Feasibility of Voice over IP on the Internet

VoIP (Voice over IP) services are using the Internet infrastructure to enable new forms of communication and collaboration. A growing number of VoIP service providers such as Skype, Vonage, Broadvoice, as well as many cable services are using the Internet to offer telephone services at much lower costs. However, VoIP services rely on the user's Internet connection, and this can often translate into lower quality communication. Overlay networks offer a potential solution to this problem by improving the default Internet routing and overcome failures. To assess the feasibility of using overlays to improve VoIP on the Internet, we have conducted a detailed experimental study to evaluate the benefits of using an overlay on PlanetLab nodes for improving voice communication connectivity and performance around the world. Our measurements demonstrate that an overlay architecture can significantly improve VoIP communication across most regions and provide their greatest benefit for locations with poorer default Internet connectivity. We explore overlay topologies and show that a small number of well-connected intermediate nodes is sufficient to improve VoIP performance. We show that there is significant variation over time in the best overlay routing paths and argue for the need for adaptive routing to account for this variation to deliver the best performance

Reducing COPD Exacerbation Among African Americans Affected with COPD in a South Florida Clinic. A Quality Improvement Project

Abstract Objective: The study aimed to determine if an educational intervention will improve COPD exacerbations in African American population after a 6-week self-management program that includes the proper use of an inhaler. Methods: Ten patients were recruited in a primary care setting with the assistance of the providers. All patients had to complete a pretest and a posttest to assess their knowledge of inhaler technique, their willingness to engage in self-efficacy disease management, their symptoms improvement and treatment adherence. The Inhaler Device Assessment Tool, the Self-Efficacy for Managing Chronic Disease 6-Item Scale, the CAT test, and the TAI test were the tools used for the project. Results: In the recruited population, 70 % were females and 30% were males. In the pretest and posttest of the inhaler technique, there is an improvement difference of 1.5 %, with a mean of 3.1 % in the pretest, and 4.6 % in the posttest. The CAT test has a mean of 17.9 with a standard deviation (SD) of 7.25 in the pretest, and a mean of 9.9 and a SD of 6.21. The CAT test has difference of 8. Self-efficacy management shows a difference of 3.4 with a mean value of 53 and 56.4 in the pretest and posttest respectively. Inhaler adherence has a mean value of 46.4 in the pretest and 49.3 in the posttest with a difference of 2.9 in inhaler adherence improvement. Conclusion: The study reveals that teaching the proper use of an inhaler can improve COPD symptoms and decrease the frequency of COPD exacerbations in African American population after a 6-week self-management program, but further studies are needed to validate the findings

Mitigating the Effect of Free-Riders in BitTorrent using Trusted Agents

Even though Peer-to-Peer (P2P) systems present a cost-effective and scalable solution to content distribution, most entertainment, media and software, content providers continue to rely on expensive, centralized solutions such as Content Delivery Networks. One of the main reasons is that the current P2P systems cannot guarantee reasonable performance as they depend on the willingness of users to contribute bandwidth. Moreover, even systems like BitTorrent, which employ a tit-for-tat protocol to encourage fair bandwidth exchange between users, are prone to free-riding (i.e. peers that do not upload). Our experiments on PlanetLab extend previous research (e.g. LargeViewExploit, BitTyrant) demonstrating that such selfish behavior can seriously degrade the performance of regular users in many more scenarios beyond simple free-riding: we observed an overhead of up to 430% for 80% of free-riding identities easily generated by a small set of selfish users. To mitigate the effects of selfish users, we propose a new P2P architecture that classifies peers with the help of a small number of {\em trusted nodes} that we call Trusted Auditors (TAs). TAs participate in P2P download like regular clients and detect free-riding identities by observing their neighbors' behavior. Using TAs, we can separate compliant users into a separate service pool resulting in better performance. Furthermore, we show that TAs are more effective ensuring the performance of the system than a mere increase in bandwidth capacity: for 80\% of free-riding identities a single-TA system has a 6\% download time overhead while without the TA and three times the bandwidth capacity we measure a 100\% overhead

Search for domain wall dark matter with atomic clocks on board global positioning system satellites

Cosmological observations indicate that 85% of all matter in the Universe is dark matter (DM), yet its microscopic composition remains a mystery. One hypothesis is that DM arises from ultralight quantum fields that form macroscopic objects such as topological defects. Here we use GPS as a ~ 50,000 km aperture DM detector to search for such defects in the form of domain walls. GPS navigation relies on precision timing signals furnished by atomic clocks hosted on board GPS satellites. As the Earth moves through the galactic DM halo, interactions with topological defects could cause atomic clock glitches that propagate through the GPS satellite constellation at galactic velocities ~ 300 km/s. Mining 16 years of archival GPS data, we find no evidence for DM in the form of domain walls at our current sensitivity level. This allows us to improve the limits on certain quadratic scalar couplings of domain wall DM to standard model particles by several orders of magnitude.Comment: 7 pages (main text), and 12 pages for Supplementary Information. v3: Update titl

Functions Returning Values of Dynamic Size

Modern programming languages, such as Ada (Ichbiah 80), permit the definition of functions that return values whose size can not be determined until the function returns. This paper discusses five implementation techniques that can be used to implement this capability. Comparisons of the techniques are provided and guidelines for selecting a particular technique for a compiler are given

