Navigation Timing

W3C

W3C Candidate Recommendation 2 June 2011

This version:
http://www.w3.org/TR/2011/CR-navigation-timing-20110602/
Latest version:
http://www.w3.org/TR/navigation-timing/
Latest Editor's Draft:
http://www.w3c-test.org/webperf/specs/NavigationTiming/
Previous version:
http://www.w3.org/TR/2011/CR-navigation-timing-20110315/
Editors:
Zhiheng Wang (Google Inc.) <>

Copyright © 2011 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

Abstract

This specification defines an interface for web applications to access timing information related to navigation and elements.

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 http://www.w3.org/TR/.

This is an update to the Candidate Recommendation of Navigation Timing, taking into account feedback from implementors.

W3C publishes a Candidate Recommendation to indicate that the document is believed to be stable and to encourage implementation by the developer community.

The entrance criteria for this document to enter the Proposed Recommendation stage is to have a minimum of two independent and interoperable user agents that implementation all the features of this specification, which will be determined by passing the user agent tests defined in the test suite developed by the Working Group.

The Working Group does not expect to advance to Proposed Recommendation prior to 30 June 2011. A preliminary implementation report is available and will be updated during the Candidate Recommendation period.

Please send comments to public-web-perf@w3.org (archived) with [NavigationTiming] at the start of the subject line.

A diff document outlining the changes done since the previous publication is available. Those are based on the feedback received during the Candidate Recommendation period.

This document is produced by the Web Performance Working Group. The Web Performance Working Group is part of the Rich Web Clients Activity in the W3C Interaction Domain.

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.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

  1. 1 Introduction
  2. 2 Conformance requirements
  3. 3 Terminology
  4. 4 Navigation Timing
    1. 4.1 Introduction
    2. 4.2 The PerformanceTiming interface
    3. 4.3 The PerformanceNavigation interface
    4. 4.4 The window.performance attribute
  5. 5 Process
    1. 5.1 Processing Model
    2. 5.2 Garbage Collection
    3. 5.3 The Monotonic Clock
  6. 6 Privacy
  7. 7 Security
  8. 8 References
  9. Acknowledgements

1 Introduction

This section is non-normative.

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.

For example, the following Javascript shows a naive attempt to measure the time it takes to fully load a page:

<html>
<head>
<script type="text/javascript">

var start = new Date().getTime();
function onLoad() {
  var now = new Date().getTime();
  var latency = now - start;
  alert("page loading time: " + latency);
}

</script>
</head>
<body >
<!- Main page body goes from here. -->
</body>
</html>

The script calculates the time it takes to load the page after the first bit of JavaScript in the head is executed, but it does not give any information about the time it takes to get the page from the server.

To address the need for complete information on user experience, this document introduces the PerformanceTiming interfaces. This interface allows JavaScript mechanisms to provide complete client-side latency measurements within applications. With the proposed interface, the previous example can be modified to measure a user's perceived page load time.

The following script calculates how much time to load a page since the most recent navigation.

<html>
<head>
<script type="text/javascript">
function onLoad() {
  var now = new Date().getTime();
  var page_load_time = now - performance.timing.navigationStart;
  alert("User-perceived page loading time: " + page_load_time);
}

</script>
</head>
<body >
<!- Main page body goes from here. -->
</body>
</html>

The interface provided by this work does not intend to be used as any sort of performance benchmark for user agents.

2 Conformance requirements

All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. For readability, these words do not appear in all uppercase letters in this specification.

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.)

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 Core specifications.

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 ECMA-262, rather than the official term ECMAScript, since the term JavaScript is more widely known.

Throughout this work, time is measured in milliseconds since midnight of January 1, 1970 (UTC).

4 Navigation Timing

4.1 Introduction

This section is non-normative

This specification introduces an interface that provides Web applications with timing-related information. This specification does not cover how Web applications leverage these interfaces to collect, store and report the provided information.

