On the Interaction between Dynamic Routing in the

Native and Overlay Layers

INFOCOM 2006

Srinivasan SeetharamanMostafa Ammar

College of ComputingGeorgia Institute of Technology

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Infrastructure overlay networks offer better services by deploying intelligent routing schemes.

Uncoordinated dynamic routing in the two layers lead to many problems.

We focus on the effect of native link failures, as they trigger each layer to reroute independently

Dual Rerouting

Inter-Layer Interaction Problem

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Temporal Dynamics

Consider a native link failure in CEOnly one overlay link is affected.The native path AE is rerouted over F (ACE → ACFDE)

Native Failure

Overlay recovery: 8

Overlay rerouting: 4

Original: 2Native Rerouting: 2

Time

∞

Native Recover

y

Native Repair

Cost

A

B

C

D

F

E

H

I

G

A

E

I

G3

23

2

+

OVERLAY

NATIVE

4

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1. Overlap of functionality between layers causing large number of route flaps (oscillations)

2. Unawareness of other layer’s decisions leading to resource overloading, multiple simultaneous failures a low success rate in rerouting sub-optimal paths after rerouting

3. Lack of flexibility and control

Downside to Dual Rerouting

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Problem Statement I

Assume the two ends of each link (native & overlay) use a keepAlive protocol for link verification.

3 keepAlive messages lost Failure

Understand the effects of different parameters on the rerouting performance.

KeepAlive-time: Time between two keepAlive messages Hold-time: Time window to declare link as down Overlay link cost scheme (Ex: Native hops, Overlay

hops)

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Performance Metrics

1. Hit-time: Time taken for traffic to be recovered. = Detection time + Convergence time +Device time

(depends on timers) (protocol specific)(Negligible)

2. Success rate of recoverySuccess rate of a layer = Number of paths recovered

Number of failed overlay paths

3. Number of route flapsAverage route flaps = Number of route flaps

Number of failed overlay paths

4. Peak & Stabilized inflation (before repair)Path cost inflation = Path cost after rerouting

Path cost before failure

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Temporal Dynamics

Native Failure

Overlay recovery: 8

Overlay rerouting: 4

Original: 2Native Rerouting: 2

Time

Hit time∞

Native Recover

y

Native Repair

Cost

Overlay path AEOverlay detects first100% success rate3 route flapsPeak inflation = 8/2Stabilized inflation = 4/2

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Performance Evaluation – ns2

Using GT-ITM, we randomly generate:25 topologies = (5 overlay network) x (5 native network)

Two scenarios1. Inspect intra-domain failures in single-domain native network2. Inspect inter-domain failures in multi-domain native network

In each scenario, tabulate failure recovery statistics of all overlay paths by breaking one native link at a time

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Effect of Routing Parameters

Observations: By varying the overlay keepAlive-time, hold-time and cost scheme, we observe:

hold-time hit time (only until overlay hold-time < native hold-time)

hold-time # route flaps hold-time sub-optimality keepAlive-time hit-time

hold-time

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Conclusion I

Dual rerouting can be made optimal by adopting the following recommendations:

Overlay hold-time very close to the native hold-time.

Overlay keepAlive-time that is half that of the hold-time as it leads to an earlier detection.

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Problem Statement II

Main observation from previous simulations: “Native-rerouting yields the optimal path, albeit a bit

later”

Make the overlay layer aware of this observation and give higher precedence to native rerouting attempts

Improve overlay routing performance by adjusting the overlay layer functioning

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Three Levels of Layer Awareness

1. No awareness Dual rerouting

2. Awareness of native layer’s existence: Probabilistically Suppressed Overlay Rerouting

(PSOR):Suppress overlay rerouting attempt with probability ‘p’

Deferred Overlay Rerouting (DOR):Delay overlay recovery by time ‘d’

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3. Awareness of native layer’s parameters: Follow-on Suppressed Overlay Rerouting (FSOR)

If follow-on time < threshold ‘f’, then suppress overlay rerouting

Three Levels of Layer Awareness (contd.)

TimeOverlay layerdetects failure

Native layer detects failure

Failure

Follow-on time

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Effect of Adjusting Overlay

All three schemes are simple and offer significant control over the tradeoffs between hit-time and the other metrics.

PSOR: Least number of route flaps Least peak inflation

DSOR and FSOR behave similarly (FSOR has slightly better hit-time):

Better success rate Lower stabilized inflation

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Conclusion II

By appropriately tuning keepAlive-time hold-time suppression probability delay follow-on threshold

…we can improve results for: Hit-time # Route flaps Path cost inflation Stabilization time Success rate

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Problem Statement III

Main observation from previous simulations: “It is not possible to improve all metrics

simultaneously. Hence, performance is still bounded!”

As overlay applications proliferate, the native layer should gradually evolve to suit them

Improve overlay routing performance by adjusting the native layer functioning

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Tuning the Native keepAlive-time

We adopt a non-invasive procedure to advance the native layer rerouting

Tuning of the native layer keepAlive-time

Constraints: Tuning should not generate any extra overhead Effective detection time should be same

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Consider the following scenarios for tuning. Scenario B is vanilla Dual rerouting

Scenario A is the layer-aware overlay rerouting schemeScenario C is the tuning we recommend here

Tuning the Native keepAlive-time (contd.)

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Conclusions III

Native layer tuning we proposed achieves the best performance in all our metrics

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Summary

We propose means to mitigate the problems associated in the inter-layer interaction

We explore two directions:1. Adjusting the overlay layer functioning2. Adjusting the native layer functioning