Jérôme Blanc
email: jerome.blanc@ish-lyon.cnrs.fr

Silvio Gesell's Theory

AND ACCELERATED MONEY EXPERIMENTS

Silvio Gesell (1862-1930) proposed a system of stamped money in order to accelerate monetary circulation and to free money from interest. This was part of a global socialist system intended to free economy from rent and interest. In the 1930s, Irving Fisher, who proposed the system to President Roosevelt, and John Maynard Keynes rendered homage to Gesell's monetary proposals in the context of the economic depression. Among the experiments that took place, several were based on his ideas, notably in the Austrian town of Wörgl and in the United States. These experiments were always local and never lasted more than a few months. This article shows that trust is the main issue of this kind of monetary organization; and therefore, that such experiments can only take place successfully on a small scale.

FREE MONEY FOR SOCIAL PROGRESS : THEORY AND PRACTICE OF GESELL'S ACCELERATED MONEY

I. INTRODUCTION

Money is often the central point of theories that are designed to improve social organization. In the 1930s, the Great Depression led to a search for alternative solutions to restart the economy, or at least to slow its decline. Thousands of persons proposed plans for just this purpose. Many of these plans were based on a reform of the monetary organization, and most of them were never actually tried.

This was not the case with a pre-existent theory -Silvio Gesell's theory of free money -on which John Maynard Keynes showered fulsome praise and which Irving Fisher tried to apply in the United States. This theory was tried. Admittedly, there are key differences between Gesell's theory and the several attempts made to employ it. These differences include the fact that none of the attempts occurred at a national or worldwide level, despite the preferences of Gesell and some founders of the experiments. Moreover, the theoretical purpose of free money became simply the acceleration of monetary circulation, that is, accelerated money, thus abandoning the other dimensions of Gesell's socialist theory. However, both these experiments and their underlying ideas stress the necessity of the quick circulation of income. It is possible to accelerate the flowering of society by encouraging the circulation of currency, by setting up a monetary organization that promotes what might be called consumption money. Money is viewed as a necessary medium of exchange, but its organization is transformed in order to suppress the disastrous effects of speculation, hoarding, and usury, which are considered to be the sources of economic crises.

The purpose of this paper is not to reappraise Gesell's theory but rather to compare his theories to some issues raised by the twentieth-century experiments. I shall also examine Gesellian socialism and the theoretical context of Gesell's monetary theory as well as the debates and social experiments that grew from these ideas mainly in the period between the world wars. Finally, I discuss the practical issues that face every attempt to employ accelerated money, especially the central issue of trust, and the importance that monetary organizations remain local and confined to a single small region in order to work.

II. GESELLIAN SOCIALISM AND MONEY

Silvio Gesell (1862-1930) In Europe, The Natural Economic Order aroused wide interest during the period between the wars. Gesell acquired many followers... and even more detractors. His admirers considered him a « prophet », but among his detractors, orthodox economists in particular, he was dismissed as a « crank » 2 .

What provoked such a rejection were both Gesell's non-academic and deeply heterodox ideas, and his hysterical rhetoric, which was similar to that of many socialist writers. In fact, Gesell considered himself a socialist -not a Marxian, but a Proudhonian one.

Some economists considered Gesell as a stage in heterodox economics between the monetary theory of Knut Wicksell and that of Keynes 3 . This was in part due to the fact that he analyzed a monetary economy and stated that economic crises have a monetary origin. The crises Gesell was interested in are deflationist ones, like the Argentinean crisis of the 1880s, which gave him the substance of his theory, and the worldwide one of the 1930s (during which Keynes wrote his General Theory).

Gesell's theory was intended to change the economic organization of society and promote progress towards social justice and economic welfare, by way of freeing the economy and thus establishing what he called a natural economic order. This move towards a free economy (Freiwirtschaft) requires the freeing of land from rent and the freeing of money from interest. In an economic organization cleared of the economic privileges of landowners and moneylenders, workers would receive the whole value of their output.

Competitors would then have a level playing field, and economy would subsequently flourish.

1 See Currency Reform as a Bridge to the Social State, which Philip Pye translated in english in 1951.

2 Keynes [1936, p. 353] wrote : « Like other academic economists, I treated his profoundly original strivings as being no better than those of a crank ». See also Herland [1992]. In the many histories of economic analysis that exist today, it is very rare to find more than some indignant sentences on Gesell -and it is likely that no one would remember Gesell's name and theories if Keynes did not write about them (Keynes [1936, pp. 353-358]).

3 See Herland [1992]. Dillard [1940] considered Gesell as a step between Proudhon and Keynes. Herland stresses that this was just a step that Keynes easily topped. Only Ludwig von Mises seems to place Gesell above Keynes, but this might be linked to his rejection of Keynes, more than his acceptance of Gesell's theory.

Let us now turn to Gesell's monetary proposal of a free money. In his theoretical view of the economy, the stockpiling of wealth is identified as the danger to be avoided, because it stops the revenue flow dynamics. Gesell, as Keynes did some years after, denounced the hoarding of money, even as savings deposited in financial institutions, since high interest rates make loans difficult. Two key points appear here: hoarding and the cost of credit. Both blockade a part of the money supply from functioning properly.

Gesell identified the primary function of money in its circulation: economic dynamics are based on money as a perpetuum mobile4 . Above all, money has to have a means of circulation. How then, Gesell pondered, could the slowering of monetary flow be prevented?

An arrangement favoring monetary circulation and discouraging the stockpiling of wealth had to be set up.

It is true that many other economists developed ideas promoting the circulation function of money. Keynes, for example, advocated the lowering of interest rates. But this is not sufficient to create a monetary utopia. Gesell did inasmuch as he developed an organizational perspective of consumption money.

One can distinguish two meanings of the idea of consumption money:

1. The quick and whole spending of money. This means that income is not hoarded during the period of production that follows its distribution.

2. The disbursement of money in the very place where incomes are created. This means that income does not get used for consumption or invested elsewhere.

It is possible to mix these two meanings, but their theoretical and practical consequences are very different.

In the first meaning, one can recognize many less utopian than heterodox authors who wanted to break the cycle of hoarding and reduce the motive for saving. Keynes was among these; that is why he saw Gesell as a greater writer than Marx. Gesell advocated destroying the function of storage of wealth. This led him to advocate accelerated money5 , that is, organizing the acceleration of its circulation.

In the second meaning, the issue to solve is the localization of the formation, the distribution, and the disbursement of income. Big companies and money management induce a gap between where incomes are formed and the where they are spent. This leads to projects at the local level intended to speed up the relocalization of the economic flow. This is not necessarily a return to autarky linked to fear of outside influences. In the town of Wörgl (Austria), in 1932-33, the organization of local accelerated money on Silvio Gesell's model was simply intended to revitalize the local economy.

Hence, a consumption money can be set up by two means: by reducing or even destroying the monetary function of store of wealth and by the organization of local and even strictly closed-region economic flows. The purpose of some monetary utopias was the creation of the first sort of consumption money, but the actual application of such consumption money on a national scale appeared impossible and so it could only be applied on a local scale. Hence the attempts to put in practice accelerated money (as Gesell theorized) led to the setting up of local regional systems.

III. TOWARD THE PRINCIPLE OF AN ACCELERATED MONEY

The purpose of Gesell's monetary theory was to free money from interest rates, in order to set in motion the dynamics of credit, investment and consumption. This freeing involved an acceleration of monetary circulation, or accelerated money.

The core of Gesell's monetary theory consists of a theory of interest. Like Wicksell,

Gesell distinguishes between a monetary interest rate and a real interest rate. Gesell considers the second as linked to the first one; hence only the monetary interest rate is of importance in his analysis. In Gesell's theory, then, the monetary domain determines the real one.

The monetary interest rate consists of three parts: a risk premium linked to the loan, a premium linked to the expected rate of inflation, and, above all, a fundamental interest rate called « tribute » paid for the natural advantage of possessing money over other goods.

Gesell reasoned that the natural advantage of money is twofold: first, that money is indestructible and possessing it does not incur costs; and second, that money is the only liquid good by which everything can be obtained.

As a necessary remuneration for the advantages of money, paying a tribute establishes a lower limit to the rate of return on every business project6 . This means that someone lends capital only if its interest rate is over the cost of the tribute, and that someone purchases or invests only if he expects the selling price to be over the buying price increased by the tribute.

If the numbers do not add up, money will be hoarded and withdrawn from the economic flows7 . Hence, the capitalist is likely to break the cycle of economic activity if he abstains from any economic action.

Hoarding then becomes the cause of the crisis, and the natural advantages of money cause hoarding. How then to stimulate the circulation of money and discourage hoarding?

The lowering of the interest rate below that of the tribute (estimated to be nearly 3 to 4 percent a year, according to Gesell) would be disastrous, because it would encourage hoarding outside of banks. This is the mechanism of Keynes's awful « liquidity trap » 8 .

Gesell stated that the lowering of the interest rate at a level even above that of the tribute rate would not discourage hoarding and stimulate the circulation of money. That is why Gesell did not whish to merely lower interest rates. Rather, he wanted to free money from interest rates altogether. The lowering or even the abolition of interest is not a way to boost the economy but may be a consequence of its new organization.

The solution was to be found in the depreciation of money. That is the only way to pass beyond the mechanism of the tribute. Money, as we have seen, has two advantages over goods: no carrying costs and general liquidity. The first one is linked to the store function of money, the second to its payment function. The solution is to give money an artificial carrying cost in order to suppress the first advantage and to privilege the second one. Thus

Gesell proposed his famous stamped money system. Every month or every week notes lose a fixed percentage of their nominal value, for example a weekly rate of 0,1 percent of the nominal value of the notes, i.e. a yearly depreciation of 5,2 percent 9 . Then, in order to maintain the value of the notes, people would have to purchase stamps every week at the Post Office 10 . Stamped money means that the authorities impose on money, not prices, a stable, fixed and announced inflation.

Yet, the storage of wealth is safe in Gesell's system. While every deposit in the savings bank creates a need to buy stamps, it is always the holder of the notes who needs to buy them. In the case of deposits in savings banks, the banks have this responsibility. The saver withdraws his deposit at the same value he deposited it. Hence the savings bank is stimulated to loan its funds in order to avoid this costly need. When funds are loaned, the stamps are bought not by the depositor or the savings bank, but by the borrower who holds the money. In this system, lies not in borrowing money, but in holding money idle.

Since it is in the interest of no one to hold notes idle, this system stimulates deposits, discourages hoarding, stimulates loans, and, finally, stimulates monetary circulation in general.

8 Compare Gesell [1948, p. 191] with Keynes [1936, p. 207]. 9 The different editions of the Natural Economic Order give different periods and different depreciation rates.

The book was translated in english by Philip Pye and published in this language in 1929, 1934 and 1951. Keynes read the translation of the sixth german edition, which, with the seventh one, shows a weekly depreciation of 0,1% (5,2% a year) (Keynes [1936, p. 357]). The French edition, based on the eighth german one, shows a monthly depreciation of 0,5% (6% a year) (see Gesell [1948, p. 213 and the notes of the translator]). In any case, these depreciation rates are justified by Gesell's assessment that hoarding disappears once inflation reaches 5%. See Gesell [1948, p. 184]. 10 See Gesell [1948, p. 213].

IV. ACCELERATED MONEY DEBATES AND EXPERIMENTS

Gesell's ideas and militancy gave occasion for the birth of several groups promoting his Free Economy. Despite the academic rejection of Gesell's ideas, Keynes noted that many non-academic were infatuated with his ideas: The German hyperinflation in the twenties, the subsequent difficulties experienced in going back to the gold standard, the monetary disorders and then the great economic depression with its extraordinary unemployment in Europe and America, allowed heterodox, even utopian, ideas to flourish. Debates on Gesellian ideas occurred in the context of the lack of money in Europe from the beginning of the First World War to the middle of the twenties, and in the context of the depression of the thirties. Gesell's ideas were well discussed in Germany, France, Switzerland, Great Britain, Austria, and the United States, long before people experimented with their practical application.

The birthplace of the debates was Switzerland, where Gesell settled in 1900. Dr.

Theophile Christen, Fritz Schwartz, and Gesell created in the 1910s the Freiland-und

Freigeldbund, an organization intended to promote the Free Economy. The Popular Socialist Party supported them. The free economy projects were discussed in 1923 at the Monetary Conference convoked by the Federal Council.

An engineer and friend of Gesell, Hans Timm, founded the first experiment with stamped money. It occurred in Germany in 1930, and was based on an exchange bank and stamped notes called wära 12 . Soon the government argued that this system was hurting the issuing privilege of the Central Bank and creating a risk of inflation. The government lost a court case against the experiment, but eventually banned by decree the use of this sort of money in October 1931 13 .

Perhaps the most important experiment with accelerated money took place in Wörgl, a little town in Austria, in 1932-33 14 . It is still considered today to be the seminal experiment of its type. Founders of LETS (Local Exchange Trading Systems) all over the world frequently

11 Keynes [1936, p. 354].

12 Wära means Ware (commodity) and Währung (circulation).

13 Delannès [1938, pp. 64-65]. 14 See Baudin [1947, pp. 335-339], Delannès [1938] and the review Annales de l'économie collective, 1934.

refer to Wörgl in order to outline the historical context of LETS, despite the absence, today, of any accelerating system of exchanges derived from Gesell's ideas.

