Mobile Ad Hoc Networking (Cutting Edge Directions) || Data Dissemination in Opportunistic Networks

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<ul><li><p>12DATA DISSEMINATION INOPPORTUNISTIC NETWORKS</p><p>Chiara Boldrini and Andrea Passarella</p><p>ABSTRACT</p><p>Among the alternatives to pure general-purpose MANETs, one of the most promisingapproach is that of opportunistic networks [2]. Differently from MANETs, oppor-tunistic networks are designed to work properly even when the nodes of the networkmove. More specifically, opportunistic networks reverse the approach of MANETs,and what was before an accident to avoid (the mobility of nodes) now becomes anopportunity for communications. In fact, in an opportunistic network messages areexchanged between nodes when they come into contact, creating a multi-hop pathfrom the source to the destination of the message.</p><p>One of the most appealing applications to build upon an opportunistic network isdata dissemination. Conceptually, data dissemination systems can be seen as varia-tions of the publish/subscribe paradigm: publisher nodes generate content items andinject them into the network, subscriber nodes declare their interest in receiving cer-tain types of content (e.g., sport news, radio podcast, blog entries, etc.) and striveto get it in some ways. Nodes can usually be publishers and subscribers at the sametime. The main difference between message forwarding and content dissemination isthat the source and destination of a message are typically well known when routinga message (and clearly listed in the header of the message itself), while, in contentdissemination, content generators and content consumers might well be unaware ofeach other. Publish/subscribe systems have gained new momentum thanks to the Web2.0 User Generated Content (UCG) paradigm, with users generating their own content</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>453</p></li><li><p>454 DATA DISSEMINATION IN OPPORTUNISTIC NETWORKS</p><p>and uploading it on popular platforms like Blogger, Youtube, or Flickr. The appli-cation of the UGC paradigm to opportunistic networks is particularly appealing. Afuture of users generating content items on the fly while moving, and distributing thiscontent to the users in their proximity, can be realistically envisioned for the nextyears. In order to make this future a reality, new strategies for disseminating contentitems must be designed, while at the same time accounting for a wise usage of networkresources, which can be easily saturated in this scenario.</p><p>In this chapter we discuss the challenges connected with content disseminationin an opportunistic network and the solutions proposed in the literature. We classifycurrent proposals that address the problem of content dissemination into six maincategories, based on the specific problem targeted and the type of solution proposed.Then, we present and discuss the work that we believe best summarizes the mainfeatures of each category.</p><p>12.1 INTRODUCTION</p><p>Opportunistic networks represent one of the most interesting evolution of traditionalMobile Ad Hoc NETworks (MANET). The typical MANET scenario comprises mo-bile users with their wireless-enabled mobile devices that cooperate in an ad hocfashion to support communication without relying on any preexisting networkinginfrastructure. Specifically, in MANETs the nodes of the network become active en-tities and also become a substitute to routers, switches, and so on, in forwardingmessages. Thus, messages are delivered following a multihop path over the nodesof the MANET itself. Despite the huge research activity that they have generated,MANETs were far from being widely adopted. The main drawback of MANETswas their lack of realism in research approach [1]. From a practical standpoint, realsmall-scale implementations have been long disregarded, and real users have not beeninvolved in the MANET evaluation. From a research standpoint, MANET results weremined by excessively unrealistic assumptions. The most significant among these isthe intolerance to temporary network partitions, which actually may be very commonin a network where users move and where communication devices are expected torun out of battery, or to be out of reach, very often.</p><p>Among the alternatives to pure general-purpose MANETs, one of the most promis-ing approach is that of opportunistic networks [2]. Differently from MANETs,opportunistic networks are designed to work properly even when the nodes of thenetwork move. More specifically, opportunistic networks reverse the approach ofMANETs, and what was before an accident to avoid (the mobility of nodes) nowbecomes an opportunity for communications. In fact, in an opportunistic network,messages are exchanged between nodes when they come into contact, creating a mul-tihop path from the source to the destination of the message. The exploitation of directcontacts between nodes for message forwarding introduces, as a side effect, additionaldelays in the message delivery process. In fact, user mobility cannot be engineered:Node contacts are usually neither controllable nor scheduled, and networking pro-tocols can only wait for them to occur. For this reason, opportunistic networks fallinto the category of delay-tolerant networks [3]. For many common applications, this</p></li><li><p>INTRODUCTION 455</p><p>additional latency may be a tolerable price for the ubiquity provided by the opportunis-tic network. Web 2.0 content sharing services, for example, are already delay-tolerantin their nature, because they rely on an asynchronous communication paradigm.</p><p>One of the most appealing applications to build on top of an opportunistic networkis data dissemination. Conceptually, data dissemination systems can be seen as vari-ations of the publish/subscribe paradigm [4]: Publisher nodes generate contents andinject them into the network, and subscriber nodes declare their interest in receivingcertain types of content (e.g., sport news, radio podcast, blog entries, etc.) and striveto get it in some ways. Nodes can usually be publishers and subscribers at the sametime. The main difference between message forwarding and content dissemination isthat the source and destination of a message are typically well known when routing it(and clearly stated in the header of the message itself), while, in content dissemina-tion, content generators and content consumers might well be unaware of each other.Publish/subscribe systems have gained new momentum thanks to the Web 2.0 UserGenerated Content (UCG) paradigm, with users generating their own content anduploading it on popular platforms like Blogger, Youtube, or Flickr. The applicationof the UGC paradigm to opportunistic networks is particularly appealing. A futureof users generating content items on the fly while moving, as well as distributing thiscontent to the users in their proximity, can be realistically envisioned for the nextyears. In order to make this future a reality, new strategies for disseminating contentitems must be designed, while at the same time accounting for a wise usage of networkresources, which can be easily saturated in this scenario.</p><p>12.1.1 Motivation and Taxonomy</p><p>In this chapter we discuss the challenges connected with content dissemination inan opportunistic network and the solutions proposed in the literature. We classifycurrent proposals that address the problem of content dissemination into six maincategories, based on the specific problem targeted and the type of solution proposed.For each category, we present and discuss the work that we believe best summarizesthe approach proposed.</p><p>We start in Section 12.2 by discussing the initial work on the area which ignited theresearch on this topic. To the best of our knowledge, the PodNet Project [5] was thefirst initiative to explicitly address the problem of disseminating content in a networkmade up of users mobile devices in an opportunistic fashion. Within the PodNetproject, heuristics were defined in order to drive the selection of content items tobe cached based on the popularity of the content itself. Such heuristics enforced acooperative caching among nodes and were shown to clearly outperform the simplestrategy in which each node only keeps the content it is directly interested in.</p><p>The second category of solutions is based on the exploitation of the social char-acteristics of user behavior (Section 12.3). In this case, heuristics are proposed thattake into account the social dimension of usersthat is, the fact that people belong-ing to the same community tend to spend significant time together and to be will-ing to cooperate with each other. We take ContentPlace [6] as representative of thisapproach because it was one of the first fully-fledged solutions to incorporate the ideaof communities with a systematic approach to data dissemination.</p></li><li><p>456 DATA DISSEMINATION IN OPPORTUNISTIC NETWORKS</p><p>The third category of content dissemination approaches brings the ideas of pub-lish/subscribe overlays into the realm of opportunistic networks (Section 12.4). Pub-lish/subscribe systems are based on content-centric overlays in which broker nodesbring together the needs of both content publishers and subscribers by matching thecontent generated by publishers with the interests of subscribers and by delivering thecontent to them. How the publish/subscribe ideas can be adapted to an opportunisticenvironment is well exemplified by Yoneki et al. [7], in which the pub/sub overlay isbuilt exploiting the knowledge on the social behavior of users.</p><p>Protocols belonging to the fourth category, discussed in Section 12.5, reverse theapproach of heuristic-based protocols as they depart from the local optimization prob-lems at the basis of heuristic approaches in order to find a global, optimal solutionto the content dissemination problem. Such a global solution, typically unfeasible inpractice in real scenarios, is then approximated using a local, distributed strategy. Tothe best of our knowledge, the work by Reich and Chaintreau [8] has been the first toprovide a comprehensive analysis of a global optimization problem applied to contentdissemination in opportunistic networks.