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<Paper uid="I05-2005">
  <Title>A Novel Method for Content Consistency and Efficient Full-text Search for P2P Content Sharing Systems</Title>
  <Section position="2" start_page="0" end_page="25" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> P2P content sharing systems can distribute large amounts of contents with limited resources. By utilizing this exceptional feature, the P2P content sharing model is expected to be one of the major means for exchanging contents.</Paragraph>
    <Paragraph position="1"> However, the presently available P2P content sharing systems are mainly used to illegally copy movies and music contents. In some cases, the service providers are accused of such illegal data exchange.</Paragraph>
    <Paragraph position="2"> We have recognized that the following technical problems may result in the above mentioned misuse of P2P.</Paragraph>
    <Paragraph position="3"> First, the presently available commercial P2P content sharing systems do not provide sufficient functions to track the exchange of contents among users. Due to this, service providers cannot monitor the illegal exchange or tampering of shared contents among users.</Paragraph>
    <Paragraph position="4"> Second, the presently available commercial P2P content sharing systems only provide simple search functions, such as keyword search; therefore, they are unsuitable for contents that are either frequently updated or have text. In practice, the current P2P content sharing systems are mainly used to only share movies and music contents because these are not frequently updated. The development of an appropriate search method for the P2P content sharing system is required in order to apply them to search text contents and the latest version of contents.</Paragraph>
    <Paragraph position="5"> In order to solve these technical problems, we are developing a content consistency maintenance method and an information search technique for P2P content sharing systems. Our content consistency maintenance method consists of a technique that prevents the tampering of contents and a method that maintains consistency between the following: 1. how users exchange contents on a P2P contents sharing system and 2. how the service provider recognizes the exchange of contents.</Paragraph>
    <Paragraph position="6"> Finally, we aim to standardize the result of previous research [10].</Paragraph>
    <Paragraph position="7"> In order to handle the updates of contents, the P2P content sharing system that we are developing maintains digital signs for each version of the content. Our system uses a download protocol based on asymmetric key encryption to maintain content consistency. In order to obtain the latest version of contents, even for updated contents, this method employs links to the original and the downloaded contents. These links are managed on a central server.</Paragraph>
    <Paragraph position="8"> In order to efficiently implement a full-text search, clients connected to our system perform morphological analysis and summarization of the text to generate text information that is necessary for building a reverse index on a central server. The text information is stored on a central server when the content is updated. To reduce the load of full-text search, the search results are cached on clients. By these techniques, we can distribute the load of natural language processing among clients and rapidly search text contents with content updates.</Paragraph>
    <Paragraph position="9"> In this paper, we briefly describe the P2P content sharing system that we are developing and the techniques used in it, namely, a content consistency maintenance method and a full-text search method. We also report the result of a preliminary experiment on load balancing of full-text search by our technique.</Paragraph>
    <Paragraph position="10"> This paper is structured as follows: Section 2 describes related work. Section 3 briefly describes the  P2P content sharing system that we are developing.</Paragraph>
    <Paragraph position="11"> Sections 4 and 5 describe techniques for content consistency maintenance and full-text search, respectively. Finally, Section 6 presents the conclusion and future work.</Paragraph>
  </Section>
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