Question

I’m looking at code, and I’m not certain of its functionality. I looked at documentation for the functions however they didn’t do much justice. Here is the code:

    private void m_InitUdpSock()
    {
        m_sockBroadcast = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        m_sockBroadcast.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.Broadcast,1);

        m_localhost     = new IPEndPoint(IPAddress.Parse(m_localHostIP),BIND_PORT);
        m_remotehost    = new IPEndPoint(IPAddress.Broadcast,SCAN_PORT);

        m_sockBroadcast.Bind(m_localhost);
        m_sockBroadcast.BeginReceiveFrom(m_rxBuffer,0,m_rxBuffer.Length,SocketFlags.None,ref m_HostEp,new AsyncCallback(ReceiveBroadcastData),m_sockBroadcast);
    }

From what I understand: this function is setting up a socket to broadcast a UDP message. It is using the m_localhost to send out broadcast. The m_localhost used to be:

m_localhost     = new IPEndPoint(IPAddress.Any,BIND_PORT);

Then, I wanted to select the Network Adapter I chose to communicate with, so I retrieved IP address of the Network Adapter I wanted to use and changed the m_localhost to the first example. Then using the Bind function, it should use the specificed network adapter, and use the port, BIND_PORT, to listen to respoonse messages.

I’m hoping someone could confirm I’m indeed using the network adapter of the IP I pass localhost.

The m_remotehost is just a IPEndPoint, which will hold an IP Address of a packet that responds, and the port, SCAN_PORT, with be the outgoing broadcast port.

SCAN_PORT and BIND_PORT have comments from previous developer; however I don’t know that they are accurate. Is m_remotehost specifying the port it should use to communicate back with me? (That sounds like bad practice to me)

SCAN_PORT  // Outgoing broadcast port.
BIND_PORT  // Incoming response listen port.

Next, in BeginReceiveFrom, the documentation was clear for the most part, however the parameter the previous developer used “ref HostEp” threw me off, because it was declared as:

m_HostEp = new IPEndPoint(IPAddress.Any,0);

so when I call BeginReceiveFrom, is it still using any local IP Address to broadcast messages, which is not what I want? m_HostEp is also used in the call EndReceiveFrom(iar,ref m_HostEp).

I’m looking for clarification on:
1. BIND_PORT and SCAN_PORT
2. What is m_HostEp usage, how is it affecting my communication?

If you need any clarifications, ask!

Thanks in advance.

Answer 1

Looking at the code snippet, m_remotehost is not used so you cannot determine the meaning of SCAN_PORT from it, but apparently the broadcast messages can be sent on a different port than the one incoming messages are received on.

• BIND_PORT is the port you will receive messages on.
• m_HostEp will contain the EndPoint of the sender of the message you receive.

When learning about sockets I read http://tangentsoft.net/wskfaq/ which turned out to be a great reference if you want to know more about the things “under the hood” as broadcast networking can be tricky. It isn’t targetted specifically to c# though.