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draft-ietf-quic-http.md
---
title: Hypertext Transfer Protocol Version 3 (HTTP/3)
abbrev: HTTP/3
docname: draft-ietf-quic-http-latest
date: {DATE}
category: std
ipr: trust200902
area: Transport
workgroup: QUIC
stand_alone: yes
pi: [toc, sortrefs, symrefs, docmapping]
author:
-
ins: M. Bishop
name: Mike Bishop
org: Akamai
email: mbishop@evequefou.be
role: editor
normative:
QUIC-TRANSPORT:
title: "QUIC: A UDP-Based Multiplexed and Secure Transport"
date: {DATE}
seriesinfo:
Internet-Draft: draft-ietf-quic-transport-latest
author:
-
ins: J. Iyengar
name: Jana Iyengar
org: Fastly
role: editor
-
ins: M. Thomson
name: Martin Thomson
org: Mozilla
role: editor
QPACK:
title: "QPACK: Header Compression for HTTP over QUIC"
date: {DATE}
seriesinfo:
Internet-Draft: draft-ietf-quic-qpack-latest
author:
-
ins: C. Krasic
name: Charles 'Buck' Krasic
org: Google, Inc
-
ins: M. Bishop
name: Mike Bishop
org: Akamai Technologies
-
ins: A. Frindell
name: Alan Frindell
org: Facebook
role: editor
informative:
--- abstract
The QUIC transport protocol has several features that are desirable in a
transport for HTTP, such as stream multiplexing, per-stream flow control, and
low-latency connection establishment. This document describes a mapping of HTTP
semantics over QUIC. This document also identifies HTTP/2 features that are
subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3.
--- note_Note_to_Readers
Discussion of this draft takes place on the QUIC working group mailing list
(quic@ietf.org), which is archived at
<https://mailarchive.ietf.org/arch/search/?email_list=quic>.
Working Group information can be found at <https://github.com/quicwg>; source
code and issues list for this draft can be found at
<https://github.com/quicwg/base-drafts/labels/-http>.
--- middle
# Introduction
HTTP semantics are used for a broad range of services on the Internet. These
semantics have commonly been used with two different TCP mappings, HTTP/1.1 and
HTTP/2. HTTP/2 introduced a framing and multiplexing layer to improve latency
without modifying the transport layer. However, TCP's lack of visibility into
parallel requests in both mappings limited the possible performance gains.
The QUIC transport protocol incorporates stream multiplexing and per-stream flow
control, similar to that provided by the HTTP/2 framing layer. By providing
reliability at the stream level and congestion control across the entire
connection, it has the capability to improve the performance of HTTP compared to
a TCP mapping. QUIC also incorporates TLS 1.3 at the transport layer, offering
comparable security to running TLS over TCP, but with improved connection setup
latency (unless TCP Fast Open {{?RFC7413}}} is used).
This document defines a mapping of HTTP semantics over the QUIC transport
protocol, drawing heavily on the design of HTTP/2. This document identifies
HTTP/2 features that are subsumed by QUIC, and describes how the other features
can be implemented atop QUIC.
QUIC is described in {{QUIC-TRANSPORT}}. For a full description of HTTP/2, see
{{!RFC7540}}.
## Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 {{!RFC2119}} {{!RFC8174}}
when, and only when, they appear in all capitals, as shown here.
Field definitions are given in Augmented Backus-Naur Form (ABNF), as defined in
{{!RFC5234}}.
This document uses the variable-length integer encoding from
{{QUIC-TRANSPORT}}.
Protocol elements called "frames" exist in both this document and
{{QUIC-TRANSPORT}}. Where frames from {{QUIC-TRANSPORT}} are referenced, the
frame name will be prefaced with "QUIC." For example, "QUIC CONNECTION_CLOSE
frames." References without this preface refer to frames defined in {{frames}}.
# Connection Setup and Management
## Draft Version Identification
> **RFC Editor's Note:** Please remove this section prior to publication of a
> final version of this document.
HTTP/3 uses the token "h3" to identify itself in ALPN and Alt-Svc. Only
implementations of the final, published RFC can identify themselves as "h3".
Until such an RFC exists, implementations MUST NOT identify themselves using
this string.
Implementations of draft versions of the protocol MUST add the string "-" and
the corresponding draft number to the identifier. For example,
draft-ietf-quic-http-01 is identified using the string "h3-01".
Non-compatible experiments that are based on these draft versions MUST append
the string "-" and an experiment name to the identifier. For example, an
experimental implementation based on draft-ietf-quic-http-09 which reserves an
extra stream for unsolicited transmission of 1980s pop music might identify
itself as "h3-09-rickroll". Note that any label MUST conform to the "token"
syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are encouraged to
coordinate their experiments on the quic@ietf.org mailing list.
## Discovering an HTTP/3 Endpoint
An HTTP origin advertises the availability of an equivalent HTTP/3 endpoint via
the Alt-Svc HTTP response header field or the HTTP/2 ALTSVC frame
({{!ALTSVC=RFC7838}}), using the ALPN token defined in
{{connection-establishment}}.
For example, an origin could indicate in an HTTP response that HTTP/3 was
available on UDP port 50781 at the same hostname by including the following
header field:
~~~ example
Alt-Svc: h3=":50781"
~~~
On receipt of an Alt-Svc record indicating HTTP/3 support, a client MAY attempt
to establish a QUIC connection to the indicated host and port and, if
successful, send HTTP requests using the mapping described in this document.
Connectivity problems (e.g. firewall blocking UDP) can result in QUIC connection
establishment failure, in which case the client SHOULD continue using the
existing connection or try another alternative endpoint offered by the origin.
Servers MAY serve HTTP/3 on any UDP port, since an alternative always includes
an explicit port.
### QUIC Version Hints {#alt-svc-version-hint}
This document defines the "quic" parameter for Alt-Svc, which MAY be used to
provide version-negotiation hints to HTTP/3 clients. QUIC versions are four-byte
sequences with no additional constraints on format. Leading zeros SHOULD be
omitted for brevity.
Syntax:
~~~ abnf
quic = DQUOTE version-number [ "," version-number ] * DQUOTE
version-number = 1*8HEXDIG; hex-encoded QUIC version
~~~
Where multiple versions are listed, the order of the values reflects the
server's preference (with the first value being the most preferred version).
Reserved versions MAY be listed, but unreserved versions which are not supported
by the alternative SHOULD NOT be present in the list. Origins MAY omit supported
versions for any reason.
Clients MUST ignore any included versions which they do not support. The "quic"
parameter MUST NOT occur more than once; clients SHOULD process only the first
occurrence.
For example, suppose a server supported both version 0x00000001 and the version
rendered in ASCII as "Q034". If it also opted to include the reserved version
(from Section 15 of {{QUIC-TRANSPORT}}) 0x1abadaba, it could specify the
following header field:
~~~ example
Alt-Svc: h3=":49288";quic="1,1abadaba,51303334"
~~~
A client acting on this header field would drop the reserved version (not
supported), then attempt to connect to the alternative using the first version
in the list which it does support, if any.
## Connection Establishment {#connection-establishment}
HTTP/3 relies on QUIC as the underlying transport. The QUIC version being used
MUST use TLS version 1.3 or greater as its handshake protocol. HTTP/3 clients
MUST indicate the target domain name during the TLS handshake. This may be done
using the Server Name Indication (SNI) {{!RFC6066}} extension to TLS or using
some other mechanism.
QUIC connections are established as described in {{QUIC-TRANSPORT}}. During
connection establishment, HTTP/3 support is indicated by selecting the ALPN
token "h3" in the TLS handshake. Support for other application-layer protocols
MAY be offered in the same handshake.
While connection-level options pertaining to the core QUIC protocol are set in
the initial crypto handshake, HTTP/3-specific settings are conveyed in the
SETTINGS frame. After the QUIC connection is established, a SETTINGS frame
({{frame-settings}}) MUST be sent by each endpoint as the initial frame of their
respective HTTP control stream (see {{control-streams}}).
## Connection Reuse
Once a connection exists to a server endpoint, this connection MAY be reused for
requests with multiple different URI authority components. The client MAY send
any requests for which the client considers the server authoritative.
An authoritative HTTP/3 endpoint is typically discovered because the client has
received an Alt-Svc record from the request's origin which nominates the
endpoint as a valid HTTP Alternative Service for that origin. As required by
{{RFC7838}}, clients MUST check that the nominated server can present a valid
certificate for the origin before considering it authoritative. Clients MUST NOT
assume that an HTTP/3 endpoint is authoritative for other origins without an
explicit signal.
A server that does not wish clients to reuse connections for a particular origin
can indicate that it is not authoritative for a request by sending a 421
(Misdirected Request) status code in response to the request (see Section 9.1.2
of {{!RFC7540}}).
The considerations discussed in Section 9.1 of {{?RFC7540}} also apply to the
management of HTTP/3 connections.
# Stream Mapping and Usage {#stream-mapping}
A QUIC stream provides reliable in-order delivery of bytes, but makes no
guarantees about order of delivery with regard to bytes on other streams. On the
wire, data is framed into QUIC STREAM frames, but this framing is invisible to
the HTTP framing layer. The transport layer buffers and orders received QUIC
STREAM frames, exposing the data contained within as a reliable byte stream to
the application. Although QUIC permits out-of-order delivery within a stream
HTTP/3 does not make use of this feature.
QUIC streams can be either unidirectional, carrying data only from initiator to
receiver, or bidirectional. Streams can be initiated by either the client or
the server. For more detail on QUIC streams, see Section 2 of
{{QUIC-TRANSPORT}}.
When HTTP headers and data are sent over QUIC, the QUIC layer handles most of
the stream management. HTTP does not need to do any separate multiplexing when
using QUIC - data sent over a QUIC stream always maps to a particular HTTP
transaction or connection context.
