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Peer to peer

INF 123 – Software Architecturetdebeauv@uci.edu

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Outline

• Some theory• Napster• Gnutella/torrents• Skype• Sensor networks• I won’t talk about– NAT punching/Firewall (ICE, STUN, TURN)– Distributed system algorithms/graph walks

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SOME THEORY

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Peer to peer

• Identical components running independently on different networked hosts

• Similar to distributed systems– The topology can change– Many paths from A to B

• Each peer is client and server to other peers• Each peer provides resources: data, code, CPU,

…

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Peer 1

Peer 4Peer 2

Peer 3

Peer 4Peer 2

Peer 3

now

later

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Pros and cons

• Pros– Scaling: each node provides CPU and storage– Robust if one node fails

• Cons– Complex protocols for resource discovery– Security, trust management

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Examples

• Skype• Sensor networks (house, environment, …)• DNS caching• File sharing

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NAPSTER: HYBRID CS AND P2P

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Napster

• Open source• Resource location– Client-server– Custom protocol on top of TCP

• Resource retrieval– P2P– HTTP GET

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Communication diagram

Peer A Peer B

Content Directory

1) I have Gangnam Style

2) Who has Gangnam Style? 3) A has Gangnam Style

4) Give me Gangnam Style

5) Here is Gangnam Style

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Behind Firewall? Sequence diagram

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Protocol: I have a song

“filename”, md5, size (B), bitrate (kbps), frequency (Hz), time (s)

“C:\music\OMGPSY\gangnam.mp3” 9e107d9d372bb6826bd81d3542a419d6 1234567 128 44100 253

• To class: why md5?

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Protocol: Search query

[FILENAME CONTAINS "artist name"] MAX_RESULTS <max> [FILENAME CONTAINS "song"] [LINESPEED <compare> <link-type>] [BITRATE <compare> "<br>"] [FREQ <compare> "<freq>"] [WMA-FILE] [LOCAL_ONLY]

MAX_RESULTS 100 FILENAME CONTAINS “Gangnam Style” BITRATE “AT LEAST” “128”

MAX_RESULTS 10000 # browse • To class: query for songs from Psy?

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Achilles’ heel

• Server = bottleneck• Swamped with requests for the

location of a popular song– Answer location requests slowly

• Heavy load if the only owner is behind a Firewall– Cap number of simultaneous

transfers

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Achilles’ heel #2

• Server = single point of failure• Without it, the peers are blind and useless• Shutdown mandated by court order

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GNUTELLA: FULL P2P

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Gnutella

• Open source• Resource retrieval– P2P– HTTP GET

• Resource location, peer discovery, …– P2P!– Custom over TCP

• Purely distributed/decentralized search engine• Peer also called servent (server + client)

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Flooding

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Flood prevention

• Every message has a Time to Live flag (TTL)• Decreased by 1 every hop• When 0, don’t forward

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Peer discovery

• Send ping• When receiving ping– Send pong where it came from– And forward the ping to other nodes

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Search query

• Query message payload

• Response: QueryHit message payload

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Query hit

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Seeds

• How do I find peers when I join for the first time?– Aka bootstrapping, seeding

• Need a reliable list of nodes/seeds– Same idea as web page seeds for web crawlers

• Ship the list in the software• Also: IRC, mailing lists, websites with seeds

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SKYPE: OVERLAY P2P

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Skype supernodes

• Promote “strong” peers to supernodes– Based on topology, bandwidth, …– Trade secret

• Directory distributed and replicated in supernodes– Robust and scalable directory

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Privacy and security

• Calls relayed through supernodes– Proprietary protocol– Encrypted for privacy

• Closed-source client– Very hard to inspect the code– Even harder to hack it– Therefore: no malicious peers

http://recon.cx/en/f/vskype-part1.pdfhttp://www1.cs.columbia.edu/~salman/publications/skype1_4.pdf

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SENSOR NETWORKS

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Basic idea

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Constraints

• Each node can– Sense – Compute– Communicate with its neighbors

• Non-functional properties– Low energy consumption– Fault-tolerance– Scalability– Low latency

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Usage

• Home automation• Army communication• Forest fire, tsunami, or tornado detection• Airport weather conditions• …

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London Heathrow Airport

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Base Station (aka sink or gateway)

Analog-Digital Converter

Deploymentarchitecture

System architecture

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Communication: routing

• Flat• Hierarchical (super nodes)• Location-based

• Depends on network topology

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Communication: protocol

• Multipath – Send data through multiple paths– Good fault-tolerance, but high energy cost

• Query-based– The gateway sends a query to the network– A node with the data answers the query

• Quality of Service-based– Balance energy consumption and data quality

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Read more

• https://connect.innovateuk.org/web/network-of-sensors/article-view/-/blogs/advanced-sensor-networks-developed-for-heathrow-airport

• Routing Techniques in Wireless Sensor Networks: A Survey, Al-Karaki et al., 2004