Large-Scale Hybrid Ad Hoc Network for Mobile Platforms: Challenges and Experiences
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Large-scale hybrid ad hoc network for mobile platforms: Challenges and Experiences Nirmit Desai, Wendy Chong, Heather Achilles, Shahrokh Daijavad Thomas La Porta IBM T. J. Watson Research Center Dept. of Computer Science and Engineering Yorktown Heights, NY, USA Penn State University fnirmit.desai, wendych, hachilles, [email protected] University Park, PA, USA [email protected] Abstract—Peer-to-peer (p2p) networks and Mobile ad hoc net- populated areas, e.g., sports arenas. The following are the key works (MANET) have been widely studied. However, a real-world characteristics in these scenarios: deployment for the masses has remained elusive. Ever-increasing density of mobile devices, especially in urban areas, has given CH0. No communication infrastructure such as WiFi ac- rise to new applications of p2p communication. However, the cess points to fall back on modern smartphone platforms have limited support for such CH1. Device users are mobile, pattern of mobility is not communications. Further, the issues of battery life, range, and predictable trust remain unaddressed. A key question then is, what kinds of CH . New information may arrive at any time applications can the modern mobile platforms support and what 2 challenges remain? This paper identifies a class of applications CH3. Trustworthy information is scarce, misinformation and presents a novel center-to-peer-to-peer (c2p2p) architecture and rumours are common place called Mesh Network Alerts (MNA) to support them. We describe CH4. Small payloads suffice in many cases and informa- our experiences in deploying MNA as a real-world system to tion retains value for a few minutes millions of users for relaying severe weather information along CH . Device battery is a scarce resource, power supply with the challenges faced, and the approaches for addressing 5 them. for recharging may not be available Index Terms—peer-to-peer systems, mobile ad hoc network, CH6. Devices are owned by citizens, deployment of delay-tolerant network special-purpose devices is cost prohibitive The above needs are well-recognized in the industry as well I. INTRODUCTION as academia with several ambitious attempts to address them, Mobile devices with programmable platforms such as An- e.g., Google Loon project2 and Facebook Aquilla3, though droid and iOS have steadily grown over the last decade, with limited impact. Leveraging user mobile devices as peer surpassing the 2 billion mark1. MANETs have been widely nodes for a large-scale deployment has been another theme studied given their decentralized nature and potential for in the prior works, e.g., the Serval project [6]. Serval mesh new applications [11], [18]. Most of the prior work on p2p enables p2p communication over on-device WiFi radio, but networks has focused on analytical and simulation-based study requires root access to the device via jail-breaking. Although of MANET behavior [12], [13], [21]. Given the outstanding significant lessons have been learned through these attempts, a practical challenges in deploying a large number of physical mass-scale p2p network for such applications remains elusive. nodes, real-world implementations have been limited and A vast majority of the literature has focused on a traditional have not reached mass scale [10]. However, with the growth model of stateful, fully decentralized, reliable networking. of smartphones, large-scale real-world implementations may Specifically, the nodes maintain connectivity with peers and become feasible. This paper describes a real-world implemen- routing is optimized with techniques based on link state tation of a p2p delay-tolerant network, called Mesh Network (OLSR) or distance vectors (AODV) [5], [17] focusing on Alerts (MNA), for relaying severe weather information to optimizing the network utilization. millions of mobile device users as part of the Weather Channel Given the application characteristics above, this paper iden- app [8] on both Android and iOS platforms. tifies practical challenges associated with modern device plat- Before describing MNA, it is critical to identify applica- forms and finds novel ways to overcome them. This leads to tions that need p2p communication, given pervasive Internet MNA – a new paradigm in p2p networking that employs a connectivity and the ease of using it. Doing so enables us delay-tolerant, and zero-routing overhead architecture. Unlike to define key characteristics of such applications and focus previous works, MNA is a hybrid of a centralized and a decen- on the challenges in meeting them. This paper focuses on two tralized architecture, called center-to-peer-to-peer (c2p2p).A separate classes of applications: communication in (a) disaster- central service is leveraged as the trusted source of information affected or remote areas and (b) congested networks in densely while the information is propagated in a decentralized fashion 1https://www.statista.com/statistics/330695/number-of-smartphone-users- 2https://loon.co worldwide/ 3https://en.wikipedia.org/wiki/Facebook Aquila