Internet Engineering Task Force (IETF) M. Belshe
Request for Comments: 7540 BitGo
Category: Standards Track R. Peon
ISSN: 2070-1721 Google, Inc
M. Thomson, Ed.
Mozilla
May 2015
Hypertext Transfer Protocol Version 2 (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 perception of latency by introducing header
field compression and allowing multiple concurrent exchanges on the
same connection. It also introduces unsolicited push of
representations from servers to clients.
This specification is an alternative to, but does not obsolete, the
HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7540.
Belshe, et al. Standards Track [Page 1]
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Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
2. HTTP/2 Protocol Overview ........................................5
2.1. Document Organization ......................................6
2.2. Conventions and Terminology ................................6
3. Starting HTTP/2 .................................................7
3.1. HTTP/2 Version Identification ..............................8
3.2. Starting HTTP/2 for "http" URIs ............................8
3.2.1. HTTP2-Settings Header Field .........................9
3.3. Starting HTTP/2 for "https" URIs ..........................10
3.4. Starting HTTP/2 with Prior Knowledge ......................10
3.5. HTTP/2 Connection Preface .................................11
4. HTTP Frames ....................................................12
4.1. Frame Format ..............................................12
4.2. Frame Size ................................................13
4.3. Header Compression and Decompression ......................14
5. Streams and Multiplexing .......................................15
5.1. Stream States .............................................16
5.1.1. Stream Identifiers .................................21
5.1.2. Stream Concurrency .................................22
5.2. Flow Control ..............................................22
5.2.1. Flow-Control Principles ............................23
5.2.2. Appropriate Use of Flow Control ....................24
5.3. Stream Priority ...........................................24
5.3.1. Stream Dependencies ................................25
5.3.2. Dependency Weighting ...............................26
5.3.3. Reprioritization ...................................26
5.3.4. Prioritization State Management ....................27
5.3.5. Default Priorities .................................28
5.4. Error Handling ............................................28
5.4.1. Connection Error Handling ..........................29
5.4.2. Stream Error Handling ..............................29
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5.4.3. Connection Termination .............................30
5.5. Extending HTTP/2 ..........................................30
6. Frame Definitions ..............................................31
6.1. DATA ......................................................31
6.2. HEADERS ...................................................32
6.3. PRIORITY ..................................................34
6.4. RST_STREAM ................................................36
6.5. SETTINGS ..................................................36
6.5.1. SETTINGS Format ....................................38
6.5.2. Defined SETTINGS Parameters ........................38
6.5.3. Settings Synchronization ...........................39
6.6. PUSH_PROMISE ..............................................40
6.7. PING ......................................................42
6.8. GOAWAY ....................................................43
6.9. WINDOW_UPDATE .............................................46
6.9.1. The Flow-Control Window ............................47
6.9.2. Initial Flow-Control Window Size ...................48
6.9.3. Reducing the Stream Window Size ....................49
6.10. CONTINUATION .............................................49
7. Error Codes ....................................................50
8. HTTP Message Exchanges .........................................51
8.1. HTTP Request/Response Exchange ............................52
8.1.1. Upgrading from HTTP/2 ..............................53
8.1.2. HTTP Header Fields .................................53
8.1.3. Examples ...........................................57
8.1.4. Request Reliability Mechanisms in HTTP/2 ...........60
8.2. Server Push ...............................................60
8.2.1. Push Requests ......................................61
8.2.2. Push Responses .....................................63
8.3. The CONNECT Method ........................................64
9. Additional HTTP Requirements/Considerations ....................65
9.1. Connection Management .....................................65
9.1.1. Connection Reuse ...................................66
9.1.2. The 421 (Misdirected Request) Status Code ..........66
9.2. Use of TLS Features .......................................67
9.2.1. TLS 1.2 Features ...................................67
9.2.2. TLS 1.2 Cipher Suites ..............................68
10. Security Considerations .......................................69
10.1. Server Authority .........................................69
10.2. Cross-Protocol Attacks ...................................69
10.3. Intermediary Encapsulation Attacks .......................70
10.4. Cacheability of Pushed Responses .........................70
10.5. Denial-of-Service Considerations .........................70
10.5.1. Limits on Header Block Size .......................71
10.5.2. CONNECT Issues ....................................72
10.6. Use of Compression .......................................72
10.7. Use of Padding ...........................................73
10.8. Privacy Considerations ...................................73
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11. IANA Considerations ...........................................74
11.1. Registration of HTTP/2 Identification Strings ............74
11.2. Frame Type Registry ......................................75
11.3. Settings Registry ........................................75
11.4. Error Code Registry ......................................76
11.5. HTTP2-Settings Header Field Registration .................77
11.6. PRI Method Registration ..................................78
11.7. The 421 (Misdirected Request) HTTP Status Code ...........78
11.8. The h2c Upgrade Token ....................................78
12. References ....................................................79
12.1. Normative References .....................................79
12.2. Informative References ...................................81
Appendix A. TLS 1.2 Cipher Suite Black List .......................83
Acknowledgements ..................................................95
Authors' Addresses ................................................96
1. Introduction
The Hypertext Transfer Protocol (HTTP) is a wildly successful
protocol. However, the way HTTP/1.1 uses the underlying transport
([RFC7230], Section 6) has several characteristics that have a
negative overall effect on application performance today.
