A Multicast Wrapper Class

A multicast wrapper class has been requested by several of the visitors at CodeGuru and a few months back I was one of them. Surprisingly, there are very few resources on the Internet for multicast programming under WinSock and I think that this class that I wrote for my project will come in handy to some of you. The Code is amply commented and not too hard to follow. Points worth mentioning are discussed below and if any help is required don't hesitate to contact me.

I will assume that you are aware of the concept of multicasting and the socket API's. The class CMulticastSocket derives from MFC CAsyncSocket and will allow the programmer to use the following facilities:

    • Join a multicast host group given its multicast IP address and Port
    • Leave the host group
    • Send/receive data to/from the host group
    • Set the time to live (TTL) and Loopback options on the sending socket

Join a Group
To join a multicast group the following API call can be used

    Bool CMulticastSocket::JoinGroup (CString HostGroupIP, UINT nHostGroupPort, UINT nTTL, BOOL nLoopback)

The function returns true if successful, otherwise false.

All fields are pretty self explanatory but, for those who may try to go into the details of the code, I'll mention a technical problem that is faced with the loopback option. Not all multicast interfaces support the loopback option in which case, the loopback is by default enabled. Any packet sent to the host group will loop back to the sending interface. In short, the sender will receive any packet that he sends whether he/she likes it or not.

CMulticastSocket handles this problem internally. If the multicast interface does not support loopback option and the user wants to disable loopback, the class ignore any packets that it receives from the IP/Port address of the sending socket of the user. This way, all messages sent by you that loopback to the interface are ignored by the OnReceive function of the class.

nTTL     contains the value of Time to live for the packets. The TTL determine the number of hops of the packet can make before it dies.
 

Leave a Group
To leave a multicast hostgroup that you are connected to use

    Bool CMulticastSocket::LeaveGroup()

Returns true if successful, otherwise false.
 

Send Data to host group
    Bool CMulticastSocket::SendTo(const char* strMessage, int nSize)

Returns true if successful, otherwise false.

strMessage contains the Message to be sent.
nSize is the size of the Message
 

Receive Data from host group
    void CMulticastSocket::OnReceive(int nErrorCode)

This function overrides the CAsyncSocket OnReceive Notification Message. Within this message, there are check to ensure loopback is working correctly. The programmer can write his/her code in this function to handle the data that is received.
 

Class Data Members

A description of the class data members is as follows. This should come in particularly handy if you want to make changes to the code to suit your needs.
 

m_mrMReq The mreq structure containing the host group IP and interface
m_saHostGroup SOCKADDR_IN structure containing IP/Port of host group
m_SendSocket Another CAsyncSocket object for sending data to the host group
m_strLocalIP CString containing IP Address of the local machine (needed for forcing no loopback)
m_nLocalPort UINT containing Port Number of the receiving socket (needed for forcing no loopback)
m_strBuffer A character array containing the data received
m_strSendersIP CString containing IP of the Sender of data to the host group
m_nSendersPort UINT containing Port Number of the Sender of data to the host group
bForceNoLoopback Boolean indicating whether loopback has to be forcefully disabled or not (don't probe. For internal use only)
Data Members of CMulticastSocket

Downloads

Download Demo Project - 19 Kb
Download Source - 3Kb Date Posted : June 28, 1999