LSR Working Group                                                           Louis Chan
INTERNET-DRAFT                                                    Krzysztof Szarkowicz
Intended status: Standard Track                                       Juniper Networks
                                                                           Gyan Mishra
                                                                          Verizon Inc.

   Expires: Jan 7, 2024                                                    Jul 7, 2023


                 IGP extensions for Advertising Offset for Flex-Algorithm
                         draft-chan-lsr-igp-adv-offset-03.txt


   Abstract

      This document describes the IGP extensions to provide predictable Adjacency-SIDs
      per Flex-Algorithm [FLEXALGO] in segment routing.

      We propose some methods to allow the advertisement of additional TLV in IGP so that
      the Flex-Algorithm specific Adjacency-SIDs could be automatically derived.

      With the proposed method, the size of advertisement on per node per link basis is
      greatly reduced. Each participating router would derive the required labels
      automatically.

      Extensions for offset to derive Flex-Algorithm Prefix-SID is also included in the
      document.

   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
      http://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 Jan 7, 2024.

   Copyright Notice

      Copyright (c) 2023 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 (http://trustee.ietf.org/license-info) in effect on the date of


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      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. Conventions used in this document ........................... 3
      3. Virtual Flex-Algorithm ...................................... 3
         3.1. Multi-point to Multi-point Hierarchical QOS ............ 3
         3.2. DC interconnect ........................................ 4
      4. ISIS extension .............................................. 5
         4.1. Algorithm Offset for Adj-SID ........................... 5
         4.2. Algorithm Offset for LAN based Adj-SID ................. 6
         4.3. Algorithm Offset for Prefix SID ........................ 7
      5. OSPF extension  ............................................ 10
         5.1. Algorithm Offset for Adj-SID .......................... 10
         5.2. Algorithm Offset for LAN based Adj-SID ................ 11
         5.3. Algorithm Offset for Prefix SID ....................... 12
      6. Allowed configurations ..................................... 14
      7. Example for illustration ................................... 14
      8. Compatibility  ............................................. 15
         8.1. Legacy nodes which support only Flex-Algo ............. 15
         8.2. TI-LFA calculation .................................... 15
         8.3. Backward compatibility for VFA ........................ 16
            8.3.1. Option 1: Fallback only to base Flex-Algo ........ 16
            8.3.2. Option 2: Fallback with tunnel ................... 17
            8.3.3. Option 3: New CSPF ............................... 17
            8.3.4. Choice of options ................................ 17
      9. Error Handling  ............................................ 18
      10. Security Consideration .................................... 18
      11. References  ............................................... 18
         11.1. Normative References ................................. 18
         11.2. Informative References ............................... 18
      12. Acknowledgments  .......................................... 19

   1. Introduction


      The draft proposes methods for routers to announce Flex-Algorithm specific
      Adjacency-SID with minimal advertisement in IGP. When the other routers need a SR
      policy (aka SR-TE) or TI-LFA path, the Flex-Algorithm specific Adjacency-SID would
      be taken into the path label construction.

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      Hence, the top most label, either node or link related, in a SR policy stack would
      be used to identify a certain Flex-Algo with full identification.

      In the same draft, Virtual Flex-Algorithm (VFA) concept is introduced.



   2. Conventions used in this document

      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
      "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
      interpreted as described in RFC 2119 [RFC2119].

      In this document, these words will appear with that interpretation only when in ALL
      CAPS. Lower case uses of these words are not to be interpreted as carrying
      significance described in RFC 2119.



   3. Virtual Flex-Algorithm

      Here in this draft, Virtual Flex-Alogrithm (VFA) is introduced. There could be
      additional virtual topology which would like to share the same Flex-Algo topology
      with same metrics and constraints, and VFA would need different Prefix-SID and Adj-
      SID for identification in the forwarding plane.

      It would save resource to recalculate the shared topology for different VFA.

      The relationship between VFA and its related Flex-Algorithm could be similar to
      VLAN and port. Or, they could be in parallel relationship.

      VFA would be represented in 32 bits with the minimum value of 256.

      There are different VFA use cases that could be applied as described below. FA,
      here means Flex-Algo.

      If the network nodes shared the same SRGB range, for Prefix-SID MPLS label, it
      would looks like this

       [FA-ID][node] or [VFA-ID][node]

      The FA portion or VFA portion of MPLS label would be the identification of QOS
      across network. Since SRGB is the same, the values of FA-ID and VFA-ID are
      consistent.


