As already defined in Chapter 2, the Peer-to-Peer paradigm relies on
the design and implementation of distributed systems where each system
has (nearly) the same functionality and responsibility. By definition, these
systems have to coordinate themselves in a distributed manner without centralized
control and without the use of centralized services. Thus, scalability
should be an inherent property of Peer-to-Peer systems. Unfortunately, not
all of them have shown this to be true so far. In this chapter, we will demonstrate
this by discussing the lookup problem – a fundamental challenge for all
kinds of massively distributed and Peer-to-Peer systems