Date: Fri, 28 Sep 2018 06:13:41 -0700
From: Dmitri Tikhonov <notifications@github.com>
Reply-To: quicwg/base-drafts
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Message-ID: <quicwg/base-drafts/pull/1763/review/159853030@github.com>
In-Reply-To: <quicwg/base-drafts/pull/1763@github.com>
References: <quicwg/base-drafts/pull/1763@github.com>
Subject: Re: [quicwg/base-drafts] QPACK: Encode Largest Reference modulo
 MaxEntries (#1763)
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dtikhonov commented on this pull request.



> -safe to process the rest of the block.
+safe to process the rest of the block.  If Largest Reference is greater than
+zero, the encoder transforms it as follows before encoding:
+
+~~~
+   LargestReference = LargestReference mod 2*MaxEntries + 1
+~~~
+
+The decoder reconstructs the Largest Reference using the following algorithm:
+
+~~~
+   if LargestReference > 0:
+      LargestReference -= 1
+      CurrentWrapped = TableLargestAbsoluteIndex mod 2*MaxEntries
+
+      if CurrentWrapped >= LargestReference + MaxEntries:

It seems this range is one too long: `[ LargestReference - MaxEntries, LargestReference + MaxEntries ]` is `2 * MaxEntries + 1` long, while the actual valid range is at most `2 * MaxEntries`.  I don't yet know whether this is a problem, as I have to get used to this version of the logic.

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<p><b>@dtikhonov</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/quicwg/base-drafts/pull/1763#discussion_r221247551">draft-ietf-quic-qpack.md</a>:</p>
<pre style='color:#555'>&gt; -safe to process the rest of the block.
+safe to process the rest of the block.  If Largest Reference is greater than
+zero, the encoder transforms it as follows before encoding:
+
+~~~
+   LargestReference = LargestReference mod 2*MaxEntries + 1
+~~~
+
+The decoder reconstructs the Largest Reference using the following algorithm:
+
+~~~
+   if LargestReference &gt; 0:
+      LargestReference -= 1
+      CurrentWrapped = TableLargestAbsoluteIndex mod 2*MaxEntries
+
+      if CurrentWrapped &gt;= LargestReference + MaxEntries:
</pre>
<p>It seems this range is one too long: <code>[ LargestReference - MaxEntries, LargestReference + MaxEntries ]</code> is <code>2 * MaxEntries + 1</code> long, while the actual valid range is at most <code>2 * MaxEntries</code>.  I don't yet know whether this is a problem, as I have to get used to this version of the logic.</p>

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