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Requirements for Subscription to YANG Datastores

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7923.
Authors Eric Voit , Alexander Clemm , Alberto Gonzalez Prieto
Last updated 2022-12-08 (Latest revision 2016-05-17)
Replaces draft-voit-i2rs-pub-sub-requirements
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Susan Hares
Shepherd write-up Show Last changed 2016-01-04
IESG IESG state Became RFC 7923 (Informational)
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Responsible AD Alia Atlas
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IANA IANA review state Version Changed - Review Needed
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Interface to the Routing System (i2rs)                           E. Voit
Internet-Draft                                                  A. Clemm
Intended status: Informational                        A. Gonzalez Prieto
Expires: November 18, 2016                                 Cisco Systems
                                                            May 17, 2016

            Requirements for Subscription to YANG Datastores


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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 18, 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

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Business Drivers  . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Pub/Sub in I2RS . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Pub/Sub Variants on Network Elements  . . . . . . . . . .   5
     2.3.  Existing Generalized Pub/Sub Implementations  . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Assumptions for Subscriber Behavior . . . . . . . . . . .   7
     4.2.  Subscription Service Requirements . . . . . . . . . . . .   7
       4.2.1.  General . . . . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  Negotiation . . . . . . . . . . . . . . . . . . . . .   9
       4.2.3.  Update Distribution . . . . . . . . . . . . . . . . .   9
       4.2.4.  Transport . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.5.  Security Requirements . . . . . . . . . . . . . . . .  11
       4.2.6.  Subscription QoS  . . . . . . . . . . . . . . . . . .  12
       4.2.7.  Filtering . . . . . . . . . . . . . . . . . . . . . .  13
       4.2.8.  Assurance and Monitoring  . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 to 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 load on

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   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

   Push solutions will not displace all existing operations
   infrastructure needs.  And SNMP and MIBs will remain widely deployed
   and the defacto 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 additional load on the network and
   devices, because it is brittle in case polling cycles are missed, and
   because 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 I2RS

   Various I2RS documents highlight the need to provide Pub/Sub
   capabilities between network elements.  From [i2rs-arch], there are
   references throughout the document beginning in section 6.2.  Some
   specific examples include:

   o  section 7.6 provides high level pub/sub (notification) guidance

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   o  section 6.4.2 identifies "subscribing to an information stream of
      route changes receiving notifications about peers coming up or
      going down"

   o  section 6.3 notes that when local config preempts I2RS, external
      notification might be necessary

   In addition [i2rs-usecase] has relevant requirements.  A small subset

   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 IMs) should allow
      a subscriber to select portions of the data model.

   o  PI-REQ01: monitor the available routes installed in the RIB of
      each forwarding device, including near real time notification of
      route installation and removal.

   o  BGP-REQ10: 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 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.

   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 LSPs from the I2RS Agent.

   o  L-Data-REQ-01: I2rs must be able to collect large data set from
      the network with high frequency and resolution with minimal impact
      to the device's CPU and memory.

   And [i2rs-traceability] has Pub/Sub requirements listed in
   Section 7.4.3.

   o  I2RS Agents should support publishing I2RS trace log information
      to that feed as described in [i2rs-arch].  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

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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 which have done explicit or implicit pub/sub in the past.
   In addition, new policy notification mechanisms which 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 then may push spontaneous changes
      in their configured hardware and software[sacm-requirements]

   o  In MPLS VPNs [RFC6513] a Customer Edge router exchanges PIM
      control messages before PE Routing Adjacencies are passed.

   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 based on the need for connection-oriented/unicast
   communication.  Eventual support for multicast and broadcast
   subscription update distribution will be needed as well.

2.3.  Existing Generalized Pub/Sub Implementations

   TIBCO, RSS, CORBA, and other technologies all show precursor Pub/Sub
   technologies.  However there are new needs described in Section 4
   below which these technologies do not serve.  We need a new pub-sub

   There are at least two widely deployed generalized pub/sub
   implementations which come close to current needs: XMPP[XEP-0060] and
   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

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   scale of such object distribution for various subtree sizes and
   transport types.