In particular, HTTP/1.0 allowed only one request to be outstanding at
a time on a given TCP connection. HTTP/1.1 added request pipelining,
but this only partially addressed request concurrency and still
suffers from head-of-line blocking. Therefore, HTTP/1.0 and HTTP/1.1
clients that need to make many requests use multiple connections to a
server in order to achieve concurrency and thereby reduce latency.
Furthermore, HTTP header fields are often repetitive and verbose,
causing unnecessary network traffic as well as causing the initial
TCP [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 request and response messages on the same
connection and uses an efficient coding for HTTP header fields. It
also allows prioritization of requests, letting more important
requests complete more quickly, further improving performance.
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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.
Finally, HTTP/2 also enables more efficient processing of messages
through use of binary message framing.
2. HTTP/2 Protocol Overview
HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2
supports all of the core features of HTTP/1.1 but aims to be more
efficient in several ways.
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.
Flow control and prioritization ensure that it is possible to
efficiently use multiplexed streams. Flow control (Section 5.2)
helps to ensure that only data that can be used by a receiver is
transmitted. Prioritization (Section 5.3) ensures that limited
resources can be directed to the most important streams first.
HTTP/2 adds a new interaction mode whereby a server can push
responses to a client (Section 8.2). Server push allows 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.
Because HTTP header 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.
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2.1. Document Organization
The HTTP/2 specification is split into four parts:
o Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
initiated.
o The frame (Section 4) and stream (Section 5) layers describe the
way HTTP/2 frames are structured and formed into multiplexed
streams.
o Frame (Section 6) and error (Section 7) definitions include
details of the frame and error types used in HTTP/2.
o HTTP mappings (Section 8) and additional requirements (Section 9)
describe how HTTP semantics are expressed using frames and
streams.
While some of the frame and stream layer concepts are isolated from
HTTP, this specification does not define a completely generic frame
layer. The frame and stream layers are tailored to the needs of the
HTTP protocol and server push.
2.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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.
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.
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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 2.3 of [RFC7230]. Intermediaries act as both
client and server at different times.
The term "payload body" is defined in Section 3.3 of [RFC7230].
3. Starting HTTP/2
An HTTP/2 connection is an application-layer protocol running on top
of a TCP connection ([TCP]). The client is the TCP connection
initiator.
HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1.
HTTP/2 shares the same default port numbers: 80 for "http" URIs and
443 for "https" URIs. As a result, implementations processing
requests for target resource URIs like "http://example.org/foo" or
"https://example.com/bar" are required to first discover whether the
upstream server (the immediate peer to which the client wishes to
establish a connection) supports HTTP/2.
The means by which support for HTTP/2 is determined is different for
"http" and "https" URIs. Discovery for "http" URIs is described in
Section 3.2. Discovery for "https" URIs is described in Section 3.3.
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3.1. HTTP/2 Version Identification
The protocol defined in this document has two identifiers.
o The string "h2" identifies the protocol where HTTP/2 uses
Transport Layer Security (TLS) [TLS12]. 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.
o The string "h2c" identifies the protocol where HTTP/2 is run over
cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade
header field and in any place where HTTP/2 over TCP is identified.
The "h2c" string is reserved from the ALPN identifier space but
describes a protocol that does not use TLS.
Negotiating "h2" or "h2c" implies the use of the transport, security,
framing, and message semantics described in this document.
3.2. Starting HTTP/2 for "http" URIs
A client that makes a request for an "http" URI without prior
knowledge about support for HTTP/2 on the next hop uses the HTTP
Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by
making an HTTP/1.1 request that includes an Upgrade header field with
the "h2c" token. Such an HTTP/1.1 request MUST include exactly one
HTTP2-Settings (Section 3.2.1) header field.
For example:
GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
Requests that contain a payload body MUST be sent in their entirety
before the client can send HTTP/2 frames. This means that a large
request can block the use of the connection until it is completely
sent.
