Service Assurance for Intent-based Networking Architecture
draft-claise-opsawg-service-assurance-architecture-00

OPSAWG                                                         B. Claise
Internet-Draft                                               J. Quilbeuf
Intended status: Informational                       Cisco Systems, Inc.
Expires: May 6, 2020                                    November 3, 2019

       Service Assurance for Intent-based Networking Architecture
         draft-claise-opsawg-service-assurance-architecture-00

Abstract

   This document describes the architecture for Service Assurance for
   Intent-based Networking (SAIN).  This architecture aims at assuring
   that service instances are correctly running.  As services rely on
   multiple sub-services by the underlying network devices, getting the
   assurance of a healthy service is only possible with a holistic view
   of network devices.  This architecture not only helps to correlate
   the service degradation with the network root cause but also the
   impacted services impacted when a network component fails or
   degrades.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 6, 2020.

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
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   (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

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   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.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Decomposing a Service Instance Configuration into an
           Assurance Tree  . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Intent and Assurance Tree . . . . . . . . . . . . . . . .   9
     3.3.  Subservices . . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Building the Expression Tree from the Assurance Tree  . .  10
     3.5.  Building the Expression from a Subservice . . . . . . . .  10
     3.6.  Open Interfaces with YANG Modules . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Changes between revisions  . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  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.

   Agent (SAIN Agent): Component that communicates with a device, a set
   of devices, or another agent to build an expression tree from a
   received assurance tree and perform the corresponding computation.

   Assurance Tree: DAG representing the assurance case for one or
   several service instances.  The nodes are the service instances
   themselves and the subservices, the edges indicate a dependency
   relations.

   Collector (SAIN collector): Component that fetches the computer-
   consumable output of the agent(s) and displays it in a user friendly
   form or process it locally.

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   DAG: Directed Acyclic Graph.

   ECMP: Equal Cost Multiple Paths

   Expression Tree: Generic term for a DAG representing a computation in
   SAIN.  More specific terms are:

   o  Subservice Expressions: expression tree representing all the
      computations to execute for a subservice.

   o  Service Expressions: expression tree representing all the
      computations to execute for a service instance, i.e. including the
      computations for all dependent subservices.

   o  Global Computation Forest: expression tree representing all the
      computations to execute for all services instances in an instance
      of SAIN (i.e. all computations performed within an instance of
      SAIN).

   Impacting Dependency: Type of dependency in the assurance tree.  The
   status of the dependency is completely taken into account by the
   dependent service instance or subservice.

   Informational Dependency: Type of dependency in the assurance tree.
   Only the symptoms (i.e. for informational reasons) are taken into
   account in the dependent service instance or subservice.  In
   particular, the score is not taken into account.

   Metric: Information retrieved from a network device.

   Metric Engine: Maps metrics to a list of candidate metric
   implementations depending on the target model.

   Metric Implementation: Actual way of retrieving a metric from a
   device.

   Network Service YANG Module: describes the characteristics of
   service, as agreed upon with consumers of that service [RFC8199].

   Service Instance: A specific instance of a service.

   Orchestrator (SAIN Orchestrator): Component of SAIN in charge of
   fetching the configuration specific to each service instance and
   converting it into an assurance tree.

   Health status: Score and symptoms indicating whether a service
   instance or a subservice is heathy.  A non-maximal score MUST always
   be explained by one or more symptoms.

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   Subservice: Part of an assurance tree that assures a specific feature
   or subpart of the network system.

   Symptom: Reason explaining why a service instance or a subservice is
   not completely healthy.

2.  Introduction

   Network Service YANG Modules [RFC8199] describe the configuration,
   state data, operations, and notifications of abstract representations
   of services implemented on one or multiple network elements.

   Quoting RFC8199: "Network Service YANG Modules describe the
   characteristics of a service, as agreed upon with consumers of that
   service.  That is, a service module does not expose the detailed
   configuration parameters of all participating network elements and
   features but describes an abstract model that allows instances of the
   service to be decomposed into instance data according to the Network
   Element YANG Modules of the participating network elements.  The
   service-to-element decomposition is a separate process; the details
   depend on how the network operator chooses to realize the service.
   For the purpose of this document, the term "orchestrator" is used to
   describe a system implementing such a process.""

   In other words, orchestrators deploy Network Service YANG Modules
   through the configuration of Network Element YANG Modules.  Network
   configuration is based on those YANG data models, with protocol/
   encoding such as NETCONF/XML [RFC6241] , RESTCONF/JSON [RFC8040],
   gNMI/gRPC/protobuf, etc.  Knowing that a configuration is applied
   doesn't imply that the service is running correctly (for example the
   service might be degraded because of a failure in the network), the
   network operator must monitor the service operational data at the
   same time as the configuration.  The industry has been standardizing
   on telemetry to push network element performance information.

