GATE | GATE-CS-2015 (Set 2) | Question 65

Assume that the bandwidth for a TCP connection is 1048560 bits/sec. Let α be the value of RTT in milliseconds (rounded off to the nearest integer) after which the TCP window scale option is needed. Let β be the maximum possible window size with window scale option. Then the values of α and β are.

(A)

63 milliseconds 65535 × 2¹⁴

(B)

63 milliseconds 65535 × 2¹⁶

(C)

500 milliseconds 65535 × 2¹⁴

(D)

500 milliseconds 65535 × 2¹⁶

Answer: (C)

Explanation:

Since window size field number in TCP header is limited to 16 bits, the maximum window size is limited. When bandwidth delay product of a link is high, scaling is required to efficiently use link. TCP allows scaling of windows when bandwidth delay product is greater than 65,535 .

The bandwidth delay product for given link is 1048560 * α. Window scaling is needed when this value is more than 65535 bytes, i.e., when α is greater than 65535 * 8 / 1048560 or 0.5 seconds. Scaling is done by specifying a one byte shift count in the header options field. The true receive window size is left shifted by the value in shift count. A maximum value of 14 may be used for the shift count value. Therefore maximum window size with scaling option is 65535 × 2¹⁴

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