6.1 Extensions to the IP Service Interface

6.1. Extensions to the IP Service Interface

Multicast IP datagrams are sent using the same "Send IP" operation as unicast datagrams, with an IP host group address, rather than an individual IP address, in the destination address field. However, some extensions or additions are required or may be desirable.

First, if there is not already such a capability, the service interface should provide a way for the upper-layer protocol to specify the IP time-to-live of an outgoing multicast datagram. If the upper-layer protocol chooses not to specify a time-to-live, it should default to 1 for all multicast datagrams, so that an explicit choice is required to multicast beyond a single network.

Second, for hosts that may be attached to more than one network, the service interface should provide a way for the upper-layer protocol to identify the network interface to be used for the multicast transmission. Only one interface is used for the initial transmission, with multicast routers responsible for forwarding to any other networks, if necessary. If the upper-layer protocol chooses not to identify an outgoing interface, a default interface should be used, preferably under the control of system management.

Third (level 2 implementations only), for the case where the host itself is a member of the group to which a datagram is being sent, the service interface should provide a way for the upper-layer protocol to inhibit local delivery of that datagram; by default, a copy of the datagram is looped back. This is a performance optimization for upper-layer protocols that restrict the membership of a group to one process per host (such as a routing protocol), or that handle loopback of group communication at a higher layer (such as a multicast transport protocol).