Twist

mbelshe@chromium.org

Google, Inc

fenix@google.com

Mozilla

331 E Evelyn Street Mountain View CA 94041 US martin.thomson@gmail.com

Applications HTTPbis HTTP SPDY Web This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent messages on the same connection. It also introduces unsolicited push of representations from servers to clients. This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged. Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at . Working Group information can be found at ; that specific to HTTP/2 are at . The changes in this draft are summarized in .

The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, the HTTP/1.1 message format () has several characteristics that have a negative overall effect on application performance today. In particular, HTTP/1.0 allowed only one request to be outstanding at a time on a given TCP connection. HTTP/1.1 added request pipelining, but this only partially addressed request concurrency and still suffers from head-of-line blocking. Therefore, HTTP/1.1 clients that need to make many requests typically use multiple connections to a server in order to achieve concurrency and thereby reduce latency. Furthermore, HTTP header fields are often repetitive and verbose, causing unnecessary network traffic, as well as causing the initial TCP congestion window to quickly fill. This can result in excessive latency when multiple requests are made on a new TCP connection. HTTP/2 addresses these issues by defining an optimized mapping of HTTP's semantics to an underlying connection. Specifically, it allows interleaving of request and response messages on the same connection and uses an efficient coding for HTTP header fields. It also allows prioritization of requests, letting more important requests complete more quickly, further improving performance. The resulting protocol is more friendly to the network, because fewer TCP connections can be used in comparison to HTTP/1.x. This means less competition with other flows, and longer-lived connections, which in turn leads to better utilization of available network capacity. Finally, HTTP/2 also enables more efficient processing of messages through use of binary message framing.

HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core features of HTTP/1.1, but aims to be more efficient in several ways. The basic protocol unit in HTTP/2 is a frame. Each frame type serves a different purpose. For example, HEADERS and DATA frames form the basis of HTTP requests and responses; other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are used in support of other HTTP/2 features. Multiplexing of requests is achieved by having each HTTP request-response exchange associated with its own stream. Streams are largely independent of each other, so a blocked or stalled request or response does not prevent progress on other streams. Flow control and prioritization ensure that it is possible to efficiently use multiplexed streams. Flow control helps to ensure that only data that can be used by a receiver is transmitted. Prioritization ensures that limited resources can be directed to the most important streams first. HTTP/2 adds a new interaction mode, whereby a server can push responses to a client. Server push allows a server to speculatively send a client data that the server anticipates the client will need, trading off some network usage against a potential latency gain. The server does this by synthesizing a request, which it sends as a PUSH_PROMISE frame. The server is then able to send a response to the synthetic request on a separate stream. Frames that contain HTTP header fields are compressed. HTTP requests can be highly redundant, so compression can reduce the size of requests and responses significantly.

The HTTP/2 specification is split into four parts: Starting HTTP/2 covers how an HTTP/2 connection is initiated. The framing and streams layers describe the way HTTP/2 frames are structured and formed into multiplexed streams. Frame and error definitions include details of the frame and error types used in HTTP/2. HTTP mappings and additional requirements describe how HTTP semantics are expressed using frames and streams. While some of the frame and stream layer concepts are isolated from HTTP, this specification does not define a completely generic framing layer. The framing and streams layers are tailored to the needs of the HTTP protocol and server push.

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. 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: The endpoint initiating the HTTP/2 connection. A transport-layer connection between two endpoints. An error that affects the entire HTTP/2 connection. Either the client or server of the connection. 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. An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is remote to the primary subject of discussion. An endpoint that is receiving frames. An endpoint that is transmitting frames. The endpoint which did not initiate the HTTP/2 connection. A bi-directional flow of frames across a virtual channel within the HTTP/2 connection. An error on the individual HTTP/2 stream. Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are defined in .

An HTTP/2 connection is an application layer protocol running on top of a TCP connection (). The client is the TCP connection initiator. HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same default port numbers: 80 for "http" URIs and 443 for "https" URIs. As a result, implementations processing requests for target resource URIs like http://example.org/foo or https://example.com/bar are required to first discover whether the upstream server (the immediate peer to which the client wishes to establish a connection) supports HTTP/2. The means by which support for HTTP/2 is determined is different for "http" and "https" URIs. Discovery for "http" URIs is described in . Discovery for "https" URIs is described in .

The protocol defined in this document has two identifiers. The string "h2" identifies the protocol where HTTP/2 uses TLS. This identifier is used in the TLS application layer protocol negotiation extension (ALPN) 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 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 . Experimenters are encouraged to coordinate their experiments on the ietf-http-wg@w3.org mailing list.

A client that makes a request for an "http" URI without prior knowledge about support for HTTP/2 uses the HTTP Upgrade mechanism (). 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 header field.

For example: ]]>

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, an OPTIONS request can be used to perform the upgrade to HTTP/2, at the cost of an additional round-trip. A server that does not support HTTP/2 can respond to the request as though the Upgrade header field were absent:

HTTP/1.1 200 OK Content-Length: 243 Content-Type: text/html ...

A server MUST ignore 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 . 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.

For example: HTTP/1.1 101 Switching Protocols Connection: Upgrade Upgrade: h2c [ HTTP/2 connection ...

The first HTTP/2 frame sent by the server is a SETTINGS frame () as the server connection preface (). Upon receiving the 101 response, the client sends a connection preface, 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. 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 request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Settings header field. The HTTP2-Settings header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.

A server MUST NOT upgrade the connection to HTTP/2 if this header field is not present, or if more than one is present. A server MUST NOT send this header field. The content of the HTTP2-Settings header field is the payload of a SETTINGS frame (), encoded as a base64url string (that is, the URL- and filename-safe Base64 encoding described in , with any trailing '=' characters omitted). The ABNF production for token68 is defined in . Since the upgrade is only intended to apply to the immediate connection, a client sending HTTP2-Settings MUST also send HTTP2-Settings as a connection option in the Connection header field to prevent it from being forwarded downstream. A server decodes and interprets these values as it would any other SETTINGS frame. Acknowledgement of the SETTINGS parameters is not necessary, since a 101 response serves as implicit acknowledgment. Providing these values in the Upgrade request gives a client an opportunity to provide parameters prior to receiving any frames from the server.

A client that makes a request to an "https" URI uses TLS with the application layer protocol negotiation extension. 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.

A client can learn that a particular server supports HTTP/2 by other means. For example, 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; a server can identify such a connection 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. 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.

