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IGP Flexible Algorithms (Flex-Algorithm) In IP Networks
draft-ietf-lsr-ip-flexalgo-06

Document Type Active Internet-Draft (lsr WG)
Authors William Britto , Shraddha Hegde , Parag Kaneriya , Rejesh Shetty , Ron Bonica , Peter Psenak
Last updated 2022-05-16
Replaces draft-bonica-lsr-ip-flexalgo
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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RTGDIR Last Call Review Incomplete, due 2022-04-30
Stream WG state Submitted to IESG for Publication
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Shepherd write-up Show Last changed 2022-05-16
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Responsible AD John Scudder
Send notices to acee@cisco.com
draft-ietf-lsr-ip-flexalgo-06
LSR Working Group                                              W. Britto
Internet-Draft                                                  S. Hegde
Intended status: Standards Track                             P. Kaneriya
Expires: November 17, 2022                                     R. Shetty
                                                               R. Bonica
                                                        Juniper Networks
                                                               P. Psenak
                                                           Cisco Systems
                                                            May 16, 2022

        IGP Flexible Algorithms (Flex-Algorithm) In IP Networks
                     draft-ietf-lsr-ip-flexalgo-06

Abstract

   An IGP Flexible Algorithm (Flex-Algorithm) allows IGPs to compute
   constraint-based paths.  The base IGP Flex-Algorithm specification
   describes how it is used with Segment Routing (SR) data planes - SR
   MPLS and SRv6.

   This document extends IGP Flex-Algorithm, so that it can be used with
   regular IPv4 and IPv6 forwarding.

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 17, 2022.

Copyright Notice

   Copyright (c) 2022 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

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   (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
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Use Case Example  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Advertising Flex-Algorithm Definitions (FAD)  . . . . . . . .   3
   5.  Advertising IP Flex-Algorithm Participation . . . . . . . . .   3
     5.1.  The IS-IS IP Algorithm Sub-TLV  . . . . . . . . . . . . .   4
     5.2.  The OSPF IP Algorithm TLV . . . . . . . . . . . . . . . .   5
   6.  Advertising IP Flex-Algorthm Reachability . . . . . . . . . .   6
     6.1.  The IS-IS IPv4 Algorithm Prefix Reachability TLV  . . . .   6
     6.2.  The IS-IS IPv6 Algorithm Prefix Reachability TLV  . . . .   8
     6.3.  The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV . . .   9
     6.4.  The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV . . .  10
     6.5.  The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV  . . .  11
   7.  Calculating of IP Flex-Algorthm Paths . . . . . . . . . . . .  13
   8.  IP Flex-Algorthm Forwarding . . . . . . . . . . . . . . . . .  13
   9.  Deployment Considerations . . . . . . . . . . . . . . . . . .  13
   10. Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     14.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   An IGP Flex-Algorithm as specified in [I-D.ietf-lsr-flex-algo]
   computes a constraint-based path to:

   o  All Flex-Algorithm specific Prefix Segment Identifiers (SIDs)
      [RFC8402].

   o  All Flex-Algorithm specific SRv6 Locators [RFC8986].

   Therefore, Flex-Algorithm cannot be deployed in the absence of SR and
   SRv6.

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   This document extends Flex-Algorithm, allowing it to compute paths to
   IPv4 and IPv6 prefixes.

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

3.  Use Case Example

   Mobile networks are becoming more and more IP centric.  Each end-user
   session from a gNB (gNodeB) can be destined to a specific UPFs (User
   Plane Function) based on the session requirements.  For example, some
   sessions require high bandwidth, others need to be routed along the
   lowest latency path.  Each UPF is assigned a unique IP address.  As a
   result, traffic for different sessions is destined to a different
   destination IP address.

   By associating the prefix that contains the UPF addresses with a
   specific IP algorithm and routing the algorithm specific traffic
   according to a certain constraints, e.g., low latency, a session
   traffic is routed according to the SLA (Service Level Agreement)
   appropriate for such session.

4.  Advertising Flex-Algorithm Definitions (FAD)

   To guarantee loop free forwarding, all routers that participate in a
   Flex-Algorithm MUST agree on the Flex-Algorithm Definition (FAD).

   Selected nodes within the IGP domain MUST advertise FADs as described
   in Sections 5, 6, and 7 of [I-D.ietf-lsr-flex-algo].

