---
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:

  BREACH:
    title: "BREACH: Reviving the CRIME Attack"
    date: "July 2013"
    target: http://breachattack.com/resources/BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf
    author:
      -
        ins: Y. Gluck
      -
        ins: N. Harris
      -
        ins: A. Prado

--- 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_DO_NOT_DEPLOY_THIS_VERSION_OF_HTTP

DO NOT DEPLOY THIS VERSION OF HTTP/3 UNTIL IT IS IN AN RFC. This version is
still a work in progress. For trial deployments, please use earlier versions.

--- 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 ({{!SEMANTICS=I-D.ietf-httpbis-semantics}}) are used for a broad
range of services on the Internet. These semantics have most commonly been used
with HTTP/1.1, over a variety of transport and session layers, and with HTTP/2
over TLS. HTTP/3 supports the same semantics over a new transport protocol,
QUIC.

## Prior versions of HTTP

HTTP/1.1 ({{?HTTP11=I-D.ietf-httpbis-messaging}}) uses whitespace-delimited text
fields to convey HTTP messages.  While these exchanges are human-readable, using
whitespace for message formatting leads to parsing complexity and excessive
tolerance of variant behavior.  Because HTTP/1.x does not include a multiplexing
layer, multiple TCP connections are often used to service requests in parallel.
However, that has a negative impact on congestion control and network
efficiency, since TCP does not share congestion control across multiple
connections.

HTTP/2 ({{?HTTP2=RFC7540}}) introduced a binary framing and multiplexing layer
to improve latency without modifying the transport layer.  However, because the
parallel nature of HTTP/2's multiplexing is not visible to TCP's loss recovery
mechanisms, a lost or reordered packet causes all active transactions to
experience a stall regardless of whether that transaction was directly impacted
by the lost packet.

## Delegation to QUIC

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, QUIC has the capability to improve the performance of HTTP compared
to a TCP mapping.  QUIC also incorporates TLS 1.3 ({{?TLS13=RFC8446}}) at the
transport layer, offering comparable confidentiality and integrity to running
TLS over TCP, with the improved connection setup latency of TCP Fast Open
({{?TFO=RFC7413}}).

This document defines a mapping of HTTP semantics over the QUIC transport
protocol, drawing heavily on the design of HTTP/2.  While delegating stream
lifetime and flow control issues to QUIC, a similar binary framing is used on
each stream. Some HTTP/2 features are subsumed by QUIC, while other features are
implemented atop QUIC.

QUIC is described in {{QUIC-TRANSPORT}}.  For a full description of HTTP/2, see
{{?HTTP2}}.

# HTTP/3 Protocol Overview

HTTP/3 provides a transport for HTTP semantics using the QUIC transport protocol
and an internal framing layer similar to HTTP/2.

Once a client knows that an HTTP/3 server exists at a certain endpoint, it opens
a QUIC connection. QUIC provides protocol negotiation, stream-based
multiplexing, and flow control.  Discovery of an HTTP/3 endpoint is described in
{{discovery}}.

Within each stream, the basic unit of HTTP/3 communication is a frame
({{frames}}).  Each frame type serves a different purpose.  For example, HEADERS
and DATA frames form the basis of HTTP requests and responses
({{request-response}}).

Multiplexing of requests is performed using the QUIC stream abstraction,
described in Section 2 of {{QUIC-TRANSPORT}}.  Each request-response pair
consumes a single QUIC stream.  Streams are independent of each other, so one
stream that is blocked or suffers packet loss does not prevent progress on other
streams.

Server push is an interaction mode introduced in HTTP/2 ({{?HTTP2}}) that
permits a server to push a request-response exchange to a client in anticipation
of the client making the indicated request.  This trades off network usage
against a potential latency gain.  Several HTTP/3 frames are used to manage
server push, such as PUSH_PROMISE, MAX_PUSH_ID, and CANCEL_PUSH.

As in HTTP/2, request and response fields are compressed for transmission.
Because HPACK ({{?HPACK=RFC7541}}) relies on in-order transmission of compressed
field sections (a guarantee not provided by QUIC), HTTP/3 replaces HPACK with
QPACK ([QPACK]).  QPACK uses separate unidirectional streams to modify and track
field table state, while encoded field sections refer to the state of the table
without modifying it.

## Document Organization

The following sections provide a detailed overview of the lifecycle of an HTTP/3
connection:

- Connection Setup and Management ({{connection-setup}}) covers how an HTTP/3
  endpoint is discovered and an HTTP/3 connection is established.
- HTTP Request Lifecycle ({{http-request-lifecycle}}) describes how HTTP
  semantics are expressed using frames.
- Connection Closure ({{connection-closure}}) describes how HTTP/3 connections
  are terminated, either gracefully or abruptly.

The details of the wire protocol and interactions with the transport are
described in subsequent sections:

- Stream Mapping and Usage ({{stream-mapping}}) describes the way QUIC streams
  are used.
- HTTP Framing Layer ({{http-framing-layer}}) describes the frames used on
  most streams.
- Error Handling ({{errors}}) describes how error conditions are handled and
  expressed, either on a particular stream or for the connection as a whole.

Additional resources are provided in the final sections:

- Extensions to HTTP/3 ({{extensions}}) describes how new capabilities can be
  added in future documents.
- A more detailed comparison between HTTP/2 and HTTP/3 can be found in
  {{h2-considerations}}.

## Conventions and Terminology

{::boilerplate bcp14}

This document uses the variable-length integer encoding from
{{QUIC-TRANSPORT}}.

The following terms are used:

abort:
: An abrupt termination of a connection or stream, possibly due to an error
  condition.

client:
: The endpoint that initiates an HTTP/3 connection.  Clients send HTTP requests
  and receive HTTP responses.

connection:
: A transport-layer connection between two endpoints, using QUIC as the
  transport protocol.

connection error:
: An error that affects the entire HTTP/3 connection.

endpoint:
: Either the client or server of the connection.

frame:
: The smallest unit of communication on a stream in HTTP/3, consisting of a
  header and a variable-length sequence of bytes structured according to the
  frame type.

  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}}.

HTTP/3 connection:
: A QUIC connection where the negotiated application protocol is HTTP/3.

peer:
: An endpoint.  When discussing a particular endpoint, "peer" refers to the
  endpoint that is remote to the primary subject of discussion.

receiver:
: An endpoint that is receiving frames.

sender:
: An endpoint that is transmitting frames.

server:
: The endpoint that accepts an HTTP/3 connection.  Servers receive HTTP requests
  and send HTTP responses.

stream:
: A bidirectional or unidirectional bytestream provided by the QUIC transport.
  All streams within an HTTP/3 connection can be considered "HTTP/3 streams,"
  but multiple stream types are defined within HTTP/3.

stream error:
: An application-level error on the individual stream.

The term "content" is defined in Section 6.4 of {{!SEMANTICS}}.

Finally, the terms "resource", "message", "user agent", "origin server",
"gateway", "intermediary", "proxy", and "tunnel" are defined in Section 3 of
{{!SEMANTICS}}.

Packet diagrams in this document use the format defined in Section 1.3 of
{{QUIC-TRANSPORT}} to illustrate the order and size of fields.


# Connection Setup and Management {#connection-setup}

## Discovering an HTTP/3 Endpoint {#discovery}

HTTP relies on the notion of an authoritative response: a response that has been
determined to be the most appropriate response for that request given the state
of the target resource at the time of response message origination by (or at the
direction of) the origin server identified within the target URI.  Locating an
authoritative server for an HTTP URL is discussed in Section 4.3 of
{{!SEMANTICS}}.

The "https" scheme associates authority with possession of a certificate that
the client considers to be trustworthy for the host identified by the authority
component of the URL.

If a server presents a valid certificate and proof that it controls the
corresponding private key, then a client will accept a secured TLS session with
that server as being authoritative for all origins with the "https" scheme and a
host identified in the certificate.  The host must be listed either as the CN
field of the certificate subject or as a dNSName in the subjectAltName field of
the certificate; see {{!RFC6125}}.  For a host that is an IP address, the client
MUST verify that the address appears as an iPAddress in the subjectAltName field
of the certificate.

If the hostname or address is not present in the certificate, the client MUST
NOT consider the server authoritative for origins containing that hostname or
address.  See Section 4.3 of {{!SEMANTICS}} for more detail on authoritative
access.

A client MAY attempt access to a resource with an "https" URI by resolving the
host identifier to an IP address, establishing a QUIC connection to that address
on the indicated port, and sending an HTTP/3 request message targeting the URI
to the server over that secured connection.  Unless some other mechanism is used
to select HTTP/3, the token "h3" is used in the Application Layer Protocol
Negotiation (ALPN; see {{!RFC7301}}) extension during the TLS handshake.

Connectivity problems (e.g., blocking UDP) can result in QUIC connection
establishment failure; clients SHOULD attempt to use TCP-based versions of HTTP
in this case.

Servers MAY serve HTTP/3 on any UDP port; an alternative service advertisement
always includes an explicit port, and URLs contain either an explicit port or a
default port associated with the scheme.

### HTTP Alternative Services {#alt-svc}

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}}),
using the "h3" ALPN token.

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; if this
connection is successful, the client can send HTTP requests using the mapping
described in this document.

### Other Schemes

Although HTTP is independent of the transport protocol, the "http" scheme
associates authority with the ability to receive TCP connections on the
indicated port of whatever host is identified within the authority component.
Because HTTP/3 does not use TCP, HTTP/3 cannot be used for direct access to the
authoritative server for a resource identified by an "http" URI.  However,
protocol extensions such as {{!ALTSVC=RFC7838}} permit the authoritative server
to identify other services that are also authoritative and that might be
reachable over HTTP/3.

Prior to making requests for an origin whose scheme is not "https", the client
MUST ensure the server is willing to serve that scheme. For origins whose scheme
is "http", an experimental method to accomplish this is described in
{{?RFC8164}}. Other mechanisms might be defined for various schemes in the
future.


## Connection Establishment {#connection-establishment}

HTTP/3 relies on QUIC version 1 as the underlying transport.  The use of other
QUIC transport versions with HTTP/3 MAY be defined by future specifications.

QUIC version 1 uses TLS version 1.3 or greater as its handshake protocol.
HTTP/3 clients MUST support a mechanism to indicate the target host to the
server during the TLS handshake.  If the server is identified by a domain name
({{?DNS-TERMS=RFC8499}}), clients MUST send the Server Name Indication (SNI;
{{!RFC6066}}) TLS extension unless an alternative mechanism to indicate the
target host is used.

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

HTTP/3 connections are persistent across multiple requests.  For best
performance, it is expected that clients will not close connections until it is
determined that no further communication with a server is necessary (for
example, when a user navigates away from a particular web page) or until the
server closes the connection.

