Merge pull request #3301 from kazuho/kazuho/incremental-false-and-extensibility

allow use of ?0 and clarify the possibility to add parameters

Author: kazuho

draft-ietf-httpbis-incremental.md

@@ -100,13 +100,20 @@ intermediaries to start forwarding the message downstream before the entire
 message is received.
 
 The Incremental header field is defined as a structured field
-{{STRUCTURED-FIELDS}} of type Item.  There is just one valid value, which is of
-type Boolean: "?1".
+{{STRUCTURED-FIELDS}} of type Item.
+Only Boolean values ({{Section 3.3.6 of STRUCTURED-FIELDS}}) are valid;
+a recipient ignores the field if it contains any other type.
 
 ~~~
-Incremental = ?1
+Incremental = sf-boolean
 ~~~
 
+A true value ("?1") indicates that the sender requests intermediaries to forward
+the message incrementally, as described below.
+
+A false value ("?0") indicates the default behavior defined in {{HTTP}}, where
+intermediaries might buffer the entire message before forwarding it.
+
 Upon receiving a header section that includes an Incremental header field with a
 true value, HTTP intermediaries SHOULD NOT buffer the entire message before
 forwarding it.  Instead, intermediaries SHOULD transmit the header section
@@ -139,6 +146,10 @@ transmit message contents incrementally in both directions.  However, when devel
 bidirectional protocols over HTTP, Extended CONNECT {{?RFC8441}}{{?RFC9220}} is
 generally more consistent with HTTP's architecture.
 
+This document does not define any parameters for the Incremental header field
+value, but future documents might define parameters. Receivers MUST ignore
+unknown parameters.
+
 
 # Security Considerations