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GO Classes DPP | CN | CONGESTION CONTROL

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Consider a TCP connection using AIMD (Additive Increase Multiplicative Decrease) congestion control. The current congestion window size is $\mathbf{2 4 ~MSS}$. A timeout occurs, followed by three successful RTTs of transmission.

What is the congestion window size after these three RTTs?

1 Answer

1 1 vote

After Timeout

When a timeout occurs, TCP enters slow start phase:

  • Congestion window (cwnd) is set to $1 ~\mathrm{MSS}$
     
  • Slow start threshold (ssthresh) is set to half of previous window: $24 / 2=12 ~\mathrm{MSS}$
     

First RTT (Slow Start)

  • $\mathrm{cwnd = 1 ~MSS \rightarrow 2 ~MSS}$ (exponential increase in slow start)
     

Second RTT (Slow Start continues)

  • $\mathrm{cwnd =2 ~MSS \rightarrow 4 ~MSS}$ (exponential increase)
     
  • Still below ssthresh $\mathrm{(12 ~MSS)}$, so slow start continues.
     

Third RTT (Still in Slow Start)

  • $\mathrm{cwnd =4 ~MSS \rightarrow 8 ~MSS}$ (exponential increase)
     
  • Now $\text{cwnd} ~(8 ~\mathrm{MSS})$ is still less than $\text{ssthresh}~(12 ~\mathrm{MSS})$
     

Final Window Size

After three successful RTTs, the congestion window size is $\mathbf{8 ~MSS}$.

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