Wörgl was an impoverished town of 2,000 inhabitants; within its borders there were 1,500 unemployed. Moreover, the town was in state of bankruptcy. In July 1932, its mayor, Michael Unterguggenberger 15 , set up a system of stamped notes called « labour notes » of one to ten Austrian schillings that were issued by the municipal office. Every month, holders of labor notes had to buy stamps of 1 percent of their nominal value to maintain the notes.

The notes issued were totally backed on a schilling reserve. The municipal office paid a part of the salaries of the municipal employees. This stamped money circulated in the town and the neighbourhood, people trusting in their final reception by the municipal office, which received them in payment of taxes at par with the Austrian schilling. Moreover, it secured the convertibility between the stamped notes and the Austrian schilling, taking a 2 percent duty.

The currency movement in Wörgl sharply accelerated 16 . Some paid their taxes in advance to avoid buying the stamps. The financial situation of the town and the local economy recovered. The municipality even hired workers to perform public works projects.

The principle of savings was safe with a system of deposits, and currency depreciation ended.

However, in spite of the apparent success of the system of Wörgl 17 , and facing the desire of many towns to set up similar systems 18 , the Austrian government, goaded by the National Bank of Austria, banned the principle of local stamped money in September 1933 19 .

In the United States, some debates on and experiments with stamped money occurred in the thirties. In addition to Keynes, it seems that Irving Fisher was the only other famous economist who observed, criticized and encouraged experiments with Gesell's theory, especially the Wörgl one. Fisher even wrote a book titled Stamp Scrip in 1933. However, this aspect of Fisher's works has been blotted out of the collective memory of economists 20 .

In the context of the American deflation at the beginning of the thirties, Fisher proposed to set up a temporary but nationwide system of accelerated money, wherein depreciation would be 2 percent per week. He wanted the velocity of money to accelerate, and prices to rise 21 . 15 Unterguggenberger had been trade unionist in the Social Democrat Party. He refused to be considered as a marxist. Moreover, the social democratic government of the Austrian Tyrol refused his experiment as not being consistent with the party's projects. See Delannès [1938, pp. 66-68]. 16 According to Kennedy [1996], the 32 000 schillings issued were exchanged 463 times -generally 21 times with the Austrian schilling. 17 Beyond the idyllic views of the effects of the system, one can stress that this was a singular experiment. One may stress, along with Baudin [1947, p. 338] or von Muralt [1934], that the recovery was partly due to the flow of tourists visiting this town to see and buy the stamped money. 18 It seems that 170 Austrian towns (including Innsbrück) were planning to set up such a system, but they were waiting for a decision on the legal action against Wörgl. 19 See von Muralt [1934, pp. 321-322]. 20 For example, Schumpeter [1954] is silent on this matter. The paper of the New Palgrave on Fisher too, though it is rather exhaustive, also does not mention these proposals, studies and book of Fisher. 21 See Fisher [1933].

Fisher adopted these principles during the summer 1932. He tried unsuccessfully to convince the future President Roosevelt (who was at that time running for office) of the value of experimenting with Gesellian theories and other unorthodox projects designed to boost the economy 22 . A bill intended to allow the government to issue stamped money for a predetermined period of the deflationist crisis was brought to the US Congress, but it did not pass 23 .

Yet around 1932-33, on the basis of the Wörgl experiment, some fifteen towns set up a system stamped money [START_REF]The towns were mostly in Iowa[END_REF] . Generally, the issued notes were not backed on an equal dollar reserve, but the necessity of paying stamps of 2 percent of their nominal value every week constituted the reserve. In one year, a reserve of 104 percent of the nominal value of the issued notes was in place. There was a strange system of paying stamps at the time of every exchange, not at every fixed period of one week. This had the exactly opposite consequences than an accelerated money system, since people were then encouraged to hold their notes or to barter exchange to avoid paying any duty. Fraud was rampant. Some private organizations (chambers of commerce, groups of tradesmen) also set up such a stamping system.

There were some others experiments outside the US.S as well. In Alberta, Canada, William Aberhart, disciple of Major Douglas, founder of the theory of social credit [START_REF][END_REF] , applied a similar system, which soon failed. In Great Britain, Henry Büchi unsuccessfully tried to advance the principle of stamped money. In France, men such as Jean Barral, Pierre Mouton and Marino-Bertil Issautier advocated in favor of stamped money systems. An experiment occurred two years in Nice, France, in the thirties, but the government, on the advice of the Banque de France, soon banned it. Nevertheless, a deputy had proposed a bill to create just such a system 26 and in 1933, Edouard Daladier, President of Council, proposed such a system at a Congress of the Radical Party; however, he only provoked an outcry. In 1937, a new project was submitted to the Radical Party for the financing of large-scale public works by the issuing of stamped money not backed by preliminary reserves 27 . New debates on stamped money also occurred during the war. After the Second World War, two small

French towns set up systems of stamped money, but the founders of the systems decided to stop them before the government stepped in 28 .

V. TRUST AND ACCELERATED MONEY: THE PRACTICAL LIMITS OF THE

GESELLIAN UTOPIA

The experiments with Gesellian Theory in this century do not provide any evidence as to how an accelerated money system would work over two or three years. None lasted more than a few months: they were banned or stopped before to be banned. Moreover, they never occurred on a large scale, but always on a local regional scale. The attempts to create free money were never linked with an attempt to set up a free economy, as Gesell theorized it. Hence, the Gesellian theory of freeing money from interest and accelerating monetary circulation generates some questions concerning its viability and the proper way to set up such a system. Behind its technical problems, trust remains the main issue.

We saw that the acceleration of monetary circulation was induced by the regular depreciation of the nominal value of the means of payment. The main issue of this way of accelerating the currency is its effect on people's behavior.

Generally, the acceleration of the monetary circulation comes from a lowering in the money supply, or from a loss of trust precisely linked to the depreciation of money. One can see there the beginning of an inflationist or even hyperinflationist process. Hence such an organization of the acceleration of currency is dependent on people having confidence in their money. If this trust does not exist, there is a great risk that monetary substitutes will appear in order to avoid using money whose nominal value has to be maintained -inconvenientlyby costly stamps.

In order to maintain consumer confidence, the accelerated money system needs, from a technical point of view, to announce and plan the regular depreciation of money and to insure that there will be no effect on prices. Moreover, it has to lock the payment system so that there is no easy possibility of substituting stable money for the depreciating means of payments.

