How does Ripple synchronizes its ledger?

[Abrosim Adderiy]

Bitcoin decides on a global clock by the blockchain - every mined block is another tick of that clock.

In Ripple, there is a global ledger that keeps advancing in time - the ledger number keeps increasing all the time, and the ledger represents the state of the system at time t.

What synchronizes this ledger? How do different Ripple nodes know when they should advanced their ledger counter?