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GATE CSE 2015 | Set 1 | Question: 1‏9

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82 82 votes

Suppose two hosts use a TCP connection to transfer a large file. Which of the following statements is/are FALSE with respect to the TCP connection?

  1. If the sequence number of a segment is $m,$ then the sequence number of the subsequent segment is always $m+1.$
  2. If the estimated round trip time at any given point of time is $t$ sec, the value of the retransmission timeout is always set to greater than or equal to $t$ sec.
  3. The size of the advertised window never changes during the course of the TCP connection.
  4. The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.
  1. III only
  2. I and III only
  3. I and IV only
  4. II and IV only

11 Answers

Best answer
50 50 votes

Option B

III. False. It is the size of the receiver's buffer that's never changed. RcvWindow is the part of the receiver's buffer that's changing all the time depending on the processing capability at the receiver's side and the network traffic.

http://web.eecs.utk.edu/~qi/teaching/ece453f06/hw/hw7_sol.htm

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37 37 votes

This problem is taken from tanenbaum exercise .

I is directly taken , see 2 d) here http://web.eecs.utk.edu/~qi/teaching/ece453f06/hw/hw7_sol.htm

II is also in above link but indirectly , see  2 f) 

III  it is also direct from tanenbaum, see above link 2 b) 

IV Indirectly taken , see question number  2 c) 

28 28 votes

If the sequence number of a segment is m, then the sequence number of the subsequent segment is always m+1.

False. Sequence numbers indicate bytes, not segments.

If the estimated round trip time at any given point of time is t sec, the value of the retransmission timeout is always set to greater than or equal to t sec.

True. If RTO is set lower than RTT, Sender will have to retransmit even before the Receiver has a proper chance to acknowledge.

The size of the advertised window never changes during the course of the TCP connection.

False. Sender and Receiver's buffer sizes are fixed.

Congestion window and Advertised window are dynamic windows that change as per the current circumstances.

The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.

Unacknowledged bytes consist of the segments that are currently in the Sender Window. (If they were acknowledged, they won't be in the Sender's window, because, obviously, they're transmitted successfully. Why would we keep them in the transmission (sender) window again?)

The amount of unacknowledged data at a sender can't exceed the minimum of $cwnd$ and $rwnd$, that is:

$$LastByteSent \  – \  LastByteAcked \leq min\{cwnd, rwnd\} $$

 

And, $cwnd$ can't exceed $rwnd$ (to maintain the flow control)

Hence, the size of $rwnd$ is the upper bound of the number of unacknowledged bytes at the sender.

 

So, True.

Option B
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18 18 votes
1.False : because in tcp every byte is numbred and header contains the sequence number of 1st byte so sequence number in 2nd packet will be the next sequence number of last bit in 1st packet.

2. true:consider either the basic algorithm or jacobsn algorithm the value of timeout is always set > rtt for next transmission.

3.false: adv value in tcp header is dynamically changed by receiver each time.

4.true: window size at the receiver is always less than or equal to re1ceiver window size and thus the number of unacknowleged bytes can never exceed advertized window at that time.
12 12 votes

Option B.
Reason for I to be wrong: The sequence number of the subsequent segment depends on the number of 8-byte characters in the current segment. Transfer in TCP is byte ordered.

3 3 votes

St. 1: Not always true.

The subsequent sequence is only​​​ m+1  when we send one byte data. During connection establishment phase, we send only one byte data in SYN segments, so at that time subsequent sequence number is m+1

St. 2: True. The timeout timer is always >= RTT.

So that we have enough time to receive ACK of the sent packet.

St. 3: False. The advertised window size will change based on the buffer size at the receiver end.

St. 4: True. Sender will always send data of size less than or equal to receiver window size, as a part of flow control mechanism.

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