RFC 003: Amend Connect HTTP codes (#148)

docs/governance/rfc/003-http-codes.md

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+# 003: Amend Connect HTTP codes
+
+This RFC proposes amending the Connect protocol to translate back and forth
+between RPC error codes and HTTP codes more correctly. This fixes some
+transport-level bugs and improves interoperability with gRPC, but does change
+how Connect clients interpret malformed responses.
+
+The proposed diff to the protocol specification is in [PR 130][pr130]. This
+document focuses on the rationale for the change and the anticipated end user
+impact.
+
+## RPC → HTTP mappings
+
+We propose changing the mappings from RPC error codes to HTTP codes for two
+reasons:
+
+* In two cases, Connect's current use of HTTP 408 codes is categorically
+  incorrect - it indicates that the client may retry the request. Using this
+  code for non-idempotent RPCs is a potentially severe bug.
+* In an additional two cases, the current protocol diverges from the mappings
+  suggested in [`code.proto`][code.proto]. The `code.proto` mapping isn't used
+  by gRPC, but it _is_ used by the Google Cloud Platform front-ends,
+  gRPC-Gateway, Envoy, and a variety of other projects in the Protobuf
+  ecosystem.
+
+Most Connect users won't notice these changes, since clients read explicit RPC
+error codes from the response body. Servers will stop encouraging clients and
+proxies to auto-retry, which fixes a severe bug. Since it changes the wire
+representation of errors, this portion of the proposal will visibly affect any
+HTTP-level observability that users have in place today.
+
+## HTTP → RPC mappings
+
+We propose removing some of the mappings from HTTP codes to RPC codes, so that
+Connect's mapping becomes _identical_ to gRPC's. This helps to deliver on
+Connect's promise that users can switch RPC protocols without code changes.
+
+Again, most Connect users won't notice these changes, because clients read RPC
+error codes from the response body. However, this will change clients'
+interpretation of malformed responses (usually coming from a misbehaving
+proxy).
+
+[code.proto]: https://github.com/googleapis/googleapis/blob/master/google/rpc/code.proto
+[pr130]: https://github.com/connectrpc/connectrpc.com/pull/130