BIER Underlay Path Calculation Algorithm and Constraints
draft-ietf-bier-bar-ipa-11

BIER                                                            Z. Zhang
Internet-Draft                                             A. Przygienda
Updates: 8401, 8444 (if approved)                       Juniper Networks
Intended status: Standards Track                             A. Dolganow
Expires: 25 September 2022                                    Individual
                                                              H. Bidgoli
                                                                   Nokia
                                                             I. Wijnands
                                                              Individual
                                                                A. Gulko
                                          Edward Jones Wealth Management
                                                           24 March 2022

        BIER Underlay Path Calculation Algorithm and Constraints
                       draft-ietf-bier-bar-ipa-11

Abstract

   This document specifies general rules for the interaction between the
   BIER Algorithm (BAR) and the IGP Algorithm (IPA) used for underlay
   path calculation.  The semantics defined in this document update
   RFC8401 and RFC8444.

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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 September 2022.

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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/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Updated Definition for BAR and IPA Fields . . . . . . . . . .   3
   3.  General Rules for the BAR and IPA Interaction . . . . . . . .   3
     3.1.  When BAR Is Not Used  . . . . . . . . . . . . . . . . . .   4
     3.2.  Exceptions/Extensions to the General Rules  . . . . . . .   4
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 generic IGP algorithm (e.g.
   SPF) or could be a BIER specific one.

   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.

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   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], [RFC8444].

2.  Updated Definition for BAR and IPA Fields

   The definition for the BAR and IPA fields in [RFC8401] and [RFC8444]
   are updated as follows.

   IPA: IGP Algorithm.  Specifies a generic Routing Algorithm (RA) and
   related Routing Constraints (RC) to calculate underlay paths to reach
   other Bit-Forwarding Routers (BFRs).  Values are from the "IGP
   Algorithm Types" registry [RFC8665].

   BAR: BIER Algorithm.  Specifies a BIER-specific Algorithm (BA) and
   BIER-specific Constraints (BC) 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 [RFC8401].

   When a BAR value is defined, the corresponding BA and BC semantics
   SHOULD be specified.  For an IGP Algorithm to be used as a BIER IPA,
   its RA and RC semantics SHOULD be specified.

   None of the components of the BAR or IPA can be unknown.  If any of
   the components is not specified, it is interpreted as "NULL"
   algorithm or constraint.  For example, the IGP Algorithm 0 defined in
   [RFC8665] is treated as having a NULL RC, i.e., no constraints.

   If a BAR value is not specified in a RFC but only privately used for
   a deployment, it MUST be within the "240-254 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 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:

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   1.  Apply the BIER constraints, resulting in BC(X).

   2.  Apply the routing constraints, resulting in RC(BC(X)).

   3.  Select the algorithm AG as following:

       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 it is a network
   design issue that an operator needs to avoid when choosing BAR/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/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 - whatever RC defined by the IPA is augmented with "excluding
   BIER incapable routers", i.e., 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.  That
   is a network design issue that an operator needs to avoid when
   choosing BAR/IPA.

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

5.  IANA Considerations

   No IANA Consideration is requested in this document.

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], [RFC8444].

7.  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.

8.  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>.

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   [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>.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

   Antoni Przygienda
   Juniper Networks
   Email: prz@juniper.net

   Andrew Dolganow
   Individual
   Email: adolgano@gmail.com

   Hooman Bidgoli
   Nokia
   Email: hooman.bidgoli@nokia.com

   IJsbrand Wijnands
   Individual
   Email: ice@braindump.be

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

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