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Peer-to-Peer Intro
5.4.2005
Jani & Sami Peltotalo
Overview of P2P
• Overlay networks
• Current P2P applications
– P2P file sharing
– Instant messaging / voice over IP
– P2P distributed computing
P2P Architectures
1G P2P: Centralized Network
• Fast search/query response times
• Simple Protocol
• Provides a high degree of
performance and resilience
• Susceptible to being shutdown,
single server or server farm
• e.g. Napster
2G P2P: Decentralized Network
• Slow search/query response times
which generates large volumes of
network traffic
• Network resilience and
performance governed by users' PCs
and their network connectivity
• No central points of failure or
control
• e.g. Gnutella 0.4
3G P2P: Hybrid Architecture
• Improved search/query
response times, with less
traffic generated per
query than decentralized
networks
• The deployment of
super-peers provides a
high degree of
performance and
resilience
• No central points of
failure or control
• e.g. FastTrack, Gnutella
0.6
super-peers
4G P2P: Different type of
architectures
• BitTorrent: centralized
• eDonkey2000: semi-centralized
• Overnet: decentralized
FastTrack Overview
FastTrack
• Clients: KaZaA, iMesh, Grokster...
• MP3s & entire albums, videos, games
• Decentralized network, supernodes act as
temporary indexing servers (hierarchical
architecture)
• Control data encrypted
• Everything in HTTP request and response
messages
• Optional parallel downloading of files
FastTrack: Architecture
• Each peer is either a
supernode or is
assigned to a
supernode
• Selection criterias:
CPU, memory, network
connection
• Each SN has about
100-150 children nodes
and has 30-50 TCP
connections with other
supernodes
• SN tracks the content
and IP of its children
nodes, not content
under its neighboring
SNs
supernodes
originalnodes
FastTrack: Metadata
• When ON connects to SN, it uploads its metadata
• For each file:
–
–
–
–
File name
File size
Content Hash (MD5+CRC)
File descriptors: used for keyword matches during query
• Content Hash:
– When peer A selects file at peer B, peer A sends ContentHash in
HTTP request
– If download for a specific file fails (partially completes),
ContentHash is used to search for new copy of file
FastTrack: Overlay Maintenance
• List of potential supernodes included within
software download
• New peer goes through the list until it finds
operational supernode
– Node “pings” (5-6) supernodes on the list and
connects with the first replied SN
– Connects and obtains more up-to-date list, with 200
entries
– SNs in the updated list are “close” to ON
• If supernode goes down, node goes through the
updated list and finds new supernode
FastTrack: Queries
• Node first sends query to supernode
– Supernode responds with matches
– If x matches found, done
• Otherwise, supernode forwards query to
subset of supernodes
– If total of x matches found, done
• Otherwise, query further forwarded
– Probably by original supernode rather than
recursively
FastTrack: Parallel Downloading
and Recovery
• If file is found in multiple nodes, user can select
parallel downloading
• Identical copies identified by ContentHash
• HTTP byte-range header used to request
different portions of the file from different nodes
• Automatic recovery when server peer stops
sending file
– ContentHash is used to search for new copy of file
eDonkey2000 Overview
eDonkey2000 (ED2K)
•
•
Semi-centralized network, includes index servers
Many clients: eDonkey2000, MLDonkey, eMule,
Shareaza...
Index server: Lugdunum
Used also for legal content delivery
Files identified by hash (MD4)
Possible to search files using web, founded ed2k links
can be used to start file download
•
•
•
•
–
ed2k://|file|gentoo.linux.install-x86-minimal-2004.1 [found via
www.FileDonkey.com].iso|85764096|F1819D1C731923327E1
40F09DB7400B6|/)
ED2K
•
Communication:
–
–
–
–
•
client-connected index server: TCP
client-other index servers: UDP
index server-index server: UDP
client-client: TCP
File transfer using Multisource File
Transmission Protocol (MFTP)
– also HTTP and BitTorrent supported
ED2K: Registration
Index Server 1
Index Server 2
Index Server 3
Register to server,
tell server own shared files
Peer 4 (registered to
index server 3)
XXXX.txt
XXXX.exe
Index Server 1
XXXX.exe
YYYY.txt
YYYY.exe
Peer 2
Peer 2
Peer 2
Index Server 2
YYYY.txt
ZZZZ.exe
Peer 3
Peer 3
ZZZZ.txt
ZZZZ.exe
Peer 1
Index Server 3
ZZZZ.txt
ZZZZ.exe
Peer 4
Peer 4
YYYY.txt
YYYY.exe
XXXX.exe
Peer 2 (registered to
index server 1)
YYYY.txt
ZZZZ.exe
Peer 3 (registered to
index server 2)
ED2K: Registration Reply
Index Server 1
Index Server 2
Index Server 3
List of other index servers
known by index server 1
Peer 4 (registered to
index server 3)
ZZZZ.txt
ZZZZ.exe
XXXX.txt
XXXX.exe
Peer 1 (registered to
index server 1)
Index Server 1
XXXX.txt
XXXX.exe
YYYY.txt
YYYY.exe
Peer 1
Peer 1 & Peer 2
Peer 2
Peer 2
YYYY.txt
YYYY.exe
XXXX.exe
Peer 2 (registered to
index server 1)
YYYY.txt
ZZZZ.exe
Peer 3 (registered to
index server 2)
ED2K: File Search
Index Server 1
Index Server 2
Index Server 3
Search Files (UDP)
Search Files (TCP)
Search Files message
includes:
•keyword
•optionally
- min file size
- max file size
- availability
- etc.
