Wireless Community Networks: An Alternative Approach - Columbia

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1 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 206 ACCEPTED FROM OPEN CALL Wireless Community Networks: An Alternative Approach for Nomadic Broadband Network Access Pantelis A. Frangoudis and George C. Polyzos, Athens University of Economics and Business Vasileios P. Kemerlis, Columbia University ABSTRACT forming a scan for wireless network presence in such a setting reveals so high a number of wire- Low-cost wireless LAN equipment operating less access points (APs) in the neighborhood in unlicensed spectrum has revolutionized local that we should be more concerned about inter- area communications, introducing novel schemes ference than coverage. AP density in metropoli- for open wireless connectivity and even new tan areas has made it possible to build a business business models. The ease of deployment of based just on their existence and not their com- Wi-Fi has made it ubiquitous in densely populat- munication capabilities [1]. ed urban areas, and it was just a matter of time Wireless Community Networks (WCNs) have before wireless communities emerged. Using been developed as grassroots movements of inexpensive wireless technology to communicate, WLAN enthusiasts, who use inexpensive net- autonomous wireless internetworks have been working equipment for free interconnection, built, offering a variety of broadband services. thus creating all-wireless autonomous networks. Based on our own experience participating in Reasons for their emergence can be found in the one of the largest wireless communities world- above discussion. However, their success also wide, the Athens Wireless Metropolitan Net- depends on local factors (e.g. degree of penetra- work, we attempt to explain their structure and tion of fixed broadband access services in the mechanisms, study the reasons for their emer- area). Based on the offered wireless connectivity, gence and growth, and predict their future evo- such networks aim at providing a variety of ser- lution. We expect that future wireless vices, with free community-wide VoIP and file communities will be oriented toward providing sharing topping the list of the most frequently broadband access to nomadic users. To this end, accessed services. we have proposed that wireless network access Increased Wi-Fi coverage and the emer- be shared in a peer-to-peer, reciprocal manner. gence of WCNs raise the question of whether Thus, ubiquitous broadband Internet access can low-cost alternatives to traditional mobile ser- be achieved at a low cost, at least in metropoli- vices offered by 2.5G and 3G systems can be tan areas where there is abundant wireless cov- achieved. We focus on nomadic users, i.e. roam- erage, providing an inexpensive alternative to ing users accessing broadband services from broadband cellular services. multiple fixed locations and without necessarily sustaining uninterrupted connectivity while INTRODUCTION moving. In selected urban areas the answer is clearly yes, and the trend is in the direction of For wireless local area network communications, adequate coverage in all urban areas [2]. Before Wi-Fi emergence was a true revolution. Driven this idealistic environment is a reality, though, by their low cost and ease of deployment, IEEE issues such as the incentives of private WLAN 802.11-enabled devices became standard equip- owners to permit public access to their APs and ment for laptops and handheld devices and relevant security concerns need to be resolved. appeared as the predominant technology for As to WCNs, they are mainly exclusive; offering local wireless connectivity. Operation in unli- services to (potentially mobile) nonmembers censed spectrum facilitated Wi-Fi deployment, occurs today only due to the altruism of some since it was straightforward for commercial oper- of their participants. The architecture of such ators, academic institutions, or even plain radio networked communities and incentives that communications enthusiasts and tech-savvy users keep them operating with robustness are worth This article was written to build wireless service architectures on top of studying. while V.P. Kemerlis was it, without the need for acquiring a license. In In the remainder of this article, we first clas- at the Athens University of modern densely populated urban areas, the cov- sify public wireless access schemes and report on Economics and Business. erage that WLANs offer is ever-growing. Per- some of the most significant WCNs worldwide. 206 0163-6804/11/$25.00 2011 IEEE IEEE Communications Magazine May 2011