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 client and server each send a different connection preface. The client connection preface starts with a sequence of 24 octets, which in hex notation are:

(the string PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n). This sequence is followed by a SETTINGS frame (). 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 . The server connection preface consists of a potentially empty SETTINGS frame () 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 ) 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 of type PROTOCOL_ERROR. A GOAWAY frame () MAY be omitted in this case, since an invalid preface indicates that the peer is not using HTTP/2.

Once the HTTP/2 connection is established, endpoints can begin exchanging frames.

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

The fields of the frame header are defined as: 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. 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. 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. 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. A 31-bit stream identifier (see ). 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 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. The size of the frame header is not included when describing frame sizes. Certain frame types, such as PING, 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; this includes any frame carrying a header block (that is, HEADERS, PUSH_PROMISE, and CONTINUATION), SETTINGS, and any WINDOW_UPDATE 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 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. They are used within HTTP request and response messages as well as server push operations (see ). 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. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS, PUSH_PROMISE or CONTINUATION frames. The Cookie header field is treated specially by the HTTP mapping (see ). A receiving endpoint reassembles the header block by concatenating its fragments, then decompresses the block to reconstruct the header list. A complete header block consists of either: 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 is used 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 of type COMPRESSION_ERROR if it does not decompress a header block.

A "stream" is an independent, bi-directional sequence of frames exchanged between the client and server within an HTTP/2 connection. Streams have several important characteristics: A single HTTP/2 connection can contain multiple concurrently open streams, with either endpoint interleaving frames from multiple streams. Streams can be established and used unilaterally or shared by either the client or server. Streams can be closed by either endpoint. The order in which frames are sent on a stream is significant. Recipients process frames in the order they are received. In particular, the order of HEADERS, and DATA frames is semantically significant. Streams are identified by an integer. Stream identifiers are assigned to streams by the endpoint initiating the stream.

The lifecycle of a stream is shown in .

| |<-----------' | | R | closed | R | `-------------------->| |<--------------------' +--------+ H: HEADERS frame (with implied CONTINUATIONs) PP: PUSH_PROMISE frame (with implied CONTINUATIONs) ES: END_STREAM flag R: RST_STREAM frame ]]>

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 this purpose, 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: 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 . 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)". Receiving any frames other than HEADERS or PUSH_PROMISE on a stream in this state MUST be treated as a connection error of type PROTOCOL_ERROR. 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 ). 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 or RST_STREAM in this state. A PRIORITY frame MAY be received in this state. Receiving any type of frame other than RST_STREAM or PRIORITY on a stream in this state MUST be treated as a connection error of type PROTOCOL_ERROR. 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 or RST_STREAM on a stream in this state MUST be treated as a connection error of type PROTOCOL_ERROR. 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. 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". 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. 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 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. 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. 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 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 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 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 ). 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 of type PROTOCOL_ERROR. Frame of unknown types are ignored. An example of the state transitions for an HTTP request/response exchange can be found in . An example of the state transitions for server push can be found in and .

Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numbered stream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A stream identifier of zero (0x0) is used for connection control messages; the stream identifier zero cannot be used to establish a new stream. HTTP/1.1 requests that are upgraded to HTTP/2 (see ) are responded to with a stream identifier of one (0x1). After the upgrade completes, stream 0x1 is "half closed (local)" to the client. Therefore, stream 0x1 cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1. The identifier of a newly established stream MUST be numerically greater than all streams that the initiating endpoint has opened or reserved. This governs streams that are opened using a HEADERS frame and streams that are reserved using PUSH_PROMISE. An endpoint that receives an unexpected stream identifier MUST respond with a connection error of type PROTOCOL_ERROR. The first use of a new stream identifier implicitly closes all streams in the "idle" state that might have been initiated by that peer with a lower-valued stream identifier. For example, if a client sends a HEADERS frame on stream 7 without ever sending a frame on stream 5, then stream 5 transitions to the "closed" state when the first frame for stream 7 is sent or received. Stream identifiers cannot be reused. Long-lived connections can result in an endpoint exhausting the available range of stream identifiers. A client that is unable to establish a new stream identifier can establish a new connection for new streams. A server that is unable to establish a new stream identifier can send a GOAWAY frame so that the client is forced to open a new connection for new streams.

A peer can limit the number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameter (see ) within a SETTINGS frame. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum number of concurrent streams the server can initiate, and servers specify the maximum number of concurrent streams the client can initiate. Streams that are in the "open" state, or either of the "half closed" states count toward the maximum number of streams that an endpoint is permitted to open. Streams in any of these three states count toward the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the "reserved" states do not count toward the stream limit. Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame that causes their advertised concurrent stream limit to be exceeded MUST treat this as a stream error. An endpoint that wishes to reduce the value of SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current number of open streams can either close streams that exceed the new value or allow streams to complete.

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.

HTTP/2 stream flow control aims to allow a variety of flow control algorithms to be used without requiring protocol changes. Flow control in HTTP/2 has the following characteristics: Flow control is specific to a connection; i.e., it is "hop-by-hop", not "end-to-end". Flow control is based on window update frames. Receivers advertise how many octets they are prepared to receive on a stream and for the entire connection. This is a credit-based scheme. Flow control is directional with overall control provided by the receiver. A receiver MAY choose to set any window size that it desires for each stream and for the entire connection. A sender MUST respect flow control limits imposed by a receiver. Clients, servers and intermediaries all independently advertise their flow control window as a receiver and abide by the flow control limits set by their peer when sending. The initial value for the flow control window is 65,535 octets for both new streams and the overall connection. The frame type determines whether flow control applies to a frame. Of the frames specified in this document, only DATA frames are subject to flow control; all other frame types do not consume space in the advertised flow control window. This ensures that important control frames are not blocked by flow control. Flow control cannot be disabled. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE frame (). This document does not stipulate how a receiver decides when to send this frame or the value that it sends, nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs. Implementations are also responsible for managing how requests and responses are sent based on priority; choosing how to avoid head of line blocking for requests; and managing the creation of new streams. Algorithm choices for these could interact with any flow control algorithm.

Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxy needs to share memory between many connections, and also might have a slow upstream connection and a fast downstream one. Flow control addresses cases where the receiver is unable process data on one stream, yet wants to continue to process other streams in the same connection. Deployments that do not require this capability can advertise a flow control window of the maximum size, incrementing the available space when new data is received. This effectively disables flow control for that receiver. Conversely, a sender is always subject to the flow control window advertised by the receiver. Deployments with constrained resources (for example, memory) can employ flow control to limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of available network resources if flow control is enabled without knowledge of the bandwidth-delay product (see ). Even with full awareness of the current bandwidth-delay product, implementation of flow control can be difficult. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion. Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read and acted upon.

