Questions and Answers of Computer Network

Show the tag that is needed to make the string 'ACM' be a hyperlink to www.acm.org.
Design a form for a new company, Interburger, that allows hamburgers to be ordered via the Internet. The form should include the customer's name, address, and city, as well as a choice of size
Design a form that requests the user to type in two numbers. When the user clicks on the submit button, the server returns their sum. Write the server side as a PHP script.
For each of the following applications, tell whether it would be (1) possible and (2) better to use a PHP script or JavaScript and why. (a) Displaying a calendar for any requested month since
Write a program in JavaScript that accepts an integer greater than 2 and tells whether it is a prime number. Note that JavaScript has if and while statements with the same syntax as C and Java. The
An HTML page is as follows: "www.info-source.com/welcome.html" If the user clicks on the link, a TCP connection is opened and a series of lines is sent to the server. List all the lines sent.
The If-Modified-Since header can be used to check whether a cached page is still valid. Requests can be made for pages containing images, sound, video, and so on, as well as HTML. Do you think the
On the day of a major sporting event, such as the championship game in some popular sport, many people go to the official Web site. Is this a flash crowd in the same sense as the Florida election in
Does it make sense for a single ISP to function as a CDN? If so, how would that work? If not, what is wrong with the idea?
Under what conditions is using a CDN a bad idea?
Wireless Web terminals have low bandwidth, which makes efficient coding important. Devise a scheme for transmitting English text efficiently over a wireless link to a WAP device. You may assume that
A compact disc holds 650 MB of data. Is compression used for audio CDs? Explain your reasoning.
In Fig. 7-57(c) quantization noise occurs due to the use of 4-bit samples to represent nine signal values. The first sample, at 0, is exact, but the next few are not. What is the percent error for
Could a psychoacoustic model be used to reduce the bandwidth needed for Internet telephony? If so, what conditions, if any, would have to be met to make it work? If not, why not?
An audio streaming server has a one-way distance of 50 msec with a media player. It outputs at 1 Mbps. If the media player has a 1-MB buffer, what can you say about the position of the low-water mark
The interleaving algorithm of Fig. 7-60 has the advantage of being able to survive an occasional lost packet without introducing a gap in the playback. However, when used for Internet telephony, it
Does voice over IP have the same problems with firewalls that streaming audio does? Discuss your answer.
What is the bit rate for transmitting uncompressed 800 x 600 pixel color frames with 8 bits/pixel at 40 frames/sec?
Can a 1-bit error in an MPEG frame affect more than the frame in which the error occurs? Explain your answer.
Consider a 100,000-customer video server, where each customer watches two movies per month. Half the movies are served at 8 P.M. How many movies does the server have to transmit at once during this
Suppose that Zipf's law holds for accesses to a 10,000-movie video server. If the server holds the most popular 1000 movies on magnetic disk and the remaining 9000 on optical disk, give an expression
Break the following monoalphabetic cipher. The plaintext, consisting of letters only, is a well-known excerpt from a poem by Lewis Carroll. kfd ktbd fzm eubd kfd pzyiom mztx ku kzyg ur bzha kfthcm
Break the following columnar transposition cipher. The plaintext is taken from a popular computer textbook, so ''computer'' is a probable word. The plaintext consists entirely of letters (no spaces).
Find a 77-bit one-time pad that generates the text ''Donald Duck'' from the cipher text of Fig. 8-4.
Quantum cryptography requires having a photon gun that can, on demand, fire a single photon carrying 1 bit. In this problem, calculate how many photons a bit carries on a 100-Gbps fiber link. Assume
If Trudy captures and regenerates photons when quantum cryptography is in use, she will get some of them wrong and cause errors to appear in Bob's one-time pad. What fraction of Bob's one-time pad
A fundamental cryptographic principle states that all messages must have redundancy. But we also know that redundancy helps an intruder tell if a guessed key is correct. Consider two forms of
In Fig. 8-6, the P-boxes and S-boxes alternate. Although this arrangement is esthetically pleasing, is it any more secure than first having all the P-boxes and then all the S-boxes?
