Extensions to BIER Tree Engineering (BIER-TE) for large multicast domains and 1:1 protection

Bit Index Explicit Replication (BIER) was developed by the IETF as a new forwarding paradigm for stateless multicast packet forwarding. IP multicast (IPMC) creates a per-hop forwarding state for each IP multicast group. In contrast, ingress nodes of a BIER domain equip packets with a bitstring indicating egress nodes, and BIER nodes forward the packets according to that bitstring. As the bitstring size is limited, e.g., 256 bits, only that number of egress nodes can be addressed. To scale BIER to larger networks, the egress nodes can be assigned to subsets and addressed with with a subset-specific bitstrings including a subset ID. This approach is even compliant with fast reroute (FRR) mechanisms for BIER. BIER-TE extends BIER with tree engineering capabilities, i.e., the bitstring is repurposed to indicate both egress nodes and links, so that explicit paths can be encoded over which packets are transmitted. However, BIER’s scaling mechanism cannot be adopted out of the box for BIER-TE because the encoding of explicit paths requires that all contained links belong to the same subset. Obviously, chosen subsets for BIER-TE scaling must represent connected sub-topologies. In this work, we extend BIER-TE for scaling. When an ingress node sends a packet to an egress node in another subset, it tunnels the packet to the ingress node of that subset. We further protect the communication against link and node failures, which is particularly challenging when the ingress node of a subset fails, and which is a major contribution of this paper. The suggested protocol solution combines existing concepts like tunneling, egress protection, and BIER-TE-FRR. We identify various behaviors of BIER-TE nodes, and implement them on the P4-programmable Tofino switching ASIC to demonstrate the feasibility of the proposed approach. Finally, we evaluate and discuss the forwarding performance of the prototype.