Hamed Sadeghian
email: h.sadeghianmarnani@tudelft.nl

Hans Goosen

Andre Bossche

Barend Thijsse

Fred Van Keulen

On the size-dependent elasticity of silicon nanocantilevers: Impact of defects

Recent measurements have indicated that the elastic behavior of silicon nanocantilevers and nanowires is size-dependent. Several theoretical models have been proposed to explain this phenomenon, mainly focused on surface stress effects. However, discrepancies are found between experiments and theories, indicating that there could be other influences besides surface effects. One of the important issues, which was experimentally confirmed and has not been considered, is accounting for the fact that experimentally tested nanocantilevers and nanowires are not defect free. In this paper molecular dynamics (MD) is utilized to study the effects of defects on the elasticity of silicon. The effective Young's modulus Ẽ of [100] and [110] oriented silicon nanoplates is extracted in presence of defects, showing that such defects significantly influence the size-dependent behavior in Ẽ. The MD results are compared with the results of continuum theory, showing that continuum theory holds, even for very small defects. Taking into account the surface effects, native oxide layers together with fabrication-induced defects, the experimental measurements can be explained. The studied example involved nanocantilevers, but can be extended to nanowires.

Due to advantages of miniaturization, such as smaller size, shorter time response, higher performance and reduced energy requirements, significant research efforts have been directed towards the developments of nanoelectromechanical systems (NEMS). The building blocks of NEMS are mostly nanocantilevers, nanoplates, nanowires and nanotubes. Due to their applications, their mechanical properties are of considerable interest. Recent experimental [START_REF] Li | Ultrathin single-crystalline-silicon cantilever resonators: Fabrication technology and significant specimen size effect on young's modulus[END_REF][START_REF] Gordon | Size effects in mechanical deformation and fracture of cantilevered silicon nanowires[END_REF][START_REF] Sadeghian | Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability[END_REF][START_REF] Zhu | Mechanical properties of vapor-liquid-solid synthesized silicon nanowires[END_REF] and computational studies, including ab initio and density functional theory (DFT) [START_REF] Umeno | Ab initio study of the surface properties and ideal strength of (100) silicon thin films[END_REF][START_REF] Byeongchan | First-principles calculation of mechanical properties of si¡001¿ nanowires and comparison to nanomechanical theory[END_REF][START_REF] Lee | First-principles study of the young's modulus of si ¡001¿ nanowires[END_REF], molecular dynamics (MD) [START_REF] Park | Molecular dynamics study on size-dependent elastic properties of silicon nanocantilevers[END_REF][START_REF] Woo | Nanoplate elasticity under surface reconstruction[END_REF][START_REF] Mcdowell | Bending and tensile deformation of metallic nanowires[END_REF][START_REF] Kang | Brittle and ductile fracture of semiconductor nanowiresmolecular dynamics simulations[END_REF], and modifications to continuum theory [START_REF] Miller | Size-dependent elastic properties of nanosized structural elements[END_REF][START_REF] Wang | Size and temperature dependence of young's modulus of a silicon nano-plate[END_REF][START_REF] Wei | Finite element characterization of the size-dependent mechanical behaviour in nanosystems[END_REF][START_REF] Sadeghian | Surface stress-induced change in overall elastic behavior and self-bending of ultrathin cantilever plates[END_REF] revealed strong size-dependent mechanical properties as the characteristic dimensions of the structure approached nanometer scale.

Li et al. [START_REF] Li | Ultrathin single-crystalline-silicon cantilever resonators: Fabrication technology and significant specimen size effect on young's modulus[END_REF] measured the Ẽ of [110] silicon cantilevers with thicknesses ranging from 300 to 12 nm using the resonance frequency method. The Ẽ showed significant decrease from 167 GPa to 53 GPa. Gordon et al. [START_REF] Gordon | Size effects in mechanical deformation and fracture of cantilevered silicon nanowires[END_REF] performed a new, multipoint bending protocol in an atomic force microscope in order to extract the elastic modulus of vertically aligned Si [111] nanowires (diameter ranges from 100 to 700 nm) in an asgrown state. Zhu et al. [START_REF] Zhu | Mechanical properties of vapor-liquid-solid synthesized silicon nanowires[END_REF] measured the effective Young's modulus of silicon nanowires with diameters between 15 to 60 nm and lengths between 1.5 and 4.3 µm. The nanowires, grown by the vapor-liquid-solid process, were subjected to tensile tests in situ inside a scanning electron microscope.

The size-dependence of the effective Young's modulus Ẽ has generally been attributed to a number of effects, including surface stress effects [START_REF] Miller | Size-dependent elastic properties of nanosized structural elements[END_REF][START_REF] Sadeghian | Surface stress-induced change in overall elastic behavior and self-bending of ultrathin cantilever plates[END_REF][START_REF] Umeno | Ab initio study of the surface properties and ideal strength of (100) silicon thin films[END_REF], surface contaminations and surface oxidation [START_REF] Sadeghian | Effects of surface stress on nanocantilevers[END_REF][START_REF] Sadeghian | Surface contamination-induced resonance frequency shift of cantilevers[END_REF][START_REF] Sadeghian | Some considerations of effects-induced errors in resonant cantilevers with the laser deflection method[END_REF], nonlinear bulk elasticity [START_REF] Liang | Size-dependent elasticity of nanowires: Nonlinear effects[END_REF], impact of loading method (bending vs. extension) [START_REF] Mcdowell | Bending and tensile deformation of metallic nanowires[END_REF] and effects of different geometry and surface structure [START_REF] Mcdowell | On the elastic modulus of metallic nanowires[END_REF]. The succeeding results and discussions have made clear that a substantial gap exists between theoretical predictions and experimental measurements, revealing that although preceding factors influence Ẽ to some extent, they alone are insufficient to explain the experimentally observed trends. As an example, the experimental results of single crystal silicon [110] nanowires and nanocantilevers reported the observed size-dependence at about 100 nm [START_REF] Li | Ultrathin single-crystalline-silicon cantilever resonators: Fabrication technology and significant specimen size effect on young's modulus[END_REF][START_REF] Sadeghian | Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability[END_REF][START_REF] Zhu | Mechanical properties of vapor-liquid-solid synthesized silicon nanowires[END_REF], while the computational investigations predicts this to occur at a characteristic size of less than 10 nm [START_REF] Byeongchan | First-principles calculation of mechanical properties of si¡001¿ nanowires and comparison to nanomechanical theory[END_REF][START_REF] Park | Molecular dynamics study on size-dependent elastic properties of silicon nanocantilevers[END_REF]. Figure 1 shows a comparison between our recent experimental measurements of the size-dependent bending mode of Ẽ of silicon nanocantilevers [START_REF] Sadeghian | Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability[END_REF], the experimental results reported in [START_REF] Li | Ultrathin single-crystalline-silicon cantilever resonators: Fabrication technology and significant specimen size effect on young's modulus[END_REF] and several theoretical models. The figure shows the effects of surface elasticity in extensional mode of Ẽ from molecular dynamics (MD), a semi-continuum framework [START_REF] Sadeghian | Surface stress-induced change in overall elastic behavior and self-bending of ultrathin cantilever plates[END_REF] without and with a 2 nm thick native oxide layer. The thickness is shown on a logarithmic scale in order to clearly visualize the difference between theory and experiments. The experimental results showed that the size-dependence starts at about 150 nm. The results of semi-continuum approach indicates size effects below 15 nm, i.e. very far from what experimental results showed. Taking the native oxide layer into consideration reduces the difference, yet there is still a considerable gap. Previous work [START_REF] Sadeghian | Effects of size and defects on the elasticity of silicon nanocantilevers[END_REF] showed that different oxide's properties partially explain the distinctive drops of Ẽ, but not fully. Therefore, the question remains: why Ẽ decreases at larger thicknesses? A couple of issues have been raised by researchers in order to reduce the gap between Figure 1. Measured effective Young's modulus of silicon nanocantilevers for different thicknesses (squares [START_REF] Sadeghian | Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability[END_REF] and stars [START_REF] Li | Ultrathin single-crystalline-silicon cantilever resonators: Fabrication technology and significant specimen size effect on young's modulus[END_REF]). The triangles are the results of direct MD simulation in extensional mode [START_REF] Sadeghian | Effects of size and surface on the elasticity of silicon nanoplates: Molecular dynamics and semi-continuum approaches[END_REF]. The solid line shows the prediction of size dependence when considering surface elasticity [START_REF] Sadeghian | Surface stress-induced change in overall elastic behavior and self-bending of ultrathin cantilever plates[END_REF], and the dashed line shows the prediction when additional 2 nm native oxide layers are taken into account [START_REF] Sadeghian | Effects of size and defects on the elasticity of silicon nanocantilevers[END_REF].

