How does diplomacy work during a military deadlock?

The war with Iran is in a deadlock. Despite a back and forth of peace plans, there is no permanent ceasefire. President Trump has oscillated between a willingness to engage in diplomacy and threats to resume the American bombing campaign if he doesn’t get a deal. All this has complicated negotiations, which the U.S. and Iran are holding through intermediaries. So, how do leaders try to negotiate with countries they’re in conflict with?