Once a connection exists to a server endpoint, this connection MAY be reused for
requests with multiple different URI authority components.  Clients SHOULD NOT
open more than one HTTP/3 connection to a given host and port pair, where the
host is derived from a URI, a selected alternative service ({{!ALTSVC}}), or a
configured proxy.  A client MAY open multiple HTTP/3 connections to the same IP
address and UDP port using different transport or TLS configurations but SHOULD
avoid creating multiple connections with the same configuration.

Servers are encouraged to maintain open HTTP/3 connections for as long as
possible but are permitted to terminate idle connections if necessary.  When
either endpoint chooses to close the HTTP/3 connection, the terminating endpoint
SHOULD first send a GOAWAY frame ({{connection-shutdown}}) so that both
endpoints can reliably determine whether previously sent frames have been
processed and gracefully complete or terminate any necessary remaining tasks.

A server that does not wish clients to reuse HTTP/3 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 {{?HTTP2}}.


# HTTP Request Lifecycle

## HTTP Message Exchanges {#request-response}

A client sends an HTTP request on a request stream, which is a client-initiated
bidirectional QUIC stream; see {{request-streams}}.  A client MUST send only a
single request on a given stream.  A server sends zero or more interim HTTP
responses on the same stream as the request, followed by a single final HTTP
response, as detailed below. See Section 15 of {{!SEMANTICS}} for a description
of interim and final HTTP responses.

Pushed responses are sent on a server-initiated unidirectional QUIC stream; see
{{push-streams}}.  A server sends zero or more interim HTTP responses, followed
by a single final HTTP response, in the same manner as a standard response.
Push is described in more detail in {{server-push}}.

On a given stream, receipt of multiple requests or receipt of an additional HTTP
response following a final HTTP response MUST be treated as malformed
({{malformed}}).

An HTTP message (request or response) consists of:

1. the header field section, sent as a single HEADERS frame (see
   {{frame-headers}}),

2. optionally, the content, if present, sent as a series of DATA frames
   (see {{frame-data}}), and

3. optionally, the trailer field section, if present, sent as a single HEADERS
   frame.

Header and trailer field sections are described in Sections 6.3 and 6.5 of
{{!SEMANTICS}}; the content is described in Section 6.4 of
{{!SEMANTICS}}.

Receipt of an invalid sequence of frames MUST be treated as a connection error
of type H3_FRAME_UNEXPECTED; see {{errors}}.  In particular, a DATA frame before
any HEADERS frame, or a HEADERS or DATA frame after the trailing HEADERS frame
is considered invalid.  Other frame types, especially unknown frame types,
might be permitted subject to their own rules; see {{extensions}}.

A server MAY send one or more PUSH_PROMISE frames ({{frame-push-promise}})
before, after, or interleaved with the frames of a response message. These
PUSH_PROMISE frames are not part of the response; see {{server-push}} for more
details.  PUSH_PROMISE frames are not permitted on push streams; a pushed
response that includes PUSH_PROMISE frames MUST be treated as a connection error
of type H3_FRAME_UNEXPECTED; see {{errors}}.

Frames of unknown types ({{extensions}}), including reserved frames
({{frame-reserved}}) MAY be sent on a request or push stream before, after, or
interleaved with other frames described in this section.

The HEADERS and PUSH_PROMISE frames might reference updates to the QPACK dynamic
table. While these updates are not directly part of the message exchange, they
must be received and processed before the message can be consumed.  See
{{header-formatting}} for more details.

The "chunked" transfer encoding defined in Section 7.1 of {{?HTTP11}} MUST NOT
be used.

A response MAY consist of multiple messages when and only when one or more
interim responses (1xx; see Section 15.2 of {{!SEMANTICS}}) precede a final
response to the same request.  Interim responses do not contain content
or trailers.

An HTTP request/response exchange fully consumes a client-initiated
bidirectional QUIC stream. After sending a request, a client MUST close the
stream for sending.  Unless using the CONNECT method (see {{connect}}), 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 the final 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-initiated stream terminates without enough of the HTTP
message to provide a complete response, the server SHOULD abort its response
with the error code H3_REQUEST_INCOMPLETE; see {{errors}}.

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 the server does not need to receive the
remainder of the request, it MAY abort reading the request stream, send a
complete response, and cleanly close the sending part of the stream.  The error
code H3_NO_ERROR SHOULD be used when requesting that the client stop sending on
the request 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.  If the server sends a partial
or complete response but does not abort reading the request, clients SHOULD
continue sending the body of the request and close the stream normally.


### Field Formatting and Compression {#header-formatting}

HTTP messages carry metadata as a series of key-value pairs called HTTP fields;
see Sections 6.3 and 6.5 of {{!SEMANTICS}}. For a listing of registered HTTP
fields, see the "Hypertext Transfer Protocol (HTTP) Field Name Registry"
maintained at [](https://www.iana.org/assignments/http-fields/).

> **Note:**  This registry will not exist until {{!SEMANTICS}} is approved.
> **RFC Editor**, please remove this note prior to publication.

As in previous versions of HTTP, field names are strings containing a subset of
ASCII characters that are compared in a case-insensitive fashion.  Properties of
HTTP field names and values are discussed in more detail in Section 5.1 of
{{!SEMANTICS}}.  As in HTTP/2, characters in field names MUST be converted to
lowercase prior to their encoding.  A request or response containing uppercase
characters in field names MUST be treated as malformed ({{malformed}}).

Like HTTP/2, HTTP/3 does not use the Connection header field to indicate
connection-specific fields; in this protocol, connection-specific metadata is
conveyed by other means.  An endpoint MUST NOT generate an HTTP/3 field section
containing connection-specific fields; any message containing
connection-specific fields MUST be treated as malformed ({{malformed}}).

The only exception to this is the TE header field, which MAY be present in an
HTTP/3 request header; when it is, it MUST NOT contain any value other than
"trailers".

This means that an intermediary transforming an HTTP/1.x message to HTTP/3 will
need to remove any fields nominated by the Connection field, along with the
Connection field itself.  Such intermediaries SHOULD also remove other
connection-specific fields, such as Keep-Alive, Proxy-Connection,
Transfer-Encoding, and Upgrade, even if they are not nominated by the Connection
field.

#### Pseudo-Header Fields

Like HTTP/2, HTTP/3 employs a series of pseudo-header fields where the field
name begins with the ':' character (ASCII 0x3a).  These pseudo-header fields
convey the target URI, the method of the request, and the status code for the
response.

Pseudo-header fields are not HTTP fields.  Endpoints MUST NOT generate
pseudo-header fields other than those defined in this document; however, an
extension could negotiate a modification of this restriction; see
{{extensions}}.

Pseudo-header fields are only valid in the context in which they are defined.
Pseudo-header fields defined for requests MUST NOT appear in responses;
pseudo-header fields defined for responses MUST NOT appear in requests.
Pseudo-header fields MUST NOT appear in trailers.  Endpoints MUST treat a
request or response that contains undefined or invalid pseudo-header fields as
malformed ({{malformed}}).

All pseudo-header fields MUST appear in the header field section before regular
header fields.  Any request or response that contains a pseudo-header field that
appears in a header field section after a regular header field MUST be treated
as malformed ({{malformed}}).

The following pseudo-header fields are defined for requests:

  ":method":

  : Contains the HTTP method (Section 9 of {{!SEMANTICS}})

  ":scheme":

  : Contains the scheme portion of the target URI (Section 3.1 of
  {{!URI=RFC3986}})

  : ":scheme" is not restricted to "http" and "https" schemed URIs.  A proxy or
    gateway can translate requests for non-HTTP schemes, enabling the use of
    HTTP to interact with non-HTTP services.

  ":authority":

  : Contains the authority portion of the target URI (Section 3.2 of
    {{!URI}}). The authority MUST NOT include the deprecated "userinfo"
    subcomponent for "http" or "https" schemed URIs.

  : To ensure that the HTTP/1.1 request line can be reproduced accurately, this
    pseudo-header field MUST be omitted when translating from an HTTP/1.1
    request that has a request target in origin or asterisk form; see Section
    3.2 of {{?HTTP11}}.  Clients that generate HTTP/3 requests directly SHOULD
    use the ":authority" pseudo-header field instead of the Host field. An
    intermediary that converts an HTTP/3 request to HTTP/1.1 MUST create a Host
    field if one is not present in a request by copying the value of the
    ":authority" pseudo-header field.

  ":path":

  : Contains the path and query parts of the target URI (the "path-absolute"
    production and optionally a '?' character followed by the "query"
    production; see Sections 3.3 and 3.4 of {{!URI}}.  A request in
    asterisk form includes the value '*' for the ":path" pseudo-header field.

  : This pseudo-header field MUST NOT be empty for "http" or "https" URIs;
    "http" or "https" URIs that do not contain a path component MUST include a
    value of '/'.  The exception to this rule is an OPTIONS request for an
    "http" or "https" URI that does not include a path component; these MUST
    include a ":path" pseudo-header field with a value of '*'; see Section 3.2.4
    of {{?HTTP11}}.

All HTTP/3 requests MUST include exactly one value for the ":method", ":scheme",
and ":path" pseudo-header fields, unless it is a CONNECT request; see
{{connect}}.

If the ":scheme" pseudo-header field identifies a scheme that has a mandatory
authority component (including "http" and "https"), the request MUST contain
either an ":authority" pseudo-header field or a "Host" header field.  If these
fields are present, they MUST NOT be empty.  If both fields are present, they
MUST contain the same value.  If the scheme does not have a mandatory authority
component and none is provided in the request target, the request MUST NOT
contain the ":authority" pseudo-header or "Host" header fields.

An HTTP request that omits mandatory pseudo-header fields or contains invalid
values for those pseudo-header fields is malformed ({{malformed}}).

HTTP/3 does not define a way to carry the version identifier that is included in
the HTTP/1.1 request line.

For responses, a single ":status" pseudo-header field is defined that carries
the HTTP status code; see Section 15 of {{!SEMANTICS}}.  This pseudo-header
field MUST be included in all responses; otherwise, the response is malformed
({{malformed}}).

HTTP/3 does not define a way to carry the version or reason phrase that is
included in an HTTP/1.1 status line.

#### Field Compression

HTTP/3 uses QPACK field compression as described in [QPACK], a variation of
HPACK that allows the flexibility to avoid compression-induced head-of-line
blocking.  See that document for additional details.

To allow for better compression efficiency, the "Cookie" field ({{!RFC6265}})
MAY be split into separate field lines, each with one or more cookie-pairs,
before compression. If a decompressed field section contains multiple cookie
field lines, these MUST be concatenated into a single byte string using the
two-byte delimiter of 0x3b, 0x20 (the ASCII string "; ") before being passed
into a context other than HTTP/2 or HTTP/3, such as an HTTP/1.1 connection, or a
generic HTTP server application.

