Post on 28-Mar-2015
University of Nevada, Reno
Router-level Internet Topology Mapping
CS790 PresentationModified from Dr. Gunes’ slides
by Talha OZ
Outline• Introduction• Internet Topology Measurement
– Topology Discovery Issues– Impact of IP Alias Resolution
• Topology Discovery– Resolving Anonymous Routers
• Graph-based Induction Technique
– Resolving Alias IP Addresses• Analytical and Probe-based Alias Resolution
– Resolving Genuine Subnets• Dynamic Subnet Inference
• SummaryInternet Topology Discovery 2
Internet
• Web of interconnected networks– Grows with no central authority– Autonomous Systems optimize local communication efficiency– The building blocks are engineered and studied in depth– Global entity has not been characterized
• Most real world complex-networks have non-trivial properties.
• Global properties can not be inferred from local ones– Engineered with large technical diversity– Range from local campuses to transcontinental backbone
providers3
Internet Measurements• Understand topological and functional characteristics of the
Internet– Essential to design, implement, protect, and operate underlying
network technologies, protocols, services, and applications
• Need for Internet measurements arises due to commercial, social, and technical issues– Realistic simulation environment for developed products,– Improve network management– Robustness with respect to failures/attacks– Comprehend spreading of worms/viruses– Know social trends in Internet use– Scientific discovery
• Scale-free (power-law), Small-world, Rich-club, Dissasortativity,…
Internet Topology Discovery 4
Internet Topology Measurement
• Types of Internet topology maps– Autonomous System (AS) level maps– Router level maps
• A router level Internet map consists of– Nodes: End-hosts and routers – Links: Point-to-point or multi-access links
• Router level Internet topology discovery– A process of identifying nodes and links among them
Internet Topology Discovery 5Lumenta Jan 06CAIDA Jan 08CAIDA Jan 00
6
Current Schema
Internet Topology MeasurementBackground
Internet topology measurement studies Involves topology collection / construction / analysis
• Current state of the research activities• Distributed topology data collection studies/platforms
– iPlane, Skitter, Dimes, DipZoom, …– 20M path traces with over 20M nodes (daily)
• Topology discovery issues1. Sampling2. Anonymous routers3. Alias IP addresses4. Genuine subnets
Internet Topology Discovery 7
Internet Topology MeasurementsProbing
• Direct probing
• Indirect probing
Internet Topology Discovery 8
A DB CIPB TTL=64
IPB
IPD TTL=64
IPD
Vantage Point
A DB C
Vantage Point
IPB
IPD TTL=2IPD TTL=1
IPC
Internet Topology MeasurementTopology Collection (traceroute)
• Probe packets are carefully constructed to elicit intended response from a probe destination
• traceroute probes all nodes on a path towards a given destination– TTL-scoped probes obtain ICMP error messages from routers on the path– ICMP messages includes the IP address of intermediate routers as its source
• Merging end-to-end path traces yields the network map
Internet Topology Discovery 9
S DA B C
DestinationTTL=1
IPA
TTL=2
IPB
TTL=3
IPC
TTL=4
IPD
Vantage Point
Internet Topology Measurement:Background
Internet Topology Mapping 10
S
L
U
H
C
N
W
A
s.2
l.1
s.3
u.1
l.3
u.3
h.1
k.3
h.2
h.3
a.3
u.2k.1 c.4
a.1 a.2
w.3c.3
w.1c.2
n.1 n.3
w.2
l.2
K
c.1
k.2
dh.4
Trace to Seattle
h.4
l.3
s.2
Trace to NY
h.4
a.3
w.3
n.3
Internet2 backbone
Internet Topology Measurement:Background
Internet Topology Mapping 11
S
L
UC
N
A
s.2
l.1
s.3
u.1
l.3
h.1
k.3
h.2
a.3
u.2k.1 c.4
a.1 a.2
w.3c.3
w.1c.2
n.1 n.3
w.2
l.2
K
c.1
k.2
h.3
dh.4
s.1e f
n.2
H
W
u.3
Internet Topology MeasurementTopology Collection
Internet Topology Discovery 12
Internet2 backbone
