Frame Format / 帧格式

All frames begin with a fixed 9-octet header followed by a variable-length payload.

+-----------------------------------------------+
|                 Length (24)                   |
+---------------+---------------+---------------+
|   Type (8)    |   Flags (8)   |
+-+-------------+---------------+-------------------------------+
|R|                 Stream Identifier (31)                      |
+=+=============================================================+
|                   Frame Payload (0...)                      ..
+---------------------------------------------------------------+
                        Figure 1: Frame Layout

所有的帧都以固定的9字节的首部开始，后面接可变长度的有效载荷。

 +-----------------------------------------------+
 |                 Length (24)                   |
 +---------------+---------------+---------------+
 |   Type (8)    |   Flags (8)   |
 +-+-------------+---------------+-------------------------------+
 |R|                 Stream Identifier (31)                      |
 +=+=============================================================+
 |                   Frame Payload (0...)                      ...
 +---------------------------------------------------------------+
                            图 1：帧格式

The fields of the frame header are defined as:

  • 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 boolean flags specific to the frame type.

    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.

帧首部字段定义如下：

  • Length: 帧有效载荷的长度，以24位无符号整数表示。除非接收端通过SETTINGS_MAX_FRAME_SIZE设置了更大的值，否则不能发送Length值大于2^14 (16,384)的帧。

    帧首部的9字节长度不计入该值。

  • Type: 8bit的帧类型。帧类型决定了帧的格式和语义。必须忽略和丢弃任何未知的帧类型。

  • Flags: 为帧类型保留的8bit布尔标识字段。

    针对确定的帧类型赋予标识特定的语义。与确定的帧类型语义不相符的标识必须被忽略，并且在发送时必须是未设置的(0x0)。

  • R: 1bit的保留字段。未定义该bit的语义。当发送时，该bit必须是未设置的(0x0)；当接收时，必须忽略该bit。

  • Stream Identifier: 流标识符(参见 5.1.1节 )是一个31bit的无符号整数。值0x0是保留的，表明帧是与整体的连接相关的，而不是和单独的流相关。

帧有效载荷结构和内容完全取决于帧类型。

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