Benchmarking Working Group                       M. Konstantynowicz, Ed.
Internet-Draft                                             V. Polak, Ed.
Intended status: Informational                             Cisco Systems
Expires: January 13, 2022                                  July 12, 2021


      Multiple Loss Ratio Search for Packet Throughput (MLRsearch)
                      draft-ietf-bmwg-mlrsearch-01

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 in a simple way of getting the same result
   sooner, so more repetitions can be done to describe the
   replicability.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on January 13, 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  MLRsearch Background  . . . . . . . . . . . . . . . . . . . .   5
   3.  MLRsearch Overview  . . . . . . . . . . . . . . . . . . . . .   6
   4.  Sample Implementation . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Input Parameters  . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Initial Phase . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Non-Initial Phases  . . . . . . . . . . . . . . . . . . .  11
   5.  FD.io CSIT Implementation . . . . . . . . . . . . . . . . . .  15
     5.1.  Additional details  . . . . . . . . . . . . . . . . . . .  15
       5.1.1.  FD.io CSIT Input Parameters . . . . . . . . . . . . .  17
     5.2.  Example MLRsearch Run . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Terminology

   o  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 (octets).

   o  Packet size: same as frame size, both terms used interchangeably.





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   o  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.

   o  System Under Test (SUT): System under test (SUT) is a part of the
      whole test setup whose performance is to be benchmarked.  The
      complete test setup contains other parts, whose performance is
      either already established, or not affecting the benchmarking
      result.

   o  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 and/or separately
      for each measured direction.  In most cases bi-directional tests
      use the same (symmetric) load in both directions.

   o  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.

   o  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.

   o  Effective loss ratio: A corrected value of measured packet loss
      ratio chosen to avoid difficulties if SUT exhibits decreasing loss
      with increasing load.  Maximum of packet loss ratios measured at
      the same duration on all loads smaller than (and including) the
      current one.

   o  Target loss ratio: A packet loss ratio value acting as an imput
      for search.  The search is finding tight enough lower and upper
      bound in intended load, so that the lower bound has smaller or
      equal loss ratio, and upper bound has strictly larger loss ratio.



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      For the tighterst upper bound, the effective loss ratio is the
      same as packet loss ratio.  For the tightest lower bound, the
      effective loss ratio can be higher than the packet loss ratio, but
      still not larger than the target loss ratio.

   o  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.

   o  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 octets) and inter-frame gap (12 octets).  For
      bi-directional tests, bandwidth throughput rate should be reported
      as aggregate for both directions.  Expressed in bits-per-second
      (bps).

   o  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).

   o  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).

   o  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).

   o  Trial: a single measurement step.  See [RFC2544] section 23.

   o  Trial duration: amount of time over which packets are transmitted
      in a single measurement step.




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2.  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, each rate is associated with a
   distinct Packet Loss Ratio (PLR) criterion.

   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 with NDR and PDR pair as an example.

   Similarly to other throughput search approaches like binary search,
   MLRsearch is effective for SUTs/DUTs with PLR curve that is non-
   decreasing with growing offered load.  It may not be as effective for
   SUTs/DUTs with abnormal PLR curves, although it will always converge
   to some value.

   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:

   o  Traffic transmitted by traffic generator and received by SUT/DUT
      has the same packet rate in each direction, in other words the
      offered load is symmetric.

   o  SUT/DUT packet processing capacity is the same in both directions,
      resulting in the same packet loss under load.





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   MLRsearch can be applied even without those assumptions, but in that
   case the aggregate loss ratio is less useful as a metric.

   MLRsearch can be used for network transactions consisting of more
   than just one packet, or anything else that has intended load as
   input and loss ratio as output (duration as input is optional).  This
   text uses mostly packet-centric language.

3.  MLRsearch Overview

   The main properties of MLRsearch:

   o  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 are defined by following inputs:

         +  Target PLRs (e.g. 0.0 and 0.005 when searching for NDR and
            PDR).

