Peer-to-Peer Systems. Internet Computing Workshop Tom Chothia

Transcription

1 Peer-to-Peer Systems Internet Computing Workshop Tom Chothia

2 Plagiarism Reminder Plagiarism is a very serious offense. Never submit work by other people without clearly stating who wrote it. If you did this in a workplace you would be fired, and possibly prosecuted.

3 Getting Help with the Exercises Lab sessions: Get 1-1 help from a tutor. These are not being used: tutors are sitting in the labs waiting to help you. Your tutor at weekly meetings. Marco is running help sessions on Thursday at 12:30pm in Room 245 Reminder: you should be spending 20 hours a week on this module.

4 Today s Lecture Peer to Peer Systems Peer to Peer designs Napster Gnutella Bittorrent Smart Phone Programming Setting up Eclipse for Android.

5 Client Server Client-Server model: Works very well most of the time. But: Puts all the load on one server. If the server fails, so does everything else. Client Client Client Client Client Client Client Client Client

6 Peer to Peer No central server. Each peer acts as both a client and a server. Spreads the load. Many different kinds of P2P.

7 Types of P2P Network: Centralised / De-Centralised Centralized P2P has a coordinator that directs the activity of the peers. Using the coordinator makes starting and searching the network easy. The coordinator may control the network. Single point of failure. E.g. Napster, BBC iplayer (P2P version).

8 Types of P2P Network: Centralised / De-Centralised Decentralized P2P networks do not have a single coordinator. Much harder to set up. No single point of failure Better Privacy E.g. Gnutella

9 Types of P2P Network: Fully Connected Nodes connect to all other nodes. Makes message passing easy. Does not scalable.

10 Types of P2P Network: Not Fully Connected Nodes only connects to a few others. Most common. Scalable. Easy to maintain. Not as efficient.

11 File Sharing P2P Networks Napster: The first big file sharing network. Online between Central coordinator. Gnutella: Most popular file-sharing network in mid 2000s. No central coordinator. Unstructured Network. BitTorrent: Lots of little P2P networks. Single tracker coordinates each download. Rewards up-loaders.

12 Napster I Peer A asks the database for x. F x C x? A,E E H G E D B x? A x? x The database tells A which peer has the file. A downloads the file from that peer. A tells the database it has the file. Other peers can then download the file from A x? I have x

13 Napster User Figures

14 Gnutella First truly decentralised file-sharing network. Developed by Nullsoft in One day after launch Nullsoft parent company (AOL) stop the download. Many clients, including Limewire, Morpheus, BearShare, Limewire shut down by court order on Oct. 26 th 2010.

15 Original Gnutella In Gnutella, peers record the IP address of other peers. A searcher sends a request to all of its neighbours. A This is forwarded to all of there neighbours, up to a fixed number of hops.

16 Original Gnutella The search request includes A s IP address. Any peer with the requested file contacts A directly. A Peer A may then request the file.

17 Ultrapeers (UPs) Searches slow for huge networks. Nodes assigned as: Ultrapeers: 10s of connections to Ups Leaves: ~3 connections to Ups Ultrapeers maintain a Query Routing Table popular Queries can be sent directly to the right node.

18 Dealing with NAT/Firewalls One issue with any P2P networks is that many users are behind NAT or Firewalls. These users cannot accept incoming connections. In most home P2P networks only a small number peers accept incoming connections.

19 BitTorrent: Setting Up Tracker A Web Server T Peer A wants to share a find. A must find a tracker to coordinate the download. A T A makes a torrent file, The name of the file. The hash of the data. The IP address of the tracker. A must put the torrent somewhere people can find it. E.g., on a web server. A tells the tracker is wants to share the file.

20 BitTorrent: Sharing Tracker A,B Web Server T Peer B finds the torrent file. B finds the tracker IP and connects to it. T B A A The tracker: tells B the IP of the file sharers. And adds B address to the list of sharers B connects to A and then gets some pieces of the file.

21 BitTorrent: Sharing Tracker A, B,C Web Server T Peer C connects in the same way. A will try to hand out different copies of the file. T B A A may disconnect. T C A,B T D B,C A C may get the rest of the file from B. D can connect and the sharing continues without A.

22 Monitoring of file sharer Bittorrent is largely used for illegal file sharing. Some firms monitor torrents to find the IP address of the file sharers. Indirect monitoring: ask the tracker Direct monitoring: connect to a peer then record their IP

23 Indirect Monitoring Trackers will give out IP address Monitor repeatedly asks tracker for maximum number of peers Very unreliable Piatek et al. got a take down notice sent to their printer. Rumor that some trackers add random IP addresses.

24 Direct Monitoring Connecting directly to a peer provides some evidence that it really is file sharing. Monitors can be detected and therefore blocked. Ask TA Chris Novakovic, if you want more information.

25 Case Study: BBC s iplayer The BBC wants to provide TV programs via the Web. Streaming all that data costs a lot. Therefore the BBC tried out a peer-to-peer system for downloads. They used a centrally controlled coordinator and digital rights management.

26 Case Study: BBC s iplayer The idea is: If I m only downloading then I cost the BBC money (they pay for bandwidth). If I upload as much as I download then I cost the BBC very little. If I upload more than I download then I am providing a service for the BBC. In theory, a company can save millions.

27 Case Study: BBC s iplayer In fact: iplayer users like to stream TV, not download. The P2P system can t provide the exact information at the exact time to watch live. In December 2008 the BBC changed their download service to direct download via HTTP. Key point: P2P has great potential, but it has it s problems

28 Skype Skype uses private P2P system. Voice over IP system (VoIP). Where possible user connect directly to each other. Allows Skype to have over 500 million users. Skype clients for all major operating systems and smart phones.

29 Why Smart Phone Programming? Smart Phones many peoples main connection to the Internet. Smart phone apps are going to be big business. Good area for your M.Sc. projects.

30 Android vs. iphone Android: Open source smart phone platform from Google et al. Program in Java. Easy to get started. Open App store Most apps free iphone Closed source owned by Apple. Uses GUI tools and Objective C Need a certificate to run code on phones Apple controlled app store People pay for apps.

31 Other phone platforms Blackberry, Very secure, used by business, not much third party support. Windows Mobile Not very popular Symbian, Intelligent System Architecture (Nokia OS), Java ME Not so smart phones

32 Developing for Android Get Eclipse Get Android SDK index.html Install and configure the Eclipse Android plugin