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

   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 which
   dictate how often or under what conditions YANG information updates
   might be sent.

   A Subscription is a contract between a Subscription Service and a
   Subscriber that stipulates the data to be pushed and the associated

   A datastore is defined in [RFC6241].

   An Update provides object changes which have occurred within
   subscribed YANG subtree(s).  An Update must include the current
   status of (data) node instances which according to any filtering are
   reportably different from the previously provided state.  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

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   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
   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 reestablish any
   subscriptions it previously created via signalling over the transport
   session.  I.e., There is no requirement for the life span of such
   signaled Subscriptions extend beyond the life span of the transport

   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,
   timeout, and terminate a Subscription.

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   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
   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

   For "on-change" updates, the Subscription Service MUST support a
   dampening period that needs to pass before the first or subsequent
   "on-change" updates are sent.  The dampening period SHOULD be
   configurable as part of the subscription request.

   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 terminating 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 CLI, and/or others.

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   When this occurs, the Subscription Service MUST notify the Subscriber
   and update 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

   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.

   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 too short update intervals 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 whether it was
   removed, added, or changed.

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   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 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.

   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 Subsription Service will
   applicable for emerging protocols such as those being defined in
   [i2rs-usecase].  The need for such transport flexibility drives the
   following requirements.

   A Subscription Service SHOULD support different transports.

   A Subscription Service SHOULD support different encodings of payload.

   It MUST be possible for Receivers to associate the update with a
   specific Subscription.

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   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 [RFC5539] 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 [i2rs-usecase] would apply these
   requirements in conjunction with those documented within
   [i2rs-environment-security] and [i2rs-protocol-security] 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

   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 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 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.

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   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 the Receiver.

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.  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.  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.)  Reliability

   A Subscription Service MAY send Updates over Best Effort and Reliable

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

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

   The Subscription Service SHOULD support the relative prioritization
   of Subscriptions so that dequeuing and discarding of push updates can
   consider this if there is insufficient bandwidth between Publisher
   and 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 corresponding object(s) within a

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   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 which are
      exactly equal to a provided string, not equal to the string, or
      containing a string.

   o  For numeric based object properties, Filter values which are =,
      !=, <, <=, >, >= 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.

   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 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 ithe 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;

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   o  Incomplete negotiations with the Subscriber.

5.  Security Considerations

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

6.  IANA Considerations

   This document has no actions for IANA.

7.  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.

8.  References

8.1.  Normative References

              Atlas, A., "An Architecture for the Interface to the
              Routing System", February 2016,

              Clarke, J., Salgueiro, G., and C. Pignataro, "Interface to
              the Routing System (I2RS) Traceability: Framework and
              Information Model", February 2016,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,

   [RFC5539]  Badra, M., "NETCONF over Transport Layer Security (TLS)",
              RFC 5539, DOI 10.17487/RFC5539, May 2009,

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   [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,

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <>.

8.2.  Informative References

              Migault, D., Halpern, J., and S. Hares, "I2RS Environment
              Security Requirements", April 2016,

              Hares, S., Migault, D., and J. Halpern, "I2RS Security
              Related Requirements", May 2016,

              Hares, S. and M. Chen, "Summary of I2RS Use Case
              Requirements", March 2016,

   [OMG-DDS]  "Data Distribution Service for Real-time Systems, version
              1.2", January 2007, <>.

              Cam Winget, N., "Secure Automation and Continuous
              Monitoring (SACM) Requirements", March 2016,

              Millard, P., "XEP-0060: Publish-Subscribe", July 2010,
              <XEP-0060: Publish-Subscribe>.

Authors' Addresses

   Eric Voit
   Cisco Systems


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   Alexander Clemm
   Cisco Systems


   Alberto Gonzalez Prieto
   Cisco Systems


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