Some Key Terms for Network Fault and Problem Management

Introduction Successful operation of large or busy networks depends on effective network management. This requires a virtuous circle of network control, network observability, network analytics, network assurance, and back to network control. Network fault and problem management is an important aspect of network management and control solutions. It deals with the detection, reporting, inspection, isolation, correlation, and management of events within the network. The intention of this document is to focus on those events that have a negative effect on the network's ability to forward traffic according to expected behavior and so deliver services, the ability to control and operate the network, and other faults that reduce the quality or reliability of the delivered service. Fault and problem management extends to include actions taken to determine the causes of problems and to work toward recovery of expected network behavior. A number of work efforts within the IETF seek to provide components of a fault management system, such as YANG data models or management protocols. It is important that a common terminology is used so that there is a clear understanding of how the elements of the management and control solutions fit together, and how faults and problems will be handled. This document sets out some terms that are fundamental to a common understanding of network fault and problem management. While "faults" and "problems" are concepts that apply at all levels of technology in the Internet, the scope of this document is restricted to the network layer and below, hence this document is specifically about "network fault and problem management." The concept of "incidents" is also touched on in this document, where an incident results from one or more problems and is the disruption of a network service. Note that some useful terms are defined in and . The definitions in this document are informed by those documents, but they are not dependent on that prior work.

Usage of Terms The terms defined in this document are principally intended for consistent use within the IETF in the scope of network fault and problem management. Where similar concepts are described in other bodies, an attempt has been made to harmonize with those other descriptions, but there is care needed where terms are not used consistently between bodies or where terms are applied outside the network layer. If other bodies find the terminology defined in this document useful, they are free to use it. The purpose of this document is to define the following terms for use in other documents. Other terms are defined to enable those definitions and may also be used by other documents, although that is not the principal purpose of their definitions here.

Event
State
Fault
Problem
Symptom
Cause
Alert
Alarm

When other documents make use of the terms as defined in this document, it is suggested here that such uses should use capitalization of the terms as in this document to help distinguish them from colloquial uses, and should include an early section listing the terms inherited from this document with a citation.

Terminology This section contains key terms. It is split into three subsections.

contains terms that help to set the context for network fault and problem management systems.
includes specific and detailed core terms that will be used in other documents that describe elements of the network fault and problem management systems.
provides three further terms that may be helpful.

Context Terminology This section includes some terminology that helps describe the context for the rest of this work. The terms may be viewed as a cascaded sequence of processes, starting with Network Telemetry and building to Network Observability. The definitions are deliberately kept relatively terse. Further documents may expand on these terms without loss of specificity. Such contextualization (if any) should be highlighted clearly in those documents.

Network Telemetry:: This is defined in and describes the process of collecting operational network data categorized according to the network plane (e.g., layer 3, layer 2, layer 1) from which it was derived. Data collected through the Network Telemetry process does not contain any data related to service definitions (i.e., "intent" per Section 3.1 of ).
Network Monitoring:: This is the process of keeping a continuous record of functions related to a network topology. It involves tracking various aspects such as traffic patterns, device health, performance metrics, and overall network behaviour. This approach differentiates network monitoring from resource or device monitoring, which focuses on individual components or resources ().
Network Analytics:: Network Analytics is the process of deriving analytical insights from operational network data. A process could be executed by a piece of software, a system, or a human that analyzes operational data and outputs new analytical data related to the operational data, for example, a symptom.
Network Observability:: The Network Observability process is the enablement of network behavioral assessment through analysis of observed operational network data (logs, alarms, traces, etc.) with the aim of detecting symptoms of network behavior, and to identify anomalies and their causes. Network Observability begins with information gathered using Network Monitoring tools and that may be further enriched with other operational data. The expected outcome of the observability processes is identification and analysis of deviations in observed state versus the expected state of a network.

Thus, there is a cascaded sequence where the following relationships apply.

