RTT HTTP Header: Syntax, Directive, Examples

In milliseconds, the RTT HTTP Header Client hint request header field tells you how long it will take for the application layer to get back to you. The RTT HTTP Header hint, unlike the transport layer RTT HTTP Header, includes the time it takes for the server to process. The RTT HTTP Header value is rounded to the nearest 25 milliseconds to avoid fingerprinting. There are many other ways an attacker could get the same round-trip information. The hint lets a server choose what information to send based on how quickly the network responds or how long it takes. Among other things, it might decide to send fewer resources. In responses, the Vary header is used to indicate that a different resource is sent for each value of the header (see HTTP Caching > Varying responses). Even if RTT HTTP Header is used to configure what resources are sent, consider leaving it out of the Vary header. It is likely to change frequently, rendering the resource uncachable. There is only one value for using the RTT HTTP Header. The value using the RTT HTTP Header is the number. An example of the RTT HTTP Header is written below. 

RTT: 143

The RTT HTTP Header Request Header can be seen above. In this article, the RTT HTTP Header Syntax, Directives, and Uses examples will be processed. 

What is RTT HTTP Header?

The RTT HTTP Header Client hint request-header field provides the application layer’s approximate round trip time in milliseconds. Unlike transport layer RTT, the RTT HTTP Header hint includes server processing time. To prevent fingerprinting, the RTT HTTP Headeralue is rounded to the nearest 25 milliseconds; there are numerous other mechanisms an attacker could use to obtain similar round-trip information. The hint enables a server to select which information to send based on network responsiveness/latency. It could, for example, choose to send fewer resources.

What is the Syntax of RTT HTTP Header?

The RTT HTTP Header uses only one value in its syntax. The syntax for using the RTT HTTP Header is written below. 

RTT: <number>

What is the Directive of RTT HTTP Header?

The RTT HTTP Header can only contain one directive. The “number” specifies the approximate time required for the round trip in milliseconds, rounded to the closest 25 milliseconds. An example directive for using the RTT HTTP Header is given below. 

RTT: 125

How to use RTT HTTP Header?

The RTT Client hint request header field is used to specify the application layer’s approximate round trip time in milliseconds. Unlike transmission layer RTT, the RTT hint includes server processing time. To avoid fingerprinting, the RTT number is rounded to the nearest 25 milliseconds. Numerous more techniques exist for an attacker to gain identical round-trip information. The hint enables a server to selectively send data based on the network’s responsiveness/latency. For instance, it may decide to deploy fewer resources.

Examples of RTT HTTP Header Use

For instance, a server must first opt-in to receive the RTT header by transmitting the Accept-CH response header containing RTT.

Accept-CH: RTT

The client may then return an RTT header on subsequent requests.

RTT: 125

What is the Specification Document for RTT HTTP Header?

There is only one specification document for the Save-Data HTTP Header which is the Network Information API. The Network Information API articles mention the RTT HTTP Header value and examples. It also mentions the RTT HTTP Header in the Effective Connection Types as one of the categories. Additionally, this article discusses the RTT HTTP Header’s definition and usage.

What is the type of RTT  HTTP Header?

The RTT HTTP Header is a request header type that specifies the application layer’s approximate round trip time in milliseconds. Unlike transmission layer RTT, the RTT hint includes server processing time.

What are the similar HTTP Headers to the RTT  HTTP Header?

There are other similar HTTP to the RTT HTTP Header. The examples are listed below. 

  • Downlink HTTP Header: The Downlink HTTP Header field contains the approximate bandwidth of the client’s connection to the server, expressed in megabits per second (Mbps). Comparable to the RTT HTTP Header, another request header type.
  • ECT HTTP Header: The effective connection type is indicated in the ECT HTTP Header Client hint request header field: slow-2g, 2g, 3g, or 4g. Comparable to the RTT HTTP Header, another request header type.
  • Save-Data HTTP Header: The Save-Data network client hint request-header field contains a boolean value indicating the client’s preference for decreased data consumption. This could be due to high transmission charges, slow connection speeds, or other factors, whereas the RTT HTTP Header contains information about server processing time.

Which Browsers Support RTT HTTP Header? 

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

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

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

RTT HTTP Header
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