Hypertext Transfer Protocol version 2

The HTTP/2 specification is split into four parts: ¶

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

While some of the frame and stream layer concepts are isolated from HTTP, the intent is not to define a completely generic framing layer. The framing and streams 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 initiating the HTTP/2 connection.

connection:

A transport-level 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 bytes 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 which did not initiate the HTTP/2 connection.

stream:

A bi-directional flow of frames across a virtual channel 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].¶

The protocol defined in this document has two identifiers. ¶

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

Negotiating "h2" or "h2c" implies the use of the transport, security, framing and message semantics described in this document.¶

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

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

Examples and text throughout the rest of this document use "h2" as a matter of editorial convenience only. Implementations of draft versions MUST NOT identify using this string.¶

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

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

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

For example:

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

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

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

A server MUST ignore a "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 can accept 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.¶

The first HTTP/2 frame sent by the server is a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client sends a connection preface (Section 3.5), which includes a SETTINGS frame.¶

The HTTP/1.1 request that is sent prior to upgrade is assigned stream identifier 1 and is assigned default priority values (Section 5.3.5). Stream 1 is implicitly half closed from the client toward the server, 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 without prior knowledge about support for HTTP/2 uses TLS [TLS12] with the application layer protocol negotiation extension [TLSALPN].¶

HTTP/2 over TLS uses the "h2" application token. The "h2c" token MUST NOT be sent by a client or selected by a server.¶

Once TLS negotiation is complete, both the client and the server 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 MAY immediately send HTTP/2 frames to a server that is known to support HTTP/2, after the connection preface (Section 3.5). A server can identify such a connection by the use of the "PRI" method in 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 [TLSALPN].¶

Prior support for HTTP/2 is not a strong signal that a given server will support HTTP/2 for future connections. It is possible for server configurations to change; for configurations to differ between instances in clustered server; or network conditions to change.¶

Upon establishment of a TCP connection and determination that HTTP/2 will be used by both peers, each endpoint MUST send a connection preface as a final confirmation and to establish the initial SETTINGS parameters for the HTTP/2 connection.¶

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

• The client connection preface is selected so that a large proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do not attempt to process further frames. Note that this does not address the concerns raised in [TALKING].

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

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.¶

Clients and servers MUST terminate the TCP connection if either peer does not begin with a valid connection preface. A GOAWAY frame (Section 6.8) can be omitted if it is clear that the peer is not using HTTP/2.¶

All frames begin with a fixed 8-octet header followed by a payload of between 0 and 16,383 octets.¶

The fields of the frame header are defined as: ¶

R:

A reserved 2-bit field. The semantics of these bits are undefined and the bits MUST remain unset (0) when sending and MUST be ignored when receiving.

Length:

The length of the frame payload expressed as an unsigned 14-bit integer. The 8 octets of the frame header are not included in this value.

Type:

The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown.

Flags:

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

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

R:

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

Stream Identifier:

A 31-bit stream identifier (see Section 5.1.1). The value 0 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 maximum size of a frame payload varies by frame type. The absolute maximum size of a frame payload is 2¹⁴-1 (16,383) octets, meaning that the maximum frame size is 16,391 octets. All implementations MUST be capable of receiving and minimally processing frames up to this maximum size.¶

Certain frame types, such as PING (Section 6.7), impose additional limits on the amount of payload data allowed.¶

If a frame size exceeds any defined limit, or is too small to contain mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR error. 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 WINDOW_UPDATE frame with a stream identifier of 0.¶

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

Header sets are collections of zero or more header fields. When transmitted over a connection, a header set 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.¶

HTTP Header Compression does not preserve the relative ordering of header fields. Header fields with multiple values are encoded into a single header field using a special delimiter (see Section 8.1.2.3), this preserves the relative order of values for that header field.¶

The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section 8.1.2.4).¶

A receiving endpoint reassembles the header block by concatenating its fragments, then decompresses the block to reconstruct the header set.¶

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, using a single compression context for the entire connection. 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 MUST have the END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE or CONTINUATION frames MUST have 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 MUST 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 frames that affect those transitions only. In this regard, CONTINUATION frames do not result in state transitions and are effectively part of the HEADERS or PUSH_PROMISE that they follow.¶

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. In this state, no frames have been exchanged.

