RTT · Round-Trip Time
The there-and-back network delay; it sets the interactive-inference latency budget and, between sites, decides whether a training run can remain synchronous at all.
Current numbers
~5 µs/kmone-way fiber propagation floor (SMF-28, ~4.9 µs/km); ~10 ms RTT per 1,000 km — the latency physics scale-across cannot engineer around