Midterm Winter 2011 - Grading Policy
Part A
-- Each correct choice is given 2 points. Wrong answers -> do not take off any
point.
1 F
6 F
11 F
2 F
7 F
12 T
3 T
8 T
13 T
4 T
9 F
14 F
5 F
10 T
15 F
Part B
Each question is given 5 points.
Question 1
Answer:

No, since e(3,3) occurs before the cut but e(2,4) after the cut.
-- If answer Yes, take off 5 points. If say No, give 2.5 points. If explain
correctly, give 2.5 points.
Question 2
Answer:

Uses a time server to synchronize clocks
Time server keeps the reference time (say UTC)
A client asks the time server for time, the server responds with its
current time, and the client uses the received value T to set its clock

But network round-trip time introduces errors…
Let RTT = response-received-time – request-sent-time (measurable at
client),
If we know (a) min = minimum client-server one-way transmission time and
(b) that the server timestamped the message at the last possible instant
before sending it back
Then, the actual time could be between [T+min, T+RTT— min]

Client sets its clock to halfway between T+min and T+RTT— min (i.e.,
T+RTT/2)
-- If explain about the algorithm correctly, give 4 points. If explaination is
not clear enough, give 2 points. If mention correctly the [T+min, T+RTT— min],
gives 1 point.
Question 3
Answer:


Extending partial order to total order
Global timestamps:
(Ta, Pa) where Ta is the local timestamp and Pa is the process id.
(Ta,Pa) < (Tb,Pb) iff
(Ta < Tb) or ( (Ta = Tb) and (Pa < Pb))
at
Total order is consistent with partial order.
-- If show using process id to achieve the global total ordering, give 5 points.
If not, but have some explanation about timestamp, give 2 points.
Question 4
Answer:



Disjoint address spaces
More vulnerable to failure
Consume more time (mostly due to communication delays)
-- Any of the above reasons can earn 2.5 points. Not clear explanation -> take
off 1 point for each reason.
Question 5
Answer:


The Bully algorithm and the Ring-based algorithm
Trade-off: message overhead vs fault tolerance
-- Give 2 algorithms -> give 3 points (1.5 ponits for each). Must dicuss about
trade-offs considered when designing a general algorithm for leader election, not
for two algorithms above. If do correctly like this, give 2 points. And must
discuss trade-off by giving both sides. If just talk about message overhead, take
off 1 point.
Question 6
Answer:


Not all layerings are valid since lower layers may be required to support
higher layers.
Give an example here.
-- This is a hard question to differentiate students who study well. Give 2
points if say No. Take off 5 points if say Yes. Explaining why earns 1 point.
Give an example correctly with micro-protocol shown in Horus -> give 2 points.
Question 7
Answer:



Ensures that failures do not result in incomplete delivery of multicast
messages or holes in the causal delivery order(failure atomicity).
Ensures that, if two processes observe the same two consecutive
membership changes, receive the same set of regular multicast messages
between the two changes. A view change acts as a barrier across which no
multicast can pass.
Does not constrain the behavior of faulty or isolated processes.
-- Give at least 3 roles above. If 1 role is missing, take off 1 point. If two
missing, take off 3 points. Not clear explanation, take off 1 point.
Question 8


Not strong complete but strong accurancy.
Explaination is needed.
-- Give 2.5 points for each correct choice with explanation. If no explaination,
take off 1 point for each.
Part C
Each question is given 5 points.
Question 1
Answer:
a
b
c
d
e
f
g
h
k
l
m
n
o
p
q
r
s
t
1a
1
2
3
4
6
1
2
3
4
5
1
4
7
8
2
3
5
6
1
2
3
4
5
0
0
0
1
1
0
0
4
4
1
1
4
4
1b
0 0
1 0
1 0
1 0
5 1
1 0
2 1
3 1
4 1
5 1
0 1
3 2
3 3
3 4
0 0
0 0
1 0
1 0
0
0
0
0
2
0
0
0
2
2
0
0
4
4
1
2
3
4
1d
Yes
No
No
-- In 1a and 1b, for each event described wrong, take off 1/2 points. But if
there are >= 5 events described wrong, then take off all 5 points. In 1b, there
are some students don’t update the clock by +1, but if the events are consistent
then still give 5 points. This is only for 1b since we haven’t mentioned that the
clock is updated by 1 in 1b.
In 1c, for each cut correctly drawn, give 2.5 points.
In 1d, take off 1 point if there is one wrong choice, take off 3 points if there
are 3 wrong choices.
Question 2
Answer: