Question $3$

$($a$)$ Network congestion management requires for TCP to evaluate the Retransmission TimeOut $($ RTO $)$ interval using Jacobson's algorithm. The algorithm dynamically calculates the round trip time $(RTT)$ and its mean deviation $D,$ which are then used to update the current RTO. The current estimates of the $RTT$ and $D$ are: $RT{T}_0=50ms,D_0=5ms,$ and the three subsequent measured RTTs are: $M_1=45ms,M_2=55ms,$ and $M_3=50ms.$

$($i$)$ Apply Jacobson's algorithm and calculate the new updated RTOs. Compare the

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final updated RTO with its initial value $RT{O}_0$ and explain these results. Assume the same smoothing factor $==0\mathrm{.}875$ for calculations of $RTT$ and $D,$ respectively.

$($ii$)$ Discuss what will happen if TCP will set up its retransmission timer with a time$-$out

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interval considerably smaller or considerably larger than the evaluated RTO.

$($b$)$ Consider TCP in a transmitting host which is running dynamic evaluation of the congestion window $W_c$ to prevent network congestion. Suppose at some moment of time $t=0$ the current calculated congestion window size is $W_{c0}=32$ kilobytes, and the retransmission time$-$out $($RTO$)$ has occurred at this moment. The receive sliding window is $W_s=20$ kilobytes. Assume that the maximum segment size is $MSS=1$ kilobyte, and the round$-$trip time is $RTT=10ms.$

$($i$)$ Explain and describe in detail which algorithm$($s$)$ will be used by TCP for evaluation

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of the congestion window $W_c$ beginning from the moment $t=0$ until the first full window will be sent. Support your explanations with a sketch of variation of $W_c$ with time $($measured in RTT units$).$

$($ii$)$ Find how long it will take before the first full window will be sent.

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$($c$)$ A TCP machine is sending the maximum size IPv$4$ packet of $W=65,535$ bytes over a

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$B=1G\frac{b}{s}$ optical fibre link which has a $t_{1d}=1$msec one$-$way propagation delay. Calculate an upper bound for the link efficiency $L{E}_{UB}.$

Question $3$ continues...

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CE$707-7-$AU

Question $3($continued$)$

$($d$)$ Routing of packets in the global Internet relies on a hierarchical approach.

$($i$)$ List the four classes of routers used in the global Internet and explain the purpose of

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each of them.

$($ii$)$ Explain why the Routing Information Protocol $($RIP$)$ and the Open Shortest Path

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First $($OSPF$)$ protocol could not be used over the entire Internet.

$($iii$)$ Explain why the Internet is divided into Tiers, Autonomous Systems, and Areas.

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