Tech-invite3GPPspaceIETF RFCsSIP
in Index   Prev   Next

RFC 8640

Dynamic Subscription to YANG Events and Datastores over NETCONF

Pages: 19
Proposed Standard

Top   ToC   RFC8640 - Page 1
Internet Engineering Task Force (IETF)                           E. Voit
Request for Comments: 8640                                 Cisco Systems
Category: Standards Track                                       A. Clemm
ISSN: 2070-1721                                                Futurewei
                                                      A. Gonzalez Prieto
                                                       E. Nilsen-Nygaard
                                                             A. Tripathy
                                                           Cisco Systems
                                                          September 2019

    Dynamic Subscription to YANG Events and Datastores over NETCONF


This document provides a Network Configuration Protocol (NETCONF) binding to the dynamic subscription capability of both subscribed notifications and YANG-Push. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at
Top   ToC   RFC8640 - Page 2
Copyright Notice

   Copyright (c) 2019 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.
Top   ToC   RFC8640 - Page 3

Table of Contents

1. Introduction ....................................................3 2. Terminology .....................................................3 3. Compatibility with <create-subscription> as Defined in RFC 5277 ........................................................4 4. Mandatory XML, Event Stream, and Datastore Support ..............4 5. NETCONF Connectivity and Dynamic Subscriptions ..................4 6. Notification Messages ...........................................5 7. Dynamic Subscriptions and RPC Error Responses ...................5 8. Security Considerations .........................................7 9. IANA Considerations .............................................7 10. References .....................................................7 10.1. Normative References ......................................7 10.2. Informative References ....................................8 Appendix A. Examples ...............................................9 A.1. Event Stream Discovery ......................................9 A.2. Dynamic Subscriptions ......................................10 A.3. Subscription State Notifications ...........................15 A.4. Filter Examples ............................................17 Acknowledgments ...................................................19 Authors' Addresses ................................................19

1. Introduction

This document specifies the binding of a stream of events that form part of a dynamic subscription to the Network Configuration Protocol (NETCONF) [RFC6241]. Dynamic subscriptions are defined in [RFC8639]. In addition, as [RFC8641] is itself built upon [RFC8639], this document enables a NETCONF client to request via a dynamic subscription, and receive, updates from a YANG datastore located on a NETCONF server. This document assumes that the reader is familiar with the terminology and concepts defined in [RFC8639].

2. Terminology

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. The following terms are defined in [RFC8639]: dynamic subscription, event stream, notification message, publisher, receiver, subscriber, and subscription. This document does not define any additional terms.
Top   ToC   RFC8640 - Page 4

3. Compatibility with <create-subscription> as Defined in RFC 5277

A publisher is allowed to concurrently support dynamic subscription RPCs as defined in [RFC8639] at the same time as the <create-subscription> RPC defined in [RFC5277]. However, a single NETCONF transport session MUST NOT support both this specification and a subscription established by the <create-subscription> RPC defined in [RFC5277]. To protect against any attempts to use a single NETCONF transport session in this way: o A solution MUST reply with the <rpc-error> element [RFC6241] containing the "error-tag" value of "operation-not-supported" if a <create-subscription> RPC is received on a NETCONF session where an established subscription per [RFC8639] exists. o A solution MUST reply with the <rpc-error> element [RFC6241] containing the "error-tag" value of "operation-not-supported" if an "establish-subscription" request has been received on a NETCONF session where the <create-subscription> RPC [RFC5277] has successfully created a subscription. If a publisher supports this specification but not subscriptions via [RFC5277], the publisher MUST NOT advertise "urn:ietf:params:netconf:capability:notification:1.0".

4. Mandatory XML, Event Stream, and Datastore Support

The "encode-xml" feature of [RFC8639] MUST be supported. This indicates that XML is a valid encoding for RPCs, state change notifications, and subscribed content. A NETCONF publisher supporting event stream subscription via [RFC8639] MUST support the "NETCONF" event stream identified in that document.