   A network administrator needs to monitor her network and services as
   a whole, independently of the use cases or the management protocols.
   With different protocols come different data models, and different
   ways to model the same type of information.  When network
   administrators deal with multiple protocols, the network management
   must perform the difficult and time-consuming job of mapping data
   models: the model used for configuration with the model used for
   monitoring.  This problem is compounded by a large, disparate set of
   data sources (MIB modules, YANG models [RFC7950], IPFIX information
   elements [RFC7011], syslog plain text [RFC3164], TACACS+
   [I-D.ietf-opsawg-tacacs], RADIUS [RFC2138], etc.).  In order to avoid
   this data model mapping, the industry converged on model-driven
   telemetry to stream the service operational data, reusing the YANG

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   models used for configuration.  Model-driven telemetry greatly
   facilitates the notion of closed-loop automation whereby events from
   the network drive remediation changes back into the network.

   However, it proves difficult for network operators to correlate the
   service degradation with the network root cause.  For example, why
   does my L3VPN fail to connect?  Why is this specific service slow?
   The reverse, i.e. which services are impacted when a network
   component fails or degrades, is even more interesting for the
   operators.  For example, which service(s) is(are) impacted when this
   specific optic dBM begins to degrade?  Which application is impacted
   by this ECMP imbalance?  Is that issue actually impacting any other
   customers?

   Intent-based approaches are often declarative, starting from a
   statement of the "The service works correctly" and trying to enforce
   it.  Such approaches are mainly suited for greenfield deployments.

   Instead of approaching intent from a declarative way, this framework
   focuses on already defined services and tries to infer the meaning of
   "The service works correctly".  To do so, the framework works from an
   assurance tree, deduced from the service definition and from the
   network configuration.  This assurance tree is decomposed into
   components, which are then assured independently.  The root of the
   assurance tree represents the service to assure, and its children
   represent components identified as its direct dependencies; each
   component can have dependencies as well.

   When a service is degraded, the framework will highlight where in the
   assurance service tree to look, as opposed to going hop by hop to
   troubleshoot the issue.  Not only can can this framework help to
   correlate service degradation with network root cause/symptoms, but
   it can deduce from the assurance tree the number and type of services
   impacted by a component degradation/failure.  This added value
   informs the operational team where to focus its attention for maximum
   return.

3.  Architecture

   SAIN aims at assuring that service instances are correctly running.
   The goal of SAIN is to assure that service instances are operating
   correctly and if not, to pinpoint what is wrong.  More precisely,
   SAIN computes a score for each service instance and outputs symptoms
   explaining that score, especially why the score is not maximal.  The
   score augmented with the symptoms is called the health status

   As an example of a service, let us consider a point-to-point L2VPN
   connection (i.e. pseudowire).  Such a service would take as

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   parameters the two ends of the connection (device, interface or
   subinterface, and address of the other end) and configure both
   devices (and maybe more) so that a L2VPN connection is established
   between the two devices.  Examples of symptoms might be "Interface
   has high error rate" or "Interface flapping", or "Device almost out
   of memory".

   The overall architecture of our solution is presented in Figure 1.
   The assurance tree along some other configuration options is sent to
   the SAIN agents who are responsible for building the expression tree
   and computing the statuses in a distributed manner.  The collector is
   in charge of collecting and displaying the current status of the
   assured service instances.

   Network            +-----------------+       +-------------------+
   Service  --------> | (SAIN)          |       | (SAIN)            |
   Instance           | Orchestrator    |       | Collector         |
   Configuration      +-----------------+       +-------------------+
                          |                        ^
                          | Configuration          | health Status
                          | (assurance tree)       | (score + symptoms)
                          V                        | streamed
                   +-------------------+           | via telemetry
                   |+-------------------+          |
                   ||+-------------------+         |
                   +|| (SAIN)            |---------+
                    +| agent             |
                     +-------------------+
                               ^ ^ ^
                               | | |
                               | | |  Metric Collection
                               V V V
         +-------------------------------------------------------------+
         | Network                                                     |
         |                                                             |
         +-------------------------------------------------------------+

                        Figure 1: SAIN Architecture

   In order to produce the score assigned to a service instance, the
   architecture performs the following tasks:

   o  Analyze the configuration pushed to the network device(s) for
      configuring the service instance and decide: which information is

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      needed from the device(s), such a piece of information being
      called a metric, which operations to apply to the metrics for
      computing the health status.

   o  Stream (via telemetry [RFC8641]) operational and config metric
      values when possible, else continuously fetch.

   o  Continuously compute the health status of the service instances,
      based on the metric values.