   3.1. Multi-point to Multi-point Hierarchical QOS

      R1-----+       +-----R3
             C1-----C2
      R2-----+       +-----R4

      In this diagram, this is a multipoint VPN application.

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      R1 could be on VFA600, and R2 on VFA601.
      Both VFA600 and VFA601 is based on FA129 (Flex-Algo 129).

      FA129 is the parent of VFA600 and VFA601 in terms of hierarchical QOS (H-QOS)
      Router C2 could apply H-QOS when sending to R3 for traffic from R1 and R2.

      e.g. FA129   10Gbps in total
           VFA600  6Gbps/4Gbps for PIR/CIR
           VFA601  8Gbps/4Gbps for PIR/CIR

      This resembles some operations like VP shaping and VC shaping in ATM days.

      This draft only deals with how to label the packets, and how the QOS is enforced
      and signaled is out of the current scope.

      Here in this example, there are only two levels of hierarchy. It could have
      multiple levels with more VFA. e.g. geographical region levels. And, FA129 is used
      in this example, but in fact, Algo 0 could also be used with VFA concept.

   3.2. DC interconnect

       R1----DC1-----DC2----R2

      DC1 and DC2 are data center gateway routers.

      Between R1 and R2, it could run multiple VFA, like VFA600 and VFA601

      But between DC1 and DC2, it only runs FA129.

      If using compatibility mode operation (described in section 8.3), VFA600 and VFA601
      packets are tunneled through FA129 between DC1 and DC2.

      Transport labels are stacked in this case. E.g. Top transport label is from FA129,
      and the next transport label is from VFA600.

      At R1 and R2, individual QOS could be applied to VFA600 or VFA601 packets with the
      corresponding labels. VFA600 is a 2Gbps service, and VFA601 is a 4Gbps service.
      Both are low latency services within FA129.

      The application is similar in using VLAN trunk port and VLANs.












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   4. ISIS extension

      With reference to RFC8667, the information could be advertised in Router
      Capabilities as sub-TLV in Section 3.

   4.1. Algorithm Offset for Adj-SID


       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       |    Algorithm  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Virtual Flex-Algorithm                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           SID/Label Base offset/Index offset (variable)       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~           Flags/ Algo/ VFA/ Base offset/ Index offset         ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      where:

      Type:                 TDB

      Length:               variable; multiple advertisements

      Flags:                1 octet field of the following flags:

                            0 1 2 3 4 5 6 7
                           +-+-+-+-+-+-+-+-+
                           | |B|V|L|       |
                           +-+-+-+-+-+-+-+-+

                     where:

                   B-Flag:  Backup Flag.  If set, the Adj-SID is eligible
                            for protection (e.g., using IP Fast Reroute
                            (IPFRR) or MPLS Fast Reroute (MPLS-FRR)) as
                            described in [RFC8402].


                   V-Flag:  Value Flag.  If set, then it carries
                            a label value. If not set, it is an index value.

                   L-Flag:  Local Flag.  Always set to 1


       Algorithm:             1 octet
                              Algorithm or Flex-Algo value is either 0 or between 128 and
                              255.
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     Virtual Flex-Algorithm:  4 octets
                              Value 0 means that it is not VFA
                              Value >=256 means VFA identification number, and the base
                              topology is denoted by Algorithm
                              Value 1 to 127 are reserved
                              Value 128 to 255 are invalid

     Base Offset:             3 octets
                              Local label base for Adjacency-SIDs for given Flex-
                              Algorithm. The derived Adjacency-SID is the sum of the base
                              offset and the Algo 0 label

     Index Offset:            4 octets
                              If Adjacency-SID is advertised as index, this provides an
                              offset index value. The new label is sum of (SRGB base +
                              index + index offset)

      The format of Base Offset and Index Offset is the same as RFC8667. The choice of
      advertising Base Offset or Index Offset must match the advertisement of original
      Adjacency-SID method from the same router.

      Flags F, S and P are inherited from individual Adj-sid advertisement of Algo 0


   4.2. Algorithm Offset for LAN based Adj-SID


          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    |   Algorithm   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                   Virtual Flex-Algorithm                      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                  Neighbor System-ID (ID length octets)        |
         +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |        SID/Label Base Offset/Index Offset (variable)          ~
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         ~ Flags/ Algo/VFA/NB Sys-ID/ SID/Label/Index Offset (variable)  ~
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      where:

      Type:                 TDB

      Length:               variable; multiple advertisements

      Flags:                1 octet field of the following flags:

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                            0 1 2 3 4 5 6 7
                           +-+-+-+-+-+-+-+-+
                           | |B|V|L|       |
                           +-+-+-+-+-+-+-+-+

                     where:

              B-Flag:      Backup Flag.  If set, the Adj-SID is eligible
                           for protection (e.g., using IP Fast Reroute
                           (IPFRR) or MPLS Fast Reroute (MPLS-FRR)) as
                           described in [RFC8402].

