UUID格式：8-4-4-4-12 – 为什么？
为什么UUID的格式是“8-4-4-4-12”（数字）？ 我已经find了原因，但找不到要求的决定。
UUID格式为hexstring的示例：58D5E212-165B-4CA0-909B-C86B9CEE0111
它由time, version, clock_seq_hi, clock_seq_lo, node分开，如下面的rfc所示。
从IETF RFC4122：
4.1.2. Layout and Byte Order To minimize confusion about bit assignments within octets, the UUID record definition is defined only in terms of fields that are integral numbers of octets. The fields are presented with the most significant one first. Field Data Type Octet Note # time_low unsigned 32 0-3 The low field of the bit integer timestamp time_mid unsigned 16 4-5 The middle field of the bit integer timestamp time_hi_and_version unsigned 16 6-7 The high field of the bit integer timestamp multiplexed with the version number clock_seq_hi_and_rese unsigned 8 8 The high field of the rved bit integer clock sequence multiplexed with the variant clock_seq_low unsigned 8 9 The low field of the bit integer clock sequence node unsigned 48 10-15 The spatially unique bit integer node identifier In the absence of explicit application or presentation protocol specification to the contrary, a UUID is encoded as a 128-bit object, as follows: The fields are encoded as 16 octets, with the sizes and order of the fields defined above, and with each field encoded with the Most Significant Byte first (known as network byte order). Note that the field names, particularly for multiplexed fields, follow historical practice. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time_low | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | time_mid | time_hi_and_version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |clk_seq_hi_res | clk_seq_low | node (0-1) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | node (2-5) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
格式在IETF RFC4122的第3节中定义。输出格式定义在“UUID = …”的位置。
3.-名字空间注册模板
命名空间ID：UUID注册信息：注册date：2003-10-01
JTC 1 / SC6（ASN.1报告人组）名称空间的声明注册人：
语法结构声明：UUID是一个在空间和时间上都是唯一的标识符，相对于所有UUID的空间。 由于UUID的大小是固定的，并且包含时间字段，因此值可能会翻转（根据所使用的具体algorithm，AD 3400左右）。 UUID可用于多种用途，从标记寿命极短的对象到在networking中可靠地识别非常持久的对象。
The internal representation of a UUID is a specific sequence of bits in memory, as described in Section 4. To accurately represent a UUID as a URN, it is necessary to convert the bit sequence to a string representation. Each field is treated as an integer and has its value printed as a zero-filled hexadecimal digit string with the most significant digit first. The hexadecimal values "a" through "f" are output as lower case characters and are case insensitive on input. The formal definition of the UUID string representation is provided by the following ABNF [7]: UUID = time-low "-" time-mid "-" time-high-and-version "-" clock-seq-and-reserved clock-seq-low "-" node time-low = 4hexOctet time-mid = 2hexOctet time-high-and-version = 2hexOctet clock-seq-and-reserved = hexOctet clock-seq-low = hexOctet node = 6hexOctet hexOctet = hexDigit hexDigit hexDigit = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" / "a" / "b" / "c" / "d" / "e" / "f" / "A" / "B" / "C" / "D" / "E" / "F"
