On Selfish Routing In InternetLike Evironments

1
On Selfish Routing In Internet-Like Evironments

• Lili Qiu (Microsoft Research)
• Yang Richard Yang (Yale University)
• Yin Zhang (ATT Research)
• Scott Shenker (ICSI)

2
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 todays IP routing (e.g., delay,
  loss rate)
• Selfish by nature (i.e., optimize user-centric
  performance without considering system-wide
  criteria)

3
Motivation (Cont.)

• Theory front
• Roughgarden et al. showed selfish routing can
  result in serious performance degradation due to
  lack of cooperation

4
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

L(x)1
src
dest
L(x)x
5
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
Outline

• Overview
• Network model
• Evaluation Methodology
• Performance results
• Physical routing
• Overlay routing
• Multiple overlays
• Interaction with traffic engineering
• Summary and future work

7
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
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
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
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
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)

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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 Jacobs relaxation algorithm
• Computing the equilibria of multiple overlays
• Use the relaxation algorithm to guarantee the
  convergence

13
Outline

• Overview
• Network model
• Evaluation Methodology
• Performance Evaluation
• Source routing
• Overlay routing
• Multiple overlays
• Interaction with traffic engineering
• Summary and future work

14
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
Selfish Source Routing Latency

• Effects of network topologies (M/M/1, load scale
  factor1, OC3 bandwidth)

Selfish routing yields close to optimal latency,
much better than compliant routing
16
Selfish Source Routing Network Load

• Effects of network topologies

Selfish routing tends to overload links.
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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

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Outline

• Overview
• Network model
• Evaluation Methodology
• Performance results
• Source routing
• Overlay routing
• Multiple overlays
• Interaction with traffic engineering
• Conclusion and future work

19
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)

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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

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Selfish Overlay Routing (Full Overlay Coverage)

• overlay-src with opt-weight and hop-count
  performsimilarly as source routing
• overlay-src with random-weight performs much
  worse.

22
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
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.
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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

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Outline

• Overview
• Network model
• Evaluation Methodology
• Performance results
• Source routing
• Overlay routing
• Multiple overlays
• Interaction with traffic engineering
• Conclusion and future work

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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

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Interactions among Competing Overlays (Cont.)

• Effects of network-level routing

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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

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Outline

• Overview
• Network model
• Evaluation Methodology
• Performance results
• Source routing
• Overlay routing
• Multiple overlays
• Interactions with traffic engineering
• Conclusion and future work

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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?

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Specification of Vertical Interactions

• Interactive process between two players
• Traffic engineering
• Given traffic matrix Tt, where Tt(s,d) denotes
  traffic from source s to destination d in time
  slot t
• Compute routing matrix Rt for the underlay
• Objective avoid overloading network
• Selfish routing
• Given routing matrix Rt for the underlay
• Produce new traffic matrix Tt
• Objective minimize latency

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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(t1)

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Vertical Interaction with OSPF Optimizations
OSPF route optimization interacts poorly with
selfish routing
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Vertical Interaction with MPLS Optimization
MPLS optimization interacts with selfish routing
more effectively
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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

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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

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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
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)