TELCOM2310_Final_Fall2008_Sols

TELCOM2310_Final_Fall2008_Sols - University of Pittsburgh...

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TELCOM 2310 Final Exam Fall 2008 Page 1 of 7 University of Pittsburgh Name: …………………………………. TELCOM 2310: Computer Networks Final Exam Fall 2008 Part 1. True/False questions (1 pts each) Let C1 be a block cipher that operates on blocks of bits of size k1 and C2 a block cipher that operates on blocks of bits of size k2. For any block cipher C and sequence of bits X, we will denote C(X) the ciphertext obtained by applying C to X. 1. If C1(X) = C2(X) for any sequence of bits X, then k1=k2 (F) 2. If k1=k2, then C1( C2(X) ) = C2( C1(X) ) for any sequence of bits X. (F) 3. Let C1(v1) and C2(v2) be two block ciphers that use CBC with C1 having v1 as its initialization vector and C2 having v2 as its initialization vector. Both C1 and C2 operate following the model in Figure 8.5 (Page 689) without bit scrambling : 5 X, C1(X) = C2(X) v1 = v2 (F, you can easily infer this from question 1) 4. Consider the fixed-delay strategy of multimedia applications and consider two playout schedules p- r and p’-r. If p’ > p then the schedule p’-r will necessarily have (strictly) less missed playouts than the schedule p-r. (F, p’-r may have as many or less missed playouts than p-r) 5. The fixed playout strategy of multimedia applications requires changes in the transport layer at the end hosts. (F) 6. Consider two multimedia clients MC1 and MC2 accessing the same audio file F at a remote multimedia server S. MC1 uses a fixed playout delay strategy with an intial playout delay of p-r. MC2 uses an adptive playout delay strategy with a constant u for the weight given to the recent network delays (i.e., the estimate of the average network delay is as given in page 613). If MC1 and MC2 receive all the packets at exactly the same times r1, r2, etc., then MC1 and MC2 will finish playing the file F at exactly the same time. (F) 7. A multimedia client using an adptive playout delay strategy never experiences missed playouts. (F) 8. A host that requests a multimedia object through a Content Distribution Network (CDN) may experience worse performance than when it requests the object directly from the origin server. (T) 9. Public key encryption may be used in a multicast RTP session. (F) 10. Suppose Alice and Bob are communicating over an SSL session. Suppose an attacker, who does not have any of the shared keys, inserts a bogus TCP segments into a packet stream with correct TCP checksum and sequence numbers (and correct IP addresses and port numbers). SSL at the receiving side will not pass the payload of the bogus packet to the receiving application. (F) 11. The DNS has to be modified to support a CDN. (F) 12. Because an ATM network first establishes a VC between the sender and the receiver, the sender is able to transmit at a constant rate throughout the duration of the VC. (F) 13. ARP packets do not involve the network layer because ARP is a link layer protocol. (T)
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TELCOM 2310 Final Exam Fall 2008 Page 2 of 7 14. Consider an attacker A trying to decipher a cipherext T of length n encrypted using a block cipher. In a brute force attack, the number of possible permutations that the attacker A would have to try
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TELCOM2310_Final_Fall2008_Sols - University of Pittsburgh...

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