Appendix A. HTTP/2 Frames

This appendix is a quick reference for the HTTP/2 framing layer. Each section includes the frame type number, the binary layout for the frame, a description of the frame, and a list of flags specific to that frame.

The Frame Header

As described in Chapter 5, each frame starts with the same nine bytes:

See Table 5-1 for a description of each field.

DATA

DATA frames contain arbitrary sequences of octets. This is a fancy way of saying these frames contain the requested/sent objects. Object data is split up across one or more frames depending on the maximum frame length. The padding length field and the padding itself are conditionally included to hide the size of the message for security purposes:

DATA Frame Fields

Name	Length	Description
Pad Length	1 byte	Indicates the length of the padding field. Will only be present if the `PADDED` flag is set in the frame header.
Data	Variable	The content of the frame.
Padding	Variable	Length set by the Pad Length field. All bytes are set to zero.

DATA Frame Flags

Name	Bit	Description
END_STREAM	0x1	Indicates this is the frame in the stream
PADDED	0x8	Indicates that the Pad Length and Padding fields are used

HEADERS

HEADERS frames are used to start streams and send message headers to an endpoint:

HEADERS Frame Fields

Name	Length	Description
Pad Length	1 byte	Indicates the length of the padding field. Will only be present if the `PADDED` flag is set in the frame header.
E	1 bit	Indicates whether the stream dependency is exclusive. Only present if the `PRIORITY` flag is set.
Stream Dependency	31 bits	Indicates which stream this stream is dependent on, if any. Only present if the `PRIORITY` flag is set.
Weight	1 byte	Indicates the relative priority of the stream. Only present if the `PRIORITY` flag is set.
Header Block Fragment	Variable	The headers for the message.
Padding	Variable	Length set by the Pad Length field. All bytes are set to zero.

HEADERS Frame Flags

Name	Bit	Description
`END_STREAM`	0x1	Indicates this is the frame in the stream.
`END_HEADERS`	0x4	Indicates this is the last HEADERS frame in the stream. If this is flag not set it implies a CONTINUATION frame is next.
PADDED	0x8	Indicates that the Pad Length and Padding fields are used.
`PRIORITY`	0x20	When set it indicates that the E, Stream Dependency, and weight fields are used.

PRIORITY

The PRIORITY frame is sent to indicate the priority of the stream. It can be sent multiple times and will change the advised priority if previously set:

PRIORITY Frame Fields

Name	Length	Description
E	1 bit	Indicates whether the stream dependency is exclusive.
Stream Dependency	31 bits	Communicates the stream this stream is dependent on, if any.
Weight	1 byte	Indicates the relative priority of the stream.

A PRIORITY frame does not have any frame-specific flags.

RST_STREAM

RST_STREAM is used by either end of a stream to terminate the stream immediately. This is usually in response to an error condition.

The Error Code field in the frame is used to communicate the reason for the reset. See section 7 of RFC 7540¹ for a listing of those fields:

SETTINGS

The SETTINGS frame is a sequence of key/value pairs. The number of pairs is defined by the frame length divided by the individual setting length (six):

SETTINGS Parameters

Name	ID	Default	Description
`SETTINGS_HEADER_TABLE_SIZE`	0x1	4096	Changes the maximum size of the header table used for HPACK.
`SETTINGS_ENABLE_PUSH`	0x2	1	If set to 0 the peer may not send a PUSH_PROMISE frame.
`SETTINGS_MAX_CONCURRENT_STREAMS`	0x3	No limit	Indicates the maximum number of streams that the sender will allow.
`SETTINGS_INITIAL_WINDOW_SIZE`	0x4	65353	Indicates the sender’s initial window size for flow control.
`SETTINGS_MAX_FRAME_SIZE`	0x5	16384	Indicates the maximum frame size the sender is willing to receive. This value must be between this initial value and 16,777,215 (2^24-1).
`SETTINGS_MAX_HEADER_LIST_SIZE`	0x6	No limit	This setting is used to advise a peer of the maximum size of the header the sender is willing to accept.

When an endpoint receives and processes a SETTINGS frame, it must return a SETTINGS frame with the ACK flag (0x1) set in the frame header. This is the only flag defined for the SETTINGS frame. In this way the sender gets acknowledgment that the endpoint received the new SETTINGS and should be abiding by them.

PUSH_PROMISE

The PUSH_PROMISE frame is sent by a server to indicate to the client that it is about to send an object that the client has not explicitly requested. It is effectively the complement to a HEADERS frame sent by the client:

PUSH_PROMISE Frame Fields

Name	Length	Description
Pad Length	1 byte	Indicates the length of the padding field. Will only be present if the PADDED flag is set in the frame header.
R	1 bit	A single reserved bit. Do not set this or else.
Promised Stream ID	31 bits	Communicates the stream ID that the sender will use (will always be an even number as this is coming from the server by definition).
Header Block Fragment	Varible	The pushed headers for the message.
Padding	Variable	Length set by the Pad Length field. All bytes are set to zero.

PUSH_PROMISE Frame Flags

Name	Bit	Description
`END_HEADERS`	0x4	Indicates this is the last HEADERS frame in the stream. If this is not set it implies a CONTINUATION frame is next.
PADDED	0x8	Indicates that the Pad Length and Padding fields are used.

PING

The PING frame is intended to be used to measure the round-trip time between endpoints. The frame has one flag, ACK (0x1). When an endpoint receives a PING frame without an ACK sent, it must send a PING frame back with the ACK flag set and the same opaque data. It should be noted that PING frames are not associated with any particular stream (they are connection level), and thus should have their stream identifier set to 0x0:

GOAWAY

The GOAWAY frame is used to gracefully shut down a connection. This is a connection-level frame and must be sent on stream identifier 0x0. By sending a GOAWAY frame, the endpoint is able to clearly define to the receiver what it has and has not received and what problem (if any) may have contributed to the GOAWAY. In the case of a problem, the error code will be set to one of the codes defined in section 7 of RFC 7540, and the Last Stream ID will be set to the highest stream ID that was processed. If there has been no error but the endpoint is about to tear down the connection (browser tab closed, connection timer exceeded, etc.), it sends the NO_ERROR (0x0) code and sets the Last Stream ID to 2^31-1:

GOAWAY Frame Fields

Name	Length	Description
R	1 bit	A single reserved bit.
Last Stream ID	31 bits	The highest number stream ID the GOAWAY sender has received/processed. By sending this the receiver knows precisely what the sender has and has not received.
Error Code	4 bytes	The h2-defined error code or `NO_ERROR` code in the case of a successful shutdown.
Additional Debug Data	Variable	Opaque data the sender may send to indicate more information into the state or any problems.

WINDOW_UPDATE

The WINDOW_UPDATE frame is used for stream flow control. Sending a WINDOW_UPDATE frame to a peer tells that peer how many bytes the sender is willing to accept at that time. Flow control can apply to individual streams or all streams on a connection (Stream ID 0x0). Note that a WINDOW_UPDATE on a specific stream applies to the connection-level flow control as well: