Differentiated MultimedDifferentiated Multimedia Web Services Using Qia Web Services Using Quality Aware Transcodinuality Aware Transcodin

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S. Chandra, C.Schlatter Ellis and A.VahdatS. Chandra, C.Schlatter Ellis and A.Vahdat

InfoCom 2000,InfoCom 2000,IEEE Journal on Selected areas in CIEEE Journal on Selected areas in C

ommunications, Dec 2000ommunications, Dec 2000

OutlineOutline

IntroductionIntroduction Quality Aware TranscodingQuality Aware Transcoding Experiment objective and designExperiment objective and design ResultsResults ConclusionConclusion

IntroductionIntroduction

Web service to provide low-latency access to its cWeb service to provide low-latency access to its contents is constrained by available network banontents is constrained by available network bandwidthdwidth

Introduce a mechanism for managing network rIntroduce a mechanism for managing network resources at the application levelesources at the application level

Use transcoding to allow web servers to customiUse transcoding to allow web servers to customize the size of objects on a webpage by dynamicalze the size of objects on a webpage by dynamically varying the size of multimedia objects on a pely varying the size of multimedia objects on a per-client basis.r-client basis.

Quality vs. Size tradeoffs of JPEG imagesQuality vs. Size tradeoffs of JPEG images Evaluate performance of several bandwidth manEvaluate performance of several bandwidth man

agement policiesagement policies

IntroductionIntroduction

It is possible to use informed transcoding teIt is possible to use informed transcoding techniques to provide differentiated service anchniques to provide differentiated service and to dynamically allocate bandwidth among d to dynamically allocate bandwidth among different client classes, while delivering a hidifferent client classes, while delivering a high degree information content for all clients.gh degree information content for all clients.

Quality Aware TranscodingQuality Aware Transcoding Focus on transcode reduce bandwidth requiFocus on transcode reduce bandwidth requi

rements on the serverrements on the server Quantify the quality vs. size tradeoff for tranQuantify the quality vs. size tradeoff for tran

scoding JPEG imagesscoding JPEG images Changing JPEG compression metric: JPEG QChanging JPEG compression metric: JPEG Q

uality Factor, correspond to quality lossuality Factor, correspond to quality loss Use adjusting JPEG compression metric as tUse adjusting JPEG compression metric as t

he informed transcoding technique, this reshe informed transcoding technique, this result is equally valid for any transcoding with ult is equally valid for any transcoding with well understood tradeoff characteristicswell understood tradeoff characteristics

Experiment objective and Experiment objective and designdesign

Objective:Objective: bandwidth bandwidth

consumption consumption control control

differentiated differentiated service for service for preferred and preferred and ordinary clientsordinary clients

System System ArchitectureArchitecture

Experiment objective and Experiment objective and designdesign

Performance measures:Performance measures: Bandwidth ConsumedBandwidth Consumed

maintain within pre-defined levelmaintain within pre-defined level Image Quality FactorImage Quality Factor

tradeoff quality for sizetradeoff quality for sizeretain as much information quality as retain as much information quality as

possiblepossible Client Access latencyClient Access latency

acceptable client latency, 10s as acceptable client latency, 10s as thresholdthreshold

Experiment objective and Experiment objective and designdesign

Web Service PoliciesWeb Service Policies TraditionalTraditional

without bandwidth control mechanismwithout bandwidth control mechanisms. Use the s. Use the modified Apache Web servermodified Apache Web server

ModbandwidthModbandwidthbandwidth limiting web service that pribandwidth limiting web service that pri

oritizes the oritizes the network packets, delaying pnetwork packets, delaying packets with low priority ackets with low priority requests (Apache requests (Apache mod_bandwidth module)mod_bandwidth module)

DenialDenialtemporarily denies requests under heatemporarily denies requests under hea

vily loaded vily loaded conditions (HTTP error code conditions (HTTP error code 503 : Service 503 : Service Unavailable)Unavailable)

Experiment objective and Experiment objective and designdesign

TranscodingTranscodingtranscode an image to a number of variationtranscode an image to a number of variationssEnsure that any loss in image quality is Ensure that any loss in image quality is efficiefficientent, definded as a transcoding that loses at l, definded as a transcoding that loses at least as much in image size as the loss in imaeast as much in image size as the loss in image information qualityge information quality

grayscale

85 KB

10 KBOrigin

116KB

Crop

40KB

5 % quality

16% size

Original

17KB

50% quality

49% SizeEffective

Ineffective

Experiment objective and Experiment objective and designdesign

Experimental WorkloadExperimental Workload Access TracesAccess Traces

client access trace that captures request client access trace that captures request arrival times.arrival times.

