Network Working Group                                     P. Psenak, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                                S. Hegde
Expires: April 10, 2021                           Juniper Networks, Inc.
                                                             C. Filsfils
                                                           K. Talaulikar
                                                     Cisco Systems, Inc.
                                                                A. Gulko
                                                              Individual
                                                         October 7, 2020


                         IGP Flexible Algorithm
                      draft-ietf-lsr-flex-algo-12

Abstract

   IGP protocols traditionally compute best paths over the network based
   on the IGP metric assigned to the links.  Many network deployments
   use RSVP-TE based or Segment Routing based Traffic Engineering to
   steer traffic over a path that is computed using different metrics or
   constraints than the shortest IGP path.  This document proposes a
   solution that allows IGPs themselves to compute constraint-based
   paths over the network.  This document also specifies a way of using
   Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets
   along the constraint-based paths.

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 April 10, 2021.








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Copyright Notice

   Copyright (c) 2020 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Flexible Algorithm  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Flexible Algorithm Definition Advertisement . . . . . . . . .   6
     5.1.  ISIS Flexible Algorithm Definition Sub-TLV  . . . . . . .   6
     5.2.  OSPF Flexible Algorithm Definition TLV  . . . . . . . . .   7
     5.3.  Common Handling of Flexible Algorithm Definition TLV  . .   9
   6.  Sub-TLVs of ISIS FAD Sub-TLV  . . . . . . . . . . . . . . . .  10
     6.1.  ISIS Flexible Algorithm Exclude Admin Group Sub-TLV . . .  10
     6.2.  ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV .  11
     6.3.  ISIS Flexible Algorithm Include-All Admin Group Sub-TLV .  12
     6.4.  ISIS Flexible Algorithm Definition Flags Sub-TLV  . . . .  12
     6.5.  ISIS Flexible Algorithm Exclude SRLG Sub-TLV  . . . . . .  13
   7.  Sub-TLVs of OSPF FAD TLV  . . . . . . . . . . . . . . . . . .  14
     7.1.  OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . .  14
     7.2.  OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV .  14
     7.3.  OSPF Flexible Algorithm Include-All Admin Group Sub-TLV .  15
     7.4.  OSPF Flexible Algorithm Definition Flags Sub-TLV  . . . .  15
     7.5.  OSPF Flexible Algorithm Exclude SRLG Sub-TLV  . . . . . .  16
   8.  ISIS Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . .  17
   9.  OSPF Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . .  18
   10. Advertisement of Node Participation in a Flex-Algorithm . . .  19
     10.1.  Advertisement of Node Participation for Segment Routing   19
     10.2.  Advertisement of Node Participation for Other
            Applications . . . . . . . . . . . . . . . . . . . . . .  19
   11. Advertisement of Link Attributes for Flex-Algorithm . . . . .  20
   12. Calculation of Flexible Algorithm Paths . . . . . . . . . . .  20
     12.1.  Multi-area and Multi-domain Considerations . . . . . . .  22
   13. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . .  23
     13.1.  Segment Routing MPLS Forwarding for Flex-Algorithm . . .  23



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     13.2.  SRv6 Forwarding for Flex-Algorithm . . . . . . . . . . .  24
     13.3.  Other Applications' Forwarding for Flex-Algorithm  . . .  25
   14. Operational considerations  . . . . . . . . . . . . . . . . .  25
     14.1.  Inter-area Considerations  . . . . . . . . . . . . . . .  25
     14.2.  Usage of SRLG Exclude Rule with Flex-Algorithm . . . . .  26
     14.3.  Max-metric consideration . . . . . . . . . . . . . . . .  26
   15. Backward Compatibility  . . . . . . . . . . . . . . . . . . .  27
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     17.1.  IGP IANA Considerations  . . . . . . . . . . . . . . . .  27
       17.1.1.  IGP Algorithm Types Registry . . . . . . . . . . . .  27
       17.1.2.  IGP Metric-Type Registry . . . . . . . . . . . . . .  27
     17.2.  Flexible Algorithm Definition Flags Registry . . . . . .  28
     17.3.  ISIS IANA Considerations . . . . . . . . . . . . . . . .  28
       17.3.1.  Sub TLVs for Type 242  . . . . . . . . . . . . . . .  28
       17.3.2.  Sub TLVs for for TLVs 135, 235, 236, and 237 . . . .  29
       17.3.3.  Sub-Sub-TLVs for Flexible Algorithm Definition Sub-
                TLV  . . . . . . . . . . . . . . . . . . . . . . . .  29
     17.4.  OSPF IANA Considerations . . . . . . . . . . . . . . . .  30
       17.4.1.  OSPF Router Information (RI) TLVs Registry . . . . .  30
       17.4.2.  OSPFv2 Extended Prefix TLV Sub-TLVs  . . . . . . . .  30
       17.4.3.  OSPFv3 Extended-LSA Sub-TLVs . . . . . . . . . . . .  30
       17.4.4.  OSPF Flexible Algorithm Definition TLV Sub-TLV
                Registry . . . . . . . . . . . . . . . . . . . . . .  31
       17.4.5.  Link Attribute Applications Registry . . . . . . . .  32
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     19.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   An IGP-computed path based on the shortest IGP metric must often be
   replaced by a traffic-engineered path due to the traffic requirements
   which are not reflected by the IGP metric.  Some networks engineer
   the IGP metric assignments in a way that the IGP metric reflects the
   link bandwidth or delay.  If, for example, the IGP metric is
   reflecting the bandwidth on the link and the application traffic is
   delay sensitive, the best IGP path may not reflect the best path from
   such an application's perspective.

   To overcome this limitation, various sorts of traffic engineering
   have been deployed, including RSVP-TE and SR-TE, in which case the TE
   component is responsible for computing paths based on additional
   metrics and/or constraints.  Such paths need to be installed in the
   forwarding tables in addition to, or as a replacement for, the
   original paths computed by IGPs.  Tunnels are often used to represent



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   the engineered paths and mechanisms like one described in [RFC3906]
   are used to replace the native IGP paths with such tunnel paths.

   This document specifies a set of extensions to ISIS, OSPFv2, and
   OSPFv3 that enable a router to advertise TLVs that identify (a)
   calculation-type, (b) specify a metric-type, and (c) describe a set
   of constraints on the topology, that are to be used to compute the
   best paths along the constrained topology.  A given combination of
   calculation-type, metric-type, and constraints is known as a
   "Flexible Algorithm Definition".  A router that sends such a set of
   TLVs also assigns a Flex-Algorithm value to the specified combination
   of calculation-type, metric-type, and constraints.

   This document also specifies a way for a router to use IGPs to
   associate one or more SR Prefix-SIDs or SRv6 locators with a
   particular Flex-Algorithm.  Each such Prefix-SID or SRv6 locator then
   represents a path that is computed according to the identified Flex-
   Algorithm.

2.  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
   [BCP14] [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   This section defines terms that are often used in this document.

