Bring back the Exclusive Flag

draft-ietf-quic-http.md

@@ -599,6 +599,11 @@ the orphan placeholder if progress can be made on descendants of the root.  The
 structure of the dependency tree changes as PRIORITY frames modify the
 dependency links between other prioritized elements.
 
+An exclusive flag allows for the insertion of a new level of dependencies.  The
+exclusive flag causes the prioritized element to become the sole dependency of
+its parent, causing other dependencies to become dependent on the exclusive
+element.
+
 All dependent streams are allocated an integer weight between 1 and 256
 (inclusive), derived by adding one to the weight expressed in the PRIORITY
 frame.
@@ -620,9 +625,8 @@ When a prioritized element is first created, it has a default initial weight of
 orphan placeholder; pushes are dependent on the client request on which the
 PUSH_PROMISE frame was sent.
 
-Requests may override the default initial values by including a PRIORITY frame
-(see {{frame-priority}}) at the beginning of the stream. These priorities
-can be updated by sending a PRIORITY frame on the control stream.
+Priorities can be updated by sending a PRIORITY frame (see {{frame-priority}})
+on the control stream.
 
 ### Placeholders
 
@@ -1024,7 +1028,7 @@ comparison between HTTP/2 and HTTP/3 frames is provided in {{h2-frames}}.
 | -------------- | -------------- | -------------- | ----------- | ------------------------ |
 | DATA           | No             | Yes            | Yes         | {{frame-data}}           |
 | HEADERS        | No             | Yes            | Yes         | {{frame-headers}}        |
-| PRIORITY       | Yes            | Yes (1)        | No          | {{frame-priority}}       |
+| PRIORITY       | Yes            | No             | No          | {{frame-priority}}       |
 | CANCEL_PUSH    | Yes            | No             | No          | {{frame-cancel-push}}    |
 | SETTINGS       | Yes (1)        | No             | No          | {{frame-settings}}       |
 | PUSH_PROMISE   | No             | Yes            | No          | {{frame-push-promise}}   |
@@ -1127,22 +1131,14 @@ request using the corresponding stream ID, a server push using a Push ID (see
 {{frame-push-promise}}), or a placeholder using a Placeholder ID (see
 {{placeholders}}).
 