Indeed, the inherent problem with Gesell's system is the existence, in modern economies, of different means of payment. In Gesell's system, only one sort of means of payment exists (stamped notes) since deposits cannot be used as bank money. Yet neither in Gesell's times nor today is this assumption realistic. If bank money does exist, it can replace the stamped notes, not only as a store of wealth, but also as a means of payment, since bank money is not saddled with carrying costs.

That is why Keynes wrote that the system of stamped notes « would clearly need to apply as well to some forms at least of bank-money » 29 . The recent proposals derived from Gesell's ideas take this issue into account. Every agent would have two accounts: one current account assigned to bank payments and bearing the depreciation of money, and one account 29 Keynes [1936, p. 357].

assigned to savings, not allowing bank payments and preserving the stability of currency value 30 .

Such a system is likely to succeed so long as the following conditions are fulfilled:

1) The system has to have strict regulations forbidding the use of substitutes to the depreciating means of payment. The lack of control over such substitutes would lead to the destruction of the accelerated money system by the behavior of agents looking for stable means of payment. However, Keynes stated that, in contradiction to Gesell's assumption, there is a continuum of variously liquid assets able to serve as money substitutes 31 . From a theoretical point of view, this is enough to destroy any attempt to set up an accelerated money system. It seems that this difficulty prevents this sort of project from being set up on a large scale.

2) Actually, trust is a key to the system. Trust is enough to let it succeed, and the lack of trust severely endangers its success.

On a large scale, an accelerated money system is likely to last only if a complete shift in thinking occurs. This can appear with the worsening of the economic and social contextan assumption on which some present utopias are based. Such a revolution would allow a transformation of the economic system. This necessity of a revolution in thought is another way to say that trust is necessary to the system, but that trust in such a system is quite unlikely to appear in a normal context on a large scale.

We can observe that the attempts to put stamped money systems into practice occurred in very specific contexts. The example of Wörgl's stamped money shows that the survival of such a system requires a united, small, and compliant population, who are fully aware of the advantages of such a system. Hence, in Wörgl, before paying a part of the salaries of the municipal employees with the stamped notes, the employees agreed to be paid with this money 32 . This means that an accelerated money system would probably be viable on a small scale only, and, on a larger scale, for a limited time only and under conditions attractive enough to incite the whole population to play the game 33 .

30 Kennedy [1996, p. 47].

31 Keynes [1936, p. 358]. 32 Von Muralt [1934, p. 316]. 33 See Fisher [1933, p. 59].

VI. CONCLUSION

Any experiments with accelerated money seem doomed to stay local, that is, in a network limited by space and / or purposes, whose participants trust themselves and the monetary authority. The necessity of a voluntary adherence to a network serves to limit of such an experiment and to make new members alert of the consequences of internal money.

Hence, it appears that any attempt to set up an accelerated money system (the first sort of « consumption money ») will eventually lead to a local currency (the second sort of « consumption money »).

In More than 1,000 LETS exist today, counting more than 50,000 members around the world.

LETS make hoarding both impossible and undesirable. The only wealth in such systems is in the capacity of exchange (knowledge, goods, services), and debit accounts are an accountancy necessity. LETS attempt to recreate a social cohesion on a local scale on the basis of a compensation system of personal accounts. One notable example of a LETS, that of Saint-Quentin en Yvelines, France, has been creating a sort of accelerated money by a system of taxation on positive accounts, a peculiar system that applies some Gesellian ideas.

  , born in Germany, worked as a successful importer in Argentina at the end of the nineteenth century. He began to study economics when a huge monetary crisis occurred in Argentina in the 1880s. In 1891 he published Die Reformation im Münzwesen als Brücke zum socialen Staat 1 . He returned to Europe and started writing the Natural Economic Order, a major work based on his 1891 book. It was published in 1911, reprinted several times, and translated into several languages. English translations were published in 1929 and in 1934, the latter based on the sixth German edition.

  the context of the rising poverty and unemployment of Europe in the eighties and nineties, the ideas of a Free Economy and especially of Free Money are back. Organizations promoting the sort of « economic order » Gesell theorized still exist in Germany and Scandinavia, as well as Great Britain or Mexico. New debates about accelerated money are also focusing on how to manage an economy in an ecologically sound fashion 34 . Though local currencies were not born in Wörgl, this experiment served as a reference long after it was banned. Today, many experiments with local currencies hark back to Wörgl, even when they are not accelerated in a similar fashion. Two kinds of local currencies may be distinguished: currencies used in towns, and currencies used in small, closed groups. Local currencies, such as Ithaca Hour, were recently tried out in nearly fourty U.S. towns. Despite many differences, Local Exchange Trading Systems (LETS) are based on similar purposes. These small, closed groups have developed all over the world since 1983.

«

  In the post-war years his devotees bombarded me with copies of his works[...]. The last decade of his life was spent in Berlin and Switzerland and devoted to propaganda. Gesell, drawing to himself the semi-religious fervour which had formerly centred round Henry George, became the revered prophet of a cult with many thousand disciples throughout the world. [...] Since his death in 1930 much of the peculiar type of fervour which doctrines such as his are capable of exciting has been diverted to other (in my opinion less eminent) prophets » 11 .

See Herland [1977] on Boisguilbert, Proudhon and Gesell.

We will employ this term rather than Keynes's « stamped money », since it is more precise. The purpose of the stamping system is the acceleration of the circulation of money.

Gesell [1948, pp. 172-175]. Some years after, Keynes found in it a proof for his concept of the marginal efficiency of capital.

Gesell's idea implies that the buyer (in his mind, a capitalist) is able to wait before buying because money has no carrying costs, while the seller has to sell quickly because of the carrying costs of his goods. Many authors considered that the buyer has an advantage over the seller, without developing such a tribute theory. See, for example,Simmel [1978, p. 214] orKeynes [1936, pp. 225, 355-356].

Margrit Kennedy, for example, is the German author of a book entitled Freeing money from inflation and interest rates. In it, she proposed a transformation of society by means of monetary, land and fiscal reforms. See also for example[START_REF] Cotten | L'argent : monnaie de singe, monnaie de sage ? Propositions pour une mutation économique inéluctable, ou comment remettre l'économie au service de l'homme[END_REF].


Arnaud Legrand
email: arnaud.legrand@ens-lyon.fr

Martin Quinson
email: martin.quinson@ens-lyon.fr

Automatic deployment of the Network Weather Service using the Effective Network View

The monitoring infrastructure constitutes a key component of any Grid scheduler. The Network Weather Service (NWS) is the most commonly used tool to fulfill this need. Unfortunately, users have to deploy the NWS manually, which can be very tedious and error-prone. This paper characterizes the NWS deployment requirements and introduces a method based on the Effective Network View (ENV) network mapper to automatically perform this task. We also present the resulting deployment on our lab's LAN.

