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Versions: 00 01

NETCONF Working Group                                        H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                                 E. Voit
Expires: January 17, 2019                                  Cisco Systems
                                                           July 16, 2018


                         Concise YANG Telemetry
                 draft-birkholz-yang-core-telemetry-01

Abstract

   This document defines CoAP operations that implement the capabilities
   of YANG Datastore Subscriptions and YANG Customized Subscriptions for
   the CoAP Management Interface (CoMI).  The '/s' resource, as defined
   in CoMI, is extended analogously to include a set of sub-resources,
   each of them representing an observable resource identified by its
   subscription-id.  Specific additions include but are not limited to
   new FETCH Body definitions and simplified subtree subscriptions to
   intermediate data nodes in YANG datastore modules using SID.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 January 17, 2019.

Copyright Notice

   Copyright (c) 2018 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
   (https://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



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   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  CoAP Terminology  . . . . . . . . . . . . . . . . . . . .   6
   3.  Summary of the Additions to CoMI  . . . . . . . . . . . . . .   7
     3.1.  Telemetry-Specific CoMI datastores  . . . . . . . . . . .   7
   4.  Subscription Content and State  . . . . . . . . . . . . . . .   8
     4.1.  Selection Filter  . . . . . . . . . . . . . . . . . . . .   9
   5.  Subscription Characteristics  . . . . . . . . . . . . . . . .   9
     5.1.  Subscription Trigger  . . . . . . . . . . . . . . . . . .  10
       5.1.1.  CoMI Configured Subscriptions . . . . . . . . . . . .  10
       5.1.2.  Dynamic Subscriptions . . . . . . . . . . . . . . . .  11
     5.2.  Subscription Interval . . . . . . . . . . . . . . . . . .  11
       5.2.1.  Periodic Subscription . . . . . . . . . . . . . . . .  11
       5.2.2.  On-Change Subscription  . . . . . . . . . . . . . . .  11
     5.3.  Subscription Type . . . . . . . . . . . . . . . . . . . .  12
       5.3.1.  Stream Subscription . . . . . . . . . . . . . . . . .  13
       5.3.2.  Datastore Subscription  . . . . . . . . . . . . . . .  13
   6.  Resilient Subscriptions . . . . . . . . . . . . . . . . . . .  13
   7.  Subscription Management (better word?)  . . . . . . . . . . .  13
     7.1.  YANG RPCs . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  NETCONF Access Control Model [RFC6536bis] . . . . . . . .  13
   8.  Selection Filters . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  SID for subtree Selection Filter  . . . . . . . . . . . .  13
     8.2.  CBOR-YANG for XPATH-like Selection Filter . . . . . . . .  13
   9.  Update Triggers for Periodic Subscriptions  . . . . . . . . .  14
     9.1.  Interval  . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.2.  Anchor Time . . . . . . . . . . . . . . . . . . . . . . .  14
   10. Update Triggers for On-Change Subscriptions . . . . . . . . .  14
     10.1.  dampening period . . . . . . . . . . . . . . . . . . . .  14
     10.2.  change-type  . . . . . . . . . . . . . . . . . . . . . .  14
     10.3.  no sync-on-start . . . . . . . . . . . . . . . . . . . .  14
   11. YANG Telemetry Operations for COMI  . . . . . . . . . . . . .  14
     11.1.  Extension of the CoMI Event Stream Resource  . . . . . .  14
     11.2.  Extension of the CoMI Datastore Resource . . . . . . . .  15
     11.3.  Extension of the YANG Subscription Mechanism . . . . . .  15
   12. Upcoming Features and Stories . . . . . . . . . . . . . . . .  15
   13. IANA considerations . . . . . . . . . . . . . . . . . . . . .  15
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   16. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   17. Normative References  . . . . . . . . . . . . . . . . . . . .  16



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The YANG management interface for constrained devices and networks,
   called CoAP Management Interface (CoMI), is defined in
   [I-D.ietf-core-comi] and covers the capabilities as defined by YANG
   1.1 [RFC7950].  The most essential characteristics of CoMI are the
   use of:

   o  the Constrained Application Protocol (CoAP [RFC7252]) to
      facilitate an HTTP-esque interaction model,

   o  the Concise Binary Object Representation (CBOR [RFC7049]) to
      facilitate an efficient content encoding using
      [I-D.ietf-core-yang-cbor], and

   o  YANG identifier strings are represented as numbers called YANG
      Schema Item Identifier (SID), also defined in
      [I-D.ietf-core-yang-cbor].

   This document defines additions to CoMI called Concise YANG Telemetry
   that:

   o  enrich its capabilities to subscribe to a variety of YANG modeled
      notifications--based on YANG Customized Subscriptions to a
      Publisher's Event Streams
      [I-D.ietf-netconf-subscribed-notifications], and

   o  enable subscriptions to changes data note values in modules
      provided by the YANG datastore (or parts of them)--based on YANG
      Datastore Subscription [I-D.ietf-netconf-yang-push].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology

   Due to the utilization of CoAP, the interaction model of CoMI is
   quite similar to RESTCONF [RFC8040].  RESTCONF supports subscriptions
   to a YANG datastore via notification statements in YANG modules,
   which--when subscribed to via the base subscription YANG RPC defined
   in [RFC7950]--result in Series [I-D.bormann-t2trg-stp] of Server Sent
   Events [W3C.REC-eventsource-20150203]}.  A corresponding Event Stream
   specification for NETCONF [RFC6241] Event Notifications is defined in



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   section 3.2.3 of [RFC5277].  To simplify corresponding terminology
   (and especially consolidate the impedance mismatch of the terms
   Notifications and Events), this document defines the following new
   term:

   YANG Telemetry:  A Series of YANG modeled data items, potentially
      composing Update Records, sent from a YANG datastore [RFC8342] to
      a YANG receiver [I-D.ietf-netconf-subscribed-notifications] either
      solicited or unsolicited in a fashion that can guarantee well-
      defined levels of Visibility with respect to Data Node Value
      changes.

   Please note: while the focus of YANG is typically on management and
   operations, the scope of YANG Telemetry extends into the Security
   Area with respect to Security Events.  Because of this, YANG
   Telemetry characteristics that address security requirements, such as
   Visibility and Resilient YANG Subscriptions, are addressed in this
   document.

   The definition of YANG Telemetry is based on the following
   terminology:

   Data Node Value:  Defined in CoMI as the value assigned to a data
      node instance.  Data node values are serialized into the payload
      according to the rules defined in section 4 of
      [I-D.ietf-core-yang-cbor].  Changes in Data Node Values are
      conveyed via data node updates referred to as Update Records.

   Update Record:  Defined in [I-D.ietf-netconf-yang-push], in the
      context of Concise YANG Telemetry, an Update Record composes a
      single YANG data item in a Series of YANG Notifications or YANG
      Notification Bundle Messages conveying the changes to a YANG
      datastore's module's Data Node Values.

   Resilient Subscription:  A subscription
      [I-D.ietf-netconf-subscribed-notifications] used to create Concise
      YANG Telemetry that continues to function in a given scope of CoMI
      Clients, or discoverable homes (see CoAP Call Home, respectively),
      if the current receiver of Concise YANG Telemetry is rendered
      unavailable.

      This subscription characteristic enables a CoMI datastore to
      detect loss of a specific CoMI Client that is the target of
      Concise YANG Telemetry, provided that the CoMI Client is part of a
      group of CoMI Clients that supports this kind of fail-over
      mechanism.  This subscription characteristic can have a
      deteriorating effect on the level of assurance with respect to




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      Visibility and can therefore result in missed Updated Records,
      which a "new home" has to be notified about.

   Series:  Series Transfer Pattern are described in
      [I-D.bormann-t2trg-stp] as the conveyance of a sequence of
      associated data items over time, where a client is able to obtain
      the Series and to learn about new items.

      YANG Event Stream Subscriptions or a YANG Datastore Subscription
      creates an specific Series Transfer Pattern composed of individual
      YANG Notifications or YANG Notification Bundle Messages that
      include related updated records.

   Subscription Characteristics:  The set of attributes associated with
      an active YANG Subscription.

      The set of attributes is defined by the data definition statements
      [RFC7950] defined by the augments in YANG Customized Subscriptions
      to a Publisher's Event Streams
      [I-D.ietf-netconf-subscribed-notifications], YANG Datastore
      Subscription [I-D.ietf-netconf-yang-push] and corresponding CoAP
      Tokens for Concise YANG Telemetry.

   Visibility:  A level of assurance that Update Records will be
      received by a YANG Client.

