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計算機網絡代寫 - COMP3250 Computer Networks
時間:2020-10-12
Department of Computing Macquarie University COMP3250 Computer Networks Assignment Two 2020 Worth: 60 Marks Weight: 15% Lecturer In-Charge: Nardin Hanna Due Date: Week 12, Thursday 29th Oct 2020 at 5 pm. Topics Covered: ? TCP ? Security ? IPv6 1. TCP (15 Marks) Host A and B are communicating over a TCP connection, and Host B has already received from A all bytes up to byte 126. Suppose Host A then sends two segments to Host B back?to-back. The first and second segments contain 80 and 40 bytes of data, respectively. In the first segment, the sequence number is 127, the source port number is 3024, and the destination port number is 80. Host B sends an acknowledgement whenever it receives a segment from Host A. a. (3 marks) In the second segment sent from Host A to B, what are the sequence number, source port number, and destination port number? b. (3 marks) If the first segment arrives before the second segment, in the acknowledgement of the first arriving segment, what is the acknowledgement number, the source port number, and the destination port number? c. (4 marks) If the second segment arrives before the first segment, in the acknowledgement of the first arriving segment, what is the acknowledgement number? d. (5 marks) Suppose the two segments sent by A arrive in order at B. B sends acknowledgements for both segments, but the first acknowledgement is lost and the second acknowledgement arrives after the first timeout interval. Draw a timing diagram showing these segments and all other segments and acknowledgements sent (you can assume no other segment loss). For each segment in your diagram, provide the sequence number and the number of bytes of data; for each acknowledgement that you add, show the acknowledgement number. 2. Security (25 marks) Question 1 (15 marks) Outline a security architecture for the distributed environment shown below, where users wish to access enterprise services from various remote locations such as home, airport and other branch offices. a. (5 marks) Analyse the security threats which can arise in such an environment. State any assumptions that you are making. (300-400 words) b. (5 marks) Specify the types of security services that would be needed to counteract these security threats and what type of security mechanisms could support these services. (300- 400 words) c. (5 marks) Identify the types of security components that can be used to provide these security services and mechanisms and where they would be placed. (300-400 words) Question 2 (10 marks) a. (5 marks) IPSec can be used to prevent replay attacks on IP datagrams. How does IPSec accomplish this goal? Explain your answer in detail. (200-250 words) b. (5 marks) In IPSec, the ESP header provides both authentication as well as confidentiality services whereas the AH header simply provides authentication service. Why would you still want to apply both AH and ESP on a packet? (150-200 words) 3. IPV6 (20 marks) Enumerate and evaluate at least five differences between IPv4 and IPv6. A starting point for this would be the differences in the IP headers. The most basic example is that the address space has expanded from 32 bits to 128 bits. This addresses the enumeration part which is syntax oriented (the syntax). But you should evaluate it as well. That means finding out why they might have made such a change (the reason). You should also investigate the effect this has on the protocol and how it works (the semantics). Is a change made to tidy up a detail, for performance (to make routing easier or faster), etc. Your approach to this should be to gain some insight into the IP protocol and indeed into the design and evolution of protocols in general, what mistakes can be made and how to avoid them. Each difference you evaluate should take between 150 and 250 words. You should use tables and graphics and structure your answer well so that a reader can quickly get an overview and then read the detail. Assignment and Submission Guidelines Assessment For all questions in this assignment not only content but also presentation will affect your mark. You will lose marks (and not necessarily only a small portion) if there are problems with the presentation, particularly with clarity. This means that your answers to each question should be a coherent statement and that the spelling and grammar of your submission will be considered in assessing its presentation. Note ? Answers must be within the specified word limit. This is an absolute word limit and no excess will be allowed. ? Assumptions (if any) must be stated clearly in your answers. Remember, there may not be one right answer for some of the questions. Rather, your explanations do need to present your case clearly. The explanations you provide do not have to be long, concise is preferred to meandering. For full marks, your answers should all be clear, coherent, and correct. The standards of marking described in the unit outline will be applied to this assignment as relevant to the assignment topics. In addition, the following standards will be applied in marking this assignment: ? Spelling and grammar: o Assignment submissions with more than 4 spelling or grammatical errors will not achieve a grade higher than distinction; submissions with more than 8 such errors will not achieve a grade higher than credit. ? Clarity: o Ambiguous or poorly worded answers will receive a grade no more than a pass for the individual question. o Minor issues of clarity will receive a grade no more than credit for the individual question. ? Correctness of approach taken, and answer obtained: o Incorrect answers with the correct logic or approach will receive no more than a pass for the individual question. o Correct answers with incorrect logic or approach will receive no more than pass for the individual question. o Incorrect answers with no explanation of the approach taken or with the incorrect approach will receive a fail grade for the individual question. The questions will be marked individually, the marks totalled, and a final grade assigned that is no more than indicated by the total marks, and no more than allowed by the standards specified above and in the unit outline. Bibliography Guidelines In your bibliography be careful to identify the source of all material that you use and to clearly identify any quotations that you make. Any quotations that are not properly identified will be considered as plagiarism and will attract a penalty. For more information on how to write bibliographies, you can find resources on the Internet such as How to Write a Bibliography (an introduction) or Bibliography Guides. We strictly recommend ACM and IEEE bibliography style. Font Guidelines It is strongly recommended to use Times New Roman with a font size 12. Submission ? Online submission via Turnitin. Assignments will normally be marked and returned online. There are no hardcopy submissions for written assignments. Ensure you submit the correct file. The submission process shows you a complete preview of your entire assignment after you have uploaded it but before you have submitted it. Carefully check through every single page to ensure everything is there and the correct version has been uploaded, and only then press CONFIRM. Multiple submissions may be possible via Turnitin prior to the final due date and time of an assessment task and originality reports will be made available to the students. Teaching staff will use the report to judge whether plagiarism has occurred and whether penalties should apply for breaches of the Academic Honesty Policy. Any similar text identified by Turnitin will be considered carefully to see if it is indeed a breach of the Academic Honesty Policy

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