If concurrency of an initial request with subsequent requests is
important, an OPTIONS request can be used to perform the upgrade to
HTTP/2, at the cost of an additional round trip.
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A server that does not support HTTP/2 can respond to the request as
though the Upgrade header field were absent:
HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html
...
A server MUST ignore an "h2" token in an Upgrade header field.
Presence of a token with "h2" implies HTTP/2 over TLS, which is
instead negotiated as described in Section 3.3.
A server that supports HTTP/2 accepts the upgrade with a 101
(Switching Protocols) response. After the empty line that terminates
the 101 response, the server can begin sending HTTP/2 frames. These
frames MUST include a response to the request that initiated the
upgrade.
For example:
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c
[ HTTP/2 connection ...
The first HTTP/2 frame sent by the server MUST be a server connection
preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5).
Upon receiving the 101 response, the client MUST send a connection
preface (Section 3.5), which includes a SETTINGS frame.
The HTTP/1.1 request that is sent prior to upgrade is assigned a
stream identifier of 1 (see Section 5.1.1) with default priority
values (Section 5.3.5). Stream 1 is implicitly "half-closed" from
the client toward the server (see Section 5.1), since the request is
completed as an HTTP/1.1 request. After commencing the HTTP/2
connection, stream 1 is used for the response.
3.2.1. HTTP2-Settings Header Field
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
one "HTTP2-Settings" header field. The HTTP2-Settings header field
is a connection-specific header field that includes parameters that
govern the HTTP/2 connection, provided in anticipation of the server
accepting the request to upgrade.
HTTP2-Settings = token68
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A server MUST NOT upgrade the connection to HTTP/2 if this header
field is not present or if more than one is present. A server MUST
NOT send this header field.
The content of the HTTP2-Settings header field is the payload of a
SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
the URL- and filename-safe Base64 encoding described in Section 5 of
[RFC4648], with any trailing '=' characters omitted). The ABNF
[RFC5234] production for "token68" is defined in Section 2.1 of
[RFC7235].
Since the upgrade is only intended to apply to the immediate
connection, a client sending the HTTP2-Settings header field MUST
also send "HTTP2-Settings" as a connection option in the Connection
header field to prevent it from being forwarded (see Section 6.1 of
[RFC7230]).
A server decodes and interprets these values as it would any other
SETTINGS frame. Explicit acknowledgement of these settings
(Section 6.5.3) is not necessary, since a 101 response serves as
implicit acknowledgement. Providing these values in the upgrade
request gives a client an opportunity to provide parameters prior to
receiving any frames from the server.
3.3. Starting HTTP/2 for "https" URIs
A client that makes a request to an "https" URI uses TLS [TLS12] with
the application-layer protocol negotiation (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 3.5).
3.4. Starting HTTP/2 with Prior Knowledge
A client can learn that a particular server supports HTTP/2 by other
means. For example, [ALT-SVC] describes a mechanism for advertising
this capability.
A client MUST send the connection preface (Section 3.5) and then MAY
immediately send HTTP/2 frames to such a server; servers can identify
these connections by the presence of the connection preface. This
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only affects the establishment of HTTP/2 connections over cleartext
TCP; implementations that support HTTP/2 over TLS MUST use protocol
negotiation in TLS [TLS-ALPN].
Likewise, the server MUST send a connection preface (Section 3.5).
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.
3.5. 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 immediately upon receipt of a 101
(Switching Protocols) response (indicating a successful upgrade) or
as the first application data octets of a TLS connection. If
starting an HTTP/2 connection with prior knowledge of server support
for the protocol, the client connection preface is sent upon
connection establishment.
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.
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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 parameters 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 parameters 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 payload.
+-----------------------------------------------+
| Length (24) |
+---------------+---------------+---------------+
| Type (8) | Flags (8) |
+-+-------------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+=+=============================================================+
| Frame Payload (0...) ...
+---------------------------------------------------------------+
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. 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.
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Type: The 8-bit type of the frame. The frame type determines the
format and semantics of the frame. Implementations MUST ignore
and discard any frame that has a type that is unknown.
Flags: An 8-bit field reserved for boolean flags specific to the
frame type.
Flags are assigned semantics specific to the indicated frame type.
Flags that have no defined semantics for a particular frame type
MUST be ignored and MUST be left unset (0x0) when sending.
R: A reserved 1-bit field. The semantics of this bit are undefined,
and the bit MUST remain unset (0x0) when sending and MUST be
ignored when receiving.
Stream Identifier: A stream identifier (see Section 5.1.1) expressed
as an unsigned 31-bit integer. The value 0x0 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 is dependent entirely
on the frame type.
