RFC 7540 - Hypertext Transfer Protocol Version 2 (HTTP/2)

This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). 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 exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.¶

5. Streams and Multiplexing

A "stream" is an independent, bidirectional 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.

5.1. Stream States

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

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

       send:   endpoint sends this frame
       recv:   endpoint receives this frame

       H:  HEADERS frame (with implied CONTINUATIONs)
       PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
       ES: END_STREAM flag
       R:  RST_STREAM frame

Figure 2: Stream States

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

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

Streams have the following states: ¶

idle:

All streams start in the "idle" state.

The following transitions are valid from this state:

Sending or receiving a HEADERS frame causes the stream to become "open". The stream identifier is selected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediately become "half-closed".
Sending a PUSH_PROMISE frame on another stream reserves the idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (local)".
Receiving a PUSH_PROMISE frame on another stream reserves an idle stream that is identified for later use. The stream state for the reserved stream transitions to "reserved (remote)".
Note that the PUSH_PROMISE frame is not sent on the idle stream but references the newly reserved stream in the Promised Stream ID field.

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

reserved (local):

A stream in the "reserved (local)" state is one that has been promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame reserves an idle stream by associating the stream with an open stream that was initiated by the remote peer (see Section 8.2).

In this state, only the following transitions are possible:

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

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

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

reserved (remote):

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

In this state, only the following transitions are possible:

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

An endpoint MAY send a PRIORITY frame in this state to reprioritize the reserved stream. An endpoint MUST NOT send any type of frame other than RST_STREAM, WINDOW_UPDATE, or PRIORITY in this state.

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

open:

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

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

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

half-closed (local):

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

A stream transitions from this state to "closed" when a frame that contains an END_STREAM flag is received or when either peer sends a RST_STREAM frame.

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

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

half-closed (remote):

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

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

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

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

closed:

The "closed" state is the terminal state.

An endpoint MUST NOT send frames other than PRIORITY on a closed stream. An endpoint that receives any frame other than PRIORITY after receiving a RST_STREAM MUST treat that as a stream error (Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint that receives any frames after receiving a frame with the END_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED, unless the frame is permitted as described below.

WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATA or HEADERS frame containing an END_STREAM flag is sent. Until the remote peer receives and processes RST_STREAM or the frame bearing the END_STREAM flag, it might send frames of these types. Endpoints MUST ignore WINDOW_UPDATE or RST_STREAM frames received in this state, though endpoints MAY choose to treat frames that arrive a significant time after sending END_STREAM as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

PRIORITY frames can be sent on closed streams to prioritize streams that are dependent on the closed stream. Endpoints SHOULD process PRIORITY frames, though they can be ignored if the stream has been removed from the dependency tree (see Section 5.3.4).

If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAM might have already sent — or enqueued for sending — frames on the stream that cannot be withdrawn. An endpoint MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. An endpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after this time as being in error.

Flow-controlled frames (i.e., DATA) received after sending RST_STREAM are counted toward the connection flow-control window. Even though these frames might be ignored, because they are sent before the sender receives the RST_STREAM, the sender will consider the frames to count against the flow-control window.

An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causes a stream to become "reserved" even if the associated stream has been reset. Therefore, a RST_STREAM is needed to close an unwanted promised stream.

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

An example of the state transitions for an HTTP request/response exchange can be found in Section 8.1. An example of the state transitions for server push can be found in Sections 8.2.1 and 8.2.2.¶

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 of zero cannot be used to establish a new stream.¶

HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) 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 (Section 5.4.1) 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.¶

5.1.2. Stream Concurrency

A peer can limit the number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) 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 in 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 its advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR or REFUSED_STREAM. The choice of error code determines whether the endpoint wishes to enable automatic retry (see Section 8.1.4) for details).¶

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.¶

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 WINDOW_UPDATE frame (Section 6.9).¶

5.2.1. Flow-Control Principles

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. Both types of flow control are between the endpoints of a single hop and not over the entire end-to-end path.
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 (Section 6.9). 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.¶

