Appendix A. Design Rationale

A.1. The "Exclude" Filter Mode

The "Exclude" filter mode was introduced to support "Source-Specific Multicast" (SSM) while still maintaining compatibility with the existing "Any-Source Multicast" (ASM) model.

In ASM, a host joins a group G and receives traffic from all sources sending to G. This is equivalent to EXCLUDE({}, G). In SSM, a host joins a specific channel (S, G) and receives traffic only from source S sent to group G. This is equivalent to INCLUDE({S}, G).

The EXCLUDE mode allows a host to block specific sources from an ASM group, which is useful for filtering out unwanted traffic.

A.2. The "Include" Filter Mode

The "Include" filter mode is the primary mode for SSM. It allows a host to explicitly specify the set of sources it wishes to receive. This simplifies the multicast routing protocol, as the router only needs to build a source-specific tree.

A.3. State Change Reports

State Change Reports are sent to ensure reliability. By repeating the reports, the probability of them being lost is reduced. This is important because IGMP runs over an unreliable transport (IP).