code: func EncodeSACKPermittedOption(b []byte) int {
  	if len(b) < 2 {
  		return 0
  	}
  
  	b[0], b[1] = TCPOptionSACKPermitted, 2
  	return int(b[1])
  }
docs: // EncodeSACKPermittedOption encodes a SACKPermitted option into the provided
  // buffer. If the buffer is smaller than required it just returns without
  // encoding anything. It returns the number of bytes written to the provided
  // buffer.