A Universally Unique IDentifier (UUID) URN Namespace
https://tools.ietf.org/html/rfc4122.html
A UUID is 128 bits long, and requires no central
registration process.
One of the main reasons for using UUIDs is that no centralized
authority is required to administer them (although one format uses
IEEE 802 node identifiers, others do not).
10 million per second per machine
A UUID is an identifier that is unique across both space and time,
with respect to the space of all UUIDs.
Generating a UUID does not require that a registration authority
be contacted. One algorithm requires a unique value over space
for each generator. This value is typically an IEEE 802 MAC
address, usually already available on network-connected hosts.
The address can be assigned from an address block obtained from
the IEEE registration authority.
去中心化检验的的借助mac地址和时间戳实现的唯一性
mac地址有中心化管理的机制
唯一性的保证的前提是有协议的限制，依赖于中心化的机制。