         +  Final trial duration.

         +  Measurement resolution.

   o  Initial Phase:

      *  Measure MRR over initial trial duration.

      *  Measured MRR is used as an input to the first intermediate
         phase.

   o  Multiple Intermediate Phases:

      *  Trial duration:

         +  Start with initial trial duration in the first intermediate
            phase.

         +  Converge geometrically towards the final trial duration.



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      *  Track all previous trial measurement results:

         +  Duration, offered load and loss ratio are tracked.

         +  Effective loss ratios are tracked.

            -  While in practice, real loss ratios can decrease with
               increasing load, effective loss ratios never decrease.
               This is achieved by sorting results by load, and using
               the effective loss ratio of the previous load if the
               current loss ratio is smaller than that.

         +  The algorithm queries the results to find best lower and
            upper bounds.

            -  Effective loss ratios are always used.

         +  The phase ends if all target loss ratios have tight enough
            bounds.

      *  Search:

         +  Iterate over target loss ratios in increasing order.

         +  If both upper and lower bound are in measurement results for
            this duration, apply bisect until the bounds are tight
            enough, and continue with next loss ratio.

         +  If a bound is missing for this duration, but there exists a
            bound from the previous duration (compatible with the other
            bound at this duration), re-measure at the current duration.

         +  If a bound in one direction (upper or lower) is missing for
            this duration, and the previous duration does not have a
            compatible bound, compute the current "interval size" from
            the second tightest bound in the other direction (lower or
            upper respectively) for the current duration, and choose
            next offered load for external search.

         +  The logic guarantees that a measurement is never repeated
            with both duration and offered load being the same.

         +  The logic guarantees that measurements for higher target
            loss ratio iterations (still within the same phase duration)
            do not affect validity and tightness of bounds for previous
            target loss ratio iterations (at the same duration).

      *  Use of internal and external searches:



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         +  External search:

            -  It is a variant of "exponential search".

            -  The "interval size" is multiplied by a configurable
               constant (powers of two work well with the subsequent
               internal search).

         +  Internal search:

            -  A variant of binary search that measures at offered load
               between the previously found bounds.

            -  The interval does not need to be split into exact halves,
               if other split can get to the target width goal faster.

               o  The idea is to avoid returning interval narrower than
                  the current width goal.  See sample implementation
                  details, below.

   o  Final Phase:

      *  Executed with the final test trial duration, and the final
         width goal that determines resolution of the overall search.

   o  Intermediate Phases together with the Final Phase are called Non-
      Initial Phases.

   o  The returned bounds stay within prescribed min_rate and max_rate.

      *  When returning min_rate or max_rate, the returned bounds may be
         invalid.

         +  E.g. upper bound at max_rate may come from a measurement
            with loss ratio still not higher than the target loss ratio.

   The main benefits of MLRsearch vs. binary search include:

   o  In general MLRsearch is likely to execute more trials overall, but
      likely less trials at a set final trial duration.

   o  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.

   o  In all cases MLRsearch yields the same or similar results to
      binary search.



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   o  Note: both binary search and MLRsearch are susceptible to
      reporting non-repeatable results across multiple runs for very bad
      behaving cases.

   Caveats:

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

      *  Re-measurement at higher duration can trigger a long external
         search.  That never happens in binary search, which uses the
         final duration from the start.

4.  Sample Implementation

   Following is a brief description of a sample MLRsearch
   implementation, which is a simplified version of the existing
   implementation.

4.1.  Input Parameters

   1.  *max_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.

   2.  *min_rate* - minimum packet transmit rate to be used for
       measurements.  MLRsearch fails if lower transmit rate needs to be
       used to meet search criteria.

   3.  *final_trial_duration* - required trial duration for final rate
       measurements.

   4.  *initial_trial_duration* - trial duration for initial MLRsearch
       phase.