   Flexible Algorithm Definition (FAD) - the set consisting of (a)
   calculation-type, (b) metric-type, and (c) a set of constraints.

   Flexible Algorithm - a numeric identifier in the range 128-255 that
   is associated via configuration with the Flexible-Algorithm
   Definition.

   Local Flexible Algorithm Definition - Flexible Algorithm Definition
   defined locally on the node.

   Remote Flexible Algorithm Definition - Flexible Algorithm Definition
   received from other nodes via IGP flooding.

   Flexible Algorithm Participation - per application configuration
   state that expresses whether the node is participating in a
   particular Flexible Algorithm.




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   IGP Algorithm - value from the the "IGP Algorithm Types" registry
   defined under "Interior Gateway Protocol (IGP) Parameters" IANA
   registries.  IGP Algorithms represents the triplet (Calculation Type,
   Metric, Constraints), where the second and third elements of the
   triple MAY be unspecified.

   ABR - Area Border Router.  In ISIS terminology it is also known as
   L1/L2 router.

   ASBR - Autonomous System Border Router.

4.  Flexible Algorithm

   Many possible constraints may be used to compute a path over a
   network.  Some networks are deployed as multiple planes.  A simple
   form of constraint may be to use a particular plane.  A more
   sophisticated form of constraint can include some extended metric as
   described in [RFC8570].  Constraints which restrict paths to links
   with specific affinities or avoid links with specific affinities are
   also possible.  Combinations of these are also possible.

   To provide maximum flexibility, we want to provide a mechanism that
   allows a router to (a) identify a particular calculation-type, (b)
   metric-type, (c) describe a particular set of constraints, and (d)
   assign a numeric identifier, referred to as Flex-Algorithm, to the
   combination of that calculation-type, metric-type, and those
   constraints.  We want the mapping between the Flex-Algorithm and its
   meaning to be flexible and defined by the user.  As long as all
   routers in the domain have a common understanding as to what a
   particular Flex-Algorithm represents, the resulting routing
   computation is consistent and traffic is not subject to any looping.

   The set consisting of (a) calculation-type, (b) metric-type, and (c)
   a set of constraints is referred to as a Flexible-Algorithm
   Definition.

   Flexible-Algorithm is a numeric identifier in the range 128-255 that
   is associated via configuratin with the Flexible-Algorithm
   Definition.

   IANA "IGP Algorithm Types" registry defines the set of values for IGP
   Algorithms.  We propose to allocate the following values for Flex-
   Algorithms from this registry:

      128-255 - Flex-Algorithms






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5.  Flexible Algorithm Definition Advertisement

   To guarantee the loop-free forwarding for paths computed for a
   particular Flex-Algorithm, all routers that (a) are configured to
   participate in a particular Flex-Algorithm, and (b) are in the same
   Flex-Algorithm definition advertisement scope MUST agree on the
   definition of the Flex-Algorithm.

5.1.  ISIS Flexible Algorithm Definition Sub-TLV

   The ISIS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used
   to advertise the definition of the Flex-Algorithm.

   The ISIS FAD Sub-TLV is advertised as a Sub-TLV of the ISIS Router
   Capability TLV-242 that is defined in [RFC7981].

   ISIS FAD Sub-TLV has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Length     |Flex-Algorithm |  Metric-Type  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Calc-Type   |    Priority   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Sub-TLVs                             |
   +                                                               +
   |                            ...                                |

   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: 26

      Length: variable, dependent on the included Sub-TLVs

      Flex-Algorithm: Single octet value between 128 and 255 inclusive.

      Metric-Type: Type of metric to be used during the calculation.
      Following values are defined:

         0: IGP Metric

         1: Min Unidirectional Link Delay as defined in [RFC8570],
         section 4.2, encoded as application specific link attribute as




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         specified in [I-D.ietf-isis-te-app] and Section 11 of this
         document.

         2: Traffic Engineering Default Metric as defined in [RFC5305],
         section 3.7, encoded as application specific link attribute as
         specified in [I-D.ietf-isis-te-app] and Section 11 of this
         document.

      Calc-Type: value from 0 to 127 inclusive from the "IGP Algorithm
      Types" registry defined under "Interior Gateway Protocol (IGP)
      Parameters" IANA registries.  IGP algorithms in the range of 0-127
      have a defined triplet (Calculation Type, Metric, Constraints).
      When used to specify the Calc-Type in the FAD Sub-TLV, only the
      Calculation Type defined for the specified IGP Algorithm is used.
      The Metric/Constraints MUST NOT be inherited.  If the required
      calculation type is Shortest Path First, the value 0 SHOULD appear
      in this field.

      Priority: Value between 0 and 255 inclusive that specifies the
      priority of the advertisement.

      Sub-TLVs - optional sub-TLVs.

   The ISIS FAD Sub-TLV MAY be advertised in an LSP of any number, but a
   router MUST NOT advertise more than one ISIS FAD Sub-TLV for a given
   Flexible-Algorithm.  A router receiving multiple ISIS FAD Sub-TLVs
   for a given Flexible-Algorithm from the same originator SHOULD select
   the first advertisement in the lowest numbered LSP.

   The ISIS FAD Sub-TLV has an area scope.  The Router Capability TLV in
   which the FAD Sub-TLV is present MUST have the S-bit clear.

   ISIS L1/L2 router MAY be configured to re-generate the winning FAD
   from level 2, without any modification to it, to level 1 area.  The
   re-generation of the FAD Sub-TLV from level 2 to level 1 is
   determined by the L1/L2 router, not by the originator of the FAD
   advertisement in the level 2.  In such case, the re-generated FAD
   Sub-TLV will be advertised in the level 1 Router Capability TLV
   originated by the L1/L2 router.

   L1/L2 router MUST NOT re-generate any FAD Sub-TLV from level 1 to
   level 2.

5.2.  OSPF Flexible Algorithm Definition TLV

   OSPF FAD TLV is advertised as a top-level TLV of the RI LSA that is
   defined in [RFC7770].




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   OSPF FAD TLV has the following format:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Flex-Algorithm |   Metric-Type |   Calc-Type   |    Priority   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Sub-TLVs                           |
     +                                                               +
     |                               ...                             |

     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: 16

      Length: variable, dependent on the included Sub-TLVs

      Flex-Algorithm:: Flex-Algorithm number.  Value between 128 and 255
      inclusive.

      Metric-Type: Type of metric to be used during the calculation.
      Following values are defined:

         0: IGP Metric

         1: Min Unidirectional Link Delay as defined in [RFC7471],
         section 4.2, encoded as application specific link attribute as
         specified in [I-D.ietf-ospf-te-link-attr-reuse] and Section 11
         of this document.

         2: Traffic Engineering metric as defined in [RFC3630], section
         2.5.5, encoded as application specific link attribute as
         specified in [I-D.ietf-ospf-te-link-attr-reuse] and Section 11
         of this document.

      Calc-Type: as described in Section 5.1

      Priority: as described in Section 5.1

      Sub-TLVs - optional sub-TLVs.





