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概念的？？　いきなり36ページ当たりに

　Note that the proxy MAY
　forward the request on to another proxy or directly to the server
specified by the absoluteURI. In order to avoid request loops, a
proxy MUST be able to recognize all of its server names, including
any aliases, local variations, and the numeric IP address. An example
Request-Line would be:

GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

とかいうのがある気がするんだが。資料から必要な情報だけ抜き出すコツは
早めにつかんでおいたほうが良いよ。