when transaction is committed in postgres

Hello,I read the introduction of transaction on the postgresql official website. It notes that transaction is committed when commit call.
Suppose that , I execute begin、execute command and then commit using some postgres client such as jdbc or odbc, what happens if the postgresql server receive commit command and response commit message failure,like tcp shakehand?If it is still to be committed, the client has no way to know whether the transaction is committed successfully or not.How does postgresql handle such situation

Thanks advances
Richard