path: root/docs/ietf/draft-vpolak-mkonstan-bmwg-mlrsearch-02.md

---
title: Multiple Loss Ratio Search for Packet Throughput (MLRsearch)
# abbrev: MLRsearch
docname: draft-vpolak-mkonstan-bmwg-mlrsearch-02
date: 2019-07-08

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
#  - sortrefs
#  - symrefs
  toc: yes
  sortrefs:   # defaults to yes
  symrefs: yes

author:
      -
        ins: M. Konstantynowicz
        name: Maciek Konstantynowicz
        org: Cisco Systems
        role: editor
        email: mkonstan@cisco.com
      -
        ins: V. Polak
        name: Vratko Polak
        org: Cisco Systems
        role: editor
        email: vrpolak@cisco.com

normative:
  RFC2544:
  RFC8174:

informative:
  FDio-CSIT-MLRsearch:
    target: https://docs.fd.io/csit/rls1904/report/introduction/methodology_data_plane_throughput/methodology_mlrsearch_tests.html
    title: "FD.io CSIT Test Methodology - MLRsearch"
    date: 2019-06
  PyPI-MLRsearch:
    target: https://pypi.org/project/MLRsearch/
    title: MLRsearch 0.2.0, Python Package Index
    date: 2018-08

--- abstract

This document proposes changes to [RFC2544], specifically to packet
throughput search methodology, by defining a new search algorithm
referred to as Multiple Loss Ratio search (MLRsearch for short). Instead
of relying on binary search with pre-set starting offered load, it
proposes a novel approach discovering the starting point in the initial
phase, and then searching for packet throughput based on defined packet
loss ratio (PLR) input criteria and defined final trial duration time.
One of the key design principles behind MLRsearch is minimizing the
total test duration and searching for multiple packet throughput rates
(each with a corresponding PLR) concurrently, instead of doing it
sequentially.

The main motivation behind MLRsearch is the new set of challenges and
requirements posed by NFV (Network Function Virtualization),
specifically software based implementations of NFV data planes. Using
[RFC2544] in the experience of the authors yields often not repetitive
and not replicable end results due to a large number of factors that are
out of scope for this draft. MLRsearch aims to address this challenge and
define a common (standard?) way to evaluate NFV packet throughput
performance that takes into account varying characteristics of NFV
systems under test.

