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: [email protected]
Thank you!
Comments or Questions: [email protected]