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Meeyoung Cha, Sue Moon, Chong-Dae Park

Aman Shaikh

Placing Relay Nodes forIntra-Domain Path Diversity

To appear in IEEE INFOCOM 2006

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• Link and router failures are frequent. • Routing protocols are used to detect such

failures and route around them.– Convergence time is in the order of seconds or

minutes. – End-to-end connections experience long outages.

• How to increase reliability and robustness of mission-critical services against temporary end-to-end path outages?

Routing Instability in the Internet

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• Take advantage of path diversity provided by the network topology.

Overlay path – use a node inside the network to relay packets over an alternate path that is different from the default routing path. ex) RON [Anderson et al., SOSP 2001]

Detour [Savage et al., IEEE Micro 1999]

Use disjoint overlay paths along with the default routing path to route around failures.

Path Diversity and Overlay Networks

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• Previous work has focused on selecting good relay nodes assuming relay nodes are already deployed.

• As an ISP, we consider the problem of placing relay nodes well. – Find a fixed set of relay nodes that offer as

much path diversity as possible to all OD pairs.

We assume:· Intra-domain setting [Shortest Path First

Routing]· Relays are simply routers with relaying

capability· Overlay paths use single relay nodes

Objective of Our Work

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disjoint overlay path

Destination(egress router)

default path

relays

ISP Network

Disjoint overlay path gives maximum robustness against single link failures!

Path Diversity – Disjoint Overlay Path

Origin(ingress router)

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• Completely disjoint overlay paths are often not possible. - Existing path diversity: Equal Cost Multi-Paths (ECMP)

Impact of ECMP on Overlay Path Selection

Intra-PoP

AR

AR BR

BR

BR

BR AR

AR

Inter-PoP

(AR: Access Router, BR: Border Router)

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Partially Disjoint Overlay Path

We may need to allow partially disjoint paths.

Such overlap makes networks less resilient to failures. We introduce the notion of penalty to quantify the quality degradation of overlay paths when paths overlap.

o d

r

default path

overlay path

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Penalty for Overlapped Links

0.5

0.5

0.25

0.25

0.5 0.75

0.125

0.125

0.875

0.125

1.0o d

• Impact of a single link failure on a path- prob. a packet routed from o to d encounters a failed link l Io,d,l = P[ path od fails | link l fails ]

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• Consider overlay path (ord) is used with default one (od).

Penalty – fraction of traffic carried on overlapped link

Penalty Measures

o d

r

• Penalty of a relay r for OD pair (o,d) – prob. both packets routed (1) directly from o to d and (2)

indirectly from o to d via r encounter a single link failure Po,d(r) = P[ both ord and od fail | single link failure ]

• Penalty of a relay set R of size k – sum of minimum penalty of all OD pairs using relays in R

∑o,d min( Po,d(r) | r in R )

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• How to find a relay set R of size k with minimum penalty

• Optimal solution– Exhaustive search, 0-1 Integer Programming (IP)– Too expensive

• Greedy selection heuristic– Start with 0 relays – Iteratively make a greedy choice that yields minimal penalty– Repeat until k relays are selected

• Local search heuristic– Start with k set of random relays– Repeat single swaps if penalty is reduced

Placement Algorithms

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• Overall performance– Tradeoff between number of relays and reduction in

penalty– Comparison of metric-sensitive heuristics against

optimal and other possible heuristics (random, degree-based)

• Sensitivity to network dynamics– Using three-month topology snapshots, we examine

whether relays selected remain effective as topology changes.

– Using six-month network event logs, we calculate fraction of traffic that would have been protected from failures by using relays.

Evaluation Overview

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• We use an operational tier-1 ISP backbone and daily topology snapshots and event logs. Topology - 100 routers, 200 links Hypothetical traffic matrix

- assumes equal amount of traffic between OD pairs

• For results on other topologies (1 real, 3 inferred, 6 synthetic), please refer to our technical report athttp://an.kaist.ac.kr/~mycha/docs/CS-TR-2005-214.pdf

Data Sets

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Sensitivity to Network Dynamics

Relay nodes by initial placement are nearly as good as daily relocation: relatively insensitive to network dynamics.

5% of nodes are selected as relays

10% of nodes are selected as relays

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Hypothetical Traffic Loss from Failure Event Logs

complete protectionfor 75.3% failures

less than 1% of trafficlost for 92.8% failures

(failu

re e

vents

)

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• This is the first work to consider relay placement for path diversity in intra-domain routing.

• We quantify the penalty of using partially disjoint overlay paths; and propose two heuristics for relay node placement.

• We evaluate our methods on diverse dataset. – Relays by our method perform consistently better than

other heuristics and are near-optimal.– A small number of relay nodes (less than 10%) is

effective over the entire course of several months.– Relays are relatively insensitive to network dynamics.

Conclusions

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• Relay architecture and practical considerations– loose source routing option in routers/attaching

servers to routers– reflecting real traffic matrix

• Relay placement in inter-domain setting– inter-domain routing is based on BGP’s path

selection– very challenging: AS path inference, AS path

asymmetries, and realistic traffic matrix estimation

• Lower layer path diversity – at physical layer, disjoint IP layer paths may run

over the same optical fiber– how to incorporate fiber map into our algorithm?

Further Works

END