Comparison of BitTorrent with Traditional Content Distribution - - PowerPoint PPT Presentation

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Comparison of BitTorrent with Traditional Content Distribution - - PowerPoint PPT Presentation

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Comparison of BitTorrent with Traditional Content Distribution Networks (CDNs) ENSC 835 High Performance Networks Catherine Chan Sean Puttergill Roadmap Introduction & Motivation Overview of Related Work Technical

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Comparison of BitTorrent with Traditional Content Distribution Networks (CDNs)

ENSC 835 – High Performance Networks Catherine Chan Sean Puttergill

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Roadmap

  • Introduction & Motivation
  • Overview of Related Work
  • Technical Details
  • Implementation & Simulation in ns-2
  • References
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Motivation

  • Traditional CDN

– client-server based – fixed infrastructure – network of HTTP and/or FTP mirror sites

– server to download from selected by locality

– all upload cost placed on server – does not scale – not free!

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Introduction

  • BitTorrent

– peer-to-peer (P2P) – ad-hoc – sophisticated protocol with numerous optimisations to increase efficiency – fairness - downloaders required to upload – balance upload & download rate through choking

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Overview of Related Work

  • “Traditional” P2P networks

– Gnutella, Kazaa/FastTrack, etc.

  • Konspire-2b

– broadcast, random first, copy and forward

  • Logistical backbone (L-bone)

– running on Internet2 – developed at University of Tennessee – file stored in logistical network depots (L-bone) – XML encoded metadata file (exNode) maps segments

  • f file to L-bone storage locations
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Technical Details

  • .torrent metafile

– contains file info & tracker URL – served from regular Web server

  • Tracker node

– maintains list of peers & gathers statistics – returns random subset of peers when queried

  • Seed node

– starts off with complete file

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foo.mpg.torrent Seed Node Tracker Node Peer A Web Server Peer B Peer C

http://www.bar.com/foo.mpg.torrent

Peer D Peer E Peer F foo.mpg List of Peer IPs

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foo.mpg.torrent Seed Node Tracker Node Peer A Web Server Peer B Peer C

Tracker replies with list of peers = {C,E,F}

Peer D Peer E Peer F foo.mpg

peer introduces itself

List of Peer IPs

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foo.mpg.torrent Seed Node Tracker Node Peer A Web Server Peer B Peer C Peer D Peer E Peer F foo.mpg List of Peer IPs

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Protocol Highlights

  • File fragmentation

– content file split into fixed sized pieces – pieces further subdivided into sub-pieces – SHA1 hash checks data integrity of pieces

  • Pipelining

– peer keeps multiple sub-piece requests pending – offsets overhead of TCP slow-start

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Piece Selection

  • Peers advertise the set of pieces they have
  • How to choose which piece to get next?
  • Strict priority

– always finish a piece before seeking another

– rapidly obtain complete pieces to share with others

  • Rarest First

– choose rare pieces amongst group of peers – helps ensure peer has pieces others want – rapidly disseminates pieces from seed node

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Piece Selection continued

  • Rarest First exception

– randomly choose first piece – get a piece quickly so node has something to share

  • Endgame Mode

– once all remaining sub-pieces are actively being requested then send requests to all peers – send cancels as sub-pieces come in – prevents a very slow peer with remaining sub- pieces from delaying completion of download

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Choking

  • Enforces balance of upload & download
  • Temporary refusal to upload to another peer
  • Peers maximise own download rate
  • Uses tit-for-tat

– peers reciprocate uploading to peers who they themselves have successfully downloaded from

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Choking continued

  • Peer starts out with all peers choked
  • Decides which subset of peers to unchoke
  • Choking decisions made every 10s

– avoids thrashing

  • Decisions based on estimated download rate

– uses 20s rolling average

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Optimistic Unchoking

  • attempts to find better peers
  • periodically replace one unchoked peer with

a new peer chosen regardless of download rate history

  • optimistic unchoke round robins amongst

choked peers

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Peer A Peer B Peer C Peer D Peer E Peer F Peer G Peer H Peer I Peer J Peer K Peer L

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Peer A Peer B Peer C Peer D Peer E Peer F Peer G Peer H Peer I Peer J Peer K Peer L

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Anti-snubbing

  • Occasionally peer will be choked by all

peers it was previously downloading from

  • If after 1 minute no new pieces obtained

then assume snubbed by peer

  • When snubbed, stop uploading to peer
  • Instead do an additional optimistic unchoke
  • Results in faster restoration of download rate
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Implementation & Simulation

  • Simulate using ns-2 (v2.26)
  • Model a traditional CDN
  • Model a BitTorrent network

– implement piece selection and choking – abstract other protocol details

  • Simulate downloading of single fixed size file
  • Metrics

– bandwidth efficiency – elapsed time until all peers complete download

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Network Topology

Peer I Peer H Peer G Peer F Peer E Peer D Peer A Peer B Peer C ISP A ISP B ISP C

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Traditional CDN

  • Place CDN server in each AS
  • Assume file already replicated on all CDN

servers

  • Peers download file from closest server
  • Download via HTTP
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BitTorrent

  • Add Seed Node & Tracker Node to network
  • Peers require multiple TCP connections so

follow GnutApp example

  • Peers remain connected for a random

amount of time after their download completes

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References

1. Bram Cohen, "BitTorrent: Protocol Specification", November 10, 2003. http://www.bitconjurer.org/BitTorrent/protocol.html 2. Jason Rohrer, "Konspire2b: a revolution in mass-scale content distribution", November 10, 2003. http://konspire.sourceforge.net 3. Sir Lancelot, "Bit Torrent Head Quarters", October 23, 2003. http://www.cen.uiuc.edu/~halm/index.html 4. Unknown Author for Webpage, "Wiki theory.org - Bit Torrent FAQ", November 8, 2003. http://wiki.theory.org/index.php/BitTorrentFAQ 5. Sameer G, "A Brand New File Sharing App", November 10, 2003. http://www.rediff.com/netguide/2003/jun/19konspire.htm 6. Guillaume Pierre, "Globule: an Open-Source Content Distribution Network", November 6, 2003. http://www.globule.org

7. A Bassi, M Beck, T Moore, J S Plank, Logistical Computing & Internetworking Lab, “The Logistical Backbone: Scalable Infrastructure for Global Data Grids”, December 2002 http://loci.cs.utk.edu/modules.php?name=Content&pa=showpage&pid=2_4