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    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - Navigation Timing API reporting below 0 for response time (responseEnd - responseStart)"
   href="https://bugs.webkit.org/show_bug.cgi?id=188620">188620</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>Navigation Timing API reporting below 0 for response time (responseEnd - responseStart)
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>WebKit
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>WebKit Nightly Build
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>iPhone / iPad
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>iOS 11
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>Major
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P2
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>WebCore Misc.
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>webkit-unassigned@lists.webkit.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>msartintarm@gmail.com
          </td>
        </tr></table>
      <p>
        <div>
        <pre>I work on a site with millions of hits per day. We collect latency data and make assertions on the results client-side before using them in aggregate metrics.

We are receiving invalid metrics from the Safari browser for various iPhone / iPad devices using iOS 10 and 11.

The following interval is showing up as below 0 in many instances:

`performance.timing.responseEnd - performance.timing.responseStart`

In some cases the number is significantly below 0. It doesn't seem like it would ever make sense for this number to be below 0.

We collect samples of performance.timing data for which these errors occur. Here is a sample where responseEnd - responseStart is -28412:

{"navigationStart":1534229958311,"unloadEventStart":0,"unloadEventEnd":0,"redirectStart":0,"redirectEnd":0,"fetchStart":1534229958311,"domainLookupStart":1534229958311,"domainLookupEnd":1534229958311,"connectStart":1534229958311,"connectEnd":1534229958311,"secureConnectionStart":0,"requestStart":1534229958313,"responseStart":1534229958435,"responseEnd":1534229930023,"domLoading":1534229958326,"domInteractive":1534229958585,"domContentLoadedEventStart":1534229958585,"domContentLoadedEventEnd":1534229958585,"domComplete":1534229961509,"loadEventStart":1534229961510,"loadEventEnd":1534229961510}


        sub_family: "AppleWebKit"
        platform: "iPad"
        device: "iPad"
        os: "CPU OS 10_3_3 like Mac OS X"
        browser: "Safari"
        browser_version: "10.0"
        browser_engine_version: "603.3.8"

Let me know if there is any additional analysis / info I can attach that would be helpful.</pre>
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      </p>


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