      There might be reasons, such as resource exhaustion or dampening
      settings, that result in Update Records being lost in transit or
      not being emitted by the YANG datastore.  Sequential Message-IDs
      or specific YANG Notifications that report, e.g., about past
      events of resource exhaustion will inform the YANG Client about
      the characteristics of the loss of Update Records.

   YANG Receiver:  A subset of clients as defined in [RFC8342].  A YANG
      Receiver is the entity that can receive and process - in contrast
      to access, which is mandatory in order to create a subscription,
      but not mandatory in order to receive the corresponding telemetry
      stream - YANG-defined data originating from a server, over some
      network management protocol.

      YANG Receiver is a representation agnostic term.  Specific network
      protocols that operate on YANG Servers via YANG Clients use
      representations, such as XML, JSON, or CBOR.

   YANG Data Item:  An instance of YANG a modules Data Node Values or
      the changes to Data Node Values, conveyed as data in motion,
      serialized in a specific representation, such as XML, JSON, or




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      CBOR.  A Data Node Update as defined in
      [I-D.ietf-netconf-yang-push] is a YANG data item.

   YANG Datastore:  Defined in [RFC8342], a datastore is a conceptual
      place to store and access information.  A datastore might be
      implemented, for example, using files, a database, flash memory
      locations, or combinations thereof.

   YANG Notification:  Defined in [RFC8342], a Notification is a server-
      initiated message indicating that a certain event has been
      recognized by the server.

   YANG Notification (Bundle) Message:  An encapsulation header for one
      or more YANG notifications as defined in
      [I-D.ietf-netconf-notification-messages].  The message header
      includes a specific set of well-known objects, which are of
      potential use to networking layers prior being interpreted by some
      receiving application layer process.

      Examples of header object include, but are not limited to:
      timestamps, signatures, or evidence about the integrity of the
      agents creating messages or notifications.

   YANG Subscription:  A Subscription in the context of YANG is defined
      in [I-D.ietf-netconf-subscribed-notifications] as a contract with
      a publisher, stipulating which information one or more receivers
      wish to have pushed from the publisher without the need for
      further solicitation.

      In the context of Concise YANG Telemetry, this is a parent term
      that encompasses the concepts of YANG Datastore Subscriptions and
      YANG Customized Subscriptions.

2.1.  CoAP Terminology

   In addition to the illustration of the scope of YANG Telemetry above,
   this section highlights the most important terms that are vital to
   the functionality of Concise YANG Telemetry with respect to the
   Constrained Application Protocol:

   CoAP Call Home:  A procedure similar to the one defined in [RFC8071]
      NETCONF Call Home and RESTCONF Call Home, in which a YANG
      datastore can trigger a YANG Client to initiate a YANG
      Subscription (typically by taking on the role of a CoAP client and
      indicating to a CoAP node that is including a YANG Client to
      initiate a YANG Subscription).





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   CoAP Client:  The entity that emits CoAP requests to a CoAP Node with
      server capabilities.  In the context of Concise YANG Telemetry, a
      YANG client, for example. that is creating dynamic subscriptions
      is a CoAP Client.

   CoAP Token:  A token used to match a CoAP response with a CoAP
      request [RFC7252].  It is intended for use as a client-local
      identifier to differentiate between concurrent requests (similar
      to a request ID).

      A CoAP Token must be known, or at least deterministically
      inferable by a YANG Client in order to receive a Series of CoAP
      responses that compose Concise YANG Telemetry.

3.  Summary of the Additions to CoMI

   This documents defines the binding of YANG Datastore Subscriptions
   and YANG Customized Subscriptions to the CoAP Management Interface.
   In summary, these additions include:

   o  a CoAP POST operation to create, modify, delete or kill Telemetry
      subscription state,

   o  a CoAP iPATCH operation to create, modify, delete or kill modify
      one or more Telemetry subscription states,

   o  a CoAP GET operation including the Observe option to receive a
      Telemetry stream,

   o  a CoAP FETCH operation including the Observe option to receive
      multiple Telemetry streams,

   o  an extension of the "/s" resource that includes sub-resource, such
      as subscription identifiers and corresponding SID instances, and

   o  the capability to direct GET and FETCH operations including the
      Observe option at resources that are sub-resources of "/c".