#### Header Size Constraints

An HTTP/3 implementation MAY impose a limit on the maximum size of the message
header it will accept on an individual HTTP message.  A server that receives a
larger header section than it is willing to handle can send an HTTP 431 (Request
Header Fields Too Large) status code ({{?RFC6585}}).  A client can discard
responses that it cannot process.  The size of a field list is calculated based
on the uncompressed size of fields, including the length of the name and value
in bytes plus an overhead of 32 bytes for each field.

If an implementation wishes to advise its peer of this limit, it can be conveyed
as a number of bytes in the SETTINGS_MAX_FIELD_SECTION_SIZE parameter. An
implementation that has received this parameter SHOULD NOT send an HTTP message
header that exceeds the indicated size, as the peer will likely refuse to
process it.  However, an HTTP message can traverse one or more intermediaries
before reaching the origin server; see Section 3.6 of {{!SEMANTICS}}.  Because
this limit is applied separately by each implementation which processes the
message, messages below this limit are not guaranteed to be accepted.

### Request Cancellation and Rejection {#request-cancellation}

Once a request stream has been opened, the request MAY be cancelled by either
endpoint.  Clients cancel requests if the response is no longer of interest;
servers cancel requests if they are unable to or choose not to respond.  When
possible, it is RECOMMENDED that servers send an HTTP response with an
appropriate status code rather than canceling a request it has already begun
processing.

Implementations SHOULD cancel requests by abruptly terminating any
directions of a stream that are still open.  This means resetting the
sending parts of streams and aborting reading on receiving parts of streams;
see Section 2.4 of [QUIC-TRANSPORT].

When the server cancels a request without performing any application processing,
the request is considered "rejected."  The server SHOULD abort its response
stream with the error code H3_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.

Servers MUST NOT use the H3_REQUEST_REJECTED error code for requests that were
partially or fully processed.  When a server abandons a response after partial
processing, it SHOULD abort its response stream with the error code
H3_REQUEST_CANCELLED.

Client SHOULD use the error code H3_REQUEST_CANCELLED to cancel requests.  Upon
receipt of this error code, a server MAY abruptly terminate the response using
the error code H3_REQUEST_REJECTED if no processing was performed.  Clients MUST
NOT use the H3_REQUEST_REJECTED error code, except when a server has requested
closure of the request stream with this error code.

If a stream is canceled 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. Only
idempotent actions such as GET, PUT, or DELETE can be safely retried; a client
SHOULD NOT automatically retry a request with a non-idempotent method unless it
has some means to know that the request semantics are idempotent
independent of the method or some means to detect that the original request was
never applied.  See Section 9.2.2 of {{!SEMANTICS}} for more details.

### Malformed Requests and Responses {#malformed}

A malformed request or response is one that is an otherwise valid sequence of
frames but is invalid due to:

- the presence of prohibited fields or pseudo-header fields,
- the absence of mandatory pseudo-header fields,
- invalid values for pseudo-header fields,
- pseudo-header fields after fields,
- an invalid sequence of HTTP messages,
- the inclusion of uppercase field names, or
- the inclusion of invalid characters in field names or values.

A request or response that is defined as having content when it contains a
Content-Length header field (Section 6.4.1 of {{!SEMANTICS}}),
is malformed if the value of a Content-Length header field does not equal the
sum of the DATA frame lengths received. A response that is defined as never
having content, even when a Content-Length is present, can have a non-zero
Content-Length field even though no content is included in DATA frames.

Intermediaries that process HTTP requests or responses (i.e., any intermediary
not acting as a tunnel) MUST NOT forward a malformed request or response.
Malformed requests or responses that are detected MUST be treated as a stream
error ({{errors}}) of type H3_MESSAGE_ERROR.

For malformed requests, a server MAY send an HTTP response indicating the error
prior to closing or resetting the stream.  Clients MUST NOT accept a malformed
response.  Note that these requirements are intended to protect against several
types of common attacks against HTTP; they are deliberately strict because being
permissive can expose implementations to these vulnerabilities.


## The CONNECT Method {#connect}

The CONNECT method requests that the recipient establish a tunnel to the
destination origin server identified by the request-target; see Section 9.3.6 of
{{!SEMANTICS}}.  It 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 and HTTP/3, the CONNECT method is used to establish
a tunnel over a single stream.

A CONNECT request MUST be constructed as follows:

- The ":method" pseudo-header field is set to "CONNECT"
- The ":scheme" and ":path" pseudo-header fields are omitted
- The ":authority" pseudo-header field contains the host and port to connect to
  (equivalent to the authority-form of the request-target of CONNECT requests;
  see Section 3.2.3 of {{?HTTP11}})

The request stream remains open at the end of the request to carry the data to
be transferred.  A CONNECT request that does not conform to these restrictions
is malformed; see {{malformed}}.

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 Section 15.3 of {{!SEMANTICS}}.

All DATA frames on the stream correspond to data sent or received on the TCP
connection. The payload of 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.

Once the CONNECT method has completed, only DATA frames are permitted to be sent
on the stream.  Extension frames MAY be used if specifically permitted by the
definition of the extension.  Receipt of any other known frame type MUST be
treated as a connection error of type H3_FRAME_UNEXPECTED; see {{errors}}.

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 close the send stream
that it sends to the client. TCP connections that 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 by abruptly terminating the stream. 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 H3_CONNECT_ERROR; see
{{errors}}.  Correspondingly, if a proxy detects an error with the stream or the
QUIC connection, it MUST close the TCP connection.  If the underlying TCP
implementation permits it, the proxy SHOULD send a TCP segment with the RST bit
set.

## HTTP Upgrade

HTTP/3 does not support the HTTP Upgrade mechanism (Section 7.8 of
{{!SEMANTICS}}) or 101 (Switching Protocols) informational status code (Section
15.2.2 of {{!SEMANTICS}}).

## Server Push

Server push is an interaction mode that permits a server to push a
request-response exchange to a client in anticipation of the client making the
indicated request.  This trades off network usage against a potential latency
gain.  HTTP/3 server push is similar to what is described in Section 8.2 of
{{?HTTP2}}, but uses different mechanisms.

Each server push is assigned a unique Push ID by the server.  The Push ID is
used to refer to the push in various contexts throughout the lifetime of the
HTTP/3 connection.

The Push ID space begins at zero, and ends at a maximum value set by the
MAX_PUSH_ID frame; see {{frame-max-push-id}}.  In particular, a server is not
able to push until after the client sends a MAX_PUSH_ID frame.  A client sends
MAX_PUSH_ID frames to control the number of pushes that a server can promise.  A
server SHOULD use Push IDs sequentially, beginning from zero.  A client MUST
treat receipt of a push stream as a connection error of type H3_ID_ERROR
({{errors}}) when no MAX_PUSH_ID frame has been sent or when the stream
references a Push ID that is greater than the maximum Push ID.

The Push ID is used in one or more PUSH_PROMISE frames ({{frame-push-promise}})
that carry the header section of the request message.  These frames are sent on
the request stream that generated the push.  This allows the server push to be
associated with a client request.  When the same Push ID is promised on multiple
request streams, the decompressed request field sections MUST contain the same
fields in the same order, and both the name and the value in each field MUST be
identical.

The Push ID is then included with the push stream that ultimately fulfills
those promises; see {{push-streams}}.  The push stream identifies the Push ID of
the promise that it fulfills, then contains a response to the promised request
as described in {{request-response}}.

Finally, the Push ID can be used in CANCEL_PUSH frames; see
{{frame-cancel-push}}.  Clients use this frame to indicate they do not wish to
receive a promised resource.  Servers use this frame to indicate they will not
be fulfilling a previous promise.

Not all requests can be pushed.  A server MAY push requests that have the
following properties:

- cacheable; see Section 9.2.3 of {{!SEMANTICS}}
- safe; see Section 9.2.1 of {{!SEMANTICS}}
- does not include a request body or trailer section

The server MUST include a value in the ":authority" pseudo-header field for
which the server is authoritative; see {{connection-reuse}}.

Clients SHOULD send a CANCEL_PUSH frame upon receipt of a PUSH_PROMISE frame
carrying a request that is not cacheable, is not known to be safe, that
indicates the presence of a request body, or for which it does not consider the
server authoritative.  Any corresponding responses MUST NOT be used or cached.

Each pushed response is associated with one or more client requests.  The push
is associated with the request stream on which the PUSH_PROMISE frame was
received.  The same server push can be associated with additional client
requests using a PUSH_PROMISE frame with the same Push ID on multiple request
streams.  These associations do not affect the operation of the protocol, but
MAY be considered by user agents when deciding how to use pushed resources.

Ordering of a PUSH_PROMISE frame in relation to certain parts of the response is
important. The server SHOULD send PUSH_PROMISE frames prior to sending HEADERS
or DATA frames that reference the promised responses.  This reduces the chance
that a client requests a resource that will be pushed by the server.

Due to reordering, push stream data can arrive before the corresponding
PUSH_PROMISE frame.  When a client receives a new push stream with an
as-yet-unknown Push ID, both the associated client request and the pushed
request header fields 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.

Push stream data can also arrive after a client has canceled a push. In this
case, the client can abort reading the stream with an error code of
H3_REQUEST_CANCELLED. This asks the server not to transfer additional data and
indicates that it will be discarded upon receipt.

Pushed responses that are cacheable (see Section 3 of
{{!CACHING=I-D.ietf-httpbis-cache}}) can be stored by the client, if it
implements an HTTP cache.  Pushed responses are considered successfully
validated on the origin server (e.g., if the "no-cache" cache response directive
is present; see Section 5.2.2.3 of {{!CACHING}}) at the time the pushed response
is received.

Pushed responses that are not cacheable MUST NOT be stored by any HTTP cache.
They MAY be made available to the application separately.

# 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 QUIC
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 HTTP/3 connection for new requests if the existing
connection has been idle for longer than the idle timeout negotiated during the
QUIC handshake, and SHOULD do so if approaching the idle timeout; see Section
10.1 of {{QUIC-TRANSPORT}}.

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 10.1.2 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 initiate a graceful connection close.  Endpoints initiate the
graceful shutdown of an HTTP/3 connection by sending a GOAWAY frame
({{frame-goaway}}). The GOAWAY frame contains an identifier that indicates to
the receiver the range of requests or pushes that were or might be processed in
this connection.  The server sends a client-initiated bidirectional Stream ID;
the client sends a Push ID ({{server-push}}).  Requests or pushes with the
indicated identifier or greater are rejected ({{request-cancellation}}) by the
sender of the GOAWAY. This identifier MAY be zero if no requests or pushes were
processed.

The information in the GOAWAY frame enables a client and server to agree on
which requests or pushes were accepted prior to the shutdown of the HTTP/3
connection. Upon sending a GOAWAY frame, the endpoint SHOULD explicitly cancel
(see {{request-cancellation}} and {{frame-cancel-push}}) any requests or pushes
that have identifiers greater than or equal to that indicated, in order to clean
up transport state for the affected streams. The endpoint SHOULD continue to do
so as more requests or pushes arrive.

