HTTPbis Working GroupM. Belshe
Internet-DraftTwist
Intended status: Standards TrackR. Peon
Expires: August 21, 2015Google, Inc
M. Thomson, Editor
Mozilla
February 17, 2015

Hypertext Transfer Protocol version 2

draft-ietf-httpbis-http2-17

Abstract

This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP). 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.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at <https://lists.w3.org/Archives/Public/ietf-http-wg/>.

Working Group information can be found at <https://tools.ietf.org/wg/httpbis/>; that specific to HTTP/2 are at <https://http2.github.io/>.

The changes in this draft are summarized in Appendix B.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

This Internet-Draft will expire on August 21, 2015.

Copyright Notice

Copyright © 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.


1. Introduction

The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, how 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.

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 leads 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.

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 framing (Section 4) and streams (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 framing layer. The framing and streams 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.
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 serve HTTP responses.
stream:
A bi-directional 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.

3.1 HTTP/2 Version Identification

The protocol defined in this document has two identifiers.

  • The string "h2" identifies the protocol where HTTP/2 uses TLS [TLS12]. This identifier is used in the TLS application layer protocol negotiation extension (ALPN) [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 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.

[rfc.comment.1: RFC Editor's Note: please remove the remainder of this section prior to the publication of a final version of this document.]

Only implementations of the final, published RFC can identify themselves as "h2" or "h2c". Until such an RFC exists, implementations MUST NOT identify themselves using these strings.

Implementations of draft versions of the protocol MUST add the string "-" and the corresponding draft number to the identifier. For example, draft-ietf-httpbis-http2-11 over TLS is identified using the string "h2-11".

Non-compatible experiments that are based on these draft versions MUST append the string "-" and an experiment name to the identifier. For example, an experimental implementation of packet mood-based encoding based on draft-ietf-httpbis-http2-09 might identify itself as "h2-09-emo". Note that any label MUST conform to the "token" syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are encouraged to coordinate their experiments on the ietf-http-wg@w3.org mailing list.

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 an 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.

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 SETTINGS frame (Section 6.5) as the server connection preface (Section 3.5). 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

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 HTTP2-Settings 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 acknowledgment. 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 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 are:

0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

(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.

  • 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 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 214 (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. Implementations MUST ignore and discard any frame that has a type that is unknown.

Flags:

An 8-bit field reserved for frame-type specific boolean flags.

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 214 (16,384) and 224-1 (16,777,215) octets, inclusive.

All implementations MUST be capable of receiving and minimally processing frames up to 214 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 a FRAME_SIZE_ERROR error if a frame exceeds the size defined in SETTINGS_MAX_FRAME_SIZE, any limit defined for the frame type, or it 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.

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. They are used within HTTP request and response messages as well as 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, then decompresses the block to reconstruct the header list.

A complete header block consists of either:

Header compression is stateful. One compression context and one decompression context is 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 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, bi-directional sequence of frames exchanged between the client and server within an HTTP/2 connection. Streams have several important characteristics:

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. 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).

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 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. A receiver can ignore WINDOW_UPDATE frames in this state, 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 for a stream that is in this state, other than WINDOW_UPDATE, PRIORITY or RST_STREAM, 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.

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.

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 Section 8.2.1 and Section 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 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.

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 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 their 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).

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.

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 (231-1), and by maintaining 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. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion. Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read and acted upon.

5.3 Stream priority

A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. At any other time, the PRIORITY frame (Section 6.3) can be used to change the priority of a stream.

The purpose of prioritization is to allow an endpoint to express how it would prefer its peer allocate resources when managing concurrent streams. Most importantly, priority can be used to select streams for transmitting frames when there is limited capacity for sending.

Streams can be prioritized by marking them as dependent on the completion of other streams (Section 5.3.1). Each dependency is assigned a relative weight, a number that is used to determine the relative proportion of available resources that are assigned to streams dependent on the same stream.

Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee any particular processing or transmission order for the stream relative to any other stream. An endpoint cannot force a peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only ever a suggestion.

