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IGP Color-Aware Routing
draft-lin-lsr-igp-car-02

Document Type Active Internet-Draft (individual)
Authors Changwang Lin , Mengxiao Chen , Liyan Gong
Last updated 2024-11-27
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draft-lin-lsr-igp-car-02
Network Working Group                                            C. Lin
Internet Draft                                                  M. Chen
Intended status: Standards Track                   New H3C Technologies
Expires: May 28, 2025                                           L. Gong
                                                           China Mobile
                                                      November 27, 2024

                          IGP Color-Aware Routing
                         draft-lin-lsr-igp-car-02

Abstract

   This document describes an IGP based routing solution to establish
   end-to-end intent-aware paths across a multi-domain service provider
   transport 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
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   This Internet-Draft will expire on May 28, 2025.

Copyright Notice

   Copyright (c) 2024 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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   (http://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.....................................3
   2. IGP CAR Solution...............................................3
      2.1. SR Policy based IGP CAR...................................4
      2.2. Flex-Algo based IGP CAR...................................5
      2.3. Hybrid IGP CAR............................................7
   3. Advertisement of IGP CAR Route.................................9
      3.1. IS-IS CAR Sub-TLV.........................................9
         3.1.1. IS-IS CAR MPLS-SR Encapsulation Sub-sub-TLV.........10
         3.1.2. IS-IS CAR SRv6 Encapsulation Sub-sub-TLV............11
      3.2. OSPF CAR Sub-TLV.........................................11
         3.2.1. OSPF CAR MPLS-SR Encapsulation Sub-sub-TLV..........12
         3.2.2. OSPF CAR SRv6 Encapsulation Sub-sub-TLV.............13
   4. Resolving of IGP CAR Route....................................13
      4.1. Resolving Over SR Policy.................................15
      4.2. Resolving Over Flex-Algo.................................15
   5. Security Considerations.......................................16
   6. IANA Considerations...........................................16
   7. References....................................................16
      7.1. Normative References.....................................16
      7.2. Informational References.................................16
   Authors' Addresses...............................................18

1. Introduction

   [I-D.hr-spring-intentaware-routing-using-color] describes the scope,
   set of use-cases and requirements for a distributed routing based
   solution to establish end-to-end intent-aware paths spanning multi-
   domain packet networks. [I-D.ietf-idr-bgp-car] proposes a BGP based
   solution called "BGP Color-Aware Routing" (BGP CAR). [I-D.ietf-idr-
   bgp-ct] also proposes a BGP based solution called "BGP Classful
   Transport" (BGP CT).

   Network operators often organize networks into multiple smaller
   network domains, and each network domain typically runs an IGP. In
   some scenarios, only the PE nodes run the BGP protocol, while the

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   other nodes (including the ASBR nodes) run only the IGP protocol, as
   shown in Figure 1.

                                BGP
               ......................................
               :                                    :
               : +----------+----------+----------+ :
               : |          |          |          | :
               : |       ASBR121    ASBR231       | :
               : |          |          |          | :
              PE1+    D1    |    D2    |    D3    +PE2
                 |          |          |          |
                 |       ABSR122    ASBR232       |
                 |          |          |          |
                 +----------+----------+----------+

                 |---IGP1---|---IGP2---|---IGP3---|

              Figure 1: Multi IGP Domain Network Example

   The BGP based solutions proposed by [I-D.ietf-idr-bgp-car] and [I-D.
   ietf-idr-bgp-ct] require the ASBR nodes to run BGP and signal BGP
   routes for CAR path. However, some network operators may not want to
   change existing routing protocol deployments.

   This document describes an IGP based routing solution to establish
   end-to-end intent-aware paths, without requirement for the ASBR
   nodes to run BGP.

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. IGP CAR Solution

   The following sub-sections illustrate example scenarios of Colored
   Service Route Steering over E2E IGP CAR resolving over different
   intra-domain mechanisms.

   The examples use MPLS-SR for the transport data plane. The main
   difference on SRv6 data plane is the replacement of CAR labels with
   CAR SIDs.

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   The OSPF and IS-IS extensions for advertising CAR route will be
   described in Section 3. The resolving of CAR route will be described
   in Section 4.