the experiment and the theory [START_REF] Agrawal | Multiscale experiments: State of the art and remaining challenges[END_REF][START_REF] Park | Theme article -mechanics of crystalline nanowires[END_REF]. One of the most important issues, which has not been considered yet, is accounting for the fact that experimentally tested nanostructures are not defect free in contrast to the perfect single crystal structures studied using atomistic simulations [START_REF] Agrawal | Multiscale experiments: State of the art and remaining challenges[END_REF][START_REF] Mcdowell | On the elastic modulus of metallic nanowires[END_REF][START_REF] Park | Theme article -mechanics of crystalline nanowires[END_REF]. Recent experimental observations have shown that reducing the thickness of silicon nanocantilevers would not only increase the surfaceto-volume ratio but also the density of fabrication-induced defects [START_REF] Naumova | Properties of silicon nanolayers on insulator[END_REF][START_REF] Sadeghian | Effects of size and defects on the elasticity of silicon nanocantilevers[END_REF]. Figure 2(a) shows a SEM of so-called HF defects in a 14 nm thick silicon device layer. The HF defects are observed as circles with the defect at the center. Due to convenience, HF etching is often used to test for the existence of these pinhole structures, and the resulting observed defects are called "HF defects". The details of the HF defects were published elsewhere [START_REF] Sadeghian | Effects of size and defects on the elasticity of silicon nanocantilevers[END_REF]. Using AFM, defects can be observed without HF modification. The AFM image is shown in Figure 2(b). The white light interferometric picture, shown in Figure 2(c), shows the appearance of defects on the surface of the cantilever as well.

It can be speculated that the HF defects can cause a decrease in Ẽ and thus contributes to the discrepancy between experimental measurements and theoretical investigations. In this paper, using MD calculations we investigate the influence of defects on the sizea.

b. c. dependent elasticity of silicon nanocantilevers. A recently developed modified embedded atom method (MEAM) potential for silicon [START_REF] Timonova | Sputter erosion of si(001) using a new silicon meam potential and different thermostats[END_REF] was used. Silicon nanoplates were created with initial atomic positions corresponding to the bulk diamond-cubic crystal. The nanoplates were oriented along the [100] and [110] directions. Three different lateral dimensions of nanoplates were studied, large (40×30 unit cells), medium (20×15 unit cells) and small (10×8 unit cells). The thickness was kept as 4 unit cells. Periodic boundary conditions were applied as follows: the plate fills the x and y directions (length and width) completely, but does not fill the z (thickness) direction completely. Thus, there are sufficiently large vacuum regions between the periodic images in z direction. Consequently, the plates do not interact with each other, so that the top and bottom of the plates represent two free surfaces. Therefore, the system mimics a single nanoplate. The simulation cells were then fully relaxed to a minimum energy state at zero Kelvin, while zero pressure was applied in all directions. A time frame of 40 ps was found to be sufficient for the energy minimization. The surfaces display (2 × 1)-type reconstruction along [110] or [ 110] directions, consisting of rows of dimers to eliminate one dangling bond per surface atom. At this stage defects with a cylindrical shape of different sizes were created by removing the atoms inside the cylinder. Each nanoplate has one defect at its center and the radius of the defects varied. Moreover, nanoplates with patterned defects were also studied. The configurations were again fully relaxed to the minimum energy state. Figure 3(a-d) shows the relaxed [100] and [110] nanoplates with single defects and patterned defects. Because removing atoms will create free surfaces inside the defect, atoms inside the defect also reconstruct.

After reaching equilibrium configurations, the nanoplates underwent quasi-static tensile loading along the x -direction and were again fully relaxed to the local energy minimum following each strain step. The virial stresses [START_REF] Tsai | The virial theorem and stress calculation in molecular dynamics[END_REF] were calculated for each level of strain and Ẽ was found from the slope of the stress-strain curve. This was repeated for different nanoplates with different sizes of defects. Figure 4 shows

E d E 0 versus A d A 0
, where E d and A d are the effective Young's modulus of the nanoplate with defect and the surface area of defect, and E 0 and A 0 are the effective Young's modulus and surface area of the nanoplate without defect. For extremely small defects, the number of atoms removed is not only a function of the defect size, but also of the position of the center of the defect. When the center of the defects is moved within the dimensions of a crystal, the number of removed atoms is changed, consequently the effective Young's modulus is changed (See inset in Figure 5). This is the reason that in Figure 4 the extremely small defects have seemingly noisy (±0.01) values of E d E 0 . Re-plotting Figure 4 with respect to the number of removed atoms results in Figure 5. N d is the number of removed atoms, and N t is the total number of atoms, respectively. It is noteworthy that creating the defect did not change the average total energy per atom significantly, except for the atoms near the surface of the defects, which have higher excess energy. The virial stresses of nanoplates with defects, compared to the uniform virial stress in a nanoplate without defects, are not uniformly distributed, which is due to the stress concentration near the defects. From the changes in the virial stresses ( σ) of each individual atom with respect to the stain (ε), the stiffness contribution of each atom to the Ẽ was determined ( σ ε ). This is shown in Figure 6 for [100] and 7 for [110] nanoplates. Substantial changes are found when the size of the defects is increased.

As shown in figures 4 and 5 the MD results were compared with an analytical solution based on continuum theory [START_REF] Shevlyakov | Relative stiffness of irregularly perforated plates[END_REF], shown as a solid line. Comparison between the MD results and the analytical solution shows a very good agreement, even for very small defects. Both MD and continuum results indicate that the Ẽ linearly decreases with increasing defect volume fraction. Moreover, the MD results confirm an identical decrease of Ẽ in different directions, which follows from the analytical solution as well. Since the reduction of the Ẽ due to the defects is significant, therefore, it has to be considered as an additional factor contributing to the decrease in the Ẽ. Moreover, the native oxide present on a silicon surface would also include the defects present in the silicon. As shown with the analytical solution and the MD results, the effects of defects on Ẽ depends strongly on the relative surface ratio of the defects A d A 0 and not on the direction or the material. Therefore, it can be assumed that the effect of defects on the native oxide layers is similar to that of the silicon. By doing so, the effect of defects can now be included in the resultant Ẽ, affected by surface elasticity and native oxide layers. Figure 8 shows the effect of defects of various densities on Ẽ as function of thickness. 0% corresponds to Ẽ of silicon cantilevers with 2 nm thick native oxide layer and surface elasticity without defect. The defect density ( A d A 0 ), experimentally observed in this work and previous work [START_REF] Naumova | Properties of silicon nanolayers on insulator[END_REF] varies between 0.005 to 0.12%, depending on the thickness of the silicon device layer. Figure 4 was used to calculate E d E 0 where E 0 is now the effective Young's modulus including the surface elasticity and the native oxide layer. This was done for different native oxide scenarios, as reported in previous work [START_REF] Sadeghian | Effects of size and defects on the elasticity of silicon nanocantilevers[END_REF][START_REF] Gordon | Size effects in mechanical deformation and fracture of cantilevered silicon nanowires[END_REF][START_REF] Hsin | Elastic properties and buckling of silicon nanowires[END_REF][START_REF] Gavrilenko | Measurement of thickness of native silicon dioxide with a scanning electron microscope[END_REF]. The results are shown in Table 1. Ẽ, including the effects of surface elasticity, native oxide layers and fabricationinduced defects was calculated for the thicknesses that were measured experimentally. The uncertainties in the experimental data are due to the errors of the measurements, reported elsewhere [START_REF] Sadeghian | Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability[END_REF]. The variations in the theoretical values are due to the variations in the defect density. It can be seen that taking the fabrication-induced defects into account can explain the observed discrepancy between the experimental measurements and theoretical predictions. As an example, for native oxide of 2 nm thickness and E Ox of 75 Gpa, decreasing the thickness from 93 to 40 nm, decreases Ẽ from 135.81-142.81 to 84.33-94.73, similar to what was experimentally observed. In summary, molecular dynamics calculations have been carried out to determine the effects of fabrication-induced defects on the effective Young's modulus of silicon nanocantilevers. The calculations were motivated by a disparity in the experimental measurements and theoretical investigations of the effective Young's modulus. The conclusion is that the size effect observed in silicon nanocantilevers can be explained by a contri- bution of surface effects, native oxide layers and fabrication-induced defects. Taking these into account, the gap between the experimental measurements and computational investigation can be closed and a similar trend was observed in the theory. Moreover, the effect of defects is only influenced by the ratio between surface area of the defects and total surface of the nanoplate, similar to what analytical solution based on the continuum theory predicts. Future experimental research on quantifying the size effects on the thickness and the mechanical properties of native oxide layer has to be done. The studied example involved silicon nanocantilevers, but can be extended to nanowires.

