CS 1652 Final Exam
Fall 2001
Name _______________________________________
Instructions:
1. The exam is closed booked, closed notes.
2. You may not use a calculator.
3. All questions refer to the Kurose and Ross textbook. For example,
for the purposes of this test, assume Kurose and Ross explanation
of the TCP congestion control heuristics for any question dealing
with congestion control.
4. Answer on the exam sheet in the space provided.
5. The test is probably long, don’t dawdle.
6. Get to the point quickly in your answers.
1. (2 points) Within the context of cyclic redundancy checks (CRC) how
does the receiver detect bit errors?
2. (4 points) In CDMA host A wants to send host B the message 1
using code –1, 1, 1, 1, -1 and host C wants to send the message –1
to host D using the code –1, 1, -1, -1, -1. Show what A and C
transmit, what C and D receive, and what B and D view as the
decoded message.
3. (4 points) Assume that you have a 100 Mbps Ethernet segment where
the speed of the medium is 300,000,000 meters per second. How long
can the Ethernet segment be so that a node will be sure that it will
detect a collision? Recall the minimum sized frame is 576 bits. Show
your work.
4. (6 points) Assume you have a 100 Mbps physical medium on which
1000 users are trying to share. We consider two situations. In the
lightest situation, only 1 user is trying to use the medium. In the
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heaviest situation, all 1000 users are trying to use the medium. For
each of the following MAC protocols, give the effective throughput
that you would expect for each of the lightest and heaviest situations.
Throughput is the number of bits per second that arrive at a receiver
that the bits were intended for.
a. TDMA
b. Slotted Aloha. You may assume that the broadcast probability
is picked optimally in each situation.
5. (6 points) Assume that the host with IP address 222.222.222.221
sends a datagram packet to the host with IP address 111.111.111.112,
and then the host 222.222.222.222 sends a datagram to host
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111.111.111.112. Give a linear list of the steps that are involved at the
network and link layer. Assume that the ARP tables are initially
empty.
6. (4 points) List 4 of the 6 fields in an Ethernet frame. For each of your
answers explain what the field is used for and why it is necessary.
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7. (4 points) Give pseudo-code for Ethernet’s CSMA/CD protocol.
8. (2 points) What is an ad-hoc wireless network?
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9. (3 points) Explain the Hidden Terminal Problem.
10. (4 points) Give sender-side pseudo code for the 802.11 CSMA/CA
protocol.
11. (3 points) What problem is “byte stuffing” trying to solve. How does
“byte stuffing” solve the problem?
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12. (3 points) Explain what a meta-file is within the context of your
browser accessing a video file. What problem are meta-files fixing,
and how do they fix this problem.
13. (2 points) Which of following protocols mostly closely resembles the
protocol for a VCR: RTP, RTSP, RSVP, RTCP, RSA
14. (3 points) What problem is an interleaved audio signal trying to fix,
and how does it fix this problem.
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15. (8 points) State the 4 principles for providing QoS support for
networked applications. For each principle, explain why that
principle is necessary.
16. (2 points) Consider weighted fair queuing with 4 queues with weights
12, 4, 12, 45. Assume that the throughput of the router is 10 Mbps.
What throughput of the first queue.
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17. (3 points) Within the diffserv framework, which of the following QoS
guarantees could be implemented for some class of traffic (you need
not justify you answer):
a. The end to end delay is below some specified threshold.
b. The traffic class gets at least ? of the resources on each
intermediate router
c. Routers should prioritize packets within the class by their TTL
fields
d. The routers should give packets in this class priority over all
other packets
e. The packets will be routed along the path with the fewest hops.
18. (4 points) Will a packet flow that conforms to a leaky bucket
specification (r, b) ever have to wait at a leaky bucket policer with
parameters r and b. Justify your answer.
19. (2 points) What is IP spoofing?
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20. (3 points) Give an example of a symmetric key cryptographic system that is provable secure as long as the shared key is in fact secret. Justify your claim.
21. (4 points) Consider a potential problem that might arise in online dating where people are sending others photos of supermodels with claims that these are photos of themselves. One company consider making money from this by having singles come to a company office, pay a fee, have their picture taken, show proper ID, and then have the picture and name posted on one of the company’s servers. That way people could have reasonable confidence of the name to picture correspondence. However, the company decides that it cannot make a profit if it must maintain servers. Explain how the company can use standard cryptographic techniques to solve this problem without have to maintain any central server site for the photographs.
22. (4 points) Consider the following protocol for Internet commerce. In the clear the consumer asks to buy a particular product at a particular price. In the clear the merchant confirms the availability of the product at that price. The consumer obtains the merchants RSA public key from a trusted authority. The consumer encodes his/her debit card number and sends this to the merchant. At which point the merchant debits the consumers account (or fails to do so), and then either sends either an ACK or a NACK to the consumer. What is the major security flaw in this protocol? Explain how to fix this flaw using standard cryptographic techniques.
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