





Internet Engineering Task Force (IETF)                       J. Tantsura
Request for Comments: 8491                                  Apstra, Inc.
Category: Standards Track                                    U. Chunduri
ISSN: 2070-1721                                      Huawei Technologies
                                                               S. Aldrin
                                                            Google, Inc.
                                                             L. Ginsberg
                                                           Cisco Systems
                                                           November 2018


             Signaling Maximum SID Depth (MSD) Using IS-IS

Abstract

   This document defines a way for an Intermediate System to
   Intermediate System (IS-IS) router 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 ID (SID) stack can be
   supported in a given network.  This document only defines one type of
   MSD: Base MPLS Imposition.  However, it defines an encoding that can
   support other MSD types.  This document focuses on MSD use in a
   network that is Segment Routing (SR) enabled, but MSD may also be
   useful when SR is not enabled.

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/rfc8491.












Tantsura, et al.             Standards Track                    [Page 1]

RFC 8491                Signaling MSD Using IS-IS          November 2018


Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) 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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Node MSD Advertisement  . . . . . . . . . . . . . . . . . . .   4
   3.  Link MSD Advertisement  . . . . . . . . . . . . . . . . . . .   5
   4.  Procedures for Defining and Using Node and Link MSD
       Advertisements  . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Base MPLS Imposition MSD  . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

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

   [PCEP-EXT] defines how to signal MSD in the Path Computation Element
   Communication Protocol (PCEP).  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 (Distribution



Tantsura, et al.             Standards Track                    [Page 2]

RFC 8491                Signaling MSD Using IS-IS          November 2018


   of Link-State and TE Information Using Border Gateway Protocol)
   [RFC7752] defines a way to expose topology and associated attributes
   and capabilities of the nodes in that topology to a centralized
   controller.  MSD signaling by BGP-LS has been defined in [MSD-BGP].
   Typically, BGP-LS is configured on a small number of nodes that do
   not necessarily act as head-ends.  In order for BGP-LS to signal MSD
   for all the nodes and links in the network for which MSD is relevant,
   MSD capabilities SHOULD be advertised by every Intermediate System to
   Intermediate System (IS-IS) router in the network.

   Other types of MSDs are known to be useful.  For example, [ELC-ISIS]
   defines Entropy Readable Label Depth (ERLD), which is used by a head-
   end to insert an Entropy Label (EL) at a depth where it can be read
   by transit nodes.

   This document defines an extension to IS-IS used to advertise one or
   more types of MSDs at node and/or link granularity.  It also creates
   an IANA registry for assigning MSD-Type identifiers and defines the
   Base MPLS Imposition MSD-Type.  In the future, it is expected that
   new MSD-Types will be defined to signal additional capabilities,
   e.g., entropy labels, SIDs that can be imposed through recirculation,
   or SIDs associated with another data plane such as IPv6.

   MSD advertisements MAY be useful even if Segment Routing itself is
   not enabled.  For example, in a non-SR MPLS network, MSD defines the
   maximum label depth.

1.1.  Terminology

   BMI:  Base MPLS Imposition is the number of MPLS labels that can be
         imposed inclusive of all service/transport/special labels.

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

   SID:  Segment Identifier is defined in [RFC8402].

   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






Tantsura, et al.             Standards Track                    [Page 3]

RFC 8491                Signaling MSD Using IS-IS          November 2018


   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.

1.2.  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.  Node MSD Advertisement

   The Node MSD sub-TLV is defined within the body of the IS-IS Router
   CAPABILITY TLV [RFC7981] to carry the provisioned SID depth of the
   router originating the IS-IS Router CAPABILITY TLV.  Node MSD is the
   smallest MSD supported by the node on the set of interfaces
   configured for use by the advertising IGP instance.  MSD values may
   be learned via a hardware API or may be provisioned.

                         0                   1
                         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |    Type       |   Length      |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |   MSD-Type    | MSD-Value     |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        //     ...................     //
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |   MSD-Type    | MSD-Value     |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: Node MSD Sub-TLV

   Type: 23

   Length: variable (multiple of 2 octets); represents the total length
   of the Value field

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

   MSD-Type: value defined in the "IGP MSD-Types" registry created by
   the IANA Considerations section of this document Section 6





Tantsura, et al.             Standards Track                    [Page 4]

RFC 8491                Signaling MSD Using IS-IS          November 2018


   MSD-Value: number in the range of 0-255 (for all MSD-Types, 0
   represents the lack of ability to support a SID 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 IS-IS instance.)

   This sub-TLV is optional.  The scope of the advertisement is specific
   to the deployment.

   If there exist multiple Node MSD advertisements for the same MSD-Type
   originated by the same router, the procedures defined in [RFC7981]
   apply.  These procedures may result in different MSD values being
   used, for example, by different controllers.  This does not, however,
   create any interoperability issue.

3.  Link MSD Advertisement

   The Link MSD sub-TLV is defined for TLVs 22, 23, 25, 141, 222, and
   223 to carry the MSD of the interface associated with the link.  MSD
   values may be signaled by the forwarding plane or may be provisioned.

                        0                   1
                        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |    Type       |   Length      |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |   MSD-Type    | MSD-Value     |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        //     ...................     //
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        |   MSD-Type    | MSD-Value     |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 2: Link MSD Sub-TLV

   Type: 15

   Length: variable (multiple of 2 octets); represents the total length
   of the Value field

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

   MSD-Type: value defined in the "IGP MSD-Types" registry created by
   the IANA Considerations section of this document Section 6





Tantsura, et al.             Standards Track                    [Page 5]

RFC 8491                Signaling MSD Using IS-IS          November 2018


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

   This sub-TLV is optional.

