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

The maximum size of a frame payload varies by frame type. The absolute maximum size of a frame is 2

Certain frame types, such as Section 6.7), impose additional limits on the amount of payload data allowed. Likewise, additional size limits can be set by specific application uses (see Section 9).

If a frame size exceeds any defined limit, or is too small to contain mandatory frame data, the endpoint MUST send a connection error [ConnectionErrorHandler].

4.3. Header Compression and Decompression

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

Header sets are collections of zero or more header fields. When transmitted over a connection, a header set is serialized into a header block using HTTP Header Compression [COMPRESSION]. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS [HEADERS], PUSH_PROMISE [PUSH_PROMISE] or CONTINUATION [CONTINUATION] frames.

HTTP Header Compression does not preserve the relative ordering of header fields. Header fields with multiple values are encoded into a single header field using a special delimiter, see Section 8.1.3.3.

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

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

Header block fragments can only be sent as the payload of connection error [ConnectionErrorHandler] of type

5. Streams and Multiplexing

5.1. Stream States

                       +--------+
                 PP    |        |    PP
              ,--------|  idle  |--------.
             /         |        |         \
            v          +--------+          v
     +----------+          |           +----------+
     |          |          | H         |          |
 ,---| reserved |          |           | reserved |---.
 |   | (local)  |          v           | (remote) |   |
 |   +----------+      +--------+      +----------+   |
 |      |          ES  |        |  ES          |      |
 |      | H    ,-------|  open  |-------.      | H    |
 |      |     /        |        |        \     |      |
 |      v    v         +--------+         v    v      |
 |   +----------+          |           +----------+   |
 |   |   half   |          |           |   half   |   |
 |   |  closed  |          | R         |  closed  |   |
 |   | (remote) |          |           | (local)  |   |
 |   +----------+          |           +----------+   |
 |        |                v                 |        |
 |        |  ES / R    +--------+  ES / R    |        |
 |        `----------->|        |<-----------'        |
 |  R                  | closed |                  R  |
 `-------------------->|        |<--------------------'
                       +--------+

Figure 1: Stream States

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

Streams have the following states:

In the absence of more specific guidance elsewhere in this document, implementations SHOULD treat the receipt of a message that is not expressly permitted in the description of a state as a connection error [ConnectionErrorHandler] of type

5.1.1. Stream Identifiers

Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numbered stream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A stream identifier of zero (0x0) is used for connection control messages; the stream identifier zero MUST NOT be used to establish a new stream.

A stream identifier of one (0x1) is used to respond to the HTTP/1.1 request which was specified during Upgrade (see Section 3.2). 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 connection error [ConnectionErrorHandler] of type

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

Stream identifiers cannot be reused. Long-lived connections can result in 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.

5.1.2. Stream Concurrency

A peer can limit the number of concurrently active streams using the

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 Section 6.5.2).

Streams in either of the "reserved" states do not count as open, even if a small amount of application state is retained to ensure that the promised stream can be successfully used.

5.2. Flow Control

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

HTTP/2 provides for flow control through use of the

5.2.1. Flow Control Principles

HTTP/2 stream flow control aims to allow for future improvements to flow control algorithms without requiring protocol changes. Flow control in HTTP/2 has the following characteristics:

Flow control is hop-by-hop, not end-to-end.
Flow control is based on window update frames. Receivers advertise how many bytes 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 bytes 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
Flow control cannot be disabled.
HTTP/2 standardizes only the format of the Section 6.9). This does not stipulate how a receiver decides when to send this frame or the value that it sends. Nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs.

Implementations are also responsible for managing how requests and responses are sent based on priority; choosing how to avoid head of line blocking for requests; and managing the creation of new streams. Algorithm choices for these could interact with any flow control algorithm.

5.2.2. Appropriate Use of Flow Control

Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxy needs to share memory between many connections, and also might have a slow upstream connection and a fast downstream one. Flow control addresses cases where the receiver is unable 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 of the maximum size, incrementing the available space when new data is received. Sending data is always subject to the flow control window advertised by the receiver.

Deployments with constrained resources (for example, memory) MAY 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 [RFC1323]).

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

5.3. Stream priority

The endpoint establishing a new stream can assign a priority for the stream. Priority is represented as an unsigned 31-bit integer. 0 represents the highest priority and 2

The purpose of this value is to allow an endpoint to express the relative priority of a stream. An endpoint can use this information to preferentially allocate resources to a stream. Within HTTP/2, priority can be used to select streams for transmitting frames when there is limited capacity for sending. For instance, an endpoint might enqueue frames for all concurrently active streams. As transmission capacity becomes available, frames from higher priority streams might be sent before lower priority streams.

Explicitly setting the priority for a stream does not guarantee any particular processing or transmission order for the stream relative to any other stream. Nor is there any mechanism provided by which the initiator of a stream can force or require a receiving endpoint to process concurrent streams in a particular order.

Unless explicitly specified in the Section 6.2) during stream creation, the default stream priority is 2

Pushed streams [PushResources] have a lower priority than their associated stream. The promised stream inherits the priority value of the associated stream plus one, up to a maximum of 2

5.4. Error Handling

HTTP/2 framing permits two classes of error:

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

A list of error codes is included in Section 7.