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   When multiple OSPF FAD TLVs, for the same Flexible-Algorithm, are
   received from a given router, the receiver MUST use the first
   occurrence of the TLV in the Router Information LSA.  If the OSPF FAD
   TLV, for the same Flex-Algorithm, appears in multiple Router
   Information LSAs that have different flooding scopes, the OSPF FAD
   TLV in the Router Information LSA with the area-scoped flooding scope
   MUST be used.  If the OSPF FAD TLV, for the same algorithm, appears
   in multiple Router Information LSAs that have the same flooding
   scope, the OSPF FAD TLV in the Router Information (RI) LSA with the
   numerically smallest Instance ID MUST be used and subsequent
   instances of the OSPF FAD TLV MUST be ignored.

   The RI LSA can be advertised at any of the defined opaque flooding
   scopes (link, area, or Autonomous System (AS)).  For the purpose of
   OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED.  The
   Autonomous System flooding scope SHOULD NOT be used by default unless
   local configuration policy on the originating router indicates domain
   wide flooding.

5.3.  Common Handling of Flexible Algorithm Definition TLV

   This section describes the protocol-independent handling of the FAD
   TLV (OSPF) or FAD Sub-TLV (ISIS).  We will refer to it as FAD TLV in
   this section, even though in case of ISIS it is a Sub-TLV.

   The value of the Flex-Algorithm MUST be between 128 and 255
   inclusive.  If it is not, the FAD TLV MUST be ignored.

   Only a subset of the routers participating in the particular Flex-
   Algorithm need to advertise the definition of the Flex-Algorithm.

   Every router, that is configured to participate in a particular Flex-
   Algorithm, MUST select the Flex-Algorithm definition based on the
   following ordered rules.  This allows for the consistent Flex-
   Algorithm definition selection in cases where different routers
   advertise different definitions for a given Flex-Algorithm:

      1.  From the advertisements of the FAD in the area (including both
      locally generated advertisements and received advertisements)
      select the one(s) with the highest priority value.

      2.  If there are multiple advertisements of the FAD with the same
      highest priority, select the one that is originated from the
      router with the highest System-ID, in the case of ISIS, or Router
      ID, in the case of OSPFv2 and OSPFv3.  For ISIS, the System-ID is
      described in [ISO10589].  For OSPFv2 and OSPFv3, standard Router
      ID is described in [RFC2328] and [RFC5340] respectively.




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   A router that is not configured to participate in a particular Flex-
   Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex-
   Algorithm.

   A router that is not participating in a particular Flex-Algorithm is
   allowed to advertise FAD for such Flex-Algorithm.  Receiving routers
   MUST consider FAD advertisement regardless of the Flex-Algorithm
   participation of the FAD originator.

   Any change in the Flex-Algorithm definition may result in temporary
   disruption of traffic that is forwarded based on such Flex-Algorithm
   paths.  The impact is similar to any other event that requires
   network-wide convergence.

   If a node is configured to participate in a particular Flexible-
   Algorithm, but the selected Flex-Algorithm definition includes
   calculation-type, metric-type, constraint, flag, or Sub-TLV that is
   not supported by the node, it MUST stop participating in such
   Flexible-Algorithm.  That implies that it MUST NOT announce
   participation for such Flexible-Algorithm as specified in Section 10
   and it MUST remove any forwarding state associated with it.

   Flex-Algorithm definition is topology independent.  It applies to all
   topologies that a router participates in.

6.  Sub-TLVs of ISIS FAD Sub-TLV

6.1.  ISIS Flexible Algorithm Exclude Admin Group Sub-TLV

   The Flexible Algorithm definition can specify 'colors' that are used
   by the operator to exclude links during the Flex-Algorithm path
   computation.

   The ISIS Flexible Algorithm Exclude Admin Group Sub-TLV is used to
   advertise the exclude rule that is used during the Flex-Algorithm
   path calculation as specified in Section 12.

   The ISIS Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-
   TLV) is a Sub-TLV of the ISIS FAD Sub-TLV.  It has the following
   format:











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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Extended Admin Group                     |
   +-                                                             -+
   |                            ...                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 1

      Length: variable, dependent on the size of the Extended Admin
      Group.  MUST be a multiple of 4 octets.

      Extended Administrative Group: Extended Administrative Group as
      defined in [RFC7308].

   The ISIS FAEAG Sub-TLV MUST NOT appear more then once in an ISIS FAD
   Sub-TLV.  If it appears more then once, the ISIS FAD Sub-TLV MUST be
   ignored by the receiver.

6.2.  ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV

   The Flexible Algorithm definition can specify 'colors' that are used
   by the operator to include links during the Flex-Algorithm path
   computation.

   The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV is used
   to advertise include-any rule that is used during the Flex-Algorithm
   path calculation as specified in Section 12.

   The format of the ISIS Flexible Algorithm Include-Any Admin Group
   Sub-TLV is identical to the format of the FAEAG Sub-TLV in
   Section 6.1.

   The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
   2.

   The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
   appear more then once in an ISIS FAD Sub-TLV.  If it appears more
   then once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.








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6.3.  ISIS Flexible Algorithm Include-All Admin Group Sub-TLV

   The Flexible Algorithm definition can specify 'colors' that are used
   by the operator to include link during the Flex-Algorithm path
   computation.

   The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV is used
   to advertise include-all rule that is used during the Flex-Algorithm
   path calculation as specified in Section 12.

   The format of the ISIS Flexible Algorithm Include-All Admin Group
   Sub-TLV is identical to the format of the FAEAG Sub-TLV in
   Section 6.1.

   The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV Type is
   3.

   The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
   appear more then once in an ISIS FAD Sub-TLV.  If it appears more
   then once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.

6.4.  ISIS Flexible Algorithm Definition Flags Sub-TLV

   The ISIS Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
   is a Sub-TLV of the ISIS FAD Sub-TLV.  It has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Flags                             |
   +-                                                             -+
   |                            ...                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 4

      Length: variable, non-zero number of octets of the Flags field

      Flags:

                 0 1 2 3 4 5 6 7...
                +-+-+-+-+-+-+-+-+...
                |M| | |          ...
                +-+-+-+-+-+-+-+-+...




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         M-flag: when set, the Flex-Algorithm specific prefix metric
         MUST be used, if advertised with the prefix.  This flag is not
         applicable to prefixes advertised as SRv6 locators.

   Bits are defined/sent starting with Bit 0 defined above.  Additional
   bit definitions that may be defined in the future SHOULD be assigned
   in ascending bit order so as to minimize the number of bits that will
   need to be transmitted.

   Undefined bits MUST be transmitted as 0.

   Bits that are NOT transmitted MUST be treated as if they are set to 0
   on receipt.

   The ISIS FADF Sub-TLV MUST NOT appear more then once in an ISIS FAD
   Sub-TLV.  If it appears more then once, the ISIS FAD Sub-TLV MUST be
   ignored by the receiver.

