| RFC 8650 | RESTCONF Transport for Event Notifications | November 2019 |
| Voit, et al. | Standards Track | [Page] |
This document provides a RESTCONF binding to the dynamic subscription capability of both subscribed notifications and YANG-Push.¶
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 athttps://www.rfc-editor.org/info/rfc8650.¶
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 (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 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.¶
Mechanisms to support event subscription and YANG-Push are defined in[RFC8639]. Enhancements to[RFC8639] that enable YANG datastore subscription and YANG-Push are defined in[RFC8641]. This document provides a transport specification for dynamic subscriptions over RESTCONF[RFC8040]. Requirements for these mechanisms are captured in[RFC7923].¶
The streaming of notifications that encapsulate the resulting information push is done via the mechanism described inSection 6.3 of [RFC8040].¶
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 use the definitions from[RFC8639]: dynamic subscription, event stream, notification message, publisher, receiver, subscriber, and subscription.¶
Other terms reused include datastore, which is defined in[RFC8342], and HTTP/2 stream, which maps to the definition of "stream" within[RFC7540],Section 2.¶
This section provides specifics on how to establish and maintain dynamic subscriptions over RESTCONF[RFC8040]. Subscribing to event streams is accomplished in this way via RPCs defined within[RFC8639],Section 2.4. The RPCs are done via RESTCONF POSTs. YANG datastore subscription is accomplished via augmentations to[RFC8639] as described within[RFC8641],Section 4.4.¶
As described inSection 6.3 of [RFC8040], a GET needs to be performed on a specific URI on the publisher. Subscribers cannot predetermine the URI against which a subscription might exist on a publisher, as the URI will only exist after the "establish-subscription" RPC has been accepted. Therefore, the POST for the "establish-subscription" RPC replaces the GET request for the "location" leaf that is used in[RFC8040] to obtain the URI. The subscription URI will be determined and sent as part of the response to the "establish-subscription" RPC, and a subsequent GET to this URI will be done in order to start the flow of notification messages back to the subscriber. As specified inSection 2.4.1 of [RFC8639], a subscription does not move to the active state until the GET is received.¶
For a dynamic subscription, when a RESTCONF session doesn't already exist, a new RESTCONF session is initiated from the subscriber.¶
As stated inSection 2.1 of [RFC8040], a subscriberMUST establish the HTTP session over TLS[RFC8446] in order to secure the content in transit.¶
Without the involvement of additional protocols, HTTP sessions by themselves do not support quick recognition of the loss of the communication path to the publisher. Where quick recognition of the loss of a publisher is required, a subscriberSHOULD use a TLS heartbeat[RFC6520], just from subscriber to publisher, to track HTTP session continuity.¶
Loss of the heartbeatMUST result in the teardownof any subscription-related TCP sessions between those endpoints. A subscriber can then attempt to re-establish the dynamic subscription by using the procedure described inSection 3.4.¶
Subscribers can learn which event streams a RESTCONF server supports by querying the "streams" container of ietf-subscribed-notifications.yang in[RFC8639]. Support for the "streams" container of ietf-restconf-monitoring.yang in[RFC8040] is not required. In the case when the RESTCONF binding specified by this document is used to convey the "streams" container from ietf-restconf-monitoring.yang (i.e., that feature is supported), any event streams contained therein are also expected to be present in the "streams" container of ietf-restconf-monitoring.yang.¶
Subscribers can learn which datastores a RESTCONF server supports by followingSection 2 of [RFC8527].¶
Specific HTTP response codes as defined inSection 6 of [RFC7231] will indicate the result of RESTCONF RPC requests with the publisher. An HTTP status code of 200 is the proper response to any successful RPC defined within[RFC8639] or[RFC8641].¶
If a publisher fails to serve the RPC request for one of the reasons indicated inSection 2.4.6 of [RFC8639] orAppendix A of [RFC8641], this will be indicated by an appropriate error code, as shown below, transported in the HTTP response.¶