   As said above, the goal of SAIN is to produce a health status for
   each service instance to assure, by collecting some metrics and
   applying operations on them.  To meet that goal, the service is
   decomposed into an assurance tree formed by subservices linked
   through dependencies.  Each subservice is then turned into
   expressions that are combined according to the dependencies between
   the subservices in order to obtain the expression tree which details
   how to fetch the metrics and how to compute the health status for
   each service instances.  The expression tree is then implemented by
   the SAIN agents.  The architecture also exports the health status of
   each subservice.

3.1.  Decomposing a Service Instance Configuration into an Assurance
      Tree

   In order to structure the assurance of a service instance, the
   service instance is decomposed into so-called subservices.  Each
   subservice focuses on a specific feature or subpart of the network
   system.

   The decomposition into subservices is at the heart of this
   architecture, for the following reasons.

   o  The result of this decomposition is the assurance case of a
      service instance, that can be represented is as a graph (called
      assurance tree) to the operator.

   o  Subservices provide a scope for particular expertise and thereby
      enable contribution from external experts.  For instance, the
      subservice dealing with the optics health should be reviewed and
      extended by an expert in optical interfaces.

   o  Subservices that are common to several service instances are
      reused for reducing the amount of computation needed.

   The assurance tree of a service instance is a DAG representing the
   structure of the assurance case for the service instance.  The nodes
   of this graph are service instances or subservice instances.  Each

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   edge of this graph indicates a dependency between the two nodes at
   its extremities: the service or subservice at the source of the edge
   depends on the service or subservice at the destination of the edge.

   Figure 2 depicts a simplistic example of the assurance tree for a
   tunnel service.  The node at the top is the service instance, the
   nodes below are its dependencies.  In the example, the tunnel service
   instance depends on the peer1 and peer2 tunnel interfaces, which in
   turn depend on the respective physical interfaces, which finally
   depend on the respective peer1 and peer2 devices.  The tunnel service
   instance also depends on the IP connectivity that depends on the IS-
   IS routing protocol.

                             +------------------+
                             | Tunnel           |
                             | Service Instance |
                             +-----------------+
                                       |
                   +-------------------+-------------------+
                   |                   |                   |
            +-------------+     +-------------+     +--------------+
            | Peer1       |     | Peer2       |     | IP           |
            | Tunnel      |     | Tunnel      |     | Connectivity |
            | Interface   |     | Interface   |     |              |
            +-------------+     +-------------+     +--------------}
                   |                   |                  |
            +-------------+     +-------------+     +-------------+
            | Peer1       |     | Peer2       |     | IS-IS       |
            | Physical    |     | Physical    |     | Routing     |
            | Interface   |     | Interface   |     | Protocol    |
            +-------------+     +-------------+     +-------------+
                   |                   |
            +-------------+     +-------------+
            |             |     |             |
            | Peer1       |     | Peer2       |
            | Device      |     | Device      |
            +-------------+     +-------------+

                     Figure 2: Assurance Tree Example

   Depicting the assurance tree helps the operator to understand (and
   assert) the decomposition.  The assurance tree shall be maintained
   during normal operation with addition, modification and removal of
   service instances.  A change in the network configuration or topology
   shall be reflected in the assurance tree.  As a first example, a
   change of routing protocol from IS-IS to OSPF would change the

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   assurance tree accordingly.  As a second example, assuming that ECMP
   is in place for the source router for that specific tunnel; in that
   case, multiple interfaces must now be monitored, on top of the
   monitoring the ECMP health itself.

3.2.  Intent and Assurance Tree

   The SAIN orchestrator analyzes the configuration of a service
   instance to:

   o  Try to capture the intent of the service instance, i.e. what is
      the service instance trying to achieve,

   o  Decompose the service instance into subservices representing the
      network features on which the service instance relies.

   The SAIN orchestrator must be able to analyze configuration from
   various devices and produce the assurance tree.

   To schematize what a SAIN orchestrator does, assume that the
   configuration for a service instance touches 2 devices and configure
   on each device a virtual tunnel interface.  Then:

   o  Capturing the intent would start by detecting that the service
      instance is actually a tunnel between the two devices, and stating
      that this tunnel must be functional.  This is the current state of
      SAIN, however it does not completely capture the intent which
      might additionally include, for instance, on the latency and
      bandwidth requirements of this tunnel.

   o  Decompose the service instance into subservices is what the
      assurance tree depicted in Figure 2 does.