Traces• d - H - L - S - e• d - H - A - W - N - f• e - S - L - H - d• e - S - U - K - C - N - f• f - N - C - K- H - d• f - N - C - K - U - S - e
S
L
U
K
C
H
A
W
N
e
d
f
Topology Sampling Issues
• Sampling to discover networks – Infer characteristics of the topology
• Different studies considered – Effect of sample size [Barford 01]– Sampling bias [Lakhina 03]– Path accuracy [Augustin 06] – Sampling approach [Gunes 07]– Utilized protocol [Gunes 08]
• ICMP echo request• TCP syn • UDP port unreachable
Internet Topology Discovery 13
Topology Sampling Approaches
• Sampling techniques– Path sampling
• Diameter
– Edge sampling• Capacity
– Node sampling• Degree characteristics
• Sampling approach– (n,n) – traceroute based topology
• Returns the Internet map among n vantage points
– (k,m) – traceroute based topology where k<<m (k=n)• Returns the Internet map between k sources and m destinations
Internet Topology Discovery 14
Path sampling vs Node sampling
(k,m)-sampling vs (n,n)-sampling
Historical Perspective on ResponsivenessData Set
• ICMP path traces from skitter– 1st collection cycle of each year (from 1999 to 2008)
• Skitter had updates to destination IP addresses– major update in the system in 2004
• Processing– Alias IP addresses
• Analytical Alias Resolver (AAR) [Gunes-06]• Analytical and Probe Based Alias Resolver (APAR) [Gunes-09]
– Anonymous routers• Graph Based Induction (GBI) [Gunes-08]
Internet Topology Discovery 15
Current Practices in Responsiveness Data Set
• 536,743 destination IP addresses– from skitter and iPlane projects
• Between 7-11 April 2008
• Probes– ICMP echo request– TCP SYN– UDP to random ports
• Direct probes– ping
• Indirect probes– tracerouteInternet Topology Discovery 16
Current Practices in Responsiveness Direct probes
Internet Topology Discovery 17
Probe Responsive(%)
ICMP 81.9
TCP 67.3
UDP 59.9
Anonymous(%)
18.1
32.7
40.1
Router(%)
End-host(%)
84.6 77.9
70.4 62.8
64.7 50.3
537 K IPs 320 K 217 K
Current Practices in Responsiveness Direct probes (domain)
Internet Topology Discovery 18
Probe Anonymous(%)
ICMP 18.1
TCP 32.7
UDP 40.1
.net(%)
.com(%)
.edu(%)
.org(%)
.gov(%)
7.7 13.6 11.1 4.5 7.1
23.3 27.4 16.8 22.7 17
36.5 38.3 42.7 35.6 37.2
537 K IPs 5 K 1.7 K25.5 K 10.1 K 0.5 K
Current Practices in Responsiveness Indirect probes
Internet Topology Discovery 19
Probe Reached(%)
Nodes(thousand)
Anonymous(%)
ICMP 93.1 1,005 68.7
TCP 73.4 965 72.3
UDP 45.0 1,479 86.0
Nodes(thousand)
Anonymous(%)
45 9.7
35 12.5
41 9.4
Initial Final306 K traces
Current Practices in Responsiveness
Internet Topology Discovery 20
Nodes that respond to indirect probes might not respond to direct probes
Nodes are most responsive to ICMP probes (%82) least responsive to UDP probes (%60)
End hosts are less responsive than routers
Responsiveness is similar for different domains
Anonymous Router Resolution Problem
Internet Topology Discovery 21
• Anonymous routers do not respond to traceroute probes and appear as a in path traces
– Same router may appear as a in multiple traces.– Anonymous nodes belonging to the same router should be resolved.
• Anonymity Types1. Ignore all ICMP packets2. ICMP rate-limiting3. Ignore ICMP when congested4. Filter ICMP at border5. Private IP address
Anonymous Router Resolution Problem
Internet Topology Discovery 22
Internet2 backboneS
L
U
K
C
H
A
W
N
e
d
Traces• d - - L - S - e• d - - A - W - - f• e - S - L - - d• e - S - U - - C - - f• f - - C - - - d• f - - C - - U - S - e
f
Anonymous Router Resolution Problem
Internet Topology Discovery 23
U K C N
L H A W
S
d
e
f
Sampled network
d
e
fS U
L
C
AW
Resulting network
Traces• d - - L - S - e• d - - A - W - - f• e - S - L - - d• e - S - U - - C - - f• f - - C - - - d• f - - C - - U - S - e
Alias Resolution
• Each interface of a router has an IP address.
