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computer networking

Questions and Answers of Computer Networking

A natural question is whether we can use a nonce and public key cryptography to solve the end-point authentication problem in Section 8.4. Consider the following natural protocol: (1) Alice sends the
How long does it take a packet of length 1000 bytes to propagate over a link of distance 2,500 km. propagation speed 2.5 • 108 m/s. and transmission raw 2 M bps? More generally, how long does it
Refer to Figure 3.55, which illustrates the convergence of TCP’ s AIMD algorithm. Suppose that instead of a multiplicative decrease, TCP decreased the window size by a constant amount. Would the
Consider again the scenario from P19 above. Give the flow tables entries at packet switches s1 and s3, such that any arriving data-grams with a source address of h3 or h4 are routed to the
For the network in Figure 6.19. the router has two ARP modules, each with its own ARP table. Is it possible that the same MAC address appears in both tables?Figure 6.19
Visit the Transmission Versus Propagation Delay applet at the companion Web site. Among the rates, propagation delay, and packet sizes available, find a combination for which the sender finishes
What field in the IP header can be used to ensure that a packet is forwarded through no more than N routers?
Visit a host that uses DHCP to obtain its IP address, network mask, default router, and IP address of its local DNS server. List these values.
Consider again the SDN Open Flow network shown in Figure 4.30. Suppose that the desired forwarding behavior for datagrams arriving from hosts h3 or h4 at s2 is follows:Any datagrarns arriving from
Consider again the SDN Open Flow network shown in Figure 4.30. Suppose we want switch s2 to function as a firewall. Specify the flow table in s2 that implements the following firewall behaviors
Consider Figure 6.15. How many sub networks are there, in the addressing sense of Section 4.3?
Graph the efficiency of slotted ALOHA and pure ALOHA as a function of p for the following values of N:a. N = 15.b. N = 25.c. N=35.
As a mobile node gets farther and farther away from a base station, what are two actions that a base station could take to ensure that the loss probability of a transmitted frame does not increase?
Describe the format of the 802.15.1 Bluetooth frame. You will have to do some reading outside of the text to find this information. Is there anything in the frame format that inherently limits the
Consider again the figure in P11, showing packet audio transmission and reception times.a. Compute the estimated delay for packets 2 through 8, using the formula for di from Section 9.3.2. Use a
Repeat Example 25.4 using the concurrent approach.
Repeat Problem 25.16 using the concurrent approach.
If a student in a college can monopolize access to a piece of software, causing other students to wait for a long time, which area of network management has failed?
Repeat Example 29.2 in the text, but assume that node N5 needs to find the responsible node for key k16.
When a sealed letter is sent from Alice to Bob, is this an example of using cryptography or steganography for confidentiality?
When a letter is sent from Bob to Alice in a language that only the two can understand, is this an example of cryptography or steganography?
Alice has found a way to write secretly to Bob. Each time, she takes a new text, such as an article from the newspaper, but inserts one or two spaces between the words. A single space means a binary
Use anonymous FTP to connect to ftp.rfc-editor.org (directory innotes), and retrieve the RFC index. Also retrieve the protocol specifications for TCP, IP, and UDP.
Use a Web search tool to locate useful, general, and noncommercial information about the following topics: MBone, ATM, MPEG, IPv6, and Ethernet.
The Unix utility whois can be used to find the domain name corresponding to an organization or vice versa. Read the man page documentation for whois and experiment with it. Try whois princeton.edu
Calculate the total time required to transfer a 1000-kB file in the following cases, assuming an RTT of 100 ms, a packet size of 1 kB data, and an initial 2 × RTT of “handshaking” before data
Calculate the total time required to transfer a 1.5 MB file in the following cases, assuming an RTT of 80 ms, a packet size of 1 kB data, and an initial 2 × RTT of “handshaking” before data are
Consider a point-to-point link 2 km in length. At what bandwidth would propagation delay (at a speed of 2×108 m/sec) equal transmit delay for 100-byte packets? What about 512-byte packets?
Consider a point-to-point link 50 km in length. At what bandwidth would propagation delay (at a speed of 2×108 m/sec) equal transmit delay for 100-byte packets? What about 512-byte packets?
What properties of postal addresses would be likely to be shared by a network addressing scheme? What differences might you expect to find? What properties of telephone numbering might be shared by a
One property of addresses is that they are unique; if two nodes had the same address, it would be impossible to distinguish between them. What other properties might be useful for network addresses
Give an example of a situation in which multicast addresses might be beneficial.
What differences in traffic patterns account for the fact that STDM is a cost-effective form of multiplexing for a voice telephone network and FDM is a cost-effective form of multiplexing for
How “wide” is a bit on a 1-Gbps link? How long is a bit in copper wire, where the speed of propagation is 2.3 × 108 m/s?
How long does it take to transmit x kB over a y-Mbps link? Give your answer as a ratio of x and y.
