feat(ietf): MLRsearch draft-06

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Signed-off-by: Vratko Polak <vrpolak@cisco.com>

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+---
+
+title: Multiple Loss Ratio Search
+abbrev: MLRsearch
+docname: draft-ietf-bmwg-mlrsearch-06
+date: 2024-03-04
+
+ipr: trust200902
+area: ops
+wg: Benchmarking Working Group
+kw: Internet-Draft
+cat: info
+
+coding: us-ascii
+pi:    # can use array (if all yes) or hash here
+  toc: yes
+  sortrefs:   # defaults to yes
+  symrefs: yes
+
+author:
+      -
+        ins: M. Konstantynowicz
+        name: Maciek Konstantynowicz
+        org: Cisco Systems
+        email: mkonstan@cisco.com
+      -
+        ins: V. Polak
+        name: Vratko Polak
+        org: Cisco Systems
+        email: vrpolak@cisco.com
+
+normative:
+  RFC1242:
+  RFC2285:
+  RFC2544:
+  RFC9004:
+
+informative:
+  TST009:
+    target: https://www.etsi.org/deliver/etsi_gs/NFV-TST/001_099/009/03.04.01_60/gs_NFV-TST009v030401p.pdf
+    title: "TST 009"
+  FDio-CSIT-MLRsearch:
+    target: https://csit.fd.io/cdocs/methodology/measurements/data_plane_throughput/mlr_search/
+    title: "FD.io CSIT Test Methodology - MLRsearch"
+    date: 2023-10
+  PyPI-MLRsearch:
+    target: https://pypi.org/project/MLRsearch/1.2.1/
+    title: "MLRsearch 1.2.1, Python Package Index"
+    date: 2023-10
+
+--- abstract
+
+This document proposes extensions to [RFC2544] throughput search by
+defining a new methodology called Multiple Loss Ratio search
+(MLRsearch). MLRsearch aims to minimize search duration,
+support multiple loss ratio searches,
+and enhance result repeatability and comparability.
+
+The primary reason for extending [RFC2544] is to address the challenges
+and requirements presented by the evaluation and testing
+of software-based networking systems' data planes.
+
+To give users more freedom, MLRsearch provides additional configuration options
+such as allowing multiple shorter trials per load instead of one large trial,
+tolerating a certain percentage of trial results with higher loss,
+and supporting the search for multiple goals with varying loss ratios.
+
+--- middle
+
+{::comment}
+    As we use Kramdown to convert from Markdown,
+    we use this way of marking comments not to be visible in the rendered draft.
+    https://stackoverflow.com/a/42323390
+    If another engine is used, convert to this way:
+    https://stackoverflow.com/a/20885980
+{:/comment}
+
+# Purpose and Scope
+
+The purpose of this document is to describe Multiple Loss Ratio search
+(MLRsearch), a data plane throughput search methodology optimized for software
+networking DUTs.
+
+Applying vanilla [RFC2544] throughput bisection to software DUTs
+results in several problems:
+
+- Binary search takes too long as most trials are done far from the
+  eventually found throughput.
+- The required final trial duration and pauses between trials
+  prolong the overall search duration.
+- Software DUTs show noisy trial results,
+  leading to a big spread of possible discovered throughput values.
+- Throughput requires a loss of exactly zero frames, but the industry
+  frequently allows for small but non-zero losses.
+- The definition of throughput is not clear when trial results are inconsistent.
+
+
+To address the problems mentioned above,
+the MLRsearch library employs the following enhancements:
+
+- Allow multiple shorter trials instead of one big trial per load.
+  - Optionally, tolerate a percentage of trial results with higher loss.
+- Allow searching for multiple search goals, with differing loss ratios.
+  - Any trial result can affect each search goal in principle.
+- Insert multiple coarse targets for each search goal, earlier ones need
+  to spend less time on trials.
+  - Earlier targets also aim for lesser precision.
+  - Use Forwarding Rate (FR) at maximum offered load
+    [RFC2285] (section 3.6.2) to initialize the initial targets.
+- Take care when dealing with inconsistent trial results.
+  - Reported throughput is smaller than the smallest load with high loss.
+  - Smaller load candidates are measured first.
+- Apply several load selection heuristics to save even more time
+  by trying hard to avoid unnecessarily narrow bounds.
+
+Some of these enhancements are formalized as MLRsearch specification,
+the remaining enhancements are treated as implementation details,
+thus achieving high comparability without limiting future improvements.
+
+MLRsearch configuration options are flexible enough to
+support both conservative settings and aggressive settings.
+Where the conservative settings lead to results
+unconditionally compliant with [RFC2544],
+but longer search duration and worse repeatability.
+Conversely, aggressive settings lead to shorter search duration
+and better repeatability, but the results are not compliant with [RFC2544].
+
+No part of [RFC2544] is intended to be obsoleted by this document.
+
+# Identified Problems
+
+This chapter describes the problems affecting usability
+of various performance testing methodologies,
+mainly a binary search for [RFC2544] unconditionally compliant throughput.
+
+## Long Search Duration
+
+The emergence of software DUTs, with frequent software updates and a
+number of different frame processing modes and configurations,
+has increased both the number of performance tests
+required to verify the DUT update and the frequency of running those tests.
+This makes the overall test execution time even more important than before.
+
+The current [RFC2544] throughput definition restricts the potential
+for time-efficiency improvements.
+A more generalized throughput concept could enable further enhancements
+while maintaining the precision of simpler methods.
+
+The bisection method, when unconditionally compliant with [RFC2544],
+is excessively slow.
+This is because a significant amount of time is spent on trials
+with loads that, in retrospect, are far from the final determined throughput.
+
+[RFC2544] does not specify any stopping condition for throughput search,
+so users already have an access to a limited trade-off
+between search duration and achieved precision.
+However, each full 60-second trials doubles the precision,
+so not many trials can be removed without a substantial loss of precision.
+
+## DUT in SUT
+
+[RFC2285] defines:
+- DUT as
+  - The network forwarding device to which stimulus is offered and
+    response measured [RFC2285] (section 3.1.1).
+- SUT as
+  - The collective set of network devices to which stimulus is offered
+    as a single entity and response measured [RFC2285] (section 3.1.2).
+
+[RFC2544] specifies a test setup with an external tester stimulating the
+networking system, treating it either as a single DUT, or as a system
+of devices, an SUT.
+
+In the case of software networking, the SUT consists of not only the DUT
+as a software program processing frames, but also of
+a server hardware and operating system functions,
+with server hardware resources shared across all programs
+and the operating system running on the same server.
+
+Given that the SUT is a shared multi-tenant environment
+encompassing the DUT and other components, the DUT might inadvertently
+experience interference from the operating system
+or other software operating on the same server.
+
+Some of this interference can be mitigated.
+For instance,
+pinning DUT program threads to specific CPU cores
+and isolating those cores can prevent context switching.
+
+Despite taking all feasible precautions, some adverse effects may still impact
+the DUT's network performance.
+In this document, these effects are collectively
+referred to as SUT noise, even if the effects are not as unpredictable
+as what other engineering disciplines call noise.
+
+DUT can also exhibit fluctuating performance itself, for reasons
+not related to the rest of SUT; for example due to pauses in execution
+as needed for internal stateful processing.
+In many cases this
+may be an expected per-design behavior, as it would be observable even
+in a hypothetical scenario where all sources of SUT noise are eliminated.
+Such behavior affects trial results in a way similar to SUT noise.
+As the two phenomenons are hard to distinguish,
+in this document the term 'noise' is used to encompass
+both the internal performance fluctuations of the DUT
+and the genuine noise of the SUT.
