Resources Included in the PerformanceResourceTiming Interface

All resource Requests fetched by a non-null client MUST be included as PerformanceResourceTiming objects in the client's global object's Performance Timeline, unless excluded from the timeline as part of the processing model. Resources that are retrieved from relevant application caches or local resources MUST be included as PerformanceResourceTiming objects in the Performance Timeline [[PERFORMANCE-TIMELINE-2]]. Resources for which the fetch was initiated, but was later aborted (e.g. due to a network error) MAY be included as PerformanceResourceTiming objects in the Performance Timeline and MUST contain initialized attribute values for processed substeps of the processing model.

The rest of this section is non-normative.

Examples:

• 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 IFRAME src 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 an HTML IMG element has a data: URI as its source [[RFC2397]], then this resource will not be included as a PerformanceResourceTiming object in the Performance Timeline. By definition data: URI contains embedded data and does not require a fetch.
• If a resource fetch was aborted due to a networking error (e.g. DNS, TCP, or TLS error), then the fetch MAY 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 SHOULD NOT not be included as a PerformanceResourceTiming object in the Performance Timeline.

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 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.

duration

The duration attribute MUST return a {{DOMHighResTimeStamp}} equal to the difference between responseEnd and startTime, respectively.

DOMHighResTimeStamp is defined in [[HR-TIME-2]].

[Exposed=(Window,Worker)]
interface PerformanceResourceTiming : PerformanceEntry {
    readonly        attribute DOMString           initiatorType;
    readonly        attribute DOMString           nextHopProtocol;
    readonly        attribute DOMHighResTimeStamp workerStart;
    readonly        attribute DOMHighResTimeStamp redirectStart;
    readonly        attribute DOMHighResTimeStamp redirectEnd;
    readonly        attribute DOMHighResTimeStamp fetchStart;
    readonly        attribute DOMHighResTimeStamp domainLookupStart;
    readonly        attribute DOMHighResTimeStamp domainLookupEnd;
    readonly        attribute DOMHighResTimeStamp connectStart;
    readonly        attribute DOMHighResTimeStamp connectEnd;
    readonly        attribute DOMHighResTimeStamp secureConnectionStart;
    readonly        attribute DOMHighResTimeStamp requestStart;
    readonly        attribute DOMHighResTimeStamp responseStart;
    readonly        attribute DOMHighResTimeStamp responseEnd;
    readonly        attribute unsigned long long  transferSize;
    readonly        attribute unsigned long long  encodedBodySize;
    readonly        attribute unsigned long long  decodedBodySize;
    [Default] object toJSON();
};

When toJSON is called, run [[WEBIDL]]'s default toJSON operation.

On getting, the initiatorType attribute MUST return one of the following DOMString values:

• The same value as the localName of that element [[DOM]], if the request is a result of processing the element;
• "css", if the request is a result of processing a CSS url() directive [[CSS-SYNTAX-3]], such as @import url() or background: url();
• "navigation", if the request is a navigation request;
• "xmlhttprequest", if the request is a result of processing an XMLHttpRequest object [[XHR]];
• "fetch", if the request is the result of processing the Fetch method [[FETCH]];
• "beacon", if the request is the result of processing the sendBeacon method [[BEACON]];
• "other", if none of the above conditions match.

On getting, the attribute nextHopProtocol returns the network protocol used to fetch the resource, as identified by the ALPN Protocol ID [[RFC7301]]; resources retrieved from relevant application caches or local resources, return an empty string. When a proxy is configured, if a tunnel connection is established then this attribute MUST return the ALPN Protocol ID of the tunneled protocol, otherwise it MUST return the ALPN Protocol ID of the first hop to the proxy. In order to have precisely one way to represent any ALPN protocol ID, the following additional constraints apply: octets in the ALPN protocol MUST NOT be percent-encoded if they are valid token characters except "%", and when using percent-encoding, uppercase hex digits MUST be used.

Formally registered ALPN protocol IDs are documented by IANA. In case the user agent is using an experimental, non-registered protocol, the user agent MUST use the ALPN negotiated value if any. If ALPN was not used for protocol negotiations, the user agent MAY use another descriptive string.

