I'm opening this PR to (hopefully!) stimulate a discussion. In brief,
I'd like to amend the gRPC-Web protocol docs to encourage
implementations to follow HTTP semantics and compare HTTP field names
case-insensitively.
The gRPC-Web specification is a nicely-designed way for proxies to
expose standard HTTP/2 gRPC servers to clients using less
tightly-controlled HTTP stacks, such as web browsers. To serve that
goal, it seems valuable to have the gRPC-Web specification follow [RFC
9110 (HTTP
Semantics)](https://www.rfc-editor.org/rfc/rfc9110.html#name-field-names).
Like previous RFCs, 9110 specifies that "field names are
case-insensitive." However, the current gRPC-Web specification requires
that servers and proxies "use lower-case header/trailer names" on the
wire. In principle, mandating casing on the wire is normal for HTTP/2
and fine (if unusual) for HTTP/1.1; however, it encourages
implementations to violate HTTP semantics and require lower-case names
when _reading_ headers and trailers.
I'd like to loosen the gRPC-Web specification to permit any casing on
the wire for HTTP/1.1. I'd also like to emphasize that gRPC-Web
implementations ought to follow standard HTTP semantics when _reading_
fields and compare names case-insensitively. Implementations that can't
treat names case-insensitively without breaking backward compatibility
should instead normalize field names to lowercase. Among the
Google-maintained gRPC implementations, at least `grpc-go` and
`grpc-java` already compare names case-insensitively (even though
they're HTTP/2-only). `grpc-dart` does the opposite and compares names
case-sensitively. `grpc-web` is sometimes case-insensitive (when reading
`grpc-status` and `grpc-message` from trailers-only responses) and
sometimes case-sensitive (when hand-parsing a block of length-prefixed
trailers).
The proposed amendment does not affect the correctness of Envoy (which
may continue to use lower-case field names). It partially affects
`grpc-web`, which would require a small patch to always normalize names.
(Both patched and unpatched versions of `grpc-web` would work with
Envoy.) `grpc-dart` would need to either begin treating field names
case-insensitively or normalize names, depending on what's possible in
Dart without breaking backward compatibility.
Relates to https://github.com/improbable-eng/grpc-web/issues/228,
https://github.com/bufbuild/connect-go/issues/453/, and
https://github.com/grpc/grpc-dart/issues/594.
Remove the section for b64 encoded trailers. As gRPC-Web uses CRLF to separate the trailers in the trailers frame, a binary trailer need to reserve in a base64 encoded format for their values. This cannot be skipped even base64 is applied on the whole trailers frame per client's request via content-type: application/grpc-web-text.
1. Clarify about trailer-only responses
2. Switch to http/1.1 header block format for in-body trailer encoding
3. Clarify how compatibility is supported (using U-A and Server headers respectively)
Akshay Shah