Hypertext Transfer Protocol version 2.0

HTTP/2.0 frames share a common base format consisting of an 8-byte header followed by 0 to 65535 bytes of data.¶

The fields of the frame header are defined as: ¶

Length:

The length of the frame data expressed as an unsigned 16-bit integer. The 8 bytes of the frame header are not included in this value.

Type:

The 8-bit type of the frame. The frame type determines how the remainder of the frame header and data are interpreted. Implementations MUST ignore unsupported and unrecognized frame types.

Flags:

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

The least significant bit (0x1) - the FINAL bit - is defined for all frame types as an indication that this frame is the last the endpoint will send for the identified stream. Setting this flag causes the stream to enter the half-closed state (Section 3.4.3). Implementations MUST process the FINAL bit for all frames whose stream identifier field is not 0x0. The FINAL bit MUST NOT be set on frames that use a stream identifier of 0.

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

R:

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

Stream Identifier:

A 31-bit stream identifier (see Section 3.4.1). A value 0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream.

The structure and content of the remaining frame data is dependent entirely on the frame type.¶

Implementations with limited resources might not be capable of processing large frame sizes. Such implementations MAY choose to place additional limits on the maximum frame size. However, all implementations MUST be capable of receiving and processing frames containing at least 8192 octets of data. [rfc.comment.2: Ed. Question: Does this minimum include the 8-byte header or just the frame data?]¶

An implementation MUST terminate a stream immediately if it is unable to process a frame due it's size. This is done by sending an RST_STREAM frame (Section 3.8.3) containing the FRAME_TOO_LARGE error code.¶

[rfc.comment.3: Issue 28: Need a way to signal the maximum frame size; no way to RST_STREAM on non-stream-related frames.] ¶

There is no coordination or shared action between the client and server required to create a stream. Rather, new streams are established by sending a frame whose stream identifier field references a previously unused stream identifier.¶

All streams are identified by an unsigned 31-bit integer. Streams initiated by a client use odd numbered stream identifiers; those initiated by the server use even numbered stream identifiers. A stream identifier of zero MUST NOT be used to establish a new stream.¶

The identifier of a newly established stream MUST be numerically greater than all previously established streams from that endpoint within the HTTP/2.0 connection, unless the identifier has been reserved using a PUSH_PROMISE (Section 3.8.5) frame. An endpoint that receives an unexpected stream identifier MUST respond with a connection error (Section 3.5.1) of type PROTOCOL_ERROR.¶

A peer can limit the total number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameters within a SETTINGS frame. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer. 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. Peer endpoints MUST NOT exceed this limit. All concurrently active streams initiated by an endpoint, including streams that are half-open (Section 3.4.3) in any direction, count toward that endpoint's limit.¶

Stream identifiers cannot be reused within a connection. Long-lived connections can cause an endpoint to exhaust 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.¶

Either endpoint can request the early termination of an unwanted stream by sending an RST_STREAM frame (Section 3.5.2) with an error code of either REFUSED_STREAM (if no frames have been processed) or CANCEL (if at least one frame has been processed). Such termination might not take effect immediately as the peer might have sent additional frames on the stream prior to receiving the termination request.¶

The endpoint establishing a new stream can assign a priority for the stream. Priority is represented as an unsigned 31-bit integer. 0 represents the highest priority and 2³¹-1 represents the lowest priority.¶

The purpose of this value is to allow the initiating endpoint to request that frames for the stream be processed with higher priority relative to any other concurrently active streams. That is, if an endpoint receives interleaved frames for multiple streams, the endpoint ought to make a best-effort attempt at processing frames for higher priority streams before processing those for lower priority streams.¶

Explicitly setting the priority for a stream does not guarantee any particular processing order for the stream relative to any other stream. Nor is there is any mechanism provided by which the initiator of a stream can force or require a receiving endpoint to process frames from one stream before processing frames from another.¶

When an endpoint sends a frame for a stream with the FINAL flag set, the stream is considered to be half-closed for that endpoint. Subsequent frames MUST NOT be sent by that endpoint for the half closed stream for the remaining duration of the HTTP/2.0 connection. When both endpoints have sent frames with the FINAL flag set, the stream is considered to be fully closed.¶

