Optimisation des DHT à partir des propriétés physiques, logiques et
sociologiques des clients
Pierre Fraigniaud CNRS
LRI, Univ. Paris-Sudhttp://www.lri.fr/~pierre
Plan
• Distributed Hash Table (DHT)• Structural properties• Sociological properties• Conclusion
Principles of DHTs
DHT • File, data, etc name• Typically: name space = [0,1[• h(file_name) = 0.10110001101
• User name • User name [0,1[• h(my_IP@) = 0.0011010110
Correspondence
01Users = { }
user x Data stored by x
Overlay network
01
x
y
zx knows the IP@ of y and z
Lookup
01
x
h(Andrei Rublev)
Node insertion
01Entry point
Examples
• CAN (D-dimensional meshes)• Chord (hypercube)• Viceroy (butterfly)• D2B, Koorde (de Bruijn)• …
Structural Properties
Desirable properties
• Small number of hops for lookup:i.e., small diameter and efficient
routing• Quick updates: i.e., small degree• Small congestion: i.e., small probability of contention
From the network point of view
Taking the inter-node distance in Internet into account!
It does not mean that closely related nodes must be close in the Overlay.
stretch = maxall routeslength(Internet route)
length(overlay route)
Solution
Theorem (Abraham & Malkhi)Under some conditions on the
physical network,……there exists an overlay network
with strech 1+ε, degree and diameter O(log n).
From the user point of view
Taking the user interests into account!
Closely related users aim at being close in the Overlay.
How to measure proximity between users?
Requests types
• Typo: h(André Roublef) vs. h(Andrei Rublev) • Structure:
Prefix search, interval, etc• Data-base type requests
Sociological Properties
Connect users sharing common interets
• Gnutella enhanced with additional links…
• Every user keeps links only with users sharing common interest (cf. Maay)
Structure of user connections
• Scale-free structure: Degree distribution = power law
Prob( deg(x)=k ) ≈ k-a
• Guided walk in scale-free graphs• Random walk• Shortest path• Neighbor with largest degree first
Rumors and legendsPath length
Network size
Random walk
Shortest path
Neighbor withhighest degree first
Using small world properties
• Milgram’s experiment six degrees of separation between indivitual
• Kleinberg’s augmented meshes capture this phenomenon
• DHT Symphony (!)• Why not just doing greedy routing?
Conclusion
Conclusion: users sociological properties seem to have more impact on DHT’s than network structural properties
Unfortunately sociological properties are difficult to model and to measure
Warning: this conclusion might be not true in other contexts, e.g., ad hoc, global computing, etc.