A client can assign a priority for a new stream by including prioritization information in the HEADERS frame that opens the stream. For an existing stream, the PRIORITY frame 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 (). Each dependency is assigned a relative weight, a number that is used to determine the relative proportion of available resources that are assigned to streams dependent on the same stream. Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee any particular processing or transmission order for the stream relative to any other stream. An endpoint cannot force a peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only ever a suggestion. Providing prioritization information is optional, so default values are used if no explicit indicator is provided ().

Each stream can be given an explicit dependency on another stream. Including a dependency expresses a preference to allocate resources to the identified stream rather than to the dependent stream. A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, the non-existent stream 0 forms the root of the tree. A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependent is a parent stream. A dependency on a stream that is not currently in the tree - such as a stream in the "idle" state - results in that stream being given a default priority. When assigning a dependency on another stream, the stream is added as a new dependency of the parent stream. Dependent streams that share the same parent are not ordered with respect to each other. For example, if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, this results in a dependency order of A followed by B, C, and D in any order.

/|\ B C B D C ]]>

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

D B C / \ B C ]]>

Inside the dependency tree, a dependent stream SHOULD only be allocated resources if all of the streams that it depends on (the chain of parent streams up to 0x0) are either closed, or it is not possible to make progress on them. A stream cannot depend on itself. An endpoint MUST treat this as a stream error of type PROTOCOL_ERROR.

All dependent streams are allocated an integer weight between 1 and 256 (inclusive). Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, if stream B depends on stream A with weight 4, and C depends on stream A with weight 12, and if no progress can be made on A, stream B ideally receives one third of the resources allocated to stream C.

Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to become dependent on the identified parent stream. Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency with the exclusive flag for a reprioritized stream moves all the dependencies of the new parent stream to become dependent on the reprioritized stream. If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved to be dependent on the reprioritized stream's previous parent. The moved dependency retains its weight.

For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and F depends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationships stay the same, except for F, which becomes dependent on A if the reprioritization is exclusive. F B C ==> F A OR A / \ | / \ /|\ D E E B C B C F | | | F E E (intermediate) (non-exclusive) (exclusive) ]]>

When a stream is removed from the dependency tree, its dependencies can be moved to become dependent on the parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight of the dependency of the closed stream proportionally based on the weights of its dependencies. Streams that are removed from the dependency tree cause some prioritization information to be lost. Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream. However, if the common dependency is removed from the tree, those streams share resources with streams at the next highest level. For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streams C and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion of resources. For equal starting weights, C receives one third, rather than one half, of available resources. It is possible for a stream to become closed while prioritization information that creates a dependency on that stream is in transit. If a stream identified in a dependency has no associated priority information, then the dependent stream is instead assigned a default priority. This potentially creates suboptimal prioritization, since the stream could be given a priority that is different to what is intended. To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streams become closed. The longer state is retained, the lower the chance that streams are assigned incorrect or default priority values. This could create a large state burden for an endpoint, so this state MAY be limited. An endpoint MAY apply a fixed upper limit on the number of closed streams for which prioritization state is tracked to limit state exposure. The amount of additional state an endpoint maintains could be dependent on load; under high load, prioritization state can be discarded to limit resource commitments. In extreme cases, an endpoint could even discard prioritization state for active or reserved streams. If a fixed limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS. An endpoint receiving a PRIORITY frame that changes the priority of a closed stream SHOULD alter the dependencies of the streams that depend on it, if it has retained enough state to do so.

Providing priority information is optional. Streams are assigned a non-exclusive dependency on stream 0x0 by default. Pushed streams initially depend on their associated stream. In both cases, streams are assigned a default weight of 16.

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 .

A connection error is any error which prevents further processing of the framing layer, or which corrupts any connection state. An endpoint that encounters a connection error SHOULD first send a GOAWAY frame () with the stream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includes an error code that indicates why the connection is terminating. After sending the GOAWAY frame, the endpoint MUST close the TCP connection. It is possible that the GOAWAY will not be reliably received by the receiving endpoint (see ). In the event of a connection error, GOAWAY only provides a best effort attempt to communicate with the peer about why the connection is being terminated. An endpoint can end a connection at any time. In particular, an endpoint MAY choose to treat a stream error as a connection error. Endpoints SHOULD send a GOAWAY frame when ending a connection, providing that circumstances permit it.

A stream error is an error related to a specific stream that does not affect processing of other streams. An endpoint that detects a stream error sends a RST_STREAM frame () that contains the stream identifier of the stream where the error occurred. The RST_STREAM frame includes an error code that indicates the type of error. A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by the remote peer. These frames can be ignored, except where they modify connection state (such as the state maintained for header compression, or flow control). Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more than a round-trip time. This behavior is permitted to deal with misbehaving implementations. An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM frame, to avoid looping.

If the TCP connection is closed or reset while streams remain in open or half closed states, then the endpoint MUST assume that those streams were abnormally interrupted and could be incomplete.

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, new settings, or new error codes. Registries are established for managing these extension points: frame types, settings and error codes. 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 are not permitted; these MUST be treated as a connection error of type PROTOCOL_ERROR. 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 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.

This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frame type serves a distinct purpose either in the establishment and management of the connection as a whole, or of individual streams. The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain a synchronized view of the connection state, successful communication within the connection will no longer be possible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected by the use any given frame.

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: 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. 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 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: 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. Bit 4 being set indicates that the Pad Length field and any padding that it describes is present. DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error 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 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 frame payload, the recipient MUST treat this as a connection error of type PROTOCOL_ERROR. A frame can be increased in size by one octet by including a Pad Length field with a value of zero. Padding is a security feature; see .

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

The HEADERS frame payload has the following fields: 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. A single bit flag indicates that the stream dependency is exclusive, see . This field is only present if the PRIORITY flag is set. A 31-bit stream identifier for the stream that this stream depends on, see . This field is only present if the PRIORITY flag is set. An 8-bit weight for the stream, see . Add one to the value to obtain a weight between 1 and 256. This field is only present if the PRIORITY flag is set. A header block fragment. Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending and ignored when receiving. The HEADERS frame defines the following flags: Bit 1 being set indicates that the header block 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. 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. Bit 3 being set indicates that this frame contains an entire header block 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 of type PROTOCOL_ERROR. Bit 4 being set indicates that the Pad Length field and any padding that it describes is present. Bit 6 being set indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see . The payload of a HEADERS frame contains a header block fragment. A header block that does not fit within a HEADERS frame is continued in a CONTINUATION frame. 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 of type PROTOCOL_ERROR. The HEADERS frame changes the connection state as described in . The HEADERS frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames. 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. Priorization fields in HEADERS frames subsequent to the first on a stream reprioritize the stream.

