Update of draft-vpolak-mkonstan-bmwg-mlrsearch-01->02.

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----
-title: Multiple Loss Ratio Search for Packet Throughput (MLRsearch)
-# abbrev: MLRsearch
-docname: draft-vpolak-mkonstan-bmwg-mlrsearch-01
-date: 2019-03-27
-
-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:
-
-
---- 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
-
-* NDR - Non-Drop Rate, a packet throughput metric with Packet Loss Ratio
-  equal zero (a zero packet loss), expressed in packets-per-second
-  (pps). NDR packet throughput has an associated metric oftentimes
-  referred to as NDR bandwidth expressed in bits-per-second (bps), and
-  calculated as a product of:
-  * NDR packet rate for specific packet (frame) size, and
-  * Packet (L2 frame size) size in bits plus any associated L1 overhead.
-* PLR - Packet Loss Ratio, a packet loss metric calculated as a ratio of
-  (packets_transmitted - packets_received) to packets_transmitted, over
-  the test trial duration.
-* PDR - Partial-Drop Rate, a packet throughput metric with Packet Loss
-  Ratio greater than zero (a non-zero packet loss), expressed in
-  packets-per-second (pps). PDR packet throughput has an associated
-  metric oftentimes referred to as PDR bandwidth expressed in bits-per-
-  second (bps), and calculated as a product of:
-  * PDR packet rate for specific packet (frame) size, and
-  * Packet (L2 frame size) size in bits plus any associated L1 overhead.
-
-# MLRsearch Background
-
-Multiple Loss Rate search (MLRsearch) is a packet throughput search
-algorithm suitable for deterministic (as opposed to probabilistic)
-systems. MLRsearch discovers multiple packet throughput rates in a
-single search, each rate associated with a distinct Packet Loss Ratio
-(PLR) criteria.
-
-Two popular names for particular PLR criteria are Non-Drop Rate (NDR,
-with PLR=0, zero packet loss) and Partial Drop Rate (PDR, with PLR>0,
-non-zero packet loss). MLRsearch discovers NDR and PDR in a single
-search reducing required execution time compared to separate binary
-searches for NDR and PDR. 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 standard NDR/PDR binary search, while guaranteeing the same or
-similar results.
-(TODO: Specify "standard" in the previous sentence.)
-
-If needed, MLRsearch can be easily adopted to discover more throughput
-rates with different pre-defined PLRs.
-
-Unless otherwise noted, all throughput rates are *always* bi-directional
-aggregates of two equal (symmetric) uni-directional packet rates
-received and reported by an external traffic generator.
-
-# 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.
-* Initial phase:
-  * Uses link rate as a starting transmit rate and discovers the Maximum
-    Receive Rate (MRR) used as an input to the first intermediate phase.
-* Intermediate phases:
-  * Start with initial trial duration (in the first phase) and converge
-    geometrically towards the final trial duration (in the final phase).
-  * Track two values for NDR and two for PDR.
-    * The values are called (NDR or PDR) lower_bound and upper_bound.
-    * Each value comes from a specific trial measurement
-      (most recent for that transmit rate),
-      and as such the value is associated with that measurement's duration and loss.
-    * A bound can be invalid, for example if NDR lower_bound
-      has been measured with nonzero loss.
-    * Invalid bounds are not real boundaries for the searched value,
-      but are needed to track interval widths.
-    * Valid bounds are real boundaries for the searched value.
-    * Each non-initial phase ends with all bounds valid.
-  * Start with a large (lower_bound, upper_bound) interval width and
-    geometrically converge towards the width goal (measurement resolution)
-    of the phase. Each phase halves the previous width goal.
-  * Use 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 - "binary search", 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 search trials overall, but
-  less trials at a set final duration.
-* In well behaving cases it greatly reduces (>50%) the overall duration
-  compared to a single PDR (or NDR) binary search 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 packet transmit rate to be used by
-   external traffic generator, limited by either the actual Ethernet
-   link rate or traffic generator NIC model capabilities. Sample
-   defaults: 2 * 14.88 Mpps for 64B 10GE link rate,
-   2 * 18.75 Mpps for 64B 40GE NIC maximum 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 maximum rate and discovers 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:
-   * *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_strial_duration and final_trial_duration.
-   * *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.
-   * *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, uper_bound = MRR.
-     Note that the initial phase is likely to create intervals with invalid bounds.
-   * *do*: According to the procedure described in point 2,
-     either exit the phase (by jumping to 1.g.),
-     or prepare new transmit rate to measure with.
-   * *do*: Perform the trial measurement at the new transmit rate
-     and trial_duration, compute its loss ratio.
-   * *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.c.), taking the updated intervals as new input.
-   * *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 1.d.):
-   * 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.
-     * 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.
-   * 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
-       (NDR lower bound + NDR upper bound) / 2.
-     * Else, *if* PDR interval does not meet the current phase width goal,
-       prepare for internal search. The new transmit rate is
-       (PDR lower bound + PDR upper bound) / 2.
-   * 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.
-   * *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.
-
-# Known Implementations
-
-The only known working implementation of MLRsearch is in Linux Foundation
-FD.io CSIT project. https://wiki.fd.io/view/CSIT. https://git.fd.io/csit/.
-
-## 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
-
-..
-
-# Security Considerations
-
-..
-
-# Acknowledgements
-
-..
-
---- back