P2P Content Distribution BitTorrent and Spotify Amir H. Payberah - - PowerPoint PPT Presentation

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P2P Content Distribution BitTorrent and Spotify Amir H. Payberah - - PowerPoint PPT Presentation

May 27, 2023 242 likes •1.07k views

P2P Content Distribution BitTorrent and Spotify Amir H. Payberah amir@sics.se Amirkabir University of Technology (Tehran Polytechnic) Amir H. Payberah (Tehran Polytechnic) P2P Content Distribution 1393/8/27 1 / 62 Amir H. Payberah (Tehran

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P2P Content Distribution BitTorrent and Spotify

Amir H. Payberah

amir@sics.se

Amirkabir University of Technology (Tehran Polytechnic)

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Possible Solutions for Content Distribution

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Client-Server Model

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Client-Server Model

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The Client-Server Model Problems

◮ Scalability?

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The Client-Server Model Problems

◮ Scalability? ◮ Single Point of failure?

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The Client-Server Model Problems

◮ Scalability? ◮ Single Point of failure?

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Client-Server Systems

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The Client-Server Model Problem

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Scalable and Fault-Tolerant Client-Server Model

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Peer-to-Peer Model

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Peer-to-Peer (P2P) Model

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P2P Challenges

◮ Churn in the system ◮ Free-riding problem ◮ Bottleneck in the overlay network ◮ Connectivity problem, e.g., NAT

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How To Discover Data?

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Possible Solutions - First Generation

◮ Central directory

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Possible Solutions - Second Generation

◮ Flooding

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Possible Solutions - Third Generation

◮ Distributed Hash Table (DHT)

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P2P Content Distribution Applications - File Sharing

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P2P Content Distribution Applications - Media Streaming

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BitTorrent

◮ BitTorrent is a system for efficient and scalable replication of large

amounts of static data.

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BitTorrent Players

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Files

◮ Files are broken into pieces of size between 64KB and 1MB.

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.torrent Files

◮ Metadata ◮ Contains:

  • URL of tracker
  • Information about the file,

e.g., filename, length, ...

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The Core Idea

◮ A peer obtains .torrent file.

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The Core Idea

◮ A peer obtains .torrent file. ◮ It, then, connects to the tracker.

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The Core Idea

◮ A peer obtains .torrent file. ◮ It, then, connects to the tracker. ◮ The tracker tells the peers from

which other peers to download the pieces of the file.

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The Core Idea

◮ A peer obtains .torrent file. ◮ It, then, connects to the tracker. ◮ The tracker tells the peers from

which other peers to download the pieces of the file.

◮ Peers use this information to communicate with each other.

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The Core Idea

◮ A peer obtains .torrent file. ◮ It, then, connects to the tracker. ◮ The tracker tells the peers from

which other peers to download the pieces of the file.

◮ Peers use this information to communicate with each other. ◮ The peers send information about the file and themselves to tracker.

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What About Free Riders?

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Tit-For-Tat

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Question

◮ From which peers download the pieces?

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Peer Selection

◮ Use choking algorithm to choose peer to download pieces. ◮ Decision to choke/unchoke based on tit-for-tat.

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Discover More Cooperating Peers

◮ Optimistic unchoking ◮ Allocate an upload slot to a randomly chosen uncooperative peer

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Snubbed Peers

◮ If all its peers choke it. ◮ Increase the number of optimistic unchokes.

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Question

◮ Which piece?

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Piece Selection

◮ Rarest first: common parts left for later

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Piece Selection

◮ Rarest first: common parts left for later ◮ Random first piece: start-up need to get a complete piece

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Piece Selection

◮ Rarest first: common parts left for later ◮ Random first piece: start-up need to get a complete piece ◮ Endgame mode: broadcast for all remaining blocks

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BitTorrent Extension

◮ Distributed tracker ◮ Peer-exchange

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Spotify

◮ Active users: over 50 million ◮ Number of songs: over 20 million ◮ Number of songs added per day: over 20000 ◮ Number of playlists: over 1.5 billion created so far ◮ Available in 58 countries ◮ Legal

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The Core Idea

◮ Request first piece from Spotify servers.

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The Core Idea

◮ Request first piece from Spotify servers. ◮ Meanwhile, search P2P network for remainder.

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The Core Idea

◮ Request first piece from Spotify servers. ◮ Meanwhile, search P2P network for remainder. ◮ Switch back and forth between Spotify servers and peers as needed.

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The Core Idea

◮ Request first piece from Spotify servers. ◮ Meanwhile, search P2P network for remainder. ◮ Switch back and forth between Spotify servers and peers as needed. ◮ Towards end of a track, start prefetching the next one.

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Main Problem in Using Spotify P2P Network

Peer Discovery

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Peer Discovery

◮ Sever-side tracker (BitTorrent style)

  • Only remembers 20 peers per track.
  • Returns 10 (online) peers to client on query.

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Peer Discovery

◮ Sever-side tracker (BitTorrent style)

  • Only remembers 20 peers per track.
  • Returns 10 (online) peers to client on query.

◮ Broadcast query in small (2 hops) neighborhood in overlay (Gnutella

style)

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Peer Discovery

◮ Sever-side tracker (BitTorrent style)

  • Only remembers 20 peers per track.
  • Returns 10 (online) peers to client on query.

◮ Broadcast query in small (2 hops) neighborhood in overlay (Gnutella

style)

◮ LAN peer discovery

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Downloading in P2P

◮ Ask for most urgent pieces first.

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Downloading in P2P

◮ Ask for most urgent pieces first. ◮ If a peer is slow, re-request from new peers.

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Downloading in P2P

◮ Ask for most urgent pieces first. ◮ If a peer is slow, re-request from new peers. ◮ When buffers are low, download from central server as well.

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Downloading in P2P

◮ Ask for most urgent pieces first. ◮ If a peer is slow, re-request from new peers. ◮ When buffers are low, download from central server as well. ◮ If buffers are very low, stop uploading.

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Spotify vs. BitTorrent

◮ One (well, three) P2P overlay for all tracks (not per-torrent).

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Spotify vs. BitTorrent

◮ One (well, three) P2P overlay for all tracks (not per-torrent). ◮ Does not inform peers about downloaded blocks.

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Spotify vs. BitTorrent

◮ One (well, three) P2P overlay for all tracks (not per-torrent). ◮ Does not inform peers about downloaded blocks. ◮ Downloads blocks in order.

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Spotify vs. BitTorrent

◮ One (well, three) P2P overlay for all tracks (not per-torrent). ◮ Does not inform peers about downloaded blocks. ◮ Downloads blocks in order. ◮ Does not enforce fairness (such as tit-for-tat).

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Spotify vs. BitTorrent

◮ One (well, three) P2P overlay for all tracks (not per-torrent). ◮ Does not inform peers about downloaded blocks. ◮ Downloads blocks in order. ◮ Does not enforce fairness (such as tit-for-tat). ◮ Informs peers about urgency of request.

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Caching

◮ Player caches tracks it has played. ◮ Use 10% of free space (capped at 10GB) ◮ Least Recently Used policy for cache eviction. ◮ Over 50% of data comes from local cache.

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Spotify Data Usage

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Spotify Says Goodbye to P2P

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Summary

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Summary

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Questions?

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