2.1. SR Policy based IGP CAR

   Figure 2 illustrates an example scenario of Colored Service Route
   Steering over E2E IGP CAR based on SR Policy.

                                         RD:V/v via E2
                                 +--+    vpn label:30030
    .............................|RR|<............................
    :                            +--+    Color C1                :
    :                                                            :
   +:-------------------+--------------------+-------------------:+
   |:                   |                    |                   :|
   |:   Redist: Pfx=E2  |    Redist: Pfx=E2  |                   :|
   |:    (C1,LI=8002)<~~|~~~  (C1,LI=8002)<~~|~~~                :|
   |:                 +---+                +---+ SR Policy(C1,E2):|
   |: |-------------->|121|--------------->|231|---------------| :|
   |: |   SR Policy   +---+    SR Policy   +---+               | :|
   |: |   (C1,121)      |      (C1,231)      |      CAR route: v :|
   |--+                 |                    |          Pfx=E2 +--|
   |E1|                 |                    |          C=C1   |E2|
   |--+ Redist: Pfx=E2  |    Redist: Pfx=E2  |          LI=8002+--|
   |  |  (C1,LI=8002)<~~|~~~  (C1,LI=8002)<~~|~~~              ^  |
   |  |               +---+                +---+               |  |
   |  |-------------->|122|--------------->|232|---------------|  |
   |      SR Policy   +---+    SR Policy   +---+    SR Policy     |
   |      (C1,122)      |      (C1,232)      |      (C1,E2)       |
   |                    |                    |                    |
   |       IGP 1        |       IGP 2        |       IGP 3        |
   +--------------------+--------------------+--------------------+
   iPE                iASBR                eASBR                ePE

   +------+             +------+             +------+
   |  S1  |             |  S2  |             |  S3  |
   +------+             +------+             +------+
   +------+             +------+             +------+
   |168002|             |168002|             |168002|
   +------+             +------+             +------+
   +------+             +------+             +------+
   |30030 |             |30030 |             |30030 |
   +------+             +------+             +------+

               Figure 2: SR Policy based IGP CAR Path

   With reference to the topology above:

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   o SR Policy provides intent in each domain.

   o Egress PE E2 advertises a VPN route RD:V/v colored with (color
      extended community) C1 to steer traffic to SR Policy. VPN route
      propagates via RR(s) to ingress PE E1.

   o IGP CAR route of prefix E2 with color C1 and label 168002 (index
      is 2 and SRGB is 168000) is advertised by Egress PE E2, and
      redistributed by ASBRs.

   o On each ASBR and the Ingress PE E1, CAR label 168002 is resolved
      over SR Policy of the domain.

   o Ingress PE E1 steers colored VPN route RD:V/v into SR Policy
      according to color.

   Packet forwarding:

   @E1:  IPv4 VRF V/v => PUSH <S1, 168002, 30030>
   @E1:  MPLS Table: S1 => forward via SR Policy to 121
   @121: MPLS Table: 168002 => PUSH S2
   @121: MPLS Table: S2 => forward via SR Policy to 231
   @231: MPLS Table: 168002 => PUSH S1
   @231: MPLS Table: S3 => forward via SR Policy to E2
   @E2:  MPLS Table: 168002 => POP => MPLS Table: 30030 => POP and
   lookup the IP DA in the VRF

2.2. Flex-Algo based IGP CAR

   Figure 3 illustrates an example scenario of Colored Service Route
   Steering over E2E IGP CAR based on Flex-Algo (FA).