+
+A simple model of SUT performance consists of an idealized noiseless performance,
+and additional noise effects.
+For a specific SUT, the noiseless performance is assumed to be constant,
+with all observed performance variations being attributed to noise.
+The impact of the noise can vary in time, sometimes wildly,
+even within a single trial.
+The noise can sometimes be negligible, but frequently
+it lowers the observed SUT performance as observed in trial results.
+
+In this model, SUT does not have a single performance value, it has a spectrum.
+One end of the spectrum is the idealized noiseless performance value,
+the other end can be called a noiseful performance.
+In practice, trial result
+close to the noiseful end of the spectrum happens only rarely.
+The worse the performance value is, the more rarely it is seen in a trial.
+Therefore, the extreme noiseful end of the SUT spectrum is not observable
+among trial results.
+Also, the extreme noiseless end of the SUT spectrum
+is unlikely to be observable, this time because some small noise effects
+are likely to occur multiple times during a trial.
+
+Unless specified otherwise, this document's focus is
+on the potentially observable ends of the SUT performance spectrum,
+as opposed to the extreme ones.
+
+When focusing on the DUT, the benchmarking effort should ideally aim
+to eliminate only the SUT noise from SUT measurements.
+However,
+this is currently not feasible in practice, as there are no realistic enough
+models available to distinguish SUT noise from DUT fluctuations,
+based on the author's experience and available literature.
+
+Assuming a well-constructed SUT, the DUT is likely its
+primary performance bottleneck.
+In this case, we can define the DUT's
+ideal noiseless performance as the noiseless end of the SUT performance spectrum,
+especially for throughput.
+However, other performance metrics, such as latency,
+may require additional considerations.
+
+Note that by this definition, DUT noiseless performance
+also minimizes the impact of DUT fluctuations, as much as realistically possible
+for a given trial duration.
+
+This document aims to solve the DUT in SUT problem
+by estimating the noiseless end of the SUT performance spectrum
+using a limited number of trial results.
+
+Any improvements to the throughput search algorithm, aimed at better
+dealing with software networking SUT and DUT setup, should employ
+strategies recognizing the presence of SUT noise, allowing the discovery of
+(proxies for) DUT noiseless performance
+at different levels of sensitivity to SUT noise.
+
+## Repeatability and Comparability
+
+[RFC2544] does not suggest to repeat throughput search.
+And from just one
+discovered throughput value, it cannot be determined how repeatable that value is.
+Poor repeatability then leads to poor comparability,
+as different benchmarking teams may obtain varying throughput values
+for the same SUT, exceeding the expected differences from search precision.
+
+[RFC2544] throughput requirements (60 seconds trial and
+no tolerance of a single frame loss) affect the throughput results
+in the following way.
+The SUT behavior close to the noiseful end of its performance spectrum
+consists of rare occasions of significantly low performance,
+but the long trial duration makes those occasions not so rare on the trial level.
+Therefore, the binary search results tend to wander away from the noiseless end
+of SUT performance spectrum, more frequently and more widely than shorter
+trials would, thus causing poor throughput repeatability.
+
+The repeatability problem can be addressed by defining a search procedure
+that identifies a consistent level of performance,
+even if it does not meet the strict definition of throughput in [RFC2544].
+
+According to the SUT performance spectrum model, better repeatability
+will be at the noiseless end of the spectrum.
+Therefore, solutions to the DUT in SUT problem
+will help also with the repeatability problem.
+
+Conversely, any alteration to [RFC2544] throughput search
+that improves repeatability should be considered
+as less dependent on the SUT noise.
+
+An alternative option is to simply run a search multiple times, and report some
+statistics (e.g. average and standard deviation).
+This can be used
+for a subset of tests deemed more important,
+but it makes the search duration problem even more pronounced.
+
+## Throughput with Non-Zero Loss
+
+[RFC1242] (section 3.17) defines throughput as:
+    The maximum rate at which none of the offered frames
+    are dropped by the device.
+
+Then, it says:
+    Since even the loss of one frame in a
+    data stream can cause significant delays while
+    waiting for the higher level protocols to time out,
+    it is useful to know the actual maximum data
+    rate that the device can support.
+
+However, many benchmarking teams accept a small,
+non-zero loss ratio as the goal for their load search.
+
+Motivations are many:
+
+- Modern protocols tolerate frame loss better,
+  compared to the time when [RFC1242] and [RFC2544] were specified.
+
+- Trials nowadays send way more frames within the same duration,
+  increasing the chance of a small SUT performance fluctuation
+  being enough to cause frame loss.
+
+- Small bursts of frame loss caused by noise have otherwise smaller impact
+  on the average frame loss ratio observed in the trial,
+  as during other parts of the same trial the SUT may work more closely
+  to its noiseless performance, thus perhaps lowering the trial loss ratio
+  below the goal loss ratio value.
+
+- If an approximation of the SUT noise impact on the trial loss ratio is known,
+  it can be set as the goal loss ratio.
+
+Regardless of the validity of all similar motivations,
+support for non-zero loss goals makes any search algorithm more user-friendly.
+[RFC2544] throughput is not user-friendly in this regard.
+
+Furthermore, allowing users to specify multiple loss ratio values,
+and enabling a single search to find all relevant bounds,
+significantly enhances the usefulness of the search algorithm.
+
+Searching for multiple search goals also helps to describe the SUT performance
+spectrum better than the result of a single search goal.
+For example, the repeated wide gap between zero and non-zero loss loads
+indicates the noise has a large impact on the observed performance,
+which is not evident from a single goal load search procedure result.
+
+It is easy to modify the vanilla bisection to find a lower bound
+for the intended load that satisfies a non-zero goal loss ratio.
+But it is not that obvious how to search for multiple goals at once,
+hence the support for multiple search goals remains a problem.
+
+## Inconsistent Trial Results
+
+While performing throughput search by executing a sequence of
+measurement trials, there is a risk of encountering inconsistencies
+between trial results.
+
+The plain bisection never encounters inconsistent trials.
+But [RFC2544] hints about the possibility of inconsistent trial results,
+in two places in its text.
+The first place is section 24, where full trial durations are required,
+presumably because they can be inconsistent with the results
+from shorter trial durations.
+The second place is section 26.3, where two successive zero-loss trials
+are recommended, presumably because after one zero-loss trial
+there can be a subsequent inconsistent non-zero-loss trial.
+
+Examples include:
+
+- A trial at the same load (same or different trial duration) results
+  in a different trial loss ratio.
+- A trial at a higher load (same or different trial duration) results
+  in a smaller trial loss ratio.
+
+Any robust throughput search algorithm needs to decide how to continue
+the search in the presence of such inconsistencies.
+Definitions of throughput in [RFC1242] and [RFC2544] are not specific enough
+to imply a unique way of handling such inconsistencies.
+
+Ideally, there will be a definition of a new quantity which both generalizes
+throughput for non-zero-loss (and other possible repeatability enhancements),
+while being precise enough to force a specific way to resolve trial result
+inconsistencies.
+But until such a definition is agreed upon, the correct way to handle
+inconsistent trial results remains an open problem.
+
+# MLRsearch Specification
+
+This chapter focuses on technical definitions needed for evaluating
+whether a particular test procedure adheres to MLRsearch specification.
+
+For motivations, explanations, and other comments see other chapters.
+
+## MLRsearch Architecture
+
+MLRsearch architecture consists of three main components:
+the manager, the controller, and the measurer.
+For definitions of the components, see the following sections.
