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3.5 CH2-3

P2P Networks

P2P目的：解决伸缩性/鲁棒性问题，分享个人clients的资源（storage and bandwidth）

P2P systems

  • Games:Xbox
  • File Sharing:BitTorrent
  • Streaming:Octoshape,Adobe 10.1 later,PPLive

The lookup problem:find where a particular file is stored

P2P phases

Phase 1

Centralized Database:Napster

Application-level, client-server protocol over TCP A centralized index system that maps files (songs) to machines that are alive and with files Steps: ◼ Connect to Napster server ◼ Upload your list of files (push) to server ◼ Give server keywords to search the full list ◼ Select “best” of hosts with answers

Phase 2

Decentralized Flooding:Gnutella

Controlled Flooding:ER(还有DQ，DQ+，不用了解)

Phase 3

DHT(DIstributed hash tables),应用广泛

Overview

Chord:Storage using a ring

Extreme:

  • Routing tables large O(N),Lookups fast O(1)
  • Or Routing tables small O(1),Lookups slow O(N)

Solutions:finger tables(例题见PPT)（有其他的DHT算法）

Chord summary

DHT summary

Phase 4

BitTorrent:A P2P file sharing protocol（详见PPT）

洪流（Torrent）:参与一个特定文件分发的所有对等方的集合

最稀缺优先/疏通（种子站）