2 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 207 Access point WiFi interface with Mesh software router Point-to-point link Mobile user/client directional antenna Internet Internet Internet (a) (b) Figure 1. Architectural alternatives for Wireless Community Networks. A wireless mesh architecture is shown in (a), while the hotspot- based alternative is shown in (b). We describe the architecture of community wire- Commercial Initiative Following the above less mesh networks. Then we discuss the circum- trend, commercial players have entered the stances under which WCNs emerged and scene, offering mediation services for the devel- evolved, respectively. We present the incentive opment of wireless communities. FON [9], for mechanisms that regulate their operation and instance, has proposed a private hotspot shar- study their future. We discuss our views and pro- ing scheme, where WLAN owners can either posal for WLAN hotspot sharing and conclude share their WLANs for a small monetary com- the article. pensation or in exchange for similar service when they are away from their own WLAN. FON takes care of user registration and authen- A CLASSIFICATION OF PUBLIC tication and withholds a fraction of the money paid to the hotspot micro-operator for the pro- WIRELESS ACCESS SCHEMES vided service. Here we characterize WCNs based on the initia- Notably, British Telecom has recently part- tive behind their emergence and their architec- nered1 with FON so that hundreds of thousands ture. of BTs subscribers share their home broadband lines over Wi-Fi with other community members. INITIATIVES Community Initiative A major focus of this Municipal Initiative Municipalities often work is on wireless communities that are the set up APs in public spaces, offering inexpen- result of collective efforts of individual volun- sive Internet access to citizens. To achieve this, teers and function on a not-for-profit basis. Suc- they may get into agreements with private com- cessful communities have emerged, members of panies, permitting them to deploy their wireless which use IEEE 802.11/Wi-Fi technologies to set solutions. Authentication with the operator of up a wireless backhaul to connect to one-anoth- the network, as well as a fee for the service, er, enjoying a variety of broadband services, may be required. This model has been adopted such as VoIP, online games, FTP, or Web access by the municipality of Philadelphia [10], as well [3][7]. Sometimes community members operate as the City of London, which has set up a deal public hotspots to offer wireless access to with The Cloud, a European Wi-Fi hotspot passers-by, attaching them to the community aggregator. network, or even offering Internet access through community-owned Internet gateways. DESIGN ALTERNATIVES In a similar fashion, an individual WLAN Wireless Mesh WCNs that have sprung owner may open his private hotspot for public from private initiatives often have this structure. access without anticipating monetary compensa- Multi-interface nodes set up a wireless mesh. tion. Instead, he is either driven by pure altru- Some also act as gateways to the public Internet ism, or expects that his offering will be and can be reached over the wireless backbone. reciprocated by other community members when We offer a more in-depth discussion of this he is roaming near foreign hotspots [8]. architecture in Fig. 1a. 1 http://www.bt.com/btfon IEEE Communications Magazine May 2011 207

3 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 208 Network Location Founded Size Architecture Initiative SeattleWireless Seattle, WA, USA 2000 ~80 nodes Mesh Community AWMN Athens, Greece 2002 2473 nodes Mesh Community CUWiN Urbana, IL, USA 2002 48 nodes Mesh Community Berlins Freifunk Berlin, Germany 2002 316 nodesa Mesh Community Wireless Leiden Leiden, Netherlands 2002 73 nodes Mesh Community NetEquality Roofnet Portland, OR, USA 2007 126 nodes Mesh Community NYCwireless New York City, USA 2001 145 nodes Hotspot-based Community Wireless Philadelphia Philadelphia, PA, USA 2007 15 miles2 Hotspot-based Municipal FON Worldwide 2006 ~700,000 hotspotsb Hotspot-based Commercial a Circa 2007 [6] b Based on information available from http://www.fon.com, as of September 2009. Table 1. Wireless communities around the world (as of September 2009). Hotspot-based Architecture Hotspot-based is to provide a free citywide all-wireless network WCNs typically target nomadic users who use in the city of Leiden and offer free broadband wireless hotspots to access the Internet (Fig. 1b). Internet access to nearby villages, where no fast Here, deploying a wireless backhaul is not the Internet alternatives exist. norm. Municipality-initiated WCNs usually have Recently, the Freifunk community has gained this structure. Sometimes, hotspot-based WCNs much