Finish test for Cache Control Directives

abbrev/acronym.tex

@@ -23,3 +23,4 @@
 \newacronym {ISP}       {ISP}       	{Internet Service Provider}
 \newacronym {SQL}	{SQL}		{Structured Query Language}
 \newacronym {CDN}	{CDN}		{Content Delivery Network}
+\newacronym {ETag}	{ETag}		{Entity Tag}

chapters/methods.tex

@@ -779,7 +779,34 @@ attack.
 
 Cache Control Directives can be supplied in the Cache-Control \gls{HTTP} header,
 allowing rules about storing, updating and deletion of resources in the cache to
-be defined. 
+be defined. Cache Control Directives make heavy use of \emph{\glspl{ETag}} and
+\emph{Last-Modified \gls{HTTP} Headers} to determine whether a cached resource
+is stale and needs to be updated. Commonly, a collision-resistant hash function
+is used to generate a unique hash of a cached resource which is sent along with
+the resource in the first \gls{HTTP} request. The resource and the hash—which is
+stored in the \gls{ETag} header—is then cached by the client. On subsequent
+retrievals of the same \gls{URL}, the client checks for an expire date on the
+requested \gls{URL} via the Cache-Control and Expire headers. If the \gls{URL}
+has expired, the client sends a request with the \emph{If-None-Match} field set
+with the \gls{ETag}. The server then compares the \gls{ETag} received by the
+client with the generated \gls{ETag} of the resource on the server side. If the
+two values match (i.e., the resource has not changed), the server can send back
+an \gls{HTTP} 304 Not-Modified status. Otherwise, the answer contains a full
+\gls{HTTP} response with the modified resource and the newly generated
+\gls{ETag}, which the client can cache again. Usage of \glspl{ETag} can
+therefore improve performance and cache consistency while at the same time
+reducing bandwidth usage.
+
+As with most other tracking methods, unique identifiers can be stored inside the
+\gls{ETag} header because it offers a storage capacity of 81864 bits. Once the
+identifier has been placed in the \gls{ETag} header, the server can answer
+requests to check for an updated resource always with an \gls{HTTP} 301
+Not-Modified header, effectively persisting the unique identifier in the
+client's cache. During their 2011 survey of QuantCast.com's top 100 U.S. based
+websites \citeauthor{ayensonFlashCookiesPrivacy2011} found \texttt{hulu.com} to
+be using \glspl{ETag} as backup for tracking cookies that are set by
+\texttt{KISSmetrics} (an analytics platform). This allowed cookies to be
+respawned once they had been cleared by checking the \gls{ETag} header.
 
 \subsection{DNS Cache}
 \label{subsec:dns cache}