Endpoints MUST NOT initiate new requests or promise new pushes on the connection
after receipt of a GOAWAY frame from the peer.  Clients MAY establish a new
connection to send additional requests.

Some requests or pushes might already be in transit:

  - Upon receipt of a GOAWAY frame, if the client has already sent requests with
    a Stream ID greater than or equal to the identifier contained in the GOAWAY
    frame, those requests will not be processed.  Clients can safely retry
    unprocessed requests on a different HTTP connection.  A client that is
    unable to retry requests loses all requests that are in flight when the
    server closes the connection.

    Requests on Stream IDs less than the Stream ID in a GOAWAY frame from the
    server might have been processed; their status cannot be known until a
    response is received, the stream is reset individually, another GOAWAY is
    received, or the connection terminates.

    Servers MAY reject individual requests on streams below the indicated ID if
    these requests were not processed.

  - If a server receives a GOAWAY frame after having promised pushes with a Push
    ID greater than or equal to the identifier contained in the GOAWAY frame,
    those pushes will not be accepted.

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.

An endpoint MAY send multiple GOAWAY frames indicating different identifiers,
but the identifier in each frame MUST NOT be greater than the identifier in any
previous frame, since clients might already have retried unprocessed requests on
another HTTP connection.  Receiving a GOAWAY containing a larger identifier than
previously received MUST be treated as a connection error of type H3_ID_ERROR;
see {{errors}}.

An endpoint that is attempting to gracefully shut down a connection can send a
GOAWAY frame with a value set to the maximum possible value (2^62-4 for servers,
2^62-1 for clients). This ensures that the peer stops creating new requests or
pushes. After allowing time for any in-flight requests or pushes to arrive, the
endpoint can send another GOAWAY frame indicating which requests or pushes it
might accept before the end of the connection. This ensures that a connection
can be cleanly shut down without losing requests.

A client has more flexibility in the value it chooses for the Push ID in a
GOAWAY that it sends.  A value of 2^62 - 1 indicates that the server can
continue fulfilling pushes that have already been promised. A smaller value
indicates the client will reject pushes with Push IDs greater than or equal to
this value.  Like the server, the client MAY send subsequent GOAWAY frames so
long as the specified Push ID is no greater than any previously sent value.

Even when a GOAWAY indicates that a given request or push will not be processed
or accepted upon receipt, the underlying transport resources still exist.  The
endpoint that initiated these requests can cancel them to clean up transport
state.

Once all accepted requests and pushes have been processed, the endpoint 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
H3_NO_ERROR 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 indicating
that the application layer has terminated the connection.  The application error
code in this frame indicates to the peer why the connection is being closed.
See {{errors}} for error codes that can be used when closing a connection in
HTTP/3.

Before closing the connection, a GOAWAY frame 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 that
interrupts connectivity.

If a connection terminates without a GOAWAY frame, clients MUST assume that any
request that was sent, whether in whole or in part, might have been processed.


# 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 fields 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 to the entire HTTP/3 connection context.

## Bidirectional Streams {#request-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.  These streams are referred to as request streams.

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 server SHOULD configure non-zero minimum values for the
number of permitted streams and the initial stream flow control window.  So as
to not unnecessarily limit parallelism, at least 100 requests SHOULD be
permitted at a time.

HTTP/3 does not use server-initiated bidirectional streams, though an extension
could define a use for these streams.  Clients MUST treat receipt of a
server-initiated bidirectional stream as a connection error of type
H3_STREAM_CREATION_ERROR ({{errors}}) unless such an extension has been
negotiated.

## 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
Unidirectional Stream Header {
  Stream Type (i),
}
~~~~~~~~~~
{: #fig-stream-header title="Unidirectional Stream Header"}

Two stream types are defined in this document: control streams
({{control-streams}}) and push streams ({{push-streams}}).  [QPACK] defines two
additional stream types.  Other stream types can be defined by extensions to
HTTP/3; see {{extensions}} for more details.  Some stream types are reserved
({{stream-grease}}).

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 excessively restrict the number of streams or the flow control
window of these streams will increase the chance that the remote peer reaches
the limit early and becomes blocked. In particular, implementations should
consider that remote peers may wish to exercise reserved stream behavior
({{stream-grease}}) with some of the unidirectional streams they are permitted
to use. To avoid blocking, the transport parameters sent by both clients and
servers MUST allow the peer to create at least one unidirectional stream for the
HTTP control stream plus the number of unidirectional streams required by
mandatory extensions (three being the minimum number required for the base
HTTP/3 protocol and QPACK), and SHOULD provide at least 1,024 bytes of flow
control credit to each stream.

Note that an endpoint is not required to grant additional credits to create more
unidirectional streams if its peer consumes all the initial credits before
creating the critical unidirectional streams. Endpoints SHOULD create the HTTP
control stream as well as the unidirectional streams required by mandatory
extensions (such as the QPACK encoder and decoder streams) first, and then
create additional streams as allowed by their peer.

If the stream header indicates a stream type that 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 abort reading of the stream with
an error code of H3_STREAM_CREATION_ERROR or a reserved error code
({{http-error-codes}}), but MUST NOT consider such streams to be a connection
error of any kind.

Implementations MAY send stream types before knowing whether the peer supports
them.  However, stream types that 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 H3_MISSING_SETTINGS. Only one control
stream per peer is permitted; receipt of a second stream claiming to be a
control stream MUST be treated as a connection error of type
H3_STREAM_CREATION_ERROR.  The sender MUST NOT close the control stream, and the
receiver MUST NOT request that the sender close the control stream.  If either
control stream is closed at any point, this MUST be treated as a connection
error of type H3_CLOSED_CRITICAL_STREAM.  Connection errors are described in
{{errors}}.

A pair of unidirectional streams is used rather than a single bidirectional
stream.  This allows either peer to send data as soon as it is able.  Depending
on whether 0-RTT is enabled on the QUIC connection, either client or server
might be able to send stream data first after the cryptographic handshake
completes.

### Push Streams

Server push is an optional feature introduced in HTTP/2 that allows a server to
initiate a response before a request has been made.  See {{server-push}} for
more details.

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 by zero or more interim HTTP
responses followed by a single final HTTP response, as defined in
{{request-response}}.  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 connection error of type H3_STREAM_CREATION_ERROR; see
{{errors}}.

~~~~~~~~~~ drawing
Push Stream Header {
  Stream Type (i) = 0x01,
  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 H3_ID_ERROR; see
{{errors}}.

### Reserved Stream Types {#stream-grease}

Stream types of the format `0x1f * N + 0x21` for non-negative 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 sending
implementation chooses.  When sending a reserved stream type, the implementation
MAY either terminate the stream cleanly or reset it.  When resetting the stream,
either the H3_NO_ERROR error code or a reserved error code
({{http-error-codes}}) SHOULD be used.


# 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 their permitted stream
types; see {{stream-frame-mapping}} for an overview.  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}}        |
| 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}}    |
| Reserved       | Yes            | Yes            | Yes         | {{frame-reserved}}       |
{: #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.

Note that, unlike QUIC frames, HTTP/3 frames can span multiple packets.

## Frame Layout

All frames have the following format:

~~~~~~~~~~ drawing
HTTP/3 Frame Format {
  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 in bytes 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
H3_FRAME_ERROR; see {{errors}}.

When a stream terminates cleanly, if the last frame on the stream was truncated,
this MUST be treated as a connection error of type H3_FRAME_ERROR; see
{{errors}}. Streams that terminate abruptly may be reset at any point in a
frame.

## Frame Definitions {#frames}

### DATA {#frame-data}

DATA frames (type=0x0) convey arbitrary, variable-length sequences of bytes
associated with HTTP request or response content.

DATA frames MUST be associated with an HTTP request or response.  If a DATA
frame is received on a control stream, the recipient MUST respond with a
connection error of type H3_FRAME_UNEXPECTED; see {{errors}}.

~~~~~~~~~~ drawing
DATA Frame {
  Type (i) = 0x0,
  Length (i),
  Data (..),
}
~~~~~~~~~~
{: #fig-data title="DATA Frame"}

### HEADERS {#frame-headers}

The HEADERS frame (type=0x1) is used to carry an HTTP field section, encoded
using QPACK. See [QPACK] for more details.

~~~~~~~~~~  drawing
HEADERS Frame {
  Type (i) = 0x1,
  Length (i),
  Encoded Field Section (..),
}
~~~~~~~~~~
{: #fig-headers title="HEADERS Frame"}

HEADERS frames can only be sent on request or push streams.  If a HEADERS frame
is received on a control stream, the recipient MUST respond with a connection
error ({{errors}}) of type H3_FRAME_UNEXPECTED.

### 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 {{server-push}}), encoded as a variable-length
integer.

When a client sends CANCEL_PUSH, it is indicating that it does not wish to
receive the promised resource.  The server SHOULD abort sending the resource,
but the mechanism to do so depends on the state of the corresponding push
stream.  If the server has not yet created a push stream, it does not create
one.  If the push stream is open, the server SHOULD abruptly terminate that
stream.  If the push stream has already ended, the server MAY still abruptly
terminate the stream or MAY take no action.

A server sends CANCEL_PUSH to indicate that it will not be fulfilling a promise
which was previously sent.  The client cannot expect the corresponding promise
to be fulfilled, unless it has already received and processed the promised
response. Regardless of whether a push stream has been opened, a server
SHOULD send a CANCEL_PUSH frame when it determines that promise will not be
fulfilled.  If a stream has already been opened, the server can
abort sending on the stream with an error code of H3_REQUEST_CANCELLED.

Sending a CANCEL_PUSH frame has no direct effect on the state of existing push
streams. A client SHOULD NOT send a CANCEL_PUSH frame when it has already
received a corresponding push stream.  A push stream could arrive after a client
has sent a CANCEL_PUSH frame, because a server might not have processed the
CANCEL_PUSH. The client SHOULD abort reading the stream with an error code of
H3_REQUEST_CANCELLED.

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 connection
error of type H3_FRAME_UNEXPECTED.

~~~~~~~~~~  drawing
CANCEL_PUSH Frame {
  Type (i) = 0x3,
  Length (i),
  Push ID (..),
}
~~~~~~~~~~
{: #fig-cancel-push title="CANCEL_PUSH Frame"}

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
{{server-push}}.  If a CANCEL_PUSH frame is received that references a Push ID
greater than currently allowed on the connection, this MUST be treated as a
connection error of type H3_ID_ERROR.

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 due to reordering.  If a
server receives a CANCEL_PUSH frame for a Push ID that has not yet been
mentioned by a PUSH_PROMISE frame, this MUST be treated as a connection error of
type H3_ID_ERROR.


### 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 an entire HTTP/3 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. If an
endpoint receives a second SETTINGS frame on the control stream, the endpoint
MUST respond with a connection error of type H3_FRAME_UNEXPECTED.

SETTINGS frames MUST NOT be sent on any stream other than the control stream.
If an endpoint receives a SETTINGS frame on a different stream, the endpoint
MUST respond with a connection error of type H3_FRAME_UNEXPECTED.

SETTINGS parameters are not negotiated; they describe characteristics of the
sending peer that 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 field
section, while servers are more cautious about request size.

The same setting identifier MUST NOT occur more than once in the SETTINGS frame.
A receiver MAY treat the presence of duplicate setting identifiers as a
connection error of type H3_SETTINGS_ERROR.

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
Setting {
  Identifier (i),
  Value (i),
}

SETTINGS Frame {
  Type (i) = 0x4,
  Length (i),
  Setting (..) ...,
}
~~~~~~~~~~~~~~~
{: #fig-ext-settings title="SETTINGS Frame"}

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_FIELD_SECTION_SIZE (0x6):
  : The default value is unlimited.  See {{header-size-constraints}} for usage.

