The following technical report is available from
http://aib.informatik.rwth-aachen.de/:
A Counterexample to the Fully Mixed Nash Equilibrium Conjecture
Simon Fischer, Berthold Vöcking
AIB 2005-11
We study a well-known resource allocation game introduced by
Koutsoupias and Papadimitriou. It was conjectured by Gairing et al.
that the fully mixed Nash equilibrium is the worst Nash equilibrium
for this game. The known algorithms for approximating the so-called
"price of anarchy" w.r.t. mixed equilibria rely on this conjecture.
We present a counterexample to the conjecture showing that fully
mixed equilibria cannot be used to approximate the price of anarchy
within reasonable factors.
The following technical report is available from
http://aib.informatik.rwth-aachen.de/:
Automata and Logics for Message Sequence Charts
Benedikt Bollig
AIB 2005-10
A message-passing automaton is an abstract model for the
implementation of a distributed system whose components communicate
via message exchange and thereby define a collection of communication
scenarios called message sequence charts. In this thesis, we study
several variants of message-passing automata in a unifying framework.
We classify their expressiveness in terms of state-space properties,
synchronization behavior, and acceptance mode and also compare them to
algebraic characterizations of sets of message sequence charts, among
them the classes of recognizable and rational languages.
We then focus on finite-state devices with global acceptance condition
that communicate via a priori unbounded channels. We show them to be
exactly as expressive as the existential fragment of monadic
second-order logic over message sequence charts and to be strictly
weaker than full monadic second-order logic. It turns out that
message-passing automata cannot be complemented and that they cannot
be determinized in general. Those results rely on a new proof
technique, which allows to apply graph acceptors as introduced by
Thomas to the framework of message sequence charts.
The following technical report is available from
http://aib.informatik.rwth-aachen.de/:
Byzantine Fault Tolerance on General Hybrid Adversary Structures
Klaus Kursawe, Felix C. Freiling
AIB 2005-09
Adversary structures are a generalization of the classical ``at most
$t$-out-of-$n$'' threshold failure model which is used in many published
Byzantine-tolerant protocols. An adversary structure basically lists all
coalitions of parties whose corruption the protocol should tolerate. Using
adversary structures it is possible to encode dependent failure models,
such as ``either all Linux machines fail or all Windows machines but not both
at the same time''. We describe a general technique that allows to transform
an algorithm designed for the threshold model into an algorithm
that works for general adversary structures. Our technique is based on
several (partly informal) rules which describe how the algorithm and its
proof must be augmented so that general adversary structures can be
tolerated. We demonstrate the applicability of our approach by transforming
an asynchronous Byzantine-tolerant reliable broadcast protocol into one
that tolerates Byzantine adversary structures. We also consider similar
transformations for hybrid failures (combinations of different fault models)
and discuss ways to map adversary structures to the real world and manage
them efficiently.
