INF 123
SW ARCH, DIST SYS & INTEROP
LECTURE 12
Prof. Crista Lopes
Objectives
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
Understanding of Peer-to-Peer architectures
Solid knowledge of well-known P2P systems
Node 1
Node 2
Peer-to-Peer
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State and behavior are distributed among peers which
can act as either clients or servers.
Peers: independent components, having their own state
and control thread.
Connectors: Network protocols, often custom.
Data Elements: Network messages
Topology: Network (may have redundant connections
between peers); can vary arbitrarily and dynamically
Supports decentralized computing with flow of control
and resources distributed among peers. Highly robust
in the face of failure of any given node. Scalable in
terms of access to resources and computing power. But
caution on the protocol!
Issues to consider in P2P
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
Locating resources
Retrieving resources
Napster
The system that made P2P (in)famous
Case Study: Napster
“The Napster”
Case Study: Napster
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Resource localization was centralized
Resource retrieval was P2P
Protocol: custom over TCP/IP
Spec
Case Study: Napster
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Notification of song: [Client  Napster]
"<filename>" <md5> <size> <bitrate>
<frequency> <time>
Example:
"C:\random band - random song.mp3"
b92870e0d41bc8e698cf2f0a1ddfeac7 443332
128 44100 60
Case Study: Napster
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Search query: [Client  Napster]
[FILENAME CONTAINS "artist name"] MAX_RESULTS
<max> [FILENAME CONTAINS "song"] [LINESPEED
<compare> <link-type>] [BITRATE <compare>
"<br>"] [FREQ <compare> "<freq>"] [WMA-FILE]
[LOCAL_ONLY]
Example:
FILENAME CONTAINS ”random"
MAX_RESULTS 75
FILENAME CONTAINS ”song"
BITRATE "AT LEAST" "128"
Case Study: Napster
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Query Response:
[Napster  Client]
"<filename>" <md5> <size> <bitrate>
<frequency> <length> <nick> <ip> <link-type>
[weight]
Example:
”C:\random band - random song.mp3"
7d733c1e7419674744768db71bff8bcd
2558199 128 44100 159 lefty 3437166285 4
Case Study: Napster
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Retrieving song (no firewall):
GET
<nick> "<filename>"
[Client  Client]
Example:
lefty "C:\random band - random song.mp3"
Case Study: Napster
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Retrieval Response: [Client  Client]
<nick> <ip> <port> "<filename>" <md5>
<linespeed> (if file exists)
or
<nick> "<filename>" (if file doesn’t exist)
Example:
lefty 4877911892 6699
"C:\random band - random song.mp3"
10fe9e623b1962da85eea61df7ac1f69 3
Case Study: Napster
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Retrieving song (firewall):
[Client  Napster Client  Client]
SEND
<nick> "<filename>"
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Example:
lefty "C:\random band - random song.mp3"
Napster’s Aquilles Heel

“The Napster” central server
 Single
point of failure
 Shutdown mandated by court order
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Without the central server, the peers were useless
Gnutella
The textbook P2P architecture
Case Study: Gnutella
Case Study: Gnutella
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Resource localization is decentralized
 Gnutella
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is, essentially, a decentralized search system
Resource retrieval is P2P
Protocols: custom over TCP/IP + HTTP
Spec
Case Study: Gnutella
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Node discovery done off-band channel
 List
shipped with software
 IRC
 Mailing

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lists
Only need 1 neighbor node to connect to the node
network
Gnutella nodes = “servents”
Case Study: Gnutella
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Search: flooding (originally)
Case Study: Gnutella
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Connection to peer:
GNUTELLA CONNECT/<protocol version string>\n\n
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Response:
GNUTELLA OK\n\n
Case Study: Gnutella
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Gnutella Protocol Descriptor
Descriptor
ID
Payload
Descriptor
Code
Type
0x00
Ping
0x01
Pong
0x80
Query
0x81
QueryHit
0x40
Push
TTL
Hops
Payload
Length
Case Study: Gnutella
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Ping messages
 Used

to discover other servents
Pong messages
 Responses
to ping messages
 May be cached; receiver may send may many pong
messages to to ping request
 Payload:
Case Study: Gnutella
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Query messages
 Used
to find resources
 Payload:

QueryHit messages
 Responses
 Payload:
to query messages
Case Study: Gnutella
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[Normal] File download is done via HTTP GET
GET /get/<File Index>/<File Name>/ HTTP/1.0\r\n
Connection: Keep-Alive\r\n
Range: bytes=0-\r\n
User-Agent: Gnutella\r\n3
\r\n
HTTP 200 OK\r\n
Server: Gnutella\r\n
Content-type: application/binary\r\n
Content-length: 4356789\r\n
\r\n
file data
Case Study: Gnutella
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Routing Protocol rules:
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Pong messages may only be sent along the same path as corresponding pings
QueryHit messages may only be sent along the same path as corresponding
queries
Push messages may only be sent along the same path that carried the incoming
QueryHits.
A servent will forward incoming Ping and Query messages to all of its directly
connected servents, except the one that delivered the incoming Ping or Query.
A servent will decrement a descriptor header’s TTL field, and increment its Hops
field, before it forwards the descriptor to any directly connected servent. If,
after decrementing the header’s TTL field, the TTL field is found to be zero, the
descriptor is not forwarded along any connection.
A servent receiving a message with the same Payload Descriptor and Descriptor
ID as one it has received before, should avoid forwarding the message to any
connected servent.
Case Study: Gnutella
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Ping/Pong routing
Case Study: Gnutella
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Query/QueryHit/Push routing
Skype
Hybrid P2P and Client-Server
Proprietary Protocols, not much documentation
Case Study: Skype
Case Study: Skype
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A mixed client-server and peer-to-peer architecture
addresses the discovery problem.
Replication and distribution of the directories, in the
form of supernodes, addresses the scalability problem
and robustness problem encountered in Napster.
Promotion of ordinary peers to supernodes based upon
network and processing capabilities addresses another
aspect of system performance: “not just any peer” is
relied upon for important services.
A proprietary protocol employing encryption provides
privacy for calls that are relayed through supernode
intermediaries.
Restriction of participants to clients issued by Skype,
and making those clients highly resistant to inspection or
modification, prevents malicious clients from entering the
network.
Summary

Understanding of Peer-to-Peer architectures
 node
discovery
 resource retrieval
 how to deal with firewalls

Solid knowledge of well-known P2P systems
 Napster
 Gnutella
 Skype
(briefly)