Security and Cooperation in Wireless
Networks
Exercise Sheet 5
WS 12/13
Youssef Shehadeh
Telematics Group,
University of Goettingen,
shehadeh@cs.uni-goettingen.de
February 1, 2013
A. Selfishness in Packet Forwarding.
(1) Draw the dependency graph of the following network. How many dependency loops are there?
B. Secure protocols for Behavior Enforcement
(1) If the system admits a Forwarding-Dominant protocol, then it converges to a unique solution.
Explain why this property makes Forwarding-Dominant protocols more desirable than protocols
that achieve Nash Equilibrium.
(2) Consider the VCG payment scheme with the following network:
a. Assume now that the packet forwarding cost over a link is computed via a protocol
between the nodes at both ends of the link (i.e. mutual computation). Consider the
following scenario: v2 cheats in the link cost establishment protocol and increases the
cost of v2  v3 to 5 while the real cost is 2. How does this affect v2's and v3's rewards?
Does the cheating influences the probability of belonging to the LCP?
b. What happens if v3 reacts by decreasing the cost of v2  v3 to 3? Conclude on the
truthfulness of VCG with such a link cost establishment protocol.
c. Explain how the secure link cost protocol using test signals makes VCG truthful.
C. Wireless operators in shared spectrum
(1) Consider again the forwarding game discussed in the slides (with 4 strategies: CC,CD,DC,DD):
We want to model this game through game theory. Assume that each node gets a benefit b (b>10)
when its packet is received at the base station. The utility of this node is then equal to the
difference between the benefit and the transmission cost (a ~2)..
Write the normal form of this game and identify the Nash equilibria.
(2) Assume that in the above Figure, the distance between the base station to the left and the sensor
next to it is 2 instead of 1 (the other distances are unchanged). Write the costs in a Table. What
can you say about cooperation in this scenario?
(3) For two operators in a shared spectrum, would you expect the normalized payoff difference to be
higher or lower for a lower path loss a. Explain why.
(4) Operators in a shared spectrum: In the case of 10 data users, we observe that the Pareto optimal
points form a straight line. Why? What kind of game is this?