Re: [quicwg/base-drafts] Implicit acknowledgment via header block acknowledgment (#1399)

[afrind <[e-mail]>]

afrind commented on this pull request.



> @@ -491,7 +491,14 @@ instruction begins with the '1' one-bit pattern. The instruction specifies the
 total number of dynamic table inserts and duplications since the last Table
 State Synchronize, encoded as a 7-bit prefix integer.  The encoder uses this
 value to determine which table entries are vulnerable to head-of-line blocking.
-A decoder MAY coalesce multiple synchronization updates into a single update.
+
+A decoder MAY coalesce multiple synchronization updates into a single update.  A
+decoder MAY rely on Header Acknowledgement instructions to indirectly
+acknowledge changes to the dynamic table.  Relying on Header Acknowledgment
+instructions exclusively leads to blocking if the encoder waits for an entry to

I don't think it would lead to blocking, but it would lead to never using the dynamic table?

> @@ -523,6 +530,12 @@ blocks within a stream have been fully processed.
 ~~~~~~~~~~
 {:#fig-header-ack title="Header Acknowledgement"}
 
+An encoder MUST treat receipt of a Header Acknowledgment as also acknowledging
+any dynamic table entries that the header block referenced.  That is, this
+instruction is also processed as a Table Size Synchronize instruction with a
+value matching the Largest Reference of the corresponding header block.

You mean TSS will be incremental from the largest acknowledged largest reference or most recent TSS?

> @@ -491,7 +491,14 @@ instruction begins with the '1' one-bit pattern. The instruction specifies the
 total number of dynamic table inserts and duplications since the last Table
 State Synchronize, encoded as a 7-bit prefix integer.  The encoder uses this
 value to determine which table entries are vulnerable to head-of-line blocking.
-A decoder MAY coalesce multiple synchronization updates into a single update.
+
+A decoder MAY coalesce multiple synchronization updates into a single update.  A
+decoder MAY rely on Header Acknowledgement instructions to indirectly
+acknowledge changes to the dynamic table.  Relying on Header Acknowledgment
+instructions exclusively leads to blocking if the encoder waits for an entry to
+be acknowledged before using it.  This happens if the encoder wants to avoid the
+risk of blocking at the decoder, or the decoder sets the
+SETTINGS_QPACK_BLOCKED_STREAMS lower than the number of active streams.

I had to read this many times to figure out what you are saying - you are explaining why the encoder might be waiting for acknowledgement before using the headers in the dynamic table.  I think it might be more clear if you change the second bit to something like

"... or the encoder has already reached the decoder's limit for blocked streams".

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