Accurate and efficient SLA compliance monitoring Service level agreements (SLAs) define performance guarantees made by service providers, e.g, in terms of packet loss, delay, delay variation, and network availability. In this paper, we describe a new active measurement methodology to accurately monitor whether measured network path characteristics are in compliance with performance targets specified in SLAs. Specifically, (1) we describe a new methodology for estimating packet loss rate that significantly improves accuracy over existing approaches; (2) we introduce a new methodology for measuring mean delay along a path that improves accuracy over existing methodologies, and propose a method for obtaining confidence intervals on quantiles of the empirical delay distribution without making any assumption about the true distribution of delay; (3) we introduce a new methodology for measuring delay variation that is more robust than prior techniques; and (4) we extend existing work in network performance tomography to infer lower bounds on the quantiles of a distribution of performance measures along an unmeasured path given measurements from a subset of paths. We unify active measurements for these metrics in a discrete time-based tool called SLAM. The unified probe stream from SLAM consumes lower overall bandwidth than if individual streams are used to measure path properties. We demonstrate the accuracy and convergence properties of SLAM in a controlled laboratory environment using a range of background traffic scenarios and in one-and two-hop settings, and examine its accuracy improvements over existing standard techniques. Network service level agreements (SLAs) detail the contractual obligations between service providers and their customers. It is Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. August 27-31, 2007 , Kyoto, Japan. increasingly common for SLAs to specify transport-level performance assurances using metrics such as packet loss, delay, delay variation, and network availability [2, 3, 4, 33] . Meeting SLA guarantees results in revenue for the ISP. However, failing to meet SLA guarantees can result in credits to the customer. The implications of not meeting SLA guarantees are therefore serious: a disruption in service can result in significant revenue loss to both the customer and provider. SLA compliance monitoring, assessing whether performance characteristics are within specified bounds, is therefore critical to both parties. We believe that SLAM represents a significant step forward for SLA compliance monitoring using active measurements. However, there are a number of issues that remain. First, there are additional issues to consider in the network-wide setting. For example, a deployment strategy must be developed to coordinate probe streams so that links internal to the network are not carrying "too much" measurement traffic. Another key question is: given a daily (or based on some other time scale) budget of probes that may be used to monitor compliance with a SLA, what are the considerations for optimizing the probe process? Should the probing period be over a 9: Performance of the DV matrix in a two-hop setting (r 2 ) using the star topology. Time series plot shown for CBR UDP traffic scenario. Curves show DV matrix metric for route r 2 and separately for links e 1 and e 4 that comprise r 2 .. relatively long time scale (e.g., the entire interval of interest), thus potentially limiting the accuracy of estimates, or should the probing period be over a shorter time scale, potentially improving estimation accuracy but at the cost of not probing over the entire interval, thus potentially missing important events? We have assumed in this paper that perfect accuracy is the goal for compliance monitoring. However, for some SLAs, a tradeoff (if it is predictable) between accuracy and measurement overhead may be appropriate. Next, our examples of distributional inference have focussed on delay. We plan to more closely examine loss in the future. Finally, while measuring availability in a simple path-oriented scenario is rather straightforward, simple application of performance tomography to infer network-wide availability may not be sufficient in the face of routing changes. In summary, this paper introduces a new methodology for SLA compliance monitoring using active measurements, including new methods for measuring end-to-end packet loss, mean delay, and delay variation. We propose a new method for obtaining confidence intervals on the empirical delay distribution. We also describe a new methodology for inferring lower bounds on the quantiles of a distribution of a performance metric along a path in a networkwide setting from a subset of known paths. We implemented these measurement methods in a tool called SLAM that unifies the various probe streams resulting in lower overall probe volume. We evaluated the capabilities of the tool in a controlled laboratory environment using a range of traffic conditions and in one-and two- True loss rates are plotted using 10 second intervals. SLAM estimates are plotted using 30 second intervals. Plots shown for longlived TCP traffic in the dumbbell topology (top) and self-similar traffic on route r 2 in the star topology (bottom). The upper and lower bars for SLAM indicate estimates of one standard deviation above and below the mean using the variance formulation of [37] .. hop settings. Our results show that SLAM's delay and loss rate estimates are much more accurate than estimates obtained through standard probe methodologies. Furthermore, we illustrated the convergence and robustness properties of the loss, delay, and delay variation estimates of SLAM which make it useful in an operational setting. General aspects and structure of SLAs have been discussed in [27, 33] . Performance assurances provided by SLAs range from network path availability, to transport-level metrics, to applicationspecific metrics. These guarantees may be based on a variety of statistics of the particular metric, such as the mean, median, or a high quantile, computed over various time scales. Examples of the kinds of guarantees offered by service providers are available online [2, 3, 4] .. To ensure that SLA performance targets are met with high probability, service providers collect measurements either passively within the network, by injecting measurement probes into the network, or by using a combination of both [6, 13, 18, 42] . While active measurement-based compliance monitoring has received some attention in the past, e.g., [18] , there has been little validation in realistic environments where a reliable basis for comparison can be established. There has been limited work addressing the accuracy of some active measurement approaches; exceptions are found in [10, 31, 35] . The issue of accuracy clearly has serious implications for SLA compliance monitoring. Other efforts have been limited in focus to estimation and optimization of a single metric, e.g., [16, 19] . Our work takes an active measurement approach, focusing on simultaneous, or multi-objective, measurement of transport-level performance metrics. We further differentiate our work through validation in a controlled, realistic testbed.. In general, there has been a great deal of work on active measurements of end-to-end delay, delay variation, and loss, e.g., [7, 8, 11, 19, 21, 28, 29, 30, 40, 41] . IETF standardization efforts for active measurement of delay, delay variation, loss, and reordering have taken place within the IETF IPPM working group [7, 8, 21, 30] . Regarding delay, our method for distribution quantile estimation is distinguished from the earlier work of Choi et al. [16] in that we do not require the quantile of interest to be specified a priori, and that we do not make any assumption regarding the underlying delay distribution. As a result, our method is robust to abrupt changes in underlying network conditions. Lastly, we note that our formulation of a delay variation measurement methodology stands apart from the related IPPM [21] and real-time protocol (RTP) [32] specifications in that rather than considering highly localized variations in delay (e.g., between consecutive probe packets), we consider delay variations over streams of packets.