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 to 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 (2³¹-1) and can maintain this window by sending a WINDOW_UPDATE frame when any 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 [RFC7323]).¶

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.¶

5.3. Stream Priority

A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. At any other time, the PRIORITY frame (Section 6.3) can be used to change the priority of a stream.¶

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

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

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

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

5.3.1. Stream Dependencies

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 (Section 5.3.5).¶

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.¶

    A                 A
   / \      ==>      /|\
  B   C             B D C

Figure 3: Example of Default Dependency Creation

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.¶

                      A
    A                 |
   / \      ==>       D
  B   C              / \
                    B   C

Figure 4: Example of Exclusive Dependency Creation

Inside the dependency tree, a dependent stream SHOULD only be allocated resources if either all of the streams that it depends on (the chain of parent streams up to 0x0) are 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 (Section 5.4.2) of type PROTOCOL_ERROR.¶

5.3.2. Dependency Weighting

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, stream C depends on stream A with weight 12, and no progress can be made on stream A, stream B ideally receives one-third of the resources allocated to stream C.¶

5.3.3. Reprioritization

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 causes 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.

    x                x                x                 x
    |               / \               |                 |
    A              D   A              D                 D
   / \            /   / \            / \                |
  B   C     ==>  F   B   C   ==>    F   A       OR      A
     / \                 |             / \             /|\
    D   E                E            B   C           B C F
    |                                     |             |
    F                                     E             E
               (intermediate)   (non-exclusive)    (exclusive)

Figure 5: Example of Dependency Reordering

5.3.4. Prioritization State Management

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 (Section 5.3.5). This potentially creates suboptimal prioritization, since the stream could be given a priority that is different from 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.¶

Similarly, streams that are in the "idle" state can be assigned priority or become a parent of other streams. This allows for the creation of a grouping node in the dependency tree, which enables more flexible expressions of priority. Idle streams begin with a default priority (Section 5.3.5).¶

The retention of priority information for streams that are not counted toward the limit set by SETTINGS_MAX_CONCURRENT_STREAMS could create a large state burden for an endpoint. Therefore, the amount of prioritization state that is retained MAY be limited.¶

The amount of additional state an endpoint maintains for prioritization 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 limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS. Implementations SHOULD also attempt to retain state for streams that are in active use in the priority tree.¶

If it has retained enough state to do so, 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.¶

5.3.5. Default Priorities

All streams are initially assigned a non-exclusive dependency on stream 0x0. Pushed streams (Section 8.2) initially depend on their associated stream. In both cases, streams are assigned a default weight of 16.¶

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 that prevents further processing of the frame layer or corrupts any connection state.¶

An endpoint that encounters a connection error SHOULD first send a GOAWAY frame (Section 6.8) 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 for an error condition, the endpoint MUST close the TCP connection.¶

It is possible that the GOAWAY will not be reliably received by the receiving endpoint ([RFC7230], Section 6.6 describes how an immediate connection close can result in data loss). 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.¶

5.4.2. Stream Error Handling

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 (Section 6.4) 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 (Section 4.3) 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.¶

To avoid looping, an endpoint MUST NOT send a RST_STREAM in response to a RST_STREAM frame.¶

5.4.3. Connection Termination

If the TCP connection is closed or reset while streams remain in "open" or "half-closed" state, then the affected streams cannot be automatically retried (see Section 8.1.4 for details).¶

5.5. Extending HTTP/2

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

This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or header fields.¶

Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), or new error codes (Section 7). Registries are established for managing these extension points: frame types (Section 11.2), settings (Section 11.3), and error codes (Section 11.4).¶

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

Extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. Note that treating any frames other than DATA frames as flow controlled is such a change in semantics and can only be done through negotiation.¶

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

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 in the establishment and management either 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.¶

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

 +---------------+
 |Pad Length? (8)|
 +---------------+-----------------------------------------------+
 |                            Data (*)                         ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

Figure 6: DATA Frame Payload

The DATA frame contains the following fields: ¶

Pad Length:: An 8-bit field containing the length of the frame padding in units of octets. This field is conditional (as signified by a "?" in the diagram) and is only present if the PADDED flag is set.
Data:: Application data. The amount of data is the remainder of the frame payload after subtracting the length of the other fields that are present.
Padding:: Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending. A receiver is not obligated to verify padding but MAY treat non-zero padding as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The DATA frame defines the following flags: ¶

END_STREAM (0x1):: When set, bit 0 indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half-closed" states or the "closed" state (Section 5.1).
PADDED (0x8):: When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.

