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Rice University | SPIN.rice.edu 3 Network Expansion Grown in size and importance Crucial for commerce, government, research, … ARPANET 1969 NSFNET 1993
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pathChirp & STAB Measuring Available Bandwidth and
Locating Bottlenecks in Packet Networks
Vinay Ribeiro
Rolf Riedi, Richard Baraniuk
Rice Universityspin.rice.edu
Rice University | SPIN.rice.edu 2
Packet Networks
• Data transmitted as packets
• Routers forward packets until destination
• Routers buffer packets in queues
• Link bandwidth = maximum data transmission rate (bits/sec)
link
Rice University | SPIN.rice.edu 3
Network Expansion
• Grown in size and importance
• Crucial for commerce, government, research, …
ARPANET 1969 NSFNET 1993
Rice University | SPIN.rice.edu 4
Study Network Properties
• Properties– connectivity between routers– bandwidth used on different
links– queuing delays– statistical properties of packet
arrivals
• Improve network performance– Network design– Use bandwidth resources efficiently– Reduce delays– Assist network-aware applications
Rice University | SPIN.rice.edu 5
Obtaining Network Information is Hard
• Different parts of Internet owned by different organizations
• Information sharing difficult– Commerical interests/trade secrets– Privacy
• Direct measurement – Router performance affected with too much measurement– Tapping links, extra infrastructure, expensive
• Sheer volume of information– Cannot measure everything
Difficulties faced by network administrator
Difficulties faced by network user
differentorganizations
Rice University | SPIN.rice.edu 6
Edge-Based Probing
• Inject probe packets into network
• Infer internal properties from packet delay
End-to-end packet delay = speed of light propagation + queuing delay
probe packets
Rice University | SPIN.rice.edu 7
Probing “Uncertainty Principle”
• Large volume of probe packets– Accurate inference of network properties– Inefficient use of precious bandwidth resources
• Small volume of probe packets– Less accurate inference– Efficient use of resources
• Balance tradeoff in accuracy vs. efficiency
Rice University | SPIN.rice.edu 8
Available Bandwidth
• Link available bandwidth = unused bandwidth on a link
• Path available bandwidth = smallest available bandwidth of all links of a path
• Available bandwidth is time-varying
• Goal: end-to-end probing to estimate path available bandwidth
Link bandwidth = 100MbpsBandwidth used to transmit packets = 30MbpsLink available bandwidth = 70Mbps
70Mbps 30Mbps 50Mbps 20Mbps 60Mbps
Link available bandwidths
Example:
Rice University | SPIN.rice.edu 9
Applications• Server selection
• Route selection (e.g. BGP, overlay networks)
• Service verification
• Tuning transport protocols
• UDP-storm attack detection
• Early warning of meltdown
Rice University | SPIN.rice.edu 10
Probing Tool Requirements
• Fast, real-time estimate
• Accurate
• Efficient, introduce light probing load
• No topology assumptions (e.g. link bandwidths)• No topology assumptions (e.g. link bandwidths)
Rice University | SPIN.rice.edu 11
Self-Induced Congestion
• Advantages– No topology information required
• Transition point gives estimate of available bandwidth
Probing bit rate > available bandwidth delay increases (queues start filling up)
Probing bit rate < available bandwidth no delay increase (queues do not fill up)
time
time
probepackets
low probing rate
high probing rate
Rice University | SPIN.rice.edu 12
Chirp Packet Trains
• Exponentially decrease packet spacing within packet train
• Simultaneously probe at wide range of probing rates
• Efficient: few packets
Example: Chirp of 25 packets with =1.2 has probing range 1--100Mbps
(bits/sec)
Rice University | SPIN.rice.edu 13
Available Bandwidth estimation with pathChirp
• Segment delay profile into increasing/decreasing regions
• Apply principle of self-induced congestion to each region
• Average over different regions for per-chirp estimate
• Final estimate: moving-average of per-chirp estimates
Rice University | SPIN.rice.edu 14
Gigabit Testbed Experiment
• CAIDA/CalNGI bandwidth estimation lab
• Vary available bandwidth using cross-traffic generator
• pathChirp tracks available bandwidth well
Mbp
s
time (seconds)
Rice University | SPIN.rice.edu 15
Thin Links
• Thin link – link with less available bandwidth than all preceding links
• Sub-path available bandwidth A[1,m] = smallest available bandwidth among first m links
• Goal: use end-to-end probing to locate thin links in space and track changes in location over time
70Mbps 30Mbps 50Mbps 20Mbps 60Mbps
Link available bandwidths
Rice University | SPIN.rice.edu 16
Applications
• Science: where does congestion occur and why?