popularity. Freifunk mesh networks have are built relying on the private contributions of sprung up in various German cities, as well as in individual WLAN owners, who share their fixed cities in Austria and Switzerland. In Berlin, Frei- broadband lines over Wi-Fi. We have witnessed funk counted 316 concurrent participating nodes both commercially initiated [9] and not-for-profit on average, according to a 2007 study [6]. [8] such attempts. The MIT Roofnet [12] mesh network started as a research project focusing on wireless multi- hop routing and IEEE 802.11 protocol perfor- WIRELESS COMMUNITIES mance, while offering Internet access to nearby residents. It is now less vibrant (approximately AROUND THE WORLD 20 active nodes), but its technology is used from Here we report on some of the most significant other wireless community projects. WCNs worldwide and categorize them as described earlier. Our findings are summarized in Table 1. The reported network dimensions ARCHITECTURE OF A are based on data found on publicly available COMMUNITY WIRELESS MESH node maps and measurement studies [6]. We have chosen representative WCNs and NODE TYPES tried to include the most well-known and influ- There are two types of nodes. Backbone nodes ential ones. SeattleWireless [3], for instance, has are those that the backhaul of the network is been at the forefront of the WCN movement built upon. They typically have more than two since the early 2000s. network interfaces and run routing software. Also, NYCwireless [11] and the CUWiN Some backbone nodes operate omni-directional Foundation [5] are active in advocating the use antennas and function also as APs. Client nodes of open wireless technologies, fostering the can then attach to these APs and use the net- growth of WCNs and developing software for works infrastructure. Clients can be considered community wireless projects. They are operated the leaves of the network. by nonprofit organizations, whose interest has Typically, WCN participants discouraged new recently been drawn to developing free wireless users from operating client nodes, since clients access solutions for under-served communities, usually do not contribute to the networks opera- both in the US and in other countries. tion and coverage. In practice, being a client is The Athens Wireless Metropolitan Network usually the first step a user takes when joining (AWMN) [4] is one of the largest community such a community. mesh networks in the world. Currently it has more than 9000 registered nodes, with more LINKS than 2400 of them being active. Wireless Leiden The backbone of the network is based on direc- [7] is a similar effort in the Netherlands. Its aim tional point-to-point links. Although IEEE 208 IEEE Communications Magazine May 2011

4 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 209 1 Directional antenna for an IEEE 802.11a point-to-point backbone link 2 Access point targeting the nearby park (IEEE 802.11b) 3 Linux based mesh router 1 1 1 2 3 (a) (b) Figure 2. Pictures from MMlabs AWMN node. Three of our backbone links, as well as our AP, are visible in (a). IEEE 802.11 wireless interfaces are attached to hand-made antennas. In (b), the Linux-powered rooftop PC, which hosts our wireless interfaces and per- forms routing, is shown. 802.11 was designed as a broadcast protocol for members share their fixed broadband connec- local communication, it is widely used for setting tions with the community, so that Internet access up long-distance links, assisted by directional is achieved through WCN-to-Internet proxies. antennas. In the beginning, 802.11b was the pro- For other services see [13]. tocol of choice, but with the advent of 802.11a, it was replaced in the backbone in order to mini- mize contention and interference from clients THE BIRTH OF and APs. IEEE 802.11b is still used by APs. A recent study [13] of the evolution of AWMN WIRELESS COMMUNITIES revealed that many backbone nodes maintain up The appearance of WCNs dates back to the late to six backbone links. The more links per node, 1990s-early 2000s, when IEEE 802.11 was intro- the higher path redundancy is achieved. duced. Radio technology enthusiasts were the first to embrace the new technology and experi- ADDRESSING AND ROUTING ment with it for long distance interconnection. Mesh-based WCNs have the same functionality, Thus, communities of interconnected peers underlying mechanisms and applications as the emerged in a period when fixed wireline infra- public Internet. They are based on an IP layer structure could not always support them. One and follow a private addressing scheme. In most can consider the first wireless community net- of them (e.g., AWMN), a central authority tack- works as the evolution