p2p content distribution network

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P2P Content Distribution BitTorrent and Spotify Amir H. Payberah [email protected] Amirkabir University of Technology (Tehran Polytechnic) Amir H. Payberah (Tehran Polytechnic) P2P Content Distribution 1393/8/27 1 / 62

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P2P Content DistributionBitTorrent and Spotify

Amir H. [email protected]

Amirkabir University of Technology(Tehran Polytechnic)

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Possible Solutions forContent Distribution

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Client-Server Model

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Client-Server Model

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The Client-Server Model Problems

I Scalability?

I Single Point of failure?

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The Client-Server Model Problems

I Scalability?

I Single Point of failure?

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The Client-Server Model Problems

I Scalability?

I 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

I Churn in the system

I Free-riding problem

I Bottleneck in the overlay network

I Connectivity problem, e.g., NAT

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How To Discover Data?

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Possible Solutions - First Generation

I Central directory

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Possible Solutions - Second Generation

I Flooding

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Possible Solutions - Third Generation

I 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

I BitTorrent is a system for efficient and scalable replication of largeamounts of static data.

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BitTorrent Players

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Files

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

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.torrent Files

I Metadata

I Contains:• URL of tracker• Information about the file,

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

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The Core Idea

I A peer obtains .torrent file.

I It, then, connects to the tracker.

I The tracker tells the peers fromwhich other peers to download thepieces of the file.

I Peers use this information to communicate with each other.

I The peers send information about the file and themselves to tracker.

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The Core Idea

I A peer obtains .torrent file.

I It, then, connects to the tracker.

I The tracker tells the peers fromwhich other peers to download thepieces of the file.

I Peers use this information to communicate with each other.

I The peers send information about the file and themselves to tracker.

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The Core Idea

I A peer obtains .torrent file.

I It, then, connects to the tracker.

I The tracker tells the peers fromwhich other peers to download thepieces of the file.

I Peers use this information to communicate with each other.

I The peers send information about the file and themselves to tracker.

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The Core Idea

I A peer obtains .torrent file.

I It, then, connects to the tracker.

I The tracker tells the peers fromwhich other peers to download thepieces of the file.

I Peers use this information to communicate with each other.

I The peers send information about the file and themselves to tracker.

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The Core Idea

I A peer obtains .torrent file.

I It, then, connects to the tracker.

I The tracker tells the peers fromwhich other peers to download thepieces of the file.

I Peers use this information to communicate with each other.

I 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

I From which peers download the pieces?

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Peer Selection

I Use choking algorithm to choose peer to download pieces.

I Decision to choke/unchoke based on tit-for-tat.

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Discover More Cooperating Peers

I Optimistic unchoking

I Allocate an upload slot to a randomly chosen uncooperative peer

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Snubbed Peers

I If all its peers choke it.

I Increase the number of optimistic unchokes.

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Question

I Which piece?

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Piece Selection

I Rarest first: common parts left for later

I Random first piece: start-up need to get a complete piece

I Endgame mode: broadcast for all remaining blocks

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Piece Selection

I Rarest first: common parts left for later

I Random first piece: start-up need to get a complete piece

I Endgame mode: broadcast for all remaining blocks

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Piece Selection

I Rarest first: common parts left for later

I Random first piece: start-up need to get a complete piece

I Endgame mode: broadcast for all remaining blocks

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BitTorrent Extension

I Distributed tracker

I Peer-exchange

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Spotify

I Active users: over 50 million

I Number of songs: over 20 million

I Number of songs added per day: over 20000

I Number of playlists: over 1.5 billion created so far

I Available in 58 countries

I Legal

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The Core Idea

I Request first piece from Spotify servers.

I Meanwhile, search P2P network for remainder.

I Switch back and forth between Spotify servers and peers as needed.

I Towards end of a track, start prefetching the next one.

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The Core Idea

I Request first piece from Spotify servers.

I Meanwhile, search P2P network for remainder.

I Switch back and forth between Spotify servers and peers as needed.

I Towards end of a track, start prefetching the next one.

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The Core Idea

I Request first piece from Spotify servers.

I Meanwhile, search P2P network for remainder.

I Switch back and forth between Spotify servers and peers as needed.

I Towards end of a track, start prefetching the next one.

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The Core Idea

I Request first piece from Spotify servers.

I Meanwhile, search P2P network for remainder.

I Switch back and forth between Spotify servers and peers as needed.

I 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

I Sever-side tracker (BitTorrent style)• Only remembers 20 peers per track.• Returns 10 (online) peers to client on query.

I Broadcast query in small (2 hops) neighborhood in overlay (Gnutellastyle)

I LAN peer discovery

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Peer Discovery

I Sever-side tracker (BitTorrent style)• Only remembers 20 peers per track.• Returns 10 (online) peers to client on query.

I Broadcast query in small (2 hops) neighborhood in overlay (Gnutellastyle)

I LAN peer discovery

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Peer Discovery

I Sever-side tracker (BitTorrent style)• Only remembers 20 peers per track.• Returns 10 (online) peers to client on query.

I Broadcast query in small (2 hops) neighborhood in overlay (Gnutellastyle)

I LAN peer discovery

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Downloading in P2P

I Ask for most urgent pieces first.

I If a peer is slow, re-request from new peers.

I When buffers are low, download from central server as well.

I If buffers are very low, stop uploading.

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Downloading in P2P

I Ask for most urgent pieces first.

I If a peer is slow, re-request from new peers.

I When buffers are low, download from central server as well.

I If buffers are very low, stop uploading.

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Downloading in P2P

I Ask for most urgent pieces first.

I If a peer is slow, re-request from new peers.

I When buffers are low, download from central server as well.

I If buffers are very low, stop uploading.

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Downloading in P2P

I Ask for most urgent pieces first.

I If a peer is slow, re-request from new peers.

I When buffers are low, download from central server as well.

I If buffers are very low, stop uploading.

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Spotify vs. BitTorrent

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

I Does not inform peers about downloaded blocks.

I Downloads blocks in order.

I Does not enforce fairness (such as tit-for-tat).

I Informs peers about urgency of request.

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Spotify vs. BitTorrent

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

I Does not inform peers about downloaded blocks.

I Downloads blocks in order.

I Does not enforce fairness (such as tit-for-tat).

I Informs peers about urgency of request.

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Spotify vs. BitTorrent

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

I Does not inform peers about downloaded blocks.

I Downloads blocks in order.

I Does not enforce fairness (such as tit-for-tat).

I Informs peers about urgency of request.

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Spotify vs. BitTorrent

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

I Does not inform peers about downloaded blocks.

I Downloads blocks in order.

I Does not enforce fairness (such as tit-for-tat).

I Informs peers about urgency of request.

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Spotify vs. BitTorrent

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

I Does not inform peers about downloaded blocks.

I Downloads blocks in order.

I Does not enforce fairness (such as tit-for-tat).

I Informs peers about urgency of request.

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Caching

I Player caches tracks it has played.

I Use 10% of free space (capped at 10GB)

I Least Recently Used policy for cache eviction.

I 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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