3.1. HTTP/2 Version Identification

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

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

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

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

3.2. Starting HTTP/2 for "https" URIs

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

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

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

3.3. Starting HTTP/2 with Prior Knowledge

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

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

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

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

3.4. HTTP/2 Connection Preface

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

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

  0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

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

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

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

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

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

4.1. Frame Format

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

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

  Flags (8),

  Reserved (1),
  Stream Identifier (31),

  Frame Payload (..),
}

The fields of the frame header are defined as:¶

Length:

The length of the frame payload expressed as an unsigned 24-bit integer in units of octets. Values greater than 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. Frames defined in this document are listed in Section 6. Implementations MUST ignore and discard frames of unknown types.¶

Flags:

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

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

Reserved:

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

Stream Identifier:

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

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

4.2. Frame Size

The size of a frame payload is limited by the maximum size that a receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This setting can have any value between 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.¶

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

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

4.3. Field Section Compression and Decompression

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

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

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

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

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

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

A complete field section consists of either:¶

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

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

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

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

5.1. Stream States

The lifecycle of a stream is shown in Figure 2.¶

send:

endpoint sends this frame¶

recv:

endpoint receives this frame¶

H:

HEADERS frame (with implied CONTINUATION frames)¶

ES:

END_STREAM flag¶

R:

RST_STREAM frame¶

PP:

PUSH_PROMISE frame (with implied CONTINUATION frames); state transitions are for the promised stream¶

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

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

Streams have the following states:¶

idle:

All streams start in the "idle" state.¶

The following transitions are valid from this state:¶

• Sending a HEADERS frame as a client, or receiving a HEADERS frame as a server, causes the stream to become "open". The stream identifier is selected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediately become "half-closed".¶
• Sending a PUSH_PROMISE frame on another stream reserves the idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (local)". Only a server may send PUSH_PROMISE frames.¶
• Receiving a PUSH_PROMISE frame on another stream reserves an idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (remote)". Only a client may receive PUSH_PROMISE frames.¶
• Note that the PUSH_PROMISE frame is not sent on the idle stream but references the newly reserved stream in the Promised Stream ID field.¶
• Opening a stream with a higher-valued stream identifier causes the stream to transition immediately to a "closed" state; note that this transition is not shown in the diagram.¶

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

reserved (local):

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

In this state, only the following transitions are possible:¶

• The endpoint can send a HEADERS frame. This causes the stream to open in a "half-closed (remote)" state.¶
• Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.¶

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

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

reserved (remote):

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

In this state, only the following transitions are possible:¶

• Receiving a HEADERS frame causes the stream to transition to "half-closed (local)".¶
• Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.¶

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

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

open:

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

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

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

half-closed (local):

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

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

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

PRIORITY frames can be received in this state.¶

half-closed (remote):

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

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

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

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

closed:

The "closed" state is the terminal state.¶

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

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

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

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

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

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

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

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

5.2. Flow Control

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

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

5.3. Prioritization

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

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

5.4. Error Handling

HTTP/2 framing permits two classes of errors:¶

• An error condition that renders the entire connection unusable is a connection error.¶
• An error in an individual stream is a stream error.¶

A list of error codes is included in Section 7.¶

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

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

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

5.5. Extending HTTP/2

HTTP/2 permits extension of the protocol. Within the limitations described in this section, protocol extensions can be used to provide additional services or alter any aspect of the protocol. Extensions are effective only within the scope of a single HTTP/2 connection.¶

This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or fields (see Section 16 of [HTTP]).¶

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

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

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

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

This document doesn't mandate a specific method for negotiating the use of an extension but notes that a setting (Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUST be defined in such a fashion that the extension is initially disabled.¶

6.1. DATA

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

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

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

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

  Reserved (1),
  Stream Identifier (31),

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

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

Pad Length:

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

Data:

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

Padding:

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

The DATA frame defines the following flags:¶

PADDED (0x08):

When set, the PADDED flag indicates that the Pad Length field and any padding that it describes are present.¶

END_STREAM (0x01):

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

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

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

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

6.2. HEADERS

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

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

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

  Reserved (1),
  Stream Identifier (31),

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

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

Pad Length:

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

Exclusive:

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

Stream Dependency:

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

Weight:

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

Field Block Fragment:

A field block fragment (Section 4.3).¶

Padding:

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

The HEADERS frame defines the following flags:¶

PRIORITY (0x20):

When set, the PRIORITY flag indicates that the Exclusive, Stream Dependency, and Weight fields are present.¶

