Mobile Ad Hoc Networking (Cutting Edge Directions) || Applications in Delay-Tolerant and Opportunistic Networks

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<ul><li><p>PART III</p><p>OPPORTUNISTIC NETWORKING</p></li><li><p>9APPLICATIONS IN DELAY-TOLERANTAND OPPORTUNISTIC NETWORKS</p><p>Teemu Karkkainen, Mikko Pitkanen, and Joerg Ott</p><p>ABSTRACT</p><p>The characteristics of delay-tolerant and opportunistic communication require fun-damentally different approaches at the network layer for routing and forwarding.Unpredictable or large latencies require task divisions according to traditional layer-ing to be reconsidered: Larger messages take the place of series of individual packetsexchanged within end-to-end transport connections as virtually instant feedback loopsbetween endpoints disappear. This (forced) move to message-based communicationbrings along new opportunities for application protocol design; at the same time,the lack of instant feedback and of instant infrastructure access creates a new set ofchallenges. In this chapter we will explore these challanges and opportunities. Sinceapplication design and development is more about practice than theory, this chapterextensively uses examples from real DTN applications and experiments.</p><p>In this chapter we will explore the challenges that face application designers work-ing on DTNs. We will first introduce typical scenarios in Section 9.1, and we willthen look at some of the typical challenges in Section 9.2. We then look in more depthat some of the critical issues including security, legacy system support, and user in-terfaces in Section 9.3. Section 9.4 presents four in-depth case studies that look atdifferent aspects of DTN applications.</p><p>Mobile Ad Hoc Networking: Cutting Edge Directions, Second Edition. Edited by Stefano Basagni,Marco Conti, Silvia Giordano, and Ivan Stojmenovic. 2013 by The Institute of Electrical and Electronics Engineers, Inc. Published 2013 by John Wiley &amp; Sons, Inc.</p><p>317</p></li><li><p>318 APPLICATIONS IN DELAY-TOLERANT AND OPPORTUNISTIC NETWORKS</p><p>9.1 APPLICATION SCENARIOS</p><p>Delay-tolerant networking techniques can be applied in a large variety of scenarios.The key differentiating factor between the scenarios is the amount of predictabil-ity and control over the contacts between the message carriers. We can identify arange of scenarios from highly opportunistic connectivity in urban area scenariosto completely predictable connectivity in challenged area scenarios. Various mixedscenarios, such as remote area and disaster scenarios, incorporate elements fromboth, but they are not discussed in this chapter in any more detail.</p><p>9.1.1 Challenged Area Scenarios</p><p>At one end of the spectrum are challenged area scenarios where contacts are com-pletely predictable and controlled. The most obvious class of these types of scenariosare space networks, which gave rise to the whole field of delay-tolerant networking.</p><p>Outside of space networking, similar characteristics are found closer to home onvarious industrial scenarios, such as underground mines or shipyards. Applications inthese scenarios are typically used for monitoring and controlling production processes.Following case study discusses a real-world deployment scenario in a mine.</p><p>9.1.1.1 Case Study: Mining. There are 30004000 operating mines worldwide.Lifetime of a mine depends on the amount and accessibility of ore and may varyfrom a few years to tens of years. Within these mines, there may be between 10 and1000 pieces of mining equipmentdrills, loaders, roof bolters, and other special-purpose machinesas well as personnel operating in two to three shifts per day.</p><p>The mining process is often divided into development and production phases. Inthe development phase the underground infrastructure and the tunnels to the ore bodyare created. During the production phase the ore is excavated from the solid rock andtransported to the surface.</p><p>Both phases have a cycle where different work methods follow each other andeach work method uses dedicated type(s) of machine(s). In underground mines, thereare typically tens of active work locations in different stages of the development orthe production cycles. Managing the equipment fleet and the locations in an efficientway is challenging.</p><p>The need to manage a fleet of equipment and personnel operating in an undergroundmine necessitates robust communications in an environment where radio propagationis severely limited by the topology of the tunnels and where wired communicationinfrastructure often cannot be built in the areas that are actively being worked on.Figure 9.1 shows an example of such a scenario.</p><p>Communications are required in two main areas: (1) voice communication (forwhich various technical solutions are in operation today) and (2) data exchange formining operations and monitoring which we address in this paper. The latter requirestransmitting measurement and operations data collected by mining equipment to acontrol room and conveying instructions from the control room to the equipment andpersonnel.