22C: 196: 001 Peer-to-peer Networks
Spring 2008, Assignment 2
50 points
Assigned March 25, 2008, Due April 1, 2008, in class
Please submit the answers in a typewritten form. Good documentation is essential. Your
solution should reflect your own understanding of the topic.
Question 1. (20 points) Review the topology of CAN. Now, consider an adversary
trying to inflict damage into a CAN network. On a 2D torus, the adversary can
completely cut you off by taking down your four neighbors. If you are the architect of
CAN. Then what can you do to protect the system fro m such adversarial attacks th at
can take down up to k nodes at any time (and k is known to you).
Question 2. (20 points) Recall Gummadi et al.’s paper (number [25] in the reading
list) on measurements and modeling. Now answer the following questions:
(a) What is a Zipf distribution?
(b) Why did the Zipf-ian req uest distribution show a flattened head when web-
objects were replaced by immutable objects?
(c) What was the impact of a changing client pool (compared to a stagnant client
pool?)
(d) Why does a shared cache show a low hit ratio when new objects and new users
were not added?
(e) What does an improved hit ratio mean to the users and the network
administrators ?
Question 3. (10 points) We discussed about a self-stabilizing mutual exclusion
algorithm in the class. In a unidirectional ring of n processes 0, 1, 2, ... , n-1 each
process can remain in one of the k possible states from the set {0,1,2, ..., k-1}. We
consider the shared-memory model of computation: A process i, in addition to
reading its own state s[i], can read the state s[i-1 mod n] of its predecessor process
i-1 mod n. Depending on whether a boolean function ( called a guard) of t hese two
states is true, process i may choose to modify its own state.
We will call a process with an enabled guard a "privileged process," or a
process "holding a token. The ring is in a legal configuration, when the following two
conditions hold:
1 The number of processes with an enabled guard is exactly one.
2 In an infinite behavior, a guard of each process is enabled infinitely often.
A process executes its critical section, when it has an enabled guard. A process that
has an enabled guard but does not want to execute its critical section, simply
executes an action to pass the privilege to its neighbor. Transient failures may
transform the system to an illegal configuration. The problem is to design a protocol