+
+The architecture also implies the presence of other components, such as the SUT.
+
+These components can be seen as abstractions present in any testing procedure.
+
+### Measurer
+
+The measurer is the component that performs one trial
+as described in [RFC2544] section 23.
+
+Specifically, one call to the measurer accepts a trial load value
+and trial duration value, performs the trial, and returns
+the measured trial loss ratio, and optionally a different duration value.
+
+It is the responsibility of the measurer to uphold any requirements
+and assumptions present in MLRsearch specification
+(e.g. trial forwarding ratio not being larger than one).
+Implementers have some freedom, for example in the way they deal with
+duplicated frames, or what to return if the tester sent zero frames towards SUT.
+Implementations are RECOMMENDED to document their behavior
+related to such freedoms in as detailed a way as possible.
+
+Implementations MUST document any deviations from RFC documents,
+for example if the wait time around traffic
+is shorter than what [RFC2544] section 23 specifies.
+
+### Controller
+
+The controller selects trial load and duration values
+to achieve the search goals in the shortest expected time.
+
+The controller calls the measurer multiple times,
+receiving the trial result from each call.
+After exit condition is met, the controller returns
+the overall search results.
+
+The controller's role in optimizing trial load and duration selection
+distinguishes MLRsearch algorithms from simpler search procedures.
+
+For controller inputs, see later section Controller Inputs.
+For controller outputs, see later section Controller Outputs.
+
+### Manager
+
+The controller gets initiated by the manager once, and subsequently calls
+
+The manager is the component that initializes SUT, the traffic generator
+(tester in [RFC2544] terminology), the measurer and the controller
+with intended configurations.
+It then calls the controller once, and receives its outputs.
+
+The manager is also responsible for creating reports in the appropriate format,
+based on information in controller outputs.
+
+## Units
+
+The specification deals with physical quantities, so it is assumed
+each numeric value is accompanied by an appropriate physical unit.
+
+The specification does not state which unit is appropriate,
+but implementations MUST make it explicit which unit is used
+for each value provided or received by the user.
+
+For example, load quantities (including the conditional throughput)
+returned by the controller are defined to be based on a single-interface
+(unidirectional) loads.
+For bidirectional traffic, users are likely
+to expect bidirectional throughput quantities, so the manager is responsible
+for making its report clear.
+
+## SUT
+
+As defined in [RFC2285]:
+The collective set of network devices to which stimulus is offered
+as a single entity and response measured.
+
+## Trial
+
+A trial is the part of the test described in [RFC2544] section 23.
+
+### Trial Load
+
+The trial load is the intended constant load for a trial.
+
+Load is the quantity implied by Constant Load of [RFC1242],
+Data Rate of [RFC2544] and Intended Load of [RFC2285].
+All three specify this value applies to one (input or output) interface.
+
+### Trial Duration
+
+Trial duration is the intended duration of the traffic for a trial.
+
+In general, this quantity does not include any preparation nor waiting
+described in section 23 of [RFC2544].
+
+However, the measurer MAY return a duration value that deviates
+from the intended duration.
+This feature can be beneficial for users
+who wish to manage the overall search duration,
+rather than solely the traffic portion of it.
+The manager MUST report
+how the measurer computes the returned duration values in that case.
+
+### Trial Forwarding Ratio
+
+The trial forwarding ratio is a dimensionless floating point value
+that ranges from 0.0 to 1.0, inclusive.
+It is calculated by dividing the number of frames
+successfully forwarded by the SUT
+by the total number of frames expected to be forwarded during the trial.
+
+Note that, contrary to loads, frame counts used to compute
+trial forwarding ratio are aggregates over all SUT output ports.
+
+Questions around what is the correct number of frames
+that should have been forwarded is outside of the scope of this document.
+E.g. what should the measurer return when it detects
+that the offered load differs significantly from the intended load.
+
+### Trial Loss Ratio
+
+The trial loss ratio is equal to one minus the trial forwarding ratio.
+
+### Trial Forwarding Rate
+
+The trial forwarding rate is a derived quantity, calculated by
+multiplying the trial load by the trial forwarding ratio.
+
+It is important to note that while similar, this quantity is not identical
+to the Forwarding Rate as defined in [RFC2285] section 3.6.1,
+as the latter is specific to one output interface,
+whereas the trial forwarding ratio is based
+on frame counts aggregated over all SUT output interfaces.
+
+## Traffic profile
+
+Any other specifics (besides trial load and trial duration)
+the measurer needs in order to perform the trial
+are understood as a composite called the traffic profile.
+All its attributes are assumed to be constant during the search,
+and the composite is configured on the measurer by the manager
+before the search starts.
+
+The traffic profile is REQUIRED by [RFC2544]
+to contain some specific quantities, for example frame size.
+Several more specific quantities may be RECOMMENDED.
+
+Depending on SUT configuration, e.g. when testing specific protocols,
+additional values need to be included in the traffic profile
+and in the test report.
+See other IETF documents.
+
+## Search Goal
+
+The search goal is a composite consisting of several attributes,
+some of them are required.
+Implementations are free to add their own attributes.
+
+A particular set of attribute values is called a search goal instance.
+
+Subsections list all required attributes and one recommended attribute.
+Each subsection contains a short informal description,
+but see other chapters for more in-depth explanations.
+
+The meaning of the attributes is formally given only by their effect
+on the controller output attributes (defined in later in section Search Result).
+
+Informally, later chapters give additional intuitions and examples
+to the search goal attribute values.
+Later chapters also give motivation to formulas of computation of the outputs.
+
+### Goal Final Trial Duration
+
+A threshold value for trial durations.
+This attribute is REQUIRED, and the value MUST be positive.
+
+Informally, while MLRsearch is allowed to perform trials shorter than this,
+but results from such short trials have only limited impact on search results.
+
+The full relation needs definitions is later subsections.
+But for example, the conditional throughput
+(definition in subsection Conditional Throughput)
+for this goal will be computed only from trial results
+from trials at least as long as this.
+
+### Goal Duration Sum
+
+A threshold value for a particular sum of trial durations.
+This attribute is REQUIRED, and the value MUST be positive.
+
+This uses the duration values returned by the measurer.
+
+Informally, even when looking only at trials done at this goal's
+final trial duration, MLRsearch may spend up to this time measuring
+the same load value.
+If the goal duration sum is larger than
+the goal final trial duration, it means multiple trials need to be measured
+at the same load.
+
+### Goal Loss Ratio
+
+A threshold value for trial loss ratios.
+REQUIRED attribute, MUST be non-negative and smaller than one.
+
+Informally, if a load causes too many trials with trial loss ratios
+larger than this, the conditional throughput for this goal
+will be smaller than that load.
+
+### Goal Exceed Ratio
+
+A threshold value for a particular ratio of duration sums.
+REQUIRED attribute, MUST be non-negative and smaller than one.
+
+The duration sum values come from the duration values returned by the measurer.
+
+Informally, the impact of lossy trials is controlled by this value.
+The full relation needs definitions is later subsections.
+
+But for example, the definition of the conditional throughput
+(given later in subsection Conditional Throughput)
+refers to a q-value for a quantile when selecting
+which trial result gives the conditional throughput.
+The goal exceed ratio acts as the q-value to use there.
+
+Specifically, when the goal exceed ratio is 0.5 and MLRsearch happened
+to use the whole goal duration sum (using full-length trials),
+it means the conditional throughput is the median of trial forwarding rates.
+
+### Goal Width
+
+A value used as a threshold for telling when two trial load values
+are close enough.