</p></li><li><p>APPLICATION SCENARIOS 319</p><p>Figure 9.1 One production level of a mine with WiFi access points (light brown actual, reddevelopment plans).</p><p>In some development stages (e.g., production drilling) the mining equipment maybe out of reach of any communications for days and the operators for entire shifts.Being able to transmit data between the work site and the control room during theselong blackout times would allow increasing efficiency and control of the miningoperation, which in turn may lead to lower operational costs and better exploitationof the ore.</p><p>Technologies currently used for data communications in mines include wirelessnetworking (WiFi), data over leaky feeder, and manual transfer with USB memorystick.</p><p>WiFi infrastructure can be built within the mine to provide communications be-tween wireless devices and the fixed mine network. However, the topology of manymines severely limits the propagation of radio waves leading to the need to installand maintain a network of hundreds of interconnected access points. Furthermore,drifts are frequently blasted, which makes permanent installations in developmentareas impossible.</p><p>Leaky feeder is a coaxial cable that leaksthat is, emits and receives high-frequency radio waves. It is used in many mines for voice communication. Datacommunication over leaky feeder cable is low bandwidth, unstable and expensive.Furthermore, it may not be possible to install the cable within areas that are beingactively worked on, and a line of sight is required.</p><p>Completely manual transfer of data can be achieved by physically carrying dataon USB memory sticks (for example) between the mining equipment and the controlroom. Communication is limited in frequency to once per shift and is subject to humanerrors (such as simply losing or forgetting to deliver the memory stick) and the riskof malfunctions, for example, due to dirt.</p><p>It has been shown that a communication system can be built in mines to enablefrequent data transfer between mining equipment and the control room without theneed for fixed infrastructure by exploiting store-carry-forward networking [1].</p><p>Although such networks are closed and tightly controlled, security considerationscannot be completely omitted. First, the need for easier and more cost-effective remote</p></li><li><p>320 APPLICATIONS IN DELAY-TOLERANT AND OPPORTUNISTIC NETWORKS</p><p>monitoring and controlling has meant that truly air-gapped production networks arebecoming rareand even air-gapped networks have been compromised by malware,such as Stuxnet, capable of hopping the gap on a USB stick. Coupled with weaklysecured outward facing services, this provides an attack vector for remotely compro-mising the system. Second, for business reasons, vendors might want to secure theirapplication messaging against reading and tampering by competitors equipment.</p><p>9.1.2 Urban Area Scenarios</p><p>Mobile devices and particularly mobile phones gain increasing importance asprimary means to access content and services on the Internet. This is achieved bytaking advantage of the seemingly ubiquitous connectivity offered in urban areas bytodays cellular operators. However, in practice, pervasive mobile access over cellularis hampered in numerous ways.</p><p>First, cellular coverage may not be as ubiquitous as expected, as even in well-provisioned countries and cities it is not unusual for calls or data connection to getdisrupted. Second, cellular access may be undesirable due to the high costs associatedwith roaming charges or traffic volume based pricing models or due to the user nottrusting the infrastructure operator. Finally, even if cellular connectivity is theoreti-cally available and affordable, there are practical limitations in the current networks.For example, heavy queuing can be used inside the networkfor example, to reducedata loss on the wireless link. This leads to significant round-trip times that impactthe user experience especially when accessing resources such as large web pagescomprising many objects [2].</p><p>Moreover, the current trend of gearing mobile handsets towards Internet usagehas led to devices such as the iPhone with data-hungry applications assuming andusing always-on connectivity at rapidly increasing rates. The result is increasingcongestion and noticeably reduced performance of cellular networks, especially whenusers are forced to compete for the shared communication resource without guaranteedbandwidth allocations as was observed in cellular networks [2].</p><p>If we stop looking at mobile communication technologies simply as a means toconnect a mobile device to some rigid infrastructure network and we abolish thestrict consumeroperator service model, we can begin to see the true potential oftodays mobile devices and the additional communication opportunities arising fromdirect device-to-device contacts. Mobile users are not only consumers but also primeproducers of content, often eager to share their experiences and impressions withothers. Some of this content may have short temporal relevance or small spatialrelevance making it unsuitable for centralized storage in an infrastructure server.Furthermore, copies of popular content, such as front pages of news websites, arelikely to exist on devices close to the user making it much cheaper and faster toaccess them locally rather than over an infrastructure network connection.