



Internet Engineering Task Force (IETF)                       J. Tantsura
Request for Comments: 8814                                  Apstra, Inc.
Category: Standards Track                                    U. Chunduri
ISSN: 2070-1721                                   Futurewei Technologies
                                                           K. Talaulikar
                                                           Cisco Systems
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        N. Triantafillis
                                                     Amazon Web Services
                                                             August 2020


 Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol -
                               Link State

Abstract

   This document defines a way for a Border Gateway Protocol - Link
   State (BGP-LS) speaker to advertise multiple types of supported
   Maximum SID Depths (MSDs) at node and/or link granularity.

   Such advertisements allow entities (e.g., centralized controllers) to
   determine whether a particular Segment Identifier (SID) stack can be
   supported in a given network.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8814.

Copyright Notice

   Copyright (c) 2020 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
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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
     1.1.  Conventions Used in This Document
       1.1.1.  Terminology
       1.1.2.  Requirements Language
   2.  Advertisement of MSD via BGP-LS
   3.  Node MSD TLV
   4.  Link MSD TLV
   5.  IANA Considerations
   6.  Manageability Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   When Segment Routing (SR) [RFC8402] paths are computed by a
   centralized controller, it is critical that the controller learns the
   Maximum SID Depth (MSD) that can be imposed at each node/link on a
   given SR path.  This ensures that the Segment Identifier (SID) stack
   depth of a computed path doesn't exceed the number of SIDs the node
   is capable of imposing.

   [RFC8664] defines how to signal MSD in the Path Computation Element
   Protocol (PCEP).  The OSPF and IS-IS extensions for the signaling of
   MSD are defined in [RFC8476] and [RFC8491], respectively.

   However, if PCEP is not supported/configured on the head-end of an SR
   tunnel or a Binding-SID anchor node, and the controller does not
   participate in IGP routing, it has no way of learning the MSD of
   nodes and links.  BGP-LS [RFC7752] defines a way to expose topology
   and associated attributes and capabilities of the nodes in that
   topology to a centralized controller.

   This document defines extensions to BGP-LS to advertise one or more
   types of MSDs at node and/or link granularity.  Other types of MSDs
   are known to be useful.  For example, [OSPF-ELC] and [ISIS-ELC]
   define Entropy Readable Label Depth (ERLD), which is used by a head-
   end to insert an Entropy Label (EL) at a depth that can be read by
   transit nodes.

   In the future, it is expected that new MSD-Types will be defined to
   signal additional capabilities, e.g., ELs, SIDs that can be imposed
   through recirculation, or SIDs associated with another data plane
   such as IPv6.  MSD advertisements may be useful even if SR itself is
   not enabled.  For example, in a non-SR MPLS network, MSD defines the
   maximum label depth.

1.1.  Conventions Used in This Document

1.1.1.  Terminology

   MSD:  Maximum SID Depth - the number of SIDs supported by a node or a
      link on a node

   PCE:  Path Computation Element

   PCEP:  Path Computation Element Protocol

   SID:  Segment Identifier as defined in [RFC8402]

   SR:  Segment Routing

   Label Imposition:  Imposition is the act of modifying and/or adding
      labels to the outgoing label stack associated with a packet.  This
      includes:

      *  replacing the label at the top of the label stack with a new
         label

      *  pushing one or more new labels onto the label stack

      The number of labels imposed is then the sum of the number of
      labels that are replaced and the number of labels that are pushed.
      See [RFC3031] for further details.

1.1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Advertisement of MSD via BGP-LS

   This document describes extensions that enable BGP-LS speakers to
   signal the MSD capabilities [RFC8491] of nodes and their links in a
   network to a BGP-LS consumer of network topology such as a
   centralized controller.  The centralized controller can leverage this
   information in computation of SR paths based on their MSD
   capabilities.  When a BGP-LS speaker is originating the topology
   learnt via link-state routing protocols such as OSPF or IS-IS, the
   MSD information for the nodes and their links is sourced from the
   underlying extensions as defined in [RFC8476] and [RFC8491],
   respectively.

   The extensions introduced in this document allow for advertisement of
   different MSD-Types, which are defined elsewhere and were introduced
   in [RFC8491].  This enables sharing of MSD-Types that may be defined
   in the future by the IGPs in BGP-LS.

3.  Node MSD TLV

   The Node MSD ([RFC8476] [RFC8491]) is encoded in a new Node Attribute
   TLV [RFC7752] to carry the provisioned SID depth of the router
   identified by the corresponding Router-ID.  Node MSD is the smallest
   MSD supported by the node on the set of interfaces configured for
   use.  MSD values may be learned via a hardware API or may be
   provisioned.  The following format is used:

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: Node MSD TLV Format

   Where:

      Type:  266

      Length:  variable (multiple of 2); represents the total length of
         the value field in octets.

      Value:  consists of one or more pairs of a 1-octet MSD-Type and
         1-octet MSD-Value.

         MSD-Type:  one of the values defined in the "IGP MSD-Types"
            registry defined in [RFC8491].

         MSD-Value:  a number in the range of 0-255.  For all MSD-Types,
            0 represents the lack of ability to impose an MSD stack of
            any depth; any other value represents that of the node.
            This value MUST represent the lowest value supported by any
            link configured for use by the advertising protocol
            instance.

4.  Link MSD TLV

   The Link MSD ([RFC8476] [RFC8491]) is defined to carry the MSD of the
   interface associated with the link.  It is encoded in a new Link
   Attribute TLV [RFC7752] using the following format:

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Link MSD TLV Format

   Where:

      Type:  267

      Length:  variable (multiple of 2); represents the total length of
         the value field in octets.

      Value:  consists of one or more pairs of a 1-octet MSD-Type and
         1-octet MSD-Value.

