PIM-SSM
IP version 4 (IPv4) addresses in the 232/8 (232.0.0.0 to 232.255.255.255) range are designated as source-specific multicast (SSM) destination addresses and are reserved for use by source-specific applications and protocols. For IP version 6 (IPv6), the address prefix FF3x::/32 is reserved for source-specific multicast use.
The SSM destination address 232.0.0.0 is reserved, and it must not be used as a destination address. Similarly, FF3x::4000:0000 is also reserved. The goal of reserving these two addresses is to preserve one invalid SSM destination for IPv4 and IPv6, which can be useful in an implementation as a null value.
The address range 232.0.0.1 – 232.0.0.255 is currently reserved for allocation by IANA.
The policy for allocating the rest of the SSM addresses to sending applications is strictly locally determined by the sending host.
When allocating SSM addresses dynamically, a host or host operating system MUST NOT allocate sequentially starting at the first allowed address. It is RECOMMENDED to allocate SSM addresses to applications randomly, while ensuring that allocated addresses are not given simultaneously to multiple applications (and avoiding the reserved addresses).
As described in the post “Layer 2 Multicast Addresses“, the mapping of an IP packet with an SSM destination address onto a link-layer multicast address does not take into account the datagram’s full source IP address (on commonly-used link layers like Ethernet). If all hosts started at the first allowed address, then with high probability, many source-specific channels on shared-medium local area networks would use the same link-layer multicast address. As a result, traffic destined for one channel subscriber would be delivered to another’s IP module, which would then have to discard the datagram.
Packet Forwarding
A router that receives an IP datagram with a source-specific destination address MUST silently drop it unless a neighboring host or router has communicated a desire to receive packets sent from the source and to the destination address of the received packet.
With PIM-SSM, successful establishment of an (S,G) forwarding path from the source S to any receiver depends on hop-by-hop forwarding of the explicit join request from the receiver toward the source. The protocol(s) and algorithms that are used to select the forwarding path for this explicit join must provide a loop-free path.
Protocol Behavior
A network can concurrently support SSM in the SSM address range and any-source multicast in the rest of the multicast address space, and it is expected that this will be commonplace. In such a network, a router may receive a non-source-specific, or “(*,G)” in conventional terminology, request for delivery of traffic in the SSM range from a neighbor that does not implement source-specific multicast in a manner compliant with this document. A router that receives such a non-source-specific request for data in the SSM range MUST NOT use the request to establish forwarding state and MUST NOT propagate the request to other neighboring routers. A router MAY log an error in such a case. This applies both to any request received from a host (e.g., an IGMPv1 or IGMPv2 [IGMPv2] host report) and to any request received from a routing protocol (e.g., a PIM-SM (*,G) join).
(The majority of the text above was taken from RFC4607.)
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