Design an attack on DES based on the knowledge that the plaintext consists exclusively of upper case ASCII letters, plus space, comma, period, semicolon, carriage return, and line feed. Nothing is
In the text we computed that a cipher-breaking machine with a billion processors that could analyze a key in 1 picosecond would take only 1010 years to break the 128-bit version of AES. However,
AES supports a 256-bit key. How many keys does AES-256 have? See if you can find some number in physics, chemistry, or astronomy of about the same size. Use the Internet to help search for big
Suppose that a message has been encrypted using DES in cipher text block chaining mode. One bit of cipher text in block Ci is accidentally transformed from a 0 to a 1 during transmission. How much
Now consider cipher text block chaining again. Instead of a single 0 bit being transformed into a 1 bit, an extra 0 bit is inserted into the cipher text stream after block Ci. How much plaintext will
Compare cipher block chaining with cipher feedback mode in terms of the number of encryption operations needed to transmit a large file. Which one is more efficient and by how much?
Using the RSA public key cryptosystem, with a = 1, b = 2, etc., a. If p = 7 and q = 11, list five legal values for d. b. If p = 13, q = 31, and d = 7, find e. c. Using p = 5, q = 11, and d = 27,
Suppose a user, Maria, discovers that her private RSA key (d 1, n 1) is same as the public RSA key (e 2, n 2) of another user, Frances. In other words, d 1 = e 2 and n 1 = n 2. Should Maria consider
Consider the use of counter mode, as shown in Fig. 8-15, but with IV = 0. Does the use of 0 threaten the security of the cipher in general?
The signature protocol of Fig. 8-18 has the following weakness. If Bob crashes, he may lose the contents of his RAM. What problems do this cause and what can he do to prevent them?
In Fig. 8-20, we see how Alice can send Bob a signed message. If Trudy replaces P, Bob can detect it. But what happens if Trudy replaces both P and the signature?
Digital signatures have a potential weakness due to lazy users. In e-commerce transactions, a contract might be drawn up and the user asked to sign its SHA-1 hash. If the user does not actually
A math class has 20 students. What is the probability that at least two students have the same birthday? Assume that nobody was born on leap day, so there are 365 possible birthdays.
After Ellen confessed to Marilyn about tricking her in the matter of Tom's tenure, Marilyn resolved to avoid this problem by dictating the contents of future messages into a dictating machine and
Consider the failed attempt of Alice to get Bob's public key in Fig. 8-23. Suppose that Bob and Alice already share a secret key, but Alice still wants Bob's public key. Is there now a way to get it
Alice wants to communicate with Bob, using public-key cryptography. She establishes a connection to someone she hopes is Bob. She asks him for his public key and he sends it to her in plaintext along
Suppose that a system uses PKI based on a tree-structured hierarchy of CAs. Alice wants to communicate with Bob, and receives a certificate from Bob signed by a CA X after establishing a
Can IPsec using AH be used in transport mode if one of the machines is behind a NAT box? Explain your answer.
Give one advantage of HMACs over using RSA to sign SHA-1 hashes.
Give one reason why a firewall might be configured to inspect incoming traffic. Give one reason why it might be configured to inspect outgoing traffic. Do you think the inspections are likely to be
The WEP packet format is shown in Fig. 8-31. Suppose that the checksum is 32 bits, computed by XORing all the 32-bit words in the payload together. Also suppose that the problems with RC4 are
Suppose an organization uses VPN to securely connect its sites over the Internet. Is there a need for a user, Jim, in this organization to use encryption or any other security mechanism to
Change one message in protocol of Fig. 8-34 in a minor way to make it resistant to the reflection attack. Explain why your change works.
The Diffie-Hellman key exchange is being used to establish a secret key between Alice and Bob. Alice sends Bob (719, 3 191). Bob responds with (543). Alice's secret number, x, is 16. What is the
If Alice and Bob have never met, share no secrets, and have no certificates, they can nevertheless establish a shared secret key using the Diffie-Hellman algorithm. Explain why it is very hard to
In the protocol of Fig. 8-39, why is A sent in plaintext along with the encrypted session key? In the protocol of Fig. 8-39, we pointed out that starting each plaintext message with 32 zero bits is
In the Needham-Schroeder protocol, Alice generates two challenges, RA and RA2. This seems like overkill. Would one not have done the job?
Suppose an organization uses Kerberos for authentication. In terms of security and service availability, what is the effect if AS or TGS goes down?
In the public-key authentication protocol of Fig. 8-43, in message 7, RB is encrypted with KS. Is this encryption necessary, or would it have been adequate to send it back in plaintext? Explain your
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 capture and store all the information on the card as well as
Give two reasons why PGP compresses messages.
Assuming that everyone on the Internet used PGP, could a PGP message be sent to an arbitrary Internet address and be decoded correctly by all concerned? Discuss your answer.
The attack shown in Fig. 8-47 leaves out one step. The step is not needed for the spoof to work, but including it might reduce potential suspicion after the fact. What is the missing step?
