Signalling ERLD using BGP-LS
draft-ietf-idr-bgp-ls-segment-routing-rld-02

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2.  Conventions used in this document

2.1.  Terminology

   BGP-LS: Distribution of Link-State and TE Information using Border
   Gateway Protocol

   ERLD: Entropy capable Readable Label Depth

   PCC: Path Computation Client

   PCE: Path Computation Element

   PCEP: Path Computation Element Protocol

   SID: Segment Identifier

   SR: Segment routing

3.  Problem Statement

   In existing technology both ISIS [4] and OSPF [3] have proposed
   extensions to signal the RLD (Readable Label Depth) and ELC (Entropy
   Label Capability) of a node.  However, if a network SDN controller is
   connected to the network through a BGP-LS session and not through
   ISIS or OSPF technology, then both RLD and ELC needs to be signalled
   using BGP-LS encoding.  This document describes the extension BGP-LS
   requires to transport the combined RLD and ELC into an ERLD (Entropy
   capable Readable Label Depth) attribute.

   A network SDN controller having awareness of the ERLD can for example
   use it as a constraint on path computation to make sure that high
   bandwidth LSPs are not placed on LAG (Link Aggregation Group),
   containing links with smaller member bandwidth, if they know the
   Entropy Label cannot be processed by the node at the ingress to the
   link.

4.  Origination of ERLD in BGP-LS

   Both ISIS [4] and OSPF [3] have proposed extensions to signal the RLD
   (Readable Label Depth) and ELC (Entropy Label Capability) for a node.
   A BGP-LS router exporting the IGP LSDB, MUST NOT encode the IGP RLD
   value in an BGP-LS ERLD attribute, if the associated node ELC is not
   signalled.

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5.  ERLD support by a node

   Node ERLD is encoded in a new Node Attribute TLV, as defined in
   RFC7752 [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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     ERLD      |
     +-+-+-+-+-+-+-+-+

                                 Figure 1

      Type : A 2-octet field specifying code-point of the new TLV type.
      Code-point: TBA from BGP-LS Node Descriptor, Link Descriptor,
      Prefix Descriptor, and Attribute TLVs registry

      Length: A 2-octet field that indicates the length of the value
      portion

      ERLD: Node ERLD is a number in the range of 0-254.  The value of 0
      represents lack of ability to read a label stack of any depth, any
      other value represents the readable label depth of the node.

6.  Security Considerations

   This document does not introduce security issues beyond those
   discussed in RFC7752 [2]

7.  Acknowledgements

   Thanks to discussions with Acee Lindem, Jeff Tantsura, Stephane
   Litkowski, Bruno Decraene, Kireeti Kompella, John E.  Drake and
   Carlos Pignataro to bring the concept of combining ELC and RLD into a
   single ERLD signalled parameter more suitable for SDN controller
   based networks.

8.  IANA Considerations

   This document requests assigning a new code-points from the BGP-LS
   Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
   TLVs registry as specified in section 5.

   Note: placeholder IANA request

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   Request Node ERLD codepoint

   BGP-LS TLV Code Point: TBD1

   ISIS TLV 242/TBD2

   Note: There is nothing in IANA from draft draft-ietf-isis-mpls-elc

   Note: Draft talks only about ELC/RLD and that is mismatch with ERLD

   OSPF RI TLV TBD5

   OSPF ELC in Non-OSPF functionality Capability Bits (TBD6)

9.  References

9.1.  Normative References

   [1]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://xml.resource.org/public/rfc/html/rfc2119.html>.

   [2]        Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

9.2.  Informative References

   [3]        Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "draft-ietf-ospf-mpls-elc", January 2018.

   [4]        Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "draft-ietf-isis-mpls-elc", January 2018.

Authors' Addresses

   Gunter Van de Velde (editor)
   Nokia
   Antwerp
   BE

   Email: gunter.van_de_velde@nokia.com

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   Wim Henderickx
   Nokia
   Belgium

   Email: wim.henderickx@nokia.com

   Matthew Bocci
   Nokia
   Shoppenhangers Road
   Maidenhead, Berks
   UK

   Email: matthew.bocci@nokia.com

   Keyur Patel
   Arrcus
   USA

   Email: keyur@arrcus.com

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