3.1.  Telemetry-Specific CoMI datastores

   The CoMI architecture (and YANG in general) assumes that both YANG
   client and YANG datastores (server) retain or have access to
   knowledge about the same YANG specification (see Figure 1 in
   [I-D.ietf-core-comi]).  This is not necessarily true for a YANG
   Telemetry capable CoMI server.  Highly constrained nodes can emit
   Series of subscribed notifications without previous solicitation:
   this allows them to create well-formed YANG-modeled Telemetry from
   hard-coded building blocks of YANG-modeled data, which are in



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   compliance to YANG modules.  In consequence, while taking on the role
   of a YANG datastore, a YANG Subscription capable CoMI server SHOULD
   be capable to process YANG queries, but MAY not be due to the lack of
   corresponding functions or knowledge of the resulting structures
   received.

   As these flavors of YANG datastores are not necessarily able to
   create CoMI responses based on client request, it is likely that
   highly constrained datastores initiate a Call-Home procedure (see
   [RFC8071]) acting as if a request was already received (see
   Configured Subscription above), soliciting a very specific request
   they can fulfill (dynamic subscription) or rendezvous via a
   discoverable YANG Zero Touch entity.  In all these usage scenarios,
   the datastore intends to create device specific YANG Telemetry to be
   conveyed to corresponding YANG clients.

   In essence, incorporating a complete YANG module on a CoMI datastore
   that is capable of YANG Subscriptions is not required to enable the
   initiation of Concise YANG Telemetry within a very specific scope.
   This kin of Telemetry-Specific CoMI datastore is therefore not a
   fully YANG 1.1 compliant datastore, but able to create valid YANG
   modeled YANG Data Items.

4.  Subscription Content and State

   Two generic YANG notification statements for Update Records are
   introduced by YANG Datastore Subscriptions
   [I-D.ietf-netconf-yang-push] augments to enable the following
   capabilities:

   push-update:  A notification that includes a complete (and
      potentially filtered) update of data node values of YANG datastore
      nodes per the terms of a subscription.

   push-change-update:  A notification that includes an incremental (and
      potentially filtered) update of data node values of YANG datastore
      nodes since the last (change-)update notification.

   Every Update Record in a Series that is generated in the context of a
   subscription is emitted per the characteristics of the subscription
   state maintained by the CoMI datastore.  Subscription state can be
   created on the CoMI datastore during manufacturing, onboarding,
   enrollment, deployment, or maintenance of the CoMI datastore.  Most
   typically, subscription state is created by a YANG Client (e.g. a
   Network Management System) via a dynamic subscription.






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4.1.  Selection Filter

   A vital part of the subscription state that defines the content of a
   YANG Telemetry stream is the filter expression associated with the
   subscription characteristics.  A filter expression enables a CoMI
   datastore to emit only a subset of potential notification content;
   reducing the volume of data in motion, significantly.

   Three types of Filter Expressions enable a CoMI datastore to emit
   filtered subsets of data node value updates:

   Subtree Filter Expression:  A SID pointing to a specific data node in
      a YANG module (including notification statements) is used to
      create update records that include updates about the identified
      data node and its potential child nodes.  Effectively, a single
      SID points to the root node of the subtree update records are
      created for.

   XPATH Filter Expression:  A more detailed selection of SIDs and
      corresponding data node values that update records are created
      for.  The corresponding representation of XPATH Filter Expressions
      for COMI is defined in [I-D.ietf-core-yang-cbor].

   Conditional SID Selectors (experimental):  The SID concept introduced
      by CoMI and represented via YANG modeled date conveyed using CBOR
      allows for a simplified Filter Expression data model that retains
      most of the capabilities of an XPATH Filter Expression, while
      using a significantly simpler model definition.

5.  Subscription Characteristics

   Distinct YANG Telemetry streams are defined by the following three
   primary subscription characteristics:

   1.  Subscription Trigger (dynamic / configured)

   2.  Subscription Interval (periodic / on-change)

   3.  Subscription Type (stream / datastore)

   These characteristics define how subscription state is created and
   how the resulting Telemetry streams behave.  In general,
   corresponding subscription state can be created by a YANG client via
   the "establish-subscription" RPC as defined in
   [I-D.ietf-netconf-subscribed-notifications].






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5.1.  Subscription Trigger

   There are three options how to establish a YANG Telemetry stream via
   YANG Subscriptions:

   1.  a YANG client starts to receive a Telemetry stream from a YANG
       datastore, without previous solicitation.  To enable this,
       persistent subscription characteristics must have been configured
       on the YANG datastore earlier (e.g. during onboarding), but may
       be left unknown to the YANG Client.  They then have to be
       acquired or inferred via procedures that are--at the time of this
       writing--out-of-scope of this document.  In essence,
       corresponding CoAP Tokens are unknown to the YANG Client when the
       first YANG Data Item is received from the datastore.