Prioritization information can be omitted from messages. Defaults are used prior to any explicit values being provided (Section 5.3.5).

5.3.1 Stream Dependencies

Each stream can be given an explicit dependency on another stream. Including a dependency expresses a preference to allocate resources to the identified stream rather than to the dependent stream.

A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, the non-existent stream 0 forms the root of the tree.

A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependent is a parent stream. A dependency on a stream that is not currently in the tree - such as a stream in the "idle" state - results in that stream being given a default priority (Section 5.3.5).

When assigning a dependency on another stream, the stream is added as a new dependency of the parent stream. Dependent streams that share the same parent are not ordered with respect to each other. For example, if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, this results in a dependency order of A followed by B, C, and D in any order.

    A                 A
   / \      ==>      /|\
  B   C             B D C

Figure 3: Example of Default Dependency Creation

An exclusive flag allows for the insertion of a new level of dependencies. The exclusive flag causes the stream to become the sole dependency of its parent stream, causing other dependencies to become dependent on the exclusive stream. In the previous example, if stream D is created with an exclusive dependency on stream A, this results in D becoming the dependency parent of B and C.

                      A
    A                 |
   / \      ==>       D
  B   C              / \
                    B   C

Figure 4: Example of Exclusive Dependency Creation

Inside the dependency tree, a dependent stream SHOULD only be allocated resources if all of the streams that it depends on (the chain of parent streams up to 0x0) are either closed, or it is not possible to make progress on them.

A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.

5.3.2 Dependency Weighting

All dependent streams are allocated an integer weight between 1 and 256 (inclusive).

Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, if stream B depends on stream A with weight 4, and C depends on stream A with weight 12, and if no progress can be made on A, stream B ideally receives one third of the resources allocated to stream C.

5.3.3 Reprioritization

Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to become dependent on the identified parent stream.

Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency with the exclusive flag for a reprioritized stream moves all the dependencies of the new parent stream to become dependent on the reprioritized stream.

If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved to be dependent on the reprioritized stream's previous parent. The moved dependency retains its weight.

For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and F depends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationships stay the same, except for F, which becomes dependent on A if the reprioritization is exclusive.

    ?                ?                ?                 ?
    |               / \               |                 |
    A              D   A              D                 D
   / \            /   / \            / \                |
  B   C     ==>  F   B   C   ==>    F   A       OR      A
     / \                 |             / \             /|\
    D   E                E            B   C           B C F
    |                                     |             |
    F                                     E             E
               (intermediate)   (non-exclusive)    (exclusive)

Figure 5: Example of Dependency Reordering

5.3.4 Prioritization State Management

When a stream is removed from the dependency tree, its dependencies can be moved to become dependent on the parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight of the dependency of the closed stream proportionally based on the weights of its dependencies.

Streams that are removed from the dependency tree cause some prioritization information to be lost. Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream. However, if the common dependency is removed from the tree, those streams share resources with streams at the next highest level.

For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streams C and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion of resources. For equal starting weights, C receives one third, rather than one half, of available resources.

It is possible for a stream to become closed while prioritization information that creates a dependency on that stream is in transit. If a stream identified in a dependency has no associated priority information, then the dependent stream is instead assigned a default priority (Section 5.3.5). This potentially creates suboptimal prioritization, since the stream could be given a priority that is different to what is intended.

To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streams become closed. The longer state is retained, the lower the chance that streams are assigned incorrect or default priority values.

Similarly, streams that are in the "idle" state can be assigned priority or become a parent of other streams. This allows for the creation of a grouping node in the dependency tree, which enables more flexible expressions of priority. Idle streams begin with a default priority (Section 5.3.5).

The retention of priority information for streams that are not counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could create a large state burden for an endpoint. Therefore the amount of prioritization state that is retained MAY be limited.

The amount of additional state an endpoint maintains for prioritization could be dependent on load; under high load, prioritization state can be discarded to limit resource commitments. In extreme cases, an endpoint could even discard prioritization state for active or reserved streams. If a limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS. Implementations SHOULD also attempt to retain state for streams that are in active use in the priority tree.

