Towards A Content-Based Aggregation Network

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Towards A Content-Based Aggregation Network

• By
• Shagun Kakkar
• May 29, 2002

2
Outline

• Motivation
• Introduction
• Current Approaches
• Problems with Current Approaches
• Overview of our Approach
• Conclusion

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Motivation

• To design a content-based aggregation in
  peer-to-peer file sharing network

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Introduction

• Peer-to-Peer (P2P) networks have the ability of
  file sharing directly between people
• Major attraction of this system is allowing users
  to share content without having an intermediary
  party
• P2P is still a very vague term, covering systems
  such as Napster(centralized system) ,Gnutella and
  Freenet(decentralized system).

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Peer-to-Peer Network

• Overlay network comprised purely of end systems
  without any intermediate systems
• Decentralized approach
• Similar to Freenet, not Napster

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Current Approaches

• Gnutella
• Takes radical approach to decentralization
• Does not use any form of indexing
• Characterized as application-layer networks
• Different from IP routing
• - we address content i.e., files being shared
  between users
• Concept of content-based routing
• - allows to address the real problem of P2P
  system rather than trying to emulate
  IP-style networking

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Contd

• Content-based routing
• the content is exposed to the network transport
  mechanism, to influence the routing of
  messages
• no information other than content is used
• producers generate messages but with no
  particular destinations intended

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Contd

• Freenet
• Use index based on meta data called descriptive
  strings
• In order to retrieve a piece from Freenet
  network, the user must first derive, or know a
  key which matches the file associated with the
  item.
• This request is sent to the querying nodes
  neighbor, which forwards the request to its
  neighbor

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Contd

• In Freenet, knowing or deriving a key is a severe
  weakness as it does not facilitate searching for
  an item of content
• Addresses content-location problem

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Contd

• CAN and Chord
• Address content-location in a peer-to-peer
  environment.
• They are distributed hash tables
• Provide techniques to ensure the operation of
  hash tables when nodes join or leave the network
• Effective in providing a content location service

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Problems with Current Approaches

• Problem with scalability
• Gnutella
• absence of indexing
• Freenet
• no mechanism to query a group which represents a
  sub-set of meta data, e.g., all MP3 files
• does not assist locating an item of content at
  the first place

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Our Approach

• Overview
• To achieve scalability
• - reduce the amount of search requests that
  the system
  sends out in order to locate content
• - concept of aggregation with a hierarchical
  scheme is proposed

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Definitions

• Aggregation point
• In a content-based routing, the aggregation
  point advertises aggregated content to other
  aggregation points in the overlay network
• Helper node
• If a node perform this function, when a new
  node wishes to join the overlay network, the
  helper node can provide information about other
  nodes in the network

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Contd

• When a node wishes to join a network, it contact
  the helper node
• When a node wishes to advertise a file, it should
  consult the helper node for finding existing
  aggregation point of that type
• If the aggregation point is not found, the
  advertising node can become aggregation point

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Contd

• Route Aggregation in Overlay Networks
• Introduce helper node to overcome the problem of
  boot - strapping
• (when a node first comes online, it does not
  necessarily know the addresses of other nodes in
  the network)
• When a node comes online, it attempts to select
  an aggregation group from the history list of
  aggregation points provided by helper node
• Node then makes some policy-based measurements
  e.g., delay latency and based on these makes a
  decision of which group to join
• Enables the system to aggregate on network
  performance

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The hierarchy in an Aggregation Point based
peer-to-peer network
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Contd

• Meta data - Towards Content-based Aggregation
• To enable content-based aggregation
• every shared file is accompanied by a meta data
• these files are encoded in XML and comprise of
  various fields, organized according to their
  order of significance
• these fields used for obtaining different levels
  of aggregation
• the top level of hierarchy, the data format
  (normally a file extension), serves as a first
  classification

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Examples of Meta Data
The meta data examples, one describing an mp3
file with a recording of the Dminor Organ
Toccata and Fuge by J.S Bach and the other an
ASCII file version of D.R. Hofstadters Gödel
Escher Bach
Meta data represented in XML
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Dealing with change in the system

• Meta data is used as the basis of aggregation in
  our network
• For e.g., the first field of meta data is the
  file type
• ltASCIIgt or ltmp3gt
• Threshold is used for determining when an
  aggregation group should be split

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Contd

• Strategy used
• Aggregation point choose to create a new
  aggregation point based upon a lower precedence
  field of meta data
• This strategy used when the number of items of
  content in a group expand beyond threshold

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Conclusion

• Introduced a scheme for content-based
  aggregation, for achieving scalability, without
  relying on centralized resources
• Future work further elaboration of the scheme
  along with implementing the system

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Reference

• R. Gold, D. Tidhar Towards a Content-based
  Aggregation Network. Proceedings of the
  International Conference on Peer-to-Peer
  Computing, Linköping, 2001.