   2.  a YANG datastore initiates contact with a YANG client via a
       Rendezvous, Join, or Call Home procedure and triggers the
       creation of a Telemetry stream by the YANG Client.  In this case,
       the subscription characteristics are provisioned by the YANG
       Client and come into effect after a YANG datastore successfully
       discovered the corresponding YANG Client that is its home.  In
       essence, the corresponding CoAP Tokens are created by the CoAP
       Client and are conveyed to the YANG datastore via the traditional
       CoAP interaction model.

   3.  a YANG client starts to receive a Telemetry stream from a YANG
       datastore, after being solicited to do so.  In this case,
       persistent subscription characteristics are known by a YANG
       Client and a YANG datastore before deployment or acquired after
       their deployment.  In essence, corresponding CoAP Tokens and
       Subscription State are already known by the YANG Client and the
       YANG datastore when the first YANG Data Item is received from the
       datastore.

5.1.1.  CoMI Configured Subscriptions

   CoAP defines a strict coupling of request and corresponding response
   messages via the CoAP Token.  Every CoAP Request MUST include a CoAP
   Token that is generated by the CoAP Client.  Analogously, every CoAP
   response that is associated with that request MUST include the
   corresponding CoAP Token in order for the CoAP Request not to be
   discarded.

   In order to enable this type of YANG Subscription, one or more CoMI
   Clients have to retain or gain knowledge about the corresponding CoAP
   Tokens (via declarative guidance, a distribution mechanism, or by
   inferring them via a Call Home procedure), for which they are
   intended to receive Concise YANG Telemetry by.  This implies the



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   existence of a deployed solution that enables a secure and resilient
   distribution of corresponding CoAP Tokens in a group of CoMI Clients.
   The exact architecture of this solution is--at the time of this
   writing--out-of-scope of this document.

5.1.2.  Dynamic Subscriptions

   This subscription trigger requires knowledge about potential YANG
   datastores to subscribe to by a YANG Client.  This subscription
   characteristics have to be pre-configured or discoverable by the YANG
   Client.  A typical procedure to facilitate a dynamic subscription is
   the Call Home discovery mechanism.

5.2.  Subscription Interval

   The subscription interval is a specific Subscription Characteristic
   that defines the events that trigger emission of an Update Record in
   the context of a YANG Subscription.  There are two types of
   subscription interval: periodic subscription and on-change
   subscription.

5.2.1.  Periodic Subscription

   A periodic subscription uses a timer in order to emit an Update
   Record in the context of a YANG Subscription.  This type of
   Subscription Characteristic is intended to be used if Data Node
   Values change with a regular, predicable cadence.  A typically
   example of Data Node Values that benefit from this type of
   Subscription Characteristic is used are PDU Counters.

5.2.2.  On-Change Subscription

   An on-change subscription uses the event of a Data Node Value change
   to emit an Update Record in the context of a YANG Subscription.  This
   type of Subscription Characteristic is intended to be used if Data
   Node Values changes only occasionally, but conveyance of information
   about that change in a timely fashion is required.  Typical examples
   are, deployment of a new IEEE 802.1AR LDevID, or the modification of
   an ACL by a logged in user.

5.2.2.1.  On-Change Subscription Prerequisites

   An on-change subscription capability MUST be explicitly annotated in
   a YANG module definition in order to prevent a meaningless or harmful
   association of Subscription Characteristics to a YANG Subscription.
   E.g. it is advisable not to allow for on-change subscriptions to data
   definition statements that provide Data Node Values representing
   rapidly changing counters.



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   The actual syntax and corresponding semantics of data definition
   statements that are intended to allow for associating an on-change
   Subscription Characteristic with a YANG Datastore Subscription to a
   YANG module is out-of-scope of [I-D.ietf-netconf-yang-push] or
   [I-D.ietf-netconf-subscribed-notifications] and--in consequence--also
   out-of-scope of this document.  Mechanisms being proposed include
   [draft-lengyel-netconf-notification-capabilities-01].

5.2.2.2.  Visibility

   In usage scenarios that require a high level of assurance with
   respect to Visibility (most prominently security-related events) it
   is vital for a YANG Client to gain knowledge about a deterioration of
   Visibility of Update Records.

