GATE CS 2023 Set 1 — Question 50
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Computer Networks → Transport Layer → TCP Segment Format
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Question
Suppose you are asked to design a new reliable byte-stream transport protocol like TCP. This protocol, named myTCP, runs over a 100 Mbps network with Round Trip Time of 150 milliseconds and the maximum segment lifetime of 2 minutes.Which of the following is/are valid lengths of the Sequence Number field in the myTCP header?
Correct answer
(B) 32 bits; (C) 34 bits; (D) 36 bits
Solution
To ensure that sequence numbers do not wrap around within the Maximum Segment Lifetime (MSL), the sequence number space must be large enough to uniquely identify every byte sent during that duration.1. Calculate the data rate in bytes per second:
Bandwidth () = 100 Mbps = bits per second.
in bytes/sec = bytes/sec.2. Calculate the total bytes sent during the MSL:
Maximum Segment Lifetime () = 2 minutes = 120 seconds.
Total bytes = bytes.3. Determine the required number of bits ():
The sequence number space must be greater than or equal to the total bytes sent to avoid wrap-around.Evaluating powers of 2:
Any length is a valid length for the sequence number field to prevent wrap-around within the MSL.
Bandwidth () = 100 Mbps = bits per second.
in bytes/sec = bytes/sec.2. Calculate the total bytes sent during the MSL:
Maximum Segment Lifetime () = 2 minutes = 120 seconds.
Total bytes = bytes.3. Determine the required number of bits ():
The sequence number space must be greater than or equal to the total bytes sent to avoid wrap-around.Evaluating powers of 2:
Any length is a valid length for the sequence number field to prevent wrap-around within the MSL.
- (A) 30 bits: Invalid ()
- (B) 32 bits: Valid ()
- (C) 34 bits: Valid ()
- (D) 36 bits: Valid ()
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