   If the ISIS FADF Sub-TLV is not present inside the ISIS FAD Sub-TLV,
   all the bits are assumed to be set to 0.

6.5.  ISIS Flexible Algorithm Exclude SRLG Sub-TLV

   The Flexible Algorithm definition can specify Shared Risk Link Groups
   (SRLGs) that the operator wants to exclude during the Flex-Algorithm
   path computation.

   The ISIS Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG) is used to
   advertise the exclude rule that is used during the Flex-Algorithm
   path calculation as specified in Section 12.

   The ISIS FAESRLG Sub-TLV is a Sub-TLV of the ISIS FAD Sub-TLV.  It
   has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Shared Risk Link Group Value             |
   +-                                                             -+
   |                            ...                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 5





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      Length: variable, dependent on number of SRLG values.  MUST be a
      multiple of 4 octets.

      Shared Risk Link Group Value: SRLG value as defined in [RFC5307].

   The ISIS FAESRLG Sub-TLV MUST NOT appear more then once in an ISIS
   FAD Sub-TLV.  If it appears more then once, the ISIS FAD Sub-TLV MUST
   be ignored by the receiver.

7.  Sub-TLVs of OSPF FAD TLV

7.1.  OSPF Flexible Algorithm Exclude Admin Group Sub-TLV

   The Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is
   a Sub-TLV of the OSPF FAD TLV.  It's usage is described in
   Section 6.1.  It has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Extended Admin Group                     |
   +-                                                             -+
   |                            ...                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 1

      Length: variable, dependent on the size of the Extended Admin
      Group.  MUST be a multiple of 4 octets.

      Extended Administrative Group: Extended Administrative Group as
      defined in [RFC7308].

   The OSPF FAEAG Sub-TLV MUST NOT appear more then once in an OSPF FAD
   TLV.  If it appears more then once, the OSPF FAD TLV MUST be ignored
   by the receiver.

7.2.  OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV

   The usage of this Sub-TLVs is described in Section 6.2.

   The format of the OSPF Flexible Algorithm Include-Any Admin Group
   Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
   Section 7.1.




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   The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
   2.

   The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
   appear more then once in an OSPF FAD TLV.  If it appears more then
   once, the OSPF FAD TLV MUST be ignored by the receiver.

7.3.  OSPF Flexible Algorithm Include-All Admin Group Sub-TLV

   The usage of this Sub-TLVs is described in Section 6.3.

   The format of the OSPF Flexible Algorithm Include-Any Admin Group
   Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
   Section 7.1.

   The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
   3.

   The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
   appear more then once in an OSPF FAD TLV.  If it appears more then
   once, the OSPF FAD TLV MUST be ignored by the receiver.

7.4.  OSPF Flexible Algorithm Definition Flags Sub-TLV

   The OSPF Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
   is a Sub-TLV of the OSPF FAD TLV.  It has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Flags                             |
   +-                                                             -+
   |                            ...                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 4

      Length: variable, dependent on the size of the Flags field.  MUST
      be a multiple of 4 octets.

      Flags:







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                 0 1 2 3 4 5 6 7...
                +-+-+-+-+-+-+-+-+...
                |M| | |          ...
                +-+-+-+-+-+-+-+-+...

         M-flag: when set, the Flex-Algorithm specific prefix metric
         MUST be used, if advertised with the prefix.  This flag is not
         applicable to prefixes advertised as SRv6 locators.

   Bits are defined/sent starting with Bit 0 defined above.  Additional
   bit definitions that may be defined in the future SHOULD be assigned
   in ascending bit order so as to minimize the number of bits that will
   need to be transmitted.

   Undefined bits MUST be transmitted as 0.

   Bits that are NOT transmitted MUST be treated as if they are set to 0
   on receipt.

   The OSPF FADF Sub-TLV MUST NOT appear more then once in an OSPF FAD
   TLV.  If it appears more then once, the OSPF FAD TLV MUST be ignored
   by the receiver.

   If the OSPF FADF Sub-TLV is not present inside the OSPF FAD TLV, all
   the bits are assumed to be set to 0.

7.5.  OSPF Flexible Algorithm Exclude SRLG Sub-TLV

   The OSPF Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG Sub-TLV) is
   a Sub-TLV of the OSPF FAD TLV.  Its usage is described in
   Section 6.5.  It has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Shared Risk Link Group Value                |
   +-                                                             -+
   |                            ...                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 5

      Length: variable, dependent on the number of SRLGs.  MUST be a
      multiple of 4 octets.




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      Shared Risk Link Group Value: SRLG value as defined in [RFC4203].

   The OSPF FAESRLG Sub-TLV MUST NOT appear more then once in an OSPF
   FAD TLV.  If it appears more then once, the OSPF FAD TLV MUST be
   ignored by the receiver.

8.  ISIS Flexible Algorithm Prefix Metric Sub-TLV

   The ISIS Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the
   advertisement of a Flex-Algorithm specific prefix metric associated
   with a given prefix advertisement.

   The ISIS FAPM Sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237 and
   has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Length     |Flex-Algorithm |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   where:

      Type: 6

      Length: 5 octets

      Flex-Algorithm: Single octet value between 128 and 255 inclusive.

      Metric: 4 octets of metric information

   The ISIS FAPM Sub-TLV MAY appear multiple times in its parent TLV.
   If it appears more then once with the same Flex-Algorithm value, the
   first instance MUST be used and any subsequent instances MUST be
   ignored.

   If a prefix is advertised with a Flex-Algorithm prefix metric larger
   then MAX_PATH_METRIC as defined in [RFC5305] this prefix MUST NOT be
   considered during the Flexible-Algorithm computation.

   The usage of the Flex-Algorithm prefix metric is described in
   Section 12.

   The ISIS FAPM Sub-TLV MUST NOT be advertised as a sub-TLV of the ISIS
   SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions].  The ISIS SRv6
   Locator TLV includes the Algorithm and Metric fields which MUST be
   used instead.  If the FAPM Sub-TLV is present as a sub-TLV of the



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   ISIS SRv6 Locator TLV in the received LSP, such FAPM Sub-TLV MUST be
   ignored.

9.  OSPF Flexible Algorithm Prefix Metric Sub-TLV

   The OSPF Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the
   advertisement of a Flex-Algorithm specific prefix metric associated
   with a given prefix advertisement.

   The OSPF Flex-Algorithm Prefix Metric (FAPM) Sub-TLVis a Sub-TLV of
   the:

      - OSPFv2 Extended Prefix TLV [RFC7684]

      - Following OSPFv3 TLVs as defined in [RFC8362]:

         Intra-Area Prefix TLV

         Inter-Area Prefix TLV

         External Prefix TLV

   OSPF FAPM Sub-TLV has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Flex-Algorithm |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Metric                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

      Type: 3 for OSPFv2, 26 for OSPFv3

      Length: 8 octets

      Flex-Algorithm: Single octet value between 128 and 255 inclusive.