4.2. Frame Size
The size of a frame payload is limited by the maximum size that a
receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This
setting can have any value between 2^14 (16,384) and 2^24-1
(16,777,215) octets, inclusive.
All implementations MUST be capable of receiving and minimally
processing frames up to 2^14 octets in length, plus the 9-octet frame
header (Section 4.1). The size of the frame header is not included
when describing frame sizes.
Note: Certain frame types, such as PING (Section 6.7), impose
additional limits on the amount of 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 header
block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and
CONTINUATION), SETTINGS, and any frame with a stream identifier of 0.
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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. Header Compression and Decompression
Just as in HTTP/1, a header field in HTTP/2 is a name with one or
more associated values. Header fields are used within HTTP request
and response messages as well as in server push operations (see
Section 8.2).
Header lists are collections of zero or more header fields. When
transmitted over a connection, a header list is serialized into a
header block using HTTP header compression [COMPRESSION]. The
serialized header block is then divided into one or more octet
sequences, called header block fragments, and transmitted within the
payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or
CONTINUATION (Section 6.10) frames.
The Cookie header field [COOKIE] is treated specially by the HTTP
mapping (see Section 8.1.2.5).
A receiving endpoint reassembles the header block by concatenating
its fragments and then decompresses the block to reconstruct the
header list.
A complete header block consists of either:
o a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
set, or
o a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
and one or more CONTINUATION frames, where the last CONTINUATION
frame has the END_HEADERS flag set.
Header compression is stateful. One compression context and one
decompression context are used for the entire connection. A decoding
error in a header block MUST be treated as a connection error
(Section 5.4.1) of type COMPRESSION_ERROR.
Each header block is processed as a discrete unit. Header 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
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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
header block to be logically equivalent to a single frame.
Header block fragments can only be sent as the 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 header 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 header block.
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:
o A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple
streams.
o Streams can be established and used unilaterally or shared by
either the client or server.
o Streams can be closed by either endpoint.
o The order in which frames are sent on a stream is significant.
Recipients process frames in the order they are received. In
particular, the order of HEADERS and DATA frames is semantically
significant.
o Streams are identified by an integer. Stream identifiers are
assigned to streams by the endpoint initiating the stream.
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5.1. Stream States
The lifecycle of a stream is shown in Figure 2.
+--------+
send PP | | recv PP
,--------| idle |--------.
/ | | \
v +--------+ v
+----------+ | +----------+
| | | send H / | |
,------| reserved | | recv H | reserved |------.
| | (local) | | | (remote) | |
| +----------+ v +----------+ |
| | +--------+ | |
| | recv ES | | send ES | |
| send H | ,-------| open |-------. | recv H |
| | / | | \ | |
| v v +--------+ v v |
| +----------+ | +----------+ |
| | half | | | half | |
| | closed | | send R / | closed | |
| | (remote) | | recv R | (local) | |
| +----------+ | +----------+ |
| | | | |
| | send ES / | recv ES / | |
| | send R / v send R / | |
| | recv R +--------+ recv R | |
| send R / `----------->| |<-----------' send R / |
| recv R | closed | recv R |
`----------------------->| |<----------------------'
+--------+
send: endpoint sends this frame
recv: endpoint receives this frame
H: HEADERS frame (with implied CONTINUATIONs)
PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
ES: END_STREAM flag
R: RST_STREAM frame
Figure 2: Stream States
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.
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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 or receiving a HEADERS frame 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)".
* 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)".
* 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.
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.
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 8.2).
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In this state, only the following transitions are possible:
* The endpoint can send a HEADERS frame. This causes the stream
to open in a "half-closed (remote)" state.
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.
An endpoint MUST NOT send any type of frame other than HEADERS,
RST_STREAM, or PRIORITY in this state.
A PRIORITY or WINDOW_UPDATE frame MAY be received in this state.
Receiving any type of frame other than RST_STREAM, PRIORITY, or
WINDOW_UPDATE on a stream in this state MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
reserved (remote):
A stream in the "reserved (remote)" state has been reserved by a
remote peer.
In this state, only the following transitions are possible:
* Receiving a HEADERS frame causes the stream to transition to
"half-closed (local)".
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.
An endpoint MAY send a PRIORITY frame in this state to
reprioritize the reserved stream. 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
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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 that
contains an END_STREAM flag is received 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 bearing the END_STREAM
flag is sent.
PRIORITY frames received in this state are used to reprioritize
streams that depend on the identified stream.
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 that contains an END_STREAM flag or when either peer sends a
RST_STREAM frame.
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closed:
The "closed" state is the terminal state.