An endpoint receiving a PRIORITY frame that changes the priority of a closed stream SHOULD alter the dependencies of the streams that depend on it, if it has retained enough state to do so.

5.3.5 Default Priorities

All streams are initially assigned a non-exclusive dependency on stream 0x0. Pushed streams (Section 8.2) initially depend on their associated stream. In both cases, streams are assigned a default weight of 16.

5.4 Error Handling

HTTP/2 framing permits two classes of error:

  • 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.

5.4.1 Connection Error Handling

A connection error is any error which prevents further processing of the framing layer, or which 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 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 (see [RFC7230], Section 6.6). In the event of a connection error, GOAWAY only provides a best effort attempt to communicate with the peer about why the connection is being terminated.

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

5.4.2 Stream Error Handling

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

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

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

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

5.4.3 Connection Termination

If the TCP connection is closed or reset while streams remain in open or half closed states, then the affected streams cannot be automatically retried (see Section 8.1.4 for details).

5.5 Extending HTTP/2

HTTP/2 permits extension of the protocol. Protocol extensions can be used to provide additional services or alter any aspect of the protocol, within the limitations described in this section. 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 header fields.

Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), or new error codes (Section 7). Registries are established for managing these extension points: frame types (Section 11.2), settings (Section 11.3) and error codes (Section 11.4).

Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation. However, extension frames that appear in the middle of a header block (Section 4.3) are not permitted; these MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

Extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. Note that treating any frame other than DATA frames as flow controlled is such a change in semantics, and 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 either in the establishment and management of the connection as a whole, or of 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 any given frame.

6.1 DATA

DATA frames (type=0x0) 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 payloads.

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.

 +---------------+
 |Pad Length? (8)|
 +---------------+-----------------------------------------------+
 |                            Data (*)                         ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

Figure 6: DATA Frame Payload

The DATA frame contains the following 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:

END_STREAM (0x1):
Bit 0 being set 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).
PADDED (0x8):
Bit 3 being set indicates that the Pad Length field and any padding that it describes is present.

DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, 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)" states. The entire DATA frame payload is included in flow control, including Pad Length and Padding fields if present. If a DATA frame is received whose stream is not in "open" or "half closed (local)" state, the recipient MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.

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

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

6.2 HEADERS

The HEADERS frame (type=0x1) is used to open a stream (Section 5.1), and additionally carries a header block fragment. HEADERS frames can be sent on a stream in the "open" or "half closed (remote)" states.

 +---------------+
 |Pad Length? (8)|
 +-+-------------+-----------------------------------------------+
 |E|                 Stream Dependency? (31)                     |
 +-+-------------+-----------------------------------------------+
 |  Weight? (8)  |
 +-+-------------+-----------------------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

Figure 7: HEADERS Frame Payload

The HEADERS frame payload has the following 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.
E:
A single bit flag indicates that the stream dependency is exclusive, see Section 5.3. This field is only present if the PRIORITY flag is set.
Stream Dependency:
A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3. This field is only present if the PRIORITY flag is set.
Weight:
An unsigned 8-bit integer representing a priority weight for the stream, see Section 5.3. Add one to the value to obtain a weight between 1 and 256. This field is only present if the PRIORITY flag is set.
Header Block Fragment:
A header block fragment (Section 4.3).
Padding:
Padding octets.

The HEADERS frame defines the following flags:

END_STREAM (0x1):

Bit 0 being set indicates that the header block (Section 4.3) is the last that the endpoint will send for the identified stream.

A HEADERS frame carries the END_STREAM flag that 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.

END_HEADERS (0x4):

Bit 2 being set indicates that this frame contains an entire header 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.

PADDED (0x8):

Bit 3 being set indicates that the Pad Length field and any padding that it describes is present.

PRIORITY (0x20):

Bit 5 being set indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see Section 5.3.

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

HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose stream identifier field is 0x0, 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 HEADERS frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).

