wdiff rfc9113v1.txt rfc9113.txt

Internet Engineering Task Force (IETF) M. Thomson, Ed.
Request for Comments: 9113 Mozilla
Obsoletes: 7540, 8740 C. Benfield, Ed.
Category: Standards Track Apple Inc.
ISSN: 2070-1721 March June 2022

HTTP/2

Abstract

This specification describes an optimized expression of the semantics
of the Hypertext Transfer Protocol (HTTP), referred to as HTTP
version 2 (HTTP/2). HTTP/2 enables a more efficient use of network
resources and a reduced latency by introducing field compression and
allowing multiple concurrent exchanges on the same connection.

This document obsoletes RFCs 7540 and 8740.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9113.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. HTTP/2 Protocol Overview
2.1. Document Organization
2.2. Conventions and Terminology
3. Starting HTTP/2
3.1. HTTP/2 Version Identification
3.2. Starting HTTP/2 for "https" URIs
3.3. Starting HTTP/2 with Prior Knowledge
3.4. HTTP/2 Connection Preface
4. HTTP Frames
4.1. Frame Format
4.2. Frame Size
4.3. Field Section Compression and Decompression
4.3.1. Compression State
5. Streams and Multiplexing
5.1. Stream States
5.1.1. Stream Identifiers
5.1.2. Stream Concurrency
5.2. Flow Control
5.2.1. Flow-Control Principles
5.2.2. Appropriate Use of Flow Control
5.2.3. Flow-Control Performance
5.3. Prioritization
5.3.1. Background on Priority in RFC 7540
5.3.2. Priority Signaling in This Document
5.4. Error Handling
5.4.1. Connection Error Handling
5.4.2. Stream Error Handling
5.4.3. Connection Termination
5.5. Extending HTTP/2
6. Frame Definitions
6.1. DATA
6.2. HEADERS
6.3. PRIORITY
6.4. RST_STREAM
6.5. SETTINGS
6.5.1. SETTINGS Format
6.5.2. Defined Settings
6.5.3. Settings Synchronization
6.6. PUSH_PROMISE
6.7. PING
6.8. GOAWAY
6.9. WINDOW_UPDATE
6.9.1. The Flow-Control Window
6.9.2. Initial Flow-Control Window Size
6.9.3. Reducing the Stream Window Size
6.10. CONTINUATION
7. Error Codes
8. Expressing HTTP Semantics in HTTP/2
8.1. HTTP Message Framing
8.1.1. Malformed Messages
8.2. HTTP Fields
8.2.1. Field Validity
8.2.2. Connection-Specific Header Fields
8.2.3. Compressing the Cookie Header Field
8.3. HTTP Control Data
8.3.1. Request Pseudo-Header Fields
8.3.2. Response Pseudo-Header Fields
8.4. Server Push
8.4.1. Push Requests
8.4.2. Push Responses
8.5. The CONNECT Method
8.6. The Upgrade Header Field
8.7. Request Reliability
8.8. Examples
8.8.1. Simple Request
8.8.2. Simple Response
8.8.3. Complex Request
8.8.4. Response with Body
8.8.5. Informational Responses
9. HTTP/2 Connections
9.1. Connection Management
9.1.1. Connection Reuse
9.2. Use of TLS Features
9.2.1. TLS 1.2 Features
9.2.2. TLS 1.2 Cipher Suites
9.2.3. TLS 1.3 Features
10. Security Considerations
10.1. Server Authority
10.2. Cross-Protocol Attacks
10.3. Intermediary Encapsulation Attacks
10.4. Cacheability of Pushed Responses
10.5. Denial-of-Service Considerations
10.5.1. Limits on Field Block Size
10.5.2. CONNECT Issues
10.6. Use of Compression
10.7. Use of Padding
10.8. Privacy Considerations
10.9. Remote Timing Attacks
11. IANA Considerations
11.1. HTTP2-Settings Header Field Registration
11.2. The h2c Upgrade Token
12. References
12.1. Normative References
12.2. Informative References
Appendix A. Prohibited TLS 1.2 Cipher Suites
Appendix B. Changes from RFC 7540
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

The performance of applications using the Hypertext Transfer Protocol
[HTTP]
(HTTP, [HTTP]) is linked to how each version of HTTP uses the
underlying transport, and the conditions under which the transport
operates.

Making multiple concurrent requests can reduce latency and improve
application performance. HTTP/1.0 allowed only one request to be
outstanding at a time on a given TCP [TCP] connection. HTTP/1.1
[HTTP/1.1] added request pipelining, but this only partially
addressed request concurrency and still suffers from application-
layer head-of-line blocking. Therefore, HTTP/1.0 and HTTP/1.1
clients use multiple connections to a server to make concurrent
requests.

Furthermore, HTTP fields are often repetitive and verbose, causing
unnecessary network traffic as well as causing the initial TCP
congestion window to quickly fill. This can result in excessive
latency when multiple requests are made on a new TCP connection.

HTTP/2 addresses these issues by defining an optimized mapping of
HTTP's semantics to an underlying connection. Specifically, it
allows interleaving of messages on the same connection and uses an
efficient coding for HTTP fields. It also allows prioritization of
requests, letting more important requests complete more quickly,
further improving performance.

The resulting protocol is more friendly to the network because fewer
TCP connections can be used in comparison to HTTP/1.x. This means
less competition with other flows and longer-lived connections, which
in turn lead to better utilization of available network capacity.
Note, however, that TCP head-of-line blocking is not addressed by
this protocol.

Finally, HTTP/2 also enables more efficient processing of messages
through use of binary message framing.

This document obsoletes RFCs 7540 and 8740. Appendix B lists notable
changes.

2. HTTP/2 Protocol Overview

HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2
supports all of the core features of HTTP but aims to be more
efficient than HTTP/1.1.

HTTP/2 is a connection-oriented application-layer protocol that runs
over a TCP connection [TCP]. ([TCP]). The client is the TCP connection
initiator.

The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each
frame type serves a different purpose. For example, HEADERS and DATA
frames form the basis of HTTP requests and responses (Section 8.1);
other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are
used in support of other HTTP/2 features.

Multiplexing of requests is achieved by having each HTTP request/
response exchange associated with its own stream (Section 5).
Streams are largely independent of each other, so a blocked or
stalled request or response does not prevent progress on other
streams.

Effective use of multiplexing depends on flow control and
prioritization. Flow control (Section 5.2) ensures that it is
possible to efficiently use multiplexed streams by restricting data
that is transmitted to what the receiver is able to handle.
Prioritization (Section 5.3) ensures that limited resources are used
most effectively. This revision of HTTP/2 deprecates the priority
signaling scheme from [RFC7540].

Because HTTP fields used in a connection can contain large amounts of
redundant data, frames that contain them are compressed
(Section 4.3). This has especially advantageous impact upon request
sizes in the common case, allowing many requests to be compressed
into one packet.

Finally, HTTP/2 adds a new, optional interaction mode whereby a
server can push responses to a client (Section 8.4). This is
intended to allow a server to speculatively send data to a client
that the server anticipates the client will need, trading off some
network usage against a potential latency gain. The server does this
by synthesizing a request, which it sends as a PUSH_PROMISE frame.
The server is then able to send a response to the synthetic request
on a separate stream.

2.1. Document Organization

The HTTP/2 specification is split into four parts:

* Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
initiated.

* The frame (Section 4) and stream (Section 5) layers describe the
way HTTP/2 frames are structured and formed into multiplexed
streams.

* Frame (Section 6) and error (Section 7) definitions include
details of the frame and error types used in HTTP/2.

* HTTP mappings (Section 8) and additional requirements (Section 9)
describe how HTTP semantics are expressed using frames and
streams.

While some of the frame- and stream-layer concepts are isolated from
HTTP, this specification does not define a completely generic frame
layer. The frame and stream layers are tailored to the needs of the
HTTP protocol.
HTTP.

2.2. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate. Hexadecimal literals are prefixed
with "0x" to distinguish them from decimal literals.

This specification describes binary formats using the conventions
described in Section 1.3 of RFC 9000 [QUIC]. Note that this format
uses network byte order and that high-valued bits are listed before
low-valued bits.

The following terms are used:

client: The endpoint that initiates an HTTP/2 connection. Clients
send HTTP requests and receive HTTP responses.

connection: A transport-layer connection between two endpoints.

connection error: An error that affects the entire HTTP/2
connection.

endpoint: Either the client or server of the connection.

frame: The smallest unit of communication within an HTTP/2
connection, consisting of a header and a variable-length sequence
of octets structured according to the frame type.

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/2 connection. Servers
receive HTTP requests and send HTTP responses.

stream: A bidirectional flow of frames within the HTTP/2 connection.

stream error: An error on the individual HTTP/2 stream.

Finally, the terms "gateway", "intermediary", "proxy", and "tunnel"
are defined in Section 3.7 of [HTTP]. Intermediaries act as both
client and server at different times.

The term "content" as it applies to message bodies is defined in
Section 6.4 of [HTTP].

3. Starting HTTP/2

Implementations that generate HTTP requests need to discover whether
a server supports HTTP/2.

HTTP/2 uses the "http" and "https" URI schemes defined in Section 4.2
of [HTTP], with the same default port numbers as HTTP/1.1 [HTTP/1.1].
These URIs do not include any indication about what HTTP versions an
upstream server (the immediate peer to which the client wishes to
establish a connection) supports.

The means by which support for HTTP/2 is determined is different for
"http" and "https" URIs. Discovery for "https" URIs is described in
Section 3.2. HTTP/2 support for "http" URIs can only be discovered
by out-of-band means and requires prior knowledge of the support as
described in Section 3.3.

3.1. HTTP/2 Version Identification

The protocol defined in this document has two identifiers. Creating
a connection based on either implies the use of the transport,
framing, and message semantics described in this document.

* The string "h2" identifies the protocol where HTTP/2 uses
Transport Layer Security (TLS); see Section 9.2. This identifier
is used in the TLS Application-Layer Protocol Negotiation (ALPN)
extension [TLS-ALPN] field and in any place where HTTP/2 over TLS
is identified.

The "h2" string is serialized into an ALPN protocol identifier as
the two-octet sequence: 0x68, 0x32.

* The "h2c" string was previously used as a token for use in the
HTTP Upgrade mechanism's Upgrade header field (Section 7.8 of
[HTTP]). This usage was never widely deployed and is deprecated
by this document. The same applies to the HTTP2-Settings header
field, which was used with the upgrade to "h2c".

3.2. Starting HTTP/2 for "https" URIs

A client that makes a request to an "https" URI uses TLS [TLS13] with
the ALPN extension [TLS-ALPN].

HTTP/2 over TLS uses the "h2" protocol identifier. The "h2c"
protocol identifier MUST NOT be sent by a client or selected by a
server; the "h2c" protocol identifier describes a protocol that does
not use TLS.

Once TLS negotiation is complete, both the client and the server MUST
send a connection preface (Section 3.4).

3.3. Starting HTTP/2 with Prior Knowledge

A client can learn that a particular server supports HTTP/2 by other
means. For example, a client could be configured with knowledge that
a server supports HTTP/2.

A client that knows that a server supports HTTP/2 can establish a TCP
connection and send the connection preface (Section 3.4) followed by
HTTP/2 frames. Servers can identify these connections by the
presence of the connection preface. This only affects the
establishment of HTTP/2 connections over cleartext TCP; HTTP/2
connections over TLS MUST use protocol negotiation in TLS [TLS-ALPN].

Likewise, the server MUST send a connection preface (Section 3.4).

Without additional information, prior support for HTTP/2 is not a
strong signal that a given server will support HTTP/2 for future
connections. For example, it is possible for server configurations
to change, for configurations to differ between instances in
clustered servers, or for network conditions to change.

3.4. HTTP/2 Connection Preface

In HTTP/2, each endpoint is required to send a connection preface as
a final confirmation of the protocol in use and to establish the
initial settings for the HTTP/2 connection. The client and server
each send a different connection preface.

The client connection preface starts with a sequence of 24 octets,
which in hex notation is:

0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

That is, the connection preface starts with the string "PRI *
HTTP/2.0\r\n\r\nSM\r\n\r\n". This sequence MUST be followed by a
SETTINGS frame (Section 6.5), which MAY be empty. The client sends
the client connection preface as the first application data octets of
a connection.

| Note: The client connection preface is selected so that a large
| proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries
| do not attempt to process further frames. Note that this does
| not address the concerns raised in [TALKING].

The server connection preface consists of a potentially empty
SETTINGS frame (Section 6.5) that MUST be the first frame the server
sends in the HTTP/2 connection.

The SETTINGS frames received from a peer as part of the connection
preface MUST be acknowledged (see Section 6.5.3) after sending the
connection preface.

To avoid unnecessary latency, clients are permitted to send
additional frames to the server immediately after sending the client
connection preface, without waiting to receive the server connection
preface. It is important to note, however, that the server
connection preface SETTINGS frame might include settings that
necessarily alter how a client is expected to communicate with the
server. Upon receiving the SETTINGS frame, the client is expected to
honor any settings established. In some configurations, it is
possible for the server to transmit SETTINGS before the client sends
additional frames, providing an opportunity to avoid this issue.

Clients and servers MUST treat an invalid connection preface as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR. A GOAWAY
frame (Section 6.8) MAY be omitted in this case, since an invalid
preface indicates that the peer is not using HTTP/2.

4. HTTP Frames

Once the HTTP/2 connection is established, endpoints can begin
exchanging frames.

4.1. Frame Format

All frames begin with a fixed 9-octet header followed by a variable-
length frame payload.

HTTP Frame {
Length (24),
Type (8),

Flags (8),

Reserved (1),
Stream Identifier (31),

Frame Payload (..),
}

Figure 1: Frame Layout

The fields of the frame header are defined as:

Length: The length of the frame payload expressed as an unsigned
24-bit integer in units of octets. Values greater than 2^14
(16,384) MUST NOT be sent unless the receiver has set a larger
value for SETTINGS_MAX_FRAME_SIZE.

The 9 octets of the frame header are not included in this value.

Type: The 8-bit type of the frame. The frame type determines the
format and semantics of the frame. Frames defined in this
document are listed in Section 6. Implementations MUST ignore and
discard frames of unknown types.

Flags: An 8-bit field reserved for boolean flags specific to the
frame type.

Flags are assigned semantics specific to the indicated frame type.
Unused flags are those that have no defined semantics for a
particular frame type. Unused flags MUST be ignored on receipt
and MUST be left unset (0x0) (0x00) when sending.

Reserved: A reserved 1-bit field. The semantics of this bit are
undefined, and the bit MUST remain unset (0x0) (0x00) when sending and
MUST be ignored when receiving.

Stream Identifier: A stream identifier (see Section 5.1.1) expressed
as an unsigned 31-bit integer. The value 0x0 0x00 is reserved for
frames that are associated with the connection as a whole as
opposed to an individual stream.

The structure and content of the frame payload are dependent entirely
on the frame type.

4.2. Frame Size

The size of a frame payload is limited by the maximum size that a
receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This
setting can have any value between 2^14 (16,384) and 2^24-1
(16,777,215) octets, inclusive.

All implementations MUST be capable of receiving and minimally
processing frames up to 2^14 octets in length, plus the 9-octet frame
header (Section 4.1). The size of the frame header is not included
when describing frame sizes.

| Note: Certain frame types, such as PING (Section 6.7), impose
| additional limits on the amount of frame payload data allowed.

An endpoint MUST send an error code of FRAME_SIZE_ERROR if a frame
exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, exceeds any
limit defined for the frame type, or is too small to contain
mandatory frame data. A frame size error in a frame that could alter
the state of the entire connection MUST be treated as a connection
error (Section 5.4.1); this includes any frame carrying a field block
(Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION),
SETTINGS, a
SETTINGS frame, and any frame with a stream identifier of 0.

Endpoints are not obligated to use all available space in a frame.
Responsiveness can be improved by using frames that are smaller than
the permitted maximum size. Sending large frames can result in
delays in sending time-sensitive frames (such as RST_STREAM,
WINDOW_UPDATE, or PRIORITY), which, if blocked by the transmission of
a large frame, could affect performance.

4.3. Field Section Compression and Decompression

Field section compression is the process of compressing a set of
field lines (Section 5.2 of [HTTP]) to form a field block. Field
section decompression is the process of decoding a field block into a
set of field lines. Details of HTTP/2 field section compression and
decompression are defined in [COMPRESSION], which, for historical
reasons, refers to these processes as header compression and
decompression.

Each field block carries all of the compressed field lines of a
single field section. Header sections also include control data
associated with the message in the form of pseudo-header fields
(Section 8.3) that use the same format as a field line.

| Note: RFC 7540 [RFC7540] used the term "header block" in place
| of the more generic "field block".

Field blocks carry control data and header sections for requests,
responses, promised requests, and pushed responses (see Section 8.4).
All these messages, except for interim responses and requests
contained in PUSH_PROMISE (Section 6.6) frames, can optionally
include a field block that carries a trailer section.

A field section is a collection of field lines. Each of the field
lines in a field block carries a single value. The serialized field
block is then divided into one or more octet sequences, called field
block fragments. The first field block fragment is transmitted
within the frame payload of HEADERS (Section 6.2) or PUSH_PROMISE
(Section 6.6), each of which could be followed by CONTINUATION
(Section 6.10) frames to carry subsequent field block fragments.

The Cookie header field [COOKIE] is treated specially by the HTTP
mapping (see Section 8.2.3).

