RFC 9272

Internet Engineering Task Force (IETF)                          Z. Zhang
Request for Comments: 9272                                 A. Przygienda
Updates: 8401, 8444                                     Juniper Networks
Category: Standards Track                                    A. Dolganow
ISSN: 2070-1721                                               Individual
                                                              H. Bidgoli
                                                            IJ. Wijnands
                                                                A. Gulko
                                          Edward Jones Wealth Management
                                                          September 2022

   Underlay Path Calculation Algorithm and Constraints for Bit Index
                      Explicit Replication (BIER)


   This document specifies general rules for the interaction between the
   BIER Algorithm (BAR) and the IGP Algorithm (IPA) used for underlay
   path calculation within the Bit Index Explicit Replication (BIER)
   architecture.  The semantics defined in this document update RFC 8401
   and RFC 8444.  This document also updates the "BIER Algorithm"
   registry established in RFC 8401.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

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   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Updated Definitions for IPA and BAR Fields
   3.  General Rules for the BAR and IPA Interaction
     3.1.  When BAR Is Not Used
     3.2.  Exceptions or Extensions to the General Rules
   4.  Examples
   5.  IANA Considerations
   6.  Security Considerations
   7.  Normative References

   Authors' Addresses

1.  Introduction

   In the Bit Index Explicit Replication (BIER) architecture [RFC8279],
   packets with a BIER encapsulation header are forwarded to the
   neighbors on the underlay paths towards Bit-Forwarding Egress Routers
   (BFERs) that are represented by bits set in the BIER header's
   BitString.  The paths are calculated in the underlay topology for
   each sub-domain following a calculation algorithm specific to the
   sub-domain.  The topology or algorithm may or may not be congruent
   with unicast.  The algorithm could be a BIER-specific algorithm or
   could be a generic IGP one, e.g., Shortest Path First (SPF).

   In [RFC8401] and [RFC8444], an 8-bit BAR (BIER Algorithm) field and
   8-bit IPA (IGP Algorithm) field are defined to signal the BIER-
   specific algorithm and generic IGP Algorithm, respectively, and only
   value 0 is allowed for both fields in those two documents.

   This document specifies general rules for the interaction between the
   BIER Algorithm (BAR) and the IGP Algorithm (IPA) used for underlay
   path calculation when other BAR and/or IPA values are used.  The
   semantics defined in this document update [RFC8401] and [RFC8444].
   This document also updates the "BIER Algorithm" registry defined in
   [RFC8401] by renaming the "Experimental Use" range to "Private or
   Experimental Use".

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Updated Definitions for IPA and BAR Fields

   The definitions for the IPA and BAR fields in Section 6.1 of
   [RFC8401] and Section 2.1 of [RFC8444] are updated as follows.

   IPA:  IGP Algorithm.  Specifies a generic Routing Algorithm and
      related Routing Constraints to calculate underlay paths to reach
      other Bit-Forwarding Routers (BFRs).  Values are from the "IGP
      Algorithm Types" registry.  One octet.

   BAR:  BIER Algorithm.  Specifies a BIER-specific Algorithm and BIER-
      specific Constraints used to either modify, enhance, or replace
      the calculation of underlay paths to reach other BFRs as defined
      by the IPA value.  Values are allocated from the "BIER Algorithm"
      registry.  One octet.

      When a BAR value is defined, the corresponding BIER-specific
      Algorithm (BA) and BIER-specific Constraint (BC) semantics SHOULD
      be specified.  For an IGP Algorithm to be used as a BIER IPA, its
      Routing Algorithm (RA) and Routing Constraint (RC) semantics
      SHOULD be specified.  If any of these semantics is not specified,
      it MUST be interpreted as the "NULL" algorithm or constraint.  For
      example, the IGP Algorithm 0 defined in [RFC8665] is treated as
      having a NULL RC, i.e., no constraints (see Section 3).

