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<h1>Compressing HTTP contents using Content-Encoding</h1>
<h2><a id="communication"></a>Communication in HTTP</h2>
<p>For the transmission of web contents between server and client the <strong>Hypertext Transfer Protocol</strong> <small>(<a href="http://www.w3.org/Protocols/rfc2616/rfc2616.html"><img class="linkicon" height="15" width="16" alt="arrow" title="external" src="extern.gif" />HTTP</a>)</small> is being used.</p>
<p>The communication works the following way:</p>
<ol>
<li>The client is sending a request to the server. This requests mainly consists of <strong>HTTP headers</strong> that describe which data the client wants to receive.<br />
To some degree the client is able to <em>directly influence</em> the data content <small>(like specifying the URL of the requested page)</small>; other parts of its request supply the server with additional information about which type of data the client would <em>prefer to receive</em>, if the server has to offer several data variations.</li>
<li>The server evaluates all available information to decide which data with what kind of attributes were requested and subsequently attempts to serve these data in a form most suitable to the client's request.<br />
This data packet again contains HTTP headers but mainly <strong>data</strong> - which nevertheless can only be partially interpretable without consideration of the HTTP headers.</li>
</ol>
<p>The second aspect of determining the desired content is named <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec12.html#sec12"><img class="linkicon" height="15" width="16" alt="arrow" title="external" src="extern.gif" />Content Negotiation</a> in its general form. Delivering compressed page content in the form described here is based on this general principle as well.</p>
<h2><a id="encoding"></a>Encodings in HTTP</h2>
<p>HTTP 1.1 defines in <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.5"><img class="linkicon" height="15" width="16" alt="arrow" title="external" src="extern.gif" />chapter 3.5.</a> so-called <strong>content encodings</strong> that should be supported be each browser compliant to this standard.</p>
<p>For communication between server and client these encodings have the following meaning:</p>
<ol>
<li>This client <em>may</em> specify a liste of accepted content encodings in the HTTP header <code>Accept-Encoding:</code>. The special encoding <code>identity</code> <small>(describing an encoding that doesn't change the content)</small> will be used implicitly as default value.</li>
<li>The server <em>should</em> deliver the content of the requested document using an encoding accepted by this client.<br />
If the server isn't able to do so the it <em>should</em> reject the request with the <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.7"><img class="linkicon" height="15" width="16" alt="arrow" title="external" src="extern.gif" />status code <code>406 Not Acceptable</code></a>; nevertheless it <em>may</em> deliver contents not acceptable for this client according to its request <small>(and whose processing might cause unsolvable problems for the Client)</small>.<br />
The Apache webserver uses a defensive strategy in this respect: If it cannot serve a page according to the client's requests but has several other variants on offer it dynamically generates a HTML document containing a short description of the situation and links to all documents in question. <small>(This is the case as well if this very document is requested by a browser that accepts none of the languages <code>de</code> und <code>en</code> supported by myself according to its current configuration; I have deliberately not defined a default value to make this problem aware to foreign language readers of these pages.)</small></li>
</ol>
<p>If a server serves a file containing an encoding to a client not capable of handling it then this clients might not be able to handle it correctly.</p>
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<p id="mail">(<a href="mailto:michael.schroepl@gmx.de?subject=mod_gzip">Michael Schröpl</a>, 2002-07-12)</p>
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