4.2 The PerformanceTiming interface

interface PerformanceTiming {
  readonly attribute unsigned long long navigationStart;
  readonly attribute unsigned long long unloadEventStart;
  readonly attribute unsigned long long unloadEventEnd;
  readonly attribute unsigned long long redirectStart;
  readonly attribute unsigned long long redirectEnd;
  readonly attribute unsigned long long fetchStart;
  readonly attribute unsigned long long domainLookupStart;
  readonly attribute unsigned long long domainLookupEnd;
  readonly attribute unsigned long long connectStart;
  readonly attribute unsigned long long connectEnd;
  readonly attribute unsigned long long secureConnectionStart;
  readonly attribute unsigned long long requestStart;
  readonly attribute unsigned long long responseStart;
  readonly attribute unsigned long long responseEnd;
  readonly attribute unsigned long long domLoading;
  readonly attribute unsigned long long domInteractive;
  readonly attribute unsigned long long domContentLoadedEventStart;
  readonly attribute unsigned long long domContentLoadedEventEnd;
  readonly attribute unsigned long long domComplete;
  readonly attribute unsigned long long loadEventStart;
  readonly attribute unsigned long long loadEventEnd;
};

navigationStart attribute

This attribute must return the time immediately after the user agent finishes prompting to unload the previous document. If there is no previous document, this attribute must return the same value as fetchStart.

unloadEventStart attribute

If the previous document and the current document have the same origin, this attribute must return the time immediately before the user agent starts the unload event of the previous document. If there is no previous document or the previous document has a different origin than the current document, this attribute must return zero.

unloadEventEnd attribute

If the previous document and the current document have the same origin, this attribute must return the time immediately after the user agent finishes the unload event of the previous document. If there is no previous document or the previous document has a different origin than the current document or the unload is not yet completed, this attribute must return zero.

If there are HTTP redirects or equivalent when navigating and not all the redirects or equivalent are from the same origin, both unloadEventStart and unloadEventEnd must return the zero.

redirectStart attribute

If there are HTTP redirects or equivalent when navigating and if all the redirects or equivalent are from the same origin, this attribute must return the starting time of the fetch that initiates the redirect. Otherwise, this attribute must return zero.

redirectEnd attribute

If there are HTTP redirects or equivalent when navigating and all redirects and equivalents are from the same origin, this attribute must return the time immediately after receiving the last byte of the response of the last redirect. Otherwise, this attribute must return zero.

fetchStart attribute

If the new resource is to be fetched using HTTP GET or equivalent, fetchStart must return the time immediately before the user agent starts checking any relevant application caches. Otherwise, it must return the time when the user agent starts fetching the resource.

domainLookupStart attribute

This attribute must return the time immediately before the user agent starts the domain name lookup for the current document. If a persistent connection [RFC 2616] is used or the current document is retrieved from relevant application caches or local resources, this attribute must return the same value as fetchStart.

domainLookupEnd attribute

This attribute must return the time immediately after the user agent finishes the domain name lookup for the current document. If a persistent connection [RFC 2616] is used or the current document is retrieved from relevant application caches or local resources, this attribute must return the same value as fetchStart.

This section is non-normative.

Checking and retrieving contents from the HTTP cache [RFC 2616] is part of the fetching process. It's covered by the requestStart, responseStart and responseEnd attributes.

Example

In case where the user agent already has the domain information in cache, domainLookupStart and domainLookupEnd represent the times when the user agent starts and ends the domain data retrieval from the cache.

connectStart attribute

This attribute must return the time immediately before the user agent start establishing the connection to the server to retrieve the document. If a persistent connection [RFC 2616] is used or the current document is retrieved from relevant application caches or local resources, this attribute must return value of domainLookupEnd.

connectEnd attribute

This attribute must return the time immediately after the user agent finishes establishing the connection to the server to retrieve the current document. If a persistent connection [RFC 2616] is used or the current document is retrieved from relevant application caches or local resources, this attribute must return the value of domainLookupEnd

If the transport connection fails and the user agent reopens a connection, connectStart and connectEnd should return the corresponding values of the new connection.

connectEnd must include the time interval to establish the transport connection as well as other time interval such as SSL handshake and SOCKS authentication.

secureConnectionStart attribute

This attribute is optional. User agents that don't have this attribute available must set it as undefined. When this attribute is available, if the scheme of the current page is HTTPS, this attribute must return the time immediately before the user agent starts the handshake process to secure the current connection. If this attribute is available but HTTPS is not used, this attribute must return zero.

requestStart attribute

This attribute must return the time immediately before the user agent starts requesting the current document from the server, or from relevant application caches or from local resources.

If the transport connection fails after a request is sent and the user agent reopens a connection and resend the request, requestStart should return the corresponding values of the new request.

