Flexible Algorithm Definition Advertisement with BGP Link-State
draft-ietf-idr-bgp-ls-flex-algo-07

Document Type Active Internet-Draft (idr WG)
Authors Ketan Talaulikar  , Peter Psenak  , Shawn Zandi  , Gaurav Dawra 
Last updated 2021-06-08 (latest revision 2021-06-07)
Replaces draft-ketant-idr-bgp-ls-flex-algo
Stream Internet Engineering Task Force (IETF)
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Inter-Domain Routing                                  K. Talaulikar, Ed.
Internet-Draft                                                 P. Psenak
Intended status: Standards Track                           Cisco Systems
Expires: December 9, 2021                                       S. Zandi
                                                                G. Dawra
                                                                LinkedIn
                                                            June 7, 2021

    Flexible Algorithm Definition Advertisement with BGP Link-State
                   draft-ietf-idr-bgp-ls-flex-algo-07

Abstract

   Flexible Algorithm is a solution that allows routing protocols (viz.
   OSPF and IS-IS) to compute paths over a network based on user-defined
   (and hence, flexible) constraints and metrics.  The computation is
   performed by routers participating in the specific network in a
   distribute manner using a Flex Algorithm definition.  This definition
   provisioned on one or more routers and propagated (viz.  OSPF and IS-
   IS flooding) through the network.

   BGP Link-State (BGP-LS) enables the collection of various topology
   information from the network.  This draft defines extensions to BGP-
   LS address-family to advertise the Flexible Algorithm Definition as a
   part of the topology information from the network.

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 December 9, 2021.

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Copyright Notice

   Copyright (c) 2021 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  BGP-LS Extensions for Flex Algo . . . . . . . . . . . . . . .   4
   3.  Flexible Algorithm Definition . . . . . . . . . . . . . . . .   5
     3.1.  Flex Algo Exclude Any Affinity  . . . . . . . . . . . . .   6
     3.2.  Flex Algo Include Any Affinity  . . . . . . . . . . . . .   7
     3.3.  Flex Algo Include All Affinity  . . . . . . . . . . . . .   7
     3.4.  Flex Algo Definition Flags  . . . . . . . . . . . . . . .   8
     3.5.  Flex Algo Exclude SRLG  . . . . . . . . . . . . . . . . .   9
     3.6.  Flex Algo Unknown . . . . . . . . . . . . . . . . . . . .  10
   4.  Flex Algorithm Prefix Metric  . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   IGP protocols (OSPF and IS-IS) traditionally compute best paths over
   the network based on the IGP metric assigned to the links.  Many
   network deployments use RSVP-TE [RFC3209] based or Segment Routing
   (SR) Policy [RFC8402] based solutions to enforce traffic over a path
   that is computed using different metrics or constraints than the
   shortest IGP path.  [I-D.ietf-lsr-flex-algo] defines the Flexible
   Algorithm solution that allows IGPs themselves to compute constraint
   based paths over the network.

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   Flexible Algorithm is called so as it allows a user the flexibility
   to define

   o  the type of calculation to be used (e.g. shortest path)

   o  the metric type to be used (e.g.  IGP metric or TE metric)

   o  the set of constraints to be used (e.g. inclusion or exclusion of
      certain links using affinities)

   The operations of the flexible algorithm solution are described in
   detail in [I-D.ietf-lsr-flex-algo] and a high-level summary of the
   same is described here for clarity.  The network operator enables the
   participation of specific nodes in the network for a specific
   algorithm and then provisions the definition of that flexible
   algorithm on one or more of these nodes.  The nodes where the
   flexible algorithm definition (FAD) is advertised then flood these
   definitions via respective IGP (IS-IS and OSPFv2/v3) mechanisms to
   all other nodes in the network.  The nodes select the definition for
   each algorithm based on the flooded information in a deterministic
   manner and thus all nodes participating in a flexible algorithm
   computation arrive at a common understanding of the type of
   calculation that they need to use.

   When using Segment Routing (SR) [RFC8402] MPLS forwarding plane
   [RFC8660], the result of a flex algorithm computation is the
   provisioning of the Prefix SIDs associated with that algorithm with
   paths based on the topology computed based on that algorithm's
   definition.  When using SR over IPv6 (SRv6) [RFC8986], the result of
   a flex algorithm computation is the provisioning of the SRv6 Locators
   associated with that algorithm with paths based on the topology
   computed based on that algorithm.  This flex algorithm computation is
   within an IGP area or level similar to the default shortest path tree
   (SPT) algorithm.