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A Note on Projecting the Cubic Lattice It is shown that, given any (n − 1)-dimensional lattice Λ, there is a vector v ∈ Z n such that the orthogonal projection of Z n onto v ⊥ is, up to a similarity, arbitrarily close to Λ. The problem arises in attempting to find the largest cylinder anchored at two points of Z n and containing no other points of Z n . Let Z n denote the cubic lattice with basis e 1 := (1, 0, . . . , 0), . . . , e n := (0, 0, . . . , 1). If we project Z n onto the (n − 1)-dimensional subspace v ⊥ := {x ∈ R n : x · v = 0} perpendicular to a vector v ∈ Z n , we obtain an (n − 1)-dimensional lattice that we denote by Λ v . We will show that, given any (n − 1)-dimensional lattice Λ, we can choose v ∈ Z n so that Λ v is arbitrarily close to a lattice that is geometrically similar to Λ. More precisely, we will establish: Theorem 1. Let Λ be an (n − 1)-dimensional lattice with Gram matrix A (with respect to some basis for R n−1 ). For any ǫ > 0, there exist a nonzero vector v ∈ Z n , a basis B for the (n − 1)-dimensional lattice Λ v and a number c such that if A v denotes the Gram matrix of B, then. The theorem is at first surprising, since A has n+1 2 degrees of freedom, whereas v has only n degrees of freedom (for the explanation see the remark following the proof of Theorem 2).. The problem arises from a question in communication theory (see §5), which calls for projections Λ v with high packing density. Since both the determinant and minimal norm of a lattice are continuous functions of the entries in the Gram matrix, so is the packing density. 1 The theorem therefore implies that the packing density of Λ v can be made arbitrarily close to that of Λ. So if we know a dense lattice in R n−1 , we can find projections that converge to it in density.. Remark. We know (see for example [2, Cor. 8] ) that if Λ is a classically integral (n − 1)-dimensional lattice then Λ can be embedded in some odd unimodular lattice K of dimension k ≤ n + 2, although for n ≥ 7 K need not be Z k . In any case this does not imply that Λ can be recovered as a projection of K.. Notation. Λ * denotes the dual lattice to Λ, A tr is the transpose of A, and A ∞ = max i,j |A i,j |. Our vectors are row vectors. For undefined terms from lattice theory see [3] .

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Authentication of currency notes through printing technique verification An image analysis based pattern classification method is proposed to authentic the printing process used in printing different texts on currency notes. Features suitable for doing this are selected and then studied to detect fraudulent samples based on the printing method. This classification is done by using Support Vector Machines and Neural Nets. The discriminatory power of the selected features in authenticating the printing process is tested using the Linear Discriminate Analysis. Experimental results show that the proposed framework provides a highly accurate framework for authenticating the printing process in bank notes. The paper currency notes being accepted as the most appropriate agent of monetary transaction the problem of counterfeit on a large scale have recently been posing a serious threat to our society. With the advancement of printing and scanning technology this problem has become acute one. Advancement in digital image processing is also playing a significant role in producing increasing number of fake banknotes every year. This paper presents a novel experiment on authenticating the printing technique in currency notes. Fraudulent currency notes often could not match the genuine printing technique while producing fake notes. The research embodied in this paper nicely shows that using the standard image analysis and pattern classification techniques an automatic method can be designed to capture many fraudulent cases. This authentication method would assist the people in forensic sciences or in financial organizations to identify fake notes. The features used in this experiment are quickly computable and therefore, the proposed method provides a quick (and possibly low cost too) solution to the problem.. Three different classifiers have been used in this study. Integration of them can also be done in future to build a more robust classification scheme. However, testing of this method using a large dataset is required to establish it as a standard practice. The method, indeed, is not only restricted to currency notes only. Printing technique is itself a security feature in most of the security paper documents. Therefore, this research provides a viable framework for machine authentication of security documents through verifying the printing technique. Magnified scan digitized images of genuine and fake currency notes (Indian rupees of denomination 500) are collected. For considering our study, we consider 100 genuine samples and another 100 samples of fake currency note images. Note that here samples are marked as genuine and fake just based on the fact that whether expected intaglio printing has been used or not to print the samples.

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On Selective Unboundedness of VASS Numerous properties of vector addition systems with states amount to checking the (un)boundedness of some selective feature (e.g., number of reversals, run length). Some of these features can be checked in exponential space by using Rackoff's proof or its variants, combined with Savitch's theorem. However, the question is still open for many others, e.g., reversal-boundedness. In the paper, we introduce the class of generalized unboundedness properties that can be verified in exponential space by extending Rackoff's technique, sometimes in an unorthodox way. We obtain new optimal upper bounds, for example for place boundedness problem, reversal-boundedness detection (several variants exist), strong promptness detection problem and regularity detection. Our analysis is sufficiently refined so as we also obtain a polynomial-space bound when the dimension is fixed. Reversal-boundedness. A standard approach to circumvent the undecidability of the reachability problem for counter automata [19] consists in designing subclasses with simpler decision problems. For instance, the reachability problem is decidable for vector addition systems with states (VASS) [15] or for lossy counter automata [1] . Among the other interesting subclasses of counter automata, reversalbounded counter automata verify that any counter has a bounded number of reversals, alternations between a nonincreasing mode and a nondecreasing mode, and vice versa. Reversal-boundedness remains a standard concept that was initially introduced in [3] for multistack automata. A major property of such operational models is that reachability sets are effectively definable in Presburger arithmetic [13] , which allows decision procedures for LTL existential model-checking and other related problems, see e.g. [5] . However, many natural problems related to verification remain undecidable for reversal-bounded counter automata, see e.g. [5, 6] , and the class of reversal-bounded counter automata is not recursive [13] . A significant breakthrough was achieved in [8] by designing a procedure to determine when a VASS is reversal-bounded (or weakly reversal-bounded as defined later), even though the decision procedure can be nonprimitive recursive in the worst-case. This means that reversal-bounded VASS can benefit from the known techniques for Presburger arithmetic in order to solve their verification problems. Selective unboundedness. In order to characterize the complexity of detecting reversal-boundedness on VASS (the initial motivation for this work), we make a detour to selective unboundedness, as explained below. Numerous properties of vector addition systems with states amounts to checking the (un)boundedness of some selective feature. Some of these features can be verified in exponential space by using Rackoff's proof or its variants, whereas the question is still open for many of them. In the paper, we advocate that many properties can be decided as soon as we are able to decide selective unboundedness, which is a generalization of place unboundedness for Petri nets (known to be equivalent to VASS). The boundedness problem was first considered in [15] and shown decidable by simply inspecting Karp and Miller trees: the presence of the infinity value ∞ (also denoted by ω) is equivalent to unboundedness. So, unboundedness is equivalent to the existence of a witness run of the form x 0 * − → x 1 π − → x 2 such that x 1 ≺ x 2 ( ≺ is the standard strict ordering on tuples of natural numbers). In [21] , it is shown that if there is such a run, there is one of length at most doubly exponential. This leads to the EXPSPACE-completeness of the boundedness problem for VASS using the lower bound from [17] and Savitch's theorem. A variant problem consists in checking whether the ith component is bounded, i.e., is there a bound B such that for every configuration reachable from x 0 , its ith component is bounded by B? Again, inspecting Karp and Miller trees reveals the answer: the presence of the infinity value ∞ at the ith position of some extended configuration is equivalent to i-unboundedness. Surprisingly, the literature often mentions this alternative problem, see e.g. [22] , but never specifies its complexity: EXPSPACE-hardness can be obtained from [17] but as far as we know, no elementary complexity upper bound has been shown. It might be explained by the fact that, if a VASS is unbounded, then there is a witness infinite run with an infinite number of distinct configurations. By contrast, it may happen that a VASS is i-unbounded but no infinite run has an infinite amount of distinct values at the ith position of the configurations of the run. In the paper, we present a generalization of place unboundedness by checking whether a set of components is simultaneously unbounded, possibly with some ordering (see Section 3.2) . This amounts to specifying in the Karp and Miller trees, the ordering with which the value ∞ appears in the different components. Our contribution. In the paper we show the following results.. 1. Detecting whether a VASS is reversal-boundedness in the sense of [13] or [8] is EXPSPACEcomplete by refining the decidability results from [8] (see Theorem 5.2). 2. We introduce the generalized unboundedness problem in which many problems can be captured such as the reversal-boundedness detection problems, the place boundedness problem, termination, strong promptness detection problem, regularity detection and many other decision problems on VASS. We show that this problem can be solved in exponential space by adapting [21] even though it does not fall into the class of increasing path formulae introduced in [2] (see Theorem 4.5). 3. Consequently, we show that regularity and strong promptness detection problems for VASS are in EXPSPACE. The EXPSPACE upper bound has been left open in [2] . Even though most of our results essentially rest on the fact that place boundedness can be solved in EXPSPACE, our slight generalization is introduced to obtain new complexity upper bound for other related problems. 4. As a by-product of our analysis and following a parameterized analysis initiated in [24, 12] , for all the above-mentionned problems, we show that fixing the dimension of the VASS allows to get a PSPACE upper bound.. The paper has also original contributions as far as proof techniques are concerned. First, simultaneous unboundedness has a simple characterization in terms of Karp and Miller trees, but we provide in the paper a witness run characterization, which allows us to provide a complexity analysis along the lines of [21] . We also provide a witness pseudo-run characterization in which we sometimes admit negative component values. This happens to be the right approach when a characterization from coverability graphs [15, 28] already exists. Apart from this unorthodox adaptation of [21] , in the counterpart of Rackoff's proof about the induction on the dimension, we provide an induction on the dimension and on the length of the properties to be verified (see Lemma 4.3) . This is a genuine breakthrough compared to [21, 24, 10, 2] . We believe this approach is still subject to extensions. We have proved the EXPSPACE-easiness of the generalized unboundedness problem (both the initialized VASS and the generalized unboundedness property are part of the inputs). For example, this allows us to provide the optimal complexity upper bound for the reversal-boundedness detection problems, place boundedness problem, strong promptness detection problem and regularity detection problem. Even though our proof technique is clearly tailored along the lines of [21] , we had to provide a series of adaptations in order to get the final EXPSPACE upper bound (and the PSPACE upper bound for fixed dimension). In particular, we advocate the use of witness pseudo-run characterizations (instead of using runs) when there exist decision procedures using coverability graphs.. Let us conclude by possible continuations. Our EXPSPACE proof can be obviously extended by replacing intervals in properties by more complex sets of integers or by adding new constraints between intermediate configurations. The robustness of our proof technique still deserves to be determined. A challenging question is to determine the complexity of checking when a reachability set obtained by an initialized VASS is semilinear. Besides, various subclasses of VASS exist for which decision problems are of lower complexity. For instance, in [20] , the boundedness problem is shown to be in PSPACE for a class of VASS with so-called bounded benefit depth. It is unclear for which subclasses of VASS, the generalized unboundedness problem can be solved in polynomial space too.