It has been proposed to foil DNS spoofing using ID prediction by having the server put in a random ID rather than using a counter. Discuss the security aspects of this approach.
The SSL data transport protocol involves two nonces as well as a pre-master key. What value, if any, does using the nonces have?
The image of Fig. 8-55(b) contains the ASCII text of five plays by Shakespeare. Would it be possible to hide music among the zebras instead of text? If so, how would it work and how much could you
Alice was a heavy user of a type 1 anonymous remailer. She would post many messages to her favorite newsgroup, alt.fanclub.alice, and everyone would know they all came from Alice because they all
Search the Internet for an interesting case involving privacy and write a 1-page report on it.
Describe the most popular wireless Internet access technologies today. Compare and contrast them.
What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM have over FDM in a circuit-switched network?
Why is it said that packet switching employs statistical multiplexing? Contrast statistical multiplexing with the multiplexing that takes place in TDM.
Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host
What is the key distinguishing difference between a tier-l ISP and a tier-2 ISP?
Suppose users share a 2 Mbps link. Also suppose each user transmits continuously at I Mbps when transmitting, but each user transmits only 20 percent of the time. (See the discussion of statistical
Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates RI =500 kbps, R2 =2 Mbps, and R) = I Mbps. a. Assuming no other traffic in the network,
The word protocol is often used to describe diplomatic relations. Give an example of a diplomatic protocol.
Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a packet switch,
List five tasks that a layer can perform. Is it possible that one (or more) of these tasks could be performed by two (or more) layers?
What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?
What is an application-layer message? A transport-layer segment? A networklayer datagram? A link-layer frame?
Which layers in the Internet protocol stack does a router process? Which layers does a link-layer switch process? Which layers does a host process?
What is the difference between a virus, a worm, and a Trojan horse?
Describe how a botnet can be created, and how it can be used for a DDoS attack.
Suppose Alice and Bob are sending packets to each other over a computer network. Suppose Trudy position. herself in the network so that she can capture all the packets sent by Alice and send whatever
What is a client program? What is a server program? Does a server program request and receive services from a client program?
List six access technologies. Classify each one as residential access, company access, or mobile access.
Is HFC transmission rate dedicated or shared among users? Are collisions possible in a downstream HFC channel? Why or why not?
What is the transmission rate of Ethernet LANs? For a given transmission rate, can each user on the LAN continuously transmit at that rate?
Dial-up modems, HFC, DSL and FITH are all used for residential access. For each of these access technologies, provide a range of transmission rates and comment on whether the transmission rate is
Design and describe an application-level protocol to be used between an automatic teller machine and a bank's centralized computer. Your protocol should allow a user 's card and password to be
In the above problem, suppose R1 =R2 =R3 =R and dproc = 0. Further suppose the packet switch does not store-and-forward packets but instead immediately transmits each bit it receives before waiting
A packet switch receives a packet and determines the outbound link to which the packet should be forwarded. When the packet arrives, one other packet is halfway done being transmitted on this
Suppose N packets arrive simultaneously to a link at which no packets are currently being transmitted or queued. Each packet is of length L and the link has transmission rate R. What is the average
Consider the queuing delay in a router buffer (preceding an outbound link). Suppose all packets are L bits, the transmission rate is R bps, and that N packets simultaneously arrive at the buffer
Consider the queuing delay in a router buffer. Let f denote traffic intensity; that is, l < 1. Suppose that the queuing delay takes the form lL/R (1 -I) for f < 1. a. Provide a formula for the total
Let a denote the rate of packets arriving at a link in packets/sec, and let μ denote the link's transmission rate in packets/sec. Based on the formula for the total delay (i.e., the queuing delay
Consider a router buffer preceding an outbound link. In this problem, you will use Little's formula, a famous formula from queuing theory. Let N denote the average number of packets in the buffer
a. Generalize the end-to-end delay formula in Section 1.4.3 for heterogeneous processing rates, transmission rates, and propagation delays. b. Repeat (a), but now also suppose that there is an
Perform a Traceroute between source and destination on the same continent at three different hours of the Jay. a. Find the average and standard deviation of the round-trip delays at each of the three
Consider an application that transmits data at a steady rate (for example, the sender generates an N-bit unit of data every k time units, where k is small and fixed). Also, when such an application
Now suppose that there are M paths between the server and the client. No two paths share any link. Path k (k = 1, . . . , M ) consists of N links with transmission rates Rk1, Rk2, .. . , RkN. If the
Suppose that each link between the server and the client has a packet loss probability p, and the packet loss probabilities for these links are independent. What is the probability that a packet

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