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Using IP Multicast Loop Option

Novell Cool Solutions: Tip
By N Subramanian

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Posted: 11 May 2004

Problem: For multicast applications, a multicast datagram is sent to a group of which the sending host is a member, a copy of the datagram is, by default, looped back by the IP layer for local delivery and handed over to the sending process.

Solution: In NetWare 6.5, the IP_MULTICAST_LOOP option allows an application to disable this loopback delivery. We need to ensure not only that the sending application receives the multicast packet via loopback, but also every other application that is listening on that shared port receives the packet as well. (This is possible when multiple applications are bound to the same port using SO_REUSEPORT option.) Thus, simply sending the packet back to the sending socket is insufficient. We need to take into account the other applications on the same host that are expecting the packet as well.

Here is the simple java multicast application script which uses this IP_MULTICAST_LOOP option.

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