      Reserved: Must be set to 0, ignored at reception.

      Metric: 4 octets of metric information

   The OSPF FAPM Sub-TLV MAY appear multiple times in its parent TLV.
   If it appears more then once with the same Flex-Algorithm value, the



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   first instance MUST be used and any subsequent instances MUST be
   ignored.

   The usage of the Flex-Algorithm prefix metric is described in
   Section 12.

10.  Advertisement of Node Participation in a Flex-Algorithm

   When a router is configured to support a particular Flex-Algorithm,
   we say it is participating in that Flex-Algorithm.

   Paths computed for a specific Flex-Algorithm MAY be used by various
   applications, each potentially using its own specific data plane for
   forwarding traffic over such paths.  To guarantee the presence of the
   application specific forwarding state associated with a particular
   Flex-Algorithm, a router MUST advertise its participation for a
   particular Flex-Algorithm for each application specifically.

10.1.  Advertisement of Node Participation for Segment Routing

   [RFC8667], [RFC8665], and [RFC8666] (IGP Segment Routing extensions)
   describe how the SR-Algorithm is used to compute the IGP best path.

   Routers advertise the support for the SR-Algorithm as a node
   capability as described in the above mentioned IGP Segment Routing
   extensions.  To advertise participation for a particular Flex-
   Algorithm for Segment Routing, including both SR MPLS and SRv6, the
   Flex-Algorithm value MUST be advertised in the SR-Algorithm TLV
   (OSPF) or sub-TLV (ISIS).

   Segment Routing Flex-Algorithm participation advertisement is
   topology independent.  When a router advertises participation in an
   SR-Algorithm, the participation applies to all topologies in which
   the advertising node participates.

10.2.  Advertisement of Node Participation for Other Applications

   This section describes considerations related to how other
   applications can advertise their participation in a specific Flex-
   Algorithm.

   Application-specific Flex-Algorithm participation advertisements MAY
   be topology specific or MAY be topology independent, depending on the
   application itself.

   Application-specific advertisement for Flex-Algorithm participation
   MUST be defined for each application and is outside of the scope of
   this document.



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11.  Advertisement of Link Attributes for Flex-Algorithm

   Various link attributes may be used during the Flex-Algorithm path
   calculation.  For example, include or exclude rules based on link
   affinities can be part of the Flex-Algorithm definition as defined in
   Section 6 and Section 7.

   Link attribute advertisements that are to be used during Flex-
   Algorithm calculation MUST use the Application-Specific Link
   Attribute (ASLA) advertisements defined in [I-D.ietf-isis-te-app] or
   [I-D.ietf-ospf-te-link-attr-reuse].  In the case of IS-IS, this
   includes use of the L-flag as defined in [I-D.ietf-isis-te-app]
   Section 4.2 subject to the constraints discussed in Section 6 of the
   [I-D.ietf-isis-te-app].  The mandatory use of ASLA advertisements
   applies to link attributes specifically mentioned in this document
   (Min Unidirectional Link Delay, TE Default Metric, Administrative
   Group, Extended Administrative Group and Shared Risk Link Group) and
   any other link attributes that may be used in support of Flex-
   Algorithm in the future.

   A new Application Identifier Bit is defined to indicate that the ASLA
   advertisement is associated with the Flex-Algorithm application.
   This bit is set in the Standard Application Bit Mask (SABM) defined
   in [I-D.ietf-isis-te-app] or [I-D.ietf-ospf-te-link-attr-reuse]:

      Bit-3: Flexible Algorithm (X-bit)

   ASLA Admin Group Advertisements to be used by the Flexible Algorithm
   Application MAY use either the Administrative Group or Extended
   Administrative Group encodings.  If the Administrative Group encoding
   is used, then the first 32 bits of the corresponding FAD sub-TLVs are
   mapped to the link attribute advertisements as specified in RFC 7308.

12.  Calculation of Flexible Algorithm Paths

   A router MUST be configured to participate in a given Flex-Algorithm
   K and MUST select the FAD based on the rules defined in Section 5.3
   before it can compute any path for that Flex-Algorithm.

   As described in Section 10, participation for any particular Flex-
   Algorithm MUST be advertised on a per-application basis.  Calculation
   of the paths for any particular Flex-Algorithm MUST be application
   specific.

   The way applications handle nodes that do not participate in
   Flexible-Algorithm is application specific.  If the application only
   wants to consider participating nodes during the Flex-Algorithm
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   all nodes that do not advertise participation for that Flex-Algorithm
   in their application-specific advertisements MUST be pruned from the
   topology.  Segment Routing, including both SR MPLS and SRv6, are
   applications that MUST use such pruning when computing Flex-Algorithm
   paths.

   When computing the path for a given Flex-Algorithm, the metric-type
   that is part of the Flex-Algorithm definition (Section 5) MUST be
   used.

   When computing the path for a given Flex-Algorithm, the calculation-
   type that is part of the Flex-Algorithm definition (Section 5) MUST
   be used.

   Various link include or exclude rules can be part of the Flex-
   Algorithm definition.  To refer to a particular bit within an AG or
   EAG we uses term 'color'.

   Rules, in the order as specified below, MUST be used to prune links
   from the topology during the Flex-Algorithm computation.

   For all links in the topology:

      1.  Check if any exclude rule is part of the Flex-Algorithm
      definition.  If such exclude rule exists, check if any color that
      is part of the exclude rule is also set on the link.  If such a
      color is set, the link MUST be pruned from the computation.

      2.  Check if any exclude SRLG rule is part of the Flex-Algorithm
      definition.  If such exclude rule exists, check if the link is
      part of any SRLG that is also part of the SRLG exclude rule.  If
      the link is part of such SRLG, the link MUST be pruned from the
      computation.

      3.  Check if any include-any rule is part of the Flex-Algorithm
      definition.  If such include-any rule exists, check if any color
      that is part of the include-any rule is also set on the link.  If
      no such color is set, the link MUST be pruned from the
      computation.

      4.  Check if any include-all rule is part of the Flex-Algorithm
      definition.  If such include-all rule exists, check if all colors
      that are part of the include-all rule are also set on the link.
      If all such colors are not set on the link, the link MUST be
      pruned from the computation.

      5.  If the Flex-Algorithm definition uses other than IGP metric
      (Section 5), and such metric is not advertised for the particular



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      link in a topology for which the computation is done, such link
      MUST be pruned from the computation.  A metric of value 0 MUST NOT
      be assumed in such case.

12.1.  Multi-area and Multi-domain Considerations

   Any IGP Shortest Path Tree calculation is limited to a single area.
   This applies to Flex-Algorithm calculations as well.  Given that the
   computing router does not have visibility of the topology of the next
   areas or domain, the Flex-Algorithm specific path to an inter-area or
   inter-domain prefix will be computed for the local area only.  The
   egress L1/L2 router (ABR in OSPF), or ASBR for inter-domain case,
   will be selected based on the best path for the given Flex-Algorithm
   in the local area and such egress ABR or ASBR router will be
   responsible to compute the best Flex-Algorithm specific path over the
   next area or domain.  This may produce an end-to-end path, which is
   sub-optimal based on Flex-Algorithm constraints.  In cases where the
   ABR or ASBR has no reachability to a prefix for a given Flex-
   Algorithm in the next area or domain, the traffic may be dropped by
   the ABR/ASBR.

