Network Working Group A. Clemm
Internet-Draft Huawei
Intended status: Informational L. Ciavaglia
Expires: January 19, 2019 Nokia
L. Granville
Federal University of Rio Grande do Sul (UFRGS)
July 18, 2018
Clarifying the Concepts of Intent and Policy
draft-clemm-nmrg-dist-intent-01
Abstract
Intent and Intent-Based Networking are taking the industry by storm.
At the same time, those terms are used loosely and often
inconsistently, in many cases overlapping with other concepts such as
"policy". This document is therefore intended to clarify the concept
of "Intent" and how it relates to other concepts. The goal is to
contribute towards a common and shared understanding of terms and
concepts which can then be used as foundation to guide further
definition of valid research and engineering problems and their
solutions.
Status of This Memo
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions and Acronyms . . . . . . . . . . . . . . . . . . 4
4. Introduction of Concepts . . . . . . . . . . . . . . . . . . 4
4.1. Service Models . . . . . . . . . . . . . . . . . . . . . 4
4.2. Policy and Policy-Based Management . . . . . . . . . . . 6
4.3. Intent and Intent-Based Management . . . . . . . . . . . 7
5. Distinguishing between Intent, Policy, and Service Models . . 8
6. Items for Discussion . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Traditionally in the IETF, interest with regard to management and
operations has focused on individual network and device features.
Standardization emphasis has generally been put on management
instrumentation that needed to be provided by a networking device. A
prime example for this is SNMP-based management and the 200+ MIBs
that have been defined by the IETF over the years. More recent
examples include YANG data model definitions for aspects such as
interface configuration, ACL configuration, or Syslog configuration.
There is a sense that managing networks by configuring myriads of
"nerd knobs" on a device-by-device basis is no longer sustainable in
modern network environments. Big challenges arise with keeping
device configurations not only consistent across a network, but
consistent with the needs of services they are supposed to enable.
At the same time, operations need to be streamlined and automated
wherever possible to not only lower operational expenses, but allow
for rapid reconfiguration of networks at sub-second time scales.
Accordingly, IETF has begun to address end-to-end management aspects
that go beyond the realm of individual devices in isolation.
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Examples include the definition of YANG models for network topology
[RFC8345] or the introduction of service models used by service
orchestration systems and controllers [RFC8309]. In addition, a lot
of interest has been fueled by the discussion about how to manage
autonomic networks as discussed in the ANIMA working group.
Autonomic networks are driven by the desire to lower operational
expenses and make management of the network as a whole exceptionally
easy, putting it at odds with the need to manage the network one
device and one feature at a time. However, while autonomic networks
are intended to exhibit "self-management" properties, they still
require input from an operator or outside system to provide
operational guidance and information about the goals, purposes, and
service instances that the network is to serve. It is in this
context that the term "intent" was coined for the first time.
This vision has since caught on with the industry in a big way,
leading to countless offerings that tout "intent-based management"
that promise network providers to manage networks holistically at a
higher level of abstraction and as a system that happens to consist
of interconnected components, as opposed to a set of independent
devices (that happen to be interconnected). Those offerings include
SDN controllers (offering a single point of control and
administration for a network) as well as network management and
Operations Support Systems (OSS).
However, it has been recognized for a long time that comprehensive
management solutions cannot operate only at the level of individual
devices and low-level configurations. In this sense, the vision of
"intent" is not entirely new. In the past, ITU-T's model of a
Telecommunications Management Network, TMN, introduced a set of
management layers that defined a management hierarchy, consisting of
network element, network, service, and business management. High-
level operational objectives would propagate in top-down fashion from
upper to lower layers. The associated abstraction hierarchy was key
to decompose management complexity into separate areas of concerns.
This abstraction hierarchy was accompanied by an information
hierarchy that concerned itself at the lowest level with device-
specific information, but that would, at higher layers, include, for
example, end-to-end service instances. Similarly, the concept of
"policy-based management" has for a long time touted the ability to
allow users to manage networks by specifying high-level management
policies, with policy systems automatically "rendering" those
policies, i.e. breaking them down into low-level configurations and
control logic.
What is missing, however, is putting these concepts into a more
current context and defining a reference model that goes beyond a
TMN. This document attempts to clarify terminology and explain how
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intent relates to other, similar concepts, in hope that a common and
shared understanding of terms and concepts can be used as a
foundation to articulate research and engineering problems and their
solutions.