Introduction

Grids are a type of parallel and distributed systems that results from the sharing and aggregation of geographically distributed resources between several organizations [START_REF]The Grid: Blueprint for a New Computing Infrastructure[END_REF]. Unlike classical parallel machines, Grids present dynamic, heterogeneous and even non-dedicated capacities. Gathering accurate, up to date and relevant informations about them is then a very challenging issue, which has to be addressed before developing schedulers.

Nowadays, the Network Weather Service (NWS) constitutes a de facto standard in the Grid community as it is used by major Grid middlewares like Globus [START_REF] Foster | Globus: A Metacomputing Infrastructure Toolkit[END_REF] or Problem Solving Environments (PSEs) like DIET [START_REF] Caron | A Scalable Approach to Network Enabled Servers[END_REF], NETSOLVE [START_REF] Casanova | Using Agent-Based Software for Scientific Computing in the Netsolve System[END_REF] or NINF [START_REF] Nakada | Design and Implementations of Ninf: towards a Global Computing Infrastructure[END_REF] to gather informations about the current state of the platform, as well as about its future evolutions.

Unfortunately, there is no automatic way to deploy the NWS yet, and this deployment has to be done manually. This task is very tedious on such a platform and its proper achievement requires accurate knowledges both about the target network on which the tools are to be deployed and about the NWS internals.

This deployment process can be decomposed in two phases: First, we have to decide what kind of organization the NWS processes should follow in a deployment planning phase. Then, this deployment should actually be applied on the platform. The first phase can in turn be decomposed in two steps since we first have to gather the target network topology before computing a deployment plan. This paper presents a simple method using the Effective Network View (ENV) network mapper [START_REF] Shao | Using effective network views to promote distributed application performance[END_REF] to collect the needed information and automatically compute a deployment plan which can then be applied using standard tools. The rest of this article is organized as follows: Section 2 presents the NWS, focusing on how network measurements are conducted and characterizing the deployment requirements. Then, we present a rapid network mapping state of the art in Section 3 and detail the ENV tool and its methodology in Section 4. Lastly, we discuss an algorithm to deploy NWS from ENV results in Section 5 and present some results obtained on the LAN of our laboratory.

Deploying the Network Weather Service

The NWS (Network Weather Service) [START_REF] Wolski | The Network Weather Service: A Distributed Resource Performance Forecasting Service for Metacomputing. Future Generation Computing Systems[END_REF] is leaded by Prof. Wolski at the University of California, Santa Barbara. This distributed system of sensors and statistical forecasters enables to centralize the current state of the platform.

It enables to monitor the latency and throughput of any TCP/IP link, the CPU load, the available free memory or the free disk space on any host. Concerning CPU load, NWS not only reports the current load, but also the timeslice a new process would get on startup. Concerning link capacities, NWS reports the end-to-end bandwidth, latency and connection time. Given a set of n computers, there is n × (n -1) paths to test since the network is not symmetric in the general case [START_REF] Paxson | Measurements and Analysis of End-to-End Internet Dynamics[END_REF].

In addition to the current state of the platform, the NWS also provides predictions about its future evolutions by applying statistical treatments on the past measurements. Regular measurements in steady state are thus mandatory, even in absence of client requests.

To ensure correct measurements, the network experiments must not interfere with each other. If two measurements were conducted on a given network link at the same time, both of them could be influenced by the bandwidth consumption of the other one, and may therefore report an availability of about the half of the real value.

Wolski et al. [START_REF] Wolski | Synchronizing network probes to avoid measurement intrusiveness with the Network Weather Service[END_REF] handles this problem by introducing the concept of measurement clique. A measurement clique is a computer sets in which network measurements are done in a mutually exclusive manner, thanks to a token-ring based algorithm: only the host having the token at a given time is granted to launch network measurements on the links involved in that clique. Unfortunately, these measurement cliques have to be defined manually.

Moreover, token-ring algorithms are known to be not very scalable, and the frequency of the measurements obviously decreases when the number of hosts in a given clique increases. Cliques must then be split in sub-cliques to ensure a sufficient network measurement frequency and increase the responsiveness of the system. These splits have to be done wisely to ensure that the tests in a given clique will never collide on any link with tests from any other cliques.

Of course, clients applications are potentially interested in any end-to-end connexion even if there is no direct measurement between two given hosts because they are in different cliques. The system should then be able to combine the conducted experiments results to deduce the missing values.

For example, given three machines A, B and C, if the machine B is the gateway connecting A and C, it is sufficient to conduct only the experiments on (AB) and on (BC). Latency between A and C can then be roughly estimated by adding the latencies measured on AB and on BC. The minimum of the bandwidths on AB and BC can be used to estimate the one on AC. These values may be less accurate than real tests, but are still interesting when no direct test result is available.

To sum up, the NWS deployment has to satisfy four constraints:

Do not let experiments interfere. All hosts connected by a given physical network must be in the same clique to ensure that the tests conducted on that link are done in a mutually exclusive manner.

Scalability concerns.

All cliques should be as small as possible so that measurement frequency is sufficient to ensure a satisfactory reactivity level to the system.

Completeness.

If no direct measurement is conducted between two hosts, the system must be able to aggregate the conducted experiments to estimate the network characteristics of their interconnection.

Reduce intrusiveness. In order to reduce the system intrusiveness to its minimum, only the needed tests should be conducted. For example, since the bandwidth is evenly shared by all hosts connected by a hub or a bus, any hosts pair of such a set have the same connectivity. Hence, it is sufficient to measure it for a pair of hosts and use the result for all possible host pairs.

Having a good knowledge about the network topology is clearly fundamental to achieve a good NWS deployment. More precisely, we need to be able to determine which interhost connexion interfere on which other because they share a physical link. We now present some different ways to gather these informations automatically.

Network mapping solution

The OSI Network Model presents seven layers of abstractions, each of them providing a different view of the network. The lower ones are closer to the hardware reality while the higher ones offer a greater abstraction. When considering the network topology, we have therefore to specify the layer considered since it will have a great impact both on the way to discover it and on its possible uses.

The most commonly used topologies are either at the layer 2 or at the layer 3. Layer 2 is the Medium Access Control or Datalink layer, corresponding to the level of the Ethernet protocol. Layer 3 is the Network one and involves protocols such as the Internet Protocol (IP). The layer 2 is therefore closer to the physical links than the layer 3, and may be used to get useful information about routers and switches that are not directly available from layer 3. On the other hand, layer 3 is closer to the user-level applications view of the network.

