Resource Timing API

The Resource Timing interfaces enable retrieving and analyzing detailed network timing data regarding the loading of an application's resource(s). An application can use the timing metrics to determine, for example, the length of time it takes to load a specific resource, such as an XMLHttpRequest, <SVG>, image, or script.

The interface's properties create a resource loading timeline with high-resolution timestamps for network events such as redirect start and end times, DNS lookup start and end times, request start, response start and end times, etc. The interface also includes other properties that provide data about the size of the fetched resource as well as the type of resource that initiated the fetch.

This document provides an overview of the Resource Timing interfaces. For more details about the interfaces including examples, see the each interface's reference page, Using Resource Timing and the references in the See also section. For a graphical representation of the resource timing processing model, see the resource timing phases figure.

The PerformanceResourceTiming interface extends the PerformanceEntry for performance entries which have an entryType of "resource".

High-resolution time stamps

Several of the Resource Timing properties return high-resolution timestamps. These timestamps have a DOMHighResTimeStamp type, and as its name implies, they represent a high-resolution point in time. This type is a double, and its value is a discrete point in time or the difference in time between two discrete points in time.

The unit of DOMHighResTimeStamp is milliseconds and should be accurate to 5 µs (microseconds). However, If the browser is unable to provide a time value accurate to 5 microseconds (because, for example, due to hardware or software constraints), the browser can represent a the value as a time in milliseconds accurate to a millisecond.

Resource loading timestamps

An application can get timestamps for the various stages used to load a resource. The first property in the processing model is startTime which returns the timestamp immediately before the resource loading process begins. The fetchStart timestamps follows and redirect processing (if applicable) and preceeds DNS lookup. The next stages are connectStart and connectEnd which are the timestamps immediately before and after connecting to the server, respectively. The last three timestamps are, in order: requestStart - the timestamp before the browser starts requesting the resource from the server; responseStart - the timestamp after the browser receives the first byte of the response from the server; and responseEnd - the timestamp after the browser receives the last byte of the resource. If the resource is loaded via a secure connection a secureConnectionStart timestamp will be available between the connection start and end events.

When CORS is in effect, many of these values are returned as zero unless the server's access policy permits these values to be shared. This requires the server providing the resource to send the Timing-Allow-Origin HTTP response header with a value specifying the origin or origins which are allowed to get the restricted timestamp values.

The properties which are returned as 0 by default when loading a resource form a domain other than the one of the web page itself: redirectStart, redirectEnd, domainLookupStart, domainLookupEnd, connectStart, connectEnd, secureConnectionStart, requestStart, and responseStart.

The PerformanceResourceTiming interface also includes several network timing properties. The redirectStart and redirectEnd properties return timestamps for redirect start and end times, respectively. Likewise, the The domainLookupStart and domainLookupEnd properties return timestamps for DNS lookup start and end times, respectively.

This would be a nice place to have a diagram showing the relationships between these segments of the resource loading time.

Resource size

The PerformanceResourceTiming interface has three properties that can be used to obtain size data about a resource. The transferSize property returns the size (in octets) of the fetched resource including the response header fields plus the response payload body.

The encodedBodySize property returns the size (in octets) received from the fetch (HTTP or cache), of the payload body, before removing any applied content-codings. decodedBodySize returns the size (in octets) received from the fetch (HTTP or cache) of the message body, after removing any applied content-codings.

Other properties

The nextHopProtocol property returns the network protocol used to fetch the resource.

The initiatorType property returns the type of resource that initiated the performance entry such as "css" for a CSS resource, "xmlhttprequest" for an XMLHttpRequest and "img" for an image (such as a JPEG).

If the current context is a worker, the workerStart property can be used to obtain a DOMHighResTimeStamp when the worker was started.

Methods

The Resource Timing API includes two methods that extend the Performance interface. The clearResourceTimings() method removes all "resource" type performance entries from the browser's resource performance entry buffer. The setResourceTimingBufferSize() method sets the resource performance entry buffer size to the specified number of resource performance entries.

The PerformanceResourceTiming interface's toJSON() method returns a JSON serialization of a "resource" type performance entry.

Implementation status

As shown in the PerformanceResourceTiming interface's Browser Compatibility table, most of these interfaces are broadly implemented by desktop browsers. However, note that some properties have little to no implementation so see each property's "Browser compatibility" section for more specific interoperability data.

To test your browser's support for these interfaces, run the perf-api-support application.