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

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

Excellent explanation.

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Flow control also uses fast tramsmission rule.

Even if we are not apllying congestion policies say rcvd<cwd,in this case also if some frame's ack is lost then also tcp transmits after 3 dup acks,please check and tell.

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can somebody expalain s3 statment in detail

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@ Moin Mukhtar

In fast retransmit whenever the sender gets 3 ack of same sequence number it can safely assume that that packet has been lost and it retransmits the packet without waiting for the time out.

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D option is wrong

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Slow start phase too consider receiver's window size then how it cannot support flow control?

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

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

Can you explain what is fast recovery?

@Danish

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For Flow Control it uses  Sliding Window Protocol  Algorithms 

I have read that n/w layer implements flow control by Sliding Window Protocol. But TCP is a transport layer protocol. ?

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@amitqy 

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