If token held by another process, server queues request until token becomes available. When critical region exited, process sends Release message to central server. Must elect new server. Processes waiting for reply. Single server can become a bottleneck.
Process sends request to enter CR to all other processes.
Contains name of critical region, process number and time. If receiver is in CR it queues request. When process exits CR it sends OK to all processes on its queue. If any process crashes it will not respond to requests. Sender resends requests periodically then assumes dest.