Fully distributed peer-to-peer systems do not present the single points of failure that led to the demise of central MP3 servers and Napster. It is natural to ask how robust these systems are and what form potential attacks could take. We observe the following weaknesses in Gnutella-like systems:
· Free riding
· Lack of anonymity
2.4.1 Free Riding
Peer-to-peer systems are often thought of as fully decentralized networks with copies of objects uniformly distributed among the hosts. While this is possible in principle, in practice, it is not the case. Recent measurements of libraries shared by gnutella peers indicate that the majority of content is provided by a tiny fraction of the hosts [1]. In effect, although gnutella appears to be a peer-to-peer network of cooperating hosts, in actual fact it has evolved to effectively be another largely centralized system – see Fig. 2. Free riding (i.e. downloading objects without sharing them) by many gnutella users appears to be main cause of this development. Widespread free riding removes much of the power of network dynamics and may reduce a peer-to-peer network into a simple unidirectional distribution system from a small number of sources to a large number of destinations. Of course, if this is the case, then the vulnerabilities that we observed in centralized systems (e.g. FTP-servers) are present again. Free riding and the emergence of super-peers have several causes:
Peer-to-peer file sharing assumes that a significant fraction of users adhere to the somewhat post-capitalist idea of sacrificing their own resources for the “common good” of the network. Most free-riders do not seem to adopt this idea. For example, with 56 kbps modems still being the network connection for most users, allowing uploads constitutes a tangible bandwidth sacrifice. One approach is to make collaboration mandatory. For example, Freenet [6] clients are required to contribute some disk space. However, enforcing such requirements without a central infrastructure is difficult.
Existing infrastructure is another reason for the existence of super-peers. There are vast differences in the resources available to different types of hosts. For example, a T3 connection provides the combined bandwidth of about one thousand 56 kbps telephone connections.
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