   To allow the optimal end-to-end path for an inter-area or inter-
   domain prefix for any Flex-Algorithm to be computed, the FAPM has
   been defined in Section 8 and Section 9.

   If the FAD selected based on the rules defined in Section 5.3
   includes the M-flag, an ABR or ASBR MUST include the FAPM (Section 8,
   Section 9) when advertising the prefix between areas or domains.
   Such metric will be equal to the metric to reach the prefix for a
   given Flex-Algorithm in a source area or domain.  This is similar in
   nature to how the metric is set when prefixes are advertised between
   areas or domains for the default algorithm.

   If the FAD selected based on the rules defined in Section 5.3
   includes the M-flag, the FAPM MUST be used during calculation of
   prefix reachability for the inter-area and external prefixes.  If the
   FAPM for the Flex-Algorithm is not advertised with the inter-area or
   external prefix reachability advertisement, the prefix MUST be
   considered as unreachable for that Flex-Algorithm.

   Flex-Algorithm prefix metrics MUST NOT be used during the Flex-
   Algorithm computation unless the FAD selected based on the rules
   defined in Section 5.3 includes the M-Flag, as described in
   (Section 6.4 or Section 7.4).

   If the FAD selected based on the rules defined in Section 5.3 does
   not includes the M-flag, it is NOT RECOMMENDED to use the Flex-
   Algorithm for inter-area or inter-domain prefix reachability.  The



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   reason is that without the explicit Flex-Algorithm Prefix Metric
   advertisement, it is not possible to conclude whether the ABR or ASBR
   has reachability to the inter-area or inter-domain prefix for a given
   Flex-Algorithm in the next area or domain.  Sending the Flex-Algoritm
   traffic for such prefix towards the ABR or ASBR may result in traffic
   looping or black-holing.

   The FAPM MUST NOT be advertised with ISIS L1 or L2 intra-area, OSPFv2
   intra-area, or OSPFv3 intra-area routes.  If the FAPM is advertised
   for these route-types, it MUST be ignored during the prefix
   reachability calculation.

   The M-flag in FAD is not applicable to prefixes advertised as SRv6
   locators.  The ISIS SRv6 Locator TLV includes the Algorithm and
   Metric fields [I-D.ietf-lsr-isis-srv6-extensions].  When the ISIS
   SRv6 Locator is advertised between areas or domains, the metric field
   in the Locator TLV MUST be used irrespective of the M-flag in the FAD
   advertisement.

13.  Flex-Algorithm and Forwarding Plane

   This section describes how Flex-Algorithm paths are used in
   forwarding.

13.1.  Segment Routing MPLS Forwarding for Flex-Algorithm

   This section describes how Flex-Algorithm paths are used with SR MPLS
   forwarding.

   Prefix SID advertisements include an SR-Algorithm value and, as such,
   are associated with the specified SR-Algorithm.  Prefix-SIDs are also
   associated with a specific topology which is inherited from the
   associated prefix reachability advertisement.  When the algorithm
   value advertised is a Flex-Algorithm value, the Prefix SID is
   associated with paths calculated using that Flex-Algorithm in the
   associated topology.

   A Flex-Algorithm path MUST be installed in the MPLS forwarding plane
   using the MPLS label that corresponds to the Prefix-SID that was
   advertised for that Flex-algorithm.  If the Prefix SID for a given
   Flex-algorithm is not known, the Flex-Algorithm specific path cannot
   be installed in the MPLS forwarding plane.

   Traffic that is supposed to be routed via Flex-Algorithm specific
   paths, MUST be dropped when there are no such paths available.

   Loop Free Alternate (LFA) paths for a given Flex-Algorithm MUST be
   computed using the same constraints as the calculation of the primary



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   paths for that Flex-Algorithm.  LFA paths MUST only use Prefix-SIDs
   advertised specifically for the given algorithm.  LFA paths MUST NOT
   use an Adjacency-SID that belongs to a link that has been pruned from
   the Flex-Algorithm computation.

   If LFA protection is being used to protect a given Flex-Algorithm
   paths, all routers in the area participating in the given Flex-
   Algorithm SHOULD advertise at least one Flex-Algorithm specific Node-
   SID.  These Node-SIDs are used to steer traffic over the LFA computed
   backup path.

13.2.  SRv6 Forwarding for Flex-Algorithm

   This section describes how Flex-Algorithm paths are used with SRv6
   forwarding.

   In SRv6 a node is provisioned with topology/algorithm specific
   locators for each of the topology/algorithm pairs supported by that
   node.  Each locator is an aggregate prefix for all SIDs provisioned
   on that node which have the matching topology/algorithm.

   The SRv6 locator advertisement in IGPs
   ([I-D.ietf-lsr-isis-srv6-extensions]
   [I-D.ietf-lsr-ospfv3-srv6-extensions]) includes the MTID value that
   associates the locator with a specific topology.  SRv6 locator
   advertisements also includes an Algorithm value that explicitly
   associates the locator with a specific algorithm.  When the algorithm
   value advertised with a locator represents a Flex-Algorithm, the
   paths to the locator prefix MUST be calculated using the specified
   Flex-Algorithm in the associated topology.

   Forwarding entries for the locator prefixes advertised in IGPs MUST
   be installed in the forwarding plane of the receiving SRv6 capable
   routers when the associated topology/algorithm is participating in
   them.  Forwarding entries for locators associated with Flex-
   Algorithms in which the node is not participating MUST NOT be
   installed in the forwarding palne.

   When the locator is associated with a Flex-Algorithm, LFA paths to
   the locator prefix MUST be calculated using such Flex-Algorithm in
   the associated topology, to guarantee that they follow the same
   constraints as the calculation of the primary paths.  LFA paths MUST
   only use SRv6 SIDs advertised specifically for the given Flex-
   Algorithm.

   If LFA protection is being used to protect locators associated with a
   given Flex-Algorithm, all routers in the area participating in the
   given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm



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   specific locator and END SID per node and one END.X SID for every
   link that has not been pruned from such Flex-Algorithm computation.
   These locators and SIDs are used to steer traffic over the LFA-
   computed backup path.

13.3.  Other Applications' Forwarding for Flex-Algorithm

   Any application that wants to use Flex-Algorithm specific forwarding
   needs to install some form of Flex-Algorithm specific forwarding
   entries.

   Application-specific forwarding for Flex-Algorithm MUST be defined
   for each application and is outside of the scope of this document.

