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Distributed Systems > Chapter 12: P2P Applications > Flashcards

Chapter 12: P2P Applications Flashcards

1
Q

Spotify.com, 2004

A
  • Commercially deployed system, KTU start-up (Sweden)
  • Peer-assisted on-demand music streaming
  • Legal and licensed content only
  • Large catalogue of music (over 15 million tracks)
  • Available in U.S. & 7 European countries, over 10 million users, 1.6 million subscribers
  • Fast (median playback latency of 265 ms)
  • Proprietary client software for desktop & phone (not p2p)
  • Business model: Ad-funded and free & monthly subscription, no ads, premium content, higher quality streaming
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2
Q

Overview of Spotify Protocol

A
  • Proprietary protocol
  • Designed for on-demand music streaming (share to peers)
  • Only Spotify can add tracks
  • 96–320 kbps audio streams (most are Ogg Vorbis q5, 160 kbps)
  • Relatively simple and straightforward design
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3
Q

Why a peer-to-peer protocol? Spotify

A
  • Improve scalability of service
  • Decrease load on servers and network resources
  • Explicit design goal:
    • Use of peer-to-peer should not decrease overall performance (i.e., playback latency & stutter)
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4
Q

Spotify P2P overlay structure

A
  • Nodes have fixed maximum degree (60)
  • Neighbour eviction by heuristic evaluation of utility
  • ( multi objective [Pound , Upload ( opacity] ) ks Utility function
  • Limited overlay routing (search requests)
  • Looks for and connects to new peers when streaming new track
  • Overlay becomes (weakly) clustered by interest
  • Client only downloads data user needs
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5
Q

Finding peers Spotify

A
  • Sever-side tracker (cf. BitTorrent)
    • Only remembers 20 peers per track
    • Returns 10 (online) peers to client on query
  • Clients broadcast query in small (2 hops) neighbourhood in overlay (cf. Gnutella)
  • Client uses both mechanisms for every track
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6
Q

Spotify Protocol

A
  • (Almost) everything encrypted
  • (Almost) everything over TCP
  • Persistent connection to server while logged in
  • Multiplex messages over a single TCP connection
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7
Q

Spotify Caches

A
  • Client (player) caches tracks it has played
  • Default policy is to use 10% of free space (capped at 10 GB)
  • Caches are often larger (56% are over 5 GB)
  • Least Recently Used policy for cache eviction hk
  • Over 50% of data comes from local cache
  • Cached files are served in peer-to-peer overlay (if track completely downloaded)
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8
Q

Spotify - Streaming a Track

A
  • Tracks are decomposed into 16 kB chunks
  • Request first piece of track from Spotify servers
  • Meanwhile, search for peers that cache track
  • Download data in-order (chunk by chunk via TCP)
  • Ask for most urgent pieces first
  • Towards end of a track, start prefetching next track
  • If a peer is slow, re-request data from new peers
  • If local buffer is sufficiently filled, only download from peer-to-peer overlay play
  • If buffer is getting low, stop uploading
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9
Q

Spotify Security through obscurity

A
  • Music data lies encrypted in caches
  • Client must be able to access music data
  • Reverse engineers should not be able to access music data
  • Details are secret and client code is obfuscated
  • Security through obscurity is a bad idea
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10
Q

Spotify Key points

A
  • Simplicity of architecture, protocol, design
  • Peer-assisted, i.e., rely on centralized server
  • Use of peer-to-peer techniques for scalability and avoid heavy, over-provisioned infrastructure
  • Use of centralized tracker
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11
Q

BitTorrent High Level

A
  • Pull-based, swarming approach
    • Each file is split into smaller pieces (& sub-pieces)
    • Peers request desired pieces from neighboring peers
    • Pieces are not downloaded in sequential order
  • Encourages contribution by all peers
  • Based on a tit-for-tat model
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12
Q

BitTorrent use cases

A
  • File-sharing
  • What uses does BitTorrent support?
    • Downloading (licensed only ) movies, music, etc.
  • Update distribution among servers at Facebook et al.
  • Distribution of updates and releases (e.g., World of War.)
  • •…
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13
Q

BitTorrent Swarm

A
  • Swarm
    • Set of peers downloading the same file
    • Organized as a randomly connected mesh of peers
  • Each peer knows list of pieces downloaded by neighboring peers
  • Peer requests pieces it does not own from neighbors
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14
Q