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
ACL: Access Control List
Intent: An abstract, high-level policy used to operate a network
[RFC7575].
Policy: A rule, or set of rules, that governs the choices in
behavior of a system.
PDP: Policy Decision Point
PEP: Policy Enforcement Point
Service Model: A model that represents a service that is provided
by a network to a user.
4. Introduction of Concepts
The following subsections provide an overview of the concepts of
service models, of policies respectively policy-based management, and
of intent respectively intent-based management. While the
descriptions are intentionally kept brief and do not provide detailed
tutorials, they should convey the bigger picture of the purpose of
each concept and provide a sense where those concepts are similar and
where they differ. With this background, the differences between
them are subsequently summarized in in another section.
4.1. Service Models
A service model is a model that represents a service that is provided
by a network to a user. Per [RFC8309], a service model describes a
service and its parameters in a portable way that can be used
independent of the equipment and operating environment on which the
service is realized. Two subcategories are distinguished: a
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"Customer Service Model" describes an instance of a service as
provided to a customer, possibly associated with a service order. A
"Service Delivery Model" describes how a service is instantiated over
existing networking infrastructure.
An example of a service could be a Layer 3 VPN service [RFC8299], a
Network Slice, or residential Internet access. Service models
represent service instances as entities in their own right. Services
have their own parameters, actions, and lifecycles. Typically,
service instances can be bound to end users, who might be billed for
the service.
Instantiating a service typically involves multiple aspects:
o Resources need to be allocated, such as IP addresses, interfaces,
bandwidth, or memory.
o How to map services to the resources needs to be defined.
Multiple mappings are often possible, which to select may depend
on context (such as which type of access is available to connect
the end user with the service).
o Bindings need to be maintained between upper- and lower-level
objects.
They involve a system, such as a controller, that provides
provisioning logic. Orchestration itself is generally conducted
using a "push" model, in which the controller/manager initiates the
operations as required, pushing down the specific configurations to
the device. The device itself typically remains agnostic to the
service or the fact that its resources or configurations are part of
a service/concept at a higher layer.
Instantiated service models map to instantiated lower-layer network
and device models. Examples include instances of paths, or instances
of specific port configurations. The service model typically also
models dependencies and layering of services over lower-layer
networking resources that are used to provide services. This
facilitates management by allowing to follow dependencies for
troubleshooting activities, to perform impact analysis in which
events in the network are assessed regarding their impact on services
and customers Services are typically orchestrated and provisioned
top-to-bottom, which also facilitates keeping track of the assignment
of network resources.
Service models also associate with other data that does not concern
the network but provides business context. This includes things such
as customer data (such as billing information), service orders and
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service catalogues, tariffs, service contracts, and Service Level
Agreements (SLAs) including contractual agreements regarding
remediation actions.
4.2. Policy and Policy-Based Management
Policy-based management (PBM) is a management paradigm that separates
the rules that govern the behavior of a system from the functionality
of the system. It promises to reduce maintenance costs of
information and communication systems while improving flexibility and
runtime adaptability. It is today present at the heart of a
multitude of management architectures and paradigms including SLA-
driven, Business-driven, autonomous, adaptive, and self-* management
[Boutaba07]. The interested reader is asked to refer to the rich set
of existing literature which includes this and many other references.
In the following, we an only provide a much-abridged and distilled
overview.
At the heart of policy-based management is the concept of a policy.
Multiple definitions of policy exist: "Policies are rules governing
the choices in behavior of a system" [Sloman94]. "Policy is a set of
rules that are used to manage and control the changing and/or
maintaining of the state of one or more managed objects"
[Strassner03]. Common to most definitions is the definition of a
policy as a "rule". Typically, the definition of a rule consists of
an event (whose occurrence triggers a rule), a set of conditions
(that get assessed and that must be true before any actions are
actually "fired"), and finally a set of one or more actions that are
carried out when the condition holds.
Policy-based management can be considered an imperative management
paradigm: Policies specify precisely what needs to be done when.
Using policies, management can in effect be defined as a set of
simple control loops. This makes policy-based management a suitable
technology to implement autonomic behavior that can exhibit self-*
management properties including self-configuration, self-healing,
self-optimization, and self-protection. In effect, policies define
simple control loops typically used to define management as a set of
simple control loops.