-When a client initiates a request, a PRIORITY frame MAY be sent as the first
-frame of the stream, creating a dependency on an existing element.  In order to
-ensure that prioritization is processed in a consistent order, any subsequent
-PRIORITY frames for that request MUST be sent on the control stream.  A
-PRIORITY frame received after other frames on a request stream MUST be treated
-as a connection error of type HTTP_UNEXPECTED_FRAME.
-
-If, by the time a new request stream is opened, its priority information
-has already been received via the control stream, the PRIORITY frame
-sent on the request stream MUST be ignored.
+In order to ensure that prioritization is processed in a consistent order,
+PRIORITY frames MUST be sent on the control stream.
 
 ~~~~~~~~~~  drawing
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-|PT |DT | Empty |         [Prioritized Element ID (i)]        ...
+|PT |DT |X|Empty|          Prioritized Element ID (i)         ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                [Element Dependency ID (i)]                  ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@@ -1155,22 +1151,25 @@ The PRIORITY frame payload has the following fields:
 
   PT (Prioritized Element Type):
   : A two-bit field indicating the type of element being prioritized (see
-    {{prioritized-element-types}}). When sent on a request stream, this MUST be
-    set to `11`.  When sent on the control stream, this MUST NOT be set to `11`.
+    {{priority-element-types}}).  This MUST NOT be set to `11`.
 
   DT (Element Dependency Type):
   : A two-bit field indicating the type of element being depended on (see
-    {{element-dependency-types}}).
+    {{priority-element-types}}).
+
+  X (Exclusive Flag):
+  : A single-bit flag indicating that the dependency is exclusive (see
+    {{priority}}).
 
   Empty:
-  : A four-bit field which MUST be zero when sent and has no semantic value on
+  : A three-bit field which MUST be zero when sent and has no semantic value on
     receipt.
 
   Prioritized Element ID:
   : A variable-length integer that identifies the element being prioritized.
     Depending on the value of Prioritized Type, this contains the Stream ID of a
-    request stream, the Push ID of a promised resource, a Placeholder ID of a
-    placeholder, or is absent.
+    request stream, the Push ID of a promised resource, or a Placeholder ID of a
+    placeholder.
 
   Element Dependency ID:
   : A variable-length integer that identifies the element on which a dependency
@@ -1184,25 +1183,17 @@ The PRIORITY frame payload has the following fields:
     element (see {{!HTTP2}}, Section 5.3). Add one to the value to obtain a
     weight between 1 and 256.
 
-The values for the Prioritized Element Type ({{prioritized-element-types}}) and
-Element Dependency Type ({{element-dependency-types}}) imply the interpretation
-of the associated Element ID fields.
-
-| PT Bits | Type Description | Prioritized Element ID Contents |
-| ------- | ---------------- | ------------------------------- |
-| 00      | Request stream   | Stream ID                       |
-| 01      | Push stream      | Push ID                         |
-| 10      | Placeholder      | Placeholder ID                  |
-| 11      | Current stream   | Absent                          |
-{: #prioritized-element-types title="Prioritized Element Types"}
-
-| DT Bits | Type Description | Element Dependency ID Contents |
-| ------- | ---------------- | ------------------------------ |
-| 00      | Request stream   | Stream ID                      |
-| 01      | Push stream      | Push ID                        |
-| 10      | Placeholder      | Placeholder ID                 |
-| 11      | Root of the tree | Absent                         |
-{: #element-dependency-types title="Element Dependency Types"}
+The values for the Prioritized Element Type and Element Dependency Type
+({{priority-element-types}}) imply the interpretation of the associated Element
+ID fields.
+
+| Type Bits | Type Description | Element ID Contents |
+| --------- | ---------------- | ------------------- |
+| 00        | Request stream   | Stream ID           |
+| 01        | Push stream      | Push ID             |
+| 10        | Placeholder      | Placeholder ID      |
+| 11        | Root of the tree | Absent              |
+{: #priority-element-types title="Element Types of a PRIORITY frame"}
 
 Note that unlike in {{!HTTP2}}, the root of the tree cannot be referenced
 using a Stream ID of 0, as in QUIC stream 0 carries a valid HTTP request.  The
@@ -1213,12 +1204,7 @@ on the stream on which it is sent or its position in the stream. These
 situations MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.
 The following situations are examples of invalid PRIORITY frames:
 
-- A PRIORITY frame sent on a request stream with the Prioritized Element Type
-  set to any value other than `11`
-- A PRIORITY frame sent on a request stream which expresses a dependency on a
-  request with a greater Stream ID than the current stream
-- A PRIORITY frame sent on a control stream with the Prioritized Element Type
-  set to `11`
+- A PRIORITY frame with the Prioritized Element Type set to `11`.
 - A PRIORITY frame which claims to reference a request, but the associated ID
   does not identify a client-initiated bidirectional stream
 
@@ -1229,8 +1215,8 @@ A PRIORITY frame that references a non-existent Push ID, a Placeholder ID
 greater than the server's limit, or a Stream ID the client is not yet permitted
 to open MUST be treated as a connection error of type HTTP_LIMIT_EXCEEDED.
 
-A PRIORITY frame received on any stream other than a request or control stream
-MUST be treated as a connection error of type HTTP_WRONG_STREAM.
+A PRIORITY frame received on any stream other than the control stream MUST be
+treated as a connection error of type HTTP_WRONG_STREAM.
 
 PRIORITY frames received by a client MUST be treated as a connection error of
 type HTTP_UNEXPECTED_FRAME.
@@ -1992,19 +1978,10 @@ commutative, both sender and receiver must apply them in the same order to
 ensure that both sides have a consistent view of the stream dependency tree.
 
 To achieve in-order delivery of priority changes in HTTP/3, PRIORITY frames are
-sent as the first frame on a request stream or on the control stream.  Because
-ordering between these streams is not guaranteed, the assumption that the
-PRIORITY frame on the request stream, if any, always comes first is explicitly
-stated.
-
-Exclusive prioritization has been removed, since implicitly reprioritizing other
-streams which might or might not exist yet is prone to race conditions even with
-the assumptions used to permit initial PRIORITY frames on request streams.
-
-HTTP/3 permits the prioritization of requests, pushes and placeholders that each
-exist in separate identifier spaces. The HTTP/3 PRIORITY frame replaces the
-stream dependency field with fields that can identify the element of interest
-and its dependency.
+sent on the control stream.  HTTP/3 permits the prioritization of requests,
+pushes and placeholders that each exist in separate identifier spaces. The
+HTTP/3 PRIORITY frame replaces the stream dependency field with fields that can
+identify the element of interest and its dependency.
 
 ### Header Compression Differences