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

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

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

6.2. HEADERS

The HEADERS frame (type=0x1) is used to open a stream (Section 5.1), and additionally carries a header block fragment. HEADERS frames can be sent on a stream in the "idle", "reserved (local)", "open", or "half-closed (remote)" state.¶

 +---------------+
 |Pad Length? (8)|
 +-+-------------+-----------------------------------------------+
 |E|                 Stream Dependency? (31)                     |
 +-+-------------+-----------------------------------------------+
 |  Weight? (8)  |
 +-+-------------+-----------------------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

Figure 7: HEADERS Frame Payload

The HEADERS frame payload has the following fields: ¶

Pad Length:: An 8-bit field containing the length of the frame padding in units of octets. This field is only present if the PADDED flag is set.
E:: A single-bit flag indicating that the stream dependency is exclusive (see Section 5.3). This field is only present if the PRIORITY flag is set.
Stream Dependency:: A 31-bit stream identifier for the stream that this stream depends on (see Section 5.3). This field is only present if the PRIORITY flag is set.
Weight:: An unsigned 8-bit integer representing a priority weight for the stream (see Section 5.3). Add one to the value to obtain a weight between 1 and 256. This field is only present if the PRIORITY flag is set.
Header Block Fragment:: A header block fragment (Section 4.3).
Padding:: Padding octets.

The HEADERS frame defines the following flags: ¶

END_STREAM (0x1):: When set, bit 0 indicates that the header block (Section 4.3) is the last that the endpoint will send for the identified stream.
A HEADERS frame carries the END_STREAM flag that signals the end of a stream. However, a HEADERS frame with the END_STREAM flag set can be followed by CONTINUATION frames on the same stream. Logically, the CONTINUATION frames are part of the HEADERS frame.
END_HEADERS (0x4):: When set, bit 2 indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.
A HEADERS frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
PADDED (0x8):: When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.
PRIORITY (0x20):: When set, bit 5 indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see Section 5.3.

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

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

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

The HEADERS frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1). Padding that exceeds the size remaining for the header block fragment MUST be treated as a PROTOCOL_ERROR.¶

Prioritization information in a HEADERS frame is logically equivalent to a separate PRIORITY frame, but inclusion in HEADERS avoids the potential for churn in stream prioritization when new streams are created. Prioritization fields in HEADERS frames subsequent to the first on a stream reprioritize the stream (Section 5.3.3).¶

6.3. PRIORITY

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

 +-+-------------------------------------------------------------+
 |E|                  Stream Dependency (31)                     |
 +-+-------------+-----------------------------------------------+
 |   Weight (8)  |
 +-+-------------+

Figure 8: PRIORITY Frame Payload

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

E:: A single-bit flag indicating that the stream dependency is exclusive (see Section 5.3).
Stream Dependency:: A 31-bit stream identifier for the stream that this stream depends on (see Section 5.3).
Weight:: An unsigned 8-bit integer representing a priority weight for the stream (see Section 5.3). Add one to the value to obtain a weight between 1 and 256.

