1.7 ASM versus SSM
The original vision for multicast in RFC 1112 supported both one-to-many and many-to-many communication models and has come to be known as Any-Source Multicast (ASM). Radio and television, as we have already discussed, are obvious examples of the one-to-many model. Applications such as online gaming and videoconferencing, in which some or all of the participants become sources, are examples of the many-to-many model. To support the many-to-many model, the network is responsible for source discovery. When a host expresses interest in a group, the network must determine all of the sources of that group and deliver them to the receiving host.
The mechanisms that provide this control plane of source discovery contribute the majority of the complexity surrounding interdomain multicast. However, applications that are believed to possess the greatest potential for commercial viability on the Internet use the one-to-many model. Since the bulk of the complexity is providing the least important functionality, the "ratio of annoyance" is disproportionately high in ASM.
It recently has been suggested that by abandoning the many-to-many model, multicast could deliver more "bang for the buck" on the Internet. By focusing on the one-to-many model, the most appealing of multicast applications could be supported while vastly reducing the amount of complexity required. Source-Specific Multicast (SSM) is a service model that supports multicast delivery from only one specified source to its receivers.
By sacrificing functionality that many may consider less important on the Internet, the network no longer needs to provide the control plane for source discovery. This control plane is now the responsibility of receivers. Typically, the application layer (via a mouse click, for example) informs the receiver who the source is. When the receiver informs its directly connected router that it is interested in joining a group, it specifies the source as well as the group. This last-hop router is then able to join the SPT directly, instead of having to join the RPT.
SSM eliminates the need for RPTs, RPs, and Multicast Source Discovery Protocol (MSDP), radically simplifying the mechanisms needed to deliver multicast. Best of all, this service model is realized through a subset of functionality already present in existing protocols. Very little needs to be added.
It is important to note that ASM and SSM are service models, not protocols. Different protocols are implemented and configured to deliver the service model. For example, SSM is a service model that is realized through a subset of functionality of PIM-SM and IGMPv3. The first five chapters of this book examine interdomain multicast generally from an ASM point of view because ASM is much more interesting from a protocol perspective. With a clear understanding of ASM, the operation and benefits of SSM become apparent. Chapter 6 describes SSM in detail.