Network Working Group A. Clemm
Internet-Draft Futurewei Technologies, Inc.
Intended status: Informational E. Voit
Expires: April 21, 2018 Cisco Systems
X. Liu
Jabil
I. Bryskin
T. Zhou
G. Zheng
Huawei
H. Birkholz
Fraunhofer SIT
October 18, 2017
Smart filters for Push Updates - Problem Statement
draft-clemm-netconf-push-smart-filters-ps-00
Abstract
This document defines a problem statement for Smart Filters for Push
Updates. Smart Filters for Push Updates (referred to simply as
"Smart Filters" in the context of this document) allows to filter
push updates based on values of pushed objects and/or state, such as
previous updates. Smart Filters provide an important building block
for service assurance and network automation.
Status of This Memo
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This Internet-Draft will expire on April 21, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 3
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
YANG-Push [yang-push] allows client applications to subscribe to
continuous datastore updates without needing to poll. YANG-Push
subscriptions allow client applications to select which datanodes are
of interest. For this purpose, filters that act as node selectors
are offered. However, what is currently not supported are filters
that filter updates based on values, such as sending updates only
when the value falls within a certain range. Also not supported are
filters that would require additional state, such as sending updates
only when the value exceeds a certain threshold for the first time
but not again until the threshold is cleared. We refer to such
filters as "smart filters", with further subcategories of "smart
stateless filters" and "smart stateful filters", respectively.
Smart filters involve more complex subscription and implementation
semantics than the simple selection filters that are currently
offered as part of YANG-Push. They involve post processing of
updates that goes beyond basic update generation for polling
avoidance and place additional intelligence at the server. Because
of this, smart filter functionality was not included in the YANG-Push
specification, although it was recognized that YANG-Push could be
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extended to include such functionality if needed. This is the
purpose of this specification.
Smart filters facilitate service assurance, because they allow client
applications to focus on "outliers" and updates that signify
exceptions and conditions of interest have the biggest operational
significance. They save network resources by avoiding the need to
stream updates that would be discarded anyway, and allow applications
to scale better since larger networks imply a larger amount of smart
filtering operations delegated away from the application to the
network. Smart filters also facilitate network automation as they
constitute an important ingredient to specify triggers for automated
actions.
2. Key Words
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.
3. Definitions and Acronyms
Smart Filter: A filter that involves some processing, such as
comparing values or differentiating behavior depending on state.
TCA: Threshold Crossing Alert.
YANG-Push: A server capability that allows client applications to
subscribe to network management datastore updates.
4. Problem Statement
YANG-Push provides client applications with the ability to subscribe
to continuous updates from network management datastores, obviating
the need to perform polling and resulting in more robust and
efficient applications. However, many applications do not require
every update, only updates that are of certain interest.
For example, an update concerning interface utilization may be only
needed when a certain utilization level is breached. Sending
continuous updates when utilization is low might divert processing
resources away from updates regarding interfaces whose utilization
level may reach a critical point that requires attention. Doing so
will require a filter based on an object value. Even sending
continuous updates when utilization is high may be too much and
counterproductive. It may be sufficient to send an update when a
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threshold is breached to raise a flag of attention, but then not to
continue sending updates while the condition still persists but
simply let the client application know when the threshold is cleared.
This behavior cannot be accomplished simply by a value-based filter,
but requires additional state to be maintained (so that the server
has a memory whether or not the condition of a breached threshold has
already been reported in prior update cycles).
What is needed are "Smart Filters" that provide the ability to apply
filters based on object values, possibly also state. Smart Filters
are useful for Service Assurance applications that need to monitor
operational data for values that fall outside normal operational
ranges. They are also useful for network automation, in which
automated actions are automatically triggered based on when certain
events in the network occur while certain conditions hold. A YANG-
Push subscription with a smart filter can in effect act as a source
for such events. Combined with an optional check for a condition
when an event is observed, this can serve as the basis of action
triggers.
Of course, it is possible to conceive filters that are very smart and
powerful yet also very complex. While filters as defined in YANG-
Push may be a tad too simple for the applications envisioned here, it
is important to keep filters still simple enough to ensure broad
implementation and support by networking devices. The smart filters
defined in this effort intend to apply the "90/10" rule, aiming at
the sweet spot that addresses 90% of use cases and deployment
scenarios that can be addressed using 10% of the complexity. Where
those filters are not sufficient, additional filters can be
introduced outside this document.