The PRIORITY frame does not define any flags.¶

The PRIORITY frame always identifies a stream. If a PRIORITY frame is received with a stream identifier of 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

The PRIORITY frame can be sent on a stream in any state, though it cannot be sent between consecutive frames that comprise a single header block (Section 4.3). Note that this frame could arrive after processing or frame sending has completed, which would cause it to have no effect on the identified stream. For a stream that is in the "half-closed (remote)" or "closed" state, this frame can only affect processing of the identified stream and its dependent streams; it does not affect frame transmission on that stream.¶

The PRIORITY frame can be sent for a stream in the "idle" or "closed" state. This allows for the reprioritization of a group of dependent streams by altering the priority of an unused or closed parent stream.¶

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

6.4. RST_STREAM

The RST_STREAM frame (type=0x3) allows for immediate termination of a stream. RST_STREAM is sent to request cancellation of a stream or to indicate that an error condition has occurred.¶

 +---------------------------------------------------------------+
 |                        Error Code (32)                        |
 +---------------------------------------------------------------+

Figure 9: RST_STREAM Frame Payload

The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 7). The error code indicates why the stream is being terminated.¶

The RST_STREAM frame does not define any flags.¶

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

RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received with a stream identifier of 0x0, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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

6.5. SETTINGS

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

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

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

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

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

ACK (0x1):: When set, bit 0 indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more information, see Section 6.5.3 ("Settings Synchronization").

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

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

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

6.5.1. SETTINGS Format

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.¶

 +-------------------------------+
 |       Identifier (16)         |
 +-------------------------------+-------------------------------+
 |                        Value (32)                             |
 +---------------------------------------------------------------+

Figure 10: Setting Format

6.5.2. Defined SETTINGS Parameters

The following parameters are defined: ¶

SETTINGS_HEADER_TABLE_SIZE (0x1):: 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 (see [COMPRESSION]). The initial value is 4,096 octets.
SETTINGS_ENABLE_PUSH (0x2):: This setting can be used to disable server push (Section 8.2). 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 (Section 5.4.1) 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 (Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_CONCURRENT_STREAMS (0x3):: 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 is more appropriate.
SETTINGS_INITIAL_WINDOW_SIZE (0x4):: 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 (see Section 6.9.2).
Values above the maximum flow-control window size of 2³¹-1 MUST be treated as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.
SETTINGS_MAX_FRAME_SIZE (0x5):: 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 (Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_HEADER_LIST_SIZE (0x6):: 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.¶

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 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 (Section 5.4.1) of type SETTINGS_TIMEOUT.¶

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.¶

 +---------------+
 |Pad Length? (8)|
 +-+-------------+-----------------------------------------------+
 |R|                  Promised Stream ID (31)                    |
 +-+-----------------------------+-------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+
 |                           Padding (*)                       ...
 +---------------------------------------------------------------+

Figure 11: PUSH_PROMISE Payload Format

The PUSH_PROMISE frame payload has the following fields: ¶

Pad Length:: An 8-bit field containing the length of the frame padding in units of octets. This field is only present if the PADDED flag is set.
R:: A single reserved bit.
Promised Stream ID:: An unsigned 31-bit integer that identifies the stream that is reserved by the PUSH_PROMISE. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see "new stream identifier" in Section 5.1.1).
Header Block Fragment:: A header block fragment (Section 4.3) containing request header fields.
Padding:: Padding octets.

The PUSH_PROMISE frame defines the following flags: ¶

END_HEADERS (0x4):: When set, bit 2 indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames.
A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
PADDED (0x8):: When set, bit 3 indicates that the Pad Length field and any padding that it describes are present.

PUSH_PROMISE frames MUST only be sent on a peer-initiated stream that is in either the "open" or "half-closed (remote)" state. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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

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

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

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

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

The PUSH_PROMISE frame can include padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).¶

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.¶

 +---------------------------------------------------------------+
 |                                                               |
 |                      Opaque Data (64)                         |
 |                                                               |
 +---------------------------------------------------------------+

Figure 12: PING Payload Format

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

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

The PING frame defines the following flags: ¶

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

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

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

6.8. GOAWAY

The GOAWAY frame (type=0x7) is used to initiate shutdown of a connection or to signal serious error conditions. GOAWAY allows an endpoint to gracefully stop accepting new streams while still finishing processing of previously established streams. This enables administrative actions, like server maintenance.¶

