Network Working Group A. Clemm
Internet-Draft A. Gonzalez Prieto
Intended status: Experimental E. Voit
Expires: April 30, 2015 Cisco Systems
October 27, 2014
Subscribing to datastore push updates
draft-netmod-clemm-datastore-push-00.txt
Abstract
This document defines a subscription and push mechanism for
datastores. This mechanism allows client applications to request
updates from a datastore, which are then pushed by the server to the
client per a subscription policy, without requiring additional client
requests.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 4
3. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Subscription Model . . . . . . . . . . . . . . . . . . . 5
3.2. Negotiation of Subscription Policies . . . . . . . . . . 7
3.3. Push Data Stream and Transport Mapping . . . . . . . . . 8
3.4. Other considerations . . . . . . . . . . . . . . . . . . 9
3.4.1. Authorization . . . . . . . . . . . . . . . . . . . . 9
3.4.2. Subscription status and subscription monitoring . . . 9
3.4.3. Implementation considerations . . . . . . . . . . . . 10
4. YANG module . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Normative References . . . . . . . . . . . . . . . . . . 17
6.2. Informative References . . . . . . . . . . . . . . . . . 17
1. Introduction
YANG datastores, i.e. datastores that contain data modeled according
using YANG [RFC6020], are not restricted to configuration data, but
can also contain operational data. It is therefore reasonable to
expect that data in YANG datastores will increasingly be used to
support applications that are not focused on managing configurations
but that are, for example, related to service assurance.
Service assurance applications typically involve monitoring
operational state of networks and devices; of particular interest are
changes that this data undergoes over time. Likewise, there are
applications in which data and objects from one datastore need to be
made available to applications in other systems and to remote
datastores [peermount-req], requiring mechanisms that allow remote
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systems to become quickly aware of any updates to allow to validate
and maintain cross-network integrity and consistency.
Traditional approaches rely heavily on polling, in which data is
periodically explicitly retrieved by a client from a server.
There are various issues associated with polling-based management:
o It introduces additional load on network and devices. Each
polling cycle requires a separate yet arguably redundant request
that results in an interrupt, requires parsing, consumes
bandwidth.
o It lacks robustness. Polling cycles may be missed, requests may
be delayed or get lost, often particularly in cases when the
network is under stress and hence exactly when the need for the
data is the greatest.
o Data may be difficult to calibrate and compare. Polling cycles
may undergo slight fluctuations, resulting in intervals of
different lengths which makes data hard to compare. Likewise,
pollers may have difficulty issuing requests that reach all
devices at the same time, resulting in offset polling intervals
which again make data hard to compare.
More effective is an alternative in which an application can request
to be automatically updated of current content of the datastore (such
as a subtree, or data in a subtree that meets a certain filter
condition), and in which the server subsequently pushes those
updates.
The need to perform polling-based management is typically considered
an important shortcoming of management applications that rely on MIBs
polled using SNMP [RFC1157]. However, without a provision to support
a push-based alternative, there is no reason to believe that
management applications that operate on YANG datastores using
protocols such as NETCONF [RFC6241] or RESTCONF [restconf] will be
any more effective, as they would follow the same request/response
pattern.
While YANG allows to define notifications, such notifications are
generally intended to indicate the occurrence of certain well-
specified event conditions, such as a the onset of an alarm condition
or the occurrence of an error. Likewise, a capability to define
configuration change events has been defined in [RFC5277]. However,
these change events pertain only to configuration information, not to
operational state. RFC 5277 furthermore predates YANG and does not
provide tie-in with YANG-defined datastore contents.
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Service Assurance applications are not the only applications
benefiting from a push- and subscription-based alternative to
polling. Another example is Peer Mount [peermount]. Peer Mount
allows a datastore to incorporate data from remote datastores by
reference, resulting in virtual datastores that are federated across
a network and offer different local views. Various use cases
indicate the usefulness of introducing caching in conjunction with
Peer Mount, which benefits greatly if updates can automatically be
pushed from a mount server to a mount client.