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 marks the associated stream for later use. The stream state for the reserved stream transitions to "reserved (local)".
• Receiving a PUSH_PROMISE frame marks the associated stream as reserved by the remote peer. The state of the stream becomes "reserved (remote)".

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 frames other than HEADERS or RST_STREAM in this state.

A PRIORITY frame MAY be received in this state. Receiving any frames other than RST_STREAM, or PRIORITY 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 other type of frame other than RST_STREAM or PRIORITY.

Receiving any other type of frame other than HEADERS or RST_STREAM 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. Only WINDOW_UPDATE, PRIORITY and RST_STREAM frames can be sent in this state.

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.

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

PRIORITY frames received in this state are used to reprioritize streams that depend on the current 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 it performs flow control.

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

A stream 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 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 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 frame, 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 message that is not expressly permitted in the description of a state as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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. For an existing stream, the PRIORITY frame (Section 6.3) can be used to change the priority.¶

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

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

[rfc.comment.2: Note that stream dependencies have not yet been validated in practice. The theory might be fairly sound, but there are no implementations currently sending these. If it turns out that they are not useful, or actively harmful, implementations will be requested to avoid creating stream dependencies.] ¶

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

Prioritization information can be specified explicitly for streams as they are created using the HEADERS frame, or changed using the PRIORITY frame. Providing prioritization information is optional, so default values are used if no explicit indicator is 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. Protocol extensions can be used to provide additional services or alter any aspect of the protocol, within the limitations described in this section. Extensions are effective only within the scope of a single HTTP/2 connection.¶

Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), new error codes (Section 7), or new header fields that start with a colon (:). Of these, registries are established for 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, extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. Note that treating any frame other than DATA frames as flow controlled is such a change in semantics, and can only be done through negotiation.¶

This document doesn't mandate a specific method for negotiating the use of an extension, but notes that a setting (Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUST be defined so 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 arbitrary padding. Padding can be added to DATA frames to obscure the size of messages.¶

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 optional 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 and ignored when receiving.

The DATA frame defines the following flags: ¶

END_STREAM (0x1):

Bit 1 being set indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half closed" states or the "closed" state (Section 5.1).

END_SEGMENT (0x2):

Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames.

PADDED (0x8):

Bit 4 being set indicates that the Pad Length field is present.

DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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

Note:

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

Use of padding is a security feature; as such, its use demands some care, see Section 10.7.¶

The HEADERS frame (type=0x1) carries name-value pairs. It is used to open a stream (Section 5.1). HEADERS frames can be sent on a stream in the "open" or "half closed (remote)" states.¶

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 optional and is only present if the PADDED flag is set.

E:

A single bit flag indicates that the stream dependency is exclusive, see Section 5.3. This field is optional and 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 optional and is only present if the PRIORITY flag is set.

Weight:

An 8-bit weight for the stream, see Section 5.3. Add one to the value to obtain a weight between 1 and 256. This field is optional and is only present if the PRIORITY flag is set.

Header Block Fragment:

A header block fragment (Section 4.3).

Padding:

Padding octets.

The HEADERS frame defines the following flags: ¶

END_STREAM (0x1):

Bit 1 being set indicates that the header block (Section 4.3) is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of "half closed" states (Section 5.1).

A HEADERS frame that is followed by CONTINUATION frames carries the END_STREAM flag that signals the end of a stream. A CONTINUATION frame cannot be used to terminate a stream.

END_SEGMENT (0x2):

Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames.

END_HEADERS (0x4):

Bit 3 being set indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.

