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Placing Relay Nodes for
Intra-Domain Path Diversity
Meeyoung Cha, Sue Moon, Chong-Dae Park
Aman Shaikh
To appear in IEEE INFOCOM 2006
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Routing Instability in the Internet
• 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?
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Path Diversity and Overlay Networks
• 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.
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Objective of Our Work
• 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
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Path Diversity – Disjoint Overlay Path
ISP Network
Destination
(egress router)
relays
Origin
(ingress router)
Disjoint overlay path gives maximum
robustness against single link failures!
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Impact of ECMP on Overlay Path Selection
• Completely disjoint overlay paths are often not possible.
- Existing path diversity: Equal Cost Multi-Paths (ECMP)
AR
AR
BR
BR
Inter-PoP
BR
BR
AR
AR
Intra-PoP
(AR: Access Router, BR: Border Router)
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Partially Disjoint Overlay Path
We may need to allow partially disjoint paths.
overlay path
o
r
default path
d
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.
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Penalty for Overlapped Links
• 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 od fails | link l fails ]
0.25
0.125
0.125
0.5
o
1.0
0.25
0.125
0.5
d
0.875
0.5
0.75
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Penalty Measures
• Consider overlay path (ord) is used with default one
(od).
r
o
d
Penalty – fraction of traffic carried on overlapped link
• 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 ord and od 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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Placement Algorithms
• 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
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Evaluation Overview
• 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.
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Data Sets
• 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 at
http://an.kaist.ac.kr/~mycha/docs/CS-TR-2005-214.pdf
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Sensitivity to Network Dynamics
5% of nodes are selected as relays
10% of nodes are selected as relays
Relay nodes by initial placement are nearly as good as daily
relocation: relatively insensitive to network dynamics.
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Hypothetical Traffic Loss from Failure Event Logs
(failure events)
less than 1% of traffic
lost for 92.8% failures
complete protection
for 75.3% failures
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Conclusions
• 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.
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Further Works
• 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?
END
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