Features And Formatters

Features participate in message processing. Formatters provide text representations. Neither contract requires the application to understand a concrete protocol packet type.

Feature Contract

Member	Responsibility
`Id`, `Name`, `Description`	Identifies and describes the feature.
`Priority`	Exposes priority metadata; the current builder does not sort features by this value.
`Register(connection)`	Attaches feature-specific state when a connection is constructed.
`Unregister(connection)`	Releases feature-specific state when the connection is disposed.
`ProcessRx(message, connection, cancel)`	Processes a received message at that connection level.
`ProcessTx(message, connection, cancel)`	Processes a message on its outgoing path.

The current pipeline runs features in their registration order. A feature returns the original message, a replacement message, or null to stop processing that message at the current stage.

Features can run at endpoint, port, and router levels. Check the supplied connection type when behavior belongs to one particular level. A TX feature is a processing hook, not evidence that bytes have already been written.

Feature calls for one message at a given connection level are awaited sequentially. Built-in physical endpoints also await RX forwarding through their parent port and router, so slow features apply backpressure to the receive pipeline. The synchronous compatibility RX publisher used by virtual ports and custom synchronous publishers instead holds up to 100 pending messages in FIFO order and drops a new arrival when that queue is full. See Connections for queue and event-stage details.

Honor the supplied cancellation token in asynchronous feature work. Connection shutdown can stop waiting for a feature that ignores cancellation, but cannot stop the feature's own task or release resources that task retains. Feature instances are shared by connections created from the same factory, so independent connections can invoke one feature concurrently; keep per-connection state isolated and release it in Unregister.

Endpoint Identity Tags

EndpointIdTagFeature sets an endpoint identity tag on received messages. Register it through the feature builder:

using Asv.IO;
using R3;

var factory = Protocol.Create(builder =>
{
    builder.Protocols.RegisterExampleProtocol();
    builder.Features.RegisterEndpointIdTagFeature();
});

await using var link = factory.CreateVirtualConnection(printMessagesToLog: false);
using var subscription = link.Client.OnRxMessage.Subscribe(message =>
{
    Console.WriteLine(message.GetEndpointId());
});
await link.Server.Send(new ExampleMessage3 { SenderId = 1 });

The feature currently populates the message tag on RX, not TX. Treat the tag as optional. For an endpoint-level observer, retaining the endpoint from the subscription is a direct way to associate both RX and TX with a connection.

Broadcasting

BroadcastingFeature<TMessage> can retransmit matching incoming messages through the router. It uses connection/message tags to avoid sending a message back to its source endpoint. Additional loop protection is disabled by default; enable it explicitly through BroadcastingFeatureOptions.IsLoopProtectionEnabled. Its bounded cache compares serialized-message CRCs within a time window measured with Stopwatch, independently of the protocol's TimeProvider. A general router does not broadcast received traffic automatically.

Broadcasting changes traffic. A passive inspector should normally observe endpoint events rather than implement forwarding as part of its capture logic.

Formatters

IProtocolMessageFormatter declares CanPrint, Print, and Order. The factory sorts registered formatters by Order; the protocol formatter selects the first capable formatter. PacketFormatting selects inline or indented output.

The built-in simple formatter and JSON formatter are convenient text views. A formatted object is not an exact recording of bytes received on a transport. Re-serializing a message is also different from capturing its original frame, especially when diagnosing damaged or unsupported packets.

Source Files

src/Asv.IO/Protocol/Features/IProtocolFeature.cs.
src/Asv.IO/Protocol/Connection/ProtocolConnection.cs.
src/Asv.IO/Protocol/Features/EndpointIdTag and Features/Broadcast.
src/Asv.IO/Protocol/Formatters.

See Connections for event stages and Serialization for binary representation.

26 October 2026