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computer science
computer networking

Questions and Answers of Computer Networking

Suppose that a message has been encrypted using DES in counter mode. One bit of cipher text in block Ci is accidentally transformed from a 0 to a 1 during transmission. How much plain text will be
Using the RSA public key cryptosystem, with a = 1, b = 2 . . . y = 25, z = 26.(a) If p = 5 and q = 13, list five legal values for d.(b) If p = 5, q = 31, and d = 37, find e.(c) Using p = 3, q = 11,
Alice and Bob use RSA public key encryption in order to communicate between them. Trudy finds out that Alice and Bob shared one of the primes used to determine the number n of their public key pairs.
A math class has 25 students. Assuming that all of the students were born in the first half of the year—between January 1st and June 30th— what is the probability that at least two students have
Alice wants to send a message to Bob using SHA-1 hashes. She consults with you regarding the appropriate signature algorithm to be used. What would you suggest?
Suppose an organization uses VPN to securely connect its sites over the Internet. Jim, a user in the organization, uses the VPN to communicate with his boss, Mary. Describe one type of communication
The Diffie-Hellman key exchange is being used to establish a secret key between Alice and Bob. Alice sends Bob (227, 5, 82). Bob responds with (125). Alice’s secret number, x, is 12, and Bob’s
Two users can establish a shared secret key using the Diffie-Hellman algorithm, even if they have never met, share no secrets, and have no certificates(a) Explain how this algorithm is susceptible to
Is it possible to multicast a PGP message? What restrictions would apply?
Point-of-sale terminals that use magnetic-stripe cards and PIN codes have a fatal flaw: a malicious merchant can modify his card reader to log all the information on the card and the PIN code in
Alice is using the public-key authentication protocol of Fig. 8-43 to authenticate communication with Bob. However, when sending message 7, Alice forgot to encrypt RB. Trudy now knows the value of
Imagine that you have trained your St. Bernard, Bernie, to carry a box of three 8-mm tapes instead of a flask of brandy. (When your disk fills up, you consider that an emergency.) These tapes each
Besides bandwidth and latency, what other parameter is needed to give a good characterization of the quality of service offered by a network used for (i) Digitized voice traffic? (ii) Video
What are two reasons for using layered protocols? What is one possible disadvantage of using layered protocols?
The president of the Specialty Paint Corp. gets the idea to work with a local beer brewer to produce an invisible beer can (as an anti-litter measure). The president tells her legal department to
The Internet is roughly doubling in size every 18 months. Although no one really knows for sure, one estimate put the number of hosts on it at 600 million in 2009. Use these data to compute the
Mobile phone network operators need to know where their subscribers’ mobile phones (hence their users) are located. Explain why this is bad for users. Now give reasons why this is good for users.
An image is 1600 × 1200 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem channel? Over a 1-Mbps cable modem? Over a 10-Mbps
Suppose the algorithms used to implement the operations at layer k is changed. How does this impact operations at layers k − 1 and k + 1?
Suppose there is a change in the service (set of operations) provided by layer k. How does this impact services at layers k-1 and k+1?
Provide a list of reasons for why the response time of a client may be larger than the best-case delay.
What are the disadvantages of using small, fixed-length cells in ATM?
In the Internet File Server example (Figure 6-6), can the connect( ) system call on the client fail for any reason other than listen queue being full on the server? Assume that the network is
In Figure 6-20, suppose a new flow E is added that takes a path from R1 to R2 to R6. How does the max-min bandwidth allocation change for the five flows?
Suppose that you are measuring the time to receive a segment. When an interrupt occurs, you read out the system clock in milliseconds. When the segment is fully processed, you read out the clock
John wants to have an original domain name and uses a randomized program to generate a secondary domain name for him. He wants to register this domain name in the com generic domain. The domain name
A large law firm, which has many employees, provides a single email address for each employee. Each employee’s email address is @lawfirm.com. However, the firm did not explicitly define the format
Suppose that you want to send an MP3 file to a friend, but your friend’s ISP limits the size of each incoming message to 1 MB and the MP3 file is 4 MB. Is there a way to handle this situation by
Suppose that John just set up an auto-forwarding mechanism on his work email address, which receives all of his business-related emails, to forward them to his personal email address, which he shares
In any standard, such as RFC 5322, a precise grammar of what is allowed is needed so that different implementations can inter work. Even simple items have to be defined carefully. The SMTP headers
Is the vacation agent part of the user agent or the message transfer agent? Of course, it is set up using the user agent, but does the user agent actually send the replies? Explain your answer.
IMAP allows users to fetch and download email from a remote mailbox. Does this mean that the internal format of mailboxes has to be standardized so any IMAP program on the client side can read the
Consider the Chord circle of Fig. 7-71. Suppose that node 18 suddenly goes online. Which of the finger tables shown in the figure are affected? how?Figure 7-71 of successor 4 31 (30 29 Start Actual 3
Is it possible that when a user clicks on a link with Firefox, a particular helper is started, but clicking on the same link in Internet Explorer causes a completely different helper to be started,
Imagine that someone in the math department at Stanford has just written a new document including a proof that he wants to distribute by FTP for his colleagues to review. He puts the program in the
In Fig. 7-22, www.aportal.com keeps track of user preferences in a cookie. A disadvantage of this scheme is that cookies are limited to 4 KB, so if the preferences are extensive, for example, many
Write an HTML page that includes a link to the email address username@DomainName. com. What happens when a user clicks this link?
