IPv6 PIM-SSM
The ASM model includes IPv6 PIM-DM and IPv6 PIM-SM. The SSM model can be implemented by leveraging part of the IPv6 PIM-SM technique. It is also called "IPv6 PIM-SSM."
The SSM model provides a solution for source-specific multicast. It maintains the relationship between hosts and devices through MLDv2.
In actual applications, part of MLDv2 and IPv6 PIM-SM techniques are adopted to implement the SSM model. In the SSM model, because receivers have located an IPv6 multicast source, no RP or RPT is required. No source registration process is required for discovering IPv6 multicast sources in other IPv6 PIM domains.
IPv6 PIM-SSM mechanisms include neighbor discovery, DR election, and SPT building.
Neighbor discovery
IPv6 PIM-SSM uses the same neighbor discovery mechanism as IPv6 PIM-SM. For more information, see "Neighbor discovery."
DR election
IPv6 PIM-SSM uses the same DR election mechanism as IPv6 PIM-SM. For more information, see "DR election."
SPT building
The decision to build an RPT for IPv6 PIM-SM or an SPT for IPv6 PIM-SSM depends on whether the IPv6 multicast group that the receiver host joins is in the IPv6 SSM group range. The IPv6 SSM group range reserved by IANA is FF3x::/32, where "x" represents any legal address scope.
Figure 118: SPT building in IPv6 PIM-SSM
As shown in Figure 118, Host B and Host C are receivers. They send MLDv2 report messages to their DRs to express their interest in the multicast information that the IPv6 multicast source S sends to the IPv6 multicast group G.
After receiving a report message, the DR first checks whether the group address in the message is in the IPv6 SSM group range and does the following:
If the group address is in the IPv6 SSM group range, the DR sends a subscribe message hop by hop toward the IPv6 multicast source S. All devices along the path from the DR to the IPv6 multicast source create an (S, G) entry to build an SPT. The SPT is rooted the IPv6 multicast source S and has the receivers as its leaves. This SPT is the transmission channel in IPv6 PIM-SSM.
If the group address is not in the IPv6 SSM group range, the receiver-side DR sends a (*, G) join message to the RP. The IPv6 multicast source registers with the source-side DR.
In IPv6 PIM-SSM, the term "subscribe message" refers to a join message.