Re: [quicwg/base-drafts] Use "Insert Count" rather than "Largest Reference" (#2111)

[Martin Thomson <[e-mail]>]

martinthomson commented on this pull request.

We never say whether we are using 0-based or 1-based indexing, but in using the term, we implicitly established that we use 1-based indexing.  Except that the draft is inconsistent throughout.  The static table starts at 0, relative indexing is 0-based (ish), but the largest reference is implicitly 1-based.  If I want to refer to the first entry, I send a 1. 

That's very confusing.  To me at least, maybe because I didn't ever internalize the bizarre internal logic that drove HPACK.

Changing to a count means that this is not relevant.  The indexing we do in the draft is all 0-based, as it should be.  But then there is less of a dependency on that because we're counting, not indexing, in the trickiest part.

>  
 In order to identify which dynamic table entries can be safely used
 without a stream becoming blocked, the encoder tracks the absolute index of the
-decoder's Largest Known Received entry.
+decoder's Known Received Count entry.

I failed to update this text.  I will do that.

>  
 n = count of entries inserted
 d = count of entries dropped
 ~~~~~
 {: title="Example Dynamic Table Indexing - Control Stream"}
 
 Because frames from request streams can be delivered out of order with
-instructions on the encoder stream, relative indices are relative to the Base
-Index at the beginning of the header block (see {{header-prefix}}). The Base
-Index is an absolute index. When interpreting the rest of the frame, the entry
-identified by Base Index has a relative index of zero.  The relative indices of
-entries do not change while interpreting headers on a request or push stream.
+instructions on the encoder stream, relative indices are relative to the Base at
+the beginning of the header block (see {{header-prefix}}). The Base is encoded
+as a value relative to the Insert Count, so it can be expressed as the number of
+entries inserted. The dynamic table entries up to the Base can be referenced.

Yeah, I tried to fix this without changing it too much and that turned out badly.  I've a rewrite for you.

>  
 n = count of entries inserted
 d = count of entries dropped
 ~~~~~
 {: title="Example Dynamic Table Indexing - Control Stream"}
 
 Because frames from request streams can be delivered out of order with
-instructions on the encoder stream, relative indices are relative to the Base
-Index at the beginning of the header block (see {{header-prefix}}). The Base
-Index is an absolute index. When interpreting the rest of the frame, the entry
-identified by Base Index has a relative index of zero.  The relative indices of
-entries do not change while interpreting headers on a request or push stream.
+instructions on the encoder stream, relative indices are relative to the Base at
+the beginning of the header block (see {{header-prefix}}). The Base is encoded
+as a value relative to the Insert Count, so it can be expressed as the number of
+entries inserted. The dynamic table entries up to the Base can be referenced.
+The most recently inserted entry within the limit set by the Base has a relative
+index of 0.
+
+Though new entries could be added while processing a header block, the relative
+indices of entries do not change.

I was referring to parallel processing.  My implementation allows for that possibility.

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