The way in which the updates are to occur can be directed by policy.
For example, a client may request to be updated periodically in
certain intervals, or whenever data changes occur.
Because not every server may support every requested interval for
every piece of data, it is furthermore necessary for a server to be
able to indicate whether or not it is capable of supporting a
requested subscription, and possibly allow to negotiate subscription
parameters.
Finally, a mechanism is needed to communicate the updates themselves.
One option is to use existing NETCONF and RESTCONF mechanisms, by
defining special notifications with which to carry those updates.
Other alternatives are conceivable, such as use of a dedicated
publish/subscribe mechanism that provides an alternative to a NETCONF
or RESTCONF transport.
This document specifies a YANG data model for the configuration and
management of subscriptions to data in YANG datastores. It also
defines a notification that can be used to carry data updates and
thus serve as push mechanism.
2. Definitions and Acronyms
Data node: An instance of management information in a YANG datastore.
Datastore: A conceptual store of instantiated management information,
with individual data items represented by data nodes which are
arranged in a hierarchical manner.
Data subtree: An instantiated data node and the data nodes that are
hierarchically contained within it.
Mount client: The system at which a mount point resides, into which
the remote subtree is mounted.
Mount point: A data node that receives the root node of the remote
datastore being mounted.
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Mount server: The server with which the mount client communicates and
which provides the mount client with access to the mounted
information. Can be used synonymously with mount target.
Mount target: A remote server whose datastore is being mounted.
NACM: NETCONF Access Control Model
NETCONF: Network Configuration Protocol
Peer Mount: An extension to the YANG management framework that allows
local YANG datastores to incorporate data from remote (peer) YANG
datastores.
RPC: Remote Procedure Call
Remote datastore: A datastore residing at a remote node
SNMP: Simple Network Management Protocol
URI: Uniform Resource Identifier
YANG: A data definition language for NETCONF
3. Solution Overview
This document specifies a solution that allows clients to subscribe
to information updates in a YANG datastore, which are subsequently
pushed from the server to the client. The solution encompasses
several components:
o The configuration and management of the subscriptions.
o An ability to negotiate subscription parameters where a
subscription policy desired by a client cannot be supported.
o The datastream of the push updates.
In addition, there are a number of additional considerations, such as
the tie-in of the mechanisms with security mechanisms. Each of those
aspects will be discussed in the following subsections.
3.1. Subscription Model
Yang allows modeling the content of notifications. The contents are
a set of explicitly stated data nodes forming a hierarchy. For
modeling updates in a datastore, a new generic notification is
introduced, the "push-update". This notification has the following
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semantics. The contents of the notification are not explicitly
stated. They are the union of the data nodes in the yang modules
supported by the server, excluding the following statements:
"mandatory", "must", "min-elements", "max-elements", "when", and
"default". Note that the notification contents are dynamic,
depending on the modules supported by the server.
Subscriptions to the "push-update" are initiated by clients. Servers
respond to a subscription request explicitly positively or
negatively. Negative responses include information describing the
reason for the subscription rejection.
Datastore-push subscriptions are defined using a data model. This
model is based on the subscriptions defined in [RFC-5277], which is
also reused in RESTCONF. The model is extended with a subscription
type a set of parameters for each type. The complete set of
subscription parameters is:
o The name of the stream to subscribe to. The stream is called
"push-update".
o The identity of the subscriber.
o An optional filter. It describes the subset of stream events of
interest to the subscriber. The server should only send to the
subscriber the events that match the filter, when present. The
absence of a filter indicates that all events in the stream are of
interest to the subscriber and all events in it must be sent to
the subscriber. Two filtering mechanisms are considered: subtree
filtering and Xpath filtering, with the semantics described in
[RFC5277].
o An optional start time. Used to trigger replays starting at the
provided time. Its semantics are those in [RFC5277].
o An optional stop time. Used to limit temporarily the events of
interest. Its semantics are those in [RFC5277].
o A notification trigger definition. The trigger can be periodic or
based on change. For periodic subscriptions, the trigger is
defined by the interval with which to push updates. For on-change
subscriptions, the trigger is defined using the dampening interval
with which to push repeated changes, an indicator forthe magnitude
of changes, etc.
