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Computation of RTO in adaptive scheme [USE SLIDE SHOW TO VIEW SLIDES]

Computation of RTO in adaptive scheme [USE SLIDE SHOW TO VIEW SLIDES]. Retransmission timer is set to a Retransmission Timeout (RTO) value . RTO is calculated based on the RTT measurements.

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Computation of RTO in adaptive scheme [USE SLIDE SHOW TO VIEW SLIDES]

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  1. Computation of RTO in adaptive scheme [USE SLIDE SHOW TO VIEW SLIDES] • Retransmission timer is set to a Retransmission Timeout (RTO) value. • RTO is calculated based on the RTT measurements. • The RTT measurements are smoothed by the following estimators A (mean RTT value) and D (smooted mean deviation of RTT): Err = M - A A= A+ g Err=A(1-g)+gM D= D+ h (|Err|-D)=D(1-h)+ h|Err| RTO = A + 4D (latest formula) (book also says A+2D for initial value; we’ll use A+4D) The gains are set to h=1/4 and g=1/8 • In the formula for computing the new smoothed mean RTT A, 0.125 times the newly measured value (M) is added to 0.875 times the old smoothed value of A

  2. Example of RTO computation (adaptive) • Assume A=1, D=1 (initial values) Err = 2 -1 =1 (since M, the measured RTT is 2) A = 1 + 0.125×1= 1.125; D = 1+0.25 (1-1)=1 RTO = A+4D=1.125+4 = 5.125 This is why in the figure below when segment 2 is lost, it is retransmitted after 5.125 sec.

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