This interface does not include an attribute to represent the completion of sending the request, e.g., requestEnd.

responseStart attribute

This attribute must return the time immediately after the user agent receives the first byte of the response from the server, or from relevant application caches or from local resources.

responseEnd attribute

This attribute must return the time immediately after the user agent receives the last byte of the current document or immediately before the transport connection is closed, whichever comes first. The document here can be received either from the server, relevant application caches or from local resources.

domLoading attribute

This attribute must return the time immediately before the user agent sets the current document readiness to "loading".

domInteractive attribute

This attribute must return the time immediately before the user agent sets the current document readiness to "interactive".

domContentLoadedEventStart attribute

This attribute must return the time immediately before the user agent fires the DOMContentLoaded event at the Document.

domContentLoadedEventEnd attribute

This attribute must return the time immediately after the document's DOMContentLoaded event completes.

domComplete attribute

This attribute must return the time immediately before the user agent sets the current document readiness to "complete".

If the current document readiness changes to the same state multiple times, domLoading, domInteractive, domContentLoadedEventStart, domContentLoadedEventEnd and domComplete must return the time of the first occurrence of the corresponding document readiness change.

loadEventStart attribute

This attribute must return the time immediately before the load event of the current document is fired. It must return zero when the load event is not fired yet.

loadEventEnd attribute

This attribute must return the time when the load event of the current document is completed. It must return zero when the load event is not fired or is not completed.

4.3 The PerformanceNavigation interface

interface PerformanceNavigation {
  const unsigned short TYPE_NAVIGATE = 0;
  const unsigned short TYPE_RELOAD = 1;
  const unsigned short TYPE_BACK_FORWARD = 2;
  const unsigned short TYPE_RESERVED = 255;
  readonly attribute unsigned short type;
  readonly attribute unsigned short redirectCount;
};

type attribute

This attribute must return the type of the last non-redirect navigation in the current browsing context. It must have one of the following navigation type values.

TYPE_NAVIGATE: Navigation started by clicking on a link, or entering the URL in the user agent's address bar, or form submission, or initializing through a script operation other than the ones used by TYPE_RELOAD and TYPE_BACK_FORWARD as listed below.
TYPE_RELOAD: Navigation through the reload operation or the location.reload() method.
TYPE_BACK_FORWARD: Navigation through a history traversal operation.
TYPE_RESERVED: Any navigation types not defined by values above.

Client-side redirects, such as those using the Refresh pragma directive, are not considered HTTP redirects or equivalent by this spec. In those cases, the type attribute should return appropriate value, such as TYPE_RELOAD if reloading the current page, or TYPE_NAVIGATE if navigating to a new URL.

redirectCount attribute

This attribute must return the number of redirects since the last non-redirect navigation under the current browsing context. If there is no redirect or there is any redirect that is not from the same origin as the destination document, this attribute must return zero.

4.4 The window.performance attribute

interface Performance {
  readonly attribute PerformanceTiming timing;
  readonly attribute PerformanceNavigation navigation;
};

[Supplemental]
interface Window {
  [Replaceable] readonly attribute Performance performance;
};

The window.performance attribute provides a hosting area for performance related attributes. Specifically, the window.performance.timing attribute represents the timing information related to the browsing contexts since the last non-redirect navigation. Each browsing context must have a unique window.performance.timing attribute. PerformanceTiming objects in the timing attribute may be sorted by the chronological order of the corresponding browsing context.

Following is an example of how to use the interface in an HTML page: 

var t = performance.timing;
var n = performance.navigation;
if (t.loadEventEnd > 0) {
  var page_load_time = t.loadEventEnd - t.navigationStart;
  if (n.type == n.TYPE_NAVIGATE) {
    alert (page_load_time);
  }
}

5 Process

5.1 Processing Model

All the attributes in window.performance.timing and window.performance.navigation should not be written to until the Window object of the current document is created, even though their attributes are referred to in the following steps to facilitate description.

User agents may provide users the option of disabling the window.performance.timing and window.performance.navigation interfaces. When these interfaces are disabled, both window.performance.timing and window.performance.navigation must return a null value.

Example

A user agent may maintain instances of the PerformanceTiming and PerformanceNavigation interfaces until the Window object associated with the current document is created, when window.performance.timing and window.performance.navigation are replaced with these instances.

Illustration

This section is non-normative.

The following graph illustrates the timing attributes defined by the PerformanceTiming interface and the PerformanceNavigation interface with or without redirect, respectively. Attributes underlined may not be available in navigation involving documents from different origins. User agents may perform internal processing in between timings, which allow for non-normative intervals between timings.

