title: When does nginx $upstream_response_time start/stop specifically
body: <p>Does anyone know when, specifically, the clock for <code>$upstream_response_time</code> begins and ends?</p>
  
  <p>The documentation seems a bit vague:</p>
  
  <blockquote>
    <p>keeps time spent on receiving the response from the upstream server; the time is kept in seconds with millisecond resolution. Times of several responses are separated by commas and colons like addresses in the $upstream_addr variable.</p>
  </blockquote>
  
  <p>There is also an <code>$upstream_header_time</code> value, which adds more confusion.</p>
  
  <ol>
  <li><p>I assume <code>$upstream_connect_time</code> stops once the connection is established, but <strong>before</strong> it is accepted upstream?</p></li>
  <li><p>After this what does <code>$upstream_response_time</code> include?</p>
  
  <ul>
  <li>Time spent waiting for upstream to accept?</li>
  <li>Time spent sending the request?</li>
  <li>Time spent sending the response header?</li>
  </ul></li>
  </ol>