1. System & Network Reading Group On Selfish Routing In Internet-Like Evironments Lili Qiu (Microsoft Research) Yang Richard Yang (Yale University) Yin Zhang (AT&T Research) Scott Shenker (ICSI)

2. System & Network Reading Group Motivation Practical front –Recent studies (e.g., Detour/RON) showed that default routing path is often sub-optimal –Possible causes of routing inefficiency Routing hierarchy Routing policy Different routing objectives used by ISPs Stability problem in routing protocols, such as BGP … –A recent trend: end hosts choose routes Source routing (e.g., Nimrod) Overlay routing (e.g., Detour or RON) –Characteristics of routing by end hosts Improve over today’s IP routing (e.g., delay, loss rate) Selfish by nature (i.e., optimize user-centric performance without considering system-wide criteria)

3. System & Network Reading Group Motivation (Cont.) Theory front –Roughgarden et al. showed selfish routing can result in serious performance degradation due to lack of cooperation

4. System & Network Reading Group Example: Selfish Routing May Yield Sub-Optimal Performance Selfish routing –All traffic go through the lower link –Total latency = 1 Optimal routing (i.e., minimize total latency) –Traffic split equally between the two links –Total latency = ¾ The performance degradation can be unbounded for non-linear latency functions srcdest L(x)=1 L(x)=x

5. System & Network Reading Group Open Issues How does selfish routing perform in Internet-like environments? –Realistic network topologies –Realistic traffic demands –Realistic network delay functions How does selfish overlay routing perform? How does selfish traffic co-exist with the remaining traffic that uses traditional routing protocols? How does users’ selfish routing interact with underlying network control process (e.g., traffic engineering)

6. System & Network Reading Group Outline Overview Network model Evaluation Methodology Performance results –Physical routing –Overlay routing –Multiple overlays –Interaction with traffic engineering Summary and future work

7. System & Network Reading Group Overview Approach –Use a game-theoretic approach to answer the above open issues –Focus on intra-domain scenarios Recent advances in topology mapping and traffic estimation Compare with theoretical results –Focus on equilibrium behavior Compare the performance of traffic equilibria with the global optima and default IP routing Based on realistic topologies, traffic demands, latency functions

8. System & Network Reading Group Network Model Physical network –Directed graph G=(V,E) –Latency of each edge is a function of its load (e.g., M/M/1) Demands –demand(i,j): the amount of traffic from a source i to a destination j Overlays –A set of overlay nodes, overlay links, and a set of demands –The physical route corresponding to an overlay link is dictated by network-level routing –Consider mesh-like overlay topologies Users –Each user decides how its traffic should be routed –Objective: min latency

9. System & Network Reading Group Network Model (Cont.) Route controller –Uses network-level routing OSPF: shortest-path with equal-weight splitting, with the following weight settings –Hop-count –Random-weight –Optimized-compliant weight: minimize network cost when assuming all traffic is compliant (i.e., following the routes determined by the network) [FRT02] »Network cost: a piece-wise linear convex function of network load over all links MPLS: general multi-commodity flow routing

10. System & Network Reading Group Evaluation Methodology Network topology –A large tier-1 ISP topology, referred as ISPTopo –Rocketfuel topologies –Random power-law topologies Traffic demands –Real traffic demands from the ISPTopo –Synthetic traffic demands Link latency functions –M/M/1, M/D/1, P/M/1, P/D/1, BPR Performance metrics –Average latency –Maximum link utilization –Network costs: piece-wise linear, increasing, convex function [FRT02]

11. System & Network Reading Group Different Routing Schemes Physical routing –Source routing (i.e., selfish routing studied in previous theoretical work) –Optimal routing Overlay routing –Overlay source routing (i.e., selfish routing with routing constraints) –Overlay optimal routing Compliant routing (i.e., normal Internet routing)

12. System & Network Reading Group Approach to Computing the Traffic Equilibria General approach –Simulation-based: too expensive –We use a game-theoretic approach to compute the traffic equilibria directly Computing the equilibria of physical routing –linear-approximation algorithm, a variant of Frank-Wolfe algorithm Computing the equilibria of overlay routing –Symmetric: Modified linear approximation algorithm –Asymmetric: Jacob’s relaxation algorithm Computing the equilibria of multiple overlays –Use the relaxation algorithm to guarantee the convergence

13. System & Network Reading Group Outline Overview Network model Evaluation Methodology Performance Evaluation –Source routing –Overlay routing –Multiple overlays –Interaction with traffic engineering Summary and future work

14. System & Network Reading Group Selfish Source Routing Questions –Are Internet-like environments among the worst- case? –What is the system-wide cost for selfish source routing? Dimensions –Performance metrics: latency & network load –Effects of network topologies –Effects of network load –Effects of latency functions

15. System & Network Reading Group Selfish Source Routing: Latency Effects of network topologies (M/M/1, load scale factor=1, OC3 bandwidth) Selfish routing yields close to optimal latency, much better than compliant routing

16. System & Network Reading Group Selfish Source Routing: Network Load Effects of network topologies Selfish routing tends to overload links.