5.4.1. Connection Error Handling

A connection error is any error which prevents further processing of the framing layer or which corrupts any connection state.

An endpoint that encounters a connection error SHOULD first send a Section 6.8) with the stream identifier of the last stream that it successfully received from its peer. The

It is possible that the

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

5.4.2. Stream Error Handling

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

An endpoint that detects a stream error sends a Section 6.4) that contains the stream identifier of the stream where the error occurred. The

A header compression [HeaderBlock]).

Normally, an endpoint SHOULD NOT send more than one

An endpoint MUST NOT send a

5.4.3. Connection Termination

If the TCP connection is torn down while streams remain in open or half closed states, then the endpoint MUST assume that the stream was abnormally interrupted and could be incomplete.

6. Frame Definitions

This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frame type serves a distinct purpose either in the establishment and management of the connection as a whole, or of individual streams.

The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain a synchronized view of the connection state, successful communication within the connection will no longer be possible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected by the use any given frame. Accordingly, while it is expected that new frame types will be introduced by extensions to this protocol, only frames defined by this document are permitted to alter the connection state.

6.1. DATA

DATA frames (type=0x0) convey arbitrary, variable-length sequences of octets associated with a stream. One or more DATA frames are used, for instance, to carry HTTP request or response payloads.

DATA frames MAY also contain arbitrary padding. Padding can be added to DATA frames to hide the size of messages.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | [Pad High(8)] | [Pad Low (8)] |          Data (*)             .
 +---------------+---------------+-------------------------------+
 .                            Data (*)                         ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

DATA Frame Payload

The DATA frame contains the following fields:

Pad High:: An 8-bit field containing an amount of padding in units of 256 octets. This field is optional and is only present if the PAD_HIGH flag is set. This field, in combination with Pad Low, determines how much padding there is on a frame.
Pad Low:: An 8-bit field containing an amount of padding in units of single octets. This field is optional and is only present if the PAD_LOW flag is set. This field, in combination with Pad High, determines how much padding there is on a frame.
Data:: Application data. The amount of data is the remainder of the frame payload after subtracting the length of the other fields that are present.
Padding:: Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending and ignored when receiving.

The DATA frame defines the following flags:

END_STREAM (0x1):: Bit 1 being set indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half closed" states or the "closed" state [StreamStates].
END_SEGMENT (0x2):: Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames.
PAD_LOW (0x10):: Bit 5 being set indicates that the Pad Low field is present.
PAD_HIGH (0x20):: Bit 6 being set indicates that the Pad High field is present. This bit MUST NOT be set unless the PAD_LOW flag is also set. Endpoints that receive a frame with PAD_HIGH set and PAD_LOW cleared MUST treat this as a connection error [ConnectionErrorHandler] of type

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 [ConnectionErrorHandler] of type

DATA frames are subject to flow control and can only be sent when a stream is in the "open" or "half closed (remote)" states. Padding is not excluded from flow control. 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 [StreamErrorHandler] of type

The total number of padding octets is determined by multiplying the value of the Pad High field by 256 and adding the value of the Pad Low field. Both Pad High and Pad Low fields assume a value of zero if absent. If the length of the padding is greater than the length of the remainder of the frame payload, the recipient MUST treat this as a connection error [ConnectionErrorHandler] of type

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

Use of padding is a security feature; as such, its use demands some care, see Section 10.6.

6.2. HEADERS

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

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | [Pad High(8)] | [Pad Low (8)] |X|      [Priority (31)]      ...
 +---------------+---------------+-+-----------------------------+
 ...[Priority]                   | Header Block Fragment (*)   ...
 +-------------------------------+-------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

HEADERS Frame Payload

The HEADERS frame payload has the following fields:

Pad High:: Padding size high bits. This field is only present if the PAD_HIGH flag is set.
Pad Low:: Padding size low bits. This field is only present if the PAD_LOW flag is set.
X:: A single reserved bit. This field is optional and is only present if the PRIORITY flag is set.
Priority:: Prioritization information for the stream, see Section 5.3. This field is optional and is only present if the PRIORITY flag is set.
Header Block Fragment:: A header block fragment [HeaderBlock].
Padding:: Padding octets.

The HEADERS frame defines the following flags:

The payload of a HEADERS frame contains a header block fragment [HeaderBlock]. A header block that does not fit within a HEADERS frame is continued in a CONTINUATION frame [CONTINUATION].

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 [ConnectionErrorHandler] of type

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

The HEADERS frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames [DATA].

6.3. PRIORITY

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

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|                        Priority (31)                        |
 +-+-------------------------------------------------------------+

PRIORITY Frame Payload

The payload of a PRIORITY frame contains a single reserved bit and a 31-bit priority.

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 [ConnectionErrorHandler] of type

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

6.4. RST_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.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Error Code (32)                        |
 +---------------------------------------------------------------+

RST_STREAM Frame Payload

The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code [ErrorCodes]. The error code indicates why the stream is being terminated.

The RST_STREAM frame does not define any flags.

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

RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received with a stream identifier of 0x0, the recipient MUST treat this as a connection error [ConnectionErrorHandler] of type

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 [ConnectionErrorHandler] of type

6.5. SETTINGS

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate. The parameters are either constraints on peer behavior or preferences.

