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Today, HTTP headers all need to be sent before a single bit of HTTP body is sent to the browser.

This is especially problematic with new technologies such as React 18 Streaming where certain headers, such as caching headers and 103 Early Hints, can be determined with certainty only at the end of the HTTP stream. Ideally these late headers would be sent to the browser just before ending the stream.

Are there efforts from spec working groups or browser vendors to enable headers to be sent during/after the HTTP body?

After doing research, it seems that there is no spec work about this, but I wonder if there is a browser vendor working on this? (Some browser folks are active here on StackOverflow.)

Context: I'm the author of vite-plugin-ssr and react-streaming.

brillout
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There is a specification for Trailer fields for use with Chunked Encoding (Http 1.1, https://httpwg.org/specs/rfc7230.html#header.trailer).

The HTTP2 spec (which does not support Chunked Encoding) directly allows for a headers frame following the Data frames that contain the http body https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.

Library support may vary as most http libraries attempt to abstract away the differences in the underlying protocols. In Javascript you will be interested in enabling trailing headers in the cross-browser standard fetch API. The MDN docs suggest that support is coming with reference to a trailers field on the Response object: https://developer.mozilla.org/en-US/docs/Web/API/Response.

Richard Woods
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  • As of recently (May 2022) all browser vendors seem to support the Trailer header: https://stackoverflow.com/a/72271958/270274. Node.js also seems to support it: https://nodejs.org/api/http2.html#responseaddtrailersheaders. – brillout May 17 '22 at 10:03