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Versions: (draft-ppsenak-ospf-sr-flex-algo) 00

Network Working Group                                     P. Psenak, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                                S. Hegde
Expires: November 16, 2018                        Juniper Networks, Inc.
                                                             C. Filsfils
                                                           K. Talaulikar
                                                     Cisco Systems, Inc.
                                                                A. Gulko
                                                         Thomson Reuters
                                                            May 15, 2018


                         IGP Flexible Algorithm
                    draft-ietf-lsr-flex-algo-00.txt

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
   enforce 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 Prefix-SIDs 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 November 16, 2018.








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Copyright Notice

   Copyright (c) 2018 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
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   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 notation . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Flexible Algorithm  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Flexible Algorithm Definition Advertisement . . . . . . . . .   5
     5.1.  ISIS Flexible Algorithm Definition Sub-TLV  . . . . . . .   5
     5.2.  OSPF Flexible Algorithm Definition TLV  . . . . . . . . .   7
     5.3.  Common Handling of Flexible Algorithm Definition TLV  . .   8
   6.  Sub-TLVs of ISIS FAD Sub-TLV  . . . . . . . . . . . . . . . .   9
     6.1.  ISIS Flexible Algorithm Exclude Admin Group Sub-TLV . . .   9
     6.2.  ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV .  10
     6.3.  ISIS Flexible Algorithm Include-All Admin Group Sub-TLV .  10
   7.  Sub-TLVs of OSPF FAD TLV  . . . . . . . . . . . . . . . . . .  10
     7.1.  OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . .  11
     7.2.  OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV .  11
     7.3.  OSPF Flexible Algorithm Include-All Admin Group Sub-TLV .  11
   8.  Advertisement of Node Participation in a Flex-Algorithm . . .  12
     8.1.  Advertisement of Node Participation for Segment Routing .  12
     8.2.  Advertisement of Node Participation for Other
           Applications  . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Advertisement of Link Attributes for Flex-Algorithm . . . . .  13
   10. Calculation of Flexible Algorithm Paths . . . . . . . . . . .  13
   11. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . .  15
     11.1.  Segment Routing MPLS Forwarding for Flex-Algorithm . . .  15
     11.2.  Other Applications' Forwarding for Flex-Algorithm  . . .  15
   12. Backward Compatibility  . . . . . . . . . . . . . . . . . . .  16
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     14.1.  IGP IANA Considerations  . . . . . . . . . . . . . . . .  16
       14.1.1.  IGP Algorithm Types Registry . . . . . . . . . . . .  16
       14.1.2.  Flexible Algorithm Definition Metric-Type Registry .  16



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     14.2.  ISIS IANA Considerations . . . . . . . . . . . . . . . .  17
       14.2.1.  Sub TLVs for Type 242  . . . . . . . . . . . . . . .  17
       14.2.2.  Sub-Sub-TLVs for Flexible Algorithm Definition Sub-
                TLV  . . . . . . . . . . . . . . . . . . . . . . . .  17
     14.3.  OSPF IANA Considerations . . . . . . . . . . . . . . . .  18
       14.3.1.  OSPF Router Information (RI) TLVs Registry . . . . .  18
       14.3.2.  OSPF Flexible Algorithm Definition TLV Sub-TLV
                Registry . . . . . . . . . . . . . . . . . . . . . .  18
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     16.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   An IGP computed path based on the shortest IGP metric must often be
   replaced by 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 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 the path 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
   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 send TLVs that (a) describe a set of
   constraints on the topology, (b) identify calculation-type, and (c)
   metric-type 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
   specific value, Flex-Algorithm, 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 Segment Routing Prefix-SIDs with a particular
   Flex-Algorithm.  Each such Prefix-SID then represents a path that is
   computed according to the identified Flex-Algorithm.



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2.  Requirements notation

   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 - 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 provisioning 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.

   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 not exist.

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 [RFC7810].  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)



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   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 it's
   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 provisioning 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

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

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

   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:













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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     |Flex-Algorithm |  Metric-Type  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Calc-Type   |    Priority   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Sub-TLVs                             |
   +                                                               +
   |                            ...                                |

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

   where:

      Type: TBD, suggested value 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 [RFC7810].