Timing attributes

  1. If the navigation is aborted for any of the following reasons, abort these steps without changing the attributes in window.performance.timing and window.performance.navigation.
    1. The navigation is aborted due to the sandboxed navigation browsing context flag or the sandboxed top-level navigation browsing context flag, or a preexist attempt to navigate the browsing context.
    2. The navigation is caused by fragment identifiers within the page.
    3. The new resource is to be handled by some sort of inline content.
    4. The new resource is to be handled using a mechanism that does not affect the browsing context.
    5. The user refuses to allow the document to be unloaded.
  2. Immediately after the user agent prompts to unload the previous document, record the current time as navigationStart.
  3. Record the current navigation type in window.performance.navigation.type if it has not been set:
    1. If the navigation was started by clicking on a link, or entering the URL in the user agent's address bar, or form submission, or initializing through a script operation other than the location.reload() method, let the navigation type be TYPE_NAVIGATE.
    2. If the navigation was started either as a result of a meta refresh, or the location.reload() method, or other equivalent actions, let the navigation type be TYPE_RELOAD.
    3. If the navigation was started as a result of history traversal, let the navigation type be TYPE_BACK_FORWARD.
    4. Otherwise, let the navigation type be TYPE_RESERVED.
  4. If the current document and the previous document are from different origins, set both unloadEventStart and unloadEventEnd to 0 then go to step 6. Otherwise, record unloadEventStart as the time immediately before the unload event.
  5. Immediately after the unload event is completed, record the current time as unloadEventEnd.
  6. If the new resource is to be fetched using HTTP GET or equivalent, immediately before a user agent checks with the relevant application caches, record the current time as fetchStart. Otherwise, immediately before a user agent starts the fetching process, record the current time as fetchStart.
  7. Let domainLookupStart, domainLookupEnd, connectStart and connectEnd be the same value as fetchStart.
  8. If the resource is fetched from the relevant application cache or local resources, including the HTTP cache, go to step 13.
  9. If no domain lookup is required, go to step 11. Otherwise, immediately before a user agent starts the domain name lookup, record the time as domainLookupStart.
  10. 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 rest of the steps.
  11. 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 rest of the steps.
  12. In step 11, a user agent should also carry out these additional steps if it supports the secureConnectionStart attribute:
    1. If the scheme of the current document is HTTPS, the user agent must record the time as secureConnectionStart immediately before the handshake process to secure the connection.
    2. If the scheme of the current document is not HTTPS, the user agent must set the value of secureConnectionStart to 0.
  13. Immediately before a user agent starts sending request for the document, record the current time as requestStart.
  14. Record the time as responseStart immediately after the user agent receives the first byte of the response.
  15. Record the time as responseEnd immediately after receiving the last byte of the response.

    Return to step 11 if the user agent fails to send the request or receive the entire response, and needs to reopen the connection.

    Example

    When persistent connection [RFC 2616] is enabled, a user agent may first try to re-use an open connect to send the request while the connection can be asynchronously closed. In such case, connectStart, connectEnd and requestStart should represent timing information collected over the re-open connection.

  16. If the fetched resource results in an HTTP redirect or equivalent, then
    1. if the current document and the document that is redirected to are not from the same origin, set redirectStart, redirectEnd, unloadEventStart, unloadEventStart and redirectCount to 0. Then, return to step 6 with the new resource.
    2. if there is previous redirect involving documents that are not from the same origin, set redirectStart, redirectEnd, unloadEventStart, unloadEventStart and redirectCount to 0. Then, return to step 6 with the new resource.
    3. Increment redirectCount by 1.
    4. If the value of redirectStart is 0, let it be the value of fetchStart.
    5. Let redirectEnd be the value of responseEnd.
    6. Set all the attributes in window.performance.timing to 0 except navigationStart, redirectStart, redirectEnd, unloadEventStart and unloadEventEnd.
    7. Return to step 6 with the new resource.
  17. Record the time as domLoading immediately before the user agent sets the current document readiness to "loading".
  18. Record the time as domInteractive immediately before the user agent sets the current document readiness to "interactive".
  19. Record the time as domContentLoadedEventStart immediately before the user agent fires the DOMContentLoaded event at the document.
  20. Record the time as domContentLoadedEventEnd immediately after the DOMContentLoaded event completes.
  21. Record the time as domComplete immediately before the user agent sets the current document readiness to "complete".
  22. Record the time as loadEventStart immediately before the user agent fires the load event.
  23. Record the time as loadEventEnd immediately after the user agent completes the load event.