   In order to request complete Visibility of every Data Node Value
   change via a corresponding Update Record, the dampening-period (see
   section 4.2. in [I-D.ietf-netconf-yang-push] MUST be set to 0.  The
   use of YANG Notification Bundle Messages
   [I-D.ietf-netconf-notification-messages] can mitigate the
   deteriorating impact of a dampening-period higher than 0, but can
   still result in missed Update Records in an constrained-node
   environment.

   If the conveyance of an on-change Update Record failed, lost records
   can be recognized as a sequence number of pushed updates is included
   with each sequential update record.

   If the assambly of an Update Record is incomplete, datastore MUST
   include an "updates-not-sent" flag in the next Update Record.

   There are usage scenarios, in which complete Visibility of every
   Change to Data Node Values, but only the information that there was a
   Data Node Values change occurred is appropriate.  In these cases, a
   dampening-period higher than 0 should be used.  A prominent example
   are "interface-flapping" events.

5.3.  Subscription Type

   The YANG Telemetry subscription trigger mechanisms illustrated above
   creates subscription state between a YANG client and a YANG
   datastore.  As long as this subscription state between these two
   entities persists, a datastore can emit Series of YANG notifications
   to a YANG client, if appropriate conditions are met, e.g. the YANG
   client expects solicited event notifications coming from the
   datastore due to a dynamic subscription.





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   YANG Datastore Subscription [I-D.ietf-netconf-yang-push] and YANG
   Subscribed Notifications [I-D.ietf-netconf-subscribed-notifications]
   extend this mechanism by enabling subscriptions to changes of YANG
   module data node state in a YANG datastore resulting in two types of
   sources - or two different types of YANG Notification Series
   [I-D.bormann-t2trg-stp], respectively:

   o  event stream telemetry

   o  datastore changes telemetry

5.3.1.  Stream Subscription

5.3.2.  Datastore Subscription

6.  Resilient Subscriptions

   In usage scenarios with a group of more than one CoMI Client a CoMI
   datastore can potentially convey Concise YANG Telemetry to, a YANG
   Subscription can be maintained in a more resilient manner.  Emitting
   a CoAP response in a confirmable message enables a CoMI datastore to
   detect that a corresponding CoMI Client became unavailable (due to
   missing confirmation messages).  In order to create a Resilient
   Subscription, a detected loss of a CoMI Client MUST immediately re-
   trigger the CoAP Call Home procedure in order to discover an
   equivalent "new home" to send the corresponding Concise YANG
   Telemetry Stream to.  The maximum interval between confirmable
   message as a part of the Concise YANG Telemetry stream is 24 hours.
   The interval can be chosen smaller and appropriate to the
   requirements of the usage scenario.  Theoretically - but not
   necessarily advisable in a constrained-node environment - every CoAP
   response can be send in a confirmable message.

7.  Subscription Management (better word?)

7.1.  YANG RPCs

7.2.  NETCONF Access Control Model [RFC6536bis]

8.  Selection Filters

8.1.  SID for subtree Selection Filter

8.2.  CBOR-YANG for XPATH-like Selection Filter







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9.  Update Triggers for Periodic Subscriptions

9.1.  Interval

9.2.  Anchor Time

10.  Update Triggers for On-Change Subscriptions

10.1.  dampening period

   o  requires bundled messages, in order to maintain visibility

   o  applies to update record creation, not transmission

10.2.  change-type

   o  create, delete, change (where is the complete list?)

10.3.  no sync-on-start

   o  full set at start of sub is omitted

11.  YANG Telemetry Operations for COMI

   Every subscription-id is created by the YANG datastore and is used in
   the corresponding subscription state to provide the root identifier,
   by which dedicated subscription characteristics are associated with
   an established subscription.  In consequence, the basic interaction
   model of Concise YANG Telemetry is split into two operations that are
   initiated by the YANG client in sequence:

   o  a POST operation on /c executing the establish-subscription RPC
      corresponding to the included request body with content-type
      "application/yang-value+cbor" that returns the subscription-id (or
      an error response) in an "application/yang-value+cbor" response

   o  a GET Observe operation on the event stream resource /s/
      subscription-id or a FETCH operation on /s including a FETCH body
      with content-format "application/yang-selectors+cbor" and one or
      more subscription-id as content.