Network Telemetry is the process of collecting operational data from a network.
Network Monitoring is the process of creating/keeping a record of data gathered in Network Telemetry.
Network Analytics is the process of deriving insight through the data recorded in Network Monitoring.
Network Observability is the process of enabling behavioral assessment of a network through Network Analytics.

Core Terms The terms are presented below in an order that is intended to flow such that it is possible to gain understanding reading top to bottom. The figures and explanations in may aid understanding the terms set out here.

Resource:

An element of a network system. Resource is a recursive concept so that a Resource may be a collection of other Resources (for example, a network node comprises a collection of network interfaces).

Characteristic:

Observable or measurable aspect or behavior associated with a Resource.

A Characteristic may be considered to be built on facts (see 'Value', below) and the contexts and descriptors that identify and give meaning to the facts.
The term "Metric" is another word for a measurable Characteristic which may also be thought of as analogous to a 'variable'.

Value:

A Value is a measure of a Characteristic associated with a Resource. It may be in the form of a categorization (e.g., high or low), an integer (e.g., a count or gauge), or a reading of a continuous variable (e.g., an analog measurement), etc.

Change:

In the context of Network Monitoring, a Change is the variation in the Value of a Characteristic associated with a Resource and may arise over a period of time.

Not all Changes are noteworthy (i.e., they do not have Relevance).
Perception of Change depends upon Detection, the sampling rate/accuracy/detail, and perspective.
It may be helpful to qualify this as "Value Change" because the English word "change" is often heavily used.

Event:

The variation in Value of a Characteristic of a Resource at a distinct moment in time (i.e., the period is negligible).

Compared with a Change, which may be over a period of time, an Event happens at a distinct moment in time. Thus, an Event may be the observation of a Change.

Condition:

A Condition is an interpretation of the Values of a set of one or more Characteristics of a Resource (with respect to working order or some other aspect relevant to the Resource purpose/application), for example "low available memory". Thus, it is the output of a function applied to a set of one or more variables.

State:

A particular Condition that a Resource has (i.e., it is in a State) at a specific time. For example, a router may report the total amount of memory it has, and how much is free. These are the Values of two Characteristics of a Resource. These Values can be interpreted to determine the Condition of the Resource, and that may determine the State of the router, such as shortage of memory.

While a State may be observed at a specific moment in time, it is actually determined by summarizing measurement over time in a process sometimes called State compression.
It may be helpful to qualify this as "Resource State" to make clear the distinction between this and other uses of "state" such as "protocol state".
This term may be contrasted with "Operational State" as used in . For example, the state of a link might be up/down/degraded, but the operational state of link would include a collection of Values of Characteristics of the link.

Detect (hence Detected, Detection):

To notice the presence of something (State, Change, Event, activity, etc.).

Hence also to notice a Change (from the perspective of an observer such as a monitoring system).

Relevance:

Consideration of an Event, State, or Value (through the application of policy, relative to a specific perspective, intent, and in relation to other Events, States, and Values) to determine whether it is of note to the system that controls or manages the network. Note, for example, that not all Changes are Relevant.

This term may also be used as "Relevant Event", "Relevant State", or "Relevant Value".

Occurrence:

A Relevant Event or a particular Relevant Change.

An Occurrence may be an aggregation or abstraction of multiple fine-grain Events or Changes.
An Occurrence may occur at any macro or micro scale because Resources are a recursive concept, and may be perceived depending on the scope of observation (i.e., according to the level of Resource recursion that is examined). That is, Occurrences, themselves are a recursive concept.

Fault:

An Occurrence (i.e., an Event or a Change) that is not desired/required (as it may be indicative of a current or future undesired State). Thus, a Fault happens at a moment in time. A Fault can potentially be associated with a Cause. See for a more detailed discussion of network faults.

Note that there is a distinction between a Fault and a Problem that depends on context. For example, in a connectivity service where redundancy is present, a link down is a Problem, but from the perspective of managing the network resources, a link down is a Fault. Likewise, for example, in a router with two power supplies, if the backup power supply fails leaving the primary unprotected, this is a Problem.