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

PADDED (0x8):

Bit 4 being set indicates that the Pad Length field is present.

PRIORITY (0x20):

Bit 6 being set indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see Section 5.3.

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

HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

The HEADERS frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).¶

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

The payload of a PRIORITY frame contains the following fields: ¶

E:

A single bit flag indicates that the stream dependency is exclusive, see Section 5.3.

Stream Dependency:

A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3.

Weight:

An 8-bit weight for the identified stream dependency, 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 is associated with an existing 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 of the "reserved (remote)", "open", "half closed (local)", "half closed (remote)", or "closed" states, 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 current stream. For a stream that is in the "half closed (remote)" or "closed" - state, this frame can only affect processing of the current stream and not frame transmission.¶

The PRIORITY frame is the only frame that can be sent for a stream in the "closed" state. This allows for the reprioritization of a group of dependent streams by altering the priority of a parent stream, which might be closed. However, a PRIORITY frame sent on a closed stream risks being ignored due to the peer having discarded priority state information for that stream.¶

The RST_STREAM frame (type=0x3) allows for abnormal termination of a stream. When sent by the initiator of a stream, it indicates that they wish to cancel the stream or that an error condition has occurred. When sent by the receiver of a stream, it indicates that either the receiver is rejecting the stream, requesting that the stream be cancelled, or 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. 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.¶

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge the receipt of those parameters. Individually, a SETTINGS parameter can also be referred to as a "setting".¶

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

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

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

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

ACK (0x1):

Bit 1 being set indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more info, see Settings Synchronization (Section 6.5.3).

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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 optional and is only present if the PADDED flag is set.

R:

A single reserved bit.

Promised Stream ID:

This unsigned 31-bit integer identifies the stream the endpoint intends to start sending frames for. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier (Section 5.1.1)).

Header Block Fragment:

A header block fragment (Section 4.3) containing request header fields.

Padding:

Padding octets.

The PUSH_PROMISE frame defines the following flags: ¶

END_HEADERS (0x4):

Bit 3 being set indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.

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

PADDED (0x8):

Bit 4 being set indicates that the Pad Length field is present.

PUSH_PROMISE frames MUST be associated with an existing, peer-initiated stream. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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

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

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

Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream state to become indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a stream that is neither "open" nor "half closed (local)" as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST treat the receipt of a PUSH_PROMISE that promises an illegal stream identifier (Section 5.1.1) (that is, an identifier for a stream that is not currently in the "idle" state) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

The PUSH_PROMISE frame includes optional 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 data in the payload. A sender can include any value it chooses and use those bytes in any fashion.¶

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

The PING frame defines the following flags: ¶

ACK (0x1):

Bit 1 being set indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.

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

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

The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this connection. GOAWAY can be sent by either the client or the server. Once sent, the sender will ignore frames sent on any new streams with identifiers higher than the included last stream identifier. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection, although a new connection can be established for new streams.¶

The purpose of this frame is to allow an endpoint to gracefully stop accepting new streams, while still finishing processing of previously established streams. This enables administrative actions, like server maintainence.¶

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 stream which was or might be processed on the sending endpoint in this connection. 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 can know whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on.¶

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

The GOAWAY frame does not define any flags.¶

The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a GOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

Note:

In this context, "processed" means that some data from the stream was passed to some higher layer of software that might have taken some action as a result.

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

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

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

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

A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last stream identifier set to 2³¹-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that no further requests can be initiated. After waiting at least one round trip time, the server can send another GOAWAY frame with an updated last stream identifier. This ensures that a connection can be cleanly shut down without losing requests.¶

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

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

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

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 bytes 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 (0x7fffffff) bytes.¶

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.¶

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

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

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 on an existing stream, 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):

Bit 3 being set indicates that this frame ends a header block (Section 4.3).

