Towards A Content-Based Aggregation Network

1. Towards A Content-Based Aggregation Network By Shagun Kakkar May 29, 2002 University of Cincinnati

2. Outline • Motivation • Introduction • Current Approaches • Problems with Current Approaches • Overview of our Approach • Conclusion University of Cincinnati

3. Motivation • To design a content-based aggregation in peer-to-peer file sharing network University of Cincinnati

4. 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). University of Cincinnati

5. Peer-to-Peer Network • Overlay network comprised purely of end systems without any intermediate systems • Decentralized approach • Similar to Freenet, not Napster University of Cincinnati

6. 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 University of Cincinnati

7. 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 University of Cincinnati

8. 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 node’s neighbor, which forwards the request to its neighbor University of Cincinnati

9. 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 University of Cincinnati

10. 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 University of Cincinnati

11. 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 University of Cincinnati

12. 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 University of Cincinnati

13. 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 University of Cincinnati

14. 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 University of Cincinnati

15. 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 University of Cincinnati

16. The hierarchy in an Aggregation Point based peer-to-peer network University of Cincinnati

17. 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 University of Cincinnati

18. Examples of Meta Data The meta data examples, one describing an mp3 file with a recording of the D’minor Organ Toccata and Fuge by J.S Bach and the other an ASCII file version of D.R. Hofstadter’s Gödel Escher Bach Meta data represented in XML University of Cincinnati

19. 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 <ASCII> or <mp3> • Threshold is used for determining when an aggregation group should be split University of Cincinnati

20. 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 University of Cincinnati

21. 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 University of Cincinnati

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