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Tomography-based Overlay Network
Monitoring
Hugo Angelmar
Slides courtesy of (Yan Chen, David Bindel, and
Randy H. Katz)
Motivation
• Infrastructure ossification led to thrust of
overlay and P2P applications
• Such applications flexible on paths and targets,
thus can benefit from E2E distance monitoring
– Overlay routing/location
– VPN management/provisioning
– Service redirection/placement …
• Requirements for E2E monitoring system
–
–
–
–
Scalable & efficient: small amount of probing traffic
Accurate: capture congestion/failures
Incrementally deployable
Easy to use
Existing Work
• General Metrics: RON (n2 measurement)
• Latency Estimation
– Clustering-based: IDMaps, Internet Isobar, etc.
– Coordinate-based: GNP, ICS, Virtual Landmarks
• Network tomography
– Focusing on inferring the characteristics of physical
links rather than E2E paths
– Limited measurements -> under-constrained system,
unidentifiable links
Problem Formulation
Given an overlay of n end hosts and O(n2) paths,
how to select a minimal subset of paths to
monitor so that the loss rates/latency of all
other paths can be inferred.
Assumptions:
• Topology measurable
• Can only measure the E2E path, not the link
Our Approach
topology
Overlay Network
Operation Center
measurements
End hosts
Select a basis set of k paths that fully describe
O(n2) paths (k «O(n2))
• Monitor the loss rates of k paths, and infer the
loss rates of all other paths
• Applicable for any additive metrics, like latency
A
1
p1
3
Modeling of Path Space
D
2
C
B
Path loss rate p, link loss rate l
1  p1  (1  l1)(1  l 2)
 log( 1  l1) 
log( 1  p1)  log( 1  l1)  log( 1  l 2)  1 1 0 log( 1  l 2)
log( 1  l 3) 
 x1 
1 1 0 x2  b1
 x3
A
1
p1
3
Putting All Paths Together
D
2
C
B
Totally r = O(n2) paths, s links, s «r
Gx  b, where path matrix G {0 | 1}rs
link loss rate vector x  
…
s1
, path loss rate vector b  
=
r 1
Sample Path Matrix
A
b2
1
b1
3
D
2
B
b3
C
1 1 0
G  0 0 1
1 1 1
 x1   b1 
G  x 2   b 2 
 x3  b3 
x2
(1,1,0)
(1,-1,0)
path/row space
(measured)
null space
(unmeasured)
x1
x3
• x1 - x2 unknown => cannot
compute x1, x2
1
0 b1 / 2
( x1  x2 )  
• Set of vectors [1 1 0]T
0  b / 2
xG 
1

x
3 
1




2
form null space
0
1  b2 
• To separate identifiable vs.
1
unidentifiable components:
( x1  x2 )  
x = xG + xN
xN 
 1

2
 0 
Intuition through Topology Virtualization
Virtual links:
• Minimal path
segments
whose loss
rates uniquely
identified
• Can fully
describe all
paths
• xG is composed
of virtual links
x2
(1,-1,0)
null space
(unmeasured)
A
b2
1
b1
3
2
x1
x3
Virtualization

D
b3
(1,1,0)
path/row space
(measured)
C
1
2
Virtual links
B
All E2E paths are in path
space, i.e., GxN = 0
1
0 b1 / 2
( x1  x2 )  
0  b / 2
xG 
1