This work was financially supported by the Dutch national research program on micro technology, MicroNed (Project code: IV-C-2).

Figure 2 .

 2 Figure 2. (a) SEM of HF defects in a 14 nm-thick SOI wafer. The BOX layer below the HF defect is etched by HF solution. (b) AFM image of defects without HF modification (shape is caused by AFM probe) and (c) White light interferometric picture of a 40 nm thick cantilever with defects.

Figure 3 .

 3 Figure 3. Snapshots of (a) a fully relaxed defect-free [100] silicon nanoplate, (b) [100] silicon nanoplates with one defect at the center, (c) Patterned defects on [100] silicon nanoplate and (d) [110] nanoplate with one defect. Coloring is according to energy. Red colors denotes atoms of higher energy.

Figure 4 .

 4 Figure 4. E d E0 versus A d A0 for different sizes and directions of nanoplates. The thickness of the nanoplates is 4 unit cells.

AFigure 5 .

 5 Figure 5. E d E0 versus N d N0 for different sizes and directions of nanoplates. The inset shows the E d E0 as a function of relative position of the center of the defect to the center of the nanoplate.

Figure 6 .

 6 Figure 6. snapshots of middle layers of silicon [100] nanoplates with different ratios of removed atoms and the total number of atoms. Each individual atom is colored by its stiffness contribution. Scales are normalized in which 0 corresponds to the color with lowest contribution to the stiffness and 100 for highest, respectively.

Figure 7 .

 7 Figure 7. snapshots of middle layers of silicon [110] nanoplates with different ratios of removed atoms and the total number of atoms. Each individual atom is colored by its stiffness contribution. Scales are normalized in which 0 corresponds to the color with lowest contribution to the stiffness and 100 for highest, respectively.

Figure 8 .

 8 Figure 8. Ẽ versus thickness of the cantilever for different defect area fractions.

Table 1 .

 1 Comparison between experimental Ẽ data and those theoretically calculated, taking into account the surface elasticity, native oxide scenarios and fabricationinduced defects. t Ox and E Ox are the thickness and the Young's modulus of native oxide layer.

	Thickness (nm)	93	57	40
	Experimental data (GPa) 139.5 ± 8.4	100.1 ± 10.8	85.2 ± 12.7
	t Ox (nm)	E Ox (GPa)
	2	75	150.91-150.99 119.88-134.16 102.31-114.51
	5	75	135.81-142.81 103.53-115.93 84.33-94.73
	2	50	146.22-154.82 117.84-128.84 97.80-109.40
	5	50	130.16-135.16 95.13-107.33	69.08-81.88


K Denieffe

Cmo Mahony

P D Maguire

D Gahan

M B Hopkins

Retarding Field Energy Analyzer Ion Current Calibration and Transmission

Keywords: ion beam, retarding field energy analyzer, calibration, retarding grid, ion energy distribution, ion current density, process control

Accurate measurement of ion current density and ion energy distributions (IED) is often critical for plasma processes in both industrial and research settings. Retarding field energy analyzers (RFEA) have been used to measure IEDs because they are considered accurate, relatively simple and cost effective. However, their usage for critical measurement of ion current density is less common due to difficulties in estimating the proportion of incident ion current reaching the current collector through the RFEA retarding grids. In this paper an RFEA has been calibrated to measure ion current density from an ion beam at pressures ranging from 0.5 to 50.0 mTorr. A unique method is presented where the currents generated at each of the retarding grids and the RFEA upper face are measured separately, allowing the reduction in ion current to be monitored and accounted for at each stage of ion transit to the collector. From these I-V measurements a physical model is described. Subsequently, a mathematical description is extracted which includes parameters to account for grid transmissions, upper face secondary electron emission and collisionality. Pressure-dependant calibration factors can be calculated from least mean square best fits of the collector current to the model allowing quantitative measurement of ion current density.

Introduction

Accurate measurements of ion energy distributions (IED) and ion current densities (J i ) of plasma discharges and ion beam sources are important for optimisation and control of thin film deposition and etching processes and the design of plasma sources. In the case of deposition, the ion bombarding energy and flux have a direct effect on the deposition rate and properties of the resulting film [START_REF] Keudell | Interaction of hydrogen plasmas with hydrocarbon films, investigated by infrared spectroscopy using an optical cavity substrate[END_REF]. We recently reported detailed mass spectrometry, mass selected ion energy and flux measurements during the growth of carbon-based films [START_REF] Baby | Plasma Sources Sci Technol be[END_REF]. However use of complex mass -energy analysis instrumentation is problematic in many plasma processing environments. There is also an increasing demand for accurate in-situ control sensors for etch processes used in the manufacture of magnetic data storage devices and silicon wafer technologies as these industries move to ever reducing critical dimensions [START_REF] Chen | High Moment Materials and Fabrication Processes for Shielded Perpendicular Write Head Beyond 200 Gb/in 2[END_REF]. The small size and low cost of Retarding Field Energy Analyzers (RFEA) compared with other IED measurement tools mean that it is a useful diagnostic device for both industrial process control and basic research. Compared to Mass-Energy Analysis (MEA) these devices are much less complex and do not require differential pumping in applications below 100 mTorr [START_REF] Gahan | RFEA and Langmuir probe comparison[END_REF]. In addition RFEAs have also been used to measure electron energy distribution functions (EEDF) by other groups [START_REF] Gahan | RFEA and Langmuir probe comparison[END_REF]. To date, RFEA's have been used mainly for IED measurement [START_REF] Hopwood | Ion bombardment energy distributions in a radio frequency induction plasma[END_REF]6,[START_REF] Mizutani | Ion energy and angular distribution at the radio frequency biased electrode in an inductively coupled plasma apparatus[END_REF][START_REF] Mizutani | Ion energy distribution at an rf-biased electrode in an inductively coupled plasma affected by collisions in a sheath[END_REF][START_REF] Mizutani | Ion energy and mass analyzer at radio frequency electrode in a plasma chamber[END_REF], with only limited application to in-situ determination of ion flux density [START_REF] Gahan | Retarding field analyzer for ion energy distribution measurements at a radio-frequency biased electrode[END_REF] due to the lack of a clear consensus on how best to calibrate its ion current density measurements.

Here we describe the calibration of current measurements made with a retarding field energy analyzer (RFEA). In a 1 second publication [START_REF] Denieffe | CoFe and NiFe alloy sputter yield measurements[END_REF] we report the sputter yields of thin film ferrous alloys in an argon ion beam facility based on our calibrated RFEA.

An RFEA is essentially a stack of electrically isolated grids and an ion collector, all positioned along the ion beam path.

Selectable potentials can be applied to the grids to reject beam plasma electrons, to allow energy analysis and to suppress internal secondary electron emission before ion collection.

