
 within a subscription.
 It MUST be possible to attach one or more Subtree and/or object
 element and attribute Filters to a subscription.  Mandatory Filter
 types include:
 o  For character-based object properties, Filter values that are
    exactly equal to a provided string, not equal to the string, or
    containing a string.
 o  For numeric object properties, Filter values that are =, !=, <,
    <=, >, or >= a provided number.
 It SHOULD be possible for Filtering criteria to evaluate more than
 one property of a particular subscribed object as well as apply
 multiple Filters against a single object.

Voit, et al. Informational [Page 14] RFC 7923 YANG Subscription Requirements June 2016

 It SHOULD be possible to establish query match criteria on additional
 objects to be used in conjunction with Filtering criteria on a
 subscribed object.  (For example, if A has changed and B=1, then Push
 A.)  Query match capability may be done on objects within the
 datastore even if those objects are not included within the
 subscription.  This of course assumes that the subscriber has read
 access to those objects.
 For on-change subscription updates, an object MUST pass a Filter
 through a Filter if it has changed since the previous update.  This
 includes if the object has changed multiple times since the last
 update, and if the value happens to be the exact same value as the
 last one sent.

4.2.8. Assurance and Monitoring

 It MUST be possible to fetch the state of a single subscription from
 a Subscription Service.
 It MUST be possible to fetch the state of all subscriptions of a
 particular Subscriber.
 It MUST be possible to fetch a list and status of all subscription
 requests over a period of time.  If there is a failure, some failure
 reasons might include:
 o  Improper security credentials provided to access the target node;
 o  Target node referenced does not exist;
 o  Subscription type requested is not available upon the target node;
 o  Out of resources, or resources not available;
 o  Incomplete negotiations with the Subscriber.

5. Security Considerations

 There are no additional security considerations beyond the
 requirements listed in Section 4.2.5.

Voit, et al. Informational [Page 15] RFC 7923 YANG Subscription Requirements June 2016

6. References

6.1. 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>.
 [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
            DOI 10.17487/RFC2328, April 1998,
            <http://www.rfc-editor.org/info/rfc2328>.
 [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>.
 [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
            BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
            2012, <http://www.rfc-editor.org/info/rfc6513>.
 [RFC7589]  Badra, M., Luchuk, A., and J. Schoenwaelder, "Using the
            NETCONF Protocol over Transport Layer Security (TLS) with
            Mutual X.509 Authentication", RFC 7589,
            DOI 10.17487/RFC7589, June 2015,
            <http://www.rfc-editor.org/info/rfc7589>.
 [RFC7921]  Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
            Nadeau, "An Architecture for the Interface to the Routing
            System", RFC 7921, DOI 10.17487/RFC7921, June 2016,
            <http://www.rfc-editor.org/info/rfc7921>.
 [RFC7922]  Clarke, J., Salgueiro, G., and C. Pignataro, "Interface to
            the Routing System (I2RS) Traceability: Framework and
            Information Model", RFC 7922, DOI 10.17487/RFC7922, June
            2016, <http://www.rfc-editor.org/info/rfc7922>.

Voit, et al. Informational [Page 16] RFC 7923 YANG Subscription Requirements June 2016

6.2. 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.
 [OMG-DDS]  Object Management Group (OMG), "Data Distribution Service
            for Real-time Systems, Version 1.2", January 2007,
            <http://www.omg.org/spec/DDS/1.2/>.
 [SACMREQ]  Nancy, N. and L. Lorenzin, "Security Automation and
            Continuous Monitoring (SACM) Requirements", Work in
            Progress, draft-ietf-sacm-requirements-13, March 2016.
 [USECASE]  Hares, S. and M. Chen, "Summary of I2RS Use Case
            Requirements", Work in Progress, draft-ietf-i2rs-usecase-
            reqs-summary-02, March 2016.
 [XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
            Subscribe", XSF XEP-0060, July 2010,
            <http://xmpp.org/extensions/xep-0060.html>.

Voit, et al. Informational [Page 17] RFC 7923 YANG Subscription Requirements June 2016

Acknowledgments

 We wish to acknowledge the helpful contributions, comments, and
 suggestions that were received from Ambika Tripathy and Prabhakara
 Yellai as well as the helpfulness of related end-to-end system
 context info from Nancy Cam Winget, Ken Beck, and David McGrew.

Authors' Addresses

 Eric Voit
 Cisco Systems
 Email: evoit@cisco.com
 Alexander Clemm
 Cisco Systems
 Email: alex@cisco.com
 Alberto Gonzalez Prieto
 Cisco Systems
 Email: albertgo@cisco.com

Voit, et al. Informational [Page 18]

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