+
+RECOMMENDED attribute, positive.
+Implementations without this attribute
+MUST give the manager other ways to control the search exit condition.
+
+Absolute load difference and relative load difference are two popular choices,
+but implementations may choose a different way to specify width.
+
+Informally, this acts as a stopping condition, controlling the precision
+of the search.
+The search stops if every goal has reached its precision.
+
+## Controller Inputs
+
+The only REQUIRED input for controller is a set of search goal instances.
+MLRsearch implementations MAY use additional input parameters for the controller.
+
+The order of instances SHOULD NOT have a big impact on controller outputs,
+but MLRsearch implementations MAY base their behavior on the order
+of search goal instances.
+
+The search goal instances SHOULD NOT be identical.
+MLRsearch implementation MAY allow identical instances.
+
+## Goal Result
+
+Before defining the output of the controller,
+it is useful to define what the goal result is.
+
+The goal result is a composite object consisting of several attributes.
+A particular set of attribute values is called a goal result instance.
+
+Any goal result instance can be either regular or irregular.
+MLRsearch specification puts requirements on regular goal result instances.
+Any instance that does not meet the requirements is deemed irregular.
+
+Implementations are free to define their own irregular goal results,
+but the manager MUST report them clearly as not regular according to this section.
+
+All attribute values in one goal result instance
+are related to a single search goal instance,
+referred to as the given search goal.
+
+Some of the attributes of a regular goal result instance are required,
+some are recommended, implementations are free to add their own.
+
+The subsections define two required and one optional attribute
+for a regular goal result.
+
+A typical irregular result is when all trials at the maximal offered load
+have zero loss, as the relevant upper bound does not exist in that case.
+
+### Relevant Upper Bound
+
+The relevant upper bound is the smallest intended load value that is classified
+at the end of the search as an upper bound (see Appendix A)
+for the given search goal.
+This is a REQUIRED attribute.
+
+Informally, this is the smallest intended load that failed to uphold
+all the requirements of the given search goal, mainly the goal loss ratio
+in combination with the goal exceed ratio.
+
+### Relevant Lower Bound
+
+The relevant lower bound is the largest intended load value
+among those smaller than the relevant upper bound
+that got classified at the end of the search
+as a lower bound (see Appendix A) for the given search goal.
+This is a REQUIRED attribute.
+
+For a regular goal result, the distance between the relevant lower bound
+and the relevant upper bound MUST NOT be larger than the goal width,
+if the implementation offers width as a goal attribute.
+
+Informally, this is the largest intended load that managed to uphold
+all the requirements of the given search goal, mainly the goal loss ratio
+in combination with the goal exceed ratio, while not being larger
+than the relevant upper bound.
+
+### Conditional Throughput
+
+The conditional throughput (see Appendix B)
+as evaluated at the relevant lower bound of the given search goal
+at the end of the search.
+This is a RECOMMENDED attribute.
+
+Informally, this is a typical forwarding rate expected to be seen
+at the relevant lower bound of the given search goal.
+But frequently just a conservative estimate thereof,
+as MLRsearch implementations tend to stop gathering more data
+as soon as they confirm the result cannot get worse than this estimate
+within the goal duration sum.
+
+## Search Result
+
+The search result is a single composite object
+that maps each search goal to a corresponding goal result.
+
+In other words, search result is an unordered list of key-value pairs,
+where no two pairs contain equal keys.
+The key is a search goal instance, acting as the given search goal
+for the goal result instance in the value portion of the key-value pair.
+
+The search result (as a mapping)
+MUST map from all the search goals present in the controller input.
+
+## Controller Outputs
+
+The search result is the only REQUIRED output
+returned from the controller to the manager.
+
+MLRsearch implementation MAY return additional data in the controller output.
+
+# Further Explanations
+
+This chapter focuses on intuitions and motivations
+and skips over some important details.
+
+Familiarity with the MLRsearch specification is not required here,
+so this chapter can act as an introduction.
+For example, this chapter starts talking about the tightest lower bounds
+before it is ready to talk about the relevant lower bound from the specification.
+
+## MLRsearch Versions
+
+The MLRsearch algorithm has been developed in a code-first approach,
+a Python library has been created, debugged, and used in production
+before the first descriptions (even informal) were published.
+In fact, multiple versions of the library were used in the production
+over the past few years, and later code was usually not compatible
+with earlier descriptions.
+
+The code in (any version of) MLRsearch library fully determines
+the search process (for given configuration parameters),
+leaving no space for deviations.
+MLRsearch, as a name for a broad class of possible algorithms,
+leaves plenty of space for future improvements, at the cost
+of poor comparability of results of different MLRsearch implementations.
+
+There are two competing needs.
+There is the need for standardization in areas critical to comparability.
+There is also the need to allow flexibility for implementations
+to innovate and improve in other areas.
+This document defines the MLRsearch specification
+in a manner that aims to fairly balances both needs.
+
+## Exit Condition
+
+[RFC2544] prescribes that after performing one trial at a specific offered load,
+the next offered load should be larger or smaller, based on frame loss.
+
+The usual implementation uses binary search.
+Here a lossy trial becomes
+a new upper bound, a lossless trial becomes a new lower bound.
+The span of values between (including both) the tightest lower bound
+and the tightest upper bound forms an interval of possible results,
+and after each trial the width of that interval halves.
+
+Usually the binary search implementation tracks only the two tightest bounds,
+simply calling them bounds.
+But the old values still B remain valid bounds,
+just not as tight as the new ones.
+
+After some number of trials, the tightest lower bound becomes the throughput.
+[RFC2544] does not specify when (if ever) should the search stop.
+
+MLRsearch library introduces a concept of goal width.
+The search stops
+when the distance between the tightest upper bound and the tightest lower bound
+is smaller than a user-configured value, called goal width from now on.
+In other words, the interval width at the end of the search
+has to be no larger than the goal width.
+
+This goal width value therefore determines the precision of the result.
+As MLRsearch specification requires a particular structure of the result,
+the result itself does contain enough information to determine its precision,
+thus it is not required to report the goal width value.
+
+This allows MLRsearch implementations to use exit conditions
+different from goal width.
+
+## Load Classification
+
+MLRsearch keeps the basic logic of binary search (tracking tightest bounds,
+measuring at the middle), perhaps with minor technical clarifications.
+The algorithm chooses an intended load (as opposed to the offered load),
+the interval between bounds does not need to be split
+exactly into two equal halves,
+and the final reported structure specifies both bounds.
+
+The biggest difference is that to classify a load
+as an upper or lower bound, MLRsearch may need more than one trial
+(depending on configuration options) to be performed at the same intended load.
+
+As a consequence, even if a load already does have few trial results,
+it still may be classified as undecided, neither a lower bound nor an upper bound.
+
+An explanation of the classification logic is given in the next chapter,
+as it relies heavily on other sections of this chapter.
+
+For repeatability and comparability reasons, it is important that
+given a set of trial results, all implementations of MLRsearch
+classify the load equivalently.
+
+## Loss Ratios
+
+The next difference is in the goals of the search.
+[RFC2544] has a single goal,
+based on classifying full-length trials as either lossless or lossy.
+
+As the name suggests, MLRsearch can search for multiple goals,
+differing in their loss ratios.
+The precise definition of the goal loss ratio will be given later.
+The [RFC2544] throughput goal then simply becomes a zero goal loss ratio.
+Different goals also may have different goal widths.
+
+A set of trial results for one specific intended load value
+can classify the load as an upper bound for some goals, but a lower bound
+for some other goals, and undecided for the rest of the goals.