## Bidirectional Streams
All client-initiated bidirectional streams are used for HTTP requests and
responses. A bidirectional stream ensures that the response can be readily
correlated with the request. This means that the client's first request occurs
on QUIC stream 0, with subsequent requests on stream 4, 8, and so on. In order
to permit these streams to open, an HTTP/3 client SHOULD send non-zero values
for the QUIC transport parameters `initial_max_stream_data_bidi_local`. An
HTTP/3 server SHOULD send non-zero values for the QUIC transport parameters
`initial_max_stream_data_bidi_remote` and `initial_max_bidi_streams`. It is
recommended that `initial_max_bidi_streams` be no smaller than 100, so as to not
unnecessarily limit parallelism.
These streams carry frames related to the request/response (see
{{request-response}}). When a stream terminates cleanly, if the last frame on
the stream was truncated, this MUST be treated as a connection error (see
HTTP_MALFORMED_FRAME in {{http-error-codes}}). Streams which terminate abruptly
may be reset at any point in the frame.
HTTP/3 does not use server-initiated bidirectional streams; clients MUST omit or
specify a value of zero for the QUIC transport parameter
`initial_max_bidi_streams`.
## Unidirectional Streams
Unidirectional streams, in either direction, are used for a range of purposes.
The purpose is indicated by a stream type, which is sent as a variable-length
integer at the start of the stream. The format and structure of data that
follows this integer is determined by the stream type.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Type (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-stream-header title="Unidirectional Stream Header"}
Some stream types are reserved ({{stream-grease}}). Two stream types are
defined in this document: control streams ({{control-streams}}) and push streams
({{push-streams}}). Other stream types can be defined by extensions to HTTP/3;
see {{extensions}} for more details.
The performance of HTTP/3 connections in the early phase of their lifetime is
sensitive to the creation and exchange of data on unidirectional streams.
Endpoints that set low values for the QUIC transport parameters
`initial_max_uni_streams` and `initial_max_stream_data_uni` will increase the
chance that the remote peer reaches the limit early and becomes blocked. In
particular, the value chosen for `initial_max_uni_streams` should consider that
remote peers may wish to exercise reserved stream behaviour ({{stream-grease}}).
To reduce the likelihood of blocking, both clients and servers SHOULD send a
value of three or greater for the QUIC transport parameter
`initial_max_uni_streams`, and a value of 1,024 or greater for the QUIC
transport parameter `initial_max_stream_data_uni`.
If the stream header indicates a stream type which is not supported by the
recipient, the remainder of the stream cannot be consumed as the semantics are
unknown. Recipients of unknown stream types MAY trigger a QUIC STOP_SENDING
frame with an error code of HTTP_UNKNOWN_STREAM_TYPE, but MUST NOT consider such
streams to be an error of any kind.
Implementations MAY send stream types before knowing whether the peer supports
them. However, stream types which could modify the state or semantics of
existing protocol components, including QPACK or other extensions, MUST NOT be
sent until the peer is known to support them.
A sender can close or reset a unidirectional stream unless otherwise specified.
A receiver MUST tolerate unidirectional streams being closed or reset prior to
the reception of the unidirectional stream header.
### Control Streams
A control stream is indicated by a stream type of `0x00`. Data on this stream
consists of HTTP/3 frames, as defined in {{frames}}.
Each side MUST initiate a single control stream at the beginning of the
connection and send its SETTINGS frame as the first frame on this stream. If
the first frame of the control stream is any other frame type, this MUST be
treated as a connection error of type HTTP_MISSING_SETTINGS. Only one control
stream per peer is permitted; receipt of a second stream which claims to be a
control stream MUST be treated as a connection error of type
HTTP_WRONG_STREAM_COUNT. The sender MUST NOT close the control stream. If the
control stream is closed at any point, this MUST be treated as a connection
error of type HTTP_CLOSED_CRITICAL_STREAM.
A pair of unidirectional streams is used rather than a single bidirectional
stream. This allows either peer to send data as soon they are able. Depending
on whether 0-RTT is enabled on the connection, either client or server might be
able to send stream data first after the cryptographic handshake completes.
### Push Streams
A push stream is indicated by a stream type of `0x01`, followed by the Push ID
of the promise that it fulfills, encoded as a variable-length integer. The
remaining data on this stream consists of HTTP/3 frames, as defined in
{{frames}}, and fulfills a promised server push. Server push and Push IDs are
described in {{server-push}}.
Only servers can push; if a server receives a client-initiated push stream, this
MUST be treated as a stream error of type HTTP_WRONG_STREAM_DIRECTION.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Push ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-push-stream-header title="Push Stream Header"}
Each Push ID MUST only be used once in a push stream header. If a push stream
header includes a Push ID that was used in another push stream header, the
client MUST treat this as a connection error of type HTTP_DUPLICATE_PUSH.
### Reserved Stream Types {#stream-grease}
Stream types of the format `0x1f * N + 0x21` for integer values of N are
reserved to exercise the requirement that unknown types be ignored. These
streams have no semantics, and can be sent when application-layer padding is
desired. They MAY also be sent on connections where no data is currently being
transferred. Endpoints MUST NOT consider these streams to have any meaning upon
receipt.
The payload and length of the stream are selected in any manner the
implementation chooses.
# HTTP Framing Layer {#http-framing-layer}
HTTP frames are carried on QUIC streams, as described in {{stream-mapping}}.
HTTP/3 defines three stream types: control stream, request stream, and push
stream. This section describes HTTP/3 frame formats and the streams types on
which they are permitted; see {{stream-frame-mapping}} for an overiew. A
comparison between HTTP/2 and HTTP/3 frames is provided in {{h2-frames}}.
| Frame | Control Stream | Request Stream | Push Stream | Section |
| -------------- | -------------- | -------------- | ----------- | ------------------------ |
| DATA | No | Yes | Yes | {{frame-data}} |
| HEADERS | No | Yes | Yes | {{frame-headers}} |
| PRIORITY | Yes | Yes (1) | 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}} |
| GOAWAY | Yes | No | No | {{frame-goaway}} |
| MAX_PUSH_ID | Yes | No | No | {{frame-max-push-id}} |
| DUPLICATE_PUSH | No | Yes | No | {{frame-duplicate-push}} |
{: #stream-frame-mapping title="HTTP/3 frames and stream type overview"}
Certain frames can only occur as the first frame of a particular stream type;
these are indicated in {{stream-frame-mapping}} with a (1). Specific guidance
is provided in the relevant section.
## Frame Layout
All frames have the following format:
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame Payload (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-frame title="HTTP/3 frame format"}
A frame includes the following fields:
Type:
: A variable-length integer that identifies the frame type.
Length:
: A variable-length integer that describes the length of the Frame Payload.
Frame Payload:
: A payload, the semantics of which are determined by the Type field.
Each frame's payload MUST contain exactly the fields identified in its
description. A frame payload that contains additional bytes after the
identified fields or a frame payload that terminates before the end of the
identified fields MUST be treated as a connection error of type
HTTP_MALFORMED_FRAME.
## Frame Definitions {#frames}
### DATA {#frame-data}
DATA frames (type=0x0) convey arbitrary, variable-length sequences of bytes
associated with an HTTP request or response payload.
DATA frames MUST be associated with an HTTP request or response. If a DATA
frame is received on either control stream, the recipient MUST respond with a
connection error ({{errors}}) of type HTTP_WRONG_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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-data title="DATA frame payload"}
### HEADERS {#frame-headers}
The HEADERS frame (type=0x1) is used to carry a header block, compressed using
QPACK. See [QPACK] for more details.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Block (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-headers title="HEADERS frame payload"}
HEADERS frames can only be sent on request / push streams.
### PRIORITY {#frame-priority}
The PRIORITY (type=0x2) frame specifies the client-advised priority of a
request, server push or placeholder.
A PRIORITY frame identifies an element to prioritize, and an element upon which
it depends. A Prioritized ID or Dependency ID identifies a client-initiated
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 stream 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.
~~~~~~~~~~ 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)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Element Dependency ID (i)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Weight (8) |
+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-priority title="PRIORITY frame payload"}
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`.
DT (Element Dependency Type):
: A two-bit field indicating the type of element being depended on (see
{{element-dependency-types}}).
Empty:
: A four-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.
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 absent. For details of
dependencies, see {{priority}} and {{!RFC7540}}, Section 5.3.
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.
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"}
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. 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. A PRIORITY frame with Empty bits not set to zero MAY 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
of a PRIORITY frame identifying a stream of any other type as a connection error
of type HTTP_MALFORMED_FRAME.
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 an HTTP_LIMIT_EXCEEDED 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 connection error of
type HTTP_UNEXPECTED_FRAME.
### CANCEL_PUSH {#frame-cancel-push}
The CANCEL_PUSH frame (type=0x3) is used to request cancellation of a server
push prior to the push stream being received. The CANCEL_PUSH frame identifies
a server push by Push ID (see {{frame-push-promise}}), encoded as a
variable-length integer.
When a server receives this frame, it aborts sending the response for the
identified server push. If the server has not yet started to send the server
push, it can use the receipt of a CANCEL_PUSH frame to avoid opening a push
stream. If the push stream has been opened by the server, the server SHOULD
send a QUIC RESET_STREAM frame on that stream and cease transmission of the
response.
A server can send the CANCEL_PUSH frame to indicate that it will not be
fulfilling a promise prior to creation of a push stream. Once the push stream
has been created, sending CANCEL_PUSH has no effect on the state of the push
stream. A QUIC RESET_STREAM frame SHOULD be used instead to abort transmission
of the server push response.
A CANCEL_PUSH frame is sent on the control stream. Receiving a CANCEL_PUSH
frame on a stream other than the control stream MUST be treated as a stream
error of type HTTP_WRONG_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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Push ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-cancel-push title="CANCEL_PUSH frame payload"}
The CANCEL_PUSH frame carries a Push ID encoded as a variable-length integer.
The Push ID identifies the server push that is being cancelled (see
{{frame-push-promise}}).
If the client receives a CANCEL_PUSH frame, that frame might identify a Push ID
that has not yet been mentioned by a PUSH_PROMISE frame.
### SETTINGS {#frame-settings}
The SETTINGS frame (type=0x4) conveys configuration parameters that affect how
endpoints communicate, such as preferences and constraints on peer behavior.
Individually, a SETTINGS parameter can also be referred to as a "setting"; the
identifier and value of each setting parameter can be referred to as a "setting
identifier" and a "setting value".
