Hypertext Transfer Protocol Version 2 (HTTP/2)

The HTTP/2 specification is split into four parts:

• Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is initiated.
• The frame (Section 4) and stream (Section 5) layers describe the way HTTP/2 frames are structured and formed into multiplexed streams.
• Frame (Section 6) and error (Section 7) definitions include details of the frame and error types used in HTTP/2.
• HTTP mappings (Section 8) and additional requirements (Section 9) describe how HTTP semantics are expressed using frames and streams.

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

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 send HTTP responses.

stream:

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

stream error:

An error on the individual HTTP/2 stream.

Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are defined in Section 2.3 of [RFC7230]. Intermediaries act as both client and server at different times.

The term "payload body" is defined in Section 3.3 of [RFC7230].

The protocol defined in this document has two identifiers.

• The string "h2" identifies the protocol where HTTP/2 uses Transport Layer Security (TLS) [TLS12]. This identifier is used in the TLS application-layer protocol negotiation (ALPN) extension [TLS-ALPN] field and in any place where HTTP/2 over TLS is identified.

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

• 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.

A client that makes a request for an "http" URI without prior knowledge about support for HTTP/2 on the next hop uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by making an HTTP/1.1 request that includes an Upgrade header field with the "h2c" token. Such an HTTP/1.1 request MUST include exactly one HTTP2-Settings (Section 3.2.1) header field.

For example:

GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

Requests that contain a payload body MUST be sent in their entirety before the client can send HTTP/2 frames. This means that a large request can block the use of the connection until it is completely sent.

If concurrency of an initial request with subsequent requests is important, an OPTIONS request can be used to perform the upgrade to HTTP/2, at the cost of an additional round trip.

A server MUST ignore an "h2" token in an Upgrade header field. Presence of a token with "h2" implies HTTP/2 over TLS, which is instead negotiated as described in Section 3.3.

A server that supports HTTP/2 accepts the upgrade with a 101 (Switching Protocols) response. After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the upgrade.

For example:

HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c

[ HTTP/2 connection ...

The first HTTP/2 frame sent by the server MUST be a server connection preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client MUST send a connection preface (Section 3.5), which includes a SETTINGS frame.

The HTTP/1.1 request that is sent prior to upgrade is assigned a stream identifier of 1 (see Section 5.1.1) with default priority values (Section 5.3.5). Stream 1 is implicitly "half-closed" from the client toward the server (see Section 5.1), since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the response.

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).

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.

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.

That is, the connection preface starts with the string PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n). This sequence MUST be followed by a SETTINGS frame (Section 6.5), which MAY be empty. The client sends the client connection preface immediately upon receipt of a 101 (Switching Protocols) response (indicating a successful upgrade) or as the first application data octets of a TLS connection. If starting an HTTP/2 connection with prior knowledge of server support for the protocol, the client connection preface is sent upon connection establishment.

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.

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

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 boolean flags specific to the frame type.

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

R:

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

Stream Identifier:

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

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

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 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.

Just as in HTTP/1, a header field in HTTP/2 is a name with one or more associated values. Header fields are used within HTTP request and response messages as well as in server push operations (see Section 8.2).

Header lists are collections of zero or more header fields. When transmitted over a connection, a header list is serialized into a header block using HTTP header compression [COMPRESSION]. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6), or CONTINUATION (Section 6.10) frames.

The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section 8.1.2.5).

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

A complete header block consists of either:

• 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 cleared and one or more CONTINUATION frames, where the last CONTINUATION frame has the END_HEADERS flag set.

Header compression is stateful. One compression context and one decompression context are used for the entire connection. A decoding error in a header block MUST be treated as a connection error (Section 5.4.1) of type COMPRESSION_ERROR.

Each header block is processed as a discrete unit. Header blocks MUST be transmitted as a contiguous sequence of frames, with no interleaved frames of any other type or from any other stream. The last frame in a sequence of HEADERS or CONTINUATION frames has the 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.

The lifecycle of a stream is shown in Figure 2.

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. In this state, a receiver can ignore WINDOW_UPDATE frames, which might arrive for a short period after a frame bearing the END_STREAM flag is sent.

PRIORITY frames received in this state are used to reprioritize streams that depend on the identified stream.

half-closed (remote):

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

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

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

A stream can transition from this state to "closed" by sending a frame that contains an END_STREAM flag or when either peer sends a RST_STREAM frame.

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 Sections 8.2.1 and 8.2.2.

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).

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 to 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 a suggestion.

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

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.

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 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 frames 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.

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.

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 (as signified by a "?" in the diagram) 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):

When set, bit 0 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):

When set, bit 3 indicates that the Pad Length field and any padding that it describes are 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)" 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 "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.

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 "idle", "reserved (local)", "open", or "half-closed (remote)" state.