              V-Flag:      Value Flag.  If set, then it carries
                           a label value. If not set, it is an index value.

              L-Flag:      Local Flag.  Always set to 1


      Algorithm:              1 octet. Same as Section 4.1

      Virtual Flex-Algorithm: 4 octets. Same as Section 4.1

      Neighbor System-ID:     IS-IS System-ID of length "ID Length" as
                              defined in [ISO10589].

      Base Offset:            3 octets. Same as Section 4.1

      Index Offset:           4 octets. Same as Section 4.1

      Other flags:            inherited from Algo 0. See Section 4.1

   4.3. Algorithm Offset for Prefix SID

      Prefix-SID for Algorithm could also be generated by adding an index offset value or
      a base offset for label.

















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      This advertisement is an sub-TLV in RFC8667 Section 2.

       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       |    Algorithm  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Virtual Flex-Algorithm                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        SID/Label Base offset/ Index offset                    ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~      Flags/ Algorithm/ VFA/ Base Offset/ Index offset         ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Type:                TDB

      Length:              variable

      Flags:               1 octet field of the following flags:

                            0 1 2 3 4 5 6 7
                           +-+-+-+-+-+-+-+-+
                           |R|N|   |V|L|   |
                           +-+-+-+-+-+-+-+-+

                 where:

              R-Flag:      Re-advertisement Flag.  If set, then the prefix
                           to which this Prefix-SID is attached has been
                           propagated by the router from either another
                           level (i.e., from Level-1 to Level-2 or the
                           opposite) or redistribution (e.g., from another
                           protocol).

              N-Flag:      Node-SID Flag.  If set, then the Prefix-SID
                           refers to the router identified by the prefix.
                           Typically, the N-Flag is set on Prefix-SIDs that
                           are attached to a router loopback address.  The
                           N-Flag is set when the Prefix-SID is a Node-SID
                           as described in [RFC8402].

              V-Flag:      Value Flag.  If set, then it carries
                           a label value. If not set, it is an index value.

              L-Flag:      Local Flag.  Always set to 0

    Algorithm:               1 octet
                             Algorithm/Flex-Algo value is either 0 or between 128
                             and 255

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    Virtual Flex-Algorithm:  4 octets
                             Value 0 means that it is not VFA
                             Value >=256 means VFA identification number, and the base
                             topology is denoted by Algorithm
                             Value 1 to 127 is reserved
                             Value 128 to 255 are invalid


    Base offset:             3 octets
                             Label base for Prefix-SIDs for given Algorithm
                             The derived Prefix-SID is the sum of the base
                             Offset and the Algo 0 label

    Index Offset:            4 octets
                             Index offset counting from Algo 0
                             The new label for Prefix is sum of (SRGB base +
                             index + index offset)

      The format of Base offset and Index Offset is the same as RFC8667. The choice of
      advertising Base Offset or Index Offset MUST match the advertisement of original
      Prefix-SID method from the same router.

      Flags P and E are inherited from Algo 0 advertisement. Use of Flag E might need
      some investigation.

      Flags R and N are under discussion to be removed from this draft.



























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   5. OSPF extension

      The following TLVs are advertised in the Router Information Opaque LSA (defined in
      [RFC7770]).  These TLVs are applicable to both OSPFv2 and OSPFv3.

   5.1. Algorithm Offset for Adj-SID

          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     |    Reserved   |   MT-ID       |  Weight       |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Algorithm   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                   Virtual Flex-Algorithm                      |
         +---------------------------------------------------------------+
         |          SID/Label Offset/Index Offset (variable)             ~
         +---------------------------------------------------------------+
         ~  Flags/R/MT-ID/Algo/Weight/SID/Label/Index Offset(variable)   ~
         +---------------------------------------------------------------+


       where:

            Type:       TBD

            Length:     Variable

            Flags:      Single-octet field containing the following flags:

                        0 1 2 3 4 5 6 7
                        +-+-+-+-+-+-+-+-+
                        |B|V|L|         |
                        +-+-+-+-+-+-+-+-+

               where:

            B-Flag:     Backup Flag.  If set, the Adj-SID is eligible
                        for protection (e.g., using IP Fast Reroute
                        (IPFRR) or MPLS Fast Reroute (MPLS-FRR)) as
                        described in [RFC8402].