Setting identifiers of the format `0x1f * N + 0x21` for non-negative 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.

Setting identifiers which were used in HTTP/2 where there is no corresponding
HTTP/3 setting have also been reserved ({{iana-settings}}). These settings MUST
NOT be sent, and their receipt MUST be treated as a connection error of type
H3_SETTINGS_ERROR.

Additional settings can be defined by extensions to HTTP/3; see {{extensions}}
for more details.

#### Initialization {#settings-initialization}

An HTTP implementation MUST NOT send frames or requests that would be invalid
based on its current understanding of the peer's settings.

All settings begin at an initial value.  Each endpoint SHOULD use these initial
values to send messages before the peer's SETTINGS frame has arrived, as packets
carrying the settings can be lost or delayed.  When the SETTINGS frame arrives,
any settings are changed to their new values.

This removes the need to wait for the SETTINGS frame before sending messages.
Endpoints MUST NOT require any data to be received from the peer prior to
sending the SETTINGS frame; settings MUST be sent as soon as the transport is
ready to send data.

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.  1-RTT keys will always become available prior to
the packet containing SETTINGS being processed by QUIC, even if the server sends
SETTINGS immediately.  Clients SHOULD NOT wait indefinitely for SETTINGS to
arrive before sending requests, but SHOULD process received datagrams in order
to increase the likelihood of processing SETTINGS before sending the first
request.

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 SHOULD store the
settings the server provided in the HTTP/3 connection where resumption
information was provided, but MAY opt not to store settings in certain cases
(e.g., if the session ticket is received before the SETTINGS frame). A client
MUST comply with stored settings -- or default values, if no values are stored
-- when attempting 0-RTT. Once a server has provided new settings, clients MUST
comply with those values.

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.  If the server cannot determine that
the settings remembered by a client are compatible with its current settings, it
MUST NOT accept 0-RTT data.  Remembered settings are compatible if a client
complying with those settings would not violate the server's current settings.

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.  The server MUST include all settings that differ from
their default values.  If a server accepts 0-RTT but then sends settings that
are not compatible with the previously specified settings, this MUST be treated
as a connection error of type H3_SETTINGS_ERROR. If a server accepts 0-RTT but
then sends a SETTINGS frame that omits a setting value that the client
understands (apart from reserved setting identifiers) that was previously
specified to have a non-default value, this MUST be treated as a connection
error of type H3_SETTINGS_ERROR.


### PUSH_PROMISE {#frame-push-promise}

The PUSH_PROMISE frame (type=0x5) is used to carry a promised request header
field section from server to client on a request stream, as in HTTP/2.

~~~~~~~~~~  drawing
PUSH_PROMISE Frame {
  Type (i) = 0x5,
  Length (i),
  Push ID (i),
  Encoded Field Section (..),
}
~~~~~~~~~~
{: #fig-push-promise title="PUSH_PROMISE Frame"}

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}}) and CANCEL_PUSH frames
  ({{frame-cancel-push}}).

Encoded Field Section:
: QPACK-encoded 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}}). A client MUST treat receipt of a
PUSH_PROMISE frame that contains a larger Push ID than the client has advertised
as a connection error of H3_ID_ERROR.

A server MAY use the same Push ID in multiple PUSH_PROMISE frames. If so, the
decompressed request header sets MUST contain the same fields in the same order,
and both the name and the value in each field MUST be exact matches. Clients
SHOULD compare the request header sections for resources promised multiple
times. If a client receives a Push ID that has already been promised and detects
a mismatch, it MUST respond with a connection error of type
H3_GENERAL_PROTOCOL_ERROR. If the decompressed field sections match exactly, the
client SHOULD associate the pushed content with each stream on which a
PUSH_PROMISE frame was received.

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 PUSH_PROMISE frame that
uses a Push ID that they have already consumed and discarded are forced to
ignore the promise.

If a PUSH_PROMISE frame is received on the control stream, the client MUST
respond with a connection error of type H3_FRAME_UNEXPECTED; see {{errors}}.

A client MUST NOT send a PUSH_PROMISE frame.  A server MUST treat the receipt of
a PUSH_PROMISE frame as a connection error of type H3_FRAME_UNEXPECTED; see
{{errors}}.

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 an HTTP/3
connection by either endpoint.  GOAWAY allows an endpoint to stop accepting new
requests or pushes while still finishing processing of previously received
requests and pushes.  This enables administrative actions, like server
maintenance.  GOAWAY by itself does not close a connection.

~~~~~~~~~~  drawing
GOAWAY Frame {
  Type (i) = 0x7,
  Length (i),
  Stream ID/Push ID (..),
}
~~~~~~~~~~
{: #fig-goaway title="GOAWAY Frame"}

The GOAWAY frame is always sent on the control stream.  In the server to client
direction, 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 H3_ID_ERROR.

In the client to server direction, the GOAWAY frame carries a Push ID encoded as
a variable-length integer.

The GOAWAY frame applies to the entire connection, not a specific stream.  A
client MUST treat a GOAWAY frame on a stream other than the control stream as a
connection error of type H3_FRAME_UNEXPECTED; see {{errors}}.

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 PUSH_PROMISE and CANCEL_PUSH frames.
Consequently, this also limits the number of push streams that the server can
initiate in addition to the limit maintained by the QUIC transport.

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 H3_FRAME_UNEXPECTED.

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 H3_FRAME_UNEXPECTED.

