The VLDB Journal (2007)
ohm · Erik Buchmann
Free riding-aware forwarding in Content-Addressable Networks
Received: 2 February 2005 / Accepted: 27 September 2005 / Published online: 27 January 2006
Abstract Research on P2P data structures has tacitly
assumed that peers readily participate in the work, i.e., are
cooperative. But such participation is voluntary, and free
riding is the dominant strategy. This article describes a pro-
tocol that renders free riding unattractive, for one particular
P2P data structure. The protocol is based on feedback that
adjacent nodes exchange. This induces transitive logical
networks of nodes that rule out uncooperative peers. The
protocol uses proofs of work to deter free riding. To show
that cooperative behavior dominates, we have come up with
a cost model that quantiﬁes the overall cost of peers, de-
pending on their degree of cooperativeness and many other
parameters. The cost model tells us that we can achieve
a good discrimination against peers that are less coopera-
tive, with moderate additional cost for cooperative peers.
Extensive experiments conﬁrm the validity of our approach.
Keywords Peer-to-peer · Distributed hashtables · Free
riding · Incentives · Reputation
Peer-to-Peer data structures (P2P data structures) address a
core issue of data management research, namely administer-
ing huge sets of (key, value)-pairs. A P2P system consists of
nodes, a.k.a. peers. Peers may issue queries, but they are also
supposed to participate in the work, i.e., storage of data and
evaluation of queries in the context of P2P data structures.
By participating in the work, peers share the infrastructure
costs (disk space, energy, network bandwidth etc.). P2P data
structures do not have a centralized instance, a.k.a. coordina-
tor, which monitors and controls the peers. So far, research
at Karlsruhe (TH), Germany.
at Magdeburg, Germany
on P2P data structures has tacitly assumed that peers read-
ily participate in the work. But experience with P2P systems
that are operational, notably ﬁle-sharing systems, indicates
that this is not realistic . Peers seek to minimize their
costs. Free riding is the dominant behavior in the economic
sense. Existing technology does not solve the problem for
P2P data structures: Payment mechanisms  are vulnera-
ble and have high infrastructure costs. Certiﬁed code or sim-
ilar solutions  typically require a centralized certiﬁcation
instance, i.e., are not P2P. Proposals against free riding in
mobile environments [5, 21] are not applicable either. There,
peers can observe the behavior of other peers in the same ra-
dio network cell and infer their degree of cooperativeness.
Our objective is the design of protocols for P2P data
structures that render free riding unattractive. This article fo-
cuses on the evaluation of queries. It does so for Content-
Addressable Networks (CAN) , a prominent P2P data
structure. With the protocol envisioned, peers will only an-
swer queries issued by reliable nodes
, i.e., nodes that have
correctly processed all recent incoming queries. New nodes
or nodes with an unclear status must prove their reliability
ﬁrst, before beneﬁting from the system. At the same time,
the costs of the protocol shall not be much higher than the
ones of existing protocols. The design of such a protocol is
difﬁcult, for various reasons:
– In contrast to other P2P scenarios different from P2P
data structures (cf. [8, 11]), it is not only one peer, but a
sequence of peers that processes a query. This makes the
problem much more difﬁcult, as this article will show.
In our setting, peers cannot readily observe the behav-
ior of other nodes: if a query remains unanswered, the
issuer cannot say which other peer has not cooperated.
From a slightly different perspective, peers hide their in-
tentions. A statement like “Connection refused” will not
be generated if a node does not participate in the work.
Hence, standard recovery mechanisms for CAN  like
expanded ring search or ﬂooding will not work.
In the context of this article, reliable and cooperative are syn-
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