x
  3   1 
2
0
1  b2 
b  Gx  GxG  GxN  GxG
More Examples
1 1 0
G

1
0
1


1
1’
2
Rank(G)=2
1
1
G
0

0
0
0
1

1
Rank(G)=3
1
0
1
0
0
1
0
1
1’

2’
3
1
2
2’
3’
4
3
2
1
1

2
3
4’ Virtualization
Real links (solid) and all of the overlay
paths (dotted) traversing them
Virtual links
Algorithms
GxG  b
…
…
=
=
• Select k = rank(G) linearly
independent paths to
monitor
– Use QR decomposition
– Leverage sparse matrix: time
O(rk2) and memory O(k2)
• E.g., 10 minutes for n = 350
(r = 61075) and k = 2958
• Compute the loss rates of
other paths
– Time O(k2) and memory O(k2)
How many measurements saved ?
k « O(n2) ?
For a power-law Internet topology
• When the majority of end hosts are on the overlay
k = O(n) (with proof)
• When a small portion of end hosts are on overlay
– If Internet a pure hierarchical structure (tree): k = O(n)
– If Internet no hierarchy at all (worst case, clique):
k = O(n2)
– Internet has moderate hierarchical structure [TGJ+02]
For reasonably large n, (e.g., 100), k = O(nlogn)
(extensive linear regression tests on both synthetic and real topologies)
Practical Issues
• Topology measurement errors tolerance
– Care about path loss rates than any interior links
– Poor router alias resolution
=> assign similar loss rates to the same links
– Unidentifiable routers
=> add virtual links to bypass
• Measurement load balancing on end hosts
– Randomly order the paths for scan and selection of
• Topology Changes
– Efficient algorithms for incrementally update of
for adding/removing end hosts & routing changes
G
G
Topology Changes
• Basic building block: add/remove one path
– Incremental changes: O(k2) time (O(n2k2) for re-scan)
– Add path: check linear dependency with old basis set, G
– Delete path p : hard when p  G
The essential info described by p :
a vector y, such that y  G, but y  {G  p}
if {G  p}  y  0, add any path x to G,
where x {G  p} and x  y  0
• Add/remove end hosts , Routing changes
• Topology relatively stable in order of a day
=> incremental detection
Evaluation
• Simulation
– Topology
• BRITE: Barabasi-Albert, Waxman, hierarchical: 1K – 20K nodes
• Real topology from Mercator: 284K nodes
– Fraction of end hosts on the overlay: 1 - 10%
– Loss rate distribution (90% links are good)
• Good link: 0-1% loss rate; bad link: 5-10% loss rates
• Good link: 0-1% loss rate; bad link: 1-100% loss rates
– Loss model:
• Bernouli: independent drop of packet
• Gilbert: busty drop of packet
– Path loss rate simulated via transmission of 10K pkts
• Experiments on PlanetLab
Evaluation
• Extensive Simulations
• Experiments on PlanetLab
– 51 hosts, each from different
organizations
– 51 × 50 = 2,550 paths
– On average k = 872
US (40)
• Results Highlight
– Avg real loss rate: 0.023
– Absolute error mean: 0.0027
90% < 0.014
– Relative error mean: 1.1
90% < 2.0
– On average 248 out of 2550
paths have no or incomplete
routing information
– No router aliases resolved
# of
hosts
Areas and Domains
Europe (6)
International
(11)
.edu
33
.org
3
.net
2
.gov
1
.us
1
France
1
Sweden
1
Denmark
1
Germany
1
UK
2
Taiwan
1
Hong Kong
1
Asia (2)
Canada
2
Australia
1
Experiments on Planet Lab
• 51 hosts, each from
different organizations
– 51 × 50 = 2,550 paths
• Simultaneous loss rate
measurement
US (40)
– 300 trials, 300 msec each
– In each trial, send a 40-byte
UDP pkt to every other host
• Simultaneous topology
measurement
– Traceroute
• Experiments: 6/24 – 6/27
– 100 experiments in peak
hours
# of
hosts
Areas and Domains
Europe (6)
International
(11)
.edu
33
.org
3
.net
2
.gov
1
.us
1
France
1
Sweden
1
Denmark
1
Germany
1
UK
2
Taiwan
1
Hong Kong
1
Asia (2)
Canada
2
Australia
1
Sensitivity Test of Sending Frequency
• Big jump for # of lossy paths when the sending rate is over
12.8 Mbps
PlanetLab Experiment Results
• Loss rate distribution
loss
rate
[0, 0.05)
%
95.9%
lossy path [0.05, 1.0] (4.1%)
[0.05, 0.1)
[0.1, 0.3)
[0.3, 0.5)
[0.5, 1.0)
1.0
15.2%
31.0%
23.9%
4.3%
25.6%
• Metrics
– Absolute error |p – p’ |:
• Average 0.0027 for all paths, 0.0058 for lossy paths
– Relative error [BDPT02]
 p( ) p' ( ) 
F ( p, p' )  max 
,

