According the paper  (Section 4), timestamp ordering (T/O) is a technique whereby a serialization order is selected a priori and transaction execution is forced to obey this order. In the following, it reads
Each transaction is assigned a unique timestamp by its TM (transaction manager). The TM attaches the timestamp to all
dm-writesissued on behalf of the transaction, and DMs (data manager) are required to process conflicting operations in timestamp order.
However, this paper did not explain
Question: How to generate the globally unique timestamps for transactions in distributed database systems (assuming no centralized coordination)? What are the commonly used techniques (with references) in this area?
Specifically, is the
<local clock, site-id> pair sufficient and appropriate as a timestamp mechanism? If not, why?
 Concurrency Control in Distributed Database Systems by Philip A. Bernstein and Nathan Goodman. Computing Surveys, Vol.13, No., 2, June 1981.