This specification defines an interface for web applications to access the complete timing information for resources in a document.
Status of This Document
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
Implementers are encouraged to look at the latest version of Resource Timing before implementing this Candidate Recommendation.
This document was published by the Web Performance Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to
public-web-perf@w3.org (subscribe,
archives) with [ResourceTiming] at the start of your email's subject.
W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community. This Candidate Recommendation is expected to advance to Proposed Recommendation no earlier than 27 April 2017. All comments are welcome.
Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
User latency is an important quality benchmark for Web Applications. While JavaScript-based mechanisms can provide comprehensive instrumentation for user latency measurements within an application, in many cases, they are unable to provide a complete end-to-end latency picture. This document introduces the PerformanceResourceTiming interface to allow JavaScript mechanisms to collect complete timing information related to resources on a document. Navigation Timing 2 [NAVIGATION-TIMING-2] extends this specification to provide additional timing information associated with a navigation.
For example, the following JavaScript shows a simple attempt to measure the time it takes to fetch a resource:
Example 1
<!doctype html><html><head></head><bodyonload="loadResources()"><script>functionloadResources()
{
var start = newDate().getTime();
var image1 = new Image();
var resourceTiming = function() {
var now = newDate().getTime();
var latency = now - start;
alert("End to end qresource fetch: " + latency);
};
image1.onload = resourceTiming;
image1.src = 'https://www.w3.org/Icons/w3c_main.png';
}
</script><imgsrc="https://www.w3.org/Icons/w3c_home.png"></body></html>
Though this script can measure the time it takes to fetch a resource, it cannot break down the time spent in various phases. Further, the script cannot easily measure the time it takes to fetch resources described in markup.
To address the need for complete information on user experience, this document introduces the PerformanceResourceTiming interface. This interface allows JavaScript mechanisms to provide complete client-side latency measurements within applications. With this interface, the previous example can be modified to measure a user's perceived load time of a resource.
The following script calculates the amount of time it takes to fetch every resource in the page, even those defined in markup. This example assumes that this page is hosted on https://www.w3.org. One could further measure the amount of time it takes in every phase of fetching a resource with the PerformanceResourceTiming interface.
Example 2
<!doctype html><html><head></head><bodyonload="loadResources()"><script>functionloadResources()
{
var image1 = new Image();
image1.onload = resourceTiming;
image1.src = 'https://www.w3.org/Icons/w3c_main.png';
}
functionresourceTiming()
{
var resourceList = window.performance.getEntriesByType("resource");
for (i = 0; i < resourceList.length; i++)
{
if (resourceList[i].initiatorType == "img")
{
alert("End to end resource fetch: "+ resourceList[i].responseEnd - resourceList[i].startTime);
}
}
}
</script><imgid="image0"src="https://www.w3.org/Icons/w3c_home.png"></body></html>
2. Conformance
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, and SHOULD are to be interpreted as described in [RFC2119].
Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("MUST", "SHOULD", "
MAY", etc) used in introducing the algorithm.
Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on user agents.
Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)
The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WebIDL]
3. Terminology
The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo".
The term DOM is used to refer to the API set made available to scripts in Web applications, and does not necessarily imply the existence of an actual
Document object or of any other Node objects as defined in the DOM specification. [DOM]
A DOM attribute is said to be getting when its value is being retrieved (such as by author script), and is said to be setting when a new value is assigned to it.
The term JavaScript is used to refer to ECMA262, rather than the official term ECMAScript, since the term JavaScript is more widely known. [ECMA-262]
The term resource is used to refer to elements and any other user-initiated fetches throughout this specification. For example, a resource could originate from XMLHttpRequest objects [[XMLHttpRequest], HTML elements [HTML51] such as iframe, img, script, object, embed, and link with the link type of stylesheet, and SVG elements [SVG11] such as svg.
The term cross-origin is used to mean non
same origin.
Throughout this work, all time values are measured in milliseconds since the start of navigation of the document [HR-TIME-2]. For example, the start of navigation of the document occurs at time 0.
The term current time refers to the number of milliseconds since the start of navigation of the document until the current moment in time.
Note
This definition of time is based on the High Resolution Time specification [
HR-TIME-2] and is different from the definition of time used in the Navigation Timing specification [
NAVIGATION-TIMING], where time is measured in milliseconds since midnight of January 1, 1970 (UTC).
If the same canonical URL is used as the src attribute of two HTML IMG elements, the fetch of the resource initiated by the first HTML IMG element SHOULD be included as a PerformanceResourceTiming object in the Performance Timeline. The user agent might not re-request the URL for the second HTML IMG element, instead using the existing download it initiated for the first HTML IMG element. In this case, the fetch of the resource by the first
IMG element would be the only occurrence in the Performance Timeline.
