In Principio On Mars, SIN is taken to an extreme. Logically, Urbit is a single broadcast network - a single big Ethernet wire. Everyone sees everyone else's packets. You don't send a packet. You "release" it. As a matter of practical optimization, of course, routing is necessary, but it is opaque to the receiver. If there is a routing envelope, it is stripped before processing. An easy way to see this is to compare an Urbit packet, or card, to an IP packet. An IP packet has a header, which is visible both to the routing infrastructure and the application stack. The header, of course, contains the infamous source address. Urbit removes not just the source address, but also the rest of the header. An Urbit card is all payload - it has no region seen both by receiver and router. "Urbit: functional programming from scratch". Moron Lab. January 13, 2010. In the classical stack, a basic result of protocol design is that you can’t have exactly-once message delivery . To put it in the terms we in Section 3.2: you can’t build a bus on a network. With permanent