   A flex algorithm specific metric MAY be advertised along with the
   prefix as described in [I-D.ietf-lsr-flex-algo] to enable end-to-end
   optimal path computation for prefixes across multiple areas/domains
   in the flex algorithm computation for the SR-MPLS forwarding plane.

   The BGP-LS extensions for SR are defined in
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] and
   [I-D.ietf-idr-bgpls-srv6-ext].  They include the

   o  SR Algorithm TLV to indicate the participation of a node in a flex
      algorithm computation

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   o  Prefix SID TLV to indicate the association of the Prefix-SIDs to a
      specific flex algorithm for SR-MPLS forwarding

   o  SRv6 Locator TLV to indicate the Locator for specific flex
      algorithm for SRv6 forwarding

   Thus a controller or a Path Computation Engine (PCE) is aware of the
   IGP topology across multiple domains which includes the above
   information related to the flexible algorithm.  This draft defines
   extensions to BGP-LS for carrying the FAD information so that it
   enables the controller/PCE to learn the mapping of the flex algorithm
   number to its definition in each area/domain of the underlying IGP.
   The controller/PCE also learns the type of computation used and the
   constraints for the same.  This information can then be leveraged by
   it for setting up SR Policy paths end to end across domains by
   leveraging the appropriate Flex Algorithm specific SIDs in its
   Segment List [I-D.ietf-spring-segment-routing-policy]. e.g. picking
   the Flex Algorithm Prefix SID (in case of SR-MPLS) or End SID (in
   case of SRv6) of ABRs/ASBRs corresponding to a definition that
   optimizes on the delay metric enables the PCE/controller to build an
   end to end low latency path across IGP domains with minimal SIDs in
   the SID list.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  BGP-LS Extensions for Flex Algo

   The BGP-LS [RFC7752] specifies the Node NLRI for the advertisement of
   nodes along with their attributes using the BGP-LS Attribute, the
   Link NLRI for the advertisement of links along with their attributes
   using the BGP-LS Attribute and the Prefix NLRI for the advertisement
   of prefixes along with their attributes using the BGP-LS Attribute.

   The FAD advertised by a node is considered as its node level
   attributes and advertised as such.

   Various link attributes like affinities and SRLGs used during the
   Flex-Algorithm path calculations in IS-IS and OSPF are advertised in
   those protocols using the Application Specific Link Attribute (ASLA)
   advertisements as described in [I-D.ietf-lsr-flex-algo].  The BGP-LS
   extensions for ASLA advertisements
   [I-D.ietf-idr-bgp-ls-app-specific-attr] MUST be used for the

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   advertisement of these Flex-Algorithm application-specific link
   attributes from the underlying IGP protocols using the Flexible
   Algorithm application specific bit defined in
   [I-D.ietf-lsr-flex-algo].

   The Flexible Algorithm Prefix Metric (FAPM) are considered as prefix
   attributes and advertised as such.

3.  Flexible Algorithm Definition

   This document defines a new optional BGP-LS Attribute TLV associated
   with the Node NLRI called the Flexible Algorithm Definition (FAD) TLV
   and its format is as follows:

      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       ...                            //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: Flex Algorithm Definition TLV

     where:

   o  Type: 1039

   o  Length: variable.  Minimum of 4 octets.

   o  Flex-Algorithm : 1 octet value in the range between 128 and 255
      inclusive which is the range defined for Flexible Algorithms in
      the IANA "IGP Parameters" registries under the "IGP Algorithm
      Types" registry [I-D.ietf-lsr-flex-algo].

   o  Metric-Type : 1 octet value indicating the type of metric used in
      the computation.  Values allowed come from the IANA "IGP
      Parameters" registries under the "Flexible Algorithm Definition
      Metric-Type" registry [I-D.ietf-lsr-flex-algo].

   o  Calculation-Type : 1 octet value in the range between 0 and 127
      inclusive which is the range defined for the standard algorithms
      in the IANA "IGP Parameters" registries under the "IGP Algorithm
      Types" registry [I-D.ietf-lsr-flex-algo].

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   o  Priority : 1 octet value between 0 and 255 inclusive that
      specifies the priority of the FAD.

   o  sub-TLVs : zero or more sub-TLVs may be included as described
      further in this section.

   The FAD TLV can only be added to the BGP-LS Attribute of the Node
   NLRI if the corresponding node originates the underlying IGP TLV/sub-
   TLV as described below.  This information is derived from the
   protocol specific advertisements as below.

   o  IS-IS, as defined by the ISIS Flexible Algorithm Definition sub-
      TLV in [I-D.ietf-lsr-flex-algo].

   o  OSPFv2/OSPFv3, as defined by the OSPF Flexible Algorithm
      Definition TLV in [I-D.ietf-lsr-flex-algo].

