The use of the P2P approach in e-learning systems promotes this kind of collaboration and changes the e-learning system
from a static framework, in which users access resources from one or more centralised systems managed by a single author-
ity, to a dynamic framework in which resources are provided by each user of the system.
Ariadne is based on the P2P model inspired by IEEE LOMster [37–39], a project with the goal to support LO sharing and
reuse. Among its facilities, LOMster allows users to add metadata to their learning objects in addition to a system generated
description in order to improve resources sharing. LOMster uses JXTA as a P2P framework adopting the associated publishing
mechanism to locate peers in the network (peers have to previously register). To find resources, a peer executes a limited
flooding sending its query to all registered nodes [40].
Edutella [41] is a well-known P2P network designed for distributed environments. The project defines a framework that
provides a mechanism to manage peer access, a reference model called ‘‘Edutella Common Data Model” allowing the sharing
of queries and messages, but each peer refers only to local data representation and a query model [42].
P2P based e-learning systems allow collaboration, spreading of information and experience [9,43]. Of course, some mech-
anisms – often based on ontologies – must be present in order to integrate the exchanged data allowing users to both query
the system and extract meaningful information. Simple but effective approaches store information about a resource in sev-
eral peers to increase reliability, but each user might also share a managed resource thereby extending the whole system
[44,42].
In traditional distributed environments, service providers and requesters are usually known to each other. Often shared
information in the environment tells which parties can provide what kind of services and which parties are entitled to make
use of those services. Thus, trust between parties is a straightforward question. In spite of this, the above systems adopt dif-
ferent approaches when building learning paths, but only preliminary studies can be found on trust negotiation in e-learning
systems.
Trust was studied in social sciences, business and psychology before it became central to computer science research; the
survey [11] offers a complete overview of most recent issues concerning trust. In [45], the first work providing a formal mod-
el of trust, the value of trust is chosen within the range [1,1], covering from complete distrust to full trust. In [46], however,
the use of a range with negative values to model distrust is somehow criticised, mainly due to algorithmic-related issues (not
necessarily real values for trust matrix eigenvector); authors also debate whether a trust score of 0 translates as distrust or as
no opinion. We choose to assign trust values in the range [0,1] since rather than explicitly distrust the peer i (i.e. using values
in the [1,0] range), peers commonly prefer not to express any opinion about i (i.e. a value of 0 is simply indicated).
The paper [21] shows the question of trust in the computer science context as a propagation of a social matter (‘‘in today’s
connected world, it is possible and common to interact with unknown people...”). The metric they define for trust evaluation is
based on their previous work, MoleTrust, that predicts the trust score of source user on target user by walking the social net-
work starting from the source user and by propagating trust along trust edges; intuitively the trust score of a user depends
on the trust statements of other users on them and their trust scores.
In recent decades, recommender systems have gained an important role in today’s networked worlds because they pro-
vide some tools to support decision making in selecting reliable from unreliable. Reliability is often expressed through a trust
value with which each agent labels its neighbours; [47,48] explore this, but they do not investigate the topic of formation of
trust based on real world social studies. Some recent works have suggested to combine distributed recommendation systems
with trust and reputation mechanisms [47,12].
Ziegler and Golbeck [49,16] believe that trust and user similarity are correlated, in particular that the notion of trust
should reflect user similarity.
Therefore, a reputation system is an important tool in any network, and assume a central role in emerging P2P networks
where many people interact with many others in a sort of ‘‘democratic” fashion. Some authors discuss decentralised methods
that approximate ranks according to local history and a notion of a neighbourhood [50] where trust is calculated taking
advantage of small-world properties often emerging in networks that mimic the real world. In the P2P area EigenTrust
[51] distributed implementation of PageRank [52] is proposed that also needs also a distributed structure to store data
and imposes a pre-trust of all nodes belonging to the net thus reducing the ‘‘de-centralisation”.
A preliminary attempt at trust negotiation in e-learning systems is introduced in [53] in which the trust negotiation is
managed using PeerTrust a
policy
language tailored for negotiating and establishing trust in a distributed environment. It
is used in a Edutella-based system [54] to asses the trust of LO.
3. The proposed e-learning framework
The model of the e-learning trust- and recommendation-aware framework we propose is informally introduced in the
following (in Section 3.2 formal definitions are provided).
Our model is based on the P2P paradigm, the topology of this P2P network is depicted in Fig. 1 where circles represent
peers and squares represent resources. Edges used to build the network can be categorised as follows:
1. The solid directed edges joining peers represent trust relationships, i.e. X had some direct experience with Z and assigned
a trustworthiness to Z, and similarly occurred for peers Z with respect to Y. The trustworthiness is a value in the range
[0,1] used to label the edge (these weights are omitted in the figure). In addition, several expertise ranks regarding dis-
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