[quicwg/base-drafts] Greasing for Unidirectional Stream Types (#1490)

[Mike Bishop <[e-mail]>]

#1446 added greasing values for various elements in HTTP/QUIC (settings, frame types, etc.) where the peer is required to ignore unknown values.  #1359 added unidirectional stream types, and there was a suggestion from @LPardue to grease those as well.

However, given that an unknown stream type is defined to be an error (and implementations are cautioned that peers can promote these errors to connection errors), sending grease values and mandating that receivers not treat them any differently will be a bit fragile.

Options I see:
- Prohibit unknown unidirectional streams from being treated as an error at all; ignore them.  Unfortunate for wasted data transfer; you really want a STOP_SENDING here.  This is also an attack vector.  And if you expect some valid unidirectional streams, you can't just clamp down on the number of allowed unidirectional streams using MAX_STREAM_ID.
- Prohibit unknown unidirectional streams from being treated as a connection error.  This seems an unfortunate exception case, especially since it opens the same attack vector as the previous.
- Special-case the grease types on the receiver, which rather defeats the purpose of having grease at all.
- Don't grease these.

Unless someone has a bright idea, I'm currently inclined toward the last option.

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