SETTINGS frames always apply to a connection, never a single stream. A SETTINGS
frame MUST be sent as the first frame of each control stream (see
{{control-streams}}) by each peer, and MUST NOT be sent subsequently or on any
other stream. If an endpoint receives a SETTINGS frame on a different stream,
the endpoint MUST respond with a connection error of type HTTP_WRONG_STREAM. If
an endpoint receives a second SETTINGS frame, the endpoint MUST respond with a
connection error of type HTTP_UNEXPECTED_FRAME.
SETTINGS parameters are not negotiated; they describe characteristics of the
sending peer, which can be used by the receiving peer. However, a negotiation
can be implied by the use of SETTINGS - each peer uses SETTINGS to advertise a
set of supported values. The definition of the setting would describe how each
peer combines the two sets to conclude which choice will be used. SETTINGS does
not provide a mechanism to identify when the choice takes effect.
Different values for the same parameter can be advertised by each peer. For
example, a client might be willing to consume a very large response header,
while servers are more cautious about request size.
Parameters MUST NOT occur more than once in the SETTINGS frame. A receiver MAY
treat the presence of the same parameter more than once as a connection error of
type HTTP_MALFORMED_FRAME.
The payload of a SETTINGS frame consists of zero or more parameters. Each
parameter consists of a setting identifier and a value, both encoded as QUIC
variable-length integers.
~~~~~~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~~~~~~
{: #fig-ext-settings title="SETTINGS parameter format"}
An implementation MUST ignore the contents for any SETTINGS identifier it does
not understand.
#### Defined SETTINGS Parameters {#settings-parameters}
The following settings are defined in HTTP/3:
SETTINGS_MAX_HEADER_LIST_SIZE (0x6):
: The default value is unlimited. See {{header-formatting}} for usage.
SETTINGS_NUM_PLACEHOLDERS (0x9):
: The default value is 0. However, this value SHOULD be set to a non-zero
value by servers. See {{placeholders}} for usage.
Setting identifiers of the format `0x1f * N + 0x21` for integer values of N are
reserved to exercise the requirement that unknown identifiers be ignored. Such
settings have no defined meaning. Endpoints SHOULD include at least one such
setting in their SETTINGS frame. Endpoints MUST NOT consider such settings to
have any meaning upon receipt.
Because the setting has no defined meaning, the value of the setting can be any
value the implementation selects.
Additional settings can be defined by extensions to HTTP/3; see {{extensions}}
for more details.
#### Initialization
An HTTP implementation MUST NOT send frames or requests which would be invalid
based on its current understanding of the peer's settings. All settings begin
at an initial value, and are updated upon receipt of a SETTINGS frame. For
servers, the initial value of each client setting is the default value.
For clients using a 1-RTT QUIC connection, the initial value of each server
setting is the default value. When a 0-RTT QUIC connection is being used, the
initial value of each server setting is the value used in the previous session.
Clients MUST store the settings the server provided in the session being resumed
and MUST comply with stored settings until the current server settings are
received.
A server can remember the settings that it advertised, or store an
integrity-protected copy of the values in the ticket and recover the information
when accepting 0-RTT data. A server uses the HTTP/3 settings values in
determining whether to accept 0-RTT data.
A server MAY accept 0-RTT and subsequently provide different settings in its
SETTINGS frame. If 0-RTT data is accepted by the server, its SETTINGS frame MUST
NOT reduce any limits or alter any values that might be violated by the client
with its 0-RTT data.
### PUSH_PROMISE {#frame-push-promise}
The PUSH_PROMISE frame (type=0x5) is used to carry a promised request header
set from server to client on a request stream, as in HTTP/2.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Push ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Block (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-push-promise title="PUSH_PROMISE frame payload"}
The payload consists of:
Push ID:
: A variable-length integer that identifies the server push operation. A Push
ID is used in push stream headers ({{server-push}}), CANCEL_PUSH frames
({{frame-cancel-push}}), DUPLICATE_PUSH frames ({{frame-duplicate-push}}), and
PRIORITY frames ({{frame-priority}}).
Header Block:
: QPACK-compressed request header fields for the promised response. See [QPACK]
for more details.
A server MUST NOT use a Push ID that is larger than the client has provided in a
MAX_PUSH_ID frame ({{frame-max-push-id}}) and MUST NOT use the same Push ID in
multiple PUSH_PROMISE frames. A client MUST treat receipt of a PUSH_PROMISE
that contains a larger Push ID than the client has advertised or a Push ID which
has already been promised as a connection error of type HTTP_MALFORMED_FRAME.
If a PUSH_PROMISE frame is received on either control stream, the recipient MUST
respond with a connection error ({{errors}}) of type HTTP_WRONG_STREAM.
See {{server-push}} for a description of the overall server push mechanism.
### GOAWAY {#frame-goaway}
The GOAWAY frame (type=0x7) is used to initiate graceful shutdown of a
connection by a server. GOAWAY allows a server to stop accepting new requests
while still finishing processing of previously received requests. This enables
administrative actions, like server maintenance. GOAWAY by itself does not
close a connection.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-goaway title="GOAWAY frame payload"}
The GOAWAY frame is always sent on the control stream. It carries a QUIC Stream
ID for a client-initiated bidirectional stream encoded as a variable-length
integer. A client MUST treat receipt of a GOAWAY frame containing a Stream ID
of any other type as a connection error of type HTTP_WRONG_STREAM.
Clients do not need to send GOAWAY to initiate a graceful shutdown; they simply
stop making new requests. A server MUST treat receipt of a GOAWAY frame on any
stream as a connection error ({{errors}}) of type HTTP_UNEXPECTED_FRAME.
The GOAWAY frame applies to the connection, not a specific stream. A client
MUST treat a GOAWAY frame on a stream other than the control stream as a
connection error ({{errors}}) of type HTTP_UNEXPECTED_FRAME.
See {{connection-shutdown}} for more information on the use of the GOAWAY frame.
### MAX_PUSH_ID {#frame-max-push-id}
The MAX_PUSH_ID frame (type=0xD) is used by clients to control the number of
server pushes that the server can initiate. This sets the maximum value for a
Push ID that the server can use in a PUSH_PROMISE frame. Consequently, this
also limits the number of push streams that the server can initiate in addition
to the limit set by the QUIC MAX_STREAMS frame.
The MAX_PUSH_ID frame is always sent on the control stream. Receipt of a
MAX_PUSH_ID frame on any other stream MUST be treated as a connection error of
type HTTP_WRONG_STREAM.
A server MUST NOT send a MAX_PUSH_ID frame. A client MUST treat the receipt of
a MAX_PUSH_ID frame as a connection error of type HTTP_UNEXPECTED_FRAME.
The maximum Push ID is unset when a connection is created, meaning that a server
cannot push until it receives a MAX_PUSH_ID frame. A client that wishes to
manage the number of promised server pushes can increase the maximum Push ID by
sending MAX_PUSH_ID frames as the server fulfills or cancels server pushes.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Push ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-max-push title="MAX_PUSH_ID frame payload"}
The MAX_PUSH_ID frame carries a single variable-length integer that identifies
the maximum value for a Push ID that the server can use (see
{{frame-push-promise}}). A MAX_PUSH_ID frame cannot reduce the maximum Push ID;
receipt of a MAX_PUSH_ID that contains a smaller value than previously received
MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.
### DUPLICATE_PUSH {#frame-duplicate-push}
The DUPLICATE_PUSH frame (type=0xE) is used by servers to indicate that an
existing pushed resource is related to multiple client requests.
The DUPLICATE_PUSH frame is always sent on a request stream. Receipt of a
DUPLICATE_PUSH frame on any other stream MUST be treated as a connection error
of type HTTP_WRONG_STREAM.
A client MUST NOT send a DUPLICATE_PUSH frame. A server MUST treat the receipt
of a DUPLICATE_PUSH frame as a connection error of type HTTP_UNEXPECTED_FRAME.
~~~~~~~~~~ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Push ID (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~~~~~~~~~~
{: #fig-duplicate-push title="DUPLICATE_PUSH frame payload"}
The DUPLICATE_PUSH frame carries a single variable-length integer that
identifies the Push ID of a resource that the server has previously promised
(see {{frame-push-promise}}).
This frame allows the server to use the same server push in response to multiple
concurrent requests. Referencing the same server push ensures that a promise
can be made in relation to every response in which server push might be needed
without duplicating request headers or pushed responses.
Allowing duplicate references to the same Push ID is primarily to reduce
duplication caused by concurrent requests. A server SHOULD avoid reusing a Push
ID over a long period. Clients are likely to consume server push responses and
not retain them for reuse over time. Clients that see a DUPLICATE_PUSH that
uses a Push ID that they have since consumed and discarded are forced to ignore
the DUPLICATE_PUSH.
### Reserved Frame Types {#frame-grease}
Frame types of the format `0x1f * N + 0x21` for integer values of N are reserved
to exercise the requirement that unknown types be ignored ({{extensions}}).
These frames have no semantics, and can be sent when application-layer padding
is desired. They MAY also be sent on connections where no data is currently
being transferred. Endpoints MUST NOT consider these frames to have any meaning
upon receipt.
The payload and length of the frames are selected in any manner the
implementation chooses.
# HTTP Request Lifecycle
## HTTP Message Exchanges {#request-response}
A client sends an HTTP request on a client-initiated bidirectional QUIC
stream. A client MUST send only a single request on a given stream.
A server sends one or more HTTP responses on the same stream as the request,
as detailed below.
An HTTP message (request or response) consists of:
1. the message header (see {{!RFC7230}}, Section 3.2), sent as a single HEADERS
frame (see {{frame-headers}}),
2. the payload body (see {{!RFC7230}}, Section 3.3), sent as a series of DATA
frames (see {{frame-data}}),
3. optionally, one HEADERS frame containing the trailer-part, if present (see
{{!RFC7230}}, Section 4.1.2).
A server MAY interleave one or more PUSH_PROMISE frames (see
{{frame-push-promise}}) with the frames of a response message. These
PUSH_PROMISE frames are not part of the response; see {{server-push}} for more
details.
