Merge pull request #3 from davidben/unrecognized-frames

Clarify receivers should ignore unrecognized ALPS frames.

draft-vvv-httpbis-alps.md

@@ -83,27 +83,40 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
 document are to be interpreted as described in BCP 14 {{RFC2119}} {{!RFC8174}}
 when, and only when, they appear in all capitals, as shown here.
 
-# Use of ALPS in HTTP
+# Use of ALPS in HTTP/2
 
 If ALPS is successfully negotiated during the TLS handshake for an HTTP/2
 connection, the ALPS payload for both peers SHALL be a sequence of HTTP/2
-frames.  Frames SHALL NOT be present in ALPS unless they are explicitly allowed
-to be there; this document only allows the SETTINGS frame ({{!RFC7540}},
-Section 6.5.1).  Sending a SETTINGS frame in ALPS supersedes the requirement to
-send a SETTINGS frame at the beginning of the connection.  All settings
-exchanged via ALPS SHALL be automatically treated as acknowledged.
+frames.  The sender MUST NOT send a frame in ALPS unless the corresponding
+value in the "Allowed in ALPS" column in the "HTTP/2 Frame Type" registry
+({{iana}}) is "Yes". This document only allows the SETTINGS frame
+({{!RFC7540}}, Section 6.5.1). Frames defined in later documents may be allowed
+in ALPS, so the receiver MUST ignore unrecognized frames in the ALPS payload.
+
+Sending a SETTINGS frame in ALPS supersedes the requirement to send a SETTINGS
+frame at the beginning of the connection.  All settings exchanged via ALPS
+SHALL be automatically treated as acknowledged. Since settings exchanged
+through ALPS are always available at the beginning of the connection, HTTP/2
+extensions MAY opt to require those to be sent through ALPS.
+
+# Use of ALPS in HTTP/3
 
 If ALPS is successfully negotiated during TLS handshake for an HTTP/3
 connection, the ALPS payload for both peers SHALL be a sequence of HTTP/3
-frames.  Frames SHALL NOT be present in ALPS unless they are explicitly allowed
-to be there; this document only allows the SETTINGS frame ([HTTP3], Section
-7.2.4).  Sending a SETTINGS frame in ALPS supersedes the requirement to send a
-SETTINGS frame at the beginning of the control stream.
+frames.  The sender MUST NOT send a frame in ALPS unless the corresponding
+value in the "Allowed in ALPS" column in the "HTTP/3 Frame Type" registry
+({{iana}}) is "Yes". This document only allows the SETTINGS frame ([HTTP3],
+Section 7.2.4).  Frames defined in later documents may be allowed in ALPS, so
+the receiver MUST ignore unrecognized frames in the ALPS payload.
+
+Sending a SETTINGS frame in ALPS supersedes the requirement to send a SETTINGS
+frame at the beginning of the control stream. Since settings exchanged through
+ALPS are always available at the beginning of the connection, HTTP/3 extensions
+MAY opt to require those to be sent through ALPS.  Such extensions are exempt
+from the initialization requirements of the Section 7.2.4.2 of [HTTP3].
 
-Since settings exchanged through ALPS are always available at the beginning of
-the connection, some HTTP extensions may opt to require those to be sent
-through ALPS.  Such extensions are exempt from the initialization requirements
-of the Section 7.2.4.2 of [HTTP3].
+[[OPEN ISSUE: Unlike HTTP/2, HTTP/3 sends exactly one SETTINGS frame, so we
+need to be a bit more precise here.]]
 
 # Security Considerations
 
@@ -113,14 +126,15 @@ authenticated; thus, if a server relies on TLS client authentication and
 considers its settings private, it MUST NOT use the mechanism defined in this
 document.
 
-# IANA Considerations
+# IANA Considerations {#iana}
 
-IANA will add an "Allowed in ALPS" column to the "HTTP/2 Frames" section of the
-"Hypertext Transfer Protocol version 2 (HTTP/2) Parameters" registry, with a
-value set to "Yes" for SETTINGS (0x4), and to "No" for all other previously
-defined settings.
+IANA will add an "Allowed in ALPS" column to the "HTTP/2 Frame Type" registry
+{{RFC7540}}, with a value set to "Yes" for SETTINGS (0x4), and to "No" for all
+other previously defined settings.
 
-TODO: Add HTTP/3 once IANA has an HTTP/3 registry.
+IANA will add an "Allowed in ALPS" column to the "HTTP/3 Frame Type" registry
+{{HTTP3}}, with a value set to "Yes" for SETTINGS (0x4), and to "No" for all
+other previously defined settings.
 
 --- back