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

SETTINGS frames MUST be sent at the start of a connection, and MAY be sent at any other time by either endpoint over the lifetime of the connection.

Implementations MUST support all of the settings defined by this specification and MAY support additional settings defined by extensions to this protocol. Unsupported or unrecognized settings MUST be ignored. New settings MUST NOT be defined or implemented in a way that requires endpoints to understand them in order to communicate successfully.

Each setting in a SETTINGS frame replaces the existing value for that setting. Settings are processed in the order in which they appear, and a receiver of a SETTINGS frame does not need to maintain any state other than the current value of settings. Therefore, the value of a setting is the last value that is seen by a receiver. This permits the inclusion of the same settings multiple times in the same SETTINGS frame, though doing so does nothing other than waste connection capacity.

The SETTINGS frame defines the following flag:

ACK (0x1):: Bit 1 being set indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error [ConnectionErrorHandler] of type Settings Synchronization [SettingsSync].

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a settings frame MUST be zero. If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error [ConnectionErrorHandler] of type

The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST be treated as a connection error [ConnectionErrorHandler] of type

6.5.1. Setting Format

The payload of a SETTINGS frame consists of zero or more settings. Each setting consists of an unsigned 8-bit setting identifier, and an unsigned 32-bit value.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Identifier (8) |                 Value (32)                  ...
 +---------------+-----------------------------------------------+
 ...Value        |
 +---------------+

Setting Format

6.5.2. Defined Settings

The following settings are defined:

An endpoint that receives a SETTINGS frame with any other setting identifier MUST treat this as a connection error [ConnectionErrorHandler] of type

6.5.3. Settings Synchronization

Most values in SETTINGS benefit from or require an understanding of when the peer has received and applied the changed setting 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 settings as soon as possible upon receipt.

The values in the SETTINGS frame MUST be applied in the order they appear, with no other frame processing between values. Once all values have been applied, the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. The sender of altered settings applies changes upon receiving a SETTINGS frame with the ACK flag set.

If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error [ConnectionErrorHandler] of type

6.6. PUSH_PROMISE

The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a set of headers that provide additional context for the stream. Section 8.2 contains a thorough description of the use of PUSH_PROMISE frames.

PUSH_PROMISE MUST NOT be sent if the

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|                Promised-Stream-ID (31)                      |
 +-+-------------------------------------------------------------+
 |                 Header Block Fragment (*)                   ...
 +---------------------------------------------------------------+

PUSH_PROMISE Payload Format

The payload of a PUSH_PROMISE includes a "Promised-Stream-ID". This unsigned 31-bit integer identifies the stream the endpoint intends to start sending frames for. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier [StreamIdentifiers]).

Following the "Promised-Stream-ID" is a header block fragment [HeaderBlock].

PUSH_PROMISE frames MUST be associated with an existing, peer-initiated stream. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error [ConnectionErrorHandler] of type

The PUSH_PROMISE frame defines the following flags:

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

Recipients of PUSH_PROMISE frames can choose to reject promised streams by returning a

The PUSH_PROMISE frame modifies the connection state as defined in Section 4.3.

A PUSH_PROMISE frame modifies the connection state in two ways. The inclusion of a header block [HeaderBlock] potentially modifies the compression state. 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 [StreamIdentifiers] (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 [ConnectionErrorHandler] of type illegal stream identifier [StreamIdentifiers] (that is, an identifier for a stream that is not currently in the "idle" state) as a connection error [ConnectionErrorHandler] of type

6.7. PING

The PING frame (type=0x6) is a mechanism for measuring a minimal round-trip time from the sender, as well as determining whether an idle connection is still functional. PING frames can be sent from any endpoint.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                      Opaque Data (64)                         |
 |                                                               |
 +---------------------------------------------------------------+

PING Payload Format

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

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

The PING frame defines the following flags:

ACK (0x1):: Bit 1 being set indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.

PING frames are not associated with any individual stream. If a PING frame is received with a stream identifier field value other than 0x0, the recipient MUST respond with a connection error [ConnectionErrorHandler] of type

Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error [ConnectionErrorHandler] of type

6.8. GOAWAY

The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this connection. GOAWAY can be sent by either the client or the server. Once sent, the sender will ignore frames sent on new streams for the remainder of the connection. 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 (perhaps for a reboot or maintenance), while still finishing processing of previously established streams.

There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last stream which was processed on the sending endpoint in this connection. If the receiver of the GOAWAY used streams that are newer than the indicated stream identifier, they were not processed by the sender and the receiver may treat the streams as though they had never been created at all (hence the receiver may want to re-create the streams 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 where it stopped working. An endpoint might choose to close a connection without sending GOAWAY for misbehaving peers.

After sending a GOAWAY frame, the sender can discard frames for new streams. However, any frames that alter connection state cannot be completely ignored. For instance, Section 4.3); similarly DATA frames MUST be counted toward the connection flow control window.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|                  Last-Stream-ID (31)                        |
 +-+-------------------------------------------------------------+
 |                      Error Code (32)                          |
 +---------------------------------------------------------------+
 |                  Additional Debug Data (*)                    |
 +---------------------------------------------------------------+

GOAWAY Payload Format

The GOAWAY frame does not define any flags.