The maximum Push ID is unset when an HTTP/3 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
MAX_PUSH_ID Frame {
  Type (i) = 0xd,
  Length (i),
  Push ID (i),
}
~~~~~~~~~~
{: #fig-max-push title="MAX_PUSH_ID Frame"}

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 {{server-push}}.  A
MAX_PUSH_ID frame cannot reduce the maximum Push ID; receipt of a MAX_PUSH_ID
frame that contains a smaller value than previously received MUST be treated as
a connection error of type H3_ID_ERROR.

### Reserved Frame Types {#frame-reserved}

Frame types of the format `0x1f * N + 0x21` for non-negative integer values of N
are reserved to exercise the requirement that unknown types be ignored
({{extensions}}).  These frames have no semantics, and MAY be sent on any stream
where frames are allowed to be sent. This enables their use for
application-layer padding.  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.

Frame types that were used in HTTP/2 where there is no corresponding HTTP/3
frame have also been reserved ({{iana-frames}}).  These frame types MUST NOT be
sent, and their receipt MUST be treated as a connection error of type
H3_FRAME_UNEXPECTED.


# Error Handling {#errors}

When a stream cannot be completed successfully, QUIC allows the application to
abruptly terminate (reset) that stream and communicate a reason; see Section 2.4
of {{QUIC-TRANSPORT}}.  This is referred to as a "stream error."  An HTTP/3
implementation can decide to close a QUIC stream and communicate the type of
error.  Wire encodings of error codes are defined in {{http-error-codes}}.
Stream errors are distinct from HTTP status codes which indicate error
conditions.  Stream errors indicate that the sender did not transfer or consume
the full request or response, while HTTP status codes indicate the result of a
request that was successfully received.

If an entire connection needs to be terminated, QUIC similarly provides
mechanisms to communicate a reason; see Section 5.3 of {{QUIC-TRANSPORT}}.  This
is referred to as a "connection error."  Similar to stream errors, an HTTP/3
implementation can terminate a QUIC connection and communicate the reason using
an error code from {{http-error-codes}}.

Although the reasons for closing streams and connections are called "errors,"
these actions do not necessarily indicate a problem with the connection or
either implementation. For example, a stream can be reset if the requested
resource is no longer needed.

An endpoint MAY choose to treat a stream error as a connection error under
certain circumstances, closing the entire connection in response to a condition
on a single stream.  Implementations need to consider the impact on outstanding
requests before making this choice.

Because new error codes can be defined without negotiation (see {{extensions}}),
use of an error code in an unexpected context or receipt of an unknown error
code MUST be treated as equivalent to H3_NO_ERROR.  However, closing a stream
can have other effects regardless of the error code; for example, see
{{request-response}}.

## HTTP/3 Error Codes {#http-error-codes}

The following error codes are defined for use when abruptly terminating streams,
aborting reading of streams, or immediately closing HTTP/3 connections.

H3_NO_ERROR (0x100):
: No error.  This is used when the connection or stream needs to be closed, but
  there is no error to signal.

H3_GENERAL_PROTOCOL_ERROR (0x101):
: Peer violated protocol requirements in a way that does not match a more
  specific error code, or endpoint declines to use the more specific error code.

H3_INTERNAL_ERROR (0x102):
: An internal error has occurred in the HTTP stack.

H3_STREAM_CREATION_ERROR (0x103):
: The endpoint detected that its peer created a stream that it will not accept.

H3_CLOSED_CRITICAL_STREAM (0x104):
: A stream required by the HTTP/3 connection was closed or reset.

H3_FRAME_UNEXPECTED (0x105):
: A frame was received that was not permitted in the current state or on the
  current stream.

H3_FRAME_ERROR (0x106):
: A frame that fails to satisfy layout requirements or with an invalid size
  was received.

H3_EXCESSIVE_LOAD (0x107):
: The endpoint detected that its peer is exhibiting a behavior that might be
  generating excessive load.

H3_ID_ERROR (0x108):
: A Stream ID or Push ID was used incorrectly, such as exceeding a limit,
  reducing a limit, or being reused.

H3_SETTINGS_ERROR (0x109):
: An endpoint detected an error in the payload of a SETTINGS frame.

H3_MISSING_SETTINGS (0x10a):
: No SETTINGS frame was received at the beginning of the control stream.

H3_REQUEST_REJECTED (0x10b):
: A server rejected a request without performing any application processing.

H3_REQUEST_CANCELLED (0x10c):
: The request or its response (including pushed response) is cancelled.

H3_REQUEST_INCOMPLETE (0x10d):
: The client's stream terminated without containing a fully-formed request.

H3_MESSAGE_ERROR (0x10e):
: An HTTP message was malformed and cannot be processed.

H3_CONNECT_ERROR (0x10f):
: The TCP connection established in response to a CONNECT request was reset or
  abnormally closed.

H3_VERSION_FALLBACK (0x110):
: The requested operation cannot be served over HTTP/3.  The peer should
  retry over HTTP/1.1.

Error codes of the format `0x1f * N + 0x21` for non-negative integer values of N
are reserved to exercise the requirement that unknown error codes be treated as
equivalent to H3_NO_ERROR ({{extensions}}). Implementations SHOULD select an
error code from this space with some probability when they would have sent
H3_NO_ERROR.

# 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 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.  However, where a known frame type is required to be in a specific
location, such as the SETTINGS frame as the first frame of the control stream
(see {{control-streams}}), an unknown frame type does not satisfy that
requirement and SHOULD be treated as an error.

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.  Coordinating when such a revised layout comes
into effect could prove complex.  As such, allocating new identifiers for
new definitions of existing protocol elements is likely to be more effective.

This document does not 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.


# Security Considerations

The security considerations of HTTP/3 should be comparable to those of HTTP/2
with TLS.  However, many of the considerations from Section 10 of {{?HTTP2}}
apply to [QUIC-TRANSPORT] and are discussed in that document.

## Server Authority

HTTP/3 relies on the HTTP definition of authority. The security considerations
of establishing authority are discussed in Section 17.1 of {{!SEMANTICS}}.

## Cross-Protocol Attacks

The use of ALPN in the TLS and QUIC handshakes establishes the target
application protocol before application-layer bytes are processed.  This ensures
that endpoints have strong assurances that peers are using the same protocol.

This does not guarantee protection from all cross-protocol attacks.  Section
21.5 of {{QUIC-TRANSPORT}} describes some ways in which the plaintext of QUIC
packets can be used to perform request forgery against endpoints that don't use
authenticated transports.

## Intermediary Encapsulation Attacks

The HTTP/3 field encoding allows the expression of names that are not valid
field names in the syntax used by HTTP (Section 5.1 of {{!SEMANTICS}}).
Requests or responses containing invalid field names MUST be treated as
malformed ({{malformed}}).  An intermediary therefore cannot translate an HTTP/3
request or response containing an invalid field name into an HTTP/1.1 message.

Similarly, HTTP/3 can transport field values that are not valid. While most
values that can be encoded will not alter field parsing, carriage return (CR,
ASCII 0xd), line feed (LF, ASCII 0xa), and the zero character (NUL, ASCII 0x0)
might be exploited by an attacker if they are translated verbatim. Any request
or response that contains a character not permitted in a field value MUST be
treated as malformed ({{malformed}}).  Valid characters are defined by the
"field-content" ABNF rule in Section 5.5 of {{!SEMANTICS}}.

## Cacheability of Pushed Responses

Pushed responses do not have an explicit request from the client; the request is
provided by the server in the PUSH_PROMISE frame.

Caching responses that are pushed is possible based on the guidance provided by
the origin server in the Cache-Control header field. However, this can cause
issues if a single server hosts more than one tenant.  For example, a server
might offer multiple users each a small portion of its URI space.

Where multiple tenants share space on the same server, that server MUST ensure
that tenants are not able to push representations of resources that they do not
have authority over.  Failure to enforce this would allow a tenant to provide a
representation that would be served out of cache, overriding the actual
representation that the authoritative tenant provides.

Clients are required to reject pushed responses for which an origin server is
not authoritative; see {{server-push}}.

## Denial-of-Service Considerations

An HTTP/3 connection can demand a greater commitment of resources to operate
than an HTTP/1.1 or HTTP/2 connection.  The use of field compression and flow
control depend on a commitment of resources for storing a greater amount of
state.  Settings for these features ensure that memory commitments for these
features are strictly bounded.

The number of PUSH_PROMISE frames is constrained in a similar fashion.  A client
that accepts server push SHOULD limit the number of Push IDs it issues at a
time.

Processing capacity cannot be guarded as effectively as state capacity.

The ability to send undefined protocol elements that the peer is required to
ignore can be abused to cause a peer to expend additional processing time.  This
might be done by setting multiple undefined SETTINGS parameters, unknown frame
types, or unknown stream types.  Note, however, that some uses are entirely
legitimate, such as optional-to-understand extensions and padding to increase
resistance to traffic analysis.

Compression of field sections also offers some opportunities to waste processing
resources; see Section 7 of [QPACK] for more details on potential abuses.

All these features -- i.e., server push, unknown protocol elements, field
compression -- have legitimate uses.  These features become a burden only when
they are used unnecessarily or to excess.

An endpoint that does not monitor this behavior exposes itself to a risk of
denial-of-service attack.  Implementations SHOULD track the use of these
features and set limits on their use.  An endpoint MAY treat activity that is
suspicious as a connection error of type H3_EXCESSIVE_LOAD ({{errors}}), but
false positives will result in disrupting valid connections and requests.

### Limits on Field Section Size

A large field section ({{request-response}}) can cause an implementation to
commit a large amount of state.  Header fields that are critical for routing can
appear toward the end of a header field section, which prevents streaming of the
header field section to its ultimate destination.  This ordering and other
reasons, such as ensuring cache correctness, mean that an endpoint likely needs
to buffer the entire header field section.  Since there is no hard limit to the
size of a field section, some endpoints could be forced to commit a large amount
of available memory for header fields.

An endpoint can use the SETTINGS_MAX_FIELD_SECTION_SIZE
({{header-size-constraints}}) setting to advise peers of limits that might apply
on the size of field sections. This setting is only advisory, so endpoints MAY
choose to send field sections that exceed this limit and risk having the request
or response being treated as malformed.  This setting is specific to an HTTP/3
connection, so any request or response could encounter a hop with a lower,
unknown limit.  An intermediary can attempt to avoid this problem by passing on
values presented by different peers, but they are not obligated to do so.

A server that receives a larger field section than it is willing to handle can
send an HTTP 431 (Request Header Fields Too Large) status code ({{?RFC6585}}).
A client can discard responses that it cannot process.

### CONNECT Issues

The CONNECT method can be used to create disproportionate load on a proxy,
since stream creation is relatively inexpensive when compared to the creation
and maintenance of a TCP connection.  A proxy might also maintain some resources
for a TCP connection beyond the closing of the stream that carries the CONNECT
request, since the outgoing TCP connection remains in the TIME_WAIT state.
Therefore, a proxy cannot rely on QUIC stream limits alone to control the
resources consumed by CONNECT requests.

## Use of Compression

Compression can allow an attacker to recover secret data when it is compressed
in the same context as data under attacker control. HTTP/3 enables compression
of fields ({{header-formatting}}); the following concerns also apply to the use
of HTTP compressed content-codings; see Section 8.5.1 of {{!SEMANTICS}}.

There are demonstrable attacks on compression that exploit the characteristics
of the web (e.g., {{BREACH}}).  The attacker induces multiple requests
containing varying plaintext, observing the length of the resulting ciphertext
in each, which reveals a shorter length when a guess about the secret is
correct.

Implementations communicating on a secure channel MUST NOT compress content that
includes both confidential and attacker-controlled data unless separate
compression contexts are used for each source of data.  Compression MUST NOT be
used if the source of data cannot be reliably determined.

Further considerations regarding the compression of fields sections are
described in {{QPACK}}.

## Padding and Traffic Analysis

Padding can be used to obscure the exact size of frame content and is provided
to mitigate specific attacks within HTTP, for example, attacks where compressed
