Sharing and hoarding


Are the digital commons tragic?

A new study, entitled Free Riding on Gnutella, reveals lots of taking and little giving among users of the file sharing system Gnutella. The study presents important data but draws questionable conclusions from it.

Conducted by researchers of the Internet Ecologies Area at Xerox's Palo Alto Research Laboratories (PARC) this study is based on a 24 hour traffic analysis of a single node in the Gnutella Network. Through this traffic analysis, the researchers "established that 70% of Gnutella users share no files, and 90% of the users answer no queries." Effectively, this means that only 30% of the users contribute any files to the common resource base.

The study goes on to say that even among those who do contribute, the concentration at the top is heavy. The top 10% of hosts contribute 87% of all files, with close to half of all files (40%) provided by the top 1%. Furthermore, 90% of all users either provided no files or the files they provided were never requested. The files that were actually of interest, hence downloaded by others, concentrated on only 10% of all hosts.

This data questions some commonly held assumptions about the nature of a distributed system such as Gnutella. First, the system is much less distributed than the number of hosts indicates. A relatively small number of hosts constitute, in effect, a central repository for a large part of all files, particularly the popular files. Second, this concentration (re)introduces into the system a number of vulnerabilities that were thought to be avoided by its supposedly distributed nature.

The system is more vulnerable to censorship or hacking attacks (Distributed Denial of Service attacks, for example) than usually claimed because it is possible to identify the relatively small number of hosts that contribute the majority of resources. 40% of the resources, as the study shows, were contributed by only 314 hosts. While this is significantly more than the single central directory of Napster, it still might not be too difficult to enforce copyright/intellectual property/censorship laws against most of them, once a clear legal regime has been established and precedences have been set.

This heavy concentration of resources might introduce another weakness: the unequal use of bandwidth throughout the system. If only 10% of hosts contribute those files that are actually downloaded, then this small number of hosts will have to carry 100% of the bandwidth used in the system, potentially introducing bottle necks, slowing down transmission, and burdening the most valuable contributors with the lion share of the (bandwidth) costs. Hence the system punishes those who contribute the most.

The researchers conclude: "These findings have serious implications for the future development of Gnutella and its many variants. In order for distributed systems with no central monitoring to succeed, a large amount of voluntary cooperation is required, a requirement that is very hard to fulfill in systems with large user populations that remain anonymous." Consequently, an open file sharing system is likely to be affected by "the tragedy of the digital commons."

The tragedy of the commons

The tragedy of the commons refers to a tendency of freely available resources to degrade over time. in his classic 1968 article Garret Hardin gave the archetypal example of this:

Picture a pasture open to all. It is to be expected that each herdsman will try to keep as many cattle as possible on the commons [and] the inherent logic of the commons remorselessly generates tragedy. As a rational being, each herdsman seeks to maximize his gain. Explicitly or implicitly, more or less consciously, he asks, "What is the utility to me of adding one more animal to my herd?" This utility has one negative and one positive component.

1. The positive component is a function of the increment of one animal. Since the herdsman receives all the proceeds from the sale of the additional animal, the positive utility is nearly + 1.

2. The negative component is a function of the additional overgrazing created by one more animal. Since, however, the effects of overgrazing are shared by all the herdsmen, the negative utility for any particular decision-making herdsman is only a fraction of - 1.

Adding together the component partial utilities, the rational herdsman concludes that the only sensible course for him to pursue is to add another animal to his herd. And another.... But this is the conclusion reached by each and every rational herdsman sharing a commons. Therein is the tragedy. Each man is locked into a system that compels him to increase his herd without limit -- in a world that is limited. Ruin is the destination toward which all men rush, each pursuing his own best interest in a society that believes in the freedom of the commons. Freedom in a commons brings ruin to all.

The difficulty of this example, apart from assuming a rather limited rationality, is that it is not clear how it applies to the sharing of digital goods, if it applies at all. Rishab Ghosh, in his Cooking-Pot Markets, argues why it might not apply: With a cooking-pot made of iron, what comes out is little more than what went in -- albeit processed by fire -- so a limited quantity must be shared by the entire community. The Internet cooking-pots are quite different, naturally. They take in whatever is produced, and give out their entire contents to whoever wants to consume. The digital cooking-pot is obviously a vast cloning machine, dishing out not single morsels but clones of the entire pot. But seen one at a time, every potful of clones is as valuable to the consumer as were the original products that went in.

The difference stems from the fact that taking from the digital resource base does not diminish what is available to others. Since digital data is not tangible, giving it away to others does not imply being dispossed of it. As a consequence digital commons can tolerate a much higher degree of consumption than physical commons, where everything that is consumed needs to be replaced before it is available.

The fact that none of the file sharing systems has been negatively affected by the "tragedy of the commons" suggests that it might not apply to digital goods. So far, every increase in diversity of the files available is an increase of the attractiveness of the system for all users, even if the number of users grows quicker than the number of files. This suggests that these systems are much more stable than implied by the PARC study and much less in need to be converted into "market-based architecture."

While these systems are unlikely to collapse under their own weight, the study does indicate that the centralization, introduced by the extremely unequal distribution of resources throughout the network, makes the system vulnerable to hostile attacks, both on a technical as well as on a legal level. This shows, that even cleverly designed system cannot guarantee the free flow of information in an environment that is either not willing to support this goal or downright hostile to it.

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