--- middle

# Terminology

* Frame size: size of an Ethernet Layer-2 frame on the wire, including
  any VLAN tags (dot1q, dot1ad) and Ethernet FCS, but excluding Ethernet
  preamble and inter-frame gap. Measured in bytes.
* Packet size: same as frame size, both terms used interchangeably.
* Inner L2 size: for tunneled L2 frames only, size of an encapsulated
  Ethernet Layer-2 frame, preceded with tunnel header, and followed by
  tunnel trailer. Measured in Bytes.
* Inner IP size: for tunneled IP packets only, size of an encapsulated
  IPv4 or IPv6 packet, preceded with tunnel header, and followed by
  tunnel trailer. Measured in Bytes.
* Device Under Test (DUT): In software networking, "device" denotes a
  specific piece of software tasked with packet processing. Such device
  is surrounded with other software components (such as operating system
  kernel). It is not possible to run devices without also running the
  other components, and hardware resources are shared between both. For
  purposes of testing, the whole set of hardware and software components
  is called "system under test" (SUT). As SUT is the part of the whole
  test setup performance of which can be measured by [RFC2544] methods,
  this document uses SUT instead of [RFC2544] DUT. Device under test
  (DUT) can be re-introduced when analysing test results using whitebox
  techniques, but this document sticks to blackbox testing.
* System Under Test (SUT): System under test (SUT) is a part of the
  whole test setup whose performance is to be benchmarked. The complete
  methodology contains other parts, whose performance is either already
  established, or not affecting the benchmarking result.
* Bi-directional throughput tests: involve packets/frames flowing in
  both transmit and receive directions over every tested interface of
  SUT/DUT. Packet flow metrics are measured per direction, and can be
  reported as aggregate for both directions (i.e. throughput) and/or
  separately for each measured direction (i.e. latency). In most cases
  bi-directional tests use the same (symmetric) load in both directions.
* Uni-directional throughput tests: involve packets/frames flowing in
  only one direction, i.e. either transmit or receive direction, over
  every tested interface of SUT/DUT. Packet flow metrics are measured
  and are reported for measured direction.
* Packet Loss Ratio (PLR): ratio of packets received relative to packets
  transmitted over the test trial duration, calculated using formula:
  PLR = ( pkts_transmitted - pkts_received ) / pkts_transmitted.
  For bi-directional throughput tests aggregate PLR is calculated based
  on the aggregate number of packets transmitted and received.
* Packet Throughput Rate: maximum packet offered load DUT/SUT forwards
  within the specified Packet Loss Ratio (PLR). In many cases the rate
  depends on the frame size processed by DUT/SUT. Hence packet
  throughput rate MUST be quoted with specific frame size as received by
  DUT/SUT during the measurement. For bi-directional tests, packet
  throughput rate should be reported as aggregate for both directions.
  Measured in packets-per-second (pps) or frames-per-second (fps),
  equivalent metrics.
* Bandwidth Throughput Rate: a secondary metric calculated from packet
  throughput rate using formula: bw_rate = pkt_rate * (frame_size +
  L1_overhead) * 8, where L1_overhead for Ethernet includes preamble (8
  Bytes) and inter-frame gap (12 Bytes). For bi-directional tests,
  bandwidth throughput rate should be reported as aggregate for both
  directions. Expressed in bits-per-second (bps).
* Non Drop Rate (NDR): maximum packet/bandwith throughput rate sustained
  by DUT/SUT at PLR equal zero (zero packet loss) specific to tested
  frame size(s). MUST be quoted with specific packet size as received by
  DUT/SUT during the measurement. Packet NDR measured in
  packets-per-second (or fps), bandwidth NDR expressed in
  bits-per-second (bps).
* Partial Drop Rate (PDR): maximum packet/bandwith throughput rate
  sustained by DUT/SUT at PLR greater than zero (non-zero packet loss)
  specific to tested frame size(s). MUST be quoted with specific packet
  size as received by DUT/SUT during the measurement. Packet PDR
  measured in packets-per-second (or fps), bandwidth PDR expressed in
  bits-per-second (bps).
* Maximum Receive Rate (MRR): packet/bandwidth rate regardless of PLR
  sustained by DUT/SUT under specified Maximum Transmit Rate (MTR)
  packet load offered by traffic generator. MUST be quoted with both
  specific packet size and MTR as received by DUT/SUT during the
  measurement. Packet MRR measured in packets-per-second (or fps),
  bandwidth MRR expressed in bits-per-second (bps).
* Trial: a single measurement step.
* Trial duration: amount of time over which packets are transmitted and
  received in a single throughput measurement step.

# MLRsearch Background

Multiple Loss Ratio search (MLRsearch) is a packet throughput search
algorithm suitable for deterministic systems (as opposed to
probabilistic systems). MLRsearch discovers multiple packet throughput
rates in a single search, with each rate associated with a distinct
Packet Loss Ratio (PLR) criteria.

For cases when multiple rates need to be found, this property makes
MLRsearch more efficient in terms of time execution, compared to
traditional throughput search algorithms that discover a single packet
rate per defined search criteria (e.g. a binary search specified by
[RFC2544]). MLRsearch reduces execution time even further by relying on
shorter trial durations of intermediate steps, with only the final
measurements conducted at the specified final trial duration. This
results in the shorter overall search execution time when compared to a
traditional binary search, while guaranteeing the same results for
deterministic systems.

In practice two rates with distinct PLRs are commonly used for packet
throughput measurements of NFV systems: Non Drop Rate (NDR) with PLR=0
and Partial Drop Rate (PDR) with PLR>0. The rest of this document
describes MLRsearch for NDR and PDR. If needed, MLRsearch can be easily
adapted to discover more throughput rates with different pre-defined
PLRs.

