使用Unity3d NetworkTransport.Receive连接ID导致消息误发送给自己

Hey there! Let's figure out why your server is sending that "pong" message to itself instead of the client—this is a super common gotcha with Unity's low-level NetworkTransport API, so we can sort this out quickly.

The Core Issue: Mixing Up Connection IDs #

9 times out of 10, this happens because you're using the wrong ID when calling NetworkTransport.Send() on the server. Here's what's going wrong and how to fix it:

1. Don't Confuse Host ID with Connection ID

When you start your server, you get a hostId from NetworkTransport.AddHost()—this is just the identifier for the server's listening socket, not the ID you use to send messages to a client.

When a client connects, the server will receive a ConnectEvent, and you need to call NetworkTransport.AcceptConnection() to get a valid connectionId for that specific client. This is the ID you must use when sending messages back.

2. Example of Wrong vs. Correct Code

Let's look at the mistake first:

// ❌ Wrong: Using the server's hostId as the connectionId
byte error;
byte[] pongData = Encoding.UTF8.GetBytes("pong");
NetworkTransport.Send(serverHostId, serverHostId, reliableChannel, pongData, pongData.Length, out error);

This sends the message to the server's own host socket, hence why it's receiving it itself.

Here's the corrected version, using the client's actual connection ID from the received event:

// ✅ Correct: Using the client's connectionId from the DataEvent/ConnectEvent
void Update()
{
    int recHostId;
    int recConnectionId;
    int recChannelId;
    byte[] recBuffer = new byte[1024];
    int bufferSize = 1024;
    int dataSize;
    byte error;

    NetworkEventType eventType = NetworkTransport.Receive(
        out recHostId, out recConnectionId, out recChannelId, 
        recBuffer, bufferSize, out dataSize, out error
    );

    switch (eventType)
    {
        case NetworkEventType.ConnectEvent:
            // Accept the connection to get a valid client connection ID
            int acceptedConnId = NetworkTransport.AcceptConnection(recConnectionId, out error);
            if (error == 0)
            {
                Debug.Log($"Client connected: {acceptedConnId}");
            }
            break;
        case NetworkEventType.DataEvent:
            string receivedMsg = Encoding.UTF8.GetString(recBuffer, 0, dataSize);
            if (receivedMsg == "ping")
            {
                // Send pong using the client's connectionId from the DataEvent
                byte[] pongData = Encoding.UTF8.GetBytes("pong");
                NetworkTransport.Send(
                    recHostId, recConnectionId, reliableChannel, 
                    pongData, pongData.Length, out error
                );

                if (error != 0)
                {
                    Debug.LogError($"Send error: {(NetworkError)error}");
                }
            }
            break;
    }
}

3. Always Validate Connection IDs

• Make sure you're storing valid client connection IDs (from AcceptConnection()) and using those for all outgoing messages to clients.
• Check the error output parameter every time you call NetworkTransport.Send()—if you get InvalidConnectionId, that means you're using an ID that doesn't correspond to an active client connection.

Quick Recap #

• Use the connectionId from the received DataEvent or the accepted connection ID from ConnectEvent when sending replies.
• Never use the server's hostId as a connection ID for sending messages.
• Always accept connections properly with NetworkTransport.AcceptConnection() before communicating with a client.

内容的提问来源于stack exchange，提问作者Jeffrey W.