</p><p>The fifth category (Section 12.6) is characterized by the exploitation of a broadbandwireless infrastructure in conjunction with the opportunistic network of user devices.The idea here is to partially relieve the burden of disseminating content from the infras-tructure by exploiting opportunistic content dissemination among users. We choosethe work by Whitbeck et al. [9] as representative of this category because of the tightinteraction that it proposes between the infrastructure and the opportunistic network.</p><p>Finally, the sixth category hosts proposals that tackle the dissemination problemusing an analogy with unstructured p2p systems (Section 12.7). To the best of ourknowledge, the work by Zhou et al. [10] is one of the most significant in this area,which formulates the dissemination problem by means of p2p universal swarms andprovides solid theoretical results regarding the advantage of cooperative strategiesagainst greedy approaches.</p><p>12.2 INITIAL IDEAS: PODNET</p><p>Initial research efforts on content dissemination in opportunistic networks were madewithin the PodNet project [5]. The aim of the PodNet project is to develop a con-tent distribution system that builds up from the mobile users that opportunisticallyparticipate to the network. The scenario considered by PodNet is that of one or moreAccess Points that are able to retrieve content from the Internet and to send it tothose nodes that are in radio range. Coverage provided by the Access Points might belimited, thus content items are also disseminated by the mobile users of the networkin an opportunistic fashion. In addition, content may also be generated by the usersthemselves, according to the Web 2.0 User Generated Content paradigm.</p><p>12.2.1 Content Organization</p><p>PodNet borrows the representation of content as a set of channels from Web syndi-cation [11,12]. Syndication provides a structured way for making content available</p></li><li><p>INITIAL IDEAS: PODNET 457</p><p>Request for discovery channel</p><p>Discovery channel returned</p><p>Requests for subscribed channels</p><p>Entries returned</p><p>Requests for entries to be stored in the public cache</p><p>Entries returned</p><p>NODE A NODE B</p><p>Figure 12.1 PodNet message exchange.</p><p>on the Internet. Content items are organized into channels, based on the informationthey carry. Each item is an entry of the channel. For example, a blog can be classifiedas a channel, and each new post becomes an entry of the channel. Upon generationof a new channel, the content producer generates a feed (which is an XML file) thatlists the available entries for the channel. Depending on the type of content that itgenerates, each channel can have different requirements. For breaking news channels,the freshness of the entries is most important. On the contrary, music channel entriesremain interesting for months or years after their original publication.</p><p>Entries are exchanged by nodes during pairwise contacts. Besides the user-definedchannels described above, a discovery channel is defined in the PodNet system as acontrol channel that lists the channels cached by the node itself. Consider two nodesthat establish a contact (Figure 12.1). The one that is interested in retrieving newcontent items asks the other for its discovery channel. By reading the information pro-vided through the discovery channel, the node will be informed of the entries availableon the peering node and it will make decisions about which entries to download.</p><p>12.2.2 Content-Centric Dissemination Strategies</p><p>In the simplest case, nodes only download and store those content items that are in-teresting to them. There is no intentional content dissemination in this case, but thenet result is that content items happen to travel across the network based on the inter-ests of users. This can be considered a baseline, unintentional content disseminationprocess. Lenders et al. [13] evaluate the performance improvement that is introducedwhen nodes not only download the entries they are interested to, but also cooperatewith other nodes by making available the unused portion of their cache to entries thatmight be of interest to other nodes. Cache is thus split into two spaces: a private space,reserved to content items of subscribed channels, and a public cache (Figure 12.2).</p></li><li><p>458 DATA DISSEMINATION IN OPPORTUNISTIC NETWORKS</p><p>Channel 1 EntryEntry</p><p>Channel 2 EntryEntry</p><p>Channel 3 EntryEntry</p><p>Channel 5 EntryEntry</p><p>Channel 6 EntryEntry</p><p>Channel 7 EntryEntry</p><p>PrivateCache</p><p>PublicCache</p><p>Figure 12.2 Cache and content organization.</p><p>More specifically, Lenders et al. investigate the problem of which entries should behosted in the public cache in order to best serve the other nodes.</p><p>The popularity of channels among mobile users is assumed to be different: Thereare channels that have more subscribers, other channels that have less subscribers.Statistics on the p...</p></li></ul>

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