Color Operation with BGP Label Unicast

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Author Louis Chan 
Last updated 2021-03-07
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IDR Working Group                                                          Louis Chan 
Intended status: Experimental                                        Juniper Networks 
Expires: Sep 6, 2021                                                      Mar 6, 2021 
                        Color Operation with BGP Label Unicast 


      This document specifies how to carry colored path advertisement via an enhancement 
      to the existing protocol BGP Label Unicast. It would allow backward compatibility 
      with RFC8277. 

      The targeted solution is to use stack of labels advertised via BGP Label Unicast 
      2.0 for end to end traffic steering across multiple IGP domains. The operation is 
      similar to Segment Routing.  

      This proposed protocol will convey the necessary reachability information to the 
      ingress PE node to construct an end to end path. 

      Another two problems addressed here are the interworking with Flex-Algo, and the 
      MPLS label space limit problem. 

      Please note that there is a major change of protocol format starting from version 
      01 draft. Except the optional BGP capability code, these rest of BGP attributes 
      used in this draft are defined in previous RFC or in use today in other scenario. 

   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 

      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 Sep 6, 2021. 

   Copyright Notice 

      Copyright (c) 2017 IETF Trust and the persons identified as the document authors.  
      All rights reserved. 

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      This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating 
      to IETF Documents ( 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 
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      warranty as described in the Simplified BSD License. 


   Table of Contents 

      1. Introduction ......................................................... 2 
      2. Conventions used in this document .................................... 4 
      3. Carrying Label Mapping Information with Color and Label Stack ........ 4 
         3.1. Use of Add-path to advertise multiple color paths ............... 4 
         3.2. Color extended community for BGP Labeled Unicast ................ 4 
         3.3. Color extended community for service prefixes ................... 5 
         3.4. Color Slicing Capability ........................................ 6 
      4. Uniqueness of path entries ........................................... 7 
      5. AIGP consideration ................................................... 8 
      6. Explicit Withdraw of a <path-id, color(s), prefix> ................... 8 
      7. Error Handling Procedure ............................................. 8 
      8. Controller Compatibility ............................................. 8 
      9. Interworking with Flex Algo .......................................... 8 
      10. Label stacking to increase label space .............................. 9 
      11. Security Considerations ............................................. 9 
      12. IANA Considerations ................................................. 9 
      13. References  ......................................................... 9 
         13.1. Normative References ........................................... 9 
         13.2. Informative References ......................................... 9 
      14. Acknowledgments  ................................................... 10 
   1. Introduction 

      The proposed protocol is aimed to solve interdomain traffic steering, with 
      different transport services in mind. One application is low latency service across 
      multiple IGP domains, which could scale up to 100k or more routers network. 

      BGP is a flexible protocol. With additional of color attribute to BGP Label 
      Unicast, a path with specific color would be given a meaning in application - a low 
      latency path, a fully protected path, or a path for diversity.  

      The stack of labels would mean an end to end path across domains through each ABR 
      or ASBR. Each ABR or ASBR will take one label from the stack, and hence pick the 
      forwarding path to next ABR, ASBR, or the final destination. 

      And the label in the stack may be derived from any of the below 

      - Prefix SID 
      - Binding SID for RSVP LSP  
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      - Binding SID for SR-TE LSP 
      - Local assigned label 

      The enhancement to the original RFC8277 is to add color extended community, with 
      multiple advertisement allowed. The result is similar to multi-topology BGP-LU with 
      different colors.  

      With Add-path [RFC7911] feature, non color RIB and colored RIB could be advertised 
      to the BGP neighbors without new additional attributes. Add-path capability is 
      required advertise multiple paths with same prefix but different colors. 

      A new [BGP-CAP] should be required to enforce such slicing operation during 

      On the other hand, to enable the service prefixes to be mapped accordingly, the 
      L3VPN, L2VPN, EVPN and IP prefix with BGP signaling, the color extended community 
      is also added there. In the PE node, the service prefixes with color will be 
      matched to a transport tunnel with the same color. 

      The following is an example. Between PE1 and PE2, there is a VPN service running 
      with label 16, which is associated with color 100. 


      PE1 will send the following labels with a color 100 path plus VPN label 

      [2001 13001 801 16], where 

      2001 - SR label to reach ABR1 

      13001 - a Binding-SID label for ABR1-ABR2 tunnel. Underlying tunnel type is RSVP-TE 

      801 - a Binding-SID label for ABR2-PE2 tunnel. Underlying tunnel type is SR-TE 

      16 - a VPN label, which is signaled via other means 

      [2001 13001 801] - denotes the label stack for this color 100 path to reach PE2 

      The document here is going to describe how PE1 gains enough information to build 
      this label stack across routing domains. 

      If PE1 wants to reach PE2 with another colored path, say color 200, the label stack 
      could be different.  

      At the same time, this architecture is also controller friendly, since all the 
      notation is Segment Routing compatible, like use of Binding-SID. 