content includes both attacker-controlled plaintext and secret data (e.g.,
{{BREACH}}).

Where HTTP/2 employs PADDING frames and Padding fields in other frames to make a
connection more resistant to traffic analysis, HTTP/3 can either rely on
transport-layer padding or employ the reserved frame and stream types discussed
in {{frame-reserved}} and {{stream-grease}}.  These methods of padding produce
different results in terms of the granularity of padding, how padding is
arranged in relation to the information that is being protected, whether padding
is applied in the case of packet loss, and how an implementation might control
padding.

Reserved stream types can be used to give the appearance of sending traffic even
when the connection is idle.  Because HTTP traffic often occurs in bursts,
apparent traffic can be used to obscure the timing or duration of such bursts,
even to the point of appearing to send a constant stream of data.  However, as
such traffic is still flow controlled by the receiver, a failure to promptly
drain such streams and provide additional flow control credit can limit the
sender's ability to send real traffic.

To mitigate attacks that rely on compression, disabling or limiting compression
might be preferable to padding as a countermeasure.

Use of padding can result in less protection than might seem immediately
obvious.  Redundant padding could even be counterproductive.  At best, padding
only makes it more difficult for an attacker to infer length information by
increasing the number of frames an attacker has to observe.  Incorrectly
implemented padding schemes can be easily defeated.  In particular, randomized
padding with a predictable distribution provides very little protection;
similarly, padding payloads to a fixed size exposes information as payload sizes
cross the fixed-sized boundary, which could be possible if an attacker can
control plaintext.

## Frame Parsing

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.

## Early Data

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.

## Migration

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.

## Privacy Considerations

Several characteristics of HTTP/3 provide an observer an opportunity to
correlate actions of a single client or server over time.  These include the
value of settings, the timing of reactions to stimulus, and the handling of any
features that are controlled by settings.

As far as these create observable differences in behavior, they could be used as
a basis for fingerprinting a specific client.

HTTP/3's preference for using a single QUIC connection allows correlation of a
user's activity on a site.  Reusing connections for different origins allows
for correlation of activity across those origins.

Several features of QUIC solicit immediate responses and can be used by an
endpoint to measure latency to their peer; this might have privacy implications
in certain scenarios.

# IANA Considerations

This document registers a new ALPN protocol ID ({{iana-alpn}}) and creates new
registries that manage the assignment of codepoints in HTTP/3.

## Registration of HTTP/3 Identification String {#iana-alpn}

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

## New Registries {#iana-policy}

New registries created in this document operate under the QUIC registration
policy documented in Section 22.1 of {{QUIC-TRANSPORT}}.  These registries all
include the common set of fields listed in Section 22.1.1 of {{QUIC-TRANSPORT}}.
These registries \[SHALL be/are] collected under a "Hypertext Transfer Protocol
version 3 (HTTP/3) Parameters" heading.

The initial allocations in these registries created in this document are all
assigned permanent status and list a change controller of the IETF and a contact
of the HTTP working group (ietf-http-wg@w3.org).

### 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 registry follows the QUIC
registry policy; see {{iana-policy}}.  Permanent registrations in this registry
are assigned using the Specification Required policy ({{!RFC8126}}), except for
values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned
using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of
{{!RFC8126}}.

While this registry is separate from the "HTTP/2 Frame Type" registry defined in
{{?HTTP2}}, 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.

In addition to common fields as described in {{iana-policy}}, permanent
registrations in this registry MUST include the following field:

Frame Type:
: A name or label for the frame type.

Specifications of frame types MUST include a description of the frame layout and
its semantics, including any parts of the frame that are conditionally present.

The entries in {{iana-frame-table}} are registered by this document.

| ---------------- | ------ | -------------------------- |
| Frame Type       | Value  | Specification              |
| ---------------- | :----: | -------------------------- |
| DATA             |  0x0   | {{frame-data}}             |
| HEADERS          |  0x1   | {{frame-headers}}          |
| Reserved         |  0x2   | N/A                        |
| 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}}      |
| ---------------- | ------ | -------------------------- |
{: #iana-frame-table title="Initial HTTP/3 Frame Types"}

Each code of the format `0x1f * N + 0x21` for non-negative integer values of N
(that is, 0x21, 0x40, ..., through 0x3ffffffffffffffe) MUST NOT be assigned by
IANA and MUST NOT appear in the listing of assigned values.

### Settings Parameters {#iana-settings}

This document establishes a registry for HTTP/3 settings.  The "HTTP/3 Settings"
registry governs a 62-bit space.  This registry follows the QUIC registry
policy; see {{iana-policy}}.  Permanent registrations in this registry are
assigned using the Specification Required policy ({{!RFC8126}}), except for
values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned
using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of
{{!RFC8126}}.

While this registry is separate from the "HTTP/2 Settings" registry defined in
{{?HTTP2}}, 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.

In addition to common fields as described in {{iana-policy}}, permanent
registrations in this registry MUST include the following fields:

Setting Name:
: A symbolic name for the setting.  Specifying a setting name is optional.

Default:
: The value of the setting unless otherwise indicated. A default SHOULD be the
  most restrictive possible value.

The entries in {{iana-setting-table}} are registered by this document.

| ---------------------------- | ------ | ------------------------- | --------- |
| Setting Name                 |  Value | Specification             | Default   |
| ---------------------------- | :----: | ------------------------- | --------- |
| Reserved                     |  0x2   | N/A                       | N/A       |
| Reserved                     |  0x3   | N/A                       | N/A       |
| Reserved                     |  0x4   | N/A                       | N/A       |
| Reserved                     |  0x5   | N/A                       | N/A       |
| MAX_FIELD_SECTION_SIZE       |  0x6   | {{settings-parameters}}   | Unlimited |
| ---------------------------- | ------ | ------------------------- | --------- |
{: #iana-setting-table title="Initial HTTP/3 Settings"}

Each code of the format `0x1f * N + 0x21` for non-negative integer values of N
(that is, 0x21, 0x40, ..., through 0x3ffffffffffffffe) MUST NOT be assigned by
IANA and MUST NOT appear in the listing of assigned values.

### Error Codes {#iana-error-codes}

This document establishes a registry for HTTP/3 error codes. The "HTTP/3 Error
Code" registry manages a 62-bit space.  This registry follows the QUIC registry
policy; see {{iana-policy}}.  Permanent registrations in this registry are
assigned using the Specification Required policy ({{!RFC8126}}), except for
values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned
using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of
{{!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.  Use of values that
are registered in the "HTTP/2 Error Code" registry is discouraged.

In addition to common fields as described in {{iana-policy}}, this registry
includes two additional fields.  Permanent registrations in this registry MUST
include the following field:

Name:
: A name for the error code.

Description:
: A brief description of the error code semantics.

The entries in {{iana-error-table}} are registered by this document. These
error codes were selected from the range that operates on a Specification
Required policy to avoid collisions with HTTP/2 error codes.

| --------------------------------- | ---------- | ---------------------------------------- | ---------------------- |
| Name                              | Value      | Description                              | Specification          |
| --------------------------------- | ---------- | ---------------------------------------- | ---------------------- |
| H3_NO_ERROR                       | 0x0100     | No error                                 | {{http-error-codes}}   |
| H3_GENERAL_PROTOCOL_ERROR         | 0x0101     | General protocol error                   | {{http-error-codes}}   |
| H3_INTERNAL_ERROR                 | 0x0102     | Internal error                           | {{http-error-codes}}   |
| H3_STREAM_CREATION_ERROR          | 0x0103     | Stream creation error                    | {{http-error-codes}}   |
| H3_CLOSED_CRITICAL_STREAM         | 0x0104     | Critical stream was closed               | {{http-error-codes}}   |
| H3_FRAME_UNEXPECTED               | 0x0105     | Frame not permitted in the current state | {{http-error-codes}}   |
| H3_FRAME_ERROR                    | 0x0106     | Frame violated layout or size rules      | {{http-error-codes}}   |
| H3_EXCESSIVE_LOAD                 | 0x0107     | Peer generating excessive load           | {{http-error-codes}}   |
| H3_ID_ERROR                       | 0x0108     | An identifier was used incorrectly       | {{http-error-codes}}   |
| H3_SETTINGS_ERROR                 | 0x0109     | SETTINGS frame contained invalid values  | {{http-error-codes}}   |
| H3_MISSING_SETTINGS               | 0x010a     | No SETTINGS frame received               | {{http-error-codes}}   |
| H3_REQUEST_REJECTED               | 0x010b     | Request not processed                    | {{http-error-codes}}   |
| H3_REQUEST_CANCELLED              | 0x010c     | Data no longer needed                    | {{http-error-codes}}   |
| H3_REQUEST_INCOMPLETE             | 0x010d     | Stream terminated early                  | {{http-error-codes}}   |
| H3_MESSAGE_ERROR                  | 0x010e     | Malformed message                        | {{http-error-codes}}   |
| H3_CONNECT_ERROR                  | 0x010f     | TCP reset or error on CONNECT request    | {{http-error-codes}}   |
| H3_VERSION_FALLBACK               | 0x0110     | Retry over HTTP/1.1                      | {{http-error-codes}}   |
| --------------------------------- | ---------- | ---------------------------------------- | ---------------------- |
{: #iana-error-table title="Initial HTTP/3 Error Codes"}

Each code of the format `0x1f * N + 0x21` for non-negative integer values of N
(that is, 0x21, 0x40, ..., through 0x3ffffffffffffffe) MUST NOT be assigned by
IANA and MUST NOT appear in the listing of assigned values.

### 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 registry follows the
QUIC registry policy; see {{iana-policy}}.  Permanent registrations in this
registry are assigned using the Specification Required policy ({{!RFC8126}}),
except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are
assigned using Standards Action or IESG Approval as defined in Section 4.9 and
4.10 of {{!RFC8126}}.

In addition to common fields as described in {{iana-policy}}, permanent
registrations in this registry MUST include the following fields:

Stream Type:
: A name or label for the stream type.

Sender:
: Which endpoint on an HTTP/3 connection may initiate a stream of this type.
  Values are "Client", "Server", or "Both".

Specifications for permanent registrations MUST include a description of the
stream type, including the layout and semantics of the stream contents.

The entries in the following table are registered by this document.

| ---------------- | ------ | -------------------------- | ------ |
| Stream Type      | Value  | Specification              | Sender |
| ---------------- | :----: | -------------------------- | ------ |
| Control Stream   |  0x00  | {{control-streams}}        | Both   |
| Push Stream      |  0x01  | {{server-push}}            | Server |
| ---------------- | ------ | -------------------------- | ------ |

Each code of the format `0x1f * N + 0x21` for non-negative integer values of N
(that is, 0x21, 0x40, ..., through 0x3ffffffffffffffe) MUST NOT be assigned by
IANA and MUST NOT appear in the listing of assigned values.


--- back

# Considerations for Transitioning from HTTP/2 {#h2-considerations}

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 where QUIC differs from TCP,
either to take advantage of QUIC features (like streams) or to accommodate
important shortcomings (such as a lack of total ordering). These differences
make HTTP/3 similar to HTTP/2 in key aspects, such as the relationship of
requests and responses to streams. However, the details of the HTTP/3 design are
substantially different from HTTP/2.

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 same
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.

In contrast to HTTP/2, stream concurrency in HTTP/3 is managed by QUIC.  QUIC
considers a stream closed when all data has been received and sent data has been
acknowledged by the peer.  HTTP/2 considers a stream closed when the frame
containing the END_STREAM bit has been committed to the transport. As a result,
the stream for an equivalent exchange could remain "active" for a longer period
