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.



> -the `SETTINGS_NUM_PLACEHOLDERS` setting. Because the server commits to
-maintaining these placeholders in the prioritization tree, clients can use them
-with confidence that the server will not have discarded the state. Clients MUST
-NOT send the `SETTINGS_NUM_PLACEHOLDERS` setting; receipt of this setting by a
-server MUST be treated as a connection error of type
+In HTTP/3, by using the `SETTINGS_NUM_PLACEHOLDERS` setting, the server
+advertises the number of client-controlled placeholders for which it is
+committing to maintain state.  Client-controlled placeholders are given their
+own number space that spans between zero and 2^62-1.  Clients can use the
+client-controlled placeholders with an ID less than the value of this setting
+with the confidence that the server will not have discarded the state.  The
+orphan placeholder cannot be prioritized or referenced by the client.
+
+Servers are RECOMMENDED to support at least 32 placeholders.  Those that do not
+SHOULD refrain from supporting placeholders at all, setting
+`SETTINGS_NUM_PLACEHOLDERS` to zero.

This is the outcome of the discussion (see above). The intent is:
* recommend providing at least 32 placeholders
* acknowledge that 0 is a sensible value when that not possible

The rational is to make it possible for a client to create a priority tree that works reasonably well, assuming that it uses no more than 32 placeholders in these two conditions.

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