      The above architecture could be used in conjunction with Flex-Algo [FLEXAGLO] where 
      one color could represent a Flex Algorithm. e.g. color 128 equals to Algo 128 

      When using with Flex Algo in huge network, there could be label space limit. The 
      MPLS label 20 bits long and the maximum label space is around 1 million. In order 
      to represent more IPv4 or IPv6 nodes, label stacking method is recommended. One IP 

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      loopback address could be represent by one or more labels. In this case, (20 bits x 
      n) of label address space is possible. 


   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. Carrying Label Mapping Information with Color and Label Stack 


   3.1. Use of Add-path to advertise multiple color paths 

      The use of Path Identifier is to allow multiple advertisement of the same prefix 
      but with different colors or null color. 
      The extended NLRI format would be like this 
                        | Path Identifier (4 octets)     | 
                        | Length (1 octet)               | 
                        | Label (3 octets)               ~ 
                        ~ Label (3 octets)               | 
                        | Prefix (variable)              | 

   3.2. Color extended community for BGP Labeled Unicast 

      The addition of Color Extended Community is an opaque extended community from 
      RFC4360 and RFC5512. The draft allows multiple color values advertisement.  

                   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 
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                  |       0x03    |     0x0b      |C|O|        Reserved     |X|X|X| 
                  |                          Color Value                          ~ 
                  ~       0x03    |     0x0b      |C|O|        Reserved     |X|X|X| 
                  |                          Color Value                          | 
                            Figure 1: Color value advertisement format 
      Both in BGP update and MP_UNREACH_NLRI message, multiple color extended communities 
      could be included. It means that multiple colors, indicating different kind of 
      services, could share the same label stack. With the use of Path-ID, the multiple 
      colors are considered as one bundled update. Any subsequent update is based on 
      If color extended community is not present in a BGP update message, it would be 
      treated as normal BGP-LU without any color. 

      3 bits of XXX is reserved here for the draft.  

      The meaning for XXX is interpreted as sub-slice of color, with 0 to 7 in decimal, 
      or 000b and 111b in binary. These sub-slice could be used in either of the 
      following case. 

      a) Primary path and fallback paths in order of preference 
        0 - primary path 
        1 - first and most preferred backup path  
        7 - least preferred backup path 
      b) ECMP paths up to 8, since all paths should be active in forwarding plane. 

      Color value 0 is reserved for future interoperability purpose.  

      Color value 1 - 31 are not recommended to use, and this range is reserved for 
      future use. 


   3.3. Color extended community for service prefixes 

      The same format of color extended community is advertised with service prefixes, 
      which could be VPN prefixes or IP prefixes. The order of the color extended 
      community could be interpreted as  

      - Order of primary and fallback colors 
      - Or, ECMP of equal split between color paths 

      The above would be interpreted by the receiving PE upon its local configuration.  

      It is optional to enable sub-slice notation.  
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      But if sub-slice bits are used, it will be used to map directly to each of the sub-
      slice path. If sub-slice path is not available for mapping, it should just fallback 
      to resolving by color. 







   3.4. Color Slicing Capability 
      The Color Slicing Capability is a BGP capability [RFC5492], with Capability Code xx 
      The color slicing capability is an optional but preferred to have capability. It 
      could be configurable parameters at both side of BGP session but with assumption of 
      BGP add-path support [RFC7911]. If the specific BGP capability is not negotiated, 
      it is assumed version 0 without sub-slice notation. In this case, multiple paths 
      with color attribute are advertised through BGP add-path. 
      The Capability Length field of this capability is variable.  The Capability Value 
      field consists of one or more of the following tuples: 
                      | Address Family Identifier (2 octets)           | 
                      | Subsequent Address Family Identifier (1 octet) | 
                      | version (1 octet)                              | 
                      | Reserved (3 octet)                             | 
         The meaning and use of the fields are as follows: 
            Address Family Identifier (AFI): 
               This field is the same as the one used in [RFC4760]. 
            Subsequent Address Family Identifier (SAFI): 
               This field is the same as the one used in [RFC4760]. 
            This field is for capability negotiation. 
                   0 1 2 3 4 5 6 7  
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                  |v v v v|     |s| 
      Each of 4 bits of v represents a flag of version from 0 to 4, where LSB denotes 
      support of version 1, and MSB denotes version 4. Version 0 is the default mode of 
      operation, which is described in this document. To determine the common capability 
      between the two BGP PEER, logical AND function to use determine the highest 
      denominator of protocol version. 
      For example, if BGP receive 0b0110 from its peer and perform AND function with its 
      own capability 0b0010, the result is 0b0010. Version 2 is selected.  
      The other examples are 
      - 0b0110 AND 0b0110, version 3 is selected 
      - 0b0100 AND 0b0010, version 0 is selected 
      Version 1 (0b0001) is reserved. 
        S-flag is the indication of use of sub-slice. Set to 1 if sub-slice notation is 
      enforced. If either side is set to 0 for S-flag, sub-slice is not in use. 
               This field is reserved for future use. 