The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a connection error [ConnectionErrorHandler] of type

The last stream identifier in the GOAWAY frame contains the highest numbered stream identifier for which the sender of the GOAWAY frame has received frames on and might have taken some action on. All streams up to and including the identified stream might have been processed in some way. The last stream identifier is set to 0 if no streams were processed.

Note: In this case, "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, this value is effectively the highest 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.

The last stream ID MUST be 0 if no streams were acted upon.

If an endpoint maintains the connection and continues to exchange frames, ignored frames MUST be counted toward flow control limits [FlowControl] or update header compression state [HeaderBlock]. Otherwise, flow control or header compression state can become unsynchronized.

The GOAWAY frame also contains a 32-bit error code [ErrorCodes] that contains the reason for closing the connection.

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

6.9. WINDOW_UPDATE

The WINDOW_UPDATE frame (type=0x8) is used to implement flow control.

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 stream error [StreamErrorHandler] or connection error [ConnectionErrorHandler] of type

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |X|              Window Size Increment (31)                     |
 +-+-------------------------------------------------------------+

WINDOW_UPDATE Payload Format

The payload of a WINDOW_UPDATE frame is one reserved bit, plus an unsigned 31-bit integer indicating the number of bytes that the sender can transmit in addition to the existing flow control window. The legal range for the increment to the flow control window is 1 to 2

The WINDOW_UPDATE frame does not define any flags.

The WINDOW_UPDATE frame can be specific to a stream or to the entire connection. In the former case, the frame's stream identifier indicates the affected stream; in the latter, the value "0" indicates that the entire connection is the subject of the frame.

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

A receiver that receives a flow controlled frame MUST always account for its contribution against the connection flow control window, unless the receiver treats this as a connection error [ConnectionErrorHandler]. 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.

6.9.1. The Flow Control Window

Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow control window is a simple integer value that indicates how many bytes of data the sender is permitted to transmit; as such, its size is a measure of the buffering capability 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 (for example, an empty data frame) MAY be sent if there is no available space in either flow control window.

For flow control calculations, the 8 byte 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 2

Flow controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completely asynchronous with respect to each other. This property allows a receiver to aggressively update the window size kept by the sender to prevent streams from stalling.

6.9.2. Initial Flow Control Window Size

When a HTTP/2 connection is first established, new streams are created with an initial flow control window size of 65,535 bytes. The connection flow control window is 65,535 bytes. Both endpoints can adjust the initial window size for new streams by including a value for

Prior to receiving a

An endpoint MUST treat a change to connection error [ConnectionErrorHandler] of type

A change to

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

6.9.3. Reducing the Stream Window Size

A receiver that wishes to use a smaller flow control window than the current size can send a new

A receiver has two options for handling streams that exceed flow control limits:

The receiver can immediately send
The receiver can accept the streams and tolerate the resulting head of line blocking, sending WINDOW_UPDATE frames as it consumes data.

If a receiver decides to accept streams, both sides MUST recompute the available flow control window based on the initial window size sent in the

6.10. CONTINUATION

The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments [HeaderBlock]. Any number of CONTINUATION frames can be sent on an existing stream, as long as the preceding frame on the same stream is one of

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | [Pad High(8)] | [Pad Low (8)] |  Header Block Fragment (*)    .
 +---------------+---------------+-------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

CONTINUATION Frame Payload

The CONTINUATION frame payload has the following fields:

Pad High:: Padding size high bits. This field is only present if the PAD_HIGH flag is set.
Pad Low:: Padding size low bits. This field is only present if the PAD_LOW flag is set.
Header Block Fragment:: A header block fragment [HeaderBlock].
Padding:: Padding octets.

The CONTINUATION frame defines the following flags:

The payload of a CONTINUATION frame contains a header block fragment [HeaderBlock].

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

CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error [ConnectionErrorHandler] of type PROTOCOL_ERROR.

A CONTINUATION frame MUST be preceded by a connection error [ConnectionErrorHandler] of type

The CONTINUATION frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames [DATA].

7. Error Codes

Error codes are 32-bit fields that are used in

Error codes share a common code space. Some error codes only apply to specific conditions and have no defined semantics in certain frame types.

The following error codes are defined:

NO_ERROR (0):: The associated condition is not as a result of an error. For example, a
PROTOCOL_ERROR (1):: The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available.
INTERNAL_ERROR (2):: The endpoint encountered an unexpected internal error.
FLOW_CONTROL_ERROR (3):: The endpoint detected that its peer violated the flow control protocol.
SETTINGS_TIMEOUT (4):: The endpoint sent a SETTINGS frame, but did not receive a response in a timely manner. See Settings Synchronization [SettingsSync].
STREAM_CLOSED (5):: The endpoint received a frame after a stream was half closed.
FRAME_SIZE_ERROR (6):: The endpoint received a frame that was larger than the maximum size that it supports.
REFUSED_STREAM (7):: The endpoint refuses the stream prior to performing any application processing, see Section 8.1.4 for details.
CANCEL (8):: Used by the endpoint to indicate that the stream is no longer needed.
COMPRESSION_ERROR (9):: The endpoint is unable to maintain the compression context for the connection.
CONNECT_ERROR (10):: The connection established in response to a CONNECT request [CONNECT] was reset or abnormally closed.
ENHANCE_YOUR_CALM (11):: The endpoint detected that its peer is exhibiting a behavior over a given amount of time that has caused it to refuse to process further frames.
INADEQUATE_SECURITY (12):: The underlying transport has properties that do not meet the minimum requirements imposed by this document (see Section 9.2) or the endpoint.

