Seek more clarify on Largest Reference/Base Index encoding

draft-ietf-quic-qpack.md

@@ -550,18 +550,36 @@ the block.  Blocking decoders use the Largest Reference to determine when it is
 safe to process the rest of the block.
 
 `Base Index` is used to resolve references in the dynamic table as described in
-{{indexing}}.  To save space, Base Index is encoded relative to Largest
-Reference using a one-bit sign flag.
+{{indexing}}.
 
-    baseIndex = largestReference + deltaBaseIndex
+To save space, Base Index is encoded as a relative to the Base Index using a
+one-bit sign and the `Delta Base Index` value.  A sign bit set to 0 indicates
+that the Base Index has a greater absolute index than the Largest Reference; the
+value of Delta Base Index is added to the Largest Reference to determine the
+absolute value of the Base Index.  A sign bit of 1 indicates that the Base Index
+is smaller than the Base Index.
 
-If the encoder inserted entries to the table while the encoding the block,
-Largest Reference will be greater than Base Index, so deltaBaseIndex will be
-negative and encoded with S=1.  If the block did not reference the most recent
-entry in the table and did not insert any new entries, Largest Reference will be
-less than Base Index, so deltaBaseIndex will be positive and encoded with S=0.
-When Largest Reference and Base Index are equal, deltaBaseIndex is 0 and encoded
-with S=0.
+That is, after turning a sign bit of 1 to -1 and 0 to 1, the absolute Base Index
+is:
+
+~~~
+    baseIndex = largestReference + sign * deltaBaseIndex
+~~~
+
+If the encoder inserted entries to the table while the encoding the header
+block, Largest Reference will be greater than Base Index, so the encoded
+difference is negative and the sign bit will be 1.  If the header block did not
+reference the most recent entry in the table and did not insert any new entries,
+Base Index will be greater than the Largest Reference, so the delta will be
+positive and the sign bit will be 0.
+
+When Largest Reference and Base Index are equal, the Delta Base Index is encoded
+with a zero sign bit.  A sign bit set to 1 when the Delta Base Index is 0 MUST
+be treated as a decoder error.
+
+A header block that does not reference the dynamic table can use any value for
+Base Index and Largest Reference; sending zero values for both fields is the
+most efficient.
 
 
 ### Instructions
@@ -697,7 +715,7 @@ represented as an 8-bit prefix string literal.
 
 # Encoding Strategies
 
-## Single pass encoding
+## Single Pass Encoding
 
 An encoder making a single pass over a list of headers must choose Base Index
 before knowing Largest Reference.  When trying to reference a header inserted to