The "h3" ALPN ID is defined for the final version of the HTTP/3 protocol in the HTTP/3 Internet Draft.

Note that the nextHopProtocol attribute is intended to identify the network protocol in use for the fetch regardless of how it was actually negotiated; that is, even if ALPN is not used to negotiate the network protocol, this attribute still uses the ALPN Protocol ID's to indicate the protocol in use.

On getting, the workerStart attribute MUST return as follows:

1. If the active service worker of the context object's relevant settings object is not null and if the resource passes the timing allow check algorithm or the resource Request's destination does not equal "document":

   1. the time immediately before the user agent fires an event named `fetch` at the active worker if the worker is available.
   2. the time immediately before the user agent runs the worker required to service the request.
2. zero, otherwise.

Note that according to the definition of time origin in workers, it is possible that some attributes of navigation preload requests will have negative {{DOMHighResTimeStamp}} values.

On getting, the redirectStart attribute MUST return as follows:

1. 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 the resource passes the timing allow check algorithm.
2. zero, otherwise.

On getting, the redirectEnd attribute MUST return as follows:

1. 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 the resource passes the timing allow check algorithm.
2. zero, otherwise.

On getting, the fetchStart attribute MUST return as follows:

1. The time immediately before the user agent starts to fetch the final resource in the redirection, if there are HTTP redirects or equivalent.
2. The time immediately before the user agent starts to fetch the resource otherwise.

On getting, the domainLookupStart attribute MUST return as follows:

1. Zero, if the resource fails the timing allow check algorithm.
2. The same value as fetchStart, if no domain lookup was required to fetch the resources (e.g. if a persistent connection [[RFC7230]] was used or in case the resource was retrieved from relevant application caches or local resources).
3. The time immediately after the user agent starts the domain data retrieval from the domain information cache, if the user agent has the domain information in cache.
4. The time immediately before the user agent starts the domain name lookup for the resource, otherwise.

On getting, the domainLookupEnd attribute MUST return as follows:

1. Zero, if the resource fails the timing allow check algorithm.
2. The same value as fetchStart, if no domain lookup was required to fetch the resources (e.g. if a persistent connection [[RFC7230]] was used or in case the resource was retrieved from relevant application caches or local resources).
3. 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.
4. The time immediately after the user agent finishes the domain name lookup for the resource, otherwise.

On getting, the connectStart attribute MUST return as follows:

1. Zero, if the resource fails the timing allow check algorithm.
2. The same value as fetchStart, if a persistent connection [[RFC7230]] is used or the resource is retrieved from relevant application caches or local resources.
3. The time immediately before the user agent start establishing the connection to the server to retrieve the resource, otherwise.

   If the transport connection fails and the user agent reopens a connection, connectStart SHOULD return the corresponding value of the new connection.

On getting, the connectEnd attribute MUST return as follows:

1. Zero, if the resource fails the timing allow check algorithm.
2. The same value as fetchStart, if a persistent connection [[RFC7230]] is used or the resource is retrieved from relevant application caches or local resources.
3. The time immediately after the user agent finish establishing the connection to the server to retrieve the resource, otherwise.
   • The returned time MUST include the time interval to establish the transport connection, as well as other time intervals such as SOCKS authentication. It MUST include the time interval to complete enough of the TLS handshake to request the resource.
   • If the user agent used TLS False Start [[RFC7918]] for this connection, this interval MUST NOT include the time needed to receive the server's Finished message.
   • If the user agent sends the request with early data [[RFC8470]] without waiting for the full handshare to complete, this interval MUST NOT include the time needed to receive the server's ServerHello message.
   • If the user agent waits for full handshake completion to send the request, this interval includes the full TLS handshake even if other requests were sent using early data on this connection.

   Example: Suppose the user agent establishes an HTTP/2 connection over TLS 1.3 to send a GET request and a POST request. It sends the ClientHello at time t1 and then sends the GET request with early data. The POST request is not safe [[RFC7231]] (section 4.2.1), so the user agent waits to complete the handshake at time t2 before sending it. Although both requests used the same connection, the GET request reports a connectEnd value of t1, while the POST request reports a connectEnd value for t2.