of amateur radio. les addressing issues. Each newly-registered As wireless technology matured and gained node is assigned a private IP address range. popularity and as more advanced WLAN stan- Every community follows its own routing dards emerged, WCNs started to grow. Howev- scheme, often involving BGP, much like the er, one of the major factors that contributed to Internet itself. From the BGP viewpoint, each their growth was the low penetration of broad- WCN node can be considered a single band access technologies in some countries. autonomous system. In some cases, experimenta- In addition, some wireless communities [12] tion with routing protocols suited for wireless ad have been developed as experimental testbeds hoc networks, such as OLSR, is carried out. for wireless research. In these cases, free wire- less Internet connectivity is a side effect, while UNDERLYING TECHNOLOGIES the main goal of such WCNs is to test novel Open source software and often hand-crafted wireless technologies and evaluate current hardware are used for interconnection. Linux- WLAN standards. powered embedded rooftop equipment is used for controlling the operation of each node, while much experimentation involves the design and GROWTH construction of low-cost custom-made antennas. One of the reasons AWMN has grown to be one Figure 2 depicts part of the infrastructure of the of the largest such networks worldwide was the AWMN node that we have set up on the rooftop fact that, in Greece, DSL penetration used to be of one of AUEBs buildings. one of the lowest in the European Union before 2005. Actually, back in 2002, when the first SERVICES AND APPLICATIONS AWMN node was set up, there was a single The most prevalent services are file sharing and operator offering DSL to corporate clients, with community-wide VoIP. Also, on some occasions, download speeds of at most 128Kbps and a very IEEE Communications Magazine May 2011 209

5 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 210 high price. Setting up long-distance wireless links Most WCNs are open for participation, but There is no direct using IEEE 802.11 to achieve cheap citywide have some structure and their deployment is not broadband connectivity seemed to be a one-way completely anarchic. In order for the system to and measurable gain street for tech-savvy users. At the time, the most operate without disruptions, nodes should follow for an AP to offer successful applications were fast file sharing and some rules, especially concerning addressing and connectivity to online gaming, as well as Internet connectivity routing. Also, they should not behave selfishly, through AWMN-to-Internet gateways (when since that could potentially cause congestion to clients. However, in some participants shared their fixed broadband the backbone links, as well as saturation of some the long run, there is Internet lines with the community). services. In case a user does not behave, there is The network evolved and gained publicity high probability that he will eventually be exclud- a potential gain for through word-of-mouth and dissemination activi- ed from the network. At a technical level, non- the community as a ties of its members, which included demonstra- conformant behavior can be easily detected from whole, since clients tions at technology expos and universities and ones neighbors and reported to the community. participation in festivals and other public venues, Exclusion can also easily be implemented; are anticipated to where free wireless connectivity was offered neighboring users can simply shut down the links bring more content through the AWMN infrastructure. they share with the user that causes anomalies to A key technological shift was the adoption of the WCNs operation. Thus, the latter will be and are expected to the new IEEE 802.11a variant for setting up disconnected and isolated from the community. gradually upgrade point-to-point backbone links. Moving from In fact, we have witnessed similar incidents with- their nodes to back- 2.4GHz to the less congested 5GHz band, inter- in AWMN. ference was limited. Nodes with multiple collo- Incentives for the maintenance of the first bone ones. cated interfaces had more non-overlapping two links of a backbone node are quite straight- channels to choose from for each of their radio forward WCN participation. Additional links interfaces, while links of higher throughput add redundancy, network bandwidth, and repu- could be achieved. Backbone nodes could sus- tation in the community. There is expressed tain more links, which offered path redundancy mutual interest for the two link endpoints to set and resilience. Citywide VoIP and video confer- one up. As soon as one of the two nodes