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Consider instance of TCP AIMD protocol where the Window size at start is cwnd = 2, and the threshold is ssthresh = 16. Timeout occurs during the transmission of 7th packet. What is the value of cwnd at the end of 14RTT ?
in Computer Networks by Loyal (6.9k points) | 137 views
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My Approach:-

TIME           CWND             SSTHRESH

    0                  1                          16

    1                  2                          16

    2                  4                          16

Now during the transmmision of 7th packet the value of cwnd becomes = 6 and timeout occurs so cwnd = 1 and ssthresh = 3

    3                  1                          3

    4                  2                          3

    5                  3                          3

    6                  4                          3

    7                  5                          3

    8                  6                          3

    9                  7                          3

  10                  8                          3

  11                  9                          3

  12                10                          3

  13                11                          3

  14                12                          3

Hence 12 is the answer
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Is my approach valid ?
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This is wrong.... I am getting ssthresh = 9.5
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PLEASE POINT MY MISTAKE :)
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see the answer.
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Na462 can you tell me that what's the answer given ???

1 Answer

+1 vote
this is the seq:

trans started ->       cwind

1 -> 2

2 ->4

3 ->8

4->16

5->17

6->18

7->19

new ssthresh = 9.5 => 9

new cwind = 1

8 -> 1

9->2

10->4

11->8

12->9

13->10

14->11 (14th transmission has started at cwind =11)

so when 14th transmission has ended we have 14RTT  and cwind increased from 11 to 12
by Active (1.1k points)
edited by
0

I didnot get u. It simply mentions timeout occurs during transmission of 7th packet. It does not mention every 7th packet

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ans is 13 bro

here it's given that Window size at start is cwnd = 2

therefore, slow start is start with 2 mss not 1 mss

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Sorry, this not about slow start and CA. It is about timeout. In case of loss due to timeout  cwind = 1MSS.
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cwnd = 2 means it start with 2 segment

2 4 8 ...... that's how series is executed
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it start with 2, 4, 8 but after timeout it starts with 1, 2, 4, 8...
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no , after timeout again it starts with 2 ,4 ,8
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please mention the source ???? .... i hv read it starts from 1,2,4
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source :- Forouzan

>>>when there Time out occurs  then TCP  acts strongly :

A:- It sets the valuee of threshold to one half the current window size.

B:- it sets cwnd the size of one segment .

C: it strats slow -start phase again.

>>> when detection is by ACK's i.e. three ACK's received then :

A:- - It sets the valuee of threshold to one half the current window size.

B: sets CWND to threshold.

C: congestion avoidance phase starts.

 

In This question starting window size is two . That's why it must be two , four, eight ...

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Answer in the above Forozoun is B ?? isn't it

B option mentions that cwind = 1 segment

so it is repeating from 1,2, 4...   after timeout

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nephron

Hmm I was wrong...thank you

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