ECS173A程序写作、Computer Networks程序辅导

” ECS173A程序写作、Computer Networks程序辅导EEC152A/ECS173A – Computer Networks Summer Session I 202024-hour take-home Final Assessment: July 30, 2020Duration: 24 hours (July 30, 1:30pm – July 31, 1:30pm)a) Use the supplementary document (.docx file) provided with this exam as your answersheet. You can edit this document to include your answers. Submit a .pdf file.b) The answers should be typed. For diagrams and long calculations, you may hand-writeyour answers but make sure they are readable.c) Submit your answer file in Gradescope (similar to Homeworks). No late submissionallowed. Start the submission process early; do not wait till last minute.d) Be sure to sign the statement of academic integrity in the following page.e) This assignment is open book, open notes. Discussion with peers in any form is notallowed and plagiarism will not be tolerated.f) If you are using materials from other sources (e.g., book or online resources), do notdirectly copy; write in your own words. Cite any source.g) Justify your answers by qualitative or quantitative arguments or illustrations. Writeanswers briefly and precisely. Excessive description only confuses the reader.h) Show your reasoning clearly. If your reasoning is correct, but your final answer iswrong, you will receive partial credit. If you just show the answer without reasoning,and your answer is wrong, you may receive no points at all.Good luck!Do not write in this boxQuestion 1 2 3 4 5 6 7 TotalPoints 20 20 20 15 15 25 15 130ScoreSifat Ferdousi ECS152A/EEC173A SS120ECS173A作业写作、Computer Networks作业辅导Statement of academic integrity (please sign below)As a student at UC Davis, I hold myself to a high standard of integrity, and bysigning/accepting the statement below I reaffirm my pledge to act ethically byhonoring the UC Davis Code of Academic Conduct. I will also encourage otherstudents to avoid academic misconduct.I acknowledge that the work I submit is my individual effort. I did not consult withor receive any help from any person or other source. I also did not provide helpto others. I may work with others only if the instructor gave specific instructions,and only to the extent allowed by the instructor.I understand that suspected misconduct on this assignment/exam will bereported to the Office of Student Support and Judicial Affairs and, if established,will result in disciplinary sanctions up through Dismissal from the University anda grade penalty up to a grade of F for the course.I understand that if I fail to acknowledge or sign this statement, an instructor maynot grade this work and may assign a grade of 0 of F.Signature (sign/type above this line)Sifat Ferdousi ECS152A/EEC173A SS1203Problem 1 [20 points]In each of the following questions, justify your answers by qualitative or quantitative argumentsor illustrations. Answer briefly and precisely.(a) [4 points] Suppose a computer is moved from one department to another. Does itsphysical address need to change? Does the IP address need to change? Why or why not?(b) [4 points] Network administrators must often choose between switches and routers wheninstalling an interconnection device. When should an institutional network (for example, auniversity campus network or a corporate campus network) use switches, and whenshould it use routers?(c) [4 points] How does the local MAC-layer retransmission help enhance the throughputperformance of the end-to-end TCP connection?(d) [4 points] Why do HTTP and SMTP run on top of TCP rather than UDP? Why do DNS andDHCP run on top of UDP rather than TCP?(e) [4 points] When the loss rate is high and round trip time is long, which one is better: GoBack-Nor Selective-Repeat? Why?Sifat Ferdousi ECS152A/EEC173A SS1204Problem 2 [20 points](a) [10 points] Consider an end system A connected to a router by a link of rate R1 = 1 Mbps; andthis router is connected by the Internet to another router, which is connected to an endsystem B by a link of rate R2 = 10 Mbps. Let the round-trip time (RTT) between the two endsystems be 100 milliseconds. This problem will study the TCP performance between the twoend systems.1. [2 points] If the congestion window (cwnd) is static and always fixed at W = 1000 bytes,what is the end-to-end throughput?2. [2 points] Now consider the case where cwnd follows a saw-tooth pattern: with linearincrease under no congestion. Congestion always occurs when cwnd exactly reaches apeak of W = 1000 bytes and cwnd is halved to W/2 when congestion occurs. Draw thesaw-tooth pattern of cwnd.3. [2 points] For part 2, what is the end-to-end throughput?4. [2 points] Can we increase the throughputs in parts 1 and 3 by increasing R1 to 10 Mbps?Justify.5. [2 points] What is rwnd? What flow-control assumption is made in the abovecalculations?