Topology Control - Peer-to-Peer Computing: Applications, Architecture, Protocols, and Challenges - page 54

Chapter 4
Topology Control
4.1 Introduction
One defining feature of any P2P system is that the composition of the
system is highly dynamic and time-varying in nature. In essence, peers join
and leave the system at will. Such population dynamics is also sometimes
highly unpredictable. A major consequence is that the network topology can
change dramatically over time: some high bandwidth links might come and
go, or turn into low bandwidth links without notice. Thus, the performance
of the participating peers, in terms of downloading/uploading rates, might
be adversely affected by the topology changes. To combat these performance
degradations that are unavoidable in a practical P2P system, a topology control
component is necessary.
As P2P networking is all about exchanging information and data e -
ciently without the help of centralized infrastructure, it is crucial to have an
effective topology, one that facilitates fast and robust communications among
peers. For example, it would be ideal if peers always form a highly e cient
DHT network so that redundancies in file data communication are minimized.
However, it is inherently di cult to establish and maintain an effective net-
work topology. This is because in the first place, peers are autonomous and
thus, it is di cult to enforce topology establishment and maintenance rules.
Secondly, peers come and go and such peer churns also make maintaining an
effective topology very di cult. Consequently, topology control is a challenging
and very important research problem.
Indeed, even if a DHT is used (e.g., Chord), it is mandatory for every par-
ticipating peer to comply with the maintenance rules such as transfer of data
to neighboring peers, which need to accept this responsibility uncondition-
ally. Thus, a major challenge in an effective topology control scheme is that
each peer, while rationally selfish, is willing to execute some locally optimizing
rules, so as to help maintain an effective global topology.
Topology control is arguably even more important in an unstructured net-
work because in such a network, peer dynamics can easily render an initially
good topology very ineffective. Fortunately, in many seemingly random net-
works, nice global properties emerge, including small-world, power law, etc.
Milgram [Milgram, 1967] pioneered the formal investigation of the “small-
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