source selectable path diversity via routing deflectionssource routing `senders determine packet...

Source Selectable Path Diversity via Routing Deflections

Xiaowei Yang and David Wetherall

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OutlineOutlineDeflection Routing

Routing Rules

Tag Architecture

I d R lInter-domain Rules

EvaluationEvaluation

Conclusion

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Source routingSource routingSenders determine packet routes.

Improve the performance and reliability of networksImprove the performance and reliability of networksLower latency

Force packets to follow a single path.

A id d i bl Avoid undesirable routes.Bypass faulty nodes.

Provide path diversity.p yKeep all possible routes to the destination.

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Source routingSource routingProblems

Cannot use preferred routesCannot use preferred routes.Hard to maintain up-to-date maps

Conflict with the ISP routing policyEnd-systems specify paths.

Security issueSource address spoofing in LSRR (loose source and record route)Source address spoofing in LSRR (loose source and record route)

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Deflection routingDeflection routingPath diversity in deflection routing

Routers Routers Have a set of paths constructed by deflection rules.Deflect packets independently.Forward packets along non-shortest path if it’s not desirable.

End-systems Affect the choice of deflection by tagging packets.y gg g p

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Deflection routingDeflection routingFeatures

Routing rulesRouting rulesConstruct diverse paths with loop-free connectivity.Build on the shortest path machinery.

ISP f i dlISP-friendly

Tag architectureEnd-systems tag packets to express routing preference.y g p p g p

Routers use tags in path selection.

How to carry tags?Tags are encoded in IP or carried in a shim protocol layer.g p y

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Routing Rules

Tag Architecture



Conclusion

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Deflection RulesDeflection RulesDeflection Rules

Generate a deflection set of neighboring nodes.Based on shortest-path machineryKey issues:

Correctnessloop-free (safety condition)Reach the destination (liveness condition)

EffectivenessSimulation result

NotionsCost(ni): minimum cost to reach a destination from ni

li: links between ni-1 and ni

G\li: network graph without li links

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G\li: network graph without li links

2095

856 700

6391295

366 587

233260548

366

1893 902846

1176

Cost(Seattle)=2095+639=2734C t(S l ) 1295+639 1934

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Cost(Sunnyvale)=1295+639=1934

2095

856639

1295 233

700

260548

366

1893 902846

587

1176

Cost(Seattle)= 2095+639=2734

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Cost(LA)=366+1295+639=2300

20952095

856639

1295 233

700

260548

366

1893 902846

587

1176

Cost(Indianapolis)= 548+902+1176=2626

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Cost(Indianapolis) 548 902 1176 2626Cost(Kansas City)=902+1176=2078


Routing Rules

Tag Architecture



Conclusion

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Tag ArchitectureTag ArchitectureEach packet carries a tag to select a path from deflection set.set.Tag properties

Lack of global meaningg gWith tag values, each ISP can select a path according to its own policy.

Default path has zero.

Different tags should select different pathsDifferent tags should select different paths.Available paths should be diverse enough to detour faults.Different tags may select a default path for backwards compatibility.

Two approachesShim layer tag encodingIP t di

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IP tag encoding

Shim Layer Tag EncodingShim Layer Tag Encoding

Shim layer tag is located between IP and transport layer.Shim layer tag is located between IP and transport layer.TTL range: portion of the path where tag selection is used.Default path conditions

out of TTL rangeout of TTL rangezero value of tagIP fragments

Only Initial fragment has shim headerOnly Initial fragment has shim header.How to map tag values to paths?

Tag Mapping Procedure

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IP Tag EncodingIP Tag Encoding

TTL field for tag selection rangeCommon initial TTL values: 30, 32, 60, 64, 128, 255Internet paths rarely exceed 40 hopsRarely used TTL space: 128 ~ 215Tag selection range: 160-200Turn off tag selection at other values

Disadvantages:Overloading of IP cannot guarantee backwards-compatible.Traceroute cannot be used.

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Other proposals cannot use IP identification field.

Mapping Tags to Deflection pathsMapping Tags to Deflection pathsAssumption

A deflection set has K membersA deflection set has K members.The tag number is T.

Tag Mapping Procedureg pp gGive random numbers 1~K to members.