When an HTTP error code is returned, the RPC replyMUST include an <rpc-error> element perSection 7.1 of [RFC8040] with the following parameter values:¶
| Error identity | Uses "error-tag" | HTTP code |
|---|---|---|
| dscp-unavailable | invalid-value | 400 |
| encoding-unsupported | invalid-value | 400 |
| filter-unsupported | invalid-value | 400 |
| insufficient-resources | resource-denied | 409 |
| no-such-subscription | invalid-value | 404 |
| replay-unsupported | operation-not-supported | 501 |
| Error identity | Uses "error-tag" | HTTP code |
|---|---|---|
| cant-include | operation-not-supported | 501 |
| datastore-not-subscribable | invalid-value | 400 |
| no-such-subscription-resync | invalid-value | 404 |
| on-change-unsupported | operation-not-supported | 501 |
| on-change-sync-unsupported | operation-not-supported | 501 |
| period-unsupported | invalid-value | 400 |
| update-too-big | too-big | 400 |
| sync-too-big | too-big | 400 |
| unchanging-selection | operation-failed | 500 |
| RPC | Select an identity with a base |
|---|---|
| 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 |
Each error identity will be inserted as the "error-app-tag" using JSON encoding following the form <modulename>:<identityname>. An example of such a valid encoding would be "ietf-subscribed-notifications:no-such-subscription".¶
In the case of error responses to an "establish-subscription" or "modify-subscription" request, there is the option to include an "error-info" node. This node may contain hints for parameter settings that might lead to successful RPC requests in the future. Tables4 and5 show the yang-data structures that may be returned.¶
| Target: | Return hints in yang-data structure |
|---|---|
| event stream | establish-subscription-stream-error-info |
| datastore | establish-subscription-datastore-error-info |
| Target: | Returns hints in yang-data structure |
|---|---|
| event stream | modify-subscription-stream-error-info |
| datastore | modify-subscription-datastore-error-info |
The yang-data included within "error-info"SHOULD NOT include the optional leaf "reason", as such a leaf would be redundant with information that is already placed within the "error-app-tag".¶
In case of an <rpc-error> as a result of a "delete-subscription", a "kill-subscription", or a "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 parameters need to be provided.¶
Note that "error-path"[RFC8040] does not need to be included with the <rpc-error> element, as subscription errors are generally associated with the choice of RPC input parameters.¶
The call flow for Server-Sent Events (SSE) is defined inFigure 1. The logical connections denoted by (a) and (b) can be a TCP connection or an HTTP/2 stream (if HTTP/2 is used, multiple HTTP/2 streams can be carried in one TCP connection). Requests to RPCs as defined in[RFC8639] or[RFC8641] are sent on a connection indicated by (a). A successful "establish-subscription" will result in an RPC response returned with both a subscription identifier that uniquely identifies a subscription, as well as a URI that uniquely identifies the location of subscription on the publisher (b). This URI is defined via the "uri" leaf in the data model inSection 7.¶
An HTTP GET is then sent on a separate logical connection (b) to the URI on the publisher. This signals the publisher to initiate the flow of notification messages that are sent in SSE[W3C-20150203] as a response to the GET. There cannot be two or more simultaneous GET requests on a subscription URI: any GET request received while there is a current GET request on the same URIMUST be rejected with HTTP error code 409.¶
As described inSection 6.4 of [RFC8040], RESTCONF serversSHOULD NOT send the "event" or "id" fields in the SSE event notifications.¶
+--------------+ +--------------+| Subscriber | | Publisher || | | || Logical | | Logical || Connection | | Connection || (a) (b) | | (a) (b) |+--------------+ +--------------+ | RESTCONF POST (RPC:establish-subscription) | |--------------------------------------------->| | HTTP 200 OK (ID,URI)| |<---------------------------------------------| | |HTTP GET (URI) | | |--------------------------------------------->| | | HTTP 200 OK| | |<---------------------------------------------| | | SSE (notif-message)| | |<---------------------------------------------| | RESTCONF POST (RPC:modify-subscription) | | |--------------------------------------------->| | | | HTTP 200 OK| | |<---------------------------------------------| | | | SSE (subscription-modified)| | |<------------------------------------------(c)| | | SSE (notif-message)| | |<---------------------------------------------| | RESTCONF POST (RPC:delete-subscription) | | |--------------------------------------------->| | | | HTTP 200 OK| | |<---------------------------------------------| | | | | | | | | | (a) (b) (a) (b)