The PRIORITY frame (type=0x2) specifies the sender-advised priority of a stream. 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: A single bit flag indicates that the stream dependency is exclusive, see . A 31-bit stream identifier for the stream that this stream depends on, see . An 8-bit weight for the identified stream dependency, see . 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 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. 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. 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 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 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: 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 of type FRAME_SIZE_ERROR. For more info, see 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 of type PROTOCOL_ERROR. The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST be treated as a connection error of type PROTOCOL_ERROR.

The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and an unsigned 32-bit value.

The following parameters are defined: Allows the sender to inform the remote endpoint of the maximum size of the header compression table used to decode header blocks, in octets. The encoder can select any size equal to or less than this value by using signaling specific to the header compression format inside a header block. The initial value is 4,096 octets. This setting can be use to disable server push. An endpoint MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0. An endpoint that has both set this parameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connection error of type PROTOCOL_ERROR. The initial value is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error of type PROTOCOL_ERROR. Indicates the maximum number of concurrent streams that the sender will allow. This limit is directional: it applies to the number of streams that the sender permits the receiver to create. Initially there is no limit to this value. It is recommended that this value be no smaller than 100, so as to not unnecessarily limit parallelism. A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as special by endpoints. A zero value does prevent the creation of new streams, however this can also happen for any limit that is exhausted with active streams. Servers SHOULD only set a zero value for short durations; if a server does not wish to accept requests, closing the connection could be preferable. Indicates the sender's initial window size (in octets) for stream level flow control. The initial value is 216-1 (65,535) octets. This setting affects the window size of all streams, including existing streams, see . Values above the maximum flow control window size of 231-1 MUST be treated as a connection error of type FLOW_CONTROL_ERROR. Indicates the size of the largest frame payload that the sender is willing to receive, in octets. The initial value is 214 (16,384) octets. The value advertised by an endpoint MUST be between this initial value and the maximum allowed frame size (224-1 or 16,777,215 octets), inclusive. Values outside this range MUST be treated as a connection error of type PROTOCOL_ERROR. This advisory setting informs a peer of the maximum size of header list that the sender is prepared to accept, in octets. The value is based on the uncompressed size of header fields, including the length of the name and value in octets plus an overhead of 32 octets for each header field. For any given request, a lower limit than what is advertised MAY be enforced. The initial value of this setting is unlimited. An endpoint that receives a SETTINGS frame with any unknown or unsupported identifier MUST ignore that setting.

Most values in SETTINGS benefit from or require an understanding of when the peer has received and applied the changed parameter values. In order to provide such synchronization timepoints, the recipient of a SETTINGS frame in which the ACK flag is not set MUST apply the updated parameters as soon as possible upon receipt. The values in the SETTINGS frame MUST be processed in the order they appear, with no other frame processing between values. Unsupported parameters MUST be ignored. Once all values have been processed, the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon receiving a SETTINGS frame with the ACK flag set, the sender of the altered parameters can rely on the setting having been applied. If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error of type SETTINGS_TIMEOUT.

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. contains a thorough description of the use of PUSH_PROMISE frames.

The PUSH_PROMISE frame payload has the following fields: 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. A single reserved bit. 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). A header block fragment containing request header fields. Padding octets. The PUSH_PROMISE frame defines the following flags: Bit 3 being set indicates that this frame contains an entire header block 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 of type PROTOCOL_ERROR. Bit 4 being set indicates that the Pad Length field and any padding that it describes 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 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 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 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 (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 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 (that is, an identifier for a stream that is not currently in the "idle" state) as a connection error of type PROTOCOL_ERROR. The PUSH_PROMISE frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames.

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: 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 of type PROTOCOL_ERROR. Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error 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 maintainance. There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last peer-initiated stream which was or might be processed on the sending endpoint in this connection. For instance, if the server sends a GOAWAY frame, the identified stream is the highest numbered stream initiated by the client. If the receiver of the GOAWAY has sent data on streams with a higher stream identifier than what is indicated in the GOAWAY frame, those streams are not or will not be processed. The receiver of the GOAWAY frame can treat the streams as though they had never been created at all, thereby allowing those streams to be retried later on a new connection. Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote 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 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. 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 ); 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 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 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 or connection error 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 (0x7fffffff) 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 of type PROTOCOL_ERROR; errors on the connection flow control window MUST be treated as a connection error. 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 . 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. 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.

Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow control window is a simple integer value that indicates how many octets of data the sender is permitted to transmit; as such, its size is a measure of the buffering capacity of the receiver. Two flow control windows are applicable: the stream flow control window and the connection flow control window. The sender MUST NOT send a flow controlled frame with a length that exceeds the space available in either of the flow control windows advertised by the receiver. Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow control window. For flow control calculations, the 9 octet frame header is not counted. After sending a flow controlled frame, the sender reduces the space available in both windows by the length of the transmitted frame. The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up space in flow control windows. Separate WINDOW_UPDATE frames are sent for the stream and connection level flow control windows. A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount specified in the frame. A sender MUST NOT allow a flow control window to exceed 231-1 octets. If a sender receives a WINDOW_UPDATE that causes a flow control window to exceed this maximum it MUST terminate either the stream or the connection, as appropriate. For streams, the sender sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code; for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code. Flow controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completely asynchronous with respect to each other. This property allows a receiver to aggressively update the window size kept by the sender to prevent streams from stalling.

