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Re: HTTP should be able to transfer part of a document

From: Adrian Colley <aecolley@sse.ie>
Date: Wed, 08 Mar 1995 11:24:20 +0000
Message-Id: <12503081117.AA15500@terpsichore.sse.ie>
To: http-wg (will serve files for coffee) <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
In message <1DE9E260DDF@ksvi.mff.cuni.cz> of Wed, 08 Mar 1995 11:17:04
  +0100, Adam Dingle <DINGLE@ksvi.mff.cuni.cz> wrote:

> One possibility would be to address partial HTTP documents using URI
> fragment identifiers, so that, for example, a hypertext page could
> refer to an individual file within a tar archive, or to a group of
> several paragraphs within the text of a book.  For example, we might
> use a syntax such as
> http://www.cuni.cz/a/b/xxx#(1500000,1600000)
> to mean bytes 1500000 through 1600000 of the given document.

WN already uses a syntax like http://www.cuni.cz/a/b/xxx;bytes=1500000-1600000
to return ranges.

> Of course, such number-based addressing is somewhat dangerous to use in a
> static URL,

Well, you could protect it with if-modified-since.  If you want a link
which will get the index to any .zip file on a remote machine even after it
changes, then you have to decide where to put the zip-interpreting code.

Putting it on the server is obviously nice because it'll help the users of
all the non-hacked clients.  In general, however, someone's going to want
to use a format not supported by someone else's server.

Adding it to the client means a kind of request-wrapping processor has to be
defined.  So, here's how I think it might work:

1. Client gets a document via http for display in the normal way.  The
   server responds with Content-type: application/x-zip.  The client
   recognises this type from its table of content-type interpreters, and
   fires up the Zip Interpreting Algorithm.
2. Under the zip-specific code, the client aborts the http request [*] and
   starts a new one, retrieving the portion of the zip file which contains
   the index [&].
3. When it arrives, it's decoded into a menu of files.  Where links into
   the zipfile are needed, you can use a URL like:

     internal-zip:name=readme.txt&start=577&end=16320/http://host/big.zip

   (meaning: bytes 577-16320 of http://host/big.zip, to be interpreted as
    a file named readme.txt).  As you can probably guess, I just made this
    format up.  But since I'm proud of it, let's take a closer look at
    its structure:

    <method>:<arguments>/<recursive-url>

    The <arguments> are encoded identically to those used in form responses,
    and are arguments to the client-side processing identified by <method>.
    The <recursive-url> identifies the input data to the <method>.  (This
    allows you to browse tar files inside zip files.)

    For referring to the readme.txt in a static URL, you might use

     internal-zip:name=readme.txt/http://host/big.zip

    (leaving out the byte ranges).  This would cause extra processing in
    the client to get the index first, but would survive if the zipfile
    changed [%]

[*] Server implementors may now yelp in pain
[&] Of course, you have to check that the server can return byte-ranges,
    perhaps through a server-extensions line in the original reply.
[%] In fact, if the internal-zip algorithm is stateful or can use a cache,
    the byte ranges can be left out completely even in the generated menu.

Comments?

-- 
Adrian.Colley@sse.ie   <g=Adrian;s=Colley;o=SSE;p=SSE;a=EIRMAIL400;c=ie>
phones:- work: +353-1-6769089; fax: +353-1-6767984; home: +353-1-6606239
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Received on Wednesday, 8 March 1995 03:26:25 EST

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