Re: [quicwg/base-drafts] QPACK: Encode Largest Reference modulo MaxEntries (#1763)

[Mike Bishop <[e-mail]>]

MikeBishop approved 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 makes sense -- if it can be either `MaxEntries` greater *or* `MaxEntries` lesser, then you have `2*MaxEntries + 1` items, counting the one you're currently on.  There's always ambiguity at the far end -- the end values in the range are equivalent -- but because those ambiguous entries are always MaxEntries away, they can't be sent without evicting something unacknowledged or referencing something already evicted, both of which are prohibited.

> +### Calculating Table Size
+
+The size of the dynamic table is the sum of the size of its entries.
+
+The size of an entry is the sum of its name's length in octets (as defined in
+{{string-literals}}), its value's length in octets, and 32.
+
+The size of an entry is calculated using the length of its name and value
+without any Huffman encoding applied.
+
+`MaxEntries` is the maximum number of entries that the dynamic table can have.
+The smallest entry has empty name and value strings and has the size of 32.
+The MaxEntries is calculated as
+
+~~~
+   MaxEntries = floor( MaxTableSize / 32 )

I was concerned that the doubling could cause this to give us one less entry than we need, but after playing with some edge table sizes for a few minutes, this seems to work.

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