14.  Operational considerations

14.1.  Inter-area Considerations

   The scope of the FA computation is an area, so is the scope of the
   FAD.  In ISIS, the Router Capability TLV in which the FAD Sub-TLV is
   advertised MUST have the S-bit clear, which prevents it to be flooded
   outside of the level in which it was originated.  Even though in OSPF
   the FAD Sub-TLV can be flooded in an RI LSA that has AS flooding
   scope, the FAD selection is performed for each individual area in
   which it is being used.

   There is no requirement for the FAD for a particular Flex-Algorithm
   to be identical in all areas in the network.  For example, traffic
   for the same Flex-Algorithm may be optimized for minimal delay (e.g.,
   using delay metric) in one area or level, while being optimized for
   available bandwidth (e.g., using IGP metric) in another area or
   level.

   As described in Section 5.1, ISIS allows the re-generation of the
   winning FAD from level 2, without any modification to it, into a
   level 1 area.  This allows the operator to configure the FAD in one
   or multiple routers in the level 2, without the need to repeat the
   same task in each level 1 area, if the intent is to have the same FAD
   for the particular Flex-Algorithm across all levels.  This can
   similarly be achieved in OSPF by using the AS flooding scope of the
   RI LSA in which the FAD Sub-TLV for the particular Flex-Algoritm is
   advertised.

   Re-generation of FAD from a level 1 area to the level 2 area is not
   supported in ISIS, so if the intent is to regenerate the FAD between
   ISIS levels, the FAD MUST be defined on router(s) that are in level
   2.  In OSPF, the FAD definition can be done in any area and be




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   propagated to all routers in the OSPF routing domain by using the AS
   flooding scope of the RI LSA.

14.2.  Usage of SRLG Exclude Rule with Flex-Algorithm

   There are two different ways in which SRLG information can be used
   with Flex-Algorithm:

      In a context of a single Flex-Algorithm, it can be used for
      computation of backup paths, as described in
      [I-D.ietf-rtgwg-segment-routing-ti-lfa].  This usage does not
      require association of any specific SRLG constraint with the given
      Flex-Algorithm definition.

      In the context of multiple Flex-Algorithms, it can be used for
      creating disjoint sets of paths by pruning the links belonging to
      a specific SRLG from the topology on which a specific Flex-
      Algorithm computes its paths.  This usage:

         Facilitates the usage of already deployed SRLG configurations
         for setup of disjoint paths between two or more Flex-
         Algorithms.

         Requires explicit association of a given Flex-Algorithm with a
         specific set of SRLG constraints as defined in Section 6.5 and
         Section 7.5.

   The two usages mentioned above are orthogonal.

14.3.  Max-metric consideration

   Both ISIS and OSPF have a mechanism to set the IGP metric on a link
   to a value that would make the link either non-reachable or to serve
   as the link of last resort.  Similar functionality would be needed
   for the Min Unidirectional Link Delay and TE metric, as these can be
   used to compute Flex-Algorithm paths.

   The link can be made un-reachable for all Flex-Algorithms that use
   Min Unidirectional Link Delay as metric, as described in Section 5.1,
   by removing the Flex-Algorithm ASLA Min Unidirectional Link Delay
   advertisement for the link.  The link can be made the link of last
   resort by setting the delay value in the Flex-Algorithm ASLA delay
   advertisement for the link to the value of 16,777,215 (2^24 - 1).

   The link can be made un-reachable for all Flex-Algorithms that use TE
   metric, as described in Section 5.1, by removing the Flex-Algorithm
   ASLA TE metric advertisement for the link.  The link can be made the
   link of last resort by setting the TE metric value in the Flex-



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   Algorithm ASLA delay advertisement for the link to the value of (2^24
   - 1) in ISIS and (2^32 - 1) in OSPF.

15.  Backward Compatibility

   This extension brings no new backward compatibility issues.

16.  Security Considerations

   This draft adds two new ways to disrupt IGP networks:

      An attacker can hijack a particular Flex-Algorithm by advertising
      a FAD with a priority of 255 (or any priority higher than that of
      the legitimate nodes).

      An attacker could make it look like a router supports a particular
      Flex-Algorithm when it actually doesn't, or vice versa.

   Both of these attacks can be addressed by the existing security
   extensions as described in [RFC5304] and [RFC5310] for ISIS, in
   [RFC2328] and [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552]
   for OSPFv3.

17.  IANA Considerations

17.1.  IGP IANA Considerations

17.1.1.  IGP Algorithm Types Registry

   This document makes the following registrations in the "IGP Algorithm
   Types" registry:

      Type: 128-255.

      Description: Flexible Algorithms.

      Reference: This document (Section 4).

17.1.2.  IGP Metric-Type Registry

   IANA is requested to set up a registry called "IGP Metric-Type
   Registry" under a "Interior Gateway Protocol (IGP) Parameters" IANA
   registries.  The registration policy for this registry is "Standards
   Action" ([RFC8126] and [RFC7120]).

   Values in this registry come from the range 0-255.





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   This document registers following values in the "IGP Metric-Type
   Registry":

      Type: 0

      Description: IGP metric

      Reference: This document (Section 5.1)

      Type: 1

      Description: Min Unidirectional Link Delay as defined in
      [RFC8570], section 4.2, and [RFC7471], section 4.2.

      Reference: This document (Section 5.1)

      Type: 2

      Description: Traffic Engineering Default Metric as defined in
      [RFC5305], section 3.7, and Traffic engineering metric as defined
      in [RFC3630], section 2.5.5

      Reference: This document (Section 5.1)

17.2.  Flexible Algorithm Definition Flags Registry

   IANA is requested to set up a registry called "ISIS Flexible
   Algorithm Definition Flags Registry" under a "Interior Gateway
   Protocol (IGP) Parameters" IANA registries.  The registration policy
   for this registry is "Standards Action" ([RFC8126] and [RFC7120]).

   This document defines the following single bit in Flexible Algorithm
   Definition Flags registry:

           Bit #   Name
           -----   ------------------------------
           0       Prefix Metric Flag (M-flag)


   Reference: This document (Section 6.4, Section 7.4).

17.3.  ISIS IANA Considerations

17.3.1.  Sub TLVs for Type 242

   This document makes the following registrations in the "sub-TLVs for
   TLV 242" registry.




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      Type: 26.

      Description: Flexible Algorithm Definition.

      Reference: This document (Section 5.1).

17.3.2.  Sub TLVs for for TLVs 135, 235, 236, and 237

   This document makes the following registrations in the "Sub-TLVs for
   for TLVs 135, 235, 236, and 237" registry.

      Type: 6

      Description: Flexible Algorithm Prefix Metric.

      Reference: This document (Section 8).

17.3.3.  Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV

   This document creates the following Sub-Sub-TLV Registry:

      Registry: Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV

      Registration Procedure: Expert review

      Reference: This document (Section 5.1)

   This document defines the following Sub-Sub-TLVs in the "Sub-Sub-TLVs
   for Flexible Algorithm Definition Sub-TLV" registry:

      Type: 1

      Description: Flexible Algorithm Exclude Admin Group

      Reference: This document (Section 6.1).