If the src attribute of a HTML IMG element is changed via script, both the fetch of the original resource as well as the fetch of the new URL would be included as PerformanceResourceTiming objects in the Performance Timeline.
If an HTML IFRAME element is added via markup without specifying a src attribute, the user agent may load the about:blank document for the IFRAME. If at a later time the src attribute is changed dynamically via script, the user agent may
fetch the new URL resource for the IFRAME. In this case, only the
fetch of the new URL would be included as a PerformanceResourceTiming object in the Performance Timeline.
If an XMLHttpRequest is generated twice for the same canonical URL, both fetches of the resource would be included as a PerformanceResourceTiming object in the Performance Timeline. This is because the fetch of the resource for the second XMLHttpRequest cannot reuse the download issued for the first XMLHttpRequest.
If an HTML IFRAME element is included on the page, then only the resource requested by IFRAMEsrc attribute is included as a PerformanceResourceTiming object in the Performance Timeline. Sub-resources requested by the IFRAME document will be included in the IFRAME document's Performance Timeline and not the parent document's Performance Timeline.
If a resource fetch was aborted due to a networking error (e.g. DNS, TCP, or TLS error), then the fetch would be included as a PerformanceResourceTiming object in the Performance Timeline with initialized attribute values up to the point of failure - e.g. a TCP handshake error should report DNS timestamps for the request, and so on.
If a resource fetch is aborted because it failed a fetch precondition (e.g. mixed content, CORS restriction, CSP policy, etc), then this resource will not be included as a PerformanceResourceTiming object in the Performance Timeline.
4.3 The PerformanceResourceTiming Interface
The PerformanceResourceTiming interface participates in the
Performance Timeline and extends the following attributes of the
PerformanceEntry interface:
name
This attribute MUST return the resolved URL of the requested resource. This attribute MUST NOT change even if the fetch redirected to a different URL.
entryType
The entryType attribute MUST return the DOMString "resource".
startTime
The startTime attribute MUST return a DOMHighResTimeStamp [HR-TIME-2] with the time immediately before the user agent starts to queue the resource for fetching. If there are HTTP redirects or equivalent when fetching the resource, and if all the redirects or equivalent are from the same origin as the current document or the timing allow check algorithm passes, this attribute MUST return the same value as redirectStart. Otherwise, this attribute MUST return the same value as fetchStart.
"css", if the initiator is a CSS resource downloaded by the url() syntax [CSS-SYNTAX-3], such as @import url() or background: url().
"xmlhttprequest", if the initiator is an XMLHttpRequest object [XHR].
On getting, the redirectStart attribute MUST return as follows:
The time immediately before the user agent starts to fetch the resource that initiates the redirect, if there are HTTP redirects or equivalent when fetching the resource and all the redirects or equivalent pass the timing allow check algorithm.
zero, otherwise.
On getting, the redirectEnd attribute MUST return as follows:
The time immediately after receiving the last byte of the response of the last redirect, if there are HTTP redirects or equivalent when fetching the resource and all the redirects or equivalent pass the timing allow check algorithm.
zero, otherwise.
On getting, the fetchStart attribute MUST return as follows:
The time immediately before the user agent starts to fetch the final resource in the redirection, if there are HTTP redirects or equivalent.
The time immediately before the user agent starts to fetch the resource otherwise.
On getting, the domainLookupStart attribute MUST return as follows:
The time immediately after the user agent before the domain data retrieval from the domain information cache, if the user agent has the domain information in cache.
The time immediately before the user agent starts the domain name lookup for the resource, if the last non-redirected fetch of the resource passes the timing allow check algorithm.
zero, otherwise.
On getting, the domainLookupEnd attribute MUST return as follows:
The time immediately after the user agent ends the domain data retrieval from the domain information cache, if the user agent has the domain information in cache.
The time immediately before the user agent finishes the domain name lookup for the resource, if the last non-redirected fetch of the resource passes the timing allow check algorithm.
zero, otherwise.
On getting, the connectStart attribute MUST return as follows:
The time immediately before the user agent start establishing the connection to the server to retrieve the resource, if the last non-redirected fetch of the resource passes the timing allow check algorithm.
If the transport connection fails and the user agent reopens a connection,
connectStartSHOULD return the corresponding value of the new connection.
zero, otherwise.
On getting, the connectEnd attribute MUST return as follows:
The time immediately after the user agent start establishing the connection to the server to retrieve the resource, if the last non-redirected fetch of the resource passes the timing allow check algorithm.
The returned time MUST include the time interval to establish the transport connection, as well as other time intervals such as SSL handshake and SOCKS authentication.
If the transport connection fails and the user agent reopens a connection, connectEndSHOULD return the corresponding value of the new connection.
zero, otherwise.
The secureConnectionStart attribute is optional. On getting, the attribute MUST return as follows:
The time immediately before the user agent starts the handshake process to secure the current connection, if a secure transport is used and the last non-redirected fetch of the resource passes the timing allow check algorithm.
zero, otherwise.