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 indicating 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):

When set, bit 0 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):

When set, bit 2 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):

When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.

PRIORITY (0x20):

When set, bit 5 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). Padding that exceeds the size remaining for the header block fragment MUST be treated as a PROTOCOL_ERROR.

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).

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

The payload of a PRIORITY frame contains the following fields:

E:

A single-bit flag indicating 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; it does not affect frame transmission on that stream.

The PRIORITY frame can be sent for a stream in the "idle" or "closed" state. 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.

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.

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.

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):

When set, bit 0 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 information, see Section 6.5.3 ("Settings Synchronization").

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). 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.

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.

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" in 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):

When set, bit 2 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):

When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.

PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that is in either the "open" or "half-closed (remote)" state. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, 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. First, the inclusion of a header block (Section 4.3) potentially modifies the state maintained for header compression. Second, 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) 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 PUSH_PROMISE frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).

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.

In addition to the frame header, PING frames MUST contain 8 octets of opaque 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):

When set, bit 0 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.

The GOAWAY frame (type=0x7) 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 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 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.

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.

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, 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 initiating further requests is prohibited. After allowing time for any in-flight stream creation (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 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 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.

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.

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. 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.

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.

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

The CONTINUATION frame defines the following flag:

END_HEADERS (0x4):

When set, bit 2 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.

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),
2. one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see [RFC7230], Section 3.2),
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 the 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 receiving a final (non-informational) status code MUST treat the corresponding request or response as malformed (Section 8.1.2.6).

An HTTP request/response exchange fully consumes a single stream. A request starts with the HEADERS frame that puts the stream into an "open" state. The request ends with a frame bearing END_STREAM, which causes the stream to become "half-closed (local)" for the client and "half-closed (remote)" for the server. A response starts with a HEADERS frame and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.

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 header 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). 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.

HTTP/2 allows a server to pre-emptively send (or "push") responses (along with corresponding "promised" requests) to a client in association with a previous client-initiated request. This can be useful when the server knows the client will need to have those responses available in order to fully process the response to the original request.

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 cacheable (see [RFC7231], Section 4.2.3), MUST be safe (see [RFC7231], Section 4.2.1), and MUST NOT include a request body. Clients that receive a promised request that is not cacheable, that is not known to be safe, or that indicates the presence of a request body MUST reset the promised stream with a stream error (Section 5.4.2) of type PROTOCOL_ERROR. Note 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 [RFC7234], Section 3) 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 ([RFC7234], Section 5.2.2)) while the stream identified by the promised stream ID 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. Clients MUST reject any attempt to change the SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) 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 is used to establish a tunnel over a single HTTP/2 stream to a remote host for similar purposes. The HTTP header field mapping works as defined in Section 8.1.2.3 ("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 [RFC7230], Section 5.3)).

A CONNECT request that does not conform to these restrictions is malformed (Section 8.1.2.6).

A proxy that supports CONNECT establishes a TCP connection [TCP] to the server 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 [RFC7231], Section 4.3.6.

After the initial HEADERS frame sent by each peer, all subsequent DATA frames correspond to data sent on the TCP connection. The 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 bearing the END_STREAM flag. 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.

HTTP/2 connections are persistent. For best performance, it is expected that clients will not close connections until it is determined that no further communication with a server is necessary (for example, when a user navigates away from a particular web page) or until the server closes the connection.

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

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

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

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

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. HTTP/2 clients MUST indicate the target domain name when negotiating TLS.

Deployments of HTTP/2 that negotiate TLS 1.3 or higher need only support and use the SNI extension; deployments of TLS 1.2 are subject to the requirements in the following sections. Implementations are encouraged to provide defaults that comply, but it is recognized that deployments are ultimately responsible for compliance.

HTTP/2 relies on the HTTP/1.1 definition of authority for determining whether a server is authoritative in providing a given response (see [RFC7230], Section 9.1). This relies on local name resolution for the "http" URI scheme and the authenticated server identity for the "https" scheme (see [RFC2818], Section 3).

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

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

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

The cleartext version of HTTP/2 has minimal protection against cross-protocol attacks. The connection preface (Section 3.5) contains a string that is designed to confuse HTTP/1.1 servers, but no special protection is offered for other protocols. A server that is willing to ignore parts of an HTTP/1.1 request containing an Upgrade header field in addition to the client connection preface could be exposed to a cross-protocol attack.

The HTTP/2 header field encoding allows the expression of names that are not valid field names in the Internet Message Syntax used by HTTP/1.1. Requests or responses containing invalid header field names MUST be treated as malformed (Section 8.1.2.6). An intermediary therefore cannot translate an HTTP/2 request or response containing an invalid field name into an HTTP/1.1 message.

