A Multi-perspective Analysis of Carrier-Grade NAT Deployment

As ISPs face IPv4 address scarcity they increasingly turn to network address translation (NAT) to accommodate the address needs of their customers. Recently, ISPs have moved beyond employing NATs only directly at individual customers and instead begun deploying Carrier-Grade NATs (CGNs) to apply address translation to many independent and disparate endpoints spanning physical locations, a phenomenon that so far has received little in the way of empirical assessment. In this work we present a broad and systematic study of the deployment and behavior of these middleboxes. We develop a methodology to detect the existence of hosts behind CGNs by extracting non-routable IP addresses from peer lists we obtain by crawling the BitTorrent DHT. We complement this approach with improvements to our Netalyzr troubleshooting service, enabling us to determine a range of indicators of CGN presence as well as detailed insights into key properties of CGNs. Combining the two data sources we illustrate the scope of CGN deployment on today's Internet, and report on characteristics of commonly deployed CGNs and their effect on end users

HII Region Ionization of the Interstellar Medium: A Case Study of NGC 7538

Using data from the Green Bank Telescope, we analyze the radio continuum (free-free) and radio recombination line (RRL) emission of the compact HII region NGC 7538 (Sharpless 158). We detect extended radio continuum and hydrogen RRL emission beyond the photodissociation region (PDR) toward the north and east, but a sharp decrease in emission toward the south and west. This indicates that a non-uniform PDR morphology is affecting the amount of radiation "leaking" through the PDR. The strongest carbon RRL emission is found in the western PDR that appears to be dense. We compute a leaking fraction $f_R = 15 \pm 5$ % of the radio continuum emission measured in the plane of the sky which represents a lower limit when accounting for the three-dimensional geometry of the region. We detect an average $^4\textrm{He}^+/\textrm{H}^+$ abundance ratio by number of $0.088 \pm 0.003$ inside the HII region and a decrease in this ratio with increasing distance from the region beyond the PDR. Using Herschel Space Observatory data, we show that small dust temperature enhancements to the north and east of NGC 7538 coincide with extended radio emission, but that the dust temperature enhancements are mostly contained within a second PDR to the east. Unlike the giant HII region W43, the radiation leaking from NGC 7538 seems to only affect the local ambient medium. This suggests that giant HII regions may have a large effect in maintaining the ionization of the interstellar medium.Comment: Accepted for publication in ApJ (15 pages, 10 figures, 2 tables

Demonstration of rapid and sensitive module leak certification for Space Station Freedom

A leak detection and quantification demonstration using perflurocarbon tracer (PFT) technology was successfully performed at the NASA Marshall Space Flight Center on January 25, 1991. The real-time Dual Trap Analyzer (DTA) at one-half hour after the start of the first run gave an estimated leak rate of 0.7 mL/min. This has since been refined to be 1.15 (+ or -) 0.09 mL/min. The leak rates in the next three runs were determined to be 9.8 (+ or -) 0.7, -0.4 (+ or -) 0.3, and 76 (+ or -) 6 mL/min, respectively. The theory on leak quantification in the steady-state and time-dependent modes for a single zone test facility was developed and applied to the above determinations. The laboratory PFT analysis system gave a limit-of-detection (LOD) of 0.05 fL for ocPDCH. This is the tracer of choice and is about 100-fold better than that for the DTA. Applied to leak certification, the LOD is about 0.00002 mL/s (0.000075 L/h), a 5 order-of-magnitude improvement over the original leak certification specification. Furthermore, this limit can be attained in a measurement period of 3 to 4 hours instead of days, weeks, or months. A new Leak Certification Facility is also proposed to provide for zonal (three zones) determination of leak rates. The appropriate multizone equations, their solutions, and error analysis have already been derived. A new concept of seal-integrity certification has been demonstrated for a variety of controlled leaks in the range of module leak testing. High structural integrity leaks were shown to have a linear dependence of flow on (Delta)p. The rapid determination of leak rates at different pressures is proposed and is to be determined while subjecting the module to other external force-generating parameters such as vibration, torque, solar intensity, etc

Using Lyman-alpha to detect galaxies that leak Lyman continuum

We propose to infer ionising continuum leaking properties of galaxies by looking at their Lyman-alpha line profiles. We carry out Lyman-alpha radiation transfer calculations in two models of HII regions which are porous to ionising continuum escape: 1) the so-called "density bounded" media, in which massive stars produce enough ionising photons to keep the surrounding interstellar medium transparent to the ionising continuum, i.e almost totally ionised, and 2) "riddled ionisation-bounded" media, surrounded by neutral interstellar medium, but with holes, i.e. with a covering factor lower than unity. The Lyman-alpha spectra emergent from these configurations have distinctive features: 1) a "classical" asymmetric redshifted profile in the first case, but with a small shift of the maximum of the profile compare to the systemic redshift (Vpeak < 150 km/s); 2) a main peak at the systemic redshift in the second case (Vpeak = 0 km/s), with, as a consequence, a non-zero Lyman-alpha flux bluewards the systemic redshift. Assuming that in a galaxy leaking ionising photons, the Lyman-alpha component emerging from the leaking star cluster(s) dominates the total Lyman-alpha spectrum, the Lyman-alpha shape may be used as a pre-selection tool to detect Lyman continuum (LyC) leaking galaxies, in objects with well determined systemic redshift, and high spectral resolution Lyman-alpha spectra (R >= 4000). The examination of a sample of 10 local starbursts with high resolution HST-COS Lyman-alpha spectra and known in the literature as LyC leakers or leaking candidates, corroborates our predictions. Observations of Lyman-alpha profiles at high resolution should show definite signatures revealing the escape of Lyman continuum photons from star-forming galaxies.Comment: A&A in pres

Knowledge-First Theories of Justification

Knowledge-first theories of justification give knowledge priority when it comes to explaining when and why someone has justification for an attitude or an action. The emphasis of this entry is on knowledge-first theories of justification for belief. As it turns out there are a number of ways of giving knowledge priority when theorizing about justification, and in what follows I offer an opinionated survey of more than a dozen existing options that have emerged in the last two decades since the publication of Timothy Williamson’s Knowledge and Its Limits. I first trace several of the general theoretical motivations that have been offered for putting knowledge first in the theory of justification. I then go on to examine existing knowledge-first theories of justification and their standing objections. These objections are largely, but not exclusively, concerned with the extensional adequacy of knowledge-first theories of justification. There are doubtless more ways of giving knowledge priority in the theory of justification than I cover here, but the resulting survey will be instructive as it highlights potential shortcomings that would-be knowledge-first theorists of justification may wish either to avoid or else to be prepared with a suitable error theory