Additional requirements for dynamic subscriptions over SSE include:¶
A publisherMUST terminate a subscription in the following cases:¶
A publisherMAY terminate a subscription at any time as stated inSection 1.3 of [RFC8639].¶
Qos treatment for event streams is described inSection 2.3 of [RFC8639]. In addition, if HTTP/2 is used, the publisherMUST:¶
For dynamic subscriptions with the same Differentiated Services Code Point (DSCP) value to a specific publisher, it is recommended that the subscriber sends all URI GET requests on a common HTTP/2 session (if HTTP/2 is used). Conversely, a subscriber cannot use a common HTTP/2 session for subscriptions with different DSCP values.¶
Notification messages transported over RESTCONF will be encoded according to[RFC8040],Section 6.4.¶
The YANG module defined inSection 7 has one leaf that augments three nodes of[RFC8639].¶
module: ietf-restconf-subscribed-notifications augment /sn:establish-subscription/sn:output: +--ro uri? inet:uri augment /sn:subscriptions/sn:subscription: +--ro uri? inet:uri augment /sn:subscription-modified: +--ro uri? inet:uri¶
This module references[RFC8639].¶
<CODE BEGINS> file "ietf-restconf-subscribed-notifications@2019-11-17.yang"module ietf-restconf-subscribed-notifications { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:" + "ietf-restconf-subscribed-notifications"; prefix rsn; import ietf-subscribed-notifications { prefix sn; } import ietf-inet-types { prefix inet; } organization "IETF NETCONF (Network Configuration) Working Group"; contact "WG Web: <https://datatracker.ietf.org/wg/netconf/> WG List: <mailto:netconf@ietf.org> Editor: Eric Voit <mailto:evoit@cisco.com> Editor: Alexander Clemm <mailto:ludwig@clemm.org> Editor: Reshad Rahman <mailto:rrahman@cisco.com>"; description "Defines RESTCONF as a supported transport for subscribed event notifications. Copyright (c) 2019 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC 8650; see the RFC itself for full legal notices."; revision 2019-11-17 { description "Initial version"; reference "RFC 8650: Dynamic Subscription to YANG Events and Datastores over RESTCONF"; } grouping uri { description "Provides a reusable description of a URI."; leaf uri { type inet:uri; config false; description "Location of a subscription-specific URI on the publisher."; } } augment "/sn:establish-subscription/sn:output" { description "This augmentation allows RESTCONF-specific parameters for a response to a publisher's subscription request."; uses uri; } augment "/sn:subscriptions/sn:subscription" { description "This augmentation allows RESTCONF-specific parameters to be exposed for a subscription."; uses uri; } augment "/sn:subscription-modified" { description "This augmentation allows RESTCONF-specific parameters to be included as part of the notification that a subscription has been modified."; uses uri; }}<CODE ENDS>¶This document registers the following namespace URI in the "ns" subregistry of the "IETF XML Registry"[RFC3688]:¶
This document registers the following YANG module in the "YANG Module Names" registry[RFC6020]:¶
The YANG module specified in this document defines a schema for data that is designed to be accessed via network management transports such as NETCONF[RFC6241] or RESTCONF[RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH)[RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS[RFC8446].¶
The Network Configuration Access Control Model (NACM)[RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content.¶
The one new data node introduced in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to this data node. These are the subtrees and data nodes and their sensitivity/vulnerability:¶
Container: "/subscriptions"¶
The subscription URI is implementation specific and is encrypted via the use of TLS. Therefore, even if an attacker succeeds in guessing the subscription URI, a RESTCONF username[RFC8040] with the required administrative permissions must be used to be able to access or modify that subscription. It is recommended that the subscription URI values not be easily predictable.¶
