Suppose that, when a TCP segment is sent more than once, we take SampleRTT to be

Question

Suppose that, when a TCP segment is sent more than once, we take SampleRTT to be the time between the original transmission and the ACK, as in Figure 5.10(a). Show that if a connection with a 1-packet window loses every other packet (i.e., each packet is transmitted twice), then EstimatedRTT increases to infinity. Assume TimeOut = EstimatedRTT; both algorithms presented in the text always set TimeOut even larger. (Hint: EstimatedRTT = EstimatedRTT+ ×(SampleRTTEstimatedRTT).)

Sender Receiver Sender Receiver onginal transmission Original transmission Retransmission ACK SC Retransmission ACK. (a) (b) FIGURE 5.10 Associating the AOK with a original transmission versus(b)retransmision.

Explanation / Answer

Let us consider the real RTT (for successful transmissions) be 1.0 unit. By hypothesis, every

Packet times out once and then the retransmission is acknowledged after 1.0

unit ; this means that each SampleRTT measurement is TimeOut+1 = EstimatedRTT+1.

We then have,

EstimatedRTT = × EstimatedRTT + × SampleRTT

= EstimatedRTT + ×(SampleRTT EstimatedRTT).

EstimatedRTT +

Thus it follows that the Nth EstimatedRTT is greater than or equal(>=) to N.

Without the assumption, TimeOut = EstimatedRTT

we still have SampleRTT EstimatedRTT 1 and so the above argument still applies.

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