Prioritization information in a HEADERS frame is logically equivalent to a separate PRIORITY frame, but inclusion in HEADERS avoids the potential for churn in stream prioritization when new streams are created. Prioritization fields in HEADERS frames subsequent to the first on a stream reprioritize the stream (Section 5.3.3).

6.3 PRIORITY

The PRIORITY frame (type=0x2) specifies the sender-advised priority of a stream (Section 5.3). It can be sent at any time for any stream, including idle or closed streams.

 +-+-------------------------------------------------------------+
 |E|                  Stream Dependency (31)                     |
 +-+-------------+-----------------------------------------------+
 |   Weight (8)  |
 +-+-------------+

Figure 8: PRIORITY Frame Payload

The payload of a PRIORITY frame contains the following fields:

E:
A single bit flag indicates that the stream dependency is exclusive, see Section 5.3.
Stream Dependency:
A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3.
Weight:
An unsigned 8-bit integer representing a priority weight for the stream, see Section 5.3. Add one to the value to obtain a weight between 1 and 256.

The PRIORITY frame does not define any flags.

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

The PRIORITY frame can be sent on a stream in any state, though it cannot be sent between consecutive frames that comprise a single header block (Section 4.3). Note that this frame could arrive after processing or frame sending has completed, which would cause it to have no effect on the identified stream. For a stream that is in the "half closed (remote)" or "closed" - state, this frame can only affect processing of the identified stream and its dependent streams and not frame transmission on that stream.

The PRIORITY frame can be sent for a stream in the "idle" or "closed" states. This allows for the reprioritization of a group of dependent streams by altering the priority of an unused or closed parent stream.

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

6.4 RST_STREAM

The RST_STREAM frame (type=0x3) 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.

 +---------------------------------------------------------------+
 |                        Error Code (32)                        |
 +---------------------------------------------------------------+

Figure 9: RST_STREAM Frame Payload

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, with the exception of PRIORITY. However, after sending the RST_STREAM, the sending endpoint MUST be prepared to receive and process additional frames sent on the stream that might have been sent by the peer prior to the arrival of the RST_STREAM.

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

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

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

6.5 SETTINGS

The SETTINGS frame (type=0x4) 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 parameters. Individually, a SETTINGS parameter can also be referred to as a "setting".

SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which are used by the receiving peer. Different values for the same parameter 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 parameters defined by this specification.

Each parameter in a SETTINGS frame replaces any existing value for that parameter. Parameters 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 its parameters. Therefore, the value of a SETTINGS parameter is the last value that is seen by a receiver.

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

ACK (0x1):
Bit 0 being set indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the 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 info, see Settings Synchronization (Section 6.5.3).

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, 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 payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and an unsigned 32-bit value.

 +-------------------------------+
 |       Identifier (16)         |
 +-------------------------------+-------------------------------+
 |                        Value (32)                             |
 +---------------------------------------------------------------+

Figure 10: Setting Format

6.5.2 Defined SETTINGS Parameters

The following parameters are defined:

SETTINGS_HEADER_TABLE_SIZE (0x1):

Allows the sender to inform the remote endpoint of the maximum size of the header compression table used to decode header blocks, in octets. The encoder can select any size equal to or less than this value by using signaling specific to the header compression format inside a header block, see [COMPRESSION]. The initial value is 4,096 octets.

SETTINGS_ENABLE_PUSH (0x2):

This setting can be use to disable server push (Section 8.2). An endpoint MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0. An endpoint 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 is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

SETTINGS_MAX_CONCURRENT_STREAMS (0x3):

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

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

SETTINGS_INITIAL_WINDOW_SIZE (0x4):

Indicates the sender's initial window size (in octets) for stream level flow control. The initial value is 216-1 (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 231-1 MUST be treated as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

SETTINGS_MAX_FRAME_SIZE (0x5):

Indicates the size of the largest frame payload that the sender is willing to receive, in octets.