Peer 4
Peer 1
Peer 2
Peer 3
ED2K: File Search Reply
Index Server 1
Index Server 2
Index Server 3
Search File
Results (UDP)
Search File Results (TCP)
Search File Results message
includes one or more file info:
•file hash
•client IP and port (optional?)
•file name
Peer 4
Peer 1
Peer 2
Peer 3
ED2K: File Downloading 1/3
Index Server 1
Index Server 2
Index Server 3
Get Sources (UDP)
Get Sources (TCP)
•Done if Search File Results
message(s) don’t include
client IP and port pair(s) or
ED2K link is used to start
downloading
•includes:
- file hash
Peer 4
Peer 1
Peer 2
Peer 3
ED2K: File Downloading 2/3
Index Server 1
Index Server 2
Index Server 3
Found
Sources (UDP)
Found Sources (TCP)
Found Sources message
includes:
•file hash
•address list
- client IP and port
Peer 4
Peer 1
Peer 2
Peer 3
ED2K: File Downloading 3/3
Index Server 1
Index Server 2
Index Server 3
Peer 4
File requests and
downloading
Peer 1
Peer 2
Peer 3
BitTorrent Overview
BitTorrent
•
•
•
•
Centralized network, includes tracker
.torrent files
Google search for .torrents
Legal material available
BitTorrent: Get .torrent
HTTP Server
Tracker
Seed 1
GET .torrent file
.torrent file
Leecher 1
Downloader
In .torrent file:
• file size
• file name
• hash of file (SHA1)
• url of tracker
Seed 2
BitTorrent: Get Peer List
HTTP Server
Tracker
Seed 1
GET-announce
Response-peer list
Leecher 1
Downloader
Seed 2
BitTorrent: Query File Pieces
HTTP Server
Tracker
Seed 1
GET pieces of file
Leecher 1
Downloader
Seed 2
BitTorrent: File Pieces
HTTP Server
Tracker
Info about
download
status
Seed 1
pieces of file
Leecher 1
Leecher 2
Seed 2
BitTorrent: Status Information
HTTP Server
Tracker
Info about
complete
download
Seed 1
Seed 3
Seed 4
Seed 2
Skype Overview
Skype
• Skype is a P2P VoIP client developed by the people who
did KaZaA
• Allows its users to place voice calls and send text
messages to other users of Skype clients
• Two types of nodes in the overlay network, ordinary
hosts (OH) and super nodes (SN)
• OH is a Skype application that can be used to place
voice calls and send text messages
• SN is an ordinary host’s end-point on the Skype network
• Any node with a public IP address having sufficient CPU,
memory, and network bandwidth is a candidate to
become a SN
Skype
• OH must connect to a SN and must
register itself with the Skype login server
for a successful login
• 7 bootstrap super nodes
• The host cache (HC) is a list of super node
IP address and port pairs that OH builds
and refreshes regularly
• HC contains a maximum of 200 entries
Skype Network
Skype Login
Server
Super Nodes
Message exchange
during login
Skype
• Uses its Global Index technology to search
for a user
• Firewall traversal: First UDP, second TCP,
third TCP port 80 (HTTP), fourth TCP port
443 (HTTPS)
• Call signaling is carried always over TCP
NAT and Firewall Traversal
-If caller is behind portrestricted NAT, call
signaling (TCP) is
forwarded through a
node, which has a
public IP address
-If either caller or callee
or both are behind portrestricted NAT voice
traffic (UDP) is
forwarded through the
same node
- If both caller
and callee have
a public IP
address, call
signaling (TCP)
and voice traffic
(UDP) flow
directly between
them
- If both caller and callee are behind
port-restricted NAT and UDPrestricted firewall, then signaling
traffic and voice traffic is forwarded
through another node over TCP