The access permission considerations for the RPCs "modify-subscription", "resync-subscription", "delete-subscription", and "kill-subscription" are described inSection 3.4.¶
If a buggy or compromised RESTCONF subscriber sends a number of "establish-subscription" requests, then these subscriptions accumulate and may use up system resources. In such a situation, the publisherMAY also suspend or terminate a subset of the active subscriptions from that RESTCONF subscriber in order to reclaim resources and preserve normal operation for the other subscriptions.¶
This section is non-normative. To allow easy comparison, this section mirrors the functional examples shown with NETCONF over XML within[RFC8640]. In addition, HTTP/2 vs HTTP/1.1 headers are not shown as the contents of the JSON encoded objects are identical within.¶
The subscription URI values used in the examples in this section are purely illustrative, and are not indicative of the expected usage that is described inSection 9.¶
The DSCP values are only for example purposes and are all indicated in decimal since the encoding is JSON[RFC7951].¶
The following figure shows two successful "establish-subscription" RPC requests as per[RFC8639]. The first request is given a subscription identifier of 22, and the second, an identifier of 23.¶
+------------+ +-----------+ | Subscriber | | Publisher | +------------+ +-----------+ | | |establish-subscription | |------------------------------>| (a) | HTTP 200 OK, id#22, URI#1 | |<------------------------------| (b) |GET (URI#1) | |------------------------------>| (c) | HTTP 200 OK,notif-mesg (id#22)| |<------------------------------| | | | | |establish-subscription | |------------------------------>| | HTTP 200 OK, id#23, URI#2| |<------------------------------| |GET (URI#2) | |------------------------------>| | | | | | notif-mesg (id#22)| |<------------------------------| | HTTP 200 OK,notif-mesg (id#23)| |<------------------------------| | |
To provide examples of the information being transported, example messages for interactions inFigure 2 are detailed below:¶
POST /restconf/operations /ietf-subscribed-notifications:establish-subscription{ "ietf-subscribed-notifications:input": { "stream-xpath-filter": "/example-module:foo/", "stream": "NETCONF", "dscp": 10 }}As the publisher was able to fully satisfy the request, the publisher sends the subscription identifier of the accepted subscription and the URI:¶
HTTP status code - 200{ "id": 22, "uri": "https://example.com/restconf/subscriptions/22"}Upon receipt of the successful response, the subscriber does a GET to the provided URI to start the flow of notification messages. When the publisher receives this, the subscription is moved to the active state (c).¶
GET /restconf/subscriptions/22
While not shown inFigure 2, if the 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 inFigure 3 proved unacceptable, the publisher may have returned:¶
HTTP status code - 400{ "ietf-restconf:errors" : { "error" : [ { "error-type": "application", "error-tag": "invalid-value", "error-severity": "error", "error-app-tag": "ietf-subscribed-notifications:dscp-unavailable" } ] }}The subscriber can use this information in future attempts to establish a subscription.¶
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.¶
+------------+ +-----------+ | Subscriber | | Publisher | +------------+ +-----------+ | | | notification message (id#23)| |<-----------------------------| | | |modify-subscription (id#23) | |----------------------------->| (d) | HTTP 400 error (with hint)| |<-----------------------------| (e) | | |modify-subscription (id#23) | |----------------------------->| | HTTP 200 OK | |<-----------------------------| | | | notif-mesg (id#23)| |<-----------------------------| | |
If the subscription being modified inFigure 7 is a datastore subscription as per[RFC8641], the modification request made in (d) may look like that shown inFigure 8. As can be seen, the modifications being attempted are the application of a new XML Path Language (XPath) filter as well as the setting of a new periodic time interval.¶
