Solutions for the Third Problem Set COSC 6360

Solutions for the Third Problem Set COSC 6360 Fall 2014

First question n What is the function of the i-node map in a log-structured file system? (10 points)

Answer n What is the function of the i-node map in a logstructured file system? (10 points) ¨ 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.

Second question n How would you modify the Kerberos protocol to provide secure encrypted communication between the client’s workstation and the server it accesses? (20 points)

Answer n How would you modify the Kerberos protocol to provide secure encrypted communication between the client’s workstation and the server it accesses? (20 points) It would be trivial because the client c and the server s already share a secret key Kc, s ¨ This key could be use to encrypt all messages between the client c and the server s ¨

Third question—Part A n Explain how SSH provides mutual authentication between the client’s workstation and the server. (10 points)

Answer n Explain how SSH lets the client’s workstation authenticate the server it attempts to connect to. (10 points) When the client contacts a server, the server replies with its public host key and its public server key ¨ Client can check whether the public host key is the same key as the one it was provided ¨

Third question—Part B n Under which circumstances is this not possible? (20 points)

Answer n Under which circumstances is this not possible? (20 points) ¨ Whenever the client cannot consult a trusted list of servers' public keys n By far the most frequent case!

Fourth question n n Over the last five to ten years, many corporations have replaced their RAID level 5 disk arrays by RAID level 6 arrays even when handling double disk failures was not considered a major issue. Can you explain this paradox? (20 points)

Answer n n Over the last five to ten years, many corporations have replaced their RAID level 5 disk arrays by RAID level 6 arrays even when handling double disk failures was not considered a major issue. Can you explain this paradox? (20 points) ¨ They want to protect their data against a single disk failure and the presence of irrecoverable read errors on the other disks.

Fifth question n A RAID level 6 array has ten 2 TB disks. What is its actual storage capacity? (5 points) ¨ n Answer: ____ Terabytes How many write operations will the array perform when we update a single data block? (5 points) ¨ Answer: ____ write operations

Answer n A RAID level 6 array has ten 2 TB disks. What is its actual storage capacity? (5 points) ¨ n Answer: (10 - 2)× 2 = 16 Terabytes How many write operations will the array perform when we update a single data block? (5 points) ¨ Answer: three write operations n (a data block and two parity blocks)