BIER Payload Label

Introduction BIER is a multicast technology that achieves efficient replication without incurring per-flow/tunnel states inside a BIER domain. specifies its encapsulation. specifies:

. Note that the sub-domain-id must be inferred from the BIFT-id." ]]> The codepoints for "Proto field" in the above quote are registered in the IANA "BIER Next Protocol Identifiers" registry, with the following values for MPLS payload (among other values that are omitted here): Value Description Reference 1 MPLS packet with downstream-assigned label at top of stack [RFC8296] 2 MPLS packet with upstream-assigned label at top of stack [RFC8296] MPLS downstream-assigned labels are defined in . introduces upstream-assigned labels as a mechanism to support multicasting of MPLS packets, such that a single MPLS packet can be received by more than one LSR. To avoid the need for all receiving LSR to coordinate the assignment of such multicasted packet, the label is assigned by the upstream (sending) LSR. specifies "Context-Specific Label Spaces" as the common MPLS forwarding plane mechanism to support upstream assigned MPLS label for multicast, as well as other use-cases that can not be supported by downstream-assigned labels, such as per-platform and per-interface label spaces defined in . After was published, which specifies BIER header Proto field values only for upstream and downstream labels, new MPLS label semantics where introduced: introduced "SR Global Block" (SRGB), and introduced "Domain-wide Control Block" (DCB). Hence, these new semantics of MPLS labels where not considered by BIER Next Protocol Identifiers. Both type of new MPLS labels can be multicast, aka: replicated and hence received by more than one receiver. In terms of this could make them upstream-assigned. However, these MPLS labels are "globally" or rather domain-wide assigned, so there is no need for the use of a Context-Specific Label Space as required for upstream-assigned labels. In the MPLS LSR forwarding plane, such Context-Specific Label Spaces are more expensive, because they require more expensive context specific lookups of MPLS labels. Therefore it is desirable that new label semantics that can use the default label space are signalled such that they will be looked up from the default label space. Both SRGB and DCB labels where designed to use the default label space. Because the BIER Next Protocol Identifier value is meant to support (only) the necessary demultiplexing of packets in the forwarding plane on a BFER (but no further control plane semantics of the label used), it does not make sense to introduce new codepoints values for new MPLS label semantics when they are intended to share pre-existing forwarding plane semantics; specifically the default label space as used for downstream-assigned labels (codepoint 1), or a Context-Specific Label Space as used for upstream-assigned labels (codepoint 2). Additional codepoints for these cases would only complicate MPLS forwarding planes unnecessary and make introduction of new MPLS label semantics that can share these pre-existing forwarding plane demultiplexing paths unnecessary complex. For this reason, this document changes the definitions of the two existing codepoints as follows: Code point 1 indicates a label from the default label space. It is intended to be used by all label semantics that can use the default label space. Code point 2 indicates a label from a context-specific label space. It is intended to be used only by those label semantics that can not use code point 1 because different sending LSR in a domain may have different bindings for the same label. Both of these definitions are a superset of and hence backward-compatible with the existing definitions from , and . They are just not defining the use of the codepoints based on the the label semantics but on the LFIB required to support the semantic. In addition, this document clarifies that SRGB and DCB labels are using the default label space and hence need to be signalled in the BIER header via codepoint 1 and how future semantics need to use existing or new BIER Next Protocol Identifier values.

Specification

Update to RFC8296 The Description for codepoint 1 and 2 in the IANA BIER Next Protocol Identifier table are changed as follows: Value Description Reference 1 MPLS packet with the top of stack label to be looked up in the Default LFIB [THISRFC] 2 MPLS packet with the top of stack label to be looked up in the Context-specific LFIB for the BFIR [THISRFC] Downstream-assigned labels , "SR Global Block" (SRGB, ) and "Domain-wide Control Block" (DCB), label use the Default LFIB for forwarding and to thus require use of value 1. Upstream-assigned labels , as used by , , and require the use of a context-specific LFIB and do thus require use of value 2. New semantics for MPLS labels beyond the aforementioned (upstream, downstream, SRGB, DCB) SHOULD re-use one of these two pre-existing values unless they can not co-exist with the pre-existing MPLS label semantics in the forwarding plane. In that case, a new entry needs to be alloced in the registry.

Updates to RFC8402 and RFC9573 This document specifies that "SR Global Block" (SRGB) labels as specified in ) and "Domain-wide Control Block" (DCB) labels as specified in have to be indicated through a value of 1 (default LFIB) when used as a top op stack label in an BIER packet MPLS payload. This was underspecified in both RFCs.