5. NETCONF Connectivity and Dynamic Subscriptions

Management of dynamic subscriptions occurs via RPCs as defined in [RFC8641] and [RFC8639]. For a dynamic subscription, if the NETCONF session involved with the "establish-subscription" terminates, the subscription MUST be terminated. For a dynamic subscription, any "modify-subscription", "delete-subscription", or "resync-subscription" RPCs MUST be sent using the same NETCONF session upon which the referenced subscription was established.
Top   ToC   RFC8640 - Page 5

6. Notification Messages

Notification messages transported over NETCONF MUST be encoded in a <notification> message as defined in [RFC5277], Section 4. And per the <eventTime> object definition provided in [RFC5277], <eventTime> is populated with the event occurrence time. For dynamic subscriptions, all notification messages MUST use the NETCONF transport session used by the "establish-subscription" RPC.

7. Dynamic Subscriptions and RPC Error Responses

When an RPC error occurs as defined in [RFC8639], Section 2.4.6 and [RFC8641], Appendix A, the NETCONF RPC reply MUST include an <rpc-error> element per [RFC6241] with the error information populated as follows: o An "error-type" node of "application". o An "error-tag" node, where the value is a string that corresponds to an identity associated with the error. For the mechanisms specified in this document, this "error-tag" will correspond to the error identities in either (1) [RFC8639], Section 2.4.6, for general subscription errors: error identity uses error-tag ---------------------- ----------------------- dscp-unavailable invalid-value encoding-unsupported invalid-value filter-unsupported invalid-value insufficient-resources resource-denied no-such-subscription invalid-value replay-unsupported operation-not-supported or (2) [RFC8641], Appendix A.1, for subscription errors specific to YANG datastores: error identity uses error-tag --------------------------- ----------------------- cant-exclude operation-not-supported datastore-not-subscribable invalid-value no-such-subscription-resync invalid-value on-change-unsupported operation-not-supported on-change-sync-unsupported operation-not-supported period-unsupported invalid-value update-too-big too-big sync-too-big too-big unchanging-selection operation-failed
Top   ToC   RFC8640 - Page 6
   o  An "error-severity" of "error" (this MAY be included).

   o  An "error-app-tag" node, where the value is a string that
      corresponds to an identity associated with the error, as defined
      in [RFC8639], Section 2.4.6 for general subscriptions and
      [RFC8641], Appendix A.1 for datastore subscriptions.  The specific
      identity to use depends on the RPC for which the error occurred.
      Each error identity will be inserted as the "error-app-tag"
      following the form <modulename>:<identityname>.  An example of
      such a valid encoding would be
      "ietf-subscribed-notifications:no-such-subscription".  Viable
      errors for different RPCs are as follows:

         RPC                     has base identity
         ----------------------  ----------------------------
         establish-subscription  establish-subscription-error
         modify-subscription     modify-subscription-error
         delete-subscription     delete-subscription-error
         kill-subscription       delete-subscription-error
         resync-subscription     resync-subscription-error

   o  In the case of error responses to an "establish-subscription" or
      "modify-subscription" request, there is the option of including an
      "error-info" node.  This node may contain XML-encoded data with
      hints for parameter settings that might lead to successful RPC
      requests in the future.  The yang-data structures from [RFC8639]
      and [RFC8641] that may be returned are as follows:

      establish-subscription returns hints in yang-data structure
      ---------------------- -------------------------------------------
      target: event stream   establish-subscription-stream-error-info
      target: datastore      establish-subscription-datastore-error-info

      modify-subscription    returns hints in yang-data structure
      ---------------------- ----------------------------------------
      target: event stream   modify-subscription-stream-error-info
      target: datastore      modify-subscription-datastore-error-info

      The yang-data included in "error-info" SHOULD NOT include the
      optional leaf "reason", as such a leaf would be redundant with
      information that is already placed in the "error-app-tag".