O O O O O O O O B I I I I O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I I O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Quantum measurements and entropic bounds on information transmission While a positive operator valued measure gives the probabilities in a quantum measurement, an instrument gives both the probabilities and the a posteriori states. By interpreting the instrument as a quantum channel and by using the monotonicity theorem for relative entropies many bounds on the classical information extracted in a quantum measurement are obtained in a unified manner. In particular, it is shown that such bounds can all be stated as inequalities between mutual entropies. This approach based on channels gives rise to a unified picture of known and new bounds on the classical information (Holevo's, Shumacher-Westmoreland-Wootters', Hall's, Scutaru's bounds, a new upper bound and a new lower one). Some examples clarify the mutual relationships among the various bounds. A problem which appears in the field of quantum communication and in quantum statistics is the following: a collection of statistical operators, with some a priori probabilities, describes the possible states of a quantum system and an observer wants to decide by means of a quantum measurement in which of these states the system is. The quantity of information extracted by the measurement is the classical mutual information I c of the input/output joint distribution; interesting upper and lower bounds for I c , due to the quantum nature of the measurement, are given in the literature [1, 2, 3, 4, 5, 6, 7] . Usually the measurement is described by a generalized observable or positive operator valued (POV) measure which allows to obtain the probabilities for the outcomes of the measurement. However, with respect to a POV measure, a more detailed level of description of a Also: Istituto Nazionale di Fisica Nucleare, Sezione di Milano. E-mail: Alberto.Barchielli@polimi.it b Also: Istituto Nazionale di Fisica Nucleare, Sezione di Milano. E-mail: Giancarlo.Lupieri@mi.infn.it 1 the quantum measurement is represented by a different mathematical object, the instrument [8, 9, 10] : given a state (the preparation) as input, it gives as output not only the probabilities of the outcomes but also the state after the measurement, conditioned on the observed outcome (the a posteriori state). We can think the instrument to be a channel: from a quantum state (the pre-measurement state) to a quantum/classical state (a posteriori state plus probabilities). The mathematical formalization of the idea that an instrument is a channel is central in our paper and allows for a unified approach to various bounds for I c and for related quantities [11, 12] .. To maintain things at a sufficiently simple mathematical level, we shall develop and present all the results in the case of a finite-dimensional Hilbert space, a finite alphabet and an instrument with finite outcomes.. In Section 2 we introduce the notion of instrument and we show how to associate a channel to it; some inequalities on various relative entropies are deduced from Ulhmann's monotonicity theorem. From such inequalities we obtain in Section 3 some bounds on the quantity of information I c which can be extracted by using an instrument as decoding apparatus; more precisely, we obtain the bound of Holevo [1] (31), a slight generalization of the bound of Shumacher, Westmoreland, Wootters (SWW) [4] (39) and the new inequalities (35), (47) . From the SWW bound we obtain in a straight way also a result by Groenewold, Lindblad, Ozawa [13, 14, 15] on the positivity of the quantum information gain given by an instrument. We also show how such bounds can be stated as inequalities between mutual entropies (the relative entropy of a bipartite state with respect to its marginals). In Section 4 we generalize a transformation due to Hall [5] , we introduce a new instrument and we obtain another set of bounds on I c : Hall's bound (72), a strengthening of it (76), Scutaru's bound [3] (79) and the new inequality (82). All the bounds of Sections 3 and 4 concern a fixed instrument and the associated POV measure; we can say that they quantify the performances of the measurement procedure with respect to the initial ensemble. In Section 5 we give a summary and some examples of the various bounds.

O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


From

Hippocratic

Databases

to

Secure

Tropos:

A

Computer-Aided

Re-Engineering

Approach

Privacy

protection

is

a

growing

concern

in

the

marketplace.

Yet,

privacy

requirements

and

mechanisms

are

usually

retro-fitted

into

a

pre-existing

design

which

may

not

be

able

to

accommodate

them

due

to

potential

conflicts

with

functional

requirements.We

propose

a

procedure

for

automatically

extracting

privacy

requirements

from

databases

supporting

access

control

mechanisms

for

personal

data

(hereafter

Hippocratic

databases)

and

representing

them

in

the

Tropos

modeling

framework

where

tools

are

available

for

checking

the

correctness

and

consistency

of

privacy

requirements.

The

procedure

is

illustrated

with

a

case

study.

