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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
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9 Answers

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10 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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2 votes
Option -B

Window size is not static .It always dynamic.

Sequence number in TCP is always random because of less probability of collission.
1 votes
1 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.

0 votes
0 votes
TCP sequence number of a segment is the byte number of the first byte in the segment. For example, if the segment contains 500 bytes which are from 1000 to 1499, then sequence number of the segment would be 1000 and sequence number of next segment would be 1500.

Receiver window changes when TCP data is processed by application layer of receiver side.
Answer:

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