RFC 9113 - HTTP/2
RFC 9113
HTTP/2
June 2022
Thomson & Benfield
Standards Track
[Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
9113
Obsoletes:
7540
8740
Category:
Standards Track
Published:
June 2022
ISSN:
2070-1721
Authors:
M. Thomson,
Ed.
Mozilla
C. Benfield,
Ed.
Apple Inc.
RFC 9113
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
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) 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
The performance of applications using the Hypertext Transfer Protocol
(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
). 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 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 (0x00) when sending.
Reserved:
A reserved 1-bit field. The semantics of this bit are undefined, and the bit
MUST
remain unset (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 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
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
), 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
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
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
HEADERS
frame (with implied
CONTINUATION
frames)
ES
END_STREAM flag
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.
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
frame also causes the promised
stream to become "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 (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
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 (local)" or "reserved (remote)". When
a stream transitions out of the "idle" state, all streams in the "idle" state that might have been opened by the peer with a 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:
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.
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.
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.
The initial value for the flow-control window is 65,535 octets for both new streams
and the overall connection.
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.
An endpoint can choose to disable its own flow control, but an endpoint cannot ignore
flow-control signals from its peer.
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 product (see
RFC7323
).
Even with full awareness of the current 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 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.
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
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=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) = 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 (0x08):
When set, the PADDED flag indicates that the Pad Length field and any padding that it describes
are present.
END_STREAM (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 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=0x01) is used to
open a stream
Section 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) = 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:
field block fragment
Section 4.3
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 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 (0x08):
When set, the PADDED flag indicates that the Pad Length field and any padding that it
describes are present.
END_HEADERS (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 (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
CONTINUATION frame
Section 6.10
HEADERS frames
MUST
be associated with a stream. If a HEADERS frame is received whose
Stream Identifier field is 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
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.
6.3.
PRIORITY
The PRIORITY frame (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) = 0x05,
Type (8) = 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 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=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) = 0x04,
Type (8) = 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 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=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 (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 (0x00). If an endpoint receives a SETTINGS
frame whose Stream Identifier field is anything other than 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) = 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 (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 (0x02):
This setting can be used to enable or disable server push. A server
MUST NOT
send a
PUSH_PROMISE
frame if it receives
this parameter set to a value of 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, 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 (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 (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 (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 (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=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) = 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.
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:
field block fragment
Section 4.3
containing the request control
data and a header section.
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 PUSH_PROMISE frame defines the following flags:
PADDED (0x08):
When set, the PADDED flag indicates that the Pad Length field and any padding that it
describes are present.
END_HEADERS (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 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 (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.
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.
6.7.
PING
The PING frame (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) = 0x08,
Type (8) = 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 (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 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=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) = 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
GOAWAY
frame with a stream identifier other than 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
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=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
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) = 0x04,
Type (8) = 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
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" 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.
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=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) = 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 (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
with a Stream Identifier field 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
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 (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 (0x01):
The endpoint detected an unspecific protocol error. This error is for use when a more
specific error code is not available.
INTERNAL_ERROR (0x02):
The endpoint encountered an unexpected internal error.
FLOW_CONTROL_ERROR (0x03):
The endpoint detected that its peer violated the flow-control protocol.
SETTINGS_TIMEOUT (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 (0x05):
The endpoint received a frame after a stream was half-closed.
FRAME_SIZE_ERROR (0x06):
The endpoint received a frame with an invalid size.
REFUSED_STREAM (0x07):
The endpoint refused the stream prior to performing any application processing (see
Section 8.7
for details).
CANCEL (0x08):
The endpoint uses this error code to indicate that the stream is no longer needed.
COMPRESSION_ERROR (0x09):
The endpoint is unable to maintain the field section compression context for the
connection.
CONNECT_ERROR (0x0a):
The connection established in response to a
CONNECT
request
Section 8.5
was reset or abnormally closed.
ENHANCE_YOUR_CALM (0x0b):
The endpoint detected that its peer is exhibiting a behavior that might be generating
excessive load.
INADEQUATE_SECURITY (0x0c):
The underlying transport has properties that do not meet minimum security
requirements (see
Section 9.2
).
HTTP_1_1_REQUIRED (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:
one
HEADERS
frame (followed by zero or
more
CONTINUATION
frames) containing
the header section (see
Section 6.3
of [
HTTP
),
zero or more
DATA
frames containing the
message content (see
Section 6.4
of [
HTTP
), and
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, 0x00), line feed (ASCII LF,
0x0a), or carriage return (ASCII CR, 0x0d) at any position.
A field value
MUST NOT
start or end with an ASCII whitespace character (ASCII SP or
HTAB, 0x20 or 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
uses a semicolon (";") to delimit
cookie-pairs (or "crumbs"). This header field contains multiple values, but does not use
a COMMA (",") as a separator, 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 an entity 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 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 [
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 [
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 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 <
>.
