$1.$ Suppose a $4000$ bytes TCP message $($including the TCP header$)$ is passed to IP for delivery across two networks of the Internet.$$ The first network has an MTU of $1500$ bytes; the second has an MTU of $512$ bytes.$$ Each network$$s MTU gives the size of the largest IP datagram that can be carried in a link$-$layer frame.$$

$($a$)$ Give the sizes, offsets, values of offset fields in the IP headers of the sequence of fragments delivered to the network layer at the destination host.$$ Assume all IP headers are $20$ bytes.$$ Note, the IP requires that fragmentation should always happen on $8-$byte boundaries.

$($b$)$ Path MTU is the smallest MTU of any link on the current path $($route$)$ between two hosts.$$ Assume that we could discover the path MTU of the path used, and that we use this value as the MTU for all the path segments.$$ Give the sizes of the sequence of IP packets delivered to the network layer at the destination host.$$

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$2.$ IP addresses. Company X and Company Y both connect to the same ISP. Company X is assigned the prefix $121\mathrm{.}77\mathrm{.}0\mathrm{.}0/18$ and Company Y is assigned the prefix $121\mathrm{.}77\mathrm{.}64\mathrm{.}0/18.$ The ISP has a single $3-$port router: port $1$ connects to Company X$,$ port $2$ connects to Company Y$,$ and port $3$ connects to the rest of the Internet.

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a$)$ Draw and complete $($as best you can$)$ the contents of the forwarding table in the ISP$$s router.

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b$)$ What aggregated prefix does the router advertize to the rest of the Internet so that packets can reach both Company X and Company Y$?$

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$3.$ Suppose a route has built up the routing table shown in the following table.

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SubnetNumber

SubnetMask

Next hop

$128\mathrm{.}96\mathrm{.}39\mathrm{.}0$

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

Interface $0$

$128\mathrm{.}96\mathrm{.}39\mathrm{.}128$

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

Interface $1$

$128\mathrm{.}96\mathrm{.}40\mathrm{.}0$

$255\mathrm{.}255\mathrm{.}255\mathrm{.}128$

R$2$

$192\mathrm{.}4\mathrm{.}153\mathrm{.}0$

$255\mathrm{.}255\mathrm{.}255\mathrm{.}192$

R$3$

Default

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R$4$

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The router can deliver packets directly over interfaces $0$ and $1,$ or it can forward packets to routers R$2,$ R$3,$ or R$4.$ Describe what the router does with a packet addressed to each of the following destinations:

$($a$)128\mathrm{.}96\mathrm{.}39\mathrm{.}135$

$($b$)128\mathrm{.}96\mathrm{.}39\mathrm{.}40$

$($c$)128\mathrm{.}96\mathrm{.}40\mathrm{.}150$

$($d$)128\mathrm{.}96\mathrm{.}40\mathrm{.}16$

$($e$)192\mathrm{.}4\mathrm{.}153\mathrm{.}200$

$($f$)192\mathrm{.}4\mathrm{.}153\mathrm{.}32$

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$4.$The following table is a routing table using CIDR.$$ Address bytes are in hexadecimal.$$

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Net$/$MaskLength

Next hop

C$4\mathrm{.}50\mathrm{.}00\mathrm{.}00/12$

A

C$4\mathrm{.}5$E$\mathrm{.}10\mathrm{.}00/20$

B

C$4\mathrm{.}60\mathrm{.}00\mathrm{.}00/12$

C

C$4\mathrm{.}68\mathrm{.}00\mathrm{.}00/14$

D

$80\mathrm{.}00\mathrm{.}00\mathrm{.}00/1$

E

$40\mathrm{.}00\mathrm{.}00\mathrm{.}00/2$

F

$00\mathrm{.}00\mathrm{.}00\mathrm{.}00/2$

G

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State to what next hop the following will be delivered.

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$($a$)$ C$4\mathrm{.}5$E$\mathrm{.}1$A$\mathrm{.}87$

$($b$)$ C$4\mathrm{.}5$E$\mathrm{.}32\mathrm{.}09$

$($c$)$ C$4\mathrm{.}69\mathrm{.}22\mathrm{.}89$

$($d$)2$E$\mathrm{.}43\mathrm{.}91\mathrm{.}12$

$($e$)9$A$\mathrm{.}88\mathrm{.}77\mathrm{.}66$

$($f$)$ C$4\mathrm{.}62\mathrm{.}31\mathrm{.}2$E

$($g$)$ C$4\mathrm{.}6$A$\mathrm{.}31\mathrm{.}2$E

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$5.$ An organization has a class C network $212\mathrm{.}192\mathrm{.}88\mathrm{.}0$ and wants to form subnets for six departments, which host as follows:

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A$.45$ hosts

B$.20$ hosts

C$.55$ hosts

D$.25$ hosts

E$.10$ hosts

F$.22$ hosts

G$.9$ hosts

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There are $180$ hosts in all.$$ Design a possible arrangement of subnets to make each department in a different subnet.$$ For each subnet, give subnet number, subnet mask and range of IP addresses. $1.$ Using the example network below, give the virtual circuit tables for all the switches after each of the following connections is established. Assume that the sequence of connections is cumulative; that is$,$ the first connection is still up when the second connection is established, and so on$.$ Also assume that the VCI assignment always picks the lowest unused VCI on each link, starting with $0,$ and that a VCI is consumed for both directions of a virtual circuit.

$($a$)$ Host A connects to host C $.$

$($b$)$ Host D connects to host B $.$

$($c$)$ Host D connects to host I.

$($d$)$ Host A connects to host B$.$

$($e$)$ Host F connects to host J$.$

$($f$)$ Host H connects to host A $.2.$ Consider the arrangement of learning bridges shown in the following figure. Assuming all are initially empty, give the forwarding tables for each of the bridges $\backslash (\backslash$mathrm$\{$B$\}1-\backslash$mathrm$\{$B$\}3\backslash )$ after the following transmissions:

$(1)$ A sends to $\backslash ($ B $\backslash )$; $(2)\backslash ($ B $\backslash )$ sends to $\backslash ($ A $\backslash )$; $(3)\backslash ($ A $\backslash )$ sends to $\backslash ($ X $\backslash )-$

$(4)\backslash ($ X $\backslash )$ sends to $\backslash ($ A $\backslash )$; $(5)\backslash ($ D $\backslash )$ sends to $\backslash ($ Y $\backslash )$; $(6)\backslash ($ E $\backslash )$ sends to $\backslash (\backslash$underline$\{\backslash$underline$\{$D$\}\}\backslash )8.$ Link$-$state routing protocols

This question explores how to set the $($configurable$)$ link weights in link$-$state routing protocols like OSPF inside a single Autonomous System $($AS$)$ to achieve AS$-$wide goals.

a$)$ How should the network operators set the link weights if their goal is to minimize the number of hops each packet traverses to reach its destination?

b$)$ How should the operators set the link weights to minimize the end$-$to$-$end delay the traffic experiences? Assume the network is lightly loaded, so queuing delay is insignificant.

c$)$ In the picture below, the nodes are routers, the edges are links, and the integers correspond to the link weight on each direction of the link. $($That is$,$ the link a $-$ b and the link b $-$a both have weight $10.)$ Put arrows on the edges to show the shortest path from every node to the destination node $\backslash (\backslash$mathbf$\{$d$\}\backslash ).$ That is$,$ show the "sink tree" leading to node $\backslash (\backslash$mathbf$\{$d$\}\backslash ).$

d$)$ Suppose the link i$-$h is overloaded with traffic. Identify a sing $5.$