defects, encing within the community, as well as file shar- the link becomes inoperative. ing, have thrived since. Increased capacity, better The case is different for client-to-AP links. quality of service, and the fact that backbone There is no direct and measurable gain for an nodes could now sustain more links caused a AP to offer connectivity to clients. However, in population boom in the WCN. A direct conse- the long run, there is a potential gain for the quence was the expansion of its coverage to community as a whole, since clients are antici- most of the suburbs of Athens. pated to bring more content and are expected to gradually upgrade their nodes to backbone ones. Thus, serving clients is considered a means of INCENTIVES FOR SUSTAINABLE attracting attention to the community and gradu- WIRELESS COMMUNITIES ally recruiting more resources. INCENTIVES FOR PARTICIPATION BUILDING REPUTATION Why would one participate in a WCN? Usually Interestingly, senior WCN members and mem- it is the enthusiasm stemming from experimenta- bers who own powerful 2 nodes are highly tion with new technologies or the joy of creation esteemed in the community. Because their nodes offered by building ones own equipment and are important for the operation of the network, configuring ones node. Also, the services pro- their decisions may affect many users. vided by the community are tempting, especially Building up reputation within a WCN can when the respective services over commercial easily be explained. In such communities, partici- (fixed) broadband infrastructure are of worse pation anonymity is not easy (or even possible) quality, or even non-existent. to achieve, nor is it desirable. Recruiting new Those who believe that broadband connectiv- members is an incremental process, where older ity should be unleashed and the barriers imposed members introduce new ones to the community, by the oligopoly of ISPs be removed and that offering them connectivity. broadband access should be a public good usual- The infrastructure-based nature of the net- ly are the first to join WCNs. Under this per- work is such that long-term relationships among spective, WCNs can be considered a modern members are built; nodes are fixed and links are technological movement. Operation in unli- typically permanent. Most of the time, owners of censed spectrum facilitates their growth and, nodes that share a link know each other person- coupled with the extensive use of open source ally, while the community encourages socializa- software, shapes their free and self-organizing tion among its members. Contribution to the character. It is atypical of their participants to community is directly attributed to its initiators, think of community-initiated WCNs as the vehi- while unacceptable behavior is detected, report- 2 Here we refer to back- cle to make profit or develop a new business. ed and, potentially, punished, as described earli- bone nodes with many er. links, hosting multiple ser- CONFORMANCE AND CONTRIBUTION vices or operating an AP. As far as WCNs such as AWMN are concerned, They are important for the contribution to the community and abiding with A LOOK TO THE FUTURE OF WCNS network, since much of its the explicit or implicit participation rules is To gain insight into how WCNs will evolve, one traffic flows through them enforced in a tit-for-tat manner. Users that do has to reconsider the factors that led to their and many other nodes rely not conform to such rules can be effectively emergence and booming growth. on them for connectivity. excluded from the community. First, low-cost broadband services, once one 210 IEEE Communications Magazine May 2011

6 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 211 of the highlights of WCNs, have now become a PEER-TO-PEER HOTSPOT SHARING commodity in most modern metropolitan areas. To lower the entry No more is joining a WCN the only way to We have proposed [8] that wireless Internet enjoy high-quality multimedia services or fast bandwidth be exchanged in a reciprocal manner; barrier to the system, file sharing. one shares his Internet connection with anony- no registration with On the positive side, though, promising new mous passers-by over Wi-Fi with the anticipation central authorities is wireless technologies, such as the IEEE 802.11n that he will enjoy the same benefit from another standard, are about to be integrated. These may peer when mobile. That is, our approach to the required, nor any revitalize interest in WCNs and cause a new problem is based on the peer-to-peer paradigm. strong user identifi- wave of development in the area. On the other Private WLAN owners have an incentive to con- hand, an issue that is generally still unresolved is tribute