Peer-to-Peer Networks
Hongli Luo
CEIT, IPFW
 Topics
 Application architecture
 P2P file sharing
 P2P networks:
•
•
•
•
Napster
Gnutella
KaAzA
Bittorrent
Application architectures
 Client-server
 Peer-to-peer (P2P)
 Hybrid of client-server and P2P
Client-server architecture
server:
 always-on host
 permanent IP address
 server farms for scaling
clients:


client/server


communicate with server
may be intermittently
connected
may have dynamic IP
addresses
do not communicate directly
with each other
Client-server architecture
 Applications based on client-server architecture are
infrastructure intensive.
 Service provider must pay connection and bandwidth
costs for transmission of data over the Internet
 Applications





Search engines – Google
Web-based e-mail – Yahoo Mail
Social networking – MySpace and Facebook
Video sharing – YouTube
Amazon? - use cloud computing
 Server farm
 Used when a single server is incapable of keeping up with all
the requests from clients
 A cluster of hosts is used to create a powerful virtual server
P2P architecture

no always-on server
 arbitrary end systems directly
communicate
 peers are intermittently
connected and change IP
addresses
 Peers are not owned by
service provider
Highly scalable but difficult to
manage
peer-peer
P2P architecture
 Applications
 File distribution – BitTorrent
 File search/sharing – eMule and LimeWire
 Internet telephony – Skype
 IPTV – PPLive
 Advantages
 Self-scalability
 Cost effective – requires no significant server infrastructure
and server bandwidth
 Disadvantage
 Difficult to manage
 security
Hybrid of client-server and P2P
Skype
voice-over-IP P2P application
 centralized server: finding address of remote party:
 client-client connection: direct (not through server)
Instant messaging
 chatting between two users is P2P

• User-to-user messages are sent directly between user hosts
without passing through intermediate servers

centralized service: client presence detection/location
• user registers its IP address with central server when it
comes online
• user contacts central server to find IP addresses of
buddies
• Servers are used to track IP addresses of USERS
Processes communicating
Process: program running within a host.
 within same host, two processes communicate using interprocess communication (defined by OS).
 processes in different hosts communicate by exchanging
messages
P2P: applications with P2P architectures have client processes &
server processes
 In P2P file sharing, peer downloading the file is client , the
peer uploading the file is server.
 Client process: process that initiates communication
 Server process: process that waits to be contacted
P2P file sharing
 Problem: How to distribute a large file from a single
server to a large number of hosts?
 Solutions

Client-server file distribution
• the server send a copy of the file to each of the hosts
• Burden on the server and server bandwidth

P2P file distribution
• each peer can redistribute any portion of the file it has received
to any other peers
• The most popular protocol BitTorrent – consists of 30% of the
Internet backbone traffic
Server Distributing a Large File
 Assumptions
 Internet core has abundant bandwidth
 All of the bottlenecks are in network access
 The server and clients are not participating in any other
network applications

Distribution time D – the time it takes to get a copy of
the file to all N peers.
Server Distributing a Large File
 Server sending a large file to N receivers
 Large file with F bits
 Single server with upload rate us
 Download rate di for receiver i
 Server transmission to N receivers
 Server needs to transmit NF bits
 Takes at least NF/us time
 Receiving the data
 Slowest receiver receives at rate dmin= mini{di}
 Takes at least F/dmin time
 Distribution time for client-server
Dcs >= max{NF/us, F/dmin}
Speeding Up the File Distribution
 Distribution time increases linearly with the number of
peers N.
 Increase the upload rate from the server



Higher link bandwidth at the one server
Multiple servers, each with their own link
Requires deploying more infrastructure
 Alternative: have the receivers help
 Receivers get a copy of the data
 And then redistribute the data to other receivers
 To reduce the burden on the server
Peers Help Distributing a Large File
F bits
d4
u4
upload rate us
Internet
d3
d1
u1
Upload rates ui
Download rates di
d2
u2
u3
Peers Help Distributing a Large File
 Start with a single copy of a large file
 Large file with F bits and server upload rate us
 Peer i with download rate di and upload rate ui
 Two components of distribution latency
 Server must send each bit: min time F/us
 Slowest peer receives each bit: min time F/dmin
 Total upload time using all upload resources
 Total number of bits: NF
 Total upload bandwidth us + sumi(ui)
 Distribution time for peer-2-peer
Dp2p>= max{F/us, F/dmin, NF/(us+sumi(ui))}
Comparing Client-server, P2P architectures
assumes that all peers have the same upload rate u
F/u =1 hour, us = 10u, dmin>=us
The minimum distribution time is less than 1 hour for any number of peers N.
Minimum Distribution Time
3.5
P2P
Client-Server
3
2.5
2
1.5
1
0.5
0
0
5
10
20
15
N
25
30
35
Comparing the Two Models
 Download time
 Client-server: Dcs = max{NF/us, F/dmin}
 Peer-to-peer:
Dp2p = max{F/us, F/dmin, NF/(us+sumi(ui))}
assume each peer redistribute a bit as soon as it receives
the bit
 Peer-to-peer is self-scaling
 Much lower demands on server bandwidth
 Distribution time grows only slowly with N
 But…
 Peers may come and go
 Peers need to find each other
 Peers need to be willing to help each other
P2P file sharing
Example
 Alice runs P2P client
application on her notebook
computer
 intermittently connects to
Internet; gets new IP
address for each connection
 asks for “Hey Jude”
 application displays other
peers that have copy of Hey
Jude.
 Alice chooses one of the
peers, Bob.
 file is copied from Bob’s PC
to Alice’s notebook
 while Alice downloads, other
users uploading from Alice.
 Alice’s peer is both a client
and a transient server.
All peers are servers = highly
scalable!
Challenges of Peer-to-Peer
 Peers come and go
 Peers are intermittently connected
 May come and go at any time
 Or come back with a different IP address
 How to locate the relevant peers?
 Peers that are online right now
 Peers that have the content you want
 How to motivate peers to stay in system?
 Why not leave as soon as download ends?
 Why bother uploading content to anyone else?
Locating the Relevant Peers
 Mapping of information to host locations
 File-sharing system: file tracking, map file to IP of the peer
 Instant messaging : presence tracking, map username to IP
 Three main approaches for indexing and searching
files