SNMP and BGP

The layer 2 of the OSI model is the one where LAN are defined and configured. It is therefore possible to ask directly to the network components about their configuration as expressed by network administrators using for example the Simple Network Management Protocol (SNMP). Albeit, some old or dumb switches and routers do not answer to SNMP requests while other equipments requires the use of proprietary tools and protocols such as the Cisco's Discovery Protocol1 or Bay Networks' Optivity Enterprise2 .

Completing this view for a WAN can be done by using the Border Gateway Protocol (BGP), which is used to exchange routing informations between the autonomous systems composing Internet.

The Remos [START_REF] Dinda | The architecture of the remos system[END_REF] tool uses the SNMP protocol to construct the local network topology, plus some simple benchmarks to gather informations about WAN [START_REF] Miller | Collecting network status information for network-aware applications[END_REF]. It is then able to reconstruct the part of the network where dumb routers do not answer to the requests in order to get a complete topology.

The advantage of this approach is that such tools directly access to the network configuration as expressed by the network administrator. They are therefore very quick and not intrusive. On the other hand, the use of those protocols is almost always restricted to the authorized users. This is due to two major reasons: security, since it is possible to conduct Deny Of Service attacks by flooding the routers of requests, and privacy, since ISP generally do not like to publicly expose the possible bottlenecks of their networks.

As a matter of fact, this limitation is simply not acceptable in a metacomputing context. Since Grid platforms traditionally involve several well established and large organizations such as universities, obtaining the grant to use level 2 protocols can be very time consuming, and even reveal impossible due to human factors.

But on the other hand, any solution based on level 3 and above are more complicated to design and use because for example of the VLAN technology. It allows to present a logical view of the network differing from the physical reality to the higher layers. It enables network administrators to split the physical network into several logical ones. This is for example used in our lab to separate for security reasons in several networks the machines administrated only by the staff from the laptops and such on which the users may become root. Extra provisions are needed to take such things into account when mapping the network.

Tomographic approach

Tomography is a methodology used for example in medical imaging to reconstruct a 3D view of an object from several 2D ones taken from different point of view. Likewise, several solutions allow to rebuild a complete view of the network by merging the local views obtained from different hosts. Those methods mainly differ on the methodology used to get the local views to be merged.

The classical ping tool reports the round trip time between two hosts. Projects like IDMaps [START_REF] Francis | Idmaps: A global internet host distance estimation service[END_REF] or Global Network Positioning [START_REF] Ng | Predicting internet network distance with coordiantes-based approaches[END_REF] use this method to compute a network topology by clustering the network using this distance metric. This is clearly not sufficient for our needs. We need more information on links, such as their bandwidth (which IDMaps may sometimes provide) and how they would be shared between several streams using them concurrently.

The layer 3 of the OSI model is the one where internetwork connectivity is configured. To avoid infinite loops, all packets are given on creation a given Time To Live (TTL). This value is decreased by each router transmitting the packet, and when it becomes zero, the packet is destroyed and an error is signified to the emitter. This feature is used by the classical traceroute tool (and by projects developing wrappers to traceroute such as TopoMon [START_REF] Burger | TOPOMON: A monitoring tool for grid network topology[END_REF] or Lumeta [START_REF] Burch | Internet mapping project[END_REF]). Since most routers indicate their address in the error message generated when the TTL becomes zero, traceroute can discover most hops on a given network path by sending several packets with increasing TTLs.

This approach has several drawbacks. First, since routers can return different addresses, combining the paths can be non-trivial. Then traceroute gives no information on how concurrent transfers interact when sharing a given link, not even the available bandwidth. In fact, traceroute does not report the information relevant in our context: It focuses on the path followed in the network by the packets while we are interested in a more macroscopic view indicating the effects of this data movement at an application level.

Another important issue with traceroute-based network mapping solution is that it relies on the fact that routers return an error message when the TTL of a packet becomes zero. Unfortunately, this is not always the case as administrators can disable this feature to avoid Deny Of Service attacks based on flooding. Therefore, more recent work in network tomography focus on the design of new measurement methods relying only on classical packets. In [START_REF] Rabbat | Multiple source network tomography[END_REF], the authors presents a measurement methodology based only on regular packet exchanges.

As this approach does not require any specific privilege, it is very appealing in our context. Unfortunately, the obtained view of the platform does not fulfill our needs. It focuses on the physical interconnection topology and the description of the path followed by the packets while our goal is to identify the interferences between concurrent streams. These two notions are very close, but do not necessary match. If, using this methodology, two paths are reported to be independent (i.e. they do not share any network element), it is clear that data streams using these paths cannot interfere, but the contrary is not necessary true. Indeed, if the shared section is over-dimensioned and able to carry both streams without impacting on their performance, we want our tool to report the paths as independent. This goal's divergence makes this approach unusable directly in our context, even if it could constitute a first guess of the topology.

Link capacity

pathchar is the solution proposed by Jacobson (the traceroute's author) to gather not only the network organization, but also the capacity of each link. As traceroute, packets with different TTLs are transmitted, but the size of these packets also varies. Analyzing the time before the error packet is received, it is possible to infer the latency and bandwidth of each link in the path, the distribution of queue times, and the packet loss probability [START_REF] Downey | Using pathchar to estimate internet link characteristics[END_REF].

The first problem with pathchar is that it needs to assume that the sent probes will have negligible queuing delays on all encountered link. Since the probability of this event is rather low on loaded networks, this tool needs to conduct a lot of experiments to be sure that one of them respects this assumption. In current implementations, more than 1500 probes are usually used, so tests can last for hours on highly loaded routes constituted of many hops. Moreover, it only gives the capacity of encountered links and not how their bandwidth will be shared between several competing data streams.

Finally, in order to forge the packets needed for its experiments, pathchar needs to be given the super-user privileges on the machines where it runs, which is clearly unacceptable in Grid context.

Summary

We cannot get the information from where it was configured by network administrators (using SNMP or BGP) because it requires special privileges which cannot always be obtained on a Grid platform. As a consequence, it means that we cannot detect directly the use of technologies like VLAN, commonly used by network administrators to present a logical view different of the physical reality, and special provisions are needed to take this into account.

Existing layer 3 methods are not sufficient for us since they either fail to report which part of the network path are limiting (i.e. the ones on which a bandwidth shortage is possible) or need super-user privileges to be used (like pathchar does).

We decided to use ENV, presented in next section, because it captures a view of the network well adapted to our needs with no need of special privileges to run.