When an HTTP/2 connection is first established, new streams are created with an initial flow control window size of 65,535 octets. The connection flow control window is 65,535 octets. Both endpoints can adjust the initial window size for new streams by including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms part of the connection preface. The connection flow control window can only be changed using WINDOW_UPDATE frames. Prior to receiving a SETTINGS frame that sets a value for SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default initial window size when sending flow controlled frames. Similarly, the connection flow control window is set to the default initial window size until a WINDOW_UPDATE frame is received. A SETTINGS frame can alter the initial flow control window size for all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream flow control windows that it maintains by the difference between the new value and the old value. A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available space in a flow control window to become negative. A sender MUST track the negative flow control window, and MUST NOT send new flow controlled frames until it receives WINDOW_UPDATE frames that cause the flow control window to become positive. For example, if the client sends 60KB immediately on connection establishment, and the server sets the initial window size to be 16KB, the client will recalculate the available flow control window to be -44KB on receipt of the SETTINGS frame. The client retains a negative flow control window until WINDOW_UPDATE frames restore the window to being positive, after which the client can resume sending. A SETTINGS frame cannot alter the connection flow control window. An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that causes any flow control window to exceed the maximum size as a connection error of type FLOW_CONTROL_ERROR.

A receiver that wishes to use a smaller flow control window than the current size can send a new SETTINGS frame. However, the receiver MUST be prepared to receive data that exceeds this window size, since the sender might send data that exceeds the lower limit prior to processing the SETTINGS frame. After sending a SETTINGS frame that reduces the initial flow control window size, a receiver has two options for handling streams that exceed flow control limits: The receiver can immediately send RST_STREAM with FLOW_CONTROL_ERROR error code for the affected streams. The receiver can accept the streams and tolerate the resulting head of line blocking, sending WINDOW_UPDATE frames as it consumes data.

The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments. 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. The CONTINUATION frame defines the following flag: Bit 3 being set indicates that this frame ends a header block. 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 of type PROTOCOL_ERROR. The CONTINUATION frame changes the connection state as defined in . 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 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 of type PROTOCOL_ERROR.

Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons for the stream or connection error. Error codes share a common code space. Some error codes apply only to either streams or the entire connection and have no defined semantics in the other context. The following error codes are defined: The associated condition is not as a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection. The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available. The endpoint encountered an unexpected internal error. The endpoint detected that its peer violated the flow control protocol. The endpoint sent a SETTINGS frame, but did not receive a response in a timely manner. See Settings Synchronization. The endpoint received a frame after a stream was half closed. The endpoint received a frame with an invalid size. The endpoint refuses the stream prior to performing any application processing, see for details. Used by the endpoint to indicate that the stream is no longer needed. The endpoint is unable to maintain the header compression context for the connection. The connection established in response to a CONNECT request was reset or abnormally closed. The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load. The underlying transport has properties that do not meet minimum security requirements (see ). Unknown or unsupported error codes MUST NOT trigger any special behavior. These MAY be treated by an implementation as being equivalent to INTERNAL_ERROR.

HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means that, from the application perspective, the features of the protocol are largely unchanged. To achieve this, all request and response semantics are preserved, although the syntax of conveying those semantics has changed. Thus, the specification and requirements of HTTP/1.1 Semantics and Content , Conditional Requests , Range Requests , Caching and Authentication are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing , such as the HTTP and HTTPS URI schemes, are also applicable in HTTP/2, but the expression of those semantics for this protocol are defined in the sections below.

A client sends an HTTP request on a new stream, using a previously unused stream identifier. A server sends an HTTP response on the same stream as the request. An HTTP message (request or response) consists of: 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 and ), and one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see ), and zero or more DATA frames containing the message payload (see ), and optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see ). 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. 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. 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. 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.

HTTP/2 removes support for the 101 (Switching Protocols) informational status code (). The semantics of 101 (Switching Protocols) aren't applicable to a multiplexed protocol. Alternative protocols are able to use the same mechanisms that HTTP/2 uses to negotiate their use (see ).

HTTP header fields carry information as a series of key-value pairs. For a listing of registered HTTP headers, see the Message Header Field Registry maintained at .

While HTTP/1.x used the message start-line (see ) to convey the target URI and method of the request, and the status code for the response, HTTP/2 uses special pseudo-header fields beginning with ':' character (ASCII 0x3a) for this purpose. Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT generate pseudo-header fields other than those defined in this document. Pseudo-header fields are only valid in the context in which they are defined. Pseudo-header fields defined for requests MUST NOT appear in responses; pseudo-header fields defined for responses MUST NOT appear in requests. Pseudo-header fields MUST NOT appear in trailers. Endpoints MUST treat a request or response that contains undefined or invalid pseudo-header fields as malformed. Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. A request or response containing uppercase header field names MUST be treated as malformed. All pseudo-header fields MUST appear in the header block before regular header fields. Any request or response that contains a pseudo-header field that appears in a header block after a regular header field MUST be treated as malformed.

HTTP/2 does not use the Connection header field to indicate connection-specific header fields; in this protocol, connection-specific metadata is conveyed by other means. An endpoint MUST NOT generate a HTTP/2 message containing connection-specific header fields; any message containing connection-specific header fields MUST be treated as malformed. This means that an intermediary transforming an HTTP/1.x message to HTTP/2 will need to remove any header fields nominated by the Connection header field, along with the Connection header field itself. Such intermediaries SHOULD also remove other connection-specific header fields, such as Keep-Alive, Proxy-Connection, Transfer-Encoding and Upgrade, even if they are not nominated by Connection. One exception to this is the TE header field, which MAY be present in an HTTP/2 request, but when it is MUST NOT contain any value other than "trailers". HTTP/2 purposefully does not support upgrade to another protocol. The handshake methods described in are believed sufficient to negotiate the use of alternative protocols.

The following pseudo-header fields are defined for HTTP/2 requests: The :method pseudo-header field includes the HTTP method (). The :scheme pseudo-header field includes the scheme portion of the target URI (). :scheme is not restricted to http and https schemed URIs. A proxy or gateway can translate requests for non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services. The :authority pseudo-header field includes the authority portion of the target URI (). The authority MUST NOT include the deprecated userinfo subcomponent for http or https schemed URIs. To ensure that the HTTP/1.1 request line can be reproduced accurately, this pseudo-header field MUST be omitted when translating from an HTTP/1.1 request that has a request target in origin or asterisk form (see ). Clients that generate HTTP/2 requests directly SHOULD use the :authority pseudo-header field instead of the Host header field. An intermediary that converts an HTTP/2 request to HTTP/1.1 MUST create a Host header field if one is not present in a request by copying the value of the :authority pseudo-header field. The :path pseudo-header field includes the path and query parts of the target URI (the path-absolute production from and optionally a '?' character followed by the query production, see and ). A request in asterisk form includes the value '*' for the :path pseudo-header field. This pseudo-header field MUST NOT be empty for http or https URIs; http or https URIs that do not contain a path component MUST include a value of '/'. The exception to this rule is an OPTIONS request for an http or https URI that does not include a path component; these MUST include a :path pseudo-header field with a value of '*' (see ). All HTTP/2 requests MUST include exactly one valid value for the :method, :scheme, and :path pseudo-header fields, unless it is a CONNECT request. An HTTP request that omits mandatory pseudo-header fields is malformed. HTTP/2 does not define a way to carry the version identifier that is included in the HTTP/1.1 request line.

