Incentives - Peer-to-Peer Computing: Applications, Architecture, Protocols, and Challenges - page 79

conforms nicely with the simulation findings by Jun and Ahamad [Jun and
Ahamad, 2005] who observed that in BitTorrent, free-riders are not penalized
adequately while contributors are not rewarded su ciently.

5.2.1.2 Hierarchical P2P Systems

In some situations, the P2P network may be structured in a hierarchical
manner so that some specialized machines (called “super-peers”) can take up
a more important role for handling resource management tasks such as request
forwarding and routing, directory listing, etc.
Singh et al. [Singh et al., 2003] proposed a super-peer-based scheme. Specif-
ically, Singh et al. studied the impacts of super-peers in a P2P file sharing
network. Simply put, a super-peer is a special network node that serves as a
hub to provide file indexing service to other nodes. The problem is that there
is a lack of incentive for a participating node to act as a super-peer because
any node can simply join an existing super-peer to obtain good performance.
Singh et al. observed that some entities external to the P2P system, such as an
Internet Service Provider (ISP) or a content publisher, have business driven
incentives for designating some nodes to act as super-peers, and for enhancing
the capabilities of super-peers. Specifically, a super-peer can cache meta-data
only instead of the files themselves. As such, the cost of acting as a super-peer
is lower. Furthermore, with properly designed meta-data, a super-peer can
support value-added search commands (e.g., topic-based search of files).
With such facilities incorporated in each super-peer, a hierarchical P2P file
sharing network is then much more e cient than a flat P2P system which relies
on request-flooding. Consequently, all nodes in the system have the incentive
in maintaining such a P2P file sharing system. Simulation results also indicate
that the proposed super-peer scheme is effective.

5.2.1.3 Payment-Based Systems

Hauscheer et al. [Hausheer et al., 2003] suggested a token-based accounting
system that is generic and can support different pricing schemes for charging
peers in file sharing. The proposed system is depicted in Figure 5.1. The system
mandates that each user has a permanent ID authenticated by a certification
authority. Each peer has a token account keeping track of the current amount
of tokens, which are classified as local and foreign. A peer can spend its local
tokens for accessing remote files. The file owner treats such tokens as foreign
tokens, which cannot be spent but need to be exchanged with super-peers
for new local tokens. Each token has a unique ID so that it cannot be spent
multiple times.
At the beginning of each file sharing transaction, the file consumer tells
the file owner about which tokens it intends to spend. The file owner then
checks against the file consumer's account kept at the file owner's machine. If
the tokens specified are valid (i.e., they have not been spent before), then the
file consumer can send the tokens in an unsigned manner to the file owner.