PADDED (0x08):

When set, the PADDED flag indicates that the Pad Length field and any padding that it describes are present.¶

END_HEADERS (0x04):

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

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

END_STREAM (0x01):

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

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

The frame payload of a HEADERS frame contains a field block fragment (Section 4.3). A field block that does not fit within a HEADERS frame is continued in 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 0x00, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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

6.3. PRIORITY

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

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

  Unused Flags (8),

  Reserved (1),
  Stream Identifier (31),

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

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

Exclusive:

A single-bit flag.¶

Stream Dependency:

A 31-bit stream identifier.¶

Weight:

An unsigned 8-bit integer.¶

The PRIORITY frame does not define any flags.¶

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

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

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

6.4. RST_STREAM

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

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

  Unused Flags (8),

  Reserved (1),
  Stream Identifier (31),

  Error Code (32),
}

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

The RST_STREAM frame does not define any flags.¶

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

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

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

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

6.5. SETTINGS

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

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

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

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

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

ACK (0x01):

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

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

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

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

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

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

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

  Setting (48) ...,
}

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

6.6. PUSH_PROMISE

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

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

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

  Reserved (1),
  Stream Identifier (31),

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

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

Pad Length:

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

Promised Stream ID:

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

Field Block Fragment:

A field block fragment (Section 4.3) containing the request control data and a header section.¶

Padding:

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

The PUSH_PROMISE frame defines the following flags:¶

PADDED (0x08):

When set, the PADDED flag indicates that the Pad Length field and any padding that it describes are present.¶

END_HEADERS (0x04):

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

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

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

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

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

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

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

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

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

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

6.7. PING

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

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

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

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

  Opaque Data (64),
}

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

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

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

The PING frame defines the following flags:¶

ACK (0x01):

When set, the ACK flag indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.¶

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

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

6.8. GOAWAY

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

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

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

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

Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote peer can know whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on.¶

An endpoint might choose to close a connection without sending a GOAWAY for misbehaving peers.¶

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

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

  Unused Flags (8),

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

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

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

The GOAWAY frame does not define any flags.¶

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

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

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

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

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

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

A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last stream identifier set to 2³¹-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that initiating further requests is prohibited. After allowing time for any in-flight stream creation (at least one round-trip time), the server MAY send another GOAWAY frame with an updated last stream identifier. This ensures that a connection can be cleanly shut down without losing requests.¶

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

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

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

6.9. WINDOW_UPDATE

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

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

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

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

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

  Unused Flags (8),

  Reserved (1),
  Stream Identifier (31),

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

The Length, Type, Unused Flag(s), Reserved, and Stream Identifier fields are described in Section 4. The frame payload of a WINDOW_UPDATE frame is one reserved bit plus an unsigned 31-bit integer indicating the number of octets that the sender can transmit in addition to the existing flow-control window. The legal range for the increment to the flow-control window is 1 to 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 a flow-control window increment of 0 as a stream error (Section 5.4.2) of type PROTOCOL_ERROR; errors on the connection flow-control window MUST be treated as a connection error (Section 5.4.1).¶

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

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

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

6.10. CONTINUATION

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

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

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

  Reserved (1),
  Stream Identifier (31),

  Field Block Fragment (..),
}

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

The CONTINUATION frame defines the following flag:¶

END_HEADERS (0x04):

When set, the END_HEADERS flag indicates that this frame ends a field block (Section 4.3).¶

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

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

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

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

8.1. HTTP Message Framing

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

An HTTP message (request or response) consists of:¶

1. one HEADERS frame (followed by zero or more CONTINUATION frames) containing the header section (see Section 6.3 of [HTTP]),¶
2. zero or more DATA frames containing the message content (see Section 6.4 of [HTTP]), and¶
3. optionally, one HEADERS frame (followed by zero or more CONTINUATION frames) containing the trailer section, if present (see Section 6.5 of [HTTP]).¶

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

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

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

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

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

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

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

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

8.2. HTTP Fields

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

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

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

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

8.3. HTTP Control Data

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

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

Pseudo-header fields are only valid in the context in which they are defined. Pseudo-header fields defined for requests MUST NOT appear in responses; pseudo-header fields defined for responses MUST NOT appear in requests. Pseudo-header fields MUST NOT appear in a trailer section. Endpoints MUST treat a request or response that contains undefined or invalid pseudo-header fields as malformed (Section 8.1.1).¶