5.  Advertising IP Flex-Algorithm Participation

   A node may use various algorithms when calculating paths to nodes and
   prefixes.  Algorithm values are defined in the IGP Algorithm Type
   Registry [IANA-ALG].

   A node MUST participate in a Flex-Algorithm to be:

   o  Able to compute path for such Flex-Algorithm

   o  Part of the topology for such Flex-Algorithm

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   Flex-Algorithm participation MUST be advertised for each Flex-
   Algorithm data-plane independently, as specified in
   [I-D.ietf-lsr-flex-algo].  Using Flex-Algorithm for regular IPv4 and
   IPv6 prefixes represents an independent Flex-Algorithm data-plane,
   and as such, the Flex-Algorithm participation for the IP Flex-
   Algorithm data-plane MUST be signalled independently of any other
   Flex-Algorithm data-plane (e.g., SR).

   Advertisement of participation in IP Flex-Algorithm MUST NOT impact
   the router participation in default algorithm 0.

   Advertisement of participation in IP Flex-Algorithm MUST NOT impact
   the router participation signaled for other data-planes.

   The following sections describe how the IP Flex-Algorithm
   participation is advertised in IGP protocols.

5.1.  The IS-IS IP Algorithm Sub-TLV

   The ISIS [ISO10589] IP Algorithm Sub-TLV is a sub-TLV of the IS-IS
   Router Capability TLV [RFC7981] 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    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Algorithm 1   |  Algorithm 2  | Algorithm ... |  Algorithm n  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: IS-IS IP Algorithm Sub-TLV

   o  Type: IP Algorithm Sub-TLV (Value 29)

   o  Length: Variable

   o  Algorithm (1 octet): Value from 128 to 255.

   The IP Algorithm Sub-TLV MUST be propagated throughout the level and
   MUST NOT be advertised across level boundaries.  Therefore, the S bit
   in the Router Capability TLV, in which the IP Algorithm Sub-TLV is
   advertised, MUST NOT be set.

   The IP Algorithm Sub-TLV is optional.  It MUST NOT be advertised more
   than once at a given level.  A router receiving multiple IP Algorithm
   sub-TLVs from the same originator MUST select the first advertisement

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   in the lowest-numbered LSP and subsequent instances of the IP
   Algorithm Sub-TLV MUST be ignored.

   The use of IP Algorithm Sub-TLV to advertise support for algorithms
   outside the Flex-Algorithm range (128-255) is outside the scope of
   this document.

   The IP Flex-Algorithm participation advertised in the IS-IS IP
   Algorithm Sub-TLV is topology independent.  When a router advertises
   participation in the IS-IS IP Algorithm Sub-TLV, the participation
   applies to all topologies in which the advertising node participates.

5.2.  The OSPF IP Algorithm TLV

   The OSPF [RFC2328] IP Algorithm TLV is a top-level TLV of the Router
   Information Opaque LSA [RFC7770] 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Algorithm 1 | Algorithm...  |   Algorithm n |               |
      +-                                                             -+
      |                                                               |
      +                                                               +

                      Figure 2: OSPF IP Algorithm TLV

   o  Type: IP Algorithm TLV (Value TBD by IANA)

   o  Length: Variable

   o  Algorithm (1 octet): Value from 128 to 255.

   The IP Algorithm TLV is optional.  It MUST only be advertised once in
   the Router Information LSA.

   When multiple IP Algorithm TLVs are received from a given router, the
   receiver MUST use the first occurrence of the TLV in the Router
   Information LSA.  If the IP Algorithm TLV appears in multiple Router
   Information LSAs that have different flooding scopes, the IP
   Algorithm TLV in the Router Information LSA with the area-scoped
   flooding scope MUST be used.  If the IP Algorithm TLV appears in
   multiple Router Information LSAs that have the same flooding scope,
   the IP Algorithm TLV in the Router Information LSA with the
   numerically smallest Instance ID (Opaque ID for OSPFv2 or Link State

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   ID for OSPFv3) MUST be used and subsequent instances of the IP
   Algorithm TLV MUST be ignored.

   The Router Information LSA can be advertised at any of the defined
   flooding scopes (link, area, or Autonomous System (AS)).  For the
   purpose of IP Algorithm TLV advertisement, area-scoped flooding is
   REQUIRED.

