OSPF Working Group                                           J. Tantsura
Internet-Draft                                                Individual
Intended status: Standards Track                             U. Chunduri
Expires: June 17, 2018                               Huawei Technologies
                                                               S. Aldrin
                                                             Google, Inc
                                                               P. Psenak
                                                           Cisco Systems
                                                       December 14, 2017


              Signaling MSD (Maximum SID Depth) using OSPF
                 draft-ietf-ospf-segment-routing-msd-07

Abstract

   This document proposes a way to signal Maximum SID Depth (MSD)
   supported by a node at node and/or link granularity by an OSPF
   Router.  In a Segment Routing (SR) enabled network a centralized
   controller that programs SR tunnels needs to know the MSD supported
   by the head-end at node and/or link granularity to impose the SID
   stack of an appropriate depth.  MSD is relevant to the head-end of a
   SR tunnel or Binding-SID anchor node where Binding-SID expansions
   might result in creation of a new SID stack.  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.

   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 June 17, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




Tantsura, et al.          Expires June 17, 2018                 [Page 1]


Internet-Draft                                             December 2017


   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.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Node MSD TLV  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Link MSD sub-TLV  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Node MSD vs Link MSD conflict resolution  . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   When Segment Routing tunnels are computed by a centralized
   controller, it is critical that the controller learns the MSD
   "Maximum SID Depth" of the node or link SR tunnel exits over, so the
   SID stack depth of a path computed doesn't exceed the number of SIDs
   the node is capable of imposing.  This document describes how to use
   OSPF to signal the MSD of a node or link to a centralized controller.

   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 a 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 which has been
   configured.  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



Tantsura, et al.          Expires June 17, 2018                 [Page 2]


Internet-Draft                                             December 2017


   necessarily act as head-ends.  In order, for BGP-LS to signal MSD for
   the all nodes and links in the network MSD is relevant, MSD
   capabilites SHOULD be distributed to every OSPF router in the
   network.

   [I-D.ietf-ospf-mpls-elc] defines Readable Label Depth Capability
   (RLDC) that is used by a head-end to insert Entropy Label (EL) at
   appropriate depth, so it could be read by transit nodes.  MSD in
   contrary signals ability to impose SID's stack of a particular depth.

   MSD of type 1 (IANA Registry), called Base MSD is used to signal the
   total number of SIDs a node is capable of imposing, to be used by a
   path computation element/controller.  In case, there are additional
   SIDs (e.g. service) that are to be imposed to the stack - this would
   be signaled with an another MSD type (TBD), no adjustment to the Base
   MSD should be made.  In the future, new MSD types could be defined to
   signal additional capabilities: entropy labels, SIDs that can be
   imposed thru recirculation, or another dataplane e.g IPv6.

1.1.  Conventions used in this document

1.1.1.  Terminology

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

   OSPF: Open Shortest Path First

   MSD: Maximum SID Depth

   PCC: Path Computation Client

   PCE: Path Computation Element

   PCEP: Path Computation Element Protocol

   SID: Segment Identifier

   SR: Segment routing

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].






Tantsura, et al.          Expires June 17, 2018                 [Page 3]


Internet-Draft                                             December 2017


2.  Terminology

   This memo makes use of the terms defined in [RFC4970].

3.  Node MSD TLV

   A new TLV within the body of the OSPF RI Opaque LSA, called Node MSD
   TLV is defined to carry the provisioned SID depth of the router
   originating the RI LSA.  Node MSD is the lowest MSD supported by the
   node.

        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 ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...


                          Figure 1: Node MSD TLV

   The Type (2 bytes) of this TLV has value of 12.

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

   Value field consists of a 1 octet sub-type (IANA Registry) and 1
   octet value.

   Sub-Type 1 (IANA Section), MSD and the Value field contains maximum
   MSD of the router originating the RI LSA.  Node Maximum MSD is a
   number in the range of 0-254. 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 lowest value supported by
   node.

   Other Sub-types other than defined above 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.







Tantsura, et al.          Expires June 17, 2018                 [Page 4]


Internet-Draft                                             December 2017


4.  Link MSD sub-TLV

   A new sub-TLV called Link MSD sub-TLV is defined to carry the
   provisioned SID depth of the interface associated with the link.

        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 ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...


                        Figure 2: Link MSD Sub-TLV

   The Type (2 bytes) of this TLV:

   For OSPFv2, the Link level MSD value is advertised as an optional
   Sub-TLV of OSPFv2 Extended Link TLV as defined in [RFC7684], and has
   value of 6.

   For OSPFv3, the Link level MSD value is advertised as an optional
   Sub-TLV of the Router-Link TLV as defined in
   [I-D.ietf-ospf-ospfv3-lsa-extend], and has value of 3 (Suggested
   value - to be assigned by IANA).

   Length is variable and similar to what is defined in Section 3.

   Value field consists of a 1 octet sub-type (IANA Registry) and 1
   octet value.

   Sub-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 0-254. 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 Sub-types other than defined above are reserved for future
   extensions.

5.  Node MSD vs Link MSD conflict resolution

   When both Node MSD and Link MSD are present, the value in the Link
   MSD MUST be used.





Tantsura, et al.          Expires June 17, 2018                 [Page 5]


Internet-Draft                                             December 2017


6.  IANA Considerations

   This document includes a request to IANA to allocate TLV type codes
   for the new TLV proposed in Section 3 of this document from OSPF
   Router Information (RI) TLVs Registry as defined by [RFC4970].  For
   THE link MSD, we request IANA to allocate new sub-TLV codes as
   proposed in Section 4 from OSPFv2 Extended Link TLV Sub-TLVs registry
   and from Router-Link TLV defined in OSPFv3 Extend-LSA Sub-TLV
   registry.

   This document also requests IANA to create a new Sub-type registry as
   proposed in Section 3, Section 4.


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

                Figure 3: MSD Sub-type Codepoints Registry

7.  Security Considerations

   This document describes a mechanism to signal Segment Routing MSD
   supported at node and/or link granularity through OSPF LSA's and does
   not introduce any new security issues.

8.  Contributors

   The following people contributed to this document:

   Les Ginsberg

   Email: ginsberg@cisco.com

9.  Acknowledgements

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

10.  References







Tantsura, et al.          Expires June 17, 2018                 [Page 6]


Internet-Draft                                             December 2017


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>.

   [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>.

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 Using
              OSPF", draft-ietf-ospf-mpls-elc-04 (work in progress),
              November 2016.

   [I-D.ietf-ospf-ospfv3-lsa-extend]
              Lindem, A., Roy, A., Goethals, D., Vallem, V., and F.
              Baker, "OSPFv3 LSA Extendibility", draft-ietf-ospf-ospfv3-
              lsa-extend-18 (work in progress), November 2017.

   [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>.





Tantsura, et al.          Expires June 17, 2018                 [Page 7]


Internet-Draft                                             December 2017


   [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
   Individual

   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





















Tantsura, et al.          Expires June 17, 2018                 [Page 8]