   In the case of an RPC error resulting from a "delete-subscription",
   "kill-subscription", or "resync-subscription" request, no
   "error-info" needs to be included, as the "subscription-id" is the
   only RPC input parameter and no hints regarding this RPC input
   parameter need to be provided.
Top   ToC   RFC8640 - Page 7

8. Security Considerations

This document does not introduce additional security considerations for dynamic subscriptions beyond those discussed in [RFC8639]. But there is one consideration worthy of more refinement based on the connection-oriented nature of NETCONF. Specifically, if a buggy or compromised NETCONF subscriber sends a number of "establish- subscription" requests, then these subscriptions accumulate and may use up system resources. In such a situation, subscriptions MAY be terminated by terminating the underlying NETCONF session. The publisher MAY also suspend or terminate a subset of the active subscriptions on that NETCONF session in order to reclaim resources and preserve normal operation for the other subscriptions.

9. IANA Considerations

This document has no IANA actions.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <>. [RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008, <>. [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, <>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <>. [RFC8639] Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard, E., and A. Tripathy, "Subscription to YANG Notifications", RFC 8639, DOI 10.17487/RFC8639, September 2019, <>.
Top   ToC   RFC8640 - Page 8
   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <>.

              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation
              REC-xml-20081126, November 2008,

10.2. Informative References

[RFC8347] Liu, X., Ed., Kyparlis, A., Parikh, R., Lindem, A., and M. Zhang, "A YANG Data Model for the Virtual Router Redundancy Protocol (VRRP)", RFC 8347, DOI 10.17487/RFC8347, March 2018, <>. [XPATH] Clark, J. and S. DeRose, "XML Path Language (XPath) Version 1.0", November 1999, <>.
Top   ToC   RFC8640 - Page 9

Appendix A. Examples

This appendix is non-normative. Additionally, the subscription "id" values of 22, 23, 39, and 99 used below are just examples. In production, the actual values of "id" might not be small integers.

A.1. Event Stream Discovery

As defined in [RFC8639], an event stream exposes a continuous set of events available for subscription. A NETCONF client can retrieve the list of available event streams from a NETCONF publisher using the <get> operation against the top-level "streams" container defined in [RFC8639], Section 3.1. The following XML example [W3C.REC-xml-20081126] illustrates the retrieval of the list of available event streams: <rpc message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"> <get> <filter type="subtree"> <streams xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"/> </filter> </get> </rpc> Figure 1: <get> Request for Retrieval of Event Streams After such a request, the NETCONF publisher returns a list of available event streams as well as additional information that might exist in the container.
Top   ToC   RFC8640 - Page 10

A.2. Dynamic Subscriptions

A.2.1. Establishing Dynamic Subscriptions

Figure 2 shows two successful "establish-subscription" RPC requests as per [RFC8639]. The first request is given a subscription "id" of 22, and the second is given an "id" of 23. +------------+ +-----------+ | Subscriber | | Publisher | +------------+ +-----------+ | | | Capability Exchange | |<---------------------------->| | | | | | establish-subscription | |----------------------------->| (a) | RPC Reply: OK, id = 22 | |<-----------------------------| (b) | | | notification message (for 22)| |<-----------------------------| | | | | | establish-subscription | |----------------------------->| | notification message (for 22)| |<-----------------------------| | RPC Reply: OK, id = 23 | |<-----------------------------| | | | | | notification message (for 22)| |<-----------------------------| | notification message (for 23)| |<-----------------------------| | | Figure 2: Multiple Subscriptions over a NETCONF Session
Top   ToC   RFC8640 - Page 11
   To provide examples of the information being transported, example
   messages for interactions (a) and (b) in Figure 2 are detailed below
   (Figures 3 and 4):

<rpc message-id="102" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
    <stream-xpath-filter xmlns:ex="">

              Figure 3: "establish-subscription" Request (a)

