Packet.Ax25 0.31.0

Packet.Ax25

AX.25 v2.2 frame codec + connected-mode session runtime + inbound-connection listener.

The core AX.25 protocol library: a U/S/I frame codec, an SDL-driven connected-mode link-layer state machine, and Ax25Listener - a node-style coordinator that accepts inbound connections, originates outbound ones, and caches a session per peer. It deals in raw AX.25 frame bytes over an IAx25Transport, so it is transport-neutral (KISS, AXUDP, KISS-over-TCP - your choice). Part of Packet.NET, a .NET amateur-radio / AX.25 packet stack.

Install

dotnet add package Packet.Ax25

Quick start

Run a station that answers inbound connections, echoes received data back to the peer, and can also originate outbound links. Ax25Listener takes any IAx25Transport (from Packet.Kiss, Packet.Axudp, etc.):

using Packet.Ax25;
using Packet.Ax25.Session;
using Packet.Ax25.Transport;
using Packet.Core;

// IAx25Transport comes from a transport package (Packet.Kiss, Packet.Axudp, ...).
IAx25Transport transport = /* e.g. a KISS or AXUDP transport */;

await using var listener = new Ax25Listener(transport, new Ax25ListenerOptions
{
    MyCall = new Callsign("N0CALL", 0),
});

// A peer connected to us — wire up its inbound-data stream.
listener.SessionAccepted += (_, e) =>
{
    var session = e.Session;
    session.DataLinkSignalEmitted += (_, signal) =>
    {
        if (signal is DataLinkDataIndication data)
        {
            // Echo whatever the peer sent straight back.
            listener.SendData(session, data.Info);
        }
    };
};

await listener.StartAsync();

// Originate an outbound connection, then send some data on it.
var outbound = await listener.ConnectAsync(new Callsign("Q0PDN", 0));
listener.SendData(outbound, "hello over AX.25"u8.ToArray());

Need just the codec? Build and parse frames directly, no session machinery:

using Packet.Ax25;
using Packet.Core;

// Build a connectionless UI (unproto) frame.
var ui = Ax25Frame.Ui(
    destination: new Callsign("AA1AA", 0),
    source: new Callsign("N0CALL", 0),
    info: "73"u8,
    pid: Ax25Frame.PidNoLayer3);
byte[] kissBytes = ui.ToBytes();        // body only — a KISS TNC adds the FCS
byte[] axudpBytes = ui.ToBytesWithFcs(); // body + CRC-16-CCITT, for AXUDP / AXIP

// Parse a frame back from its on-the-wire (KISS-form) bytes.
if (Ax25Frame.TryParse(kissBytes, out var frame) && frame.IsUi)
{
    Console.WriteLine($"{frame.Source.Callsign} -> {frame.Destination.Callsign}");
}

Key types

• Ax25Frame - one AX.25 frame; codec for U/S/I/UI frames (mod-8 and extended mod-128), TryParse / ToBytes / ToBytesWithFcs, plus static factories (Ui, Sabm, Sabme, Disc, Ua, Dm, Rr, Rej, Srej, I, Xid, Test, …).
• Ax25FrameType - the wire type read off the control octet (Ax25Frame.FrameType, Ax25Frame.FrameTypeOf), valid under either modulo; Ax25Pid names the §3.4 protocol identifiers.
• Ax25LinkObserver (Packet.Ax25.Monitor) - a third party's reading of the links in heard traffic: feed it every frame on a port and it groups them by callsign pair, narrates each one in plain words ("calls M0LTE-9 again (attempt 3)", "asks GB7RDG-2 to resend from #3") with flags for resends, polls, rejects and digipeated copies, and snapshots every link's state and counters. For monitors and dashboards, not for running a link.
• Ax25Listener - owns one IAx25Transport; accepts inbound SABM, originates ConnectAsync, caches a session per peer (LRU), and exposes SessionAccepted / FrameTraced events. The node-style entry point.
• Ax25ListenerOptions - listener config: MyCall, timer overrides (T1V/T2/T3/N2/K), MaxCachedPeers, ParseOptions, Quirks, extended-mode preference.
• Ax25Session - one connected-mode link's state machine; exposes CurrentState, Context, and the DataLinkSignalEmitted upward-signal stream.
• DataLinkSignal - the upward signal record hierarchy (DataLinkConnectIndication, DataLinkDataIndication, DataLinkDisconnectIndication, DataLinkErrorIndication, …).
• Ax25ParseOptions - strict-vs-lenient parser knobs (named flags for real-world-peer leniencies); Ax25SessionQuirks - named per-session deviations matching buggy reference implementations.
• Ax25Adapter - lower-level glue wiring an Ax25Session + dispatcher to a byte sink, for callers who don't want the full listener.

See also

• Source & issues
• Packet.Ax25.Transport.Abstractions - the IAx25Transport seam the listener consumes
• Packet.Kiss - a KISS-TNC IAx25Transport
• Packet.Axudp - an AXUDP IAx25Transport
• Packet.NetRom - NET/ROM Layer 3/4 on top of AX.25

AGPL-3.0-licensed. Part of the Packet.NET stack.