Interest

in

privacy

and

privacy-aware

information

and

communication

technologies

is

growing

and

many

countries

have

promulgated

new

privacy

legislation.

Regulations

in

the

US

and

in

the

EU

are

largely

based

on

the

idea

of

"Fair

Information

Practices".

These

practices

stem

from

a

set

of

principles

established

in

by

the

Organization

for

Economic

Cooperation

and

Development

(OECD).

Most

of

these

laws

incorporate

rules

governing

collection,

use,

storage

and

distribution

of

personally

identifiable

information.

Enterprises

that

do

not

correctly

manage

customer

personally

identifiable

information

chance

some

risks.

It

is

up

to

an

organization

to

ensure

that

data

processing

operations

comply

with

legislative

requirements.

Actually,

an

enterprise

can

be

sued

if

it

uses

customer

personal

data

for

purposes

other

than

those

they

were

collected..

Central

to

the

concept

of

privacy

are

transparency

and

fairness

principles.

Transparency

means

that

organizations

should

make

known

to

individuals

which

information

has

been

collected

about

them,

and

how

it

is

used.

Fairness

means

that

information

should

be

used

only

for

the

purpose

for

which

it

is

collected.

If

the

organization

wants

to

use

personal

information

for

other

purposes,

it

must

previously

obtain

consent

from

its

owners..

In

recent

years,

researchers

have

recognized

that

traditional

access

control

models,

such

as

discretionary

access

control

mandatory

access

control

and

role-based

access

control

fail

in

protecting

privacy

(see

Ref.

Actually,

privacy

policies

focus

on

the

correct

use

of

data

(i.e.,

the

use

of

data

in

compliance

with

the

fairness

principle),

rather

than

authorization

decisions

based

on

who

is

accessing

the

data

and

which

action

is

performed

on

them

(see

Ref.

Therefore,

defining

and

enforcing

privacy

policies

require

extensions

to

traditional

access

control

models.

In

particular,

data

subject

a

and

the

purpose

for

which

data

items

can

be

collected

and

used

should

be

considered

in

the

model

(see

Ref.

also,

mechanisms

for

recording

consent

and

enforcing

minimal

disclosure

and

limited

retention

should

be

provided

by

technologies

developed

for

privacy

protection.

Many

privacy-aware

technologies

propose

new

purpose-based

access

control

models

for

enforcing

compliance

with

privacy

policies

(see

Ref.

Among

them,

Agrawal

et

al.

propose

Hippocratic

databases

that

use

purpose

as

a

central

concept

around

which

privacy

protection

is

built.

Their

aim

is

to

negotiate

the

information

exchanged

by

consumers

and

enterprises

and

enforce

enterprise

privacy

policies.

Policies

would

specify

the

purpose

for

which

information

is

collected,

who

can

receive

it,

the

length

of

time

it

can

be

retained

and

the

authorized

users

who

can

access

it.

Looking

at

such

policies

customers

would

have

the

choice

to

accept

or

deny

them.

However,

the

proposed

framework

defines

privacy

rules

at

a

logical,

rather

than

conceptual

schema.

As

such,

it

is

not

possible

to

check

the

set

of

privacy

rules

supported

by

an

organization

for

consistency

and

completeness..

The

objective

of

this

paper

is

to

propose

a

re-engineering

approach

and

algorithms

for

automatically

extracting

privacy

requirements

from

policy

statements

stored

in

existing

Hippocratic

databases.

These

are

then

represented

in

a

Requirements

Engineering

framework

where

tools

are

available

for

formal

analysis.

Specifically,

we

aim

to

re-model

privacy

concerns

captured

in

Hippocratic

databases

in

Secure

Tropos

and

check

for

their

consistency.

This

approach

has

two

advantages.

Firstly,

it

provides

a

representation

of

the

enterprise

privacy

policy

in

a

modeling

framework

where

formal

tools

are

available

for

model

checking

(see

Ref.

Secondly,

it

offers

a

unifying

view

of

systems

built

using

a

structured

Requirements

Engineering

methodology

such

as

Tropos/i*

or

KAOS

and

systems

directly

implemented

as

Hippocratic

databases.

Thus,

different

design

decisions

can

be

compared

at

a

level

suitable

for

the

designer..

Our

approach

is

illustrated

through

a

case

study

that

is

a

somewhat

more

complex

than

the

one

presented

in

Ref.

This

allows

us

to

compare

these

representations

and

identify

possible

limits

in

the

Hippocratic

DB

approach.

We

are

currently

applying

our

methodology

to

a

large

case

studies

for

capturing

technical

and

administrative

security

measures

required

by

the

Italian

privacy

legislation

for

Public

Administrations

(see

Ref.

porting

the

entire

methodology

and

automatic

verification

of

the

correctness

and

consistency

of

functional,

security

and

privacy

requirements.

The

remainder

of

the

paper

is

structured

as

follows.

Section

introduces

a

scenario

used

throughout

the

paper

as

running

example.

Section

introduces

the

Secure

Tropos

methodology

and

describes

the

basic

concepts

and

diagrams

used

to

model

privacy

requirements.

Section

digests

the

Hippocratic

DB

approach,

and

Section

identifies

and

discusses

some

of

its

limitations.

Section

proposes

algorithms

to

map

Hippocratic

databases

into

the

Secure

Tropos

framework

and

applies

those

algorithms

to

our

scenario.

Finally,

Section

discusses

related

work,

and

Section

concludes

the

paper

and

outlines

future

work.

Enterprise

privacy

policies

define

the

rules

that

control

access

to

customer

personal

information.

The

main

contribution

of

this

paper

is

a

procedure

for

extracting

privacy

requirements

from

existing

Hippocratic

DB

architectures

and

mapping

them

into

Secure

Tropos

models

where

the

correctness

and

consistency

of

such

requirements

can

be

checked.

To

this

end,

we

have

extended

Secure

Tropos

in

order

to

support

the

notion

of

purpose

since

it

is

fundamental

in

privacy-aware

technolo-gies.

Another

advantage

of

this

approach

is

that

it

offers

a

unifying

view

of

systems

directly

implemented

as

Hippocratic

databases

using

a

structured

requirements

engineering

methodology.

This

allows

system

designers

to

compare

different

design

decisions

at

a

level

that

is

suitable

for

them..

There

are

a

number

of

issues

left

for

future

work:.

Find

the

minimum

set

of

customer

data

needed

to

fulfill

a

purpose..

Specify

privacy

requirements

in

Secure

Tropos

during

requirements

analysis,

and

then

generate

from

these

a

Hippocratic

DB

system

that

implements

these

requirements..

Derive

enterprise-wide

privacy

policies

by

looking

at

several

Hippocratic

DB

systems

within

an

enterprise

and

merging

them

into

a

single

Secure

Tropos

model..

Introduce

an

actor

hierarchy

to

model

the

hierarchical

nature

of

organizational

actors

(e.g.,

company-division-department-group-individual

worker)..

Develop

trust

conflict

resolution

techniques

for

conflicts

between

social

and

individual

levels.

For

instance,

this

is

needed

when

a

bank

relies

on

a

delivery

company

that

is

not

trusted

by

the

customer.

