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Estimating Shared Congestion Among Internet

Paths

Weidong Cui, Sridhar MachirajuRandy H. Katz, Ion Stoica

Electrical Engineering and Computer Science DepartmentUniversity of California, Berkeley

{wdc, machi, randy, istoica}@EECS.Berkeley.EDU

Short Course August 2003

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Motivation• Applications using path diversity for

better performance – multimedia streaming - independent losses– parallel downloads – better throughput– overlay routing networks - backup paths for

robustness

N1

N2

N3

N4

N5

N6

N7

Congested Links

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Problem Formulation

• Problem: Given two paths in the Internet, estimate the fraction of packet drops at shared points of congestion (PoCs) using probe flows along the paths

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Existing Techniques• Traceroute will not work

– Provides no congestion-related information – Will not work with ICMP filtering

• Limitations of existing solutions– Work only with Y and iY (Inverted Y)

topologies

– Return a “Yes/No” decision– Limited Evaluations

Shared routersand PoCs

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Our Contributions• Path Independence Estimator (PIE)

– Topology-dependent (Y, iY, YiY, iYY)– Uses Stat. learning (EM) – Other parameters relatively insensitive

• A novel and extensive overlay-based measurement methodology to validate PIE

Shared routersand PoCs

YiY iYY

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Assumptions and Solution Motivation

• Assumptions– Most routers use drop-tail queuing discipline– Most traffic is TCP-based

• Motivation for PIE– Droptail Queues +TCP => Bursty Drops– Packets traversing a PoC around the same

time are likely to be dropped or not dropped together

– Count simultaneous drops of the two probe flows

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Challenges

• Determine times of traversal at shared PoCs based on sending/receiving times

• All packets during a bursty drop period may not be dropped; this could lead to false negatives

• Long bursts could lead to false positives

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PIE: How It Works

• Use CBR UDP probe flows along the 2 paths

• Classify drops along each path into bursts

• Use the sending and receiving times and topology (Y, iY, iYY, YiY) to determine the times at which drops would have occurred at the PoCs

• Calculate the number of drops in simultaneous bursts

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Classify drops into bursts• All packets during congested period at

PoC may not be dropped. Hence, use simultaneous bursts

• Use EM algorithm with Bayesian technique to determine burst interval b

Flow 1

Flow 2

Burst of Flow 1

Burst of Flow 2

Packet Drop

Transmitted Packetb

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What are simultaneous bursts?

• To determine simultaneous bursts, we need to know the times of occurrence in terms of a common clock

• For iY topology,– Common clock is at sender– Time at shared PoC ~ sending time

• For Y topology, – Common clock is at receiver– Time at shared PoC ~ intrapolated receiving

times

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Simultaneous Bursts (iYY)

• For iYY topology,– No common clock for all possible PoCs– Time at shared PoCs near sender ~

sending times– Time at shared PoCs near receiver ~

intrapolated receiving times– Hence, count drops in bursts that

are simultaneous with a burst (of the other flow) using sending OR receiving times

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Simultaneous bursts (YiY)

• For YiY topology, – Common clock ~ sending time +

delay to shared routers (and PoCs)– But, clocks of the senders may not

be synchronized– Delay to shared routers is not known– Need to determine

synchronization lag = difference in clocks of senders + difference in delays to shared routers

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Synchronization Lag

0

CBR Flow 1

CBR Flow 2

Time

Sender 1

Sender 2

PoC

PoC

0T

1

0

d1

2

0

1 2

3 4

1

d2+

0

Synchronization Lag = 3T

3

2

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Synchronization Lag

0 1 2 3 4 5 6 7

0

T

d1 d2+

CBR Flow 1

CBR Flow 2

Time

Sender 1

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6

Sender 2

0 1 2 3 4

Synchronization Lag = 3T

Note: is bounded by RTTmax/2

PoC

PoC

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Determine Synclag• Using the sending times, construct 2

sequences of 1s(drops) and 0s• Synclag is loosely bounded by 2*RTTmax

• For a given synclag, cross-correlation coefficient (CCC) of the 2 (synclag-shifted) sequences can be calculated

