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Versions: (draft-tantsura-isis-segment-routing-msd) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19

IS-IS Working Group                                          J. Tantsura
Internet-Draft                                              Apstra, Inc.
Intended status: Standards Track                             U. Chunduri
Expires: April 12, 2019                              Huawei Technologies
                                                               S. Aldrin
                                                             Google, Inc
                                                             L. Ginsberg
                                                           Cisco Systems
                                                         October 9, 2018


             Signaling MSD (Maximum SID Depth) using IS-IS
                 draft-ietf-isis-segment-routing-msd-19

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 SID (Segment ID) stack can be
   supported in a given network.  This document only defines one type of
   MSD (Base MPLS Imposition), but defines an encoding that can support
   other MSD types.  This document focuses on MSD use in a Segment
   Routing enabled network, but MSD may also be useful when Segment
   Routing is not enabled.

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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 12, 2019.








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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.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   When Segment Routing (SR) 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 of a given SR path
   to ensure that the Segment Identifier (SID) stack depth of a computed
   path does not exceed the number of SIDs the node is capable of
   imposing.

   [I-D.ietf-pce-segment-routing] 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 controller does not participate in IGP
   routing, it has no way to learn the MSD of nodes and links.  BGP-LS



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   (Distribution 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
   [I-D.ietf-idr-bgp-ls-segment-routing-msd].  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 MSD is relevant, MSD capabilities SHOULD be
   advertised by every Intermediate System to Intermediate System(IS-IS)
   router in the network.

   Other types of MSD are known to be useful.  For example,
   [I-D.ietf-isis-mpls-elc] defines Readable Label Depth Capability
   (RLDC) that is used by a head-end to insert an Entropy Label (EL) at
   a depth, that could be read by transit nodes.

   This document defines an extension to IS-IS used to advertise one or
   more types of MSD at node and/or link granularity.  It also creates
   an IANA registry for assigning MSD-type identifiers.  It also 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 dataplane e.g., 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 which can be
   imposed inclusive of all service/transport/special labels

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

   SID: Segment Identifier as 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:

   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






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   The number of labels imposed is then the sum of the number of labels
   which are replaced and the number of labels which 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 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 (allocated by IANA via the early assignment process)

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

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

   MSD-Type is a value defined in the IGP MSD-Types registry created by
   the IANA Section of this document.




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   MSD-Value is a number in the range of 0-255.  For all MSD-Types, 0
   represents lack of the ability to support 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 by (for example) different controllers - but this does not
   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 (allocated by IANA via the early assignment process)

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

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

   MSD-Type is a value defined in the MSD-Types registry created by the
   IANA Section of this document.




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   MSD-Value is a number in the range of 0-255.  For all MSD-Types, 0
   represents lack of the ability to support 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 used to select which copy is
   used 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.  When a Link MSD-type is
   not signaled but the Node MSD-type is, then the Node MSD-type value
   MUST be considered as 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.  However, in some cases 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 which can be imposed, including all service/transport/special
   labels.

   Absence of BMI-MSD advertisements indicates solely that the
   advertising node does not support advertisement of this capability.








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6.  IANA Considerations

   This document requests IANA to allocate a sub-TLV type for the new
   sub TLV proposed in Section 2 of this document from IS-IS Router
   Capability TLV Registry as defined by [RFC7981].

   IANA has allocated the following value through the early assignment
   process:


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

                            Figure 3: Node MSD

   This document requests IANA to allocate a sub-TLV type as defined in
   Section 3 from Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223
   registry.

   IANA has allocated the following value through the early assignment
   process:


      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

   This document requests creation of an IANA managed registry under the
   category of "Interior Gateway Protocol (IGP) Parameters" IANA
   registries to identify MSD-types as proposed in Section 2 and
   Section 3.  The registration procedure is "Expert Review" as defined
   in [RFC8126].  Suggested registry name is "IGP MSD-Types".  Types are
   an unsigned 8 bit number.  The following values are defined by this
   document:





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

                  Figure 6: MSD-Types Codepoints Registry

   General guidance for the Designated Experts is as defined in
   [RFC7370]

7.  Security Considerations

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

   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 also may inform an attacker of how
   to induce any of the aforementioned conditions.

8.  Contributors

   The following people contributed to this document:

   Peter Psenak

   Email: ppsenak@cisco.com

9.  Acknowledgements

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

10.  References








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10.1.  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>.

   [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>.

10.2.  Informative References

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

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



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   [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-12 (work in progress), June
              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>.

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















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