CIKM WORKSHOP REPORTThe CIKM 2006 Workshop on Information
Retrieval in Peer-to-Peer Networks
Ivana Podnar Žarko
FER, University of Zagreb, Croatiaivana.podnar@fer.hr

1Fabrizio Silvestri
ISTI-CNR, Pisa, Italy
fabrizio.silvestri@isti.cnr.it

Introduction

Information Retrieval (IR) in distributed and decentralized environments has become an active
field of research during the last years. Recently, Peer-to-Peer (P2P) networks have emerged as an
attractive architectural paradigm for IR, both for technical and economic reasons. P2P networks
are distributed and self-organizing systems that support resource sharing. For this reasons, they
are promising building blocks for next-generation search engines that will have to deal with hugeamounts of heterogeneous (e.g. textual, multimedia, audiovisual) and continuously changing data.
Moreover, P2P search is appealing from an economic perspective since it requires minimal in-place
infrastructure and maintenance, yet facilitating higher diversity in contents and search methods.
However, P2P retrieval methods still pose a lot of research challenges. Search methods are typicallylimited to simple keyword queries and the use of advanced retrieval models is quite constrained.
There is an ongoing debate within the research community on the efficiency and scalability of suchapproaches, as well as effectiveness and applicability of traditional models in this new environment.
P2PIR 2006, the third Workshop on Information Retrieval in Peer-to-Peer Networks was held
on November 11, 2006 in conjunction with CIKM 2006. This year’s workshop was a team effort
continuing the tradition of past workshops, the Workshop on Information Retrieval in Peer-to-Peer
Networks which was collocated with SIGIR in 2004 and with CIKM in 2005, and the Workshop on
Heterogeneous Distributed Information Retrieval collocated with SIGIR in 2005. The workshop
attracted around 15 researchers from the domains of IR and databases working on distributed anddecentralized IR systems.
The call for papers attracted 13 submissions from Asia, Europe, and the United States. Theprogram committee accepted 7 scientific papers presenting original solutions for resource selection and query routing, indexing techniques in decentralized environments, and implementationsof P2P retrieval engines. The workshop was opened by Fausto Rabitti, research director at
ISTI-CNR, Pisa, Italy, who gave an invited talk on the future of P2P audiovisual search andprovoked a lively discussion that continued throughout the paper presentations and the closing
panel discussion. Paper presentations were divided into the following sessions: Resource selection, Similarity search, and Systems and applications. The panel discussion was organized aroundthe usage scenarios, and economic and social factors steering the adoption of the P2P technology in the field of IR. The invited talk slides and workshop program are available online fromACM SIGIR Forum101

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http://lsirwww.epfl.ch/p2pir2006/, and all published papers are accessible online from the ACM
Digital Library.

2Presentations

The invited talk given by Fausto Rabitti on The Future of P2P Audiovisual Search motivatedthe need for effective and efficient search methods that can deal with the growing volume ofdigital content most of which is in audiovisual form. Fausto gave an overview of existing P2P
applications and their search capabilities, and commented on the limitations of the state-of-the-art
audiovisual web search engines. Similarity search was proposed as one of the promising techniques
for audiovisual search, and P2P systems as the underlying technology enabling dynamic search
scenarios. Search expressiveness, scalability, and load balancing have been identified as further
research challenges.
The first technical session on Resource selection was opened by Matthias Bender with Size
Doesn’t Always Matter: Exploiting PageRank for Query Routing in Distributed IR. Matthias presented a method for distributed authority scoring and a strategy for incorporating global PageRankscores into the query routing process which shows the potential to improve peer selection compared
to CORI. Gleb Skobeltsyn presented a query-driven full-text indexing strategy in structured P2P
networks with his talk on Distributed Cache Table: Efficient Query-Driven Processing of MultiTerm Queries in P2P Networks. Presented results show that by caching specially selected andprofitable query answers in structured P2P networks under the assumption of limited storage perpeer, a large fraction of future queries can be efficiently answered.
Three papers were presented in the session on Similarity search. Ruj Akavipat introduced amethod to evaluate a peer search network based on the concept of semantic similarity in his talkon Emerging Semantic Communities in Peer Web Search. Ruj gave a demo of 6S, an unstructured
P2P search engine for collaborative web search which was used to investigate peer clustering bymonitoring the quality of the peers neighbors over time. The analysis has shown the spontaneous
formation of semantic communities through local peer interactions. Alessandro Linari presented
the work on Efficient Peer-to-Peer Semantic Overlay Networks based on Statistical Language Models and proposed the formation of P2P network in such a way that each peer becomes a neighborof a small number of peers holding content most similar to the one it hosts. Similarity is determined using the square root of the Jensen-Shannon divergence. In his talk on M-Grid: Similarity
Searching in Grids, Vlastislav Dohnal proposed the Metric Grid (M-Grid), a distributed index
structure for similarity searching that exploits a Grid infrastructure.
The last session was related to systems and applications. In his talk on Applying Information
Retrieval Techniques to Peer-to-Peer File Sharing Systems Ophir Frieder gave an overview ofresearch directions related to P2P information retrieval at the Illinois Institute of Technology andillustrated how IR techniques can improve the search efficiency of P2P file sharing systems. The
last talk was delivered by Martin Rajman who presented ALVIS Peers: A Scalable Full-text Peerto-Peer Retrieval Engine. Martin explained a novel indexing and retrieval model designed for
scalability in structured P2P networks, and presented how it has been integrated with a solution
for content-based ranking into a fully-functional P2P full-text retrieval engine.

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3Discussions

The panel discussion was motivated by the question on whether there is a place for the P2Ptechnology within the field of IR and which factors and applications will increase the adoption ofthis technology to produce next-generation search engines.
The volume of produced data and diversity of such data was identified as the major factor
driving the need for distributed and decentralized IR solutions. There are immense amounts ofprivately owned data that will never be indexed by search engines like Google or Yahoo. Theseengines are extremely good to satisfy the needs of everyday users. However, there was a common
agrement that P2P search engines are not playing against state-of-the-art engines on the same
field, but are trying to be complementary specifically oriented towards community-based search.
Technology, social impact, and economic issues were listed as the three main factors influencing
the adoption of the P2P technology. Someone suggested that intellectual input from volunteers
where people invest there knowledge and time is important because P2P is about sharing ofresources and data. There is potential to generate new technology and to improve user experience
with e.g. online home stores that are getting more popular and that might create a successful
business model. There was no common agreement on whether we can define a successful businessmodel because the participants were mainly from academia focusing on innovation and research.
However, it was concluded that new prototypes and interesting applications are needed to attract
more users and enable large-scale experiments. Additionally, benchmark tests are necessary tocompare different systems. The discussions were closed with a proposal to initiate a wiki page for
the P2PIR community and to organize a summer school where participants can further discuss
the technology-related issues and define the benchmark tests.

4

Acknowledgments

We thank the authors and presenters, the invited speaker and the members of the program committee, and all workshop participants for their contributions to the presented material and productive
discussions. We would like to remember and acknowledge the efforts of our dear colleague Henrik Nottelmann in appreciation of his enthusiastic efforts invested in organization of the previous
P2PIR workshops.

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