Privacy

was

studied

under

the

name

of

security

(e.g.,

for

statistical

databases

in

the

and

while

now

there

is

a

clear

distinction

between

security

and

From

Hippocratic

Databases

to

Secure

Tropos

privacy

research.

Indeed,

Alan

Westin

has

defined

privacy

as

"the

right

of

individuals

to

determine

for

themselves

when,

how,

and

to

what

extent

information

about

them

is

communicated

to

others"..

The

last

decades

have

seen

an

increasing

awareness

that

privacy

plays

a

key

role

in

organizations.

The

World

Wide

Web

Consortium

developed

the

standard

to

aid

users

to

protect

their

personally

identifiable

information

when

they

visit

web

sites,

and

web

sites

to

formalize

their

privacy

policies

in

a

standard

format

that

can

be

easily

understood

by

users.

Karjoth

et

al.

propose

Platform

for

Enterprise

Privacy

Practices.

that

establishes

how

an

enterprise

should

manage

and

exchange

customer

personal

information.

Also

Enterprise

Privacy

Authorization

Language

(EPAL),

developed

by

IBM,

allows

an

enterprise

to

enforce

its

privacy

policy.

This

language

supports

enterprises

for

formalizing

their

privacy

promises

into

policies

and

associating

a

policy

to

each

information

stored

into

the

database.

Byun

et

al.

propose

a

purpose-based

access

control

extending

RBAC.

In

particular,

they

introduce

the

notion

of

purpose

together

with

purpose

hierarchies

and

a

purpose

management

model

for

reasoning

on

access

control

(see

Ref.

However,

their

hierarchies

are

based

on

the

principles

of

generalization

and

specialization,

and

so

they

are

not

expressive

enough

to

support

the

complex

strategies

that

can

be

defined

by

enterprises..

What

is

still

missing

in

these

privacy-aware

technologies

is

a

procedure

for

checking

the

consistency

of

privacy

requirements.

This

limitation

has

led

to

a

number

of

research

proposals

that

try

to

incorporate

privacy

in

the

software

engineering

process.

Liu

et

al.

propose

an

agent-oriented

modeling

framework

for

dealing

with

security

and

privacy

requirements

(see

Ref.

In

Ref.

it

is

presented

a

Requirement

Engineering

approach

extending

the

KAOS

framework,

for

modeling

security

and

privacy

goals

and

anti-goals,

and

for

deriving

attack

trees

automatically

through

antigoal

refinement.

These

methodologies,

however,

are

different

from

ours

since

they

do

not

use

purpose

as

foundation

to

model

privacy

requirements..

In

Ref.