17. System & Network Reading Group Summary: Selfish Source Routing The performance is qualitatively the same as we vary latency functions and network load Unlike the theoretical worst cases, selfish source routing yields close to optimal latency Selfish routing tends to overload links on the shortest paths

18. System & Network Reading Group Outline Overview Network model Evaluation Methodology Performance results –Source routing –Overlay routing –Multiple overlays –Interaction with traffic engineering Conclusion and future work

19. System & Network Reading Group Selfish Overlay Routing Questions –Does selfish overlay routing perform well? –How does the coverage of overlay network affect the performance? Dimensions –Effects of network topologies –Effects of amount of overlay coverage –Effects of how overlay nodes are selected (e.g., random or edge nodes)

20. System & Network Reading Group Difference between Source Routing and Overlay Routing Even if the overlay includes all network nodes, routing on an overlay is still different –Network-level routing can prevent overlay traffic from using a link by setting the corresponding entry in routing matrix to 0 (in OSPF this is achieved by assigning a large weight) –Certain physical routes cannot be implemented by any overlay routing Routing flexibility is further reduced when only a fraction of nodes belong to an overlay

21. System & Network Reading Group Selfish Overlay Routing (Full Overlay Coverage) 1)overlay-src with opt-weight and hop-count perform similarly as source routing 2)overlay-src with random-weight performs much worse.

22. System & Network Reading Group Selfish Overlay Routing (Full Overlay Coverage) Direct Link Shortest [DLS] –For any physically adjacent nodes A and B, all the traffic from A to B is routed through the direct link AB without involving any other links. (e.g., hop-count-based OSPF) For an overlay that covers all network nodes and satisfies DLS –routing on the overlay = routing on the underlay Hop-count-based OSPF and optimized OSPF weights satisfy DLS  they perform similarly as source routing Random OSPF weights violate DLS  some links are pruned, and performance degrades

23. System & Network Reading Group Selfish Overlay Routing (Partial Overlay Coverage) Overlay is formed from all edge nodes in ISPTopo The effects of partial overlay coverage is small in backbone topologies.

24. System & Network Reading Group Summary: Selfish Overlay Routing For full overlay coverage –Overlay has full routing control when the underlay satisfies DLS –The only way in which OSPF affects overlay routing is by violating DLS, which could reduce available network resources –Overlay source routing reduces latency at the expense of higher network cost The effects of partial overlay coverage are small in backbone topologies

25. System & Network Reading Group Outline Overview Network model Evaluation Methodology Performance results –Source routing –Overlay routing –Multiple overlays –Interaction with traffic engineering Conclusion and future work

26. System & Network Reading Group Interactions among Competing Overlays Question –Can multiple overlays share network resources fairly and effectively? Dimensions –Effects of network topologies –Effects of network-level routing schemes –Effects of network load and traffic distribution among overlays –Effects of the number of competing overlays

27. System & Network Reading Group Interactions among Competing Overlays (Cont.) Effects of network-level routing

28. System & Network Reading Group Summary: Interactions among Competing Overlays With reasonable OSPF weights (e.g., hop-count) –Different routing schemes co-exist without hurting each other With bad OSPF weights –Selfish overlay improves both for themselves and for compliant traffic

29. System & Network Reading Group Outline Overview Network model Evaluation Methodology Performance results –Source routing –Overlay routing –Multiple overlays –Interactions with traffic engineering Conclusion and future work

30. System & Network Reading Group Selfish Routing vs. Traffic Engineering So far we assume network is dumb (i.e., static underlay routing) In practice, the network is smart due to traffic engineering (i.e., underlay routing adapts to varying traffic) Question –Will the system reach a state with both low latency and low network cost, as selfish routing and traffic engineering each tries to optimize their objective by adapting to the other process?

31. System & Network Reading Group Specification of Vertical Interactions Interactive process between two players –Traffic engineering Given traffic matrix T t, where T t (s,d) denotes traffic from source s to destination d in time slot t Compute routing matrix R t for the underlay Objective: avoid overloading network –Selfish routing Given routing matrix R t for the underlay Produce new traffic matrix T t Objective: minimize latency

32. System & Network Reading Group One Round during Vertical Interaction T(t) = Traffic matrix when routing matrix is R(t-1) R(t) = OptimizedRoutingMatrix(T(t)) Traffic engineering installs R(t) to network Selfish routing redistributes traffic to form T(t+1)

33. System & Network Reading Group Vertical Interaction with OSPF Optimizations OSPF route optimization interacts poorly with selfish routing

34. System & Network Reading Group Vertical Interaction with MPLS Optimization MPLS optimization interacts with selfish routing more effectively

35. System & Network Reading Group Summary: Selfish Routing vs. Traffic Engineering OSPF route optimization interacts poorly with selfish routing MPLS interacts with selfish routing more effectively Despite the encouraging results from MPLS, several challenges exist –How to estimate traffic matrices accurately in presence of adaptive selfish traffic? –Large optimization problems

36. System & Network Reading Group Conclusion Formulate and evaluate selfish overlay routing Unlike the theoretical worst cases, selfish routing in Internet-like environments yields close to optimal latency –The above result is true for both source routing and overlay routing –Selfish routing can achieve good performance without hurting the traffic that is using default routing

37. System & Network Reading Group Conclusion Mismatch between selfish routing and traffic engineering –Different objectives Selfish routing: minimize e2e delay Traffic engineering: aim to balance load –Selfish routing reduces latency at the cost of increased congestion –The adaptive nature of selfish routing makes traffic demands less predictable and reduces the effectiveness of traffic engineering

38. System & Network Reading Group Future Work Study impacts of multi-AS nature of the Internet Study dynamics of selfish routing (i.e., how traffic equilibria are reached?) Improve the interactions between selfish routing and traffic engineering Study other selfish routing objectives (e.g., loss and throughput)