   The IP Flex-Algorithm participation advertised in the OSPF IP
   Algorithm TLV is topology independent.  When a router advertises
   participation in OSPF IP Algorithm TLV, the participation applies to
   all topologies in which the advertising node participates.

6.  Advertising IP Flex-Algorthm Reachability

   To be able to associate the prefix with the Flex-Algorithm, the
   existing prefix reachability advertisements can not be used, because
   they advertise the prefix reachability in default algorithm 0.
   Instead, a new IP Flex-Algorithm reachability advertisements are
   defined in IS-IS and OSPF.

   The M-flag in FAD is not applicable to IP Algorithm Prefixes.  Any IP
   Algorithm Prefix advertisement includes the Algorithm and Metric
   fields.  When an IP Algorithm Prefix is advertised between areas or
   domains, the metric field in the IP Algorithm Prefix advertisement
   MUST be used irrespective of the M-flag in the FAD advertisement.

6.1.  The IS-IS IPv4 Algorithm Prefix Reachability TLV

   A top-level TLV is defined for advertising IPv4 Flex-Algorithm Prefix
   Reachability in IS-IS - IPv4 Algorithm Prefix Reachability TLV.

   This new TLV shares the sub-TLV space defined for TLVs Advertising
   Prefix Reachability.

   The IS-IS IPv4 Algorithm Prefix Reachability 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    |R|R|R|R|    MTID               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 3: IS-IS IPv4 Algorithm Prefix Reachability TLV

   o  Type: IPv4 Algorithm Prefix Reachability TLV (Value 126).

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   o  Length: Variable.

   o  R bits (4 bits): Reserved for future use.  They MUST be set to
      zero on transmission and MUST be ignored on receipt.

   o  MTID (12 bits): Multitopology Identifier as defined in [RFC5120].
      Note that the value 0 is legal.

   Followed by one or more prefix entries of the form:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Metric                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |  Algorithm    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Pfx Length   |  Prefix (variable)...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Sub-tlv-len  |         Sub-TLVs (variable) . . .             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 4: IS-IS IPv4 Algorithm Prefix Reachability TLV

   o  Metric (4 octets): Metric information.

   o  Flags (1 octet):

                 0 1 2 3 4 5 6 7
                +-+-+-+-+-+-+-+-+
                |D|  Reserved   |
                +-+-+-+-+-+-+-+-+

         D-flag: When the Prefix is leaked from level-2 to level-1, the
         D bit MUST be set.  Otherwise, this bit MUST be clear.
         Prefixes with the D bit set MUST NOT be leaked from level-1 to
         level-2.  This is to prevent looping.

   o  Algorithm (1 octet): Associated Algorithm from 128 to 255.

   o  Prefix Len (1 octet): Prefix length measured in bits.

   o  Prefix (variable length): Prefix mapped to Flex-Algorithm.

   o  Optional Sub-TLV-length (1 octet): Number of octets used by sub-
      TLVs

   o  Optional sub-TLVs (variable length).

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   A router receiving multiple IPv4 Algorithm Prefix Reachability
   advertisements for the same prefix, from the same originator, each
   with a different Algorithm, MUST select the first advertisement in
   the lowest-numbered LSP and ignore any subsequent IPv4 Algorithm
   Prefix Reachability advertisements for the same prefix for any other
   Algorithm.

   A router receiving multiple IPv4 Algorithm Prefix Reachability
   advertisements for the same prefix, from different originators, each
   with a different Algorithm, MUST ignore all of them and MUST NOT
   install any forwarding entries based on these advertisements.  This
   situation SHOULD be logged as an error.

   In cases where a prefix advertisement is received in both a IPv4
   Prefix Reachability TLV and an IPv4 Algorithm Prefix Reachability
   TLV, the IPv4 Prefix Reachability advertisement MUST be preferred
   when installing entries in the forwarding plane.

6.2.  The IS-IS IPv6 Algorithm Prefix Reachability TLV

   The IS-IS IPv6 Algorithm Prefix Reachability TLV is identical to the
   IS-IS IPv4 Algorithm Prefix Reachability TLV, except that it has a
   unique type.  The type is 127.

   A router receiving multiple IPv6 Algorithm Prefix Reachability
   advertisements for the same prefix, from the same originator, each
   with a different Algorithm, MUST select the first advertisement in
   the lowest-numbered LSP and ignore any subsequent IPv6 Algorithm
   Prefix Reachability advertisements for the same prefix for any other
   Algorithm.