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                                         RD:V/v via E2
                                 +--+    vpn label:30030
    .............................|RR|<............................
    :                            +--+    Color C1                :
    :                                                            :
   +:-------------------+--------------------+-------------------:+
   |:                   |                    |                   :|
   |:   Redist: Pfx=E2  |    Redist: Pfx=E2  |                   :|
   |:    (C1,LI=8002)<~~|~~~  (C1,LI=8002)<~~|~~~                :|
   |:                 +---+                +---+                 :|
   |:     Pfx-sid=8121|121|    Pfx-sid=8231|231|                 :|
   |:        Algo=128 +---+       Algo=128 +---+     CAR route:  :|
   |--+                 |                    |          Pfx=E2 +--|
   |E1|                 |                    |          C=C1   |E2|
   |--+ Redist: Pfx=E2  |    Redist: Pfx=E2  |          LI=8002+--|
   |     (C1,LI=8002)<~~|~~~  (C1,LI=8002)<~~|~~~                 |
   |                  +---+                +---+     Pfx-sid=8021 |
   |      Pfx-sid=8122|122|    Pfx-sid=8232|232|        Algo=128  |
   |         Algo=128 +---+       Algo=128 +---+                  |
   |                    |                    |                    |
   |       IGP 1        |       IGP 2        |       IGP 3        |
   | FA 128(Mapping C1) | FA 128(Mapping C1) | FA 128(Mapping C1) |
   +--------------------+--------------------+--------------------+
   iPE                iASBR                eASBR                ePE

   +------+             +------+             +------+
   |168121|             |168231|             |168021|
   +------+             +------+             +------+
   +------+             +------+             +------+
   |168002|             |168002|             |168002|
   +------+             +------+             +------+
   +------+             +------+             +------+
   |30030 |             |30030 |             |30030 |
   +------+             +------+             +------+

                    Figure 3: FA based IGP CAR Path

   With reference to the topology above:

   o IGP FA 128 is running in each domain, and mapped to Color C1.

   o Egress PE E2 advertises a VPN route RD:V/v colored with (color
      extended community) C1 to steer traffic to IGP FA 128. VPN route
      propagates via RR(s) to ingress PE E1.

   o IGP CAR route of prefix E2 with color C1 and label 168002 (index
      is 8002 and SRGB is 160000) is advertised by Egress PE E2, and
      redistributed by ASBRs.

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   o On each ASBR and the Ingress PE E1, label 168002 is resolved over
      IGP FA 128 of the domain.

   o Ingress PE E1 steers colored VPN route RD:V/v into IGP FA 128
      according to the mapping relationship between FA and Color.

   Packet forwarding:

   @E1:  IPv4 VRF V/v => PUSH <168121, 168002, 30030>
   @E1:  MPLS Table: 168121 => forward via FA path to 121
   @121: MPLS Label: 168002 => PUSH 168231
   @121: MPLS Label: 168231 => forward via FA path to 231
   @231: MPLS Label: 168002 => PUSH 168021
   @231: MPLS Label: 168021 => forward via FA path to E2
   @E2:  MPLS Label: 168002 => POP => 30030 => POP and lookup the IP DA
   in the VRF

2.3. Hybrid IGP CAR

   Figure 4 illustrates an example where the same intent is provided by
   SR Policy in some domains but by Flex-Algo in some other domains.

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                                         RD:V/v via E2
                                 +--+    vpn label:30030
    .............................|RR|<............................
    :                            +--+    Color C1                :
    :                                                            :
   +:-------------------+--------------------+-------------------:+
   |:                   |                    |                   :|
   |:   Redist: Pfx=E2  |     Redist: Pfx=E2 |                   :|
   |:    (C1,LI=8002)<~~|~~~   (C1,LI=8002)<~~|~~~                :|
   |:                 +---+                +---+ SR Policy(C1,E2):|
   |: |-------------->|121|    Pfx-sid=8231|231|---------------| :|
   |: |   SR Policy   +---+       Algo=128 +---+               | :|
   |: |   (C1,121)      |                    |      CAR route: v :|
   |--+                 |                    |          Pfx=E2 +--|
   |E1|                 |                    |          C=C1   |E2|
   |--+ Redist: Pfx=E2  |     Redist: Pfx=E2 |          LI=8002+--|
   |  |  (C1,LI=8002)<~~|~~~  (C1,LI=8002)<~~|~~~              ^  |
   |  |               +---+                +---+               |  |
   |  |-------------->|122|    Pfx-sid=8232|232|---------------|  |
   |      SR Policy   +---+       Algo=128 +---+    SR Policy     |
   |      (C1,122)      |                    |      (C1,E2)       |
   |                    |                    |                    |
   |       IGP 1        |       IGP 2        |       IGP 3        |
   |       Algo 0       | FA 128(Mapping C1) |       Algo 0       |
   +--------------------+--------------------+--------------------+
   iPE                iASBR                eASBR                ePE