</p><p>In addition, the many commercial WLAN hotspots spread across urban areas,and an increasing number of WLAN community networks present an interestingopportunity for Internet access. While they may not be as thoroughly administeredas cellular networks, they offer sizeable access rates, exhibit short RTTs, and are</p></li><li><p>APPLICATION SCENARIOS 321</p><p>Figure 9.2 Opportunistic content access via WLAN hotspots.</p><p>usually underutilized. This makes them a suitable platform for mobile web access,though with the caveats that (1) their coverage is very limited and (2) access is usuallyconstrained to subscribers or community members.</p><p>Figure 9.2 illustrates a sample communication scenario consisting of severalmobile users in an urban area. The mobile users devices are capable of commu-nicating among each other opportunistically when they come into radio range. Whenin the coverage of a WLAN hotspot, the mobile nodes connect to the Internet viathe access point that provides DTN routing and, optionally, additional caching func-tionality. Geographically neighboring WLAN access points may further conspire toreplicate responses from the Internet to reach (indirectly connected) mobile nodeswho may have moved in the meantime. Mobile nodes and access points holding acopy of the sought content may reply immediately to requests. This yields a DTNmessaging overlay among the mobile nodes, the hotspots, and the Internet, whichenables mobile users to choose when to use delay-tolerant and when cellular Internetaccess.</p><p>In effect, this offloads data belonging to delay-tolerant interactions from the 3Gnetwork to the closest WLAN access points, thus helping to preserve cellular con-nectivity for real-time and more interactive applications. Several applicationsforexample, fetching RSS feeds, P2P file retrieval, or posting to blogsdo not requireimmediate feedback and thus are good candidates to benefit from the presented oppor-tunistic web access model. Obviously, the cellular network can also serve as a backupif DTN-based communication does not yield a result in the expected time frame.Altogether, complementary DTN overlays support and enhance pervasive informa-tion access for mobile users.</p></li><li><p>322 APPLICATIONS IN DELAY-TOLERANT AND OPPORTUNISTIC NETWORKS</p><p>9.2 CHALLENGES FOR APPLICATIONS OVER DTN</p><p>Application design over DTNs poses many challenges different from the ones typi-cally encountered on the Internet. These challenges arise from transport considera-tions, security, and interactivity and semantics, but there are also opportunities suchas interacting with routing. In this section we briefly describe some of the challengesand give two case studies to highlight them.</p><p>Transport Considerations. When using a DTN transport, in particular the BundleProtocol, the application designer must take into account the special characteristicsimposed by itin particular, fragmentation, congestion control, and reliability.</p><p>Fragmentation of messages may occur on the bundle layer transparently to theapplication. This can happen if a node proactively decides to fragment the messageor if the message must be reactively fragmented due to the underlying link breakingduring bundle transmission. While the bundle layer should be able to transparentlyfragment and combine fragments back to a full message, in practice this greatlyreduces the delivery probability and thus makes application interactions less likely tosucceed if the message gets fragmented [3]. This implies that the application designershould take the risk of fragmentation into account and avoid or alleviate its negativeimpact by, for example, avoiding sending messages that are large with respect to thetypical contact volumes of the underlying scenario. Such efforts are analogous tothe typical practice of limiting IP packet sizes to the Ethernet MTU size to avoid IPfragmentation in the Internet.</p><p>Congestion control in the Internet has been largely solved since 1988 through theuse of the Transmission Control Protocol, which dynamically adjusts outgoing datavolume based on feedback from the network (in the form of packet losses or explicitnotification). For many applications, managing congestion control is a simple matterof choosing to use the TCP transport; only application protocols that do not use TCPneed to worry about congestion. Unfortunately, such congestion control mechanismsdo not exist in DTNs even though some theoretical work already exists [4]. Evenworse, the traffic volume generated by an application sending a message is a func-tion of the routing protocol (replication) and the underlying scenario (node density).It is therefore up to the application designer to reduce the impact of its traffic onDTN congestion. There are two practical ways for DTN applications to achieve this;(1) limiting the amount of data sent, and (2) controlling the messag...</p></li></ul>