GATE IT 2007 | Question: 13

Question

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?

• A. $(i)$ is false, but $(ii)$ and $(iii)$ are true
• B. $(i)$ and $(iii)$ are false, but $(ii)$ is true
• C. $(i)$ and $(ii)$ are false, but $(iii)$ is true
• D. $(i), (ii)$ and $(iii)$ are false

Comments

It means the TO will be constant for entire session and will not change which is not hte case in TCP.

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$\alpha$ is also called "smoothening factor"

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RTT should be equal to or GREATER than propogation delay.... Thats why iii is false

Answer 1

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).

Comments

what does timout means ?

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Are these algos still in GATE 2020 syllabus ?

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marja tu

Answer 2

Answer: B

Jacobson's algorithm is used.

Comments

Why basic algo cannot be used according to which Timeout value is set to twice the propagation delay.

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in basic algo - timeout = 2*(round trip time)

Answer 3

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.

Answer 4

Option B is right option.