When Windows 2003 came out, Microsoft introduced the ability to broadcast messages to any machines that were listening back. All you had to do was send out a message on a particular port and IP address and any client that had set up a Multicast queue with matching port and IP address would get a copy.

Since its introduction, there have been a couple of security vulnerabilities that needed to be removed:

• Microsoft Security Bulletin MS06-052
  Vulnerability in Pragmatic General Multicast (PGM) Could Allow Remote Code Execution (919007)
  
• Microsoft Security Bulletin MS08-036
  Vulnerabilities in Pragmatic General Multicast (PGM) could allow denial of service (950762)

The second of these, MS08-036, was resolved through an undocumented change in functionality. Basically, a limit of 64kb was put on the maximum size of a message that could be broadcast using the Multicast method. Obviously this has caused a few problems for any existing MSMQ Multicast applications that expected to be able to send larger messages.

A hotfix has been developed to resolve this problem.

961605 FIX: Multicast messages larger than 64 kilobytes (KB) are not delivered as expected by using Message Queuing 3.0 after security update MS08-036 is installed

A registry change is required:

1. Open the registry with Regedit
2. Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RMCAST\Parameters\
3. Create a DWord called MaxpacketSize
4. Set the value to the desired number of bytes.
   You can set it to a value between zero and 4MB.
   If you specify anything above 4MB, it will default to 64K.
5. A reboot is needed after adding this value.