SRv6 Bitmap Multicast.

Segment routing with IPv6 as specified in RFC8689 provides a source-routing solution for next generation network requirements. More applications and use-cases are finding a better solutions using SRv6. Along with this comes the need to support multicasting and broadcasting in such networks. The various use-cases for this would be stated in the subsequent sections. Broadcasting typically needs a point-to-multipoint (p2mp) distribution with all the nodes in the network being receivers. Multicasting would imply p2mp distribution along with multipoint-to-multipoint (mp2mp) packet distribution with the participants being pre-determined via a discovery or provisioning mechanism. Bit-Index-Explicit-Replication specified in RFC8279 introduced a per-flow-state-free forwarding for multicast using a bit-indexed addressing of multicast receivers. This document introduces a bit-map based distribution schema in IPv6 networks to achieve p2mp distribution patterns. SRv6 introduced a new semantic to IPv6 address by fragmenting the address space into Locator:Function:Argument construct to achieve the desired SR functionality. This document proposes a similar treatment of IPv6 address to achieve BIER forwarding.

This document provides a new semantic to the IPv6 address as SI_LOCATOR:BITSTRING:FUNCTION:ARGUMENT. This structure is partly borrowed from SRv6. The BITSTRING part is newly introduced to address the egress routers in the BIER subdomain by its bit index. From here on this format is called as Bit-Index-6 format (BI6) BIER architecture envisages forwarding by identifying each egress router with an BFR-id. These BFR-id in forwarding translates to a Set-Identifier (SI) and Bitstring. In the IPv6 Bit-Index format, the SI is identified by the SI_LOCATOR and bitstring is encoded in the BITSTRING part of the BI6. The FUNCTION and ARGUMENT bits are part of the format. But depending on the network requirement their lengths may be set to 0 for using this bits for extended bitstring. SI_LOCATOR is defined as a routable prefix to reach the specific set of routers in a SetIdentifier. Once a BI6 packet reaches a router that is part of a SI, The bit-index based part is referred to for forwarding towards the BFER's with the BIER forwarding principles. The semantics of the FUNC and ARG bits is global in the Sub-domain. The attributes of FUNCTION and ARGUMENT bits must be pre-determined.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SI_LOCATOR | BITSTRING | FUNC | ARG | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

BIER architecture puts forward a multicast forwarding based on "Bit-Index-Explicit-Replication". This architecture defines a BIER domain in which an ingress router would encapsulate p2mp packet with a BIER header RFC8296 . This BIER packet would be replicated to the egress routers identified by the ingress in its BIER header, over an optimal per-flow-stateless tree discovered with the underlay.

In BIER architecture the multicast egress routers must be learned by the ingress router. This discovery may happen via some out-of-band mechanism beyond the scope of this document.

A network topology may have legacy devices which may not be capable of BI6 processing. When deploying BI6 the traffic may have to pass through some of these devices for loop-free forwarding. A router may come to know about the BI6 capability of a neighbouring device via the capabilities it has published in its IGP advertisement. Based on this IGP may form a map of BFER to the nearest BFR on the path to egress. If the BFR is not directly connected, then that BFR's node sid may be inserted into the SRH prior to forwarding the packet.

This specification introduces new semantics for IPv6 address. Though this draft does not need any allocations, New IANA allocations would be required for the supplimentary specifications.

This document proposes a semantic for IPv6 address. The security challenges that apply to IPv6 and in the BIER architecture applies to the intended BI6 forwarding model specified here. The further security scenarios would be added in the due course.

The Unique Local IPv6 address allocation RFC4193 provides free to use address blocks with SI_LOCATOR size of 48. This provides a maximum BI6 addressing space of 80 bit length. The Bit-index string length that can be used would be determined by the SI_locator prefix length and the need for FUNC and ARG bits. Hence if Unique local address space is used, upto 80 BFER's can be addressed.

With this specification the typical scale a BIER domain would be less than 96 egress routers. On networks with larger scale the current proposal is to have multiple subdomains and to do ingress replication for traffic bound to various subdomains.