• A router may respond withdifferent IP addresses to different queries.
• Alias Resolution is the process of grouping the interface IP addresses of each router into a single node.
• Inaccuracies in alias resolution may result in a network map that– includes artificial links/nodes – misses existing links
Internet Topology Discovery 24
.5.33
.18
.13.7
Denver
Internet Topology Discovery 25
IP Alias Resolution Problem
S
L
UC
N
W
A
s.2
l.1
s.3
u.1
l.3
u.3
h.1
k.3
h.2
a.3
u.2k.1 c.4
a.1 a.2
w.3c.3
w.1c.2
n.1n.3
w.2
l.2
K
c.1
k.2
h.3
d
h.4
s.1e f
n.2
HTraces
• d - h.4 - l.3 - s.2 - e• d - h.4 - a.3 - w.3 - n.3 - f• e - s.1 - l.1 - h.1 - d• e - s.1 - u.1 - k.1 - c.1 - n.1 - f• f - n.2 - c.2 - k.2 - h.2 - d• f - n.2 - c.2 - k.2 - u.2 - s.3 - e
IP Alias Resolution Problem
Internet Topology Discovery 26
U K C N
L H A W
S
d
e
fSampled network
Sample map without alias resolution
s.3
s.1
s.2
l.3
l.1
u.1
u.2
k.1 c.1 n.1
n.2k.2 c.2
w.3
a.3
h.2
h.4
h.1
e
d
f
n.3
Traces• d - h.4 - l.3 - s.2 - e• d - h.4 - a.3 - w.3 - n.3 - f• e - s.1 - l.1 - h.1 - d• e - s.1 - u.1 - k.1 - c.1 - n.1 - f• f - n.2 - c.2 - k.2 - h.2 - d• f - n.2 - c.2 - k.2 - u.2 - s.3 - e
Genuine Subnet Resolution
Internet Topology Discovery 27
• Alias resolution– IP addresses that belong to the same router
• Subnet resolution– IP addresses that are connected over the same medium
IP2 IP3
IP4IP1
IP6 IP5
IP2 IP3
IP1
IP2 IP3
IP1
Outline
Internet Topology Discovery 28
• Introduction• Internet Topology Measurement
– Topology Discovery Issues– Impact of IP Alias Resolution
• Topology Discovery– Resolving Anonymous Routers (Hakan’s work !)
• Graph-based Induction Technique
– Resolving Alias IP Addresses• Analytical and Probe-based Alias Resolution
– Resolving Genuine Subnets• Dynamic Subnet Inference
• Summary
Summary - Anonymous Router Resolution
Internet Topology Discovery 29
DA
C
E
GBI
DA
C
E
Underlying
DA
C
E
Collected
DA
C
E
Neighbor Matching
Responsiveness reduced in the last decade NP-hard problem Graph Based Induction Technique
Practical approach for anonymous router resolution Takes ~6 hours to process data sets of ~20M path traces
Identifies common structures Handles all anonymity types Helpful in resolving multiple anonymous routers in a locality
Outline
Internet Topology Discovery 30
• Introduction• Internet Topology Measurement
– Topology Discovery Issues– Impact of IP Alias Resolution
• Topology Discovery– Resolving Anonymous Routers
• Graph-based Induction Technique
– Resolving Alias IP Addresses• Analytical and Probe-based Alias Resolution
– Resolving Genuine Subnets• Dynamic Subnet Inference
• Summary
IP Alias Resolution Problem
• A set of collected traces– w, …,b1, a1, c1, …, x– z, …,d1, a2, e1, …, y– x, …,c2, a3, b2, …, w– y, …,e2, a4, d2, …, z
Internet Topology Discovery 31
a
c
d
b
e
a sub-graph
a1
c1
b2
b1
c2
with no alias resolution
w
z y
x
xwa3
a2
e1
d2
d1
e2
yza4
Sample map from the collected path traces
13
4
11
1 1
2
2 2
22
A router may appear with different IP addresses in different path traces• Need to resolve IP addresses belonging to the same router
IP Alias Resolution Problem
Internet Topology Discovery 32
ac1
b2
b1
c2
partial alias resolution (only router a is resolved)
xw
e1
d2d1
e2
y
z
partial alias resolution (only router a is not resolved)
a2
c
d
b
e
w
z y
x
a3
a4
a1
a
c
d
b
e
sub-graph
w
z y
x
13
4
11
1 1
2
2 2
22
IP Alias Resolution: Previous Approaches
• Source IP Address Based Method [Pansiot 98]– Relies on a particular implementation of ICMP error generation.