If an endpoint receives additional frames for a stream that was previously half-closed by the sending peer, the recipient MUST respond with a stream error (Section 3.5.2) of type STREAM_CLOSED.¶

An endpoint that has not yet half-closed a stream by sending the FINAL flag can continue sending frames on the stream.¶

It is not necessary for an endpoint to half-close a stream for which it has not sent any frames. This allows endpoints to use fully unidirectional streams that do not require explicit action or acknowledgement from the receiver.¶

Streams can be terminated in the following ways: ¶

Normal termination:

Normal stream termination occurs when both client and server have half-closed the stream by sending a frame containing a FINAL flag (Section 3.3.1).

Half-close on unidirectional stream:

A stream that only has frames sent in one direction can be tentatively considered to be closed once a frame containing a FINAL flag is sent. The active sender on the stream MUST be prepared to receive frames after closing the stream.

Abrupt termination:

Either peer can send a RST_STREAM control frame at any time to terminate an active stream. RST_STREAM contains an error code to indicate the reason for termination. A RST_STREAM indicates that the sender will transmit no further data on the stream and that the receiver is advised to cease transmission on it.

The sender of a RST_STREAM frame MUST allow for frames that have already been sent by the peer prior to the RST_STREAM being processed. If in-transit frames alter connection state, these frames cannot be safely discarded. See Stream Error Handling (Section 3.5.2) for more details.

TCP connection teardown:

If the TCP connection is torn down while un-closed streams exist, then the endpoint MUST assume that the stream was abnormally interrupted and may be incomplete.

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

An endpoint that encounters a connection error MUST first send a GOAWAY (Section 3.8.7) frame with the stream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includes an error code that indicates why the connection is terminating. After sending the GOAWAY frame, the endpoint MUST close the TCP connection.¶

It is possible that the GOAWAY will not be reliably received by the receiving endpoint. 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 if the error is recurrent. Endpoints SHOULD send a GOAWAY frame when ending a connection, as long as circumstances permit it.¶

A stream error is an error related to a specific stream identifier that does not affect processing of other streams at the framing layer.¶

An endpoint that detects a stream error sends a RST_STREAM (Section 3.8.3) frame that contains the stream identifier of the stream where the error occurred. The RST_STREAM frame includes an error code that indicates the type of error.¶

A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by the remote peer. These frames can be ignored, except where they modify connection state (such as the state maintained for header compression (Section 3.7)).¶

Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more than a round trip time. This behavior is permitted to deal with misbehaving implementations.¶

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

Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons for the stream or connection error.¶

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

The following error codes are defined: ¶

NO_ERROR (0):

The associated condition is not as a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection.

PROTOCOL_ERROR (1):

The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available.

INTERNAL_ERROR (2):

The endpoint encountered an unexpected internal error.

FLOW_CONTROL_ERROR (3):

The endpoint detected that its peer violated the flow control protocol.

INVALID_STREAM (4):

The endpoint received a frame for an inactive stream.

STREAM_CLOSED (5):

The endpoint received a frame after a stream was half-closed.

FRAME_TOO_LARGE (6):

The endpoint received a frame that was larger than the maximum size that it supports.

REFUSED_STREAM (7):

The endpoint is refusing the stream before processing its payload.

CANCEL (8):

Used by the creator of a stream to indicate that the stream is no longer needed.

COMPRESSION_ERROR (9):

The endpoint is unable to maintain the compression context for the connection.