A receiving endpoint reassembles the field block by concatenating its
fragments and then decompresses the block to reconstruct the field
section.

A complete field section consists of either:

* a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
set, or

* a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag unset
and one or more CONTINUATION frames, where the last CONTINUATION
frame has the END_HEADERS flag set.

Each field block is processed as a discrete unit. Field blocks MUST
be transmitted as a contiguous sequence of frames, with no
interleaved frames of any other type or from any other stream. The
last frame in a sequence of HEADERS or CONTINUATION frames has the
END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE
or CONTINUATION frames has the END_HEADERS flag set. This allows a
field block to be logically equivalent to a single frame.

Field block fragments can only be sent as the frame payload of
HEADERS, PUSH_PROMISE, or CONTINUATION frames because these frames
carry data that can modify the compression context maintained by a
receiver. An endpoint receiving HEADERS, PUSH_PROMISE, or
CONTINUATION frames needs to reassemble field blocks and perform
decompression even if the frames are to be discarded. A receiver
MUST terminate the connection with a connection error (Section 5.4.1)
of type COMPRESSION_ERROR if it does not decompress a field block.

A decoding error in a field block MUST be treated as a connection
error (Section 5.4.1) of type COMPRESSION_ERROR.

4.3.1. Compression State

Field compression is stateful. Each endpoint has an HPACK encoder
context and an HPACK decoder context that are used for encoding and
decoding all field blocks on a connection. Section 4 of
[COMPRESSION] defines the dynamic table, which is the primary state
for each context.

The dynamic table has a maximum size that is set by an HPACK decoder.
An endpoint communicates the size chosen by its HPACK decoder context
using the SETTINGS_HEADER_TABLE_SIZE setting; see Section 6.5.2.
When a connection is established, the dynamic table size for the
HPACK decoder and encoder at both endpoints starts at 4,096 bytes,
the initial value of the SETTINGS_HEADER_TABLE_SIZE setting.

Any change to the maximum value set using SETTINGS_HEADER_TABLE_SIZE
takes effect when the endpoint acknowledges settings (Section 6.5.3).
The HPACK encoder at that endpoint can set the dynamic table to any
size up to the maximum value set by the decoder. An HPACK encoder
declares the size of the dynamic table with a Dynamic Table Size
Update instruction (Section 6.3 of [COMPRESSION]).

Once an endpoint acknowledges a change to SETTINGS_HEADER_TABLE_SIZE
that reduces the maximum below the current size of the dynamic table,
its HPACK encoder MUST start the next field block with a Dynamic
Table Size Update instruction that sets the dynamic table to a size
that is less than or equal to the reduced maximum; see Section 4.2 of
[COMPRESSION]. An endpoint MUST treat a field block that follows an
acknowledgment of the reduction to the maximum dynamic table size as
a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it
does not start with a conformant Dynamic Table Size Update
instruction.

| Implementers are advised that reducing the value of
| SETTINGS_HEADER_TABLE_SIZE is not widely interoperable. Use of
| the connection preface to reduce the value below the initial
| value of 4,096 is somewhat better supported, but this might
| fail with some implementations.

5. Streams and Multiplexing

A "stream" is an independent, bidirectional sequence of frames
exchanged between the client and server within an HTTP/2 connection.
Streams have several important characteristics:

* A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple
streams.

* Streams can be established and used unilaterally or shared by
either endpoint.

* Streams can be closed by either endpoint.

* The order in which frames are sent is significant. Recipients
process frames in the order they are received. In particular, the
order of HEADERS and DATA frames is semantically significant.

* Streams are identified by an integer. Stream identifiers are
assigned to streams by the endpoint initiating the stream.

5.1. Stream States

The lifecycle of a stream is shown in Figure 2.

Figure 2: Stream States

send: endpoint sends this frame
recv: endpoint receives this frame
H: HEADERS frame (with implied CONTINUATION frames)
ES: END_STREAM flag
R: RST_STREAM frame
PP: PUSH_PROMISE frame (with implied CONTINUATION frames); state
transitions are for the promised stream

Note that this diagram shows stream state transitions and the frames
and flags that affect those transitions only. In this regard,
CONTINUATION frames do not result in state transitions; they are
effectively part of the HEADERS or PUSH_PROMISE that they follow.
For the purpose of state transitions, the END_STREAM flag is
processed as a separate event to the frame that bears it; a HEADERS
frame with the END_STREAM flag set can cause two state transitions.

Both endpoints have a subjective view of the state of a stream that
could be different when frames are in transit. Endpoints do not
coordinate the creation of streams; they are created unilaterally by
either endpoint. The negative consequences of a mismatch in states
are limited to the "closed" state after sending RST_STREAM, where
frames might be received for some time after closing.

Streams have the following states:

idle: All streams start in the "idle" state.

The following transitions are valid from this state:

* Sending a HEADERS frame as a client, or receiving a HEADERS
frame as a server, causes the stream to become "open". The
stream identifier is selected as described in Section 5.1.1.
The same HEADERS frame can also cause a stream to immediately
become "half-closed".

* Sending a PUSH_PROMISE frame on another stream reserves the
idle stream that is identified for later use. The stream state
for the reserved stream transitions to "reserved (local)".
Only a server may send PUSH_PROMISE frames.

* Receiving a PUSH_PROMISE frame on another stream reserves an
idle stream that is identified for later use. The stream state
for the reserved stream transitions to "reserved (remote)".
Only a client may receive PUSH_PROMISE frames.

* Note that the PUSH_PROMISE frame is not sent on the idle stream
but references the newly reserved stream in the Promised Stream
ID field.

* Opening a stream with a higher-valued stream identifier causes
the stream to transition immediately to a "closed" state; note
that this transition is not shown in the diagram.

Receiving any frame other than HEADERS or PRIORITY on a stream in
this state MUST be treated as a connection error (Section 5.4.1)
of type PROTOCOL_ERROR. If this stream is initiated by the
server, as described in Section 5.1.1, then receiving a HEADERS
frame MUST also be treated as a connection error (Section 5.4.1)
of type PROTOCOL_ERROR.

reserved (local): A stream in the "reserved (local)" state is one
that has been promised by sending a PUSH_PROMISE frame. A
PUSH_PROMISE frame reserves an idle stream by associating the
stream with an open stream that was initiated by the remote peer
(see Section 8.4).

In this state, only the following transitions are possible:

* The endpoint can send a HEADERS frame. This causes the stream
to open in a "half-closed (remote)" state.

* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.

An endpoint MUST NOT send any type of frame other than HEADERS,
RST_STREAM, or PRIORITY in this state.

A PRIORITY or WINDOW_UPDATE frame MAY be received in this state.
Receiving any type of frame other than RST_STREAM, PRIORITY, or
WINDOW_UPDATE on a stream in this state MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.

reserved (remote): A stream in the "reserved (remote)" state has
been reserved by a remote peer.

In this state, only the following transitions are possible:

* Receiving a HEADERS frame causes the stream to transition to
"half-closed (local)".

* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.

An endpoint MUST NOT send any type of frame other than RST_STREAM,
WINDOW_UPDATE, or PRIORITY in this state.

Receiving any type of frame other than HEADERS, RST_STREAM, or
PRIORITY on a stream in this state MUST be treated as a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.

open: A stream in the "open" state may be used by both peers to send
frames of any type. In this state, sending peers observe
advertised stream-level flow-control limits (Section 5.2).

From this state, either endpoint can send a frame with an
END_STREAM flag set, which causes the stream to transition into
one of the "half-closed" states. An endpoint sending an
END_STREAM flag causes the stream state to become "half-closed
(local)"; an endpoint receiving an END_STREAM flag causes the
stream state to become "half-closed (remote)".

Either endpoint can send a RST_STREAM frame from this state,
causing it to transition immediately to "closed".

half-closed (local): A stream that is in the "half-closed (local)"
state cannot be used for sending frames other than WINDOW_UPDATE,
PRIORITY, and RST_STREAM.

A stream transitions from this state to "closed" when a frame is
received with the END_STREAM flag set or when either peer sends a
RST_STREAM frame.

An endpoint can receive any type of frame in this state.
Providing flow-control credit using WINDOW_UPDATE frames is
necessary to continue receiving flow-controlled frames. In this
state, a receiver can ignore WINDOW_UPDATE frames, which might
arrive for a short period after a frame with the END_STREAM flag
set is sent.

PRIORITY frames can be received in this state.

half-closed (remote): A stream that is "half-closed (remote)" is no
longer being used by the peer to send frames. In this state, an
endpoint is no longer obligated to maintain a receiver flow-
control window.

If an endpoint receives additional frames, other than
WINDOW_UPDATE, PRIORITY, or RST_STREAM, for a stream that is in
this state, it MUST respond with a stream error (Section 5.4.2) of
type STREAM_CLOSED.

A stream that is "half-closed (remote)" can be used by the
endpoint to send frames of any type. In this state, the endpoint
continues to observe advertised stream-level flow-control limits
(Section 5.2).

A stream can transition from this state to "closed" by sending a
frame with the END_STREAM flag set or when either peer sends a
RST_STREAM frame.

closed: The "closed" state is the terminal state.

A stream enters the "closed" state after an endpoint both sends
and receives a frame with an END_STREAM flag set. A stream also
enters the "closed" state after an endpoint either sends or
receives a RST_STREAM frame.

An endpoint MUST NOT send frames other than PRIORITY on a closed
stream. An endpoint MAY treat receipt of any other type of frame
on a closed stream as a connection error (Section 5.4.1) of type
STREAM_CLOSED, except as noted below.

An endpoint that sends a frame with the END_STREAM flag set or a
RST_STREAM frame might receive a WINDOW_UPDATE or RST_STREAM frame
from its peer in the time before the peer receives and processes
the frame that closes the stream.

An endpoint that sends a RST_STREAM frame on a stream that is in
the "open" or "half-closed (local)" state could receive any type
of frame. The peer might have sent or enqueued for sending these
frames before processing the RST_STREAM frame. An endpoint MUST
minimally process and then discard any frames it receives in this
state. This means updating header compression state for HEADERS
and PUSH_PROMISE frames. Receiving a PUSH_PROMISE frames frame also cause
causes the promised stream to become "reserved", "reserved (remote)", even
when the PUSH_PROMISE frame is received on a closed stream.
Additionally, the content of DATA frames counts toward the
connection flow-control window.

An endpoint can perform this minimal processing for all streams
that are in the "closed" state. Endpoints MAY use other signals
to detect that a peer has received the frames that caused the
stream to enter the "closed" state and treat receipt of any frame
other than PRIORITY as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. Endpoints can use frames that indicate that the
peer has received the closing signal to drive this. Endpoints
SHOULD NOT use timers for this purpose. For example, an endpoint
that sends a SETTINGS frame after closing a stream can safely
treat receipt of a DATA frame on that stream as an error after
receiving an acknowledgment of the settings. Other things that
might be used are PING frames, receiving data on streams that were
created after closing the stream, or responses to requests created
after closing the stream.

In the absence of more specific rules, implementations SHOULD treat
the receipt of a frame that is not expressly permitted in the
description of a state as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. Note that PRIORITY can be sent and received in any
stream state.

The rules in this section only apply to frames defined in this
document. Receipt of frames for which the semantics are unknown
cannot be treated as an error, as the conditions for sending and
receiving those frames are also unknown; see Section 5.5.

An example of the state transitions for an HTTP request/response
exchange can be found in Section 8.8. An example of the state
transitions for server push can be found in Sections 8.4.1 and 8.4.2.

5.1.1. Stream Identifiers

Streams are identified by an unsigned 31-bit integer. Streams
initiated by a client MUST use odd-numbered stream identifiers; those
initiated by the server MUST use even-numbered stream identifiers. A
stream identifier of zero (0x0) (0x00) is used for connection control
messages; the stream identifier of zero cannot be used to establish a
new stream.

The identifier of a newly established stream MUST be numerically
greater than all streams that the initiating endpoint has opened or
reserved. This governs streams that are opened using a HEADERS frame
and streams that are reserved using PUSH_PROMISE. An endpoint that
receives an unexpected stream identifier MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.

A HEADERS frame will transition the client-initiated stream
identified by the stream identifier in the frame header from "idle"
to "open". A PUSH_PROMISE frame will transition the server-initiated
stream identified by the Promised Stream ID field in the frame
payload from "idle" to "reserved". "reserved (local)" or "reserved (remote)".
When a stream transitions out of the "idle" state, all "idle" streams in the
"idle" state that might have been opened by the peer with a lower-valued lower-
valued stream identifier immediately transition to "closed". That
is, an endpoint may skip a stream identifier, with the effect being
that the skipped stream is immediately closed.

Stream identifiers cannot be reused. Long-lived connections can
result in an endpoint exhausting the available range of stream
identifiers. A client that is unable to establish a new stream
identifier can establish a new connection for new streams. A server
that is unable to establish a new stream identifier can send a GOAWAY
frame so that the client is forced to open a new connection for new
streams.

5.1.2. Stream Concurrency

A peer can limit the number of concurrently active streams using the
SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within
a SETTINGS frame. The maximum concurrent streams setting is specific
to each endpoint and applies only to the peer that receives the
setting. That is, clients specify the maximum number of concurrent
streams the server can initiate, and servers specify the maximum
number of concurrent streams the client can initiate.

Streams that are in the "open" state or in either of the "half-
closed" states count toward the maximum number of streams that an
endpoint is permitted to open. Streams in any of these three states
count toward the limit advertised in the
SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the
"reserved" states do not count toward the stream limit.

Endpoints MUST NOT exceed the limit set by their peer. An endpoint
that receives a HEADERS frame that causes its advertised concurrent
stream limit to be exceeded MUST treat this as a stream error
(Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice
of error code determines whether the endpoint wishes to enable
automatic retry (see Section 8.7 for details).

An endpoint that wishes to reduce the value of
SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current
number of open streams can either close streams that exceed the new
value or allow streams to complete.

5.2. Flow Control

Using streams for multiplexing introduces contention over use of the
TCP connection, resulting in blocked streams. A flow-control scheme
ensures that streams on the same connection do not destructively
interfere with each other. Flow control is used for both individual
streams and the connection as a whole.

HTTP/2 provides for flow control through use of the WINDOW_UPDATE
frame (Section 6.9).

5.2.1. Flow-Control Principles

HTTP/2 stream flow control aims to allow a variety of flow-control
algorithms to be used without requiring protocol changes. Flow
control in HTTP/2 has the following characteristics:

1. Flow control is specific to a connection. HTTP/2 flow control
operates between the endpoints of a single hop and not over the
entire end-to-end path.

2. Flow control is based on WINDOW_UPDATE frames. Receivers
advertise how many octets they are prepared to receive on a
stream and for the entire connection. This is a credit-based
scheme.

3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it
desires for each stream and for the entire connection. A sender
MUST respect flow-control limits imposed by a receiver. Clients,
servers, and intermediaries all independently advertise their
flow-control window as a receiver and abide by the flow-control
limits set by their peer when sending.

4. The initial value for the flow-control window is 65,535 octets
for both new streams and the overall connection.

5. The frame type determines whether flow control applies to a
frame. Of the frames specified in this document, only DATA
frames are subject to flow control; all other frame types do not
consume space in the advertised flow-control window. This
ensures that important control frames are not blocked by flow
control.

6. An endpoint can choose to disable its own flow control, but an
endpoint cannot ignore flow-control signals from its peer.

7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE
frame (Section 6.9). This document does not stipulate how a
receiver decides when to send this frame or the value that it
sends, nor does it specify how a sender chooses to send packets.
Implementations are able to select any algorithm that suits their
needs.

Implementations are also responsible for prioritizing the sending of
requests and responses, choosing how to avoid head-of-line blocking
for requests, and managing the creation of new streams. Algorithm
choices for these could interact with any flow-control algorithm.

5.2.2. Appropriate Use of Flow Control

Flow control is defined to protect endpoints that are operating under
resource constraints. For example, a proxy needs to share memory
between many connections and also might have a slow upstream
connection and a fast downstream one. Flow control addresses cases
where the receiver is unable to process data on one stream yet wants
to continue to process other streams in the same connection.

Deployments that do not require this capability can advertise a flow-
control window of the maximum size (2^31-1) and can maintain this
window by sending a WINDOW_UPDATE frame when any data is received.
This effectively disables flow control for that receiver.
Conversely, a sender is always subject to the flow-control window
advertised by the receiver.

Deployments with constrained resources (for example, memory) can
employ flow control to limit the amount of memory a peer can consume.
Note, however, that this can lead to suboptimal use of available
network resources if flow control is enabled without knowledge of the
bandwidth-delay
bandwidth * delay product (see [RFC7323]).

Even with full awareness of the current bandwidth-delay bandwidth * delay product,
implementation of flow control can be difficult. Endpoints MUST read
and process HTTP/2 frames from the TCP receive buffer as soon as data
is available. Failure to read promptly could lead to a deadlock when
critical frames, such as WINDOW_UPDATE, are not read and acted upon.
Reading frames promptly does not expose endpoints to resource
exhaustion attacks, as HTTP/2 flow control limits resource
commitments.

5.2.3. Flow-Control Performance

If an endpoint cannot ensure that its peer always has available flow-
control window space that is greater than the peer's bandwidth-delay bandwidth *
delay product on this connection, its receive throughput will be
limited by HTTP/2 flow control. This will result in degraded
performance.

Sending timely WINDOW_UPDATE frames can improve performance.
Endpoints will want to balance the need to improve receive throughput
with the need to manage resource exhaustion risks and should take
careful note of Section 10.5 in defining their strategy to manage
window sizes.

