I just read your post and I understood what you are facing in.

The main problem is that when you are using NetMQ Router-Dealer sockets with bandwidth limiting tools like NetLimiter (set to 1 kbit/s) calling SendMulipartMessage (or even TrySendMulipartMessage with timeout) hangs indefinitely. This happens because the send operation blocks waiting for the network buffer to be available, which is serverely throttled.

Then why it happend?

NetMQ's SendMultipartMessage is a blocking call by default. If the outgoing buffer is full due to low bandwidth or slow consumers, the call will block until space is freed. even if you are using TrySendmultipartMessage with a timeout should prevent indefinite blocking, sometimes it can still hang because the underlying socket cannot send any data.

So To solve this problem ? then how can we solve this problem ?

So I think you can do like this.

First:
you can use TrySendMultipartMessage properly with a reasonable timeout and handle the failure case gracefully (e.g., retry later or drop the message).

Second:

you can implement your own message queue or backpressure system since the network is heavily throttled.

So you don't overwhelm the socket with messages faster than it can send.

Third:

you can use NetMQPoller and send messages only when the socket signals it's ready to send ( SendReady event ), to avoid blocking.

Fourth:

you can use async patterns or background workers. and Nerver block the main thread.

I will share the Example Code.

private readonly Queue<NetMQMessage> _sendQueue = new Queue<NetMQMessage>();
private RouterSocket _routerSocket;
private NetMQPoller _poller;

public void Setup()
{
    _routerSocket = new RouterSocket();
    _routerSocket.Bind("tcp://*:5555");

    _routerSocket.SendReady += (s, e) =>
    {
        if (_sendQueue.Count > 0)
        {
            var msg = _sendQueue.Peek();
            if (e.Socket.TrySendMultipartMessage(msg))
            {
                _sendQueue.Dequeue();
            }
            else
            {
                // Send failed; try again next SendReady event
            }
        }
    };

    _poller = new NetMQPoller { _routerSocket };
    _poller.RunAsync();
}

public void EnqueueMessage(NetMQMessage msg)
{
    _sendQueue.Enqueue(msg);
}

I hope this helps you.

Thanks.
Jeremy.