





Internet Engineering Task Force (IETF)                       J. Tantsura
Request for Comments: 8476                                  Apstra, Inc.
Category: Standards Track                                    U. Chunduri
ISSN: 2070-1721                                      Huawei Technologies
                                                               S. Aldrin
                                                            Google, Inc.
                                                               P. Psenak
                                                           Cisco Systems
                                                           December 2018


              Signaling Maximum SID Depth (MSD) Using OSPF

Abstract

   This document defines a way for an Open Shortest Path First (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 Segment Identifier (SID) stack can be supported in a given
   network.  This document only refers to the Signaling MSD as defined
   in RFC 8491, but it defines an encoding that can support other MSD
   types.  Here, the term "OSPF" means both OSPFv2 and OSPFv3.

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/rfc8476.














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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 ....................................................3
      1.1. Terminology ................................................4
      1.2. Requirements Language ......................................4
   2. Node MSD Advertisement ..........................................5
   3. Link MSD Sub-TLV ................................................6
   4. Procedures for Defining and Using Node and Link MSD
      Advertisements ..................................................7
   5. IANA Considerations .............................................7
   6. Security Considerations .........................................8
   7. References ......................................................9
      7.1. Normative References .......................................9
      7.2. Informative References ....................................10
   Acknowledgements ..................................................11
   Contributors ......................................................11
   Authors' Addresses ................................................11



















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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 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.

   [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
   of Link-State and TE Information Using BGP) [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 OSPF 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 OSPF used to advertise one or
   more types of MSDs at node and/or link granularity.  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.













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1.1.  Terminology

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

   BGP-LS:  Distribution of Link-State and TE Information Using BGP

   OSPF:    Open Shortest Path First

   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:

            *  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.

   PCEP:    Path Computation Element Communication Protocol

   SR:      Segment Routing

   LSA:     Link State Advertisement

   RI:      Router Information

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.










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2.  Node MSD Advertisement

   The Node MSD TLV within the body of the OSPF RI Opaque LSA [RFC7770]
   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   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 1: Node MSD TLV

   Type: 12

   Length: variable (multiple of 2 octets); 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 OSPF instance.

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

   When multiple Node MSD TLVs are received from a given router, the
   receiver MUST use the first occurrence of the TLV in the Router
   Information (RI) LSA.  If the Node MSD TLV appears in multiple RI
   LSAs that have different flooding scopes, the Node MSD TLV in the RI
   LSA with the area-scoped flooding scope MUST be used.  If the Node
   MSD TLV appears in multiple RI LSAs that have the same flooding
   scope, the Node MSD TLV in the RI LSA with the numerically smallest
   Instance ID MUST be used and other instances of the Node MSD TLV MUST
   be ignored.  The RI LSA can be advertised at any of the defined



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   opaque flooding scopes (link, area, or Autonomous System (AS)).  For
   the purpose of Node MSD TLV advertisement, area-scoped flooding is
   RECOMMENDED.

3.  Link MSD Sub-TLV

   The Link MSD 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   |  MSD-Value    |  MSD-Type...  |  MSD-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 a type of 6.

      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 a
      type of 9.

   Length: variable; same as defined in Section 2.

   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].

   The MSD-Value field contains the Link MSD of the router originating
   the corresponding LSA as specified for OSPFv2 and OSPFv3.  The Link
   MSD is 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 particular link when used as
   an outgoing interface.

   If this sub-TLV is advertised multiple times for the same link in
   different OSPF Extended Link Opaque LSAs / E-Router-LSAs originated
   by the same OSPF router, the sub-TLV in the OSPFv2 Extended Link




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   Opaque LSA with the smallest Opaque ID or in the OSPFv3 E-Router-LSA
   with the smallest Link State ID MUST be used by receiving OSPF
   routers.  This situation SHOULD be logged as an error.

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.  Per [RFC8491], the correct interpretation
   MUST be specified when an MSD-Type is defined.

5.  IANA Considerations

   This specification updates several existing OSPF registries.

   IANA has allocated TLV type 12 from the "OSPF Router Information (RI)
   TLVs" registry as defined by [RFC7770].

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

                           Figure 3: RI Node MSD

   IANA has allocated sub-TLV type 6 from the "OSPFv2 Extended Link TLV
   Sub-TLVs" registry.

      Value     Description                      Reference
      -----     ---------------                  -------------
      6         OSPFv2 Link MSD                  This document

                         Figure 4: OSPFv2 Link MSD







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   IANA has allocated sub-TLV type 9 from the "OSPFv3 Extended-LSA
   Sub-TLVs" registry.

      Value     Description                      Reference
      -----     ---------------                  -------------
      9         OSPFv3 Link MSD                  This document

                         Figure 5: OSPFv3 Link MSD

6.  Security Considerations

   Security concerns for OSPF are addressed in [RFC7474], [RFC4552], and
   [RFC7166].  Further security analysis for the OSPF protocol is done
   in [RFC6863].  Security considerations as specified by [RFC7770],
   [RFC7684], and [RFC8362] are applicable to this document.

   Implementations MUST ensure that malformed TLVs and sub-TLVs defined
   in this document are detected and do not provide a vulnerability for
   attackers to crash the OSPF router or routing process.  Reception of
   malformed TLVs or sub-TLVs SHOULD be counted and/or logged for
   further analysis.  Logging of malformed TLVs and sub-TLVs SHOULD be
   rate-limited to prevent a Denial-of-Service (DoS) attack (distributed
   or otherwise) from overloading the OSPF control plane.

   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.

   There's no DoS risk specific to this extension, and it is not
   vulnerable to replay attacks.















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7.  References

7.1.  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>.

   [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>.

   [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>.

   [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>.

   [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>.








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7.2.  Informative References

   [ELC-ISIS] Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label-stack Depth Using OSPF", Work in Progress,
              draft-ietf-ospf-mpls-elc-07, 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-14,
              October 2018.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC6863]  Hartman, S. and D. Zhang, "Analysis of OSPF Security
              According to the Keying and Authentication for Routing
              Protocols (KARP) Design Guide", RFC 6863,
              DOI 10.17487/RFC6863, March 2013,
              <https://www.rfc-editor.org/info/rfc6863>.

   [RFC7166]  Bhatia, M., Manral, V., and A. Lindem, "Supporting
              Authentication Trailer for OSPFv3", RFC 7166,
              DOI 10.17487/RFC7166, March 2014,
              <https://www.rfc-editor.org/info/rfc7166>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [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>.









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Acknowledgements

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

Contributors

   The following person contributed to this document:

   Les Ginsberg

   Email: ginsberg@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


   Peter Psenak
   Cisco Systems

   Email: ppsenak@cisco.com













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