5.3. Prioritization

In a multiplexed protocol like HTTP/2, prioritizing allocation of
bandwidth and computation resources to streams can be critical to
attaining good performance. A poor prioritization scheme can result
in HTTP/2 providing poor performance. With no parallelism at the TCP
layer, performance could be significantly worse than HTTP/1.1.

A good prioritization scheme benefits from the application of
contextual knowledge such as the content of resources, how resources
are interrelated, and how those resources will be used by a peer. In
particular, clients can possess knowledge about the priority of
requests that is relevant to server prioritization. In those cases,
having clients provide priority information can improve performance.

5.3.1. Background on Priority in RFC 7540

RFC 7540 defined a rich system for signaling priority of requests.
However, this system proved to be complex, and it was not uniformly
implemented.

The flexible scheme meant that it was possible for clients to express
priorities in very different ways, with little consistency in the
approaches that were adopted. For servers, implementing generic
support for the scheme was complex. Implementation of priorities was
uneven in both clients and servers. Many server deployments ignored
client signals when prioritizing their handling of requests.

In short, the prioritization signaling in RFC 7540 [RFC7540] was not
successful.

5.3.2. Priority Signaling in This Document

This update to HTTP/2 deprecates the priority signaling defined in
RFC 7540 [RFC7540]. The bulk of the text related to priority signals
is not included in this document. The description of frame fields
and some of the mandatory handling is retained to ensure that
implementations of this document remain interoperable with
implementations that use the priority signaling described in RFC
7540.

A thorough description of the RFC 7540 priority scheme remains in
Section 5.3 of [RFC7540].

Signaling priority information is necessary to attain good
performance in many cases. Where signaling priority information is
important, endpoints are encouraged to use an alternative scheme,
such as the scheme described in [HTTP-PRIORITY].

Though the priority signaling from RFC 7540 was not widely adopted,
the information it provides can still be useful in the absence of
better information. Endpoints that receive priority signals in
HEADERS or PRIORITY frames can benefit from applying that
information. In particular, implementations that consume these
signals would not benefit from discarding these priority signals in
the absence of alternatives.

Servers SHOULD use other contextual information in determining
priority of requests in the absence of any priority signals. Servers
MAY interpret the complete absence of signals as an indication that
the client has not implemented the feature. The defaults described
in Section 5.3.5 of [RFC7540] are known to have poor performance
under most conditions, and their use is unlikely to be deliberate.

5.4. Error Handling

HTTP/2 framing permits two classes of errors:

* An error condition that renders the entire connection unusable is
a connection error.

* An error in an individual stream is a stream error.

A list of error codes is included in Section 7.

It is possible that an endpoint will encounter frames that would
cause multiple errors. Implementations MAY discover multiple errors
during processing, but they SHOULD report at most one stream and one
connection error as a result.

The first stream error reported for a given stream prevents any other
errors on that stream from being reported. In comparison, the
protocol permits multiple GOAWAY frames, though an endpoint SHOULD
report just one type of connection error unless an error is
encountered during graceful shutdown. If this occurs, an endpoint
MAY send an additional GOAWAY frame with the new error code, in
addition to any prior GOAWAY that contained NO_ERROR.

If an endpoint detects multiple different errors, it MAY choose to
report any one of those errors. If a frame causes a connection
error, that error MUST be reported. Additionally, an endpoint MAY
use any applicable error code when it detects an error condition; a
generic error code (such as PROTOCOL_ERROR or INTERNAL_ERROR) can
always be used in place of more specific error codes.

5.4.1. Connection Error Handling

A connection error is any error that prevents further processing of
the frame layer or corrupts any connection state.

An endpoint that encounters a connection error SHOULD first send a
GOAWAY frame (Section 6.8) with the stream identifier of the last
stream that it successfully received from its peer. The GOAWAY frame
includes an error code (Section 7) that indicates why the connection
is terminating. After sending the GOAWAY frame for an error
condition, the endpoint MUST close the TCP connection.

It is possible that the GOAWAY will not be reliably received by the
receiving endpoint. In the event of a connection error, GOAWAY only
provides a best-effort attempt to communicate with the peer about why
the connection is being terminated.

An endpoint can end a connection at any time. In particular, an
endpoint MAY choose to treat a stream error as a connection error.
Endpoints SHOULD send a GOAWAY frame when ending a connection,
providing that circumstances permit it.

5.4.2. Stream Error Handling

A stream error is an error related to a specific stream that does not
affect processing of other streams.

An endpoint that detects a stream error sends a RST_STREAM frame
(Section 6.4) that contains the stream identifier of the stream where
the error occurred. The RST_STREAM frame includes an error code that
indicates the type of error.

A RST_STREAM is the last frame that an endpoint can send on a stream.
The peer that sends the RST_STREAM frame MUST be prepared to receive
any frames that were sent or enqueued for sending by the remote peer.
These frames can be ignored, except where they modify connection
state (such as the state maintained for field section compression
(Section 4.3) or flow control).

Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
for any stream. However, an endpoint MAY send additional RST_STREAM
frames if it receives frames on a closed stream after more than a
round-trip time. This behavior is permitted to deal with misbehaving
implementations.

To avoid looping, an endpoint MUST NOT send a RST_STREAM in response
to a RST_STREAM frame.

5.4.3. Connection Termination

If the TCP connection is closed or reset while streams remain in the
"open" or "half-closed" states, then the affected streams cannot be
automatically retried (see Section 8.7 for details).

5.5. Extending HTTP/2

HTTP/2 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/2 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 (see Section 16 of
[HTTP]).

Extensions are permitted to use new frame types (Section 4.1), new
settings (Section 6.5), or new error codes (Section 7). Registries
for managing these extension points are defined in Section 11 of
[RFC7540].

Implementations MUST ignore unknown or unsupported values in all
extensible protocol elements. Implementations MUST discard frames
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, extension frames that appear in
the middle of a field block (Section 4.3) are not permitted; these
MUST be treated as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

Extensions SHOULD avoid changing protocol elements defined in this
document or elements for which no extension mechanism is defined.
This includes changes to the layout of frames, additions or changes
to the way that frames are composed into HTTP messages (Section 8.1),
the definition of pseudo-header fields, or changes to any protocol
element that a compliant endpoint might treat as a connection error
(Section 5.4.1).

An extension that changes existing protocol elements or state MUST be
negotiated before being used. For example, an extension that changes
the layout of the HEADERS frame cannot be used until the peer has
given a positive signal that this is acceptable. In this case, it
could also be necessary to coordinate when the revised layout comes
into effect. For example, treating frames other than DATA frames as
flow controlled requires a change in semantics that both endpoints
need to understand, so this can only be done through negotiation.

This document doesn't mandate a specific method for negotiating the
use of an extension but notes that a setting (Section 6.5.2) 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 initial value MUST
be defined in such a fashion that the extension is initially
disabled.

6. Frame Definitions

This specification defines a number of frame types, each identified
by a unique 8-bit type code. Each frame type serves a distinct
purpose in the establishment and management of either the connection
as a whole or individual streams.

The transmission of specific frame types can alter the state of a
connection. If endpoints fail to maintain a synchronized view of the
connection state, successful communication within the connection will
no longer be possible. Therefore, it is important that endpoints
have a shared comprehension of how the state is affected by the use
of any given frame.

6.1. DATA

DATA frames (type=0x0) (type=0x00) convey arbitrary, variable-length sequences
of octets associated with a stream. One or more DATA frames are
used, for instance, to carry HTTP request or response message
contents.

DATA frames MAY also contain padding. Padding can be added to DATA
frames to obscure the size of messages. Padding is a security
feature; see Section 10.7.

DATA Frame {
Length (24),
Type (8) = 0x0, 0x00,

Unused Flags (4),
PADDED Flag (1),
Unused Flags (2),
END_STREAM Flag (1),

Reserved (1),
Stream Identifier (31),

[Pad Length (8)],
Data (..),
Padding (..2040),
}

Figure 3: DATA Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The DATA frame contains the
following additional fields:

Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is conditional and is only
present if the PADDED flag is set.

Data: Application data. The amount of data is the remainder of the
frame payload after subtracting the length of the other fields
that are present.

Padding: Padding octets that contain no application semantic value.
Padding octets MUST be set to zero when sending. A receiver is
not obligated to verify padding but MAY treat non-zero padding as
a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The DATA frame defines the following flags:

PADDED (0x8): (0x08): When set, the PADDED flag indicates that the Pad
Length field and any padding that it describes are present.

END_STREAM (0x1): (0x01): When set, the END_STREAM flag indicates that this
frame is the last that the endpoint will send for the identified
stream. Setting this flag causes the stream to enter one of the
"half-closed" states or the "closed" state (Section 5.1).

| Note: An endpoint that learns of stream closure after sending
| all data can close a stream by sending a STREAM frame with a
| zero-length Data field and the END_STREAM flag set. This is
| only possible if the endpoint does not send trailers, as the
| END_STREAM flag appears on a HEADERS frame in that case; see
| Section 8.1.

DATA frames MUST be associated with a stream. If a DATA frame is
received whose Stream Identifier field is 0x0, 0x00, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

DATA frames are subject to flow control and can only be sent when a
stream is in the "open" or "half-closed (remote)" state. The entire
DATA frame payload is included in flow control, including the Pad
Length and Padding fields if present. If a DATA frame is received
whose stream is not in the "open" or "half-closed (local)" state, the
recipient MUST respond with a stream error (Section 5.4.2) of type
STREAM_CLOSED.

The total number of padding octets is determined by the value of the
Pad Length field. If the length of the padding is the length of the
frame payload or greater, the recipient MUST treat this as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.

| Note: A frame can be increased in size by one octet by
| including a Pad Length field with a value of zero.

6.2. HEADERS

The HEADERS frame (type=0x1) (type=0x01) is used to open a stream (Section 5.1) 5.1),
and additionally carries a field block fragment. Despite the name, a
HEADERS frame can carry a header section or a trailer section.
HEADERS frames can be sent on a stream in the "idle", "reserved
(local)", "open", or "half-closed (remote)" state.

HEADERS Frame {
Length (24),
Type (8) = 0x1, 0x01,

Unused Flags (2),
PRIORITY Flag (1),
Unused Flag (1),
PADDED Flag (1),
END_HEADERS Flag (1),
Unused Flag (1),
END_STREAM Flag (1),

Reserved (1),
Stream Identifier (31),

[Pad Length (8)],
[Exclusive (1)],
[Stream Dependency (31)],
[Weight (8)],
Field Block Fragment (..),
Padding (..2040),
}

Figure 4: HEADERS Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The HEADERS frame payload has the
following additional fields:

Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is only present if the
PADDED flag is set.

Exclusive: A single-bit flag. This field is only present if the
PRIORITY flag is set. Priority signals in HEADERS frames are
deprecated; see Section 5.3.2.

Stream Dependency: A 31-bit stream identifier. This field is only
present if the PRIORITY flag is set.

Weight: An unsigned 8-bit integer. This field is only present if
the PRIORITY flag is set.

Field Block Fragment: A field block fragment (Section 4.3).

The HEADERS frame defines the following flags:

PRIORITY (0x20): When set, the PRIORITY flag indicates that the
Exclusive, Stream Dependency, and Weight fields are present.

PADDED (0x8): (0x08): When set, the PADDED flag indicates that the Pad
Length field and any padding that it describes are present.

END_HEADERS (0x4): (0x04): When set, the END_HEADERS flag indicates that
this frame contains an entire field block (Section 4.3) and is not
followed by any CONTINUATION frames.

A HEADERS frame without the END_HEADERS flag set MUST be followed
by a CONTINUATION frame for the same stream. A receiver MUST
treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

END_STREAM (0x1): (0x01): When set, the END_STREAM flag indicates that the
field block (Section 4.3) is the last that the endpoint will send
for the identified stream.

A HEADERS frame with the END_STREAM flag set signals the end of a
stream. However, a HEADERS frame with the END_STREAM flag set can
be followed by CONTINUATION frames on the same stream. Logically,
the CONTINUATION frames are part of the HEADERS frame.

The frame payload of a HEADERS frame contains a field block fragment
(Section 4.3). A field block that does not fit within a HEADERS
frame is continued in a CONTINUATION frame (Section 6.10).

HEADERS frames MUST be associated with a stream. If a HEADERS frame
is received whose Stream Identifier field is 0x0, 0x00, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

The HEADERS frame changes the connection state as described in
Section 4.3.

| Note: A frame can be increased in size by one octet by
| including a Pad Length field with a value of zero.

6.3. PRIORITY

The PRIORITY frame (type=0x2) (type=0x02) is deprecated; see Section 5.3.2. A
PRIORITY frame can be sent in any stream state, including idle or
closed streams.

PRIORITY Frame {
Length (24) = 0x5, 0x05,
Type (8) = 0x2, 0x02,

Unused Flags (8),

Reserved (1),
Stream Identifier (31),

Exclusive (1),
Stream Dependency (31),
Weight (8),
}

Figure 5: PRIORITY Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The frame payload of a PRIORITY
frame contains the following additional fields:

Exclusive: A single-bit flag.

Stream Dependency: A 31-bit stream identifier.

Weight: An unsigned 8-bit integer.

The PRIORITY frame does not define any flags.

The PRIORITY frame always identifies a stream. If a PRIORITY frame
is received with a stream identifier of 0x0, 0x00, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

Sending or receiving a PRIORITY frame does not affect the state of
any stream (Section 5.1). The PRIORITY frame can be sent on a stream
in any state, including "idle" or "closed". A PRIORITY frame cannot
be sent between consecutive frames that comprise a single field block
(Section 4.3).

A PRIORITY frame with a length other than 5 octets MUST be treated as
a stream error (Section 5.4.2) of type FRAME_SIZE_ERROR.

6.4. RST_STREAM

The RST_STREAM frame (type=0x3) (type=0x03) allows for immediate termination of
a stream. RST_STREAM is sent to request cancellation of a stream or
to indicate that an error condition has occurred.

RST_STREAM Frame {
Length (24) = 0x4, 0x04,
Type (8) = 0x3, 0x03,

Unused Flags (8),

Reserved (1),
Stream Identifier (31),

Error Code (32),
}

Figure 6: RST_STREAM Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. Additionally, the RST_STREAM
frame contains a single unsigned, 32-bit integer identifying the
error code (Section 7). The error code indicates why the stream is
being terminated.

The RST_STREAM frame does not define any flags.

The RST_STREAM frame fully terminates the referenced stream and
causes it to enter the "closed" state. After receiving a RST_STREAM
on a stream, the receiver MUST NOT send additional frames for that
stream, except for PRIORITY. However, after sending the RST_STREAM,
the sending endpoint MUST be prepared to receive and process
additional frames sent on the stream that might have been sent by the
peer prior to the arrival of the RST_STREAM.

RST_STREAM frames MUST be associated with a stream. If a RST_STREAM
frame is received with a stream identifier of 0x0, 0x00, the recipient
MUST treat this as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
If a RST_STREAM frame identifying an idle stream is received, the
recipient MUST treat this as a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.

A RST_STREAM frame with a length other than 4 octets MUST be treated
as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.

6.5. SETTINGS

The SETTINGS frame (type=0x4) (type=0x04) conveys configuration parameters that
affect how endpoints communicate, such as preferences and constraints
on peer behavior. The SETTINGS frame is also used to acknowledge the
receipt of those settings. Individually, a configuration parameter
from a SETTINGS frame is referred to as a "setting".

Settings are not negotiated; they describe characteristics of the
sending peer, which are used by the receiving peer. Different values
for the same setting can be advertised by each peer. For example, a
client might set a high initial flow-control window, whereas a server
might set a lower value to conserve resources.

A SETTINGS frame MUST be sent by both endpoints at the start of a
connection and MAY be sent at any other time by either endpoint over
the lifetime of the connection. Implementations MUST support all of
the settings defined by this specification.

Each parameter in a SETTINGS frame replaces any existing value for
that parameter. Settings are processed in the order in which they
appear, and a receiver of a SETTINGS frame does not need to maintain
any state other than the current value of each setting. Therefore,
the value of a SETTINGS parameter is the last value that is seen by a
receiver.

SETTINGS frames are acknowledged by the receiving peer. To enable
this, the SETTINGS frame defines the ACK flag:

ACK (0x1): (0x01): When set, the ACK flag indicates that this frame
acknowledges receipt and application of the peer's SETTINGS frame.
When this bit is set, the frame payload of the SETTINGS frame MUST
be empty. Receipt of a SETTINGS frame with the ACK flag set and a
length field value other than 0 MUST be treated as a connection
error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more
information, see Section 6.5.3 ("Settings Synchronization").

SETTINGS frames always apply to a connection, never a single stream.
The stream identifier for a SETTINGS frame MUST be zero (0x0). (0x00). If
an endpoint receives a SETTINGS frame whose Stream Identifier field
is anything other than 0x0, 0x00, the endpoint MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The SETTINGS frame affects connection state. A badly formed or
incomplete SETTINGS frame MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.

A SETTINGS frame with a length other than a multiple of 6 octets MUST
be treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.

6.5.1. SETTINGS Format

The frame payload of a SETTINGS frame consists of zero or more
settings, each consisting of an unsigned 16-bit setting identifier
and an unsigned 32-bit value.

