Configuring the Multicast Time to Live Property

The multicast time to live (net.slp.multicastTTL property) determines the range over which a multicast packet is propagated in your intranet. The multicast TTL is configured by setting the net.slp.multicastTTL property to an integer between 1 and 255. The default value of the multicast TTL is 255, which means, theoretically, that the packet routing is unrestricted. However, a TTL of 255 causes a multicast packet to penetrate the intranet to the border routers on the edge of your administrative domain. Correct configuration of multicast on border routers is required to prevent multicast packets from leaking into the Internet's multicast backbone, or to your ISP.

Multicast TTL scoping is similar to standard IP TTL, with the exception that a TTL comparison is made. Each interface on a router that is multicast-enabled is assigned a TTL value. When a multicast packet arrives, the router compares the TTL of the packet with the TTL of the interface. If the TTL of the packet is greater than or equal to the TTL of the interface, the packet TTL is reduced by one, as is the case with the standard IP TTL. If the TTL becomes zero, the packet is discarded. Therefore, using TTL scoping for SLP multicast requires that your routers be properly configured so that packets are limited to a particular subsection of your intranet.

You can reduce the range of multicast propagation by reducing the TTL value. If the TTL value is 1, then the packet is restricted to the subnet. If it is 32, the packet is restricted to the site. Unfortunately, the term site is not defined by RFC 1075, where multicast TTLs are discussed. Values above 32 refer to theoretical routing on the Internet, and should not be used, but values below 32 can be used to restrict multicast to a set of accessible subnets, provided the routers are properly configured with TTLs.