   The BGP-LS Attribute associated with a Node NLRI MAY include one or
   more FAD TLVs corresponding to the FAD for each algorithm that the
   particular node is advertising.

   The following sub-sections define the sub-TLVs for the FAD TLV.

3.1.  Flex Algo Exclude Any Affinity

   The Flex Algo Exclude Any Affinity sub-TLV is an optional sub-TLV
   that is used to carry the affinity constraints [RFC2702] associated
   with the FAD and enable the exclusion of links carrying any of the
   specified affinities from the computation of the specific algorithm
   as described in [I-D.ietf-lsr-flex-algo].  The affinity is expressed
   in terms of Extended Admin Group (EAG) as defined in [RFC7308].

   The 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Exclude-Any EAG (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1040

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

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   o  Exclude-Any EAG : the bitmask used to represent the affinities to
      be excluded.

   The information in the Flex Algo Exclude Any Affinity sub-TLV is
   derived from the IS-IS and OSPF protocol specific Flexible Algorithm
   Exclude Admin Group sub-TLV as defined in [I-D.ietf-lsr-flex-algo].

3.2.  Flex Algo Include Any Affinity

   The Flex Algo Include Any Affinity sub-TLV is an optional sub-TLV
   that is used to carry the affinity constraints [RFC2702] associated
   with the FAD and enable the inclusion of links carrying any of the
   specified affinities in the computation of the specific algorithm as
   described in [I-D.ietf-lsr-flex-algo].  The affinity is expressed in
   terms of Extended Admin Group (EAG) as defined in [RFC7308].

   The 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Include-Any EAG (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1041

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

   o  Include-Any EAG : the bitmask used to represent the affinities to
      be included.

   The information in the Flex Algo Include Any Affinity sub-TLV is
   derived from the IS-IS and OSPF protocol specific Flexible Algorithm
   Include-Any Admin Group sub-TLV as defined in
   [I-D.ietf-lsr-flex-algo].

3.3.  Flex Algo Include All Affinity

   The Flex Algo Include All Affinity sub-TLV is an optional sub-TLV
   that is used to carry the affinity constraints [RFC2702] associated
   with the FAD and enable the inclusion of links carrying all of the
   specified affinities in the computation of the specific algorithm as

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   described in [I-D.ietf-lsr-flex-algo].  The affinity is expressed in
   terms of Extended Admin Group (EAG) as defined in [RFC7308].

   The 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Include-All EAG (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1042

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

   o  Include-All EAG : the bitmask used to represent the affinities to
      be included.

   The information in the Flex Algo Include All Affinity sub-TLV is
   derived from the IS-IS and OSPF protocol specific Flexible Algorithm
   Include-All Admin Group sub-TLV as defined in
   [I-D.ietf-lsr-flex-algo].

3.4.  Flex Algo Definition Flags

   The Flex Algo Definition Flags sub-TLV is an optional sub-TLV that is
   used to carry the flags associated with the FAD that are used in the
   computation of the specific algorithm as described in
   [I-D.ietf-lsr-flex-algo].

   The 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Flags (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1043

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   o  Length: variable.  MUST be a multiple of 4 octets.

   o  Flags : the bitmask used to represent the flags for the FAD as
      introduced by [I-D.ietf-lsr-flex-algo] and listed in the "Flex-
      Algorithm Definition Flags" registry under the "Interior Gateway
      Protocol (IGP) Parameters" IANA registry.

   The information in the Flex Algo Definition Flags sub-TLV is derived
   from the IS-IS and OSPF protocol specific Flexible Algorithm
   Definition Flags sub-TLV as defined in [I-D.ietf-lsr-flex-algo].

3.5.  Flex Algo Exclude SRLG

   The Flex Algo Exclude SRLG sub-TLV is an optional sub-TLV that is
   used to carry the shared risk link group (SRLG) [RFC4202] information
   associated with the FAD and enable the exclusion of links that are
   associated with any of the specified SRLG in the computation of the
   specific algorithm as described in [I-D.ietf-lsr-flex-algo].  The
   SRLGs associated with a link are carried in the BGP-LS Shared Link
   Risk Group (TLV 1096) [RFC7752].

   The 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Shared Risk Link Group Values (variable)           //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1045

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

   o  SRLG Values : One or more SRLG values, each of 4 octet size, as
      defined in [RFC4202].

