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An Approach to Alleviate Link Overload as Observed on an IP Backbone. Tuesday, April 1 st Infocom 2003. Sundar Iyer 1,2 , Supratik Bhattacharrya 2 , Nina Taft 2 , Christophe Diot 2 1 Stanford University, 2 ATL SprintLabs. Contents. Introduction Pathology of link overload

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An Approach to Alleviate Link Overload as Observed on an IP Backbone

Tuesday, April 1st

Infocom 2003

Sundar Iyer1,2, Supratik Bhattacharrya2, Nina Taft2, Christophe Diot2

1Stanford University, 2ATL SprintLabs


Contents
Contents Backbone

  • Introduction

  • Pathology of link overload

  • Alleviate overload - deflection routing

  • Performance analysis


There should be no link overload
There should be no link overload Backbone

  • Overload: More than 50% utilization

  • IP backbones are

    • Overprovisioned  low average utilization

    • Have multiple paths

  • Routing algorithms

    • balance load across multiple shortest paths

    • should reduce the likelihood of overload


But there is link overload
But there is link overload Backbone

  • Shortest path routing

    • puts load on a small set of equal cost shortest paths

    • causes unequal use of link capacity

  • Unpredictable traffic

    • Short term load fluctuations e.g. hotspots

  • Failure

    • Link failures, fiber cuts, network maintenance

  • Hard to predict all factors apriori


Why bother about link overload
Why bother about link overload? Backbone

  • Operators upgrade persistently overloaded links

    • Peaks in link utilization  cannot increase average utilization

  • Severe link overload causes packet drops

  • Interactive, real-time applications make it mandatory to overcome overload


Contents1
Contents Backbone

  • Introduction

  • Pathology of link overload

  • Alleviate overload - deflection routing

  • Performance analysis


Methodology
Methodology Backbone

  • Measurement of data from the Sprint backbone

    • Analyzed 138 backbone links for 9 months

    • SNMP link utilization data polled every 5 minutes

    • The link utilization is an exponentially weighted moving average (EWMA)

    • Measurements under-estimate overload

    • Short term fluctuations are missed


Maximum load
Maximum load Backbone

Maximum Load

  • Observation 1: There is always some overloaded link


Contribution of links to overload
Contribution of links to overload Backbone

Non-Overloaded links

Overloaded links

  • Observation 2: Most of the links are not overloaded


Types of link overload

and temporary overload

Types of link overload

Periods of link overload

  • Observation 4: Often just 1-2 links are simultaneously overloaded


Causes of temporary link overload
Causes of temporary link overload Backbone

Link Utilizations

  • Observation 5: Link failures cause temporary overload

  • Observation 6: Fiber cuts cause severe overload


Contents2
Contents Backbone

  • Introduction

  • Pathology of link overload

  • Alleviate overload - deflection routing

  • Performance analysis


The case for deflection routing

Allow normal network operation most of the time Backbone

The case for deflection routing

  • Previous techniques

    • useful for long term overload

    • change normal functioning of the network

    • useful when overload is common

  • We observe that link overload

    • is relatively rare (0.1% of the time on any link)

    • are typically caused due to link failures/maintenance

    • lasts for minutes-hours on average

    • occurs on maximum of 1-2 links simultaneously

    • can be easily overcome by deflecting packets


Problem
Problem Backbone

  • Problem:

    • How can we design a simple, stateless, loop-free deflection algorithm to overcome link overload?

  • Theorem 1: (sufficiency)

    • Any deflection algorithm which deflects packets with “strictly decreasing cost” is loop-free


Explanation of theorem 1
Explanation of Theorem 1 Backbone

  • A packet is forwarded from node s to d according to the strictly decreasing cost criteria as follows

    • If shortest path not overloaded

      Forward the packet on the shortest path with cost C

    • If link to neighboring node n is not overloaded

      Forward the packet to n if n’s cost to d is  C

    • Else

      Forward the packet on the shortest path


Intuition for theorem 1

Yes Backbone

No

  • Loop-free deflection routing:

  • we do not consider the cost of reaching the deflection node

Intuition for Theorem 1

20

15

30

Router: n1

10

10

Router: s

Router: d

Router: n2

20

25

Router: n3

  • Shortest path routing:

    • forward packet on the shortest path

    • the sequence of costs to a destination is strictly decreasing


Problem1
Problem Backbone

  • Problem:

    • Can we always find loop-free deflection paths according to the strictly decreasing cost criteria?

