Question $3$

$($a$)$ Network congestion management requires for the TCP to evaluate the Retransmission

Time$-$Out $($ RTO $)$ interval using the 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

final updated RTO with its initial value $RT{O}_0$ and explain these results. Assume

the smoothing factor $=0\mathrm{.}875.$

$($ii$)$ What does the value of the smoothing factor $$ represent?

$($iii$)$ Explain what will happen if TCP will set up its retransmission timer with the time$-$

out interval considerably smaller or considerably larger than the evaluated RTO.

$($b$)$ Fast transmission of data over the long$-$haul gigabit link imposes limitation on the efficiency

of the line utilisation due to signal propagation delay.

$($i$)$ Explain briefly what is meant by the propagation$-$delay limited line. Write down

the formula for the upper bound of the link efficiency $(L{E}_{UB})$ and explain its

significance.

$($ii$)$ Consider transmission of a data frame over a $2\mathrm{.}5-G\frac{b}{s}$ optical fibre link. The link

length is $2,000km,$ and the speed of light in the fibre is $200,000k\frac{m}{s}.$ Calculate

the minimum required size of the frame in order to achieve $50\%$ of the link

efficiency utilisation.

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

$($i$)$ Explain what is meant by hierarchical routing.

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

of each of them.

$($iii$)$ Explain why the Routing Information Protocol $($RIP$)$ and the Open Shortest Path $[10\%]$

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

Question $4$

$($a$)$ Apply Dijkstra's algorithm to compute the shortest path from router R$1$ to each other

router in the network shown in Figure $4\mathrm{.}1.$ The number on each line shows the cost of the

line. Based upon these calculations write down the shortest routes from R$1$ to each other

router, the route cost values, and the first$-$hop router.