The "chunked" transfer encoding defined in Section 4.1 of {{!RFC7230}} MUST NOT
be used.
Trailing header fields are carried in an additional HEADERS frame following the
body. Senders MUST send only one HEADERS frame in the trailers section;
receivers MUST discard any subsequent HEADERS frames.
A response MAY consist of multiple messages when and only when one or more
informational responses (1xx, see {{!RFC7231}}, Section 6.2) precede a final
response to the same request. Non-final responses do not contain a payload body
or trailers.
An HTTP request/response exchange fully consumes a bidirectional QUIC stream.
After sending a request, a client MUST close the stream for sending. Unless
using the CONNECT method (see {{the-connect-method}}), clients MUST NOT make
stream closure dependent on receiving a response to their request. After sending
a final response, the server MUST close the stream for sending. At this point,
the QUIC stream is fully closed.
When a stream is closed, this indicates the end of an HTTP message. Because some
messages are large or unbounded, endpoints SHOULD begin processing partial HTTP
messages once enough of the message has been received to make progress. If a
client stream terminates without enough of the HTTP message to provide a
complete response, the server SHOULD abort its response with the error code
HTTP_INCOMPLETE_REQUEST.
A server can send a complete response prior to the client sending an entire
request if the response does not depend on any portion of the request that has
not been sent and received. When this is true, a server MAY request that the
client abort transmission of a request without error by triggering a QUIC
STOP_SENDING frame with error code HTTP_EARLY_RESPONSE, sending a complete
response, and cleanly closing its stream. Clients MUST NOT discard complete
responses as a result of having their request terminated abruptly, though
clients can always discard responses at their discretion for other reasons.
### Header Formatting and Compression {#header-formatting}
HTTP message headers carry information as a series of key-value pairs, called
header fields. For a listing of registered HTTP header fields, see the "Message
Header Field" registry maintained at
<https://www.iana.org/assignments/message-headers>.
Just as in previous versions of HTTP, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion. Properties of HTTP
header field names and values are discussed in more detail in Section 3.2 of
{{!RFC7230}}, though the wire rendering in HTTP/3 differs. As in HTTP/2, header
field names MUST be converted to lowercase prior to their encoding. A request
or response containing uppercase header field names MUST be treated as
malformed.
As in HTTP/2, HTTP/3 uses special pseudo-header fields beginning with the ':'
character (ASCII 0x3a) to convey the target URI, the method of the request, and
the status code for the response. These pseudo-header fields are defined in
Section 8.1.2.3 and 8.1.2.4 of {{!RFC7540}}. Pseudo-header fields are not HTTP
header fields. Endpoints MUST NOT generate pseudo-header fields other than
those defined in {{!RFC7540}}. The restrictions on the use of pseudo-header
fields in Section 8.1.2.1 of {{!RFC7540}} also apply to HTTP/3.
HTTP/3 uses QPACK header compression as described in [QPACK], a variation of
HPACK which allows the flexibility to avoid header-compression-induced
head-of-line blocking. See that document for additional details.
An HTTP/3 implementation MAY impose a limit on the maximum size of the header it
will accept on an individual HTTP message; encountering a larger message header
SHOULD be treated as a stream error of type `HTTP_EXCESSIVE_LOAD`. If an
implementation wishes to advise its peer of this limit, it can be conveyed as a
number of bytes in the `SETTINGS_MAX_HEADER_LIST_SIZE` parameter. The size of a
header list is calculated based on the uncompressed size of header fields,
including the length of the name and value in bytes plus an overhead of 32 bytes
for each header field.
### Request Cancellation and Rejection {#request-cancellation}
Clients can cancel requests by aborting the stream (QUIC RESET_STREAM and/or
STOP_SENDING frames, as appropriate) with an error code of
HTTP_REQUEST_CANCELLED ({{http-error-codes}}). When the client cancels a
response, it indicates that this response is no longer of interest.
Implementations SHOULD cancel requests by aborting both directions of a stream.
When the server rejects a request without performing any application processing,
it SHOULD abort its response stream with the error code HTTP_REQUEST_REJECTED.
In this context, "processed" means that some data from the stream was passed to
some higher layer of software that might have taken some action as a result. The
client can treat requests rejected by the server as though they had never been
sent at all, thereby allowing them to be retried later on a new connection.
Servers MUST NOT use the HTTP_REQUEST_REJECTED error code for requests which
were partially or fully processed. When a server abandons a response after
partial processing, it SHOULD abort its response stream with the error code
HTTP_REQUEST_CANCELLED.
When a client sends a STOP_SENDING with HTTP_REQUEST_CANCELLED, a server MAY
send the error code HTTP_REQUEST_REJECTED in the corresponding RESET_STREAM
if no processing was performed. Clients MUST NOT reset streams with the
HTTP_REQUEST_REJECTED error code except in response to a QUIC STOP_SENDING
frame that contains the same code.
If a stream is cancelled after receiving a complete response, the client MAY
ignore the cancellation and use the response. However, if a stream is cancelled
after receiving a partial response, the response SHOULD NOT be used.
Automatically retrying such requests is not possible, unless this is otherwise
permitted (e.g., idempotent actions like GET, PUT, or DELETE).
## The CONNECT Method
The pseudo-method CONNECT ({{!RFC7231}}, Section 4.3.6) is primarily used with
HTTP proxies to establish a TLS session with an origin server for the purposes
of interacting with "https" resources. In HTTP/1.x, CONNECT is used to convert
an entire HTTP connection into a tunnel to a remote host. In HTTP/2, the CONNECT
method is used to establish a tunnel over a single HTTP/2 stream to a remote
host for similar purposes.
A CONNECT request in HTTP/3 functions in the same manner as in HTTP/2. The
request MUST be formatted as described in {{!RFC7540}}, Section 8.3. A CONNECT
request that does not conform to these restrictions is malformed. The request
stream MUST NOT be closed at the end of the request.
A proxy that supports CONNECT establishes a TCP connection ({{!RFC0793}}) to the
server identified in the ":authority" pseudo-header field. Once this connection
is successfully established, the proxy sends a HEADERS frame containing a 2xx
series status code to the client, as defined in {{!RFC7231}}, Section 4.3.6.
All DATA frames on the stream correspond to data sent or received on the TCP
connection. Any DATA frame sent by the client is transmitted by the proxy to the
TCP server; data received from the TCP server is packaged into DATA frames by
the proxy. Note that the size and number of TCP segments is not guaranteed to
map predictably to the size and number of HTTP DATA or QUIC STREAM frames.
The TCP connection can be closed by either peer. When the client ends the
request stream (that is, the receive stream at the proxy enters the "Data Recvd"
state), the proxy will set the FIN bit on its connection to the TCP server. When
the proxy receives a packet with the FIN bit set, it will terminate the send
stream that it sends to the client. TCP connections which remain half-closed in
a single direction are not invalid, but are often handled poorly by servers, so
clients SHOULD NOT close a stream for sending while they still expect to receive
data from the target of the CONNECT.
A TCP connection error is signaled with QUIC RESET_STREAM frame. A proxy treats
any error in the TCP connection, which includes receiving a TCP segment with the
RST bit set, as a stream error of type HTTP_CONNECT_ERROR
({{http-error-codes}}). Correspondingly, a proxy MUST send a TCP segment with
the RST bit set if it detects an error with the stream or the QUIC connection.
## Prioritization {#priority}
HTTP/3 uses a priority scheme similar to that described in {{!RFC7540}}, Section
5.3. In this priority scheme, a given element can be designated as dependent
upon another element. This information is expressed in the PRIORITY frame
{{frame-priority}} which identifies the element and the dependency. The elements
that can be prioritized are:
- Requests, identified by the ID of the request stream
- 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.
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.
Requests may override the default initial values by including a PRIORTIY frame
(see {{frame-priority}}) at the beginning of the stream. These priorities
can be updated by sending a PRIORITY frame on the control stream.
### Placeholders
In HTTP/2, certain implementations used closed or unused streams as placeholders
in describing the relative priority of requests. This created
confusion as servers could not reliably identify which elements of the priority
tree could be discarded safely. Clients could potentially reference closed
streams long after the server had discarded state, leading to disparate views of
the prioritization the client had attempted to express.
In HTTP/3, a number of placeholders are explicitly permitted by the server using
the `SETTINGS_NUM_PLACEHOLDERS` setting. Because the server commits to
maintaining these placeholders in the prioritization tree, clients can use them
with confidence that the server will not have discarded the state. Clients MUST
NOT send the `SETTINGS_NUM_PLACEHOLDERS` setting; receipt of this setting by a
server MUST be treated as a connection error of type
`HTTP_WRONG_SETTING_DIRECTION`.
Placeholders are identified by an ID between zero and one less than the number
of placeholders the server has permitted.
Like streams, placeholders have priority information associated with them.
### Priority Tree Maintenance
Because placeholders will be used to "root" any persistent structure of the tree
which the client cares about retaining, servers can aggressively prune inactive
regions from the priority tree. For prioritization purposes, a node in the tree
is considered "inactive" when the corresponding stream has been closed for at
least two round-trip times (using any reasonable estimate available on the
server). This delay helps mitigate race conditions where the server has pruned
a node the client believed was still active and used as a Stream Dependency.
Specifically, the server MAY at any time:
- Identify and discard branches of the tree containing only inactive nodes
(i.e. a node with only other inactive nodes as descendants, along with those
descendants)
- Identify and condense interior regions of the tree containing only inactive
nodes, allocating weight appropriately
~~~~~~~~~~ drawing
x x x
| | |
P P P
/ \ | |
I I ==> I ==> A
/ \ | |
A I A A
| |
A A
~~~~~~~~~~
{: #fig-pruning title="Example of Priority Tree Pruning"}
In the example in {{fig-pruning}}, `P` represents a Placeholder, `A` represents
an active node, and `I` represents an inactive node. In the first step, the
server discards two inactive branches (each a single node). In the second step,
the server condenses an interior inactive node. Note that these transformations
will result in no change in the resources allocated to a particular active
stream.
Clients SHOULD assume the server is actively performing such pruning and SHOULD
NOT declare a dependency on a stream it knows to have been closed.
