5,547 research outputs found
Fragmentation of positronium in collision with He atoms
The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state
Minimally-intrusive frequent round trip time measurements using Synthetic Packet-Pairs
Accurate and frequent round trip time (RTT) measurements are important in testbeds and operational networks. Active measurement techniques inject probe packets that may modify the behaviour of the observed network and may produce misleading RTT estimates if the network handles probe packets differently to regular packets. Previous passive measurement techniques address these issues, but require precise time synchronisation or are limited to certain traffic types. We introduce Synthetic Packet-Pairs (SPP), a novel passive technique for RTT measurement. SPP provides frequently updated RTT measurements using any network traffic already present in the network without the need for time synchronisation. SPP accurately measures the RTT experienced by any application's traffic, even applications that do not exhibit symmetric client-server packet exchanges. We experimentally demonstrate the advantages of SPP
Client RTT and hop Count Distributions viewed from an Australian âEnemy Territoryâ Server
Network game servers experience traffic caused by actual game players and by remote clients simply probing the game serverâs current status. Game clients probe game servers for information such as the current map and number of current players on the server to enable players to fin d suitable games. The number of clients that probe a given server is orders of magnitudes higher than the number of eventual players. Network level round trip time (delay, or âla gâ) between a client and server is a very important criterion for players when deciding which server to join. Often the round trip time is roughly proportional to hop count. In this paper we document and investigate the distributions of round trip time and hop count for game clients that only probe and clients that actually play on a public game server. We also examine the geographical distributions of both groups. Our data was gathered from a Wolfenstein Enemy Territory server operating in Melbourne, Australia, in late 2004
Estimating IPv4 address space usage with capture-recapture
As of April 2013 almost 95% of the IPv4 address space has been allocated. Yet, the transition to IPv6 is still relatively slow. One reason could be existing âIPv4 reservesâ â allocated but unused IPv4 addresses. Knowing how many addresses are actively used is important to predict a potential IPv4 address market, predict the IPv6 deployment time frame, and measure progressive exhaustion after the IPv4 space is fully allocated. Unfortunately, only a fraction of hosts respond to active probes, such as âpingâ. We propose a capture-recapture method to estimate the actively used IPv4 addresses from multiple incomplete data sources, including âpingâ censuses, network traces and server logs. We estimate that at least 950â1090 million IPv4 addresses are used, which is 36â41% of the publicly routed space. We analyse how the utilisation depends on various factors, such as region, country and allocation prefix length
Dynamics of the IP time to live field in internet traffic flows
The Time To Live (TTL) field present in the IP protocol header is used to limit the lifetime of packets in the network. Previous research has measured TTL for studying path lengths and dynamics in IP networks, and for detecting route changes. How the TTL varies over short timescales of subsequent packets of traffic flows has not yet been analysed. Such knowledge is needed for passively detecting route changes based on existing traffic in the network or traffic traces, and for designing mechanisms that modulate the TTL field such as IP traceback techniques and covert channels. In this paper we analyse small time scale TTL variation in Internet traffic flows based on a number of packet traces captured at different locations in the Internet
Mitigating sampling error when measuring internet client IPv6 capabilities
Despite the predicted exhaustion of unallocated IPv4 addresses be- tween 2012 and 2014, it remains unclear how many current clients can use its successor, IPv6, to access the Internet. We propose a refinement of previous measurement studies that mitigates intrin- sic measurement biases, and demonstrate a novel web-based tech- nique using Google ads to perform IPv6 capability testing on a wider range of clients. After applying our sampling error reduction, we find that 6% of world-wide connections are from IPv6-capable clients, but only 1â2% of connections preferred IPv6 in dual-stack (dual-stack failure rates less than 1%). Except for an uptick around IPv6-day 2011 these proportions were relatively constant, while the percentage of connections with IPv6-capable DNS resolvers has in- creased to nearly 60%. The percentage of connections from clients with native IPv6 using happy eyeballs has risen to over 20
Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY
The black hole at the Galactic centre exhibits regularly flares of radiation,
the origin of which is still not understood. In this article, we study the
ability of the near-future GRAVITY infrared instrument to constrain the nature
of these events. We develop realistic simulations of GRAVITY astrometric data
sets for various flare models. We show that the instrument will be able to
distinguish an ejected blob from alternative flare models, provided the blob
inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and
the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA
Absolute differential positronium-formation cross sections
The first absolute experimental determinations of the differential cross-sections for the formation of ground-state positronium are presented for He, Ar, H2 and CO2 near 0â. Results are compared with available theories. The ratio of the differential and integrated cross-sections for the targets exposes the higher propensity for forward-emission of positronium formed from He and H2
Optical spectra of the heavy fermion uniaxial ferromagnet UGe
We report a detailed study of UGe single crystals using infrared
reflectivity and spectroscopic ellipsometry. The optical conductivity suggests
the presence of a low frequency interband transition and a narrow free-carrier
response with strong frequency dependence of the scattering rate and effective
mass. We observe sharp changes in the low frequency mass and scattering rate
below the upper ferromagnetic transition . The characteristic
changes are exhibited most strongly at an energy scale of around 12 meV (100
cm). They recover their unrenormalized value above and for 40 meV. In contrast no sign of an anomaly is seen at the lower transition
temperature of unknown nature 30 K, observed in transport and
thermodynamic experiments. In the ferromagnetic state we find signatures of a
strong coupling to the longitudinal magnetic excitations that have been
proposed to mediate unconventional superconductivity in this compound
- âŠ