Internet bandwidth, given that they place cation scheme. Par- how such broadband services will be offered for high value on the mobile network access they ticipants are mobile users at low cost. 3G-based solutions are will enjoy as good contributors. We call our identified by self- still considered expensive by many users and scheme the Peer-to-Peer Wireless Network Con- their adoption is relatively low. Basing our argu- federation (P2PWNC). issued, uncertified ments on the increased wireless coverage in To lower the entry barrier to the system, no public-secret key modern metropolitan areas, we believe that registration with central authorities is required, open wireless access schemes can become a nor any strong user identification scheme. Par- pairs. To join viable alternative for the provision of nomadic ticipants are identified by self-issued, uncertified P2PWNC, users sim- broadband network access. To support this public-secret key pairs. To join P2PWNC, users ply configure their action, traditional WCNs may need to become simply configure their APs for open access and less exclusive so that they are accessible not only install the necessary software. APs for open access strictly to their participants, but also to roaming Accounting is based on digital proofs of ser- and install the neces- non-members. vice (receipts) that mobile users provide to visit- To this end, opportunistic WLAN access ed APs. Receipts are stored in repositories sary software. based on the private contributions of individual distributed across the system; each peer main- WLAN owners will likely attract more attention tains his own repository, which represents his as a vehicle to make inexpensive ubiquitous (partial) view of the systems history of service broadband Internet a reality. WLAN owners can provisions. become micro-operators that trade bandwidth Receipt repositories (RR) are the input to for payment in kind or even money. In the fol- the reciprocity algorithm, which identifies good lowing section we present our architecture contributors and detects free riders, i.e. those toward this vision. who consume resources without contributing to the community. Each time a mobile user requests WLAN SHARING service, the reciprocity algorithm is executed by the visited peer to decide whether the visitor is a FUNDAMENTAL CHALLENGES good contributor and deserves to be reciprocat- Sharing ones Internet connection with roaming ed. individuals incurs direct and indirect costs or P2PWNC is decentralized and no registration may be forbidden by the ISP. Users may have to is mandated. Other similar schemes, such as put up with the managerial overhead of config- FON [9], do not have these properties. Also, uring their WLAN for shared access. Also, P2PWNC has some inherent privacy enhance- admitting more users to ones private network ments. The non-persistence of user identifiers, as can result in reduced service level for its owner. well as the fact that the disclosure of real-world Sometimes, WLANs are attached to metered user identities is not needed, assist in WLAN broadband lines. Sharing such a WLAN means roaming anonymity. We have also shown that a that its operator pays for every single byte decentralized architecture for secure and private uploaded or downloaded by visitors. Security voice and multimedia communications can be concerns also make individuals reluctant to open built on top of our scheme [14]. their WLANs for public access. The challenge for the system designer is to minimize the MECHANISMS AND PROTOCOLS incurred costs, while revealing the benefits for Here we present an overview of the P2PWNC users to join and contribute their resources. operations, also depicted in Fig. 3. Joining the system should be possible with minimal installation cost, both financially and Receipts, Receipt Repositories and the effort-wise. User registration should be fast and Reciprocity Algorithm The P2PWNC decentralized, to facilitate the systems organic accounting unit is a receipt, a document digitally growth and scaling. From the nomadic users signed by a service consumer (mobile user) using perspective, wireless access should have low cost his private key. It contains the provider and con- compared to its cellular alternatives. Also, roam- sumer identities, a timestamp, and information ing privacy is a desirable feature; a user may on the amount of traffic forwarded by an AP on wish to access foreign APs without disclosing behalf of the signer during a P2PWNC session. personal information to the service provider or Receipts represent debt among peers, which his network point of attachment to his communi- we assume to be transitive; if peer A has provid- cating endpoints. ed service to peer B and the latter has served Note that our focus is on low-mobility, peer C, then C indirectly owes some service to nomadic users and our emphasis is on access A. As contribution can be reciprocated when he issues rather that true mobility support. Hand- visits one of Bs or Cs APs. With each receipt ing-off efficiently between foreign visited encoding an I-owe-you relationship, a RR can WLANs is a significant challenge. be viewed as a logical directed graph, whose ver- IEEE Communications Magazine May 2011 211

