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Solution to the Third COSC 6360 Quiz for Fall 2013

This solution provides answers and explanations for the questions asked in the third COSC 6360 quiz for Fall 2013. Topics covered include Kerberos system, log-structured file system, RAID-5 array, BitTorrent rarest first rule, lottery scheduling, randomized tit-for-tat policy, and SHA-1 signatures.

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Solution to the Third COSC 6360 Quiz for Fall 2013

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  1. Solution to theThird COSC 6360 Quizfor Fall 2013 Jehan-François Pâris jfparis@uh.edu

  2. First question • In the Kerberos system, which entities share these secret keys or passwords?(5 points per correct line, no partial credit)

  3. Answer • Secret User’s WS Kerberos TGS Server SUser’spassword __X__ __X__ _____ _____Secret keyof TGS _____ _____ _____ _____Secret keyof server S _____ _____ _____ _____

  4. Answer Secret User’s WS Kerberos TGS Server SUser’spassword __X__ __X__ _____ _____Secret keyof TGS _____ __X__ __X__ _____Secret keyof server S _____ _____ _____ _____

  5. Answer Secret User’s WS Kerberos TGS Server SUser’spassword __X__ __X__ _____ _____Secret keyof TGS _____ __X__ __X__ _____Secret keyof server S _____ _____ __X__ __X__

  6. Second question • What is the function of the i-node map in a log-structured file system? (10 points) • Where and how is it stored on the disk? (5 points)

  7. Answer • The i-node map contains the addresses of the i-node blocks. • Required because i-nodes do not reside at fixed positions on the disk. • The i-node map is stored on the log along with the data blocks, the directory blocks and the i-node blocks. • Not at a a fixed location!

  8. What it means Fixed location but out-of-date Checkpoint Area I-node map blocksspread on the log Log I-node blocks alsospread on the log Log

  9. Third Question • Consider a RAID-5 array having four data blocks, namely, b0, b1, b2, and b3, and one parity block p per stripe • Assuming that block b3 suddenly becomes unavailable, how could you reconstruct its contents?(10 points)

  10. Answer b3 =b0 b1 b2 p

  11. Fourth question • It would allow intruders to replay tickets of legitimate users

  12. Fifth question • What is the purpose of the BitTorrent rarest first rule?(10 points) • When does it not apply? (5 points)

  13. Answer • The rarest first policy ensures that each downloader fetches first the pieces that most of its peers want. • It does not apply to downloaders that have not yet downloaded their first piece.

  14. Sixth question • What is the purpose of ticket transfers in lottery scheduling?(10 points) • Which problem do they solve? (5 points)

  15. Answer • Ticket transfers provide explicit transfers of tickets from one client to another • When a client waits for a reply from a server, it can temporarily transfer its tickets to that server • They eliminate priority inversions

  16. Seventh question • According to Shah et al., what is the main motivation for their randomized tit-for-tat policy? (10 points)

  17. Answer • Randomized tit-for tat lets each peer select neighbors at random at the beginning of every playback • Results in faster diffusion of new chunks among peers OR • Gives more free tries to a larger number of peers in the swarm to download chunks

  18. Eighth question • What are the main property and the main use of SHA-1 signatures? (10 points)

  19. Answer • SHA-1 is a cryptographic hash function • It guarantees that any change to the hashed data will (with very high probability) change the hash value • It is used to verify the integrity of SSH packets

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