POST /restconf/operations /ietf-subscribed-notifications:modify-subscription{ "ietf-subscribed-notifications:input": { "id": 23, "ietf-yang-push:datastore-xpath-filter": "/example-module:foo/example-module:bar", "ietf-yang-push:periodic": { "ietf-yang-push:period": 500 } }}If the publisher can satisfy both changes, the publisher sends a positive result for the RPC. If the publisher cannot satisfy either of the proposed changes, the publisher sends an RPC error response (e). The following is an example RPC error response for (e) that includes a hint. This hint is an alternative time period value that might have resulted in a successful modification:¶
HTTP status code - 400{ "ietf-restconf:errors" : { "error" : [ "error-type": "application", "error-tag": "invalid-value", "error-severity": "error", "error-app-tag": "ietf-yang-push:period-unsupported", "error-info": { "ietf-yang-push": "modify-subscription-datastore-error-info": { "period-hint": 3000 } } ] }}The following demonstrates deleting a subscription. This subscription may have been to either a stream or a datastore.¶
POST /restconf/operations /ietf-subscribed-notifications:delete-subscription{ "delete-subscription": { "id": "22" }}If the publisher can satisfy the request, the publisher replies with success to the RPC request.¶
If the publisher cannot satisfy the request, the publisher sends an <rpc-error> element indicating the modification didn't work.Figure 11 shows a valid response for an existing valid subscription identifier, but that subscription identifier was created on a different transport session:¶
HTTP status code - 404{ "ietf-restconf:errors" : { "error" : [ "error-type": "application", "error-tag": "invalid-value", "error-severity": "error", "error-app-tag": "ietf-subscribed-notifications:no-such-subscription" ] }}A publisher will send subscription state notifications according to the definitions within[RFC8639].¶
A "subscription-modified" encoded in JSON would look like:¶
{ "ietf-restconf:notification" : { "eventTime": "2007-09-01T10:00:00Z", "ietf-subscribed-notifications:subscription-modified": { "id": 39, "uri": "https://example.com/restconf/subscriptions/22" "stream-xpath-filter": "/example-module:foo", "stream": { "ietf-netconf-subscribed-notifications" : "NETCONF" } } }}¶A "subscription-completed" notification would look like:¶
{ "ietf-restconf:notification" : { "eventTime": "2007-09-01T10:00:00Z", "ietf-subscribed-notifications:subscription-completed": { "id": 39, } }}¶The "subscription-resumed" and "replay-complete" are virtually identical, with "subscription-completed" simply being replaced by "subscription-resumed" and "replay-complete".¶
A "subscription-terminated" would look like:¶
{ "ietf-restconf:notification" : { "eventTime": "2007-09-01T10:00:00Z", "ietf-subscribed-notifications:subscription-terminated": { "id": 39, "error-id": "suspension-timeout" } }}¶The "subscription-suspended" is virtually identical, with "subscription-terminated" simply being replaced by "subscription-suspended".¶
This section provides an example that illustrates the method of filtering event record contents. The example is based on the YANG notification "vrrp-protocol-error-event" as defined per the ietf-vrrp.yang module within[RFC8347]. Event records based on this specification that are generated by the publisher might appear as:¶
data: {data: "ietf-restconf:notification" : {data: "eventTime" : "2018-09-14T08:22:33.44Z",data: "ietf-vrrp:vrrp-protocol-error-event" : {data: "protocol-error-reason" : "checksum-error"data: }data: }data: }Suppose 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 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:¶
POST /restconf/operations /ietf-subscribed-notifications:establish-subscription{ "ietf-subscribed-notifications:input": { "stream": "NETCONF", "stream-xpath-filter": "/ietf-vrrp:vrrp-protocol-error-event[ protocol-error-reason='checksum-error']/", }}For more examples of XPath filters, see[XPATH].¶
Suppose the "establish-subscription" inFigure 16 was accepted. And suppose later a subscriber decided they wanted to broaden this subscription cover to 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:¶
POST /restconf/operations /ietf-subscribed-notifications:modify-subscription{ "ietf-subscribed-notifications:input": { "stream": "NETCONF", "stream-subtree-filter": { "/ietf-vrrp:vrrp-protocol-error-event" : {} } }}For more examples of subtree filters, see[RFC6241],Section 6.4.¶
We wish to acknowledge the helpful contributions, comments, and suggestions that were received from Ambika Prasad Tripathy, Alberto Gonzalez Prieto, Susan Hares, Tim Jenkins, Balazs Lengyel, Kent Watsen, Michael Scharf, Guangying Zheng, Martin Bjorklund, Qin Wu, and Robert Wilton.¶