Analyzing access made via the NLANR proxy Analyzing access made via the NLANR proxy caches, use the accesses to geocities.comcaches, use the accesses to geocities.com

Exp trace: changes access rate every 30 Exp trace: changes access rate every 30 seconds,seconds,scale trace to 12500 seconds(3.5hrs)scale trace to 12500 seconds(3.5hrs)

ImageImage CollectionCollection 3531 JPEG images total 38 MB3531 JPEG images total 38 MB

Experiment objective and Experiment objective and designdesign

Client ClassesClient Classes modify http_load to generate a configurable modify http_load to generate a configurable

percentage of requests from different clienpercentage of requests from different client classest classes

http_load informs the server about the class that http_load informs the server about the class that a request belongs to using custom HTTP headersa request belongs to using custom HTTP headers

Web ServerWeb Server Apache server computes the bandwidth trend by Apache server computes the bandwidth trend by

averaging over the past 30 mins intervalaveraging over the past 30 mins interval

Experiment objective and Experiment objective and designdesign

Server implements Transcoding mechanism:Server implements Transcoding mechanism:

If the average consumed bandwidth for the past halIf the average consumed bandwidth for the past half hour is more than target bandwidth, the server prf hour is more than target bandwidth, the server proportionately lower quality variations of images.oportionately lower quality variations of images.When the consumed bandwidth exceeds twice the tWhen the consumed bandwidth exceeds twice the target bandwidth, the server denies further requests.arget bandwidth, the server denies further requests.

Client: http_load to simulate accessClient: http_load to simulate accesscompute the individual access latenciescompute the individual access latenciesplay back the client access trace generated from the play back the client access trace generated from the NLANR proxy traces.NLANR proxy traces.

ResultsResults

Bandwidth ControlBandwidth Control Target bandwidth: 200KB/sTarget bandwidth: 200KB/s Avg. access rate: 5.7 accesses/sAvg. access rate: 5.7 accesses/s

Overshoot the target bandwidth :

Traditional: 65% of time

Modbandwidth: 0%

Denial: 35%

Transcoding: 45%

Traditional and Denial provides best control over bandwidth, but driving client latencies to unacceptable level

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Denial requests : infinite latency and Quality factor of 0

Transcoding scheme gracefully degrades the Quality factor

Modbandwidth: 90% of requests takes 10s to service

Denial, Transcoding: less than 1s

Transcoding provides useful bandwidth contTranscoding provides useful bandwidth control by gracefully degrading image quality facrol by gracefully degrading image quality factors.tors.

Transcoding does not control bandwidth effTranscoding does not control bandwidth effectively in the presence of flash crowds and ectively in the presence of flash crowds and if there’s a wide differential between target if there’s a wide differential between target and requested bandwidth.and requested bandwidth.

Denial and Modbandwidth control their banDenial and Modbandwidth control their bandwidth consumption by denying service for dwidth consumption by denying service for a large portion of the imagesa large portion of the images

ResultsResults Differentiated Web ServiceDifferentiated Web Service

Since Modbandwidth performs with unacceptablSince Modbandwidth performs with unacceptable latencies, only compare Denial and Transcodine latencies, only compare Denial and Transcoding schemesg schemes

Avg. access rate: 28.6 accesses/sAvg. access rate: 28.6 accesses/s Target bandwidth : 1MB/sTarget bandwidth : 1MB/s Avg. bandwidth demanded: 1.5MB/sAvg. bandwidth demanded: 1.5MB/s 40% of available bandwidth for the 40% of available bandwidth for the preferred clipreferred cli

entsents 20% of the clients be preferred clients20% of the clients be preferred clients

Prefer clients: Denial and Transcoding: 90%,65% less than 400KB/s

Target bandwidth is closer to Max. bandwidth of Transcoding,

the system better handles the heavy load at time =4500s

ConclusionConclusion

Explore a web service that uses informed transExplore a web service that uses informed transcoding to manage its bandwidth consumptioncoding to manage its bandwidth consumption

The service degrades the quality gracefully for The service degrades the quality gracefully for different user classes, while still managing ovedifferent user classes, while still managing overall consumed bandwidth effectively.rall consumed bandwidth effectively.

Allow server to manage its bandwidth without Allow server to manage its bandwidth without adding excessive latency or denying serviceadding excessive latency or denying service

Future work: to specify the relative importancFuture work: to specify the relative importance of various multimedia components of web pe of various multimedia components of web pages.ages.