   If the transport connection fails and the user agent reopens a connection, connectEnd SHOULD return the corresponding value of the new connection.

On getting, the secureConnectionStart attribute MUST return as follows:

1. Zero, if a secure transport is not used or if the resource fails the timing allow check algorithm.
2. The same value as fetchStart, if a persistent connection [[RFC7230]] is used or the resource is retrieved from relevant application caches or local resources.
3. The time immediately before the user agent starts the handshake process to secure the current connection, otherwise.

On getting, the requestStart attribute MUST return as follows:

1. 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 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, requestStart MUST return the corresponding values of the new request.

2. zero, otherwise.

On getting, the responseStart attribute MUST return as follows:

1. The time immediately after the user agent's HTTP parser receives the first byte of the response (e.g. frame header bytes for HTTP/2, or response status line for HTTP/1.x) from relevant application caches, or from local resources or from the server, if the resource passes the timing allow check algorithm.
2. zero, otherwise.

On getting, the responseEnd attribute MUST return the result of running the algorithm to get response end time.

When requested to run the get response end time algorithm, the user agent must run the following steps:

1. If the fetch was aborted due to a network error, return the time immediately before the user agent aborts the fetch.
2. Otherwise, return 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, local resources, or from the server.

On getting, the transferSize attribute MUST return as follows:

1. the size, in octets received from a HTTP-network fetch, consumed by the response header fields and the response payload body [[RFC7230]], if the resource passes the timing allow check algorithm.

   If there are HTTP redirects or equivalent when navigating and if all the redirects or equivalent are same origin, this attribute SHOULD include the HTTP overhead of incurred redirects.

   This attribute SHOULD include HTTP overhead (such as HTTP/1.1 chunked encoding and whitespace around header fields, including newlines, and HTTP/2 frame overhead, along with other server-to-client frames on the same stream), but SHOULD NOT include lower-layer protocol overhead (such as TLS [[RFC5246]]or TCP).

2. zero otherwise, including for resources retrieved from relevant application caches or from local resources.

On getting, the encodedBodySize attribute MUST return as follows:

1. The size, in octets, received from a HTTP-network-or-cache fetch, of the payload body [[RFC7230]], prior to removing any applied content-codings [[RFC7231]], if the resource passes the timing allow check algorithm.
2. The size, in octets, of the payload body prior to removing any applied content-codings if the resource is retrieved from relevant application caches or from local resources.
3. zero, otherwise.

On getting, the decodedBodySize attribute MUST return as follows:

1. The size, in octets, received from a HTTP-network-or-cache fetch, of the message body [[RFC7230]], after removing any applied content-codings [[RFC7231]], if the resource passes the timing allow check algorithm.
2. The size, in octets, of the payload after removing any applied content-codings, if the resource is retrieved from relevant application caches or from local resources.
3. zero, otherwise.

A user agent implementing PerformanceResourceTiming would need to include "resource" in supportedEntryTypes. This allows developers to detect support for Resource Timing.

Extensions to the Performance Interface

The user agent MAY choose to limit how many resources are included as PerformanceResourceTiming objects in the Performance Timeline [[PERFORMANCE-TIMELINE-2]]. This section extends the Performance interface to allow controls over the number of PerformanceResourceTiming objects stored.

The recommended minimum number of PerformanceResourceTiming objects is 250, though this may be changed by the user agent. setResourceTimingBufferSize can be called to request a change to this limit.

Each ECMAScript global environment has:

• A resource timing buffer size limit which should initially be 250 or greater.
• A resource timing buffer current size which is initially 0.
• A resource timing buffer full event pending flag which is initially false.
• A resource timing secondary buffer current size which is initially 0.
• A resource timing secondary buffer to store PerformanceResourceTiming objects that is initially empty.

partial interface Performance {
  undefined clearResourceTimings ();
  undefined setResourceTimingBufferSize (unsigned long maxSize);
              attribute EventHandler onresourcetimingbufferfull;
};

The Performance interface is defined in [[HR-TIME-2]].

