Packet

I’ve written up the first month of mailcast as a paper: Mailcast: one-way HF delivery of packet BBS bulletins with fountain codes. It’s long, so this post is the short version. What mailcast is UK packet BBSes pass bulletins to each other over slow connected-mode links, and every link carries every bulletin again. On HF that means fading, retries, and bulletins that take days to get through or never arrive because some BBS along the way isn’t set up to forward to the next. ...

DAPPS (Distributed Asynchronous Packet Pub/Sub) is a store-and-forward messaging layer for packet radio networks. Applications send messages to app@CALLSIGN, and DAPPS finds a path, forwards, fragments, retries and acknowledges, and delivers when it can. Real-time connectivity is not a goal. Think of it as packet mail for application developers, with delivery guarantees and a modern interface. There is a project page at /projects/dapps/ and the code is at packet-net/dapps. Why it exists Anything that wants to move data across a network of nodes, with links coming and going, has to build queues, retries and deduplication. I wanted that done once, below the application, so that someone writing an app only has to publish a message and subscribe to replies. ...

AX.25 is the link-layer protocol most of amateur packet radio runs on. The v2.2 specification ends with a set of SDL diagrams (figures C4.1 to C4.7) that draw the connected-mode state machine: states like Disconnected, Awaiting Connection, Connected and Timer Recovery, with every event, guard and action on the arrows. Each AX.25 stack I know of is somebody’s reading of those figures, typed into a switch statement. The readings differ in small ways, and when two stacks disagree there is nothing executable to check against, because the reference is a PDF. ...

APRS is widely used and inconsistently implemented. The spec is several documents written over twenty-odd years (APRS 1.0.1, the 1.1 and 1.2 addenda, now merged into APRS12c), real-world packets often bend the rules, and in places the text contradicts itself. Every decoder is somebody’s own reading of all that, and they disagree with each other. I’ve written five APRS libraries, one each in C#, Rust, Python, TypeScript and C, plus a shared set of test cases. All five give the same answer for every packet I’ve tested them with. They’re all at version 1.0.1. ...