Experience with TCP congestion control has shown that algorithms can evolve over time to become more sophisticated without requiring protocol changes. TCP congestion control and its evolution is clearly different from HTTP/2.0 flow control, though the evolution of TCP congestion control algorithms shows that a similar approach could be feasible for HTTP/2.0 flow control.¶

HTTP/2.0 stream flow control aims to allow for future improvements to flow control algorithms without requiring protocol changes. Flow control in HTTP/2.0 has the following characteristics: ¶

1. Flow control is hop-by-hop, not end-to-end.
2. Flow control is based on window update frames. Receivers advertise how many octets they are prepared to receive on a stream. This is a credit-based scheme.
3. 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 preferences as a receiver and abide by the flow control limits set by their peer when sending.
4. The initial value for the flow control window is 65536 bytes for both new streams and the overall connection.
5. 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.
6. Flow control can be disabled by a receiver. A receiver can choose to either disable flow control for a stream or connection by declaring an infinite flow control limit.
7. HTTP/2.0 standardizes only the format of the window update frame (Section 3.8.9). This does not stipulate how a receiver decides when to send this frame or the value that it sends. Nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs.

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

Flow control is defined to protect endpoints (client, server or intermediary) that are operating under resource constraints. For example, a proxy needs to share memory between many connections, and also might have a slow upstream connection and a fast downstream one. Flow control addresses cases where the receiver is unable process data on one stream, yet wants to continue to process other streams in the same connection.¶

Deployments that do not require this capability SHOULD disable flow control for data that is being received. Note that flow control cannot be disabled for sending. Sending data is always subject to the flow control window advertised by the receiver.¶

Deployments with constrained resources (for example, memory) MAY employ flow control to limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of available network resources if flow control is enabled without knowledge of the bandwidth-delay product (see [RFC1323]).¶

Even with full awareness of the current bandwidth-delay product, implementation of flow control is difficult. However, it can ensure that constrained resources are protected without any reduction in connection utilization.¶

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.¶

The DATA frame does not define any type-specific flags.¶

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 3.5.1) of type PROTOCOL_ERROR.¶

The HEADERS+PRIORITY frame (type=0x1) allows the sender to set header fields and stream priority at the same time.¶

The HEADERS+PRIORITY frame is identical to the HEADERS frame (Section 3.8.8), preceded by a single reserved bit and a 31-bit priority; see Section 3.4.2.¶

HEADERS+PRIORITY uses the same flags as the HEADERS frame, except that a HEADERS+PRIORITY frame with a CONTINUES bit MUST be followed by another HEADERS+PRIORITY frame. See HEADERS frame (Section 3.8.8) for any flags.¶

HEADERS+PRIORITY frames MUST be associated with a stream. If a HEADERS+PRIORITY frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 3.5.1) of type PROTOCOL_ERROR.¶

The HEADERS+PRIORITY frame modifies the connection state as defined in Section 3.7.¶

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

The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 3.5.3). The error code indicates why the stream is being terminated.¶

No type-flags are defined.¶

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

RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received whose stream identifier field is 0x0 the recipient MUST respond with a connection error (Section 3.5.1) of type PROTOCOL_ERROR.¶

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

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

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

A SETTINGS frame is not required to include every defined setting; senders can include only those parameters for which it has accurate values and a need to convey. When multiple parameters are sent, they SHOULD be sent in order of numerically lowest ID to highest ID. A single SETTINGS frame MUST NOT contain multiple values for the same ID. If the receiver of a SETTINGS frame discovers multiple values for the same ID, it MUST ignore all values for that ID except the first one.¶

Over the lifetime of a connection, an endpoint MAY send multiple SETTINGS frames containing previously unspecified parameters or new values for parameters whose values have already been established. Only the most recent value provided setting value applies.¶

The SETTINGS frame defines the following flag: ¶

CLEAR_PERSISTED (0x2):

Bit 2 being set indicates a request to clear any previously persisted settings before processing the settings. Clients MUST NOT set this flag.

SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a settings frame MUST be zero. If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (Section 3.5.1) of type PROTOCOL_ERROR.¶

The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a minimal set of headers that provide additional context for the stream. Section 4.3 contains a thorough description of the use of PUSH_PROMISE frames.¶

The payload of a PUSH_PROMISE includes a "Promised-Stream-ID". This unsigned 31-bit integer identifies the stream the endpoint intends to start sending frames for. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier (Section 3.4.1)).¶

PUSH_PROMISE frames MUST be associated with an existing stream. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 3.5.1) of type PROTOCOL_ERROR.¶