by peers. Such an architecture allows MNA to bypass the is responsible for constantly forwarding available information key issue of trust deficit in open decentralized systems while to other peers via advertisements. However, due to CH5, the retaining the ability of infrastructure-less communication in MNA activity must keep the device battery consumption to a many application scenarios. minimum. This is a challenge because advertising and discov- MNA is implemented on both Android and iOS as an SDK ery are power-hungry operations over the radio channels. and integrated with the Weather Channel mobile apps. With C. Testing p2p networks extensive experiments and experience of deploying to almost 10 million users, MNA represents a way forward for large- Given the heterogeneity of devices owned by users and scale p2p networks. operating system distributions (CH6), it is challenging to test This paper makes the following key contributions: whether or not MNA works as expected on a single device. Further, running test scenarios on a p2p network at large-scale • Identification of a class of applications for p2p and their key characteristics is non-trivial given that a large number of devices need to take coordinated action followed by coordinated observations • A deeper investigation of the practical challenges in supporting the above class of applications to determine whether a test passes or fails. Further, since range and mobility affect p2p communications and they are unpre- • A novel c2p2p implementation using multiple radio chan- nels on modern mobile platforms (Android and iOS) dictable (CH1), it is important to run test cases under various mobility patterns across all nodes in the network. Emulated • Experimental evaluation and real-world deployment statistics mobility frameworks such as CORE [1] and EMANE [15] fall short as the connection latency and wireless transmission In the following, Section II describes the practical chal- are specific to device hardware. This is not a challenge in lenges. Section III outlines the architectural details of MNA traditional mobile application development as the application along with platform-specific implementation issues for An- functionality is confined within a single connected device. droid and iOS in addressing the challenges. Section IV dives into the algorithms of two of the MNA protocols. Experi- D. Trust in information mental evaluation and deployment statistics are presented in As user devices advertise on unsecured wireless protocols, Section V. A deeper look at the literature and contrast to MNA it may be possible for a malicious attacker to listen for such is summarized in Section VI with conclusions in Section VII. advertisements and reverse-engineer the protocols used. Then, the attackers may generate fake messages and advertise them, II. CHALLENGES e.g., a fake tornado alert. Given a lack of trusted information We present the main challenges for modern mobile device in such scenarios (CH3), misinformation campaigns can have platforms in supporting the classes of applications described disastrous consequences. In open decentralized systems, such above. These have been uncovered via extensive experiments false messages cannot be distinguished from the real ones, and in some cases include direct feedback from the developers and MNA will end up propagating them to as many devices of Android and iOS. as possible, “poisoning” the network. In general, veracity of A. Operating system restrictions such information cannot be independently verified in open decentralized systems and previous works on peer-to-peer Due to CH , even when a user is not interacting with 2 networks do not address this challenge. an app, or worse yet, when the device is not being used at all, the devices must continue to discover peers to receive III. C2P2P SYSTEM ARCHITECTURE and forward information. Although modern mobile operating systems such as Android and iOS offer APIs to discover and advertise information to peers over WiFi and Bluetooth interfaces, peer-to-peer connections do not work reliably when the same APIs are accessed while the app is in the background. Further, on Android, each WiFi p2p connection must be explicitly approved by the user. Prior works widely document these challenges and take the approach of having special access on the devices, e.g., jail-breaking or rooting [6]. Clearly, such an approach does not scale to mass adoption. B. Power constraints Fig. 1. c2p2p system architecture Since devices may be offline when new weather information arrives, MNA on each peer must remain active at all times To address (or bypass) the challenges identified above, this to be able to discover new information as soon as it arrives. paper proposes c2p2p – a hybrid architecture that leverages Further, as there is no back up infrastructure (CH0) and a set a central service as the sole source of trusted information as of peers in range can change at any time (CH1), each peer depicted in Figure 1. An application-specific central service, e.g., weather.com backend, originates new information and have experimented with using WiFi and Bluetooth interfaces, assigns it a unique identifier.