Interface to the Routing System (i2rs)                           E. Voit
Internet-Draft                                                  A. Clemm
Intended status: Informational                        A. Gonzalez Prieto
Expires: September 4, 2015                                 Cisco Systems
                                                           March 3, 2015

            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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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 4, 2015.

Copyright Notice

   Copyright (c) 2015 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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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 . . . . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . . . .  10
       4.2.3.  Update Distribution . . . . . . . . . . . . . . . . .  10
       4.2.4.  Transport . . . . . . . . . . . . . . . . . . . . . .  11
       4.2.5.  Security Requirements . . . . . . . . . . . . . . . .  11
       4.2.6.  QoS Requirements  . . . . . . . . . . . . . . . . . .  12
       4.2.7.  Filtering . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   YANG has gained acceptance as the data definition language of choice
   for control and management related information.  Applications that
   interact with YANG datastores are extending beyond traditional
   configuration of network elements.  In many cases these applications
   are aimed at service-assurance, which involves monitoring of
   operational data and state.  The existing YANG technology ecosystem
   is proving insufficient for those applications due to:

   o  a reliance on RPC-style interactions where data is configured or
      fetched on-demand by applications.

   o  change notifications which identify a node associated with the
      config change, without the actual data updates

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   Put simply, periodic fetching of data is not an adequate solution for
   applications requiring frequent or prompt updates of remote object
   state.  Trying to impose a polling based solution to this problem
   imposes load on networks, devices, and applications.  Additionally,
   polling solutions are brittle in the face of communication glitches,
   and they have limitations in their ability to synchronize and
   calibrate retrieval intervals across a network.

   I2RS WG documents have expressed a need for more robust YANG object
   subscriptions.  Similar discussions are underway in NETMOD and
   NETCONF.  With the support of standards bodies such as OMG (DDS), standard, generic Pub/Sub mechanisms to communicate data
   updates have been defined and proven themselves in a wide variety of

   It is time to incorporate such generic object subscription mechanisms
   as part of Network Elements, and allow these mechanisms to be applied
   in the context of data that is conceptually contained in YANG
   datastores.  With such mechanisms, both controller and local Network
   Element based applications can have access to a set of consistent
   network information driven via push from peer Network Elements which
   host authoritative information.

   There are some valid IETF starting points and contexts for these
   mechanisms.  For example Netconf Event Notifications [RFC5277]
   provides a useful tool for an end-to-end solution.  However RFC5277
   does not follow the Pub/Sub paradigm, does not allow the explicit
   deletion of subscriptions, and predates YANG.  [RFC6470] defines
   configuration change notifications, but doesn't provide the actual
   configuration change.

   Because of this, the authors have put forward this requirements
   document as well as [datastore-push].  We are hoping these could
   provide a context upon which to create new solution.

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 SDN,
   imperative policy distribution, and YANG's ascent as a dominant
   programmatic interface 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 objects and object
   changes as they happen.

   These push distribution mechanisms will not replace existing
   networking protocols.  Instead they will supplement these protocols,

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   providing different response time, peering, scale, and security

   At the same time, SNMP and MIBs are still widely deployed and the de-
   facto choice for many monitoring solutions.  Those solutions do not
   require support for configuration transactions and the need to
   validate and maintain configuration consistency, hence there is less
   pressure to abandon SNMP and MIBs.  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, is brittle in case polling cycles are missed, and is hard to
   synchronize and calibrate across a network, making data obtained from
   multiple devices less comparable.  If applications need to apply
   those same interaction patterns for YANG datastores, similar issues
   can be expected.  Migration to YANG datastores by applications that
   do not have to worry about transactional integrity becomes a lot more
   compelling if those issues are addressed.

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

   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 subscribe 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.

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   o  BGP-REQ10: I2RS client SHOULD be able 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

   There are additional individual drafts such as [i2rs-pubsub-security]
   documenting the Pub/Sub needs for: time delivery sensitivity, support
   for multiple transport protocols, secure/authorized communications,
   and support for a range specification of subscribed data delivery
   content.  So the list above should not be considered exhaustive.

2.2.  Pub/Sub variants on Network Elements

   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 very small subset of
   these includes:

   o  Routing Adjacencies in MPLS VPNs [RFC6513]

   o  OSPF Route Flooding [RFC2328]

   o  Multicast topology establishment protocols (IGMP, PIM, etc.)

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   o  Audio-Video Bridging streams needing guaranteed latency
      [AVB-latency] (802.1Q-2011 Clause 35)

   o  Secure Automation and Continuous Monitoring (SACM)

   o  "Peer Mount" subscriptions for configuration verification between

   Worthy of note in the list above is the wide variety of broadcast,
   multicast, and unicast transports used.  In addition some transports
   are at L3, and some at L2.  Therefore if we are going to attempt a
   generic Pub/Sub mechanism, it will need to be structured so that it
   may support alternative transports.  Looking at the nearer term based
   on current I2RS requirements, NETCONF should be our transport
   starting point as it supports connection oriented/Unicast
   communication.  But we need to be prepared to decouple where viable
   to support Multicast and Broadcast distribution 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 technology.

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

3.  Terminology

   A Subscriber makes requests for set(s) of YANG object data.  The
   Subscriber is the owner of the Subscription.