   A router receiving multiple IPv6 Algorithm Prefix Reachability
   advertisements for the same prefix, from different originators, each
   with a different Algorithm, MUST ignore all of them and MUST NOT
   install any forwarding entries based on these advertisements.  This
   situation SHOULD be logged as an error.

   In cases where a prefix advertisement is received in both an IPv6
   Prefix Reachability TLV and an IPv6 Algorithm Prefix Reachability
   TLV, the IPv6 Prefix Reachability advertisement MUST be preferred
   when installing entries in the forwarding plane.

   In cases where a prefix advertisement is received in both IS-IS SRv6
   Locator TLV and in IS-IS IPv6 Algorithm Prefix Reachability TLV, the
   receiver MUST ignore both of them and MUST NOT install any forwarding
   entries based on these advertisements.  This situation SHOULD be
   logged as an error.

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6.3.  The OSPFv2 IP Algorithm Prefix Reachability Sub-TLV

   A new Sub-TLV of the OSPFv2 Extended Prefix TLV is defined for
   advertising IP Algorithm Prefix Reachability in OSPFv2, the OSPFv2 IP
   Algorithm Prefix Reachability Sub-TLV.

   The OSPFv2 IP Algorithm Prefix Reachability 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            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       MT-ID   |  Algorithm    |            Reserved           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Metric                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 5: OSPFv2 IP Algorithm Prefix Reachability Sub-TLV

   o  Type (2 octets) : The value is TBD.

   o  Length (2 octet): 8

   o  MT-ID (1 octet): Multi-Topology ID as defined in [RFC4915]

   o  Algorithm (1 octet): Associated Algorithm from 128 to 255.

   o  Reserved: (2 octets).  SHOULD be set to 0 on transmission and MUST
      be ignored on reception.

   o  Metric (4 octets): The algorithm specific metric value.

   An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix
   Reachability Sub-TLVs in the same OSPFv2 Extended Prefix TLV, MUST
   select the first advertisement of this Sub-TLV and MUST ignore all
   remaining occurences of this Sub-TLV in the OSPFv2 Extended Prefix
   TLV.

   An OSPFv2 router receiving multiple OSPFv2 IP Algorithm Prefix
   Reachability TLVs for the same prefix, from different originators,
   each with a different Algorithm, MUST ignore all of them and MUST NOT
   install any forwarding entries based on these advertisements.  This
   situation SHOULD be logged as an error.

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   In cases where a prefix advertisement is received in any of the LSAs
   advertising the prefix reachability for algorithm 0 and in an OSPFv2
   IP Algorithm Prefix Reachability TLV, only the prefix reachability
   advertisement for algorithm 0 MUST be used and all occurences of the
   OSPFv2 IP Algorithm Prefix Reachability TLV MUST be ignored.

6.4.  The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV

   The OSPFv3 [RFC5340] IP Algorithm Prefix Reachability Sub-TLV is
   defined for advertisement of the IP Algorithm Prefix Reachability in
   OSPFv3.

   The OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is a sub-TLV of
   the following OSPFv3 TLVs defined in [RFC8362]:

   o  Intra-Area-Prefix TLV

   o  Inter-Area-Prefix TLV

   o  External-Prefix TLV

   The format of OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is
   shown below:

     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            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Algorithm    |                 Reserved                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Metric                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 6: OSPFv3 IP Algorithm Prefix Reachability Sub-TLV

   Where:

      Type (2 octets): The value is TBD.

      Length (2 octets): 8.

      Algorithm (1 octet): Associated Algorithm from 128 to 255.

      Reserved: (3 octets).  SHOULD be set to 0 on transmission and MUST
      be ignored on reception.

      Metric (4 octets): The algorithm specific metric value.

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   When the OSPFv3 IP Algorithm Prefix Reachability Sub-TLV is present,
   the metric value in its parent TLV MUST be set to LSInfinity
   [RFC2328].  If the metric value in the parent TLV is not set to
   LSInfinity, the OSPFv3 IP Algorithm Prefix Sub-TLV MUST be ignored by
   the receiver.

   An OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix
   Reachability Sub-TLVs in the same parent TLV, MUST select the first
   advertisement of this Sub-TLV and MUST ignore all remaining
   occurences of this Sub-TLV in the parent TLV.

