Fully Decentralized Information System
P2P file sharing
Global scale application.
Example: Gnutella.
40.000 nodes, 3 Mio files.
3M nodes (Jan 2006).
Strengths
Good response time, scalable.
No infrastructure, no administration.
No single point of failure.
Weaknesses
High network traffic.
No structured search.
Free-riding.
Gnutella
Meeting Peers (Ping/Pong)
Protocol Message Types
| Type | Description | Contained Information |
|---|---|---|
Ping | Announce availability and probe for other servents. | None. |
Pong | Response to a ping. | IP address and port# of responding servent; number and total kb of files shared. |
Query | Search request | Minimum network bandwidth of responding servent; search criteria. |
QueryHit | Returned by servents that have the requested file | IP address, port# and network bandwidth of responding servent; number of results and result set. |
Push | File download requests for servents behind a firewall | Servent identifier; index of requested file; IP address and port to send file to. |
Searching
(Query/QueryHit/GET)
Structureless
Queries are flooded to neighbors, have a TTL, and are forwarded only once.
A query may obtain several responses indicating which peers provide the requested file. Among those, it selects one and is directly contacted in order to download the file.
Can we search using fewer packets?
Improvements in Message Flooding
Expanding Ring.
Start the search with small TTL (e.g. TTL = 1).
If no success iteratively increases TTL (e.g. TTL = TTL +2).
k-Random Walkers.
Forward the query to one randomly chosen neighbor only, with large TTL.
Start k random walkers.
Random walker periodically checks with the requester whether to continue.
Hybrid Gnutella: “Ultrapeers”
Ultrapeers can be installed (KaZaA) or self-promoted (Gnutella v.2).
Real Gnutella Network
Popular open-source file-sharing network.
~450,000 users as of 2003.
~2,000,000 today.
Ultrapeer-based Topology.
Queries flooded among ultrapeers.
Leaf nodes are shielded from query traffic.
Based on multiple crawlers.
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