P2P Network Architectures - Peer-to-Peer Computing: Applications, Architecture, Protocols, and Challenges - page 40

3.4 Hybrid P2P Systems
Ohnishi et al. [Ohnishi et al., 2007] proposed an interesting hybrid archi-
tecture comprising a DHT ring (e.g., Chord) of unstructured small networks,
called load balancing clusters. Specifically, as shown in Figure 3.10, the core
network is formed by a set of Super Nodes using a DHT such as Chord.
Each Super Node, in turn, manages a small unstructured network of regu-
lar nodes. The rationale is that the load of each Super Node can be shared
by a dynamic population of regular nodes, depending on the access patterns
and tra c conditions. Consider a file sharing application as an example, the
following procedure can be implemented.
1. A peer searching for a particular file computes the hash value of the
desired file.
2. The search request is then forwarded to a cached Super Node.
3. The receiving Super Node checks the request against its own range of
hash values and if the request hash value falls within range, Step (4) is
executed; otherwise, Step (5) is executed.
4. The Super Node floods the request within its own load balancing cluster
(LBC) so as to locate the file, which is then sent to the requesting peer.
5. The Super Node forwards the request to the next Super Node whose
identifier is the closest to the request hash value. Execute Step (3) at
this new Super Node.
The maintenance of this semi-structured network architecture entails many
load balancing considerations and is discussed in Chapter 4.
Lagesse and Kumar [Lagesse and Kumar, 2007] proposed a hybrid archi-
tecture, called Utility Based Clustering Architecture (UBCA), which works by
overlaying a clustering mechanism on top of an unstructured P2P network, as
shown in Figure 3.11. The main clustering criterion is incentive based—peers
form a cluster provided they can derive mutual utility gains. The design goals
of UBCA are as follows [Lagesse and Kumar, 2007]:
•Enhance availability and quality of resources;
•Encourage resource sharing in the P2P system;
•Application adaptivity; and
•Maintain underlying system's structure and decentralization.
As can be seen from Figure 3.11, UBCA intercepts the P2P application's
requests (e.g., search, insert, delete, etc.) to the underlying P2P network (e.g.,