Network Topology Measurement
Yang Chen
CS 8803
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Outline
•
•
•
•
Big Picture
ISP Topology Measurement
Statistical Results
Problems & Solutions
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Heuristics for Internet Map
Discovery
R. Govindan and H. Tangmunarunkit
INFOCOM 2000
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Why do we need the topology?
• Understand the macroscopic properties of
the Internet physical structure
• Network management
• Topology-aware algorithms
• Simulation and topology generation tools
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On-going efforts
• CAIDA Skitter
• Router View
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Fundamental: traceroute
Prober sends packets with successively increased TTL.
A router responds with ICMP time exceeded when the
probe is with TTL=1
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Fundamental: traceroute
Geographic info can help on building up the topology.
* Data
from http://www.linkwan.com/vr/
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Fundamental: Tree => map
1)Source routing
2)Multiple Vantage points
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Address scan space
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•
•
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BGP tables
Route Table
Database
Informed Random Address Probing
– A response from some IP address is considered as a
sign that some prefix P of A must contain addressable
nodes;
– If P is an addressable prefix, the neighboring prefixes
of P are also considered as possibly addressable.
(128.8/16 and 128.10/16 are neighbors of 128.9/16)
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Some results
• 150,000 interfaces and nearly 200,000 links
• Findings related to source route
– Simulation demonstrated that In relatively sparse
random networks, a few source route capable nodes
(< 5%) are sufficient to discover 90% of the links. In
fact, there are 8% routers support source route.
– Source route discovered links do not skew the
qualitative conclusion on the network statistics.
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For example: degree distribution
Similar observation on hop-pair distribution
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Measuring ISP Topologies
with Rocketfuel
N.Spring, R. Mahajan and D. Wetherall
ACM Sigcomm 2002
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ISP network infrastructure
Access
Router
Access
Router
Backbone
Router
Backbone Link
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ISP topology measurement
• An old story: the blinds and the elephant
ISP
Traceroute
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ISP topology measurement
Traceroute
Server
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Focusing on one ISP – Directed probing
Network
* 192.9.9.0
*
*
*
*
*>
*
*
*
blackrose.org
Verio
Sprint
MCI
LINX
CERFnet
IIJ
PIPEX
IAGnet
* BGP
Next Hop
204.212.44.128
205.238.48.3
144.228.240.93
204.70.4.89
194.68.130.254
134.24.127.3
202.232.1.8
158.43.133.48
131.103.20.49
(Ann Arbor)
(MAE-WEST)
(Stockton)
(San Francisco)
(London)
(San Diego)
(Japan)
(London)
(Chicago)
M/LP/Weight Path
0 234 266 237 3561 701 90 i
0 2914 1 90 i
0 1239 701 90 i
0 3561 1 90 i
0 5459 5413 1 90 i
0 1740 701 90 i
0 2497 701 90 i
0 1849 702 701 90 i
0 1225 2548 1 90 i
204.212.44.128 through AS234
205.238.48.3 through AS2914
144.228.240.93 through AS1239
204.70.4.89 through AS3561
194.68.130.254 through AS5459
134.24.127.3 through AS1740
202.232.1.8 through AS2497
158.43.133.48 through AS1849
131.103.20.49 through AS1225
table source: RouteView project
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Focusing on one ISP – Directed probing
• Traceroutes to dependent prefixes: All traceroutes to these
prefixes from any vantage point should transit the ISP.
Dependent prefixes can be readily identified from the BGP
table. All AS-paths for the prefix would contain the number of
the AS being mapped.
• Traceroutes from insiders: We call a traceroute server located
in a dependent prefix an insider. Traceroutes from insiders to
any prefix should transit the ISP.
• Traceroutes that are likely to transit the ISP based on some
AS-path are called up/down traces.
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Path/Query reduction
Share Ingress
Share egress
Same next-hop
AS number
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Impacts of directed probing
1) Fraction of useful but pruned traces from 0.1 to 7%
2) Unnecessary traces around 6% over all the ISPs
*Comparison
based on Skitter data
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Impacts of ingress reduction
Overall, ingress reduction
keeps only 12% of the traces
chosen by directed probing.
The number of vantage points that share an ingress by rank
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Impacts of egress reduction
Overall, egress reduction
keeps only 18% of the
Dependent prefix traces
chosen by directed probing.
The number of dependent prefixes that share an egress by rank
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Impacts of next-hop reduction
Overall, Next-hop AS reduction
Reduces the number of traces
to 5% of those chosen by
directed probing.
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POP sizes analysis
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Power Law
• Complementary cumulative distribution function
(CCDF) P(X>x)
• Pareto Distribution
 x
P( X  x )  
 xmin



k
P( X  x )  x
k
• Power Law
ln( y )  C ln( x )
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Router degree distribution
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Peering structure
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Difficulties in topology discovery
• Shared media
• Backup links
• Router Identification and annotation
• Alias resolution
• Completeness Validation
Currently, none of them is completely solved!
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POP hierarchy
Naming convention, DNS information and neighbor inferring
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Backbone topology
AT&T
Level 3
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Alias Problem
OR
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Alias: is it a big deal?
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Alias resolution
• Send a packet with unreachable port to
certain interfaces which are possible alias.
The corresponding ICMP port unreachable
response will contain the source address.
• IP identifier
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Completeness validation
• Comparison with Router Views
• Comparison with Skitter
• IP address space
– Search prefixes of ISP’s address space for
additional IP addresses
• Validation with ISPs
– Is “Good” enough?
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In Search of Path Diversity
in ISP Networks
P. Teixeira, K. Marzullo, S. Savage
and G. M. Voelker
IMC 2003
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Real metric instead of counting links
• Path diversity
– Metric that reflects the number of routes
available between two points in the network
• An extreme example
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Real topology speaks
Inter-PoP Path diversity in
the Sprint Network
Inter-PoP Path diversity inferred
by Rocketfuel
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Take a closer look
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Inaccuracy introduced during
probing
• Lack of vantage points
– How many points are sufficient?
• Incomplete traceroutes
– What can we do if ISP turns off traceroute
functionality?
• Changes in the path of a probe
• Incorrect DNS record
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Inaccuracy from processing probed
links
• Alias Resolution
• Adding reverse links
Missed and added links in Rocketfuel PoP topology relative to
the number of links in the Sprintreal topology
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Questions?
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