Efficiency of BIER Multicast in Large Networks

Bit Index Explicit Replication (BIER) has been introduced by the IETF to transport IP multicast (IPMC) traffic within a BIER domain. Its advantage over IPMC is improved scalability regarding the number of multicast groups. However, scaling BIER to large networks is a challenge. To that end, receivers of a BIER domain are assigned to smaller subdomains. To deliver an IPMC packet over a BIER domain, a copy is sent to any subdomain with a receiver for that packet. Consequently, some links may carry multiple copies of the same IPMC packet, which contradicts the multicast idea. In this paper, we propose and compare various algorithms to select subdomains for BIER in order to keep the overall BIER traffic low despite multiple packet copies. We apply them to investigate the traffic savings potential of IPMC and BIER relative to unicast under various conditions. We show that the traffic savings depend on network topology, network size, and the size of the multicast groups. Also the extra traffic caused by BIER depends on these factors. In spite of some redundant packets, BIER can efficiently reduce the overall traffic in most network topologies. Similarly to IPMC, BIER also avoids heavily loaded links. Finally, we demonstrate that BIER subdomains optimized for failure-free conditions do not cause extensive overload in case of single link failures.