The state of promised streams is bound to the state of the original associated stream on which the PUSH_PROMISE frame were sent. If the originating stream state changes to fully closed, all associated promised streams fully close as well. [rfc.comment.5: Ed. Note: We need clarification on this point. How synchronized are the lifecycles of streams and associated promised streams?] ¶

PUSH_PROMISE uses the same flags as the HEADERS frame, except that a PUSH_PROMISE frame with a CONTINUES bit MUST be followed by another PUSH_PROMISE frame. See HEADERS frame (Section 3.8.8) for any flags.¶

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

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

The PUSH_PROMISE frame modifies the connection state as defined in Section 3.7.¶

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.¶

PING frames consist of an arbitrary, variable-length sequence of octets. Receivers of a PING send a response PING frame with the PONG flag set and precisely the same sequence of octets back to the sender as soon as possible.¶

Processing of PING frames SHOULD be performed with the highest priority if there are additional frames waiting to be processed.¶

The PING frame defines one type-specific flag: ¶

PONG (0x2):

Bit 2 being set indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.

PING frames are not associated with any individual stream. If a PING frame is received with a stream identifier field value other than 0x0, the recipient MUST respond with a connection error (Section 3.5.1) of type PROTOCOL_ERROR.¶

The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this connection. It can be sent from the client or the server. Once sent, the sender will ignore frames sent on new streams for the remainder of the connection. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection, although a new connection can be established for new streams. The purpose of this frame is to allow an endpoint to gracefully stop accepting new streams (perhaps for a reboot or maintenance), while still finishing processing of previously established streams.¶

There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last stream which was processed on the sending endpoint in this connection. If the receiver of the GOAWAY used streams that are newer than the indicated stream identifier, they were not processed by the sender and the receiver may treat the streams as though they had never been created at all (hence the receiver may want to re-create the streams later on a new connection).¶

Endpoints should always send a GOAWAY frame before closing a connection so that the remote can know whether a stream has been partially processed or not. (For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate where it stopped working).¶

After sending a GOAWAY frame, the sender can ignore frames for new streams.¶

[rfc.comment.6: Issue: connection state that is established by those "ignored" frames cannot be ignored without the state in the two peers becoming unsynchronized.] ¶

The GOAWAY frame does not define any type-specific flags.¶

The GOAWAY frame applies to the connection, not a specific stream. The stream identifier MUST be zero.¶

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

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

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

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

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

The GOAWAY frame also contains a 32-bit error code (Section 3.5.3) that contains the reason for closing the connection.¶

The HEADERS frame (type=0x8) provides header fields for a stream. Any number of HEADERS frames can may be sent on an existing stream at any time.¶

Additional type-specific flags for the HEADERS frame are: ¶

CONTINUES (0x2):

The CONTINUES bit indicates that this frame does not contain the entire payload necessary to provide a complete set of headers.

The payload for a complete set of headers is provided by a sequence of HEADERS frames, terminated by a HEADERS frame without the CONTINUES bit. Once the sequence terminates, the payload of all HEADERS frames are concatenated and interpreted as a single block.

A HEADERS frame that includes a CONTINUES bit MUST be followed by a HEADERS 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 3.5.1) of type PROTOCOL_ERROR.

The payload of a HEADERS frame contains a Headers Block (Section 3.7).¶

The HEADERS frame is associated with an existing stream. If a HEADERS frame is received with a stream identifier of 0x0, the recipient MUST respond with a stream error (Section 3.5.2) of type PROTOCOL_ERROR.¶

The HEADERS frame changes the connection state as defined in Section 3.7.¶

The WINDOW_UPDATE frame (type=0x9) is used to implement flow control.¶

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

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

Flow control only applies to frames that are identified as being subject to flow control. Of the frame types defined in this document, this includes only DATA frame. 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 3.5.2) or connection error (Section 3.5.1) of type FLOW_CONTROL_ERROR if it is unable accept a frame.¶

The following additional flags are defined for the WINDOW_UPDATE frame: ¶

END_FLOW_CONTROL (0x2):

Bit 2 being set indicates that flow control for the identified stream or connection has been ended; subsequent frames do not need to be flow controlled.

