Some users of LBM wish to limit the scope of multicast to the local machine. This can save some NIC transmit bandwidth for cases where it is known that there are no interested receivers reachable by network connections.
Setting TTL to 1 is a well-defined way of limiting traffic to the sending LAN, but it can have undesired consequences (see Section 12). We haven't yet found a definition for the expected behavior when TTL is set to 0, but limiting the scope to the local machine would seem to be the obvious meaning. We know that multicast is not copied to local receivers through the loopback interface used for local unicast receivers. We believe that it may be copied closer to the device driver level in some kernels. Hence the particular NIC driver in use may have an impact on TTL 0 behavior. It's always best to test for traffic leakage before assuming that TTL 0 will keep it local to the sending machine.
We have run tests in our Latency Busters® Lab and found that TTL 0 will keep multicast traffic local on Linux 2.4 kernels, Solaris 8 and 10, Windows XP, AIX, and FreeBSD.
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