The initial value is 214 (16,384) octets. The value advertised by an endpoint MUST be between this initial value and the maximum allowed frame size (224-1 or 16,777,215 octets), inclusive. Values outside this range MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

SETTINGS_MAX_HEADER_LIST_SIZE (0x6):

This advisory setting informs a peer of the maximum size of header list that the sender is prepared to accept, in octets. The value is based on the uncompressed size of header fields, including the length of the name and value in octets plus an overhead of 32 octets for each header field.

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 parameters as soon as possible upon receipt.

The values in the SETTINGS frame MUST be processed in the order they appear, with no other frame processing between values. Unsupported parameters 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 parameters can rely on the setting having been applied.

If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error (Section 5.4.1) of type SETTINGS_TIMEOUT.

6.6 PUSH_PROMISE

The PUSH_PROMISE frame (type=0x5) 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 set of headers that provide additional context for the stream. Section 8.2 contains a thorough description of the use of PUSH_PROMISE frames.

 +---------------+
 |Pad Length? (8)|
 +-+-------------+-----------------------------------------------+
 |R|                  Promised Stream ID (31)                    |
 +-+-----------------------------+-------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

Figure 11: PUSH_PROMISE Payload Format

The PUSH_PROMISE frame payload has the following 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.
R:
A single reserved bit.
Promised Stream ID:
An unsigned 31-bit integer that identifies the stream that is reserved by the PUSH_PROMISE. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier (Section 5.1.1)).
Header Block Fragment:
A header block fragment (Section 4.3) containing request header fields.
Padding:
Padding octets.

The PUSH_PROMISE frame defines the following flags:

END_HEADERS (0x4):

Bit 2 being set indicates that this frame contains an entire header 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.

PADDED (0x8):

Bit 3 being set indicates that the Pad Length field and any padding that it describes is present.

PUSH_PROMISE frames MUST be associated with a peer-initiated stream that is in either the "open" or "half closed (remote)" state. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, 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 acknowledgement 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. The inclusion of a header block (Section 4.3) potentially modifies the state maintained for header compression. PUSH_PROMISE also reserves a stream for later use, causing the promised stream to enter the "reserved" 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) (that is, an identifier for a stream that is not currently in the "idle" state) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The PUSH_PROMISE frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).

6.7 PING

The PING frame (type=0x6) 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.

 +---------------------------------------------------------------+
 |                                                               |
 |                      Opaque Data (64)                         |
 |                                                               |
 +---------------------------------------------------------------+

Figure 12: PING Payload Format

In addition to the frame header, PING frames MUST contain 8 octets of data in the 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 payload. PING responses SHOULD be given higher priority than any other frame.

The PING frame defines the following flags:

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

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

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

6.8 GOAWAY

The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this connection. GOAWAY can be sent by either the client or the server. Once sent, the sender will ignore frames sent on any new streams with identifiers 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.

The purpose of this frame is to allow 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 sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last peer-initiated stream which 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.

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 GOAWAY for misbehaving peers.

 +-+-------------------------------------------------------------+
 |R|                  Last-Stream-ID (31)                        |
 +-+-------------------------------------------------------------+
 |                      Error Code (32)                          |
 +---------------------------------------------------------------+
 |                  Additional Debug Data (*)                    |
 +---------------------------------------------------------------+

Figure 13: GOAWAY Payload Format

The GOAWAY frame does not define any flags.

The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a GOAWAY frame with a stream identifier other than 0x0 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, re-attempting requests, transactions, or any protocol activity is not possible, with the exception of 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 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 231-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that no further requests can be initiated. After waiting at least one round trip time, the server can 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 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 the state maintained for header 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 header 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 payload of any GOAWAY frame. Additional debug data is intended for diagnostic purposes only and carries no semantic value. Debug information could contain security- or privacy-sensitive data. Logged or otherwise persistently stored debug data MUST have adequate safeguards to prevent unauthorized access.

6.9 WINDOW_UPDATE

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

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

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

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

 +-+-------------------------------------------------------------+
 |R|              Window Size Increment (31)                     |
 +-+-------------------------------------------------------------+

Figure 14: WINDOW_UPDATE Payload Format

The 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 231-1 (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 an 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 bearing the END_STREAM flag. This means that a receiver could receive a WINDOW_UPDATE frame on a "half closed (remote)" or "closed" stream. 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. Since the sender counts the frame toward the flow control window, if the receiver does not, the flow control window at 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.

