Gracefully Degrading Agreement in Directed Dynamic Networks

ADynNet (Gracefully Degrading Agreement in Directed Dynamic Networks) is devoted to the development of the theoretical foundations, models, algorithms and analysis techniques for relaxed distributed agreement in directed dynamic networks. Such networks are characterized by (i) sets of participants (processes) that are a priori unknown and potentially time-varying, (ii) rapidly changing uni-directional connectivity between processes, and (iii) the absence of central control. Instantiated, e.g., by (wireless) sensor networks and ad-hoc networks, such dynamic networks are becoming ubiquitous in many applications nowadays. A natural approach to build robust services despite the dynamic nature of such networks would be to use distributed consensus to agree system-wide on (fundamental) parameters like schedules, operating frequencies, operating modes etc. Unfortunately, however, in larger-scale dynamic networks, this is usually impossible, since solving consensus requires a well-connected and temporarily stable network topology.

In order to overcome this fundamental limitation, we propose to consider gracefully degrading variants of consensus, in particular, approximate agreement, where decision values may slightly deviate from each other, and k-set agreement, which may deliver up to k different decisions in case of bad network conditions that e.g. lead to k isolated network partitions. In our project, we will develop network assumptions that both allow to solve, say, k-set agreement, and have some reasonable assumption coverage in real systems. Therefore, our focus will be on weakest (necessary and sufficient) conditions and the analysis of the resulting assumption coverage. Other central part of our project is the development of solution algorithms and their correctness proofs. Particular emphasis will be put on performance of our algorithms, since there is a tradeoff between weak network conditions and the communication and memory complexity of solutions algorithms.

Overall, the project shall yield new insights into the fundamental limitations of dynamic networks as well as the development of novel algorithms that solve distributed agreement problems reliably even under very weak communication guarantees.

Keywords: Distributed algorithms, dynamic networks, distributed agreement, time-varying communication topology

Project duration: 2016-2020 (final project report)

Project head: Ulrich Schmid

Supported by the Austrian Science Fund (FWF) under project number P28182