A router contains a single time-counter. When a link interface on a router receives a time-code it checks that it is one more than the current value of the router’s time-counter. It then increments the router’s time-count and emits a TICK_OUT signal. This TICK_OUT signal propagates to the TICK_IN interfaces of all the router output ports so that they all emit the time-code. This time-code is the same value as that received by the router, since the router time-counter has been incremented. The time-code is not emitted by the link that first received the time-code.
If there is a circular connection then the router will receive a time-code with the same time value as the router time-counter. When this happens the time-code is ignored. In this way time flows forward through a network reaching all nodes but is suppressed if it flows back due to a circular connection.