1. Solutions for the Third Problem Set
COSC 6360
Fall 2014

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

3. Answer
What is the function of the i-node map in a log-structured 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.

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

5. Answer
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

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

7. Answer
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

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

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

10. Fourth question
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)

11. Answer
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.

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

13. Answer
A RAID level 6 array has ten 2TB disks. What is its actual storage capacity? (5 points)
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
(a data block and two parity blocks)

14. Sixth question
According to Shah et al., what is the best chunk selection policy within the sliding window of their policy? (10 points)

15. Answer
According to Shah et al., what is the best chunk selection policy within the sliding window of their policy? (10 points)
Rarest first