Internet DRAFT - draft-chan-lsr-igp-adv-offset

draft-chan-lsr-igp-adv-offset



    
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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