1. Impact of Neighbor Selection on Performance and Resilience of Structured P2P Networks
Sushma Maramreddy

2. Overlay networks
An overlay network is a logical network on top of one or more networks. E.g. Internet
Main purpose of these networks is to provide effective means by which a huge amount of computing links are linked and accessed.
Various distributed services can be built on top of these networks.
Structured and Unstructured overlay networks.

3. Overlay networks Contd..
Unstructured overlay network – a peer joins the network by connecting itself to any node in the network.
Structured overlay network – a peer joins the network by connecting itself to some other well-defined peers using a logical identifier.

4. Authors Byung-Gon Chun, U.C.Berkley Ben Y.Zhao, U.C.Santa Barbara
John D.Kubiatowicz, U.C.Berkley

5. Impact of neighbor selection on performance and Resilience
Introduction
Related work
Details of neighbor selection
Impact of cost functions on Performance and Resilience
Conclusion

6. Introduction
Structured overlay networks provide routing to endpoints or nodes inside the network requiring logarithmic steps at each node.
Nodes choose the neighbors based on optimization metrics.
A recent study by Gummadi has shown that neighbor selection based on network proximity significantly increase overall performance.
Problem – network imbalance.

7. Introduction Contd..
To better model neighbor selection across the networks, a generalized cost model is presented.
Most current protocols only consider network proximity in neighbor selection
This paper uses different models based on network proximity and network capacity.
Study the impact they have on lookup latency and static resilience in tree and ring geometries.

8. Related Work
Closest work was done by Gummadi. The authors quantified the impact of routing geometry on performance and resilience.
Albert shows a correlation between scale-free nature of networks and resilience to attacks and failures.
Several researchers propose optimizing the overlay construction of structured overlays using network proximity, but generally ignore CPU load, storage and bandwidth capacity.

9. Structured Overlay Construction
Each node chooses neighbors that meet logical identifier constraints (e.g., prefix matching or identifier range) and builds direct links.
These constraints are flexible such that a no. of nodes are possible for each routing table entry.
The neighbor selection problem is reduced to cost minimization problem.

10. Cost Model
Optimizing neighbor selection for node i means minimizing the sum of the cost from i to all nodes.
Cost from i to j consists of two factors: node cost and edge cost.
Node cost is the cost incurred by intermediate node-
Edge cost is the cost incurred by the network links -

11. Cost Model Contd.. N = network size t (i, j) = traffic from i, to j.
Cp (i,j) =cost of the path from i to j
V(I,J) = intermediate nodes

12. Cost Model Contd..
This model captures the heterogeneity node capacity – a function of bandwidth, computation power, disk access time and so on.
For structured networks such as Cord, Pastry and Tapestry cost function is defined as follows

13. Cost function in structured networks
b = neighbor index
nb = neighbor indexed by b
Nb = no. of neighbors
Rb = set of destination through nb
cn(i) = node cost, ce(e) = edge cost
Ce(k,l) = edge cost between k and l.

14. Neighbor selections Four neighbor selection models
Random - choose neighbors randomly
Dist – neighbors physically closest in the network
Cap – neighbors with smallest processing delay
DistCap – neighbors with smallest combined delay (sum of node processing delay and overlay link delay)

15. Cost functions studied
Cn(i) - processing delay in node i
Ce(i, nb) -direct overlay link delay between node I and node nb.

16. Simulation - set up
Simulate Tapestry and Chord protocols as representatives of tree and ring structures.
Simulations use 5100 node network topologies.
Each node in Chord forwards messages to the live neighbor closest to the destination. Look up fails if all neighbors before the destination in the namespace fail.
For tapestry each node forwards messages to the first live neighbor matching one more prefix digit. If all primary and backup links in the routing entry fail, the lookup fails.

17. Simulation results - Performance
Two different distributions of node processing delay.
Uniform and Bimodal distributions
In Uniform we assign the processing delay uniformly from a/10, 2a/10. … a where a is the max processing delay.
In Bimodal, nodes are either fast or slow. Fast nodes can process 100 messages/sec and slow nodes process 1 message/sec.

18. Simulation Results - Uniform

19. Simulation Results - Bimodal

20. Simulation Results – Static Resilience
Measure resilience as the proportion of all pairs of live nodes that can still route to each other after an external event, either randomized node failures or targeted attacks.
Assumptions - attacks focus on removing nodes with the highest in-degree in order to maximize damage to overall network reachability.
Assume nodes have an uniform processing delay distribution with a=0.5s

21. Satic Resilience
For Tapestry, examine resilience of the base protocol, base protocol plus additional backup routes (all chosen using the neighbor selection algorithms), base protocol plus backup routes chosen at random.
For chord we examine the base protocol, base protocol plus sequential neighbors.

22. Random node failures - Tapestry

23. Random Node failure - Chord

24. Targeted node attacks - Tapestry

25. Targeted node attacks - Chord

26. Simulation Results
Attacking nodes with high in-degree affects network connectivity severely.
Random shows the best attack tolerance among neighbor selections
CapDist has the worst attack tolerance than Dist, although it has better performance.
This result demonstrates tradeoff between performance and attack resilience in structured P2P overlay construction.

27. Conclusion and Future Work
Took a quantitative approach to examine the benefits and costs of considering network or physical characteristics in overlay construction
The choice of neighbor selection algorithm drives a tradeoff between performance and resilience.
If high degree nodes are attacked the impact on network connectivity is severe.
As future work investigate the resilience of different geometries under different neighbor selection algorithms.