Problem:

A State that is undesirable and that may require remedial action. A Problem cannot necessarily be associated with a Cause. The resolution of a Problem does not necessarily act on the thing that has the Problem.

Note that there is a historic aspect to the concept of a Problem. The current State may be operational, but there could have been a Fault that is unexplained, and the fact of that unexplained recent Fault is a Problem.
Note that while a Problem is unresolved it may continue to require attention. A record of resolved Problems may be maintained in a log.
Note that there may be a State which is considered to be a Problem from several perspectives. For example, consider a "loss of light" State that may cause multiple services to fail. In this example, a new State (the light recovers) may cause the Problem to be resolved from one perspective (the services are operational once more), but may leave the Problem as unresolved (because the loss of light has not been explained). Further, in this example, there could be another development (the reason for the temporary loss of light is traced to a microbend in the fiber that is repaired) resulting in that unresolved Problem now being resolved. But, in this example, this still leaves a further Problem unresolved (a microbend occurred, and that Problem is not resolved until it is understood how it occurred and a remedy is put in place to prevent recurrence).

Cause:

The Events (Detected or otherwise) that gave rise to a Fault/Problem.

Incident:

A (Network) Incident is an undesired Occurrence such as an unexpected interruption of a network service, degradation of the quality of a network service, or the below-target performance of a network service. An Incident results from one or more Problems, and a Problem may give rise to or contribute to one or more Incidents. Greater discussion of Network Incident relationships, including Customer Incidents and Incident management, can be found in .

Symptom:

An observable Value, Change, State, Event, or Condition considered as an indication of a Problem or potential Problem.

Anomaly:

A (Network) Anomaly is an unusual or unexpected Event or pattern in network data in the forwarding plane, control plane, or management plane that deviates from the normal, expected behavior. See for more details.

Alert:

An indication of a Fault.

Alarm:

As specified in , an Alarm signifies an undesirable State in a Resource that requires corrective action. From a management point of view, an Alarm can be seen as a State in its own right and the transition to this State may result in an Alert being issued. The receipt of this Alert may give rise to a continuous indication (to a human operator) highlighting the potential or actual presence of a Problem.

Other Terms Three other terms may be helpful:

Intermittent:: A State that is not continuous, but keeps recurring in some time frame.
Transient:: A State that is not continuous, and occurs once in some time frame.
Recurrent:: A Problem that is actively resolved, but returns.

Workflow Explanations This section aims to add information about the relationship between the terms defined in in the context of network fault and problem management. The text and figures here are for explanation and are not normative for the definition of terms. The relationship between Resources and Characteristics is shown in . Note that there is a 1:n relationship between Network system and Resources, and between Resources and Characteristics: this is not shown on the figure for clarity.

Resources and Characteristics The Value of a Characteristic of a Resource may change over time. Specific Changes in Value may be noticed at a specific time (as digital Changes), Detected, and treated as Events. This is shown on the left of . The center of shows how the Value of a Characteristic may change over time. The Value may be Detected at specific times or periodically and give rise to Conditions that are States (and consequently State Changes). In practice, the Characteristic may vary in an analog manner over time as shown on the right-hand side of . The Value can be read or reported (i.e., Detected) periodically leading to analog Values that may be deemed Relevant Values, or may be evaluated over time as shown in .

Characteristics and Changes shows the workflow progress for Events. As noted above, an Event is a Change in the Value of a Characteristic at a time. The Event may be evaluated (considering policy, relative to a specific perspective, with a view to intent, and in relation to other Events, States, and Values) to determine if it is an Occurrence and possibly to indicate a Change of State. An Occurrence may be undesirable (a Fault) and that can cause an Alert to be generated, may be evidence of a Problem and could directly indicate a Cause. In some cases, an Alert may give rise to an Alarm highlighting the potential or actual presence of a Problem.

