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P2P Peer-to-peer:
“direct” connections between peers Peers are all equal - both a sender and a receiver of a
content
P2P core principle Self-organizing
no central management, peers are completely independent
Large collection of resources Millions of simultaneous users, voluntary participation
Scalability scalability with respect to number of nodes
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P2P principle P2P is an overlay network (of internet)
a virtual network on top of the underlying IP network
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Overlay graph
Overlay network
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Peer-to-Peer (P2P) Systems Old ideas
1979 - USENET news service (still in use) Popular around 1999
Napster, Kazaa and Gnutella for sharing files, music.. ‘01: Skype launched (Kazaa) ‘06’, ’10’: Acquired by eBay,
Microsoft ‘01: BitTorrent launched – heavily used for file and music
sharing Still very popular today for sharing multimedia content
BitTorrent – 30% of internet traffic (mid 2000s) Skype – 663M users (2010), 700 M minutes a day
Problem: Free Riders - only consume, not contribute5
Current State of P2P P2P networks going strong, all over the world
Currently P2P accounts for almost 70% of network traffic
P2P networks currently mostly used for illegal sharing of copyrighted material Music, videos, software, …
Content providers not so happy Sue companies making P2P software (e.g.,
Napster), sue software developers (Winny), sue users sharing material
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P2P Application P2P principle applicable to many kinds of
systems Content distribution
Most current P2P targeted at one application: File sharing Users share files (e.g., music, video, software) and others
download Also often illegally shared (except BitTorrent) Example
BitTorrent, Napster, Gnutella, KaZaA
From Acadamic Chord
Communication Skype
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Napster Napster launched in 1999 by Shawn Fanning
The term “P2P was coined by Napster. In 2000:,25% of traffic out of Uni. of Wisconsin Madison, 60M
users Centralized real-time directory, distributed files, mostly MP3
music;
Based in USA; lawsuits put it out of business RIAA sues Napster, asking $100K per download Indirectly helping users to infringe copyright
Currently, paid service Pay % to songwriters and music companies as copyright required Napster protocol is open, people free to develop
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Napster Connect to Napster server
Upload list of music files that you want to share
Server stores no files
Maintain a list of
<filename, ip_address, portnum>
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Structure
Napster search Send server keywords to search with Server returns a list of hosts – <ip_address, portnum> tuples – to
client Client pings each host in the list to find transfer rates Client fetches file from best host
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Napster Problem Centralized server a source of congestion
Centralized server single point of failure
Napster.com declared to be responsible for users’ copyright violation “Indirect infringement”
Next system: Gnutella
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Gnutella Eliminate the servers Client search and retrieve amongst themselves Clients act as servers too, called servents In 2000, release by AOL, 88K users by ’03’
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How a peer join a network To join the network,
peer needs the address of another peer that is currently a member
New peer sends connect message to existing peer GNUTELLA CONNECT Reply is simply “OK”
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Gnutella search Gnutella routes different messages within the
overlay graph Gnutella protocol has 5 main message types
Query (search) QueryHit (response to query) Ping (to probe network for other peers) Pong (reply to ping, contains address of another
peer) Push (used to initiate file transfer)
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Gnutella Message Header Format
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Flooding query message Query message
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How do search results come back?