When RFEAs are used to measure an ion energy distribution (IED), an ion retarding potential is applied to a grid to separate ions arriving at the RFEA entrance aperture plane by kinetic energy. As the retarding potential is varied, ions of different energies are reflected, or transmitted to subsequent grids for collection. It could be assumed that at low pressures charged particle flux can be estimated from the product of the known optical transmission of each grid. This approach is, however, only valid for collisionless trajectories at low pressures [START_REF] Palmer | Design Implementation and first results of retarding field analyzers developed for the CESRTA program[END_REF], ). An additional method was used at a single pressure (7.5 mTorr) where T CTOT was determined directly by comparing the ion current density at the RFEA top face to the ion current at the collector (= e -L/λ , where L is the device length, λ is the ion mean free path). Reasonable consistency across the range 7.5 to 52.7 mTorr was obtained using best fit parameters P G =2.23% and cross-section σ = 6.06 10 -15 cm 2 . This cross-section was also used to and collector may be significant, even when the same material is used since surface condition can have as great an impact as surface material [START_REF] Phelps | Cross sections and swarm coefficients for nitrogen ions and neutrals in N 2 and argon ions and neutrals in Ar for energies from 0.1 eV to 10 keV[END_REF]. Also, as the authors point out, the presence of a positive plasma sheath potential produces a preferred direction of the secondary electrons away from the top face surface resulting in a larger SE emission at the top face compared with that at the collector. (iii) Collection of slow ions arriving at the reference electrode will further add to the current inventory. These ions originate from charge exchange collisions or from a diffuse beam plasma. The latter is produced by ionisation via secondary electrons released from surfaces or from ion-atom collisions.

typically
In this study, we develop a technique which involves recording currents to each RFEA grid or the collector in turn as the specific potential to that grid is swept. The RFEA top face current is also recorded. The advantage of this method is that it allows the observation of the progressive reduction in measured current at each of the grids. This improves the analyses and hence accuracy of the calculated grid transmission values and the extracted SE emission of each grid. Also, full calibration can be performed with the RFEA in-situ without needing a remote reference measurement of ion current e.g. using a Langmuir probe.

Experimental Configuration

All measurements were performed with a Copra GSE 160 ion beam source operated in a custom-built vacuum chamber. This The vacuum chamber has a volume of ~110 litres pumped using a 140 l/s turbomolecular pump (TMP) that is backed using a 25 m 3 /h rotary pump. Chamber pressure control is achieved by adjusting chamber pumping, controlled by restricting the conductance pathway to the TMP with a throttle valve. (iii) For -40 V < V G < 0 V, I G2 and I C are relatively constant, however there is a marked increase in I G1 with increasing V G .

(iv) For 0 V < V G < 30 V, the variation of positive ion collection dominates, particularly for 10 V < V G < 20 V. the four unknown variables, assumed initially to be pressure dependent, by using a least squares method to give the best fits. The initial fit showed three parameters, T G1 , T G2 and γ to be remarkably independent of pressure. The fitting was thus repeated using pressure independent values for these three

Discussion

Calculated grid transparencies

Grid transparencies T G1 and T G2 obtained using least squares fits were pressure invariant and equal at 0.32, very close to the estimated optical transmission T OPT = 0.34. The similar and pressure independent values of T G1 and T G2 are most probably due to ion optics within the RFEA. We observe ion transmissions T G1 and T G2 ~ T OPT and are not sensitive to experimental conditions. This indicates that the net divergence of a beamlet passing through an individual orifice is sufficient to spread that beamlet over many orifices at a subsequent grid.

Several factors could play a part in beamlet spread and steering: ion beam effects, ion optical effects, collisions, grid geometry and grid misalignment. Detailed consideration of these factors indicate that the net divergence required for this spread is due to the combined effects of: non-normal incidence due to electric field or beam plasma sheath distortion above G 0 , grid geometry and alignment, individual beamlet divergence and ion lens aberrations particularly at G 0 . It was necessary to vary T G0 with pressure in order to obtain a best fit. This is not surprising because unlike currents I G1 and I G2 , the current I G0 results from complex pressure dependant components such as slow beam plasma ions and subsequent SE production. In addition, measurement of ion current at the top face is unrestricted by acceptance angle, and there may be a change in ion angular distribution with pressure.

Calibrated collector current

The ion current density (J i ) can be calculated from the collector current (I C ) using the following equation and the values T GAP , T G0 , T G1 and T G2 obtained in section 3 :

For example, given that T G1 and T G2 are constant at ~ 0.32, J i calculated using I C = 200 nA obtained from figure D and the T G0 and T GAP values for 1 mTorr: This calibration is valid for the RFEA under argon ion beam processing conditions, described in section 2. If this RFEA were to be calibrated for a different process, e.g. an ICP plasma, we assume the calculated values of T G1 T G2 will remain constant at 0.32. However, we would expect the value of T G0 to differ, depending on process conditions because the current measured at grid G 0 contains components that are unique to that process (e.g. beam plasma). Thus a recalculation of T G0 using the method described in section 3 would be necessary. The values for secondary electron emission coefficients determined from the intercepts at -50 V on figure G are γ Ni ≈ 0.04 for grid G 2 and γ Cu ≈ 0.03 for the collector. The value for the aluminium top face is γ Al ≈ 0.08. These values are in broad agreement with the literature which shows many "dirty" metal surfaces have γ ~ 0.1 for the argon ion energy range investigated [START_REF] Phelps | Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons[END_REF].

Error analysis shows measurement accuracy is particularly dependant on pressure, with values of: ±11% at 0.5 mTorr, ±13% at 5.0 mTorr and ±27% at 50 mTorr. However, while fits to the I G1 , I G2 and I C data in Fig H using the function I = K P -1/3 also shows the scatter rises with pressure, the scatter values are about half the error analysis values. This suggests our error analysis gives overestimates. Calibration using calorimetric methods and/or with pulsed/chopped parallel ion beam would resolve the matter.

Conclusions

An RFEA has been calibrated to quantitatively measure ion current arriving from an argon ion beam at pressures from 0.

  below 1 mTorr for the shallow (~1 mm deep) RFEA used in this study, and for uniform fluxes incident at each grid i.e. relatively straightforward ion optics. Another alternative calibration technique is to compare RFEA ion currents to a reference ion flux, determined from a separate Faraday cup or Langmuir probe measurement. The latter technique has been used by Gahan et. al. [4] to compare the RFEA flux determined from RFEA collector current and assumed transmission to the probe saturation ion current and the Bohm flux determined from the probe I-V and extracted values of N e and T e . While RFEA and Langmuir ion current characteristics exhibit similar trends, decreasing markedly with pressure, the accuracy of the comparison is limited by standard Langmuir probe design and analysis methods. Baloniak et. al. [13] performed a calibration of the measured ion collector current (I C ) by modelling the total collisional (T CTOT ) and geometrical transmissions (T GTOT ) of a four-grid RFEA based on the Bohm current (I B ) at the RFEA orifice derived from a Langmuir Probe measurements with T CTOT and T GTOT as fit parameters(

  calculate the collisional transparency (P C ) in the additional method, which subsequently yielded a geometrical transparency of P G =3.32 %. It was suggested that uncertainty in the determination of N e is likely to lead to an overestimate of the ion current to the orifice which may account for discrepancies in geometrical transparency determined from the two methods. There are several other factors limiting the calibration accuracy based on measuring the current at the top face. (i) The current density may not be uniform across the top face, depending on the plasma conditions. (ii) Differences in secondary electron (SE) emission coefficient between top face

  consists of a 13.56 MHz plasma inductively coupled (ICP) source with a grounded extraction grid. Rf power is coupled to the plasma through a single turn induction coil and a sourceincorporated matching network. The source was operated throughout at 200 W rf power and 5.4 sccm argon flow. The RFEA was positioned centrally on the sample table (Fig A) and the RFEA casing was separated electrically from the table by an insulating holder which masked its base and sides; this allowed the current to it's top face (I F ) to be measured distinctly from the table current (I B ). The RFEA top face area A F was accurately defined by the insulating holder which prevented the ion beam striking its side. The Copra ion source produces an ion beam extracted by a circular grounded extraction grid of 84 mm diameter. The extraction potential is provided by the ion source plasma potential V PLDC , typically 10 to 20 V. A small number of fast electrons, with energy greater than V PLDC , also exit the source, but are deflected to the vacuum wall by the biased sample table. The ICP source is operated at 200 W in the H mode and under these conditions the rf component of plasma potential V PLRF is typically a few volts.. The RFEA was mounted on the biased sample table (V B = -50 V) and a separate dc power supply was used to repel beam plasma electrons during calibration. I F (Fig A)was measured outside the vacuum chamber across a resistor (R meas = 99.7 Ω). The RFEA casing potential V A is the sum of sample table potential and the small potential drop across R meas (V A = V B + V Rmeas ). The ion beam current density was found to peak on axis, is almost circularly symmetric, and reduces with distance from the source with a ~ ±15° divergence.

Figure A :

 A Figure A: Experimental schematic showing RFEA top face exposed to ion beam

Figure B :

 B Figure B: Cross-section of RFEA probe showing potentials applied to orifice and grids

Figure C :Figure D :

 CD Figure C: SEM image of a sample grid mesh. The sides of the square (L) are ~14-15 μm and solid metal (S) ~10 μm.