Event and Dependent Terms Alarm ^ | | -----> Cause | | |----------> Problem | | Fault ^ | | | Occurrence ^ | |----------> State | | Event ]]> Parallel to the workflow for Events, shows the workflow progress for States. As shown in , Change noted at a particular time gives rise to State. The State may be deemed to have Relevance considering policy, relative to a specific perspective, with a view to intent, and in relation to other Events, States, and Values. A Relevant State may be deemed a Problem, or may indicate a Problem or potential Problem. Problems may be considered based on Symptoms and may map directly or indirectly to Causes. An Incident results from one or more Problems. An Alarm may be raised as the result of a Problem, and the transition to an Alarmed state may give rise to an Alert.

State and Dependent Terms Alert ^ | ------> Incident | | | | ---> Cause | | | Problem---------> Symptom ^ | | Relevance | | State ]]> shows how Faults and Problems may be consolidated to determine the Causes. The arrows show how one item may give rise to another. A Cause can be indicated by or determined from Faults, Problems, and Symptoms. It may be that one Cause points to another, and can also be considered as a Symptom. The determination of Causes can consider multiple inputs. An Incident results from one or more Problems.

Consolidation of Symptoms and Causes | Symptom | | | | | | | --------- v | ^ --------- | ------->| Cause |<--------- | | --------- | | | ^ | | | | | | | | | --- | | | | | --------- --------- ---------- | Fault |------------------->| Problem |------->| Incident | --------- --------- ---------- ]]> shows how thresholds are important in the consideration of analog Values and Events. The arrows in the figure show how one item may give rise to or utilize another. The use of threshold-driven Events and States (and the Alerts that they might give rise to) must be treated with caution to dampen any "flapping" (so that consistent States may be observed) and to avoid overwhelming management processes or systems. Analog Values may be read or notified from the Resource and could transition a threshold, be deemed Relevant Values, or evaluated over time. Events may be counted, and the Count may cross a threshold or reach a Relevant Value. The Threshold Process may be implementation-specific and subject to policies. When a threshold is crossed and any other conditions are matched, an Event may be determined, and treated like any other Event.

Counts, Thresholds, and Values State | | ------- Relevance |------>| Count |-----------------------------> Value | ------- | ^ | | | | | | | | Relevance | | v | | | ----------- ---------------- Event | | Evaluated | | | ^ | | over time |<--------| Analog Value | | v ----------- | | | ----------- | | | | | Threshold | | | | |<----| Process |<------ | | | | |<----------------------| | | ----------- ---------------- | ^ | | | Detect Detect | | | Change at a Time Change over Time ]]>

Security Considerations This document specifies terminology and has no direct effect on the security of implementations or deployments. However, protocol solutions and management models need to be aware of several aspects:

The exposure of information pertaining to Faults and Problems may make available knowledge of the internal workings of a network (in particular its vulnerabilities) that may be of use to an attacker.
Systems that generate management information (messages, notifications, etc.) when Faults occur, may be attacked by causing them to generate so much information that the system that manages the network is swamped and unable to properly manage the network.
Reporting false information about Faults (or masking reports of Faults) may cause the system that manages the network to function incorrectly.

Privacy Considerations In general, network fault and problem management should not expose information about end-user activities or user data. The main privacy concern is for a network operator to keep control of all information about Faults to protect their privacy and the details of how the network operators operate their network.

IANA Considerations This document makes no requests for IANA action.

Acknowledgments The authors would like to thank Med Boucadair, Wanting Du, Joe Clarke, Javier Antich, Benoit Claise, Christopher Janz, Sherif Mostafa, Kristian Larsson, Dirk Hugo, Carsten Bormann, Hilarie Orman, Stewart Bryant, Bo Wu, Paul Kyzivat, Jouni Korhonen, Reshad Rahman, Rob Wilton, Mahesh Jethanandani, Tim Bray, and Paul Aitken for their helpful comments. Special thanks to the team that met at a side meeting at IETF-120 to discuss some of the thorny issues:

Benoit Claise
Watson Ladd
Brad Peters
Bo Wu
Georgios Karagiannis
Olga Havel
Vincenzo Riccobene
Yi Lin
Jie Dong
Aihua Guo
Thomas Graf
Qin Wu
Chaode Yu
Adrian Farrel