Re: [quicwg/base-drafts] Define INITIAL_PRIORITY frame and remove exclusive dependencies (#2075)

[Lucas Pardue <[e-mail]>]

LPardue commented on this pull request.



> +
+~~~~~~~~~~  drawing
+ 0                   1                   2                   3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|DT |   Empty   |           Element Dependency ID (i)         ...
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|   Weight (8)  |
++-+-+-+-+-+-+-+-+
+~~~~~~~~~~
+{: #fig-initial-priority title="INITIAL_PRIORITY frame payload"}
+
+The INITIAL_PRIORITY frame payload has the following fields:
+
+  Dependency Type:
+  : A two-bit field indicating the type of element being depended on.

One alternative would to have a static two-bit field that is always set to the value of `11`. There's no extra overhead since there's unused reserved bits.

However, I like @kazuho's proposal and think there is room to extend it further.

The PRIORITY frame definition says that 

> Element Dependency ID: A variable-length integer that identifies the element on which a dependency is being expressed. Depending on the value of Dependency Type, this contains the Stream ID of a request stream, the Push ID of a promised resource, the Placeholder ID of a placeholder, or is ignored.

IIUC the ignored case properly, wouldn't it be more efficient to make the EDID (chuckle, not [this one](https://en.wikipedia.org/wiki/Extended_Display_Identification_Data)) optional too, based on the prioritisation type. I.e. if the dependency is root of the tree, don't send an ID.

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