+
+Therefore, the load classification depends not only on trial results,
+but also on the goal.
+The overall search procedure becomes more complicated
+(compared to binary search with a single goal),
+but most of the complications do not affect the final result,
+except for one phenomenon, loss inversion.
+
+## Loss Inversion
+
+In [RFC2544] throughput search using bisection, any load with a lossy trial
+becomes a hard upper bound, meaning every subsequent trial has a smaller
+intended load.
+
+But in MLRsearch, a load that is classified as an upper bound for one goal
+may still be a lower bound for another goal, and due to the other goal
+MLRsearch will probably perform trials at even higher loads.
+What to do when all such higher load trials happen to have zero loss?
+Does it mean the earlier upper bound was not real?
+Does it mean the later lossless trials are not considered a lower bound?
+Surely we do not want to have an upper bound at a load smaller than a lower bound.
+
+MLRsearch is conservative in these situations.
+The upper bound is considered real, and the lossless trials at higher loads
+are considered to be a coincidence, at least when computing the final result.
+
+This is formalized using new notions, the relevant upper bound and
+the relevant lower bound.
+Load classification is still based just on the set of trial results
+at a given intended load (trials at other loads are ignored),
+making it possible to have a lower load classified as an upper bound,
+and a higher load classified as a lower bound (for the same goal).
+The relevant upper bound (for a goal) is the smallest load classified
+as an upper bound.
+But the relevant lower bound is not simply
+the largest among lower bounds.
+It is the largest load among loads
+that are lower bounds while also being smaller than the relevant upper bound.
+
+With these definitions, the relevant lower bound is always smaller
+than the relevant upper bound (if both exist), and the two relevant bounds
+are used analogously as the two tightest bounds in the binary search.
+When they are less than the goal width apart,
+the relevant bounds are used in the output.
+
+One consequence is that every trial result can have an impact on the search result.
+That means if your SUT (or your traffic generator) needs a warmup,
+be sure to warm it up before starting the search.
+
+## Exceed Ratio
+
+The idea of performing multiple trials at the same load comes from
+a model where some trial results (those with high loss) are affected
+by infrequent effects, causing poor repeatability of [RFC2544] throughput results.
+See the discussion about noiseful and noiseless ends
+of the SUT performance spectrum.
+Stable results are closer to the noiseless end of the SUT performance spectrum,
+so MLRsearch may need to allow some frequency of high-loss trials
+to ignore the rare but big effects near the noiseful end.
+
+MLRsearch can do such trial result filtering, but it needs
+a configuration option to tell it how frequent can the infrequent big loss be.
+This option is called the exceed ratio.
+It tells MLRsearch what ratio of trials
+(more exactly what ratio of trial seconds) can have a trial loss ratio
+larger than the goal loss ratio and still be classified as a lower bound.
+Zero exceed ratio means all trials have to have a trial loss ratio
+equal to or smaller than the goal loss ratio.
+
+For explainability reasons, the RECOMMENDED value for exceed ratio is 0.5,
+as it simplifies some later concepts by relating them to the concept of median.
+
+## Duration Sum
+
+When more than one trial is needed to classify a load,
+MLRsearch also needs something that controls the number of trials needed.
+Therefore, each goal also has an attribute called duration sum.
+
+The meaning of a goal duration sum is that when a load has trials
+(at full trial duration, details later)
+whose trial durations when summed up give a value at least this long,
+the load is guaranteed to be classified as an upper bound or a lower bound
+for the goal.
+
+As the duration sum has a big impact on the overall search duration,
+and [RFC2544] prescribes wait intervals around trial traffic,
+the MLRsearch algorithm is allowed to sum durations that are different
+from the actual trial traffic durations.
+
+## Short Trials
+
+MLRsearch requires each goal to specify its final trial duration.
+Full-length trial is a shorter name for a trial whose intended trial duration
+is equal to (or longer than) the goal final trial duration.
+
+Section 24 of [RFC2544] already anticipates possible time savings
+when short trials (shorter than full-length trials) are used.
+Full-length trials are the opposite of short trials,
+so they may also be called long trials.
+
+Any MLRsearch implementation may include its own configuration options
+which control when and how MLRsearch chooses to use shorter trial durations.
+
+For explainability reasons, when exceed ratio of 0.5 is used,
+it is recommended for the goal duration sum to be an odd multiple
+of the full trial durations, so conditional throughput becomes identical to
+a median of a particular set of forwarding rates.
+
+The presence of shorter trial results complicates the load classification logic.
+Full details are given later.
+In short, results from short trials
+may cause a load to be classified as an upper bound.
+This may cause loss inversion, and thus lower the relevant lower bound
+(below what would classification say when considering full-length trials only).
+
+For explainability reasons, it is RECOMMENDED users use such configurations
+that guarantee all trials have the same length.
+Alas, such configurations are usually not compliant with [RFC2544] requirements,
+or not time-saving enough.
+
+## Conditional Throughput
+
+As testing equipment takes the intended load as an input parameter
+for a trial measurement, any load search algorithm needs to deal
+with intended load values internally.
+
+But in the presence of goals with a non-zero loss ratio, the intended load
+usually does not match the user's intuition of what a throughput is.
+The forwarding rate (as defined in [RFC2285] section 3.6.1) is better,
+but it is not obvious how to generalize it
+for loads with multiple trial results and a non-zero goal loss ratio.
+
+MLRsearch defines one such generalization, called the conditional throughput.
+It is the forwarding rate from one of the trials performed at the load
+in question.
+Specification of which trial exactly is quite technical,
+see the specification and Appendix B.
+
+Conditional throughput is partially related to load classification.
+If a load is classified as a lower bound for a goal,
+the conditional throughput can be calculated,
+and guaranteed to show an effective loss ratio
+no larger than the goal loss ratio.
+
+While the conditional throughput gives more intuitive-looking values
+than the relevant lower bound, especially for non-zero goal loss ratio values,
+the actual definition is more complicated than the definition of the relevant
+lower bound.
+In the future, other intuitive values may become popular,
+but they are unlikely to supersede the definition of the relevant lower bound
+as the most fitting value for comparability purposes,
+therefore the relevant lower bound remains a required attribute
+of the goal result structure, while the conditional throughput is only optional.
+
+Note that comparing the best and worst case, the same relevant lower bound value
+may result in the conditional throughput differing up to the goal loss ratio.
+Therefore it is rarely needed to set the goal width (if expressed
+as the relative difference of loads) below the goal loss ratio.
+In other words, setting the goal width below the goal loss ratio
+may cause the conditional throughput for a larger loss ratio to become smaller
+than a conditional throughput for a goal with a smaller goal loss ratio,
+which is counter-intuitive, considering they come from the same search.
+Therefore it is RECOMMENDED to set the goal width to a value no smaller
+than the goal loss ratio.
+
+## Search Time
+
+MLRsearch was primarily developed to reduce the time
+required to determine a throughput, either the [RFC2544] compliant one,
+or some generalization thereof.
+The art of achieving short search times
+is mainly in the smart selection of intended loads (and intended durations)
+for the next trial to perform.
+
+While there is an indirect impact of the load selection on the reported values,
+in practice such impact tends to be small,
+even for SUTs with quite a broad performance spectrum.
+
+A typical example of two approaches to load selection leading to different
+relevant lower bounds is when the interval is split in a very uneven way.
+Any implementation choosing loads very close to the current relevant lower bound
+is quite likely to eventually stumble upon a trial result
+with poor performance (due to SUT noise).