Central directory (Napster)
Query flooding (Gnutella)
Hierarchical overlay (Kazaa, modern Gnutella)
 Design goals
 Scalability
 Simplicity
 Robustness
 Plausible deniability
Peer-to-Peer Networks: Napster
 Napster history: the rise
 January 1999: Napster version 1.0
 May 1999: company founded
 December 1999: first lawsuits
 2000: 80 million users
 Napster history: the fall
 Mid 2001: out of business due to lawsuits
 Mid 2001: dozens of P2P alternatives that were harder to
touch, though these have gradually been constrained
 2003: growth of pay services like iTunes
 Napster history: the resurrection
 2003: Napster name/logo reconstituted as a pay service
P2P: centralized directory
original “Napster” design
1) when peer connects, it informs
central server:


Bob
centralized
directory server
1
IP address
content
peers
1
2) Alice queries for “Hey Jude”
3) Alice requests file from Bob
3
1
2
1
Alice
Napster Technology: Directory Service
 User installing the software
 Download the client program
 Register name, password, local directory, etc.
 Client contacts Napster (via TCP)
 Provides a list of music files it will share
 Napster’s central server updates the directory
 Client searches on a title or performer
 Napster identifies online clients with the file
 and provides IP addresses
 Client requests the file from the chosen supplier
 Supplier transmits the file to the client
 Both client and supplier report status to Napster
Napster Technology: Properties
 Server’s directory continually updated
 Always know what music is currently available
 Point of vulnerability for legal action
 Peer-to-peer file transfer
 No load on the server
 Plausible deniability for legal action (but not enough)
 Proprietary protocol
 Login, search, upload, download, and status operations
 No security: cleartext passwords and other vulnerability
 Bandwidth issues
 Suppliers ranked by apparent bandwidth & response time
Napster: Limitations with Centralized Directory
 single point of failure
 performance bottleneck
 copyright infringement: “target”
of lawsuit is obvious
 Later P2P systems were more
distributed

Gnutella went to the other
extreme
file transfer is decentralized,
but locating content is
highly centralized
Peer-to-Peer Networks: Gnutella
 Gnutella history
 2000: J. Frankel &
T. Pepper released
Gnutella
 Soon after: many
other clients (e.g.,
Limewire)
 2001: protocol
enhancements, e.g.,
“ultrapeers”
 Query flooding
 Join: contact a few nodes
to become neighbors
 Search: ask neighbors,
who ask their neighbors
 Fetch: get file directly from
another node
Gnutella: Query Flooding
 Fully distributed
 No central server
 Many Gnutella clients
implementing protocol
 Peers form an abstract,
logical network, called
overlay network
Overlay network: graph
 Edge between peer X
and Y if there’s a TCP
connection
 All active peers and
edges is overlay
network
 An edge is not a
physical communication
link, but an abstract link
 Given peer will typically
be connected with < 10
overlay neighbors
Gnutella: Protocol
File transfer:
HTTP
 Query message sent
over existing TCP
connections
 Peers forward
Query message
 QueryHit
sent over
reverse
path
Query
QueryHit
Query
QueryHit
Scalability:
limited scope
flooding
Gnutella: limited-scope query flooding
 When an initial query message is sent out, a
peer-count field in the message is set to a
specific limit, e.g. 7.
 Each time the query message reaches a new
peer,

the peer decrements the peer-count filed before
forwarding the query to its neighbors
 A peer stops forwarding a query if the peer-
count field equals to zero
 It is possible that a peer may not be able to
locate the content
Gnutella: Peer Joining
 Joining peer X must find some other peers
 Start with a list of candidate peers
 X sequentially attempts TCP connections with candidate
peers on list until connection setup with Y
 Flooding: X sends Ping message to Y
 Ping message includes a peer-count field
 Y forwards Ping message to his overlay neighbors until the
peer-count field is zero
 All peers receiving Ping message respond with Pong
message
 The pong message includes peer’s IP address.
 X receives many Pong messages
 X can then set up additional TCP connections with the peers
Gnutella: Pros and Cons
 Advantages
 Fully decentralized
 Search cost distributed
 Disadvantages
 Search scope may be quite large
 Search time may be quite long
 High overhead, and nodes come and go often
 Employed by the popular P2P client Limewire
Peer-to-Peer Networks: KaAzA
 KaZaA history
 2001: created by Dutch company (Kazaa BV)
 First used by FastTrack, a P2P file-sharing protocol
which is used by other clients as well, including
Kazaa and Morpheus
Hierarchical Overlay
 between centralized index, query
flooding approaches
 each peer is either a group leader
(super peer) or assigned to a group
leader as an ordinary peer.