   The information in the Flex Algo SRLG Exclude sub-TLV is derived from
   the IS-IS and OSPF protocol specific Flexible Algorithm Exclude SRLG
   sub-TLV as defined in [I-D.ietf-lsr-flex-algo].

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3.6.  Flex Algo Unknown

   The OSPF and ISIS signaling for FAD allows for extensions via new
   sub-TLVs under the respective IGP's Flex Algorithm Definition TLV.
   As specified in section 5.3 of [I-D.ietf-lsr-flex-algo], it is
   important that the entire FAD be understood by anyone using it for
   computation purposes.  Therefore the FAD is different from most other
   protocol extensions where the skipping or ignoring of unknown or
   unsupported sub-TLV information does not affect the base behavior.

   The Flex Algo Unknown sub-TLV is an optional sub-TLV that is used to
   indicate the presence of unknown or unsupported FAD sub-TLVs.  The
   need for this sub-TLV arises when the BGP-LS implementation on the
   advertising node does not support one or more of the FAD sub-TLVs
   present in the IGP advertisement.

   The 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           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Protocol-ID  | sub-TLV types (variable) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: TBD

   o  Length: variable

   o  Protocol-ID: Indicates the BGP-LS Protocol-ID of the protocol from
      which the FAD is being advertised via BGP-LS.  The values are from
      the "BGP-LS Protocol-IDs" registry under the IANA BGP-LS
      Parameters registry.

   o  Sub-TLV Types : Zero or more sub-TLV types that are unknown or
      unsupported by the node originating the BGP-LS advertisement.  The
      size of each sub-TLV type depends on the protocol indicated by the
      Protocol-ID field e.g., for ISIS each sub-TLV type would be of
      size 1 byte while for OSPF each sub-TLV type would be of size 2
      bytes.

   The discussion on the use of the FAD information by the consumers of
   the BGP-LS information is beyond the scope of this document.
   However, it is RECOMMENDED that the choice of the node used for
   originating the IGP topology information into BGP-LS be made such

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   that the advertising node supports all the FAD extensions in use in
   its part of the network.  This avoids the scenario where an
   incomplete FAD gets advertised via BGP-LS.

   The node originating the advertisement SHOULD include the Flex Algo
   Unknown sub-TLV when it comes across an unsupported or unknown sub-
   TLV in the corresponding FAD in the IS-IS and OSPF advertisement.
   This serves as an indication that the FAD information in BGP-LS is
   incomplete and is not usable for computation purposes.  When
   advertising the Flex Algo Unknown sub-TLV, the protocol specific sub-
   TLV types that are unsupported or unknown SHOULD be included.  This
   information serves as a diagnostic aid.

4.  Flex Algorithm Prefix Metric

   This document defines a new optional BGP-LS Attribute TLV associated
   with the Prefix NLRI called the Flexible Algorithm Prefix Metric
   (FAPM) TLV and its format is as follows:

    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 |     Flags     |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Metric                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Type: 1044

   o  Length: 8 octets.

   o  Flex-Algorithm : 1 octet value in the range between 128 and 255
      inclusive which is the range defined for Flexible Algorithms in
      the IANA "IGP Parameters" registries under the "IGP Algorithm
      Types" registry [I-D.ietf-lsr-flex-algo].

   o  Flags: single octet value and only applicable for OSPF as defined
      in [I-D.ietf-lsr-flex-algo].  The value MUST be set to 0 for ISIS
      and ignored by the receiver.

   o  Reserved : 2 octet value that SHOULD be set to 0 by the originator
      and MUST be ignored by the receiver.

   o  Metric : 4 octets field to carry the metric information.

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   The FAPM TLV can be added to the BGP-LS Attribute of the Prefix NLRI
   originated by a node, only if the corresponding node originates the
   Prefix in along with the underlying IGP TLV/sub-TLV as described
   below.  This information is derived from the protocol specific
   advertisements as below.

   o  IS-IS, as defined by the ISIS Flexible Algorithm Prefix Metric
      sub-TLV in [I-D.ietf-lsr-flex-algo].

   o  OSPFv2/OSPFv3, as defined by the OSPF Flexible Algorithm Prefix
      Metric sub-TLV in [I-D.ietf-lsr-flex-algo].

   The BGP-LS Attribute associated with a Prefix NLRI MAY include one or
   more FAPM TLVs corresponding to the Flexible Algorithm Prefix Metric
   for each algorithm associated with that particular prefix.