11.1.  Extension of the CoMI Event Stream Resource

   A standard CoMI datastore as defined in [I-D.ietf-core-comi]
   typically uses the datastore resource "/c" to provide the YANG
   datastore tree and the resource "/s" to provide the YANG notification
   stream.  Sub-resources under "/c" are represented in the format of
   /c/sid.



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   Concise YANG Telemetry extends the scope of the "/s" resource.  Sub-
   resources under "/s" are represented as /s/key, where key is a
   numeric string representation of the subscription identifier, e.g.
   "/s/65536/".  The key representation reduces the ambiguity with
   respect to sid, which uses an URI safe base64 representation.

11.2.  Extension of the CoMI Datastore Resource

   Each subscription identifier key is instantiated as a sub-resource of
   the "/c/subid" resource, a YANG tree instance of the subscription
   characteristics yang:ietf-subscribed-notifications/subscriptions (as
   defined in YANG Datastore Subscription [I-D.ietf-netconf-yang-push],
   which augments ietf-subscribed-notification defined in
   [I-D.ietf-netconf-subscribed-notifications]) is provided here for
   each active subscription.

11.3.  Extension of the YANG Subscription Mechanism

   YANG Customized Subscriptions provides augmented RPC for
   establishing, modifying, deleting, or killing a subscription.  CoMI
   uses the same module as YANG Datastore Subscription and provides a
   corresponding interface to allow for a corresponding confirmable POST
   message to RPC resources (see [I-D.ietf-core-comi] Section 5.3.2.).

   Concise YANG Telemetry also defines the capabilities to point
   confirmable FETCH messages - including the Observe option - to sub-
   resources provided by "/c".  If the body of the FETCH message
   includes a CBOR modeled [I-D.ietf-core-yang-cbor] subtree filter
   expression, a new subscription is created and a corresponding
   subscription id is returned.  Additionally, a corresponding
   subscription sub-resource under "/s" is created.

   As usual in CoMI, iPATCH requests can be used to perform a number of
   operations on the datastore in one request, such as deleting,
   creating, and updating subscriptions.

12.  Upcoming Features and Stories

   o  definition of a module that populates /c with sub-resources
      representing the Subscription Characteristics for every active
      subscription.

13.  IANA considerations

   This document includes no requests to IANA, but solutions drafts
   incubated via this document might.





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14.  Security Considerations

   This document includes no security considerations, but solution
   drafts incubated via this document will.

15.  Acknowledgements

   Carsten Bormann, Klaus Hartke, Michel Veillette

16.  Change Log

   First version -00

17.  Normative References

   [I-D.bormann-t2trg-stp]
              Bormann, C. and K. Hartke, "The Series Transfer Pattern
              (STP)", draft-bormann-t2trg-stp-01 (work in progress),
              July 2018.

   [I-D.ietf-core-comi]
              Veillette, M., Stok, P., Pelov, A., and A. Bierman, "CoAP
              Management Interface", draft-ietf-core-comi-03 (work in
              progress), June 2018.

   [I-D.ietf-core-yang-cbor]
              Veillette, M., Pelov, A., Somaraju, A., Turner, R., and A.
              Minaburo, "CBOR Encoding of Data Modeled with YANG",
              draft-ietf-core-yang-cbor-06 (work in progress), February
              2018.

   [I-D.ietf-netconf-notification-messages]
              Voit, E., Birkholz, H., Bierman, A., Clemm, A., and T.
              Jenkins, "Notification Message Headers and Bundles",
              draft-ietf-netconf-notification-messages-03 (work in
              progress), February 2018.

   [I-D.ietf-netconf-subscribed-notifications]
              Voit, E., Clemm, A., Prieto, A., Nilsen-Nygaard, E., and
              A. Tripathy, "Customized Subscriptions to a Publisher's
              Event Streams", draft-ietf-netconf-subscribed-
              notifications-13 (work in progress), June 2018.

   [I-D.ietf-netconf-yang-push]
              Clemm, A., Voit, E., Prieto, A., Tripathy, A., Nilsen-
              Nygaard, E., Bierman, A., and B. Lengyel, "YANG Datastore
              Subscription", draft-ietf-netconf-yang-push-17 (work in
              progress), July 2018.



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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
              <https://www.rfc-editor.org/info/rfc5277>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [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,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8071]  Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
              RFC 8071, DOI 10.17487/RFC8071, February 2017,
              <https://www.rfc-editor.org/info/rfc8071>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.



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Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   Darmstadt  64295
   Germany

   Email: henk.birkholz@sit.fraunhofer.de


   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com




































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