• Try various values of synclag and calculate CCCs

• Use the synclag that maximizes the CCC of (synclag-shifted) packet drop sequences

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Prevent False Positives

• Bursts at different PoCs may have small overlap; this causes false positives

• Consider bursts only if the simultaneous portion > overlap ratio f of the total length

Flow 1

Flow 2

Burst of Flow 1

Burst of Flow 2

Packet Drop

Transmitted Packet

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Evaluation

• Hard to generate “real-world” effects with ns-2 simulations; experimental evaluation

• Need large number of flows on different paths for an experimental evaluation; Planetlab

• Not possible to verify the performance of PIE since congestion information about individual links is not available; overlay-based instantiation

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Overlay-based Instantiations

• Goal: Need 2 paths with shared and non-shared sub-paths

• Each path consists of a sequence of shared and non-shared sub-paths

• Instantiate the first and last node of each sub-path with an overlay router

S1 S2

R1 R2

M1

M2

OverlayNode

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Overlay Instantiations (contd.)

• All PoCs on shared overlay hop are shared

• But, converse may not be true!• 2 overlay hops from/to the same

node could share a few IP routers

• Drops on ambiguous (overlay) hops may be shared Overlay

NodeAmbiguousHops

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Overlay Instantiations (contd.)

• Bounds on fraction of drops at shared PoCs– Lower bound: d3/(d1+d2+d3+d4)– Upper bound: (d2+d3+d4)/(d1+d2+d3+d4)

• Also, use experiments in which there are few drops on the ambiguous overlay hops

S1

S2

R1

R2M1 M2

d2 d3 d4d1

N1

N2

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Evaluation Details

• Planetlab: 45 sites all over the world• CBR UDP probes (default: 1

packet/10ms) using setitmer function in Linux

• Also used 1ms probing by polling timer• Duration of flows: 600s• 2 experimental datasets - March and

June

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Instantiations used• 4 Overlay topologies

• 2 IP level topologies Y iY YiYYiY

Internet Internet

I topology II topology

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Outline of Results

• I topology – Base Case Result– Effect of Probing Rate

• YiY topology– Unambiguous results– All results

• Justifying our estimation of synclag• Other observations

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Base Case• For I topology, we expect PIE to output

1• CDF shows that PIE’s error < 0.3 in

80% of the experiments

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Probing Rate

• Using 1ms traces for I topology, construct probes of period 10ms and 20ms separated by [1ms,2ms] and [5ms,10ms]

• Conclusions– Burst length>5ms– 20 ms probing much worse

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YiY topology

• For YiY topology, consider cases with few ambiguous drops

• Conclusions– Overlap ratio 0.5 is good– 1 ms probing does not help (not shown)– Error < 0.3 in 88% of the experiments

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YiY Topology

• June results similar – 2 false positive; traceroute showed shared trans-Atlantic link

• CDF of all results shows estimate > minimum fraction in 80% cases

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Estimating Synchronization Lag

• Graph justifies the maximization of cross-correlation coefficient for typical I topology

• Disadvantage: 6% false positive rate (estimate>0.2) with 2 independent paths (II topology)

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Other Observations

• For Y, iY and iYY topologies we see that error < 0.3 for 80% of the experiments

• Pathological sites: – All results do not include 3 sites which

strongly exhibit random dropping– Individual flows in other failed cases do not

show bursty losses; RED is likely cause

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Conclusions

• PIE provides an estimate within 0.3 for 80% of the cases for each of Y, iY, YiY, iYY topologies

• Failed cases likely due to RED• Bursts are normally longer than 10ms;

10ms probing seems optimal (4 KBps overhead)

• Synclag estimation by maximizing CCC is mostly justified

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Future Work

• Can we reduce the probing rate?• How can we adapt PIE to work with

passive (maybe TCP-based) probes• Issues with RED:

– How can we infer a RED-based PoC– How can we estimate shared RED-based PoCs