[Webkit-unassigned] [Bug 188620] New: Navigation Timing API reporting below 0 for response time (responseEnd - responseStart)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=188620

            Bug ID: 188620
           Summary: Navigation Timing API reporting below 0 for response
                    time (responseEnd - responseStart)
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: iPhone / iPad
                OS: iOS 11
            Status: NEW
          Severity: Major
          Priority: P2
         Component: WebCore Misc.
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: msartintarm at gmail.com

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.

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