SETTINGS Frame {
Length (24),
Type (8) = 0x4, 0x04,

Unused Flags (7),
ACK Flag (1),

Reserved (1),
Stream Identifier (31) = 0,

Setting (48) ...,
}

Setting {
Identifier (16),
Value (32),
}

Figure 7: SETTINGS Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The frame payload of a SETTINGS
frame contains any number of Setting fields, each of which consists
of:

Identifier: A 16-bit setting identifier; see Section 6.5.2.

Value: A 32-bit value for the setting.

6.5.2. Defined Settings

The following settings are defined:

SETTINGS_HEADER_TABLE_SIZE (0x1): (0x01): This setting allows the sender to
inform the remote endpoint of the maximum size of the compression
table used to decode field blocks, in units of octets. The
encoder can select any size equal to or less than this value by
using signaling specific to the compression format inside a field
block (see [COMPRESSION]). The initial value is 4,096 octets.

SETTINGS_ENABLE_PUSH (0x2): (0x02): This setting can be used to enable or
disable server push (Section 8.4). push. A server MUST NOT send a PUSH_PROMISE frame
if it receives this parameter set to a value of 0. 0; see
Section 8.4. A client that has both set this parameter to 0 and
had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame
as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The initial value of SETTINGS_ENABLE_PUSH is 1. For a client,
this value indicates that it is willing to receive PUSH_PROMISE
frames. For a server, this initial value has no effect effect, and is
equivalent to the value 0. Any value other than 0 or 1 MUST be
treated as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

A server MUST NOT explicitly set this value to 1. A server MAY
choose to omit this setting when it sends a SETTINGS frame, but if
a server does include a value, it MUST be 0. A client MUST treat
receipt of a SETTINGS frame with SETTINGS_ENABLE_PUSH set to 1 as
a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

SETTINGS_MAX_CONCURRENT_STREAMS (0x3): (0x03): This setting indicates the
maximum number of concurrent streams that the sender will allow.
This limit is directional: it applies to the number of streams
that the sender permits the receiver to create. Initially, there
is no limit to this value. It is recommended that this value be
no smaller than 100, so as to not unnecessarily limit parallelism.

A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be
treated as special by endpoints. A zero value does prevent the
creation of new streams; however, this can also happen for any
limit that is exhausted with active streams. Servers SHOULD only
set a zero value for short durations; if a server does not wish to
accept requests, closing the connection is more appropriate.

SETTINGS_INITIAL_WINDOW_SIZE (0x4): (0x04): This setting indicates the
sender's initial window size (in units of octets) for stream-level
flow control. The initial value is 2^16-1 (65,535) octets.

This setting affects the window size of all streams (see
Section 6.9.2).

Values above the maximum flow-control window size of 2^31-1 MUST
be treated as a connection error (Section 5.4.1) of type
FLOW_CONTROL_ERROR.

SETTINGS_MAX_FRAME_SIZE (0x5): (0x05): This setting indicates the size of
the largest frame payload that the sender is willing to receive,
in units of octets.

The initial value is 2^14 (16,384) octets. The value advertised
by an endpoint MUST be between this initial value and the maximum
allowed frame size (2^24-1 or 16,777,215 octets), inclusive.
Values outside this range MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.

SETTINGS_MAX_HEADER_LIST_SIZE (0x6): (0x06): This advisory setting informs
a peer of the maximum field section size that the sender is
prepared to accept, in units of octets. The value is based on the
uncompressed size of field lines, including the length of the name
and value in units of octets plus an overhead of 32 octets for
each field line.

For any given request, a lower limit than what is advertised MAY
be enforced. The initial value of this setting is unlimited.

An endpoint that receives a SETTINGS frame with any unknown or
unsupported identifier MUST ignore that setting.

6.5.3. Settings Synchronization

Most values in SETTINGS benefit from or require an understanding of
when the peer has received and applied the changed parameter values.
In order to provide such synchronization timepoints, the recipient of
a SETTINGS frame in which the ACK flag is not set MUST apply the
updated settings as soon as possible upon receipt. SETTINGS frames
are acknowledged in the order in which they are received.

The values in the SETTINGS frame MUST be processed in the order they
appear, with no other frame processing between values. Unsupported
settings MUST be ignored. Once all values have been processed, the
recipient MUST immediately emit a SETTINGS frame with the ACK flag
set. Upon receiving a SETTINGS frame with the ACK flag set, the
sender of the altered settings can rely on the values from the oldest
unacknowledged SETTINGS frame having been applied.

If the sender of a SETTINGS frame does not receive an acknowledgment
within a reasonable amount of time, it MAY issue a connection error
(Section 5.4.1) of type SETTINGS_TIMEOUT. In setting a timeout, some
allowance needs to be made for processing delays at the peer; a
timeout that is solely based on the round-trip time between endpoints
might result in spurious errors.

6.6. PUSH_PROMISE

The PUSH_PROMISE frame (type=0x5) (type=0x05) is used to notify the peer
endpoint in advance of streams the sender intends to initiate. The
PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
stream the endpoint plans to create along with a field section that
provides additional context for the stream. Section 8.4 contains a
thorough description of the use of PUSH_PROMISE frames.

PUSH_PROMISE Frame {
Length (24),
Type (8) = 0x5, 0x05,

Unused Flags (4),
PADDED Flag (1),
END_HEADERS Flag (1),
Unused Flags (2),

Reserved (1),
Stream Identifier (31),

[Pad Length (8)],
Reserved (1),
Promised Stream ID (31),
Field Block Fragment (..),
Padding (..2040),
}

Figure 8: PUSH_PROMISE Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The PUSH_PROMISE frame payload
has the following additional fields:

Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is only present if the
PADDED flag is set.

Reserved: A single reserved bit.

Promised Stream ID: An unsigned 31-bit integer that identifies the
stream that is reserved by the PUSH_PROMISE. The promised stream
identifier MUST be a valid choice for the next stream sent by the
sender (see "new stream identifier" in Section 5.1.1).

Field Block Fragment: A field block fragment (Section 4.3)
containing the request control data and a header section.

The PUSH_PROMISE frame defines the following flags:

PADDED (0x8): (0x08): When set, the PADDED flag indicates that the Pad
Length field and any padding that it describes are present.

END_HEADERS (0x4): (0x04): When set, the END_HEADERS flag indicates that
this frame contains an entire field block (Section 4.3) and is not
followed by any CONTINUATION frames.

A PUSH_PROMISE frame without the END_HEADERS flag set MUST be
followed by a CONTINUATION frame for the same stream. A receiver
MUST treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that
is in either the "open" or "half-closed (remote)" state. The stream
identifier of a PUSH_PROMISE frame indicates the stream it is
associated with. If the Stream Identifier field specifies the value
0x0,
0x00, a recipient MUST respond with a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.

Promised streams are not required to be used in the order they are
promised. The PUSH_PROMISE only reserves stream identifiers for
later use.

PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of
the peer endpoint is set to 0. An endpoint that has set this setting
and has received acknowledgment MUST treat the receipt of a
PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

Recipients of PUSH_PROMISE frames can choose to reject promised
streams by returning a RST_STREAM referencing the promised stream
identifier back to the sender of the PUSH_PROMISE.

A PUSH_PROMISE frame modifies the connection state in two ways.
First, the inclusion of a field block (Section 4.3) potentially
modifies the state maintained for field section compression. Second,
PUSH_PROMISE also reserves a stream for later use, causing the
promised stream to enter the "reserved" "reserved (local)" or "reserved
(remote)" state. A sender MUST NOT send a PUSH_PROMISE on a stream
unless that stream is either "open" or "half-closed (remote)"; the
sender MUST ensure that the promised stream is a valid choice for a
new stream identifier (Section 5.1.1) (that is, the promised stream
MUST be in the "idle" state).

Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame
causes the stream state to become indeterminate. A receiver MUST
treat the receipt of a PUSH_PROMISE on a stream that is neither
"open" nor "half-closed (local)" as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. However, an endpoint that
has sent RST_STREAM on the associated stream MUST handle PUSH_PROMISE
frames that might have been created before the RST_STREAM frame is
received and processed.

A receiver MUST treat the receipt of a PUSH_PROMISE that promises an
illegal stream identifier (Section 5.1.1) as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Note that an illegal stream
identifier is an identifier for a stream that is not currently in the
"idle" state.

| Note: A frame can be increased in size by one octet by
| including a Pad Length field with a value of zero.

6.7. PING

The PING frame (type=0x6) (type=0x06) is a mechanism for measuring a minimal
round-trip time from the sender, as well as determining whether an
idle connection is still functional. PING frames can be sent from
any endpoint.

PING Frame {
Length (24) = 0x8, 0x08,
Type (8) = 0x6, 0x06,

Unused Flags (7),
ACK Flag (1),

Reserved (1),
Stream Identifier (31) = 0,

Opaque Data (64),
}

Figure 9: PING Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4.

In addition to the frame header, PING frames MUST contain 8 octets of
opaque data in the frame payload. A sender can include any value it
chooses and use those octets in any fashion.

Receivers of a PING frame that does not include an ACK flag MUST send
a PING frame with the ACK flag set in response, with an identical
frame payload. PING responses SHOULD be given higher priority than
any other frame.

The PING frame defines the following flags:

ACK (0x1): (0x01): When set, the ACK flag indicates that this PING frame is
a PING response. An endpoint MUST set this flag in PING
responses. An endpoint MUST NOT respond to PING frames containing
this flag.

PING frames are not associated with any individual stream. If a PING
frame is received with a Stream Identifier field value other than
0x0,
0x00, the recipient MUST respond with a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.

Receipt of a PING frame with a length field value other than 8 MUST
be treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.

6.8. GOAWAY

The GOAWAY frame (type=0x7) (type=0x07) is used to initiate shutdown of a
connection or to signal serious error conditions. GOAWAY allows an
endpoint to gracefully stop accepting new streams while still
finishing processing of previously established streams. This enables
administrative actions, like server maintenance.

There is an inherent race condition between an endpoint starting new
streams and the remote peer sending a GOAWAY frame. To deal with
this case, the GOAWAY contains the stream identifier of the last
peer-initiated stream that was or might be processed on the sending
endpoint in this connection. For instance, if the server sends a
GOAWAY frame, the identified stream is the highest-numbered stream
initiated by the client.

Once the GOAWAY is sent, the sender will ignore frames sent on
streams initiated by the receiver if the stream has an identifier
higher than the included last stream identifier. Receivers of a
GOAWAY frame MUST NOT open additional streams on the connection,
although a new connection can be established for new streams.

If the receiver of the GOAWAY has sent data on streams with a higher
stream identifier than what is indicated in the GOAWAY frame, those
streams are not or will not be processed. The receiver of the GOAWAY
frame can treat the streams as though they had never been created at
all, thereby allowing those streams to be retried later on a new
connection.

Endpoints SHOULD always send a GOAWAY frame before closing a
connection so that the remote peer can know whether a stream has been
partially processed or not. For example, if an HTTP client sends a
POST at the same time that a server closes a 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 might choose to close a connection without sending a
GOAWAY for misbehaving peers.

A GOAWAY frame might not immediately precede closing of the
connection; a receiver of a GOAWAY that has no more use for the
connection SHOULD still send a GOAWAY frame before terminating the
connection.

GOAWAY Frame {
Length (24),
Type (8) = 0x7, 0x07,

Unused Flags (8),

Reserved (1),
Stream Identifier (31) = 0,

Reserved (1),
Last-Stream-ID (31),
Error Code (32),
Additional Debug Data (..),
}

Figure 10: GOAWAY Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4.

The GOAWAY frame does not define any flags.

The GOAWAY frame applies to the connection, not a specific stream.
An endpoint MUST treat a GOAWAY frame with a stream identifier other
than 0x0 0x00 as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.

The last stream identifier in the GOAWAY frame contains the highest-
numbered stream identifier for which the sender of the GOAWAY frame
might have taken some action on or might yet take action on. All
streams up to and including the identified stream might have been
processed in some way. The last stream identifier can be set to 0 if
no streams were processed.

| Note: 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.

If a connection terminates without a GOAWAY frame, the last stream
identifier is effectively the highest possible stream identifier.

On streams with lower- or equal-numbered identifiers that were not
closed completely prior to the connection being closed, reattempting
requests, transactions, or any protocol activity is not possible,
except for idempotent actions like HTTP GET, PUT, or DELETE. Any
protocol activity that uses higher-numbered streams can be safely
retried using a new connection.

Activity on streams numbered lower than or equal to the last stream
identifier might still complete successfully. The sender of a GOAWAY
frame might gracefully shut down a connection by sending a GOAWAY
frame, maintaining the connection in an "open" state until all in-
progress streams complete.

An endpoint MAY send multiple GOAWAY frames if circumstances change.
For instance, an endpoint that sends GOAWAY with NO_ERROR during
graceful shutdown could subsequently encounter a condition that
requires immediate termination of the connection. The last stream
identifier from the last GOAWAY frame received indicates which
streams could have been acted upon. Endpoints MUST NOT increase the
value they send in the last stream identifier, since the peers might
already have retried unprocessed requests on another connection.

A client that is unable to retry requests loses all requests that are
in flight when the server closes the connection. This is especially
true for intermediaries that might not be serving clients using
HTTP/2. A server that is attempting to gracefully shut down a
connection SHOULD send an initial GOAWAY frame with the last stream
identifier set to 2^31-1 and a NO_ERROR code. This signals to the
client that a shutdown is imminent and that initiating further
requests is prohibited. After allowing time for any in-flight stream
creation (at least one round-trip time), the server MAY send another
GOAWAY frame with an updated last stream identifier. This ensures
that a connection can be cleanly shut down without losing requests.

After sending a GOAWAY frame, the sender can discard frames for
streams initiated by the receiver with identifiers higher than the
identified last stream. However, any frames that alter connection
state cannot be completely ignored. For instance, HEADERS,
PUSH_PROMISE, and CONTINUATION frames MUST be minimally processed to
ensure that the state maintained for field section compression is
consistent (see Section 4.3); similarly, DATA frames MUST be counted
toward the connection flow-control window. Failure to process these
frames can cause flow control or field section compression state to
become unsynchronized.

The GOAWAY frame also contains a 32-bit error code (Section 7) that
contains the reason for closing the connection.

Endpoints MAY append opaque data to the frame payload of any GOAWAY
frame. Additional debug data is intended for diagnostic purposes
only and carries no semantic value. Debug information could contain
security- or privacy-sensitive data. Logged or otherwise
persistently stored debug data MUST have adequate safeguards to
prevent unauthorized access.

6.9. WINDOW_UPDATE

The WINDOW_UPDATE frame (type=0x8) (type=0x08) is used to implement flow
control; see Section 5.2 for an overview.

Flow control operates at two levels: on each individual stream and on
the entire connection.

Both types of flow control are hop by hop, that is, only between the
two endpoints. Intermediaries do not forward WINDOW_UPDATE frames
between dependent connections. However, throttling of data transfer
by any receiver can indirectly cause the propagation of flow-control
information toward the original sender.

Flow control only applies to frames that are identified as being
subject to flow control. Of the frame types defined in this
document, this includes only DATA frames. Frames that are exempt
from flow control MUST be accepted and processed, unless the receiver
is unable to assign resources to handling the frame. A receiver MAY
respond with a stream error (Section 5.4.2) or connection error
(Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept
a frame.

WINDOW_UPDATE Frame {
Length (24) = 0x4, 0x04,
Type (8) = 0x8, 0x08,

Unused Flags (8),

Reserved (1),
Stream Identifier (31),

Reserved (1),
Window Size Increment (31),
}

Figure 11: WINDOW_UPDATE Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The frame payload of a
WINDOW_UPDATE frame is one reserved bit plus an unsigned 31-bit
integer indicating the number of octets that the sender can transmit
in addition to the existing flow-control window. The legal range for
the increment to the flow-control window is 1 to 2^31-1
(2,147,483,647) octets.

The WINDOW_UPDATE frame does not define any flags.

The WINDOW_UPDATE frame can be specific to a stream or to the entire
connection. In the former case, the frame's stream identifier
indicates the affected stream; in the latter, the value "0" indicates
that the entire connection is the subject of the frame.

A receiver MUST treat the receipt of a WINDOW_UPDATE frame with a
flow-control window increment of 0 as a stream error (Section 5.4.2)
of type PROTOCOL_ERROR; errors on the connection flow-control window
MUST be treated as a connection error (Section 5.4.1).

WINDOW_UPDATE can be sent by a peer that has sent a frame with the
END_STREAM flag set. This means that a receiver could receive a
WINDOW_UPDATE frame on a stream in a "half-closed (remote)" or
"closed" stream. state. A receiver MUST NOT treat this as an error (see
Section 5.1).

A receiver that receives a flow-controlled frame MUST always account
for its contribution against the connection flow-control window,
unless the receiver treats this as a connection error
(Section 5.4.1). This is necessary even if the frame is in error.
The sender counts the frame toward the flow-control window, but if
the receiver does not, the flow-control window at the sender and
receiver can become different.

A WINDOW_UPDATE frame with a length other than 4 octets MUST be
treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.

6.9.1. The Flow-Control Window

Flow control in HTTP/2 is implemented using a window kept by each
sender on every stream. The flow-control window is a simple integer
value that indicates how many octets of data the sender is permitted
to transmit; as such, its size is a measure of the buffering capacity
of the receiver.

Two flow-control windows are applicable: the stream flow-control
window and the connection flow-control window. The sender MUST NOT
send a flow-controlled frame with a length that exceeds the space
available in either of the flow-control windows advertised by the
receiver. Frames with zero length with the END_STREAM flag set (that
is, an empty DATA frame) MAY be sent if there is no available space
in either flow-control window.

