IDR Working Group                                            J. Tantsura
Internet-Draft                                              Apstra, Inc.
Intended status: Standards Track                             U. Chunduri
Expires: August 23, December 3, 2019                                      Huawei USA                         Futurewei Technologies
                                                           K. Talaulikar
                                                           Cisco Systems
                                                               G. Mirsky
                                                               ZTE Corp.
                                                            S. Sivabalan
                                                                   Cisco
                                                        N. Triantafillis
                                                            Apstra, Inc.
                                                       February 19,
                                                            June 1, 2019

 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-
                                 State
              draft-ietf-idr-bgp-ls-segment-routing-msd-04
              draft-ietf-idr-bgp-ls-segment-routing-msd-05

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

Status of This Memo

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   This Internet-Draft will expire on August 23, December 3, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3   4
   2.  Problem Statement . .  Advertisement of MSD via BGP-LS . . . . . . . . . . . . . . .   4
   3.  Node MSD TLV  . . . . .   3
   3.  MSD supported by a node . . . . . . . . . . . . . . . . . . .   4
   4.  Link MSD supported on a link TLV  . . . . . . . . . . . . . . . . . . .   4 . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5   6
   7.  Acknowledgements  Contributors  . . . . . . . . . . . . . . . . . . . . . .   5 . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7   8

1.  Introduction

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

   [I-D.ietf-pce-segment-routing] defines how to use
   BGP-LS to signal the MSD of a node or link to a centralized
   controller.

   PCEP SR in the Path
   Computation Element Protocol (PCEP).  The OSPF and IS-IS extensions draft [I-D.ietf-pce-segment-routing] signals
   for signaling of MSD are defined in SR PCE Capability TLV [RFC8476] and METRIC Object. [RFC8491]
   respectively.

   However, if PCEP is not supported/configured on the head-end of a SR
   tunnel or a Binding-SID anchor node node, and controller does not
   participate in IGP routing, it has no way to learn of learning the MSD of
   nodes and links which has been
   configured. links.  BGP-LS [RFC7752] defines a way to expose advertise
   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 MSD are known to be useful.  For example,
   [I-D.ietf-ospf-mpls-elc] and [I-D.ietf-isis-mpls-elc] define Readable
   Label Depth Capability (RLDC) that 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

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

   MSD: Maximum SID Depth

   PCC: Path Computation Client

   PCE: Path Computation Element

   PCEP: Path Computation Element Protocol

   SID: Segment Identifier

   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:

   o  replacing the label at the top of the label stack with a new
      label.

   o  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.  Problem Statement

   In existing technology only PCEP has extension  Advertisement of MSD via BGP-LS

   This document describes extensions that enable BGP-LS speakers to
   signal the MSD (SR
   PCE Capability TLV/ METRIC Object capabilities of nodes and their links in a network to
   a BGP-LS consumer of network topology such as defined a centralized
   controller.  The centralized controller can leverage this information
   in
   [I-D.ietf-pce-segment-routing],If PCEP is not supported by the node
   (head-end computation of the SR tunnel) controller has no way to learn the paths and their instantiation on network nodes
   based on their MSD of capabilities.  When a BGP-LS speaker is
   originating the node/link configured. topology learnt via link-state routing protocols like
   OSPF or IS-IS, the MSD information for the nodes and IS-IS their links is
   sourced from the underlying extensions are as defined in:

   [RFC8476], in [RFC8476] and
   [RFC8491]

3. respectively.  The BGP-LS speaker may also advertise the
   MSD supported by a information for the local node and its links when not running any
   link-state IGP protocol e.g. when running BGP as the only routing
   protocol.

   The extensions introduced in this document allow for advertisement of
   different MSD-Types.  This document does not define these MSD-Types
   but leverages the definition, guidelines and the code-point registry
   specified 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

   Node MSD is encoded in a new Node Attribute TLV, as defined in 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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Sub-Type and Value ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: Node attribute format

   Type : A 2-octet field specifying code-point of the new MSD TLV type.
   Code-point:(TBD1) from BGP-LS Node Descriptor, Link Descriptor,
   Prefix Descriptor, and Attribute TLVs registry Format

   Where:

   o  Type: 266

   o  Length: A 2-octet field that indicates variable (multiple of 2); represents the total length of
      the value
   portion

   Sub-Type field in octets.

   o  Value : consists of one or more pairs of a 1-octet MSD-Type and value fields are as
      1-octet MSD-Value.