   An OSPFv3 router receiving multiple OSPFv3 IP Algorithm Prefix
   Reachability TLVs for the same prefix, from different originators,
   each with a different Algorithm, MUST ignore all of them and MUST NOT
   install any forwarding entries based on these advertisements.  This
   situation SHOULD be logged as an error.

   In cases where a prefix advertisement is received in any of the LSAs
   advertising the prefix reachability for algorithm 0 and in an OSPFv3
   OSPFv3 IP Algorithm Prefix Reachability Sub-TLV, only the prefix
   reachability advertisement for algorithm 0 MUST be used and all
   occurences of the OSPFv3 IP Algorithm Prefix Reachability TLV MUST be
   ignored.

   In cases where a prefix advertisement is received in both an OSPFv3
   SRv6 Locator TLV and in an OSPFv3 IP Algorithm Prefix Reachability
   Sub-TLV, the receiver MUST ignore both of them and MUST NOT install
   any forwarding entries based on these advertisements.  This situation
   SHOULD be logged as an error.

6.5.  The OSPF IP Flexible Algorithm ASBR Metric Sub-TLV

   [I-D.ietf-lsr-flex-algo] defines the OSPF Flexible Algorithm ASBR
   Metric Sub-TLV (FAAM) that is used by an OSPFv2 or an OSPFv3 ABR to
   advertise a Flex-Algorithm specific metric associated with the
   corresponding ASBR LSA.

   As described in [I-D.ietf-lsr-flex-algo] each data-plane signals the
   participation independently.  IP Flex-Algorithm participation is
   signaled independent of the Segment Routing (SR) Flex-Algorithm
   participation.  As a result, the calculated topologies for SR and IP
   Flex-Algorithm could be different.  Such difference prevents the
   usage of FAAM for the purpose of the IP Flex-Algorithm.

   The OSPF IP Flexible Algorithm ASBR Metric (IPFAAM) Sub-TLV is
   defined for the advertisement of the IP Flex-Algorithm specific
   metric associated with an ASBR by the ABR.

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   The IPFAAM Sub-TLV is a Sub-TLV of the:

      - OSPFv2 Extended Inter-Area ASBR TLV as defined in
      [I-D.ietf-lsr-flex-algo]

      - OSPFv3 Inter-Area-Router TLV defined in [RFC8362]

   The OSPF IPFAAM 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Algorithm   |                   Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Metric                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

         Figure 7: OSPF IP Flexible Algorithm ASBR Metric Sub-TLV

      Type (2 octets): TBD for OSPFv2, TBD for OSPFv3.

      Length (2 octets): 8.

      Algorithm (1 octet): Associated Algorithm from 128 to 255.

      Reserved: (3 octets).  SHOULD be set to 0 on transmission and MUST
      be ignored on reception.

      Metric (4 octets): The algorithm specific metric value.

   The usage of the IPFAAM Sub-TLV is similar to the usage of the FAAM
   Sub-TLV defined in [I-D.ietf-lsr-flex-algo], but it is used to
   advertise IP Flex-Algorithm metric.

   An OSPF ABR MUST include the OSPF IPFAAM Sub-TLVs as part of the ASBR
   reachability advertisement between areas for every IP Flex-Algorithm
   in which it participates and the ASBR is reachable in.

   The FAAM Sub-TLV as defined in [I-D.ietf-lsr-flex-algo] MUST NOT be
   used during IP Flex-Algorithm path calculation, the IPFAAM Sub-TLV
   MUST be used instead.

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7.  Calculating of IP Flex-Algorthm Paths

   The IP Flex-Algorthm is considered as yet another data-plane of the
   Flex-Algorithm as described [I-D.ietf-lsr-flex-algo].

   Participation for the IP Flex-Algorithm is signalled as described in
   Section 5 and is specific to the IP Flex-Algorithm data-plane.

   Calculation of IP Flex-Algorithm paths follows what is described in
   [I-D.ietf-lsr-flex-algo].  This computation uses the IP Flex-
   Algorithm data-plane participation and is independent of the Flex-
   Algorithm calculation done for any other Flex-Algorithm data-plane
   (e.g., SR, SRv6).

   The IP Flex-Algorithm data-plane only considers participating nodes
   during the Flex-Algorithm calculation.  When computing paths for a
   given Flex-Algorithm, all nodes that do not advertise participation
   for the IP Flex-Algorithm, as described in Section 5, MUST be pruned
   from the topology.