Some user agents maintain the DOM structure of the document in memory during navigation operations such as forward and backward. In those cases, the window.performance.timing and window.performance.navigation objects must not be altered during the navigation.

5.2 Garbage Collection

There are implied strong references from the window object to its window.performance.timing and window.performance.navigation objects.

5.3 The Monotonic Clock

This section is non-normative.

The values of the timing attributes should monotonically increasing at the rate time actually passes at. However, wallclock time can be adjusted by system users or other mechanisms such as NTP. Implementations that is purely based on the wallclock is prone to large jumps forwards and backwards as the wallclock is adjusted, so the timing attributes can not guarantee to be monotonic.

To prevent wallclock adjustment from skewing the timing attributes, user agents are recommended to record the wallclock time at the beginning of the navigation and define other timing attributes in terms of a monotonic clock measuring time elapsed from the beginning of the navigation.

6 Privacy

This section is non-normative.

6.1 Information disclosure

There is the potential for disclosing an end-user's browsing and activity history by using carefully crafted timing attacks. For instance, the unloading time reveals how long the previous page takes to execute its unload handler, which could be used to infer the user's login status. These attacks have been mitigated by enforcing the same origin policy when timing information involving the previous navigation is accessed.

The relaxed same origin policy doesn't provide sufficient protection against unauthorized visits across documents. In shared hosting, an untrusted third party is able to host an HTTP server at the same IP address but on a different port.

6.2 Cross-directory access

Different pages sharing one host name, for example contents from different authors hosted on sites with user generated content are considered from the same origin because there is no feature to restrict the access by pathname. Navigating between these pages allows a latter page to access timing information of the previous one, such as timing regarding redirection and unload event.

7 Security

This section is non-normative.

7.1 Detecting proxy servers

In case a proxy is deployed between the user agent and the web server, the time interval between the connectStart and the connectEnd attributes indicates the delay between the user agent and the proxy instead of the web server. With that, web server can potentially infer the existence of the proxy. For SOCKS proxy, this time interval includes the proxy authentication time and time the proxy takes to connect to the web server, which obfuscate the proxy detection. In case of an HTTP proxy, the user agent might not have any knowledge about the proxy server at all so it's not always feasible to mitigate this attack.

7.2 Tampering by replacing timing objects

The window.performance object is replaceable to avoid conflicts with existing pages using the same object. By doing that, it is possible for third-party to replace the object and scripts relying on the interface described in this work would break.

8 References

[IETF RFC 2119]
Key words for use in RFCs to Indicate Requirement Levels, Scott Bradner, Author. Internet Engineering Task Force, March 1997. Available at http://www.ietf.org/rfc/rfc2119.txt.
[IETF RFC 2616]
Hypertext Transfer Protocol -- HTTP/1.1, R. Fielding et al., Authors. Internet Engineering Task Force, June 1999. Available at http://www.ietf.org/rfc/rfc2616.txt.
[DOM Level 3 Core]
Document Object Model Level 3 Core Specification, A. Le Hors, et al., Editors. World Wide Web Consortium, 7 April 2004. This version of the Document Object Model Level 3 Core Recommendation is http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407. The latest version of DOM Level 3 Core is available at http://www.w3.org/TR/DOM-Level-3-Core.
[ECMA-262]
ECMAScript Language Specification, 5th Edition. ECMA International, Standard ECMA-262, December 2009. This version of the ECMAScript Language is available from http://www.ecma-international.org/publications/standards/Ecma-262.htm.
[HTML5]
HTML5, Ian Hickson, Editor. World Wide Web Consortium, May 2011. This version of the HTML5 is available from http://www.w3.org/TR/html5/. The latest editor's draft is available at http://dev.w3.org/html5/spec/.

Acknowledgements

I would like to offer my sincere thanks to all the people that I have been in touch with regarding this draft, including Anderson Quach, Alex Russell, Alois Reitbauer, Annie Sullivan, Christian Biesinger, Darin Fisher, Eric Lawrence, James Simonsen, Jason Sobel, Jason Weber, Jonas Sicking, Kyle Scholz, Lenny Rachitsky, Nic Jansma, Richard Rabbat, Sergey Novikov, Sigbjorn Vik, Steve Souders, Tony Gentilcore for their reviews and feedback.