Suppose a 100-Mbps point-to-point link is being set up between Earth and a new lunar colony. The distance from the moon to the Earth is approximately 385,000 km, and data travel over the link at the
Suppose a 128-kbps point-to-point link is set up between Earth and a rover on Mars. The distance from Earth to Mars (when they are closest together) is approximately 55 Gm, and data travel over the
For each of the following operations on a remote file server, discuss whether they are more likely to be delay-sensitive or bandwidthsensitive.(a) Open a file.(b) Read the contents of a file.(c)
Calculate the latency (from first bit sent to last bit received) for the following:(a) 10-Mbps Ethernet with a single store-and-forward switch in the path and a packet size of 5000 bits. Assume that
Calculate the latency (from first bit sent to last bit received) for:(a) 1-Gbps Ethernet with a single store-and-forward switch in the path and a packet size of 5000 bits. Assume that each link
Calculate the effective bandwidth for the following cases. For (a) and (b), assume there is a steady supply of data to send; for (c), simply calculate the average over 12 hours.(a) 10-Mbps Ethernet
Calculate the bandwidth × delay product for the following links. Use one-way delay, measured from first bit sent to first bit received.(a) 10-Mbps Ethernet with a delay of 10 µs.(b) 10-Mbps
Hosts A and B are each connected to a switch S via 10-Mbps links as in Figure 1.21. The propagation delay on each link is 20 µs. S is a store-and-forward device; it begins retransmitting a received
Suppose a host has a 1-MB file that is to be sent to another host. The file takes 1 second of CPU time to compress 50%, or 2 seconds to compress 60%.(a) Calculate the bandwidth at which each
Suppose that a certain communications protocol involves a perpacket overhead of 100 bytes for headers and framing. We send 1 million bytes of data using this protocol; however, one byte is
Assume you wish to transfer an n-B file along a path composed of the source, the destination, seven point-to-point links, and five switches.Suppose each link has a propagation delay of 2 ms, a
Consider a network with a ring topology, link bandwidths of 100 Mbps, and propagation speed 2 × 108 m/s. What would the circumference of the loop be to exactly contain one 250-byte packet, assuming
Compare the channel requirements for voice traffic with the requirements for the real-time transmission of music in terms of bandwidth, delay, and jitter. What would have to improve? By
For the following, assume that no data compression is done; this would in practice almost never be the case. For (a)–(c), calculate the bandwidth necessary for transmitting in real-time:(a) video
For the following, as in the previous problem, assume that no data compression is done. Calculate the bandwidth necessary for transmitting in real-time:(a) HDTV high-definition video at a resolution
Discuss the relative performance needs of the following applications in terms of average bandwidth, peak bandwidth, latency, jitter, and loss tolerance:(a) file server.(b) print server.(c) digital
Suppose a shared medium M offers to hosts A1,A2,...,AN in roundrobin fashion an opportunity to transmit one packet; hosts that have nothing to send immediately relinquish M. How does this differ
Consider a simple protocol for transferring files over a link. After some initial negotiation, A sends data packets of size 1 kB to B; B then replies with an acknowledgment. A always waits for each
Suppose hosts A and B are connected by a link. Host A continuously transmits the current time from a high-precision clock, at a regular rate, fast enough to consume all the available bandwidth. Host
Obtain and build the simplex-talk sample socket program shown in the text. Start one server and one client, in separate windows. While the first client is running, start 10 other clients that connect
Modify the simplex-talk socket program so that each time the client sends a line to the server, the server sends the line back to the client.The client (and server) will now have to make alternating
Modify the simplex-talk socket program so that it uses UDP as the transport protocol rather than TCP. You will have to change SOCK_STREAM to SOCK_DGRAM in both client and server. Then, in the server,
Investigate the different options and parameters one can set for a TCP connection. (Do “man tcp” on Unix.) Experiment with various parameter settings to see how they affect TCP performance.
The Unix utility ping can be used to find the RTT to various Internet hosts. Read the man page for ping, and use it to find the RTT to www.cs.princeton.edu in New Jersey and www.cisco.com in
The Unix utility traceroute, or its Windows equivalent tracert, can be used to find the sequence of routers through which a message is routed. Use this to find the path from your site to some others.
Use traceroute, above, to map out some of the routers within your organization (or to verify none are used).
Show the NRZ, Manchester, and NRZI encodings for the bit pattern shown in Figure 2.36. Assume that the NRZI signal starts out low. Figure 2.36 Bits NRZ Clock Manchester NRZI I 1 0 0 1 1 1 1 1 0 0
Show the 4B/5B encoding and the resulting NRZI signal for the following bit sequence:1110 0101 0000 0011.
Show the 4B/5B encoding and the resulting NRZI signal for the following bit sequence:1101 1110 1010 1101 1011 1110 1110 1111.
In the 4B/5B encoding (Table 2.2), only two of the 5-bit codes used end in two 0s. How many possible 5-bit sequences are there (used by the existing code or not) that meet the stronger restriction of
Assuming a framing protocol that uses bit stuffing, show the bit sequence transmitted over the link when the frame contains the following bit sequence:110101111101011111101011111110.Mark the stuffed
Suppose the following sequence of bits arrives over a link:1101011111010111110010111110110.Show the resulting frame after any stuffed bits have been removed.Indicate any errors that might have been
Suppose the following sequence of bits arrive over a link:011010111110101001111111011001111110.Show the resulting frame after any stuffed bits have been removed.Indicate any errors that might have
Suppose you want to send some data using a byte-oriented framing protocol, and the last 2 bytes of your data are DLE and ETX. What sequence of bytes would be transmitted immediately prior to the CRC?
