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FIN: 1 bit

  1.   Indicates that this is the final fragment in a message.  The first<br />      fragment MAY also be the final fragment.

RSV1, RSV2, RSV3: 1 bit each

  1.   MUST be 0 unless an extension is negotiated that defines meanings<br />      for non-zero values.  If a nonzero value is received and none of<br />      the negotiated extensions defines the meaning of such a nonzero<br />      value, the receiving endpoint MUST _Fail the WebSocket<br />      Connection_.

Opcode: 4 bits

  1.   Defines the interpretation of the "Payload data".  If an unknown<br />      opcode is received, the receiving endpoint MUST _Fail the<br />      WebSocket Connection_.  The following values are defined.
  3.   *  %x0 denotes a continuation frame
  5.   *  %x1 denotes a text frame
  7.   *  %x2 denotes a binary frame
  9.   *  %x3-7 are reserved for further non-control frames
  11.   *  %x8 denotes a connection close
  13.   *  %x9 denotes a ping
  15.   *  %xA denotes a pong
  17.   *  %xB-F are reserved for further control frames

Mask: 1 bit

  1.   Defines whether the "Payload data" is masked.  If set to 1, a<br />      masking key is present in masking-key, and this is used to unmask<br />      the "Payload data" as per [Section 5.3](https://tools.ietf.org/html/rfc6455#section-5.3).  All frames sent from<br />      client to server have this bit set to 1.

Payload length: 7 bits, 7+16 bits, or 7+64 bits

  1.   The length of the "Payload data", in bytes: if 0-125, that is the<br />      payload length.  If 126, the following 2 bytes interpreted as a<br />      16-bit unsigned integer are the payload length.  If 127, the<br />      following 8 bytes interpreted as a 64-bit unsigned integer (the<br />      most significant bit MUST be 0) are the payload length.  Multibyte<br />      length quantities are expressed in network byte order.  Note that<br />      in all cases, the minimal number of bytes MUST be used to encode<br />      the length, for example, the length of a 124-byte-long string<br />      can't be encoded as the sequence 126, 0, 124.  The payload length<br />      is the length of the "Extension data" + the length of the<br />      "Application data".  The length of the "Extension data" may be<br />      zero, in which case the payload length is the length of the<br />      "Application data".

参考