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

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

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

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

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

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

 +-+-------------------------------------------------------------+
 |R|                  Last-Stream-ID (31)                        |
 +-+-------------------------------------------------------------+
 |                      Error Code (32)                          |
 +---------------------------------------------------------------+
 |                  Additional Debug Data (*)                    |
 +---------------------------------------------------------------+

Figure 13: 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 GOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

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

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

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

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

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

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

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

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

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

6.9. WINDOW_UPDATE

The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see Section 5.2 for an overview.¶

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

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

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

 +-+-------------------------------------------------------------+
 |R|              Window Size Increment (31)                     |
 +-+-------------------------------------------------------------+

Figure 14: 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 octets that the sender can transmit in addition to the existing flow-control window. The legal range for the increment to the flow-control window is 1 to 2³¹-1 (2,147,483,647) octets.¶

The WINDOW_UPDATE frame does not define any flags.¶

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

A receiver MUST treat the receipt of a WINDOW_UPDATE frame with an flow-control window increment of 0 as a stream error (Section 5.4.2) of type PROTOCOL_ERROR; errors on the connection flow-control window MUST be treated as a connection error (Section 5.4.1).¶

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

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

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

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 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 an error code of FLOW_CONTROL_ERROR; for the connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR is sent.¶

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 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 also 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.¶

In addition to changing the flow-control window for streams that are not yet active, a SETTINGS frame can alter the initial flow-control window size for streams with active flow-control windows (that is, streams in the "open" or "half-closed (remote)" state). 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 60 KB immediately on connection establishment and the server sets the initial window size to be 16 KB, the client will recalculate the available flow-control window to be -44 KB 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 (Section 5.4.1) of type FLOW_CONTROL_ERROR.¶

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 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 MAY continue to process streams that exceed flow-control limits. Allowing streams to continue does not allow the receiver to immediately reduce the space it reserves for flow-control windows. Progress on these streams can also stall, since WINDOW_UPDATE frames are needed to allow the sender to resume sending. The receiver MAY instead send a RST_STREAM with an error code of FLOW_CONTROL_ERROR for the affected streams.¶

6.10. CONTINUATION

The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments (Section 4.3). Any number of CONTINUATION frames can be sent, as long as the preceding frame is on the same stream and is a HEADERS, PUSH_PROMISE, or CONTINUATION frame without the END_HEADERS flag set.¶

 +---------------------------------------------------------------+
 |                   Header Block Fragment (*)                 ...
 +---------------------------------------------------------------+

Figure 15: CONTINUATION Frame Payload

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

The CONTINUATION frame defines the following flag: ¶

END_HEADERS (0x4):: When set, bit 2 indicates that this frame ends a header block (Section 4.3).
If the END_HEADERS bit is not set, this frame MUST be followed by another CONTINUATION frame. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

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

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

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

8. HTTP Message Exchanges

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 [RFC7231], Conditional Requests [RFC7232], Range Requests [RFC7233], Caching [RFC7234], and Authentication [RFC7235] are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing [RFC7230], 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.¶

8.1. HTTP Request/Response Exchange

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

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

for a response only, zero or more HEADERS frames (each followed by zero or more CONTINUATION frames) containing the message headers of informational (1xx) HTTP responses (see [RFC7230], Section 3.2 and [RFC7231], Section 6.2),
one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see [RFC7230], Section 3.2),
zero or more DATA frames containing the payload body (see [RFC7230], Section 3.3), and
optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see [RFC7230], Section 4.1.2).