8. HTTP Message Exchanges

HTTP/2 is intended to be as compatible as possible with current web-based applications. This means that, from the perspective of the server business logic or application API, the features of HTTP are unchanged. To achieve this, all of the application request and response header semantics are preserved, although the syntax of conveying those semantics has changed. Thus, the rules from HTTP/1.1 ([HTTP-p1], [HTTP-p2], [HTTP-p4], [HTTP-p5], [HTTP-p6], and [HTTP-p7]) apply with the changes in the sections below.

8.1. HTTP Request/Response Exchange

A client sends an HTTP request on a new stream, using a previously unused stream identifier [StreamIdentifiers]. A server sends an HTTP response on the same stream as the request.

An HTTP request or response each consist of:

a
one contiguous sequence of zero or more
zero or more
optionally, a contiguous sequence that starts with a

The last frame in the sequence bears an END_STREAM flag, though a

Other frames MAY be interspersed with these frames, but those frames do not carry HTTP semantics. In particular,

Trailing header fields are carried in a header block that also terminates the stream. That is, a sequence starting with a

An HTTP request/response exchange fully consumes a single stream. A request starts with the

8.1.1. Informational Responses

The 1xx series of HTTP response status codes ([HTTP-p2]) are not supported in HTTP/2.

The most common use case for 1xx is using a Expect header field with a 100-continue token (colloquially, "Expect/continue") to indicate that the client expects a 100 (Continue) non-final response status code, receipt of which indicates that the client should continue sending the request body if it has not already done so.

Typically, Expect/continue is used by clients wishing to avoid sending a large amount of data in a request body, only to have the request rejected by the origin server.

HTTP/2 does not enable the Expect/continue mechanism; if the server sends a final status code to reject the request, it can do so without making the underlying connection unusable.

Note that this means HTTP/2 clients sending requests with bodies may waste at least one round trip of sent data when the request is rejected. This can be mitigated by restricting the amount of data sent for the first round trip by bandwidth-constrained clients, in anticipation of a final status code.

Other defined 1xx status codes are not applicable to HTTP/2; the semantics of 101 (Switching Protocols) is better expressed using a distinct frame type, since they apply to the entire connection, not just one stream. Likewise, 102 (Processing) is no longer necessary, because HTTP/2 has a separate means of keeping the connection alive.

This difference between protocol versions necessitates special handling by intermediaries that translate between them:

An intermediary that gateways HTTP/1.1 to HTTP/2 MUST generate a 100 (Continue) response if a received request includes and Expect header field with a 100-continue token ([HTTP-p2]), unless it can immediately generate a final status code. It MUST NOT forward the 100-continue expectation in the request header fields.
An intermediary that gateways HTTP/2 to HTTP/1.1 MAY add an Expect header field with a 100-continue expectation when forwarding a request that has a body; see [HTTP-p2] for specific requirements.
An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all other 1xx informational responses.

8.1.2. Examples

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 contiguous sequence of

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

Similarly, a response that includes only response header fields is transmitted as a sequence of

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

An HTTP POST request that includes request header fields and payload data is transmitted as one

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

                                 DATA
                                   + END_STREAM
                                     {binary data}

A response that includes header fields and payload data is transmitted as a

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

                                 DATA
                                   + END_STREAM
                                     {binary data}

Trailing header fields are sent as a header block after both the request or response header block and all the

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

                                HEADERS
                                  + END_STREAM
                                  + END_HEADERS
                                    foo: bar

8.1.3. HTTP Header Fields

HTTP/2 request and response header fields carry information as a series of key-value pairs. This includes the target URI for the request, the status code for the response, as well as HTTP header fields.

HTTP header field names are strings of ASCII characters that are compared in a case-insensitive fashion. 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 [malformed].

HTTP/2 does not use the Connection header field to indicate "hop-by-hop" header fields; in this protocol, connection-specific metadata is conveyed by other means. As such, a HTTP/2 message containing Connection MUST be treated as malformed [malformed].

This means that an intermediary transforming a 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 a HTTP/2 request, but when it is MUST NOT contain any value other than "trailers".

Note:: HTTP/2 purposefully does not support upgrade to another protocol. The handshake methods described in Section 3 are believed sufficient to negotiate the use of alternative protocols.

8.1.3.1. Request Header Fields

HTTP/2 defines a number of header fields starting with a colon ':' character that carry information about the request target: :method, :scheme, and :path header fields, unless this is a CONNECT request [CONNECT]. An HTTP request that omits mandatory header fields is malformed [malformed].

All HTTP/2 requests MUST include exactly one valid value for the

Header field names that start with a colon are only valid in the HTTP/2 context. These are not HTTP header fields. Implementations MUST NOT generate header fields that start with a colon, but they MUST ignore any header field that starts with a colon. In particular, header fields with names starting with a colon MUST NOT be exposed as HTTP header fields.

HTTP/2 does not define a way to carry the version identifier that is included in the HTTP/1.1 request line.