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Avoiding excessive traffic To avoid duplicate transmissions, each peer
maintains a list of recently received messages Query forwarded to all neighbors except peer from
which received Each Query (identified by DescriptorID) forwarded
only once QueryHit routed back only to peer from which Query
received with same DescriptorID Duplicates with same DescriptorID and Payload
descriptor (msg type) are dropped QueryHit with DescriptorID for which Query not
seen is dropped18
After receiving QueryHit messages Requestor chooses “best” QueryHit responder
Initiates HTTP request directly to responder’s ip+port
Responder then replies with file packets after this message:
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Dealing with Firewalls Requestor sends Push to responder asking for
file transfer
Responder establishes a TCP connection at ip_address, port specified. Sends
Requestor then sends GET to responder (as before) and file is transferred as explained earlier
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PING-PONG Peers initiate Ping’s periodically Ping’s flooded out like Query’s, Pong’s routed
along reverse path like QueryHit’s Pong replies used to update set of
neighboring peers To keep neighbor lists fresh in spite of peers
joining, leaving and failing
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Problem Flooding a query is extremely inefficient
Wastes lot of network and peer resources Repeated searches with same keywords Solution:
Gnutella’s network management not efficient Periodic PING/PONGs consume lot of resources Ping/Pong constituted 50% traffic
Modem-connected hosts do not have enough bandwidth for passing Gnutella traffic
Another solution: FastTrack System
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FastTrack Hybrid between Gnutella and Napster Takes advantage of “healthier” participants in
the system Underlying technology in Kazaa, KazaaLite,
Grokster Like Gnutella, but with some peers designated
as supernodes
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FastTrack (2) A supernode stores a directory listing a subset of
nearby (<filename,peer pointer>), similar to Napster servers
Supernode membership changes over time Any peer can become (and stay) a supernode,
provided it has earned enough reputation Kazaalite: participation level (=reputation) of a user
between 0 and 1000, initially 10, then affected by length of periods of connectivity and total number of uploads
More sophisticated Reputation schemes invented, especially based on economics (See P2PEcon workshop)
A peer searches by contacting a nearby supernode
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Strength Combines good points from Napster and
Gnutella Efficient searching under each supernode Flooding restricted to supernodes only Result: Efficient searching with “low” resource
usage
Most popular network Lot of content, lot of users Currently most file sharing networks adopted this
architecture
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BitTorrent Developed by Bram Cohen in 2001
Written in Python, available on many platforms
BitTorrent is a new approach for sharing large files distributed directories, distributed files Each file divided as chunks
Each chunk contains 32 KB – 256 KB Each chunks can traverse different paths
BitTorrent widely used also for legal content For example, Linux distributions, software patches, Official
movie Currently lots of illegal content on BitTorrent too…
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Topology of BitTorrent Overlay graph
(1) physical (2) neighboring peer (3) peering relationship
A tracker a server which tracks the currently active clients serves as a centralized directory
Topology can be changed regularly Tracker factors: content, distance, peer churn,
randomization
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BitTorrent: Players Three entities needed to start distribution of a file
Terminology: A “torrent” file: the metadata about the file Seed: Client with a complete copy of the file Leecher: Client still downloading the file
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BitTorrent Start Up New client gets torrent-file and gets peer list
from tracker
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BitTorrent Operation
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Summary of BitTorrent operation A new peer A receives a.torrent file from one of the BitTorrent web
servers, including the name, size, and number of chunks of a particular file, together with the IP address and port number of the corresponding tracker.
It then registers with the right tracker. It will also periodically send keep-alive messages to the tracker.
The tracker sends to peer A a list of potential peers (peer set = 50 peers).
Peer A selects a subset (following the tit-for-tat and randomization rules) and establishes connections with these five peers.
Peer A downloads chunks from peers in peer set and provides them with its own chunks (possible to parallel) Chunks typically 256 KB Starting with the rarest chunks.
Every now and then, each peer updates its peer list.
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Peering construction methods Tracker suggests a set of 50 peers Let new peer picks 5 peers (at this
time!) for exchanging chunks Exchanging contents evenly
between them (Rarely chunk first)
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Peer serves 4 peers in peer set simultaneously (tit-for-tat) Seeks 4 best downloaders in last time slot if it’s a seed Seeks 4 best uploaders in last time slot if it’s a leecher
The fifth peer selected at 50% randomly (randomization)
Choking: Limit number of neighbors to which concurrent uploads or download <= a number
Strength Tit-for-Tat
A peer serves peers that serve it Encourages cooperation, discourage free-riding
Rarely chunk first Prefer early download of blocks that are least
replicated among neighbors Avoid the problem that most of peers have most of the
chuck but all must wait for the few rare chunks
Randomization avoids unfairness of little upload capacity nodes
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Weakness File needs to be quite large
256 KB chunks Rarest first needs large number of chunks
Everyone must contribute Low-bandwidth clients have a disadvantage
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How can BitTorrent be free? It leverages peer uplink capacities to send
chunks of files to each other without deploying many media servers.
P2P is used for sharing content in BitTorrent.
Scalable? Add many nodes as the network scale up without
a bottleneck
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P2P versus client-server architecture Client-server architecture
Each client requests data from the server Not help each other
P2P Peer is both a sender and a receiver of a content each peer helps each other in a distributed
manner Data transmission is distributed Although control plane for signaling is centralized
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Summary Most existing P2P networks built on searching,
however Searching does not scale in same way Either centralized system with all its problems Distributed system with all its problems Hybrid systems cannot guarantee discovery either
Alternatively, use addressing instead of searching Distributed hash tables (DHTs) - efficient searching
and object location in P2P network Example
Chord, CAN, Plaxton, Pastry, Tapestry
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Reference Kangasharju: Peer-to-Peer Networks Brinton, Christopher; Chiang, Mung (2013-06-
10). Networks Life: 20 Questions and Answers