TCP connection between peer and its
group leader.
TCP connections between some
pairs of group leaders.
 Super peer – higher bandwidth,
higher availabilities, greater
responsibilities
 A super peer may have a few
hundred ordinary peer as children.
 group leader tracks content in its
children
ordinary peer
group-leader peer
neighoring relationships
in overlay network
Peer-to-Peer Networks: KaAzA
 Smart query flooding
 Join: on start, the client contacts a super-node (and may
later become one)
 Publish: client sends list of files to its super-node
 Search:
• send query to super-node,
• the super-nodes flood queries among themselves
• Limited-scope flooding in the overlay networks of super
peers

Fetch:
• get file directly from peer(s);
• can fetch from multiple peers at once
P2P Case Study: BitTorrent (1)
 BitTorrent history and motivation
 2002: B. Cohen debuted BitTorrent
 Key motivation: popular content
 Focused on efficient fetching, not searching
• Distribute same file to many peers
• Single publisher, many downloaders
accounted for roughly 27% to 55% of all Internet
traffic (depending on geographical location) as of
February 2009. (Wiki)
 Preventing free-riding
 The collection of all peers participating in the
distribution of a particular file is called a torrent

BitTorrent: Simultaneous Downloading
 Divide large file into many pieces
 Replicate different pieces on different peers
 A peer with a complete piece can trade with other
peers
 Peer can (hopefully) assemble the entire file
 Allows simultaneous downloading
 Retrieving different parts of the file from different
peers at the same time
 And uploading parts of the file to peers
 Important for very large files
P2P Case Study: BitTorrent (1)
 P2P file distribution
tracker: tracks peers
participating in torrent
obtain list
of peers
trading
chunks
peer
torrent: group of
peers exchanging
chunks of a file
BitTorrent: Tracker
 Infrastructure node
 Keeps track of peers participating in the torrent
 Peers register with the tracker
 Peer registers when it arrives
 Peer periodically informs tracker it is still there
 Tracker selects peers for downloading
 Returns a random set of peers
 Including their IP addresses
 So the new peer knows who to contact for data
 Peer connects to subset of peers (“neighbors”)
BitTorrent: Chunks
 Large file divided into smaller pieces
 Fixed-sized chunks
 Typical chunk size of 256 Kbytes
 When a peer joins the torrent, it has no chunks, but
will accumulate them over time
 Allows simultaneous transfers


Downloading chunks from different neighbors
Uploading chunks to other neighbors
 Learning what chunks your neighbors have
 Periodically asking them for a list
 File done when all chunks are downloaded
 Once peer has entire file, it may (selfishly) leave or
(altruistically) remain as a seed
BitTorrent: Pulling Chunks
 at any given time, different peers have
different subsets of file chunks
 periodically, a peer (Alice) asks each
neighbor for list of chunks that they have.
 Alice issues requests for her missing
chunks
 Rarest first
BitTorrent: Chunk Request Order
 Which chunks to request?
 Could download in order
 Like an HTTP client does
 Problem: many peers have the early chunks
 Peers have little to share with each other
 Limiting the scalability of the system
 Problem: eventually nobody has rare chunks
 E.g., the chunks need the end of the file
 Limiting the ability to complete a download
 Solutions: random selection and rarest first
BitTorrent: Rarest Chunk First
 Which chunks to request first?
 The chunk with the fewest available copies
 I.e., the rarest chunk first
 Benefits to the peer
 Avoid starvation when some peers depart
 Benefits to the system
 Avoid starvation across all peers wanting a file
 Balance load by equalizing number of copies of
chunks
Free-Riding Problem in P2P Networks
 Vast majority of users are free-riders
 Most share no files and answer no queries
 Others limit number of connections or upload
speed
 A few “peers” essentially act as servers
 A few individuals contributing to the public good
 Making them hubs that basically act as a server
 BitTorrent prevent free riding
 Allow the fastest peers to download from you
 Occasionally let some free riders download
Bit-Torrent: Preventing Free-Riding
 Peer has limited upload bandwidth
 And must share it among multiple peers
 Prioritizing the upload bandwidth:
 gives priority to the neighbors that are currently supplying
her data at the highest rate.
 Rewarding the top four neighbors
 continuously measures the rate at which it receives the bits
 Reciprocates by sending to the top four peers
 Recompute and reallocate top four peers every 10 seconds
 Optimistic unchoking
 Randomly try a new neighbor every 30 seconds
 So new neighbor has a chance to be a better partner
 newly chosen peer may join top 4