Re: [quicwg/base-drafts] allow PRIORITY frames referring to placeholders exceeding `SETTING_NUM_PLACEHOLDERS` (#2761)

[Kazuho Oku <[e-mail]>]

kazuho commented on this pull request.



> @@ -1202,9 +1202,17 @@ The following situations are examples of invalid PRIORITY frames:
 A PRIORITY frame with Empty bits not set to zero MAY be treated as a connection
 error of type HTTP_MALFORMED_FRAME.
 
-A PRIORITY frame that references a non-existent Push ID, a Placeholder ID
-greater than the server's limit, or a Stream ID the client is not yet permitted
-to open MUST be treated as a connection error of type HTTP_LIMIT_EXCEEDED.
+A PRIORITY frame that references a non-existent Push ID, or a Stream ID the
+client is not yet permitted to open MUST be treated as a connection error of
+type HTTP_LIMIT_EXCEEDED.
+
+A PRIORITY frame MAY reference a Placeholder ID that is equal to or greater than
+the server's advertised value of the `SETTINGS_NUM_PLACEHOLDERS` settings.  When
+receiving a PRIORITY frame with its prioritized element set to such a
+placeholder, the server SHOULD discard the frame without making any change to
+the priority tree.  When receiving a PRIORITY frame with its element dependency
+set to such a placeholder, the server SHOULD update the priority tree with the
+root of the tree being the dependency instead.

I agree that this is debatable.

The reason I've chosen "root" as the starting point of this PR is because it is the behavior defined in HTTP/2; I think we should try to retain similarity if possible (unless / until we decide to adopt a very different design). Note that orphan placeholder is still useful regardless of this; the placeholder minimizes the negative impact when PRIORITY frames on the control stream gets lost.

Hopefully, this point becomes a less of an issue once we recommend some sensible minimum for SETTINGS_NUM_PLACEHOLDERS; essentially guiding servers to support either none or at least 32 (or something alike).

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