Similarly to other throughput search approaches like binary search,
MLRsearch is effective for SUTs/DUTs with PLR curve that is continuously
flat or increasing with growing offered load. It may not be as
effective for SUTs/DUTs with abnormal PLR curves.

MLRsearch relies on traffic generator to qualify the received packet
stream as error-free, and invalidate the results if any disqualifying
errors are present e.g. out-of-sequence frames.

MLRsearch can be applied to both uni-directional and bi-directional
throughput tests.

For bi-directional tests, MLRsearch rates and ratios are aggregates of
both directions, based on the following assumptions:

* Packet rates transmitted by traffic generator and received by SUT/DUT
  are the same in each direction, in other words the load is symmetric.
* SUT/DUT packet processing capacity is the same in both directions,
  resulting in the same packet loss under load.

# MLRsearch Overview

The main properties of MLRsearch:

* MLRsearch is a duration aware multi-phase multi-rate search algorithm:
  * Initial Phase determines promising starting interval for the search.
  * Intermediate Phases progress towards defined final search criteria.
  * Final Phase executes measurements according to the final search
    criteria.
  * Final search criteria is defined by following inputs:
    * PLRs associated with NDR and PDR.
    * Final trial duration.
    * Measurement resolution.
* Initial Phase:
  * Measure MRR over initial trial duration.
  * Measured MRR is used as an input to the first intermediate phase.
* Multiple Intermediate Phases:
  * Trial duration:
    * Start with initial trial duration in the first intermediate phase.
    * Converge geometrically towards the final trial duration.
  * Track two values for NDR and two for PDR:
    * The values are called lower_bound and upper_bound.
    * Each value comes from a specific trial measurement:
      * Most recent for that transmit rate.
      * As such the value is associated with that measurement's duration
        and loss.
    * A bound can be valid or invalid:
      * Valid lower_bound must conform with PLR search criteria.
      * Valid upper_bound must not conform with PLR search criteria.
      * Example of invalid NDR lower_bound is if it has been measured
        with non-zero loss.
      * Invalid bounds are not real boundaries for the searched value:
        * They are needed to track interval widths.
      * Valid bounds are real boundaries for the searched value.
      * Each non-initial phase ends with all bounds valid.
      * Bound can become invalid if it re-measured at longer trial
        duration in sub-sequent phase.
  * Search:
    * Start with a large (lower_bound, upper_bound) interval width, that
      determines measurement resolution.
    * Geometrically converge towards the width goal of the phase.
    * Each phase halves the previous width goal.
  * Use of internal and external searches:
    * External search:
      * Measures at transmit rates outside the (lower_bound,
        upper_bound) interval.
      * Activated when a bound is invalid, to search for a new valid
        bound by doubling the interval width.
      * It is a variant of "exponential search".
    * Internal search:
      * A "binary search" that measures at transmit rates within the
        (lower_bound, upper_bound) valid interval, halving the interval
        width.
* Final Phase:
  * Executed with the final test trial duration, and the final width
    goal that determines resolution of the overall search.
* Intermediate Phases together with the Final Phase are called
  Non-Initial Phases.

The main benefits of MLRsearch vs. binary search include:

* In general MLRsearch is likely to execute more trials overall, but
  likely less trials at a set final trial duration.
* In well behaving cases, e.g. when results do not depend on trial
  duration, it greatly reduces (>50%) the overall duration compared to a
  single PDR (or NDR) binary search over duration, while finding
  multiple drop rates.
* In all cases MLRsearch yields the same or similar results to binary
  search.
* Note: both binary search and MLRsearch are susceptible to reporting
  non-repeatable results across multiple runs for very bad behaving
  cases.

Caveats:

* Worst case MLRsearch can take longer than a binary search e.g. in case of
  drastic changes in behaviour for trials at varying durations.

# Sample Implementation

Following is a brief description of a sample MLRsearch implementation
based on the open-source code running in FD.io CSIT project as part of a
Continuous Integration / Continuous Development (CI/CD) framework.

