Makeup Exam

8/16/2019 Makeup Exam

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CS5800 Makeup Exam Questions

1. Consider the Figure below, for which there is an institutional network connected to the

Internet. Suppose that the average object size is 30,000 bits and that the average request

rate from the institution’s br owser to the origin servers is 6 requests per second. Also

suppose that the amount of time it takes from when the router on the Internet side of the

access link forwards an HTTP request until it receives the responses in two seconds on

average. Model the total average response time as the sum of the average access delay

(that is, the delay from the Internet router to institution router) and the average Internet

delay. For the average access delay, you may use δ/(1-δβ), where δ is the average time

required to send an object over the access link and β is the arrival rate of objects to the

access link.

(a) Find the total average response time

(b) Now suppose a cache is installed in the institutional LAN. Suppose the hit rate is

0.5. Find the total response time.

2. For your start-up company that needs web (say, www.mystartup.com) and email (e.g.,

[email protected]) services, (a) which DNS RRs (resource records) should be

stored in a top-level domain (.com) DNS server? (b) which DNS RRs (resource records)should be stored in an authoritative DNS server? Use the following information to fill in

the blanks.

localdns.mystartup.com's IP address = 174.222.32.67

www1.mystartup.com's IP address = 174.222.32.76

smtp.mystartup.com's IP address = 174.222.32.69


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(a) In a TLD DNS server

(mystartup.com, _________________________, _______)

(localdns.mystartup.com, _________________________, _______)

(b) In an authoritative DNS server

(www.mystartup.com, www1.mystartup.com, _______)(www1.mystartup.com, _________________________, _______)

(mystartup.com, smtp.mystartup.com, _______)

(smtp.mystartup.com, _________________________, _______)

3. Compare Selective Repeat and TCP (no delayed ACK). Assume that the timeout

values for all three protocols are sufficiently long such that 5 consecutive data

segments and their corresponding ACKs can be received (if not lost in the channel) by

the receiving host (host B) and the sending host (host A) respectively. Suppose Host

A sends 5 data segments to Host B, and the 3rd

and 4th

segments (sent from A) are lost.In the end, all 5 data segments have been correctly received by Host B.

(a) With Selective Repeat, how many segments has Host A sent in total and how

many ACKs has Host B sent in total? What are their sequence numbers?

(b) With TCP, how many segments has Host A sent in total and how many ACKs has

Host B sent in total? What are their sequence numbers?

4. Below is a list of requests for network address allocations to a service provider. The

service provider allocates addresses in the order of organization 1, 2, 3 and 4,

beginning at 234.195.0.0.

Organization 1: 350 hosts




Show the resulting address allocations with CIDR notation.

CIDR Address Specification Range of Addresses Allocated

Organization 1

Organization 2

Organization 3

Organization 4


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5. Consider sending a 4,000 bytes datagram into a link that has an MTU of 1,100 bytes.

Suppose the original datagram is stamped with the identification number 23.

(a) How many fragments are generated?

(b) What are the values in the various fields in each IP datagram(s) generated related

to fragmentation?

6. The table below is a routing table using CIDR. Address bytes are denoted in binary. The

notation '/12' in 11000000.01100000.00000000.00000000/12 denotes a netmask with 12

leading 1-bits, that is 11111111.11110000. 00000000. 00000000.

Netmask/Length NextHop

11000100.01011010.00000010.00000000/23 A

11000100.01011010.00000100.00000000/22 B

11000100.01011010.10100000.00000000/19 C

11000100.01011010.01000000.00000000/18 D11000100.01011010.00000000.00000000/16 E

11000000.01011000.00000000.00000000/14 F

10000000.00000000.00000000.00000000/1 G

Select to what next hop the packets with the following destination address will be

delivered.

(a) 11000100. 01011010.00000011.11111100

(b) 11000100.01011010.11101100.10001111

7. Consider the network shown below.

Show the operation of Distance vector algorithm (with their tables) for computing the

shortest path at all nodes, x, y, and z.


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8. Consider the network shown below.

Show the operation of Dijkstra’s algorithm.

N' D(v), p(v) D(u), p(u) D(w), p(w) D(y), p(y) D(z), y(z)

x

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