  • Theorem 2: (sufficiency)

    • A network with redundant equal length paths always has a loop-free deflection path if the link weights are in a ratio 1 + 1/(d-1), where d is the diameter of the network


Requirements
Requirements Backbone

  • Intuition:

    • All link weights are in the range [Wmin ,Wminx]

    • the minimum cost of the shortest path is dWmin

    • the maximum cost of the deflection path is (d-1)Wminx

    • (d-1)Wminx  dWmin  x  1 + 1/(d-1)

  • Criteria for Theorem 2

    • Need equal length shortest paths between any two nodes

    • Weights need to be within a bounded ratio “1 + 1/(d-1)”

    • The diameter d of the network should be small


Topology considerations inter pop network

ANA-1 Backbone

SJ-1

NYC-1

CHI-1

RTP-1

FW-1

CHI-4

ANA-4

NYC-4

RTP-4

SJ-4

FW-4

SJ-2

RTP-2

NYC-2

CHI-2

FW-2

ANA-2

ANA-3

RTP-3

CHI-3

NYC-3

SJ-3

FW-3

Topology ConsiderationsInter-PoP Network

PoP

New York

PoP

San Jose

PoP

Chicago

Small diameter, d=3

Redundant equal length paths are guaranteed

PoP

Anaheim

PoP

RTP

PoP

Fort-Worth

Large inter-POP weights are within ratio


Topology considerations complete network

Perfect Mesh in PoPs Backbone

SJ-1

NYC-1

RTP-1

FW-1

NYC-4

RTP-4

SJ-4

FW-4

SJ-2

NYC-2

RTP-2

FW-2

Redundant equal length paths not guaranteed

RTP-3

FW-3

NYC-3

SJ-3

Small (wmax) Intra-POP Weights

Topology ConsiderationsComplete Network

CHI-1

CHI-2

CHI-4

CHI-3

Diameter is larger

ANA-1

ANA-2

ANA-4

ANA-3

Large Inter-POP Weights


Problem2
Problem Backbone

  • Inter-PoP Network: PoPs as a single ‘logical node’

    + All criteria for theorem 2 are satisfied

  • The complete network

    - Equal length redundant paths does not exist

    - Diameter of the network is not small

    - Maximum intra-PoP link weight wmax is unrelated and very small compared to inter-PoP link weights

  • Problem

    - Cannot satisfy theorem 2 for the complete network


Practical deflection routing algorithm solution clumping a pop

Inter- Backbone

PoP

Intra-

PoP

Practical deflection routing algorithmSolution: Clumping a PoP

  • A packet is forwarded from node s to d as follows, where wgain = wmax

    • If shortest path not overloaded

      Forward the packet on the shortest path (with cost C)

    • If link to neighboring node n is not overloaded

      Forward the packet to n if n’s cost to d is C –wgain

    • Else if link to (intra-PoP) node n’ is not overloaded

      Forward the packet if its cost to d is  C +wmax

    • Forward the packet on the shortest path


Theorem 3
Theorem 3 Backbone

  • Theorem 3:

    • The practical deflection routing algorithm has no inter-PoP loops

  • Comments

    • The sequence of costs strictly decreases across PoPs

    • This is in keeping with the idea of ‘PoPs’

  • Link failures

    • The algorithm is extended by setting wgain = (n-1)wmax


Contents3
Contents Backbone

  • Introduction

  • Pathology of link overload

  • Alleviate overload - deflection routing

  • Performance analysis


Simulations
Simulations Backbone

  • Simulation parameters

    • 14 node inter-PoP network and 4-5 node intra-PoP network

    • Estimated traffic matrix with gravity models & link measurements

    • Deflection threshold was set to 45%

    • Deflection based on fast EWMA

    • Simulations for link failures and fiber cuts


Link overload due to a fiber cut
Link overload due to a fiber cut Backbone

  • Deflection routing decreases the maximum load amongst all links in the backbone


Conclusions
Conclusions Backbone

  • Deflection routing algorithm

    • Based on practical considerations and overload pathology

    • Exploits backbone architecture, meshed topology

    • Mandates a condition on weights which is not too restrictive

    • Is loop-free across PoPs

  • Note

    • Needs a redundant backbone network with equal-length paths

    • Useful when average utilization is low

  • Future Work

    • Stability needs to be investigated


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