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Consider the following statements regarding the slow start phase of the TCP congestion control algorithm. Note that cwnd stands for the TCP congestion window and MSS window denotes the Maximum Segments Size:

  1. The cwnd increases by 2MSS on every successful acknowledgment
  2. The cwnd approximately doubles on every successful acknowledgment
  3. The cwnd increases by 1MSS every round trip time
  4. The cwnd approximately doubles every round trip time

ref link:

Here  we double the congestion window after receiving the ACK right, without getting the ack how we blindly double in RTT time. If that statement is true I will get option 2

Can someone please clarify on above statement

in Computer Networks by Loyal (6.7k points) | 49 views
Not getting your doubt.
That depends , if system is incorporating only one composite acknowledgement then yes , on every successful acknowledgement cwnd doubles (which is precisely the RTT) .

But if for every segment an acknowledgement  is sent , then don't you think cwnd would increase at a very drastic rate?

@Utkarsh Joshi 

Option 4 says "The cwnd approximately doubles every round trip time" 

where as Option 2  "The cwnd approximately doubles on every successful acknowledgment".

The congestion window should be double which is right, but shouldn't this be happening only after receiving the successful acknowledgement ??

@prashant jha 1 I agree with you on this "But if for every segment an acknowledgement  is sent , then don't you think cwnd would increase at a very drastic rate?" --> But if the ack was not available within RTT we still double the congestion window right, is that valid??


CWND increases by 1 for each ack so doubles after each rtt

No it is done only on successful receipt of acknowledgement . Your confusion seems valid here @Hemanth_13

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