For HTTP/2 responses, a single :status pseudo-header field is defined that carries the HTTP status code field (see ). This pseudo-header field MUST be included in all responses, otherwise the response is malformed. HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.

The Cookie header field can carry a significant amount of redundant data. The Cookie header field uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This header field doesn't follow the list construction rules in HTTP (see ), which prevents cookie-pairs from being separated into different name-value pairs. This can significantly reduce compression efficiency as individual cookie-pairs are updated. To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields, each with one or more cookie-pairs. If there are multiple Cookie header fields after decompression, these MUST be concatenated into a single octet string using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ") before being passed into a non-HTTP/2 context, such as an HTTP/1.1 connection, or a generic HTTP server application.

Therefore, the following two lists of Cookie header fields are semantically equivalent.

A malformed request or response is one that is an otherwise valid sequence of HTTP/2 frames, but is otherwise invalid due to the presence of extraneous frames, prohibited header fields, the absence of mandatory header fields, or the inclusion of uppercase header field names. A request or response that includes an entity body can include a content-length header field. A request or response is also malformed if the value of a content-length header field does not equal the sum of the DATA frame payload lengths that form the body. A response that is defined to have no payload, as described in , can have a non-zero content-length header field, even though no content is included in DATA frames. Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUST NOT forward a malformed request or response. Malformed requests or responses that are detected MUST be treated as a stream error of type PROTOCOL_ERROR. For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response. Note that these requirements are intended to protect against several types of common attacks against HTTP; they are deliberately strict, because being permissive can expose implementations to these vulnerabilities.

This section shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests and responses. An HTTP GET request includes request header fields and no body and is therefore transmitted as a single HEADERS frame, followed by zero or more CONTINUATION frames containing the serialized block of request header fields. The HEADERS frame in the following has both the END_HEADERS and END_STREAM flags set; no CONTINUATION frames are sent:

+ END_STREAM Accept: image/jpeg + END_HEADERS :method = GET :scheme = https :path = /resource host = example.org accept = image/jpeg ]]>

Similarly, a response that includes only response header fields is transmitted as a HEADERS frame (again, followed by zero or more CONTINUATION frames) containing the serialized block of response header fields.

+ END_STREAM Expires: Thu, 23 Jan ... + END_HEADERS :status = 304 etag = "xyzzy" expires = Thu, 23 Jan ... ]]>

An HTTP POST request that includes request header fields and payload data is transmitted as one HEADERS frame, followed by zero or more CONTINUATION frames containing the request header fields, followed by one or more DATA frames, with the last CONTINUATION (or HEADERS) frame having the END_HEADERS flag set and the final DATA frame having the END_STREAM flag set:

- END_STREAM Content-Type: image/jpeg - END_HEADERS Content-Length: 123 :method = POST :path = /resource {binary data} :scheme = https CONTINUATION + END_HEADERS content-type = image/jpeg host = example.org content-length = 123 DATA + END_STREAM {binary data} ]]> Note that data contributing to any given header field could be spread between header block fragments. The allocation of header fields to frames in this example is illustrative only.

A response that includes header fields and payload data is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames, followed by one or more DATA frames, with the last DATA frame in the sequence having the END_STREAM flag set:

- END_STREAM Content-Length: 123 + END_HEADERS :status = 200 {binary data} content-type = image/jpeg content-length = 123 DATA + END_STREAM {binary data} ]]>

Trailing header fields are sent as a header block after both the request or response header block and all the DATA frames have been sent. The HEADERS frame starting the trailers header block has the END_STREAM flag set.

- END_STREAM Transfer-Encoding: chunked + END_HEADERS Trailer: Foo :status = 200 content-length = 123 123 content-type = image/jpeg {binary data} trailer = Foo 0 Foo: bar DATA - END_STREAM {binary data} HEADERS + END_STREAM + END_HEADERS foo = bar ]]>

An informational response using a 1xx status code other than 101 is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames: - END_STREAM + END_HEADERS :status = 103 extension-field = bar ]]>

In HTTP/1.1, an HTTP client is unable to retry a non-idempotent request when an error occurs, because there is no means to determine the nature of the error. It is possible that some server processing occurred prior to the error, which could result in undesirable effects if the request were reattempted. HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has not been processed: The GOAWAY frame indicates the highest stream number that might have been processed. Requests on streams with higher numbers are therefore guaranteed to be safe to retry. The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the stream is being closed prior to any processing having occurred. Any request that was sent on the reset stream can be safely retried. Requests that have not been processed have not failed; clients MAY automatically retry them, even those with non-idempotent methods. A server MUST NOT indicate that a stream has not been processed unless it can guarantee that fact. If frames that are on a stream are passed to the application layer for any stream, then REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame MUST include a stream identifier that is greater than or equal to the given stream identifier. In addition to these mechanisms, the PING frame provides a way for a client to easily test a connection. Connections that remain idle can become broken as some middleboxes (for instance, network address translators, or load balancers) silently discard connection bindings. The PING frame allows a client to safely test whether a connection is still active without sending a request.

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. Pushing additional message exchanges in this fashion 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. Promised requests MUST be cacheable (see ), MUST be safe (see ) and MUST NOT include a request body. Clients that receive a promised request that is not cacheable, unsafe or that includes a request body MUST reset the stream with a stream error of type PROTOCOL_ERROR. Pushed responses that are cacheable (see ) can be stored by the client, if it implements a HTTP cache. Pushed responses are considered successfully validated on the origin server (e.g., if the "no-cache" cache response directive is present) 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. 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 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 of type PROTOCOL_ERROR.