         2: TE default metric as defined in [RFC5305].

      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 flooded only in a given level or
   throughout the domain.  In the latter case the S-flag is set as



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   described in [RFC7981].  It is recommended that domain-wide flooding
   NOT be the default behavior.

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].

   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: TBD, suggested value 16

      Length: variable, dependent on the included Sub-TLVs

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

      Metric-Type: as described in Section 5.1

      Calc-Type: as described in Section 5.1

      Priority: as described in Section 5.1

      Sub-TLVs - optional sub-TLVs.

   When multiple OPSF 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



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   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.

   Not every router configured to participate in a particular Flex-
   Algorithm need a local definition of such Flex-Algorithm.  Only a
   subset of the routers participating in the particular Flex-Algorithm
   need the local 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.

      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 case of ISIS or Router ID in
      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.

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




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   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 or constraint 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 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.

   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:

    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].






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   ISIS FAEAG Sub-TLV MAY 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 link during the Flex-Algorithm path
   computation.

   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 Section 10.

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

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

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

6.3.  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.

   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 Section 10.

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

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

   ISIS Flexible Algorithm Include-All Admin Group Sub-TLV MAY 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








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7.1.  OSPF Flexible Algorithm Exclude Admin Group Sub-TLV

   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].

   OSPF FAEAG Sub-TLV MAY 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.

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

   OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MAY NOT
   appear more then once in an OPSF 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.





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   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.

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

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

8.  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 the data 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.

8.1.  Advertisement of Node Participation for Segment Routing

   [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions] (IGP Segment
   Routing extensions) describe how SR-Algorithm is used to define how
   the best path is computed by the IGP.

   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, 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.

8.2.  Advertisement of Node Participation for Other Applications

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





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   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.

9.  Advertisement of Link Attributes for Flex-Algorithm

   Various link include or exclude rules can be part of the Flex-
   Algorithm definition.  These rules use Admin Groups (AG) as defined
   in [RFC7308] and [RFC5305], or Extended Administrative Groups (EAG)
   as defined in [RFC7308].

   To advertise a link affinity in a form of the AG or EAG that is used
   during Flex-Algorithm calculation, an Application Specific Link
   Attributes sub-TLV as described in [I-D.ietf-isis-te-app], or sub-TLV
   of Extended Link TLV as described in
   [I-D.ietf-ospf-te-link-attr-reuse] MUST be used.  The advertisement
   MUST indicate that it is usable by the Flex-Algorithm application.

10.  Calculation of Flexible Algorithm Paths

   A router MUST be configured to participate in a given Flex-Algorithm
   K before it can compute any path for that Flex-Algorithm.

   A router which participates in a given Flex Algorithm MUST use the
   FAD selected based on the rules defined in Section Section 5.3.

   As described in Section 8, 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
   calculation, then when computing paths for a given Flex-Algorithm,
   all nodes that do not advertise participation for that Flex-Algorithm
   in the application specific advertisements MUST be pruned from the
   topology.  MPLS Segment Routing is an application that MUST use such
   pruning when computing Flex-Algorithm paths.

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




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   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 particular bit within an AG or EAG
   we uses term 'color'.

   Rules, in the order as specified below, MUST be used to prune link
   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 exist, the link MUST be pruned from the computation.

      2.  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
      such color does not exist, the link MUST be pruned from the
      computation.

      3.  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 not all such colors are set on the link, the link MUST be
      pruned from the computation.

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

   Any IGP Shortest Path Tree calculation is limited to a single area.
   Same applies to Flex-Algorithm calculations.  Given that the
   computing router may not have the visibility to the topology of
   remote areas, the Flex-Algorithm specific path to an inter-area
   prefix will only be computed for the local area only.  The egress L1/
   L2 router (ABR in OSPF) will be selected based on the best path for
   the given Flex-Algorithm in the local area and such egress L1/L2 (ABR
   in OSPF) router will be responsible to compute the best Flex-
   Algorithm specific path over the next area.  This may produce an end-
   to-end path, which is sub-optimal based on Flex-Algorithm
   constraints.  If the best end-to-end path for a given Flex-Algorithm
   needs to be used for inter-area destinations, paths for such



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   destinations need to be computed by the entity that has the
   topological information about all areas.

