# Enhanced Multicast Repair Fast Reroute Mechanism for Smart Sensors IoT and Network Infrastructure
## 4 M-REP FRR Mechanism
### Description of the Original M-REP Mechanism
\begin{table}
\begin{tabular}{c c} \hline
**Protected Flow** & Unicast flow of packets with specified source and destination IP addresses (source, destination). The M-REP protects packets of a secured flow from losses during network failures. The unicast source address specifies a sending host, the unicast destination address specifies the receiving host. \\
**M-REP Address** & Special reserved multicast address used exclusively by the M-REP mechanism. The address represents a multicast G address of PIM-DM (S, G) pair, that is reserved and preconfigured for each protected flow. Each protected flow has unique G multicast address. \\
**M-REP Flow** & A multicast (S, G) flow that encapsulates packets of a protected flow in the event of a failure. The S address is the original source IP address of the protected flow. The G address is the multicast M-REP address. Together, they define a multicast distribution (Source, M-REP address) pair. \\
**Received Packet** & A packet of a protected flow received by a router. The router identifies the packet based on configured IP addresses of a protected flow. \\
**Received Multicast Packet** & A packet of the M-REP flow received by a router. The packet is identified by its M-REP address destination address. \\
**Protected Interface** & Router output interface selected according to the unicast routing table used. The interface is used for the routing of a protected flow (destination). \\
**Failure of Protected Interface** & Loss of connectivity on the protected interface. \\  & A router interface that first receives a multicast packet with the specified destination M-REP address (M-REP address). This interface has a similar role to the RFP interface in the ”original” PIM-DM specification. Each router may have at most one RFP interface per M-REP address. \\
**Reverse-path forwarding (RFP) Interface** & Interface has a similar role to the RFP interface in the ”original” PIM-DM specification. Each router may have at most one RFP interface per M-REP address. \\
**Connected Destination** & The network that contains the host with a protected flow destination address. The D router is directly connected to this network by one of its interfaces. \\
**M-REP Requirements** & Point-to-point routers. The original destination of the original unicast communication must be directly connected to router D. \\ \hline \end{tabular}
\end{table} 
Table 2: Terminology used by the Multicast Repair (M-REP) mechanism. Incorrect selection of the RPF interface would prevent routers, with the M-REP mechanism implemented, to accept and forward the multicast M-REP flow. For this reason, we created the following modification to the RPF interface selection:  
For a router that is not suitable for the M-REP multicast (S, G) flow, let an RPF interface be the one that allows the first multicast packet of a suitable multicast (S, G) flow. The original PIM-DM communication processing and sending mechanism, as well as legacy PIM-DM RPF selection, are not modified. The revised RPF check will only apply to the specific range of multicast group addresses reserved for the M-REP mechanism. The term \"first packet\" used in the modified RPF rule (the rule of first-arrival) refers to a multicast packet, the processing of which leads to the creation of a new multicast table entry for a specific (S, G) pair. This principle is in accordance with the standard rule of the PIM-DM protocol that the first multicast packet of a specific (S, G) pair requires the creation of a new entry in the multicast routing table. This record does not exist before the arrival of the first multicast packet of a specific multicast (S, G) pair. After selecting a RPF interface, previous routers forward multicast packets from their other interfaces and active PIM-DMs. Each router has exactly one RPF interface for a specific multicast M-REP (S, G) pair. An alternative path created by the M-REP IPFRR mechanism is random because its formation is conditional on the arrival of M-REP multicast packets to individual routers.