Computer Networks-->Retransmission Timer Management--> ME ENGINEERING LECTURER NOTES

Retransmission Timer Management

Three Techniques to calculate retransmission timer (RTO):

lRTT Variance Estimation

lExponential RTO Backoff

lKarn’s Algorithm

RTT Variance Estimation
(Jacobson’s Algorithm)

3 sources of high variance in RTT

lIf data rate relative low, then transmission delay will be relatively large, with larger variance due to variance in packet size

lLoad may change abruptly due to other sources

lPeer may not acknowledge segments immediately

 Jacobson’s Algorithm

SRTT(K + 1) = (1 – g) × SRTT(K) + g × RTT(K + 1)

SERR(K + 1) = RTT(K + 1) – SRTT(K)

SDEV(K + 1) = (1 – h) × SDEV(K) + h ×|SERR(K + 1)|

RTO(K + 1) = SRTT(K + 1) + f × SDEV(K + 1)

g = 0.125 

h = 0.25

f = 2  or  f = 4   (most current implementations use f = 4)

Two Other Factors

Jacobson’s algorithm can significantly improve TCP performance, but:

lWhat RTO to use for retransmitted segments?

 ANSWER: exponential RTO backoff algorithm

lWhich round-trip samples to use as input to Jacobson’s algorithm?

ANSWER: Karn’s algorithm

Exponential RTO Backoff

lIncrease RTO each time the same segment retransmitted – backoff process

lMultiply RTO by constant:

RTO = q × RTO

lq = 2 is called binary exponential backoff

Which Round-trip Samples?

lIf an ack is received for retransmitted segment, there are 2 possibilities:

lAck is for first transmission

lAck is for second transmission

lTCP source cannot distinguish 2 cases

lNo valid way to calculate RTT:

–From first transmission to ack, or

–From second transmission to ack?

–Karn’s Algorithm

lDo not use measured RTT to update SRTT and SDEV

lCalculate backoff RTO when a retransmission occurs

lUse backoff RTO for segments until an ack arrives for a segment that has not been retransmitted