Early-Data header
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
Experimental:This is anexperimental technology
Check theBrowser compatibility table carefully before using this in production.
The HTTPEarly-Datarequest header is set by an intermediary to indicate that the request has been conveyed inTLS early data, and also indicates that the intermediary understands the425 Too Early status code.
If a client has interacted with a server recently, early data (also known as zero round-trip time(0-RTT) data) allows the client to send data to a server in the first round trip of a connection, without waiting for the TLShandshake to complete.This reduces latency for repeat connections between a client and server, but has security implications, as early data is susceptible to replay attacks.
TheEarly-Data header isnot set by the originator of the request (i.e., a browser).
| Header type | Request header |
|---|---|
| Forbidden request header | No |
Syntax
Early-Data: 1Examples
A GET request with an Early-Data header
A client that wants to use early data can send HTTP requests immediately after sending the TLSClientHello.Sending a request in early data implies that the client is willing to retry a request in response to a425 Too Early status code, so theEarly-Data header is not included:
GET /resource HTTP/1.1Host: example.comAn intermediary that forwards a request prior to the completion of the TLS handshake with its client sends it with theEarly-Data header set to1:
GET /resource HTTP/1.1Host: example.comEarly-Data: 1Specifications
| Specification |
|---|
| Using Early Data in HTTP # header |