(b) [10 points] Imagine a TCP session over wireless where the congestion window is fixed at 5segments (congestion control is turned off and no fast retransmits). Segments may get lostbut are not reordered. The receiver has infinite buffer and it sends an acknowledgment assoon as it receives a segment, i.e., acknowledgments are not deferred. Similarly, sendertransmits a segment as soon as it is allowed to. Each segment carries 1000 bytes and the timeto transmit a segment is 2 ms. Assume that transmission of ACK takes negligible time. Notethat the retransmission timer for a segment is started after the last bit of the segment is sent.Assume Go-Back-5, and cumulative ACK are used.1. [4 points] Suppose two data segments with byte sequence numbers 3000 and 15000 arelost once during the transmission. How many segments get retransmitted under each ofthe following conditions?A. Round trip time = 100 ms, Timeout = 101 msB. Round trip time = 100 ms, Timeout = 152 ms2. [4 points] Suppose acknowledgments corresponding to the above data segments are lostinstead of the data segments. How many segments get retransmitted under the aboveconditions?A. Round trip time = 100 ms, Timeout = 101 msB. Round trip time = 100 ms, Timeout = 152 ms3. [2 points] Briefly describe how TCP is partly GBN and partly SR?Sifat Ferdousi ECS152A/EEC173A SS1205Problem 3 [20 points](a) [5 points] Consider the operation of a learning switch in the context of a network in which 6nodes labeled A through F are star connected into an Ethernet switch.Suppose that (i) B sends a frame to E, (ii) E replies with a frame to B, (iii) A sends a frame to B, and(iv) B replies with a frame to A. The switch table is initially empty. Show the state of the switchtable and after each of these events. For each of these events, identify the link(s) on which thetransmitted frame will be forwarded.Event Switch Table Link(s) on which transmitted frame willMAC Address Interface be forwarded for each event.(i) B 2(ii) E(iii) A(iv)(b) [15 points] Youd like to send the data bits 1011010010 using the Generator polynomial.1. [5 points] What is the degree of the Generator polynomial? What is the CRC codegenerated by the sender? Show each step.2. [5 points] What bits are transmitted to the receiver? Show the receivers side of thecalculation. Show each step.3. [5 points] What is the probability that there were errors in the transmission and thereceiver doesnt detect them?Sifat Ferdousi ECS152A/EEC173A SS1206Problem 4 [15 points](a) [9 points] We explore how to set the (configurable) link weights in link-state routing protocolslike OSPF and IS-IS inside a single Autonomous System (AS) to achieve AS-wide goals.1. [3 points] How should the network operators set the link weights if their goal is tominimize the number of hops each packet traverses to reach its destination? Justify youranswer.2. [3 points] How should the operators set the link weights to minimize the end-to-end delaythe traffic experiences? Assume the network is lightly loaded, so queuing delay isinsignificant.3. [3 points] How should the network operators set the link weights to combine differentmetrics (e.g., cost, delay, congestion, etc.) into one weight value which will be used bythe routing algorithms to find the best path?(b) [6 points] In the picture below, the nodes are routers, the edges are links, and the integerscorrespond to the link weight on each direction of the link. The arrows on the edges to showthe shortest path from every node to the destination node d.Suppose the link f-e is overloaded with traffic. Identify a single weight change (on just one link)that would divert traffic from source f to destination d away from the f-e edge without affectingthe path between any other source-destination pairs. Avoid any reliance on how routers choosebetween multiple paths with the same (smallest) cost.Sifat Ferdousi ECS152A/EEC173A SS1207Problem 5 [15 points](a) [5 points] In each of the following questions, mark all the statements that are