   In order for SAIN to be applied, the configuration necessary for each
   service instance should be identifiable and thus should come from a
   "service-aware" source.  While the figure 1 makes a distinction
   between the SAIN orchestrator and a different component providing the
   service instance configuration, in practice those two components are
   mostly likely combined.  The internals of the orchestrator are
   currently out of scope of this standardization.

3.3.  Subservices

   A subservice corresponds to subpart or a feature of the network
   system that is needed for a service instance to function properly.
   In the context of SAIN, subservice is actually a shortcut for
   subservice assurance, that is the method for assuring that a
   subservice behaves correctly.

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   A subservice is characterized by a list of metrics to fetch and a
   list of computations to apply to these metrics in order to produce a
   health status.  Subservices, as services, have high-level parameters
   which defines which object should be assured.

3.4.  Building the Expression Tree from the Assurance Tree

   From the assurance tree is derived a so-called expression tree, which
   is actually a DAG whose sources are constants or metrics and other
   nodes are operators.  The expression tree encodes all the operations
   needed to produce heath statuses from the collected metrics.

   Subservices shall be device independent.  To justify this, let's
   consider the interface operational status.  Dependending on the
   device capabilities, this status can be collected by an industry-
   accepted YANG module (IETF, Openconfig), by a vendor-specific YANG
   module, or even by a MIB module.  If the subservice was dependent on
   the mechanism to collect the operational status, then we would need
   multiple subservice definitions in order to support all different
   mechanisms.

   In order to keep subservices independent from metric collection
   method, or, expressed differently, to support multiple combinations
   of platforms, OSes, and even vendors, the framework introduces the
   concept of "metric engine".  The metric engine maps each device-
   independent metric used in the subservices to a list of device-
   specific metric implementations that precisely define how to fetch
   values for that metric.  The mapping is parameterized by the
   characteristics (model, OS version, etc.) of the device from which
   the metrics are fetched.

3.5.  Building the Expression from a Subservice

   Additionally, to the list of metrics, each subservice defines a list
   of expressions to apply on the metrics in order to compute the health
   status of the subservice.  The definition or the standardization of
   those expressions (also known as heuristic) is currently out of scope
   of this standardization.

3.6.  Open Interfaces with YANG Modules

   The interfaces between the architecture components are open thanks to
   YANG module(I-D.claise-opsawg-service-assurance-yang) defines objects
   for assuring network services based on their decomposition into so-
   called subservices, according to the SAIN architecture.

   This module is intended for the following use cases:

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   o  Assurance tree configuration:

      *  Subservices: configure a set of subservices to assure, by
         specifying their types and parameters.

      *  Dependencies: configure the dependencies between the
         subservices, along with their type.

   o  Assurance telemetry: export the health status of the subservices,
      along with the observed symptoms.

4.  Security Considerations

   TO BE COMPLETED

5.  IANA Considerations

   This document includes no request to IANA.

6.  Open Issues

      -Security Considerations to be completed

7.  References

7.1.  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>.

7.2.  Informative References

   [I-D.ietf-opsawg-tacacs]
              Dahm, T., Ota, A., dcmgash@cisco.com, d., Carrel, D., and
              L. Grant, "The TACACS+ Protocol", draft-ietf-opsawg-
              tacacs-15 (work in progress), September 2019.

   [RFC2138]  Rigney, C., Rubens, A., Simpson, W., and S. Willens,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2138, DOI 10.17487/RFC2138, April 1997,
              <https://www.rfc-editor.org/info/rfc2138>.

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   [RFC3164]  Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
              DOI 10.17487/RFC3164, August 2001,
              <https://www.rfc-editor.org/info/rfc3164>.

   [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,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
              "Specification of the IP Flow Information Export (IPFIX)
              Protocol for the Exchange of Flow Information", STD 77,
              RFC 7011, DOI 10.17487/RFC7011, September 2013,
              <https://www.rfc-editor.org/info/rfc7011>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8199]  Bogdanovic, D., Claise, B., and C. Moberg, "YANG Module
              Classification", RFC 8199, DOI 10.17487/RFC8199, July
              2017, <https://www.rfc-editor.org/info/rfc8199>.

   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <https://www.rfc-editor.org/info/rfc8641>.

Appendix A.  Changes between revisions

   v00 - v01

   o  Placeholder for next version.

Acknowledgements

   The authors would like to thank ...

Authors' Addresses

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   Benoit Claise
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   1831 Diegem
   Belgium

   Email: bclaise@cisco.com

   Jean Quilbeuf
   Cisco Systems, Inc.
   1, rue Camille Desmoulins
   92782 Issy Les Moulineaux
   France

   Email: jquilbeu@cisco.com

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