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On the scale and performance of cooperative Web proxy caching Abstract Cooperative caching -the sharing and coordination of cache state among multiple communicating caches -has been shown to improve the performance of file and virtualmemory systems in a high-speed, local-area network environment [3, 17] . For example, when a file-page miss occurs, the local file cache may transfer the page from the file cache on another node. Cooperative caching works in this environment because network transfer time is much smaller than the disk access time required to service a miss.. Internet proxy caching has become a commonplace approach for improving the performance of Web browsers. Typically, the proxy sits in front of an entire company or organization. By caching requests for a group of users, a proxy can quickly return documents previously accessed by Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage, and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. SOSP-17 12/1999 Kiawah Island, SC (~)1999 ACM 1-58113-140-2/99/0012... $5.00 other clients. Ultimately, though, the hit rate of the proxy is a function of the size of the population it manages -a size often dictated by political, organizational, or geographic considerations. An obvious question, then, is whether multiple proxies should cooperate with each other in order to increase total client population, improve hit ratios, and reduce document-access latency. Whether such cooperative proxy caching is a useful architecture for improving performance depends on a number of factors. These include the sharing patterns of documents across organizations, the ratio of inter-proxy communication time to server fetch time, and the scale at which cooperation is undertaken.. Several cooperative-caching protocols have been proposed [9, 16, 28, 30, 33] ; however, few studies have examined cooperative Web caching from a systemic viewpoint. As a result, we know neither the environments in which cooperative caching :is useful (if any) nor its potential performance benefits. Answering such questions has been difficult in the past, because studying proxy cooperation requires simultaneous traces from multiple proxies.. In this paper, we take a two-pronged approach to exploring the limits and potentials of cooperative proxy caching. As the first approach, we collect and analyze traces from two environments: the University of Washington and the Microsoft Corporation. As a key component of our university trace, we identify each client in terms of its membership in one of about 200 university departments or programs. This gives us the equivalent of a simultaneous trace of 200 diverse, independent organizations, permitting us to analyze document sharing among those organizations and to measure the potential benefits of cooperation among organizationbased proxies. We examine latency and bandwidth benefits of proxy caching for this data, as well. We then use the Microsoft trace of employee traffic to the Internet to explore the potential of cooperation between larger organizations. To do this, we analyze traces from Microsoft and the university that we collected over the same time period and processed with the same anonymization function. This permits a direct computation of the degree of document sharing, and hence the benefit of sharing, between two proxies each handling tens of thousands of clients.. As the second approach, we develop an analytic model of Web behavior that extends beyond the limits of our trace results. The model permits us to examine the impact of larger population sizes, to explore the tradeoffs among various cooperative-caching schemes, and to speculate on the performance implications of future trends.. Our results show the benefits of cooperative caching among collections of small organizations. However, we show that cooperative caching is unlikely to have significant benefits for larger organizations or populations. That is, with current sharing patterns, there is little point in designing highly scalable cooperative-caching schemes; all reasonable schemes will have similar performance in the low-end population range where cooperative caching works. Thus, the crucial problem that must be solved to improve Web performance is how to increase document cacheability.. The paper is organized as follows. The next section examines previous work and its relationship to our study. Section 3 describes our traces and presents and analyzes data from the traces. Section 4 develops an analytic model of steady-state Web proxy caching, and uses the model to study the performance of large-scale proxy caches. Section 5 then uses the model to compare cooperative caching schemes, and Section 6 summarizes and concludes. In this section, we analyzed traces from two environments to explore the bounds of cooperative-caching performance. From this data, we saw that:. 1. The behavior of cooperative caching is characterized by two different regions of the hit rate vs. population curve. For smaller populations, hit rate increases rapidly with population; it is in this region that cooperative caching can be used effectively. However, these population sizes can be handled by a single proxy. Therefore, cooperative caching is only necessary to adapt to proxy assignments made for political or geographical reasons.. 2. For larger populations (beyond the knee of the population vs. hit rate curve), cooperative caching is unlikely to provide significant benefit. We demonstrate this using our simultaneous traces of the Microsoft and university populations: a four-fold increase to a large university population via cooperative caching netted only a 2.7% increase in cacheable hit rate.. 3. Although there exist organizations with significant locality, such as the Microsoft population, our clustering study shows that cooperative caching specialized to interest groups is unlikely to be effective.. 4. While some of these general trends have been observed before, the inevitable conclusion for cooperative caching has not been explicitly noted.. Despite these negative conclusions, it is still possible that cooperative caching will perform better, either in steadystate or for different (possibly future) Web characteristics. In the domain where cooperative caching is effective, we also wish to understand which of the proposed cooperativecaching algorithms will be most beneficial. We explore these questions in the next two sections.. In this section, we developed an analytic model and used it to examine the steady-state performance of cooperativecaching schemes. Our model extends the results of our trace to a wider range of parameter values, including document popularity and document rate of change. We show again that relatively small populations achieve most of the performance benefits of cooperative caching. This paper studied cooperative proxy caching in local-and wide-area environments. We used a combination of tracebased analysis and analytic modelling to evaluate cooperative caching, and proxy caching in general, at a wide range of population sizes, document characteristics, and access patterns. At a high level, our results show that:. 1. In the absence of significant changes in client behavior, there is little point in continuing to expend effort on the design and evaluation of highly scalable, cooperative-caching schemes. The scale at which cooperative caching makes sense (viz., up to the level of a medium-sized city) is sufficiently small that reasonable schemes will achieve most of the benefit.. 2. The largest benefit for cooperative caching is achieved for relatively small populations. This is demonstrated by our analysis of cooperation among small organizations within the university environment. Our simultaneous traces of UW and Microsoft confirmed the marginal benefit of cooperative caching among organizations with populations of 20K clients or more. Scaling beyond such populations provides only minor improvement and therefore makes sense only in very high-bandwidth, low-latency environments.. 3. Performance at the population level at which cooperative caching works effectively is basically limited by document cacheability. Therefore, increasing cacheability of documents is the main challenge for research aimed at improving Web cache behavior.. 4. Cluster-based analysis of client access patterns indicates that cooperative-caching organizations based on mutual interest offer no obvious advantages over randomly assigned or organization-based groupings.. Fundamentally, the usefulness of cooperative Web proxy caching depends upon the scale at which it is being applied. From our trace data of users at the University of Washington and Microsoft Corporation, cooperative Web proxy caching is an effective architecture for small individual caches that together comprise user populations in the tens of thousands. At such small scales, any reasonable cooperative caching scheme will serve. But cooperative caching is not required for user populations of this size. If it is administratively and politically feasible, a single proxy cache can provide the same benefits with fewer resources and less overhead.. Whether or not they use cooperative caching locally, large organizations should use proxy caching for their user populations. A key issue is whether these large organizational caches benefit from cooperating. Experiments with our steady-state model indicate that cooperation among the organizational caches within a medium to large city will still provide benefit, although an incremental benefit, over cooperative caching at small scales. Assuming that bandwidth within a city is plentiful and latencies are small, the overhead of cooperative caching would be low and therefore worth the secondary benefits that such caching provides. In principle, the organizational caches within a city can use a hash-based scheme to maximize storage efficiency. In practice, however, given the cheap cost of disks, using a hash-based scheme to spread load is more important than storage efficiency. Extrapolating to yet larger scales, such as the state level and even the west coast of the U.S., our model results indicate that cooperative caching among cities would provide very limited additional benefit, particularly given the increased latencies among caches.. Finally, we note that our results on cooperative caching are based upon Web workload behavior currently observed. Fundamental shifts in Web workloads might change these resuits. For example, the workloads we have examined consist primarily of static documents. But we have also observed a growing presence in Web workloads of streaming multimedia traffic [37] , and streaming multimedia objects have different characteristics than static objects. Their average size is orders of magnitude larger, so cooperative caching for storage efficiency becomes more appealing. Furthermore, last-byte latency is not a critical performance metric for streaming data. Instead, reducing jitter and making more effective use of the network become more important. Lastly, given the sizes of streaming objects, and the relatively long period of time over which they are transferred over the network, transport optimizations like multicast might prove more effective. Web tracing and caching are highly active research areas. Recent studies of Web traffic include analyses of Web access traces from the perspective of browsers [11, 26] , proxies [2, 5, 6, 7, 10, 13, 14, 18, 20, 23, 30] , and servers [1, 4, 29] . Earlier tracing studies were limited in request rate, number of requests, and diversity of population. The most recent tracing studies have been larger and more diverse. In addition to static analysis, some studies have also used trace-driven cache simulation to characterize the locality and sharing properties of very large traces [2, 6, 14, 18, 20, 23] and to study the effects of cookies, aborted connections, and persistent connections on the performance of proxy caching [6, 18] .. Researchers have studied the temporal locality of Web proxy traces and examined how hit-ratio depends, asymptotically, on cache size and the number of requests. Several interesting findings have been identified. First, it has been well-documented that, for most traces, the relative frequency with which Web pages are requested follows a Zipflike distribution, where the number of requests to the i th most popular document is proportional to 1/i '~ for some constant a [2, 5, 8, 11, 19, 24] . Second, for infinite-sized caches, it has been shown empirically and analytically that the hit ratio for a Web proxy grows logarithmically with the client population of the proxy and the number of requests seen by the proxy [5, 8, 14, 20, 24] .. There has also been extensive work on cooperative Web caching as a technique to reduce access latency and bandwidth consumption. Cooperative Web caching proposals include hierarchical schemes like Harvest and Squid [9, 32] , hash-based schemes [21, 35] , directory-based schemes [16, 27, 33] and multicast-based schemes [28, 34] . Although each of these research efforts included a performance evaluation of the protocols proposed and a discussion of algorithm scalability, only [22] presents empirical evaluations of cooperation for small populations, and none present empirical or analytical evaluations of the effectiveness of their schemes for the large client populations found in a wide-area setting.. Using client traces, Krishnan et al. studied the utility of cooperation among three Bell Labs proxies with a small user population [22] . They concluded that cooperative Web caching can be useful, but that a cache manager was necessary to dynamically determine when to cooperate, because of the extra server load imposed by cooperation. This paper expands on these previous research efforts. We use trace-based analysis to quantify the potential advantages and drawbacks of inter-proxy cooperation for smalland medium-sized organizations. We use analytic modelling to examine cooperative-caching performance in wider-area environments. Our model is inspired by the model of Breslau et al. [5] . We have augmented their assumptions with a model-of document rate of change, client request rate, and a small number of other parameters that allow us to study the steady-state behavior of cooperative-caching schemes as a function of the characteristics of the population they serve.

O O B O B O O B B O O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O B O O O B O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O B B O O B O O O O O B O O O O O O O O O O O O O B O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O B O O O O O O O O B O O O O O B O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B B O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O B B O O O O O O O B O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O O B O O B B O O O O B O O O B O B B B O O O O O B O O O O O O O O O O O O O O O O B O O B O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O B O O O O B O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B B O O O O B O O O O O O O O O O O O O O O O O O O O O B B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O O B B O O O O O O O O O O O O O B O O O O O O O B O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O B O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B B O O O B O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O O O O O B B B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B B O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O B O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B B O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O B O O O O O O O B O B O O O O O O O O O B O O O O O B O O O O O B O O B O B O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O