8.1.3.2. Response Header Fields

A single :status header field is defined that carries the HTTP status code field (see [HTTP-p2]). This header field MUST be included in all responses, otherwise the response is malformed [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.

8.1.3.3. Header Field Ordering

HTTP Header Compression [COMPRESSION] does not preserve the order of header fields. The relative order of header fields with different names is not important. However, the same header field can be repeated to form a comma-separated list (see [HTTP-p1]), where the relative order of header field values is significant. This repetition can occur either as a single header field with a comma-separated list of values, or as several header fields with a single value, or any combination thereof.

To preserve the order of a comma-separated list, the ordered values for a single header field name appearing in different header fields are concatenated into a single value. A zero-valued octet (0x0) is used to delimit multiple values.

After decompression, header fields that have values containing zero octets (0x0) MUST be split into multiple header fields before being processed.

Header fields containing multiple values MUST be concatenated into a single value unless the ordering of that header field is known to be not significant.

The special case of set-cookie - which does not form a comma-separated list, but can have multiple values - does not depend on ordering. The set-cookie header field MAY be encoded as multiple header field values, or as a single concatenated value.

8.1.3.4. Compressing the Cookie Header Field

The Cookie header field [COOKIE] 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 [HTTP-p1]), 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 "; ").

The Cookie header field MAY be split using a zero octet (0x0), as defined in Section 8.1.3.3. When decoding, zero octets MUST be replaced with the cookie delimiter ("; ").

8.1.3.5. Malformed Requests and Responses

A malformed request or response is one that uses a valid sequence of HTTP/2 frames, but is otherwise invalid due to the presence of 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

Intermediaries that process HTTP requests or responses (i.e., all intermediaries other than those acting as tunnels) MUST NOT forward a malformed request or response.

Implementations that detect malformed requests or responses need to ensure that the stream ends. For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response.

8.1.4. Request Reliability Mechanisms in HTTP/2

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:

Clients MUST NOT treat requests that have not been processed as having failed. Clients MAY automatically retry these requests, including 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

In addition to these mechanisms, the

8.2. Server Push

HTTP/2 enables a server to pre-emptively send (or "push") multiple associated resources to a client in response to a single request. This feature becomes particularly helpful when the server knows the client will need to have those resources available in order to fully process the originally requested resource.

Pushing additional resources is optional, and is negotiated only between individual endpoints. The

A client cannot push resources. Clients and servers MUST operate as though the server has disabled connection error [ConnectionErrorHandler]. Clients MUST reject any attempt to change this setting by treating the message as a connection error [ConnectionErrorHandler] of type

A server can only push requests that are safe (see [HTTP-p2]), cacheable (see [HTTP-p6]) and do not include a request body.

8.2.1. Push Requests

Server push is semantically equivalent to a server responding to a request. The

Pushed resources are always associated with an explicit request from a client. The Section 5.1.1).

The header fields in request header fields [HttpRequest]. The server MUST include a method in the :method header field that is safe and cacheable. If a client receives a :method header field identifies a method that is not safe, it MUST respond with a stream error [StreamErrorHandler] of type

The server SHOULD send Section 6.6) frames prior to sending any frames that reference the promised resources. This avoids a race where clients issue requests for resources prior to receiving any

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

8.2.2. Push Responses

After sending the response [HttpResponse] 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 Section 8.1. This stream becomes "half closed" to the client [StreamStates] after the initial

Once a client receives a

If the client determines, for any reason, that it does not wish to receive the pushed resource from the server, or if the server takes too long to begin sending the promised resource, the client can send an

A client can use the

Clients receiving a pushed response MUST validate that the server is authorized to push the resource using the same-origin policy ([RFC6454]). For example, a HTTP/2 connection to example.com is generally www.example.org.

8.3. The CONNECT Method

The HTTP pseudo-method CONNECT ([HTTP-p2]) is used to convert an HTTP/1.1 connection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLS session with a 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. The HTTP header field mapping works as mostly as defined in Request Header Fields [HttpRequest], 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 [HTTP-p1]).

A proxy that supports CONNECT, establishes a TCP connection [TCP] to the server identified in the :authority header field. Once this connection is successfully established, the proxy sends a [HTTP-p2].

After the initial stream error [StreamErrorHandler] if received.

The TCP connection can be closed by either peer. The END_STREAM flag on a

A TCP connection error is signaled with stream error [StreamErrorHandler] of type

9. Additional HTTP Requirements/Considerations

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

9.1. Connection Management

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 origin ([RFC6454]) concurrently. A client can create additional connections as replacements, either to replace connections that are near to exhausting the available stream identifiers [StreamIdentifiers], or to replace connections that have encountered errors [ConnectionErrorHandler].

Clients MAY use a single connection for more than one origin when each origin's hostname resolves to the same IP address, and they share the same port. For "https" scheme origins, the server's certificate MUST be valid for each origin's hostname. The considerations in RFC 6125 [RFC6125] for verification of identity apply.

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

9.2. Use of TLS Features

Implementations of HTTP/2 MUST support TLS 1.2 [TLS12]. The general TLS usage guidance in [TLSBCP] SHOULD be followed, with some additional restrictions that are specific to HTTP/2.

The TLS implementation MUST support the Server Name Indication (SNI) [TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target domain name when negotiating TLS.