+For an implementation choosing loads very close
+to the current relevant upper bound, this is unlikely,
+as it examines more loads that can see a performance
+close to the noiseless end of the SUT performance spectrum.
+
+However, as even splits optimize search duration at give precision,
+MLRsearch implementations that prioritize minimizing search time
+are unlikely to suffer from any such bias.
+
+Therefore, this document remains quite vague on load selection
+and other optimization details, and configuration attributes related to them.
+Assuming users prefer libraries that achieve short overall search time,
+the definition of the relevant lower bound
+should be strict enough to ensure result repeatability
+and comparability between different implementations,
+while not restricting future implementations much.
+
+Sadly, different implementations may exhibit their sweet spot of
+the best repeatability for a given search duration
+at different goals attribute values, especially concerning
+any optional goal attributes such as the initial trial duration.
+Thus, this document does not comment much on which configurations
+are good for comparability between different implementations.
+For comparability between different SUTs using the same implementation,
+refer to configurations recommended by that particular implementation.
+
+## [RFC2544] compliance
+
+The following search goal ensures unconditional compliance with
+[RFC2544] throughput search procedure:
+
+- Goal loss ratio: zero.
+
+- Goal final trial duration: 60 seconds.
+
+- Goal duration sum: 60 seconds.
+
+- Goal exceed ratio: zero.
+
+The presence of other search goals does not affect the compliance
+of this goal result.
+The relevant lower bound and the conditional throughput are in this case
+equal to each other, and the value is the [RFC2544] throughput.
+
+If the 60 second quantity is replaced by a smaller quantity in both attributes,
+the conditional throughput is still conditionally compliant with
+[RFC2544] throughput.
+
+# Logic of Load Classification
+
+This chapter continues with explanations,
+but this time more precise definitions are needed
+for readers to follow the explanations.
+The definitions here are wordy, implementers should read the specification
+chapter and appendices for more concise definitions.
+
+The two related areas of focus in this chapter are load classification
+and the conditional throughput, starting with the latter.
+
+The section Performance Spectrum contains definitions
+needed to gain insight into what conditional throughput means.
+The rest of the subsections discuss load classification,
+they do not refer to Performance Spectrum, only to a few duration sums.
+
+For load classification, it is useful to define good and bad trials.
+A trial is called bad (according to a goal) if its trial loss ratio
+is larger than the goal loss ratio.
+The trial that is not bad is called good.
+
+## Performance Spectrum
+
+There are several equivalent ways to explain
+the conditional throughput computation.
+One of the ways relies on an object called the performance spectrum.
+First, two heavy definitions are needed.
+
+Take an intended load value, a trial duration value, and a finite set
+of trial results, all trials measured at that load value and duration value.
+The performance spectrum is the function that maps
+any non-negative real number into a sum of trial durations among all trials
+in the set that has that number as their forwarding rate,
+e.g. map to zero if no trial has that particular forwarding rate.
+
+A related function, defined if there is at least one trial in the set,
+is the performance spectrum divided by the sum of the durations
+of all trials in the set.
+That function is called the performance probability function, as it satisfies
+all the requirements for probability mass function function
+of a discrete probability distribution,
+the one-dimensional random variable being the trial forwarding rate.
+
+These functions are related to the SUT performance spectrum,
+as sampled by the trials in the set.
+
+As for any other probability function, we can talk about percentiles
+of the performance probability function, including the median.
+The conditional throughput will be one such quantile value
+for a specifically chosen set of trials.
+
+Take a set of all full-length trials performed at the relevant lower bound,
+sorted by decreasing forwarding rate.
+The sum of the durations of those trials
+may be less than the goal duration sum, or not.
+If it is less, add an imaginary trial result with zero forwarding rate,
+such that the new sum of durations is equal to the goal duration sum.
+This is the set of trials to use.
+The q-value for the quantile
+is the goal exceed ratio.
+If the quantile touches two trials,
+the larger forwarding rate (from the trial result sorted earlier) is used.
+The resulting quantity is the conditional throughput of the goal in question.
+
+First example.
+For zero exceed ratio, when goal duration sum has been reached.
+The conditional throughput is the smallest forwarding rate among the trials.
+
+Second example.
+For zero exceed ratio, when goal duration sum has not been reached yet.
+Due to the missing duration sum, the worst case may still happen,
+so the conditional throughput is zero.
+This is not reported to the user,
+as this load cannot become the relevant lower bound yet.
+
+Third example.
+Exceed ratio 50%, goal duration sum two seconds,
+one trial present with the duration of one second and zero loss.
+The imaginary trial is added with the duration
+of one second and zero forwarding rate.
+The median would touch both trials, so the conditional throughput
+is the forwarding rate of the one non-imaginary trial.
+As that had zero loss, the value is equal to the offered load.
+
+Note that Appendix B does not take into account short trial results.
+
+### Summary
+
+While the conditional throughput is a generalization of the forwarding rate,
+its definition is not an obvious one.
+
+Other than the forwarding rate, the other source of intuition
+is the quantile in general, and the median the the recommended case.
+
+In future, different quantities may prove more useful,
+especially when applying to specific problems,
+but currently the conditional throughput is the recommended compromise,
+especially for repeatability and comparability reasons.
+
+## Single Trial Duration
+
+When goal attributes are chosen in such a way that every trial has the same
+intended duration, the load classification is simpler.
+
+The following description looks technical, but it follows the motivation
+of goal loss ratio, goal exceed ratio, and goal duration sum.
+If the sum of the durations of all trials (at the given load)
+is less than the goal duration sum, imagine best case scenario
+(all subsequent trials having zero loss) and worst case scenario
+(all subsequent trials having 100% loss).
+Here we assume there are as many subsequent trials as needed
+to make the sum of all trials equal to the goal duration sum.
+As the exceed ratio is defined just using sums of durations
+(number of trials does not matter), it does not matter whether
+the "subsequent trials" can consist of an integer number of full-length trials.
+
+In any of the two scenarios, we can compute the load exceed ratio,
+As the duration sum of good trials divided by the duration sum of all trials,
+in both cases including the assumed trials.
+
+If even in the best case scenario the load exceed ratio would be larger
+than the goal exceed ratio, the load is an upper bound.
+If even in the worst case scenario the load exceed ratio would not be larger
+than the goal exceed ratio, the load is a lower bound.
+
+Even more specifically.
+Take all trials measured at a given load.
+The sum of the durations of all bad full-length trials is called the bad sum.
+The sum of the durations of all good full-length trials is called the good sum.
+The result of adding the bad sum plus the good sum is called the measured sum.
+The larger of the measured sum and the goal duration sum is called the whole sum.
+The whole sum minus the measured sum is called the missing sum.
+The optimistic exceed ratio is the bad sum divided by the whole sum.
+The pessimistic exceed ratio is the bad sum plus the missing sum,
+that divided by the whole sum.
+If the optimistic exceed ratio is larger than the goal exceed ratio,
+the load is classified as an upper bound.
+If the pessimistic exceed ratio is not larger than the goal exceed ratio,
+the load is classified as a lower bound.
+Else, the load is classified as undecided.
+
+The definition of pessimistic exceed ratio is compatible with the logic in
+the conditional throughput computation, so in this single trial duration case,
+a load is a lower bound if and only if the conditional throughput
+effective loss ratio is not larger than the goal loss ratio.
+If it is larger, the load is either an upper bound or undecided.
+
+## Short Trial Scenarios
+
+Trials with intended duration smaller than the goal final trial duration
+are called short trials.
+The motivation for load classification logic in the presence of short trials
+is based around a counter-factual case: What would the trial result be
+if a short trial has been measured as a full-length trial instead?
