Thomas Kunz
Systems and Computer Engineering
362
Adaptive Retransmission
nJacobson/Karels Algorithm
nNew calculation for average RTT
–Diff =
SampleRTT - EstimatedRTT
–EstimatedRTT
= EstimatedRTT + (d x Diff)
–Deviation =
Deviation + d(|Diff|- Deviation)
–where d is a fraction between 0 and 1
nConsider variance when setting timeout value
–TimeOut = m x EstimatedRTT + f x
Deviation
–where m = 1 and f =
4
nNotes
–algorithm
only as good as granularity of clock (500ms on Unix)
–accurate
timeout mechanism important to congestion control