   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 where a Publisher pushes updates.  In
   general, the Receiver and Subscriber will be the same entity.  A

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   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 made by the
   Subscriber of a Publisher and targeted to a Receiver.  A Subscription
   MAY include constraints which dictates how often or under what
   conditions YANG subtree 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 YANG datastore is a conceptual datastore that contains hierarchical
   data defined in YANG data models.  It is what is referred in existing
   RFCs as "Netconf datastore".  However, as the same datastore is no
   longer tied to Netconf as a specific transport, the term "YANG
   datastore" is deemed more appropriate.

   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 which 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 Property Filters where
   an object should only be published if it has propert(ies) meeting
   specified Filter criteria.  For "on-change" notifications, passing
   through the Filter requires that a subscribed object is now different
   that from the previous Push, AND at least one of the YANG objects
   being evaluated has changed since the last Update.

4.  Requirements

   Many of the requirements within this section have been morphed from
   OMG's DDS and's 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.

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   However In order to frame the desired behavior of the Subscription
   Service, it is important to specify key input constraints.

4.2.  Subscription Service Requirements

   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 QoS criteria to the Subscription Service
   such that if the Subscription Service is unable to meet those
   criteria, the Subscription should not be established.

   When a Subscriber needs to restart, it is acceptable for the
   Subscriber to have to resubscribe.  There is no requirement for the
   life span of the Subscription to extend beyond the life span of the

   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 re-lease a
   Subscription from a Subscription Service.

4.2.1.  General

   A Subscription Service MUST support the ability to create, renew,
   timeout, and terminate a Subscription.

   A Subscription Service MUST be able to support and independently
   track one or more Subscription Requests by the same Subscriber.

   A Subscription Service MUST be able to support an add/change/delete
   of one or more YANG subtrees as part of the same Subscription

   A Subscription Service MUST support Subscriptions against operational
   datastores, configuration datastores, or both.

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   A Subscription Service MUST be able support a Subtree Filter 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, the Subscription Service MUST publish only data nodes that
   meet the filter criteria.

   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 SHOULD be able to interpret Subscription
   Requests QoS Policy requests, and only establish a Subscription if it
   is possible to meet the QoS those QoS Policy requests.

   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.
   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

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   subscribers with an indication 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 of periodic

   o  The interval, for periodic publication policy

   o  The dampening period, for on-change update policy

   o  Any filters associated with a subtree subscription

   A Subscription Service SHOULD be able to negotiate QoS criteria for a
   Subscription.  Examples of 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

   In cases where a Subscription Request cannot be fulfilled, the
   Subscription Service MUST include in its decline a set of 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, the minimum acceptable
   interval period SHOULD be included.  If on-change updates were
   requested with a dampening period, the minimum acceptable dampening
   period SHOULD be included, or an indication whether only periodic
   updates are supported along with the minimum acceptable interval
   period for the data set being subscribed to.

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 will not know what has actually undergone change.]
   The updates for each object needs to include an indication 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 of 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

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   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).

   A Subscription Service MAY, as an option, support a persistence/
   replay capability.

4.2.4.  Transport

   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.

   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

   As part of the Subscription establishment, there MUST be mutual
   authentication between the Subscriber and the Subscription Service.

   When there are multiple Subscribers, it SHOULD be possible to provide
   cryptographic authentication in such a way that no Subscriber can
   pose as the original Subscription Service.

   Versioning MUST be supported.

   A Subscription could be used to attempt to retrieve information that
   a client has not authorized access to.  Therefore it is important
   that data pushed based on Subscriptions is authorized in the same way
   that regular data retrieval operations are.  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.

   Subscription requests, including requests to create, terminate,
   suspend, and resume Subscriptions MUST be properly authorized.

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   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
   would deplete its resources.  It is preferable to decline a
   Subscription when originally requested, rather than having to
   terminate it prematurely later.

4.2.6.  QoS Requirements

   A Subscription Service SHOULD be able to negotiate the following QoS
   parameters with a Subscriber: Dampening, Reliability, Deadline,
   Bundling.  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
   transports.  Coherence

   Every update to a subscribed object MUST be sent to the Receiver in
   sequential order.  Presentation

   The Subscription Service SHOULD 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.

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   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

   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.

4.2.7.  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

   It MUST be possible to attach one or more Subtree and/or Property
   Filters to a subscription.  Mandatory Property 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 Property Filtering criteria to evaluate
   more than one property of a particular subscribed object as well as
   apply multiple filters against a single property.

   It SHOULD be possible to establish query match criteria on additional
   objects to be used in conjunction with Property Filtering criteria on
   a subscribed object.  (For example: if A has changed AND B=1, then
   Push A.)  (Note: 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.)

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5.  Acknowledgements

   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 from [i2rs-pubsub-security] from Nancy Cam Winget, Ken Beck,
   and David McGrew.

6.  References

6.1.  Normative References

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

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, July 2008.

   [RFC6470]  Bierman, A., "Network Configuration Protocol (NETCONF)
              Base Notifications", RFC 6470, February 2012.

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

6.2.  Informative References

              Jeffree, T., "802.1Qav - Forwarding and Queuing
              Enhancements for Time-Sensitive Streams", December 2009,

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

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

              Clemm, A., Gonzalez Prieto, A., and E. Voit, "Subscribing
              to datastore push updates", October 2014,

              Voit, E., "Requirements for Peer Mounting of YANG subtrees
              from Remote Datastores", October 2014,

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Authors' Addresses

   Eric Voit
   Cisco Systems


   Alex Clemm
   Cisco Systems


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   Alberto Gonzalez Prieto
   Cisco Systems


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