p
'
(

)
p
(

)


where p( )  max(  , p), p' ( )  max(  , p' )
– Lossy path inference: coverage and false positive ratio
• On average k = 872 out of 2550
Accuracy Results for One Experiment
• 95% of absolute error < 0.0014
• 95% of relative error < 2.1
Accuracy Results for All Experiments
• For each experiment, get its 95% absolute & relative errors
• Most have absolute error < 0.0135 and relative error < 2.0
Lossy Path Inference Accuracy
• 90 out of 100 runs have coverage over 85% and false
positive less than 10%
• Many caused by the 5% threshold boundary effects
Topology/Dynamics Issues
• Out of 13 sets of pair-wise traceroute …
• On average 248 out of 2550 paths have no or
incomplete routing information
• No router aliases resolved
Conclusion: robust against topology measurement
errors
• Simulation on adding/removing end hosts and
routing changes also give good results
Performance Improvement with Overlay
• With single-node relay
• Loss rate improvement
– Among 10,980 lossy paths:
– 5,705 paths (52.0%) have loss rate reduced by 0.05 or more
– 3,084 paths (28.1%) change from lossy to non-lossy
• Throughput improvement
– Estimated with
1.5
throughput 
rtt lossrate
– 60,320 paths (24%) with non-zero loss rate, throughput computable
– Among them, 32,939 (54.6%) paths have throughput improved,
13,734 (22.8%) paths have throughput doubled or more
• Implications: use overlay path to bypass congestion or
failures
Adaptive Overlay Streaming Media
Stanford
5. Alert +
New Overlay Path
OVERLAY NETWORK
OPERATION CENTER
2. Register trigger
4. Detect congestion /
failure
UC Berkeley
X
CLIENT
UC San Diego
1. Setup
connection
SERVER
3. Network congestion /
failure
7. Skip-free streaming
media recovery
6. Setup New Path
OVERLAY RELAY
NODE
HP Labs
•
•
•
•
Implemented with Winamp client and SHOUTcast server
Congestion introduced with a Packet Shaper
Skip-free playback: server buffering and rewinding
Total adaptation time < 4 seconds
Adaptive Streaming Media Architecture
MEDIA
SOURCE
OVERLAY NETWORK
OPERATION CENTER
SERVER
CLIENT
Winamp client
Triggering /
alert + new path
SHOUTcast
Server
RELAY
Overlay Layer
Buffering Layer
Winamp Video/Audio Filter
Byte Counter
Path Management
RELAY
Path Management
TCP/IP Layer
Calculated
concatenation
point
TCP/IP Layer
Client 4 Client 3 Client 2 Client 1
From
SHOUTcast
server
BUFFER
Byte
Counter
Overlay
Layer
TCP/IP Layer
Internet
Client 1
OVERLAY RELAY NODE
Client 2
INTERNET
Client 3
Client 4
Conclusions
• A tomography-based overlay network monitoring
system
– Given n end hosts, characterize O(n2) paths with a
basis set of O(nlogn) paths
– Selectively monitor O(nlogn) paths to compute the
loss rates of the basis set, then infer the loss rates
of all other paths
• Both simulation and real Internet experiments
promising
• Built adaptive overlay streaming media system
on top of monitoring services
– Bypass congestion/failures for smooth playback
within seconds