io.aeron.driver

Class OptimalMulticastDelayGenerator

java.lang.Object

io.aeron.driver.OptimalMulticastDelayGenerator

All Implemented Interfaces:

FeedbackDelayGenerator

public class OptimalMulticastDelayGenerator
extends java.lang.Object
implements FeedbackDelayGenerator

Feedback delay used for NAKs as well as for some retransmission use cases.

Generates delay based on Optimal Multicast Feedback
http://tools.ietf.org/html/rfc5401#page-13

maxBackoffT is max interval for delay

C version of the code:

 double RandomBackoff(double maxBackoffT, double groupSize)
 {
     double lambda = log(groupSize) + 1;
     double x = UniformRand(lambda / maxBackoffT) + lambda / (maxBackoffT * (exp(lambda) - 1));
     return ((maxBackoffT / lambda) * log(x * (exp(lambda) - 1) * (maxBackoffT / lambda)));
 }
 

where UniformRand(x) is uniform distribution from 0..max

In this implementations calculation:

• the groupSize is a constant (could be configurable as system property)
• maxBackoffT is a constant (could be configurable as system property)
• GRTT is a constant (could be configurable as a system property)

N (the expected number of feedback messages per RTT) is:
N = exp(1.2 * L / (2 * maxBackoffT / GRTT))

Assumptions:

maxBackoffT = K * GRTT (K >= 1)

Recommended K:

• K = 4 for situations where responses come from multiple places (i.e. for NAKs, multiple retransmitters)
• K = 6 for situations where responses come from single places (i.e. for NAKs, source only retransmit)

Constructor Summary

Constructors

Constructor and Description
OptimalMulticastDelayGenerator(double maxBackoffT, double groupSize, double gRtt) Create new feedback delay generator based on estimates.

Method Summary

Modifier and Type	Method and Description
double	calculatedN() Return the estimated number of feedback messages per RTT.
long	generateDelay() Generate a new delay value
double	generateNewOptimalDelay() Generate a new randomized delay value in the units of backoff and gRtt.
static double	uniformRandom(double max) Return uniform random value in the range 0..max

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface io.aeron.driver.FeedbackDelayGenerator

shouldFeedbackImmediately

Constructor Detail

OptimalMulticastDelayGenerator

public OptimalMulticastDelayGenerator(double maxBackoffT,
                                      double groupSize,
                                      double gRtt)

Create new feedback delay generator based on estimates. Pre-calculating some parameters upfront.

maxBackoffT and gRtt must be expressed in the same units.

Parameters:

maxBackoffT - of the delay interval

groupSize - estimate

gRtt - estimate

Method Detail

generateDelay

public long generateDelay()

Generate a new delay value

Specified by:

generateDelay in interface FeedbackDelayGenerator

Returns:

delay value in nanoseconds

generateNewOptimalDelay

public double generateNewOptimalDelay()

Generate a new randomized delay value in the units of backoff and gRtt.

Returns:

delay in units of backoff and RTT

calculatedN

public double calculatedN()

Return the estimated number of feedback messages per RTT.

Returns:

number of estimated feedback messages in units of backoff and RTT

uniformRandom

public static double uniformRandom(double max)

Return uniform random value in the range 0..max

Parameters:

max - of the random range

Returns:

random value