It is proposed that Smart Filters for Push Updates will provide
support for the following features:
o Support for smart filter extensions to YANG-Push subscriptions.
The targeted model takes a "base" YANG-Push subscription and
subjects updates to an additional filtering stage that is based on
an object's value.
* Filters that match or compare the object value against a fixed
term or expression.
* Filters that match or compare an object value against the value
of another object. (This feature is useful particularly in
conjunction with possible extensions that allow to compute
aggregates. Such an ability opens the possibility to compare
an object's current value to its mean, for example.)
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o Support for refined on-change update semantics that allow client
to distinguish whether object values were omitted or included
because the object was created or deleted, or because the object's
value fell outside filter range.
o Support for selected stateful filters:
* This includes specifically support for generalized "threshold
crossing alert" filters, or filters that provide an update only
when an object's value passes a filter for the first time, and
not again until the object's value passes a counter filter. In
effect, the support involves attaching filter and counter
filter to an object, including a switch at the object
indicating which filter is in effect, and providing a
distinction in the update which filter (e.g. onset of clear)
was applied.
* It may include additional filters, such a "recent high water
mark" filters that allow to specify a time horizon until the
current high water mark clears. A recent high water mark
filter sends an update to an object only if its new value is
greater than the last value that had been previously reported.
In order to constrain complexity, it is proposed that the following
items will be outside the scope, subject to discussion by the Working
Group:
o Filters that involve freely programmable logic.
o Filters that aggregate or otherwise process information over time.
An example would be filters that compute an aggregate over a time
series of data, for example, an object's average or top percentile
value. (One way in which this can be accomplished is by defining
a separate YANG module that allows to specify aggregates
independent of any filtering, then asking users to subscribe to
updates of the aggregate objects and applying filters there. The
definition of such an aggregation module goes beyond the scope of
the work defined here.)
o Filters that aggregate object's values with those of other
objects, such as the maximum or average from objects over a list,
or that operate on a function of other objects. An example would
be an object for interface utilization that gets computed from
objects for interface speed and interface packet rate, with the
packet rate object itself potentially computed from counter
snapshots that are taken at different times. (One way in which
this can be accomplished is by defining a separate YANG module
that allows to define objects that compute such functions akin to
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the Expression MIB [RFC2982], then asking users to subscribe to
updates of those objects and applying filters there.)
5. IANA Considerations
Not applicable
6. Security Considerations
The application of smart filters requires a certain amount of
processing resources at the server. An attacker could attempt to
attack a server by creating YANG-push subscriptions with a large
number of complex smart filters in an attempt to diminish server
resources. Server implementations can guard against such scenarios
in several ways. For one, they can implement NACM [RFC6536] in order
to require proper authorization for requests to be made. Second,
server implementations can reject requests made for a a larger number
of smart filters than the implementation can reasonably sustain.
7. Normative References
[notif-sub]
Voit, E., Clemm, A., Gonzalez Prieto, A., Nilsen-Nygaard,
E., and A. Tripathy, "Custom subscription to event
notifications", July 2017,
<https://datatracker.ietf.org/doc/
draft-ietf-netconf-subscribed-notifications/>.
[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>.
[RFC2982] Kavasseri, R., Ed., "Distributed Management Expression
MIB", RFC 2982, DOI 10.17487/RFC2982, October 2000,
<https://www.rfc-editor.org/info/rfc2982>.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration
Protocol (NETCONF) Access Control Model", RFC 6536,
DOI 10.17487/RFC6536, March 2012,
<https://www.rfc-editor.org/info/rfc6536>.
[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>.
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[yang-push]
Clemm, A., Voit, E., Gonzalez Prieto, A., Tripathy, A.,
Nilsen-Nygaard, E., Bierman, A., and B. Lengyel,
"Subscribing to YANG datastore push updates", August 2017,
<https://datatracker.ietf.org/doc/
draft-ietf-netconf-yang-push/>.
Authors' Addresses
Alexander Clemm
Futurewei Technologies, Inc.
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: ludwig@clemm.org
Eric Voit
Cisco Systems
Email: evoit@cisco.com
Xufeng Liu
Jabil
Email: Xufeng_Liu@jabil.com
Igor Bryskin
Huawei
Email: Igor.Bryskin@huawei.com
Tianran Zhou
Huawei
Email: zhoutianran@huawei.com
Guangying Zheng
Huawei
Email: zhengguangying@huawei.com
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Henk Birkholz
Fraunhofer SIT
Email: henk.birkholz@sit.fraunhofer.de
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