## Server Push
HTTP/3 server push is similar to what is described in HTTP/2 {{!RFC7540}}, but
uses different mechanisms.
Each server push is identified by a unique Push ID. This Push ID is used in a
single PUSH_PROMISE frame (see {{frame-push-promise}}) which carries the request
headers, possibly included in one or more DUPLICATE_PUSH frames (see
{{frame-duplicate-push}}), then included with the push stream which ultimately
fulfills those promises.
Server push is only enabled on a connection when a client sends a MAX_PUSH_ID
frame (see {{frame-max-push-id}}). A server cannot use server push until it
receives a MAX_PUSH_ID frame. A client sends additional MAX_PUSH_ID frames to
control the number of pushes that a server can promise. A server SHOULD use Push
IDs sequentially, starting at 0. A client MUST treat receipt of a push stream
with a Push ID that is greater than the maximum Push ID as a connection error of
type HTTP_LIMIT_EXCEEDED.
The header of the request message is carried by a PUSH_PROMISE frame (see
{{frame-push-promise}}) on the request stream which generated the push. This
allows the server push to be associated with a client request. Ordering of a
PUSH_PROMISE in relation to certain parts of the response is important (see
Section 8.2.1 of {{!RFC7540}}). Promised requests MUST conform to the
requirements in Section 8.2 of {{!RFC7540}}.
The same server push can be associated with additional client requests using a
DUPLICATE_PUSH frame (see {{frame-duplicate-push}}). Ordering of a
DUPLICATE_PUSH in relation to certain parts of the response is similarly
important.
When a server later fulfills a promise, the server push response is conveyed on
a push stream (see {{push-streams}}). The push stream identifies the Push ID of
the promise that it fulfills, then contains a response to the promised request
using the same format described for responses in {{request-response}}.
Due to reordering, DUPLICATE_PUSH frames or push stream data can arrive before
the corresponding PUSH_PROMISE frame. When a client receives a DUPLICATE_PUSH
frame for an as-yet-unknown Push ID, the request headers of the push are not
immediately available. The client can either delay generating new requests for
content referenced following the DUPLICATE_PUSH frame until the request headers
become available, or can initiate requests for discovered resources and cancel
the requests if the requested resource is already being pushed. When a client
receives a new push stream with an as-yet-unknown Push ID, both the associated
client request and the pushed request headers are unknown. The client can
buffer the stream data in expectation of the matching PUSH_PROMISE. The client
can use stream flow control (see section 4.1 of {{QUIC-TRANSPORT}}) to limit the
amount of data a server may commit to the pushed stream.
If a promised server push is not needed by the client, the client SHOULD send a
CANCEL_PUSH frame. If the push stream is already open or opens after sending the
CANCEL_PUSH frame, a QUIC STOP_SENDING frame with an appropriate error code can
also be used (e.g., HTTP_PUSH_REFUSED, HTTP_PUSH_ALREADY_IN_CACHE; see
{{errors}}). This asks the server not to transfer additional data and indicates
that it will be discarded upon receipt.
# Connection Closure
Once established, an HTTP/3 connection can be used for many requests and
responses over time until the connection is closed. Connection closure can
happen in any of several different ways.
## Idle Connections
Each QUIC endpoint declares an idle timeout during the handshake. If the
connection remains idle (no packets received) for longer than this duration, the
peer will assume that the connection has been closed. HTTP/3 implementations
will need to open a new connection for new requests if the existing connection
has been idle for longer than the server's advertised idle timeout, and SHOULD
do so if approaching the idle timeout.
HTTP clients are expected to request that the transport keep connections open
while there are responses outstanding for requests or server pushes, as
described in Section 19.2 of {{QUIC-TRANSPORT}}. If the client is not expecting
a response from the server, allowing an idle connection to time out is preferred
over expending effort maintaining a connection that might not be needed. A
gateway MAY maintain connections in anticipation of need rather than incur the
latency cost of connection establishment to servers. Servers SHOULD NOT actively
keep connections open.
## Connection Shutdown
Even when a connection is not idle, either endpoint can decide to stop using the
connection and let the connection close gracefully. Since clients drive request
generation, clients perform a connection shutdown by not sending additional
requests on the connection; responses and pushed responses associated to
previous requests will continue to completion. Servers perform the same
function by communicating with clients.
Servers initiate the shutdown of a connection by sending a GOAWAY frame
({{frame-goaway}}). The GOAWAY frame indicates that client-initiated requests
on lower stream IDs were or might be processed in this connection, while
requests on the indicated stream ID and greater were rejected. This enables
client and server to agree on which requests were accepted prior to the
connection shutdown. This identifier MAY be lower than the stream limit
identified by a QUIC MAX_STREAM_ID frame, and MAY be zero if no requests were
processed. Servers SHOULD NOT increase the QUIC MAX_STREAM_ID limit after
sending a GOAWAY frame.
Clients MUST NOT send new requests on the connection after receiving GOAWAY;
a new connection MAY be established to send additional requests.
Some requests might already be in transit. If the client has already sent
requests on streams with a Stream ID greater than or equal to that indicated in
the GOAWAY frame, those requests will not be processed and MAY be retried by the
client on a different connection. The client MAY cancel these requests. It is
RECOMMENDED that the server explicitly reject such requests (see
{{request-cancellation}}) in order to clean up transport state for the affected
streams.
Requests on Stream IDs less than the Stream ID in the GOAWAY frame might have
been processed; their status cannot be known until a response is received, the
stream is reset individually, or the connection terminates. Servers MAY reject
individual requests on streams below the indicated ID if these requests were not
processed.
Servers SHOULD send a GOAWAY frame when the closing of a connection is known
in advance, even if the advance notice is small, so that the remote peer can
know whether a request has been partially processed or not. For example, if an
HTTP client sends a POST at the same time that a server closes a QUIC
connection, the client cannot know if the server started to process that POST
request if the server does not send a GOAWAY frame to indicate what streams it
might have acted on.
A client that is unable to retry requests loses all requests that are in flight
when the server closes the connection. A server MAY send multiple GOAWAY frames
indicating different stream IDs, but MUST NOT increase the value they send in
the last Stream ID, since clients might already have retried unprocessed
requests on another connection. A server that is attempting to gracefully shut
down a connection SHOULD send an initial GOAWAY frame with the last Stream ID
set to the current value of QUIC's MAX_STREAM_ID and SHOULD NOT increase the
MAX_STREAM_ID thereafter. This signals to the client that a shutdown is
imminent and that initiating further requests is prohibited. After allowing
time for any in-flight requests (at least one round-trip time), the server MAY
send another GOAWAY frame with an updated last Stream ID. This ensures that a
connection can be cleanly shut down without losing requests.
Once all accepted requests have been processed, the server can permit the
connection to become idle, or MAY initiate an immediate closure of the
connection. An endpoint that completes a graceful shutdown SHOULD use the
HTTP_NO_ERROR code when closing the connection.
If a client has consumed all available bidirectional stream IDs with requests,
the server need not send a GOAWAY frame, since the client is unable to make
further requests.
## Immediate Application Closure
An HTTP/3 implementation can immediately close the QUIC connection at any time.
This results in sending a QUIC CONNECTION_CLOSE frame to the peer; the error
code in this frame indicates to the peer why the connection is being closed.
See {{errors}} for error codes which can be used when closing a connection.
Before closing the connection, a GOAWAY MAY be sent to allow the client to retry
some requests. Including the GOAWAY frame in the same packet as the QUIC
CONNECTION_CLOSE frame improves the chances of the frame being received by
clients.
## Transport Closure
For various reasons, the QUIC transport could indicate to the application layer
that the connection has terminated. This might be due to an explicit closure
by the peer, a transport-level error, or a change in network topology which
interrupts connectivity.
If a connection terminates without a GOAWAY frame, clients MUST assume that any
request which was sent, whether in whole or in part, might have been processed.
# Extensions to HTTP/3 {#extensions}
HTTP/3 permits extension of the protocol. Within the limitations described in
this section, protocol extensions can be used to provide additional services or
alter any aspect of the protocol. Extensions are effective only within the
scope of a single HTTP/3 connection.
This applies to the protocol elements defined in this document. This does not
affect the existing options for extending HTTP, such as defining new methods,
status codes, or header fields.
Extensions are permitted to use new frame types ({{frames}}), new settings
({{settings-parameters}}), new error codes ({{errors}}), or new unidirectional
stream types ({{unidirectional-streams}}). Registries are established for
managing these extension points: frame types ({{iana-frames}}), settings
({{iana-settings}}), error codes ({{iana-error-codes}}), and stream types
({{iana-stream-types}}).
Implementations MUST ignore unknown or unsupported values in all extensible
protocol elements. Implementations MUST discard frames and unidirectional
streams that have unknown or unsupported types. This means that any of these
extension points can be safely used by extensions without prior arrangement or
negotiation.
Extensions that could change the semantics of existing protocol components MUST
be negotiated before being used. For example, an extension that changes the
layout of the HEADERS frame cannot be used until the peer has given a positive
signal that this is acceptable. In this case, it could also be necessary to
coordinate when the revised layout comes into effect.
This document doesn't mandate a specific method for negotiating the use of an
extension but notes that a setting ({{settings-parameters}}) could be used for
that purpose. If both peers set a value that indicates willingness to use the
extension, then the extension can be used. If a setting is used for extension
negotiation, the default value MUST be defined in such a fashion that the
extension is disabled if the setting is omitted.
# Error Handling {#errors}
QUIC allows the application to abruptly terminate (reset) individual streams or
the entire connection when an error is encountered. These are referred to as
"stream errors" or "connection errors" and are described in more detail in
{{QUIC-TRANSPORT}}. An endpoint MAY choose to treat a stream error as a
connection error.
This section describes HTTP/3-specific error codes which can be used to express
the cause of a connection or stream error.
## HTTP/3 Error Codes {#http-error-codes}
The following error codes are defined for use in QUIC RESET_STREAM frames,
STOP_SENDING frames, and CONNECTION_CLOSE frames when using HTTP/3.
HTTP_NO_ERROR (0x00):
: No error. This is used when the connection or stream needs to be closed, but
there is no error to signal.