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

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

HTTP/2 uses DATA frames to carry message payloads. The chunked transfer encoding defined in Section 4.1 of [RFC7230] MUST NOT be used in HTTP/2.¶

Trailing header fields are carried in a header block that also terminates the stream. Such a header block is a sequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where the HEADERS frame bears an END_STREAM flag. Header blocks after the first that do not terminate the stream are not part of an HTTP request or response.¶

A HEADERS frame (and associated CONTINUATION frames) can only appear at the start or end of a stream. An endpoint that receives a HEADERS frame without the END_STREAM flag set after receiving a final (non-informational) status code MUST treat the corresponding request or response as malformed (Section 8.1.2.6).¶

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

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

8.1.1. Upgrading from HTTP/2

HTTP/2 removes support for the 101 (Switching Protocols) informational status code ([RFC7231], Section 6.2.2).¶

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 Section 3).¶

8.1.2. HTTP Header Fields

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 <https://www.iana.org/assignments/message-headers>.¶

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 (Section 8.1.2.6).¶

8.1.2.1. Pseudo-Header Fields

While HTTP/1.x used the message start-line (see [RFC7230], Section 3.1) to convey the target URI, the 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 (Section 8.1.2.6).¶

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 (Section 8.1.2.6).¶

8.1.2.2. Connection-Specific Header Fields

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 an HTTP/2 message containing connection-specific header fields; any message containing connection-specific header fields MUST be treated as malformed (Section 8.1.2.6).¶

The only exception to this is the TE header field, which MAY be present in an HTTP/2 request; when it is, it MUST NOT contain any value other than "trailers".¶

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 the Connection header field.¶

8.1.2.3. Request Pseudo-Header Fields

The following pseudo-header fields are defined for HTTP/2 requests: ¶

The :method pseudo-header field includes the HTTP method ([RFC7231], Section 4).
The :scheme pseudo-header field includes the scheme portion of the target URI ([RFC3986], Section 3.1).
: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 ([RFC3986], Section 3.2). 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 [RFC7230], Section 5.3). 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 and optionally a '?' character followed by the query production (see Sections 3.3 and 3.4 of [RFC3986]). 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 [RFC7230], Section 5.3.4).

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 (Section 8.3). An HTTP request that omits mandatory pseudo-header fields is malformed (Section 8.1.2.6).¶

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

8.1.2.4. Response Pseudo-Header Fields

For HTTP/2 responses, a single :status pseudo-header field is defined that carries the HTTP status code field (see [RFC7231], Section 6). This pseudo-header field MUST be included in all responses; otherwise, the response is malformed (Section 8.1.2.6).¶

HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.¶

8.1.2.5. Compressing the Cookie Header Field

The Cookie header field [COOKIE] uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This header field doesn't follow the list construction rules in HTTP (see [RFC7230], Section 3.2.2), 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.

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

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

8.1.2.6. Malformed Requests and Responses

A malformed request or response is one that is an otherwise valid sequence of HTTP/2 frames but is 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 a payload 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 [RFC7230], Section 3.3.2, 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 (Section 5.4.2) 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.¶

8.1.3. 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 payload 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.¶

  GET /resource HTTP/1.1           HEADERS
  Host: example.org          ==>     + 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.¶

  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 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:¶

  POST /resource HTTP/1.1          HEADERS
  Host: example.org          ==>     - 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:¶

  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}

An informational response using a 1xx status code other than 101 is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames.¶

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.¶

The following example includes both a 100 (Continue) status code, which is sent in response to a request containing a "100-continue" token in the Expect header field, and trailing header fields:

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

  HTTP/1.1 200 OK                  HEADERS
  Content-Type: image/jpeg   ==>     - END_STREAM
  Transfer-Encoding: chunked         + END_HEADERS
  Trailer: Foo                         :status = 200
                                       content-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

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: ¶

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.¶

8.2. Server Push

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

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

Promised requests MUST be cacheable (see [RFC7231], Section 4.2.3), MUST be safe (see [RFC7231], Section 4.2.1), and MUST NOT include a request body. Clients that receive a promised request that is not cacheable, that is not known to be safe, or that indicates the presence of a request body MUST reset the promised stream with a stream error (Section 5.4.2) of type PROTOCOL_ERROR. Note this could result in the promised stream being reset if the client does not recognize a newly defined method as being safe.¶

Pushed responses that are cacheable (see [RFC7234], Section 3) can be stored by the client, if it implements an HTTP cache. Pushed responses are considered successfully validated on the origin server (e.g., if the "no-cache" cache response directive is present ([RFC7234], Section 5.2.2)) while the stream identified by the promised stream ID is still open.¶

