UW-Madison COMPSCI 838 Topic - Cabernet - A Content Delivery Network for Moving Vehicles

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Computer Science and Artificial Intelligence LaboratoryTechnical Reportmassachusetts institute of technology, cambridge, ma 02139 usa — www.csail.mit.eduMIT-CSAIL-TR-2008-003 January 17, 2008Cabernet: A Content Delivery Network for Moving VehiclesJakob Eriksson, Hari Balakrishnan, and Sam MaddenCabernet: A Content Delivery Network for Moving VehiclesJakob Eriksson, Hari Balakrishnan and Sam MaddenMIT Computer Science and Artificial Intelligence Laboratory{jakob, hari, madden}@csail.mit.eduAbstractThis paper describes the design, implementation, andevaluation of Cabernet, a system to deliver data to andfrom moving vehicles using open 802.11 (WiFi) accesspoints encountered opportunistically during travel. Net-work connectivity in Cabernet is both fleeting (accesspoints are typically within range for a few seconds) andintermittent (because the access points don’t provide con-tinuous coverage), and suffers from high packet loss ratesover the wireless channel. On the positive side, in the ab-sence of losses, achievable data rates over WiFi can reachmany megabits per second. Unfortunately, current proto-cols don’t establish end-to-end connectivity fast enough,don’t cope well with intermittent connectivity, and don’thandle high packet loss rates well enough to achieve thispotential throughput.Cabernet incorporates two new techniques to improvedata delivery throughput: QuickWifi, a streamlined client-side process to establish end-to-end connectivity quickly,reducing the mean time to establish connectivity from12.9 seconds to less than 366 ms and CTP, a transportprotocol that distinguishes congestion on the wired por-tion of the path from losses over the wireless link to re-liably and efficiently deliver data to nodes in cars. Wehave deployed the system on a fleet of 10 taxis, each run-ning several hours per day in the Boston area. Our ex-periments show that CTP improves throughput by a factorof 2× over TCP and that QuickWifi increases the num-ber of connections by a factor of 4× over unoptimizedapproaches. Thus, Cabernet is perhaps the first practicalsystem capable of delivering data to moving vehicles overexisting short-range WiFi radios, with a mean transfer ca-pacity of approximately 38 megabytes/hour per car, or amean rate of 87 kbit/s.1 IntroductionThis paper describes the design, implementation, and ex-perimental evaluation of Cabernet, a content delivery net-work for vehicles moving in and around cities. Cabernetdelivers data to and from cars using open 802.11 (WiFi)access points (APs) that the cars connect to opportunisti-cally while they travel. Cabernet is well-suited for appli-cations that deliver messages (e.g., traffic updates, park-ing information, event and store information, email) andfiles (e.g., maps, documents, web objects, songs, movies,etc.) to users in cars, as well as applications that delivermessages and data from devices and sensors on cars toInternet hosts [8, 14]. These applications do not requireinteractive end-to-end connectivity between a sender andreceiver, but can tolerate some delay (say, seconds to sev-eral minutes).The network infrastructure in Cabernet consists of WiFiAPs connected using back-haul wired or wireless linksto the Internet, as well as nodes on the Internet runningCabernet software. The cars carry an embedded com-puter running Cabernet software that scans for participat-ing APs, associates with a suitable one, obtains an IP ad-dress, and communicates with Cabernet nodes on the In-ternet. When a car connects via a WiFi AP, it can poten-tially transfer data at the same rates as static clients con-nected to the same network. However, as cars move, theirconnectivity is both fleeting, usually lasting only a fewseconds at urban speeds, and intermittent, with gaps fromdozens of seconds up to minutes before the next time itobtains connectivity. In addition, we observe that packetloss rates over the wireless channel are both high (often20%) and vary over the duration of a single AP associa-tion. Our goal is to develop techniques that will allow carsto obtain high data transfer throughputs as they move inand around cities overcoming these problems, so that theycan take advantage of the high raw transfer rates that arepotentially achievable whenever they are connected.Running stock implementations of current proto-cols (IEEE 802.11 authentication and association, ARP,DHCP, TCP, HTTP) on the cars is sub-optimal for threereasons. First, current client implementations take toolong to scan and associate with an AP, acquire an IP ad-dress, and establish end-to-end connectivity in the faceof the high packet loss rates in this environment. Second,current data transfer protocols do not work well when con-nectivity is only a few seconds long and loss rates are high(as is the case for moving cars). Third, current protocolsdon’t cope well with short-lived, intermittent connections.Cabernet incorporates novel techniques to overcomethese weaknesses. To reduce the time between when the1wireless channel to an AP is usable and when Internetconnectivity through the AP is actually achieved, we in-troduce QuickWifi, a streamlined process that combinesall the different protocols involved in obtaining connec-tivity (across the 802.11 and IP layers) into a single statemachine. To improve end-to-end throughput over lossywireless links, we propose CTP, an alternative to TCP,which avoids the problem by measuring congestion onlyon the wired section of each path.We make two main contributions. First, we describe thedesign and prototype implementation of Cabernet’s archi-tecture and protocols, focusing on QuickWifi and CTP.Second, we present the results of several experiments thatevaluate the performance of the implemented system ona 25-node real-world vehicular testbed running on taxisin the Boston area that we have deployed and maintainedover the past year.Except where noted, our measurement results wereobtained using nodes installed in 10 taxis operating inthe Boston metropolitan area. Each node is a Soekris4801 equipped with an Ubiquity Networks SR2 (Atheroschipset) 802.11b/g radio, a 3 dBi omni-directional an-tenna mounted inside the passenger compartment, and aGPS receiver. Although Cabernet supports data transfersin both directions, our evaluation in this paper focuses ontransfers from Internet hosts to the car.We carefully consider only those time periods in whichthe cars are actually moving: if a car stops for more than2 minutes, that time period is


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UW-Madison COMPSCI 838 Topic - Cabernet - A Content Delivery Network for Moving Vehicles

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