BitTorrent terminology

A
  • Seed: Peer with the entire file
    • Original Seed: First seed for a file
  • Leech: Peer downloading the file
    • Leech becomes a seed, once file downloaded, if the peer stays online & continues by protocol
  • Sub-piece: Further subdivision of a piece
    • “Unit for requests” is a sub-piece (16 kB)
    • Peer uploads piece only after assembling it completely
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15
Q

How a node enters a swarm for file “popeye.mp4” Bitorrent

A
  • File popeye.mp4.torrent hosted at a (well- known) web server
  • The .torrent has address of tracker for file
  • The tracker, which runs on a web server as well, keeps track of all peers downloading file
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16
Q

Contents of .torrent file

A
  • URL of tracker
  • Piece length – Usually 256 KB
  • SHA-1 hashes of each piece in file - Why?
  • Hash helps to check if file downloaded is the right one
17
Q

Bitorrent 3 Phases

A
  • Beginning: First piece
    • First, pick a random piece (random first policy)
      • When peer starts to download, request random piece
      • When first complete piece assembled, switch to rarest- first
      • Get first piece to quickly participate in swarm with upload
      • Rare pieces are only available at few peers (slows
  • Middle: Piece 1 to n-1
    • Strict priority policy
      • Always complete download of piece (sub-pieces) before starting a new piece
      • Getting a complete piece as quickly as possible
  • End-game: Piece n
    • When requests sent for all sub-pieces of last

piece, re-send requests to all peers

* **Upon** **download** of **entire** **piece**, **cancel** **request** for downloading **sub**-**pieces** **from** **other** **peers**
* **Speeds** **up** **completion** of download, **otherwise last piece could delay completion of download**
18
Q

Tit-for-tat as incentive to upload ​Bitorrent

A
  • Want to encourage all peers to contribute
  • Peer A said to choke peer B if A decides not to upload to B
  • Each peer (say A) unchokes at most 4 interested peers at any time
    • The three with the largest download rates from A
      • Where the tit-for-tat comes in
    • Another randomly chosen (optimistic unchoke)
      • Periodically discover better choices
19
Q

Why BitTorrent took off

A
  • Working implementation (Bram Cohen) with simple well- defined interfaces for publishing content
  • Open specification
  • Many competitors got sued & shut down (Napster, KaZaA)
  • Simple approach
    • Doesn’t do “search” per se
    • Users use well-known, trusted sources to locate content
20
Q

Pros & cons of BitTorrent

A

Pros:

  • Proficient in utilizing partially downloaded files
  • Discourages“freeloading”
    • By rewarding fastest uploaders
  • Encourages diversity through “rarest-first”
    • Extends lifetime of swarm
  • Works well for “hot content”

​Cons:

  • Assumes all interested peers active at same time
  • Performance deteriorates if swarm “cools off”
  • Too much overhead to disseminate small files
21
Q

Spotify vs. BitTorrent

A

Spotify:

  • Live listening of streamed track
  • One peer-to-peer overlay for all tracks
  • Does not inform peers about downloaded blocks
  • Downloads blocks in order
  • Does not enforce fairness
  • Informs peers about urgency of request
  • Supports search

Bitorrent:

  • Batch download of large files
  • Essentially, a swarm (overlay) per torrent
  • Exchange of downloaded blocks with peers
  • Random download order
  • Tit-for-tat (game theoretic roots)
  • No notion of urgency
  • No search feature
22
Q

Skype network

A
  • Super Nodes: Any node with a public IP address having sufficient CPU, memory and network bandwidth is candidate to become a super node C know who is online)
  • Ordinary Host: Host needs to connect to super node and must register itself with the Skype login server
  • Login server and PSTN gateway (not shown) are only centralized components
23
Q

Impact of Skype

A
  • Skype has shown, at least has suggested, the following
    • Signaling, the most unique property of traditional phone systems, can now be accomplished effortlessly with self-organizing P2P networks
    • P2P overlay networks can scale up to handle large- scale connection-oriented real-time services such as voice
24
Q

Common principles

A
  • Peer-assisted, hybrid architecture
  • Use of tracker, as directory (centralized)
  • Peer-to-peer capability to limit infrastructure investments & achieve scalability
  • Use of “heavier” TCP, due to inherent guarantees
  • Simplicity of design, as simple as possible

Distributed Systems (21 decks)

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