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add comment about Jacobson's formula for RTT estimation

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This commit is contained in:
Stephen Birarda 2016-10-18 11:24:08 -07:00
parent 1041443796
commit d5a450ad30
1 changed files with 16 additions and 5 deletions
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21
libraries/networking/src/udt/TCPVegasCC.cpp
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@ -49,15 +49,26 @@ bool TCPVegasCC::onACK(SequenceNumber ack, p_high_resolution_clock::time_point r
} }
if (_ewmaRTT == -1) { if (_ewmaRTT == -1) {
// first RTT sample - set _ewmaRTT to the value and set the variance to half the value
_ewmaRTT = lastRTT; _ewmaRTT = lastRTT;
_rttVariance = lastRTT / 2; _rttVariance = lastRTT / 2;
} else { } else {
static const int RTT_ESTIMATION_ALPHA_NUMERATOR = 8; // This updates the RTT using exponential weighted moving average
static const int RTT_ESTIMATION_VARIANCE_ALPHA_NUMERATOR = 4; // This is the Jacobson's forumla for RTT estimation
// http://www.mathcs.emory.edu/~cheung/Courses/455/Syllabus/7-transport/Jacobson-88.pdf
_ewmaRTT = (_ewmaRTT * (RTT_ESTIMATION_ALPHA_NUMERATOR - 1) + lastRTT) / RTT_ESTIMATION_ALPHA_NUMERATOR; // Estimated RTT = (1 - x)(estimatedRTT) + (x)(sampleRTT)
_rttVariance = (_rttVariance * (RTT_ESTIMATION_VARIANCE_ALPHA_NUMERATOR - 1) // (where x = 0.125 via Jacobson)
+ abs(lastRTT - _ewmaRTT)) / RTT_ESTIMATION_VARIANCE_ALPHA_NUMERATOR;
// Deviation = (1 - x)(deviation) + x |sampleRTT - estimatedRTT|
// (where x = 0.25 via Jacobson)
static const int RTT_ESTIMATION_ALPHA = 8;
static const int RTT_ESTIMATION_VARIANCE_ALPHA = 4;
_ewmaRTT = (_ewmaRTT * (RTT_ESTIMATION_ALPHA - 1) + lastRTT) / RTT_ESTIMATION_ALPHA;
_rttVariance = (_rttVariance * (RTT_ESTIMATION_VARIANCE_ALPHA- 1)
+ abs(lastRTT - _ewmaRTT)) / RTT_ESTIMATION_VARIANCE_ALPHA;
} }
// add 1 to the number of ACKs during this RTT // add 1 to the number of ACKs during this RTT