   +------+             +------+             +------+
   |  S1  |             |168231|             |  S3  |
   +------+             +------+             +------+
   +------+             +------+             +------+
   |168002|             |168002|             |168002|
   +------+             +------+             +------+
   +------+             +------+             +------+
   |30030 |             |30030 |             |30030 |
   +------+             +------+             +------+

                      Figure 4: Hybrid IGP CAR Path

   With reference to the topology above:

   o SR Policy provides intent in domain 1 and domain 3. IGP FA 128 is
      running only in domain 2, and mapped to Color C1.

   o IGP CAR route of prefix E2 with color C1 and label 168002 (index
      is 2 and SRGB is 168000) is advertised by Egress PE E2, and
      redistributed by ASBRs.

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   o On ASBR 231 and the Ingress PE E1, label 168002 is resolved over
      SR Policy. On ASBR 121, label 168002 is resolved over IGP FA 128

   Packet forwarding:

   @E1:  IPv4 VRF V/v => PUSH <S1, 168002, 30030>
   @E1:  MPLS Table: S1 => forward via SR Policy to 121
   @121: MPLS Label: 168002 => PUSH 168231
   @121: MPLS Label: 168231 => forward via FA path to 231
   @231: MPLS Table: 168002 => PUSH S1
   @231: MPLS Table: S3 => forward via SR Policy to E2
   @E2:  MPLS Label: 168002 => POP => 30030 => POP and lookup the IP DA
   in the VRF

3. Advertisement of IGP CAR Route

   The advertisement of IGP CAR route is as following:

     Prefix TLV: E
       CAR Sub-TLV: C
         CAR Encapsulation Sub-sub-TLV: T

   The new-defined CAR sub-TLV is attached to a prefix E, indicating a
   color-aware path of color C and encapsulation T towards that prefix.
   On the MPLS-SR data plane, T is a label or index. On the SRv6 data
   plane, T is an SRv6 SID.

   If there are multiple color-aware path for different intents towards
   the same E. Multiple CAR sub-TLVs with different colors will be
   advertised.

     Prefix TLV: E
      CAR Sub-TLV: C1 + T1
      CAR Sub-TLV: C2 + T2
      CAR Sub-TLV: C3 + T3
      ...

3.1. IS-IS CAR Sub-TLV

   The IS-IS CAR Sub-TLV is defined in this document to advertise CAR
   information for prefixes in IS-IS. The IS-IS CAR Sub-TLV is
   applicable to TLVs 27, 135, 235, 236, and 237.

   The 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    |             Flags             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Color                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Sub-sub-TLV-len|        Sub-sub-TLVs (variable)...             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: 1 octet. The length value is variable.

   o Flags: 2 octets. No flags are defined in this document. Undefined
      flags MUST be set to 0 by the sender, and any unknown flags MUST
      be ignored by the receiver.

   o Color: 4 octets. Contains color value associated with the prefix.

   o Sub-sub-TLV-length: 1 octet. Number of octets used by sub-sub-
      TLVs.

   o Sub-sub-TLVs: Carrying the encapsulation information. IS-IS CAR
      MPLS-SR Encapsulation sub-sub-TLV and IS-IS CAR SRv6
      Encapsulation sub-sub-TLV are defined in Section 3.1.1 and 3.1.2.
      Other encapsulations may be defined in the future.

3.1.1. IS-IS CAR MPLS-SR Encapsulation Sub-sub-TLV

    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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SID/Label (variable)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: 1.

   o Length: 3 or 4.

   o SID/Label: If the length is set to 3, then the 20 rightmost bits
      represent an MPLS label. If the length is set to 4, then the
      value is a 32-bit index.

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3.1.2. IS-IS CAR SRv6 Encapsulation Sub-sub-TLV

    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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (128 bits) . . .                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: 2.

   o Length: 16.

   o SID: 16 octets. This field encodes the SRv6 SID.

