Delivering messages with no internet, no servers, and no SIM

Every messenger you use has a hidden dependency: a working network path to a datacenter. Drop into a basement, a packed stadium, a moving train through a tunnel, an exam hall with jammers, or a remote area with no plan, and the app is just a spinner. The people you want to reach are often standing a few meters away, but your message still has to travel to a server on another continent and back. When that path is gone, so is the app. Kabootar is my attempt to remove that dependency entirely. It is a messenger with no backend at all. Your phone forms a peer-to-peer mesh with other phones nearby, and messages hop device to device over Bluetooth and Wi-Fi until they reach the recipient. No internet, no servers, no SIM. It is built in Flutter, and the routing core is plain Dart. The core idea: delay-tolerant networking The insight that makes this work is refusing to assume the recipient is reachable right now . Normal networking is connection-oriented: open a path end to end, then send. If there is no path, there is no delivery. Kabootar instead treats the network as a delay-tolerant network (DTN). A message does not need a live end-to-end path at the moment you hit send. It needs a chain of carriers that will exist over time . You hand your message to whoever is nearby. They hold onto it, carry it as they walk around, and pass it along to the next phone they meet. Eventually a carrier bumps into the recipient and the message lands, even if that is minutes later and both you and the recipient have long since walked away. This is store-and-forward, the same shape as a durable, at-least-once message queue, except the queue is running across a swarm of phones instead of inside a datacenter. How a message actually travels The routing strategy is epidemic routing: flooding. When you send a message, it spreads to everyone in range like a rumor. Each device that receives it re-broadcasts it onward, so the message replicates through the crowd, taking every path at once. That red