Similarly, HTTP/2 allows header field values that are not valid. While most of the values that can be encoded will not alter header field parsing, carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa), and the zero character (NUL, ASCII 0x0) might be exploited by an attacker if they are translated verbatim. Any request or response that contains a character not permitted in a header field value MUST be treated as malformed (Section 8.1.2.6). Valid characters are defined by the field-content ABNF rule in Section 3.2 of [RFC7230].

Pushed responses do not have an explicit request from the client; the request is provided by the server in the PUSH_PROMISE frame.

Caching responses that are pushed is possible based on the guidance provided by the origin server in the Cache-Control header field. However, this can cause issues if a single server hosts more than one tenant. For example, a server might offer multiple users each a small portion of its URI space.

Where multiple tenants share space on the same server, that server MUST ensure that tenants are not able to push representations of resources that they do not have authority over. Failure to enforce this would allow a tenant to provide a representation that would be served out of cache, overriding the actual representation that the authoritative tenant provides.

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

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

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

Processing capacity cannot be guarded as effectively as state capacity.

The SETTINGS frame can be abused to cause a peer to expend additional processing time. This might be done by pointlessly changing SETTINGS parameters, setting multiple undefined parameters, or changing the same setting multiple times in the same frame. WINDOW_UPDATE or PRIORITY frames can be abused to cause an unnecessary waste of resources.

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

Header compression also offers some opportunities to waste processing resources; see Section 7 of [COMPRESSION] for more details on potential abuses.

Limits in SETTINGS parameters 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.

All these features — i.e., SETTINGS changes, small frames, header compression — have legitimate uses. These features become a burden only when they are used unnecessarily or to excess.

An endpoint that doesn't monitor this behavior exposes itself to a risk of denial-of-service attack. 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.

Compression can allow an attacker to recover secret data when it is compressed in the same context as data under attacker control. HTTP/2 enables compression of header fields (Section 4.3); the following concerns also apply to the use of HTTP compressed content-codings ([RFC7231], Section 3.1.2.1).

There are demonstrable attacks on compression that exploit the characteristics of the web (e.g., [BREACH]). The attacker induces multiple requests containing varying plaintext, observing the length of the resulting ciphertext in each, which reveals a shorter length when a guess about the secret is correct.

Implementations communicating on a secure channel MUST NOT compress content that includes both confidential and attacker-controlled data unless separate compression dictionaries are used for each source of data. Compression MUST NOT be used if the source of data cannot be reliably determined. Generic stream compression, such as that provided by TLS, MUST NOT be used with HTTP/2 (see Section 9.2).

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

Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might be provided by TLS [TLS12]. 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 payloads to a fixed size exposes information as 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.

Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions of a single client or server over time. These include the value of settings, the manner in which flow-control windows are managed, the way priorities are allocated to streams, the timing of reactions to stimulus, and the handling of any features that are controlled by settings.

As far as these create observable differences in behavior, they could be used as a basis for fingerprinting a specific client, as defined in Section 1.8 of [HTML5].

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

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

This document creates two registrations for the identification of HTTP/2 (see Section 3.3) in the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLS-ALPN].

The "h2" string identifies HTTP/2 when used over TLS:

Protocol:

HTTP/2 over TLS

Identification Sequence:

0x68 0x32 ("h2")

Specification:

This document

The "h2c" string identifies HTTP/2 when used over cleartext TCP:

Protocol:

HTTP/2 over TCP

Identification Sequence:

0x68 0x32 0x63 ("h2c")

Specification:

This document

This document establishes a registry for HTTP/2 frame type codes. The "HTTP/2 Frame Type" registry manages an 8-bit space. The "HTTP/2 Frame Type" registry operates under either of the "IETF Review" or "IESG Approval" policies [RFC5226] for values between 0x00 and 0xef, with values between 0xf0 and 0xff being reserved for Experimental Use.

New entries in this registry require the following information:

Frame Type:

A name or label for the frame type.

Code:

The 8-bit code assigned to the frame type.

Specification:

A reference to a specification that includes a description of the frame layout, its semantics, and flags that the frame type uses, including any parts of the frame that are conditionally present based on the value of flags.

The entries in the following table are registered by this document.

This document establishes a registry for HTTP/2 settings. The "HTTP/2 Settings" registry manages a 16-bit space. The "HTTP/2 Settings" registry operates under the "Expert Review" policy [RFC5226] for values in the range from 0x0000 to 0xefff, with values between and 0xf000 and 0xffff being reserved for Experimental Use.

New registrations are advised to provide the following information:

Name:

A symbolic name for the setting. Specifying a setting name is optional.

Code:

The 16-bit code assigned to the setting.

Initial Value:

An initial value for the setting.

Specification:

An optional reference to a specification that describes the use of the setting.

The entries in the following table are registered by this document.