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

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

CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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 message headers (see [RFC7230], Section 3.2), and
2. zero or more DATA frames containing the message payload (see [RFC7230], Section 3.3), and
3. optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see [RFC7230], Section 4.1.2).

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

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

Otherwise, frames MAY be interspersed on the stream between these frames, but those frames do not carry HTTP semantics. In particular, HEADERS frames (and any CONTINUATION frames that follow) other than the first and optional last frames in this sequence do not carry HTTP semantics.¶

Trailing header fields are carried in a header block that also terminates the stream. That 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.¶

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 and ends with a frame bearing END_STREAM, which causes the stream to become "half closed" for the client. A response starts with a HEADERS frame and ends with a frame bearing END_STREAM, optionally followed by CONTINUATION frames, which places the stream in the "closed" state.¶

HTTP/2 enables a server to pre-emptively send (or "push") one or more associated responses to a client in response to a single request. This feature becomes particularly helpful when the server knows the client will need to have those responses available in order to fully process the response to the original request.¶

Pushing additional responses is optional, and is negotiated between individual endpoints. The SETTINGS_ENABLE_PUSH setting can be set to 0 to indicate that server push is disabled.¶

Because pushing responses is effectively hop-by-hop, an intermediary could receive pushed responses from the server and choose not to forward those on to the client. In other words, how to make use of the pushed responses is up to that intermediary. Equally, the intermediary might choose to push additional responses 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.¶

A server can only push responses that are cacheable (see [RFC7234], Section 3); promised requests MUST be safe (see [RFC7231], Section 4.2.1) and MUST NOT include a request body.¶

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 mostly as defined in Request Header Fields (Section 8.1.2.1), with a few differences. Specifically: ¶

• The :method header field is set to CONNECT.
• The :scheme and :path header fields MUST be omitted.
• The :authority 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 proxy that supports CONNECT establishes a TCP connection [TCP] to the server identified in the :authority 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 are 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 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 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-level 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 support TLS 1.2 [TLS12] 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.¶

A client is only able to accept HTTP/2 responses from servers that are authoritative for those resources. This is particularly important for server push (Section 8.2), where the client validates the PUSH_PROMISE before accepting the response.¶

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).¶

A client MUST discard responses provided by a server that is not authoritative for those resources.¶

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 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 which 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.¶

HTTP/2 header field names and values are encoded as sequences of octets with a length prefix. This enables HTTP/2 to carry any string of octets as the name or value of a header field. An intermediary that translates HTTP/2 requests or responses into HTTP/1.1 directly could permit the creation of corrupted HTTP/1.1 messages. An attacker might exploit this behavior to cause the intermediary to create HTTP/1.1 messages with illegal header fields, extra header fields, or even new messages that are entirely falsified.¶

Header field names or values that contain characters not permitted by HTTP/1.1, including carriage return (ASCII 0xd) or line feed (ASCII 0xa) MUST NOT be translated verbatim by an intermediary, as stipulated in [RFC7230], Section 3.2.4.¶

Translation from HTTP/1.x to HTTP/2 does not produce the same opportunity to an attacker. Intermediaries that perform translation to HTTP/2 MUST remove any instances of the obs-fold production from header field values.¶

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) are never cached or used.¶

An HTTP/2 connection can demand a greater commitment of resources to operate than a 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. 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 frame to end a stream.¶

Header compression also offers some opportunities to waste processing resources; see Section 8 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.¶

HTTP/2 enables greater use of compression for both header fields (Section 4.3) and entity bodies. Compression can allow an attacker to recover secret data when it is compressed in the same context as data under attacker control.¶

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.¶

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 (see for example, [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 size 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. This includes the value of settings, the manner in which flow control windows are managed, the way priorities are allocated to streams, timing of reactions to stimulus, and handling of any optional features.¶

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

This document creates two registrations for the identification of HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLSALPN].¶

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 types 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, it's 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:

A stable reference to a specification that describes the use of the setting.

