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Versions: (draft-tantsura-idr-bgp-ls-segment-routing-msd) 00 01 02 03 04 05 06 07 08

IDR Working Group                                            J. Tantsura
Internet-Draft                                              Apstra, Inc.
Intended status: Standards Track                             U. Chunduri
Expires: March 14, 2020                           Futurewei Technologies
                                                           K. Talaulikar
                                                           Cisco Systems
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        N. Triantafillis
                                                            Apstra, Inc.
                                                      September 11, 2019


 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-
                                 State
              draft-ietf-idr-bgp-ls-segment-routing-msd-07

Abstract

   This document defines a way for a Border Gateway Protocol Link-State
   (BGP-LS) speaker 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.

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 March 14, 2020.








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Copyright Notice

   Copyright (c) 2019 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.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   4
   2.  Advertisement of MSD via BGP-LS . . . . . . . . . . . . . . .   4
   3.  Node MSD TLV  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Link MSD TLV  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   When Segment Routing (SR) [RFC8402] 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.  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.

   [I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path
   Computation Element Protocol (PCEP).  The OSPF and IS-IS extensions
   for signaling of MSD are defined in [RFC8476] and [RFC8491]
   respectively.




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   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 of learning the MSD of
   nodes and links.  BGP-LS [RFC7752] defines a way to advertise
   topology and associated attributes and capabilities of the nodes in
   that topology to a centralized controller.  This document defines
   extensions to BGP-LS to advertise one or more types of MSDs at node
   and/or link granularity.

   Other types of MSD are known to be useful.  For example,
   [I-D.ietf-ospf-mpls-elc] and [I-D.ietf-isis-mpls-elc] define 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.

   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.

1.1.  Conventions used in this document

1.1.1.  Terminology

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

   MSD: Maximum SID Depth

   PCC: Path Computation Client

   PCE: Path Computation Element

   PCEP: Path Computation Element Protocol

   SID: Segment Identifier

   SR: Segment routing

   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.





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   o  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.

1.1.2.  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.  Advertisement of MSD via BGP-LS

   This document describes extensions that enable BGP-LS speakers to
   signal the MSD capabilities (described in [RFC8491] ) of nodes and
   their links in a network to a BGP-LS consumer of network topology
   such as a centralized controller.  The centralized controller can
   leverage this information in computation of SR paths and their
   instantiation on network nodes based on their MSD capabilities.  When
   a BGP-LS speaker is originating the topology learnt via link-state
   routing protocols like OSPF or IS-IS, the MSD information for the
   nodes and their links is sourced from the underlying extensions as
   defined in [RFC8476] and [RFC8491] respectively.  The BGP-LS speaker
   may also advertise the MSD information for the local node and its
   links when not running any link-state IGP protocol e.g. when running
   BGP as the only routing protocol.

   The extensions introduced in this document allow for advertisement of
   different MSD-Types.  This document does not define these MSD-Types
   but leverages the definition, guidelines and the code-point registry
   specified in [RFC8491].  This enables sharing of MSD-Types that may
   be defined in the future by the IGPs in BGP-LS.

3.  Node MSD TLV

   Node MSD is encoded in a new Node Attribute TLV [RFC7752] using the
   following format:












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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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    MSD-Type   |  MSD-Value    |  MSD-Type...  |  MSD-Value... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: Node MSD TLV Format

   Where:

   o  Type: 266

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

   o  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 IANA registry
         titled "IGP MSD-Types" under the "Interior Gateway Protocol
         (IGP) Parameters" registry created by [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 protocol instance.

4.  Link MSD TLV

   Link MSD is encoded in a new Link Attribute TLV [RFC7752] using the
   following format:

      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 TLV Format

   Where:

   o  Type: 267



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

   o  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 IANA registry
         titled "IGP MSD-Types" under the "Interior Gateway Protocol
         (IGP) Parameters" registry created by [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 link when used as
         an outgoing interface.

5.  IANA Considerations

   This document requests assigning code-points from the registry "BGP-
   LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
   TLVs" based on table below.  Early allocation for these code-points
   have been done by IANA.

       +------------+-----------------+---------------------------+
       | Code Point |   Description   |     IS-IS TLV/Sub-TLV     |
       +------------+-----------------+---------------------------+
       |    266     | Node MSD        | 242/23                    |
       |    267     | Link MSD        | (22,23,25,141,222,223)/15 |
       +------------+-----------------+---------------------------+


6.  Manageability Considerations

   The new protocol extensions introduced in this document augment the
   existing IGP topology information that is distributed via [RFC7752].
   Procedures and protocol extensions defined in this document do not
   affect the BGP protocol operations and management other than as
   discussed in the Manageability Considerations section of [RFC7752].
   Specifically, the malformed attribute tests for syntactic checks in
   the Fault Management section of [RFC7752] now encompass the new BGP-
   LS Attribute TLVs defined in this document.  The semantic or content
   checking for the TLVs specified in this document and their
   association with the BGP-LS NLRI types or their BGP-LS Attribute is
   left to the consumer of the BGP-LS information (e.g. an application
   or a controller) and not the BGP protocol.