5.  IANA Considerations

   This document requests assigning code-points from the registry "BGP-
   LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
   TLVs" <https://www.iana.org/assignments/bgp-ls-parameters/bgp-ls-
   parameters.xhtml#node-descriptor-link-descriptor-prefix-descriptor-
   attribute-tlv> based on the table below which reflects the values
   assigned via the early allocation process.  The column "IS-IS TLV/
   Sub-TLV" defined in the registry does not require any value and
   should be left empty.

    +------------+----------------------------------------+----------+
    | Code Point |         Description                    | Length   |
    +------------+----------------------------------------+----------+
    |   1039     | Flex Algorithm Definition TLV          | variable |
    |   1040     | Flex Algo Exclude Any Affinity sub-TLV | variable |
    |   1041     | Flex Algo Include Any Affinity sub-TLV | variable |
    |   1042     | Flex Algo Include All Affinity sub-TLV | variable |
    |   1043     | Flex Algo Definition Flags sub-TLV     | variable |
    |   1044     | Flex Algorithm Prefix Metric TLV       | variable |
    |   1045     | Flex Algorithm Exclude SRLG sub-TLV    | variable |
    |   TBD      | Flex Algorithm Unknown sub-TLV         | variable |
    +------------+----------------------------------------+----------+

6.  Manageability Considerations

   The new protocol extensions introduced in this document augment the
   existing IGP topology information that was distributed via [RFC7752].
   Procedures and protocol extensions defined in this document do not
   affect the BGP protocol operations and management other than as
   discussed in the Manageability Considerations section of [RFC7752].

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   Specifically, the malformed NLRIs attribute tests in the Fault
   Management section of [RFC7752] now encompass the new TLVs for the
   BGP-LS NLRI in this document.

   The extensions specified in this document do not specify any new
   configuration or monitoring aspects in BGP or BGP-LS.  The
   specification of BGP models is an ongoing work based on
   [I-D.ietf-idr-bgp-model].

7.  Security Considerations

   The procedures and protocol extensions defined in this document do
   not affect the BGP security model.  See the "Security Considerations"
   section of [RFC4271] for a discussion of BGP security.  Also, refer
   to [RFC4272] and [RFC6952] for analyses of security issues for BGP.
   Security considerations for acquiring and distributing BGP-LS
   information are discussed in [RFC7752].  The TLVs introduced in this
   document are used to propagate the IGP Flexible Algorithm extensions
   defined in [I-D.ietf-lsr-flex-algo].  It is assumed that the IGP
   instances originating these TLVs will support all the required
   security (as described in [I-D.ietf-lsr-flex-algo]) in order to
   prevent any security issues when propagating the TLVs into BGP-LS.
   The advertisement of the node and prefix attribute information
   defined in this document presents no significant additional risk
   beyond that associated with the existing node and prefix attribute
   information already supported in [RFC7752].

8.  Acknowledgements

   The authors would like to thank Les Ginsberg, Amalesh Maity and Y F
   Siu for his reviews and contributions to this work.

9.  References

9.1.  Normative References

   [I-D.ietf-idr-bgp-ls-app-specific-attr]
              Talaulikar, K., Psenak, P., and J. Tantsura, "Application-
              Specific Attributes Advertisement with BGP Link-State",
              draft-ietf-idr-bgp-ls-app-specific-attr-06 (work in
              progress), May 2021.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-16 (work in progress), May 2021.

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   [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>.

   [RFC4202]  Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4202, DOI 10.17487/RFC4202, October 2005,
              <https://www.rfc-editor.org/info/rfc4202>.

   [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>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

9.2.  Informative References

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-18
              (work in progress), April 2021.

   [I-D.ietf-idr-bgp-model]
              Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
              YANG Model for Service Provider Networks", draft-ietf-idr-
              bgp-model-10 (work in progress), November 2020.

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              Bernier, D., and B. Decraene, "BGP Link State Extensions
              for SRv6", draft-ietf-idr-bgpls-srv6-ext-07 (work in
              progress), March 2021.

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   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-13 (work in progress),
              May 2021.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, DOI 10.17487/RFC2702, September 1999,
              <https://www.rfc-editor.org/info/rfc2702>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <https://www.rfc-editor.org/info/rfc6952>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

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Authors' Addresses

   Ketan Talaulikar (editor)
   Cisco Systems
   India

   Email: ketant@cisco.com

   Peter Psenak
   Cisco Systems
   Slovakia

   Email: ppsenak@cisco.com

   Shawn Zandi
   LinkedIn
   USA

   Email: szandi@linkedin.com

   Gaurav Dawra
   LinkedIn
   USA

   Email: gdawra.ietf@gmail.com

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