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.¶

The payload of a WINDOW_UPDATE frame is a 32-bit value indicating the additional number of bytes that the sender can transmit in addition to the existing flow control window. The legal range for this field is 1 to 2³¹ - 1 (0x7fffffff) bytes; the most significant bit of this value is reserved.¶

At the application level, HTTP uses name-value pairs in its header fields. Because HTTP/2.0 merges the existing HTTP header fields with HTTP/2.0 headers, there is a possibility that some HTTP applications already use a particular header field name. To avoid any conflicts, all header fields introduced for layering HTTP over HTTP/2.0 are prefixed with ":". ":" is not a valid sequence in HTTP/1.* header field naming, preventing any possible conflict.¶

The client initiates a request by sending a HEADERS+PRIORITY frame. Requests that do not contain a body MUST set the FINAL flag, indicating that the client intends to send no further data on this stream, unless the server intends to push resources (see Section 4.3). HEADERS+PRIORITY frame does not contain the FINAL flag for requests that contain a body. The body of a request follows as a series of DATA frames. The last DATA frame sets the FINAL flag to indicate the end of the body.¶

The header fields included in the HEADERS+PRIORITY frame contain all of the HTTP header fields associated with an HTTP request. The definitions of these headers are largely unchanged relative to HTTP/1.1, with a few notable exceptions: ¶

• The HTTP/1.1 request-line has been split into two separate header fields named :method and :path, whose values specify the HTTP method for the request and the request-target, respectively. The HTTP-version component of the request-line is removed entirely from the headers.
• The host and optional port portions of the request URI (see [RFC3986], Section 3.2), is specified using the new :host header field. [rfc.comment.7: Ed. Note: it needs to be clarified whether or not this replaces the existing HTTP/1.1 Host header.]
• A new :scheme header field has been added to specify the scheme portion of the request-target (e.g. "https")
• All header field names MUST be lowercased, and the definitions of all header field names defined by HTTP/1.1 are updated to be all lowercase.
• The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-Encoding header fields are no longer valid and MUST not be sent.

All HTTP Requests MUST include the ":method", ":path", ":host", and ":scheme" header fields.¶

Header fields whose names begin with ":" (whether defined in this document or future extensions to this document) MUST appear before any other header fields.¶

If a client sends a HEADERS+PRIORITY frame that omits a mandatory header, the server MUST reply with a HTTP 400 Bad Request reply. [rfc.comment.8: Ed: why PROTOCOL_ERROR on missing ":status" in the response, but HTTP 400 here?] ¶

If a server receives a request where the sum of the data frame payload lengths does not equal the size of the Content-Length header field, the server MUST return a 400 (Bad Request) error.¶

Although POSTs are inherently chunked, POST requests SHOULD also be accompanied by a Content-Length header field. First, it informs the server of how much data to expect, which the server can use to track overall progress and provide appropriate user feedback. More importantly, some HTTP server implementations fail to correctly process requests that omit the Content-Length header field. Many existing clients send a Content-Length header field, and some server implementations have come to depend upon its presence.¶

A client provides priority in requests as a hint to the server. A server SHOULD attempt to provide responses to higher priority requests before lower priority requests. A server could send lower priority responses during periods that higher priority responses are unavailable to ensure better utilization of a connection.¶

If the server receives a data frame prior to a HEADERS+PRIORITY frame the server MUST treat this as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.¶

The server responds to a client request using the same stream identifier that was used by the request. An HTTP response begins with a HEADERS frame. An HTTP response body consists of a series of DATA frames. The last data frame contains a FINAL flag to indicate the end of the response. A response that contains no body (such as a 204 or 304 response) consists only of a HEADERS frame that contains the FINAL flag to indicate no further data will be sent on the stream.¶

• The response status line is unfolded into name-value pairs like other HTTP header fields and must be present:

  ":status":

  The HTTP response status code (e.g. "200" or "200 OK")

• All header field names starting with ":" (whether defined in this document or future extensions to this document) MUST appear before any other header fields.
• All header field names MUST be all lowercase.
• The Connection, Keep-Alive, Proxy-Connection, and Transfer-Encoding header fields are not valid and MUST not be sent.
• Responses MAY be accompanied by a Content-Length header field for advisory purposes. This allows clients to learn the full size of an entity prior to receiving all the data frames. This can help in, for example, reporting progress.
• If a client receives a response where the sum of the data frame payload length does not equal the size of the Content-Length header field, the client MUST ignore the content length header field. [rfc.comment.9: Ed: See issue 46.]