FigureF

  Figure F shows I-V plots for grids 1, 2 and the collector for grid voltages ranging from -100 V to +100 V. Note that the RFEA PSU is configured such that supplied positive current to a grid is a positive value. Therefore, increasing positive ion

2 Figure G :ForFigure I :

 2GI Figure G: Example ion current extrapolations for Figure F to determine I G1 , I G2 , I C and secondary electron emission coefficients. To aid viewing the collector current is magnified 10 x and grid 2 by five.

Figure J :

 J Figure J: Pressure T G0 , derived from least squares fit.

Figure K :

 K Figure K: Variation of current density at the RFEA top face and calculated from I C vs V B

  5 to 50.0 mTorr. The difficulty in this type of calibration is in determining with confidence how much ion current is transmitted from the apertures through the RFEA to the collector. A unique method has been presented where the currents generated at each of the retarding grids and the RFEA top surface are measured separately, allowing the reduction in ion current at each grid along the beam path to be monitored and accounted for. A physical model has developed from these I-V measurements which accounts for grid transmissions and pressure dependant losses within the RFEA. In addition, top face secondary electron emission and collisionality has been addressed. By fitting the collector current to the model, the variability of grid G 0 transmission was found to be the dominant pressure dependant parameter This can be been attributed to pressure dependant current components at the top face, namely slow ions from a weak beam plasma and from ions trajectories which fall outside the RFEA orifice angular acceptance. The calibration method developed here allows accurate, quantitative measurement of ion currents for use with RFEAs in higher pressure processes where collisions within the RFEA are expected.


Juan Pablo Timpanaro
email: juanpablo.timpanaro@inria.fr

Thibault Cholez
email: thibault.cholez@inria.fr

Isabelle Chrisment
email: isabelle.chrisment@loria.fr

Olivier Festor
email: olivier.festor@inria.fr

When KAD meets BitTorrent -Building a Stronger P2P Network

Keywords: KAD, BitTorrent, DHT, Performance, Security, P2P architecture

The current wave of evolution that leads BitTorrent towards a fully decentralized architecture is both promising and risky. Related work demonstrates that BitTorrent's Mainline DHT is exposed to several identified security issues. In parallel, the KAD DHT has been the core of intense research and was improved over years. In this paper, we present a study that motivates the integration of both worlds. We provide a performance comparison of both DHTs in terms of publishing efficiency. We investigate the security threats and show that the current BitTorrent Mainline DHT is much more vulnerable to attacks than KAD. On the other hand, we demonstrate that the file download service provided by BitTorrent outperforms the one of KAD. Given the strengths and weaknesses of both DHTs, we propose a design in which the two P2P networks can be merged to form a fully distributed, efficient and safe P2P eco-system.

I. INTRODUCTION

According to a recent study on Internet traffic [START_REF]Ipoque. Internet Study[END_REF], P2P applications still generate a large part of it. Among these P2P applications, BitTorrent is the biggest source being ranked first with over 5 million simultaneous users. Despite its success, BitTorrent has recently suffered from several legal issues and complaints from music and movie companies, including legal actions to successfully shut-down major trackers sites, such as The Pirate Bay or Mininova. This accelerated the evolution of the network towards a fully distributed approach offering the same level of service than a central tracker. That is precisely the purpose of the Mainline DHT embedded in the major Bit-Torrent clients since 2006. Alternatively, a second distributed implementation by the Azureus client was developed, however, it is only used by the Vuze client (previously Azureus). For this reason, we chose to study the Mainline DHT, which is widely implemented.

The same study [START_REF]Ipoque. Internet Study[END_REF] shows that eDonkey is the second most popular P2P network with around 2 million users. The main client of the eDonkey network, eMule, introduced in 2004 a new fully decentralized P2P network called KAD which was also designed to be compatible with eDonkey. Since the shutdown of the major eDonkey servers in the past years after lawsuits, KAD gained popularity. Thanks to its fully distributed architecture and the open-source nature of its clients (eMule and aMule), KAD has been widely studied and improved. The DHT is mature and evolved towards a robust DHT able to deal with both scale and security.

Given these facts, we argue that BitTorrent could easily benefit from the KAD DHT performance and features for indexing contents while KAD could benefit from the performance of the BitTorrent download protocol, both networks taking advantage of this collaboration. We provide the scientific evidence to motivate this shift. For this purpose, we make the following contributions:

• We compare the performance and properties of both BitTorrent Mainline DHT and KAD DHT.

• We investigate the security mechanisms in place in KAD and compare them with those in Mainline DHT.

• We compare the download protocols performance of both networks

• We propose a new architecture which merges the strengths of KAD at a DHT level (security and indexation scheme) and the BitTorrent download protocol.

To our knowledge, this is the first work that brings together a comparison between these two networks through real world experiments. It is also the first that offers an integration design for both worlds.

The document is organized as follows. In section II, we introduce both KAD and BitTorrent P2P architectures. In section III we summarize the work done on both performance evaluation and security for either BitTorrent and KAD. Our joint evaluations regarding performance and security are made in section IV and V respectively. Focus on the download protocols is made in section VI. The design of an integrated architecture merging both worlds is finally proposed in section VII. A discussion is presented in section VIII and finally a conclusion on the proposed integration and future works are given in IX.

II. BACKGROUND ON KAD AND BITTORRENT

While BitTorrent and KAD are both P2P networks providing a file sharing application, they use two different architectures to achieve this purpose. The file download service can be mainly divided in three steps: first, a user must retrieve through a search engine a list of possible files being given a set of keywords, second the client must retrieve the sources of the selected files, third it has to initiate connections towards those peers to download the file.

A. BitTorrent architecture

In the regular BitTorrent architecture, the first step is done by using the search engine of a website indexing and delivering torrent files. A torrent file is a collection of several data: the hash of the shared file and its pieces, the IP address of the tracker server in charge of listing the peers sharing that file. The second step is done by contacting the tracker to discover the peers composing the swarm, which is a file-centered entity composed of both seeders (peers that have already completed the download) and leechers (peers currently downloading) interested in a file. Finally, the client connects to these peers in order to download the file following the specific BitTorrent protocol for peers and pieces selection, including the tit-fortat mechanism, which are actually the core of the BitTorrent protocol.

Retrieving the first peers to join the swarm is normally achieved through a central tracker but BitTorrent introduced DHTs so as to distribute this service. Currently, two distributed trackers can be found for BitTorrent, both based on a Kademlia distributed hash table. The Azureus DHT came first, and it is only used by this client. In second place, the Mainline client introduced its DHT a short time after and it was adopted by the other major clients. Both provide a simple indexation mechanism, in which, being given the ID of a torrent file 1 a peer can perform a DHT lookup to retrieve the list of peers already sharing the file. Like the central tracker, once the first peers have been retrieved, the new peer can join the swarm and share with the others.

B. KAD architecture

KAD is also based on the Kademlia protocol but achieve every step of the file sharing service in a fully distributed way thanks to its double-indexation mechanism. The first level associates keywords with files whilst the second associates files with sources (peers sharing the file). When a file is shared by a peer, the raw data and all the keywords composing its name are hashed separately with a MD4 function generating a KADID for each piece of information. Those KADIDs are then published into the DHT. The information concerning the file (fileID, filename...) are published towards the hash of each keyword (keywordID) to link them to the file. Second, the peer publishes its own information (KADID, IP address, port) towards the hash of the file to be referenced as a potential source. While the second level of indexation is similar to the service proposed by in BitTorrent's DHT, the first level indexing files with keywords is particular to KAD and allows to avoid any central component. Considering the search side, KAD DHT allows a peer to retrieve a list of files being given a set of keywords and a list of peers being given a fileID.

C. Kademlia DHT

As mentioned, the current widely deployed DHT are based on the Kademlia reference design [START_REF] Maymounkov | Kademlia: A Peer-to-Peer Information System Based On The XOR Metric[END_REF] even if they differ 1 The torrent's ID is obtained by hashing the torrent file itself or using a magnet link.

between each-other in their implementation. Each node of Kademlia and each stored information has an identifier (commonly of 160 bits) setting its position in the address space of the DHT. All routing tasks are based on the XOR metric used to evaluate the distance between two peers, or between a peer and a key. Routing is done in an iterative way and with parallel lookups by using lookup requests to discover new peers close to a specific address. The routing table is composed of groups of contacts (called a K-Bucket) organized in a binary tree so that the closer an ID is to the current node, the more peers it knows for this part of the DHT. The peers able to index information are those that are close enough to the published hash. This distance is called the "tolerance zone" and is set to the first common 8 bits (most significant). After accepting a publication request for a given resource, a peer is in charge of indexing this specific content, and to answer the related search requests.