The TLS implementation MUST disable compression. TLS compression can lead to the exposure of information that would not otherwise be revealed [RFC3749]. 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.

Implementations MUST negotiate ephemeral cipher suites (DHE or ECDHE) with a minimum size of 2048 bits (DHE) or security level of 128 bits (ECDHE). Clients MUST accept DHE sizes of up to 4096 bits.

An implementation that negotiates a TLS connection that does not meet the requirements in this section, or any policy-based constraints, SHOULD NOT negotiate HTTP/2. Removing HTTP/2 protocols from consideration could result in the removal of all protocols from the set of protocols offered by the client. This causes protocol negotiation failure, as described in [TLSALPN].

Due to implementation limitations, it might not be possible to fail TLS negotiation based on all of these requirements. An endpoint MUST terminate a HTTP/2 connection that is opened on a TLS session that does not meet these minimum requirements with a connection error [ConnectionErrorHandler] of type

Implementations are encouraged not to negotiate TLS cipher suites with known vulnerabilities, such as [RC4].

9.3. GZip Content-Encoding

Clients MUST support gzip compression for HTTP response bodies. Regardless of the value of the accept-encoding header field, a server MAY send responses with gzip or deflate encoding. A compressed response MUST still bear an appropriate content-encoding header field.

10. Security Considerations

10.1. Server Authority and Same-Origin

This specification uses the same-origin policy ([RFC6454]) to determine whether an origin server is permitted to provide content.

A server that is contacted using TLS is authenticated based on the certificate that it offers in the TLS handshake (see [RFC2818]). A server is considered authoritative for an "https" resource if it has been successfully authenticated for the domain part of the origin of the resource that it is providing.

A server is considered authoritative for an "http" resource if the connection is established to a resolved IP address for the domain in the origin of the resource.

A client MUST NOT use, in any way, resources provided by a server that is not authoritative for those resources.

10.2. Cross-Protocol Attacks

When using TLS, we believe that HTTP/2 introduces no new cross-protocol attacks. TLS encrypts the contents of all transmission (except the handshake itself), making it difficult for attackers to control the data which could be used in a cross-protocol attack.

10.3. Intermediary Encapsulation Attacks

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.

An intermediary that performs translation into HTTP/1.1 cannot alter the semantics of requests or responses. In particular, header field names or values that contain characters not permitted by HTTP/1.1, including carriage return (U+000D) or line feed (U+000A) MUST NOT be translated verbatim, as stipulated in [HTTP-p1].

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.

10.4. Cacheability of Pushed Resources

Pushed resources are responses without an explicit request from the client. Request header fields are provided by the server in the

Caching resources 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 resources for which an origin server is not authoritative are never cached or used.

10.5. Denial of Service Considerations

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. Processing capacity cannot be guarded in the same fashion.

The

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

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

Limits in settings cannot be reduced instantaneously, which leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In particular, immediately after establishing a connection, limits set by a server are not known to clients and could be exceeded without being an obvious protocol violation.

In all these cases, there are legitimate reasons to use these protocol mechanisms. 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 [ConnectionErrorHandler] of type

10.6. Use of Padding

Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might be provided by TLS [TLS12]. Redundant padding could even be counterproductive. Correct application can depend on having specific knowledge of the data that is being padded.

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

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

Use of padding can result in less protection than might seem immediately obvious. At best, padding only makes it more difficult for an attacker to infer length information by increasing the number of frames an attacker has to observe. Incorrectly implemented padding schemes can be easily defeated. In particular, randomized padding with a predictable distribution provides very little protection; or 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 NOT remove padding; though an intermediary could remove padding and add differing amounts if the intent is to improve the protections padding affords.

11. Privacy Considerations

HTTP/2 aims to keep connections open longer between clients and servers in order to reduce the latency when a user makes a request. The maintenance of these connections over time could be used to expose private information. For example, a user using a browser hours after the previous user stopped using that browser may be able to learn about what the previous user was doing. This is a problem with HTTP in its current form as well, however the short lived connections make it less of a risk.

12. IANA Considerations

A string for identifying HTTP/2 is entered into the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLSALPN].

This document establishes registries for error codes. This new registry is entered into a new "Hypertext Transfer Protocol (HTTP) 2 Parameters" section.

This document registers the HTTP2-Settings header field for use in HTTP.

This document registers the PRI method for use in HTTP, to avoid collisions with the connection header [ConnectionHeader].

12.1. Registration of HTTP/2 Identification String

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

Protocol:: HTTP/2
Identification Sequence:: 0x68 0x32 ("h2")
Specification:: This document (RFCXXXX)

12.2. Error Code Registry

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

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

New registrations are advised to provide the following information:

Error Code:: The 32-bit error code value.
Name:: A name for the error code. Specifying an error code name is optional.
Description:: A description of the conditions where the error code is applicable.
Specification:: An optional reference for a specification that defines the error code.

An initial set of error code registrations can be found in Section 7.

12.3. HTTP2-Settings Header Field Registration

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

Header field name:: HTTP2-Settings
Applicable protocol:: http
Status:: standard
Author/Change controller:: IETF
Specification document(s):: Section 3.2.1 of this document
Related information:: This header field is only used by an HTTP/2 client for Upgrade-based negotiation.

12.4. PRI Method Registration

This section registers the PRI method in the HTTP Method Registry [HTTP-p2].

