GATE IT 2008 | Question: 68

Question

Which of the following statements are TRUE?

• S1:  TCP handles both congestion and flow control
• S2:  UDP handles congestion but not flow control
• S3:  Fast retransmit deals with congestion but not flow control
• S4:  Slow start mechanism deals with both congestion and flow control

• A. $S1$, $S2$ and $S3$ only
• B. $S1$ and $S3$ only
• C. $S3$ and $S4$ only
• D. $S1$, $S3$ and $S4$ only

Comments

s2 and s4 are only correct

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s3 is also correct because in slow start we consider both the receiver window size and congestion window size. so it takes care of both flow control and congestion control

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see the gate ques...explaination in ans

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you are correct . ss deals with congestion control only

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Imp point -> Slow start is way Sender tries to gauge network capacity.

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Slow start does take into account the receiver's advertised window. And is this is not precisely "flow control"? Shouldn't this make option d the right choice?

Reference- Wikipedia.

Caption

 

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https://www.brianstorti.com/tcp-flow-control/

good read!

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What is fast retransmit?

If possible please give some easy to understand resources to learn  about it.

Thanks.

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@luc_Bloodstone Fast retransmission is a congestion control algorithm that makes it possible for sender to quickly retransmit lost packets. Without fast retransmission, sender can retransmit only after timeout timer has expired. With fast transmission, reciever can send negative acknowledgements for the packets that it does not receive properly. If sender receives three negative acknowledgements for a packet, it will retransmit it without waiting for timeout timer.

 

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Since TCP uses cumulative acknowledgements and in the case where one packet is lost and 3 packets following it are successfully received and acknowledged by the receiver. Then in the acknowledgments missing packet sequence number is send. So we can say that there is 3 duplicate acknowledgments.

Sender on receiving 3 duplicate acknowledgment can retransmit packet without waiting for timeout timer, hence this technique known as fast retransmit.

Answer 1

(S1)  TCP handles both congestion and flow control $\Rightarrow$ True.
 It uses congestion window for congestion control & Advertisement window for flow control
(S2)  UDP handles congestion but not flow control $\Rightarrow$ UDP does not handle
congestion but also not handle flow control.
(S3)  Fast retransmit deals with congestion but not flow control$\Rightarrow$ Yes.
Fast Retransmit is technique for detecting out of Order Datagram & Sending it.
It is congestion control technique and has no relation with Flow control
(S4)  Slow start mechanism deals with both congestion and flow control $\Rightarrow$ False.
It has nothing to do with Flow control. Flow control is taken care by Advertisement window.
Slow start is way Sender tries to gauge network capacity !
Answer (B) S1 and S3 only.

Comments

But slow start phase does consider receiver window size which helps in maintaining the flow control.

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Fast retransmit quickly resends lost packets without waiting for a timeout.

• If the sender gets 3 duplicate ACKs, it assumes a packet is lost.
• The sender retransmits immediately to avoid delays.
  This keeps data transfer fast and smooth.

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Doubt in option C, in selective repeat protocol, when out of order packets are received by receiver the receiver send a nagative ack for first out of order packet, when the sender receive the negative ack it immediately retransmits the packet , why it is not considered as a fast retransmission?

Answer 2

Tcp handles both Congestion Control and Flow Control. For Flow Control it uses Sliding Window Protocol Algorithms and for Congestion Control it uses four congestion control algorithms: slow start, congestion avoidance, fast re-transmit, and fast recovery.

So Option (B)

Comments

@amitqy 

Forouzan's page 729

 

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Fast Recovery 

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can you like explain me s3 and s4

Answer 3

The correct answer is:

B. S1 and S3 only

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Explanation of Each Statement:

S1: TCP handles both congestion and flow control

• True:
  • Congestion Control: TCP uses mechanisms like slow start, congestion avoidance, fast retransmit, and fast recovery to manage congestion in the network.
  • Flow Control: TCP uses a sliding window mechanism to ensure that the sender does not overwhelm the receiver with more data than it can handle.

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S2: UDP handles congestion but not flow control

• False:
  • UDP does not handle congestion or flow control.
  • It is a lightweight protocol designed for fast, connectionless communication, leaving these responsibilities to the application layer.

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S3: Fast retransmit deals with congestion but not flow control

• True:
  • Fast Retransmit: This is part of TCP's congestion control mechanisms. It retransmits lost packets quickly based on duplicate acknowledgments to reduce the impact of congestion.
  • It does not handle flow control, as it is specifically designed to manage congestion.

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S4: Slow start mechanism deals with both congestion and flow control

• False:
  • Slow Start: This is a congestion control mechanism in TCP that gradually increases the congestion window size to avoid overwhelming the network.
  • It deals with congestion control only, not flow control.

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Summary

• S1: True (TCP handles both congestion and flow control)
• S2: False (UDP handles neither congestion nor flow control)
• S3: True (Fast retransmit deals with congestion)
• S4: False (Slow start deals only with congestion)

Thus, the correct answer is B. S1 and S3 only.

Answer 4

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