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.
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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: 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