            V-Flag:     Value Flag.  If set, then it carries
                        a label value. If not set, it is an index value.

            L-Flag:     Local Flag.  Always set to 1


            Other bits: Reserved.  These MUST be zero when sent and are
                        ignored when received.

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    MT-ID:                   Multi-Topology ID (as defined in [RFC4915]

    Weight:                  Weight used for load-balancing purposes.  The use of the
                             weight is defined in [RFC8402].


    Algorithm:               1 octets
                             Algorithm/Flex-Algo value is either 0 or between 128 and
                             255.

    Virtual Flex-Algorithm:  4 octets
                             Value 0 means that it is not VFA
                             Value >=256 means VFA identification number, and the base
                             topology is denoted by Algorithm
                             Value 1 to 127 is reserved
                             Value 128 to 255 are invalid


    Base Offset:             3 octets
                             Local label base for Adjacency-SIDs for given Flex-
                             Algorithm. The derived Adjacency-SID is the sum of the base
                             offset and the Algo 0 label

    Index Offset:            4 octets
                             If Adjacency-SID is advertised as index, this provides an
                             offset index value. The new label is sum of (SRGB base +
                             index + index offset)

      The format of Base Offset and Index Offset is the same as RFC8665 or RFC8666. The
      choice of advertising Base Offset or Index Offset must match the advertisement of
      original Adjacency-SID method from the same router.


   5.2. Algorithm Offset for LAN based Adj-SID

          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     |    Reserved   |     MT-ID     |    Weight     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Algorithm   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                   Virtual Flex-Algorithm                      |
         +---------------------------------------------------------------+
         |                        Neighbor ID                            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          SID/Label Offset/Index Offset (variable)             ~
         +---------------------------------------------------------------+
         ~ Flags/R/MT-ID/Weight/Algo/NHR ID/S/Lab/Index Offset(variable) ~
         +---------------------------------------------------------------+

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      where:

            Type:              TBD

            Length:            Variable

            Flags:             Same as 5.1

            Reserved:          Same as 5.1

            MT-ID:             Same as 5.1

            Weight:            Same as 5.1

         Algorithm:            Same as 5.1

      Virtual Flex-Algorithm:  Same as 5.1

       Neighbor ID:            The Router ID of the neighbor for which the LAN
                               Adjacency SID is advertised

       Base offset:            Same as 5.1
      Index Offset:            Same as 5.1





   5.3. Algorithm Offset for Prefix SID

          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    |   Reserved    |      MT-ID    |    Algorithm  |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                   Virtual Flex-Algorithm                      |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          SID/Index Offset/Label Offset(variable)              ~
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         ~     Flags/R/MT-ID/Algo/SID/Index/Label Offset(variable)       ~
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



         where:

            Type:            TDB

            Length:          Variable

            Flags:           Single-octet field.  The following flags are defined:

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                             0  1  2  3  4  5  6  7
                            +--+--+--+--+--+--+--+--+
                            |  |  |  |E |V |L |  |  |
                            +--+--+--+--+--+--+--+--+


            E-Flag:         Explicit Null Flag.  If set, any upstream neighbor
                            of the Prefix-SID originator MUST replace the Prefix-SID
                            with the Explicit NULL label (0 for IPv4) before
                            forwarding the packet.

            V-Flag:         Value Flag.  If set, then it carries
                            a label value. If not set, it is an index value.

            L-Flag:         Local Flag.  Always set to 0

            Other bits:     Reserved.  These MUST be zero when sent and are
                            ignored when received.


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


    Algorithm:                1 octet
                              Algorithm/Flex-Algo value is either 0 or between 128 and 255

    Virtual Flex-Algorithm:   4 octets
                              Value 0 means that it is not VFA
                              Value >=256 means VFA identification number, and the base
                              topology is denoted by Algorithm
                              Value 1 to 127 is reserved
                              Value 128 to 255 are invalid

    Base offset:              3 octets
                              Label base for Prefix-SIDs for given Algorithm
                              The derived Prefix-SID is the sum of the base
                              Offset and the Algo 0 label

    Index Offset:             4 octets
                              Index offset counting from Algo 0
                              The new label for Prefix is sum of (SRGB base +
                              index + index offset)

      The format of Base offset and Index Offset is the same as RFC8665 or RFC8666. The
      choice of advertising Base Offset or Index Offset MUST match the advertisement of
      original Prefix-SID method from the same router.







