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Architecture and Pragmatics of Server-directed Transcoding

Björn Knutsson and Honghui Lu, U. Penn

Jeffrey Mogul, HP WRL

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Transcoding proxies

Problems that transcoding proxies solve Complex server content Highly variable network bandwidths Highly variable client capabilities: screen size, color

depth, encoding

Origin Server

TranscodingProxy

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Problem with transcoding proxies

JPEG, 600x800 Q=50, 48617 bytes

JPEG, 200x267 Q=34, 5815 bytes

JPEG, 600x800 Q=3, 5830 bytes

Q=50, 5831 bytes

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Server-directed transcoding (SDT)

Autonomous transcoding Proxy employs heuristics Risks loosing the meaning of the content Risks loosing opportunities for aggressive transcoding

Server-directed transcoding [Mogul WCW ’00] Server explicitly tells the proxy what to do Respects the end-to-end argument

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Contributions

A specific protocol design

An applet architecture

A proxy architecture

Interesting transcoding applets

Experimental results

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Basic architecture

Server-directives as mobile code – applets

Reference to applets attached to the HTTP response

Origin Server Transcoding

ProxyClient

Request

Transcoded response

Request

Response +

directives

Fetch applet

Size and reusability of applets?

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HTTP protocol extensions

SDT-applet: http://example.com/crop.jarSDT-parms: crop-origin=140;300, crop-size=100,100

New HTTP response headers

No restrictions on the syntax of SDT-parms SDT-parms are interpreted by applets, not by proxies

Mechanism for reusing applets

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Basic image transformations

Cropping

Scaling

Lossy compression

Color-depth reduction

De-animation

Format conversion

New basic transformations may be introduced …

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Complex image transformations

An image must be adapted for many clients

Small screen Cropping, scaling

Low bandwidth Cropping, color-depth reduction, scaling Lossy compression, de-animation

Small screen and low bandwidth?

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What should an applet look like?

Big, all inclusive applet Control complexity

Small, per image applet Reusability

Category-specific applet One applet that applies to a set of images

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Example applet: 'crop-interp'

For images with one important subject and surroundings that just add flavour

First interpolate between a set of crop boxes

Shrinks the inner most cropped image if necessary

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Other category-specific applets

Landscape panorama Apply lossy compression first Then crop …

Banner ad Reduce color depth first De-animation, scaling …

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Who is in control?

Seems the server controls everything, via the applet

But, what if Server – image cannot be scaled to < 100x100 Client – cannot display images > 50x50 Proxy – does not have enough CPU cycles for this!

Who decides what transformations to apply?

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Who is in control of what?

Origin server controls the semantics

Client controls the presentation

Proxy controls the resource used for transcoding, and network traffic

Server controls are delegated to its applet running at proxy

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Rules for resolving conflicts

Any party has a veto Server or proxy can decide not to transcode an image The client can decide to reject an image

Server is free to compromise semantics in order to meet client and proxy preferences, if the server is willing

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Proxy environment

Transcodingapplet

response

Transcodedresponse

SDT-parms

Client parameters

Proxy parameters

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Revisit ‘crop-interp’

For images with a main subject

Simple control

Crop-interp(input image, output image, SDT_params, /* crop boxes */ client parameters, proxy parameters)

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The pragmatics of an applet

Applet size is still large Image I/O Bit manipulations for basic transformations

Performance is bad Compute intensive Interpreted code is slow

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The pragmatics of an applet

Separate common, basic functions from the control

Function applet

Control applet 2Control applet 1

Implement basic functions with native code

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Prototype proxy implementation

Basic, un-optimized implementation No caching of either image or transcoding applet Sequential store and forward Implemented in Perl

Supports Perl applets

Pre-installs native functions (ImageMagick)

Long-term : augment Squid to support Java applets

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Experimental environment

Penn CIS Web server

Proxy: IBM ThinkPad, 1.2GHz Pentium III, 384MB RAM

10 Mbps Ethernet between server and proxy

Simulate various proxy-to-client bandwidths using Linux Traffic Shaper

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Transcoding overhead

“crop-interp”: 2KB Perl code

Original image: 600x800, 65982 bytes

Target size: 200x267, 5831 bytes

Download applet 30ms

Execute applet 86ms

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Effect on end-to-end latency (ms)

56 Kbps 768 Kbps

No proxy

600x8009680 510

Proxy transcoding

200x2761120 211

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Summary of contributions

A specific protocol design

An applet architecture

A proxy architecture

Interesting transcoding applets

Experimental results Improves end-to-end latency even on 768 Kbps links

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Future work

Applet design Size, reusability, performance

Proxy execution environment Native functions API

Security issues Resource consumption of transcoding applets

Caching issues cache replacement policy Matching client requests with the cached items

Transform contents other than images Extend to support CDNs and streaming media

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Related work

Proxy transcoding Brooks et al. 1995, Fox et al. 1996 Chandra and Ellis 1998

Introducing code into proxy Active Cache, Cao et al. 1998 Active Names, Vahdat et al. 1999

Other related ideas Active networking IBM WebSphere ICAP