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
                                                                  Nokia
                                                           IJ. Wijnands
                                                             Individual
                                                               A. Gulko
                                         Edward Jones Wealth Management
                                                         September 2022


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

Abstract

  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
  https://www.rfc-editor.org/info/rfc9272.

Copyright Notice

  Copyright (c) 2022 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (https://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  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
  Acknowledgements
  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",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "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
     registry.

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
  IPA.

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,
             <https://www.rfc-editor.org/info/rfc2119>.

  [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,
             <https://www.rfc-editor.org/info/rfc8279>.

  [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,
             <https://www.rfc-editor.org/info/rfc8401>.

  [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,
             <https://www.rfc-editor.org/info/rfc8444>.

  [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,
             <https://www.rfc-editor.org/info/rfc8665>.

Acknowledgements

  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: [email protected]


  Antoni Przygienda
  Juniper Networks
  Email: [email protected]


  Andrew Dolganow
  Individual
  Email: [email protected]


  Hooman Bidgoli
  Nokia
  Email: [email protected]


  IJsbrand Wijnands
  Individual
  Email: [email protected]


  Arkadiy Gulko
  Edward Jones Wealth Management
  Email: [email protected]

You are viewing proxied material from gopher.fnord.one. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.