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docs/router/transports/udp.md

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+# UDP Transport
+
+The UDP transport provides connectionless, fire-and-forget messaging suitable for broadcast notifications and scenarios where delivery guarantees are not critical.
+
+## Overview
+
+| Property | Value |
+|----------|-------|
+| Plugin Assembly | `StellaOps.Router.Transport.Udp.dll` |
+| Transport Name | `udp` |
+| Default Port | 5102 |
+| Security | Network isolation (no encryption) |
+| Use Case | Broadcast, metrics, fire-and-forget events |
+
+## Configuration
+
+### router.yaml
+
+```yaml
+Router:
+  Transport:
+    Type: udp
+    Udp:
+      Host: "0.0.0.0"
+      Port: 5102
+      ReceiveBufferSize: 65536
+      SendBufferSize: 65536
+      MulticastGroup: null         # Optional multicast group
+      MulticastTtl: 1
+      EnableBroadcast: false
+```
+
+### microservice.yaml
+
+```yaml
+routers:
+  - host: gateway.internal
+    port: 5102
+    transportType: Udp
+    priority: 1
+```
+
+### Environment Variables
+
+```bash
+ROUTER__TRANSPORT__TYPE=udp
+ROUTER__TRANSPORT__UDP__HOST=0.0.0.0
+ROUTER__TRANSPORT__UDP__PORT=5102
+```
+
+## Options Reference
+
+| Option | Type | Default | Description |
+|--------|------|---------|-------------|
+| `Host` | string | `0.0.0.0` | Bind address |
+| `Port` | int | `5102` | UDP port number |
+| `ReceiveBufferSize` | int | `65536` | Socket receive buffer size |
+| `SendBufferSize` | int | `65536` | Socket send buffer size |
+| `MulticastGroup` | string | `null` | Multicast group address (e.g., `239.1.2.3`) |
+| `MulticastTtl` | int | `1` | Multicast time-to-live |
+| `EnableBroadcast` | bool | `false` | Allow broadcast to `255.255.255.255` |
+| `MaxDatagramSize` | int | `65507` | Maximum UDP datagram size |
+
+## Use Cases
+
+### Fire-and-Forget Events
+
+For events where acknowledgment is not required:
+
+```csharp
+[StellaEndpoint("POST", "/events/metrics", FireAndForget = true)]
+public sealed class MetricsEndpoint : IStellaEndpoint<MetricsBatch, EmptyResponse>
+{
+    public Task<EmptyResponse> HandleAsync(MetricsBatch request, CancellationToken ct)
+    {
+        // Process metrics - no response expected
+        return Task.FromResult(new EmptyResponse());
+    }
+}
+```
+
+### Multicast Broadcasting
+
+For broadcasting to multiple listeners:
+
+```yaml
+Router:
+  Transport:
+    Type: udp
+    Udp:
+      MulticastGroup: "239.0.1.1"
+      MulticastTtl: 2
+```
+
+Services join the multicast group to receive broadcasts:
+
+```csharp
+// All services with this config receive broadcasts
+routers:
+  - host: "239.0.1.1"
+    port: 5102
+    transportType: Udp
+```
+
+## Performance Characteristics
+
+| Metric | Typical Value |
+|--------|---------------|
+| Latency (p50) | < 0.5ms |
+| Latency (p99) | < 2ms |
+| Throughput | 150,000+ pps |
+| Memory per socket | ~1KB |
+
+*Note: No delivery guarantees; packets may be lost under load*
+
+## Limitations
+
+1. **No delivery guarantees**: Packets may be dropped
+2. **No ordering guarantees**: Packets may arrive out of order
+3. **Size limits**: Maximum datagram size ~65KB (64KB practical limit)
+4. **No acknowledgments**: Sender doesn't know if message was received
+5. **No fragmentation handling**: Large messages must fit in single datagram
+
+## When to Use UDP
+
+**Good fit:**
+- Real-time metrics and telemetry
+- Service discovery announcements
+- Health check broadcasts
+- Low-latency gaming or streaming metadata
+
+**Not recommended:**
+- Transaction processing
+- Data that must not be lost
+- Large payloads (use TCP/TLS instead)
+
+## Security Considerations
+
+UDP transport does **not** provide encryption or authentication. Consider:
+
+- Network segmentation
+- Firewall rules to restrict UDP traffic
+- Application-level message signing if integrity is needed
+- DTLS wrapper for encrypted UDP (not built-in)
+
+## Troubleshooting
+
+### Messages Not Received
+
+1. Check firewall allows UDP traffic on the configured port
+2. Verify receiver is bound to correct address/port
+3. Check for buffer overflow (increase `ReceiveBufferSize`)
+4. Monitor for packet loss with network tools
+
+### Multicast Not Working
+
+1. Verify multicast routing is enabled on network
+2. Check `MulticastTtl` is sufficient for network topology
+3. Ensure IGMP snooping is properly configured
+4. Verify all receivers joined the multicast group
+
+### High Packet Loss
+
+1. Reduce message rate
+2. Increase buffer sizes
+3. Check for network congestion
+4. Consider switching to TCP for reliable delivery
+
+## See Also
+
+- [TCP Transport](./tcp.md) - Reliable delivery
+- [Transport Overview](./README.md)
+- [Router Architecture](../ARCHITECTURE.md)