III. RELATED WORK

Considering BitTorrent as a two-components application, the central-tracker and the swarm, we can argue that it has been the core study in many research works. However, almost all the studies on BitTorrent are focused on the swarm part and the tracker side but very few on BitTorrents two DHTs, the Mainline DHT and the Azureus DHT. Among those, Wolchok et al. [START_REF] Wolchok | Crawling BitTorrent DHTs for Fun And Profit[END_REF] conducted a monitoring study on the Azureus DHT. They clearly show how this DHT can be crawled thanks to a Sybil attack, so as to rebuild from scratch a BitTorrent search engine as well as to monitor pirate's behavior. While monitoring the Azureus DHT, the authors of [START_REF] Falkner | Profiling A Million User DHT[END_REF] considered the performance of the lookup algorithm and propose new parameters for it reducing the lookup time at the cost of a moderate overhead. Finally, Crosby et al. conducted a detailed comparative study of the two DHTs in [START_REF] Crosby | An Analysis Of BitTorrent's Two Kademlia-based DHTs[END_REF]. They point out the difference of performances between the two DHTs despite the fact that they are both based on Kademlia and implement the same service. They also highlight many design and implementation problems affecting both the security and performance of these DHTs.

Regarding the security of BitTorrent's DHTs, even fewer studies exist. In our previous work [START_REF] Pablo | BitTorrent's Mainline DHT Security Assessment[END_REF] we showed how the Mainline DHT network is wide open to attacks that can dangerously hurt the network, putting in jeopardy users privacy as well as the network performance itself. Recently, Jetter et al. [START_REF] Jetter | Quantitative Analysis of The Sybil Attack And Effective Sybil Resistance In Peerto-Peer Systems[END_REF] proposed a self-registration mechanism, as a way to avoid a Sybil attack in BitTorrent DHTs. They limit the number of peers'ID per IP, so as to avoid an attacker to launch several peers from a single machine. However, their solution require a jump to a new network, avoiding backward compatibility.

Considering KAD, its performance has been first considered in [START_REF] Stutzbach | Improving Lookup Performance Over A Widely-Deployed DHT[END_REF] in which the authors highlight the role of K-buckets in the efficiency and reliability of the routing table. Then [START_REF] Steiner | Evaluating And Improving The Content Access In KAD[END_REF] proposed a similar study but considered alternative parameters, like the number of contacts per lookup or the time window separating the lookup phase from the service. In [START_REF] Carra | Building a Reliable P2P System Out of Unreliable P2PClients: The Case of KAD[END_REF] they characterize the churn and use this model to design a new publication policy taking into account the reliability of each peer based on its session time. More recently, [START_REF] Hun | Why Kad Lookup Fails[END_REF] investigated the efficiency of the lookup process in KAD, and proposed a new approach to improve the consistency of the results.

KAD also suffered from a vast range of attacks during the past years, resulting in a wide set of proposed protections, which hardened the security of KAD clients. On the one hand, Steiner et al. [START_REF] Steiner | Exploiting KAD: Possible Uses And Misuses[END_REF] present how KAD can be misused by the well-known Sybil attack. They showed how this attack can take the control over information stored in the DHT, compromise the privacy of users and be used to launch a DDoS attack without further effort from a single computer. On the other hand, Wang et al. [START_REF] Wang | Attacking The KAD Network[END_REF] proposed an alternative attack, called reflection attack, in which all the entries in the target's routing table contain the target's IP. They proposed an identity authentication to avoid this hijacking, currently implemented by KAD clients. In [START_REF] Michael Kohnen | Conducting And Optimizing Eclipse Attacks In the KAD Peer-to-Peer Network[END_REF], the authors describe another way to eclipse a content by making the lookup process run indefinitely until timeout.

Finally, we assessed in our previous work [START_REF] Cholez | Evaluation of Sybil Attacks Protection Schemes in KAD[END_REF] some protection mechanisms introduced in KAD. These mechanisms include flood protection, IP verification and identity verification, and make a KAD client resilient to most known-attacks, including the Sybil attack [START_REF] Steiner | Exploiting KAD: Possible Uses And Misuses[END_REF] and contacts overwriting [START_REF] Wang | Attacking The KAD Network[END_REF] attacks. KAD's developers considered the network's flaws and turned it into a strong and mature P2P network. Despite that, some remaining flaws still exist involving more complex distributed attacks targeting specific contents on the KAD DHT as the one we presented in [START_REF] Cholez | Monitoring and Controlling Content Access in KAD[END_REF]. But we also proposed an efficient way [START_REF] Cholez | Efficient DHT Attack Mitigation Through Peers' ID Distribution[END_REF] to detect peers trying to attack a DHT entry (which forge their KADID accordingly) and also the countermeasures to avoid them.

IV. DHTS PERFORMANCE COMPARISON

This section aims to point out the main performance characteristics of each DHT network. Despite the fact that they are based on Kademlia, the freedom let in the implementation can lead to major performance variations, as previously shown between the two BitTorrent's DHT [?].

We will focus our attention on three main features:

• Time to publish information.

• Overhead during publishing.

• Lifetime of the stored information.

We have chosen to measure the performance regarding the time it takes to publish, but not the time to search, since both operations are expected to be symmetric. In KAD the routing procedure is the same when storing or searching a given information, with a different service message type regarding the service the client is performing. In BitTorrent, both services use the same announce request, used both to publish a torrent and retrieve a list of peers already sharing it.

In all cases, the last version of aMule (2.

A. Publishing new content

Publishing content is one of the basic function in any DHT. It mainly consists in finding the adequate peers, the closest ones in Kademlia, for that particular content and storing in them a link to the owner of this content.

The time it takes to publish a new content is a good metric to measure how well the routing algorithm performs. Therefore we conduct an experiment, aiming to measure this property, in which 1000 random IDs were published in both DHT networks during 24 hours.

In the Mainline DHT there is a type of message, Announce Peer, which publishes a content and retrieves a list of peers already sharing that content. This message has the same functionality that the Announce message in a central-tracker approach (the Announce request is sent to the tracker to join the swarm and, in the meantime, introducing the current peer and retrieving the list of seeders-leechers).

Regarding KAD, a Store message is used (for both keywords and files) in order to publish a given information, whilst a Search message provides the functionality to retrieve the sources (or files) associated to this content, opposite to the Mainline DHT, in which the same message does both operations.

An extra consideration needs to be made regarding the group of closest peers in which we will publish the content, normally called replica set. In KAD the replica set is formed by 10 peers, whilst in Mainline is a group of 8 peers. Therefore, to be fair enough, we will compute the time it takes a full announce in Mainline DHT (publishing in the closest 8 nodes) and the time it takes to store a content (Keyword or File) in KAD in the first 8 peers.

Figure 1 shows the experiment's results. It can be observed that both DHTs perform similar, both in the range of 30-40 seconds. Although Mainline DHT might perform a few seconds faster, KAD includes a 3-seconds time-window between it retrieves the closest peers and it sends the publish requests. It has been proved by STEINER ET AL. in [START_REF] Steiner | Evaluating And Improving The Content Access In KAD[END_REF] that this time-window can be reduced from 3 seconds to 0.5 seconds, and the routing algorithms will perform faster, without losing performance. Taking into account this time reduction, KAD and Mainline present similar times regarding the publish process.

B. Overhead during publishing a new content

Considering the time to publish a content is a good metric, however the number of messages used in the process needs to be taken into consideration. We compute the messages sent for the 1000 random Ids for both DHTs.

As illustrated in figure 2, the KAD DHT produces between 25 and 30 messages in average for each publication, whilst in the Mainline DHT this value claims to 40 in average. It is important to point out that these messages include routing messages and service messages. The routing messages are used to locate the closest peers, and once those peers were located, then the service requests are issued. The number of service requests is fixed, and depends on the number of peers in the replica set, therefore the difference between the number of messages is based on routing messages.

Additionally, every routing response in KAD contains 3 contacts, opposite to the Mainline DHT, in which a routing response contains 8 contacts. These extra contacts produce an unnecessary overhead, since only a sub-set of them are used to keep routing, whilst the rest are disregarded.

