CORNELL CS 4410 - 2015fa_prelim2_soln (11 pages)

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2015fa_prelim2_soln



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2015fa_prelim2_soln

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Pages:
11
School:
Cornell University
Course:
Cs 4410 - Operating Systems
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CS 4410 Operating Systems Prelim 2 Fall 2015 Profs Bracy and Van Renesse NAME NetID This is a closed book examination You have 120 minutes No electronic devices of any kind are allowed You must fill in your name and NETID above and at the top of each odd numbered page If you fail to do so we will take off 1 point for each omission Show your incomplete work for partial credit Make any other assumptions as necessary and document them Brevity is key Please write your solutions within the provided boxes as much as possible Write clearly Use the scratch paper at the end if you need to practice your answer first Question Points Possible 0 Omitting Name or NetID 2 1 Multiple Choice 20 2 Alternating Bit Protocol 20 3 Hand over Hand 20 4 Oldie but Goodie 20 5 All s Well That Ends Well 20 20 pts 1 Multiple Choice Questions 2 a Networking Which of the following are true of UDP Select all that apply A UDP is reliable B UDP is ordered C UDP has a smaller header than TCP D UDP controls network congestion E UDP is the predominant protocol for all web traffic 3 b RAID Recall that RAID Level 1 mirrors disks In the picture there are four stripes each of which is mirrored once Which of the following statements about this RAID Level 1 set up are true Select all that apply A RAID Level 1 can always detect when a single bit flips B RAID Level 1 can always detect and correct when a single bit flips C RAID Level 1 can always detect when two bits flip note that the two bits may or may not be on different disks D RAID Level 1 can always detect and correct when two bits flip E RAID Level 1 supports a 2x read performance over an un mirrored disk even while detecting bit flip errors 2 c Caching and Page Tables Which of the following statements about caching and page tables are true Select all that apply A A Page Table is a cache for memory B A Level 2 Cache is a cache for a Level 1 cache C A 2 Level Page Table is a cache for a 1 Level Page Table D A TLB is a cache for a Page Table E DRAM can be used as a cache for disk 3 d RPC Which of the following statements about Remote Procedure Calls RPC are true Select all that apply A RPCs require the programmer to construct correctly formatted network messages B A Remote Procedure Call could support a Python program running on Linux to provide a service to a C program running on Windows C After the client and server stubs are compiled the server program must then define the server s interface using an Interface Definition Language IDL D If you look at code that invokes a Remote Procedure Call the call would be indistinguishable from a local procedure call Your Answer C Your Answer A Your Answer D E Your Answer B D 2 1 Continued NETID 2 e Page Tables In a clip of the movie The Social Network that was shown in class the Harvard professor asks a question about single level page tables paraphrasing assuming PTEs have 8 status bits what would those status bits be Mark Zuckerberg answered correctly according to the professor 1 valid bit 1 modify bit 1 reference bit and 5 permission bits The professor s question is Select all that apply A impossible to answer as there is not enough information B a very difficult question that only a serious geek could answer C so easy that anyone with a basic knowledge of computer science could have answered it D really outdated 3 f Page Tables Which of the following statements about page tables are true Select all that apply A Multi level page tables always require more space than single level page tables B Multi level page tables generally have a slower look up time than single level page tables C All page table structures that are not a simple single level page table have the fundamental structure of an array of arrays D Page tables need to be invalidated on a context switch Your Answer A Your Answer B 2 g File Systems Which of the following statements about Unix like File Systems UFS are true Select all that apply A A File System Consistency Checker detects random bit flips in the data blocks of the file system B When completed every block in a consistent File System must be marked as either free or in use C In a correctly structured Directory System two distinct paths cannot lead to the same file D A Directory can consist of indirect blocks and data blocks just like a File Your Answer 3 h Networking Which of the following statements about Ethernet s Carrier Sense Multiple Access Collision Detection CSMA CD Protocol are true Select all that apply A multiple hosts use CSMA CD to share the same Ethernet physical network B CSMA CD has senders sense whether the Ethernet is currently in use C CSMA CD has senders sense to determine whether a collision has transpired D CSMA CD prevents any single host from monopolizing the network E CSMA CD guarantees packet delivery Your Answer B D A B C 3 20pts 2 Alternating Bit Protocol This question is to test your understanding of retransmission protocols such as TCP Suppose there are two computers X and Y connected by a single physical network link Packets can flow in both directions Packets can get lost but they can t get re ordered or damaged While unreliable if one computer keeps retransmitting the same packet with the same contents eventually at least one copy will arrive at the other computer The minimum latency on the link the time between sending a packet and receiving it is 1 millisecond and the maximum packet size is 101 bytes The bandwidth is unlimited Note that because of the set up packets do not need addresses a packet sent on one end of the link is automatically destined for the other The length of a packet p is given by function length p Pat designs an alternating bit protocol for reliable communication from X to Y X and Y both maintain a sequence number that counts the number of packets sent and received respectively A packet has two fields a 1 byte header and a payload of at most 100 bytes Having only limited size the header cannot store the entire sequence number In this case the 1 byte header stores the sequence number mod 2 that is the header only contains the least significant bit of the sequence number Packets from X to Y are data packets and packets from Y to X are acknowledgment packets The send function on X is as below The corresponding receive function on Y is var send seq initially 0 var recv seq initially 0 fun reliable send payload if length payload MTU 1 return ERROR payload too large fun reliable receive Keep …


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