Server push is semantically equivalent to a server responding to a request; however, in this case that request is also sent by the server, as a PUSH_PROMISE frame. The PUSH_PROMISE frame includes a header block that contains a complete set of request header fields that the server attributes to the request. It is not possible to push a response to a request that includes a request body. Pushed responses are always associated with an explicit request from the client. The PUSH_PROMISE frames sent by the server are sent on that explicit request's stream. The PUSH_PROMISE frame also includes a promised stream identifier, chosen from the stream identifiers available to the server (see ). The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid and complete set of request header fields. The server MUST include a method in the :method header field that is safe and cacheable. If a client receives a PUSH_PROMISE that does not include a complete and valid set of header fields, or the :method header field identifies a method that is not safe, it MUST respond with a stream error of type PROTOCOL_ERROR. The server SHOULD send PUSH_PROMISE () frames prior to sending any frames that reference the promised responses. This avoids a race where clients issue requests prior to receiving any PUSH_PROMISE frames. For example, if the server receives a request for a document containing embedded links to multiple image files, and the server chooses to push those additional images to the client, sending push promises before the DATA frames that contain the image links ensures that the client is able to see the promises before discovering embedded links. Similarly, if the server pushes responses referenced by the header block (for instance, in Link header fields), sending the push promises before sending the header block ensures that clients do not request them. PUSH_PROMISE frames MUST NOT be sent by the client. PUSH_PROMISE frames can be sent by the server in response to any client-initiated stream, but the stream MUST be in either the "open" or "half closed (remote)" state with respect to the server. PUSH_PROMISE frames are interspersed with the frames that comprise a response, though they cannot be interspersed with HEADERS and CONTINUATION frames that comprise a single header block. Sending a PUSH_PROMISE frame creates a new stream and puts the stream into the “reserved (local)” state for the server and the “reserved (remote)” state for the client.

After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response on a server-initiated stream that uses the promised stream identifier. The server uses this stream to transmit an HTTP response, using the same sequence of frames as defined in . This stream becomes "half closed" to the client after the initial HEADERS frame is sent. Once a client receives a PUSH_PROMISE frame and chooses to accept the pushed response, the client SHOULD NOT issue any requests for the promised response until after the promised stream has closed. If the client determines, for any reason, that it does not wish to receive the pushed response from the server, or if the server takes too long to begin sending the promised response, the client can send an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes, and referencing the pushed stream's identifier. A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit the number of responses that can be concurrently pushed by a server. Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables server push by preventing the server from creating the necessary streams. This does not prohibit a server from sending PUSH_PROMISE frames; clients need to reset any promised streams that are not wanted. Clients receiving a pushed response MUST validate that either the server is authoritative (see ), or the proxy that provided the pushed response is configured for the corresponding request. For example, a server that offers a certificate for only the example.com DNS-ID or Common Name is not permitted to push a response for https://www.example.org/doc. The response for a PUSH_PROMISE stream begins with a HEADERS frame, which immediately puts the stream into the “half closed (remote)” state for the server and “half closed (local)” state for the client, and ends with a frame bearing END_STREAM, which places the stream in the "closed" state. The client never sends a frame with the END_STREAM flag for a server push.

In HTTP/1.x, the pseudo-method CONNECT () 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 Request Header Fields, 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 ). A proxy that supports CONNECT establishes a TCP connection 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 . 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 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 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.

This section outlines attributes of the HTTP protocol that improve interoperability, reduce exposure to known security vulnerabilities, or reduce the potential for implementation variation.

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, 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, to refresh the keying material for a TLS connection, or to replace connections that have encountered errors. A client MAY open multiple connections to the same IP address and TCP port using different Server Name Indication 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 () frame so that both endpoints can reliably determine whether previously sent frames have been processed and gracefully complete or terminate any necessary remaining tasks.

Connections that are made to an origin servers, either directly or through a tunnel created using the CONNECT method MAY be reused for requests with multiple different URI authority components. A connection can be reused as long as the origin server is authoritative. For http resources, this depends on the host having resolved to the same IP address. For https resources, connection reuse additionally depends on having a certificate that is valid for the host in the URI. An origin server might offer a certificate with multiple subjectAltName attributes, or names with wildcards, one of which is valid for the authority in the URI. For example, a certificate with a subjectAltName of *.example.com might permit the use of the same connection for requests to URIs starting with https://a.example.com/ and https://b.example.com/. In some deployments, reusing a connection for multiple origins can result in requests being directed to the wrong origin server. For example, TLS termination might be performed by a middlebox that uses the TLS Server Name Indication (SNI) extension to select an origin server. This means that it is possible for clients to send confidential information to servers that might not be the intended target for the request, even though the server is otherwise authoritative. A server that does not wish clients to reuse connections can indicate that it is not authoritative for a request by sending a 421 (Misdirected Request) status code in response to the request (see ). A client that is configured to use a proxy over HTTP/2 directs requests to that proxy through a single connection. That is, all requests sent via a proxy reuse the connection to the proxy.

The 421 (Misdirected Request) status code indicates that the request was directed at a server that is not able to produce a response. This can be sent by a server that is not configured to produce responses for the combination of scheme and authority that are included in the request URI. Clients receiving a 421 (Misdirected Request) response from a server MAY retry the request - whether the request method is idempotent or not - over a different connection. This is possible if a connection is reused () or if an alternative service is selected (). This status code MUST NOT be generated by proxies. A 421 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls (see ).

Implementations of HTTP/2 MUST support TLS 1.2 for HTTP/2 over TLS. The general TLS usage guidance in SHOULD be followed, with some additional restrictions that are specific to HTTP/2. An implementation of HTTP/2 over TLS MUST use TLS 1.2 or higher with the restrictions on feature set and cipher suite described in this section. Due to implementation limitations, it might not be possible to fail TLS negotiation. An endpoint MUST immediately terminate an HTTP/2 connection that does not meet these minimum requirements with a connection error of type INADEQUATE_SECURITY.

The TLS implementation MUST support the Server Name Indication (SNI) extension to TLS. HTTP/2 clients MUST indicate the target domain name when negotiating TLS. The TLS implementation MUST disable compression. TLS compression can lead to the exposure of information that would not otherwise be revealed . Generic compression is unnecessary since HTTP/2 provides compression features that are more aware of context and therefore likely to be more appropriate for use for performance, security or other reasons. The TLS implementation MUST disable renegotiation. An endpoint MUST treat a TLS renegotiation as a connection error of type PROTOCOL_ERROR. Note that disabling renegotiation can result in long-lived connections becoming unusable due to limits on the number of messages the underlying cipher suite can encipher. A client MAY use renegotiation to provide confidentiality protection for client credentials offered in the handshake, but any renegotiation MUST occur prior to sending the connection preface. A server SHOULD request a client certificate if it sees a renegotiation request immediately after establishing a connection. This effectively prevents the use of renegotiation in response to a request for a specific protected resource. A future specification might provide a way to support this use case.

