Question $3$

$($a$)$ For a particular TCP session the current round$-$trip time, $RT{T}_{\text{c}\text{u}\text{r}},$ is $50ms,$ and at a particular

moment of time the measured acknowledgement from a data segment is after $42ms.$

$($i$)$ Apply the Jacobson algorithm to find the updated value $RT{T}_{\text{u}\text{p}\text{d}}.$ Assume in $[10\%]$

calculations the smoothing factors $==0\mathrm{.}875.$

$($ii$)$ Determine the values of the current $D_{\text{c}\text{u}\text{r}}$ and the updated $D_{\text{u}\text{p}\text{d}}$ mean deviations if the

calculated updated value of the Retransmission Time$-$Out $RT{O}_{\text{u}\text{p}\text{d}}$ is $57ms.$

$($b$)$ The dependence of the congestion window $W_c($in kilobytes$)$ on the transmission time $t($in

units of RTT$)$ for a lightly loaded internet is shown in Figure $3\mathrm{.}1,$ where it was assumed

that the maximum segment size MSS $=1$ kilobyte. Draw schematically a similar

dependence for a congested internet in which the following scenario was observed:

Between $t=0$ and $t=25$ RTT no disruptions of the transmission take place.

At $t=25$ RTT the first three duplicate acknowledgements $($ACKs$)$ have been received by

the sender host.

At $t=45$ RTT the first RTO $($Retransmission Time$-$Out$)$ of the sender$\text{'}$s acknowledgement

clock has occurred.

Between $t=45RTT$ and $t=60RTT$ the network operates without disruptions.

Explain the obtained dependence. You need to draw the dependence of $W_c$ on time $t$ for the

time interval from $t=0$ to $t=60$ RTT$.$

Question $3($continued$)$

$($c$)$ A server sends a burst of $W=600$ kilobytes of data to another host located at a distance of

$L=3000km$ over an optical fibre link operating at $B=1G\frac{b}{s}.$ The speed of light in optical

fibre is $c=200,000k\frac{m}{s}.$

$($i$)$ Calculate the upper bound of the link efficiency $(L{E}_{UB})$ for this transmission.

$($ii$)$ What should be the size of the data burst $W$ in order to achieve $L{E}_{UB}=50\%?$

Explain the obtained result and the reason why this leads to an increase of the link

efficiency $L{E}_{UB}.$

$($d$)$ Two businesses B$1$ and B$2$ intend to connect five nodes using different structural

networking models: peer$-$to$-$peer networking and client$-$server networking. B$1$ requires a

network with a good resource sharing function and low cost. B$2$ needs high network

security and good data back$-$up$.$

$($i$)$ What structural model should be employed by each business? Explain why.

$($ii$)$ Draw schematics of each of these networks.