Effective Network View

The Effective Network View (ENV [START_REF] Shao | Using effective network views to promote distributed application performance[END_REF]) project was developed by Gary Shao at University of California, San Diego. ENV is primarily designed to improve the master/slave paradigm and allows to discover the effective topology of a network from the point of view of a given host. Data acquired are then dependent from the master's choice, as explained in [START_REF] Shao | Using effective network views to promote distributed application performance[END_REF].

The main advantage of ENV is that it creates an effective profile of network configuration based only on user-level observations and thus allows to get informations on layer 2 and 3 routers without using low-level tools or protocols which could not be accessible on a Grid platform.

We now detail how ENV collects the data, taking the mapping of the ENS-Lyon network as example. This process can be split in two parts: the first is independent from the choice of the master, while the other is not.

Structural topology: Master-independent data

This constitutes a first approximation of the topology builded with traceroute, and used to guide the active tests of latter phases. Each host involved in the mapping reports the path used to get out of the Grid by executing a traceroute to a well known external destination. The part within the mapped network is used to build a tree. Hosts using the same route to get out of the studied network are clustered together as leaves on the same branch.

This tree is based on traceroute and, as explained in Section 3.2, is therefore not sufficient to detect the interferences between streams. To fulfill our needs, a few more tests, depending on the master's point of view, are conducted.

Effective topology: Master-dependent data

The second part of the data collection depends on the chosen master. These measurements can be seen as successive refinements of the network structural view. This allow to generate a new tree containing the so-called ENV networks, which contain the critical informations in our context about the layer 2 topology.

Most of these experiments use thresholds to interpret the measurement results. The value of these thresholds may have a great impact on the mapping results, and were determined experimentally and empirically by the ENV authors.

Host to host bandwidth the-doors any host

This experiment splits the clusters in order to group the machines having a comparable connexion to the master together. The bandwidth between the master M and any host A is measured separately. Then, clusters previously made are divided into groups with similar bandwidth. If the bandwidth ratio between two hosts exceeds the threshold of 3, their cluster is split to separate them.

Pairwise host bandwidth

This experiment splits the clusters based on how much the connexion between their members and the master (M ) is interfering. For each pair of machines A and B in each clus-ter, the bandwidths of M A and M B are thus tested whenever the transfers occur concurrently.

the-doors cluster This measurement is then compared to the bandwidth measured in the previous step. If the ratio of the bandwidth (M A) in normal conditions and the one obtained when paired with MB is below a threshold of 1.25, A is declared independent of B and the cluster is split to separate those hosts.

Internal host bandwidth cluster

For each cluster, bandwidth is measured between any pair of machine, within this cluster. This allows to set the local bandwidth parameters for a given cluster. This may be useful for clusters where local bandwidth is different from bandwidth achieved between the cluster and the master. For example in ENS-Lyon, the route from the-doors to the popc cluster goes trough a 10 Mbps bottleneck, whereas popc is on a local 100Mbps hub.

Jammed bandwidth

the-doors cluster For each cluster, the bandwidth to the master is measured while a transfer between two other hosts of that cluster occurs. This measure is repeated 5 times, and the average of the ratio Bandwidth/Bandwidth jammed is computed.

If the average is below the threshold of 0.7, the cluster is reported to be on a shared link. If the average is above the threshold of 0.9, the cluster is reported to be on a switched link. If the average is between those two thresholds, data gathering about this cluster stops since the values are not significant enough.

All these refinements allow ENV to determine whether structural networks are switched or shared based upon bandwidths observed between hosts. This information has a great impact on the NWS deployment, as explained in section 5. Figure 1 presents the results of ENV running on the ENS-Lyon network. Figure 1(a) shows the physical network topology while Figure 1(b) is the result of an ENV run. The presented physical topology is simplified and asymmetric routes as well as used VLANs are not pictured. The pictured effective network view is the one obtained when choosing the-doors as master.

Execution result

We can see that the vision offered by ENV is much more simplified than the physical one since it does report only the routers which cannot be suppressed from the topology without changing its structural characteristics. Besides, ENV reports the fact that popc0, myri0 and sci0 are on a 100 Mbps hub, whereas links to reach popc0 and myri0 from the-doors must go through a bottleneck at 10 Mbps.

Known ENV issues

This section presents issues discovered during these experiments and that may impact the applicability of ENV.

Master/Slave paradigm ENV was designed for master/slave computing, and this may lead to important information loss during the mapping. For example, it may prevent this tool to detect direct links between slaves since the packets emitted from the master never use them. This information loss seems to be the price to pay for rapidity, since gathering all information about the network may require a very long time to complete. Indeed, using exactly the same methodology as ENV for a whole mapping would require to first drive n * (n -1) bandwidth tests between each couple of hosts {a; b}. Then, it would require for each pair of link {a; b} and {c; d} to conduct experiments to determine whether those network path are dependent or not, i.e. whether a transmission on {a; b} impacts the bandwidth of {c; d} or not. This naive algorithm would not scale at all because of the bandwidth and time consumed. Considering that collecting information about two given links lasts half a minute (which is reasonable since the network needs to stabilize between each experiments), the whole process would last about 50 days for 20 hosts. That is why ENV does not try to completely map the network, but only focuses on a view of the network from a given point of view.

Bandwidth waste Even if focusing on the master/slave paradigm greatly improves the performance of ENV, these tests still introduce a significant bandwidth consumption. It cannot be prevented since active probes are needed to infer layer 2 informations without relying on specific tools which may not be available. Nevertheless, a given platform needs to be mapped with ENV only once, and the results could then be shared between different people. For example, administrators could publish the mapping of their network as reported by ENV, so that any user can use it without recomputing the mapping.

Asymmetric routes

The physical network in ENS-Lyon is more complex than depicted on Figure 1(a). Indeed the route between the-doors and popc goes trough a 10 Mbps link, whereas the other direction uses only 100 Mbps links. According to [START_REF] Paxson | Measurements and Analysis of End-to-End Internet Dynamics[END_REF], this situation, even if quite queer, is rather common on Internet. Since ENV bandwidth tests are conducted in only one way, the system cannot detect such issues. Solving this would imply almost a complete rewrite of ENV tests and is still to do.

Likewise, networks are usually packet driven and we cannot guarantee that the path between two given hosts always follows the same route. However, since ENV is mainly used to map local networks, we assume that this is the case for the duration of the experiment, or at least, that global measurements are not too sensitive to this variation.

Reliability and accuracy

Even if ENV provides some bandwidth estimations, their accuracy is not crucial in our context since NWS is to be deployed, and provides much more accurate and up-to-date measurements. Qualitative and topological information about the network are therefore much more important than quantitative one.

