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ianswett committed Dec 7, 2018
2 parents 95e6547 + 98d6b94 commit 02fddce
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97 changes: 61 additions & 36 deletions draft-ietf-quic-http.md
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Expand Up @@ -446,9 +446,8 @@ connection error ({{errors}}) of type HTTP_WRONG_STREAM.
~~~~~~~~~~
{: #fig-data title="DATA frame payload"}

DATA frames MUST contain a non-zero-length payload. If a DATA frame is received
with a payload length of zero, the recipient MUST respond with a stream error
({{errors}}) of type HTTP_MALFORMED_FRAME.
If a DATA frame is received with a payload length of zero, the payload of the
frame extends until the end of the stream.

### HEADERS {#frame-headers}

Expand All @@ -468,19 +467,27 @@ HEADERS frames can only be sent on request / push streams.

### PRIORITY {#frame-priority}

The PRIORITY (type=0x02) frame specifies the sender-advised priority of a
stream. In order to ensure that prioritization is processed in a consistent
order, PRIORITY frames MUST be sent on the control stream. A PRIORITY frame
sent on any other stream MUST be treated as a connection error of type
HTTP_WRONG_STREAM.
The PRIORITY (type=0x02) frame specifies the client-advised priority of a
stream.

When opening a new request stream, 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 MUST be sent on the control stream. A PRIORITY frame received
after other frames on a request stream MUST be treated as a stream error of type
HTTP_UNEXPECTED_FRAME.

Subsequent PRIORITY frames sent on the control stream might arrive before
PRIORITY frames sent on a request stream due to reordering. PRIORITY frames on
the control stream always take precedence.

~~~~~~~~~~ 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|E| Prioritized Element ID (i) ...
|PT |DT | Empty | [Prioritized Element ID (i)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Element Dependency ID (i) ...
| [Element Dependency ID (i)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Weight (8) |
+-+-+-+-+-+-+-+-+
Expand All @@ -490,30 +497,28 @@ HTTP_WRONG_STREAM.
The PRIORITY frame payload has the following fields:

Prioritized Type:
: A two-bit field indicating the type of element being prioritized.
: A two-bit field indicating the type of element being prioritized. 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`.

Dependency Type:
: A two-bit field indicating the type of element being depended on.

Empty:
: A three-bit field which MUST be zero when sent and MUST be ignored
: A four-bit field which MUST be zero when sent and MUST be ignored
on receipt.

Exclusive:
: A flag which indicates that the stream dependency is exclusive (see
{{!RFC7540}}, Section 5.3).

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, or a Placeholder ID of a
placeholder.
request stream, the Push ID of a promised resource, a Placeholder ID of a
placeholder, or is absent.

Element Dependency ID:
: A variable-length integer that identifies the element on which a dependency
is being expressed. Depending on the value of Dependency Type, this contains
the Stream ID of a request stream, the Push ID of a promised resource, the
Placeholder ID of a placeholder, or is ignored. For details of
Placeholder ID of a placeholder, or is absent. For details of
dependencies, see {{priority}} and {{!RFC7540}}, Section 5.3.

Weight:
Expand All @@ -530,17 +535,28 @@ request using the corresponding stream ID, a server push using a Push ID (see
The values for the Prioritized Element Type and Element Dependency Type 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 | Ignored |
| Type 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 |

| Type 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 |

Note that the root of the tree cannot be referenced using a Stream ID of 0, as
in {{!RFC7540}}; QUIC stream 0 carries a valid HTTP request. The root of the
tree cannot be reprioritized. A PRIORITY frame that prioritizes the root of the
tree MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.
tree cannot be reprioritized. A PRIORITY frame sent on a request stream with the
Prioritized Element Type set to any value other than `11` or which expresses a
dependency on a request with a greater Stream ID than the current stream MUST be
treated as a stream error of type HTTP_MALFORMED_FRAME. Likewise, a PRIORITY
frame sent on a control stream with the Prioritized Element Type set to `11`
MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.

When a PRIORITY frame claims to reference a request, the associated ID MUST
identify a client-initiated bidirectional stream. A server MUST treat receipt
Expand All @@ -551,6 +567,11 @@ A PRIORITY frame that references a non-existent Push ID or a Placeholder ID
greater than the server's limit MUST be treated as an HTTP_MALFORMED_FRAME
error.

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.

PRIORITY frames received by a client MUST be treated as a stream error of type
HTTP_UNEXPECTED_FRAME.

### CANCEL_PUSH {#frame-cancel-push}

Expand Down Expand Up @@ -1032,14 +1053,18 @@ another request, which expresses the preference that the latter stream (the
"dependent" request). Taken together, the dependencies across all requests in a
connection form a dependency tree.

When a client request is first sent or a placeholder first allocated, the
element is dependent on the root of the priority tree. Pushed streams are
initially dependent on the client request on which the PUSH_PROMISE frame was
sent. In all cases, elements are assigned an initial weight of 16.
When a client request is first sent, its parent and weight are determined by the
PRIORITY frame (see {{frame-priority}}) which begins the stream, if present.
Otherwise, the element is dependent on the root of the priority tree.
Placeholders are also dependent on the root of the priority tree when first
allocated. Pushed streams are initially dependent on the client request on
which the PUSH_PROMISE frame was sent. In all cases, elements are assigned an
initial weight of 16 unless an PRIORITY frame begins the stream.

The structure of the dependency tree changes as PRIORITY frames modify the
dependency links between requests. The PRIORITY frame {{frame-priority}}
identifies a prioritized element. The elements which can be prioritized are:
The structure of the dependency tree changes as PRIORITY frames on the control
stream modify the dependency links between requests. The PRIORITY frame
{{frame-priority}} identifies a prioritized element. The elements which can be
prioritized are:

- Requests, identified by the ID of the request stream
- Pushes, identified by the Push ID of the promised resource
Expand Down Expand Up @@ -1709,8 +1734,8 @@ 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.
HTTP/2 specifies priority assignments in PRIORITY frames and (optionally) in
HEADERS frames. To achieve in-order delivery of priority changes in HTTP/3,
PRIORITY frames are sent on the control stream and the PRIORITY section is
removed from the HEADERS frame.
PRIORITY frames are sent on the control stream and exclusive prioritization
has been removed.

Likewise, HPACK was designed with the assumption of in-order delivery. A
sequence of encoded header blocks must arrive (and be decoded) at an endpoint in
Expand Down

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