      Type: 2

      Description: Flexible Algorithm Include-Any Admin Group

      Reference: This document (Section 6.2).

      Type: 3

      Description: Flexible Algorithm Include-All Admin Group

      Reference: This document (Section 6.3).




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      Type: 4

      Description: Flexible Algorithm Definition Flags

      Reference: This document (Section 6.4).

      Type: 5

      Description: Flexible Algorithm Exclude SRLG

      Reference: This document (Section 6.5).

17.4.  OSPF IANA Considerations

17.4.1.  OSPF Router Information (RI) TLVs Registry

   This specification updates the OSPF Router Information (RI) TLVs
   Registry.

      Type: 16

      Description: Flexible Algorithm Definition TLV.

      Reference: This document (Section 5.2).

17.4.2.  OSPFv2 Extended Prefix TLV Sub-TLVs

   This document makes the following registrations in the "OSPFv2
   Extended Prefix TLV Sub-TLVs" registry.

      Type: 3

      Description: Flexible Algorithm Prefix Metric.

      Reference: This document (Section 9).

17.4.3.  OSPFv3 Extended-LSA Sub-TLVs

   This document makes the following registrations in the "OSPFv3
   Extended-LSA Sub-TLVs" registry.

      Type: 26

      Description: Flexible Algorithm Prefix Metric.

      Reference: This document (Section 9).





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17.4.4.  OSPF Flexible Algorithm Definition TLV Sub-TLV Registry

   This document creates the following registry:

      Registry: OSPF Flexible Algorithm Definition TLV sub-TLV

      Registration Procedure: Expert review

      Reference: This document (Section 5.2)

   The "OSPF Flexible Algorithm Definition TLV sub-TLV" registry will
   define sub-TLVs at any level of nesting for the Flexible Algorithm
   TLV and should be added to the "Open Shortest Path First (OSPF)
   Parameters" registries group.  New values can be allocated via IETF
   Review or IESG Approval.

   This document registers following Sub-TLVs in the "TLVs for Flexible
   Algorithm Definition TLV" registry:

      Type: 1

      Description: Flexible Algorithm Exclude Admin Group

      Reference: This document (Section 7.1).

      Type: 2

      Description: Flexible Algorithm Include-Any Admin Group

      Reference: This document (Section 7.2).

      Type: 3

      Description: Flexible Algorithm Include-All Admin Group

      Reference: This document (Section 7.3).

      Type: 4

      Description: Flexible Algorithm Definition Flags

      Reference: This document (Section 7.4).

      Type: 5

      Description: Flexible Algorithm Exclude SRLG

      Reference: This document (Section 7.5).



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   Types in the range 32768-33023 are for experimental use; these will
   not be registered with IANA, and MUST NOT be mentioned by RFCs.

   Types in the range 33024-65535 are not to be assigned at this time.
   Before any assignments can be made in the 33024-65535 range, there
   MUST be an IETF specification that specifies IANA Considerations that
   covers the range being assigned.

17.4.5.  Link Attribute Applications Registry

   This document registers following bit in the Link Attribute
   Applications Registry:

      Bit-3

      Description: Flexible Algorithm (X-bit)

      Reference: This document (Section 11).

18.  Acknowledgements

   This draft, among other things, is also addressing the problem that
   the [I-D.gulkohegde-routing-planes-using-sr] was trying to solve.
   All authors of that draft agreed to join this draft.

   Thanks to Eric Rosen, Tony Przygienda for their detailed review and
   excellent comments.

   Thanks to Cengiz Halit for his review and feedback during initial
   phase of the solution definition.

   Thanks to Kenji Kumaki for his comments.

   Thanks to William Britto A J. for his suggestions.

   Thanks to Acee Lindem for editorial comments.

19.  References

19.1.  Normative References

   [BCP14]    "Key words for use in RFCs to Indicate Requirement
              Levels", <https://tools.ietf.org/html/bcp14>.

   [I-D.ietf-isis-te-app]
              Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
              J. Drake, "IS-IS Application-Specific Link Attributes",
              draft-ietf-isis-te-app-19 (work in progress), June 2020.



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   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-10
              (work in progress), September 2020.

   [I-D.ietf-lsr-ospfv3-srv6-extensions]
              Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
              "OSPFv3 Extensions for SRv6", draft-ietf-lsr-
              ospfv3-srv6-extensions-01 (work in progress), August 2020.

   [I-D.ietf-ospf-te-link-attr-reuse]
              Psenak, P., Ginsberg, L., Henderickx, W., Tantsura, J.,
              and J. Drake, "OSPF Application-Specific Link Attributes",
              draft-ietf-ospf-te-link-attr-reuse-16 (work in progress),
              June 2020.

   [ISO10589]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/
              IEC 10589:2002, Second Edition, Nov 2002.

   [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>.

   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <https://www.rfc-editor.org/info/rfc4203>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <https://www.rfc-editor.org/info/rfc5307>.

   [RFC7308]  Osborne, E., "Extended Administrative Groups in MPLS
              Traffic Engineering (MPLS-TE)", RFC 7308,
              DOI 10.17487/RFC7308, July 2014,
              <https://www.rfc-editor.org/info/rfc7308>.







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   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.

   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

   [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>.

   [RFC8666]  Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
              for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
              December 2019, <https://www.rfc-editor.org/info/rfc8666>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

19.2.  Informative References

   [I-D.gulkohegde-routing-planes-using-sr]
              Hegde, S. and a. arkadiy.gulko@thomsonreuters.com,
              "Separating Routing Planes using Segment Routing", draft-
              gulkohegde-routing-planes-using-sr-00 (work in progress),
              March 2017.




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   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
              Francois, P., Voyer, D., Clad, F., and P. Camarillo,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-ietf-rtgwg-segment-routing-ti-lfa-04 (work in
              progress), August 2020.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.

   [RFC3906]  Shen, N. and H. Smit, "Calculating Interior Gateway
              Protocol (IGP) Routes Over Traffic Engineering Tunnels",
              RFC 3906, DOI 10.17487/RFC3906, October 2004,
              <https://www.rfc-editor.org/info/rfc3906>.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.






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   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
              <https://www.rfc-editor.org/info/rfc7471>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

Authors' Addresses

   Peter Psenak (editor)
   Cisco Systems
   Apollo Business Center
   Mlynske nivy 43
   Bratislava, 82109
   Slovakia

   Email: ppsenak@cisco.com


   Shraddha Hegde
   Juniper Networks, Inc.
   Embassy Business Park
   Bangalore, KA, 560093
   India

   Email: shraddha@juniper.net


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com




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   Ketan Talaulikar
   Cisco Systems, Inc.
   S.No. 154/6, Phase I, Hinjawadi
   PUNE, MAHARASHTRA   411 057
   India

   Email: ketant@cisco.com


   Arkadiy Gulko
   Individual

   Email: arkadiy.gulko@gmail.com






































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