11.  Flex-Algorithm and Forwarding Plane

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

11.1.  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 where 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
   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 enforce traffic over the LFA
   computed backup path.

11.2.  Other Applications' Forwarding for Flex-Algorithm

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



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

12.  Backward Compatibility

   This extension brings no new backward compatibility issues.

13.  Security Considerations

   This draft adds a two new ways to disrupt the 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.

14.  IANA Considerations

14.1.  IGP IANA Considerations

14.1.1.  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).

14.1.2.  Flexible Algorithm Definition Metric-Type Registry

   IANA is requested to set up a registry called "Flexible Algorithm
   Definition 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 "Flexible Algorithm
   Definition Metric-Type Registry":

      Type: 0

      Description: IGP metric

      Reference: This document (Section 5.1)

      Type: 1

      Description: Min Unidirectional Link Delay [RFC7810]

      Reference: This document (Section 5.1)

      Type: 2

      Description: TE Default Metric [RFC5305]

      Reference: This document (Section 5.1)

14.2.  ISIS IANA Considerations

14.2.1.  Sub TLVs for Type 242

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

      Type: TBD (suggested value 26).

      Description: Flexible Algorithm Definition Sub-TLV.

      Reference: This document (Section 5.1).

14.2.2.  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



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      Description: Flexible Algorithm Exclude Admin Group Sub-TLV

      Reference: This document (Section 6.1).

      Type: 2

      Description: Flexible Algorithm Include-Any Admin Group Sub-TLV

      Reference: This document (Section 6.2).

      Type: 3

      Description: Flexible Algorithm Include-All Admin Group Sub-TLV

      Reference: This document (Section 6.3).

14.3.  OSPF IANA Considerations

14.3.1.  OSPF Router Information (RI) TLVs Registry

   This specification updates the OSPF Router Information (RI) TLVs
   Registry with the following value:

   o TBD (suggested value 16) - Flexible Algorithm Definition TLV

14.3.2.  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 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 resisters following Sub-TLVs in the "TLVs for Flexible
   Algorithm Definition TLV" registry:

      Type: 1

      Description: Flexible Algorithm Exclude Admin Group Sub-TLV




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      Reference: This document (Section 7.1).

      Type: 2

      Description: Flexible Algorithm Include-Any Admin Group Sub-TLV

      Reference: This document (Section 7.2).

      Type: 3

      Description: Flexible Algorithm Include-All Admin Group Sub-TLV

      Reference: This document (Section 7.3).

   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.

15.  Contributors

   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, Les Ginsberg and 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.

16.  References

16.1.  Normative References

   [BCP14]    , <https://tools.ietf.org/html/bcp14>.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
              "IS-IS Extensions for Segment Routing", draft-ietf-isis-
              segment-routing-extensions-16 (work in progress), April
              2018.



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   [I-D.ietf-isis-te-app]
              Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
              J. Drake, "IS-IS TE Attributes per application", draft-
              ietf-isis-te-app-04 (work in progress), April 2018.

   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]
              Psenak, P., Filsfils, C., Previdi, S., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
              segment-routing-extensions-12 (work in progress), April
              2018.

   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-25 (work in progress), April 2018.

   [I-D.ietf-ospf-te-link-attr-reuse]
              Psenak, P., Lindem, A., Ginsberg, L., Henderickx, W.,
              Tantsura, J., Gredler, H., and J. Drake, "OSPFv2 Link
              Traffic Engineering (TE) Attribute Reuse", draft-ietf-
              ospf-te-link-attr-reuse-03 (work in progress), January
              2018.

   [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>.

   [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>.

   [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>.




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   [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>.

16.2.  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.

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

   [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>.





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   [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>.

   [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>.

   [RFC7810]  Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
              Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
              RFC 7810, DOI 10.17487/RFC7810, May 2016,
              <https://www.rfc-editor.org/info/rfc7810>.

   [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>.

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
   Thomson Reuters

   Email: arkadiy.gulko@thomsonreuters.com






































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