For flow-control calculations, the 9-octet frame header is not
counted.

After sending a flow-controlled frame, the sender reduces the space
available in both windows by the length of the transmitted frame.

The receiver of a frame sends a WINDOW_UPDATE frame as it consumes
data and frees up space in flow-control windows. Separate
WINDOW_UPDATE frames are sent for the stream- and connection-level
flow-control windows. Receivers are advised to have mechanisms in
place to avoid sending WINDOW_UPDATE frames with very small
increments; see Section 4.2.3.3 of [RFC1122].

A sender that receives a WINDOW_UPDATE frame updates the
corresponding window by the amount specified in the frame.

A sender MUST NOT allow a flow-control window to exceed 2^31-1
octets. If a sender receives a WINDOW_UPDATE that causes a flow-
control window to exceed this maximum, it MUST terminate either the
stream or the connection, as appropriate. For streams, the sender
sends a RST_STREAM with an error code of FLOW_CONTROL_ERROR; for the
connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR
is sent.

Flow-controlled frames from the sender and WINDOW_UPDATE frames from
the receiver are completely asynchronous with respect to each other.
This property allows a receiver to aggressively update the window
size kept by the sender to prevent streams from stalling.

6.9.2. Initial Flow-Control Window Size

When an HTTP/2 connection is first established, new streams are
created with an initial flow-control window size of 65,535 octets.
The connection flow-control window is also 65,535 octets. Both
endpoints can adjust the initial window size for new streams by
including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS
frame. The connection flow-control window can only be changed using
WINDOW_UPDATE frames.

Prior to receiving a SETTINGS frame that sets a value for
SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default
initial window size when sending flow-controlled frames. Similarly,
the connection flow-control window is set based on the default
initial window size until a WINDOW_UPDATE frame is received.

In addition to changing the flow-control window for streams that are
not yet active, a SETTINGS frame can alter the initial flow-control
window size for streams with active flow-control windows (that is,
streams in the "open" or "half-closed (remote)" state). When the
value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust
the size of all stream flow-control windows that it maintains by the
difference between the new value and the old value.

A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available
space in a flow-control window to become negative. A sender MUST
track the negative flow-control window and MUST NOT send new flow-
controlled frames until it receives WINDOW_UPDATE frames that cause
the flow-control window to become positive.

For example, if the client sends 60 KB immediately on connection
establishment and the server sets the initial window size to be 16
KB, the client will recalculate the available flow-control window to
be -44 KB on receipt of the SETTINGS frame. The client retains a
negative flow-control window until WINDOW_UPDATE frames restore the
window to being positive, after which the client can resume sending.

A SETTINGS frame cannot alter the connection flow-control window.

An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that
causes any flow-control window to exceed the maximum size as a
connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

6.9.3. Reducing the Stream Window Size

A receiver that wishes to use a smaller flow-control window than the
current size can send a new SETTINGS frame. However, the receiver
MUST be prepared to receive data that exceeds this window size, since
the sender might send data that exceeds the lower limit prior to
processing the SETTINGS frame.

After sending a SETTINGS frame that reduces the initial flow-control
window size, a receiver MAY continue to process streams that exceed
flow-control limits. Allowing streams to continue does not allow the
receiver to immediately reduce the space it reserves for flow-control
windows. Progress on these streams can also stall, since
WINDOW_UPDATE frames are needed to allow the sender to resume
sending. The receiver MAY instead send a RST_STREAM with an error
code of FLOW_CONTROL_ERROR for the affected streams.

6.10. CONTINUATION

The CONTINUATION frame (type=0x9) (type=0x09) is used to continue a sequence of
field block fragments (Section 4.3). Any number of CONTINUATION
frames can be sent, as long as the preceding frame is on the same
stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without
the END_HEADERS flag set.

CONTINUATION Frame {
Length (24),
Type (8) = 0x9, 0x09,

Unused Flags (5),
END_HEADERS Flag (1),
Unused Flags (2),

Reserved (1),
Stream Identifier (31),

Field Block Fragment (..),
}

Figure 12: CONTINUATION Frame Format

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier
fields are described in Section 4. The CONTINUATION frame payload
contains a field block fragment (Section 4.3).

The CONTINUATION frame defines the following flag:

END_HEADERS (0x4): (0x04): When set, the END_HEADERS flag indicates that
this frame ends a field block (Section 4.3).

If the END_HEADERS flag is not set, this frame MUST be followed by
another CONTINUATION frame. A receiver MUST treat the receipt of
any other type of frame or a frame on a different stream as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The CONTINUATION frame changes the connection state as defined in
Section 4.3.

CONTINUATION frames MUST be associated with a stream. If a
CONTINUATION frame is received whose with a Stream Identifier field is 0x0, of
0x00, the recipient MUST respond with a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.

A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE, PUSH_PROMISE or
CONTINUATION frame without the END_HEADERS flag set. A recipient
that observes violation of this rule MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.

7. Error Codes

Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY
frames to convey the reasons for the stream or connection error.

Error codes share a common code space. Some error codes apply only
to either streams or the entire connection and have no defined
semantics in the other context.

The following error codes are defined:

NO_ERROR (0x0): (0x00): The associated condition is not a result of an
error. For example, a GOAWAY might include this code to indicate
graceful shutdown of a connection.

PROTOCOL_ERROR (0x1): (0x01): The endpoint detected an unspecific protocol
error. This error is for use when a more specific error code is
not available.

INTERNAL_ERROR (0x2): (0x02): The endpoint encountered an unexpected
internal error.

FLOW_CONTROL_ERROR (0x3): (0x03): The endpoint detected that its peer
violated the flow-control protocol.

SETTINGS_TIMEOUT (0x4): (0x04): The endpoint sent a SETTINGS frame but did
not receive a response in a timely manner. See Section 6.5.3
("Settings Synchronization").

STREAM_CLOSED (0x5): (0x05): The endpoint received a frame after a stream
was half-closed.

FRAME_SIZE_ERROR (0x6): (0x06): The endpoint received a frame with an
invalid size.

REFUSED_STREAM (0x7): (0x07): The endpoint refused the stream prior to
performing any application processing (see Section 8.7 for
details).

CANCEL (0x8): (0x08): The endpoint uses this error code to indicate that
the stream is no longer needed.

COMPRESSION_ERROR (0x9): (0x09): The endpoint is unable to maintain the
field section compression context for the connection.

CONNECT_ERROR (0xa): (0x0a): The connection established in response to a
CONNECT request (Section 8.5) was reset or abnormally closed.

ENHANCE_YOUR_CALM (0xb): (0x0b): The endpoint detected that its peer is
exhibiting a behavior that might be generating excessive load.

INADEQUATE_SECURITY (0xc): (0x0c): The underlying transport has properties
that do not meet minimum security requirements (see Section 9.2).

HTTP_1_1_REQUIRED (0xd): (0x0d): The endpoint requires that HTTP/1.1 be
used instead of HTTP/2.

Unknown or unsupported error codes MUST NOT trigger any special
behavior. These MAY be treated by an implementation as being
equivalent to INTERNAL_ERROR.

8. Expressing HTTP Semantics in HTTP/2

HTTP/2 is an instantiation of the HTTP message abstraction (Section 6
of [HTTP]).

8.1. HTTP Message Framing

A client sends an HTTP request on a new stream, using a previously
unused stream identifier (Section 5.1.1). A server sends an HTTP
response on the same stream as the request.

An HTTP message (request or response) consists of:

1. one HEADERS frame (followed by zero or more CONTINUATION frames)
containing the header section (see Section 6.3 of [HTTP]),

2. zero or more DATA frames containing the message content (see
Section 6.4 of [HTTP]), and

3. optionally, one HEADERS frame (followed by zero or more
CONTINUATION frames) containing the trailer section, if present
(see Section 6.5 of [HTTP]).

For a response only, a server MAY send any number of interim
responses before the HEADERS frame containing a final response. An
interim response consists of a HEADERS frame (which might be followed
by zero or more CONTINUATION frames) containing the control data and
header section of an interim (1xx) HTTP response (see Section 15 of
[HTTP]). A HEADERS frame with the END_STREAM flag set that carries
an informational status code is malformed (Section 8.1.1).

The last frame in the sequence bears an END_STREAM flag, noting that
a HEADERS frame with the END_STREAM flag set can be followed by
CONTINUATION frames that carry any remaining fragments of the field
block.

Other frames (from any stream) MUST NOT occur between the HEADERS
frame and any CONTINUATION frames that might follow.

HTTP/2 uses DATA frames to carry message content. The chunked
transfer encoding defined in Section 7.1 of [HTTP/1.1] cannot be used
in HTTP/2; see Section 8.2.2.

Trailer fields are carried in a field block that also terminates the
stream. That is, trailer fields comprise a sequence starting with a
HEADERS frame, followed by zero or more CONTINUATION frames, where
the HEADERS frame bears an END_STREAM flag. Trailers MUST NOT
include pseudo-header fields (Section 8.3). An endpoint that
receives pseudo-header fields in trailers MUST treat the request or
response as malformed (Section 8.1.1).

An endpoint that receives a HEADERS frame without the END_STREAM flag
set after receiving the HEADERS frame that opens a request or after
receiving a final (non-informational) status code MUST treat the
corresponding request or response as malformed (Section 8.1.1).

An HTTP request/response exchange fully consumes a single stream. A
request starts with the HEADERS frame that puts the stream into the
"open" state. The request ends with a frame with the END_STREAM flag
set, which causes the stream to become "half-closed (local)" for the
client and "half-closed (remote)" for the server. A response stream
starts with zero or more interim responses in HEADERS frames,
followed by a HEADERS frame containing a final status code.

An HTTP response is complete after the server sends -- or the client
receives -- a frame with the END_STREAM flag set (including any
CONTINUATION frames needed to complete a field block). A server can
send a complete response prior to the client sending an entire
request if the response does not depend on any portion of the request
that has not been sent and received. When this is true, a server MAY
request that the client abort transmission of a request without error
by sending a RST_STREAM with an error code of NO_ERROR after sending
a complete response (i.e., a frame with the END_STREAM flag set).
Clients MUST NOT discard responses as a result of receiving such a
RST_STREAM, though clients can always discard responses at their
discretion for other reasons.

8.1.1. Malformed Messages

A malformed request or response is one that is an otherwise valid
sequence of HTTP/2 frames but is invalid due to the presence of
extraneous frames, prohibited fields or pseudo-header fields, the
absence of mandatory pseudo-header fields, the inclusion of uppercase
field names, or invalid field names and/or values (in certain
circumstances; see Section 8.2).

A request or response that includes message content can include a
content-length header field. A request or response is also malformed
if the value of a content-length header field does not equal the sum
of the DATA frame payload lengths that form the content, unless the
message is defined as having no content. For example, 204 or 304
responses contain no content, as does the response to a HEAD request.
A response that is defined to have no content, as described in
Section 6.4.1 of [HTTP], MAY have a non-zero content-length header
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 (Section 5.4.2) of type
PROTOCOL_ERROR.

For malformed requests, a server MAY send an HTTP response prior to
closing or resetting the stream. Clients MUST NOT accept a malformed
response.

Endpoints that progressively process messages might have performed
some processing before identifying a request or response as
malformed. For instance, it might be possible to generate an
informational or 404 status code without having received a complete
request. Similarly, intermediaries might forward incomplete messages
before detecting errors. A server MAY generate a final response
before receiving an entire request when the response does not depend
on the remainder of the request being correct.

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.

8.2. HTTP Fields

HTTP fields (Section 5 of [HTTP]) are conveyed by HTTP/2 in the
HEADERS, CONTINUATION, and PUSH_PROMISE frames, compressed with HPACK
[COMPRESSION].

Field names MUST be converted to lowercase when constructing an
HTTP/2 message.

8.2.1. Field Validity

The definitions of field names and values in HTTP prohibit some
characters that HPACK might be able to convey. HTTP/2
implementations SHOULD validate field names and values according to
their definitions in Sections 5.1 and 5.5 of [HTTP], respectively,
and treat messages that contain prohibited characters as malformed
(Section 8.1.1).

Failure to validate fields can be exploited for request smuggling
attacks. In particular, unvalidated fields might enable attacks when
messages are forwarded using HTTP/1.1 [HTTP/1.1], where characters
such as carriage return (CR), line feed (LF), and COLON are used as
delimiters. Implementations MUST perform the following minimal
validation of field names and values:

* A field name MUST NOT contain characters in the ranges 0x00-0x20,
0x41-0x5a, or 0x7f-0xff (all ranges inclusive). This specifically
excludes all non-visible ASCII characters, ASCII SP (0x20), and
uppercase characters ('A' to 'Z', ASCII 0x41 to 0x5a).

* With the exception of pseudo-header fields (Section 8.3), which
have a name that starts with a single colon, field names MUST NOT
include a colon (ASCII COLON, 0x3a).

* A field value MUST NOT contain the zero value (ASCII NUL, 0x0), 0x00),
line feed (ASCII LF, 0xa), 0x0a), or carriage return (ASCII CR, 0xd) 0x0d) at
any position.

* A field value MUST NOT start or end with an ASCII whitespace
character (ASCII SP or HTAB, 0x20 or 0x9). 0x09).

| Note: An implementation that validates fields according to the
| definitions in Sections 5.1 and 5.5 of [HTTP] only needs an
| additional check that field names do not include uppercase
| characters.

A request or response that contains a field that violates any of
these conditions MUST be treated as malformed (Section 8.1.1). In
particular, an intermediary that does not process fields when
forwarding messages MUST NOT forward fields that contain any of the
values that are listed as prohibited above.

When a request message violates one of these requirements, an
implementation SHOULD generate a 400 (Bad Request) status code (see
Section 15.5.1 of [HTTP]), unless a more suitable status code is
defined or the status code cannot be sent (e.g., because the error
occurs in a trailer field).

| Note: Field values that are not valid according to the
| definition of the corresponding field do not cause a request to
| be malformed; the requirements above only apply to the generic
| syntax for fields as defined in Section 5 of [HTTP].

8.2.2. Connection-Specific Header Fields

HTTP/2 does not use the Connection header field (Section 7.6.1 of
[HTTP]) to indicate connection-specific header fields; in this
protocol, connection-specific metadata is conveyed by other means.
An endpoint MUST NOT generate an HTTP/2 message containing
connection-specific header fields. This includes the Connection
header field and those listed as having connection-specific semantics
in Section 7.6.1 of [HTTP] (that is, Proxy-Connection, Keep-Alive,
Transfer-Encoding, and Upgrade). Any message containing connection-
specific header fields MUST be treated as malformed (Section 8.1.1).

The only exception to this is the TE header field, which MAY be
present in an HTTP/2 request; when it is, it MUST NOT contain any
value other than "trailers".

An intermediary transforming an HTTP/1.x message to HTTP/2 MUST
remove connection-specific header fields as discussed in
Section 7.6.1 of [HTTP], or their messages will be treated by other
HTTP/2 endpoints as malformed (Section 8.1.1).

| Note: HTTP/2 purposefully does not support upgrade to another
| protocol. The handshake methods described in Section 3 are
| believed sufficient to negotiate the use of alternative
| protocols.

8.2.3. Compressing the Cookie Header Field

The Cookie header field [COOKIE] uses a semicolon (";") to delimit
cookie-pairs (or "crumbs"). This header field contains multiple
values, but does not use a COMMA (",") as a separator, which prevents thereby
preventing cookie-pairs from being sent on multiple field lines (see
Section 5.2 of [HTTP]). This can significantly reduce compression
efficiency, as updates to individual cookie-pairs would invalidate
any field lines that are stored in the HPACK table.

To allow for better compression efficiency, the Cookie header field
MAY be split into separate header fields, each with one or more
cookie-pairs. If there are multiple Cookie header fields after
decompression, these MUST be concatenated into a single octet string
using the two-octet delimiter of 0x3b, 0x20 (the ASCII string "; ")
before being passed into a non-HTTP/2 context, such as an HTTP/1.1
connection, or a generic HTTP server application.

Therefore, the following two lists of Cookie header fields are
semantically equivalent.

cookie: a=b; c=d; e=f

cookie: a=b
cookie: c=d
cookie: e=f

8.3. HTTP Control Data

HTTP/2 uses special pseudo-header fields beginning with a ':'
character (ASCII 0x3a) to convey message control data (see
Section 6.2 of [HTTP]).

Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT
generate pseudo-header fields other than those defined in this
document. Note that an extension could negotiate the use of
additional pseudo-header fields; see Section 5.5.

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 a
trailer section. Endpoints MUST treat a request or response that
contains undefined or invalid pseudo-header fields as malformed
(Section 8.1.1).

All pseudo-header fields MUST appear in a field block before all
regular field lines. Any request or response that contains a pseudo-
header field that appears in a field block after a regular field line
MUST be treated as malformed (Section 8.1.1).

The same pseudo-header field name MUST NOT appear more than once in a
field block. A field block for an HTTP request or response that
contains a repeated pseudo-header field name MUST be treated as
malformed (Section 8.1.1).

8.3.1. Request Pseudo-Header Fields

The following pseudo-header fields are defined for HTTP/2 requests:

* The ":method" pseudo-header field includes the HTTP method
(Section 9 of [HTTP]).

* The ":scheme" pseudo-header field includes the scheme portion of
the request target. The scheme is taken from the target URI
(Section 3.1 of [RFC3986]) when generating a request directly, or
from the scheme of a translated request (for example, see
Section 3.3 of [HTTP/1.1]). Scheme is omitted for CONNECT
requests (Section 8.5).