• Network aware application– Route around problem spots in Internet
• Network monitoring/troubleshooting– Locating hot spots
Rice University | SPIN.rice.edu 17
Estimating Sub-Path Available Bandwidth A[1,m]
• Replace each packet by two packets: Big packet size P, small packet size p
• Key: Probing rate decreases by p/(p+P) at link m
• Self-induced congestion only up to link m
• Small packets carry timing information to receiver
1 2 m
Rice University | SPIN.rice.edu 18
Tight Link Localization with STAB
• Thin links: links at which A[1,m] decreases
• Last thin link has least available bandwidth among all links
• Implemented in Spatio-Temporal Available Bandwidth estimator (STAB)
Rice University | SPIN.rice.edu 19
Simulation• STAB tracks thin links well
Actual
Estimated
Probability that different links are thin links
topology
t=360 sec
t=180 sec
Link number m
Sub
-pat
h av
aila
ble
Ban
dwid
th A
[1,m
] (M
bps)
time (sec)
time (sec)
Sub
-pat
h av
aila
ble
Ban
dwid
th A
[1,m
] (M
bps)
Link number m
Rice University | SPIN.rice.edu 20
Probability that different links are thin links
• Locate thin links on two paths simultaneously
• Estimated thin link locations are consistent for two paths
Internet Experiment
time tim
eLink number mLink number m
Sub
-pat
h av
aila
ble
Ban
dwid
th A
[1,m
] (M
bps)
Sub
-pat
h av
aila
ble
Ban
dwid
th A
[1,m
] (M
bps)
Router data supports STABresults
UIUCRice
UIUCRice
UWiscRice
UWiscRice
Rice University | SPIN.rice.edu 21
New Research Directions• Spatio-temporal network tomography
• Wireless network probing
Rice University | SPIN.rice.edu 22
Other Projects
• Synthesis of fractal data
• Alpha-Beta analysis of Internet data
• High-speed transport protocols
Rice University | SPIN.rice.edu 23
Synthesis of Fractal Data Bytes/time time series from an Internet link
Classical Models(Markov/Poisson)
Bytes per 600ms
Bytes per 60ms
Bytes per 6ms
• Internet data is fractal --- high variability if we zoom-in or zoom-out• Fast synthesis using multifractal wavelet model
– Useful for simulations– Code available at dsp.rice.edu
• People: Matthew Crouse, Rolf Riedi, R. Baraniuk
Rice University | SPIN.rice.edu 24
Alpha-Beta Analysis of Internet Data
• Connection -- set of all packets with a unique source and destination
• Few connections (alpha) cause most of the “spikes”
• Implications for designing simulation topologies, queuing analysis, congestion control
• People: Shriram Sarvotham, Rolf Riedi, Richard Baraniuk
= +
Time series ofbytes per 500ms
Alpha component“Spiky”
Few connections
Beta componentGaussian
Most connections
Rice University | SPIN.rice.edu 25
High-Speed Transport Protocols• Transport protocols – send at maximum data rate that does
not congest network
• Current protocol (TCP-Reno) cannot utilize all the bandwidth on high-speed Giga-bit networks
• Existing solutions for high-speed networks too aggressive– Negative impact on competing TCP-Reno connections– Cannot deploy such solutions
• Hybrid protocol – Utilizes bandwidth on high-speed networks– Competes fairly with TCP-Reno connections
• People: Ryan King, Rolf Riedi, Richard Baraniuk
Rice University | SPIN.rice.edu 26
Conclusions
• pathChirp – efficient probing tool to estimate path available bandwidth
• STAB – probing tool to locate thin links in space and track changes in location over time
• Code (UNIX) – Available for download at spin.rice.edu
• Other projects – synthesis of fractal data (dsp.rice.edu), alpha-beta analysis, high-speed transport protocols