KAD optimizes the routing by responding with the 3 best contacts, which are sufficient to keep routing. We believe that Mainline's responses contains 8 contacts since the churn level of the network is higher, and therefore more contacts in the response might be stale.

C. Information lifetime

The lifetime of the information is extremely important, since it gives an approximation of the re-publishing rate needed to maintain the information in the network. A DHT network with a high called churn, needs to re-publish its data more often since the original peers which kept it in the first place might have gone off-line.

In order to measure this lifetime in both DHTs, we publish 1000 random Ids and keep the replica set. Every 30 minutes, we check how many of these peers are still alive. Figure 3 shows the measured aliveness of the peers through time.

Regarding the KAD network, it can be observed that after the first 30 minutes, 84% of the peers in the replica set are alive, and after 5 hours only 50% remain on-line. Finally after a whole day, 72% of the peers in the replica set are dead. Although these churn values might seem high, KAD adapted its publication process accordingly. In KAD there are two types of content to publish, keywords and files, respectively re-published, every 24 hours and 5 hours. The keyword-file association is permanently refreshed by all the peers sharing a given file and is unlikely to be lost. 50 % of alive peers after 5 hours is an good value that allows to retrieve the source of a file between two scheduled re-publishing.

On the other hand, Mainline DHT has higher churn levels. After the first 30 minutes, only 41% of the replica set is alive, and considering that this replica set is formed by 8 peers, it means that in average, only 3 peers are on-line. This value decreases to only 9% after one day. A difference with KAD, is that there is not a time window to re-publish a content, and it depends to the original seed to re-announce itself, otherwise the content will be lost.

As long as the seed is on-line, the client will periodically publish the torrent, similar to a periodically announce to a central-tracker. However, if the original seed does not reannounce the content, it is highly likely that new peers do not find this seed, and therefore they can not start the download. This situation is similar when the tracker goes off-line, in a central-tracker approach.

Hence, high level of churn might produce stale downloads. In order to understand this churn behaviour, we computed the population variation of both networks for 5 days. Figure 4 shows the obtained results.

The Mainline DHT network presents an important variation of its population, indicating that it is more a snatch-andgo network, contrary to KAD which presents a more stable behaviour due to the nature of its participants, which stay connected long periods of time. Clearly this high dynamism of peers is linked to the low information availability we previously obtained with our experiment.

Despite the fact we saw differences in the implementations of both DHTs, according to the Kademlia design, there is not a significant gap in terms of performance. This slightly gap is not sufficient to motivate a radical change in the current BitTorrent architecture.

We will show that Mainline DHT presents other weaknesses among security flaws and lack of some features, which will definitely motivate this change.

V. DHTS SECURITY COMPARISON This section will be organized as follow. Firstly, we will introduce the KAD protection mechanisms that the latest aMule client includes. Secondly, a Mainline client will be tested against a basic routing table attack. Finally, conclusions regarding both DHTs will be presented.

A. Protections Mechanisms in KAD

As we showed in our previous work [START_REF] Cholez | Evaluation of Sybil Attacks Protection Schemes in KAD[END_REF], KAD contains some protections at the routing table level which have been progressively introduced from (0.49a / aMule 2.2.1). Every protection adds more restrictions when adding new contacts, as presented in the following subsections.

1) Flood Protection: A flood protection mechanisms is achieved by keeping a history of all the packet received the last 12 minutes. A threshold is set as a way to restrict the maximum number of messages per peer, and in case this threshold is surpassed, the incoming messages are dropped, unless the threshold is highly overcome, in which case the peer is banned. Additionally, the client uses this protection to drop unrequested messages.

2) IP Limitations: The IP limitation aims to mitigate the Sybil attack from an attacker owning a single public IP. Before adding a new contact, the IP is checked, and in case it is already used, the contact is dropped. This makes a Sybil attack much harder to perform, since an attacker will need several public IPs to have a significant effect. This IP limitation also affects contacts from the same /24 subnet, since it is no possible to add more than ten contacts to the routing table coming from this subnet. Moreover, this ten contacts need to be in different K-buckets, so as not to allow an attacker owning a /24 subnet to position Sybils very closed to the target ID, and launch a localized attack.

3) IP Verification: Finally, an IP verification aims to avoid identity spoofing (both IP and ID) by introducing a three-way handshake before adding a contact. This three protections stack mitigates a Sybil attack from a single IP or even a /24 subnet. It is still possible to launch a distributed attack using several IPs, however we proposed in [START_REF] Cholez | Efficient DHT Attack Mitigation Through Peers' ID Distribution[END_REF] an ID distribution analysis which can successfully avoid this kind of attacks and fully compatible with the KAD network.

B. Assessing Mainline DHT

In order to establish the protection level in Mainline DHT, we ran the attack described by STEINER ET AL. in [START_REF] Steiner | Exploiting KAD: Possible Uses And Misuses[END_REF], commonly known as a routing table poisoning from a single peer, which is a basic attack. Successfully achieving this attack will prove that the network is open to the most basic vulnerabilities, and further complex attacks can be easily performed.

The idea of the attack is to add as many fake contacts, the Sybils, into the target's routing table. The target peer will eventually route requests through these Sybils, but especially respond with these to routing requests from other peers.

The attack consists in sending several ping requests (equivalent to the Hello request in KAD) to the same target peer, using an unique IP and different ports. A ping request contains the IP and port of the receiver and the ID of the sender. In our case, every ping message will contain a random ID varying from 0 bits to 159 bits in common to the target peer ID.

Figure 5 shows the number of contacts of the target peer. Firstly, we let the target peer bootstrap and reach a steady number of contacts, normally around 170 contacts, which are positioned in the first 22 buckets (sharing between 0 bits and 22 bits in common). At the minute 12 the attack begins, sending 160 ping requests. It can be observed that the routing table gets filled instantly with approximately 140 new contacts (Sybils), reaching around 310 contacts in total.

Because the target peer's first buckets are already full with normal (not Sybil) contacts, no new contacts can be added. However, launching the attack as soon as the target peer is on-line will successfully poison the first buckets, achieving a total number of 160 Sybils (one Sybil per bucket).

This first experiment consists in positioning one Sybil per bucket. However, we performed a second experiment aiming to fully fill each bucket, which means 8 Sybils per bucket. Therefore, we sent 1280 (8 * 160) ping request properly scheduled to the target peer. Notwithstanding, only one Sybil succeeded to enter a bucket, because Mainline DHT does not allow two contacts in the same bucket from the same IP. This feature protects the routing table from a full poisoning (8 Sybils per bucket), but not from a partial poisoning (1 Sybil per bucket). However, this behaviour resembles an implementation limitation, more than a real security mechanism to avoid routing table poisoning. However, in the lowest buckets (the buckets close to the target), only Sybils can be found and will be returned at the end of each lookup process which is actually the major threat.

On the other hand, figure 6 shows the number of Sybil contacts during the same attack on a KAD client. It can be observed the quick increase of Sybils during time, in the case no protection is activated. On the opposite side, when the client enables the protections, no Sybil are place in the routing table.

We have shown that the Mainline DHT is open to the basic attack from a single machine, on the contrary to KAD, in which the IP limitation takes care of this attack. We strongly believe that this lack of protection in the network is the main reason to migrate to a more secure DHT.

VI. DOWNLOAD PROTOCOL

The performance characteristics along with its security features at a DHT level make KAD an excellent network. Not only because its routing algorithm achieves good lookup times, but because it is resilient to most known attacks. The churn level measured in this network is a key feature to take into consideration, since it strongly affects the final performance, even though it does not depends on the DHT design itself.

Although the DHT level is considerably important, the final users will not perceive its performance in all cases, but the performance of the download algorithm. It is pointless to have a client that can successful publish and search for a group of sources for a given file in record time, but takes a long time to download the file. Clearly, the performance of the search procedure, a DHT-based in this case, will be overshadowed by a poor download protocol.

In order to determinate the performance of KAD and BitTorrent download protocols, we conducted the following experiment. We used 50 peers from PlanetLab's infrastructure and a single 700 MB random file, to measure the time it takes to download the file for an aMule client and a Vuze client. We ran two experiments, varying the number of initial sources, or initial seeders, in the case of BitTorrent. Both terms refer to the same concept (a peer having the entire content) and will be used indistinctly.