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, 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 HTTP 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
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
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. Both field section compression and flow control depend on a
commitment of 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 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 -- 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 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 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 [
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 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, 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 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
[CACHING]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"HTTP Caching"
STD 98
RFC 9111
DOI 10.17487/RFC9111
June 2022
[COMPRESSION]
Peon, R.
and
H. Ruellan
"HPACK: Header Compression for HTTP/2"
RFC 7541
DOI 10.17487/RFC7541
May 2015
[COOKIE]
Barth, A.
"HTTP State Management Mechanism"
RFC 6265
DOI 10.17487/RFC6265
April 2011
[HTTP]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"HTTP Semantics"
STD 97
RFC 9110
DOI 10.17487/RFC9110
June 2022
[QUIC]
Iyengar, J., Ed.
and
M. Thomson, Ed.
"QUIC: A UDP-Based Multiplexed and Secure Transport"
RFC 9000
DOI 10.17487/RFC9000
May 2021
[RFC2119]
Bradner, S.
"Key words for use in RFCs to Indicate Requirement Levels"
BCP 14
RFC 2119
DOI 10.17487/RFC2119
March 1997
[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
[RFC8174]
Leiba, B.
"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"
BCP 14
RFC 8174
DOI 10.17487/RFC8174
May 2017
[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
[RFC8470]
Thomson, M.
Nottingham, M.
, and
W. Tarreau
"Using Early Data in HTTP"
RFC 8470
DOI 10.17487/RFC8470
September 2018
[TCP]
Postel, J.
"Transmission Control Protocol"
STD 7
RFC 793
DOI 10.17487/RFC0793
September 1981
[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
[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
[TLS-EXT]
Eastlake 3rd, D.
"Transport Layer Security (TLS) Extensions: Extension Definitions"
RFC 6066
DOI 10.17487/RFC6066
January 2011
[TLS12]
Dierks, T.
and
E. Rescorla
"The Transport Layer Security (TLS) Protocol Version 1.2"
RFC 5246
DOI 10.17487/RFC5246
August 2008
[TLS13]
Rescorla, E.
"The Transport Layer Security (TLS) Protocol Version 1.3"
RFC 8446
DOI 10.17487/RFC8446
August 2018
[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
12.2.
Informative References
[ALT-SVC]
Nottingham, M.
McManus, P.
, and
J. Reschke
"HTTP Alternative Services"
RFC 7838
DOI 10.17487/RFC7838
April 2016
[BREACH]
Gluck, Y.
Harris, N.
, and
A. Prado
"BREACH: Reviving the CRIME Attack"
12 July 2013
[DNS-TERMS]
Hoffman, P.
Sullivan, A.
, and
K. Fujiwara
"DNS Terminology"
BCP 219
RFC 8499
DOI 10.17487/RFC8499
January 2019
[HTTP-PRIORITY]
Oku, K.
and
L. Pardue
"Extensible Prioritization Scheme for HTTP"
RFC 9218
DOI 10.17487/RFC9218
June 2022
[HTTP/1.1]
Fielding, R., Ed.
Nottingham, M., Ed.
, and
J. Reschke, Ed.
"HTTP/1.1"
STD 99
RFC 9112
DOI 10.17487/RFC9112
June 2022
[NFLX-2019-002]
Netflix
"HTTP/2 Denial of Service Advisory"
13 August 2019
[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
[RFC1122]
Braden, R., Ed.
"Requirements for Internet Hosts - Communication Layers"
STD 3
RFC 1122
DOI 10.17487/RFC1122
October 1989
[RFC3749]
Hollenbeck, S.
"Transport Layer Security Protocol Compression Methods"
RFC 3749
DOI 10.17487/RFC3749
May 2004
[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
[RFC6585]
Nottingham, M.
and
R. Fielding
"Additional HTTP Status Codes"
RFC 6585
DOI 10.17487/RFC6585
April 2012
[RFC7323]
Borman, D.
Braden, B.
Jacobson, V.
, and
R. Scheffenegger, Ed.
"TCP Extensions for High Performance"
RFC 7323
DOI 10.17487/RFC7323
September 2014
[RFC7540]
Belshe, M.
Peon, R.
, and
M. Thomson, Ed.
"Hypertext Transfer Protocol Version 2 (HTTP/2)"
RFC 7540
DOI 10.17487/RFC7540
May 2015
[RFC8441]
McManus, P.
"Bootstrapping WebSockets with HTTP/2"
RFC 8441
DOI 10.17487/RFC8441
September 2018
[RFC8740]
Benjamin, D.
"Using TLS 1.3 with HTTP/2"
RFC 8740
DOI 10.17487/RFC8740
February 2020
[TALKING]
Huang, L.
Chen, E.
Barth, A.
Rescorla, E.
, and
C. Jackson
"Talking to Yourself for Fun and Profit"
2011
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 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
Datatracker
RFC 9113
RFC
- Proposed Standard
Document
Document type
RFC
- Proposed Standard
June 2022
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RFC 7540
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Cory Benfield
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