   4. Uniqueness of path entries 

      a) Use of color can be considered to slice into multiple BGP Label Unicast RIB. 
      Therefore, it should be treated as unique entries for the <path-id, color(s), 

      e.g. <path-id, color(s), prefix>, [labels] 

      <123, 100,>, [1000 2000] 

      <124, 200,>, [1000 2000] 

      <222, {300,400},>, [1000 2000] 

      <223, null,>, [1000 2000] 

      All these 4 NLRI are considered different but valid entries for different color 

      b) With sub-slice notation 
        <path-id, color-sub, prefix>, [labels] 
        <901, 100-0,>, [1000 2000] 
        <902, 100-1,>, [1001 3000] 
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        <903, 100-7,>, [1002 4000] 
        These 3 NLRI are distinct, and the second and third NLRI could be used for 
        backup or ECMP purpose. 

   5. AIGP consideration 

      AIGP (RFC7311) would be also used in here to embed certain metric across. 

   6. Explicit Withdraw of a <path-id, color(s), prefix> 
      According to RFC8277, MP_UNREACH_NLRI can be used to remove binding of a <path-id, 
      color(s), prefix>. 
      If a path-id is associated with a prefix with multiple colors, the withdrawal would 
      be applied to all associated colors.  

      To withdraw color(s) partially from the same path-id advertisement, BGP update 
      should be used instead. 


   7. Error Handling Procedure 

      If BGP receiver could not handle the NLRI, it should silently discard with error 


   8. Controller Compatibility 

      The proposed architecture is compatible with controller for end to end 
      provisioning. Persistent label, like Binding-SID is recommended to be used. Hence, 
      controller could learn these labels from the network, and program specific end to 
      end path. 

      In this case, BGP-LU2 will provide a second best path to an ingress PE node, while 
      a controller, with more external information, could provide a best path from 
      overall perspective.  

      Controller could also be deployed based on domain by domain perspective. e.g. 
      Optimizing latency of a RSVP LSP, or maintain the bandwidth and loading between SR-
      TE LSPs. 


   9. Interworking with Flex Algo 

      Flex Algo is a way of network slicing, but it is only an IGP protocol. In order to 
      scale across different domains, BGP is recommended as the method to distribute the 
      information across. 

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      With color notation in this proposal, one router can distribute to another domain 
      via BGP.  

      There are two ways of mapping Flex-Algo to color attribute in BGP-LU2 

      a) Color 128 equals Flex Algo 128 
      b) Or, Color 400 is mapped to Flex Algo 128 

   10. Label stacking to increase label space 

      Due to the use of Flex-Algo [FLEXALGO], the MPLS label space might run into limit. 
      Each node will need extra labels for each Algo. 

      The idea is to use multiple labels to represent a single node. In this case, the 
      label space becomes (2^20)^n, depending on n stacking level.  

      For IPv6 address, there would be enough label space even if running with SR-MPLS. 

      For example, for node, 2 consecutive labels are used to represent the node. 

      Algo 0:       [100101 100001] 

      Algo 128:     [400101 400001] 

      How the forwarding plane treats the stacked labels is out of the discussion here. 

   11. Security Considerations 


   12. IANA Considerations 

      TBD. It will require a new BGP capability code to enable such color operation.  

      New SAFI might be required as well. 

   13. References 

   13.1. Normative References 

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

   13.2. Informative References 

      [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in BGP-4", RFC 
              3107, DOI 10.17487/RFC3107, May 2001,  


      [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended              
              Communities Attribute", RFC 4360, February 2006 

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      [RFC5512] Mohapatra, P. and E. Rosen, "The BGP Encapsulation Subsequent Address 
              Family Identifier (SAFI) and the BGP Tunnel Encapsulation Attribute", RFC 
              5512, April 2009. 


      [RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J., and D. 
              McPherson, "Dissemination of Flow Specification Rules", RFC 5575, DOI 
              10.17487/RFC5575, August 2009, 

      [RFC7311] Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro, 
                "The Accumulated IGP Metric Attribute for BGP", RFC 7311, 
                DOI 10.17487/RFC7311, August 2014, 

      [RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder, "Advertisement of 
              Multiple Paths in BGP", RFC 7911, DOI 10.17487/RFC7911, July 2016, 


      [RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address Prefixes", RFC 8277, 
              DOI 10.17487/RFC8277, October 2017,              


      [BGP-CAP] Chandra, R. and J. Scudder, "Capabilities Advertisement 

                with BGP-4", RFC 2842, May 2000. 

      [FLEXAGLO] S. Hegde, P. Psenak and etc, IGP Flexible Algorithm 



   14. Acknowledgments 

           The following people have contributed to this document: 
           Jeff Haas, Juniper Networks 
           Shraddha Hedge, Juniper Networks 
           Santosh Kolenchery, Juniper Networks 
           Shihari Sangli, Juniper Networks 
           Krzysztof Szarkowicz, Juniper Networks 
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           Yimin Shen, Juniper Networks 
        Author Address 
        Louis Chan (editor) 
           Juniper Networks 
           2604, Cityplaza One, 1111 King's Road 
           Taikoo Shing 
           Hong Kong 
           Phone: +85225876659 



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