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Versions: (draft-tantsura-isis-segment-routing-msd) 00 01 02 03 04

IS-IS Working Group                                          J. Tantsura
Internet-Draft                                                Individual
Intended status: Standards Track                             U. Chunduri
Expires: December 6, 2017                            Huawei Technologies
                                                               S. Aldrin
                                                             Google, Inc
                                                             L. Ginsberg
                                                           Cisco Systems
                                                           June 04, 2017


             Signaling MSD (Maximum SID Depth) using IS-IS
                 draft-ietf-isis-segment-routing-msd-04

Abstract

   This document proposes a way to signal Maximum SID Depth (MSD)
   supported by a node at node and/or link granularity by an IS-IS
   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 push 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.

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 December 6, 2017.

Copyright Notice

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





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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 Advertisement  . . . . . . . . . . . . . . . . . . .   4
   4.  LINK MSD Advertisement  . . . . . . . . . . . . . . . . . . .   4
   5.  Node MSD vs Link MSD conflict resolution  . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   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
   IS-IS 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.tantsura-idr-bgp-ls-segment-routing-msd].  Typically,



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   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
   the all nodes and links in the network MSD is relevant, MSD
   capabilites SHOULD be distributed to every IS-IS router in the
   network.

   [I-D.ietf-isis-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 push 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
   labels (e.g. service) that are to be pushed 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, labels that can be
   pushed 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

   IS-IS: Intermediate System to Intermediate System

   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].





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2.  Terminology

   This memo makes use of the terms defined in [RFC7981].

3.  Node MSD Advertisement

   A new sub-TLV within the body of IS-IS Router Capability TLV
   [RFC7981], Node MSD sub-TLV is defined to carry the provisioned MSD
   of the router originating the Router Capability TLV.  Node MSD is the
   lowest MSD supported by the node of any interface and can be
   provisioned in IS-IS instance.

        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 Sub-TLV

   The Type (1 byte) of this sub-TLV is 23 (Suggested value - to be
   assigned by IANA).

   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 Router Capability TLV.  Node
   Maximum MSD is a number in the range of 0-254. 0 represents lack of
   the ability to push MSD 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 sub-TLV is optional.  The scope of the
   advertisement is specific to the deployment.

4.  LINK MSD Advertisement

   A new sub-TLV - Link MSD sub-TLV is defined for TLVs 22, 23, 141,
   222, and 223 to carry the provisioned MSD of the interface associated
   with the link.




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        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 (1 byte) of this sub-TLV is 15 (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 TLV's 22, 23, 141, 222,
   and 223.  Link MSD is a number in the range of 0-254. 0 represents
   lack of the ability to push MSD of any depth; any other value
   represents that of the particular link MSD value.

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.

6.  IANA Considerations

   This document includes a request to IANA to allocate sub-TLV type
   codes for the new sub TLV proposed in Section 3 of this document from
   IS-IS Router Capability TLV Registry as defined by [RFC7981].

   Type: 23 (suggested - to be assigned by IANA)

   Description: Node MSD

   Also for link MSD, we request IANA to allocate new sub-TLV codes as
   defined in Section 4 from Sub-TLVs for TLVs 22, 23, 141, 222 and 223
   registry.

   Type: 15 (suggested - to be assigned by IANA)

   Description: Link MSD

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



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

           Figure 3: TLVs where LINK MSD Sub-TLV can be present

   This document requests the creation of a new IANA managed registry to
   identify MSD types as proposed in Section 3, Section 4.  The
   registration procedure is "Expert Review" as defined in [RFC5226].
   Suggested registry name is "MSD Sub-types".  Types are an unsigned 8
   bit number.  The following values are defined by this document


      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 4: 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 IS-IS LSPs and does
   not introduce any new security issues.

8.  Contributors

   The following people contributed to this document:

   Peter Psenak

   Email: ppsenak@cisco.com

9.  Acknowledgements

   The authors would like to thank 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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <http://www.rfc-editor.org/info/rfc5305>.

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

10.2.  Informative References

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability Using IS-
              IS", draft-ietf-isis-mpls-elc-02 (work in progress),
              October 2016.

   [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-09 (work in progress),
              April 2017.

   [I-D.tantsura-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-tantsura-idr-bgp-ls-segment-routing-
              msd-04 (work in progress), March 2017.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <http://www.rfc-editor.org/info/rfc1195>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <http://www.rfc-editor.org/info/rfc5120>.






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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [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,
              <http://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


   Les Ginsberg
   Cisco Systems

   Email: ginsberg@cisco.com
















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