   If multiple Link MSD advertisements for the same MSD-Type and the
   same link are received, the procedure to select which copy to use is
   undefined.

   If the advertising router performs label imposition in the context of
   the ingress interface, it is not possible to meaningfully advertise
   per-link values.  In such a case, only the Node MSD SHOULD be
   advertised.

4.  Procedures for Defining and Using Node and Link MSD Advertisements

   When Link MSD is present for a given MSD-Type, the value of the Link
   MSD MUST take precedence over the Node MSD.  If a Link MSD-Type is
   not signaled, but the Node MSD-Type is, then the Node MSD-Type value
   MUST be considered to be the MSD value for that link.

   In order to increase flooding efficiency, it is RECOMMENDED that
   routers with homogenous Link MSD values advertise just the Node MSD
   value.

   The meaning of the absence of both Node and Link MSD advertisements
   for a given MSD-Type is specific to the MSD-Type.  Generally, it can
   only be inferred that the advertising node does not support
   advertisement of that MSD-Type.  In some cases, however, the lack of
   advertisement might imply that the functionality associated with the
   MSD-Type is not supported.  The correct interpretation MUST be
   specified when an MSD-Type is defined.

5.  Base MPLS Imposition MSD

   Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
   labels that can be imposed, including all service/transport/special
   labels.

   The absence of BMI-MSD advertisements indicates only that the
   advertising node does not support advertisement of this capability.








Tantsura, et al.             Standards Track                    [Page 6]

RFC 8491                Signaling MSD Using IS-IS          November 2018


6.  IANA Considerations

   IANA has allocated a sub-TLV type for the new sub-TLV proposed in
   Section 2 of this document from the "Sub-TLVs for TLV 242 (IS-IS
   Router CAPABILITY TLV)" registry as defined by [RFC7981].

   IANA has allocated the following value:

      Value     Description                      Reference
      -----     ---------------                  -------------
      23        Node MSD                         This document

                            Figure 3: Node MSD

   IANA has allocated a sub-TLV type as defined in Section 3 from the
   "Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 (Extended IS
   reachability, IS Neighbor Attribute, L2 Bundle Member Attributes,
   inter-AS reachability information, MT-ISN, and MT IS Neighbor
   Attribute TLVs)" registry.

   IANA has allocated the following value:

      Value     Description                      Reference
      -----     ---------------                  -------------
      15        Link MSD                         This document

                            Figure 4: Link MSD

   Per-TLV information where Link MSD sub-TLV can be part of:

      TLV  22 23 25 141 222 223
      ---  --------------------
           y  y  y   y   y   y

      Figure 5: TLVs Where LINK MSD Sub-TLV Can Be Present
















Tantsura, et al.             Standards Track                    [Page 7]

RFC 8491                Signaling MSD Using IS-IS          November 2018


   IANA has created an IANA-managed registry titled "IGP MSD-Types"
   under the "Interior Gateway Protocol (IGP) Parameters" registry to
   identify MSD-Types as proposed in Sections 2 and 3.  The registration
   procedure is "Expert Review" as defined in [RFC8126].  Types are an
   unsigned 8-bit number.  The following values are defined by this
   document:

      Value     Name                             Reference
      -----     ---------------------            -------------
      0         Reserved                         This document
      1         Base MPLS Imposition MSD         This document
      2-250     Unassigned
      251-254   Experimental Use                 This document
      255       Reserved                         This document

                  Figure 6: MSD-Types Codepoints Registry

   General guidance for the designated experts is defined in [RFC7370].

7.  Security Considerations

   Security considerations as specified by [RFC7981] are applicable to
   this document.

   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.

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>.

   [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>.




Tantsura, et al.             Standards Track                    [Page 8]

RFC 8491                Signaling MSD Using IS-IS          November 2018


   [RFC7370]  Ginsberg, L., "Updates to the IS-IS TLV Codepoints
              Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
              <https://www.rfc-editor.org/info/rfc7370>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [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>.

8.2.  Informative References

   [ELC-ISIS] Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label Depth Using IS-IS", Work in Progress,
              draft-ietf-isis-mpls-elc-06, September 2018.

   [MSD-BGP]  Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
              "Signaling MSD (Maximum SID Depth) using Border Gateway
              Protocol Link-State", Work in Progress, draft-ietf-idr-
              bgp-ls-segment-routing-msd-02, August 2018.

   [PCEP-EXT] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              Work in Progress, draft-ietf-pce-segment-routing-13,
              October 2018.

   [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>.






Tantsura, et al.             Standards Track                    [Page 9]

RFC 8491                Signaling MSD Using IS-IS          November 2018


Acknowledgements

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

Contributors

   The following people contributed to this document:

   Peter Psenak

   Email: ppsenak@cisco.com

Authors' Addresses

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com


   Uma Chunduri
   Huawei Technologies

   Email: uma.chunduri@huawei.com


   Sam Aldrin
   Google, Inc.

   Email: aldrin.ietf@gmail.com


   Les Ginsberg
   Cisco Systems

   Email: ginsberg@cisco.com













Tantsura, et al.             Standards Track                   [Page 10]

