Interdomain Routing as Social Choice

1. Interdomain Routing as Social Choice Ronny R. Dakdouk, Semih Salihoglu, Hao Wang, Haiyong Xie, Yang Richard Yang Yale University IBC’06

2. Outline • Motivation A social choice model for interdomain routing • Implications of the model • Summary & future work

3. Motivation • Importance of Interdomain Routing • Stability • excessive churns can cause router crash • Efficiency • routes influence latency, loss rate, network congestion, etc. • Why policy-based routing? • Domain autonomy: Autonomous System (AS) • Traffic engineering objectives: latency, cost, etc.

4. BGP • The de facto interdomain routing protocol of the current Internet • Support policy-based, path-vector routing • Path propagated from destination • Import & export policy • BGP decision process selects path to use • Local preference value • AS path length • and so on…

5. Previous Studies • Policy Disputes (Dispute Wheels) may cause instability [Griffien et al. ‘99] • Economic/Business considerations may lead to stability [Gao & Rexford ‘00] • Interdomain Routing for Traffic Engineering [Wang et al. ‘05] • Design incentive-compatible mechanisms [Feigenbaum et al. ‘02]

6. What’s Missing • Efficiency (e.g., Pareto optimality) • Previous studies focus on BGP-like protocols • Increasing concern about extension of BGP or replacement (next-generation protocol) • Need a systematic methodology • Identify desired properties • Feasibility + Implementation • Implementation in strategic settings • Autonomous System may execute the protocol strategically so long as the strategic actions do not violate the protocol specification!

7. AS 1 Preference Interdomain Routing Protocol AS 1 Route ..... ..... AS N Preference AS N Route Our approach - A Black Box View of Interdomain Routing • An interdomain routing system defines a mapping (a social choice rule) • A protocol implements this mapping • Social choice rule + Implementation

8. Outline • Motivation A social choice model for interdomain routing • Implications of the model • Summary & future work

9. A Social Choice Model for Interdomain Routing • What’s the set of players? • This is easy, the ASes are the players • What’s the set common of outcomes? • Difficulty • AS cares about its own egress route, possibly some others’ routes, but not most others’ routes • The theory requires a common set of outcomes • Solution • Use routing trees or sink trees as the unifying set of outcomes

10. Routing Trees (Sink Trees) • Each AS i = 1, 2, 3 has a route to the destination (AS 0) • T(i) = AS i’s route to AS 0 • Consistency requirement: • If T(i) = (i, j) P, then T(j) = P A routing tree

11. Realizable Routing Trees • Not all topologically consistent routing trees are realizable • Import/Export policies • The common set of outcomes is the set of realizable routing trees

12. Local Routing Policies as Preference Relations • How does this work? • Example: The preference of AS i depends on its own egress route only, say, r1 > r2 • The equivalent preference: AS i is indifferent to all outcomes in which it has the same egress route • E.g: If T1(i) = r1, T2(i) = r2, T3(i) = r2, then T1 >i T2 =i T3

13. Local Routing Policies as Preference Relations (cont’) • Not just a match of theory • Can express more general local policies • Policies that depend not only on egress routes of the AS itself, but also incoming traffic patterns • AS 1 prefers its customer 3 to send traffic through it, so T1 >1 T2

14. Preference Domains • All possible combinations of preferences of individual ASes • Traditional preference domains: • Unrestricted domain • Unrestricted domain of strict preferences • Two special domains in interdomain routing • The domain of unrestricted route preference • The domain of strict route preference

15. Preference Domains (cont’) • The domain of unrestricted route preference • Requires: If T1(i) = T2(i), then T1 =i T2 • Intuition: An AS cares only about egress routes • The domain of strict route preference • Requires: If T1(i) = T2(i), then T1 =i T2 • Also requires: if T1(i)  T2(i) then T1 i T2 • Intuition: An AS further strictly differentiates between different routes

16. Interdomain Social Choice Rule (SCR) • An interdomain SCR is a mapping: • F: R=(R1,...,RN)  P  F(R) A • F incorporates the criteria of which routing tree(s) are deemed “optimal”– F(R)

17. An example

18. Some Desirable Properties of Interdomain Routing SCR • Non-emptiness • All destinations are always reachable • Uniqueness • No oscillations possible • (Strong) Pareto optimality • Efficient routing decision • Non-dictatorship • Retain AS autonomy

19. Protocol as Implementation • No central authority for interdomain routing • ASes execute routing protocols • Protocol specifies syntax and semantics of messages • May also specify some actions that should be taken for some events • Still leaves room for policy-specific actions <- strategic behavior here! • Therefore, a protocol can be modeled as implementation of an interdomain SCR

20. AS 1 Preference Interdomain Routing Protocol AS 1 Route ..... ..... AS N Preference AS N Route The Model in a Nutshell • An interdomain routing system defines a mapping (a social choice rule) • A protocol implements this mapping • Social choice rule + Implementation

21. Implications of the Model • Some results from literature • A case study of BGP from the social choice perspective

22. Some Results from Literature • On the unrestricted domain • No non-empty SCR that is non-dictatorial, strategy-proof, and has at least three possible routing trees as outcomes [Gibbard’s non-dominance theorem] • On the unrestricted route preference domain • No non-constant, single-valued SCR that is Nash-implementable • No strong-Pareto optimal and non-empty SCR that is Nash-implementable

23. A Case Study of BGP AS 1 Preference Routing Tree BGP ..... ..... AS N Preference

24. Reverse engineering BGP • Non-emptiness: X • Uniqueness: X • Non-dictatorship: X • Unanimity:  • Strong Pareto Optimality:  only on strict route preference domain

25. BGP is Manipulable! • If AS 1 and 3 follow the default BGP decision process, then AS 2 has a better strategy • Following the default BGP decision process is not a Nash equilibrium!

26. Possibility of fixing BGP • BGP is (theoretically) Nash implementable (actually, also strong implementable) • But, only in a very simple game form • The problem: the simple game form may not be followed by the ASes

27. Summary • Viewed as a black-box, interdomain routing is an SCR + implementation • Strategic implementation requirements impose stringent constraints on SCRs • The greedy BGP strategy has its merit, but is manipulable

28. What’s next? • Design of next-generation protocol (the goal!) • Stability, optimality, incentive-compatibility • Scalability • Scalability may serve as an aide (complexity may limit viable manipulation of the protocol) • A specialized theory of social choice & implementation for routing? • What is a reasonable preference domain to consider?

29. Thank you! • Comments or Questions: hao.wang@yale.edu

30. Thank you! • Comments or Questions: hao.wang@yale.edu