   As the NETCONF publisher was able to fully satisfy the request (a),
   the publisher sends the subscription "id" of the accepted
   subscription in its reply message (b):

  <rpc-reply message-id="102"

            Figure 4: A Successful "establish-subscription" (b)
Top   ToC   RFC8640 - Page 12
   If the NETCONF publisher had not been able to fully satisfy the
   request or the subscriber has no authorization to establish the
   subscription, the publisher would have sent an RPC error response.
   For instance, if the "dscp" value of 10 asserted by the subscriber in
   Figure 3 proved unacceptable, the publisher may have returned:

   <rpc-reply message-id="102"

            Figure 5: An Unsuccessful "establish-subscription"

   The subscriber can use this information in future attempts to
   establish a subscription.

A.2.2. Modifying Dynamic Subscriptions

An existing subscription may be modified. The following exchange shows a negotiation of such a modification via several exchanges between a subscriber and a publisher. This negotiation consists of a failed RPC modification request/response followed by a successful one.
Top   ToC   RFC8640 - Page 13
              +------------+                 +-----------+
              | Subscriber |                 | Publisher |
              +------------+                 +-----------+
                    |                              |
                    | notification message (for 23)|
                    |                              |
                    | modify-subscription (id = 23)|
                    |----------------------------->|  (c)
                    | RPC error (with hint)        |
                    |<-----------------------------|  (d)
                    |                              |
                    | modify-subscription (id = 23)|
                    | RPC Reply: OK                |
                    |                              |
                    | notification message (for 23)|
                    |                              |

   Figure 6: Interaction Model for Successful Subscription Modification

   If the subscription being modified in Figure 6 is a datastore
   subscription as per [RFC8641], the modification request made in (c)
   may look like that shown in Figure 7.  As can be seen, the
   modifications being attempted are the application of a new XPath
   filter as well as the setting of a new periodic time interval.

<rpc message-id="303"
    <yp:datastore-xpath-filter xmlns:ex="">

              Figure 7: Subscription Modification Request (c)
Top   ToC   RFC8640 - Page 14
   If the NETCONF publisher can satisfy both changes, the publisher
   sends a positive result for the RPC.  If the NETCONF publisher cannot
   satisfy either of the proposed changes, the publisher sends an RPC
   error response (d).  Figure 8 shows an example RPC error response for
   (d) that includes a hint.  This hint is an alternative time period
   value that might have resulted in a successful modification:

   <rpc-reply message-id="303"

           Figure 8: "modify-subscription" Failure with Hint (d)

A.2.3. Deleting Dynamic Subscriptions

Figure 9 demonstrates the deletion of a subscription. This subscription may have been to either a stream or a datastore. <rpc message-id="103" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"> <delete-subscription xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"> <id>22</id> </delete-subscription> </rpc> Figure 9: "delete-subscription"
Top   ToC   RFC8640 - Page 15
   If the NETCONF publisher can satisfy the request, the publisher
   returns a reply indicating success.

   If the NETCONF publisher cannot satisfy the request, the publisher
   sends an <rpc-error> element indicating that the modification didn't
   work.  Figure 10 shows a valid response for an existing valid
   subscription "id", but that subscription "id" was created on a
   different NETCONF transport session:

   <rpc-reply message-id="103"

             Figure 10: An Unsuccessful "delete-subscription"

A.3. Subscription State Notifications

A publisher will send subscription state notifications for dynamic subscriptions according to the definitions in [RFC8639].