   A consumer of the BGP-LS information retrieves this information over
   a BGP-LS session (refer Section 1 and 2 of [RFC7752]).  The handling
   of semantic or content errors by the consumer would be dictated by



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   the nature of its application usage and hence is beyond the scope of
   this document.

   This document only introduces new Attribute TLVs and any syntactic
   error in them would result in the BGP-LS Attribute being discarded
   with an error log.  The MSD information introduced in BGP-LS by this
   specification, may be used by BGP-LS consumer applications like a SR
   path computation engine (PCE) to learn the SR SID-stack handling
   capabilities of the nodes in the topology.  This can enable the SR
   PCE to perform path computations taking into consideration the size
   of SID Stack that the specific headend node may be able to impose.
   Errors in the encoding or decoding of the MSD information may result
   in the unavailability of such information to the SR PCE or incorrect
   information being made available to it.  This may result in the
   headend node not being able to instantiate the desired SR path in its
   forwarding and provide the SR based optimization functionality.  The
   handling of such errors by applications like SR PCE may be
   implementation specific and out of scope of this document.

   The extensions specified in this document, do not specify any new
   configuration or monitoring aspects in BGP or BGP-LS.  The
   specification of BGP models BGP and BGP-LS models is an ongoing work
   based on the [I-D.ietf-idr-bgp-model].  The management of the MSD
   features within an ietf segment-routing stack is also an ongoing work
   based on the [I-D.ietf-spring-sr-yang].  Management of the segment
   routing in IGPs is ongoing work for ISIS [I-D.ietf-isis-sr-yang] ,
   and OSPF [I-D.ietf-ospf-sr-yang].

7.  Security Considerations

   The 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.

   The document does not introduce additional security issues beyond
   discussed in [RFC7752], [RFC8476] and [RFC8491].  However, [RFC7752]
   is being revised in [I-D.ietf-idr-rfc7752bis] to provide additional
   clarification in several portions of the specification after
   receiving feedback from implementers.  One of the places that is
   being clarified is the error handling and security.  It is expected
   that after [I-D.ietf-idr-rfc7752bis] is released that implementers
   will update all BGP-LS base implementations improving the error
   handling for protocol work (including this document) that depend on
   this function.



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8.  Contributors

   Siva Sivabalan
   Cisco Systems Inc.
   Canada

   Email: msiva@cisco.com

9.  Acknowledgements

   We 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>.

   [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>.

   [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>.

   [RFC8476]  Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
              DOI 10.17487/RFC8476, December 2018,
              <https://www.rfc-editor.org/info/rfc8476>.

   [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>.

10.2.  Informative References

   [I-D.ietf-idr-bgp-model]
              Jethanandani, M., Patel, K., and S. Hares, "BGP YANG Model
              for Service Provider Networks", draft-ietf-idr-bgp-
              model-06 (work in progress), June 2019.



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   [I-D.ietf-idr-rfc7752bis]
              Talaulikar, K., Gredler, H., Medved, J., Previdi, S.,
              Farrel, A., and S. Ray, "Distribution of Link-State and
              Traffic Engineering Information Using BGP", draft-ietf-
              idr-rfc7752bis-00 (work in progress), September 2019.

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label Depth Using IS-IS", draft-ietf-isis-mpls-
              elc-08 (work in progress), September 2019.

   [I-D.ietf-isis-sr-yang]
              Litkowski, S., Qu, Y., Sarkar, P., Chen, I., and J.
              Tantsura, "YANG Data Model for IS-IS Segment Routing",
              draft-ietf-isis-sr-yang-06 (work in progress), July 2019.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and Entropy
              Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-09 (work in progress), September 2019.

   [I-D.ietf-ospf-sr-yang]
              Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
              "YANG Data Model for OSPF SR (Segment Routing) Protocol",
              draft-ietf-ospf-sr-yang-10 (work in progress), August
              2019.

   [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-16 (work in progress),
              March 2019.

   [I-D.ietf-spring-sr-yang]
              Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J.
              Tantsura, "YANG Data Model for Segment Routing", draft-
              ietf-spring-sr-yang-13 (work in progress), July 2019.

   [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>.







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   [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>.

Authors' Addresses

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com


   Uma Chunduri
   Futurewei Technologies

   Email: umac.ietf@gmail.com


   Ketan Talaulikar
   Cisco Systems

   Email: ketant@cisco.com


   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com


   Nikos Triantafillis
   Apstra, Inc.

   Email: nikos@apstra.com
















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