HTTP 2 Frame Format

All HTTP 2 frames begin with a fixed 9-octet header followed by a variable- length payload as shown in following Figure.

    +-----------------------------------------------+    |                 Length (24)                   |    +---------------+---------------+---------------+    |   Type (8)    |   Flags (8)   |    +-+-------------+---------------+-------------------------------+    |R|                 Stream Identifier (31)                      |    +=+=============================================================+    |                   Frame Payload (0...)                      ...    +---------------------------------------------------------------+

 Figure : HTTP 2 Frame Layout

So there are total six fields in HTTP 2 frame.

Now lets see each field  in detail :

Length: The length of the frame payload expressed as an unsigned 24-bit integer. Values greater than 2^14 (16,384) MUST NOT be sent unless the receiver has set a larger value for SETTINGS_MAX_FRAME_SIZE. The 9 octets of the frame header are not included in this value.

Type: The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown.

Flags: An 8-bit field reserved for frame-type specific boolean flags. Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for a particular frame type MUST be ignored, and MUST be left unset (0x0) when sending.

R: A reserved 1-bit field. The semantics of this bit are undefined and the bit MUST remain unset (0x0) when sending and MUST be ignored when receiving.

Stream Identifier: A stream identifier (see Section 5.1.1) expressed as an unsigned 31-bit integer. The value 0x0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream. The structure and content of the frame payload is dependent entirely on the frame type.

Frame Size and Payload: The size of a frame payload is limited by the maximum size that a receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This setting can have any value between 2^14 (16,384) and 2^24-1 (16,777,215) octets, inclusive. All implementations MUST be capable of receiving and minimally processing frames up to 2^14 octets in length, plus the 9 octet frame header. The size of the frame header is not included when describing frame sizes.

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