• IP Identification Based Method (ally) [Spring 03]– Relies on a particular implementation of IP identifier field,– Many routers ignore direct probes.
• DNS Based Method [Spring 04]– Relies on similarities in the host name structures
sl-bb21-lon-14-0.sprintlink.net sl-bb21-lon-8-0.sprintlink.net
– Works when a systematic naming is used.
• Record Route Based Method [Sherwood 06]– Depends on router support to IP route record processing
Internet Topology Discovery 33
Dest = A
B
Dest = A
Dest = B
A, ID=100
Dest = B
B, ID=99B, ID=103
AB
AB
Analytical Alias Resolution Approach
• Leverage IP address assignment convention to infer IP aliases– Identify symmetric path segments within the collected set
of path traces– Infer IP aliases– Use a number of checks to
• Remove false positives• Increase confidence in the identified IP aliases
Internet Topology Discovery 34
IP address Assignment PracticesPoint-to-point Links
• For a point-to-point link – use either /30 subnet or /31 subnet
• The interface IP addresses on the link are consecutive and are within /30 subnet or /31 subnet– use ↔ to represent subnet relation between two IP
addresses• Use subnet relation (↔) to infer IP aliases
Internet Topology Discovery 35
A B
192.168.1.4/30192.168.1.5 192.168.1.6
192.168.1.4 192.168.1.5 192.168.1.4/31
/30 network
/31 network
IP address Assignment PracticesMulti-access Links
• A similar relation between IP addresses belonging to the same multi-access link holds
• Example: Consider two IP addressesA:129.119.1.10 and B: 129.119.1.13
– A and B are not together in a /30 or a /31 subnet– However, they are together in /29 subnet
129.119.1.8/29
A: 129.119.1.00001010B: 129.119.1.00001101
Internet Topology Discovery 36
A B
.10 .13
129.119.1.8/29 subnet
Analytical Alias ResolutionSample traceroute pairs
Internet Topology Discovery 37
MIT
UTD
18.7.21.1
18.168.0.27
129.110.95.1
129.110.5.1
206.223.141.73
192.5.89.89
206.223.141.70
192.5.89.10
198.32.8.34
198.32.8.85198.32.8.66
198.32.8.65
198.32.8.84
198.32.8.33
192.5.89.9
206.223.141.69
192.5.89.90
206.223.141.74
18.168.0.25
no response
18.7.21.84
no response
Aliases 129.110.5.1 - 206.223.141.74
206.223.141.73 - 206.223.141.69
206.223.141.70 - 198.32.8.33
…
APARAnalytical and Probe-based Alias Resolution
• There is possibility of– incorrect subnet assumption,
• Two /30 subnets assumed as a /29,
– incorrect alignment of path traces.• IP4 and IP8 are thought of as aliases.
• To prevent false positives, some conditions are defined– Trace preservation,– Distance preservation (probing component of APAR),– Completeness,– Common neighbor.
Internet Topology Discovery 38
a sample network
a
c d
b
e f
IP1
IP2
IP9
IP3
IP4
IP8
IP7
Analytical Alias ResolutionMain Idea
• Use traceroute collected path traces only– No probing is required at this point
• Study the relations between IP addresses in different traces– Infer subnets: Use the IP address assignment
convention to infer • Point-to-point (/30 or /31) subnets, or• Multi-access (/x where x<30) subnets
from the path traces
– Infer IP aliases: Align path segments to infer IP aliases from the detected subnets
Internet Topology Discovery 39
Analytical Alias Resolution:Potential Issues
• Problems with inferring subnets accurately– False positive: two separate subnets with
consecutive /30 subnet numbers may be inferred as one /29 subnet
– False negative: a /29 subnet may be inferred as two separate /30 subnets
• Problems with inferring IP aliases accurately– False positives and false negatives possible due to
incorrectly formed subnets– Both false positives and false negatives introduce
inaccuracies to the resulting topology mapInternet Topology Discovery 40
Analytical Alias ResolutionPotential Solutions
• How to verify the accuracy of formed subnets– Accuracy condition: Two or more IP addresses from
the same subnet cannot appear in a loop-free trace (unless they are consecutive)
• Check if a newly formed subnet violates this condition for any pair of available IP addresses from this subnet in any other path trace
– Completeness condition: To infer a /x subnet among a set of IP addresses that belong the address range, require that some fraction (e.g., 50%) of these addresses appear in our data set
• Needed to increase our confidence on the inferred subnet
– Processing order: Start with subnets with higher completeness ratio
Internet Topology Discovery 41
Analytical Alias ResolutionPotential Solutions
• How to verify the accuracy of inferred IP aliases– No loop condition: No inferred IP aliases should
introduce any routing loops in any of the path traces
Example: Consider two traces• (…, a, b, c, d, …)
(…, e, f, g, h, b, i, …) (reverse trace)• Assume a subnet relation (g ↔ c)• Inferred alias pair: (b,g) ----- CAUSES LOOP!