Write an XML page for a university registrar listing multiple students, each having a name, an address, and a GPA.
In a network whose max segment is 128 bytes, max segment lifetime is 30 sec, and has 8-bit sequence numbers, what is the maximum data rate per connection?
To address the limitations of IP version 4, a major effort had to be undertaken via IETF that resulted in the design of IP version 6 and there are still is significant reluctance in the adoption of
Describe two ways to get into the SYN RCVD state of Fig. 6-39.Figure 6-39 (Start) CONNECT/SYN (Step 1 of the 3-way handshake) CLOSED CLOSE/- LISTEN/- CLOSE/- SYN/SYN + ACK LISTEN (Step 2 of the 3-way
In Fig. 6-36 we saw that in addition to the 32-bit acknowledgement field, there is an ACK bit in the fourth word. Does this really add anything? Why or why not?Figure 6-36 32 Bits Source port
Consider two networks, N1 and N2, that have the same average delay between a source A and a destination D. In N1, the delay experienced by different packets is unformly distributed with maximum delay
Several RPC implementations provide an option to the client to use RPC implemented over UDP or RPC implemented over TCP. Under what conditions will a client prefer to use RPC over UDP and under what
Some other policies for fairness in congestion control are Additive Increase Additive Decrease (AIAD), Multiplicative Increase Additive Decrease (MIAD), and Multiplicative Increase Multiplicative
One criteria for deciding whether to have a server active all the time or have it start on demand using a process server is how frequently the service provided is used. Can you think of any other
Primitives of transport service assume asymmetry between the two end points during connection establishment, one end (server) executes LISTEN while the other end (client) executes CONNECT. However,
In IP, the checksum covers only the header and not the data. Why do you suppose this design was chosen?
While IP addresses are tried to specific networks, Ethernet addresses are not. Can you think of a good reason why they are not?
A router can process 2 million packets/sec. The load offered to it is 1.5 million packets/ sec on average. If a route from source to destination contains 10 routers, how much time is spent being
The network of Fig. 5-34 uses RSVP with multicast trees for hosts 1 and 2 as shown. Suppose that host 3 requests a channel of bandwidth 2 MB/sec for a flow from host 1 and another channel of
A token bucket scheme is used for traffic shaping. A new token is put into the bucket every 5 μsec. Each token is good for one short packet, which contains 48 bytes of data. What is the maximum
Describe two major differences between the ECN method and the RED method of congestion avoidance.
Compute a multicast spanning tree for router C in the following network for a group with members at routers A, B, C, D, E, F, I, and K. в D A к н ш
Consider the network of Fig. 5-15(a). Imagine that one new line is added, between F and G, but the sink tree of Fig. 5-15(b) remains unchanged. What changes occur to Fig. 5-15(c)Figure 5-15 A A D (н
In Fig. 5-13 the Boolean OR of the two sets of ACF bits are 111 in every row. Is this just an accident here, or does it hold for all networks under all circumstances? Figure 5-13 Send flags ACK
Consider the network of Fig. 5-12(a). Distance vector routing is used, and the following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E:
In Fig. 4-48, the switch in the legacy end domain on the right is a VLAN-aware switch. Would it be possible to use a legacy switch there? If so, how would that work? If not, why not?Figure 4-48
To make VLANs work, configuration tables are needed in the bridges. What if the VLANs of Fig. 4-47 used hubs rather than switches? Do the hubs need configuration tables, too? Why or why not?Figure
It is mentioned in Section 4.8.3 that some bridges may not even be present in the spanning tree. Outline a scenario where a bridge may not be present in the spanning tree.
Consider the extended LAN connected using bridges B1 and B2 in Fig. 4-41(b). Suppose the hash tables in the two bridges are empty. List all ports on which a packet will be forwarded for the following
List some of the security concerns of an RFID system.
Suppose that there are 10 RFID tags around an RFID reader. What is the best value of Q? How likely is it that one tag responds with no collision in a given slot?
It is mentioned in Section 4.6.6 that the efficiency of a 1-slot frame with repetition encoding is about 13% at basic data rate. What will the efficiency be if a 5-slot frame with repetition encoding
Figure 4-24 shows several physical layer protocols. Which of these is closest to the Bluetooth physical layer protocol? What is the biggest difference between the two?Figure 4-24 Upper layers Logical
What is the maximum size of the data field for a 3-slot Bluetooth frame at basic rate? Explain your answer.
From Fig. 4-34, we see that a Bluetooth device can be in two piconets at the same time. Is there any reason why one device cannot be the master in both of them at the same time?Figure 4-34 Piconet 1
List two ways in which WiMAX is similar to 802.11, and two ways in which it is different from 802.11.