   5.  *final_relative_width* - required measurement resolution
       expressed as (lower_bound, upper_bound) interval width relative
       to upper_bound.

   6.  *packet_loss_ratios* - list of maximum acceptable PLR search
       criteria.

   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




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       phases may be needed to reduce the overall search duration for
       worse behaving cases.

4.2.  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.  Received rate is computed
          as intended load multiplied by pass ratio (which is one minus
          loss ratio).  This is useful when loss ratio is computed from
          a different metric than intended load.  For example, intended
          load can be in transactions (multiple packets each), but loss
          ratio is computed on level of packets, not transactions.

       *  Example: If MTR is 10 transactions per second, and each
          transaction has 10 packets, and receive rate is 90 packets per
          second, then loss rate is 10%, and MRR is computed to be 9
          transactions per second.

       If MRR is too close to MTR, MRR is set below MTR so that interval
       width is equal to the width goal of the first intermediate phase.
       If MRR is less than min_rate, min_rate is used.

   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.  If MRR2 is less than
          min_rate, min_rate is used.  If loss ratio is less or equal to
          the smallest target loss ratio, MRR2 is set to a value above
          MRR, so that interval width is equal to the width goal of the
          first intermediate phase.  MRR2 could end up being equal to




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          MTR (for example if both measurements so far had zero loss),
          which was already measured, step 3 is skipped in that case.

   3.  Third trial measures at MRR2.

       *  IN: trial_duration = initial_trial_duration.

       *  IN: offered_transmit_rate = MRR2.

       *  DO: single trial.

       *  OUT: measured loss ratio.

       *  OUT: MRR3 = measured receive rate.  If MRR3 is less than
          min_rate, min_rate is used.  If step 3 is not skipped, the
          first trial measurement is forgotten.  This is done because in
          practice (if MRR2 is above MRR), external search from MRR and
          MRR2 is likely to lead to a faster intermediate phase than a
          bisect between MRR2 and MTR.

4.3.  Non-Initial Phases

   1.  Main phase 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: Measurement results from the previous phase (previous
           duration).

       4.  Internal target ratio loop:

           1.  IN: Target loss ratio for this iteration of ratio loop.





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           2.  IN: Measurement results from all previous ratio loop
               iterations of current phase (current duration).

           3.  DO: According to the procedure described in point 2:

               1.  either exit the phase (by jumping to 1.5),

               2.  or exit loop iteration (by continuing with next
                   target loss ratio, jumping to 1.4.1),

               3.  or calculate new transmit rate to measure with.

           4.  DO: Perform the trial measurement at the new transmit
               rate and current trial duration, compute its loss ratio.

           5.  DO: Add the result and go to next iteration (1.4.1),
               including the added trial result in 1.4.2.

       5.  OUT: Measurement results from this phase.

       6.  OUT: In the final phase, bounds for each target loss ratio
           are extracted and returned.

           1.  If a valid bound does not exist, use min_rate or
               max_rate.

   2.  New transmit rate (or exit) calculation (for point 1.4.3):

       1.  If the previous duration has the best upper and lower bound,
           select the middle point as the new transmit rate.

           1.  See 2.5.3. below for the exact splitting logic.

           2.  This can be a no-op if interval is narrow enough already,
               in that case continue with 2.2.

           3.  Discussion, assuming the middle point is selected and
               measured:

               1.  Regardless of loss rate measured, the result becomes
                   either best upper or best lower bound at current
                   duration.

               2.  So this condition is satisfied at most once per
                   iteration.

               3.  This also explains why previous phase has double
                   width goal:



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                   1.  We avoid one more bisection at previous phase.

                   2.  At most one bound (per iteration) is re-measured
                       with current duration.

                   3.  Each re-measurement can trigger an external
                       search.

                   4.  Such surprising external searches are the main
                       hurdle in achieving low overal search durations.

                   5.  Even without 1.1, there is at most one external
                       search per phase and target loss ratio.

                   6.  But without 1.1 there can be two re-measurements,
                       each coming with a risk of triggering external
                       search.

