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Routing protocols for disruption-tolerant networks (DTNs) use a variety of mechanisms, including discovering the meeting probabilities among nodes, packet replication, and network coding. The primary focus of these mechanisms is to increase the likelihood of finding a path with limited information, and so these approaches have only an incidental effect on routing such metrics as maximum or average delivery delay. In this paper, we present rapid, an intentional DTN routing protocol that can optimize a specific routing metric such as the worst-case delivery delay or the fraction of packets that are delivered within a deadline. The key insight is to treat DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities which determine how packets should be replicated in the system.We evaluate rapid rigorously through a prototype deployed over a vehicular DTN testbed of 40 buses and simulations based on real traces. To our knowledge, this is the first paper to report on a routing protocol deployed on a real DTN at this scale. Our results suggest that rapid significantly outperforms existing routing protocols for several metrics. We also show empirically that for small loads RAPID is within 10% of the optimal performance.

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Abstract-Routing protocols for disruption-tolerant networks (DTNs) use a variety of mechanisms, including discovering the meeting probabilities among nodes, packet replication, and network coding. The primary focus of these mechanisms is to increase the likelihood of finding a path with limited information, and so these approaches have only an incidental effect on such routing metrics as maximum or average delivery delay. In this paper, we present RAPID, an intentional DTN routing protocol that can optimize a specific routing metric such as the worstcase delivery delay or the fraction of packets that are delivered within a deadline. The key insight is to treat DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities which determine how packets should be replicated in the system. We evaluate RAPID rigorously through a prototype deployed over a vehicular DTN testbed of 40 buses and simulations based on real traces. To our knowledge, this is the first paper to report on a routing protocol deployed on a real outdoor DTN. Our results suggest that RAPID significantly outperforms existing routing protocols for several metrics. We also show empirically that for small loads, RAPID is within 10% of the optimal performance.

We ask if the ubiquity of WiFi can be leveraged to provide cheap connectivity from moving vehicles for common applications such as Web browsing and VoIP. Driven by this question, we conduct a study of connection quality available to vehicular WiFi clients based on measurements from testbeds in two different cities. We find that current WiFi handoff methods, in which clients communicate with one basestation at a time, lead to frequent disruptions in connectivity. We also find that clients can overcome many disruptions by communicating with multiple basestations simultaneously. These findings lead us to develop ViFi, a protocol that opportunistically exploits basestation diversity to minimize disruptions and support interactive applications for mobile clients. ViFi uses a decentralized and lightweight probabilistic algorithm for coordination between participating basestations. Our evaluation using a twomonth long deployment and trace-driven simulations shows that its link-layer performance comes close to an ideal diversity-based protocol. Using two applications, VoIP and short TCP transfers, we show that the link layer performance improvement translates to better application performance. In our deployment, ViFi doubles the number of successful short TCP transfers and doubles the length of disruption-free VoIP sessions compared to an existing WiFi-style handoff protocol.

We ask if the ubiquity of WiFi can be leveraged to provide cheap connectivity from moving vehicles for common applications such as Web browsing and VoIP. Driven by this question, we conduct a study of connection quality available to vehicular WiFi clients based on measurements from testbeds in two different cities. We find that current WiFi handoff methods, in which clients communicate with one basestation at a time, lead to frequent disruptions in connectivity. We also find that clients can overcome many disruptions by communicating with multiple basestations simultaneously. These findings lead us to develop ViFi, a protocol that opportunistically exploits basestation diversity to minimize disruptions and support interactive applications for mobile clients. ViFi uses a decentralized and lightweight probabilistic algorithm for coordination between participating basestations. Our evaluation using a twomonth long deployment and trace-driven simulations shows that its link-layer performance comes close to an ideal diversity-based protocol. Using two applications, VoIP and short TCP transfers, we show that the link layer performance improvement translates to better application performance. In our deployment, ViFi doubles the number of successful short TCP transfers and doubles the length of disruption-free VoIP sessions compared to an existing WiFi-style handoff protocol.

Opportunistic connections to the Internet from open wireless access points is now commonly possible in urban areas. Vehicular networks can opportunistically connect to the Internet for several seconds via open access points. In this paper, we adapt the interactive process of web search and retrieval to vehicular networks with intermittent Internet access. Our system, called Thedu, has mobile nodes use an Internet proxy to collect search engine results and prefetch result pages. The mobile nodes download the pre-fetched web pages from the proxy. Our contribution is a novel set of techniques to make aggressive but selective prefetching practical, resulting in a significantly greater number of relevant web results returned to mobile users. In particular, we prioritize responses in the order of the usefulness of the response to the query, which allows the mobile node to download the most useful response first. To evaluate our scheme, we deployed Thedu on DieselNet, our vehicular testbed operating in a micro-urban area around Amherst, MA. Using a simulated workload, we find that users can expect four times as many useful responses to web search queries compared to not using Thedu's mechanisms. Moreover, the mean latency in receiving the first relevant response for a query is 2.7 minutes for our deployment; we expect Thedu to have even better performance in larger cities that have densely populated open APs.

Mobile Internet users have several options today, including such high-bandwidth cellular data services as 3G, which is the choice for many. However, the ubiquity and low cost of WiFi suggests an attractive alternative, namely, opportunistic use of open WiFi access points (APs) or planned municipal mesh networks. Unfortunately, for vehicular users, the intermittent nature of WiFi connectivity presents a challenge for supporting popular interactive applications, such as Web search and browsing. Our work is driven by two questions. 1) How can we enable system support for interactive web applications to tolerate disruptions in connectivity from mobile nodes? 2) Can opportunistic mobile-to-mobile (m2m) transfers enhance application performance over only using APs, and if so, under what conditions and by how much?We present Thedu, a system that enables access to Web search from moving vehicles. The key idea is to use aggressive prefetching to transform the interactive Web search application into a one-shot request/response process. We deployed a prototype of Thedu on the DieselNet testbed in Amherst, MA, which is comprised of transit buses averaging 21 on the road at a time. Our deployment results show that Thedu can deliver a fourfold increase in the number of relevant web pages. Queries from a bus receive relevant web pages with a mean delay of 2.3 minutes and within 0.55 minutes in areas with high AP density. Thedu augments AP connectivity with m2m transfers using a utility-driven DTN routing algorithm and uses caching to exploit query locality. Our analytic model and tracedriven simulations suggest that m2m routing yields little benefit over using APs alone, even under moderately dense AP deployment, such as in Amherst. With sparsely deployed APs as may be the case in rural areas, our conclusions are more mixed: m2m routing with caching improves the number of relevant responses delivered per bus by up to 58%, but the mean delay is significantly high at 6.7 minutes, calling into question its practicality for interactive applications.