Scientific collaboration dynamics in a national scientific system Abstract This paper examines the collaboration structures and dynamics of the coauthorship network of all Slovenian researchers. Its goal is to identify the key factors driving collaboration and the main differences in collaboration behavior across scientific fields and disciplines. Two approaches to modelling network dynamics are combined in this paper: the small-world model and the mechanism of preferential attachment, also known as the process of cumulative advantage. Stochastic-actor-based modelling of coauthorship network dynamics uses data for the complete longitudinal co-authorship networks for the entire Slovenian scientific community from 1996 to 2010. We confirmed the presence of clustering in all fields and disciplines. Preferential attachment is far more complex than a single global mechanism. There were two clear distinctions regarding collaboration within scientific fields and disciplines. One was that some fields had an internal national saturation inhibiting further collaboration. The second concerned the differential impact of collaboration with scientists from abroad on domestic collaboration. Scientific collaboration in modern science appears to be one of the key factors for increasing publication productivity and quality. Ziman (1994, p. 218) wrote: ''… the traditional parochial individualism of science is rapidly being transformed in what might be described as transnational collectivism''. In recent decades, structural changes appearing in science have encouraged scientific collaboration. Specialization at the individual level and the development of very sophisticated and expensive research equipment support collaboration. The development of information and communication technology and increased possibilities for the mobility of researchers also have positive effects on scientific collaboration. Another very important driver of such collaboration is research financing policy. For example, since the EU Framework Programmes (FP) were established in 1984, intra-European collaboration has increased significantly. In addition, the FP Horizon 2020 initiative for research and innovation strongly encourages scientific collaboration.. Scientific collaboration has been studied systematically since the 1960s. Different qualitative and quantitative approaches have been used to study scientific collaboration including interviews, observations, surveys, bibliometric analyses, social network analysis and simulations (e.g., Shrum and Mullins 1988; Shrum et al. 2007 ). The most commonly used approach is the bibliometric analysis of scientific co-authorship networks as the data can be easily and accurately extracted from publication databases (Pike 2010) , albeit after considerable cleaning.. Co-authorship networks and citation networks are very useful instruments for studying collaboration in science. Both have positive impacts on scientific productivity. In earlier bibliometric approaches, several studies focused on co-authorship in the social or natural sciences, but very few included comparisons between different scientific disciplines (De Stefano et al. 2011) . Scientific disciplines still represent a crucial institutional and organizational framework within which scientific activities take place. They are seen as distinct intellectual and social organizational contexts having their own norms and values forming disciplinary cultures which have evolved over time to create an ever more complicated scientific domain ). There are several international, national, and informal classifications of scientific disciplines. Since we analyzed the co-authorship networks of the Slovenian scientific system, we started with the classification into scientific fields and scientific disciplines used by the Slovenian Research Agency (henceforth: ARRS), the main policy authority in Slovenian science. It is presented in Table 1 together with the number of scientific disciplines assigned to each scientific field. The seventh scientific field (Interdisciplinary studies) never gained full recognition as a separate field in the research and development (R&D) policy context in Slovenia because R&D policy remained conservative concerning interdisciplinary-oriented research despite it having increased dramatically around the world. As a result, we ignored it in our analyses of coauthorship network dynamics.. According to analyses of the collaboration styles of researchers belonging to different scientific disciplines and fields, co-authorship represents an important differentiating indicator between them. The differences in the percentages of co-authored publications among the seven scientific fields in Slovenia are shown in Fig. 1 . There is a large gap between the average levels of co-authorship in the natural, technical, medical and biotechnical sciences, and the average co-authorship levels in the humanities and the social sciences. While six trajectories show a steady rise during 1996-2010, the trajectory for Interdisciplinary studies had considerable instability. The humanities and the social sciences had the smallest shares of co-authored publications throughout this period. However, the trend for the social sciences shows that it is slowly approaching the natural, technical, medical, and biotechnical sciences.. Slovenian researchers have studied scientific collaboration using a qualitative approach, survey analysis and bibliometric analysis of co-authorship networks using longitudinal data on the Slovenian science system in order to explore and explain their dynamics across four scientific disciplines (Ferligoj and Kronegger 2009; Mali et al. 2010 ; Kronegger et al. 2011, 2012; Groboljšek et al. 2014; Iglič et al. 2015) . The rationale for their selection of four disciplines was: (1) mathematics is an old discipline where research takes place primarily in offices; (2) physics is an old discipline where the research occurs mostly in research groups within laboratories; (3) sociology is an old discipline where research also occurs mostly in offices; and (4) biotechnology is a new laboratory discipline. The bibliometric analysis of co-authorship networks for the period from 1986 to 2005 revealed: (1) a high proportion of single-author publications within sociology and within mathematics with lower but slightly increasing levels of co-authored publications and (2) high and relatively steady levels of collaboration by physicists and biotechnologists within their disciplines and/or with authors from abroad. The high degree of scientific collaboration in laboratory sciences is coupled to a more formal division of labor. Biotechnologists in Slovenia were characterized by collaboration with researchers from other disciplines within the country, although with much greater fluctuations than in the other disciplines (Kronegger et al. 2012) . A web survey of researchers from the four scientific disciplines was designed to understand which kinds of incentives, perceptions, and personal strategies help account for collaboration from the perspectives of individual scientists (Iglič et al. 2015) . The results of the analysis showed that the differences between disciplines in the proportion of researchers active work time spent collaborating with others were much smaller when assessed through interviews compared to results from co-authorship data. Researchers in the social sciences do not necessarily collaborate less as the attributions of authorship are different in the social and natural sciences. Disciplines vary according to the nature of collaboration partners. Physicists and mathematicians from the basic sciences have much wider and far-reaching collaboration networks than sociologists or biotechnologists from applied sciences. The extensive collaboration networks of biotechnologists seem to be more limited and focused on local partners.. Disciplines differ also in their overall styles of collaboration, ranging from very informal to formal. For the former, this is due to an established division of labor within an organizational setup with clearly defined roles. Such formal arrangements are far less prevalent in the latter disciplines. Informal collaboration occurs outside an explicit organizational division of labor. The extreme form is when researchers collaborate on their own volition (see Shrum et al. 2007 ). Iglič et al. (2015) also identified the main factors driving collaboration at two levels: research-policy-related external factors and internal factors affecting the motivations of scientists regarding compatibility, cultural proximity, academic excellence, position and status. Scientists are more likely to collaborate when there are more opportunities to obtain research money, when they easily overcome status differences between senior and junior colleagues, and have a high level of agreement about what constitutes good quality research. They also collaborate more when they have a positive experience with earlier collaborations when they experience benefits from collaboration including good research results and faster individual promotion. Also, they pay attention to professional complementarity when choosing research partners.. For an even more detailed analysis of collaboration practices, a qualitative investigation among key representatives of the four scientific disciplines and research policymakers was conducted to gain an insight into their views on the importance of scientific collaboration (Groboljšek et al. 2014) . While the policy mechanisms aiming to encourage scientific collaboration are important, the interviewed scientists and policymakers believed that longterm and successful collaborations derive from researcher's efforts and their individual engagements-but only where suitable conditions have been created by policy mechanisms which encourage international mobility, along with interdisciplinary and interinstitutional/ intersectoral collaboration. Kronegger et al. (2012) combined two approaches for modelling co-authorship network dynamics. They used the small-world model (Watts and Strogatz 1998) and the mechanism of preferential attachment, also known as the process of cumulative advantage (Price 1963 (Price , 1965 Garfield and Merton 1979) . One dimension of the small world was measured by its clustering