      If a specification is not available for a specific BAR value, its
      value MUST be from the Private or Experimental Use range of the

3.  General Rules for the BAR and IPA Interaction

   For a particular sub-domain, all BFRs MUST be provisioned with and
   signal the same BAR and IPA values.  If a BFR discovers another BFR
   advertising a different BAR or IPA value for a sub-domain, it MUST
   treat the advertising router as incapable of supporting BIER for that
   sub-domain.  (One way of handling incapable routers is documented in
   Section 6.9 of [RFC8279], and additional methods may be defined in
   the future.)

   For a particular topology X that a sub-domain is associated with, a
   router MUST calculate the underlay paths according to its BAR and IPA
   values in the following way:

   1.  Apply the BIER constraints, resulting in BC(X).  If BC is NULL,
       then BC(X) is X itself.

   2.  Apply the routing constraints, resulting in RC(BC(X)).  If RC is
       NULL, then RC(BC(X)) is BC(X).

   3.  Select the algorithm AG as follows:

       a.  If BA is NULL, AG is set to RA.

       b.  If BA is not NULL, AG is set to BA.

   4.  Run AG on RC(BC(X)).

   It's possible that the resulting AG is not applicable to BIER.  In
   that case, no BIER paths will be calculated, and this is a network
   design issue that an operator needs to avoid when choosing the BAR or

3.1.  When BAR Is Not Used

   BAR value 0 is defined as "No BIER-specific algorithm is used"
   [RFC8401].  This value indicates NULL BA and BC.  Following the rules
   defined above, the IPA value alone identifies the calculation
   algorithm and constraints to be used for a particular sub-domain.

3.2.  Exceptions or Extensions to the General Rules

   Exceptions or extensions to the above general rules may be specified
   in the future for specific BAR and/or IPA values.  When that happens,
   compatibility with defined BAR and/or IPA values and semantics need
   to be specified.

4.  Examples

   As an example, one may define a new BAR with a BIER-specific
   constraint of "excluding BIER-incapable routers".  No BIER-specific
   algorithm is specified, and the BIER-specific constraint can go with
   any IPA, i.e., any RC defined by the IPA is augmented with "excluding
   BIER-incapable routers".  (Routers that do not support BIER are not
   considered when applying the IGP Algorithm.)

   If the BC and RC happen to conflict and lead to an empty topology,
   then no BIER forwarding path will be found.  For example, the BC
   could be "exclude BIER-incapable routers", and the RC could be
   "include green links only".  If all the green links are associated
   with BIER-incapable routers, it results in an empty topology.  This
   is a network design issue that an operator needs to avoid when
   choosing the BAR or IPA.

   In another example, a BAR value can be specified to use the Steiner
   tree algorithm and used together with IPA 0 (which uses an SPF
   algorithm).  According to the general rules, the BIER-specific
   algorithm takes precedence so SPF is not used.

5.  IANA Considerations

   The "BIER Algorithm" registry has been updated as follows:

   1.  The "Experimental Use" range has been renamed "Private or
       Experimental Use".

   2.  This document has been added as a reference both for the registry
       itself and for values 240-254 in the registry.

6.  Security Considerations

   This document specifies general rules for the interaction between the
   BIER Algorithm (BAR) and the IGP Algorithm (IPA) used for underlay
   path calculation.  It does not change the security aspects as
   discussed in [RFC8279], [RFC8401], and [RFC8444].

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,

   [RFC8401]  Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
              Zhang, "Bit Index Explicit Replication (BIER) Support via
              IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,

   [RFC8444]  Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
              Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
              Extensions for Bit Index Explicit Replication (BIER)",
              RFC 8444, DOI 10.17487/RFC8444, November 2018,

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,


   The authors thank Alia Atlas, Eric Rosen, Senthil Dhanaraj and many
   others for their suggestions and comments.  In particular, the
   BC/BA/RC/RA representation for the interaction rules is based on
   Alia's write-up.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

   Antoni Przygienda
   Juniper Networks
   Email: prz@juniper.net

   Andrew Dolganow
   Email: adolgano@gmail.com

   Hooman Bidgoli
   Email: hooman.bidgoli@nokia.com

   IJsbrand Wijnands
   Email: ice@braindump.be

   Arkadiy Gulko
   Edward Jones Wealth Management
   Email: arkadiy.gulko@edwardjones.com