       2.  If the previous duration has one bound best, select its
           transmit rate.  In deterministic case this is the last
           measurement needed this iteration.

       3.  If only upper bound exists in current duration results:

           1.  This can only happen for the smallest target loss ratio.

           2.  If the upper bound was measured at min_rate, exit the
               whole phase early (not investigating other target loss
               ratios).

           3.  Select new transmit rate using external search:

               1.  For computing previous interval size, use:

                   1.  second tightest bound at current duration,

                   2.  or tightest bound of previous duration, if
                       compatible and giving a more narrow interval,

                   3.  or target interval width if none of the above is
                       available.

                   4.  In any case increase to target interval width if
                       smaller.

               2.  Quadruple the interval width.

               3.  Use min_rate if the new transmit rate is lower.



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       4.  If only lower bound exists in current duration results:

           1.  If the lower bound was measured at max_rate, exit this
               iteration (continue with next lowest target loss ratio).

           2.  Select new transmit rate using external search:

               1.  For computing previous interval size, use:

                   1.  second tightest bound at current duration,

                   2.  or tightest bound of previous duration, if
                       compatible and giving a more narrow interval,

                   3.  or target interval width if none of the above is
                       available.

                   4.  In any case increase to target interval width if
                       smaller.

               2.  Quadruple the interval width.

               3.  Use max_rate if the new transmit rate is higher.

       5.  The only remaining option is both bounds in current duration
           results.

           1.  This can happen in two ways, depending on how the lower
               bound was chosen.

               1.  It could have been selected for the current loss
                   ratio, e.g. in re-measurement (2.2) or in initial
                   bisect (2.1).

               2.  It could have been found as an upper bound for the
                   previous smaller target loss ratio, in which case it
                   might be too low.

               3.  The algorithm does not track which one is the case,
                   as the decision logic works well regardless.

           2.  Compute "extending down" candidate transmit rate exactly
               as in 2.3.

           3.  Compute "bisecting" candidate transmit rate:

               1.  Compute the current interval width from the two
                   bounds.



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               2.  Express the width as a (float) multiple of the target
                   width goal for this phase.

               3.  If the multiple is not higher than one, it means the
                   width goal is met.  Exit this iteration and continue
                   with next higher target loss ratio.

               4.  If the multiple is two or less, use half of that for
                   new width if the lower subinterval.

               5.  Round the multiple up to nearest even integer.

               6.  Use half of that for new width if the lower
                   subinterval.

               7.  Example: If lower bound is 2.0 and upper bound is
                   5.0, and width goal is 1.0, the new candidate
                   transmit rate will be 4.0.  This can save a
                   measurement when 4.0 has small loss.  Selecting the
                   average (3.5) would never save a measurement, giving
                   more narrow bounds instead.

           4.  If either candidate computation want to exit the
               iteration, do as bisecting candidate computation says.

           5.  The remaining case is both candidates wanting to measure
               at some rate.  Use the higher rate.  This prefers
               external search down narrow enough interval, competing
               with perfectly sized lower bisect subinterval.

5.  FD.io CSIT Implementation

   The only known working implementation of MLRsearch is in the open-
   source code running in Linux Foundation FD.io CSIT project
   [FDio-CSIT-MLRsearch] as part of a Continuous Integration /
   Continuous Development (CI/CD) framework.

   MLRsearch is also available as a Python package in [PyPI-MLRsearch].

5.1.  Additional details

   This document so far has been describing a simplified version of
   MLRsearch algorithm.  The full algorithm as implemented in CSIT
   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.



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   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.  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_).

   3.  Intended count.

       *  The number of packets to send during the trial should be equal
          to the intended load multiplied by the duration.

          +  Also multiplied by a coefficient, if loss ratio is
             calculated from a different metric.

             -  Example: If a successful transaction uses 10 packets,
                load is given in transactions per second, byt loss ratio
                is calculated from packets, the coefficient to get
                intended count of packets is 10.

