http://http2.github.io/http2-spec/#FrameHeader section on Flags being unset

William Chan (陈智昌) <willchan@google.com> Wed, 08 May 2013 18:29 UTC

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Date: Wed, 08 May 2013 15:26:47 -0300
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From: "William Chan (陈智昌)" <willchan@google.com>
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Subject: http://http2.github.io/http2-spec/#FrameHeader section on Flags being unset
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"""
The remaining flags can be assigned semantics specific to the indicated
frame type. Flags that have no defined semantics for a particular frame
type MUST be ignored, and MUST be left unset (0) when sending.
"""

Is there a reason that it MUST be left unset? Why not allow extensibility
like we do for unknown frames? I actually don't think there's much value to
specifying behavior for unused bits.