Question $2$

$($a$)$ The IPv$4$ packet header is shown in Figure $2\mathrm{.}1$ below. The bits allocated to each field are

shown at the top of the frame.

Figure $2\mathrm{.}1$

$($i$)$ Which part of IPv$4$ packet header deals with the issue of security of transmitted

information? What are the other major functions which are implemented in this

same part of the header?

$($ii$)$ In the IPv$4$ datagram the Header Checksum is applied to the header and not to the

payload data. What is the reason for this choice?

$($iii$)$ If the size of the data present in the Options field is $280$ bits, what is the value of

IHL in this case? Would it be possible in principle to put into the Options $330$ bits?

Support your answer by the relevant calculations.

$($b$)$ The key function which is resolved by the Network Layer is the global packet addressing.

$($i$)$ List and briefly describe the five main schemes used for assigning IP addresses.

$($ii$)$ Explain the operating principles of the IPv$4$ CIDR addressing scheme. What are the

$10\%$

main advantages and disadvantages of CIDR?

$($iii$)$ A network which uses classful addressing with subnetting has a subnet mask of

$255\mathrm{.}255\mathrm{.}240\mathrm{.}0.$ Does the IP address $207\mathrm{.}240\mathrm{.}240\mathrm{.}240$ belong to a group of the

addresses subnetted with this mask? Explain why.

Question $2($continued$)$

$($c$)$ 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 $2\mathrm{.}2,$ where it was assumed

that the maximum segment size MSS $=1$ kbyte. 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. Then 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 for a time interval

between $t=0$ and $t=60$ RTT$.$