The following figure depicts the data model.
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module: ietf-datastore-push
+--rw datastore-push-subscription
+--rw stream string
+--rw subscription-id subscription-identifier
+--rw (filter)?
| +--:(substree)
| | +--rw subtree-filter
| +--:(xpath)
| +--rw xpath-filter yang:xpath1.0
+--rw (notification-trigger)
| +--:(periodic)
| | +--rw period yang:timeticks
| +--:(on-change)
| +--rw (change-policy)
| +--:(delta-policy)
| +--rw delta uint32
+--rw start-time? yang:date-and-time
+--rw stop-time? yang:date-and-time
Figure 1: Model structure
The example below illustrates a subcription for a periodic push of
all data under a container called foo.
<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<create-subscription
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<stream>push-update</stream>
<subscription-id>foo</subscription-id>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/dspush/1.0"
select="/ex:foo"/>
<period>500</period>
</filter>
</create-subscription>
</netconf:rpc>
Figure 2: Subscription example
3.2. Negotiation of Subscription Policies
A subscription rejection can be caused by the inability of the server
to provide a stream with the requested semantics. Providing "on-
change" updates for operational data can be computationally expensive
and an agent may decide not to support them or supporting them for a
small number of subscribers or for a limited set of data nodes.
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Datastore-push supports a simple negotiation between clients and
servers for subscription parameters. The negotiation is limited to a
single pair of subscription request and response. For negative
responses, the server SHOULD include in the returned error what
subscription parameters would have been accepted for the request.
The returned acceptable parameters are no guarantee for subsequent
requests for this client or others.
The example below illustrates a subcription response, where an agent
does not support frequent periodic updates, and suggests a different
sampling rate to the client.
<netconf:rpc message-id="101"
xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
<create-subscription
xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
<stream>push-update</stream>
<subscription-id>foo</subscription-id>
<filter netconf:type="xpath"
xmlns:ex="http://example.com/dspush/1.0"
select="/ex:foo"/>
<period>500</period>
</filter>
</create-subscription>
</netconf:rpc>
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<rpc-error>
<error-type>application</error-type>
<error-tag>operation-not-supported</error-tag>
<error-severity>error</error-severity>
<error-info>
<supported-subcription>
<period>3000</period>
</supported-subcription>
</error-info>
</rpc-error>
</rpc-reply>
Figure 3: Subscription negotiation example
3.3. Push Data Stream and Transport Mapping
Pushing data based on a subscription could be considered analogous to
a response to a data retrieval request, e.g. a "get" request.
However, contrary to such a request, multiple responses to the same
request may get sent over a longer period of time. Likewise, clients
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need to be able to distinguish between data updates and state update
regarding the subscription itself, for example when a subscription
can no longer be serviced.
A more suitable mechanism is therefore that of a notification.
Contrary to notifications associated with alarms and unexpected event
occurrences, push updates are solicited, i.e. tied tied to a
particular subscription which triggered the notification. (An
alternative conceptual model would consider a subscription an "opt-
in" filter on a continuous stream of updates.)
The notification contains several parameters:
o A subscription correlator, referencing the name of the
subscription on whose behalf the notification is sent.
o A data node that contains a representation of the datastore
subtree containing the updates. The subtree is filtered per
access control rules to contain only data that the subscriber is
authorized to see. Also, depending on the subscription type,
i.e., specifically for on-change subscriptions, the subtree
contains only the data nodes that contain actual changes. (This
can be simply a node of type string or, for XML-based encoding,
anyxml.)
Notifications are sent using <notification> elements as defined in
[RFC5277]. Alternative transports are conceivable but outside the
scope of this specification.
3.4. Other considerations
3.4.1. Authorization
A client may only receive updates to data that the client has proper
authorization for. Normal authorization rules apply. Data that is
being pushed therefore needs to be subjected to a filter that applies
the corresponding rules, removing any non-authorized data as
applicable.