correct.1. Which of the following statements is/are FALSE about routing protocols?i. When an OSPF router sends its link state information, it is sent only to those nodesthat are directly attached neighbors.ii. A dynamic algorithm can be run in direct response to link cost changes.iii. When a BGP router receives an advertised path from its neighbor, it must add its ownidentity to the received path and then send that new path on to all of its neighbors.iv. LS is more robust (against router malfunction) than DV because in LS, a node computesonly its own forwarding tables whereas in DV, each nodes table is used by others.2. Which of the following is/are FALSE about P2P applications?i. The server process must first be running.ii. DNS provides name to IP address mapping through a P2P model.iii. When the number of users, N, is large, P2P performance (e.g., in terms of filedistribution time) tends to outperform client-server model.iv. When the number of users, N, is small, P2P performance (e.g., in terms of filedistribution time) tends to under-perform client-server model.(b) [10 points] There are 11 terms in List A. Match them with the appropriate description in List B.As an example, the matching for the first term in List A is shown.List ANumber Item Your Response HereSifat Ferdousi ECS152A/EEC173A SS1208List B(Note: There are extra items in List B which dont match with any term in List A.)Number Description1 A program which initiates a connection with a server2 Transport protocol with minimal services3 Protocol for reliable data transfer between end hosts4 Router that hides the details of a local network from the rest of the Internet5 Algorithm that calculates the shortest path between end hosts6 Protocol that allows various different autonomous systems to communicate7 Method to code digital data with an analog signal8 Protocol using which a host obtains its IP address9 Method by which an end host determines the IP address from a host name10 Method for congestion control in TCP11 Method by which bandwidth is divided into frequency bands for communication12 Protocol that allows a node to obtain the MAC address of an interface, knowing its IP address13 Method for congestion control in TCP14 Part of a router which implements layers 1-3 functionality and is used to connect to otherrouters/switches/end systems15 Part of a router that connects its CPU, switch fabric, and line cards16 Countdown method to ensure that a packet does not circulate in the network foreverSifat Ferdousi ECS152A/EEC173A SS1209Problem 6 [25 points]Download the zip file https://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extractthe following files:a. https-ethereal-trace-2b. https-ethereal-trace-1c. dhcp-ethereal-trace-1Once you have downloaded these trace files, you can load it into Wireshark and view the traceusing the File pull down menu, choosing Open, and then selecting the required trace file.Start with https-ethereal-trace-2 file, answer the following questions (write answers and alsoprovide screenshots of the fields you are analyzing when necessary). Only for specific subparts,you will be asked to analyze a different trace file.Analysis1. [0.5×4] How many different protocols are reported? What are they?2. [1] General question (no analysis required): What is SNMP? Answer briefly in one line.3. [0.25] Of all the frames captured in the trace, how many are HTTP frames?4. [0.5×2] For the HTTP request/response messages, what is the user IP address? What is theserver IP address?5. [0.25×2] How many HTTP request messages? How many HTTP response messages?6. [0.5×2] What version of HTTP is running at the users browser? What browser type is theuser using?7. [0.5] What is the server host name?8. [0.5] What is the user requested object? Give the file name.9. [0.5] Is the object returned? How do you infer this information? Give the associated statuscode(s).a. [0.5] Answer this question for https-ethereal-trace-1 file also – is/are the user requestedobject(s) in this trace returned? Give associated status code(s).Refer back to https-ethereal-trace-2 file for the following questions:10. [0.25×2] Which frames in the trace (give the frame number) are the ARP frames? Whichframe is the query frame?a. [1] General question (no analysis required): Is ARP a link-layer protocol or a networklayerprotocol? Answer briefly in one line.b. [1] General question (no analysis