of time.  HTTP/3 servers might choose to permit a larger number of concurrent
client-initiated bidirectional streams to achieve equivalent concurrency to
HTTP/2, depending on the expected usage patterns.

Due to the presence of other unidirectional stream types, HTTP/3 does not rely
exclusively on the number of concurrent unidirectional streams to control the
number of concurrent in-flight pushes.  Instead, HTTP/3 clients use the
MAX_PUSH_ID frame to control the number of pushes received from an HTTP/3
server.

## 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 {{?HTTP2}}. However, QUIC includes many
features (e.g., flow control) that 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.

Some examples of feature adaptations are described below, as well as general
guidance to extension frame implementors converting an HTTP/2 extension to
HTTP/3.

### Prioritization Differences {#h2-diff-priority}

HTTP/2 specifies priority assignments in PRIORITY frames and (optionally) in
HEADERS frames. HTTP/3 does not provide a means of signaling priority.

Note that while there is no explicit signaling for priority, this does not mean
that prioritization is not important for achieving good performance.

### Field Compression Differences

HPACK was designed with the assumption of in-order delivery. A sequence of
encoded field sections 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.

Because this total ordering is not provided by QUIC, HTTP/3 uses a modified
version of HPACK, called QPACK.  QPACK uses a single unidirectional stream to
make all modifications to the dynamic table, ensuring a total order of updates.
All frames that contain encoded fields merely reference the table state at a
given time without modifying it.

[QPACK] provides additional details.

### Flow Control Differences

HTTP/2 specifies a stream flow control mechanism. Although all HTTP/2 frames are
delivered on streams, only the DATA frame payload is subject to flow control.
QUIC provides flow control for stream data and all HTTP/3 frame types defined in
this document are sent on streams. Therefore, all frame headers and payload are
subject to flow control.

### Guidance for New Frame Type Definitions

Frame type definitions in HTTP/3 often use the QUIC variable-length integer
encoding.  In particular, Stream IDs use this encoding, which allows 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). 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
that depend on the presence of flags need to allocate space for flags as part
of their frame payload.

Other than these issues, 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.

### Mapping Between HTTP/2 and HTTP/3 Frame Types

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. Padding is not
  defined in HTTP/3 frames.  See {{frame-headers}}.

PRIORITY (0x2):
: As described in {{h2-diff-priority}}, HTTP/3 does not provide a means of
  signaling priority.

RST_STREAM (0x3):
: RST_STREAM frames do not exist in HTTP/3, 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 frame 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 in HTTP/3, as QUIC provides equivalent
  functionality.

GOAWAY (0x7):
: GOAWAY does not contain an error code.  In the client to server direction,
  it carries a Push ID instead of a server initiated stream ID.
  See {{frame-goaway}}.

WINDOW_UPDATE (0x8):
: WINDOW_UPDATE frames do not exist in HTTP/3, since QUIC provides flow control.

CONTINUATION (0x9):
: CONTINUATION frames do not exist in HTTP/3; 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 {{?HTTP2}} 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, as the first
frame of the control stream, 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.  The superseded
settings are reserved, and their receipt is an error.  See
{{settings-parameters}} for discussion of both the retained and reserved values.

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 frame, which provides a more
  granular control over server push.  Specifying a setting with the identifier
  0x2 (corresponding to the SETTINGS_ENABLE_PUSH parameter) in the HTTP/3
  SETTINGS frame is an error.

SETTINGS_MAX_CONCURRENT_STREAMS:
: QUIC controls the largest open Stream ID as part of its flow control logic.
  Specifying a setting with the identifier 0x3 (corresponding to the
  SETTINGS_MAX_CONCURRENT_STREAMS parameter) in the HTTP/3 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 a setting with the
  identifier 0x4 (corresponding to the SETTINGS_INITIAL_WINDOW_SIZE parameter)
  in the HTTP/3 SETTINGS frame is an error.

SETTINGS_MAX_FRAME_SIZE:
: This setting has no equivalent in HTTP/3.  Specifying a setting with the
  identifier 0x5 (corresponding to the SETTINGS_MAX_FRAME_SIZE parameter) in the
  HTTP/3 SETTINGS frame is an error.

SETTINGS_MAX_HEADER_LIST_SIZE:
: This setting identifier has been renamed SETTINGS_MAX_FIELD_SECTION_SIZE.

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 that
use the full 32-bit space.  Settings ported from HTTP/2 might choose to redefine
their value to limit it to 30 bits for more efficient encoding, or to make use
of the 62-bit space if more than 30 bits are required.

Settings need to be defined separately for HTTP/2 and HTTP/3. The IDs of
settings defined in {{?HTTP2}} 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}}.

As QUIC streams might arrive out of order, endpoints are advised not to wait for
the peers' settings to arrive before responding to other streams.  See
{{settings-initialization}}.


## HTTP/2 Error Codes

QUIC has the same concepts of "stream" and "connection" errors that HTTP/2
provides. However, the differences between HTTP/2 and HTTP/3 mean that error
codes are not directly portable between versions.

The HTTP/2 error codes defined in Section 7 of {{?HTTP2}} logically map to
the HTTP/3 error codes as follows:

NO_ERROR (0x0):
: H3_NO_ERROR in {{http-error-codes}}.

PROTOCOL_ERROR (0x1):
: This is mapped to H3_GENERAL_PROTOCOL_ERROR except in cases where more
  specific error codes have been defined. Such cases include
  H3_FRAME_UNEXPECTED, H3_MESSAGE_ERROR, and H3_CLOSED_CRITICAL_STREAM defined
  in {{http-error-codes}}.

INTERNAL_ERROR (0x2):
: H3_INTERNAL_ERROR in {{http-error-codes}}.

FLOW_CONTROL_ERROR (0x3):
: Not applicable, since QUIC handles flow control.

SETTINGS_TIMEOUT (0x4):
: Not applicable, since no acknowledgment of SETTINGS is defined.

STREAM_CLOSED (0x5):
: Not applicable, since QUIC handles stream management.

FRAME_SIZE_ERROR (0x6):
: H3_FRAME_ERROR error code defined in {{http-error-codes}}.

REFUSED_STREAM (0x7):
: H3_REQUEST_REJECTED (in {{http-error-codes}}) is used to indicate that a
  request was not processed. Otherwise, not applicable because QUIC handles
  stream management.

CANCEL (0x8):
: H3_REQUEST_CANCELLED in {{http-error-codes}}.

COMPRESSION_ERROR (0x9):
: Multiple error codes are defined in [QPACK].

CONNECT_ERROR (0xa):
: H3_CONNECT_ERROR in {{http-error-codes}}.

ENHANCE_YOUR_CALM (0xb):
: H3_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):
: H3_VERSION_FALLBACK in {{http-error-codes}}.

Error codes need to be defined for HTTP/2 and HTTP/3 separately.  See
{{iana-error-codes}}.

### Mapping Between HTTP/2 and HTTP/3 Errors

An intermediary that converts between HTTP/2 and HTTP/3 may encounter error
conditions from either upstream. It is useful to communicate the occurrence of
error to the downstream but error codes largely reflect connection-local
problems that generally do not make sense to propagate.

An intermediary that encounters an error from an upstream origin can indicate
this by sending an HTTP status code such as 502, which is suitable for a broad
class of errors.

There are some rare cases where it is beneficial to propagate the error by
mapping it to the closest matching error type to the receiver. For example, an
intermediary that receives an HTTP/2 stream error of type REFUSED_STREAM from
the origin has a clear signal that the request was not processed and that the
request is safe to retry. Propagating this error condition to the client as an
HTTP/3 stream error of type H3_REQUEST_REJECTED allows the client to take the
action it deems most appropriate. In the reverse direction, the intermediary
might deem it beneficial to pass on client request cancellations that are
indicated by terminating a stream with H3_REQUEST_CANCELLED; see
{{request-cancellation}}.

Conversion between errors is described in the logical mapping. The error codes
are defined in non-overlapping spaces in order to protect against accidental
conversion that could result in the use of inappropriate or unknown error codes
for the target version. An intermediary is permitted to promote stream errors to
connection errors but they should be aware of the cost to the HTTP/3 connection
for what might be a temporary or intermittent error.

# 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-32

- Removed draft version guidance; added final version string
- Added H3_MESSAGE_ERROR for malformed messages

## Since draft-ietf-quic-http-31

Editorial changes only.

## Since draft-ietf-quic-http-30

Editorial changes only.

## Since draft-ietf-quic-http-29

- Require a connection error if a reserved frame type that corresponds to a
  frame in HTTP/2 is received (#3991, #3993)
- Require a connection error if a reserved setting that corresponds to a
  setting in HTTP/2 is received (#3954, #3955)

## Since draft-ietf-quic-http-28

- CANCEL_PUSH is recommended even when the stream is reset (#3698, #3700)
- Use H3_ID_ERROR when GOAWAY contains a larger identifier (#3631, #3634)

## Since draft-ietf-quic-http-27

- Updated text to refer to latest HTTP revisions
- Use the HTTP definition of authority for establishing and coalescing
  connections (#253, #2223, #3558)
- Define use of GOAWAY from both endpoints (#2632, #3129)
- Require either :authority or Host if the URI scheme has a mandatory
  authority component (#3408, #3475)

## Since draft-ietf-quic-http-26

- No changes

## Since draft-ietf-quic-http-25

- Require QUICv1 for HTTP/3 (#3117, #3323)
- Remove DUPLICATE_PUSH and allow duplicate PUSH_PROMISE (#3275, #3309)
- Clarify the definition of "malformed" (#3352, #3345)

## Since draft-ietf-quic-http-24

- Removed H3_EARLY_RESPONSE error code; H3_NO_ERROR is recommended instead
  (#3130,#3208)
- Unknown error codes are equivalent to H3_NO_ERROR (#3276,#3331)
- Some error codes are reserved for greasing (#3325,#3360)

## Since draft-ietf-quic-http-23

- Removed `quic` Alt-Svc parameter (#3061,#3118)
- Clients need not persist unknown settings for use in 0-RTT (#3110,#3113)
- Clarify error cases around CANCEL_PUSH (#2819,#3083)

## Since draft-ietf-quic-http-22

- Removed priority signaling (#2922,#2924)
- Further changes to error codes (#2662,#2551):
  - Error codes renumbered
  - HTTP_MALFORMED_FRAME replaced by HTTP_FRAME_ERROR, HTTP_ID_ERROR, and others
- Clarify how unknown frame types interact with required frame sequence
  (#2867,#2858)
- Describe interactions with the transport in terms of defined interface terms
  (#2857,#2805)
- Require the use of the `http-opportunistic` resource (RFC 8164) when scheme is
  `http` (#2439,#2973)
- Settings identifiers cannot be duplicated (#2979)
- Changes to SETTINGS frames in 0-RTT (#2972,#2790,#2945):
  - Servers must send all settings with non-default values in their SETTINGS
    frame, even when resuming
  - If a client doesn't have settings associated with a 0-RTT ticket, it uses
    the defaults
  - Servers can't accept early data if they cannot recover the settings the
    client will have remembered
- Clarify that Upgrade and the 101 status code are prohibited (#2898,#2889)
- Clarify that frame types reserved for greasing can occur on any stream, but
  frame types reserved due to HTTP/2 correspondence are prohibited
  (#2997,#2692,#2693)
- Unknown error codes cannot be treated as errors (#2998,#2816)

## Since draft-ietf-quic-http-21

No changes

## Since draft-ietf-quic-http-20

- Prohibit closing the control stream (#2509, #2666)
- Change default priority to use an orphan node (#2502, #2690)
- Exclusive priorities are restored (#2754, #2781)
- Restrict use of frames when using CONNECT (#2229, #2702)
- Close and maybe reset streams if a connection error occurs for CONNECT (#2228,
  #2703)
- Encourage provision of sufficient unidirectional streams for QPACK (#2100,
  #2529, #2762)
- Allow extensions to use server-initiated bidirectional streams (#2711, #2773)
- Clarify use of maximum header list size setting (#2516, #2774)
- Extensive changes to error codes and conditions of their sending
  - Require connection errors for more error conditions (#2511, #2510)
  - Updated the error codes for illegal GOAWAY frames (#2714, #2707)
  - Specified error code for HEADERS on control stream (#2708)
  - Specified error code for servers receiving PUSH_PROMISE (#2709)
  - Specified error code for receiving DATA before HEADERS (#2715)
  - Describe malformed messages and their handling (#2410, #2764)
  - Remove HTTP_PUSH_ALREADY_IN_CACHE error (#2812, #2813)
  - Refactor Push ID related errors (#2818, #2820)
  - Rationalize HTTP/3 stream creation errors (#2821, #2822)

## 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

# Acknowledgments
{:numbered="false"}

The original authors of this specification were Robbie Shade and Mike Warres.

The IETF QUIC Working Group received an enormous amount of support from many
people. Among others, the following people provided substantial contributions to
this document:

- Bence Béky
- Daan De Meyer
- Martin Duke
- Roy Fielding
- Alan Frindell
- Alessandro Ghedini
- Nick Harper
- Ryan Hamilton
- Christian Huitema
- Subodh Iyengar
- Robin Marx
- Patrick McManus
- Luca Niccolini
- <t><t><contact asciiFullname="Kazuho Oku" fullname="奥 一穂"/></t></t>
- Lucas Pardue
- Roberto Peon
- Julian Reschke
- Eric Rescorla
- Martin Seemann
- Ben Schwartz
- Ian Swett
- Willy Taureau
- Martin Thomson
- Dmitri Tikhonov
- Tatsuhiro Tsujikawa

A portion of Mike's contribution was supported by Microsoft during his
employment there.