BGP Extensions for BIER

Bit Index Explicit Replication (BIER) [RFC8279] is a new multicast forwarding architecture which doesn't require an explicit tree-building protocol and doesn't require intermediate routers to maintain any multicast state. BIER is applicable in a multi-tenant data center network environment for efficient delivery of Broadcast, Unknown-unicast and Multicast (BUM) traffic while eliminating the need for maintaining a huge amount of multicast state in the underlay. This document describes BGP extensions for advertising the BIER-specific information. More specifically, in this document, we define a new optional, non- transitive BGP attribute, referred to as the BIER attribute, to convey the BIER-specific information such as BIER Forwarding Router identifier (BFR-id), BitString Length (BSL) and so on. In addition, this document specifies procedures to prevent the BIER attribute from "leaking out" of a BIER domain. These extensions are applicable in those multi-tenant data centers where BGP instead of IGP is used as an underlay . These extensions may also be applicable to other BGP based network scenarios, e.g., as described in .

This memo makes use of the terms defined in and [RFC8279].

This draft defines a new optional, transitive BGP path attribute, referred to as the BIER attribute. This attribute can be attached to a BGP UPDATE message by the originator so as to indicate the BIER- specific information of a particular BFR which is identified by the /32 or /128 address prefix contained in the NLRI. In other words, if the BIER path attribute is present, the NLRI is treated by BIER as a "BFR-prefix". When creating a BIER attribute, a BFR needs to include one BIER TLV for every Sub-domain that it supports. The attribute type code for the BIER Attribute is TBD. The value field of the BIER Attribute contains one or more BIER TLV as shown in Figure 1.

Type: Two octets encoding the BIER TLV Type: TBD. Length: Two octets encoding the length in octets of the TLV, including the type and length fields. The length is encoded as an unsigned binary integer. (Note that the minimum length is 8, indicating that no sub-TLV is present.) Sub-domain: a one-octet field encoding the sub-domain ID corresponding to the BFR-ID. BFR-ID: a two-octet field encoding the BFR-ID. Sub-TLVs: contains one or more sub-TLV. The BIER TLV MAY appear multiple times in the BIER Path Attribute, one for each sub-domain. There MUST be no more than one BIER TLV with the same Sub-domain value; if there is, the entire BIER Path Attribute MUST be ignored. A BIER TLV may have sub-TLVs, which may have their own sub-TLVs. All those are referred to as sub-TLVs and share the same Type space, regardless of the level.

The BIER MPLS Encapsulation sub-TLV matches the OSPFv2 "BIER MPLS Encapsulation sub-TLV" as specified in Section 2.2 of . It MAY appear multiple times in the BIER TLV. The following is copied verbatim from that section:

Similar to the concept in , the BIER non-MPLS Encapsulation sub-TLV is used for non-MPLS encapsulation. It matches the OSPFv2 BIER non-MPLS Encapsulation sub TLV as specified in Section 3.2 of . The following are copied verbatim from that section. Note to RFC Editor: the following copied text must match the final text in the RFC for .

Type: TBD3 (To be assigned by IANA). Length: 4 if the Nexthop is IPv4 address and 16 if the Nexthop is IPv6 address Nexthop: 4 or 16 octets of IPv4/IPv6 address The BIER Nexthop sub-TLV MAY be included in the MPLS or non-MPLS Encapsulation sub-TLV as well as in the top level BIER TLV.

