Conflict resolution in Multimaster replication(Postgres-R)

Hello,

My basic question is: in multimaster replication, if each site goes
ahead and does the modifications issued by the transaction and then
sends the writeset to others in the group, how the ACID properties be
maintained?

Details:
Suppose there are two sites in the group, lets say, A and B and are
managing a database D. Two transactions TA and TB started in sites A
and B respectively, at nearly same time, wanted to update same row of
a table in the database. As, no locking structures and other
concurrency handling structures are replicated each will go ahead and
do the modifications in their corresponding databases and sends the
writeset. Since, both writesets contain update to the same row, will
the two transactions be rolled back or anything other than this
happens?

A more general question is: for Transactional isolation level
4(serializable level), the information such as locking of rows be
transmitted across sites? If not, what is the mechanism to address
concurrency with serializibility.

Thanks,
Srinivas