Policies typically involve a certain degree of abstraction in order
to cope with heterogeneity of networking devices. Rather than having
a device-specific policy that defines events, conditions, and actions
in terms of device-specific commands, parameters, and data models,
policy is defined at a higher-level of abstraction involving a
canonical model of systems and devices to which the policy is to be
applied. A policy agent on the device subsequently "renders" the
policy, i.e., translates the canonical model into a device-specific
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representation. This concept allows to apply the same policy across
a wide range of devices without needing to define multiple variants.
This enables operational scale and allows network operators and
authors of policies to think in higher terms of abstraction than
device specifics.
Policy-based management is typically "push-based": Policies are
pushed onto devices where they are rendered and enforced. The push
operations are conducted by a manager or controller, which is
responsible for deploying policies across the network and monitor
their proper operation. That said, other policy architectures are
possible. For example, policy-based management can also include a
pull-component in which the decision regarding which action to take
is delegated to a so-called Policy Decision Point (PDP). This PDP
can reside outside the managed device itself and has typically global
visibility and context with which to make policy decisions. Whenever
a network device observes an event that is associated with a policy,
but lacks the full definition of the policy or the ability to reach a
conclusion regarding the expected action, it reaches out to the PDP
for a decision (reached, for example, by deciding on an action based
on various conditions). Subsequently, the device carries out the
decision as returned by the PDP - the device "enforces" the policy
and hence acts as a PEP (Policy Enforcement Point). Either way, PBM
architectures typically involve a central component from which
policies are deployed across the network, and/or policy decisions
served.
4.3. Intent and Intent-Based Management
In the context of Autonomic Networks, Intent is defined as "an
abstract, high-level policy used to operate a network" [RFC7575].
According to this definition, an intent is a specific type of policy.
However, to avoid using "intent" simply as a synonym for "policy, a
clearer distinction needs to be introduced that distinguishes intent
clearly from other types of policies.
Autonomic networks are expected to "self-manage" and operate with
minimal outside intervention. However, autonomic networks are not
clairvoyant and have no way of automatically knowing particular
operational goals nor what instances of networking services to
support. In other words, they do not know what the "intent" of the
network provider is that gives the network the purpose of its being.
This still needs to be communicated by what informally constitutes
"intent".
More specifically, intent is a declaration of high-level operational
goals that a network should meet, without specifying how to achieve
them. Those goals are defined in a manner that is purely declarative
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- they specify what to accomplish or what the desired outcome for the
network operator is, not how to achieve it. This encompasses
abstraction from low-level device configurations, as well as
abstraction from particular management and control logic such as when
to spring into action.
In an autonomic network, intent should be rendered by the network
itself, i.e. translated into device specific rules and courses of
action. Ideally, it should not even be orchestrated or broken down
by a higher-level, centralized system, but by the network devices
themselves using a combination of distributed algorithms and local
device abstraction. Because intent holds for the network as a whole,
not individual devices, it needs to be automatically disseminated
across all devices in the network, which can themselves decide
whether they need to act on it. This facilitates management even
further, since it obviates the need for a higher-layer system to
break down and decompose higher-level intent, and because there is no
need to even discover and maintain an inventory of the network to be
able to manage it. Intent thus constitutes declarative policy with a
network-wide scope. A human operator defines 'what' is expected, and
the network computes a solution meeting the requirements. This
computation can occur in distributed or even decentralized fashion by
auonomic functions that reside on network nodes.
Other definitions of intent exist such as [TR523] and will be
investigated in future revisions of this document. Likewise, some
definitions of intent allow for the presence of a centralized
function that renders the intent into lower-level policies or
instructions and orchestrates them across the network. While to the
end user the concept of "intent" appears the same regardless of its
method of rendering, this interpretation opens a slippery slope of
how to clearly distinguish "intent" from other higher-layer
abstractions. Again, these notions will be further investigated in
future revisions of this document and in collaboration with NMRG.
5. Distinguishing between Intent, Policy, and Service Models
What Intent, Policy, and Service Models all have in common is the
fact that they involve a higher-layer of abstraction of a network
that does not involve device-specifics, that generally transcends
individual devices, and that makes the network easier to manage for
applications and human users compared to having to manage the network
one device at a time. Beyond that, differences emerge. Service
models have less in common with policy and intent than policy and
intent do with each other.
