COMP 416b Internet Protocols and Software

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COMP 416b Internet Protocols and Software

• Instructor Zhijun Wang
• P2P streaming

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BitTorrent (BT)

• Released in the summer of 2001 by Bram Cohen.
• P2P application that capitalizes the resources
  (disk storage bandwidth) for effective large
  files distribution.
• Unlike Kazaa/Gnutella which thrive to quickly
  locate a file, BitTorrents objective is to
  quickly replicate a file to clients.
• Uses basic ideas from game theory to largely
  eliminate the free-rider problem.
• Working extremely well.

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Basic concepts of BT

• Seed A peer that has the entire file.
• Leecher A peer that has incomplete copy of the
  file.
• A .torrent file.
• Passive component
• Lists a SHA1 hash to verify integrity of files
• Typically hosted on a web server
• A Tracker
• Active component
• Allows peers to find each other
• Returns a random list of peers connected

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Swarming Pieces and Sub-pieces

• A piece, typically 256KB is broken into 16KB
  sub-pieces.
• Until a piece is assembled, only sub-pieces for
  that piece is downloaded.
• This ensures that complete pieces assemble
  quickly.
• When transferring data over TCP, it is critical
  to always have several requests pending at once,
  to avoid a delay between pieces being sent.
• At any point in time, some number, typically 5,
  are requested simultaneously.
• On piece completion, notify all peers.

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Pieces selection

• The order of pieces is very important for good
  performance.
• A bad algorithm could result in all peers waiting
  for the same missing piece.
• Random Piece First policy
• Initially a peer had no pieces to trade, thus
  important to get a piece ASAP.
• Policy Peer starts with a random piece to
  download.
• Rarest Piece First policy
• Policy Download the pieces which are most rare
  among your peers.
• Ensures most common pieces are left for last.

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Rarest First Policy
Peer
12,7,36
12,7,14
Peer
. . .
Peer
14
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Topology P2P streaming system

• Tree topology

Server
Peers are arranged as a tree. A peer download the
data from its parent.
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Topology P2P streaming system

• Mesh topology

Server
A peer can download data from its neighbors. No
single point failure.
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IPTV system

• PPLive free P2P-based IPTV
• As of January 2006, the PPLive network provided
  200 channels with 400,000 daily users on
  average.
• The bit rates of video programs mainly range from
  250 Kbps to 400 Kbps with a few channels as high
  as 800 Kbps.
• The video content is mostly feeds from TV
  channels in Mandarin.
• The channels are encoded in two video formats
  Window Media Video (WMV) or Real Video (RMVB).
• The encoded video content is divided into chunks
  and distributed to users through the PPLive P2P
  network.

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IPTV Architecture
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PPlive process
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PPlive process

• Cached contents can be uploaded to other peers
  watching the same channel.
• This peer may also upload cached video chunks to
  multiple peers.
• Received video chunks are reassembled in order
  and buffered in queue of PPLive TV engine,
  forming local streaming file in memory.
• When the streaming file length crosses a
  predefined threshold, the PPLive TV engine
  launches media player, which downloads video
  content from local HTTP streaming server.
• After the buffer of the media player fills up to
  required level, the actual video playback starts.
• When PPLive starts, the PPLive TV engine
  downloads media content from peers aggressively
  to minimize playback start-up delay.
• When the media player receives enough content and
  starts to play the media, streaming process
  gradually stabilizes.
• The PPLive TV engine streams data to the media
  player at media playback rate.

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PPlive measurement

• One residential and one campus PC watched
  channel CCTV3
• The other residential and campus PC watched
  channel CCTV10
• Each of these four traces lasted about 2 hours.
• From the PPLive web site, CCTV3 is a popular
  channel with a 5-star popularity grade and CCTV10
  is less popular with a 3-star popularity grade.

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Session duration

• Signaling versus video sessions
• All sessions are TCP based
• The median video session is about 20 seconds and
  about 10 of video sessions last for over 15
  minutes or more.

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Video traffic breakdown
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Questions??