       *  But in practice that does not work.

          +  It could result in a fractional number of packets,

          +  so it has to be rounded in a way traffic generator chooses,

          +  which may depend on the number of traffic flows and traffic
             generator worker threads.

   4.  Attempted count.  As the real number of intended packets is not
       known exactly, the computation uses the number of packets traffic
       generator reports as sent.  Unless overriden by the next point.

   5.  Duration stretching.

       *  In some cases, traffic generator may get overloaded, causing
          it to take significantly longer (than duration) to send all
          packets.



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       *  The implementation uses an explicit stop,

          +  causing lower attempted count in those cases.

       *  The implementation tolerates some small difference between
          attempted count and intended count.

          +  10 microseconds worth of traffic is sufficient for our
             tests.

       *  If the difference is higher, the unsent packets are counted as
          lost.

          +  This forces the search to avoid the regions of high
             duration stretching.

          +  The final bounds describe the performance of not just SUT,
             but of the whole system, including the traffic generator.

   6.  Excess packets.

       *  In some test (e.g. using TCP flows) Traffic generator reacts
          to packet loss by retransmission.  Usually, such packet loss
          is already affecting loss ratio.  If a test also wants to
          treat retransmissions due to heavily delayed packets also as a
          failure, this is once again visible as a mismatch between the
          intended count and the attempted count.

       *  The CSIT implementation simply looks at absolute value of the
          difference, so it offes the same small tolerance before it
          start marking a "loss".

   7.  For result processing, we use lower bounds and ignore upper
       bounds.

5.1.1.  FD.io CSIT Input Parameters

   1.  *max_rate* - Typical values: 2 * 14.88 Mpps for 64B 10GE link
       rate, 2 * 18.75 Mpps for 64B 40GE NIC (specific model).

   2.  *min_rate* - Value: 2 * 9001 pps (we reserve 9000 pps for latency
       measurements).

   3.  *final_trial_duration* - Value: 30.0 seconds.

   4.  *initial_trial_duration* - Value: 1.0 second.

   5.  *final_relative_width* - Value: 0.005 (0.5%).



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   6.  *packet_loss_ratios* - Value: 0.0, 0.005 (0.0% for NDR, 0.5% for
       PDR).

   7.  *number_of_intermediate_phases* - Value: 2.  The value has been
       chosen based on limited experimentation to date.  More
       experimentation needed to arrive to clearer guidelines.

   8.  *timeout* - Limit for the overall search duration (for one
       search).  If MLRsearch oversteps this limit, it immediatelly
       declares the test failed, to avoid wasting even more time on a
       misbehaving SUT.  Value: 600.0 (seconds).

   9.  *expansion_coefficient* - Width multiplier for external search.
       Value: 4.0 (interval width is quadroupled).  Value of 2.0 is best
       for well-behaved SUTs, but value of 4.0 has been found to
       decrease overall search time for worse-behaved SUT
       configurations, contributing more to the overall set of different
       SUT configurations tested.

5.2.  Example MLRsearch Run

   The following list describes a search from a real test run in CSIT
   (using the default input values as above).

   o  Initial phase, trial duration 1.0 second.

   Measurement 1, intended load 18750000.0 pps (MTR), measured loss
   ratio 0.7089514628479618 (valid upper bound for both NDR and PDR).

   Measurement 2, intended load 5457160.071600716 pps (MRR), measured
   loss ratio 0.018650817320118702 (new tightest upper bounds).

   Measurement 3, intended load 5348832.933500009 pps (slightly less
   than MRR2 in preparation for first intermediate phase target interval
   width), measured loss ratio 0.00964383362905351 (new tightest upper
   bounds).

   o  First intermediate phase starts, trial duration still 1.0 seconds.

   Measurement 4, intended load 4936605.579021453 pps (no lower bound,
   performing external search downwards, for NDR), measured loss ratio
   0.0 (valid lower bound for both NDR and PDR).

