Two mode-locked femtosecond fiber lasers, connected via a $7\mathrm{km}$ fiber link, are synchronized to an rms timing jitter of $19\mathrm{fs}$ , observed over the entire Nyquist bandwidth (half of the $93\mathrm{MHz}$ repetition frequency). This result is achieved in two steps. First, active cancellation of the fiber-transmission noise reduces timing jitter caused by path length fluctuations to a record level of $16\mathrm{fs}$ . Second, using a wide bandwidth interactivity actuator, the slave laser is synchronized to the incoming stable pulse train from the reference laser to within $10\mathrm{fs}$ . These results are confirmed by an optical cross-correlation measurement performed independently of the feedback loop operated in the microwave domain.