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

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

8.4. Server Push

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

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

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

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

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

Pushed responses that are cacheable (see Section 3 of [CACHING]) can be stored by the client, if it implements an HTTP cache. Pushed responses are considered successfully validated on the origin server (e.g., if the "no-cache" cache response directive is present; see Section 5.2.2.4 of [CACHING]) while the stream identified by the promised stream identifier is still open.¶

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

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

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

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

8.5. The CONNECT Method

The CONNECT method (Section 9.3.6 of [HTTP]) is used to convert an HTTP connection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with "https" resources.¶

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

• The ":method" pseudo-header field is set to CONNECT.¶
• The ":scheme" and ":path" pseudo-header fields MUST be omitted.¶
• The ":authority" pseudo-header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT requests; see Section 3.2.3 of [HTTP/1.1]).¶

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

A proxy that supports CONNECT establishes a TCP connection [TCP] to the host and port identified in the ":authority" pseudo-header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx-series status code to the client, as defined in Section 9.3.6 of [HTTP].¶

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

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

A TCP connection error is signaled with RST_STREAM. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error (Section 5.4.2) of type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the HTTP/2 connection.¶

8.6. The Upgrade Header Field

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

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

8.7. Request Reliability

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

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

• The GOAWAY frame indicates the highest stream number that might have been processed. Requests on streams with higher numbers are therefore guaranteed to be safe to retry.¶
• The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the stream is being closed prior to any processing having occurred. Any request that was sent on the reset stream can be safely retried.¶

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

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

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

8.8. Examples

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

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

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

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

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

                                   DATA
                                     + END_STREAM
                                   {binary data}

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

                                   DATA
                                     + END_STREAM
                                   {binary data}

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

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

                                   HEADERS
                                     + END_STREAM
                                     + END_HEADERS
                                       foo = bar

9.2. Use of TLS Features

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

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

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

10.1. Server Authority

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

10.5. Denial-of-Service Considerations

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

The number of PUSH_PROMISE frames is not constrained in the same fashion. A client that accepts server push SHOULD limit the number of streams it allows to be in the "reserved (remote)" state. An excessive number of server push streams can be treated as a stream error (Section 5.4.2) of type ENHANCE_YOUR_CALM.¶

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

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

  • Providing tiny increments to flow control in WINDOW_UPDATE frames can cause a sender to generate a large number of DATA frames.¶
  • An endpoint is required to respond to a PING frame.¶
  • Each SETTINGS frame requires acknowledgment.¶
  • An invalid request (or server push) can cause a peer to send RST_STREAM frames in response.¶
• An attacker can provide large amounts of flow-control credit at the HTTP/2 layer but withhold credit at the TCP layer, preventing frames from being sent. An endpoint that constructs and remembers frames for sending without considering TCP limits might be subject to resource exhaustion.¶
• Large numbers of small or empty frames can be abused to cause a peer to expend time processing frame headers. Caution is required here as some uses of small frames are entirely legitimate, such as the sending of an empty DATA or CONTINUATION frame at the end of a stream.¶
• The SETTINGS frame might also be abused to cause a peer to expend additional processing time. This might be done by pointlessly changing settings, sending multiple undefined settings, or changing the same setting multiple times in the same frame.¶
• Handling reprioritization with PRIORITY frames can require significant processing time and can lead to overload if many PRIORITY frames are sent.¶
• Field section compression also provides opportunities for an attacker to waste processing resources; see Section 7 of [COMPRESSION] for more details on potential abuses.¶
• Limits in SETTINGS cannot be reduced instantaneously, which leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In particular, immediately after establishing a connection, limits set by a server are not known to clients and could be exceeded without being an obvious protocol violation.¶

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

An endpoint that doesn't monitor use of these features exposes itself to a risk of denial of service. Implementations SHOULD track the use of these features and set limits on their use. An endpoint MAY treat activity that is suspicious as a connection error (Section 5.4.1) of type ENHANCE_YOUR_CALM.¶

10.7. Use of Padding

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

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

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

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

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

11.1. HTTP2-Settings Header Field Registration

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

Field Name:

HTTP2-Settings¶

Status:

obsoleted¶

Reference:

Section 3.2.1 of [RFC7540]¶

Comments:

Obsolete; see Section 11.1 of this document.¶

11.2. The h2c Upgrade Token

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

Value:

h2c¶

Description:

(OBSOLETE) Hypertext Transfer Protocol version 2 (HTTP/2)¶

Expected Version Tokens:

None¶

Reference:

Section 3.1 of this document¶