7 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 212 (a) Gossiping: client updates his portable RR subset (b) Gossiping: client shows receipts to visited AP 1 UPDT RCPT RCPT 2. RCPT (c) Session establishment (d) Receipt generation (e) Session termination 1. CONN 2. QUER RCPT 2. Timeout 3. RCPT 4. CACK 3. QRSP RREQ 1. RREQ Internet Internet P2PWNC-capable WLAN AP Mobile user A peers receipt repository (RR) Part of a peers RR carried by mobile user Figure 3. P2PWNC operations. a) First, a user communicates with his home RR and updates the RR subset that he carries with him. b) Then, before requesting service from a visited AP, he presents it with the receipts he carries with him. c) Following, he issues a con- nection request, upon which the AP consults his own RR, where the reciprocity algorithm is invoked, and decides to accept the user or not. d) In the first case, Internet access is granted and a receipt request-response cycle begins. e) When a client fails to deliver a receipt on time, the session terminates and the AP forwards the last receipt to his RR. The messages exchanged during the above processes are summarized in Table 2. tices correspond to peers and its edges represent Receipt Dissemination A visited peer uses receipts and point from a service consumer to a only his own view of the systems history of service provider. transactions as input to the reciprocity algo- Such a graph is the input to the reciprocity rithm. To assist in giving potential service pro- algorithm, which is invoked each time a user viders a better view of their overall contribution requests to be served. Its output is a value express- and have better chances of getting access, visi- ing a users reputation in the eyes of a prospec- tors can also supply parts of their own RRs via a tive service provider. Given that accounting is gossiping protocol. Gossiping takes place at the decentralized and user identities are self-issued, beginning of a session; the visitor presents the the reciprocity algorithm should be intelligent AP with a subset of his own RR carried in his enough to detect free riders and prevent from mobile device. These receipts are then merged attacks to the accounting mechanism [8]. with the visited peers RR and reveal service directly or indirectly owed to the visitor. Note Session Initiation and Receipt Generation that because the receipts are cryptographically Receipts are generated on a per-session basis. A signed, invalid receipts do not help the present- prospective service consumer requests access to a ing party. A peer regularly queries his RR for P2PWNC-controlled Wi-Fi AP by presenting his updates to his portable RR subset, since RR identity. Following, the AP invokes the reciproci- contents change frequently. ty algorithm and the RR responds whether the client should be admitted to the visited WLAN. Implementation Issues P2PWNC entities If so, a receipt request-response cycle begins; the communicate using a simple ASCII-based proto- AP periodically requests an acknowledgement for col running over TCP/IP. Together with the the volume of Internet traffic forwarded on reciprocity algorithm, they have been designed behalf of the visitor thus far, and the latter and implemented with resource-constrained responds with a cumulative fresh receipt. After devices in mind, such as off-the-shelf wireless a receipt request has timed out (e.g., when the routers and PDAs. P2PWNC typically runs in visitor leaves) or upon receiving a malformed the firmware of such devices. The protocol mes- receipt (that is, one that the AP is incapable of sages exchanged among the various entities are cryptographically verifying), the session termi- summarized in Table 2. nates and the AP forwards the last receipt to his RR. This last receipt contains information about the sessions aggregate amount of traffic. Notice CONCLUSION that the receipt generation protocol ensures that WCNs have emerged as a means for inexpen- a visitors session has not been hijacked by an sive broadband connectivity in metropolitan unauthorized party. In order to sign a receipt and areas. In this article, we discussed the reasons maintain the session, the hijacker would need for their emergence and evolution and access to the visitors private key. described their architecture and the incentives 212 IEEE Communications Magazine May 2011