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 231-1 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 the error code of FLOW_CONTROL_ERROR code; for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.

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 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 that forms part of the connection preface. 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 to the default initial window size until a WINDOW_UPDATE frame is received.

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

A SETTINGS frame cannot alter the connection flow control window.

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

6.9.3 Reducing the Stream Window Size

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

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

6.10 CONTINUATION

The CONTINUATION frame (type=0x9) is used to continue a sequence of header 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.

 +---------------------------------------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+

Figure 15: CONTINUATION Frame Payload

The CONTINUATION frame payload contains a header block fragment (Section 4.3).

The CONTINUATION frame defines the following flag:

END_HEADERS (0x4):

Bit 2 being set indicates that this frame ends a header block (Section 4.3).

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

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

CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, 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 (0x0):
The associated condition is not as a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection.
PROTOCOL_ERROR (0x1):
The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available.
INTERNAL_ERROR (0x2):
The endpoint encountered an unexpected internal error.
FLOW_CONTROL_ERROR (0x3):
The endpoint detected that its peer violated the flow control protocol.
SETTINGS_TIMEOUT (0x4):
The endpoint sent a SETTINGS frame, but did not receive a response in a timely manner. See Settings Synchronization (Section 6.5.3).
STREAM_CLOSED (0x5):
The endpoint received a frame after a stream was half closed.
FRAME_SIZE_ERROR (0x6):
The endpoint received a frame with an invalid size.
REFUSED_STREAM (0x7):
The endpoint refuses the stream prior to performing any application processing, see Section 8.1.4 for details.
CANCEL (0x8):
Used by the endpoint to indicate that the stream is no longer needed.
COMPRESSION_ERROR (0x9):
The endpoint is unable to maintain the header compression context for the connection.
CONNECT_ERROR (0xa):
The connection established in response to a CONNECT request (Section 8.3) was reset or abnormally closed.
ENHANCE_YOUR_CALM (0xb):
The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load.
INADEQUATE_SECURITY (0xc):
The underlying transport has properties that do not meet minimum security requirements (see Section 9.2).
HTTP_1_1_REQUIRED (0xd):
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. HTTP Message Exchanges

HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means that, from the application perspective, the features of the protocol are largely unchanged. To achieve this, all request and response semantics are preserved, although the syntax of conveying those semantics has changed.

Thus, the specification and requirements of HTTP/1.1 Semantics and Content [RFC7231], Conditional Requests [RFC7232], Range Requests [RFC7233], Caching [RFC7234] and Authentication [RFC7235] are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are also applicable in HTTP/2, but the expression of those semantics for this protocol are defined in the sections below.

8.1 HTTP Request/Response Exchange

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

An HTTP message (request or response) consists of:

  1. for a response only, zero or more HEADERS frames (each followed by zero or more CONTINUATION frames) containing the message headers of informational (1xx) HTTP responses (see [RFC7230], Section 3.2 and [RFC7231], Section 6.2), and
  2. one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see [RFC7230], Section 3.2), and
  3. zero or more DATA frames containing the payload body (see [RFC7230], Section 3.3), and
  4. optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see [RFC7230], Section 4.1.2).

The last frame in the sequence bears an END_STREAM flag, noting that a HEADERS frame bearing the END_STREAM flag can be followed by CONTINUATION frames that carry any remaining portions of the header block.

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

HTTP/2 uses DATA frames to carry message payloads. The chunked transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be used in HTTP/2.

Trailing header fields are carried in a header block that also terminates the stream. Such a header block is a sequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where the HEADERS frame bears an END_STREAM flag. Header blocks after the first that do not terminate the stream are not part of an HTTP request or response.

A HEADERS frame (and associated CONTINUATION frames) can only appear at the start or end of a stream. An endpoint that receives a HEADERS frame without the END_STREAM flag set after receivin