Summarized differences:
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o A service model is a data model that is used to describe instances
of services that are provided to customers. A service model has
dependencies on lower models (device and network models) when
describing how the service is mapped onto underlying network and
IT infrastructure. Instantiating a service model requires
orchestration by a system; the logic for how to
orchestrate/manage/provide the service model and how to map it
onto underlying resources is not included as part of the model
itself.
o Policy is a set of rules, typically modeled around a variation of
events/conditions/actions, used to express simple control loops
that can be rendered by devices themselves, without requiring
intervention by outside system. Policy is used to define what to
do under what circumstances, but it does not specify a desired
outcome.
o Intent is a higher-level declarative policy that operates at the
level of a network, not individual devices. It is used to define
outcomes and high-level operational goals, without the need to
enumerate specific events, conditions, and actions. Ideally,
intent is rendered by the network itself; also the dissemination
of intent across the network and any required coordination between
nodes is resolved by the network itself without the need for
outside systems.
The TM Forum's Business Process Framework for network service
providers [eTOM] categorizes network operations broadly into three
categories: Fulfillment, Assurance, and Billing. Intent is generally
tied to fulfillment, broadly defined as all activities and processes
having to do with configuration of the network to fulfill a given
purpose. It is not tied to assurance, broadly defined as all
activities and processes having to do with keeping the network and
services running (including monitoring, measuring, reporting,
assessing compliance of service levels with service level objectives,
diagnostics, etc). Policy, on the other hand, aligns more closely
with assurance.
6. Items for Discussion
Arguably, given the popularity of the term intent, its use could be
broadened to encompass also known concepts ("intent-washing"). For
example, it is conceivable to introduce intent-based terms for
various concepts that, although already known, are related to the
context of intent. Each of those terms could then designate an
intent subcategory, for example:
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o Operational Intent: defines intent related to operational goals of
an operator; corresponds to the original "intent" term.
o Rule Intent: a synonym for policy rules regarding what to do when
certain events occur.
o Service intent: a synonym for customer service model [RFC8309].
o Flow Intent: A synonym for a Service Level Objective for a given
flow.
Whether to do so is an item for discussion by the Research Group.
7. IANA Considerations
Not applicable
8. Security Considerations
Not applicable
9. References
9.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
9.2. Informative References
[Boutaba07]
Boutaba, R. and I. Aib, "Policy-Based Management: A
Historical perspective. Journal of Network and Systems
Management (JNSM), Springer, Vol. 15 (4).", December 2007.
[eTOM] "GB 921 Business Process Framework, Release 17.0.1.",
February 2018.
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[RFC7575] Behringer, M., Pritikin, M., Bjarnason, S., Clemm, A.,
Carpenter, B., Jiang, S., and L. Ciavaglia, "Autonomic
Networking: Definitions and Design Goals", RFC 7575,
DOI 10.17487/RFC7575, June 2015,
<https://www.rfc-editor.org/info/rfc7575>.
[RFC8299] Wu, Q., Ed., Litkowski, S., Tomotaki, L., and K. Ogaki,
"YANG Data Model for L3VPN Service Delivery", RFC 8299,
DOI 10.17487/RFC8299, January 2018,
<https://www.rfc-editor.org/info/rfc8299>.
[RFC8309] Wu, Q., Liu, W., and A. Farrel, "Service Models
Explained", RFC 8309, DOI 10.17487/RFC8309, January 2018,
<https://www.rfc-editor.org/info/rfc8309>.
[RFC8345] Clemm, A., Medved, J., Varga, R., Bahadur, N.,
Ananthakrishnan, H., and X. Liu, "A YANG Data Model for
Network Topologies", RFC 8345, DOI 10.17487/RFC8345, March
2018, <https://www.rfc-editor.org/info/rfc8345>.
[Sloman94]
Sloman, M., "Policy Driven Management for Distributed
Systems. Journal of Network and Systems Management (JNSM),
Springer, Vol. 2 (4).", December 1994.
[Strassner03]
Strassner, J., "Policy-Based Network Management.
Elsevier.", 2003.
[TR523] "Intent NBI - Definition and Principles. ONF TR-523.",
October 2016.
Authors' Addresses
Alexander Clemm
Huawei
2330 Central Expressway
Santa Clara, CA 95050
USA
Email: ludwig@clemm.org
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Laurent Ciavaglia
Nokia
Route de Villejust
Nozay 91460
FR
Email: laurent.ciavaglia@nokia.com
Lisandro Zambenedetti Granville
Federal University of Rio Grande do Sul (UFRGS)
Av. Bento Goncalves
Porto Alegre 9500
BR
Email: granville@inf.ufrgs.br
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