Internet Topology Discovery 42
Analytical Alias ResolutionPotential Solutions
• How to verify the accuracy of inferred IP aliases– Common neighbor condition: Given two IP addresses s
and t that are candidate aliases belonging to a router R, one of the following cases should hold:1. s and t have a common neighbor in some path trace2. There exists an alias pair (b,o) such that
– b is a successor (or predecessor) of s– o is a predecessor (or successor) of t
3. involved traces are aligned such that they form two subnets, one at each side of router R
– Distance condition: Given two IP addresses s and t that are candidate aliases for a router R, s and t should be at similar distance to a vantage point
• Adds an active probing component to the solution
Internet Topology Discovery 43
EvaluationsCoverage Comparisons
• AMP: ally (1,884 pairs) and APAR (2,034 pairs)
• iPlane: ally (39,191 pairs) and APAR (50,206 pairs)
Internet Topology Discovery 44
1,003
Causing Loop Ally APAR Ally disagree
864 986 45 34
Ally APAR
Ally disagreeCausing loopSource IP based
11,070 2,5148,206
3,0586,179
iPlane 10,678 22,886
?Complete ally requires
(275K)2 probes
SummaryAnalytical and Probe-base Alias Resolution
• IP alias resolution task has a considerable effect on most of the analyzed topological characteristics– In general, false negatives have more impact than false positives.
• APAR– benefits from IP address assignment of links,– focuses on structural connections between routers,– more effective on data sets that
• include symmetric path segments• collected from large number of vantage points
– requires no/minimal probing overhead.• complements probe-based approaches
Internet Topology Discovery 45
Outline
• Introduction• Internet Topology Measurement
– Topology Discovery Issues– Impact of IP Alias Resolution
• Topology Discovery– Resolving Anonymous Routers
• Graph-based Induction Technique
– Resolving Alias IP Addresses• Analytical and Probe-based Alias Resolution
– Resolving Genuine Subnets• Dynamic Subnet Inference
• SummaryInternet Topology Discovery 46
Genuine Subnet ResolutionProblem
• Subnet resolution– Identify IP addresses that are connected over the same medium
• Improve the quality of resulting topology map
Internet Topology Discovery 47
IP2 IP3
IP1
IP2 IP3
IP1
(observed topology) (inferred topology) (underlying topology)
C D
A B
C D
A B
C D
A B
C D
A B
Subnet Resolution: Advantages
• Improve the quality of resulting topology map
vs
• Increase the scope of the map
Internet Topology Discovery 48
(observed topology) (inferred topology) (genuine topology)
C D
A B
C D
A B
C D
A B
C D
A B
C D
A B
C D
A B
Subnet Resolution: Advantages
• Improve alias resolution process– Reduce the number of probes in ally based alias resolution
• ally tool requires O(n2) probes to resolve aliases among n IP addresses.
– We could determine ally probes based on subnets
• This approach reduces the number of probes to O(n.s) where s is the average of number of IP addresses in a subnet.