         MSD-Type:  one of the values defined in the "IGP MSD-Types"
            registry defined in [RFC8491].

         MSD-Value:  a number in the range of 0-255.  For all MSD-Types,
            0 represents the lack of ability to impose an MSD stack of
            any depth; any other value represents that of the link when
            used as an outgoing interface.

5.  IANA Considerations

   IANA has assigned code points from the registry "BGP-LS Node
   Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs"
   based on the table below.

    +==========+=============+===========================+===========+
    | TLV Code | Description | IS-IS TLV/Sub-TLV         | Reference |
    | Point    |             |                           |           |
    +==========+=============+===========================+===========+
    | 266      | Node MSD    | 242/23                    | This      |
    |          |             |                           | document  |
    +----------+-------------+---------------------------+-----------+
    | 267      | Link MSD    | (22,23,25,141,222,223)/15 | This      |
    |          |             |                           | document  |
    +----------+-------------+---------------------------+-----------+

                   Table 1: BGP-LS MSD TLV Code Points

6.  Manageability Considerations

   The new protocol extensions introduced in this document augment the
   existing IGP topology information that is distributed via [RFC7752].
   Procedures and protocol extensions defined in this document do not
   affect the BGP protocol operations and management other than as
   discussed in Section 6 (Manageability Considerations) of [RFC7752].
   Specifically, the malformed attribute tests for syntactic checks in
   Section 6.2.2 (Fault Management) of [RFC7752] now encompass the new
   BGP-LS Attribute TLVs defined in this document.  The semantic or
   content checking for the TLVs specified in this document and their
   association with the BGP-LS Network Layer Reachability Information
   (NLRI) types or their BGP-LS Attribute is left to the consumer of the
   BGP-LS information (e.g., an application or a controller) and not the
   BGP protocol.

   A consumer of the BGP-LS information retrieves this information over
   a BGP-LS session (refer to Sections 1 and 2 of [RFC7752]).

   This document only introduces new Attribute TLVs, and any syntactic
   error in them would result in the BGP-LS Attribute being discarded
   [RFC7752].  The MSD information introduced in BGP-LS by this
   specification, may be used by BGP-LS consumer applications like an SR
   PCE to learn the SR SID stack handling capabilities of the nodes in
   the topology.  This can enable the SR PCE to perform path
   computations taking into consideration the size of SID stack that the
   specific head-end node may be able to impose.  Errors in the encoding
   or decoding of the MSD information may result in the unavailability
   of such information to the SR PCE, or incorrect information being
   made available to it.  This may result in the head-end node not being
   able to instantiate the desired SR path in its forwarding and provide
   the SR-based optimization functionality.  The handling of such errors
   by applications like SR PCE may be implementation specific and out of
   scope of this document.

   The extensions specified in this document do not specify any new
   configuration or monitoring aspects in BGP or BGP-LS.  The
   specification of BGP models is an ongoing work based on the
   [BGP-MODEL].

7.  Security Considerations

   The advertisement of an incorrect MSD value may have negative
   consequences.  If the value is smaller than supported, path
   computation may fail to compute a viable path.  If the value is
   larger than supported, an attempt to instantiate a path that can't be
   supported by the head-end (the node performing the SID imposition)
   may occur.  The presence of this information may also inform an
   attacker of how to induce any of the aforementioned conditions.

   The procedures and protocol extensions defined in this document do
   not affect the BGP security model.  See the "Security Considerations"
   Section of [RFC4271] for a discussion of BGP security.  Also, refer
   to [RFC4272] and [RFC6952] for analyses of security issues for BGP.
   Security considerations for acquiring and distributing BGP-LS
   information are discussed in [RFC7752].  The TLVs introduced in this
   document are used to propagate the MSD IGP extensions defined in
   [RFC8476] and [RFC8491].  It is assumed that the IGP instances
   originating these TLVs will support all the required security (as
   described in [RFC8476] and [RFC8491]) in order to prevent any
   security issues when propagating the TLVs into BGP-LS.  The
   advertisement of the node and link attribute information defined in
   this document presents no significant additional risk beyond that
   associated with the existing node and link attribute information
   already supported in [RFC7752].

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7752]  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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8476]  Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
              DOI 10.17487/RFC8476, December 2018,
              <https://www.rfc-editor.org/info/rfc8476>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

8.2.  Informative References

   [BGP-MODEL]
              Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
              YANG Model for Service Provider Networks", Work in
              Progress, Internet-Draft, draft-ietf-idr-bgp-model-09, 28
              June 2020,
              <https://tools.ietf.org/html/draft-ietf-idr-bgp-model-09>.

   [ISIS-ELC] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using IS-IS", Work in
              Progress, Internet-Draft, draft-ietf-isis-mpls-elc-13, 28
              May 2020,
              <https://tools.ietf.org/html/draft-ietf-isis-mpls-elc-13>.

   [OSPF-ELC] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using OSPF", Work in
              Progress, Internet-Draft, draft-ietf-ospf-mpls-elc-15, 1
              June 2020,
              <https://tools.ietf.org/html/draft-ietf-ospf-mpls-elc-15>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <https://www.rfc-editor.org/info/rfc6952>.

   [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>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

Acknowledgements

   We would like to thank Acee Lindem, Stephane Litkowski, Bruno
   Decraene, and Alvaro Retana for their reviews and valuable comments.

Contributors

   Siva Sivabalan
   Cisco Systems Inc.
   Canada

   Email: msiva@cisco.com


Authors' Addresses

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com


   Uma Chunduri
   Futurewei Technologies

   Email: umac.ietf@gmail.com


   Ketan Talaulikar
   Cisco Systems

   Email: ketant@cisco.com


   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com


   Nikos Triantafillis
   Amazon Web Services

   Email: nikost@amazon.com