":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.

* The ":authority" pseudo-header field conveys the authority portion
(Section 3.2 of [RFC3986]) of the target URI (Section 7.1 of
[HTTP]). The recipient of an HTTP/2 request MUST NOT use the Host
header field to determine the target URI if ":authority" is
present.

Clients that generate HTTP/2 requests directly MUST use the
":authority" pseudo-header field to convey authority information,
unless there is no authority information to convey (in which case
it MUST NOT generate ":authority").

Clients MUST NOT generate a request with a Host header field that
differs from the ":authority" pseudo-header field. A server
SHOULD treat a request as malformed if it contains a Host header
field that identifies a different an entity to that differs from the entity in
the ":authority" pseudo-header field. The values of fields need
to be normalized to compare them (see Section 6.2 of [RFC3986]).
An origin server can apply any normalization method, whereas other
servers MUST perform scheme-based normalization (see Section 6.2.3
of [RFC3986]) of the two fields.

An intermediary that forwards a request over HTTP/2 MUST construct
an ":authority" pseudo-header field using the authority
information from the control data of the original request, unless
the original request's target URI does not contain authority
information (in which case it MUST NOT generate ":authority").
Note that the Host header field is not the sole source of this
information; see Section 7.2 of [HTTP].

An intermediary that needs to generate a Host header field (which
might be necessary to construct an HTTP/1.1 request) MUST use the
value from the ":authority" pseudo-header field as the value of
the Host field, unless the intermediary also changes the request
target. This replaces any existing Host field to avoid potential
vulnerabilities in HTTP routing.

An intermediary that forwards a request over HTTP/2 MAY retain any
Host header field.

Note that request targets for CONNECT or asterisk-form OPTIONS
requests never include authority information; see Section Sections 7.1 and
7.2 of [HTTP].

":authority" MUST NOT include the deprecated userinfo subcomponent
for "http" or "https" schemed URIs.

* The ":path" pseudo-header field includes the path and query parts
of the target URI (the absolute-path production and, optionally, a
'?' character followed by the query production; see Section 4.1 of
[HTTP]). A request in asterisk form (for OPTIONS) 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 exceptions to this rule are:

- 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 7.1 of [HTTP]).

- CONNECT requests (Section 8.5), where the ":path" pseudo-header
field is omitted.

All HTTP/2 requests MUST include exactly one valid value for the
":method", ":scheme", and ":path" pseudo-header fields, unless they
are CONNECT requests (Section 8.5). An HTTP request that omits
mandatory pseudo-header fields is malformed (Section 8.1.1).

Individual HTTP/2 requests do not carry an explicit indicator of
protocol version. All HTTP/2 requests implicitly have a protocol
version of "2.0" (see Section 6.2 of [HTTP]).

8.3.2. Response Pseudo-Header Fields

For HTTP/2 responses, a single ":status" pseudo-header field is
defined that carries the HTTP status code field (see Section 15 of
[HTTP]). This pseudo-header field MUST be included in all responses,
including interim responses; otherwise, the response is malformed
(Section 8.1.1).

HTTP/2 responses implicitly have a protocol version of "2.0".

8.4. Server Push

HTTP/2 allows a server to preemptively send (or "push") responses
(along with corresponding "promised" requests) to a client in
association with a previous client-initiated request.

Server push was designed to allow a server to improve client-
perceived performance by predicting what requests will follow those
that it receives, thereby removing a round trip for them. For
example, a request for HTML is often followed by requests for
stylesheets and scripts referenced by that page. When these requests
are pushed, the client does not need to wait to receive the
references to them in the HTML and issue separate requests.

In practice, server push is difficult to use effectively, because it
requires the server to correctly anticipate the additional requests
the client will make, taking into account factors such as caching,
content negotiation, and user behavior. Errors in prediction can
lead to performance degradation, due to the opportunity cost that the
additional data on the wire represents. In particular, pushing any
significant amount of data can cause contention issues with responses
that are more important.

A client can request that server push be disabled, though this is
negotiated for each hop independently. The SETTINGS_ENABLE_PUSH
setting can be set to 0 to indicate that server push is disabled.

Promised requests MUST be safe (see Section 9.2.1 of [HTTP]) and
cacheable (see Section 9.2.3 of [HTTP]). Promised requests cannot
include any content or a trailer section. Clients that receive a
promised request that is not cacheable, that is not known to be safe,
or that indicates the presence of request content MUST reset the
promised stream with a stream error (Section 5.4.2) of type
PROTOCOL_ERROR. Note that this could result in the promised stream
being reset if the client does not recognize a newly defined method
as being safe.

Pushed responses that are cacheable (see Section 3 of [CACHE]) [CACHING]) 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 5.2.2.4 of [CACHE]) [CACHING]) while the stream identified by the
promised stream identifier is still open.

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

The server MUST include a value in the ":authority" pseudo-header
field for which the server is authoritative (see Section 10.1). A
client MUST treat a PUSH_PROMISE for which the server is not
authoritative as a stream error (Section 5.4.2) of type
PROTOCOL_ERROR.

An intermediary can receive pushes from the server and choose not to
forward them on to the client. In other words, how to make use of
the pushed information is up to that intermediary. Equally, the
intermediary might choose to make additional pushes to the client,
without any action taken by the server.

A client cannot push. Thus, servers MUST treat the receipt of a
PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. A server cannot set the SETTINGS_ENABLE_PUSH setting
to a value other than 0 (see Section 6.5.2).

8.4.1. Push Requests

Server push is semantically equivalent to a server responding to a
request; however, in this case, that request is also sent by the
server, as a PUSH_PROMISE frame.

The PUSH_PROMISE frame includes a field block that contains control
data and a complete set of request header fields that the server
attributes to the request. It is not possible to push a response to
a request that includes message content.

Promised requests are always associated with an explicit request from
the client. The PUSH_PROMISE frames sent by the server are sent on
that explicit request's stream. The PUSH_PROMISE frame also includes
a promised stream identifier, chosen from the stream identifiers
available to the server (see Section 5.1.1).

The header fields in PUSH_PROMISE and any subsequent CONTINUATION
frames MUST be a valid and complete set of request header fields
(Section 8.3.1). The server MUST include a method in the ":method"
pseudo-header field that is safe and cacheable. If a client receives
a PUSH_PROMISE that does not include a complete and valid set of
header fields or the ":method" pseudo-header field identifies a
method that is not safe, it MUST respond on the promised stream with
a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
sending any frames that reference the promised responses. This
avoids a race where clients issue requests prior to receiving any
PUSH_PROMISE frames.

For example, if the server receives a request for a document
containing embedded links to multiple image files and the server
chooses to push those additional images to the client, sending
PUSH_PROMISE frames before the DATA frames that contain the image
links ensures that the client is able to see that a resource will be
pushed before discovering embedded links. Similarly, if the server
pushes resources referenced by the field block (for instance, in Link
header fields), sending a PUSH_PROMISE before sending the header
ensures that clients do not request those resources.

PUSH_PROMISE frames MUST NOT be sent by the client.

PUSH_PROMISE frames can be sent by the server on any client-initiated
stream, but the stream MUST be in either the "open" or "half-closed
(remote)" state with respect to the server. PUSH_PROMISE frames are
interspersed with the frames that comprise a response, though they
cannot be interspersed with HEADERS and CONTINUATION frames that
comprise a single field block.

Sending a PUSH_PROMISE frame creates a new stream and puts the stream
into the "reserved (local)" state for the server and the "reserved
(remote)" state for the client.

8.4.2. Push Responses

After sending the PUSH_PROMISE frame, the server can begin delivering
the pushed response as a response (Section 8.3.2) on a server-
initiated stream that uses the promised stream identifier. The
server uses this stream to transmit an HTTP response, using the same
sequence of frames as that defined in Section 8.1. This stream
becomes "half-closed" to the client (Section 5.1) after the initial
HEADERS frame is sent.

Once a client receives a PUSH_PROMISE frame and chooses to accept the
pushed response, the client SHOULD NOT issue any requests for the
promised response until after the promised stream has closed.

If the client determines, for any reason, that it does not wish to
receive the pushed response from the server or if the server takes
too long to begin sending the promised response, the client can send
a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code
and referencing the pushed stream's identifier.

A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
the number of responses that can be concurrently pushed by a server.
Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero prevents
the server from opening the streams necessary to push responses.
However, this does not prevent the server from reserving streams
using PUSH_PROMISE frames, because "reserved" reserved streams do not count
toward the concurrent stream limit. Clients that do not wish to
receive pushed resources need to reset any unwanted reserved streams
or set SETTINGS_ENABLE_PUSH to 0.

Clients receiving a pushed response MUST validate that either the
server is authoritative (see Section 10.1) or the proxy that provided
the pushed response is configured for the corresponding request. For
example, a server that offers a certificate for only the example.com
DNS-ID (see [RFC6125]) is not permitted to push a response for
<https://www.example.org/doc>.

The response for a PUSH_PROMISE stream begins with a HEADERS frame,
which immediately puts the stream into the "half-closed (remote)"
state for the server and "half-closed (local)" state for the client,
and ends with a frame with the END_STREAM flag set, which places the
stream in the "closed" state.

| Note: The client never sends a frame with the END_STREAM flag
| set for a server push.

8.5. The CONNECT Method

The CONNECT method (Section 9.3.6 of [HTTP]) is used to convert an
HTTP connection into a tunnel to a remote host. CONNECT 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/2, the CONNECT method establishes a tunnel over a single
HTTP/2 stream to a remote host, rather than converting the entire
connection to a tunnel. A CONNECT header section is constructed as
defined in Section 8.3.1 ("Request Pseudo-Header Fields"), with a few
differences. Specifically:

* The ":method" pseudo-header field is set to CONNECT.

* The ":scheme" and ":path" pseudo-header fields MUST be 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 [HTTP/1.1]).

A CONNECT request that does not conform to these restrictions is
malformed (Section 8.1.1).

A proxy that supports CONNECT establishes a TCP connection [TCP] to
the host and port 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 9.3.6 of [HTTP].

After the initial HEADERS frame sent by each peer, all subsequent
DATA frames correspond to data sent on the TCP connection. The frame
payload of any DATA frames sent by the client is transmitted by the
proxy to the TCP server; data received from the TCP server is
assembled into DATA frames by the proxy. Frame types other than DATA
or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
MUST NOT be sent on a connected stream and MUST be treated as a
stream error (Section 5.4.2) if received.

The TCP connection can be closed by either peer. The END_STREAM flag
on a DATA frame is treated as being equivalent to the TCP FIN bit. A
client is expected to send a DATA frame with the END_STREAM flag set
after receiving a frame with the END_STREAM flag set. A proxy that
receives a DATA frame with the END_STREAM flag set sends the attached
data with the FIN bit set on the last TCP segment. A proxy that
receives a TCP segment with the FIN bit set sends a DATA frame with
the END_STREAM flag set. Note that the final TCP segment or DATA
frame could be empty.

A TCP connection error is signaled with RST_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 (Section 5.4.2) of
type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment
with the RST bit set if it detects an error with the stream or the
HTTP/2 connection.

8.6. The Upgrade Header Field

HTTP/2 does not support the 101 (Switching Protocols) informational
status code (Section 15.2.2 of [HTTP]).

The semantics of 101 (Switching Protocols) aren't applicable to a
multiplexed protocol. Similar functionality might be enabled through
the use of extended CONNECT [RFC8441], and other protocols are able
to use the same mechanisms that HTTP/2 uses to negotiate their use
(see Section 3).

8.7. Request Reliability

In general, an HTTP client is unable to retry a non-idempotent
request when an error occurs because there is no means to determine
the nature of the error (see Section 9.2.2 of [HTTP]). It is
possible that some server processing occurred prior to the error,
which could result in undesirable effects if the request were
reattempted.

HTTP/2 provides two mechanisms for providing a guarantee to a client
that a request has not been processed:

* The GOAWAY frame indicates the highest stream number that might
have been processed. Requests on streams with higher numbers are
therefore guaranteed to be safe to retry.

* The REFUSED_STREAM error code can be included in a RST_STREAM
frame to indicate that the stream is being closed prior to any
processing having occurred. Any request that was sent on the
reset stream can be safely retried.

Requests that have not been processed have not failed; clients MAY
automatically retry them, even those with non-idempotent methods.

A server MUST NOT indicate that a stream has not been processed
unless it can guarantee that fact. If frames that are on a stream
are passed to the application layer for any stream, then
REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
MUST include a stream identifier that is greater than or equal to the
given stream identifier.

In addition to these mechanisms, the PING frame provides a way for a
client to easily test a connection. Connections that remain idle can
become broken as broken, because some middleboxes (for instance, network
address translators or load balancers) silently discard connection
bindings. The PING frame allows a client to safely test whether a
connection is still active without sending a request.

8.8. Examples

This section shows HTTP/1.1 requests and responses, with
illustrations of equivalent HTTP/2 requests and responses.

8.8.1. Simple Request

An HTTP GET request includes control data and a request header with
no message content and is therefore transmitted as a single HEADERS
frame, followed by zero or more CONTINUATION frames containing the
serialized block of request header fields. The HEADERS frame in the
following has both the END_HEADERS and END_STREAM flags set; no
CONTINUATION frames are sent.

GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS
:method = GET
:scheme = https
:authority = example.org
:path = /resource
host = example.org
accept = image/jpeg

8.8.2. Simple Response

Similarly, a response that includes only control data and a response
header is transmitted as a HEADERS frame (again, followed by zero or
more CONTINUATION frames) containing the serialized block of response
header fields.

HTTP/1.1 304 Not Modified HEADERS
ETag: "xyzzy" ==> + END_STREAM
Expires: Thu, 23 Jan ... + END_HEADERS
:status = 304
etag = "xyzzy"
expires = Thu, 23 Jan ...

8.8.3. Complex Request

An HTTP POST request that includes control data and a request header
with message content is transmitted as one HEADERS frame, followed by
zero or more CONTINUATION frames containing the request header,
followed by one or more DATA frames, with the last CONTINUATION (or
HEADERS) frame having the END_HEADERS flag set and the final DATA
frame having the END_STREAM flag set:

POST /resource HTTP/1.1 HEADERS
Host: example.org ==> - END_STREAM
Content-Type: image/jpeg - END_HEADERS
Content-Length: 123 :method = POST
:authority = example.org
:path = /resource
{binary data} :scheme = https

CONTINUATION
+ END_HEADERS
content-type = image/jpeg
host = example.org
content-length = 123

DATA
+ END_STREAM
{binary data}

Note that data contributing to any given field line could be spread
between field block fragments. The allocation of field lines to
frames in this example is illustrative only.

8.8.4. Response with Body

A response that includes control data and a response header with
message content is transmitted as a HEADERS frame, followed by zero
or more CONTINUATION frames, followed by one or more DATA frames,
with the last DATA frame in the sequence having the END_STREAM flag
set:

HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM
Content-Length: 123 + END_HEADERS
:status = 200
{binary data} content-type = image/jpeg
content-length = 123

DATA
+ END_STREAM
{binary data}

8.8.5. Informational Responses

An informational response using a 1xx status code other than 101 is
transmitted as a HEADERS frame, followed by zero or more CONTINUATION
frames.

A trailer section is sent as a field block after both the request or
response field block and all the DATA frames have been sent. The
HEADERS frame starting the field block that comprises the trailer
section has the END_STREAM flag set.

The following example includes both a 100 (Continue) status code,
which is sent in response to a request containing a "100-continue"
token in the Expect header field, and a trailer section:

HTTP/1.1 100 Continue HEADERS
Extension-Field: bar ==> - END_STREAM
+ END_HEADERS
:status = 100
extension-field = bar

HTTP/1.1 200 OK HEADERS
Content-Type: image/jpeg ==> - END_STREAM
Transfer-Encoding: chunked + END_HEADERS
Trailer: Foo :status = 200
content-type = image/jpeg
123 trailer = Foo
{binary data}
0 DATA
Foo: bar - END_STREAM
{binary data}

HEADERS
+ END_STREAM
+ END_HEADERS
foo = bar

9. HTTP/2 Connections

This section outlines attributes of the HTTP protocol that improve
interoperability, reduce exposure to known security vulnerabilities,
or reduce the potential for implementation variation.

9.1. Connection Management

HTTP/2 connections are persistent. 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.

Clients SHOULD NOT open more than one HTTP/2 connection to a given
host and port pair, where the host is derived from a URI, a selected
alternative service [ALT-SVC], or a configured proxy.

A client can create additional connections as replacements, either to
replace connections that are near to exhausting the available stream
identifier space (Section 5.1.1), to refresh the keying material for
a TLS connection, or to replace connections that have encountered
errors (Section 5.4.1).

A client MAY open multiple connections to the same IP address and TCP
port using different Server Name Indication [TLS-EXT] values or to
provide different TLS client certificates but SHOULD avoid creating
multiple connections with the same configuration.

Servers are encouraged to maintain open connections for as long as
possible but are permitted to terminate idle connections if
necessary. When either endpoint chooses to close the transport-layer
TCP connection, the terminating endpoint SHOULD first send a GOAWAY
(Section 6.8) frame so that both endpoints can reliably determine
whether previously sent frames have been processed and gracefully
complete or terminate any necessary remaining tasks.

