pathChirp

Spatio-Temporal Available Bandwidth Estimation

Vinay RibeiroRolf Riedi, Richard Baraniuk

Rice University

A Bird’s Eye View of the Internet

• Data transmitted as packets• Multiple routers on end-to-end path• Routers queue bursts of packets

Edge-based Probing

• Internet provides connectivity• Lack of optimization• Difficult to obtain information from routers

Solution: inject probe packets to measure internal properties

Questions awaiting Answers

• What does the Internet topology look like?

• Where does congestion occur and why?

• Given several mirror sites to download data which one to choose?

• Is my ISP honoring the service-level agreement?

Available Bandwidth Estimation

Network Model

Packet delay = constant term (propagation,

service time) + variable term (queuing delay)

• End-to-end paths– Multi-hop– No packet reordering

• Router queues– FIFO– Constant service rate

Available Bandwidth

• Unused capacity along path

)],0[

(min],0[number queue T

TACTB iii

Available bandwidth:

• Goal: use end-to-end probing to estimate available bandwidth

Applications

• Network monitoring

• Server selection

• Route selection (e.g. BGP)

• SLA verification

• Congestion control

Available Bandwidth Probing Tool

Requirements• Fast estimate within few RTTs

• Unobtrusive introduce light probing load

• Accurate

• No topology information (e.g. link speeds)

• Robust to multiple congested links

• No topology information (e.g. link speeds)

• Robust to multiple congested links

Principle of Self-Induced Congestion

• Advantages– No topology information required– Robust to multiple bottlenecks

• TCP-Vegas uses self-induced congestion principle

Probing rate < available bw no delay increase

Probing rate > available bw delay increases

Trains of Packet-Pairs (TOPP) [Melander et al]

)( st)( rt

• Vary sender packet-pair spacing• Compute avg. receiver packet-pair spacing• Constrained regression based estimate

• Shortcoming: packet-pairs do not capture temporal queuing behavior useful for available bandwidth estimation Packet-pairs

Packet train

Pathload [Jain & Dovrolis]

• CBR packet trains • Vary rate of successive trains • Converge to available bandwidth

• Shortcoming Efficiency: only one data rate per train

Chirp Packet Trains

• Exponentially decrease packet spacing within packet train

• Wide range of probing rates

• Efficient: few packets

100Mbps-1 packets, 134.1

Chirps vs. Packet-Pairs• Each chirp train of N packets contains N-1 packet pairs at

different spacings

• Reduces load by 50% – Chirps: N-1 packet spacings, N packets– Packet-pairs: N-1 packet spacings, 2N-2 packets

• Captures temporal queuing behavior

Chirps vs. CBR Trains

• Multiple rates in each chirping train

– Allows one estimate per-chirp

– Potentially more efficient estimation

CBR Cross-Traffic Scenario

• Point of onset of increase in queuing delay gives available bandwidth

Bursty Cross-Traffic Scenario

• Goal: exploit information in queuing delay signature

PathChirp Methodology

I. Per-packet pair available bandwidth, (k=packet number)

II. Per-chirp available bandwidth

III. Smooth per-chirp estimate over sliding time window of size

kk

kkk

t

tED

kE

Self-Induced Congestion Heuristic

• Definitions: delay of packet k inst rate at packet k

kkkk

kkkk

REqq

REqq

1

1

kqkk tR size/packet

Excursions

• Must take care while using self-induced congestion principle

• Segment signature into excursions from x-axis• Valid excursions are those consisting of at least “L” packets• Apply only to valid excursions

kk RE

Setting Per-Packet Pair Available Bandwidth

• Valid excursion increasing queuing delaykk

kk

RE

RE

nk

kk

RE

RE

• Valid excursion decreasing queuing delay

nk

kk

RE

RE

•Last excursion• Invalid excursions

nk RE

pathChirp Tool

• UDP probe packets• No clock synchronization required, only uses

relative queuing delay within a chirp duration • Computation at receiver• Context switching detection• User specified average probing rate

• open source distribution at spin.rice.edu

Performance with Varying Parameters

• Vary probe size, spread factor

• Probing load const.• Mean squared error

(MSE) of estimates Result: MSE decreases with increasing probe size, decreasing spread factor

Multi-hop Experiments

• First queue is bottleneck

• Compare– No cross-traffic at

queue 2– With cross-traffic

at queue 2• Result: MSE close in

both scenarios

Internet Experiments

• 3 common hops between SLACRice and ChicagoRice paths

• Estimates fall in proportion to introduced Poisson traffic

Comparison with TOPP

30% utilization

• Equal avg. probing rates for pathChirp and TOPP

• Result: pathChirp outperforms TOPP

70% utilization

Comparison with Pathload • 100Mbps links• pathChirp uses 10

times fewer bytes for comparable accuracy

Available bandwidth

Efficiency Accuracy

pathchirp pathload pathChirp10-90%

pathloadAvg.min-max

30Mbps 0.35MB 3.9MB 19-29Mbps 16-31Mbps

50Mbps 0.75MB 5.6MB 39-48Mbps 39-52Mbps

70Mbps 0.6MB 8.6MB 54-63Mbps 63-74Mbps

Tight Link Localization

Key Definitions

iiBA min

• Goal: use end-to-end probing to locate tight link in space and over time

Path available bandwidth

Sub-path available bandwidth

Tight link: link with least available bandwidth

imiBmA

1min],1[

Applications

• Network monitoring - locating hot spots

• Network aware applications- server selection

• Science: where do Internet tight links occur and why?

Methodology

• Estimate A[1,m]

• For m>tight link, A[1,m] remains constant

imiBmA

1min],1[

Principle of Self-Induced Congestion

• Probing rate = R, path available bandwidth = A

• Advantages– No topology information required– Robust to multiple bottlenecks

R < A no delay increase

R > A delay increases

Packet Tailgating

• Large packets of size P (TTL=m)small packets of size p

• Large packets exit at hop m

• Small packets reach receiver with timing information

• Previously employed in capacity estimation

Estimating A[1,m]

• Key: Probing rate decreases by p/(p+P) at link m

• Assumption: r<A[m+1,N], no delay change after link m

R < A[1,m] no delay increase

R > A[1,m] delay increases

Tight Link Localization

• Tight link: link after which A[1,m] remains constant

• Applicable to any self-induced congestion tool: pathload, pathChirp, IGI, netest etc.

ns-2 Simulation

• Heterogeneous sources• Tight link location changes over time• pathChirp tracks tight link location change accurately

tig

ht

link

est

imat

e

Internet Experiment

• Two paths:UIUC Rice and SLACRice

• Paths share 4 common links• Same tight link estimate for both paths

SLACRice tight link

UIUCRice tight link

Comparison with MRTG Data

• A[1,m] decreases as expected• Tight link location differs from MRTG data by 1 hop

SLACRice UIUCRice

High Speed Probing

• System I/O limits probing rate• On high speed networks:

cannot estimate A using self-induced congestion

),min( ds BBA

Receiver System I/O Limitation

• Treat receiver I/O bus as an extra link

• Use packet tailgating

• If then we can estimate A[1,N-1]dBr

Sender System I/O Limitations

• Combine sources to increase net probing rate

• Issue: machine synchronization

Conclusions

• Towards spatio-temporal available bandwidth estimation

• Combine self-induced congestion and packet tailgating

• Available bandwidth and tight link localization in space and over time

• ns-2 and Internet experiments encouraging

• Solutions to system I/O bandwidth limitations

spin.rice.edu