Signaling MSD (Maximum SID Depth) using OSPF
draft-ietf-ospf-segment-routing-msd-11

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OSPF Working Group                                           J. Tantsura
Internet-Draft                                            Nuage Networks
Intended status: Standards Track                             U. Chunduri
Expires: November 8, 2018                            Huawei Technologies
                                                               S. Aldrin
                                                             Google, Inc
                                                               P. Psenak
                                                           Cisco Systems
                                                            May 07, 2018

              Signaling MSD (Maximum SID Depth) using OSPF
                 draft-ietf-ospf-segment-routing-msd-11

Abstract

   This document defines a way for an OSPF 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 stack can be
   supported in a given network.  This document defines only one type of
   MSD, but defines an encoding that can support other MSD types.  Here
   the term OSPF means both OSPFv2 and OSPFv3.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 8, 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Node MSD Advertisement  . . . . . . . . . . . . . . . . . . .   4
   3.  Link MSD sub-TLV  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Using Node and Link MSD Advertisements  . . . . . . . . . . .   6
   5.  Base MPLS Imposition MSD  . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 on a given SR path
   to insure that the SID stack depth of a computed path doesn't exceed
   the number of SIDs the node is capable of imposing.

   The PCEP SR extensions draft [I-D.ietf-pce-segment-routing] signals
   MSD in SR PCE Capability TLV and METRIC Object.  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
   [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 capabilites should be
   advertised by every OSPF router in the network.

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   Other types of MSD are known to be useful.  For example,
   [I-D.ietf-ospf-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 can be read by transit nodes.

   This document defines an extension to OSPF 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.  Although MSD
   advertisements are associated with Segment Routing, the
   advertisements MAY be present even if Segment Routing itself is not
   enabled.

1.1.  Conventions used in this document

1.1.1.  Terminology

   This memo makes use of the terms defined in [RFC7770]

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

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

   OSPF: Open Shortest Path First

   MSD: Maximum SID Depth - the number of SIDs a node or one of its
   links can support

   PCC: Path Computation Client

   PCE: Path Computation Element

   PCEP: Path Computation Element Protocol

   SR: Segment Routing

   SID: Segment Identifier

   LSA: Link state advertisement

   RI: Router Information LSA

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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
   BCP14 [RFC2119], [RFC8174] when, and only when they appear in all
   capitals, as shown here .

2.  Node MSD Advertisement

   The node MSD TLV within the body of the OSPF RI Opaque LSA is defined
   to carry the provisioned SID depth of the router originating the RI
   LSA.  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                   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 and Value ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                          Figure 1: Node MSD TLV

   The Type: TBD1

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

   Value: consists of one or more pairs of a 1 octet sub-type (IANA
   Registry) and 1 octet value.

   MSD Type 1 (IANA Section), MSD and the Value field contains the MSD
   of the originating router.  Node MSD is a number in the range of
   0-255. 0 represents lack of the ability to impose MSD stack of any
   depth; any other value represents that of the node.  This value
   SHOULD represent the minimum value supported by a node.

   Other MSD Types are reserved for future extensions.

   This TLV is applicable to OSPFv2 and to OSPFv3 [RFC5838] and is
   optional.  The scope of the advertisement is specific to the
   deployment.

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3.  Link MSD sub-TLV

   The link sub-TLV is defined to carry the MSD of the interface
   associated with the link.  MSD values may be learned via a hardware
   API or may be provisioned.

        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 and Value ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                        Figure 2: Link MSD Sub-TLV

   Type:

   For OSPFv2, the Link level MSD value is advertised as an optional
   Sub-TLV of the OSPFv2 Extended Link TLV as defined in [RFC7684], and
   has value of TBD2.

   For OSPFv3, the Link level MSD value is advertised as an optional
   Sub-TLV of the E-Router-LSA TLV as defined in [RFC8362], and has
   value of TBD3.

   Length: variable and similar to that, defined in Section 2.

   Value: consists of one or more pairs of a 1 octet MSD Type (IANA
   Registry) and 1 octet value.

