Protocol Independent Multicast and Asymmetric Routing

Originally all links in the Internet were assumed to operate bidirectionally. Like many other routing protocols, PIM (protocol independent multicast) is based on this assumption: as will be explained, PIM's concept of using the routers' unicast RIBs (routing information bases) for reverse-path-forwarding is not applicable in networks with uni-directional links. If an additional bidirectional link such as a dial-up connection exists, link-layer tunnelling can overcome these basic routing deficiencies. But in order to achieve a more efficient routing and sustain scalability we argue that multicast and unicast traffic should be distinguished either by an extended link-layer tunnelling or dual RIBs

On the Scaling of Feedback Algorithms for Very Large Multicast Groups

Feedback from multicast group members is vital for many multicast protocols. In order to avoid feedback implosion in very large groups feedback algorithms with well behaved scaling-properties must be chosen. In this paper we analyse the performance of three typical feedback algorithms described in the literature. Apart from the basic trade-off between feedback latency and response duplicates we especially focus on the algorithms' sensitivity to the quality of the group size estimation. Based on this analysis we give recommendations for the choice of well behaved feedback algorithms that are suitable for very large groups

Learning Design: reflections on a snapshot of the current landscape

The mounting wealth of open and readily available information and the swift evolution of social, mobile and creative technologies warrant a re-conceptualisation of the role of educators: from providers of knowledge to designers of learning. This need is being addressed by a growing trend of research in Learning Design. Responding to this trend, the Art and Science of Learning Design workshop brought together leading voices in the field and provided a forum for discussing its key issues. It focused on three thematic axes: practices and methods, tools and resources, and theoretical frameworks. This paper reviews some definitions of Learning Design and then summarises the main contributions to the workshop. Drawing upon these, we identify three key challenges for Learning Design that suggest directions for future research

S5 0716+714 : GeV variability study

The GeV observations by Fermi-LAT give us the opportunity to characterize the high-energy emission (100 MeV - 300 GeV) variability properties of the BL Lac object S5 0716+714. In this study, we performed flux and spectral analysis of more than 3 year long (August 2008 to April 2012) Fermi-LAT data of the source. During this period, the source exhibits two different modes of flux variability with characteristic timescales of ~75 and ~140 days, respectively. We also notice that the flux variations are characterized by a weak spectral hardening. The GeV spectrum of the source shows a clear deviation from a simple power law, and is better explained by a broken power law. Similar to other bright Fermi blazars, the break energy does not vary with the source flux during the different activity states. We discuss several possible scenarios to explain the observed spectral break.Comment: 21 pages, 10 figures, Accepted for publication in Advances in Space Research journa

dCache, scalable managed storage

End of 2007, the most challenging high energy physics experiment ever, the Large Hadron Collider(LHC)[9], at CERN, will start to produce a sustained stream of data in the order of 300MB/sec, equivalent to a stack of CDs as high as the Eiffel Tower once per week. This data is, while produced, distributed and persistently stored at several dozens of sites around the world, building the LHC data grid. The destination sites are expected to provide the necessary middle-ware, so called Storage Elements, offering standard protocols to receive the data and optionally store it at the site specific Tertiary Storage Systems. Beside its actual functionality, discussed subsequently, the Storage Element software has to be able to fit into a large variety of environments. They are known to range from sites providing a single storage box of some Tera Bytes of data and nearly no maintenance personnel up to Tier I sites with estimated disk storage capacities reaching into the Peta Byte area. Moreover, sites expected to store data permanently may want to use their already existing Hierarchical Storage Management (HSM) System to drive the robotics. This requires the Storage Element to be aware of HSM Systems and to be able to manage external file copies. The wide range of scalability, from the very small to the limits of affordable storage, is one of the primary goals of dCache, the Storage Element introduced in this presentation. By being strictly compliant to standard data transfer and control protocols, like gsiFtp, xRootd and the Storage Resource Manager protocol SRM, we are focusing on our second goal which is to make dCache available and useful beyond the borders of the High Energy Physics Community. Beside storing and preparing data for transfer, dCache provides a rich palette of functions to manage the available storage, as will be described subsequently. This includes replication of datasets on automated detection of busy storage components as well as optimization of access to tertiary storage systems

Sodium abundances in nearby disk stars

We present sodium abundances for a sample of nearby stars. All results have been derived from NLTE statistical equilibrium calculations. The influence of collisional interactions with electrons and hydrogen atoms is evaluated by comparison of the solar spectrum with very precise fits to the Na I line cores. The NLTE effects are more pronounced in metal-poor stars since the statistical equilibrium is dominated by collisions of which at least the electronic component is substantially reduced. The resulting influence on the determination of sodium abundances is in a direction opposite to that found previously for Mg and Al. The NLTE corrections are about -0.1 in thick-disk stars with [Fe/H] about -0.6. Our [Na/Fe] abundance ratios are about solar for thick- and thin-disk stars. The increase in [Na/Fe] as a function of [Fe/H] for metal-rich stars found by Edvardsson et al. (1993) is confirmed. Our results suggest that sodium yields increase with the metallicity, and quite large amounts of sodium may be produced by AGB stars. We find that [Na/Fe]ratios, together with either [Mg/Fe] ratio, kinematic data or stellar evolutionary ages, make possible the individual discrimination between thin- and thick-disk membership.Comment: 11pages, 11 figures. A&A accepte

Simultaneous Radio to (Sub-) mm-Monitoring of Variability and Spectral Shape Evolution of potential GLAST Blazars

The Large Area Telescope (LAT) instrument onboard GLAST offers a tremendous opportunity for future blazar studies. In order to fully benefit from its capabilities and to maximize the scientific return from the LAT, it is of great importance to conduct dedicated multi-frequency monitoring campaigns that will result comprehensive observations. Consequently, we initiated an effort to conduct a GLAST-dedicated, quasi-simultaneous, broad-band flux-density (and polarization) monitoring of potential GLAST blazars with the Effelsberg and OVRO radio telescopes (11cm to 7mm wavelength). Here, we present a short overview of these activities which will complement the multi-wavelengths activities of the GLAST/LAT collaboration towards the 'low-energy' radio bands. Further we will give a brief outlook including the extension of this coordinated campaign towards higher frequencies and future scientific aims.Comment: 3 pages, to appear in the Proceedings of the First GLAST Symposium, Stanford University, February 200