A. Download time with one Source

Starting with one source is the normal behaviour, since the original publisher initially uploads itself as the only source, and eventually new peers complete the download and become new sources. Figure 7 shows a graphical comparison between BitTorrent clients (Vuze clients) and KAD clients (aMule clients). While BitTorrent clients achieved a total download Additionally, figure 8 shows the percentage of clients that complete the download through time. aMule clients present a more lineal distribution, which indicates that the completeness of the download is achieved at the end by all the peers. This characteristic is expected since all the peers present the same ADSL-like bandwidth (1000kbit/s down and 300kbit/s up) and similar initial status (none of them contains the file nor pieces of it).

On the other hand, Vuze clients present a step-style distribution. This can be due to the nature of the peers selection process, however a study of the piece exchange protocol and BitTorrent's algorithms exceeds the scope of this work. Further characteristics of the BitTorrent's algorithms and an extensive analysis can be found in the work done by LEGOUT ET AL. in [START_REF] Stevens | Pushing BitTorrent Locality To The Limit. Computer Networks[END_REF].

At this point, we can observe that BitTorrent's download protocol performs better and achieve the download in 42% of the total download time regarding KAD.

However, most of the clients trying to download a file will find themselves with a higher number of sources, since in the case of popular contents, the total number of sources climb very fast. Therefore, we conducted a second experiment, with 10 sources and measured the download times.

B. Download time with ten Sources

Although ten sources is not close to the average number of sources for a popular content (around 1000 or even more for real popular content), it might be accurate enough for small torrents, or KAD files. These small torrents or files might be alive for long periods of time, despite a low number of sources.

Figure 9 shows the time it takes to download the same 700 MB file from the Vuze clients and the aMule clients. In this case Vuze clients take 224 minutes to complete the download, whilst aMule clients take 395 minutes. Even though in this case the Vuze clients performed the download in 57% of the total download time regarding KAD, BitTorrent's download protocol clearly surpassed KAD's.

Figure 10 presents the cumulative distributions of peers with their downloads complete. In this case Vuze clients present a linear-like distribution, showing that a small group of peer (14% of them) complete the download at the beginning, whilst the rest complete the download close to the end.

As a conclusion it is observed that the BitTorrent's download protocol surpassed KAD's. We measured the time to download when 2% of the peers are sources (1 source among 50 peers) and when 20% of peers are sources (10 peers out of 50). In every case, BitTorrent's clients achieve the download in 50% of the time it takes to the KAD's clients to complete it. However, this experiments did not take into account concurrent downloads, in which the final results might vary.

Although we can not generalize this behaviour for every swarm configuration, we have shown the BitTorrent's download protocol performance comparing to KAD's.

VII. MERGING THE TWO NETWORKS: PROPOSED ARCHITECTURE

After an extensive analysis of these well-known P2P networks, we proposed the architecture shown in figure 11, so as to merge the best features of each approach namely, (1) KAD's DHT security properties and its 2-level indexation solution and (2) BitTorrent's efficient download protocol.

The integrated architecture has two main components: the indexation block and the download block. The download component is based on the BitTorrent download protocol, including its tit-for-tat rewarding mechanism. The indexation component is implemented using the KAD double-indexation mechanism. A user introduces a set of keywords which defines a given content and retrieves a list of peers having that content (sources for that content). Once the user retrieves a list of peers for that content, it will join them, in order to start a download.

The KAD DHT will now also keep data from the BitTorrent network as well, allowing users to store (or search) torrents by keywords, and store (or search) sources for a given torrent. Moreover, KAD's tags can be used during the indexation process. Tags enable the user to accurately select a torrent, based on the keywords it has associated. In case of torrents, this information is already specified by the torrent's detail, so a smooth transition can be made to generate tags. Additionally, magnet links will be still useful, since the user can retrieve the list of sources, skipping the first level of indexation.

This new keyword-level indexation will complement the web-based lookup for torrents, and hopefully will replace the jungle of torrent search websites. This architecture will allow KAD users to benefit from the BitTorrent download protocol, which has been proven to achieve excellent download times, whilst maintaining a secure and strong DHT support. On the other hand, BitTorrent users can finally leave behind an almost obsolete central-tracker approach for a fully distributed tracker, based on a well-know and mature distributed hash table.

We can see a last benefit of merging those networks. When a file has been downloaded through the BitTorrent part of the merged approach, it can then automatically be shared on the regular KAD part. In that way, even if the torrent file of the content is lost (due to lost of popularity and therefore no longer reachable in the BitTorrent network), the content is still reachable through KAD own file sharing process. As a result, KAD keeps a memory of those files that once were popular in BitTorrent, even though the torrent file is no longer available.

VIII. DISCUSSION

In this section we will discuss (1) Why we do not to patch up the current implementation of the Mainline DHT, and (2) Why we do not to consider well-known solutions, like identitybased approaches or a central authority for authentication.

From a security point of view, there are no disadvantages in implementing the security protections in the Mainline DHT, even though it will require that every client implements these protections. The main reason to use the KAD DHT, and not the Mainline DHT is that the first one has the double-indexation mechanism. This feature allows the indexation of torrents, one of the main features of the proposed joint architecture.

The KAD DHT is, currently, vulnerable to distributed attacks, in which an attacker uses several nodes to achieve the same Sybil attack. However, the IDs distribution analysis proposed in [START_REF] Cholez | Efficient DHT Attack Mitigation Through Peers' ID Distribution[END_REF] successfully mitigates this attack. There are alternative mechanisms, such as the external certification service propose by Fantacci et al. [START_REF] Fantacci | Avoiding Eclipse Attacks on Kad/Kademlia: An Identity Based Approach[END_REF], the Likir framework proposed by Aiello et al. [START_REF] Luca | Tempering Kademlia with a robust identity based system[END_REF], and the set of security considerations by Sit et al. [START_REF] Sit | Security Considerations for Peer-to-Peer Distributed Hash Tables[END_REF]. However, none of these approaches consider backward compatibility, and all of them require several modifications to the DHT protocol. Although a modified DHT will be suitable to fulfill the security problems, it is not viable to migrate million of users.

We consider backward compatibility as a mayor design goal, and the IDs distribution analysis solution can be implemented in a every client, without requiring a major change in the network. Each client can progressively incorporate this protection, making the network each time more resilient.

IX. CONCLUSION

BitTorrent and KAD are two popular P2P networks used by millions of active users. Originally, BitTorrent used a central tracker to retrieve a set of peers in order to start a download, whilst in KAD this process is originally decentralized. However, in the past years and due to legal threats against central trackers, the BitTorrent community evolved to support decentralized trackers. In this paper we focused our attention on the Mainline DHT, which is the most widely used decentralized tracker in BitTorrent.

We compared the Mainline and the KAD DHT in terms of performance characteristics, security features and download properties. We demonstrated through real experiments the lack of security mechanisms in the Mainline DHT, lack which makes the network prone to the most basic P2P attacks. We also showed how KAD is resilient to these attacks. On the other hand, we reached the conclusion that BitTorrent's download protocol is around twice faster than KAD's one in a real network environment.

Finally, we presented a joint approach, merging these two great networks into a single one, integrating the KAD security mechanisms and BitTorrent's outstanding download performance. The result is a robust and strong P2P network, with a mature DHT support and an excellent download protocol. This wedding also introduces new features like keyword indexation of torrents and a memory of contents without torrent. We strongly believe that such an architecture brings together the best of both P2P networks. Future work will be based on testing the implementation of the associated implementation on a real network environment and the provision of smooth transition mechanisms.

  2.6) was used to test the KAD network. The Vuze client, version 4.5, and its Mainline DHT plug-in, version 1.3.3.1, were chosen to test the Mainline DHT.

Fig. 1 :

 1 Fig. 1: Time to Publish Random Content.

Fig. 2 :

 2 Fig. 2: Messages sent during a publish process.

Fig. 3 :

 3 Fig. 3: Measured aliveness of nodes.

Fig. 4 :

 4 Fig. 4: Measured Population.

Fig. 5 :

 5 Fig. 5: Number Sybils of contacts during attack on Mainline.

Fig. 6 :

 6 Fig. 6: Number of Sybils contacts during attack on KAD.

Fig. 7 :

 7 Fig. 7: Download Times with 1 Source.

Fig. 8 :

 8 Fig. 8: Peers with download complete (1 source).

Fig. 9 :

 9 Fig. 9: Download Times with 10 Sources.

Fig. 10 :

 10 Fig. 10: Peers with download complete (10 sources).

Fig. 11 :

 11 Fig. 11: Graphical view of the joint approach.