An endpoint MUST NOT send frames other than PRIORITY on a closed
stream. An endpoint that receives any frame other than PRIORITY
after receiving a RST_STREAM MUST treat that as a stream error
(Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint
that receives any frames after receiving a frame with the
END_STREAM flag set MUST treat that as a connection error
(Section 5.4.1) of type STREAM_CLOSED, unless the frame is
permitted as described below.
WINDOW_UPDATE or RST_STREAM frames can be received in this state
for a short period after a DATA or HEADERS frame containing an
END_STREAM flag is sent. Until the remote peer receives and
processes RST_STREAM or the frame bearing the END_STREAM flag, it
might send frames of these types. Endpoints MUST ignore
WINDOW_UPDATE or RST_STREAM frames received in this state, though
endpoints MAY choose to treat frames that arrive a significant
time after sending END_STREAM as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
PRIORITY frames can be sent on closed streams to prioritize
streams that are dependent on the closed stream. Endpoints SHOULD
process PRIORITY frames, though they can be ignored if the stream
has been removed from the dependency tree (see Section 5.3.4).
If this state is reached as a result of sending a RST_STREAM
frame, the peer that receives the RST_STREAM might have already
sent -- or enqueued for sending -- frames on the stream that
cannot be withdrawn. An endpoint MUST ignore frames that it
receives on closed streams after it has sent a RST_STREAM frame.
An endpoint MAY choose to limit the period over which it ignores
frames and treat frames that arrive after this time as being in
error.
Flow-controlled frames (i.e., DATA) received after sending
RST_STREAM are counted toward the connection flow-control window.
Even though these frames might be ignored, because they are sent
before the sender receives the RST_STREAM, the sender will
consider the frames to count against the flow-control window.
An endpoint might receive a PUSH_PROMISE frame after it sends
RST_STREAM. PUSH_PROMISE causes a stream to become "reserved"
even if the associated stream has been reset. Therefore, a
RST_STREAM is needed to close an unwanted promised stream.
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In the absence of more specific guidance elsewhere in this document,
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. Frames of unknown types
are ignored.
An example of the state transitions for an HTTP request/response
exchange can be found in Section 8.1. An example of the state
transitions for server push can be found in Sections 8.2.1 and 8.2.2.
5.1.1. Stream Identifiers
Streams are identified with an unsigned 31-bit integer. Streams
initiated by a client MUST use odd-numbered stream identifiers; those
initiated by the server MUST use even-numbered stream identifiers. A
stream identifier of zero (0x0) is used for connection control
messages; the stream identifier of zero cannot be used to establish a
new stream.
HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
responded to with a stream identifier of one (0x1). After the
upgrade completes, stream 0x1 is "half-closed (local)" to the client.
Therefore, stream 0x1 cannot be selected as a new stream identifier
by a client that upgrades from HTTP/1.1.
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.
The first use of a new stream identifier implicitly closes all
streams in the "idle" state that might have been initiated by that
peer with a lower-valued stream identifier. For example, if a client
sends a HEADERS frame on stream 7 without ever sending a frame on
stream 5, then stream 5 transitions to the "closed" state when the
first frame for stream 7 is sent or received.
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.
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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 8.1.4) 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 for the connection as a whole.
HTTP/2 provides for flow control through use of the WINDOW_UPDATE
frame (Section 6.9).
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5.2.1. Flow-Control Principles
HTTP/2 stream flow control aims to allow a variety of flow-control
algorithms to be used without requiring protocol changes. Flow
control in HTTP/2 has the following characteristics:
1. Flow control is specific to a connection. Both types of flow
control are between the endpoints of a single hop and not over
the entire end-to-end path.
2. Flow control is based on WINDOW_UPDATE frames. Receivers
advertise how many octets they are prepared to receive on a
stream and for the entire connection. This is a credit-based
scheme.
3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it
desires for each stream and for the entire connection. A sender
MUST respect flow-control limits imposed by a receiver. Clients,
servers, and intermediaries all independently advertise their
flow-control window as a receiver and abide by the flow-control
limits set by their peer when sending.
4. The initial value for the flow-control window is 65,535 octets
for both new streams and the overall connection.
5. The frame type determines whether flow control applies to a
frame. Of the frames specified in this document, only DATA
frames are subject to flow control; all other frame types do not
consume space in the advertised flow-control window. This
ensures that important control frames are not blocked by flow
control.
6. Flow control cannot be disabled.
7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE
frame (Section 6.9). This document does not stipulate how a
receiver decides when to send this frame or the value that it
sends, nor does it specify how a sender chooses to send packets.
Implementations are able to select any algorithm that suits their
needs.
Implementations are also responsible for managing how requests and
responses are sent based on priority, 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.
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