Color Operation with BGP Label Unicast

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Author Louis Chan 
Last updated 2021-08-26
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IDR Working Group                                                 Louis Chan 
Intended status: Experimental                                     Juniper Networks 
Expires: Feb 23, 2022                                             Aug 23, 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 
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   inappropriate to use Internet-Drafts as reference material or to cite them other 
   than as "work in progress." 

   This Internet-Draft will expire on Feb 23, 2022. 

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 
   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..............................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..........5 
      3.3. Color extended community for service prefixes.............6 
      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....................................9 
   10. Label stacking to increase label space........................9 
   11. Tunneling SRv6 packet via MPLS................................9 
   12. Security Considerations......................................10 
   13. IANA Considerations..........................................10 
   14. References...................................................10 
      14.1. Normative References....................................10 
      14.2. Informative References..................................10 
   15. Acknowledgments..............................................11 
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 

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   - Prefix SID 
   - Binding SID for RSVP LSP  
   - 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 

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   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 
   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)              | 

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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 
               |       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. 


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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.  

   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)                             | 

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      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  
               |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] 

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   <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] 
     <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. 

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   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. 

   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. Tunneling SRv6 packet via MPLS 


   In a SRv6 network, PE1 and PE2 is using SRv6 for VPN service. Between ABR1 and 
   ABR2, it is capable of MPLS only. The use of BGP-LU2 would be a method to provide 
   locator route mapping to MPLS tunnel between ABRs.  

   At ABR1, the mapping options could be  

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   a) Use of color attribute associated with the VPN advertisement and map to the 
     desired tunnel. 
   b) Up to the locator route. For example, use first 48 bits of SRv6 header 
     FC00:0000:nnnn::/48      ;  where nnnn is the locator portion 
   c) Making use of sub-slice information as defined in [SRV6-SUBSLICE] 
                   |   Locator  |  Sub-slice ID  |  Remainder for behavior  | 
                                |<-         Endpoint Behavior             ->| 
     Sub-slice ID could be used for mapping to different color path in MPLS. For 
     FC00:0000:nnnn:ssss::/64       ; where ssss is a sub-slice ID 
     ABR2 advertises a/64 prefix route inclusive of sub-slice ID via BGP-LU2 into 
     ABR1. Hence, traffic will be redirected to a MPLS tunnel from ABR1. 
   d) With the format described in [SRV6-SUBSLICE], a mapping could be made between 
     sub-slice ID and <color, sub-slice> mentioned in section 3.2.  



12. Security Considerations 


13. IANA Considerations 

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

   New SAFI might be required as well. 

14. References 

14.1. Normative References 

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

14.2. 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 


   [SRV6-SUBSLICE] Louis Chan, Sub-slicing for SRv6 



15. Acknowledgments 

        The following people have contributed to this document: 
        Jeff Haas, Juniper Networks 
        Shraddha Hedge, Juniper Networks 
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        Santosh Kolenchery, Juniper Networks 
        Shihari Sangli, Juniper Networks 
        Krzysztof Szarkowicz, Juniper Networks 
        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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