Malathi Veeraraghavan Originals by Jörg Liebeherr 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 - AA= 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

Malathi Veeraraghavan Originals by Jörg Liebeherr 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. R

TT =2X (packet lost)R

TO=5.125