On getting, the requestStart attribute MUST return as follows:
The time immediately before the user agent starts requesting the resource from the server, or from
relevant application caches or from local resources, if the last non-redirected fetch of the resource passes the timing allow check algorithm.
If the transport connection fails after a request is sent and the user agent reopens a connection and resend the request, requestStartMUST return the corresponding values of the new request.
zero, otherwise.
Note
On getting, the responseStart attribute MUST return as follows:
The time immediately after the user agent receives the first byte of the response from relevant application caches, or from local resources or from the server if the last non-redirected fetch of the resource passes the timing allow check algorithm.
zero, otherwise.
On getting, the responseEnd attribute MUST return as follows:
The time immediately after the user agent receives the last byte of the response or immediately before the transport connection is closed, whichever comes first. The resource here can be received either from relevant application caches, or from local resources or from the server if the last non-redirected fetch of the resource passes the timing allow check algorithm.
Server-side applications may return the Timing-Allow-Origin HTTP response header to allow the User Agent to fully expose, to the document origin(s) specified, the values of attributes that would have been zero due to the cross-origin restrictions previously specified in this section.
4.5.1 Timing-Allow-Origin Response Header
The Timing-Allow-Origin
HTTP response header field can be used to communicate a policy indicating origin(s) that are allowed to see values of attributes that would have been zero due to the cross-origin restrictions. The header's value is represented by the following ABNF [RFC5234]:
The sender MAY generate multiple Timing-Allow-Origin header fields. The recipient MAY combine multiple Timing-Allow-Origin header fields by appending each subsequent field value to the combined field value in order, separated by a comma.
The timing allow check algorithm, which checks whether a resource's timing information can be shared with the
current document, is as follows:
The following graph illustrates the timing attributes defined by the PerformanceResourceTiming interface. Attributes underlined may not be available when fetching resources from different origins. User agents may perform internal processing in between timings, which allow for non-normative intervals between timings.
If no domain lookup is required, go to step 13. Otherwise, immediately before a user agent starts the domain name lookup, record the time as domainLookupStart.
Record the time as domainLookupEnd immediately after the domain name lookup is successfully done. A user agent may need multiple retries before that. If the domain lookup fails, abort the remaining steps.
If a persistent transport connection is used to fetch the resource, let connectStart and connectEnd be the same value of domainLookupEnd. Otherwise, record the time as
connectStart immediately before initiating the connection to the server and record the time as connectEnd immediately after the connection to the server or the proxy is established. A user agent may need multiple retries before this time. If a connection can not be established, abort the remaining steps.
If supported, the user agent MUST set the secureConnectionStart attribute as follows:
When a secure transport is used, the user agent MUST record the time as secureConnectionStart immediately before the handshake process to secure the connection.
When a secure transport is not used, the user agent MUST set the value of secureConnectionStart to 0.
Immediately before a user agent starts sending the request for the resource, record the current time as requestStart.
Record the time as
responseStart immediately after the user agent receives the first byte of the response.
Record the time as responseEnd immediately after receiving the last byte of the response.
Return to step 13 if the user agent fails to send the request or receive the entire response, and needs to reopen the connection.
The value of the timing attributes MUST monotonically increase to ensure timing attributes are not skewed by adjustments to the system clock while fetching the resource. The difference between any two chronologically recorded timing attributes MUST never be negative. For all resources, including subdocument resources, the user agent MUST record the system clock at the beginning of the root document navigation and define subsequent timing attributes in terms of a monotonic clock measuring time elapsed from the beginning of the navigation.
6. Privacy and Security
This section is non-normative.
The PerformanceResourceTiming interface exposes timing information for a resource to any web page or worker that has requested that resource. To limit the access to the PerformanceResourceTiming interface, the same origin policy is enforced by default and certain attributes are set to zero, as described in 4.5Cross-origin Resources. Resource providers can explicitly allow all timing information to be collected for a resource by adding the Timing-Allow-Origin HTTP response header, which specifies the domains that are allowed to access the timing information.
Statistical fingerprinting is a privacy concern where a malicious web site may determine whether a user has visited a third-party web site by measuring the timing of cache hits and misses of resources in the third-party web site. Though the PerformanceResourceTiming interface gives timing information for resources in a document, the cross-origin restrictions prevent making this privacy concern any worse than it is today using the load event on resources to measure timing to determine cache hits and misses.
A. Acknowledgments
We would like to sincerely thank Karen Anderson, Darin Fisher, Tony Gentilcore, Nic Jansma, Kyle Scholz, Jonas Sicking, James Simonsen, Steve Souders, Annie Sullivan, Sigbjørn Vik, Jason Weber to acknowledge their contributions to this work.