9.1.1. Connection Reuse

Connections that are made to an origin server, either directly or
through a tunnel created using the CONNECT method (Section 8.5), MAY
be reused for requests with multiple different URI authority
components. A connection can be reused as long as the origin server
is authoritative (Section 10.1). For TCP connections without TLS,
this depends on the host having resolved to the same IP address.

For "https" resources, connection reuse additionally depends on
having a certificate that is valid for the host in the URI. The
certificate presented by the server MUST satisfy any checks that the
client would perform when forming a new TLS connection for the host
in the URI. A single certificate can be used to establish authority
for multiple origins. Section 4.3 of [HTTP] describes how a client
determines whether a server is authoritative for a URI.

In some deployments, reusing a connection for multiple origins can
result in requests being directed to the wrong origin server. For
example, TLS termination might be performed by a middlebox that uses
the TLS Server Name Indication (SNI) [TLS-EXT] extension to select an
origin server. This means that it is possible for clients to send
requests to servers that might not be the intended target for the
request, even though the server is otherwise authoritative.

A server that does not wish clients to reuse connections 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 15.5.20 of [HTTP]).

A client that is configured to use a proxy over HTTP/2 directs
requests to that proxy through a single connection. That is, all
requests sent via a proxy reuse the connection to the proxy.

9.2. Use of TLS Features

Implementations of HTTP/2 MUST use TLS version 1.2 [TLS12] or higher
for HTTP/2 over TLS. The general TLS usage guidance in [TLSBCP]
SHOULD be followed, with some additional restrictions that are
specific to HTTP/2.

The TLS implementation MUST support the Server Name Indication (SNI)
[TLS-EXT] extension to TLS. If the server is identified by a domain
name [DNS-TERMS], clients MUST send the server_name TLS extension
unless an alternative mechanism to indicate the target host is used.

Requirements for deployments of HTTP/2 that negotiate TLS 1.3 [TLS13]
are included in Section 9.2.3. Deployments of TLS 1.2 are subject to
the requirements in Sections 9.2.1 and 9.2.2. Implementations are
encouraged to provide defaults that comply, but it is recognized that
deployments are ultimately responsible for compliance.

9.2.1. TLS 1.2 Features

This section describes restrictions on the TLS 1.2 feature set that
can be used with HTTP/2. Due to deployment limitations, it might not
be possible to fail TLS negotiation when these restrictions are not
met. An endpoint MAY immediately terminate an HTTP/2 connection that
does not meet these TLS requirements with a connection error
(Section 5.4.1) of type INADEQUATE_SECURITY.

A deployment of HTTP/2 over TLS 1.2 MUST disable compression. TLS
compression can lead to the exposure of information that would not
otherwise be revealed [RFC3749]. Generic compression is unnecessary,
since HTTP/2 provides compression features that are more aware of
context and therefore likely to be more appropriate for use for
performance, security, or other reasons.

A deployment of HTTP/2 over TLS 1.2 MUST disable renegotiation. An
endpoint MUST treat a TLS renegotiation as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Note that disabling
renegotiation can result in long-lived connections becoming unusable
due to limits on the number of messages the underlying cipher suite
can encipher.

An endpoint MAY use renegotiation to provide confidentiality
protection for client credentials offered in the handshake, but any
renegotiation MUST occur prior to sending the connection preface. A
server SHOULD request a client certificate if it sees a renegotiation
request immediately after establishing a connection.

This effectively prevents the use of renegotiation in response to a
request for a specific protected resource. A future specification
might provide a way to support this use case. Alternatively, a
server might use an error (Section 5.4) of type HTTP_1_1_REQUIRED to
request that the client use a protocol that supports renegotiation.

Implementations MUST support ephemeral key exchange sizes of at least
2048 bits for cipher suites that use ephemeral finite field Diffie-
Hellman (DHE) (Section 8.1.2 of [TLS12]) and 224 bits for cipher
suites that use ephemeral elliptic curve Diffie-Hellman (ECDHE)
[RFC8422]. Clients MUST accept DHE sizes of up to 4096 bits.
Endpoints MAY treat negotiation of key sizes smaller than the lower
limits as a connection error (Section 5.4.1) of type
INADEQUATE_SECURITY.

9.2.2. TLS 1.2 Cipher Suites

A deployment of HTTP/2 over TLS 1.2 SHOULD NOT use any of the
prohibited cipher suites listed in Appendix A.

Endpoints MAY choose to generate a connection error (Section 5.4.1)
of type INADEQUATE_SECURITY if one of the prohibited cipher suites is
negotiated. A deployment that chooses to use a prohibited cipher
suite risks triggering a connection error unless the set of potential
peers is known to accept that cipher suite.

Implementations MUST NOT generate this error in reaction to the
negotiation of a cipher suite that is not prohibited. Consequently,
when clients offer a cipher suite that is not prohibited, they have
to be prepared to use that cipher suite with HTTP/2.

The list of prohibited cipher suites includes the cipher suite that
TLS 1.2 makes mandatory, which means that TLS 1.2 deployments could
have non-intersecting sets of permitted cipher suites. To avoid this
problem, which causes TLS handshake failures, deployments of HTTP/2
that use TLS 1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
[TLS-ECDHE] with the P-256 elliptic curve [RFC8422].

Note that clients might advertise support of cipher suites that are
prohibited in order to allow for connection to servers that do not
support HTTP/2. This allows servers to select HTTP/1.1 with a cipher
suite that is prohibited in HTTP/2. However, this can result in
HTTP/2 being negotiated with a prohibited cipher suite if the
application protocol and cipher suite are independently selected.

9.2.3. TLS 1.3 Features

TLS 1.3 includes a number of features not available in earlier
versions. This section discusses the use of these features.

HTTP/2 servers MUST NOT send post-handshake TLS 1.3
CertificateRequest messages. HTTP/2 clients MUST treat a TLS post-
handshake CertificateRequest message as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.

The prohibition on post-handshake authentication applies even if the
client offered the "post_handshake_auth" TLS extension. Post-
handshake authentication support might be advertised independently of
ALPN [TLS-ALPN]. Clients might offer the capability for use in other
protocols, but inclusion of the extension cannot imply support within
HTTP/2.

[TLS13] defines other post-handshake messages, NewSessionTicket and
KeyUpdate, which can be used as they have no direct interaction with
HTTP/2. Unless the use of a new type of TLS message depends on an
interaction with the application-layer protocol, that TLS message can
be sent after the handshake completes.

TLS early data MAY be used to send requests, provided that the
guidance in [RFC8470] is observed. Clients send requests in early
data assuming initial values for all server settings.

10. Security Considerations

The use of TLS is necessary to provide many of the security
properties of this protocol. Many of the claims in this section do
not hold unless TLS is used as described in Section 9.2.

10.1. Server Authority

HTTP/2 relies on the HTTP definition of authority for determining
whether a server is authoritative in providing a given response (see
Section 4.3 of [HTTP]). This relies on local name resolution for the
"http" URI scheme and the authenticated server identity for the
"https" scheme.

10.2. Cross-Protocol Attacks

In a cross-protocol attack, an attacker causes a client to initiate a
transaction in one protocol toward a server that understands a
different protocol. An attacker might be able to cause the
transaction to appear as a valid transaction in the second protocol.
In combination with the capabilities of the web context, this can be
used to interact with poorly protected servers in private networks.

Completing a TLS handshake with an ALPN identifier for HTTP/2 can be
considered sufficient protection against cross-protocol attacks.
ALPN provides a positive indication that a server is willing to
proceed with HTTP/2, which prevents attacks on other TLS-based
protocols.

The encryption in TLS makes it difficult for attackers to control the
data that could be used in a cross-protocol attack on a cleartext
protocol.

The cleartext version of HTTP/2 has minimal protection against cross-
protocol attacks. The connection preface (Section 3.4) contains a
string that is designed to confuse HTTP/1.1 servers, but no special
protection is offered for other protocols.

10.3. Intermediary Encapsulation Attacks

HPACK permits encoding of field names and values that might be
treated as delimiters in other HTTP versions. An intermediary that
translates an HTTP/2 request or response MUST validate fields
according to the rules in Section 8.2 before translating a message to
another HTTP version. Translating a field that includes invalid
delimiters could be used to cause recipients to incorrectly interpret
a message, which could be exploited by an attacker.

Section 8.2 does not include specific rules for validation of pseudo-
header fields. If the values of these fields are used, additional
validation is necessary. This is particularly important where
":scheme", ":authority", and ":path" are combined to form a single
URI string [RFC3986]. Similar problems might occur when that URI or
just ":path" is combined with ":method" to construct a request line
(as in Section 3 of [HTTP/1.1]). Simple concatenation is not secure
unless the input values are fully validated.

An intermediary can reject fields that contain invalid field names or
values for other reasons -- in particular, those fields that do not
conform to the HTTP ABNF grammar from Section 5 of [HTTP].
Intermediaries that do not perform any validation of fields other
than the minimum required by Section 8.2 could forward messages that
contain invalid field names or values.

An intermediary that receives any fields that require removal before
forwarding (see Section 7.6.1 of [HTTP]) MUST remove or replace those
header fields when forwarding messages. Additionally, intermediaries
should take care when forwarding messages containing Content-Length
fields to ensure that the message is well formed well-formed (Section 8.1.1).
This ensures that if the message is translated into HTTP/1.1 at any
point, the framing will be correct.

10.4. 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.

Pushed responses for which an origin server is not authoritative (see
Section 10.1) MUST NOT be used or cached.

10.5. Denial-of-Service Considerations

An HTTP/2 connection can demand a greater commitment of resources to
operate than an HTTP/1.1 connection. The use of Both field section 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 not constrained in the same
fashion. A client that accepts server push SHOULD limit the number
of streams it allows to be in the "reserved (remote)" state. An
excessive number of server push streams can be treated as a stream
error (Section 5.4.2) of type ENHANCE_YOUR_CALM.

A number of HTTP/2 implementations were found to be vulnerable to
denial of service [NFLX-2019-002]. Below is a list of known ways
that implementations might be subject to denial-of-service attacks:

* Inefficient tracking of outstanding outbound frames can lead to
overload if an adversary can cause large numbers of frames to be
enqueued for sending. A peer could use one of several techniques
to cause large numbers of frames to be generated:

- Providing tiny increments to flow control in WINDOW_UPDATE
frames can cause a sender to generate a large number of DATA
frames.

- An endpoint is required to respond to a PING frame.

- Each SETTINGS frame requires acknowledgment.

- An invalid request (or server push) can cause a peer to send
RST_STREAM frames in response.

* An attacker can provide large amounts of flow-control credit at
the HTTP/2 layer but withhold credit at the TCP layer, preventing
frames from being sent. An endpoint that constructs and remembers
frames for sending without considering TCP limits might be subject
to resource exhaustion.

* Large numbers of small or empty frames can be abused to cause a
peer to expend time processing frame headers. Caution is required
here as some uses of small frames are entirely legitimate, such as
the sending of an empty DATA or CONTINUATION frame at the end of a
stream.

* The SETTINGS frame might also be abused to cause a peer to expend
additional processing time. This might be done by pointlessly
changing settings, sending multiple undefined settings, or
changing the same setting multiple times in the same frame.

* Handling reprioritization with PRIORITY frames can require
significant processing time and can lead to overload if many
PRIORITY frames are sent.

* Field section compression also offers some provides opportunities for an
attacker to waste processing resources; see Section 7 of
[COMPRESSION] for more details on potential abuses.

* Limits in SETTINGS cannot be reduced instantaneously, which leaves
an endpoint exposed to behavior from a peer that could exceed the
new limits. In particular, immediately after establishing a
connection, limits set by a server are not known to clients and
could be exceeded without being an obvious protocol violation.

Most of the features that might be exploited for denial of service --
i.e.,
such as SETTINGS changes, small frames, field section compression --
have legitimate uses. These features become a burden only when they
are used unnecessarily or to excess.

An endpoint that doesn't monitor use of these features exposes itself
to a risk of denial of service. 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 (Section 5.4.1) of
type ENHANCE_YOUR_CALM.

10.5.1. Limits on Field Block Size

A large field block (Section 4.3) can cause an implementation to
commit a large amount of state. Field lines that are critical for
routing can appear toward the end of a field block, which prevents
streaming of fields to their ultimate destination. This ordering and
other reasons, such as ensuring cache correctness, mean that an
endpoint might need to buffer the entire field block. Since there is
no hard limit to the size of a field block, some endpoints could be
forced to commit a large amount of available memory for field blocks.

An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers
of limits that might apply on the size of uncompressed field blocks.
This setting is only advisory, so endpoints MAY choose to send field
blocks that exceed this limit and risk the request or response being
treated as malformed. This setting is specific to a 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 obliged to
do so.

A server that receives a larger field block 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 field block MUST be processed to ensure a consistent
connection state, unless the connection is closed.

10.5.2. 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 SETTINGS_MAX_CONCURRENT_STREAMS alone to limit the
resources consumed by CONNECT requests.

10.6. 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/2 enables compression of field lines (Section 4.3); the
following concerns also apply to the use of HTTP compressed content-
codings (Section 8.4.1 of [HTTP]).

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 dictionaries are used for each source of
data. Compression MUST NOT be used if the source of data cannot be
reliably determined. Generic stream compression, such as that
provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2).

Further considerations regarding the compression of header fields are
described in [COMPRESSION].

10.7. Use of Padding

Padding within HTTP/2 is not intended as a replacement for general- general
purpose padding, such as that provided by TLS [TLS13]. Redundant
padding could even be counterproductive. Correct application can
depend on having specific knowledge of the data that is being padded.

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

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]).

Use of padding can result in less protection than might seem
immediately obvious. 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 frame payloads to a fixed size exposes
information as frame payload sizes cross the fixed-sized boundary,
which could be possible if an attacker can control plaintext.

Intermediaries SHOULD retain padding for DATA frames but MAY drop
padding for HEADERS and PUSH_PROMISE frames. A valid reason for an
intermediary to change the amount of padding of frames is to improve
the protections that padding provides.

10.8. Privacy Considerations

Several characteristics of HTTP/2 provide an observer an opportunity
to correlate actions of a single client or server over time. These
include the value values of settings, the manner in which flow-control
windows are managed, the way priorities are allocated to streams, 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, as defined
in Section 3.2 of [PRIVACY].

HTTP/2's preference for using a single TCP connection allows
correlation of a user's activity on a site. Reusing connections for
different origins allows tracking across those origins.

Because the PING and SETTINGS frames solicit immediate responses,
they can be used by an endpoint to measure latency to their peer.
This might have privacy implications in certain scenarios.

10.9. Remote Timing Attacks

Remote timing attacks extract secrets from servers by observing
variations in the time that servers take when processing requests
that use secrets. HTTP/2 enables concurrent request creation and
processing, which can give attackers better control over when request
processing commences. Multiple HTTP/2 requests can be included in
the same IP packet or TLS record. HTTP/2 can therefore make remote
timing attacks more efficient by eliminating variability in request
delivery, leaving only request order and the delivery of responses as
sources of timing variability.

Ensuring that processing time is not dependent on the value of
secrets a
secret is the best defense against any form of timing attack.

11. IANA Considerations

This revision of HTTP/2 marks the HTTP2-Settings header field and the
h2c upgrade token, both defined in [RFC7540], as obsolete.

Section 11 of [RFC7540] registered the h2 and h2c ALPN identifiers
along with the PRI HTTP method. RFC 7540 also established a registry
for frame types, settings, and error codes. These registrations and
registries apply to HTTP/2 HTTP/2, but are not redefined in this document.

IANA has updated references to RFC 7540 in the following registries
to refer to this document: "TLS Application-Layer Protocol
Negotiation (ALPN) Protocol IDs", "HTTP/2 Frame Type", "HTTP/2
Settings", "HTTP/2 Error Code", and "HTTP Method Registry". The
registration of the PRI method has been updated to refer to
Section 3.4; all other section numbers have not changed.

IANA has changed the policy on those portions of the "HTTP/2 Frame
Type" and "HTTP/2 Settings" registries that were reserved for
Experimental Use [RFC8126] in RFC 7540. These portions of the registries shall
operate on the same policy as the remainder of each registry.

11.1. HTTP2-Settings Header Field Registration

This section marks the HTTP2-Settings header field registered by
Section 11.5 of [RFC7540] in the "Hypertext Transfer Protocol (HTTP)
Field Name Registry" as obsolete. This capability has been removed:
see Section 3.1. The registration is updated to include the details
as required by Section 18.4 of [HTTP]:

Field Name: HTTP2-Settings

Status: obsoleted

Reference: Section 3.2.1 of [RFC7540]

Comments: Obsolete; see Section 11.1 of this document.

11.2. The h2c Upgrade Token

This section records the h2c upgrade token registered by Section 11.8
of [RFC7540] in the "Hypertext Transfer Protocol (HTTP) Upgrade Token
Registry" as obsolete. This capability has been removed: see
Section 3.1. The registration is updated as follows:

Value: h2c

Description: (OBSOLETE) Hypertext Transfer Protocol version 2
(HTTP/2)

Expected Version Tokens: None

Reference: Section 3.1 of this document

12. References

12.1. Normative References

[CACHE]

[CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, March June 2022,
<https://www.rfc-editor.org/info/rfc9111>.

[COMPRESSION]
Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.

[COOKIE] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110,
March June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.

[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>.

[TCP] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.

[TLS-ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[TLS-ECDHE]
Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
DOI 10.17487/RFC5289, August 2008,
<https://www.rfc-editor.org/info/rfc5289>.

[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[TLS12] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[TLSBCP] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.

