Christian Scheideler

A jamming-resistant MAC protocol for single-hop wireless networks

In this paper we consider the problem of designing a medium access control (MAC) protocol for single-hop wireless networks that is provably robust against adaptive adversarial jamming. The wireless network consists of a set of honest and reliable nodes that are within the transmission range of each other. In addition to these nodes there is an adversary. The adversary may know the protocol and its entire history and use this knowledge to jam the wireless channel at will at any time. It is allowed to jam a (1-epsilon)-fraction of the time steps, for an arbitrary constant epsilon>0, but it has to make a jamming decision before it knows the actions of the nodes at the current step. The nodes cannot distinguish between the adversarial jamming or a collision of two or more messages that are sent at the same time. We demonstrate, for the first time, that there is a local-control MAC protocol requiring only very limited knowledge about the adversary and the network that achieves a constant throughput for the non-jammed time steps under any adversarial strategy above. We also show that our protocol is very energy efficient and that it can be extended to obtain a robust and efficient protocol for leader election and the fair use of the wireless channel.

Stabilization, Safety, and Security of Distributed Systems

Consensus algorithms allow a set of nodes to reach an agreement on a quantity of interest. For instance, a consensus algorithm may be used to allow a network of sensors to determine the average value of samples collected by the different sensors. Similarly, a consensus algorithm can also be used by the nodes to synchronize their clocks. Research on consensus algorithms has a long history, with contributions from different research communities, including distributed computing, control systems, and social science. In this talk, we will discuss two resilient consensus algorithms that can perform correctly despite the following two types of adversities: (i) In wireless networks, transmissions are subject to transmission errors, resulting in packet losses. We will discuss how “average consensus” can be achieved over such lossy links, without explicitly making the links reliable, for instance, via retransmissions. (ii) In a distributed setting, some of the nodes in the network may fail or may be compromised. We will discuss a consensus algorithm that can tolerate “Byzantine” failures in partially connected networks. Low-Congestion Distributed Algorithms

An O(log n) dominating set protocol for wireless ad-hoc networks under the physical interference model

Dealing with interference is one of the primary challenges to solve in the design of protocols for wireless ad-hoc networks. Most of the work in the literature assumes localized or hop-based interference models in which the effect of interference is neglected beyond a certain range from the transmitter. However, interference is a more complex phenomenon that cannot, in general, be captured by localized models, implying that protocols based on such models are not guaranteed to work in practice. This paper is the first to present and rigorously analyze a distributed dominating set protocol for wireless ad-hoc networks with O(1) approximation bound based on the physical interference model, which accounts for interference generated by all nodes in the network. The proposed protocol is fully distributed, randomized, and extensively uses physical carrier sensing to reduce message overhead. It does not need node identifiers or any kind of prior information about the system, and all messages are of constant size (in bits). We prove that, by appropriately choosing the threshold for physical carrier sensing, the protocol stabilizes within a logarithmic number of communication rounds, w.h.p., which is faster than the runtime of any known distributed protocol without prior knowledge about the system under any wireless model that does not abstract away collisions.

Towards a scalable and robust DHT

The problem of scalable and robust distributed data storage has recently attracted a lot of attention. A common approach in the area of peer-to-peer systems has been to use a distributed hash table (or DHT). DHTs are based on the concept of virtual space. Peers and data items are mapped to points in that space, and local-control rules are used to decide, based on these virtual locations, how to interconnect the peers and how to map the data to the peers.DHTs are known to be highly scalable and easy to update as peers enter and leave the system. It is relatively easy to extend the DHT concept so that a constant fraction of faulty peers can be handled without any problems, but handling adversarial peers is very challenging. The biggest threats appear to be join-leave attacks (i.e., adaptive join-leave behavior by the adversarial peers) and attacks on the data management level (i.e., adaptive insert and lookup attacks by the adversarial peers) against which no provably robust mechanisms are known so far. Join-leave attacks, for example, may be used to isolate honest peers in the system, and attacks on the data management level may be used to create a high load-imbalance, seriously degrading the correctness and scalability of the system.We show, on a high level, that both of these threats can be handled in a scalable manner, even if a constant fraction of the peers in the system is adversarial, demonstrating that open systems for scalable distributed data storage that are robust against even massive adversarial behavior are feasible.

Group Spreading: A Protocol for Provably Secure Distributed Name Service

This paper presents a method called Group Spreading that provides a scalable distributed name service that survives even massive Byzantine attacks. To accomplish this goal, this paper introduces a new methodology that essentially maintains a random distribution of all (honest and Byzantine) peers in an overlay network for any sequence of arrivals and departures of peers up to a certain rate, under a reasonable assumption that Byzantine peers are a sufficient minority. The random distribution allows to proactively protect the system from any adversarial attack within our model.

Simple routing strategies for adversarial systems

In this paper we consider the problem of delivering dynamically changing input streams in dynamically changing networks where both the topology and the input streams can change in an unpredictable way. In particular, we present two simple distributed balancing algorithms (one for packet injections and one for flow injections) and show that for the case of a single receiver these algorithms will always ensure that the number of packets or flow in the system is bounded at any time step, even for an injection process that completely saturates the capacities of the available edges and even if the network topology changes in a completely unpredictable way. We also show that the maximum number of packets or flow that can be in the system at any time is essentially best possible by providing a lower bound that holds for any online algorithm, whether distributed or not. Interestingly, our balancing algorithms do not behave well in a completely adversarial setting. We show that also in the other extreme of a static network and a static injection pattern the algorithms will converge to a point in which they achieve an average routing time that is close to the best possible average routing time that can be achieved by any strategy. This demonstrates that there are simple algorithms that can be efficient for very different scenarios.

A distributed polylogarithmic time algorithm for self-stabilizing skip graphs

Peer-to-peer systems rely on scalable overlay networks that enable efficient routing between its members. Hypercubic topologies facilitate such operations while each node only needs to connect to a small number of other nodes. In contrast to static communication networks, peer-to-peer networks allow nodes to adapt their neighbor set over time in order to react to join and leave events and failures. This paper shows how to maintain such networks in a robust manner. Concretely, we present a distributed and self-stabilizing algorithm that constructs a (variant of the) skip graph in polylogarithmic time from any initial state in which the overlay network is still weakly connected. This is an exponential improvement compared to previously known self-stabilizing algorithms for overlay networks. In addition, individual joins and leaves are handled locally and require little work.

Universal algorithms for store-and-forward and wormhole routing

In this paper we present routing algorithms that are tmiversal in the sense that they route messages along arbitrary (simple) paths in arbitrary networks. The algorithms are analyzed in terms of the number of messages being routed, the maximum number of messages that must cross any edge in the network (edge congestion), the maximum number of edges that a message must cross (dilation), the bufler size, and the bandwidth of the links. We present two main results, both of which have applications to ttnivexsal storeand-forwwd routing and universal wormhole routing. Our results yield significant performance improvements over all previously known universal routing algorithms for a wide range of parameters, and they even improve many time bounds for standard networks. In addition, we present adaptations of our main results for routing along shortest paths in arbitrary networks, and for routing in leveled networks, node-symmetric networks, edge-symmetric networks, expanders, butterflies, and meshes.

On local algorithms for topology control and routing in ad hoc networks

An ad hoc network is a collection of wireless mobile hosts forming a temporary network without the aid of any fixed infrastructure. Indeed, an important task of an ad hoc network is to determine an appropriate topology over which high-level routing protocols are implemented. Furthermore, since the underlying topology may change with time, we need to design routing algorithms that effectively react to dynamically changing network conditions.The aim of this paper is to explore the limits of communication in wireless mobile networks, concentrating on local-control algorithms for topology control and routing. We analyze the performance of the algorithms under three measures: throughput, which is the rate at which packets can be delivered, space overhead, i.e. the space necessary to buffer packets, and the total energy consumed due to packet transmissions. Energy consumption is an important performance measure for ad hoc networks since the battery power of mobile nodes is usually limited.Towards topology control, we show that for any distribution of nodes in the 2-dimensional Euclidean plane, a simple local algorithm allows to establish and maintain a connected constant degree overlay network that contains energy-efficient paths between every pair of nodes. Towards routing, we present a local routing algorithm that works for arbitrary overlay networks without transmission interference. We show that for any sequence of network changes and packet injections the algorithm is within a constant factor of the optimal, with respect to both throughput and energy, when compared to what a best possible routing algorithm can achieve under the same sequence of network changes and injection. We then combine the topology control and routing algorithms to obtain competitive wireless communication algorithms that account for transmission interference, an important performance-limiting aspect of wireless communication.

How to spread adversarial nodes?: rotate!

In this paper we study the problem of how to keep a dynamic system of nodes well-mixed even under adversarial behavior. This problem is very important in the context of distributed systems.More specifically, we consider the following game: There are n white pebbles and ε n black pebbles for some fixed constant ε < 1. Initially, all of the white pebbles are laid down in a ring, and the adversary has all of the black pebbles in its bag. In each round, the adversary can look at the entire ring and can select to add a black pebble to the ring (if its bag is not empty) or to take any black pebble from the ring and put it back into its bag (i.e. we consider adaptive adversaries). However, the adversary cannot place a black pebble into any position it likes. This is handled by a join strategy to be specified by the system. The goal is to find an oblivious join strategy, i.e. a strategy that cannot distinguish between the white and black pebbles in the ring, that integrates the black pebbles into this ring and may do some further rearrangements so that for a polynomial number of rounds the adversary will not manage to include its black pebbles into the ring so that there is a sequence of s=Θ(log n) consecutive pebbles in which at least half of the pebbles are black. If this is achieved by the join strategy, it wins. Otherwise, the adversary wins.Of course, the brute-force strategy of rearranging all of the pebbles in the ring at random after each insertion of a black pebble will achieve the stated goal, with high probability, but this would be a very expensive strategy. The challenge is to find a join strategy that needs as little randomness and as few rearrangements as possible in order to win with high probability. In this paper, we present and analyze a very simple strategy called k-rotation that chooses k-1 existing positions uniformly at random in the ring, creates a new position uniformly at random in the ring, and then rotates the new pebble and the k-1 old pebbles along these positions. Interestingly, even if the adversary has just