8 FRANGOUDIS LAYOUT 4/20/11 2:11 PM Page 213 relevant to their operation. With the abundance Message Description Direction of wireline broadband connections, future WCNs may evolve toward offering connectivity to nomadic users. Based on the increased wire- CONN P2PWNC session initiation request Client AP less coverage in metropolitan areas today, decentralized schemes such as P2PWNC can CACK P2PWNC session initiation response AP Client make WCN-based Internet access a viable low- cost alternative to the more expensive cellular RREQ Receipt request AP Client services. Client AP ACKNOWLEDGMENT RCPT Receipt AP RR We thank the student volunteers who helped set RR Client up our AUEB AWMN node and the whole P2PWNC team for their invaluable contributions Query the RR (invoke the reciprocity QUER AP RR in this area. Special thanks to Elias Efstathiou algorithm) for his valuable comments and support. QRSP Query response RR AP REFERENCES [1] Skyhook Wireless, available: http://www.skyhookwire- UPDT Client (portable) RR update request Client RR less.com. [2] G. C. Polyzos, A Peer-to-Peer Approach to Cellular Table 2. P2PWNC protocol messages. Communications, Proc. 9th ACM Intl. Symp. Modeling Analysis and Simulation of Wireless and Mobile Sys- tems (MSWiM06), Torremolinos, Spain, 2006. [3] Seattle Wireless, Available: http://www.seattlewireless. BIOGRAPHIES net. P ANTELIS A. F RANGOUDIS ([email protected]) is a Ph.D. candi- [4] Athens Wireless Metropolitan Network, available: http: date at the Department of Informatics, Athens University //www.awmn.net. of Economics and Business and member of the Mobile [5] Cuwin Community Wireless, available: http://www. Multimedia Laboratory. He holds a B.Sc. (2003) and an cuwireless.net M.Sc. (2005) in Computer Science from the same depart- [6] B. Milic and M. Malek, Analyzing Large Scale Real- ment. His current research interests include wireless net- World Wireless Multihop Network, IEEE Commun. Let- works, Internet multimedia and network security. ters, vol. 11, no. 7, July 2007, pp. 58082. [7] R. J. Kramer, A. Lopez, and A. J. Koonen, Municipal VASILEIOS P. KEMERLIS ([email protected]) is a Ph.D. can- Broadband Access Networks in the Netherlands didate in the Department of Computer Science at Columbia Three Successful Cases, and How New Europe May Ben- University, where he is also a member of the Network efit, Proc. AccessNets 06, Athens, Greece, Sept. 2006. Security Laboratory. He holds a B.Sc. with honors (2006) [8] E. C. Efstathiou, P. A. Frangoudis, and G. C. Polyzos, from the Athens University of Economics and Business, Stimulating Participation in Wireless Community Net- Greece and an M.Sc. (2010) from Columbia University, NY, works, Proc. IEEE INFOCOM, Barcelona, Spain, Apr. USA, all in Computer Science. His research interests include 2006. systems and network security, distributed systems, and [9] Fon, available: http://www.fon.com performance analysis. [10] Wireless Philadelphia Executive Committee, available: http://www.phila.gov/wireless. G EORGE C. P OLYZOS ([email protected]) is a professor of [11] NYCwireless, available: http://www.nycwireless.net. Computer Science at AUEB since 1999, has founded and is [12] J. Bicket et al., Architecture and Evaluation of An leading the Mobile Multimedia Laboratory. Previously, he Unplanned 802.11b Mesh Network, Proc. 11th ACM was Professor of Computer Science and Engineering at the Annual Intl. Conf. Mobile Computing and Networking University of California, San Diego, where he was co-direc- (Mobicom05), Cologne, Germany, 2005. tor of the Computer Systems Laboratory, member of the [13] F. Elianos et al., Structure and Evolution of A Large- Steering Committee of the UCSD Center for Wireless Com- Scale Wireless Community Network, Proc. 10th IEEE munications and Senior Fellow of the San Diego Supercom- Intl. Symp. World of Wireless, Mobile and Multimedia puter Center. He has received his Dipl. in EE from the Networks (WoWMoM 2009), Kos, Greece, 2009. National Technical University in Athens, Greece and his [14] E. C. Efstathiou, P. A. Frangoudis, and G. C. Polyzos, M.A.Sc. in EE and Ph.D. in Computer Science from the Uni- Controlled Wi-Fi Sharing in Cities: A Decentralized versity of Toronto. His current research interests include Approach Relying on Indirect Reciprocity, IEEE wireless networks, mobile multimedia communications, Trans. Mobile Comp., vol. 9, no. 8, Aug. 2010, pp. ubiquitous computing, Internet protocols and architecture, 114760. security, and system performance evaluation. IEEE Communications Magazine May 2011 213

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