3.2. OSPF CAR Sub-TLV

   The OSPF CAR Sub-TLV is defined in this document to advertise CAR
   information for prefixes in OSPFv2 and OSPFv3. The OSPF CAR Sub-TLV
   is applicable to OSPFv2 Extended Prefix TLV, OSPFv3 Inter-Area-
   Prefix TLV, OSPFv3 Intra-Area-Prefix TLV, and OSPFv3 External-Prefix
   TLV.

   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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Color                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sub-sub-TLVs (variable)...             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: 1 octet. The length value is variable.

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   o Flags: 2 octets. No flags are defined in this document.
      Undefined flags MUST be set to 0 by the sender, and any unknown
      flags MUST be ignored by the receiver.

   o Color: 4 octets. Contains color value associated with the prefix.

   o Sub-sub-TLVs: Carrying the encapsulation information. OSPF CAR
      MPLS-SR Encapsulation sub-sub-TLV and OSPF CAR SRv6 Encapsulation
      sub-sub-TLV are defined in Section 3.2.1 and 3.2.2.  Other
      encapsulations may be defined in the future.

3.2.1. OSPF CAR MPLS-SR Encapsulation Sub-sub-TLV

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         SID/Label (variable)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: 1.

   o Length: 3 or 4.

   o SID/Label: If the length is set to 3, then the 20 rightmost bits
      represent an MPLS label. If the length is set to 4, then the
      value is a 32-bit index.

   OSPF CAR MPLS-SR Encapsulation Sub-sub-TLV is applicable to the OSPF
   CAR Sub-TLV carried in OSPFv2 Extended Prefix TLV.

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3.2.2. OSPF CAR SRv6 Encapsulation Sub-sub-TLV

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (128 bits) . . .                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SID (cont . . .)                                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: 2.

   o Length: 16.

   o SID: 16 octets. This field encodes the SRv6 SID.

   OSPF CAR SRv6 Encapsulation Sub-sub-TLV is applicable to the OSPF
   CAR Sub-TLV carried in OSPFv3 Inter-Area-Prefix TLV, OSPFv3 Intra-
   Area-Prefix TLV, and OSPFv3 External-Prefix TLV.

4. Resolving of IGP CAR Route

   When the ASBR or Ingress PE receives the IGP CAR route, it will be
   resolved over the CAR paths, and then installed to CAR Route
   Database (CARDB). CARDB is a logical collection of resolved CAR
   routes. The ASBR can redistribute the CAR routes in CARDB to other
   IGP instances. The Ingress PE can use the CARDB for next-hop
   resolution of BGP colored service route.

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          IGP Instance A
   +-----------------------------+
   |IGP CAR Route                |
   |    |                        |
   |    | Resolving over CAR path|
   |    | 1. SR Policy           |
   |    | 2. Flex-Algo           |
   |    | ...                    |
   +----+------------------------+
        |
        |Installation            +--------------+
        v                     +->|IGP Instance B|
   +----+----+                |  +--------------+
   |   CAR   | Redistribution |  +--------------+
   |  Route  |----------------+->|IGP Instance C|
   | Database|                |  +--------------+
   +----+----+                |  +--------------+
        |                     +->|      ...     |
        |Next-hop Resolution     +--------------+
        v
   +----+----+
   |   BGP   |
   +---------+

          Figure 5: CAR Route Database

   Take the following IGP CAR Route on MPLS-SR data plane as an
   example:

     Advertiser: N
       Prefix TLV: E
        CAR Sub-TLV: C1 + L1
        CAR Sub-TLV: C2 + L2
        CAR Sub-TLV: C3 + L3
        CAR Sub-TLV: C4 + L4

   Assume that:

   C1 is mapped to FA 128, and N's Prefix-SID in algorithm 128 is L128.
   C2 is mapped to FA 129, and N's Prefix-SID in algorithm 129 is L129.
   C3 path is provided by SR Policy 1 (C3, N) with segment-list S1.
   C4 path is provided by SR Policy 2 (C4, N) with segment-list S2.