HTTP_WRONG_SETTING_DIRECTION (0x01):
: A client-only setting was sent by a server, or a server-only setting by a
client.
HTTP_PUSH_REFUSED (0x02):
: The server has attempted to push content which the client will not accept
on this connection.
HTTP_INTERNAL_ERROR (0x03):
: An internal error has occurred in the HTTP stack.
HTTP_PUSH_ALREADY_IN_CACHE (0x04):
: The server has attempted to push content which the client has cached.
HTTP_REQUEST_CANCELLED (0x05):
: The request or its response is cancelled.
HTTP_INCOMPLETE_REQUEST (0x06):
: The client's stream terminated without containing a fully-formed request.
HTTP_CONNECT_ERROR (0x07):
: The connection established in response to a CONNECT request was reset or
abnormally closed.
HTTP_EXCESSIVE_LOAD (0x08):
: The endpoint detected that its peer is exhibiting a behavior that might be
generating excessive load.
HTTP_VERSION_FALLBACK (0x09):
: The requested operation cannot be served over HTTP/3. The
peer should retry over HTTP/1.1.
HTTP_WRONG_STREAM (0x0A):
: A frame was received on a stream where it is not permitted.
HTTP_LIMIT_EXCEEDED (0x0B):
: A Stream ID, Push ID, or Placeholder ID greater than the current maximum for
that identifier was referenced.
HTTP_DUPLICATE_PUSH (0x0C):
: A Push ID was referenced in two different stream headers.
HTTP_UNKNOWN_STREAM_TYPE (0x0D):
: A unidirectional stream header contained an unknown stream type.
HTTP_WRONG_STREAM_COUNT (0x0E):
: A unidirectional stream type was used more times than is permitted by that
type.
HTTP_CLOSED_CRITICAL_STREAM (0x0F):
: A stream required by the connection was closed or reset.
HTTP_WRONG_STREAM_DIRECTION (0x0010):
: A unidirectional stream type was used by a peer which is not permitted to do
so.
HTTP_EARLY_RESPONSE (0x0011):
: The remainder of the client's request is not needed to produce a response.
For use in STOP_SENDING only.
HTTP_MISSING_SETTINGS (0x0012):
: No SETTINGS frame was received at the beginning of the control stream.
HTTP_UNEXPECTED_FRAME (0x0013):
: A frame was received which was not permitted in the current state.
HTTP_REQUEST_REJECTED (0x0014):
: A server rejected a request without performing any application processing.
HTTP_GENERAL_PROTOCOL_ERROR (0x00FF):
: Peer violated protocol requirements in a way which doesn't match a more
specific error code, or endpoint declines to use the more specific error code.
HTTP_MALFORMED_FRAME (0x01XX):
: An error in a specific frame type. If the frame type is `0xfe` or less, the
type is included as the last byte of the error code. For example, an error in
a MAX_PUSH_ID frame would be indicated with the code (0x10D). The last byte
`0xff` is used to indicate any frame type greater than `0xfe`.
# Security Considerations
The security considerations of HTTP/3 should be comparable to those of HTTP/2
with TLS. Note that where HTTP/2 employs PADDING frames and Padding fields in
other frames to make a connection more resistant to traffic analysis, HTTP/3 can
rely on QUIC PADDING frames or employ the reserved frame and stream types
discussed in {{frame-grease}} and {{stream-grease}}.
When HTTP Alternative Services is used for discovery for HTTP/3 endpoints, the
security considerations of {{!ALTSVC}} also apply.
Several protocol elements contain nested length elements, typically in the form
of frames with an explicit length containing variable-length integers. This
could pose a security risk to an incautious implementer. An implementation MUST
ensure that the length of a frame exactly matches the length of the fields it
contains.
The use of 0-RTT with HTTP/3 creates an exposure to replay attack. The
anti-replay mitigations in {{!HTTP-REPLAY=RFC8470}} MUST be applied when using
HTTP/3 with 0-RTT.
Certain HTTP implementations use the client address for logging or
access-control purposes. Since a QUIC client's address might change during a
connection (and future versions might support simultaneous use of multiple
addresses), such implementations will need to either actively retrieve the
client's current address or addresses when they are relevant or explicitly
accept that the original address might change.
# IANA Considerations
## Registration of HTTP/3 Identification String
This document creates a new registration for the identification of
HTTP/3 in the "Application Layer Protocol Negotiation (ALPN)
Protocol IDs" registry established in {{?RFC7301}}.
The "h3" string identifies HTTP/3:
Protocol:
: HTTP/3
Identification Sequence:
: 0x68 0x33 ("h3")
Specification:
: This document
## Registration of QUIC Version Hint Alt-Svc Parameter
This document creates a new registration for version-negotiation hints in the
"Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter" registry established in
{{!RFC7838}}.
Parameter:
: "quic"
Specification:
: This document, {{alt-svc-version-hint}}
## Frame Types {#iana-frames}
This document establishes a registry for HTTP/3 frame type codes. The "HTTP/3
Frame Type" registry governs a 62-bit space. This space is split into three
spaces that are governed by different policies. Values between `0x00` and `0x3f`
(in hexadecimal) are assigned via the Standards Action or IESG Review policies
{{!RFC8126}}. Values from `0x40` to `0x3fff` operate on the Specification
Required policy {{!RFC8126}}. All other values are assigned to Private Use
{{!RFC8126}}.
While this registry is separate from the "HTTP/2 Frame Type" registry defined in
{{RFC7540}}, it is preferable that the assignments parallel each other where the
code spaces overlap. If an entry is present in only one registry, every effort
SHOULD be made to avoid assigning the corresponding value to an unrelated
operation.
New entries in this registry require the following information:
Frame Type:
: A name or label for the frame type.
Code:
: The 62-bit code assigned to the frame type.
Specification:
: A reference to a specification that includes a description of the frame layout
and its semantics, including any parts of the frame that are conditionally
present.
The entries in the following table are registered by this document.
| ---------------- | ------ | -------------------------- |
| Frame Type | Code | Specification |
| ---------------- | :----: | -------------------------- |
| DATA | 0x0 | {{frame-data}} |
| HEADERS | 0x1 | {{frame-headers}} |
| PRIORITY | 0x2 | {{frame-priority}} |
| CANCEL_PUSH | 0x3 | {{frame-cancel-push}} |
| SETTINGS | 0x4 | {{frame-settings}} |
| PUSH_PROMISE | 0x5 | {{frame-push-promise}} |
| Reserved | 0x6 | N/A |
| GOAWAY | 0x7 | {{frame-goaway}} |
| Reserved | 0x8 | N/A |
| Reserved | 0x9 | N/A |
| MAX_PUSH_ID | 0xD | {{frame-max-push-id}} |
| DUPLICATE_PUSH | 0xE | {{frame-duplicate-push}} |
| ---------------- | ------ | -------------------------- |
Additionally, each code of the format `0x1f * N + 0x21` for integer values of N
(that is, `0x21`, `0x40`, ..., through `0x3FFFFFFFFFFFFFFE`) MUST NOT be
assigned by IANA.
## Settings Parameters {#iana-settings}
This document establishes a registry for HTTP/3 settings. The "HTTP/3 Settings"
registry governs a 62-bit space. This space is split into three spaces that are
governed by different policies. Values between `0x00` and `0x3f` (in
hexadecimal) are assigned via the Standards Action or IESG Review policies
{{!RFC8126}}. Values from `0x40` to `0x3fff` operate on the Specification
Required policy {{!RFC8126}}. All other values are assigned to Private Use
{{!RFC8126}}. The designated experts are the same as those for the "HTTP/2
Settings" registry defined in {{RFC7540}}.
While this registry is separate from the "HTTP/2 Settings" registry defined in
{{RFC7540}}, it is preferable that the assignments parallel each other. If an
entry is present in only one registry, every effort SHOULD be made to avoid
assigning the corresponding value to an unrelated operation.
New registrations are advised to provide the following information:
Name:
: A symbolic name for the setting. Specifying a setting name is optional.
Code:
: The 62-bit code assigned to the setting.
Specification:
: An optional reference to a specification that describes the use of the
setting.
The entries in the following table are registered by this document.
| ---------------------------- | ------ | ------------------------- |
| Setting Name | Code | Specification |
| ---------------------------- | :----: | ------------------------- |
| Reserved | 0x2 | N/A |
| Reserved | 0x3 | N/A |
| Reserved | 0x4 | N/A |
| Reserved | 0x5 | N/A |
| MAX_HEADER_LIST_SIZE | 0x6 | {{settings-parameters}} |
| NUM_PLACEHOLDERS | 0x9 | {{settings-parameters}} |
| ---------------------------- | ------ | ------------------------- |
Additionally, each code of the format `0x1f * N + 0x21` for integer values of N
(that is, `0x21`, `0x40`, ..., through `0x3FFFFFFFFFFFFFFE`) MUST NOT be
assigned by IANA.
## Error Codes {#iana-error-codes}
This document establishes a registry for HTTP/3 error codes. The "HTTP/3 Error
Code" registry manages a 16-bit space. The "HTTP/3 Error Code" registry
operates under the "Expert Review" policy {{?RFC8126}}.
Registrations for error codes are required to include a description
of the error code. An expert reviewer is advised to examine new
registrations for possible duplication with existing error codes.
Use of existing registrations is to be encouraged, but not mandated.
New registrations are advised to provide the following information:
Name:
: A name for the error code. Specifying an error code name is optional.
Code:
: The 16-bit error code value.
Description:
: A brief description of the error code semantics, longer if no detailed
specification is provided.
Specification:
: An optional reference for a specification that defines the error code.
