Re: [quicwg/base-drafts] HTTP/3 with strict priorities (#2700)

[ianswett <[e-mail]>]

ianswett commented on this pull request.

Thanks for all the excellent suggestions.

>  
 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.
+PRIORITY frames for that request MUST be sent on the request stream.

Agreed, it's possible I should separate that change from this one.  I know some WG members are fairly unhappy that they can't process PRIORITY frames immediately in cases when the stream hasn't arrived yet, but it's not 100% clear to me that's too painful in practice.

>  
 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.
+PRIORITY frames for that request MUST be sent on the request stream.

I've now reverted this portion of the change, thanks for the suggestion.

>  
   Weight:
-  : An unsigned 8-bit integer representing a priority weight for the prioritized
-    element (see {{!RFC7540}}, Section 5.3). Add one to the value to obtain a
-    weight between 1 and 256.
+  : An optional unsigned 8-bit integer representing a priority weight for the
+    prioritized element (see {{!RFC7540}}, Section 5.3). Add one to the value
+    to obtain a weight between 1 and 256.  When absent, indicates the resource
+    should be delivered all at once or not at all.

Thanks @rmarx suggestion taken.

>  {: #element-dependency-types title="Element Dependency Types"}
 
 Note that unlike in {{!RFC7540}}, 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
-root of the tree cannot be reprioritized.
+using 0, so the root of the tree cannot be reprioritized.
 
 The PRIORITY frame can express relationships which might not be permitted based
 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

I added back another one, but it's still a short list.  I believe one more will end up being added, so keeping the structure for now to minimize diffs.

>  - Pushes, identified by the Push ID of the promised resource
   ({{frame-push-promise}})
 - Placeholders, identified by a Placeholder ID
 
-Taken together, the dependencies across all prioritized elements in a connection
-form a dependency tree. An element can depend on another element or on the root
-of the tree. A reference to an element which is no longer in the tree is treated
-as a reference to the root of the tree. The structure of the dependency tree
-changes as PRIORITY frames modify the dependency links between prioritized
-elements.
-
-Due to reordering between streams, an element can also be prioritized which is
-not yet in the tree. Such elements are added to the tree with the requested
-priority.
+In HTTP/3, stream dependencies that were implicitly encoded by dependencies in
+HTTP/2 are explicitly encoded with strict prioritization.  This simplifies
+priority tree management, eliminates potential new race conditions introduced
+by HTTP/3's multiple streams, and improves framing efficiency with more than
+64 requests.

Removed.

>  
 When a prioritized element is first created, it has a default initial weight
-of 16 and a default dependency. Requests and placeholders are dependent on the
-root of the priority tree; pushes are dependent on the client request on which
-the PUSH_PROMISE frame was sent.
+of 16, priority of 16, and a default dependency. Requests and placeholders are

Agreed, I'll change to 0 weight, which is equivalent to omitting a weight.

>  
 When a prioritized element is first created, it has a default initial weight
-of 16 and a default dependency. Requests and placeholders are dependent on the
-root of the priority tree; pushes are dependent on the client request on which
-the PUSH_PROMISE frame was sent.
+of 16, priority of 16, and a default dependency. Requests and placeholders are
+dependent on the root of the priority tree; pushes are dependent on the client
+request on which the PUSH_PROMISE frame was sent.

Good point, will fix this.

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