   When the ASBR or Ingress PE resolved it over the CAR paths, the
   example of MPLS forwarding entries is as following:

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     +=======+==========+===========+=====================+
     | Index | In-Label | Out-Label | Next-Hop & Out-Intf |
     +=======+==========+===========+=====================+
     | 1     | L1       | <L128,L1> | FA 128 Path         |
     +-------+----------+-----------+---------------------+
     | 2     | L2       | <L129,L2> | FA 129 Path         |
     +-------+----------+-----------+---------------------+
     | 3     | L3       | <S1,L3>   | SR Policy 1 Path    |
     +-------+----------+-----------+---------------------+
     | 4     | L4       | <S2,L4>   | SR Policy 2 Path    |
     +-------+----------+-----------+---------------------+

   The example of CAR route entries installed in the CARDB is as
   following:

     +========+=======+=========+=================+
     | Key    | Encap | Proto   | Forwarding Path |
     +========+=======+=========+=================+
     | E + C1 | L1    | IS-IS 1 | MPLS Index 1    |
     +--------+-------+---------------------------+
     | E + C2 | L2    | IS-IS 1 | MPLS Index 2    |
     +--------+-------+---------------------------+
     | E + C3 | L3    | IS-IS 1 | MPLS Index 3    |
     +--------+-------+---------------------------+
     | E + C4 | L4    | IS-IS 1 | MPLS Index 4    |
     +--------+-------+---------------------------+

   The resolving of IGP CAR route SHOULD only be enabled on the ASBR
   and Ingress PE. The P nodes do not need to resolve IGP CAR routes,
   and only do SPF computation for the prefix of Egress PE, providing
   best-effort forwarding for traditional services.

4.1. Resolving Over SR Policy

   Resolving of IGP CAR Route over SR Policy can use the enhanced IGP
   shortcut mechanism in [I-D.cheng-lsr-igp-shortcut-enhancement].
   Briefly, the main point is to choose the SR Policy with the same
   color as the next-hop.

4.2. Resolving Over Flex-Algo

   When resolving of IGP CAR Route over Flex-Algo, the node will
   determine the FA to which the color is mapped, and check if the
   advertiser node is reachable in the topology of that FA. If yes, use
   the FA path as next-hop, and add the Prefix-SID or SRv6 End SID
   associated with that FA into encapsulation.

   The mapping relationship of FA and color should be pre-configured.

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5. Security Considerations

   TBD.

6. IANA Considerations

   TBD.

7. References

7.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [I-D.cheng-lsr-igp-shortcut-enhancement] Cheng, W., Gong, L., Lin,
             C., and M. Chen, "IGP Shortcut Enhancement", Work in
             Progress, Internet-Draft, draft-cheng-lsr-igp-shortcut-
             enhancement-03, 27 Feburary 2024,
             <https://datatracker.ietf.org/doc/html/draft-cheng-lsr-
             igp-shortcut-enhancement-03>.

7.2. Informational References

   [I-D.hr-spring-intentaware-routing-using-color] Hegde, S., Rao, D.,
             Sangli, S. R., Agrawal, S., Filsfils, C., Talaulikar, K.,
             Patel, K., Uttaro, J., Decraene, B., Bogdanov, A., Jalil,
             L., Alston, A., Xu, X., Gulko, A., Khaddam, M., Contreras,
             L. M., Steinberg, D., Guichard, J., Henderickx, W., and
             Co-authors, "Problem statement for Inter-domain Intent-
             aware Routing using Color", Work in Progress, Internet-
             Draft, draft-hr-spring-intentaware-routing-using-color-03,
             23 October 2023,
             <https://datatracker.ietf.org/doc/html/draft-hr-spring-
             intentaware-routing-using-color-03>.

   [I-D.ietf-idr-bgp-car] Rao, D., Agrawal, S., and Co-authors, "BGP
             Color-Aware Routing (CAR)", Work in Progress, Internet-
             Draft, draft-ietf-idr-bgp-car-07, 3 April 2024,
             <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
             car-07>.

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   [I-D.ietf-idr-bgp-ct] Vairavakkalai, K. and N. Venkataraman, "BGP
             Classful Transport Planes", Work in Progress, Internet-
             Draft, draft-ietf-idr-bgp-ct-31, 10 April 2024,
             <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
             ct-31>.

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Authors' Addresses

   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com

   Mengxiao Chen
   New H3C Technologies
   China
   Email: chen.mengxiao@h3c.com

   Liyan Gong
   China Mobile
   China
   Email: gongliyan@chinamobile.com

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