## Input Parameters

1. **maximum_transmit_rate** - Maximum Transmit Rate (MTR) of packets to
   be used by external traffic generator implementing MLRsearch,
   limited by the actual Ethernet link(s) rate, NIC model or traffic
   generator capabilities. Sample defaults: 2 * 14.88 Mpps for 64B
   10GE link rate, 2 * 18.75 Mpps for 64B 40GE NIC (specific model)
   maximum rate (lower than 2 * 59.52 Mpps 40GE link rate).
2. **minimum_transmit_rate** - minimum packet transmit rate to be used for
   measurements. MLRsearch fails if lower transmit rate needs to be
   used to meet search criteria. Default: 2 * 10 kpps (could be higher).
3. **final_trial_duration** - required trial duration for final rate
   measurements. Default: 30 sec.
4. **initial_trial_duration** - trial duration for initial MLRsearch phase.
   Default: 1 sec.
5. **final_relative_width** - required measurement resolution expressed as
   (lower_bound, upper_bound) interval width relative to upper_bound.
   Default: 0.5%.
6. **packet_loss_ratio** - maximum acceptable PLR search criteria for
   PDR measurements. Default: 0.5%.
7. **number_of_intermediate_phases** - number of phases between the initial
   phase and the final phase. Impacts the overall MLRsearch duration.
   Less phases are required for well behaving cases, more phases
   may be needed to reduce the overall search duration for worse behaving cases.
   Default (2). (Value chosen based on limited experimentation to date.
   More experimentation needed to arrive to clearer guidelines.)

## Initial Phase

1. First trial measures at configured maximum transmit rate (MTR) and
   discovers maximum receive rate (MRR).
   * IN: trial_duration = initial_trial_duration.
   * IN: offered_transmit_rate = maximum_transmit_rate.
   * DO: single trial.
   * OUT: measured loss ratio.
   * OUT: MRR = measured receive rate.
2. Second trial measures at MRR and discovers MRR2.
   * IN: trial_duration = initial_trial_duration.
   * IN: offered_transmit_rate = MRR.
   * DO: single trial.
   * OUT: measured loss ratio.
   * OUT: MRR2 = measured receive rate.
3. Third trial measures at MRR2.
   * IN: trial_duration = initial_trial_duration.
   * IN: offered_transmit_rate = MRR2.
   * DO: single trial.
   * OUT: measured loss ratio.

