Underlay Path Calculation Algorithm and Constraints for Bit Index Explicit Replication (BIER) Juniper Networks
zzhang@juniper.net
Juniper Networks
prz@juniper.net
Individual
adolgano@gmail.com
Nokia
hooman.bidgoli@nokia.com
Individual
ice@braindump.be
Edward Jones Wealth Management
arkadiy.gulko@edwardjones.com
rtg 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.
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Table of Contents
  • .  Introduction
    • .  Requirements Language
  • .  Updated Definitions for IPA and BAR Fields
  • .  General Rules for the BAR and IPA Interaction
    • .  When BAR Is Not Used
    • .  Exceptions or Extensions to the General Rules
  • .  Examples
  • .  IANA Considerations
  • .  Security Considerations
  • .  Normative References
  • Acknowledgements
  • Authors' Addresses
Introduction In the Bit Index Explicit Replication (BIER) architecture , 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 and , 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 and . This document also updates the "BIER Algorithm" registry defined in by renaming the "Experimental Use" range to "Private or Experimental Use".
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 when, and only when, they appear in all capitals, as shown here.
Updated Definitions for IPA and BAR Fields The definitions for the IPA and BAR fields in and 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 is treated as having a NULL RC, i.e., no constraints (see ). 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.
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 , 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:
    1. If BA is NULL, AG is set to RA.
    2. 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.
When BAR Is Not Used BAR value 0 is defined as "No BIER-specific algorithm is used" . 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.
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.
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.
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.
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 , , and .
Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Multicast Using Bit Index Explicit Replication (BIER) This document specifies a new architecture for the forwarding of multicast data packets. It provides optimal forwarding of multicast packets through a "multicast domain". However, it does not require a protocol for explicitly building multicast distribution trees, nor does it require intermediate nodes to maintain any per-flow state. This architecture is known as "Bit Index Explicit Replication" (BIER). When a multicast data packet enters the domain, the ingress router determines the set of egress routers to which the packet needs to be sent. The ingress router then encapsulates the packet in a BIER header. The BIER header contains a bit string in which each bit represents exactly one egress router in the domain; to forward the packet to a given set of egress routers, the bits corresponding to those routers are set in the BIER header. The procedures for forwarding a packet based on its BIER header are specified in this document. Elimination of the per-flow state and the explicit tree-building protocols results in a considerable simplification. Bit Index Explicit Replication (BIER) Support via IS-IS This document defines IS-IS extensions to support multicast forwarding using the Bit Index Explicit Replication (BIER) architecture. OSPFv2 Extensions for Bit Index Explicit Replication (BIER) Bit Index Explicit Replication (BIER) is an architecture that provides optimal multicast forwarding through a "BIER domain" without requiring intermediate routers to maintain multicast-related, per- flow state. BIER also does not require an explicit tree-building protocol for its operation. A multicast data packet enters a BIER domain at a Bit-Forwarding Ingress Router (BFIR) and leaves the BIER domain at one or more Bit-Forwarding Egress Routers (BFERs). The BFIR adds a BIER packet header to the packet. The BIER packet header contains a BitString in which each bit represents exactly one BFER to forward the packet to. The set of BFERs to which the multicast packet needs to be forwarded is expressed by the set of bits in the BIER packet header. This document describes the OSPF protocol extension (from RFC 2328) that is required for BIER with MPLS encapsulation (which is defined in RFC 8296). Support for other encapsulation types and the use of multiple encapsulation types are outside the scope of this document. OSPF Extensions for Segment Routing Segment Routing (SR) allows a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological subpaths called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). This document describes the OSPFv2 extensions required for Segment Routing.
Acknowledgements The authors thank , , 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 Juniper Networks
zzhang@juniper.net
Juniper Networks
prz@juniper.net
Individual
adolgano@gmail.com
Nokia
hooman.bidgoli@nokia.com
Individual
ice@braindump.be
Edward Jones Wealth Management
arkadiy.gulko@edwardjones.com