8.  IP Flex-Algorthm Forwarding

   The IP Algorithm Prefix Reachability advertisement as described in
   Section 5 includes the MTID value that associates the prefix with a
   specific topology.  Algorithm Prefix Reachability advertisement also
   includes an Algorithm value that explicitly associates the prefix
   with a specific Flex-Algorithm.  The paths to the prefix MUST be
   calculated using the specified Flex-Algorithm in the associated
   topology.

   Forwarding entries for the IP Flex-Algorithm prefixes advertised in
   IGPs MUST be installed in the forwarding plane of the receiving IP
   Flex-Algorithm prefix capable routers when they participate in the
   associated topology and algorithm.  Forwarding entries for IP Flex-
   Algorithm prefixes associated with Flex-Algorithms in which the node
   is not participating MUST NOT be installed in the forwarding plane.

   When the IP Flex-Algorithm prefix is associated with a Flex-
   Algorithm, LFA paths to the 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.

9.  Deployment Considerations

   IGP Flex-Algorithm can be used by many data-planes.  The original
   specification was done for SR and SRv6, this specification adds IP as
   another data-plane that can use IGP Flex-Algorithm.  Other data-
   planes may be defined in the future.  This section provides some

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   details about the coexistence of the various data-planes of an IGP
   Flex-Algorithm.

   Flex-Algorithm definition (FAD), as described in
   [I-D.ietf-lsr-flex-algo], is data-plane independent and is used by
   all Flex-Algorithm data-planes.

   Participation in the Flex-Algorithm, as described in
   [I-D.ietf-lsr-flex-algo], is data-plane specific.

   Calculation of the flex-algo paths is data-plane specific and uses
   data-plane specific participation advertisements.

   Data-plane specific participation and calculation guarantee that the
   forwarding of the traffic over the Flex-Algorithm data-plane specific
   paths is consistent between all nodes that apply the IGP Flex-
   Algorithm to the data-plane.

   Multiple data-planes can use the same Flex-Algorithm value at the
   same time and, and as such, share the FAD for it.  For example, SR-
   MPLS and IP can both use a common Flex-Algorithm.  Traffic for SR-
   MPLS will be forwarded based on Flex-algorithm specific SR SIDs.
   Traffic for IP Flex-Algorithm will be forwarded based on Flex-
   Algorithm specific prefix reachability advertisements.  Note that for
   a particular Flex-Algorithm, for a particular IP prefix, there will
   only be path(s) calculated and installed for a single data-plane.

10.  Protection

   In many networks where IGP Flexible Algorithms are deployed, IGP
   restoration will be fast and additional protection mechanisms will
   not be required.  IGP restoration may be enhanced by Equal Cost
   Multipath (ECMP).

   In other networks, operators can deploy additional protection
   mechanisms.  The following are examples:

   o  Loop Free Alternates (LFA) [RFC5286]

   o  Remote Loop Free Alternates (R-LFA) [RFC7490]

   LFA and R-LFA computations MUST be restricted to the flex-algo
   topology and the computed backup nexthops should be programmed for
   the IP flex-algo prefixes.

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11.  IANA Considerations

   This specification updates the OSPF Router Information (RI) TLVs
   Registry as follows:

         +-------+------------------+---------------------------+
         | Value | TLV Name         | Reference                 |
         +-------+------------------+---------------------------+
         | TBD   | IP Algorithm TLV | This Document Section 5.2 |
         +-------+------------------+---------------------------+

                                  Table 1

   This document also updates the ISIS "Sub-TLVs for TLV 242" registry
   as follows:

       +-------+----------------------+---------------------------+
       | Value | TLV Name             | Reference                 |
       +-------+----------------------+---------------------------+
       | 29    | IP Algorithm Sub-TLV | This Document Section 5.1 |
       +-------+----------------------+---------------------------+

                                  Table 2

   This document also updates the "IS-IS TLV Codepoints Registry"
   registry as follows:

   +-------+------------------+-----+-----+-----+-------+--------------+
   | Value | TLV Name         | IIH | LSP | SNP | Purge | Reference    |
   +-------+------------------+-----+-----+-----+-------+--------------+
   | 126   | IPv4 Algorithm   | N   | Y   | N   | N     | This         |
   |       | Prefix           |     |     |     |       | document,    |
   |       | Reachability TLV |     |     |     |       | Section 6.1  |
   | 127   | IPv6 Algorithm   | N   | Y   | N   | N     | This         |
   |       | Prefix           |     |     |     |       | document,    |
   |       | Reachability TLV |     |     |     |       | Section 6.2  |
   +-------+------------------+-----+-----+-----+-------+--------------+