Rehearsal: A Configuration Verification Tool for Puppet

Large-scale data centers and cloud computing have turned system configuration into a challenging problem. Several widely-publicized outages have been blamed not on software bugs, but on configuration bugs. To cope, thousands of organizations use system configuration languages to manage their computing infrastructure. Of these, Puppet is the most widely used with thousands of paying customers and many more open-source users. The heart of Puppet is a domain-specific language that describes the state of a system. Puppet already performs some basic static checks, but they only prevent a narrow range of errors. Furthermore, testing is ineffective because many errors are only triggered under specific machine states that are difficult to predict and reproduce. With several examples, we show that a key problem with Puppet is that configurations can be non-deterministic. This paper presents Rehearsal, a verification tool for Puppet configurations. Rehearsal implements a sound, complete, and scalable determinacy analysis for Puppet. To develop it, we (1) present a formal semantics for Puppet, (2) use several analyses to shrink our models to a tractable size, and (3) frame determinism-checking as decidable formulas for an SMT solver. Rehearsal then leverages the determinacy analysis to check other important properties, such as idempotency. Finally, we apply Rehearsal to several real-world Puppet configurations.Comment: In proceedings of ACM SIGPLAN Conference on Programming Language Design and Implementation (PLDI) 201

Aequitas: A Trusted P2P System for Paid Content Delivery

P2P file-sharing has been recognized as a powerful and efficient distribution model due to its ability to leverage users' upload bandwidth. However, companies that sell digital content on-line are hesitant to rely on P2P models for paid content distribution due to the free file-sharing inherent in P2P models. In this paper we present Aequitas, a P2P system in which users share paid content anonymously via a layer of intermediate nodes. We argue that with the extra anonymity in Aequitas, vendors could leverage P2P bandwidth while effectively maintaining the same level of trust towards their customers as in traditional models of paid content distribution. As a result, a content provider could reduce its infrastructure costs and subsequently lower the costs for the end-users. The intermediate nodes are incentivized to contribute their bandwidth via electronic micropayments. We also introduce techniques that prevent the intermediate nodes from learning the content of the files they help transmit. In this paper we present the design of our system, an analysis of its properties and an implementation and experimental evaluation. We quantify the value of the intermediate nodes, both in terms of efficiency and their effect on anonoymity. We argue in support of the economic and technological merits of the system

A Case for P2P Delivery of Paid Content

P2P file sharing provides a powerful content distribution model by leveraging users' computing and bandwidth resources. However, companies have been reluctant to rely on P2P systems for paid content distribution due to their inability to limit the exploitation of these systems for free file sharing. We present TP2, a system that combines the more cost-effective and scalable distribution capabilities of P2P systems with a level of trust and control over content distribution similar to direct download content delivery networks. TP2 uses two key mechanisms that can be layered on top of existing P2P systems. First, it provides strong authentication to prevent free file sharing in the system. Second, it introduces a new notion of trusted auditors to detect and limit malicious attempts to gain information about participants in the system to facilitate additional out-of-band free file sharing. We analyze TP2 by modeling it as a novel game between malicious users who try to form free file sharing clusters and trusted auditors who curb the growth of such clusters. Our analysis shows that a small fraction of trusted auditors is sufficient to protect the P2P system against unauthorized file sharing. Using a simple economic model, we further show that TP2 provides a more cost-effective content distribution solution, resulting in higher profits for a content provider even in the presence of a large percentage of malicious users. Finally, we implemented TP2 on top of BitTorrent and use PlanetLab to show that our system can provide trusted P2P file sharing with negligible performance overhead

Can P2P Replace Direct Download for Content Distribution

While peer-to-peer (P2P) file-sharing is a powerful and cost-effective content distribution model, most paid-for digital-content providers (CPs) rely on direct download to deliver their content. CPs such as Apple iTunes that command a large base of paying users are hesitant to use a P2P model that could easily degrade their user base into yet another free file-sharing community. We present TP2, a system that makes P2P file sharing a viable delivery mechanism for paid digital content by providing the same security properties as the currently used direct-download model.} introduces the novel notion of trusted auditors (TAs) -- P2P peers that are controlled by the system operator. TAs monitor the behavior of other peers and help detect and prevent formation of illegal file-sharing clusters among the CP's user base. TAs both complement and exploit the strong authentication and authorization mechanisms that are used in TP2 to control access to content. It is important to note that TP2 does not attempt to solve the out-of-band file-sharing or DRM problems, which also exist in the direct-download systems currently in use. We analyze TP2 by modeling it as a novel game between misbehaving users who try to form unauthorized file-sharing clusters and TAs who curb the growth of such clusters. Our analysis shows that a small fraction of TAs is sufficient to protect the P2P system against unauthorized file sharing. In a system with as many as 60\% of misbehaving users, even a small fraction of TAs can detect 99\% of unauthorized cluster formation. We developed a simple economic model to show that even with such a large fraction of malicious nodes, TP2 can improve CP's profits (which could translate to user savings) by 62 to 122\%, even while assuming conservative estimates of content and bandwidth costs. We implemented TP2 as a layer on top of BitTorrent and demonstrated experimentally using PlanetLab that our system provides trusted P2P file sharing with negligible performance overhead