level, and preferential attachment was operationalized through the collaboration of researchers within and across disciplines. They used stochastic-actor-based modelling (SAOM) of co-authorship network dynamics implemented in the SIENA program (Snijders 2005; Snijders et al. 2010) . While the presence of clustering was confirmed in all four scientific disciplines, preferential attachment was more complex than one single global mechanism. The principle of preferential attachment was only partly confirmed in the coauthorship network of physicists: researchers had fewer possibilities to establish new connections with other physicists within the Slovenian scientific community. This can be the consequence of the saturation of the network where scientific work is organized within the formalized environments around expensive and complicated technical research equipment (e.g., in laboratories). The co-authorship network of mathematicians indicated different characteristics: those scientists who collaborated with scientists outside the Slovenian community and had a higher number of articles published in journals with an impact factor had greater opportunities to establish new connections within the Slovenian mathematical community. In the case of the co-authorship networks of sociologists, collaboration with foreign scientists brought negative effects for collaboration within the national research community. At the same time, the number of articles with an impact factor had a positive effect on the formation of new co-authorship relationships within the discipline. The dynamics of biotechnologists co-authorship networks did not follow the principle of preferential attachment.. Here, based on the modelling of co-authorship network dynamics in four scientific disciplines (Kronegger et al. 2012) we specify the SAOM in a more appropriate fashion and use higher quality bibliometric data for the complete longitudinal co-authorship networks for all scientific fields and most scientific disciplines for 1996-2010 in the Slovenian system of science. The first hypothesis about the presence of clustering as a dimension of a small-world structure was confirmed emphatically. The evidence regarding the second hypothesis concerning preferential attachment as the driving mechanism of co-authorship was decidedly mixed. Yet, in the main, our results contradict the hypothesis of a single preferential attachment mechanism for the formation of collaborative ties. The current number of co-authorships inside the field or discipline has a negative effect on new tie formation in all scientific fields and nearly all scientific disciplines. Our results show the distance between researchers who collaborate matters also. Regarding this, the social sciences and humanities differ from the other four fields. Alters higher degree of collaboration outside of Slovenia has a negative effect on new tie formation in the social sciences and humanities but a positive effect in the other fields. A high degree of collaboration outside the national disciplinary field, a revised notion of preferential attachment, has a positive impact on tie formation for the natural, technical, medical and biotechnical sciences. Clearly, preferential attachment mechanisms are more subtle than can be summarized by stating that researchers who currently have a large number of co-authors will see an increase in their number of collaborators, at least for the science dynamics in smaller national scientific systems.. Another difference between the social sciences and the humanities compared with the other four fields is that publication excellence had a positive effect on the formation of collaborative ties in the social sciences and humanities but a negative effect in the other fields. However, the standard hypothesis that young researchers form new co-authorship ties with scientifically excellent older scientists was not confirmed. Researchers from all fields but the humanities were more likely to form new ties with younger colleagues. The third hypothesis was confirmed and the evidence demonstrates that the scientific fields and disciplines are affected by the organization of local institutions and publishing cultures. Overall, most of the results of Kronegger et al. (2012) were confirmed with the differences attributable to using better data containing more scientists, considering many disciplines and using a more complex and realistic SAOM specification. We note that our findings regarding the effects of small-world phenomena, preferential attachment, institutional arrangements hold for virtually all disciplines as well as for broad fields.. The differences between the two basic pools of scientific knowledge (i.e., between the natural and technical sciences, and the social sciences and humanities) according to the mechanism of preferential attachment are also the result of contextual (research policy) factors operating in Slovenia. In the former socialist era, due to ideological pressure on the social sciences and the humanities, these disciplines were less internationalized and much less oriented to publishing in highranking international journals. After the dramatic changes starting at the beginning of the 1990s, Slovenian R&D policy gradually began to introduce the criteria of (international) excellence into R&D evaluation procedures. In the first years after Slovenia's independence, governmental institutions created many initiatives, including legislation, to establish the new country including those relating to science. It seems that in adapting to the more demanding R&D evaluation criteria there were different impacts on the social sciences and humanities compared to the other fields. In part, this helps account for some of the differences between the two groups of fields. Yet, those social scientists and humanists who already engaged in excellent publication activity can be very attracted to research collaboration with their disciplinary colleagues. Generally speaking, the results of our bibliometric analysis showed that we cannot solely explain the differences in forming co-authorship ties in different parts of sciences with internal (epistemological) factors, but that also external (R&D policy) factors are involved. No single approach, be it sociological, social network analytic or based on conceptions drawn from physics, can be useful by itself. Multiple approaches are needed to account for the complex phenomenon of scientific collaboration, especially for national scientific systems. While science can be viewed as a general phenomenon it is also conditioned by local institutional contexts. Table 6 Estimated parameters for scientific disciplines according to the importance coefficients The preferential mechanism described by Barabási and Albert (1999) focuses on the formation of co-authorship ties as new nodes enter the network. It appears that, in their analyses, such ties remain in the network, which may be problematic in the case of coauthorship networks.Yet, scientists collaborating at one point in time can maintain or dissolve their co-authorship tie at a later time. Consequently, we considered the possibility that ties can be created, maintained or deleted since this is a feature that characterizes coauthorship networks. This was a major reason for our use of Stochastic Actor-Oriented Model (SAOM; /citealtsnijders2001, snijders2008), a micro-level model assuming that ties can be created and deleted in a network with an actor set that may be constant or changing. The probabilities of tie creation and deletion depend on so-called effects (explanatory variables that may depend on the network or be exogenously given), with their associated coefficients as parameters. The set of effects chosen as the model specification followed from the hypotheses as elaborated in the preceding section. 5 The clustering level was operationalized by the parameter for transitive triads to capture the tendency of actors to form ties by closing the triangles, and by the parameters 'working in the same research organizational group' and 'working in the same scientific discipline'. The last two are a better operationalization of the institutional feature of the clustering level than the one used in the study by Kronegger et al. (2012) . Preferential attachment was operationalized by five effects: degree of alter inside the co-authorship network, degree of alter with respect to collaboration with researchers outside the co-authorship network, scientific excellence, scientific age, and age similarity. Gender (male = 1) and having a PhD (having PhD = 1) were also included in the model as controls. The network was defined in three consecutive observations corresponding to the periods mentioned in Sect. 4, and a tie was defined if two researchers appeared together as authors in at least one publication. Table 6 ) . The first three parameters in the tables are standard as they a technical requirements of the stochastic-actor-based model: the rate parameter for the first transition; the rate parameter for the second transition, and the density parameter. The two rate parameters estimate the average frequency of the number of proposals for collaboration, which then may be accepted or rejected by the proposed partner within the co-authorship networks. In all scientific fields, the average number of proposals for collaboration for the second transition is higher than for the first transition. This is not the case in all scientific disciplines considered (see Table 6 ): 12 out of 49 considered disciplines had declines in their averages in the second transition compared to the first one. Using SAOM includes also an estimate of the cost of adding one more tie to the personal network of each researcher is obtained, an important characteristic seldom considered by scholars studying preferential attachment. This value is given by the third basic parameter (degree). 6 Estimates of the parameters for the degree effect are negative for all six scientific fields and all scientific disciplines. This makes sense because tie formation incurs costs in terms of time, effort, and resources. Researchers can co-author with only a limited number of different authors as each new tie represents an additional time and cost burden.

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