+Following is a brief description of the current MLRsearch
+implementation in FD.io CSIT.
+
+Input Parameters
+````````````````
+
+#. *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.
+#. *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).
+#. *final_trial_duration* - required trial duration for final rate
+ measurements. Default: 30 sec.
+#. *initial_trial_duration* - trial duration for initial MLRsearch phase.
+ Default: 1 sec.
+#. *final_relative_width* - required measurement resolution expressed as
+ (lower_bound, upper_bound) interval width relative to upper_bound.
+ Default: 0.5%.
+#. *packet_loss_ratio* - maximum acceptable PLR search criteria for
+ PDR measurements. Default: 0.5%.
+#. *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.
+
+ a. *in*: trial_duration = initial_trial_duration.
+ b. *in*: offered_transmit_rate = maximum_transmit_rate.
+ c. *do*: single trial.
+ d. *out*: measured loss ratio.
+ e. *out*: mrr = measured receive rate.
+
+2. Second trial measures at MRR and discovers MRR2.
+
+ a. *in*: trial_duration = initial_trial_duration.
+ b. *in*: offered_transmit_rate = MRR.
+ c. *do*: single trial.
+ d. *out*: measured loss ratio.
+ e. *out*: mrr2 = measured receive rate.
+
+3. Third trial measures at MRR2.
+
+ a. *in*: trial_duration = initial_trial_duration.
+ b. *in*: offered_transmit_rate = MRR2.
+ c. *do*: single trial.
+ d. *out*: measured loss ratio.
+
+Non-initial Phases
+``````````````````
+
+1. Main loop:
+
+ a. *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.
+ b. *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.
+ c. *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.
+ d. *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.
+ e. *do*: Perform the trial measurement at the new transmit rate
+ and trial_duration, compute its loss ratio.
+ f. *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.
+ g. *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.
+
+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*).
+
+(B)MRR Throughput
+-----------------
+
+Maximum Receive Rate (MRR) tests are complementary to MLRsearch tests,
+as they provide a maximum "raw" throughput benchmark for development and
+testing community. MRR tests measure the packet forwarding rate under
+the maximum load offered by traffic generator over a set trial duration,