+
+There are three main scenarios where human intuition guides
+the intended behavior of load classification.
+
+False good scenario.
+The user had their reason for not configuring a shorter goal
+final trial duration.
+Perhaps SUT has buffers that may get full at longer
+trial durations.
+Perhaps SUT shows periodic decreases in performance
+the user does not want to be treated as noise.
+In any case, many good short trials may become bad full-length trials
+in the counter-factual case.
+In extreme cases, there are plenty of good short trials and no bad short trials.
+In this scenario, we want the load classification NOT to classify the load
+as a lower bound, despite the abundance of good short trials.
+Effectively, we want the good short trials to be ignored, so they
+do not contribute to comparisons with the goal duration sum.
+
+True bad scenario.
+When there is a frame loss in a short trial,
+the counter-factual full-length trial is expected to lose at least as many
+frames.
+And in practice, bad short trials are rarely turning into
+good full-length trials.
+In extreme cases, there are no good short trials.
+In this scenario, we want the load classification
+to classify the load as an upper bound just based on the abundance
+of short bad trials.
+Effectively, we want the bad short trials
+to contribute to comparisons with the goal duration sum,
+so the load can be classified sooner.
+
+Balanced scenario.
+Some SUTs are quite indifferent to trial duration.
+Performance probability function constructed from short trial results
+is likely to be similar to the performance probability function constructed
+from full-length trial results (perhaps with larger dispersion,
+but without a big impact on the median quantiles overall).
+For a moderate goal exceed ratio value, this may mean there are both
+good short trials and bad short trials.
+This scenario is there just to invalidate a simple heuristic
+of always ignoring good short trials and never ignoring bad short trials.
+That simple heuristic would be too biased.
+Yes, the short bad trials
+are likely to turn into full-length bad trials in the counter-factual case,
+but there is no information on what would the good short trials turn into.
+The only way to decide safely is to do more trials at full length,
+the same as in scenario one.
+
+## Short Trial Logic
+
+MLRsearch picks a particular logic for load classification
+in the presence of short trials, but it is still RECOMMENDED
+to use configurations that imply no short trials,
+so the possible inefficiencies in that logic
+do not affect the result, and the result has better explainability.
+
+With that said, the logic differs from the single trial duration case
+only in different definition of the bad sum.
+The good sum is still the sum across all good full-length trials.
+
+Few more notions are needed for defining the new bad sum.
+The sum of durations of all bad full-length trials is called the bad long sum.
+The sum of durations of all bad short trials is called the bad short sum.
+The sum of durations of all good short trials is called the good short sum.
+One minus the goal exceed ratio is called the inceed ratio.
+The goal exceed ratio divided by the inceed ratio is called the exceed coefficient.
+The good short sum multiplied by the exceed coefficient is called the balancing sum.
+The bad short sum minus the balancing sum is called the excess sum.
+If the excess sum is negative, the bad sum is equal to the bad long sum.
+Otherwise, the bad sum is equal to the bad long sum plus the excess sum.
+
+Here is how the new definition of the bad sum fares in the three scenarios,
+where the load is close to what would the relevant bounds be
+if only full-length trials were used for the search.
+
+False good scenario.
+If the duration is too short, we expect to see a higher frequency
+of good short trials.
+This could lead to a negative excess sum,
+which has no impact, hence the load classification is given just by
+full-length trials.
+Thus, MLRsearch using too short trials has no detrimental effect
+on result comparability in this scenario.
+But also using short trials does not help with overall search duration,
+probably making it worse.
+
+True bad cenario.
+Settings with a small exceed ratio
+have a small exceed coefficient, so the impact of the good short sum is small,
+and the bad short sum is almost wholly converted into excess sum,
+thus bad short trials have almost as big an impact as full-length bad trials.
+The same conclusion applies to moderate exceed ratio values
+when the good short sum is small.
+Thus, short trials can cause a load to get classified as an upper bound earlier,
+bringing time savings (while not affecting comparability).
+
+Balanced scenario.
+Here excess sum is small in absolute value, as the balancing sum
+is expected to be similar to the bad short sum.
+Once again, full-length trials are needed for final load classification;
+but usage of short trials probably means MLRsearch needed
+a shorter overall search time before selecting this load for measurement,
+thus bringing time savings (while not affecting comparability).
+
+Note that in presence of short trial results,
+the comparibility between the load classification
+and the conditional throughput is only partial.
+The conditional throughput still comes from a good long trial,
+but a load higher than the relevant lower bound may also compute to a good value.
+
+## Longer Trial Durations
+
+If there are trial results with an intended duration larger
+than the goal trial duration, the precise definitions
+in Appendix A and Appendix B treat them in exactly the same way
+as trials with duration equal to the goal trial duration.
+
+But in configurations with moderate (including 0.5) or small
+goal exceed ratio and small goal loss ratio (especially zero),
+bad trials with longer than goal durations may bias the search
+towards the lower load values, as the noiseful end of the spectrum
+gets a larger probability of causing the loss within the longer trials.
+
+For some users, this is an acceptable price
+for increased configuration flexibility
+(perhaps saving time for the related goals),
+so implementations SHOULD allow such configurations.
+Still, users are encouraged to avoid such configurations
+by making all goals use the same final trial duration,
+so their results remain comparable across implementations.
+
+# Addressed Problems
+
+Now when MLRsearch is clearly specified and explained,
+it is possible to summarize how does MLRsearch specification help with problems.
+
+Here, "multiple trials" is a shorthand for having the goal final trial duration
+significantly smaller than the goal duration sum.
+This results in MLRsearch performing multiple trials at the same load,
+which may not be the case with other configurations.
+
+## Long Test Duration
+
+As shortening the overall search duration is the main motivation
+of MLRsearch library development, the library implements
+multiple improvements on this front, both big and small.
+
+Most of implementation details are not constrained by the MLRsearch specification,
+so that future implementations may keep shortening the search duration even more.
+
+One exception is the impact of short trial results on the relevant lower bound.
+While motivated by human intuition, the logic is not straightforward.
+In practice, configurations with only one common trial duration value
+are capable of achieving good overal search time and result repeatability
+without the need to consider short trials.
+
+### Impact of goal attribute values
+
+From the required goal attributes, the goal duration sum
+remains the best way to get even shorter searches.
+
+Usage of multiple trials can also save time,
+depending on wait times around trial traffic.
+
+The farther the goal exceed ratio is from 0.5 (towards zero or one),
+the less predictable the overal search duration becomes in practice.
+
+Width parameter does not change search duration much in practice
+(compared to other, mainly optional goal attributes).
+
+## DUT in SUT
+
+In practice, using multiple trials and moderate exceed ratios
+often improves result repeatability without increasing the overall search time,
+depending on the specific SUT and DUT characteristics.
+Benefits for separating SUT noise are less clear though,
+as it is not easy to distinguish SUT noise from DUT instability in general.
+
+Conditional throughput has an intuitive meaning when described
+using the performance spectrum, so this is an improvement
+over existing simple (less configurable) search procedures.
+
+Multiple trials can save time also when the noisy end of
+the preformance spectrum needs to be examined, e.g. for [RFC9004].
+
+Under some circumstances, testing the same DUT and SUT setup with different
+DUT configurations can give some hints on what part of noise is SUT noise
+and what part is DUT performance fluctuations.
+In practice, both types of noise tend to be too complicated for that analysis.
+
+MLRsearch enables users to search for multiple goals,
+potentially providing more insight at the cost of a longer overall search time.
