NEL HTTP Header: Syntax, Directive, Examples

The NEL HTTP Header is also a JSON-formatted array of objects that refers to the previously mentioned Report-To header’s reporting endpoint. The NEL HTTP Header is a Report-To header extension that instructs the user agent to send network error reports. As a result, configuring a NEL header without a Report-To header is impossible. The NEL HTTP Header value is interpreted as an array of JSON objects. An example of a NEL HTTP Header is given below. 

Report-To: {"group":"endpoint-1","max_age":10886400,"endpoints":[{"url":"https://example.uriports.com/reports"}],"include_subdomains":true}

The NEL HTTP Header Response Header can be seen above. In this article, the NEL HTTP Header Syntax, Directives, and Uses example will be processed. 

What is NEL HTTP Header?

The NEL HTTP Header is also a JSON-formatted array of objects that refers to the previously mentioned Report-To header’s reporting endpoint. The NEL HTTP Header is a Report-To header extension that instructs the user agent to send network error reports. 

What is the Syntax of the NEL HTTP Header?

The NEL HTTP Header uses multiple values in its syntax. The syntax for using the NEL HTTP Header is written below.

NEL: { "report_to": "name_of_reporting_group", "max_age": 12345, "include_subdomains": false, "success_fraction": 0.0, "failure_fraction": 1.0 }

What is the Directive of the NEL HTTP Header?

The NEL HTTP Header does not have any directives available because there is no information about its directives provided.

How to use NEL HTTP Header?

Web applications can now declare a reporting policy that the user agent can use to report network errors specific to one or more origins using Network Error Logging (NEL) HTTP Header. When a web application chooses to use NEL HTTP Header, it does so by including a NEL HTTP Header response header field in the response. User agents are instructed to log information about requests to that origin and attempt to deliver that information via the [[[REPORTING]]] group of endpoints, which have been previously configured using the policy. NEL HTTP Header reports as the name suggests, are primarily employed to document errors. However, we need to know how many successful requests are being made in order to calculate error rates across different client populations; these successful requests can also be reported through the NEL HTTP Header mechanism.

Examples of NEL HTTP Header Use

The following is an example of how to use the NEL HTTP Header.

For example, if the user agent fails to fetch a resource from https://www.example.com due to an aborted TCP connection, the user agent would queue the following report via the Reporting API.

type
"network-error"
endpoint group
the endpoint group configured by the report_to field
settings
TODO
data
{
  "referrer": "https://referrer.com/",
  "sampling_fraction": 1.0,
  "server_ip": "192.0.2.42",
  "protocol": "http/1.1",
  "elapsed_time": 321,
  "phase": "connection",
  "type": "tcp.aborted"
}

What is the Specification Document for NEL HTTP Header?

There is only one specification document for the NEL HTTP Header which is the Network Error Logging Article. The Network Error Logging Article discusses the NEL HTTP Header and its applications. Additionally, this article discusses the NEL HTTP Header’s definition and usage.

What is the type of NEL HTTP Header?

The NEL HTTP Header is a response header because it holds additional information about the response, such as its location or the server from which it came, is stored in this field. 

What is the similar HTTP Header to the NEL HTTP Header?

There is another similar HTTP to the NEL HTTP Header which is the Network Error Logging. Using the NEL HTTP response header, you can enable Network Error Logging HTTP Header. Users can choose whether or not they want their browsers to notify them when network fetches fail (or succeed) by including an experimental header in their requests.

Which Browsers Support NEL HTTP Header? 

There are multiple browsers that support NEL HTTP Header. The following browsers are listed below.

  • Chrome Browser is compatible with the NEL HTTP Header.
  • Edge Browser is compatible with the NEL HTTP Header.
  • Opera Browser is compatible with the NEL HTTP Header.
  • WebView Android Browser is compatible with the NEL HTTP Header.
  • Chrome Android Browser is compatible with the NEL HTTP Header.
  • Opera Android Browser is compatible with the NEL HTTP Header.
  • Samsung Internet Browser is compatible with the NEL HTTP Header.

You can see an image that shows cross-browsers compatibility of NEL HTTP Headers below.

NEL HTTP Header
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