Interdomain Routing as Social Choice

Ronny R. Dakdouk, Semih Salihoglu, Hao Wang, Haiyong Xie, Yang Richard

Yang

Yale University

IBC’06

Outline

Motivation

A social choice model for interdomain routing

Implications of the model

Summary & future work

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.

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…

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]

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!

Our approach - A Black Box View of Interdomain Routing

An interdomain routing system defines a mapping (a social choice rule)

A protocol implements this mappingSocial choice rule + Implementation

Interdomain Routing P

rotocol

..... .....

AS 1 Preference

AS N Preference

AS 1 Route

AS N Route

Outline

Motivation

A social choice model for interdomain routing

Implications of the model

Summary & future work

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

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 0Consistency requirement:

If T(i) = (i, j) P, then T(j) = PA routing tree

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

Local Routing Policies as Preference Relations

How does this work? Example: The preference of AS i depends o

n its own egress route only, say, r1 > r2 The equivalent preference: AS i is indiffere

nt to all outcomes in which it has the same egress route

E.g: If T1(i) = r1, T2(i) = r2, T3(i) = r2, thenT1 >i T2 =i T3

Local Routing Policies as Preference Relations (cont’)

Not just a match of theoryCan 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

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

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

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)

An example

Some Desirable Properties of Interdomain Routing SCRNon-emptiness

All destinations are always reachable

Uniqueness No oscillations possible

(Strong) Pareto optimality Efficient routing decision

Non-dictatorship Retain AS autonomy

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 im

plementation of an interdomain SCR

The Model in a Nutshell

An interdomain routing system defines a mapping (a social choice rule)

A protocol implements this mappingSocial choice rule + Implementation

Interdomain Routing P

rotocol

..... .....

AS 1 Preference

AS N Preference

AS 1 Route

AS N Route

Implications of the Model

Some results from literature

A case study of BGP from the social choice perspective

Some Results from Literature

On the unrestricted domain No non-empty SCR that is non-dictatorial, stra

tegy-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 Na

sh-implementable No strong-Pareto optimal and non-empty SCR

that is Nash-implementable

A Case Study of BGP

BGP

..... .....

AS 1 Preference

AS N Preference

Routing Tree

Reverse engineering BGP

Non-emptiness: XUniqueness: XNon-dictatorship: X

Unanimity: Strong Pareto Optimality: only on

strict route preference domain

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!

Possibility of fixing BGP

BGP is (theoretically) Nash implementable (actually, also strong implementable)

But, only in a very simple game formThe problem: the simple game form may

not be followed by the ASes

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

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?

Thank you!

Comments or Questions: hao.wang@yale.edu

Thank you!

Comments or Questions: hao.wang@yale.edu