+However, for a thorough and reliable examination of DUT-SUT interactions,
+it is necessary to employ additional methods beyond black-box benchmarking,
+such as collecting and analyzing DUT and SUT telemetry.
+
+## Repeatability and Comparability
+
+Multiple trials improve repeatability, depending on exceed ratio.
+
+In practice, one-second goal final trial duration with exceed ratio 0.5
+is good enough for modern SUTs.
+However, unless smaller wait times around the traffic part of the trial
+are allowed, too much of overal search time would be wasted on waiting.
+
+It is not clear whether exceed ratios higher than 0.5 are better
+for repeatability.
+The 0.5 value is still preferred due to explainability using median.
+
+It is possible that the conditional throughput values (with non-zero goal
+loss ratio) are better for repeatability than the relevant lower bound values.
+This is especially for implementations
+which pick load from a small set of discrete values,
+as that hides small variances in relevant lower bound values
+other implementations may find.
+
+Implementations focusing on shortening the overall search time
+are automatically forced to avoid comparability issues due to load selection,
+as they must prefer even splits wherever possible.
+But this conclusion only holds when the same goals are used.
+Larger adoption is needed before any further claims on comparability
+between MLRsearch implementations can be made.
+
+## Throughput with Non-Zero Loss
+
+Trivially suported by the goal loss ratio attribute.
+
+In practice, usage of non-zero loss ratio values
+improves the result repeatability
+(exactly as expected based on results from simpler search methods).
+
+## Inconsistent Trial Results
+
+MLRsearch is conservative wherever possible.
+This is built into the definition of conditional throughput,
+and into the treatment of short trial results for load classification.
+
+This is consistent with [RFC2544] zero loss tolerance motivation.
+
+If the noiseless part of the SUT performance spectrum is of interest,
+it should be enough to set small value for the goal final trial duration,
+and perhaps also a large value for the goal exceed ratio.
+
+Implementations may offer other (optional) configuration attributes
+to become less conservative, but currently it is not clear
+what impact would that have on repeatability.
+
+# IANA Considerations
+
+No requests of IANA.
+
+# Security Considerations
+
+Benchmarking activities as described in this memo are limited to
+technology characterization of a DUT/SUT using controlled stimuli in a
+laboratory environment, with dedicated address space and the constraints
+specified in the sections above.
+
+The benchmarking network topology will be an independent test setup and
+MUST NOT be connected to devices that may forward the test traffic into
+a production network or misroute traffic to the test management network.
+
+Further, benchmarking is performed on a "black-box" basis, relying
+solely on measurements observable external to the DUT/SUT.
+
+Special capabilities SHOULD NOT exist in the DUT/SUT specifically for
+benchmarking purposes. Any implications for network security arising
+from the DUT/SUT SHOULD be identical in the lab and in production
+networks.
+
+# Acknowledgements
+
+Some phrases and statements in this document were created
+with help of Mistral AI (mistral.ai).
+
+Many thanks to Alec Hothan of the OPNFV NFVbench project for thorough
+review and numerous useful comments and suggestions.
+
+Special wholehearted gratitude and thanks to the late Al Morton for his
+thorough reviews filled with very specific feedback and constructive
+guidelines. Thank you Al for the close collaboration over the years,
+for your continuous unwavering encouragement full of empathy and
+positive attitude.
+Al, you are dearly missed.
+
+# Appendix A: Load Classification
+
+This is the specification of how to perform the load classification.
+
+Any intended load value can be classified, according to the given search goal.
+
+The algorithm uses (some subsets of) the set of all available trial results
+from trials measured at a given intended load at the end of the search.
+All durations are those returned by the measurer.
+
+The block at the end of this appendix holds pseudocode
+which computes two values, stored in variables named optimistic and pessimistic.
+The pseudocode happens to be a valid Python code.
+
+If both values are computed to be true, the load in question
+is classified as a lower bound according to the given search goal.
+If both values are false, the load is classified as an upper bound.
+Otherwise, the load is classified as undecided.
+
+The pseudocode expects the following variables to hold values as follows:
+
+- goal_duration_sum: The duration sum value of the given search goal.
+
+- goal_exceed_ratio: The exceed ratio value of the given search goal.
+
+- good_long_sum: Sum of durations across trials with trial duration
+  at least equal to the goal final trial duration and with a trial loss ratio
+  not higher than the goal loss ratio.
+
+- bad_long_sum: Sum of durations across trials with trial duration
+  at least equal to the goal final trial duration and with a trial loss ratio
+  higher than the goal loss ratio.
+
+- good_short_sum: Sum of durations across trials with trial duration
+  shorter than the goal final trial duration and with a trial loss ratio
+  not higher than the goal loss ratio.
+
+- bad_short_sum: Sum of durations across trials with trial duration
+  shorter than the goal final trial duration and with a trial loss ratio
+  higher than the goal loss ratio.
+
+The code works correctly also when there are no trial results at the given load.
+
+~~~ python
+balancing_sum = good_short_sum * goal_exceed_ratio / (1.0 - goal_exceed_ratio)
+effective_bad_sum = bad_long_sum + max(0.0, bad_short_sum - balancing_sum)
+effective_whole_sum = max(good_long_sum + effective_bad_sum, goal_duration_sum)
+quantile_duration_sum = effective_whole_sum * goal_exceed_ratio
+optimistic = effective_bad_sum <= quantile_duration_sum
+pessimistic = (effective_whole_sum - good_long_sum) <= quantile_duration_sum
+~~~
+
+# Appendix B: Conditional Throughput
+
+This is the specification of how to compute conditional throughput.
+
+Any intended load value can be used as the basis for the following computation,
+but only the relevant lower bound (at the end of the search)
+leads to the value called the conditional throughput for a given search goal.
+
+The algorithm uses (some subsets of) the set of all available trial results
+from trials measured at a given intended load at the end of the search.
+All durations are those returned by the measurer.
+
+The block at the end of this appendix holds pseudocode
+which computes a value stored as variable conditional_throughput.
+The pseudocode happens to be a valid Python code.
+
+The pseudocode expects the following variables to hold values as follows:
+
+- goal_duration_sum: The duration sum value of the given search goal.
+
+- goal_exceed_ratio: The exceed ratio value of the given search goal.
+
+- good_long_sum: Sum of durations across trials with trial duration
+  at least equal to the goal final trial duration and with a trial loss ratio
+  not higher than the goal loss ratio.
+
+- bad_long_sum: Sum of durations across trials with trial duration
+  at least equal to the goal final trial duration and with a trial loss ratio
+  higher than the goal loss ratio.
+
+- long_trials: An iterable of all trial results from trials with trial duration
+  at least equal to the goal final trial duration,
+  sorted by increasing the trial loss ratio.
+  A trial result is a composite with the following two attributes available:
+
+  - trial.loss_ratio: The trial loss ratio as measured for this trial.
+
+  - trial.duration: The trial duration of this trial.
+
+The code works correctly only when there if there is at least one
+trial result measured at a given load.
+
+~~~ python
+all_long_sum = max(goal_duration_sum, good_long_sum + bad_long_sum)
+remaining = all_long_sum * (1.0 - goal_exceed_ratio)
+quantile_loss_ratio = None
+for trial in long_trials:
+    if quantile_loss_ratio is None or remaining > 0.0:
+        quantile_loss_ratio = trial.loss_ratio
+        remaining -= trial.duration
+    else:
+        break
+else:
+    if remaining > 0.0:
+        quantile_loss_ratio = 1.0
+conditional_throughput = intended_load * (1.0 - quantile_loss_ratio)
+~~~
+
+--- back