Multicast service models differ for receiver hosts but do not affect multicast sources. A multicast source sends multicast packets by using its own IP address as the source IP address and a group address as the destination IP address. There are two multicast models: any-source multicast (ASM) and source-specific multicast (SSM). The two models use multicast group addresses in different ranges.
The ASM model distributes multicast data based on group addresses. A group address identifies a collection of network services. Multicast packets sent from any source to this address obtain the same services. After joining a group, a host can receive multicast data sent from any source with this group address as the destination address.
For security purposes, multicast source filter policies can be configured on routers to permit or deny packets from specific multicast sources. This filters data sent to receiver hosts.
In the ASM model, each group address must be unique on the entire multicast network. An ASM group can only be used by a single application at a time. If two applications use the same ASM group, receiver hosts of the two applications receive traffic from both application sources. In this case, the network will be congested and the receiver hosts cannot function normally.
The SSM model provides service for data flows from specific sources to a specific group. Receiver hosts can specify the sources from which they want to receive data when they join a group. After joining the group, the hosts receive only the data sent from the specified sources.
The SSM model does not require globally unique group addresses, but each group address must be unique to a multicast source. Different applications on a source must use different SSM groups. Different applications on different sources can reuse SSM group addresses because each source-group pair has an (S, G) entry. This model saves multicast group addresses without congesting the network.