An initial set of setting registrations can be found in Section 6.5.2.¶

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 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

No

Idempotent

No

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 (Not Authoritative) HTTP Status code in the Hypertext Transfer Protocol (HTTP) Status Code Registry ([RFC7231], Section 8.2).¶

Status Code:

421

Short Description:

Not Authoritative

Specification:

Section 9.1.2 of this document

13.1. Normative References

[COMPRESSION]

Ruellan, H. and R. Peon, “HPACK - Header Compression for HTTP/2”, Internet-Draft draft-ietf-httpbis-header-compression-08 (work in progress), June 2014.

[COOKIE]

Barth, A., “HTTP State Management Mechanism”, RFC 6265, April 2011.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC2818]

Rescorla, E., “HTTP Over TLS”, RFC 2818, May 2000.

[RFC3986]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.

[RFC4648]

Josefsson, S., “The Base16, Base32, and Base64 Data Encodings”, RFC 4648, October 2006.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

[RFC5234]

Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

[RFC7230]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, June 2014.

[RFC7231]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, RFC 7231, June 2014.

[RFC7232]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, RFC 7232, June 2014.

[RFC7233]

Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Range Requests”, RFC 7233, June 2014.

[RFC7234]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, RFC 7234, June 2014.

[RFC7235]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Authentication”, RFC 7235, June 2014.

[TCP]

Postel, J., “Transmission Control Protocol”, STD 7, RFC 793, September 1981.

[TLS-EXT]

Eastlake, D., “Transport Layer Security (TLS) Extensions: Extension Definitions”, RFC 6066, January 2011.

[TLS12]

Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, August 2008.

[TLSALPN]

Friedl, S., Popov, A., Langley, A., and E. Stephan, “Transport Layer Security (TLS) Application Layer Protocol Negotiation Extension”, Internet-Draft draft-ietf-tls-applayerprotoneg-05 (work in progress), March 2014.

13.2. Informative References

[ALT-SVC]

Nottingham, M., McManus, P., and J. Reschke, “HTTP Alternative Services”, Internet-Draft draft-ietf-httpbis-alt-svc-01 (work in progress), April 2014.

[BCP90]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

[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]

Berjon, R., Faulkner, S., Leithead, T., Doyle Navara, E., O'Connor, E., and S. Pfeiffer, “HTML5”, W3C Candidate Recommendation CR-html5-20140204, Febuary 2014, <http://www.w3.org/TR/2014/CR-html5-20140204/>.
Latest version available at <http://www.w3.org/TR/html5/>.

[RFC1323]

Jacobson, V., Braden, B., and D. Borman, “TCP Extensions for High Performance”, RFC 1323, May 1992.

[RFC2518]

Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.

[RFC3749]

Hollenbeck, S., “Transport Layer Security Protocol Compression Methods”, RFC 3749, May 2004.

[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, May 2006.

[RFC5288]

Salowey, J., Choudhury, A., and D. McGrew, “AES Galois Counter Mode (GCM) Cipher Suites for TLS”, RFC 5288, August 2008.

[RFC6585]

Nottingham, N. and R. Fielding, “Additional HTTP Status Codes”, RFC 6585, April 2012.

[TALKING]

Huang, L-S., 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 TLS and DTLS”, Internet-Draft draft-sheffer-tls-bcp-02 (work in progress), February 2014.

Restored extensibility options.¶

Restricting TLS cipher suites to AEAD only.¶

Removing Content-Encoding requirements.¶

Permitting the use of PRIORITY after stream close.¶

Removed ALTSVC frame.¶

Removed BLOCKED frame.¶

Reducing the maximum padding size to 256 octets; removing padding from CONTINUATION frames.¶