A wireless LAN with one AP has 10 client stations. Four stations have data rates of 6 Mbps, four stations have data rates of 18 Mbps, and the last two stations have data rates of 54 Mbps. What is the
Give an example to show that the RTS/CTS in the 802.11 protocol is a little different than in the MACA protocol.
Some books quote the maximum size of an Ethernet frame as 1522 bytes instead of 1500 bytes. Are they wrong? Explain your answer.
Sketch the Manchester encoding on a classic Ethernet for the bit stream 0001110101.
What is the baud rate of classic 10-Mbps Ethernet?
Six stations, A through F, communicate using the MACA protocol. Is it possible for two transmissions to take place simultaneously? Explain your answer.
Consider five wireless stations, A, B, C, D, and E. Station A can communicate with all other stations. B can communicate with A, C and E. C can communicate with A, B and D. D can communicate with A,
What is the length of a contention slot in CSMA/CD for (a) A 2-km twin-lead cable (signal propagation speed is 82% of the signal propagation speed in vacuum)?, (b) A 40-km multi mode fiber
A 100-byte IP packet is transmitted over a local loop using ADSL protocol stack. How many ATM cells will be transmitted? Briefly describe their contents.
Compute the fraction of the bandwidth that is wasted on overhead (headers and re-transmissions) for protocol 6 on a heavily loaded 50-kbps satellite channel with data frames consisting of 40 header
The distance from earth to a distant planet is approximately 9 × 1010 m. What is the channel utilization if a stop-and-wait protocol is used for frame transmission on a 64 Mbps point-to-point link?
In the discussion of ARQ protocol in Section 3.3.3, a scenario was outlined that resulted in the receiver accepting two copies of the same frame due to a loss of acknowledgement frame. Is it possible
A 1024-bit message is sent that contains 992 data bits and 32 CRC bits. CRC is computed using the IEEE 802 standardized, 32-degree CRC polynomial. For each of the following, explain whether the
Suppose that a message 1001 1100 1010 0011 is transmitted using Internet Checksum (4-bit word). What is the value of the checksum?
Using the convolutional coder of Fig. 3-7, what is the output sequence when the input sequence is 10101010 (left to right) and the internal state is initially all zero?Figure 3-7 Output bit 1 Σ
Suppose that data are transmitted in blocks of sizes 1000 bits. What is the maximum error rate under which error detection and re-transmission mechanism (1 parity bit per block) is better than using
What is the maximum overhead in byte-stuffing algorithm?
How fast can a cable user receive data if the network is otherwise idle? Assume that the user interface is(a) 10-Mbps Ethernet(b) 100-Mbps Ethernet(c) 54-Mbps Wireless.
Using the spectral allocation shown in Fig. 2-52 and the information given in the text, how many Mbps does a cable system allocate to upstream and how many to downstream?Figure 2-52 5 42 54 88 108
In Figure 2-28, there are four stations that can transmit. Suppose four more stations are added. Provide the chip sequences of these stations.Figure 2-28 A = (-1-1 -1 +1 +1 -1 +1 +1) B = (-1 -1 +1-1
A CDMA receiver gets the following chips: (ˆ’1 +1 ˆ’3 +1 ˆ’1 ˆ’3 +1 +1). Assuming the chip sequences defined in Fig. 2-28(a), which stations transmitted, and
Suppose that A, B, and C are simultaneously transmitting 0 bits, using a CDMA system with the chip sequences of Fig. 2-28(a). What is the resulting chip sequence?Figure 2-28(a) A = (-1-1 -1 +1 +1-1
The actual layout of cells is seldom as regular that as shown in Fig. 2-45. Even the shapes of individual cells are typically irregular. Give a possible reason why this might be. How do these
In Fig. 2-40, the user data rate for OC-3 is stated to be 148.608 Mbps. Show how this number can be derived from the SONET OC-3 parameters. What will be the gross, SPE, and user data rates of an
Calculate the transmit time in the previous problem if packet switching is used instead. Assume that the packet size is 64 KB, the switching delay in the satellite and hub is 10 microseconds, and the
How long will it take to transmit a 1-GB file from one VSAT to another using a hub as shown in Figure 2-17? Assume that the up link is 1 Mbps, the down link is 7 Mbps, and circuit switching is used
SONET clocks have a drift rate of about 1 part in 109. How long does it take for the drift to equal the width of 1 bit? Do you see any practical implications of this calculation? If so, what?
What is the percent overhead on a T1 carrier? That is, what percent of the 1.544 Mbps are not delivered to the end user? How does it relate to the percent overhead in OC-1 or OC-768 lines?
What is the maximum bit rate achievable in a V.32 standard modem if the baud rate is 1200 and no error correction is used?
A modem constellation diagram similar to Fig. 2-23 has data points at the following coordinates: (1, 1), (1, ˆ’1), (ˆ’1, 1), and (ˆ’1, ˆ’1). How many bps can a
The cost of a fast microprocessor has dropped to the point where it is now possible to put one in each modem. How does that affect the handling of telephone line errors? Does it negate the need for
Is an oil pipeline a simplex system, a half-duplex system, a full-duplex system, or none of the above? What about a river or a walkie-talkie-style communication?

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