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

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 7/28/2019

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. 7/28/2019

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 7/28/2019

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 7/28/2019

Main question When and how should the overlay topology be reconfigured as the traffic patterns change? 7/28/2019

Reconfiguration Cost Topology Reconfiguration has a cost Overhead of establishing and tearing down overlay links Disruption to ongoing flows 7/28/2019

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. 7/28/2019

A Reconfiguration Policy An overlay topology reconfiguration policy is The sequence of overlay topologies used in response to changing traffic over time 7/28/2019

Policy Optimization Optimization objective: minimizing long-term overall cost 7/28/2019

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 7/28/2019

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 7/28/2019

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> 7/28/2019

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. 7/28/2019

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 7/28/2019

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 7/28/2019

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) 7/28/2019

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 7/28/2019

Performance Evaluation In small overlay network with 5 overlay nodes 7/28/2019

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 7/28/2019

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. 7/28/2019

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 7/28/2019

Questions? Please contact: Thank you! Jinliang jlfan@cc.gatech.edu Mostafa ammar@cc.gatech.edu Thank you! 7/28/2019