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GATE IT 2007 | Question: 13

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Consider the following statements about the timeout value used in TCP.

  1. The timeout value is set to the RTT (Round Trip Time) measured during TCP connection establishment for the entire duration of the connection.
  2. Appropriate RTT estimation algorithm is used to set the timeout value of a TCP connection.
  3. Timeout value is set to twice the propagation delay from the sender to the receiver.

Which of the following choices hold?

  1. $(i)$ is false, but $(ii)$ and $(iii)$ are true
  2. $(i)$ and $(iii)$ are false, but $(ii)$ is true
  3. $(i)$ and $(ii)$ are false, but $(iii)$ is true
  4. $(i), (ii)$ and $(iii)$ are false

7 Answers

Best answer
49 49 votes
  1. (i) TCP connection established in $3$ phase between $\text{SYN}$ send and $\text{SYN}$ received $(\text{SYN},\text{SYN}+\text{ACK},\text{ACK})$. After this connection establishment, data transfer takes place. Now, $\text{FIN}$ flag is called to close the connection. $\text{FIN}$ flag can close the connection after getting the $\text{ACK}$ from the receiver. If $\text{ACK}$ is not received, a timer is set which wait for the time out. So, there is no relationship between TCP connection establishment and Timeout of $\text{RTT}.$


     
  2. This is Jacobson's algorithm (Thanks @Anirudh)
    $\text{ERTT}=p\times \text{IRTT}+(1-p)\times \text{NRTT}$
    $p$ is scaling factor
    $\text{IRTT}$ initial $\text{RTT}$
    $\text{NRTT}$ is new $\text{RTT}$
    Link here

     
  3. Actually timeout value is more than twice the propagation delay from sender to receiver. Because after connection establishment and data transfer complete, then only timeout occurs. So, if we start timer at the beginning of transaction, Time Out occurs after RTT completes and after final $\text{ACK}$ comes. So, Time Out time must be more than $\text{RTT}.$

So, only (ii) is TRUE. Answer (B).

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1 flag:
✌ Edit necessary (goku4199 “Answer is correct, but little bit thing is wrong, re-transmission timeout timer(RTO) can get activated at any point of time. It’s not necessary that only after data transfer, it will get activated.”)
14 14 votes

Statement-I: It is False.
The timeout value cannot be fixed for entire duration as it will turn timer to static timer, we need dynamic timer for timeout.
Statement-II: It is True.
Basic algorithm, Jacobson's algorithm, Karl's modification; these three algorithms are to be appropriate to RTT estimation algorithm used to set timeout value dynamically.
Statement-III: It is False.
Because timeout value is set to twice the propagation delay in data link layer where hop to hop distance is known, not in TCP layer.

1 1 vote

🔍 Statement Breakdown:

i. “The timeout value is set to the RTT measured during TCP connection establishment for the entire duration of the connection.” ❌ False — TCP doesn’t freeze the timeout at the initial RTT. It continuously updates RTT estimates using algorithms like Jacobson/Karels to adapt to network conditions.

ii. “Appropriate RTT estimation algorithm is used to set the timeout value of a TCP connection.” ✅ True — TCP uses Smoothed RTT (SRTT) and RTT variance to calculate Retransmission Timeout (RTO) dynamically.

iii. “Timeout value is set to twice the propagation delay from sender to receiver.” ❌ False — TCP doesn’t use raw propagation delay. It uses RTT (which includes both forward and backward delays) and variance. Doubling propagation delay is not a standard rule.

✅ Correct Option:

B. (i) and (iii) are false, but (ii) is true

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