Anything that is close to fetchStart in resource timing API?

Web Performance Discuss Test Results
#1

Resource Timing API defines this:

“If there are no HTTP redirects or equivalent, this attribute must return the time immediately before the user agent starts to fetch the resource.”

What could be a corresponding field in the WPT HAR file? This event definitely happens before the dns lookup starts. But, the lowest start value in WPT for a given resource is _all_start which, I think, cannot be less than _dns_start and so I am not sure if WPT provides this metric at all.

#2

The resource timing definition is defining fetchStart as when the UA actually puts the request on the wire so my view it’s the earliest of _dns_start, _connect_start, and _ttfb_start in WPT terms.

If you want to verify you can use a WPT custom metric to dump the resource timing entries into a JSON string:

[resource-timing]
return JSON.stringify(window.performance.getEntriesByType("resource"));