                                  Table 3

   The above TLVs share the sub-TLV space defined in "IS-IS Sub-TLVs for
   TLVs Advertising Prefix Reachability".  This document updates the
   description of that registry by including IPv4 Algorithm Prefix
   Reachability TLV and IPv6 Algorithm Prefix Reachability TLV.  It also
   includes these TLVs in a table which lists the presence of Sub-TLVs
   in a parent TLVs as follows:

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        Type  Description                          126 127
        ----  ----------------------------------   --- ---
         1    32-bit Administrative Tag Sub-TLV     y   y
         2    64-bit Administrative Tag Sub-TLV     y   y
         3    Prefix Segment Identifier             n   n
         4    Prefix Attribute Flags                y   y
         5    SRv6 End SID                          n   n
         6    Flex-Algorithm Prefix Metric          n   n
         11   IPv4 Source Router ID                 y   y
         12   IPv6 Source Router ID                 y   y
         32   BIER Info                             n   n

   This document updates the "OSPFv2 Extended Prefix TLV Sub-TLVs"
   registry as follows:

   +-------+-----------------------------------+-----------------------+
   | Value | TLV Name                          | Reference             |
   +-------+-----------------------------------+-----------------------+
   | TBD   | OSPFv2 IP Algorithm Prefix        | This Document,        |
   |       | Reachability TLV                  | Section 6.3           |
   +-------+-----------------------------------+-----------------------+

                                  Table 4

   This document updates the "OSPFv3 Extended-LSA Sub-TLVs" registry as
   follows:

   +-------+-------------------------------------+---------------------+
   | Value | TLV Name                            | Reference           |
   +-------+-------------------------------------+---------------------+
   | TBD   | OSPFv3 IP Algorithm Prefix          | This Document,      |
   |       | Reachability Sub-TLV                | Section 6.4         |
   | TBD   | OSPFv3 IP Flexible Algorithm ASBR   | This Document,      |
   |       | Metric Sub-TLV                      | Section 6.5         |
   +-------+-------------------------------------+---------------------+

                                  Table 5

   This document updates the "OSPFv2 Extended Inter-Area ASBR Sub-TLVs"
   registry as follows:

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   +-------+------------------------------------+----------------------+
   | Value | TLV Name                           | Reference            |
   +-------+------------------------------------+----------------------+
   | 2     | OSPF IP Flexible Algorithm ASBR    | This Document,       |
   |       | Metric Sub-TLV                     | Section 6.5          |
   +-------+------------------------------------+----------------------+

                                  Table 6

12.  Security Considerations

   This document inherits security considerations from
   [I-D.ietf-lsr-flex-algo].

13.  Acknowledgements

   Thanks to Bruno Decraene for his contributions to this document.
   Special thanks to Petr Bonbon Adamec of Cesnet for supporting
   interoperability testing.

14.  References

14.1.  Normative References

   [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-19 (work in progress), April 2022.

   [ISO10589]
              IANA, "Intermediate system to Intermediate system routing
              information exchange protocol for use in conjunction with
              the Protocol for providing the Connectionless-mode Network
              Service (ISO 8473)", August 1987, <ISO/IEC 10589:2002>.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [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>.

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

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   [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>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

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   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

14.2.  Informative References

   [IANA-ALG]
              IANA, "Sub-TLVs for TLV 242 (IS-IS Router CAPABILITY
              TLV)", August 1987, <https://www.iana.org/assignments/igp-
              parameters/igp-parameters.xhtml#igp-algorithm-types>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <https://www.rfc-editor.org/info/rfc7490>.

   [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>.

   [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>.

Authors' Addresses

   William Britto
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103
   India

   Email: bwilliam@juniper.net

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   Shraddha Hegde
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103
   India

   Email: shraddha@juniper.net

   Parag Kaneriya
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103
   India

   Email: pkaneria@juniper.net

   Rejesh Shetty
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103
   India

   Email: mrajesh@juniper.net

   Ron Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, Virginia  20171
   USA

   Email: rbonica@juniper.net

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

   Email: ppsenak@cisco.com

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