GSLC · Question #98
A sequence number is a 32-bit number ranging from 1 to 4,294,967,295. When data is sent over the network, it is broken into fragments (packets) at the source and reassembled at the destination system.
The correct answer is C. 24619311. TCP Initial Sequence Numbers increment at a rate of 128,000 per second plus 64,000 per new connection, allowing the ISN to be calculated after any elapsed time and number of connections.
Question
A sequence number is a 32-bit number ranging from 1 to 4,294,967,295. When data is sent over the network, it is broken into fragments (packets) at the source and reassembled at the destination system. Each packet contains a sequence number that is used by the destination system to reassemble the data packets in the correct order. The Initial Sequence Number of your computer is 24171311 at login time. You connect your computer to a computer having the IP address 210.213.23.21. This whole process takes three seconds. What will the value of the Initial Sequence Number be at this moment?
Options
- A24171811
- B24171311
- C24619311
- D24171111
How the community answered
(35 responses)- A14% (5)
- B6% (2)
- C54% (19)
- D26% (9)
Why each option
TCP Initial Sequence Numbers increment at a rate of 128,000 per second plus 64,000 per new connection, allowing the ISN to be calculated after any elapsed time and number of connections.
24,171,811 reflects only a 500-unit increase, which does not correspond to any standard TCP ISN increment formula.
24,171,311 is the original ISN at login time and ignores the increments that occur due to elapsed time and the new connection.
The Berkeley TCP implementation increments the ISN by 128,000 for each second elapsed and by 64,000 for each new connection established. Over 3 seconds with one new connection, the total increment is (3 x 128,000) + (1 x 64,000) = 448,000. Adding this to the starting ISN gives 24,171,311 + 448,000 = 24,619,311.
24,171,111 is lower than the original ISN, which is impossible because TCP sequence numbers only increase monotonically over time.
Concept tested: TCP Initial Sequence Number increment calculation
Source: https://www.rfc-editor.org/rfc/rfc793
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