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   6. Allowed configurations

                             FA   FA    FA    VFA   VFA   VFA
      +-------------------------------------------------------+
      | Adj-SID offset    |  P  |  P  |  N  |  P  |  P  |  N  |
      | Prefix-sid offset |  N  |  P  |  N  |  N  |  P  |  P  |
      | Valid combination |  Y  |  N  |  Y  |  N  |  Y  |  Y  |
      | Note              |     |     |  #1 |     |     |  #2 |
      +-------------------------------------------------------+



      P  means the sub-tlv is present

      N  means the sub-tlv is not present

      #1 - For this Flex-Algo, it will use the adj-sid from Algo 0

      #2 - For this VFA, it will use the Adj-sid from base algorithm, which could be
      either Algo 0 or the associated Flex-Algo. Only if the base Flex-Algo has no
      advertisement of Adj-sid offset, it then would use Adj-sid of Algo 0 as fallback
      mechanism.

   7. Example for illustration

     One node advertises the following information in IGP.

     Algo 0  Prefix-sid label: 400001

     Adj-sid labels for 99 interfaces: 32, 33, 34...130

     SRGB starts with 400000

        +------------------------------------------------------------------------+
        |Value         |  Algo 0 | FA128 | VFA500| FA129 | VFA600| VFA601| VFA602|
        +------------------------------------------------------------------------+
        |Base Algo     |   n/a   |  n/a  |  128  |  n/a  |  129  | 129   | 129   |
        |Adj-sid offset|   n/a   |  NIL  |  NIL  | 2000  |  6000 | 7000  | NIL   |
        |Pre-sid offset|   n/a   |  n/a  |  5000 |  n/a  |  6000 | 7000  | 8000  |
        |Prefix-sid (N)| 400001  | 401001| 405001| 402001| 406001| 407001| 408001|
        |Intf#1        |   32    |  32   |  32   |  2032 |  6032 | 7032  |  2032 |
        |Intf#2        |   33    |  n/a  |  n/a  |  2033 |  6033 | 7033  |  2033 |
        |Intf#3        |   34    |  34   |  34   |  n/a  |   n/a |  n/a  |  n/a  |
        |.....         |   ..    |  ..   |  ..   |  ..   |   ..  |  ..   |  ..   |
        |Intf#99       |   130   |  130  |  130  |  2130 |  6130 | 7130  |  2130 |
        |Note          |         |  #1   |   #2  |   #3  |  #4   |       |  #5   |
        +------------------------------------------------------------------------+





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      Note:

      #1 There is no Adj-sid offset advertised for Flex-Algo 128. Hence FA128 shares Adj-
      sid from Algo 0.

      #2 Since FA128 has no specific Adj-sid, VFA500 follows Adj-sid from Algo 0.

      #3 There is only Adj-sid offset advertised for Flex-Algo 129. The Adj-sid would be
      added with 2000 offset. E.g. 32 becomes 2032.
      Please note that there is no Adj-sid for Intf#3 in FA129.

      #4 Both Adj-sid offset and Prefix-sid offset are advertised for VFA600. Prefix-sid
      of 406001 (400001 + 6000) and Adj-sid of 6032 to 6130 are used.

      #5 VFA602 uses Adj-sid's from FA129.

      NIL means the TLV/sub-TLV is not present.


      The QOS relationship between FA129, VFA600 and VFA601 could be in multiple forms.
      e.g.
      a. FA129 is the parent of VFA600 and VFA601. It is similar to stacked vlan.
      b. FA129, VFA600 and VFA601 are all in the same level. It is similar to parallel
         vlans.
      c. FA129 is parent of VFA600, but in parallel with VFA601. It is a mix in QOS
         relationship.

      In this draft, it only describes how the labels are assigned, and the actual QOS
      enforcement is out of the scope.

      For FA129, when the numbering system is carefully designed, labels such as, 402xxx
      for network wide, could be used to identify FA129, and apply QOS along the transit
      nodes. Label of 2xxx is for local node consumption when QOS is applied.





   8. Compatibility

   8.1. Legacy nodes which support only Flex-Algo

      For nodes that do not support Adj-sid offset, the label stack could use Adj-SID
      from Algo 0, providing that the node still support Flex-Algo Prefix SID.

      Since it cannot understand Prefix-sid offset, it will not participate in any VFA.

   8.2. TI-LFA calculation

      For TI-LFA, there would be two modes of operation, loose or strict.