      *  MSD-Type : one of the values defined in corresponding OSPF
   [RFC8476] and IS-IS [RFC8491] extensions.

4. the IANA registry
         titled "IGP MSD-Types" under the "Interior Gateway Protocol
         (IGP) Parameters" registry created by [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 on a by any link
         configured for use by the advertising protocol instance.

4.  Link MSD TLV

   Link MSD is encoded in a New new Link Attribute TLV, as defined in 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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Sub-Type and Value ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Link attribute format

   Type : A 2-octet field specifying code-point of the new MSD TLV type.
   Code-point:(TBD2) from BGP-LS Node Descriptor, Link Descriptor,
   Prefix Descriptor, and Attribute TLVs registry Format

   Where:

   o  Type: 267
   o  Length: A 2-octet field that indicates variable (multiple of 2); represents the total length of
      the value
   portion
   Sub-Type field in octets.

   o  Value : consists of one or more pairs of a 1-octet MSD-Type and value fields are as
      1-octet MSD-Value.

      *  MSD-Type : one of the values defined in corresponding OSPF
   [RFC8476] and IS-IS [RFC8491] extensions. the IANA registry
         titled "IGP MSD-Types" under the "Interior Gateway Protocol
         (IGP) Parameters" registry created by [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

   We request IANA assign code points

   This document requests assigning code-points from the registry BGP-LS "BGP-
   LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs,
   as follows: TLV
   TLVs" based on table below.  Early allocation for these code-points
   have been done by IANA.

       +------------+-----------------+---------------------------+
       | Code Point |   Description   |     IS-IS TLV/Sub-TLV Reference
   TBD1     |
       +------------+-----------------+---------------------------+
       |    266     | Node MSD        | 242/23 (this document) TBD2                    |
       |    267     | Link MSD        | (22,23,25,141,222,223)/15 (this document) |
       +------------+-----------------+---------------------------+

6.  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.

   This document does not introduce security issues beyond those
   discussed in [RFC7752], [RFC8476] and [RFC8491]

7.  Contributors
   Siva Sivabalan
   Cisco Systems Inc.
   Canada

   Email: msiva@cisco.com

8.  Acknowledgements

   We like to thank Acee Lindem, Ketan Talaulikar, Stephane Litkowski and Bruno Decraene
   for their reviews and valuable comments.

8.

9.  References

8.1.

9.1.  Normative References

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-15 (work in progress),
              February 2019.

   [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.

9.2.  Informative References

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Psenak, P., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label Depth Using IS-IS", draft-ietf-isis-mpls-
              elc-06
              elc-07 (work in progress), September 2018.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., and B. Decraene, "IS-IS Extensions for
              Segment Routing", draft-ietf-isis-segment-routing-
              extensions-22 (work in progress), December 2018. May 2019.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Psenak, P., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-07
              mpls-elc-08 (work in progress), September 2018.

   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, May 2019.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Gredler, H.,
              Shakir, R., Tantsura, J., Henderickx, W.,
              and J. Tantsura, "OSPF Hardwick, "PCEP Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-27
              draft-ietf-pce-segment-routing-16 (work in progress), December 2018.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy,
              March 2019.

   [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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-18
              (work in progress), December 2018. Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

Authors' Addresses

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com

   Uma Chunduri
   Huawei USA
   Futurewei Technologies

   Email: umac.ietf@gmail.com

   Ketan Talaulikar
   Cisco Systems

   Email: uma.chunduri@huawei.com ketant@cisco.com

   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com

   Siva Sivabalan
   Cisco

   Email: msiva@cisco.com
   Nikos Triantafillis
   Apstra, Inc.

   Email: nikos@apstra.com