Give an example of a byte/bit sequence that should never appear in a transmission for an HDLC frame.
Assume that a SONET receiver resynchronizes its clock whenever a 1 bit appears; otherwise, the receiver samples the signal in the middle of what it believes is the bit’s timeslot.(a) What relative
Show that the Internet checksum will never be 0xFFFF (that is, the final value of sum will not be 0x0000) unless every byte in the buffer is 0.Internet specifications in fact require that a checksum
Prove the Internet checksum computation shown in the text is independent of byte order (host order or network order) except that the bytes in the final checksum should be swapped later to be in the
Suppose that one byte in a buffer covered by the Internet checksum algorithm needs to be decremented (e.g., a header hop count field).Give an algorithm to compute the revised checksum without
Show that the Internet checksum can be computed by first taking the 32-bit ones’ complement sum of the buffer in 32-bit units, then taking the 16-bit ones’ complement sum of the upper and lower
Suppose we want to transmit the message 11001001 and protect it from errors using the CRC polynomial x3 + 1.(a) Use polynomial long division to determine the message that should be transmitted.(b)
Suppose we want to transmit the message 1011 0010 0100 1011 and protect it from errors using the CRC8 polynomial x8 + x2 + x1 + 1.(a) Use polynomial long division to determine the message that
The CRC algorithm as presented in this chapter requires lots of bit manipulations. It is, however, possible to do polynomial long division taking multiple bits at a time, via a table-driven method,
With 1 parity bit, we can detect all 1-bit errors. Show that at least one generalization fails, as follows:(a) Show that if messages m are 8 bits long, then there is no error detection code e = e(m)
Consider an ARQ protocol that uses only negative acknowledgments(NACKs), but no positive acknowledgments (ACKs). Describe what timeouts would need to be scheduled. Explain why an ACK-based protocol
Consider an ARQ algorithm running over a 20-km point-to-point fiber link.(a) Compute the propagation delay for this link, assuming that the speed of light is 2 × 108 m/s in the fiber.(b) Suggest a
Suppose you are designing a sliding window protocol for a 1-Mbps point-to-point link to the moon, which has a one-way latency of 1.25 seconds. Assuming that each frame carries 1 kB of data, what is
Suppose you are designing a sliding window protocol for a 1-Mbps point-to-point link to the stationary satellite evolving around the Earth at 3×104 km altitude. Assuming that each frame carries 1 kB
The text suggests that the sliding window protocol can be used to implement flow control. We can imagine doing this by having the receiver delay ACKs, that is, not send the ACK until there is free
Implicit in the stop-and-wait scenarios of Figure 2.14 is the notion that the receiver will retransmit its ACK immediately on receipt of the duplicate data frame. Suppose instead that the receiver
In stop-and-wait transmission, suppose that both sender and receiver retransmit their last frame immediately on receipt of a duplicate ACK or data frame; such a strategy is superficially reasonable
Give some details of how you might augment the sliding window protocol with flow control by having ACKs carry additional information that reduces the SWS as the receiver runs out of buffer
Describe a protocol combining the sliding window algorithm with selective ACKs. Your protocol should retransmit promptly, but not if a frame simply arrives one or two positions out of order. Your
Draw a timeline diagram for the sliding window algorithm with SWS = RWS = 3 frames for the following two situations. Use a timeout interval of about 2 × RTT.(a) Frame 4 is lost.(b) Frames 4–6 are
Draw a timeline diagram for the sliding window algorithm with SWS = RWS = 4 frames in the following two situations. Assume the receiver sends a duplicate acknowledgment if it does not receive the
Suppose that we attempt to run the sliding window algorithm with SWS = RWS = 3 and with MaxSeqNum = 5. The Nth packet DATA[N]thus actually contains N mod 5 in its sequence number field.Give an
Consider the sliding window algorithm with SWS = RWS = 3, with no out-of-order arrivals, and with infinite-precision sequence numbers.(a) Show that if DATA[6] is in the receive window, then DATA[0]
Suppose that we run the sliding window algorithm with SWS = 5 and RWS = 3, and no out-of-order arrivals.(a) Find the smallest value for MaxSeqNum. You may assume that it suffices to find the smallest
Suppose A is connected to B via an intermediate router R, as shown in Figure 2.37. The A–R and R–B links each accept and transmit only one packet per second in each direction (so two packets take
Suppose A is connected to B via an intermediate router R, as in the previous problem. The A–R link is instantaneous, but the R–B link transmits only one packet each second, one at a time (so two
Consider the situation in the previous exercise, except this time, assume that the router has a queue size of 1; that is, it can hold one packet in addition to the one it is sending (in each
Why is it important for protocols configured on top of the Ethernet to have a length field in their header, indicating how long the message is?
What kind of problems can arise when two hosts on the same Ethernet share the same hardware address? Describe what happens and why that behavior is a problem.

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