If a client receives a response with an absent or duplicated status header, the client MUST treat this as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.¶

If the client receives a data frame prior to a HEADERS frame the client MUST treat this as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.¶

Clients MUST support gzip compression. Regardless of the value of the Accept-Encoding header field, a server MAY send responses with gzip or deflate encoding. A compressed response MUST still bear an appropriate Content-Encoding header field.¶

A server pushes resources in association with a request from the client. Prior to closing the response stream, the server sends a PUSH_PROMISE for each resource that it intends to push. The PUSH_PROMISE includes header fields that allow the client to identify the resource (:scheme, :host, and :path).¶

A server can push multiple resources in response to a request, but all pushed resources MUST be promised on the response stream for the associated request. A server cannot send a PUSH_PROMISE on a new stream or a half-closed stream.¶

The server SHOULD include any header fields in a PUSH_PROMISE that would allow a cache to determine if the resource is already cached (see [HTTP-p6], Section 4).¶

After sending a PUSH_PROMISE, the server commences transmission of a pushed resource. A pushed resource uses a server-initiated stream. The server sends frames on this stream in the same order as an HTTP response (Section 4.2.3): a HEADERS frame followed by DATA frames.¶

Many uses of server push are to send content that a client is likely to discover a need for based on the content of a response representation. To minimize the chances that a client will make a request for resources that are being pushed - causing duplicate copies of a resource to be sent by the server - a PUSH_PROMISE frame SHOULD be sent prior to any content in the response representation that might allow a client to discover the pushed resource and request it.¶

The server MUST only push resources that could have been returned from a GET request.¶

Note: A server does not need to have all response header fields available at the time it issues a PUSH_PROMISE frame. All remaining header fields are included in the HEADERS frame. The HEADERS frame MUST NOT duplicate header fields from the PUSH_PROMISE frames.¶

When fetching a resource the client has 3 possibilities: ¶

1. the resource is not being pushed
2. the resource is being pushed, but the data has not yet arrived
3. the resource is being pushed, and the data has started to arrive

A client SHOULD NOT issue GET requests for a resource that has been promised. A client is instead advised to wait for the pushed resource to arrive.¶

When a client receives a PUSH_PROMISE frame from the server without a the :host, :scheme, and :path header fields, it MUST treat this as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.¶

To cancel individual server push streams, the client can issue a stream error (Section 3.5.2) of type CANCEL. After receiving a PUSH_PROMISE frame, the client is able to cancel the pushed resource before receiving any frames on the promised stream. The server ceases transmission of the pushed resource; if the server has not commenced transmission, it does not start.¶

To cancel all server push streams related to a request, the client may issue a stream error (Section 3.5.2) of type CANCEL on the associated-stream-id. By cancelling that stream, the server MUST immediately stop sending frames for any streams with in-association-to for the original stream. [rfc.comment.11: Ed: Triggering side-effects on stream reset is going to be problematic for the framing layer. Purely from a design perspective, it's a layering violation. More practically speaking, the base request stream might already be removed. Special handling logic would be required.] ¶

A client can choose to time out pushed streams if the server does not provide the resource in a timely fashion. A stream error (Section 3.5.2) of type CANCEL can be used to stop a timed out push.¶

If the server sends a HEADERS frame containing header fields that duplicate values on a previous HEADERS or PUSH_PROMISE frames on the same stream, the client MUST treat this as a stream error (Section 3.5.2) of type PROTOCOL_ERROR.¶

If the server sends a HEADERS frame after sending a data frame for the same stream, the client MAY ignore the HEADERS frame. Ignoring the HEADERS frame after a data frame prevents handling of HTTP's trailing header fields (Section 4.1.1 of [HTTP-p1]).¶