   MSD Type 1 (IANA Section), MSD and the Value field contains Link MSD
   of the router originating the corresponding LSA as specified for
   OSPFv2 and OSPFv3.  Link MSD is a number in the range of 0-255. 0
   represents lack of the ability to impose MSD stack of any depth; any
   other value represents that of the particular link MSD value.

   Other MSD Types are reserved for future extensions.

   If these TLVs are advertised multiple times, only the first instance
   of the TLV is used by receiving OSPF routers.  This situation SHOULD
   be logged as an error.

   If these TLV is advertised multiple times for the same link in
   different LSAs originated by the same OSPF router, the TLV with the

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   smallest Opaque ID/Link State ID is used by receiving OSPF routers.
   This situation MAY be logged as a warning.

4.  Using Node and Link MSD Advertisements

   When Link MSD is present for a given MSD type, the value of the Link
   MSD MUST take preference over the Node MSD.  When a Link MSD type is
   not signalled but the Node MSD type is, then the value of that Link
   MSD type MUST be considered as the corresponding Node MSD type value.
   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

   The Base MPLS Imposition MSD (BMI-MSD) signals the total number of
   MPLS labels a node is capable of imposing, including any service/
   transport labels.

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

6.  IANA Considerations

   This document requests IANA to allocate TLV type (TBD1) from the OSPF
   Router Information (RI) TLVs Registry as defined by [RFC4970].  IANA
   has allocated the value 12 through the early assignment process.
   Also, this document requests IANA to allocate a sub-TLV type (TBD2)
   from the OSPFv2 Extended Link TLV Sub-TLVs registry.  IANA has
   allocated the the value 6 through the early assignment process.
   Finally, this document requests IANA to allocate a sub-TLV type
   (TBD3) from the OSPFv3 Extended-LSA Sub-TLV registry.

   This document requests creation of an IANA managed registry under a
   new category of "Interior Gateway Protocol (IGP) Parameters" IANA
   registries to identify MSD types as proposed in Section 2, Section 3.
   The registration procedure is "Expert Review" as defined in
   [RFC8126].  The suggested registry name is "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                     This document
              255       Reserved                         This document

                  Figure 3: MSD Types Codepoints Registry

7.  Security Considerations

   Security concerns for OSPF are addressed in [RFC7474] and [RFC5310].
   Further security analysis for OSPF protocol is done in [RFC6853]
   including analysis of both the above documents.  Security
   considerations, as specified by [RFC7770] are applicable to this
   document.

   Advertisement of the additional information defined in this document
   that is false, e.g.  MSD that is incorrect may result: in a path
   computation failing and the service unavailable or instantiation of a
   path that can't be supported by the head-end (the node performing the
   imposition).

8.  Contributors

   The following people contributed to this document:

   Les Ginsberg

   Email: ginsberg@cisco.com

9.  Acknowledgements

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

10.  References

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>.

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   [RFC4970]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 4970, DOI 10.17487/RFC4970, July
              2007, <https://www.rfc-editor.org/info/rfc4970>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

10.2.  Informative References

   [I-D.ietf-idr-bgp-ls-segment-routing-msd]
              Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
              "Signaling Maximum SID Depth using Border Gateway Protocol
              Link-State", draft-ietf-idr-bgp-ls-segment-routing-msd-01
              (work in progress), October 2017.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and
              Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-05 (work in progress), January 2018.

   [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-11 (work in progress),
              November 2017.

   [RFC5838]  Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
              R. Aggarwal, "Support of Address Families in OSPFv3",
              RFC 5838, DOI 10.17487/RFC5838, April 2010,
              <https://www.rfc-editor.org/info/rfc5838>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

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   [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>.

   [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>.

Authors' Addresses

   Jeff Tantsura
   Nuage Networks

   Email: jefftant.ietf@gmail.com

   Uma Chunduri
   Huawei Technologies

   Email: uma.chunduri@huawei.com

   Sam Aldrin
   Google, Inc

   Email: aldrin.ietf@gmail.com

   Peter Psenak
   Cisco Systems

   Email: ppsenak@cisco.com

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