A BIER Forwarding Egress Router (BFER) MUST attach a BIER attribute to its own BIER prefix NLRI. The BIER attribute MUST include one BIER TLV for each BIER sub-domain that it supports. Each BIER TLV MUST include an MPLS and/or non-MPLS Encapsulation sub-TLV, and SHOULD include a BIER Nexthop sub-TLV with the Nexthop set to the BIER prefix. If the BIER Nexthop sub-TLV is not included, the BIER prefix will be used by receiving BFRs as the BIER nexthop when calculating BIFT. A BFR/BFER MAY attach a BIER proxy range sub-TLV in the BIER TLV. In this case it MUST attach a BIER attribute to its own BIER prefix NLRIs. Other than this case, a BFR that is not a BFER (i.e., its BFR-ID is 0) SHOULD NOT attach a BIER attribute to its own BIER prefix NLRIs (if a BIER attribute is attached it will not get used anyway). When a BFR re-advertises a BGP NLRI with a BIER attribute, it SHOULD set/update the BIER Nexthop sub-TLV to use its own BIER prefix, in which case it MUST replace the MPLS or non-MPLS Encapsulation sub-TLV with its own, i.e., as if the BFR is attaching the encapsulation sub-TLV for its own BIER prefix. If it does not update the BIER Nexthop sub-TLVs, it MUST NOT update MPLS or non-MPLS Encapsulation sub-TLV. It's possible that the BFR supports some but not all BSLs in the received MPLS or non-MPLS Encapsulation sub-TLVs. After updating the BIER Nexthop sub-TLV in the top BIER TLV to itself, for the BSLs that it does support, the BFR MUST remove the BIER Nexthop sub-TLV (if present) in the corresponding Encapsulation sub-TLVs. For the BSLs that it does not support, it MUST not update those Encapsulation sub-TLVs except that if a BIER Nexthop sub-TLV is not included in the Encapsulation sub-TLV, the received BIER Nexthop sub-TLV in the top BIER TLV MUST be copied into the Encapsulation sub-TLV. All impacted length fields (e.g., the Encapsulation sub-TLV Length, the top level BIER TLV Length) MUST be updated accordingly. Since the BIER attribute is an optional, transitive BGP path attribute, a non-BFR BGP speaker could still advertise the received route with a BIER attribute.

For each sub-domain, a BFR calculates the corresponding BIFTs by going through the BIER prefixes whose BIER attribute includes a BIER TLV for the sub-domain. For a non-zero BFR-id in the BIER TLV, or for each BFR-id in the BIER Proxy Range sub-TLV in the BIER TLV of a BIER prefix, a BIFT entry is created or updated. The entry's BFR Neighbor (BFR-NBR) is the Nexthop in the BIER Nexthop sub-TLV in the corresponding Encapsulation sub-TLV, or in the top level BIER TLV if the Encapsulation sub-TLV does not have a Nexthop sub-TLV. If there is no Nexthop sub-TLV at all, The entry's BFR Neighbor is the BIER prefix itself. The BIER label or BIFT-id for the entry is derived from the Label Range in the MPLS Encapsulation sub-TLV or from the BIFT-id Range in the non-MPLS Encapsulation sub-TLV. BIER traffic is sent to the BFR-NBR either natively (BIER header directly follows a layer 2 header) if the BFR-NBR is directly connected, or via a tunnel otherwise. Notice that, if a non-BFR BGP speaker re-advertises a BIER prefix (in this case it can not update the BIER attribute since it is not capable), or if a BFR BGP speaker re-advertises a BIER prefix without updating the BIER Nexthop sub-TLV, the BFR receiving the prefix will tunnel BIER traffic - the BGP speaker re-advertising the BIER prefix will not see the BIER traffic for the BIER prefix.

It's assumed by this document that the BIER domain is aligned with an Administrative Domain (AD) which may be composed of multiple ASes (either private or public ASes). Use of the BIER attribute in other scenarios is outside the scope of this document. A boundary router of the AD that supports the BIER attribute MUST support a per-EBGP-session/group policy, that indicates whether the attribute is allowed. If it is not allowed, the BIER attribute MUST NOT be sent to any EBGP peer of the session/group, and the BIER attribute received from the peer MUST be treated exactly as if it were an unrecognized non-transitive attribute. That is, "it MUST be quietly ignored and not passed along to other BGP peers".

Thanks a lot for Eric Rosen and Peter Psenak for their valuable comments on this document.

IANA is requested to assign a codepoint in the "BGP Path Attributes" registry to the BIER attribute. IANA is requested to create a registry for "BGP BIER Attribute Types" and a registry for "BGP BIER TLV sub-TLV Types". The type field for both registry consists of two octets, with possible values from 1 to 655355 (the value 0 is reserved). The allocation policy for this field is to be "First Come First Serve". Three initial values are to be allocated from the "BGP BIER TLV sub-TLV Types" for MPLS Encapsulation, non-MPLS Encapsulation, and BIER Nexthop sub-TLV respectively.

This document introduces no new security considerations beyond those already specified in [RFC4271] and [RFC8279].

This document has the following contributors:

Zheng Zhang ZTE zhang.zheng@zte.com.cn