A.3.1. "subscription-modified"

As per Section 2.7.2 of [RFC8639], a "subscription-modified" might be sent over NETCONF if the definition of a configured filter changes. A subscription state notification encoded in XML would look like: <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0"> <eventTime>2007-09-01T10:00:00Z</eventTime> <subscription-modified xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"> <id>39</id> <stream-xpath-filter xmlns:ex=""> /ex:foo </stream-xpath-filter> <stream>NETCONF</stream> </subscription-modified> </notification> Figure 11: "subscription-modified" Subscription State Notification
Top   ToC   RFC8640 - Page 16

A.3.2. "subscription-resumed" and "replay-complete"

A "subscription-resumed" would look like: <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0"> <eventTime>2007-09-01T10:00:00Z</eventTime> <subscription-resumed xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"> <id>39</id> </subscription-resumed> </notification> Figure 12: "subscription-resumed" Notification The "replay-complete" is virtually identical, with "subscription- resumed" simply being replaced by "replay-complete".

A.3.3. "subscription-terminated" and "subscription-suspended"

A "subscription-terminated" would look like: <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0"> <eventTime>2007-09-01T10:00:00Z</eventTime> <subscription-terminated xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"> <id>39</id> <reason> suspension-timeout </reason> </subscription-terminated> </notification> Figure 13: "subscription-terminated" Subscription State Notification The "subscription-suspended" is virtually identical, with "subscription-terminated" simply being replaced by "subscription- suspended".
Top   ToC   RFC8640 - Page 17

A.4. Filter Examples

This appendix provides examples that illustrate both XPath and subtree methods of filtering event record contents. The examples are based on the YANG notification "vrrp-protocol-error-event" as defined per the ietf-vrrp YANG data model in [RFC8347]. Event records based on this specification that are generated by the publisher might appear as: <notification xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0"> <eventTime>2018-09-14T08:22:33.44Z</eventTime> <vrrp-protocol-error-event xmlns="urn:ietf:params:xml:ns:yang:ietf-vrrp"> <protocol-error-reason>checksum-error</protocol-error-reason> </vrrp-protocol-error-event> </notification> Figure 14: Example VRRP Notification per RFC 8347 Suppose that a subscriber wanted to establish a subscription that only passes instances of event records where there is a "checksum-error" as part of a Virtual Router Redundancy Protocol (VRRP) protocol event. Also, assume that the publisher places such event records into the NETCONF stream. To get a continuous series of matching event records, the subscriber might request the application of an XPath filter against the NETCONF stream. An "establish- subscription" RPC to meet this objective might be: <rpc message-id="601" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"> <establish-subscription xmlns="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"> <stream>NETCONF</stream> <stream-xpath-filter xmlns="urn:ietf:params:xml:ns:yang:ietf-vrrp"> /vrrp-protocol-error-event[ vrrp:protocol-error-reason="vrrp:checksum-error"] </stream-xpath-filter> </establish-subscription> </rpc> Figure 15: Establishing a Subscription Error Reason via XPath For more examples of XPath filters, see [XPATH].
Top   ToC   RFC8640 - Page 18
   Suppose that the "establish-subscription" in Figure 15 was accepted.
   And suppose that a subscriber decided later on that they wanted to
   broaden this subscription to cover all VRRP protocol events (i.e.,
   not just those with a "checksum-error").  The subscriber might
   attempt to modify the subscription in a way that replaces the XPath
   filter with a subtree filter that sends all VRRP protocol events to a
   subscriber.  Such a "modify-subscription" RPC might look like:

 <rpc message-id="602" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">

               Figure 16: Example "modify-subscription" RPC

   For more examples of subtree filters, see [RFC6241], Section 6.4.
Top   ToC   RFC8640 - Page 19


We wish to acknowledge the helpful contributions, comments, and suggestions that were received from Andy Bierman, Yan Gang, Sharon Chisholm, Hector Trevino, Peipei Guo, Susan Hares, Tim Jenkins, Balazs Lengyel, Martin Bjorklund, Mahesh Jethanandani, Kent Watsen, Qin Wu, and Guangying Zheng.

Authors' Addresses

Eric Voit Cisco Systems Email: Alexander Clemm Futurewei Email: Alberto Gonzalez Prieto Microsoft Email: Einar Nilsen-Nygaard Cisco Systems Email: Ambika Prasad Tripathy Cisco Systems Email: