Brenton D. Walker

Analysis of simple counting protocols for delay-tolerant networks

Mobile Wireless Delay-Tolerant Networks (DTNs) are wireless networks that suffer from intermittent connectivity, but enjoy the benefit of mobile nodes that can store and forward packets or messages, and can act as relays, bringing packets and messages closer to their destination through a selective forwarding policy. Many DTN protocols compensate for the unpredictability of the network by distributing multiple message copies in the hopes that at least one will eventually be delivered. As the number of message carriers becomes large these schemes experience diminishing marginal benefits from the addition of more message carriers. We describe and analyze the Simple Counting Protocol, an extremely simple and robust method for limiting the fraction of nodes that carry a copy of a message. We examine the performance of this protocol in conjunction with several abstract mobility models and show that the protocol performs reasonably well in diverse circumstances. The Simple Counting Protocol does not assume much about node mobility, and therefore should be useful for applications where little is known about node encounter patterns. The simplicity of its implementation will hopefully make it a useful substitute for epidemic routing as a naive lower bound in protocol performance comparisons. We also show how the same simple techniques and principles can be applied in conjunction with more complex heuristic DTN protocols to reduce network resource usage, a scheme we call Intermediate Immunity.

MeshTest: Laboratory-Based Wireless Testbed for Large Topologies

Mobile, ad-hoc, wireless networks offer an interesting paradigm for ubiquitous connectivity. They have many proposed applications, and with every application comes new protocols. To test such protocols, one has two basic options: simulators and testbeds. We describe a novel wireless networks testbed called MeshTest, supporting mobile, ad-hoc, and mesh environments, with an emphasis on laboratory testing of large, outdoor environments. This paper details the testbed design, and derives mathematical relationships between attenuator settings and real-world topologies. We show how to accurately and efficiently map topologies to our testbed configuration in real time, while supporting both device mobility and multipath fading.

Delay-tolerant network experiments on the meshtest wireless testbed

Delay Tolerant Networks (DTNs) are a class of networks in which a contemporaneous end-to-end path from source to destination generally does not exist. Such networks use on a store-carry-forward communication model which relies on the mobility of nodes to transfer data between geographically separated nodes. DTN researchers have relied heavily on simulation for evaluation, due to the difficulty and expense of running live experiments with real devices running real DTN implementations. MeshTest is a laboratory-based multi-hop wireless testbed that subjects real wireless nodes running real DTN implementations to reproducible mobile scenarios. It uses shielded enclosures and an RF matrix switch to dynamically control the attenuation experienced between pairs of nodes. The testbed is an ideal platform for DTN testing, offering convenient experimental control and data management. We have installed the DTN2 Reference Implementation on wireless nodes within the testbed, and in this paper, we report on a series of experiments based on the well-known Data MULE model. Specifically, we investigate the effects of buffer limitations on the data MULEs and sensors node, velocity of the data MULEs, and bundle generation size and rate. We report results on message delivery rate and latency for varying experimental parameters. We found that an encounter between nodes does not guarantee a successful data transfer. In our experience, the quality and duration of the link, contention, and load on the nodes all influence its performance.

Using virtualization and live migration in a scalable mobile wireless testbed

Laboratory-based mobile wireless testbeds such as MeshTest and the CMU Wireless Emulator are powerful platforms that allow users to perform controlled, repeatable, mobile wireless experiments in the lab. Unfortunately such systems can only accommodate 10-20 nodes in an experiment. We have designed and built a prototype of a system that uses software virtualization and live migration to facilitate experiments involving intermittently connected networks with many multiples of the number of physical nodes available on such a testbed. Building a system like this presents many technical and research challenges. In this paper, we will describe the physical construction and software architecture of the system while providing a discussion on the research issues that we are currently addressing.

Using localized random walks to model Delay-Tolerant Networks

Mobile wireless delay-tolerant networks (DTNs) are wireless networks that suffer from intermittent connectivity, but enjoy the benefit of mobile nodes that can store, carry, and forward packets or messages, bringing them closer to their destinations through a selective forwarding policy. The evaluation of DTN routing protocols has primarily relied on simulation because most theoretical mobility models are unable to represent the mobility patterns that such protocols seek to take advantage of. In this paper we present and analyze a mobility model that we call localized random walk. This model is simple enough that it can be incorporated into mathematical models, but is spatially localized, which unlike other common mobility models, will make it possible to showcase the properties of heuristic-based DTN routing protocols. We derive the stationary spatial distribution of the mobility model, approximate what we call its spatial cross section, approximate the properties of its interaction with nodes following other mobility models, and use it to model some relatively simple DTN scenarios.

Network coded routing in delay tolerant networks: an experience report

In delay-tolerant networks, end-to-end routes are rarely available, and routing protocols must take advantage of the opportunistic interactions among nodes to deliver packets. Probabilistic routing performs well in such networks and has been the dominant focus of research in this area. However, creating efficient routing protocols is challenging because to reduce latency, one often needs to replicate messages thus increasing routing overhead. Network coding has been explored as a way to increase throughput in DTNs without a significant increase in overhead, and network coded routing approaches have shown promising results. In this paper, we report on our experience integrating both erasure coded and network coded routing into the well-adopted DTN2 Reference Implementation. We implement our routing module and evaluate it via small real-world field tests.

Nullspace-based stopping conditions for network-coded transmissions in DTNs

In a challenged network environment, where end-to-end connectivity may be a rare occurrence, delay-tolerant routing protocols must strike a balance between the increased robustness and reliability that comes with message replication and the resulting high bandwidth and storage overhead. Network coded routing, in which a node combines messages from different sources, has been shown to increase reliability in the presence of link failures with small additional overhead. A drawback of network coded routing is the lack of a natural stopping condition to control the dissemination of data. We describe an enhanced coding router that uses the mathematical structure of the orthogonal complement, or nullspace, as an improved stopping condition to eliminate redundant transmissions, and an additional technique to balance multiple coded data flows. These changes are incorporated into the DTN2 Reference Implementation and evaluated in two types of experiments. In a simple data-mule scenario, our EBR router comes very close to perfect efficiency. In a more complicated scenario with segmented communities and occasional nodes moving between them, our solutions show a drastic improvement in delivery rates.

Reality vs emulation: running real mobility traces on a mobile wireless testbed

Laboratory-based mobile wireless testbeds such as MeshTest and the CMU Wireless Emulator are powerful platforms that allow users to perform controlled, repeatable, mobile wireless experiments in the lab. Unfortunately such systems can only accommodate 10-15 nodes in an experiment. We have designed and built a scalable wireless testbed that uses software virtualization and live migration to facilitate experiments involving intermittently connected networks with many multiples of the number of physical nodes available on such a testbed. In this paper, we share our experience with using real traces from the DieselNet/DOME project on our testbed. While this trace provide GPS coordinates for where the contact occurs, we needed to deduce the overall mobility from using GPS logs for individual buses. Replicating the bus mobility on our testbed we recreate contact events and compare those to DOME traces. We also survey other traces we can use on VMT and evaluate how challenging they would be to run on the system.

Addressing scalability in a laboratory-based multihop wireless testbed

The MeshTest wireless testbed allows users to conduct repeatable mobile experiments with real radio hardware under controlled conditions. The testbed uses shielded enclosures and a matrix switch of programmable attenuators to produce multi-hop scenarios and simulate the effects of mobility and fading. Previous work focused on the theory and performance of a single-switch testbed. Connecting more than 16 nodes requires multiple matrix switches and introduces theoretical and practical challenges. In this paper we examine, in theoretical and practical terms, several potential designs for a scalable version of the MeshTest testbed. We identify one design that seems to provide the most powerful test environment, and describe our progress towards building it.We also describe our new testbed control architecture, that is an important part of making MeshTest scalable.

A demonstration of the meshtest wireless testbed for delay-tolerant network research

MeshTest is a laboratory-based multi-hop wireless testbed that can subject real wireless nodes running real DTN implementations to reproducible mobile scenarios. It uses shielded enclosures and an RF matrix switch to dynamically control the attenuation experienced between pairs of nodes. The testbed is an ideal platform for DTN testing, offering convenient experimental control and data management. We have installed the DTN2 Reference Implementation on the testbed nodes, and and have been using it to run experiments. Our current experimental scenarios are similar to the well known Data MULE and Message Ferry models, but a large variety of other experimental scenarios are possible.