The method clearResourceTimings runs the following steps:

1. Remove all PerformanceResourceTiming objects in the performance entry buffer.
2. Set resource timing buffer current size to 0.

The setResourceTimingBufferSize method runs the following steps:

1. Set resource timing buffer size limit to the maxSize parameter. If the maxSize parameter is less than resource timing buffer current size, no PerformanceResourceTiming objects are to be removed from the performance entry buffer.

The attribute onresourcetimingbufferfull is the event handler for the resourcetimingbufferfull event described below.

To check if can add resource timing entry, run the following steps:

1. If resource timing buffer current size is smaller than resource timing buffer size limit, return true.
2. Return false.

To add a PerformanceResourceTiming entry into the performance entry buffer, run the following steps:

1. Let new entry be the input PerformanceEntry to be added.
2. If can add resource timing entry returns true and resource timing buffer full event pending flag is false, run the following substeps:
   1. Add new entry to the performance entry buffer.
   2. Increase resource timing buffer current size by 1.
   3. Return.
3. If resource timing buffer full event pending flag is false, run the following substeps:
   1. Set resource timing buffer full event pending flag to true.
   2. Queue a task to run fire a buffer full event.
4. Add new entry to the resource timing secondary buffer.
5. Increase resource timing secondary buffer current size by 1.

To copy secondary buffer, run the following steps:

1. While resource timing secondary buffer is not empty and can add resource timing entry returns true, run the following substeps:
   1. Let entry be the oldest PerformanceResourceTiming in resource timing secondary buffer.
   2. Add entry to the end of performance entry buffer.
   3. Increment resource timing buffer current size by 1.
   4. Remove entry from resource timing secondary buffer.
   5. Decrement resource timing secondary buffer current size by 1.

To fire a buffer full event, run the following steps:

1. While resource timing secondary buffer is not empty, run the following substeps:
   1. Let number of excess entries before be resource timing secondary buffer current size.
   2. If can add resource timing entry returns false, then fire an event named resourcetimingbufferfull at the Performance object.
   3. Run copy secondary buffer.
   4. Let number of excess entries after be resource timing secondary buffer current size.
   5. If number of excess entries before is lower than or equals number of excess entries after, then remove all entries from resource timing secondary buffer, set resource timing secondary buffer current size to 0, and abort these steps.
2. Set resource timing buffer full event pending flag to false.

   This means that if the resourcetimingbufferfull event handler does not add more room in the buffer than it adds resources to it, excess entries will be dropped from the buffer. Developers should make sure that resourcetimingbufferfull event handlers call clearResourceTimings or extend the buffer sufficiently (by calling setResourceTimingBufferSize).

Cross-origin Resources

Cross-origin resources MUST be included as PerformanceResourceTiming objects in the Performance Timeline. If the timing allow check algorithm fails for a resource, these attributes of its PerformanceResourceTiming object MUST be set to zero: redirectStart, redirectEnd, domainLookupStart, domainLookupEnd, connectStart, connectEnd, requestStart, responseStart, secureConnectionStart, transferSize, encodedBodySize and decodedBodySize.

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.

      Timing-Allow-Origin = 1#( origin-or-null / wildcard )
    

Process

Processing Model

The following graph illustrates the timing attributes defined by the PerformanceResourceTiming interface. Attributes in parenthesis may not be available when fetching cross-origin resources. User agents may perform internal processing in between timings, which allow for non-normative intervals between timings.

For each resource whose Request has a non-null client, perform the following steps:

1. If the resource is fetched by a cross-origin stylesheet which was fetched with no-cors policy, abort the remaining steps.

   Above cross-origin exclusion should be defined via Fetch registry: CSS needs to be defined in terms of Fetch and set some kind of "opaque request flag" for no-CORS CSS subresources. In turn, Resource Timing should interface with Fetch registry to surface resource fetch events.

   The above resource exclusion is at risk as currently only one implementation passes the related test.