Internet Topology Discovery 49
Trace: IPa……...IPb ……... IPc ……... IPd
IPe
IPf
IPg
IPh
IPi
IPk IPl
subnets
Subnet Resolution: Approach
50Importance of IP Alias Resolution
129.110.0.0/16
/30
/31
/24
/24
/24/28
/29
.2
.1 .3
.4
.5
.6
129.110.12.0/29
129.110.4.0/24
129.110.6.0/28129.110.17.0/24
129.110.12.0/29
129.110.219.0/24
129.110.1.0/30
129.110.2.0/31
Genuine Subnet ResolutionTrace Preservation
Internet Topology Discovery 51
129.110.0.0/16129.110.1.1
129.110.1.2
129.110.2.0
129.110.2.1
129.110.4.1
129.110.4.83
129.110.4.217
129.110.12.1
129.110.12.2
129.110.12.6
129.110.17.1
129.110.17.135
129.110.219.1
129.
110.
0.0/
16
129.110.0.0/21
/30
/31
/24
/24
/24/28
/29
129.110.4.0/24
129.110.6.0/28129.110.17.0/24
129.110.12.0/29
129.110.219.0/24
129.110.1.0/30
129.110.2.0/31
129.
110.
4.1
129.110.1.2
129.110.2.1
129.110.12.2
129.110.12.0/29
129.110.17.0/24
129.110.4.0/24
129.110.0.0/22
Genuine Subnet ResolutionDistance Preservation
Internet Topology Discovery 52
129.110.1.1
129.110.1.2
129.110.2.0
129.110.2.1
129.110.4.1
129.110.4.83
129.110.4.217
129.110.12.1
129.110.12.2
129.110.12.6
129.110.17.1
129.110.17.135
129.110.219.1
V.P.
/30
/31
/24
/24
/24/28
/29
129.110.4.0/24
129.110.6.0/28129.110.17.0/24
129.110.12.0/29
129.110.219.0/24
129.110.1.0/30
129.110.2.0/31
2
3
3
4
2
1
2
4
5
5
4
5
3
129.110.2.0/30
129.110.4.0/24
129.110.12.0/29
129.110.17.0/24
129.110.0.0/16
129.110.1.0/31
Genuine Subnet Resolution Dynamic Subnet Inference Approach
• Inferring Subnets– Cluster IP addresses into maximal subnets up to a given size (e.g. /24)– Perform accuracy and distance analysis on candidate subnets and break them
down as necessary.IP1IP2IP3IP4IP5IP6IP7IP8IP9– Completeness: Ignore candidate subnets that have less than one quarter of
their IP addresses present.
Internet Topology Discovery 53
/25
/29
/26
/30
/31
/27A /27 subnet can have up to 25 IP addresses./24
Internet2 backbone topology on Apr 29, 2007 Inferred 116 verifiable subnets
95 exact size 12 smaller (observed IPs formed a smaller subnet) 9 bigger (false positives)
EvaluationsInternet2 backbone verification
Internet Topology Discovery 54
• 150 subnets• 547 routers• 793 IPs
R1
H1
1
R4
9
R2
R3
2
6 R5
10
11
/29 /29
R10
2
R11
10
11
/28
R2
6R6
1R1
/29
SummaryGenuine Subnet Resolution
• Identified a new step (i.e., subnet inference) to improve topology mapping studies.
• Introduced a technique to infer subnets and demonstrated its effectiveness– Detect connectivity between nodes
• An inferred /24 subnet had only a single link between two of its 73 observed IP addresses.
– Using subnets, we may reduce the number of ally probes for alias IP resolution
• e.g. 362K to 35.5K.
Internet Topology Discovery 55
Outline
• Introduction• Internet Topology Measurement
– Topology Discovery Issues– Impact of IP Alias Resolution
• Topology Discovery– Resolving Anonymous Routers
• Graph-based Induction Technique
– Resolving Alias IP Addresses• Analytical and Probe-based Alias Resolution
– Resolving Genuine Subnets• Dynamic Subnet Inference
• SummaryInternet Topology Discovery 56
Summary
• The Internet is man-made, so why do we need to measure it?– Because we still don’t really understand it
• Sometimes things go wrong– Measurement for network operations
• Detecting and diagnosing problems• What-if analysis of future changes
– Measurement for scientific discovery• Creating accurate models that represent reality• Identifying new features and phenomena
• Researchers have been sampling and analyzing Internet topology– Building network graph from raw-data is not easy.– There are several issues due to sampling
• Resolving anonymous routers, IP aliases, and genuine subnets– Huge computational and probing overhead due to very large data size
Internet Topology Discovery 57
Questions ?
Internet Topology Discovery 58