1. Dynamic Topology Configuration in Service Overlay Networks: A Study of Reconfiguration Policies
Authors: Jinliang Fan and Mostafa H. Ammar
Presented by: Srinivasan Seetharaman
Networking and Telecommunications Group
College of Computing, Georgia Tech
7/28/2019

2. Introduction Overlay Networks Example applications
Native infrastructure of Internet has become resistant to fundamental changes
Overlay networks can provide the desirable flexibility and control
Example applications
Application layer multicast
Testbeds for new technologies
Circumventing BGP faults and constraints
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3. Service Overlay Networks
1. Overlay network providers deploy a number of specially designed overlay nodes across the Internet.
They contract with underlying ISPs and buy network bandwidth between these overlay nodes
They provide value-added network services to end-systems
Traffic between end-systems is carried by and routed through the overlay networks.
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4. Dynamic Topology Configuration
Topology configurability in small time scales
a key feature of overlay networks
Our contributions:
Optimal topology reconfiguration policies: properties and approximation
evidence for the advantage of overlay networks due to their configurability
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5. Choosing Overlay Topologies
Underlying network +
Underlying network: operation cost matrix
Communication pattern: traffic matrix
Overlay topology: assuming traffic go through minimum-cost path
Choosing from all feasible overlay topologies
1) connectivity constraint
2) degree-bound constrain. Used as a generalized way to characterize and control the overall monitoring and management overhead for overlay links. We will see later in this presentation that a good reconfiguration policy can justify the use of lower degree
Application’s communication requirement
Possible overlay topologies
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6. Main question
When and how should the overlay topology be reconfigured as the traffic patterns change?
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7. Reconfiguration Cost Topology Reconfiguration has a cost
Overhead of establishing and tearing down overlay links
Disruption to ongoing flows
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8. Costs Involved Occupancy cost: traffic operation cost
Reconfiguration cost: control and rerouting overhead
Protocol dependent
Estimated with the number of changed overlay links
Overall cost
Weighting factor β is protocol specific and application specific
Occupancy cost
1. Incurred for every second when an overlay topology is used with an comm. Pattern
Reconfiguration cost includes:
Control and management overhead depends on how complex it is for the service overlay provider to interact with ISPs
Rerouting overhead
We have another slide later to show the impact of different formulation for reconfiguration cost.
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9. A Reconfiguration Policy
An overlay topology reconfiguration policy is
The sequence of overlay topologies used in response to changing traffic over time
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10. Policy Optimization
Optimization objective: minimizing long-term overall cost
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11. Two Extreme Policies Never Change Policy Always Change Policy
Optimal if Reconfiguration Cost is very high
Always Change Policy
Optimal if Reconfiguration Cost is zero
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12. General Approach
Modeling the problem of finding optimal reconfiguration policy as Markov Decision Process
Small number of nodes and Markovian process of comm. patterns
Solving using Howard’s policy iteration method
Observing properties of optimal policies
Developing approximation policies that can be also used for large, non-Markovian systems
MDP model is applicable when X(t) is a continuous Markov process
In practice, Howard’s policy iteration method is applicable when the number of nodes in the overlay network is small
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13. Traffic and Overlay Topology
Traffic changes over time
X(t) traffic matrix at time t
From a set of communication patterns {C1, … , Cs}
Feasible overlay topologies
From a set {T1, … , Tr}
Fixed number of nodes and degree bounded
State: <Traffic, Topology>
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14. Properties of Optimal Policies: A. Reconfiguration Chains
System state is represented with <C,T>
Whichever state it starts from, the system always traps into a reconfiguration chains (with a limited number of states) if it follows the optimal reconfiguration policy
Increasing aggressiveness when the reconfiguration cost is less weighted.
Average number of unique topologies in the reconfiguration chains is a good indication of system aggressiveness. This leads to the next slide.
Mostafa, it always take me a lot of effort to explain this slide. Also the figures may be too small and you may have to point the audience to the proceedings instead.
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15. Properties of Optimal Policies: B. Threshold Behavior
Mostafa, I added colors to the lines. Hope this helps.
Overall cost is a smooth line.
Occupancy cost and reconfiguration cost: threshold behavior
The threshold behavior of cost is consistent with the threshold behavior of the unique number of topos.
Left two figures shows the impact of transition rate.
Not-aggressive
Aggressive
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16. Properties of Optimal Policies: C. Internal Structure
Comm. patterns sharing the same topology form a cluster
Most clusters do not overlap
Global factors affect # of clusters
Weight of reconfiguration cost 
Average transition rate 
Local factors affect whether two comm. patterns belong to the same cluster
imbalance of occupancy time
level of coupling
similarity
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17. Approximating Optimal Policies
Mimic optimal policies and generate topologies in polynomial time
Use different types of approximation policies for scenarios with different range of β (the weight of reconfiguration cost)
β in lower extreme area. Always-Change Policy
β in higher extreme area. Never-Change Policy
β in middle area. Cluster-Based Policy
(Group the communication patterns based on clusters and identify 1 topology to be used by cluster members)
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18. Required information for constructing clusters
Percentage of long-term average occupancy time at each communication pattern
Long-term average number of transitions between every pair of communication patterns per unit time
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19. Performance Evaluation
In small overlay network with 5 overlay nodes
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20. Performance Evaluation (contd.)
In large overlay network with 40 overlay nodes
Same figure as the previous one.
Indicate the portion that we will magnify in the next slide.
Markovian
Non-Markovian
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21. Effect of Degree Bound Irrespective of degree we use
dynamic reconfiguration provides benefit.
dynamic reconfiguration is best when the reconfiguration policy cost is low.
Service provider may need to decide on a tradeoff between the complexity of dynamic reconfiguration and cost of high degree bound.
Major Observations:
Higher degree lower down the policy cost in general, but increase other costs (which can not been seen in this figure)
NCP is our base line.
The gap between two simple policies (ACP and NCP) bounds what we can achieve using dynamic reconfiguration. In that sense, dynamic reconfiguration is most desirable when high degree is not affordable.
No matter what degree we use, dynamic reconfiguration provides benefit.
No matter what degree we use, dynamic reconfiguration servers the best when the reconfiguration protocol cost is low.
Service provide may need to decide on a tradeoff between the complexity of dynamic reconfiguration and cost of high degree bound.
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22. Concluding Remarks
Topology configurability is one of the important capabilities provided
Investigated a framework for determining reconfiguration policies
Careful reconfiguration can maintain overlay performance without increasing cost
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23. Questions? Please contact: Thank you! Jinliang jlfan@cc.gatech.edu
Mostafa
Thank you!
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