Comparison of Fast-Reroute Mechanisms for BIER-Based IP Multicast

IP multicast (IPMC) delivers one-to-many traffic along distribution trees. To that end, conventional IPMC requires state in forwarding devices for each IPMC group. This limits scalability of IPMC because forwarding state in core devices may be extensive and updates are necessary when IPMC groups or the topology change. The IETF introduced Bit Index Explicit Replication (BIER) for efficient transport of IPMC traffic. BIER leverages a BIER header and IPMC-group-independent forwarding tables for forwarding of IPMC packets in a BIER domain. However, legacy devices do not support BIER. In contrary, two SDN-based implementations for OpenFlow an P4 have been published recently. In this paper, we assess BIER forwarding which may be affected by network failures. So far there is no standardized procedure to handle such situations. Two concepts have been proposed. The first approach is based on Loop-Free Alternates. It reroutes traffic to suitable neighbors in the BIER domain to steer traffic around the failure. The second approach is a tunnel-based mechanism that tunnels BIER packets to appropriate downstream nodes within the BIER distribution tree. We explain and compare both approaches, and discuss their advantages and disadvantages.