The entries in the following table are registered by this document.
| ----------------------------------- | ---------- | ---------------------------------------- | ---------------------- |
| Name | Code | Description | Specification |
| ----------------------------------- | ---------- | ---------------------------------------- | ---------------------- |
| HTTP_NO_ERROR | 0x0000 | No error | {{http-error-codes}} |
| HTTP_WRONG_SETTING_DIRECTION | 0x0001 | Setting sent in wrong direction | {{http-error-codes}} |
| HTTP_PUSH_REFUSED | 0x0002 | Client refused pushed content | {{http-error-codes}} |
| HTTP_INTERNAL_ERROR | 0x0003 | Internal error | {{http-error-codes}} |
| HTTP_PUSH_ALREADY_IN_CACHE | 0x0004 | Pushed content already cached | {{http-error-codes}} |
| HTTP_REQUEST_CANCELLED | 0x0005 | Data no longer needed | {{http-error-codes}} |
| HTTP_INCOMPLETE_REQUEST | 0x0006 | Stream terminated early | {{http-error-codes}} |
| HTTP_CONNECT_ERROR | 0x0007 | TCP reset or error on CONNECT request | {{http-error-codes}} |
| HTTP_EXCESSIVE_LOAD | 0x0008 | Peer generating excessive load | {{http-error-codes}} |
| HTTP_VERSION_FALLBACK | 0x0009 | Retry over HTTP/1.1 | {{http-error-codes}} |
| HTTP_WRONG_STREAM | 0x000A | A frame was sent on the wrong stream | {{http-error-codes}} |
| HTTP_LIMIT_EXCEEDED | 0x000B | An identifier limit was exceeded | {{http-error-codes}} |
| HTTP_DUPLICATE_PUSH | 0x000C | Push ID was fulfilled multiple times | {{http-error-codes}} |
| HTTP_UNKNOWN_STREAM_TYPE | 0x000D | Unknown unidirectional stream type | {{http-error-codes}} |
| HTTP_WRONG_STREAM_COUNT | 0x000E | Too many unidirectional streams | {{http-error-codes}} |
| HTTP_CLOSED_CRITICAL_STREAM | 0x000F | Critical stream was closed | {{http-error-codes}} |
| HTTP_WRONG_STREAM_DIRECTION | 0x0010 | Unidirectional stream in wrong direction | {{http-error-codes}} |
| HTTP_EARLY_RESPONSE | 0x0011 | Remainder of request not needed | {{http-error-codes}} |
| HTTP_MISSING_SETTINGS | 0x0012 | No SETTINGS frame received | {{http-error-codes}} |
| HTTP_UNEXPECTED_FRAME | 0x0013 | Frame not permitted in the current state | {{http-error-codes}} |
| HTTP_REQUEST_REJECTED | 0x0014 | Request not processed | {{http-error-codes}} |
| HTTP_MALFORMED_FRAME | 0x01XX | Error in frame formatting | {{http-error-codes}} |
| ----------------------------------- | ---------- | ---------------------------------------- | ---------------------- |
## Stream Types {#iana-stream-types}
This document establishes a registry for HTTP/3 unidirectional stream types. The
"HTTP/3 Stream Type" registry governs a 62-bit space. This space is split into
three spaces that are governed by different policies. Values between `0x00` and
0x3f (in hexadecimal) are assigned via the Standards Action or IESG Review
policies {{!RFC8126}}. Values from `0x40` to `0x3fff` operate on the
Specification Required policy {{!RFC8126}}. All other values are assigned to
Private Use {{!RFC8126}}.
New entries in this registry require the following information:
Stream Type:
: A name or label for the stream type.
Code:
: The 62-bit code assigned to the stream type.
Specification:
: A reference to a specification that includes a description of the stream type,
including the layout semantics of its payload.
Sender:
: Which endpoint on a connection may initiate a stream of this type. Values are
"Client", "Server", or "Both".
The entries in the following table are registered by this document.
| ---------------- | ------ | -------------------------- | ------ |
| Stream Type | Code | Specification | Sender |
| ---------------- | :----: | -------------------------- | ------ |
| Control Stream | 0x00 | {{control-streams}} | Both |
| Push Stream | 0x01 | {{server-push}} | Server |
| ---------------- | ------ | -------------------------- | ------ |
Additionally, each code of the format `0x1f * N + 0x21` for integer values of N
(that is, `0x21`, `0x40`, ..., through `0x3FFFFFFFFFFFFFFE`) MUST NOT be
assigned by IANA.
--- back
# Considerations for Transitioning from HTTP/2
HTTP/3 is strongly informed by HTTP/2, and bears many similarities. This
section describes the approach taken to design HTTP/3, points out important
differences from HTTP/2, and describes how to map HTTP/2 extensions into HTTP/3.
HTTP/3 begins from the premise that similarity to HTTP/2 is preferable, but not
a hard requirement. HTTP/3 departs from HTTP/2 primarily where necessary to
accommodate the differences in behavior between QUIC and TCP (lack of ordering,
support for streams). We intend to avoid gratuitous changes which make it
difficult or impossible to build extensions with the same semantics applicable
to both protocols at once.
These departures are noted in this section.
## Streams {#h2-streams}
HTTP/3 permits use of a larger number of streams (2^62-1) than HTTP/2. The
considerations about exhaustion of stream identifier space apply, though the
space is significantly larger such that it is likely that other limits in QUIC
are reached first, such as the limit on the connection flow control window.
## HTTP Frame Types {#h2-frames}
Many framing concepts from HTTP/2 can be elided on QUIC, because the transport
deals with them. Because frames are already on a stream, they can omit the
stream number. Because frames do not block multiplexing (QUIC's multiplexing
occurs below this layer), the support for variable-maximum-length packets can be
removed. Because stream termination is handled by QUIC, an END_STREAM flag is
not required. This permits the removal of the Flags field from the generic
frame layout.
Frame payloads are largely drawn from {{!RFC7540}}. However, QUIC includes many
features (e.g., flow control) which are also present in HTTP/2. In these cases,
the HTTP mapping does not re-implement them. As a result, several HTTP/2 frame
types are not required in HTTP/3. Where an HTTP/2-defined frame is no longer
used, the frame ID has been reserved in order to maximize portability between
HTTP/2 and HTTP/3 implementations. However, even equivalent frames between the
two mappings are not identical.
Many of the differences arise from the fact that HTTP/2 provides an absolute
ordering between frames across all streams, while QUIC provides this guarantee
on each stream only. As a result, if a frame type makes assumptions that frames
from different streams will still be received in the order sent, HTTP/3 will
break them.
For example, implicit in the HTTP/2 prioritization scheme is the notion of
in-order delivery of priority changes (i.e., dependency tree mutations): since
operations on the dependency tree such as reparenting a subtree are not
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 as the first frame on a request stream or on the
control stream and exclusive prioritization has been removed. HTTP/3 permits the
prioritisation 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.
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
the same order in which they were encoded. This ensures that the dynamic state
at the two endpoints remains in sync. As a result, HTTP/3 uses a modified
version of HPACK, described in [QPACK].
Frame type definitions in HTTP/3 often use the QUIC variable-length integer
encoding. In particular, Stream IDs use this encoding, which allow for a larger
range of possible values than the encoding used in HTTP/2. Some frames in
HTTP/3 use an identifier rather than a Stream ID (e.g. Push IDs in PRIORITY
frames). Redefinition of the encoding of extension frame types might be
necessary if the encoding includes a Stream ID.
Because the Flags field is not present in generic HTTP/3 frames, those frames
which depend on the presence of flags need to allocate space for flags as part
of their frame payload.
Other than this issue, frame type HTTP/2 extensions are typically portable to
QUIC simply by replacing Stream 0 in HTTP/2 with a control stream in HTTP/3.
HTTP/3 extensions will not assume ordering, but would not be harmed by ordering,
and would be portable to HTTP/2 in the same manner.
Below is a listing of how each HTTP/2 frame type is mapped:
DATA (0x0):
: Padding is not defined in HTTP/3 frames. See {{frame-data}}.
HEADERS (0x1):
: The PRIORITY region of HEADERS is not defined in HTTP/3 frames. A separate
PRIORITY frame is used in all cases. Padding is not defined in HTTP/3 frames.
See {{frame-headers}}.
PRIORITY (0x2):
: As described above, the PRIORITY frame references a variety of identifiers. It
is sent as the first frame on a request streams or on the control stream. See
{{frame-priority}}.
RST_STREAM (0x3):
: RST_STREAM frames do not exist, since QUIC provides stream lifecycle
management. The same code point is used for the CANCEL_PUSH frame
({{frame-cancel-push}}).
SETTINGS (0x4):
: SETTINGS frames are sent only at the beginning of the connection. See
{{frame-settings}} and {{h2-settings}}.
PUSH_PROMISE (0x5):
: The PUSH_PROMISE does not reference a stream; instead the push stream
references the PUSH_PROMISE frame using a Push ID. See
{{frame-push-promise}}.
PING (0x6):
: PING frames do not exist, since QUIC provides equivalent functionality.
GOAWAY (0x7):
: GOAWAY is sent only from server to client and does not contain an error code.
See {{frame-goaway}}.
WINDOW_UPDATE (0x8):
: WINDOW_UPDATE frames do not exist, since QUIC provides flow control.
CONTINUATION (0x9):
: CONTINUATION frames do not exist; instead, larger HEADERS/PUSH_PROMISE
frames than HTTP/2 are permitted.
Frame types defined by extensions to HTTP/2 need to be separately registered for
HTTP/3 if still applicable. The IDs of frames defined in {{!RFC7540}} have been
reserved for simplicity. Note that the frame type space in HTTP/3 is
substantially larger (62 bits versus 8 bits), so many HTTP/3 frame types have no
equivalent HTTP/2 code points. See {{iana-frames}}.
## HTTP/2 SETTINGS Parameters {#h2-settings}
An important difference from HTTP/2 is that settings are sent once, at the
beginning of the connection, and thereafter cannot change. This eliminates
many corner cases around synchronization of changes.
Some transport-level options that HTTP/2 specifies via the SETTINGS frame are
superseded by QUIC transport parameters in HTTP/3. The HTTP-level options that
are retained in HTTP/3 have the same value as in HTTP/2.
Below is a listing of how each HTTP/2 SETTINGS parameter is mapped:
SETTINGS_HEADER_TABLE_SIZE:
: See [QPACK].
SETTINGS_ENABLE_PUSH:
: This is removed in favor of the MAX_PUSH_ID which provides a more granular
control over server push.
SETTINGS_MAX_CONCURRENT_STREAMS:
: QUIC controls the largest open Stream ID as part of its flow control logic.