Nevertheless, the first problem is the possible platform evolution: many inter-dependent tests are to be run. The results given by ENV may then be corrupted if the network load evolves greatly (increasing or decreasing) between tests. There is no solution yet to this problem, except rapidity: the mapping of our platform only last a few minutes, so we can assume that the environment is stable enough to provide meaningful results in those conditions. For bigger platforms. it would be possible to map separately the different parts of the network and then merge the results together.

Moreover, experimental thresholds may be problematic, because they may be specific to platform characteristics like the media type. They may for example be adapted to LAN, but not to Terabit links. Besides, determining if the thresholds are adapted to the current platform is very difficult.

Deploying the NWS using ENV

Most common Grid testbeds are constituted several organizations inter-connected by a wide area network, each of them sharing local resources (like clusters) interconnected by local area network with the others. The resulting platform is a WAN constellation of LAN resources. The most natural solution to satisfy all constraints expressed in Section 2 is then to set up a hierarchical monitoring infrastructure, where intra-site connectivity is tested separately from the inter-site one. If needed, this hierarchy can contain more than two levels, and intra-cluster connectivity would be measured separately from the inter-cluster one.

Deployment design

This section presents a simple algorithm to determine the NWS deployment using the data gathered by ENV. For each network or subnetwork discovered by ENV, our deployment plan contains at least two cliques:

• If the network is shared, its hosts are supposed to be on the same physical link, so the latency and bandwidth of one couple of hosts is representative for any possible couple. The intra-network connectivity is then measured by a clique containing two arbitrary chosen hosts.

Note that even if this is sufficient to gather the needed information, this method reveals a NWS shortcoming: it is currently not possible to inform the system that the connexion between two hosts (AB) is representative of the connexion between two other hosts (CD), and the user has to keep track of this to ask NWS about (AB) when he wants to retrieve the characteristics of (CD).

• If the network is switched, the network characteristics between each host pair are independents and could be measured separately, but each host must be involved in at most one measurement at a given time. That is why we deploy a NWS clique containing all the hosts to make sure that only one measurement will occur at the same time on the given group of hosts.

Note that using a clique a bit too restrictive. On a switched network, the tests AB and CD would not collide if they involve different hosts, i.e. if {AB} ∩ {CD} = ∅. On the other hand, the only drawback of forbidding concurrent tests over the group is that the frequency of tests between two given hosts slightly decreases.

The resulting deployment plan for the ENS-Lyon network is depicted on Figure 2. The sci cluster is switched, so we pick all its machines to form a new clique, whereas the myri cluster is shared, so we pick only two hosts for the local clique (myri1 and myri2). myri0 and popc0 were chosen to test the network characteristics on Hub 2 while moby and canaria are used to test the Hub 1. The connection between canaria and popc0 is used to test the connexion between these hubs.

Effective deployment

Once the deployment plan has been computed, another challenging issue is to actually apply it, and launch the several parts of NWS on the different hosts with the right op-tions. Indeed, the official version of NWS offers very few support to process management and global configuration. We have to manually ssh to the right host, and pass manually the right option on the command line of each NWS process.

To solve this issue, we designed a simple NWS manager program using a configuration file shared across all involved hosts and applying the local parts on each hosts. The actual deployment of NWS is then as easy as dispatching the configuration file to the hosts (using for example NFS), and running the manager on each machines. This program parses the given configuration file and starts the NWS components with the appropriate options for the machine on which the manager runs. It thus constitutes a convenient way to apply the deployment decisions of the administrator or of an automatic deployment tool.

Conclusion

The Network Weather Service constitutes a de facto standard of Grid platform monitoring tool used by major Grid PSEs. Unfortunately, deploying this tool remains tedious since no automatic tool is provided by default, and since the manual deployment requires insight about both the target network and the NWS internals.

The main difficulty remains to accurately map the target network, and ENV seems to be the most suited tool to our needs among the existing ones. Unlike SNMP-based tools or pathchar, it does not require any specific privileges to run. This is very important on a Grid constituted of several organizations sharing their local resources, since obtaining extra privileges on all parts of the platform can reveal difficult due to human factors. Unlike classical network tomography solutions (based on ping, traceroute, or other measurement methodology) ENV is also able to quantify how each detected network is shared among several concurrent transfers, which reveals to be a crucial information to achieve a proper NWS deployment.

After having characterized the constraints a "good" NWS deployment should satisfy, we proposed a simple algorithm to deduce a deployment plan from the information provided by ENV, and have applied it on the network of the ENS-Lyon laboratory. This experiment allowed us to identify several shortcomings of the ENV. First, it considers that the routes are symmetric and only test them in one direction. According to [START_REF] Paxson | Measurements and Analysis of End-to-End Internet Dynamics[END_REF], this simplification is over-optimistic. Moreover, ENV only provides a tree view of the network to simplify and speed up the mapping process. This is well adapted to a master/slave paradigm, but is a way too limited in the general case since it will overlook some transversal links between leaves of the tree. In order to overcome these issues, we initiated a new project called ALNEM [START_REF] Legrand | An applicationlevel network mapper[END_REF] that constitutes generalization of the ENV approach with stronger theoretical basements.

the interpretation of the measurements conducted by ENV depends on thresholds whose values were determined empirically. Their values may have a great impact on the results and thus on the quality of the mapping. Yet, an accurate study of this impact is still to be done. Since the quality of those values may well depend on the targeted class of network (LAN vs. WAN), an heuristic to determine them would be necessary to meet our goal of automatic deployment of the NWS.

Other concerns revealed by this experiment involve the NWS itself. First, it does not provide process managing facilities, and we designed a simple NWS manager in charge of applying the deployment plan locally to each machine.

Moreover, the protocol used to ensure that experiments never collide on a given resource (which would lead to wrong measurements) could still be improved. Currently, it ensures that only one pair of hosts from a given group will conduct an experiment at a given time. But on a switched network, more than one experiment may be authorized if the hosts involved in each experiments are different. That is to say that a possibility to lock hosts (and not networks) is still needed. Moreover, if the network is shared (using an hub), the connexion capacities between each pair of host will be the same, and testing one pair is enough. Future works would also be necessary to explore the conditions under which network measurements may be combined (when lacking direct experiments) in order to reduce the number of monitored paths and thus improve the system scalability. Active undergoing collaboration with the NWS team should allow us to overcome some of these issues.

  Effective topology from the-doors's point of view.

Figure 1 .

 1 Figure 1. Topology of the test network.

1 Figure 2 .

 12 Figure 2. NWS deployment plan in ENS-Lyon.

http://www.cisco.com .

http://www.baynetworks.com .

Acknowledgment

The authors thank the anonymous reviewers and the Professor Rich Wolski for their insightful suggestions about and beyond this paper.