12.2. Informative References

[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.

[BREACH] Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the
CRIME Attack", 12 July 2013,
<https://breachattack.com/resources/
BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.

[DNS-TERMS]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.

[HTTP-PRIORITY]
Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", RFC 9218, DOI 10.17487/RFC9218, March June 2022,
<https://www.rfc-editor.org/info/rfc9218>.

[HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112, March
June 2022, <https://www.rfc-editor.org/info/rfc9112>.

[NFLX-2019-002]
Netflix, "HTTP/2 Denial of Service Advisory", commit
2164332, 13 August
2019, <https://github.com/Netflix/
security-bulletins/blob/master/advisories/third-
party/2019-002.md>. <https://github.com/Netflix/security-
bulletins/blob/master/advisories/third-party/2019-002.md>.

[PRIVACY] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
2004, <https://www.rfc-editor.org/info/rfc3749>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/info/rfc6585>.

[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.

[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8441] McManus, P., "Bootstrapping WebSockets with HTTP/2",
RFC 8441, DOI 10.17487/RFC8441, September 2018,
<https://www.rfc-editor.org/info/rfc8441>.

[RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740,
DOI 10.17487/RFC8740, February 2020,
<https://www.rfc-editor.org/info/rfc8740>.

[TALKING] Huang, L., Chen, E., Barth, A., Rescorla, E., and C.
Jackson, "Talking to Yourself for Fun and Profit", 2011,
<https://www.adambarth.com/papers/2011/huang-chen-barth-
rescorla-jackson.pdf>.

Appendix A. Prohibited TLS 1.2 Cipher Suites

An HTTP/2 implementation MAY treat the negotiation of any of the
following cipher suites with TLS 1.2 as a connection error
(Section 5.4.1) of type INADEQUATE_SECURITY:

* TLS_NULL_WITH_NULL_NULL
* TLS_RSA_WITH_NULL_MD5
* TLS_RSA_WITH_NULL_SHA
* TLS_RSA_EXPORT_WITH_RC4_40_MD5
* TLS_RSA_WITH_RC4_128_MD5
* TLS_RSA_WITH_RC4_128_SHA
* TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5
* TLS_RSA_WITH_IDEA_CBC_SHA
* TLS_RSA_EXPORT_WITH_DES40_CBC_SHA
* TLS_RSA_WITH_DES_CBC_SHA
* TLS_RSA_WITH_3DES_EDE_CBC_SHA
* TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA
* TLS_DH_DSS_WITH_DES_CBC_SHA
* TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA
* TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA
* TLS_DH_RSA_WITH_DES_CBC_SHA
* TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA
* TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA
* TLS_DHE_DSS_WITH_DES_CBC_SHA
* TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
* TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
* TLS_DHE_RSA_WITH_DES_CBC_SHA
* TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA
* TLS_DH_anon_EXPORT_WITH_RC4_40_MD5
* TLS_DH_anon_WITH_RC4_128_MD5
* TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA
* TLS_DH_anon_WITH_DES_CBC_SHA
* TLS_DH_anon_WITH_3DES_EDE_CBC_SHA
* TLS_KRB5_WITH_DES_CBC_SHA
* TLS_KRB5_WITH_3DES_EDE_CBC_SHA
* TLS_KRB5_WITH_RC4_128_SHA
* TLS_KRB5_WITH_IDEA_CBC_SHA
* TLS_KRB5_WITH_DES_CBC_MD5
* TLS_KRB5_WITH_3DES_EDE_CBC_MD5
* TLS_KRB5_WITH_RC4_128_MD5
* TLS_KRB5_WITH_IDEA_CBC_MD5
* TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA
* TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA
* TLS_KRB5_EXPORT_WITH_RC4_40_SHA
* TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5
* TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5
* TLS_KRB5_EXPORT_WITH_RC4_40_MD5
* TLS_PSK_WITH_NULL_SHA
* TLS_DHE_PSK_WITH_NULL_SHA
* TLS_RSA_PSK_WITH_NULL_SHA
* TLS_RSA_WITH_AES_128_CBC_SHA
* TLS_DH_DSS_WITH_AES_128_CBC_SHA
* TLS_DH_RSA_WITH_AES_128_CBC_SHA
* TLS_DHE_DSS_WITH_AES_128_CBC_SHA
* TLS_DHE_RSA_WITH_AES_128_CBC_SHA
* TLS_DH_anon_WITH_AES_128_CBC_SHA
* TLS_RSA_WITH_AES_256_CBC_SHA
* TLS_DH_DSS_WITH_AES_256_CBC_SHA
* TLS_DH_RSA_WITH_AES_256_CBC_SHA
* TLS_DHE_DSS_WITH_AES_256_CBC_SHA
* TLS_DHE_RSA_WITH_AES_256_CBC_SHA
* TLS_DH_anon_WITH_AES_256_CBC_SHA
* TLS_RSA_WITH_NULL_SHA256
* TLS_RSA_WITH_AES_128_CBC_SHA256
* TLS_RSA_WITH_AES_256_CBC_SHA256
* TLS_DH_DSS_WITH_AES_128_CBC_SHA256
* TLS_DH_RSA_WITH_AES_128_CBC_SHA256
* TLS_DHE_DSS_WITH_AES_128_CBC_SHA256
* TLS_RSA_WITH_CAMELLIA_128_CBC_SHA
* TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA
* TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA
* TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA
* TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
* TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA
* TLS_DHE_RSA_WITH_AES_128_CBC_SHA256
* TLS_DH_DSS_WITH_AES_256_CBC_SHA256
* TLS_DH_RSA_WITH_AES_256_CBC_SHA256
* TLS_DHE_DSS_WITH_AES_256_CBC_SHA256
* TLS_DHE_RSA_WITH_AES_256_CBC_SHA256
* TLS_DH_anon_WITH_AES_128_CBC_SHA256
* TLS_DH_anon_WITH_AES_256_CBC_SHA256
* TLS_RSA_WITH_CAMELLIA_256_CBC_SHA
* TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA
* TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA
* TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA
* TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
* TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA
* TLS_PSK_WITH_RC4_128_SHA
* TLS_PSK_WITH_3DES_EDE_CBC_SHA
* TLS_PSK_WITH_AES_128_CBC_SHA
* TLS_PSK_WITH_AES_256_CBC_SHA
* TLS_DHE_PSK_WITH_RC4_128_SHA
* TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA
* TLS_DHE_PSK_WITH_AES_128_CBC_SHA
* TLS_DHE_PSK_WITH_AES_256_CBC_SHA
* TLS_RSA_PSK_WITH_RC4_128_SHA
* TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA
* TLS_RSA_PSK_WITH_AES_128_CBC_SHA
* TLS_RSA_PSK_WITH_AES_256_CBC_SHA
* TLS_RSA_WITH_SEED_CBC_SHA
* TLS_DH_DSS_WITH_SEED_CBC_SHA
* TLS_DH_RSA_WITH_SEED_CBC_SHA
* TLS_DHE_DSS_WITH_SEED_CBC_SHA
* TLS_DHE_RSA_WITH_SEED_CBC_SHA
* TLS_DH_anon_WITH_SEED_CBC_SHA
* TLS_RSA_WITH_AES_128_GCM_SHA256
* TLS_RSA_WITH_AES_256_GCM_SHA384
* TLS_DH_RSA_WITH_AES_128_GCM_SHA256
* TLS_DH_RSA_WITH_AES_256_GCM_SHA384
* TLS_DH_DSS_WITH_AES_128_GCM_SHA256
* TLS_DH_DSS_WITH_AES_256_GCM_SHA384
* TLS_DH_anon_WITH_AES_128_GCM_SHA256
* TLS_DH_anon_WITH_AES_256_GCM_SHA384
* TLS_PSK_WITH_AES_128_GCM_SHA256
* TLS_PSK_WITH_AES_256_GCM_SHA384
* TLS_RSA_PSK_WITH_AES_128_GCM_SHA256
* TLS_RSA_PSK_WITH_AES_256_GCM_SHA384
* TLS_PSK_WITH_AES_128_CBC_SHA256
* TLS_PSK_WITH_AES_256_CBC_SHA384
* TLS_PSK_WITH_NULL_SHA256
* TLS_PSK_WITH_NULL_SHA384
* TLS_DHE_PSK_WITH_AES_128_CBC_SHA256
* TLS_DHE_PSK_WITH_AES_256_CBC_SHA384
* TLS_DHE_PSK_WITH_NULL_SHA256
* TLS_DHE_PSK_WITH_NULL_SHA384
* TLS_RSA_PSK_WITH_AES_128_CBC_SHA256
* TLS_RSA_PSK_WITH_AES_256_CBC_SHA384
* TLS_RSA_PSK_WITH_NULL_SHA256
* TLS_RSA_PSK_WITH_NULL_SHA384
* TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256
* TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_DHE_DSS_WITH_CAMELLIA_128_CBC_SHA256
* TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256
* TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256
* TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256
* TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256
* TLS_DHE_DSS_WITH_CAMELLIA_256_CBC_SHA256
* TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256
* TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256
* TLS_EMPTY_RENEGOTIATION_INFO_SCSV
* TLS_ECDH_ECDSA_WITH_NULL_SHA
* TLS_ECDH_ECDSA_WITH_RC4_128_SHA
* TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA
* TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA
* TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA
* TLS_ECDHE_ECDSA_WITH_NULL_SHA
* TLS_ECDHE_ECDSA_WITH_RC4_128_SHA
* TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA
* TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA
* TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
* TLS_ECDH_RSA_WITH_NULL_SHA
* TLS_ECDH_RSA_WITH_RC4_128_SHA
* TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA
* TLS_ECDH_RSA_WITH_AES_128_CBC_SHA
* TLS_ECDH_RSA_WITH_AES_256_CBC_SHA
* TLS_ECDHE_RSA_WITH_NULL_SHA
* TLS_ECDHE_RSA_WITH_RC4_128_SHA
* TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
* TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA
* TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA
* TLS_ECDH_anon_WITH_NULL_SHA
* TLS_ECDH_anon_WITH_RC4_128_SHA
* TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA
* TLS_ECDH_anon_WITH_AES_128_CBC_SHA
* TLS_ECDH_anon_WITH_AES_256_CBC_SHA
* TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA
* TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA
* TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA
* TLS_SRP_SHA_WITH_AES_128_CBC_SHA
* TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA
* TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA
* TLS_SRP_SHA_WITH_AES_256_CBC_SHA
* TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA
* TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA
* TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256
* TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384
* TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256
* TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384
* TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256
* TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384
* TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256
* TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384
* TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256
* TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384
* TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256
* TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384
* TLS_ECDHE_PSK_WITH_RC4_128_SHA
* TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA
* TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA
* TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA
* TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256
* TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384
* TLS_ECDHE_PSK_WITH_NULL_SHA
* TLS_ECDHE_PSK_WITH_NULL_SHA256
* TLS_ECDHE_PSK_WITH_NULL_SHA384
* TLS_RSA_WITH_ARIA_128_CBC_SHA256
* TLS_RSA_WITH_ARIA_256_CBC_SHA384
* TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256
* TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384
* TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256
* TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384
* TLS_DHE_DSS_WITH_ARIA_128_CBC_SHA256
* TLS_DHE_DSS_WITH_ARIA_256_CBC_SHA384
* TLS_DHE_RSA_WITH_ARIA_128_CBC_SHA256
* TLS_DHE_RSA_WITH_ARIA_256_CBC_SHA384
* TLS_DH_anon_WITH_ARIA_128_CBC_SHA256
* TLS_DH_anon_WITH_ARIA_256_CBC_SHA384
* TLS_ECDHE_ECDSA_WITH_ARIA_128_CBC_SHA256
* TLS_ECDHE_ECDSA_WITH_ARIA_256_CBC_SHA384
* TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256
* TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384
* TLS_ECDHE_RSA_WITH_ARIA_128_CBC_SHA256
* TLS_ECDHE_RSA_WITH_ARIA_256_CBC_SHA384
* TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256
* TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384
* TLS_RSA_WITH_ARIA_128_GCM_SHA256
* TLS_RSA_WITH_ARIA_256_GCM_SHA384
* TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256
* TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384
* TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256
* TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384
* TLS_DH_anon_WITH_ARIA_128_GCM_SHA256
* TLS_DH_anon_WITH_ARIA_256_GCM_SHA384
* TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256
* TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384
* TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256
* TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384
* TLS_PSK_WITH_ARIA_128_CBC_SHA256
* TLS_PSK_WITH_ARIA_256_CBC_SHA384
* TLS_DHE_PSK_WITH_ARIA_128_CBC_SHA256
* TLS_DHE_PSK_WITH_ARIA_256_CBC_SHA384
* TLS_RSA_PSK_WITH_ARIA_128_CBC_SHA256
* TLS_RSA_PSK_WITH_ARIA_256_CBC_SHA384
* TLS_PSK_WITH_ARIA_128_GCM_SHA256
* TLS_PSK_WITH_ARIA_256_GCM_SHA384
* TLS_RSA_PSK_WITH_ARIA_128_GCM_SHA256
* TLS_RSA_PSK_WITH_ARIA_256_GCM_SHA384
* TLS_ECDHE_PSK_WITH_ARIA_128_CBC_SHA256
* TLS_ECDHE_PSK_WITH_ARIA_256_CBC_SHA384
* TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384
* TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384
* TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384
* TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256
* TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384
* TLS_RSA_WITH_CAMELLIA_128_GCM_SHA256
* TLS_RSA_WITH_CAMELLIA_256_GCM_SHA384
* TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256
* TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384
* TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256
* TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384
* TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256
* TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384
* TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256
* TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384
* TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256
* TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384
* TLS_PSK_WITH_CAMELLIA_128_GCM_SHA256
* TLS_PSK_WITH_CAMELLIA_256_GCM_SHA384
* TLS_RSA_PSK_WITH_CAMELLIA_128_GCM_SHA256
* TLS_RSA_PSK_WITH_CAMELLIA_256_GCM_SHA384
* TLS_PSK_WITH_CAMELLIA_128_CBC_SHA256
* TLS_PSK_WITH_CAMELLIA_256_CBC_SHA384
* TLS_DHE_PSK_WITH_CAMELLIA_128_CBC_SHA256
* TLS_DHE_PSK_WITH_CAMELLIA_256_CBC_SHA384
* TLS_RSA_PSK_WITH_CAMELLIA_128_CBC_SHA256
* TLS_RSA_PSK_WITH_CAMELLIA_256_CBC_SHA384
* TLS_ECDHE_PSK_WITH_CAMELLIA_128_CBC_SHA256
* TLS_ECDHE_PSK_WITH_CAMELLIA_256_CBC_SHA384
* TLS_RSA_WITH_AES_128_CCM
* TLS_RSA_WITH_AES_256_CCM
* TLS_RSA_WITH_AES_128_CCM_8
* TLS_RSA_WITH_AES_256_CCM_8
* TLS_PSK_WITH_AES_128_CCM
* TLS_PSK_WITH_AES_256_CCM
* TLS_PSK_WITH_AES_128_CCM_8
* TLS_PSK_WITH_AES_256_CCM_8

| Note: This list was assembled from the set of registered TLS
| cipher suites when [RFC7540] was developed. This list includes
| those cipher suites that do not offer an ephemeral key exchange
| and those that are based on the TLS null, stream, or block
| cipher type (as defined in Section 6.2.3 of [TLS12]).
| Additional cipher suites with these properties could be
| defined; these would not be explicitly prohibited.

For more details, see Section 9.2.2.

Appendix B. Changes from RFC 7540

This revision includes the following substantive changes:

* Use of TLS 1.3 was defined based on [RFC8740], which this document
obsoletes.

* The priority scheme defined in RFC 7540 is deprecated.
Definitions for the format of the PRIORITY frame and the priority
fields in the HEADERS frame have been retained, plus the rules
governing when PRIORITY frames can be sent and received, but the
semantics of these fields are only described in RFC 7540. The
priority signaling scheme from RFC 7540 was not successful. Using
the simpler successor signaling in [HTTP-PRIORITY] is recommended.

* The HTTP/1.1 Upgrade mechanism is deprecated and no longer
specified in this document. It was never widely deployed, with
plaintext HTTP/2 users choosing to use the prior-knowledge
implementation instead.

* Validation for field names and values has been narrowed. The
validation that is mandatory for intermediaries is precisely
defined, and error reporting for requests has been amended to
encourage sending 400-series status codes.

* The ranges of codepoints for settings and frame types that were
reserved for Experimental Use are now available for general use.

* Connection-specific header fields -- which are prohibited -- are
more precisely and comprehensively identified.

* Host and ":authority" are no longer permitted to disagree.

* Rules for sending Dynamic Table Size Update instructions after
changes in settings have been clarified in Section 4.3.1.

Editorial changes are also included. In particular, changes to
terminology and document structure are in response to updates to core
HTTP semantics [HTTP]. Those documents now include some concepts
that were first defined in RFC 7540, such as the 421 status code or
connection coalescing.

Acknowledgments

Credit for non-trivial input to this document is owed to a large
number of people who have contributed to the HTTP Working Group over
the years. [RFC7540] contains a more extensive list of people that
deserve acknowledgment for their contributions.

Contributors

Mike Belshe and Roberto Peon authored the text that this document is
based on.

Authors' Addresses

Martin Thomson (editor)
Mozilla
Australia
Email: mt@lowentropy.net

Cory Benfield (editor)
Apple Inc.
Email: cbenfield@apple.com