The set of TLS cipher suites that are permitted in HTTP/2 is restricted. HTTP/2 MUST only be used with cipher suites that have ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE) or the elliptic curve variant (ECDHE). Ephemeral key exchange MUST have a minimum size of 2048 bits for DHE or security level of 128 bits for ECDHE. Clients MUST accept DHE sizes of up to 4096 bits. HTTP MUST NOT be used with cipher suites that use stream or block ciphers. Authenticated Encryption with Additional Data (AEAD) modes, such as the Galois Counter Model (GCM) mode for AES are acceptable. The effect of these restrictions is that TLS 1.2 implementations could have non-intersecting sets of available cipher suites, since these prevent the use of the cipher suite that TLS 1.2 makes mandatory. To avoid this problem, implementations of HTTP/2 that use TLS 1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 with P256 . Clients MAY advertise support of cipher suites that are prohibited by the above restrictions in order to allow for connection to servers that do not support HTTP/2. This enables a fallback to protocols without these constraints without the additional latency imposed by using a separate connection for fallback.

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

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 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 . 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 ) 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 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 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 of type ENHANCE_YOUR_CALM.

A large header block can cause an implementation to commit a large amount of state. Header fields that are critical for routing can appear toward the end of a header block, which prevents streaming of header fields to their ultimate destination. For this an other reasons, such as ensuring cache correctness, means that an endpoint might need to buffer the entire header block. Since there is no hard limit to the size of a header block, some endpoints could be forced commit a large amount of available memory for header fields. An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to advise peers of limits that might apply on the size of header blocks. This setting is only advisory, so endpoints MAY choose to send header blocks that exceed this limit and risk having the request or response being treated as malformed. This setting specific to a connection, so any request or response could encounter a hop with a lower, unknown limit. An intermediary can attempt to avoid this problem by passing on values presented by different peers, but they are not obligated to do so. A server that receives a larger header block than it is willing to handle can send an HTTP 431 (Request Header Fields Too Large) status code . A client can discard responses that it cannot process. The header block MUST be processed to ensure a consistent connection state, unless the connection is closed.

HTTP/2 enables greater use of compression for both header fields () 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., ). 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 (). Further considerations regarding the compression of header fields are described in .

Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might be provided by TLS. 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, ). 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 .

A string for identifying HTTP/2 is entered into the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in . This document establishes a registry for frame types, settings, and error codes. These new registries are entered into a new "Hypertext Transfer Protocol (HTTP) 2 Parameters" section. This document registers the HTTP2-Settings header field for use in HTTP; and the 421 (Misdirected Request) status code. This document registers the PRI method for use in HTTP, to avoid collisions with the connection preface.

This document creates two registrations for the identification of HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in . The "h2" string identifies HTTP/2 when used over TLS: HTTP/2 over TLS 0x68 0x32 ("h2") This document The "h2c" string identifies HTTP/2 when used over cleartext TCP: HTTP/2 over TCP 0x68 0x32 0x63 ("h2c") 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 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: A name or label for the frame type. The 8-bit code assigned to the frame type. 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. Frame Type Code Section DATA0x0 HEADERS0x1 PRIORITY0x2 RST_STREAM0x3 SETTINGS0x4 PUSH_PROMISE0x5 PING0x6 GOAWAY0x7 WINDOW_UPDATE0x8 CONTINUATION0x9

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 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: A symbolic name for the setting. Specifying a setting name is optional. The 16-bit code assigned to the setting. An initial value for the setting. An optional reference to a specification that describes the use of the setting. An initial set of setting registrations can be found in . Name Code Initial Value Specification HEADER_TABLE_SIZE 0x14096 ENABLE_PUSH 0x21 MAX_CONCURRENT_STREAMS 0x3(infinite) INITIAL_WINDOW_SIZE 0x465535 MAX_FRAME_SIZE 0x516384 MAX_HEADER_LIST_SIZE 0x6(infinite)

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. 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: A name for the error code. Specifying an error code name is optional. The 32-bit error code value. A brief description of the error code semantics, longer if no detailed specification is provided. An optional reference for a specification that defines the error code. The entries in the following table are registered by this document. Name Code Description Specification NO_ERROR0x0 Graceful shutdown PROTOCOL_ERROR0x1 Protocol error detected INTERNAL_ERROR0x2 Implementation fault FLOW_CONTROL_ERROR0x3 Flow control limits exceeded SETTINGS_TIMEOUT0x4 Settings not acknowledged STREAM_CLOSED0x5 Frame received for closed stream FRAME_SIZE_ERROR0x6 Frame size incorrect REFUSED_STREAM0x7 Stream not processed CANCEL0x8 Stream cancelled COMPRESSION_ERROR0x9 Compression state not updated CONNECT_ERROR0xa TCP connection error for CONNECT method ENHANCE_YOUR_CALM0xb Processing capacity exceeded INADEQUATE_SECURITY0xc Negotiated TLS parameters not acceptable

This section registers the HTTP2-Settings header field in the Permanent Message Header Field Registry. HTTP2-Settings http standard IETF of this document 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 (). PRI No No of this document 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 Hypertext Transfer Protocol (HTTP) Status Code Registry (). 421 Misdirected Request of this document

This document includes substantial input from the following individuals: Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors). Gabriel Montenegro and Willy Tarreau (Upgrade mechanism). William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, Rob Trace (Flow control). Mike Bishop (Extensibility). Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, Herve Ruellan (Substantial editorial contributions). Kari Hurtta, Tatsuhiro Tsujikawa, Greg Wilkins, Poul-Henning Kamp. Alexey Melnikov was an editor of this document during 2013. A substantial proportion of Martin's contribution was supported by Microsoft during his employment there.

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This section is to be removed by RFC Editor before publication.

Renamed Not Authoritative status code to Misdirected Request.

Pseudo-header fields are now required to appear strictly before regular ones. Restored 1xx series status codes, except 101. Changed frame length field 24-bits. Expanded frame header to 9 octets. Added a setting to limit the damage. Added a setting to advise peers of header set size limits. Removed segments. Made non-semantic-bearing HEADERS frames illegal in the HTTP mapping.

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 . Replaced abstract and introduction. Added section on starting HTTP/2.0, including upgrade mechanism. Removed unused references. Added flow control principles based on .

Adopted as base for draft-ietf-httpbis-http2. Updated authors/editors list. Added status note.