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

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

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

A client cannot push. Thus, servers MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Clients MUST reject any attempt to change the SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.¶

8.2.1. Push Requests

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 Section 5.1.1).¶

The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid and complete set of request header fields (Section 8.1.2.3). The server MUST include a method in the :method pseudo-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 pseudo-header field identifies a method that is not safe, it MUST respond with a stream error (Section 5.4.2) of type PROTOCOL_ERROR.¶

The server SHOULD send PUSH_PROMISE (Section 6.6) 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_PROMISE frames before the DATA frames that contain the image links ensures that the client is able to see that a resource will be pushed before discovering embedded links. Similarly, if the server pushes responses referenced by the header block (for instance, in Link header fields), sending a PUSH_PROMISE before sending the header block ensures that clients do not request those resources.¶

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.¶

8.2.2. Push Responses

After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response (Section 8.1.2.4) 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 (Section 5.1) 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 a RST_STREAM frame, using either the CANCEL or REFUSED_STREAM code 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 Section 10.1) 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.¶

8.3. The CONNECT Method

In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is used to convert an HTTP connection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with https resources.¶

In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host for similar purposes. The HTTP header field mapping works as defined in Section 8.1.2.3 ("Request Pseudo-Header Fields"), with a few differences. Specifically: ¶

The :method pseudo-header field is set to CONNECT.
The :scheme and :path pseudo-header fields MUST be omitted.
The :authority pseudo-header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT requests (see [RFC7230], Section 5.3)).

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

A proxy that supports CONNECT establishes a TCP connection [TCP] to the server identified in the :authority pseudo-header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6.¶

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

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

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

10.4. Cacheability of Pushed Responses

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

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

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

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

Frame Type	Code	Section
DATA	0x0	Section 6.1
HEADERS	0x1	Section 6.2
PRIORITY	0x2	Section 6.3
RST_STREAM	0x3	Section 6.4
SETTINGS	0x4	Section 6.5
PUSH_PROMISE	0x5	Section 6.6
PING	0x6	Section 6.7
GOAWAY	0x7	Section 6.8
WINDOW_UPDATE	0x8	Section 6.9
CONTINUATION	0x9	Section 6.10

Name	Code	Initial Value	Specification
HEADER_TABLE_SIZE	0x1	4096	Section 6.5.2
ENABLE_PUSH	0x2	1	Section 6.5.2
MAX_CONCURRENT_STREAMS	0x3	(infinite)	Section 6.5.2
INITIAL_WINDOW_SIZE	0x4	65535	Section 6.5.2
MAX_FRAME_SIZE	0x5	16384	Section 6.5.2
MAX_HEADER_LIST_SIZE	0x6	(infinite)	Section 6.5.2

Name	Code	Description	Specification
NO_ERROR	0x0	Graceful shutdown	Section 7
PROTOCOL_ERROR	0x1	Protocol error detected	Section 7
INTERNAL_ERROR	0x2	Implementation fault	Section 7
FLOW_CONTROL_ERROR	0x3	Flow-control limits exceeded	Section 7
SETTINGS_TIMEOUT	0x4	Settings not acknowledged	Section 7
STREAM_CLOSED	0x5	Frame received for closed stream	Section 7
FRAME_SIZE_ERROR	0x6	Frame size incorrect	Section 7
REFUSED_STREAM	0x7	Stream not processed	Section 7
CANCEL	0x8	Stream cancelled	Section 7
COMPRESSION_ERROR	0x9	Compression state not updated	Section 7
CONNECT_ERROR	0xa	TCP connection error for CONNECT method	Section 7
ENHANCE_YOUR_CALM	0xb	Processing capacity exceeded	Section 7
INADEQUATE_SECURITY	0xc	Negotiated TLS parameters not acceptable	Section 7
HTTP_1_1_REQUIRED	0xd	Use HTTP/1.1 for the request	Section 7