This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 Error Code" registry operates under the "Expert Review" policy [RFC5226].

Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated.

New registrations are advised to provide the following information:

Name:

A name for the error code. Specifying an error code name is optional.

Code:

The 32-bit error code value.

Description:

A brief description of the error code semantics, longer if no detailed specification is provided.

Specification:

An optional reference for a specification that defines the error code.

The entries in the following table are registered by this document.

This section registers the HTTP2-Settings header field in the "Permanent Message Header Field Names" registry [BCP90].

Header field name:

HTTP2-Settings

Applicable protocol:

http

Status:

standard

Author/Change controller:

IETF

Specification document(s):

Section 3.2.1 of this document

Related information:

This header field is only used by an HTTP/2 client for Upgrade-based negotiation.

This section registers the PRI method in the "HTTP Method Registry" ([RFC7231], Section 8.1).

Method Name:

PRI

Safe:

Yes

Idempotent:

Yes

Specification document(s):

Section 3.5 of this document

Related information:

This method is never used by an actual client. This method will appear to be used when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection preface.

This document registers the 421 (Misdirected Request) HTTP status code in the "HTTP Status Codes" registry ([RFC7231], Section 8.2).

Status Code:

421

Short Description:

Misdirected Request

Specification:

Section 9.1.2 of this document

This document registers the "h2c" upgrade token in the "HTTP Upgrade Tokens" registry ([RFC7230], Section 8.6).

Value:

h2c

Description:

Hypertext Transfer Protocol version 2 (HTTP/2)

Expected Version Tokens:

None

Reference:

Section 3.2 of this document

12.1. Normative References

[COMPRESSION]

Peon, R. and H. Ruellan, “HPACK: Header Compression for HTTP/2”, RFC 7541, DOI 10.17487/RFC7541, May 2015, <http://www.rfc-editor.org/info/rfc7541>.

[COOKIE]

Barth, A., “HTTP State Management Mechanism”, RFC 6265, DOI 10.17487/RFC6265, April 2011, <http://www.rfc-editor.org/info/rfc6265>.

[FIPS186]

NIST, “Digital Signature Standard (DSS)”, FIPS PUB 186-4, July 2013, <http://dx.doi.org/10.6028/NIST.FIPS.186-4>.

[RFC2119]

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

[RFC2818]

Rescorla, E., “HTTP Over TLS”, RFC 2818, DOI 10.17487/RFC2818, May 2000, <http://www.rfc-editor.org/info/rfc2818>.

[RFC3986]

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

[RFC4648]

Josefsson, S., “The Base16, Base32, and Base64 Data Encodings”, RFC 4648, DOI 10.17487/RFC4648, October 2006, <http://www.rfc-editor.org/info/rfc4648>.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>.

[RFC5234]

Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>.

[RFC7230]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.

[RFC7231]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, RFC 7231, DOI 10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/info/rfc7231>.

[RFC7232]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, RFC 7232, DOI 10.17487/RFC7232, June 2014, <http://www.rfc-editor.org/info/rfc7232>.

[RFC7233]

Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Range Requests”, RFC 7233, DOI 10.17487/RFC7233, June 2014, <http://www.rfc-editor.org/info/rfc7233>.

[RFC7234]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, RFC 7234, DOI 10.17487/RFC7234, June 2014, <http://www.rfc-editor.org/info/rfc7234>.

[RFC7235]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Authentication”, RFC 7235, DOI 10.17487/RFC7235, June 2014, <http://www.rfc-editor.org/info/rfc7235>.

[TCP]

Postel, J., “Transmission Control Protocol”, STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <http://www.rfc-editor.org/info/rfc793>.

[TLS-ALPN]

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

[TLS-ECDHE]

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

[TLS-EXT]

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

[TLS12]

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

12.2. Informative References

[ALT-SVC]

Nottingham, M., McManus, P., and J. Reschke, “HTTP Alternative Services”, Work in Progress, draft-ietf-httpbis-alt-svc-06, February 2015.

[BCP90]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004, <http://www.rfc-editor.org/info/bcp90>.

[BREACH]

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

[HTML5]

Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle Navara, E., O'Connor, E., and S. Pfeiffer, “HTML5”, W3C Recommendation REC-html5-20141028, October 2014, <http://www.w3.org/TR/2014/REC-html5-20141028/>.

[RFC3749]

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

[RFC4492]

Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, “Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)”, RFC 4492, DOI 10.17487/RFC4492, May 2006, <http://www.rfc-editor.org/info/rfc4492>.

[RFC6585]

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

[RFC7323]

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

[TALKING]

Huang, L., Chen, E., Barth, A., Rescorla, E., and C. Jackson, “Talking to Yourself for Fun and Profit”, 2011, <http://w2spconf.com/2011/papers/websocket.pdf>.

[TLSBCP]

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