Method Name:: PRI
Safe: No
Idempotent: No
Specification document(s): Section 3.5 of this document
Related information:: This method is never used by an actual client. This method will appear to be used when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection header.

13. Acknowledgements

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).
Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, Herve Ruellan (Substantial editorial contributions).
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.

14. References

14.1. Normative References

[COMPRESSION]	Ruellan, H. and R. Peon, "HPACK - Header Compression for HTTP/2", Internet-Draft draft-ietf-httpbis-header-compression-06, February 2014.
[TCP]	Postel, J., "Transmission Control Protocol ", STD 7, RFC 793, September 1981.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels ", BCP 14, RFC 2119, March 1997.
[RFC2818]	Rescorla, E., "HTTP Over TLS ", RFC 2818, May 2000.
[RFC3986]	Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC4648]	Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.
[RFC5226]	Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC5234]	Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[TLS12]	Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[TLS-EXT]	Eastlake, D., "Transport Layer Security (TLS) Extensions: Extension Definitions ", RFC 6066, January 2011.
[RFC6454]	Barth, A., "The Web Origin Concept", RFC 6454, December 2011.
[TLSALPN]	Friedl, S., Popov, A., Langley, A. and E. Stephan, "Transport Layer Security (TLS) Application Layer Protocol Negotiation Extension", Internet-Draft draft-ietf-tls-applayerprotoneg-04, January 2014.
[HTTP-p1]	Fielding, R. and J. F. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", Internet-Draft draft-ietf-httpbis-p1-messaging-26, February 2014.
[HTTP-p2]	Fielding, R. and J. F. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", Internet-Draft draft-ietf-httpbis-p2-semantics-26, February 2014.
[HTTP-p4]	Fielding, R. and J. F. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", Internet-Draft draft-ietf-httpbis-p4-conditional-26, February 2014.
[HTTP-p5]	Fielding, R., Lafon, Y. and J. F. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", Internet-Draft draft-ietf-httpbis-p5-range-26, February 2014.
[HTTP-p6]	Fielding, R., Nottingham, M. and J. F. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Caching", Internet-Draft draft-ietf-httpbis-p6-cache-26, February 2014.
[HTTP-p7]	Fielding, R. and J. F. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Authentication", Internet-Draft draft-ietf-httpbis-p7-auth-26, February 2014.
[COOKIE]	Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011.
[RFC6125]	Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS) ", RFC 6125, March 2011.

14.2. Informative References

[RFC1323]	Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for High Performance ", RFC 1323, May 1992.
[RFC3749]	Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749, May 2004.
[TALKING]	Huang, L-S., Chen, E., Barth, A., Rescorla, E. and C. Jackson, "Talking to Yourself for Fun and Profit ", 2011.
[BREACH]	Gluck, Y., Harris, N. and A. Prado, "BREACH: Reviving the CRIME Attack ", July 2013.
[RC4]	Rivest, R., "The RC4 encryption algorithm ", RSA Data Security, Inc. , March 1992.
[BCP90]	Klyne, G., Nottingham, M. and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004.
[AltSvc]	Nottingham, M., "HTTP Alternate Services ", Internet-Draft draft-nottingham-httpbis-alt-svc-01, December 2013.
[TLSBCP]	Sheffer, Y. and R. Holz, "Recommendations for Secure Use of TLS and DTLS ", Internet-Draft draft-sheffer-tls-bcp-01, September 2013.

Appendix A. Change Log (to be removed by RFC Editor before publication)

A.1. Since draft-ietf-httpbis-http2-09

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.

A.2. Since draft-ietf-httpbis-http2-08

Added cookie crumbling for more efficient header compression.

Added header field ordering with the value-concatenation mechanism.

A.3. Since draft-ietf-httpbis-http2-07

Marked draft for implementation.

A.4. Since draft-ietf-httpbis-http2-06

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.

A.5. Since draft-ietf-httpbis-http2-05

Marking the draft ready for implementation.

Renumbering END_PUSH_PROMISE flag.

Editorial clarifications and changes.

A.6. Since draft-ietf-httpbis-http2-04

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

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.

A.7. Since draft-ietf-httpbis-http2-03

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

Removed HEADERS+PRIORITY, added optional priority to

Added

A.8. Since draft-ietf-httpbis-http2-02

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.

A.9. Since draft-ietf-httpbis-http2-01

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

Removed data frame compression.

Added

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.

A.10. Since draft-ietf-httpbis-http2-00

Changed title throughout.

Removed section on Incompatibilities with SPDY draft#2.

Changed https://groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU.

Replaced abstract and introduction.

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

Removed unused references.

Added flow control principles [fc-principles] based on http://tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01.

A.11. Since draft-mbelshe-httpbis-spdy-00

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

Updated authors/editors list.

Added status note.

Authors' Addresses

Mike Belshe Twist EMail: mbelshe@chromium.org

Roberto Peon Google, Inc EMail: fenix@google.com

Martin Thomson (editor) Mozilla Suite 300 650 Castro Street Mountain View, CA 94041 US EMail: martin.thomson@gmail.com

1.1. Document Organization

1.2. Conventions and Terminology

2.1. HTTP Frames

2.2. HTTP Multiplexing

2.3. HTTP Semantics