Proposal, design and evaluation of a mechanism to limit the length of anonymous overlay network paths

An alternative to guarantee anonymity in overlay networks may be achieved by building a multi-hop path between the initiator and the destination. Random walks (also known by means of the Crowds algorithm) have been widely used for this purpose in IP networks. Therefore, we explore the use of a Crowds-based mechanism to provide anonymity in overlay networks. However, the original algorithm does not limit the length of the paths, and in an overlay network the associated costs may grow excessively. Thus, controlling the length of the Crowds-based paths is a crucial issue in this scenario. A straightforward implementation makes use of a time-to-live (TTL) field. However, this implementation will immediately reveal whether the predecessor node is the initiator or not. This paper presents a novel mechanism to control the path length without using the TTL field. We propose an analytical model to evaluate the degree of anonymity when the path length is limited using our scheme. We conclude that limiting the multi-hop path length does not have any relevant impact over the degree of anonymity. We also prove that the new mechanism does not increase the vulnerability of Crowds over the traffic analysis and predecessor attacks.