Björn Knutsson
Abstract
Proxy-based transcoding adapts Web content to be a better match for client capabilities (such as screen size and color depth) and last-hop bandwidths. Traditional transcoding breaks the end-to-end model of the Web, because the proxy does not know the semantics of the content. Server-directed transcoding preserves end-to-end semantics while supporting aggressive content transformations.We show how server-directed transcoding can be integrated into the HTTP protocol and into the implementation of a proxy. We discuss several useful transformations for image content, and present measurements of the performance impacts. Our results demonstrate that server-directed transcoding is a natural extension to HTTP, can be implemented without great complexity, and can provide good performance when carefully implemented.
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Index Terms
- Architecture and performance of server-directed transcoding
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Reviews
Subhankar Ray
An explicit form of server-directed transcoding (SDT) is described, which extends the hypertext transfer protocol (HTTP) 1.1 header without affecting the end-to-end nature of the protocol. In the HTTP request toward the content server, the client adds a feature set predicate (FSP) describing its limitations (for example, a number of colors and size of image it can handle) to the origin server (content server). The origin server adds feature set information (FSI) describing the actual image (number of colors, pixels in x/y coordinates), crop origin/size (as SDT-params information), and Java/Perl applet (as SDT-applet information) to be used by the proxy server to execute the transcoding. It also describes different proxy environments, implementation using Java/Perl, and resulting performance issues. Although it explains the improvements over autonomous transcoding proxies, the paper does not explain why implicit and more popular forms of SDT using user agent profile (UaProf) and composite capabilities/preference profile (CC/PP) are not a good idea (page 396). The HP lab and Apache Software group are managing an open source project called DELI (http://cocoon.apache.org/2.0/developing/deli.html), solving similar problems using implicit SDT. The paper also did not address the impacts of existing content and future content developers, and the upgrading process of the HTTP 1.1 server's ability to take advantage of its solutions. The application and improvement of SDT in a wireless environment where packet loss and retransmissions are common will be an interesting extension of this subject. Online Computing Reviews Service
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