   Measurement 5, intended load 5138587.208637197 pps (bisecting for
   NDR), measured loss ratio 0.0 (new tightest lower bounds).

   Measurement 6, intended load 5242656.244044665 pps (bisecting),
   measured loss ratio 0.013523745379347257 (new tightest upper bounds).



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   o  Both intervals are narrow enough.

   o  Second intermediate phase starts, trial duration 5.477225575051661
      seconds.

   Measurement 7, intended load 5190360.904111567 pps (initial bisect
   for NDR), measured loss ratio 0.0023533920869969953 (NDR upper bound,
   PDR lower bound).

   Measurement 8, intended load 5138587.208637197 pps (re-measuring NDR
   lower bound), measured loss ratio 1.2080222912800403e-06 (new
   tightest NDR upper bound).

   o  The two intervals have separate bounds from now on.

   Measurement 9, intended load 4936605.381062318 pps (external NDR
   search down), measured loss ratio 0.0 (new valid NDR lower bound).

   Measurement 10, intended load 5036583.888432355 pps (NDR bisect),
   measured loss ratio 0.0 (new tightest NDR lower bound).

   Measurement 11, intended load 5087329.903232804 pps (NDR bisect),
   measured loss ratio 0.0 (new tightest NDR lower bound).

   o  NDR interval is narrow enough, PDR interval not ready yet.

   Measurement 12, intended load 5242656.244044665 pps (re-measuring PDR
   upper bound), measured loss ratio 0.0101174866190136 (still valid PDR
   upper bound).

   o  Also PDR interval is narrow enough, with valid bounds for this
      duration.

   o  Final phase starts, trial duration 30.0 seconds.

   Measurement 13, intended load 5112894.3238511775 pps (initial bisect
   for NDR), measured loss ratio 0.0 (new tightest NDR lower bound).

   Measurement 14, intended load 5138587.208637197 (re-measuring NDR
   upper bound), measured loss ratio 2.030389804256833e-06 (still valid
   PDR upper bound).

   o  NDR interval is narrow enough, PDR interval not yet.

   Measurement 15, intended load 5216443.04126728 pps (initial bisect
   for PDR), measured loss ratio 0.005620871287975237 (new tightest PDR
   upper bound).




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   Measurement 16, intended load 5190360.904111567 (re-measuring PDR
   lower bound), measured loss ratio 0.0027629971184465604 (still valid
   PDR lower bound).

   o  PDR interval is also narrow enough.

   o  Returning bounds:

   o  NDR_LOWER = 5112894.3238511775 pps; NDR_UPPER = 5138587.208637197
      pps;

   o  PDR_LOWER = 5190360.904111567 pps; PDR_UPPER = 5216443.04126728
      pps.

6.  IANA Considerations

   No requests of IANA.

7.  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.

8.  Acknowledgements

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

9.  References







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9.1.  Normative References

   [RFC2544]  Bradner, S. and J. McQuaid, "Benchmarking Methodology for
              Network Interconnect Devices", RFC 2544,
              DOI 10.17487/RFC2544, March 1999,
              <https://www.rfc-editor.org/info/rfc2544>.

9.2.  Informative References

   [FDio-CSIT-MLRsearch]
              "FD.io CSIT Test Methodology - MLRsearch", February 2021,
              <https://docs.fd.io/csit/rls2101/report/introduction/
              methodology_data_plane_throughput/
              methodology_mlrsearch_tests.html>.

   [PyPI-MLRsearch]
              "MLRsearch 0.4.0, Python Package Index", April 2021,
              <https://pypi.org/project/MLRsearch/0.4.0/>.

Authors' Addresses

   Maciek Konstantynowicz (editor)
   Cisco Systems

   Email: mkonstan@cisco.com


   Vratko Polak (editor)
   Cisco Systems

   Email: vrpolak@cisco.com




















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