Removed per-frame GZIP compression.¶

Added BLOCKED frame (at risk).¶

Simplified priority scheme.¶

Added DATA per-frame GZIP compression.¶

Changed "connection header" to "connection preface" to avoid confusion.¶

Added dependency-based stream prioritization.¶

Added "h2c" identifier to distinguish between cleartext and secured HTTP/2.¶

Adding missing padding to PUSH_PROMISE.¶

Integrate ALTSVC frame and supporting text.¶

Dropping requirement on "deflate" Content-Encoding.¶

Improving security considerations around use of compression.¶

Adding padding for data frames.¶

Renumbering frame types, error codes, and settings.¶

Adding INADEQUATE_SECURITY error code.¶

Updating TLS usage requirements to 1.2; forbidding TLS compression.¶

Removing extensibility for frames and settings.¶

Changing setting identifier size.¶

Removing the ability to disable flow control.¶

Changing the protocol identification token to "h2".¶

Changing the use of :authority to make it optional and to allow userinfo in non-HTTP cases.¶

Allowing split on 0x0 for Cookie.¶

Reserved PRI method in HTTP/1.1 to avoid possible future collisions.¶

Added cookie crumbling for more efficient header compression.¶

Added header field ordering with the value-concatenation mechanism.¶

Marked draft for implementation.¶

Adding definition for CONNECT method.¶

Constraining the use of push to safe, cacheable methods with no request body.¶

Changing from :host to :authority to remove any potential confusion.¶

Adding setting for header compression table size.¶

Adding settings acknowledgement.¶

Removing unnecessary and potentially problematic flags from CONTINUATION.¶

Added denial of service considerations.¶

Marking the draft ready for implementation.¶

Renumbering END_PUSH_PROMISE flag.¶

Editorial clarifications and changes.¶

Added CONTINUATION frame for HEADERS and PUSH_PROMISE.¶

PUSH_PROMISE is no longer implicitly prohibited if SETTINGS_MAX_CONCURRENT_STREAMS is zero.¶

Push expanded to allow all safe methods without a request body.¶

Clarified the use of HTTP header fields in requests and responses. Prohibited HTTP/1.1 hop-by-hop header fields.¶

Requiring that intermediaries not forward requests with missing or illegal routing :-headers.¶

Clarified requirements around handling different frames after stream close, stream reset and GOAWAY.¶

Added more specific prohibitions for sending of different frame types in various stream states.¶

Making the last received setting value the effective value.¶

Clarified requirements on TLS version, extension and ciphers.¶

Committed major restructuring atrocities.¶

Added reference to first header compression draft.¶

Added more formal description of frame lifecycle.¶

Moved END_STREAM (renamed from FINAL) back to HEADERS/DATA.¶

Removed HEADERS+PRIORITY, added optional priority to HEADERS frame.¶

Added PRIORITY frame.¶

Added continuations to frames carrying header blocks.¶

Replaced use of "session" with "connection" to avoid confusion with other HTTP stateful concepts, like cookies.¶

Removed "message".¶

Switched to TLS ALPN from NPN.¶

Editorial changes.¶

Added IANA considerations section for frame types, error codes and settings.¶

Removed data frame compression.¶

Added PUSH_PROMISE.¶

Added globally applicable flags to framing.¶

Removed zlib-based header compression mechanism.¶

Updated references.¶

Clarified stream identifier reuse.¶

Removed CREDENTIALS frame and associated mechanisms.¶

Added advice against naive implementation of flow control.¶

Added session header section.¶

Restructured frame header. Removed distinction between data and control frames.¶

Altered flow control properties to include session-level limits.¶

Added note on cacheability of pushed resources and multiple tenant servers.¶

Changed protocol label form based on discussions.¶

Changed title throughout.¶

Removed section on Incompatibilities with SPDY draft#2.¶

Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <https://groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU>.¶

Replaced abstract and introduction.¶

Added section on starting HTTP/2.0, including upgrade mechanism.¶

Removed unused references.¶

Added flow control principles (Section 5.2.1) based on <https://tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.¶

Adopted as base for draft-ietf-httpbis-http2.¶

Updated authors/editors list.¶

Added status note.¶