The authorization model for data in YANG datastores is described in
the Netconf Access Control Model [RFC6536].
3.4.2. Subscription status and subscription monitoring
It is possible that a server may no longer be able to serve a
subscription that had been previously accepted. For example, a
server may have run out of resources, or internal errors may have
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occurred. When this is the case, a server needs to be able to
temporarily suspend the subscription, or even to terminate it.
For this reason, a server SHALL maintain status information for each
subscription that indicates the current status of the subscription.
In addition, a server needs to indicate any changes in status to the
subscriber through a notification. Specifically, subscribers need to
be informed of the following:
o A subscription has been temporarily suspended, including the
reason. (See subscription-suspended in the model below.)
o A subscription (that had been suspended earlier) is once again
operational. (See subscription-resumed in the model below.)
o A subscription has been abnormally terminated, including the
reason. (See subscription-terminated in the model below.)
Finally, a server might provide additional information about
subscriptions, such as statistics about the number of data updates
that were sent. However, such information is currently outside the
scope of this specification.
3.4.3. Implementation considerations
Implementation specifics are outside the scope of this specification.
That said, it should be noted that monitoring of operational state
changes inside a system can be associated with significant
implementation challenges.
Even periodic retrieval of operational state alone, to be able to
push it, can consume considerable system resources. Configuration
data may in many cases be persisted in an actual database or a
configuration file, where retrieval of the database content or the
file itself is reasonably straightforward and computationally
inexpensive. However, retrieval of operational data may, depending
on the implementation, require invocation of APIs, possibly on an
object-by-object basis, possibly involving additional internal
interrupts, etc.
For those reasons, if is important for an implementation to
understand what subscriptions it can or cannot support. It is far
preferrable to decline a subscription request, than to accept it only
to result in subsequent failure later.
Whether or not a subscription can be supported will in general be
determined by a combination of several factors, including the
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subscription policy (on-change or periodic, with on-change in general
being the more challenging of the two), the period in which to report
changes (1 second periods will consume more resources than 1 hour
periods), the amount of data in the subtree that is being subscribed
to, and the number and combination of other subscriptions that are
concurrently being serviced.
4. YANG module
<CODE BEGINS>
file "ietf-datastore-push@2014-10-27.yang"
module ietf-datastore-push {
// RFC Ed.: replace XXXX with 'ietf' and remove this note
namespace "urn:XXXX:params:xml:ns:yang:ietf-datastore-push";
prefix "datastore-push";
import ietf-yang-types { prefix yang; }
organization
"IETF";
contact
"Editor: Alexander Clemm
<mailto:alex@cisco.com>
Editor: Alberto Gonzalez Prieto
<mailto:albertgo@cisco.com>
Editor: Eric Voit
<mailto:evoit@cisco.com>";
description
"This module contains conceptual YANG specifications
for datastore push.";
revision 2014-10-27 {
description
"Initial revision.";
reference
"Datastore push.";
}
// Typedefs
typedef datastore-contents {
type string;
description
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"The encoding of the contents adheres to the subscription
parameters. It corresponds to the filtered datastore
subtree.";
}
typedef subscription-identifier {
type string {
length "1 .. max";
}
description
"A client-provided identifier for the subscription.";
}
// Identities
// Subscription error
identity subscription-errors {
description
"Base identity for subscription errors.";
}
typedef subscription-term-reason {
type identityref {
base "subscription-errors";
}
description
"Reason for a server to terminate a subscription.";
}
typedef subscription-susp-reason {
type identityref {
base "subscription-errors";
}
description
"Reason for a server to suspend a subscription.";
}
identity internal-error {
base "subscription-errors";
description
"Subscription failures caused by server internal error.";
}
identity no-resources {
base "subscription-errors";
description
"Lack of resources, e.g. CPU, memory, bandwidth";
}
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identity other {
base "subscription-errors";
description
"Fallback reason - any other reason";
}
// Notifications
notification push-update {
description
"This notification contains an update from a datastore";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the subscription because of which the