Specifying SETTINGS_MAX_CONCURRENT_STREAMS in the SETTINGS frame is an error.
SETTINGS_INITIAL_WINDOW_SIZE:
: QUIC requires both stream and connection flow control window sizes to be
specified in the initial transport handshake. Specifying
SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame is an error.
SETTINGS_MAX_FRAME_SIZE:
: This setting has no equivalent in HTTP/3. Specifying it in the SETTINGS frame
is an error.
SETTINGS_MAX_HEADER_LIST_SIZE:
: See {{settings-parameters}}.
In HTTP/3, setting values are variable-length integers (6, 14, 30, or 62 bits
long) rather than fixed-length 32-bit fields as in HTTP/2. This will often
produce a shorter encoding, but can produce a longer encoding for settings which
use the full 32-bit space. Settings ported from HTTP/2 might choose to redefine
the format of their settings to avoid using the 62-bit encoding.
Settings need to be defined separately for HTTP/2 and HTTP/3. The IDs of
settings defined in {{!RFC7540}} have been reserved for simplicity. Note that
the settings identifier space in HTTP/3 is substantially larger (62 bits versus
16 bits), so many HTTP/3 settings have no equivalent HTTP/2 code point. See
{{iana-settings}}.
## HTTP/2 Error Codes
QUIC has the same concepts of "stream" and "connection" errors that HTTP/2
provides. However, there is no direct portability of HTTP/2 error codes.
The HTTP/2 error codes defined in Section 7 of {{!RFC7540}} map to the HTTP/3
error codes as follows:
NO_ERROR (0x0):
: HTTP_NO_ERROR in {{http-error-codes}}.
PROTOCOL_ERROR (0x1):
: No single mapping. See new HTTP_MALFORMED_FRAME error codes defined in
{{http-error-codes}}.
INTERNAL_ERROR (0x2):
: HTTP_INTERNAL_ERROR in {{http-error-codes}}.
FLOW_CONTROL_ERROR (0x3):
: Not applicable, since QUIC handles flow control. Would provoke a
QUIC_FLOW_CONTROL_RECEIVED_TOO_MUCH_DATA from the QUIC layer.
SETTINGS_TIMEOUT (0x4):
: Not applicable, since no acknowledgement of SETTINGS is defined.
STREAM_CLOSED (0x5):
: Not applicable, since QUIC handles stream management. Would provoke a
QUIC_STREAM_DATA_AFTER_TERMINATION from the QUIC layer.
FRAME_SIZE_ERROR (0x6):
: HTTP_MALFORMED_FRAME error codes defined in {{http-error-codes}}.
REFUSED_STREAM (0x7):
: HTTP_REQUEST_REJECTED (in {{http-error-codes}}) is used to indicate that a
request was not processed. Otherwise, not applicable because QUIC handles
stream management. A STREAM_ID_ERROR at the QUIC layer is used for streams
that are improperly opened.
CANCEL (0x8):
: HTTP_REQUEST_CANCELLED in {{http-error-codes}}.
COMPRESSION_ERROR (0x9):
: Multiple error codes are defined in [QPACK].
CONNECT_ERROR (0xa):
: HTTP_CONNECT_ERROR in {{http-error-codes}}.
ENHANCE_YOUR_CALM (0xb):
: HTTP_EXCESSIVE_LOAD in {{http-error-codes}}.
INADEQUATE_SECURITY (0xc):
: Not applicable, since QUIC is assumed to provide sufficient security on all
connections.
HTTP_1_1_REQUIRED (0xd):
: HTTP_VERSION_FALLBACK in {{http-error-codes}}.
Error codes need to be defined for HTTP/2 and HTTP/3 separately. See
{{iana-error-codes}}.
# Change Log
> **RFC Editor's Note:** Please remove this section prior to publication of a
> final version of this document.
## Since draft-ietf-quic-http-19
- SETTINGS_NUM_PLACEHOLDERS is 0x9 (#2443,#2530)
- Non-zero bits in the Empty field of the PRIORITY frame MAY be treated as an
error (#2501)
## Since draft-ietf-quic-http-18
- Resetting streams following a GOAWAY is recommended, but not required
(#2256,#2457)
- Use variable-length integers throughout (#2437,#2233,#2253,#2275)
- Variable-length frame types, stream types, and settings identifiers
- Renumbered stream type assignments
- Modified associated reserved values
- Frame layout switched from Length-Type-Value to Type-Length-Value
(#2395,#2235)
- Specified error code for servers receiving DUPLICATE_PUSH (#2497)
- Use connection error for invalid PRIORITY (#2507, #2508)
## Since draft-ietf-quic-http-17
- HTTP_REQUEST_REJECTED is used to indicate a request can be retried (#2106,
#2325)
- Changed error code for GOAWAY on the wrong stream (#2231, #2343)
## Since draft-ietf-quic-http-16
- Rename "HTTP/QUIC" to "HTTP/3" (#1973)
- Changes to PRIORITY frame (#1865, #2075)
- Permitted as first frame of request streams
- Remove exclusive reprioritization
- Changes to Prioritized Element Type bits
- Define DUPLICATE_PUSH frame to refer to another PUSH_PROMISE (#2072)
- Set defaults for settings, allow request before receiving SETTINGS (#1809,
#1846, #2038)
- Clarify message processing rules for streams that aren't closed (#1972, #2003)
- Removed reservation of error code 0 and moved HTTP_NO_ERROR to this value
(#1922)
- Removed prohibition of zero-length DATA frames (#2098)
## Since draft-ietf-quic-http-15
Substantial editorial reorganization; no technical changes.
## Since draft-ietf-quic-http-14
- Recommend sensible values for QUIC transport parameters (#1720,#1806)
- Define error for missing SETTINGS frame (#1697,#1808)
- Setting values are variable-length integers (#1556,#1807) and do not have
separate maximum values (#1820)
- Expanded discussion of connection closure (#1599,#1717,#1712)
- HTTP_VERSION_FALLBACK falls back to HTTP/1.1 (#1677,#1685)
## Since draft-ietf-quic-http-13
- Reserved some frame types for grease (#1333, #1446)
- Unknown unidirectional stream types are tolerated, not errors; some reserved
for grease (#1490, #1525)
- Require settings to be remembered for 0-RTT, prohibit reductions (#1541,
#1641)
- Specify behavior for truncated requests (#1596, #1643)
## Since draft-ietf-quic-http-12
- TLS SNI extension isn't mandatory if an alternative method is used (#1459,
#1462, #1466)
- Removed flags from HTTP/3 frames (#1388, #1398)
- Reserved frame types and settings for use in preserving extensibility (#1333,
#1446)
- Added general error code (#1391, #1397)
- Unidirectional streams carry a type byte and are extensible (#910,#1359)
- Priority mechanism now uses explicit placeholders to enable persistent
structure in the tree (#441,#1421,#1422)
## Since draft-ietf-quic-http-11
- Moved QPACK table updates and acknowledgments to dedicated streams (#1121,
#1122, #1238)
## Since draft-ietf-quic-http-10
- Settings need to be remembered when attempting and accepting 0-RTT (#1157,
#1207)
## Since draft-ietf-quic-http-09
- Selected QCRAM for header compression (#228, #1117)
- The server_name TLS extension is now mandatory (#296, #495)
- Specified handling of unsupported versions in Alt-Svc (#1093, #1097)
## Since draft-ietf-quic-http-08
- Clarified connection coalescing rules (#940, #1024)
## Since draft-ietf-quic-http-07
- Changes for integer encodings in QUIC (#595,#905)
- Use unidirectional streams as appropriate (#515, #240, #281, #886)
- Improvement to the description of GOAWAY (#604, #898)
- Improve description of server push usage (#947, #950, #957)
## Since draft-ietf-quic-http-06
- Track changes in QUIC error code usage (#485)
## Since draft-ietf-quic-http-05
- Made push ID sequential, add MAX_PUSH_ID, remove SETTINGS_ENABLE_PUSH (#709)
- Guidance about keep-alive and QUIC PINGs (#729)
- Expanded text on GOAWAY and cancellation (#757)
## Since draft-ietf-quic-http-04
- Cite RFC 5234 (#404)
- Return to a single stream per request (#245,#557)
- Use separate frame type and settings registries from HTTP/2 (#81)
- SETTINGS_ENABLE_PUSH instead of SETTINGS_DISABLE_PUSH (#477)
- Restored GOAWAY (#696)
- Identify server push using Push ID rather than a stream ID (#702,#281)
- DATA frames cannot be empty (#700)
## Since draft-ietf-quic-http-03
None.
## Since draft-ietf-quic-http-02
- Track changes in transport draft
## Since draft-ietf-quic-http-01
- SETTINGS changes (#181):
- SETTINGS can be sent only once at the start of a connection;
no changes thereafter
- SETTINGS_ACK removed
- Settings can only occur in the SETTINGS frame a single time
- Boolean format updated
- Alt-Svc parameter changed from "v" to "quic"; format updated (#229)
- Closing the connection control stream or any message control stream is a
fatal error (#176)
- HPACK Sequence counter can wrap (#173)
- 0-RTT guidance added
- Guide to differences from HTTP/2 and porting HTTP/2 extensions added
(#127,#242)
## Since draft-ietf-quic-http-00
- Changed "HTTP/2-over-QUIC" to "HTTP/QUIC" throughout (#11,#29)
- Changed from using HTTP/2 framing within Stream 3 to new framing format and
two-stream-per-request model (#71,#72,#73)
- Adopted SETTINGS format from draft-bishop-httpbis-extended-settings-01
- Reworked SETTINGS_ACK to account for indeterminate inter-stream order (#75)
- Described CONNECT pseudo-method (#95)
- Updated ALPN token and Alt-Svc guidance (#13,#87)
- Application-layer-defined error codes (#19,#74)
## Since draft-shade-quic-http2-mapping-00
- Adopted as base for draft-ietf-quic-http
- Updated authors/editors list
# Acknowledgements
{:numbered="false"}
The original authors of this specification were Robbie Shade and Mike Warres.
A substantial portion of Mike's contribution was supported by Microsoft during
his employment there.
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