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      For strict mode, all Adj-SID involved in TI-LFA candidate path must be derived with
      offset method, when each node in the path has announced such Adj-SID offset.

      For loose mode, Adj-SID in the TI-LFA candidate path could be a mix of Adj-SID's
      with or without offset. This allows backward compatibility with routers which only
      support Flex-Algo.

      Default is loose mode.

   8.3. Backward compatibility for VFA

      There would be a few options for backward compatibility.

      All routers below support the new extensions in the draft, except R3. R3 is only
          capable of running Flex-Algo.

      Here we assume a case of one FA plus one VFA.


                     R1--------R2--------R3--------R4-------R5

       Node-sid

       FA129   402001     402002    402003    402004   402005

       VFA600  406001     406002     n/a      406004   406005


      For a packet based on VFA600 from R1 to R5, the top label would be encoded in
          one of the following fallback options.


   8.3.1. Option 1: Fallback only to base Flex-Algo

          Fallback to FA129 in this example

                 R1--------R2--------R3--------R4-------R5

      top label     406005   402005     402005    402005(or popped)

      R2 detects R3 could not support VFA600, and it allows VFA600 to fallback FA129
          node-sid for R5. It is 402005 here.

      R2 is the PLR.



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   8.3.2. Option 2: Fallback with tunnel

       It is similar to TI-LFA.

                 R1--------R2--------R3--------R4-------R5

     top labels     406005   402004     406005    406005(or popped)
                             406005

       R2 inserts a R3 understandable FA129 sid, and let the packet tunnel through
           like TI-LFA. R4 can then take 406xxx label and apply QOS for VFA600.

       R2 is the PLR.



   8.3.3. Option 3: New CSPF

      R1, R2, R4 and R5 detects that R3 is not participating in VFA600. They could spawn
      a CSPF instead of using result from FA129. However, this approach uses more
      processing power, and it is the least preferred option. It might create
      disconnection network due to misconfiguration. For example, it is only a
      misconfiguration in R3. R3 indeed could support the new extension. Now, the VFA600
      is disconnected between R1 and R5.



   8.3.4. Choice of options



      In this draft, Option 1 is mandatory for implementation and is turned on by
      default. It could be turned off for strict compliance.

      Option 2 and option 3 are optional.

      Option 1 and Option 2 could co-exist during transition phase.

      R2, adjacent to R3, is the PLR. R4 is the PLR for the reverse direction.














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   9. Error Handling

      If a node detects another node which is supposed to participate in certain VFA but
      it is not, a warning should be displayed or logged. The invalid configuration
      combinations are shown in section 6.

      For the node which is adjacent to the misconfigured node, it should possess the
      capability mentioned in Section 8 on compatibility .

   10. Security Consideration

      TBD

   11. References

   11.1. Normative References

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

   11.2. Informative References
      [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>.

      [RFC8665]     Psenak, P., Ed., Previdi, S., Ed., Filfils, C., Gredler,
                    H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
                    Extensions for Segment Routing", RFC 8665,
                    DOI 10.17487/RFC8665, December 2019,
                    <https://www.rfc-editor.org/info/rfc8665>.

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

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


      [TI-LFA]      Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
                    Decraene, B., and D. Voyer, "Topology Independent Fast
                    Reroute using Segment Routing",
                    <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-segment-
                    routing-ti-lfa>

      [FLEXAGLO]    S. Hegde, P. Psenak and etc, IGP Flexible Algorithm
                    https://datatracker.ietf.org/doc/draft-ietf-lsr-flex-algo


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      [ISO10589]    International Organization for Standardization,
                    "Information technology -- Telecommunications and
                    information exchange between systems -- Intermediate
                    system to Intermediate system intra-domain routeing
                    information exchange protocol for use in conjunction with
                    the protocol for providing the connectionless-mode network
                    service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
                    November 2002.



   12. Acknowledgments


           The following people have contributed to this document:
               Shraddha Hegde, Juniper Networks



        Authors' Addresses

        Louis Chan (Editor)
           Juniper Networks Group (Hong Kong) Limited
           Suites 3001-7
           30th Floor Tower 2
           Times Square
           1 Matheson Street
           Causeway Bay
           Hong Kong

           Phone: +852-38562100
           Email: louisc@juniper.net


           Krzysztof Grzegorz Szarkowicz
           Juniper Networks
           Parkring 10
           A-1010 Wien
           Austria

           Phone: +49 89 203012127
           Email: kszarkowicz@juniper.net


           Gyan Mishra
           Verizon Inc.

           Email: gyan.s.mishra@verizon.com





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