notification is sent.";
}
leaf datastore-contents {
type datastore-contents;
description
"This contains datastore contents
per the subscription.";
}
}
notification subscription-suspended {
description
"This notification indicates a suspension of the
subscription by the server has occurred. No further
datastore updates will be sent until subscription
resumes.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
leaf reason {
type subscription-susp-reason;
description
"Provides a reason for why the subscription was
suspended.";
}
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}
notification subscription-resumed {
description
"This notification indicates that a subscription that had
previously been suspended has resumed. Datastore updates
will once again be sent.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
}
notification subscription-terminated {
description
"This notification indicates that a subscription has been
terminated.";
leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"This references the affected subscription.";
}
leaf reason {
type subscription-term-reason;
description
"Provides a reason for why the subscription was
terminated.";
}
}
container datastore-push-subscription {
description
"Content of a yang-push subscription.";
leaf stream {
type string;
mandatory true;
description
"The name of the stream to subscribe to.";
}
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leaf subscription-id {
type subscription-identifier;
mandatory true;
description
"Identifier to use for this subscription.";
}
choice filter {
description
"Subset of stream events of interest.";
case substree {
container subtree-filter {
description
"Datastore subtree of interest.";
}
}
case xpath {
leaf xpath-filter {
type yang:xpath1.0;
mandatory true;
description
"Xpath defining the events of interest.";
}
}
}
choice notification-trigger {
mandatory true;
description
"Defines necessary conditions for sending an event to
the subscriber.";
case periodic {
description
"The agent is requested to notify periodically the
current values of the datastore or the subset
defined by the filter.";
leaf period {
type yang:timeticks;
mandatory true;
description
"Elapsed time between notifications.";
}
}
case on-change {
description
"The agent is requested to notify changes in
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values in the datastore or a subset of it defined
by a filter.";
choice change-policy {
mandatory true;
description
"Policy describing necessary conditions for
sending an event to the subscriber.";
case delta-policy {
leaf delta {
type uint32;
mandatory true;
description
"For integer, minimum difference
between current and last reports
values that can trigger an update.";
}
}
}
}
}
leaf start-time {
type yang:date-and-time;
description
"Starting time for replays.";
reference "RFC 5277, Section 2.1.1";
}
leaf stop-time {
type yang:date-and-time;
description
"Time limit for events of interest.";
reference "RFC 5277, Section 2.1.1";
}
}
}
<CODE ENDS>
5. Security Considerations
Subscriptions could be used to attempt to overload servers of YANG
datastores. For this reason, it is important that the server has the
ability to decline a subscription request if it would deplete its
resources. In addition, a server needs to be able to suspend an
existing subscription when needed. When this occur, the subscription
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status is updated accordingly and the clients are notified.
Likewise, requests for subscriptions need to be properly authorized.
A subscription could be used to retrieve data in subtrees 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
needs therefore to be filtered accordingly, just like if the data
were being retrieved on-demand. The Netconf Authorization Control
Model applies.
6. References
6.1. Normative References
[RFC1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol (SNMP)", STD 15, RFC
1157, May 1990.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, July 2008.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536, March
2012.
6.2. Informative References
[peermount]
Clemm, A., Medved, J., and E. Voit, "Mounting YANG-defined
information from remote datastores", draft-clemm-netmod-
mount-02 (work in progress), October 2014.
[peermount-req]
Voit, E., Clemm, A., Bansal, S., Tripathy, A., and P.
Yellai, "Requirements for Peer Mounting of YANG subtrees
from Remote Datastores", draft-voit-netmod-peer-mount-
requirements-00 (work in progress), September 2014.
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[restconf]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", I-D draft-ietf-netconf-restconf-03, October
2014.
Authors' Addresses
Alexander Clemm
Cisco Systems
EMail: alex@cisco.com
Alberto Gonzalez Prieto
Cisco Systems
EMail: albertgo@cisco.com
Eric Voit
Cisco Systems
EMail: evoit@cisco.com
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