Default route: 0

Ch i b P (P ≥ K)Choose a prime number P (P ≥ K).N = (T mod P) mod K

Outer mod: produces a number in right range.p g gInner mod: avoids same choice between routers given same K and T.

Members already have different IDs!

Select a path #N from the deflection set

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Select a path #N from the deflection set.


Routing Rules

Tag Architecture



Conclusion

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Inter-domain RulesInter domain RulesDeflection affects BGP and IGP.Problem: default egress point and cost metric may change Problem: default egress point and cost metric may change unexpectedly when the packet is deflected.Solution: extend cost metric

From cost(n, dst) to cost(n, nexthop(n, dst))

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Routing Rules

Tag Architecture



Conclusion

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EvaluationEvaluationGoal: Simulate tag-based deflections to characterize path diversity.Desirable result: High degree of path diversity to avoid faulty nodes on default pathsEvaluate path diversity in three metricsEvaluate path diversity in three metrics

Usable deflection paths between source and destination nodesNumber of neighbors in deflection setNumber of different deflection pathsNumber of different deflection pathsDifference between fraction paths and the shortest path

Ability to route around faulty nodesRe r ted fracti ns f the sh rtest athRe-routed fractions of the shortest path

Ability to switch peering pointsNumber of not lowest cost egress

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Number of Deflection NeighborsNumber of Deflection Neighbors

Rules 2 and 3 produce more deflections in all simulated networks.The larger networks provide more deflections.The larger networks provide more deflections.

Abilene, Exodus and Sprint have 11, 79 and 315 nodes, respectively.Large number of routers can deflect with Rules 2 and Rule 3

More than 40% of routers have multiple neighbors

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More than 40% of routers have multiple neighbors.

Number of Deflection PathsNumber of Deflection Paths

In large networks, nearly all node pairs have a deflection path.Even for Abilene, more than 80% of node pairs have a deflection path.In case of Sprint, many node pairs have close to the maximum number p y pof deflection paths, for ten-bits tag.

Rules 2 and 3 outperform Rule 1 by a wide margin.Tag mapping rules do a good job of mapping different tags to diff

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different routes.

Difference between Deflection Routes and Shortest PathsShortest Paths

Diff th l t f ti f th h t t th Difference means the largest fraction of the shortest path nodes that can be bypassed with a single deflection.

Default shortest path:Deflection paths:Deflection paths:Find the largest among all deflection path.

At least 60% of all node pairs have a node-disjoint deflection path under Rule 2 or Rule 3

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path under Rule 2 or Rule 3.

Number of Fault-Avoiding Node PairsNumber of Fault Avoiding Node Pairs

Experiment:Experiment:Pick a faulty node on default path randomly.Try to bypass the fault with ten different tags at most.

Tag values: 1~5, five random valuesCount number of node pairs that avoid a faulty node after ten tries.

A large fraction of node pairs affected by a faulty node could avoid it.In all three networks, Rule 3 stays close to the top of the lines.

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Number of Tags to Bypass a FaultNumber of Tags to Bypass a Fault

Not all faults can be bypassedNot all faults can be bypassed.Failed: node pairs needed more than ten tries.Unavoidable: node pairs that cannot avoid a faulty node.

Only one or two tags need to be tried in most cases.O y o e o two tags ee to be t e ost cases.A source can quickly find an alternative path to avoid a particular node.

There is no significant difference between trying ten tags and all tags.

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Number of switching peering pointsNumber of switching peering points

ExperimentExperimentRandomly choose peering points P where |P|=2,3,4,5.

p is a default peering point for nodes having the lowest IGP cost.Simulate the shortest path routing inside ISP.Measure number of nodes that switch peering points.

Rules 2 and 3 always provide more peering choices.As the number of peering points is larger, the fraction is higher.

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Routing Rules

Tag Architecture



Conclusion

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ConclusionConclusionPresent a practical routing system which has benefits of source-controlled routing.source controlled routing.

Deflection rules provide good path diversity.Tags are effective to access path diversity.

Moreover, deflection routing is scalable, compatible with ISP policies, and incrementally deployable.

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Q/AQ/A

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source selectable path diversity via routing deflectionssource routing `senders determine packet...

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