Re: [quicwg/base-drafts] http3: Are reserved frames allowed before SETTINGS? (#2693)

[Luca Niccolini <[e-mail]>]

In our implementation we are interpreting this in the same way: any frame on the control stream received before SETTINGS is a connection error. Even reserved frame types.

In general the way we handle this is by discarding frame types that are forbidden rather than checking frame types that are allowed on each stream. 

Here is a snippet from our code.
```
ParseResult HQControlCodec::checkFrameAllowed(FrameType type) {
  switch (type) {
    case hq::FrameType::DATA:
    case hq::FrameType::HEADERS:
    case hq::FrameType::PUSH_PROMISE:
      return HTTP3::ErrorCode::HTTP_WRONG_STREAM;
    default:
      break;
  }

  // SETTINGS MUST be the first frame on an HQ Control Stream
  if (!receivedSettings_ && type != hq::FrameType::SETTINGS) {
    return HTTP3::ErrorCode::HTTP_MISSING_SETTINGS;
  }
  // .. and a bunch of other checks here ..
```

In this way we avoid the extra effort needed to handle reserved frame types that you mentioned, if I am reading your statement correctly. 
Basically, some frame types are explicitly disallowed. All other frame types are equal, reserved or non-reserved. Do you foresee an extension possibly benefiting from sending a frame before settings ? 


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