## Non-Initial Phases

1. Main loop:
   1. IN: trial_duration for the current phase. Set to
      initial_trial_duration for the first intermediate phase; to
      final_trial_duration for the final phase; or to the element of
      interpolating geometric sequence for other intermediate phases.
      For example with two intermediate phases, trial_duration of the
      second intermediate phase is the geometric average of
      initial_trial_duration and final_trial_duration.
   2. IN: relative_width_goal for the current phase. Set to
      final_relative_width for the final phase; doubled for each
      preceding phase. For example with two intermediate phases, the
      first intermediate phase uses quadruple of final_relative_width
      and the second intermediate phase uses double of
      final_relative_width.
   3. IN: ndr_interval, pdr_interval from the previous main loop
      iteration or the previous phase. If the previous phase is the
      initial phase, both intervals have lower_bound = MRR2, upper_bound
      = MRR. Note that the initial phase is likely to create intervals
      with invalid bounds.
   4. DO: According to the procedure described in point 2., either exit
      the phase (by jumping to 1.7.), or calculate new transmit rate to
      measure with.
   5. DO: Perform the trial measurement at the new transmit rate and
      trial_duration, compute its loss ratio.
   6. DO: Update the bounds of both intervals, based on the new
      measurement. The actual update rules are numerous, as NDR external
      search can affect PDR interval and vice versa, but the result
      agrees with rules of both internal and external search. For
      example, any new measurement below an invalid lower_bound becomes
      the new lower_bound, while the old measurement (previously acting
      as the invalid lower_bound) becomes a new and valid upper_bound.
      Go to next iteration (1.3.), taking the updated intervals as new
      input.
   7. OUT: current ndr_interval and pdr_interval. In the final phase
      this is also considered to be the result of the whole search. For
      other phases, the next phase loop is started with the current
      results as an input.
2. New transmit rate (or exit) calculation (for point 1.4.):
   1. If there is an invalid bound then prepare for external search:
      * IF the most recent measurement at NDR lower_bound transmit
        rate had the loss higher than zero, then the new transmit rate
        is NDR lower_bound decreased by two NDR interval widths or the
        amount needed to hit the current width goal, whichever is
        larger.
      * Else, IF the most recent measurement at PDR lower_bound
        transmit rate had the loss higher than PLR, then the new
        transmit rate is PDR lower_bound decreased by two PDR interval
        widths.
      * Else, IF the most recent measurement at NDR upper_bound
        transmit rate had no loss, then the new transmit rate is NDR
        upper_bound increased by two NDR interval widths.
      * Else, IF the most recent measurement at PDR upper_bound
        transmit rate had the loss lower or equal to PLR, then the new
        transmit rate is PDR upper_bound increased by two PDR interval
        widths.
   2. If interval width is higher than the current phase goal:
      * Else, IF NDR interval does not meet the current phase width
        goal, prepare for internal search. The new transmit rate is a
        geometric average of NDR lower_bound and NDR upper_bound.
      * Else, IF PDR interval does not meet the current phase width
        goal, prepare for internal search. The new transmit rate is a
        geometric average of PDR lower_bound and PDR upper_bound.
   3. Else, IF some bound has still only been measured at a lower
      duration, prepare to re-measure at the current duration (and the
      same transmit rate). The order of priorities is:
      * NDR lower_bound,
      * PDR lower_bound,
      * NDR upper_bound,
      * PDR upper_bound.
   4. Else, do not prepare any new rate, to exit the phase.
      This ensures that at the end of each non-initial phase
      all intervals are valid, narrow enough, and measured
      at current phase trial duration.

## Sample MLRsearch Run

TODO add a sample MLRsearch run with values.

# Known Implementations

The only known working implementation of MLRsearch is in Linux
Foundation FD.io CSIT project [FDio-CSIT-MLRsearch]. MLRsearch is also
available as a Python package in [PyPI-MLRsearch].

## FD.io CSIT Implementation Deviations

This document so far has been describing a simplified version of
MLRsearch algorithm. The full algorithm as implemented contains
additional logic, which makes some of the details (but not general
ideas) above incorrect. Here is a short description of the additional
logic as a list of principles, explaining their main differences from
(or additions to) the simplified description, but without detailing
their mutual interaction.

1. Logarithmic transmit rate.
   * In order to better fit the relative width goal, the interval
     doubling and halving is done differently.
   * For example, the middle of 2 and 8 is 4, not 5.
2. Optimistic maximum rate.
   * The increased rate is never higher than the maximum rate.
   * Upper bound at that rate is always considered valid.
3. Pessimistic minimum rate.
   * The decreased rate is never lower than the minimum rate.
   * If a lower bound at that rate is invalid, a phase stops refining
     the interval further (until it gets re-measured).
4. Conservative interval updates.
   * Measurements above current upper bound never update a valid upper
     bound, even if drop ratio is low.
   * Measurements below current lower bound always update any lower
     bound if drop ratio is high.
5. Ensure sufficient interval width.
   * Narrow intervals make external search take more time to find a
     valid bound.
   * If the new transmit increased or decreased rate would result in
     width less than the current goal, increase/decrease more.
   * This can happen if the measurement for the other interval
     makes the current interval too narrow.
   * Similarly, take care the measurements in the initial phase create
     wide enough interval.
6. Timeout for bad cases.
   * The worst case for MLRsearch is when each phase converges to
     intervals way different than the results of the previous phase.
   * Rather than suffer total search time several times larger than pure
     binary search, the implemented tests fail themselves when the
     search takes too long (given by argument *timeout*).

# 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

Many thanks to Alec Hothan of OPNFV NFVbench project for thorough
review and numerous useful comments and suggestions.

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