2. If the resource's Request's destination equals to "document", and the Request was not triggered by process the iframe attributes or process the frame attributes, abort the remaining steps.
3. Create a new PerformanceResourceTiming object and set entryType to the DOMString resource.
4. Immediately before the user agent starts to queue the resource for retrieval, record the current time in startTime, and set nextHopProtocol to the empty DOMString.
5. Record the initiator of the resource in initiatorType.
6. Record the resolved URL of the requested resource in name.
7. Fetch start: Immediately before a user agent starts the fetching process, record the current time as fetchStart. Let domainLookupStart, domainLookupEnd, connectStart and connectEnd be the same value as fetchStart.
8. If the user agent is to reuse the data from another existing or completed fetch initiated from the current document, abort the remaining steps.
9. If there is an active worker ([[service-workers-1]]) matching the current browsing or worker context's, immediately before the user agent runs the worker record the time as workerStart, or if the worker is already available, immediately before the event named `fetch` is fired at the active worker record the time as workerStart. Otherwise, if there is no matching service worker registration, set workerStart value to zero.
10. If the resource fails the timing allow check algorithm, the user agent MUST run the following substeps:
    1. If the resource's Request's destination equals to "document", set workerStart to zero.
    2. Set redirectStart, redirectEnd, domainLookupStart, domainLookupEnd, connectStart, connectEnd, requestStart, responseStart and secureConnectionStart to zero.
    3. Go to the step labeled response end.
11. Let domainLookupStart, domainLookupEnd, connectStart and connectEnd be the same value as fetchStart.
12. If the resource is fetched from the relevant application cache or local resources, including the HTTP cache [[RFC7234]], go to the step labeled request start.
13. If no domain lookup is required, go to the step labeled connect start. Otherwise, immediately before a user agent starts the domain name lookup, record the time as domainLookupStart.
14. 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 name lookup fails and resource passes the timing allow check record the time as domainLookupEnd and go to the step labeledfinal record.
15. Connect start: 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 a successful connection to the server and record the time as connectEnd immediately after the successful connection to the server or proxy is established. A user agent may need multiple retries to establish a successful connection and should reflect the timestamps for the successful connection only. Once connection is established set the value of nextHopProtocol to the ALPN ID used by the connection. If a connection can not be established, record the time up to the connection failure as connectEnd and go to the step labeled final record.
16. The user agent MUST set the secureConnectionStart attribute as follows:
    1. When a secure transport is used, the user agent MUST record the time as secureConnectionStart immediately before the handshake process to secure the connection.
    2. When a secure transport is not used, the user agent MUST set the value of secureConnectionStart to 0.
17. Request start: Immediately before a user agent starts sending the request for the resource, record the current time as requestStart. If a user agent needed multiple retries to send the request, record the current time of the last attempt.

    Network protocols may not perform the connection establishment, secure connection establishment and request sending in a sequential manner. Therefore, developers should not expect these values to always be in a particular order.

18. Record the time as responseStart immediately after the user agent receives the first byte of the response.
19. Response end: Record the time as responseEnd immediately after receiving the last byte of the response.
    1. Return to the step labeled connect start if the user agent fails to send the request or receive the entire response, and needs to reopen the connection.
    2. Set the value of transferSize, encodedBodySize, decodedBodySize to corresponding values, subject to timing allow check algorithm.
20. Final record: If responseEnd is not set, set it to the current time. Record the difference between responseEnd and startTime in duration.
21. If the fetched resource results in an HTTP redirect or equivalent, then
    1. If the current resource fails the timing allow check algorithm, set redirectStart and redirectEnd to 0. Then, return to the step labeled Fetch start with the redirected resource.
    2. If the value of redirectStart is not set, let it be the value of fetchStart.
    3. Let redirectEnd be the value of responseEnd.
    4. Set all the attributes in the PerformanceResourceTiming object to 0 except startTime, redirectStart, redirectEnd, and initiatorType.
    5. Return to the step labeled Fetch start with the redirected resource.
22. Queue the PerformanceResourceTiming object.
23. Add the PerformanceResourceTiming object to the Request's client's global object's performance entry buffer.

This specification does not specify whether steps 20 and 21 should run before or after the load event of the resource—see issue 82 for related discussion.