required): Do net administrators create ARP tables?Answer briefly in one line.11. [0.5] What is the MAC addresses of the user?12. [0.5×2] When the user is sending an ARP query, it is looking for a MAC addresscorresponding to which IP address? To what type of network device do you think this IPaddress belongs to?Sifat Ferdousi ECS152A/EEC173A SS12010Refer to dhcp-ethereal-trace-1 file for the following questions:13. [0.5] Analyze the DHCP Offer message. Recall what information are generally included inDHCP Offer – list them. Which particular information in the DHCP Offer message in thistrace helps to verify your answer in 12?a. [0.5×2] What is the IP address of the DHCP server? Is the DHCP server integrated withthe router? How Do you infer this information?b. [0.5×2] What is the IP address offered by the DHCP server to the requesting user in thisDHCP Offer message? What is the lease time?Refer back to https-ethereal-trace-2 file for the following questions:14. [0.25×3] Which frames in the trace (give frame number) denote the TCP three-wayhandshake?15. [0.5×2] In the TCP SYNACK segment, how many flag bits are set to 1? Which one(s)?16. [0.25×2] What are the Seq#, ACK# in the third leg of the three-way handshake?17. [0.5] What is the maximum segment size?18. [0.5×2] What is the initial user window size value? Is it static?19. [0.5] What does the window size in 18 refer to: congestion window or receiver window?a. [2] General question (no analysis required): What is bandwidth-delay product? How is itrelated to TCP window size? Answer briefly.20. [0.5] Does this trace Contain TCP connection termination?21. [0.5×4] If you want to identify this TCP connection using the 4 tuples, what are the values?22. [2] As per this analysis, do we know the MAC address of the HTTP server? Is it00:06:25:da:af:73? Justify your answer.Sifat Ferdousi ECS152A/EEC173A SS12011Problem 7 Bonus Question [15 points](a) [12 points] Given the sequence of events listed below, plot the evolution of TCPs congestionwindow. Assume TCP Reno. The x-axis is Time (in RTT) and y-axis is Congestion Window Size(in segments). Put the plot in your answer script. Make sure your plot has gridlines and all thedata points are marked. Your plot should look something like Figure 3.58 in the textbook.Starting at t = 1, the initial value of cwnd is 1 and TCP enters the slow start phase with theinitial ssthresh value of 8.Give the range/interval of time during which TCP is in slow start (SS), congestion avoidance(CA), and fast recovery (FR) phases or clearly label on the plot the different phases throughoutthe evolution (either by simple marking or shading). Indicate the new value of ssthreshwhenever the value of ssthresh changes.When we say, an event occurs after t = n (but before t = n + 1), the effect of the event (e.g.,cwnd update, phase change, etc.) will be represented from t = n + 1.For the fast recovery phase, apply The Fast Recovery algorithm. You can assume that after a 3-DupAck, another duplicate ACK is received and then a new ACK is received in the subsequentRTT. Of course you can make other assumptions; in that case, state them clearly.Note: SS Phase will advance till the next nearest cwnd value in its exponential curve past thessthresh value unless cwnd value matches exactly ssthresh value. E.g., if ssthresh =2, SS will end at cwnd = 2, if ssthresh = 2.5, SS will end at cwnd = 4, and so on.Sequence of events: After t = 10, a timeout Event occurs. What is the value of cwnd at t = 10? Indicatessthresh. After t = 14, another timeout event occurs. Indicate ssthresh. What is the transition inphase? After t = 29, a 3-DupAck event occurs. At t = 30, what is the value of cwnd? Indicatessthresh. After t = 34, a timeout event occurs. What is the value of cwnd at t = 34? Indicatessthresh. After t = 38, a 3-DupAck event occurs. At this point, what was the current phase? What doesthis phase transition to? Indicate ssthresh. At t = 45 (end of our plot), What is the value of cwnd and what is the current phase?(b) [3 points] What does a loss event indicated by 3-DupAck tell you about the underlyingnetwork (in terms of congestion scenario)? For a heavily-loaded network, which loss event ismore likely to happen: a timeout or a 3-DupAck? Why does TCP Reno treat 3-DupAckdifferently than TCP Tahoe?如有需要，请加QQ：99515681 或邮箱：99515681@qq.com

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