IDR Working Group                                            J. Tantsura
Internet-Draft                                              Apstra, Inc.
Intended status: Standards Track                             U. Chunduri
Expires: November 9, 2020                         Futurewei Technologies
                                                           K. Talaulikar
                                                           Cisco Systems
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        N. Triantafillis
                                                     Amazon Web Services
                                                             May 8, 2020


 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol - Link
                                 State
              draft-ietf-idr-bgp-ls-segment-routing-msd-18

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 November 9, 2020.








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Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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.

   [RFC8664] 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 the 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 expose 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

   MSD: Maximum SID Depth - the number of SIDs supported by a node or a
   link on a node

   PCE: Path Computation Element

   PCEP: Path Computation Element Protocol

   SID: Segment Identifier as defined in [RFC8402]

   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.

   o  pushing one or more new labels onto the label stack.

   o  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.



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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 ([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 based on their MSD
   capabilities.  When a BGP-LS speaker is originating the topology
   learnt via link-state routing protocols such as 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 extensions introduced in this document allow for advertisement of
   different MSD-Types, which are defined elsewhere and were introduced
   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

   The Node MSD ([RFC8476] [RFC8491]) is encoded in a new Node Attribute
   TLV [RFC7752] to carry the provisioned SID depth of the router
   identified by the corresponding Router-ID.  Node MSD is the smallest
   MSD supported by the node on the set of interfaces configured for
   use.  MSD values may be learned via a hardware API or may be
   provisioned.  The following format is used:

      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



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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 "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 protocol instance.

4.  Link MSD TLV

   The Link MSD ([RFC8476] [RFC8491]) is defined to carry the MSD of the
   interface associated with the link.  It 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

   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 : 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 link when used as
         an outgoing interface.



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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     |   Reference       |
+------------+-----------------+---------------------------+-------------------+
|    266     | Node MSD        | 242/23                    |   This document   |
|    267     | Link MSD        | (22,23,25,141,222,223)/15 |   This document   |
+------------+-----------------+---------------------------+-------------------+


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]).

   This document only introduces new Attribute TLVs and any syntactic
   error in them would result in the BGP-LS Attribute being discarded
   [RFC7752].  The MSD information introduced in BGP-LS by this
   specification, may be used by BGP-LS consumer applications like a SR
   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 head-end 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 head-end 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.



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   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 is an ongoing work based on the
   [I-D.ietf-idr-bgp-model].

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 procedures and protocol extensions defined in this document do
   not affect the BGP security model.  See the "Security Considerations"
   section of [RFC4271] for a discussion of BGP security.  Also, refer
   to [RFC4272] and [RFC6952] for analyses of security issues for BGP.
   Security considerations for acquiring and distributing BGP-LS
   information are discussed in [RFC7752].  The TLVs introduced in this
   document are used to propagate the MSD IGP extensions defined in
   [RFC8476] [RFC8491].  It is assumed that the IGP instances
   originating these TLVs will support all the required security (as
   described in [RFC8476] [RFC8491]) in order to prevent any security
   issues when propagating the TLVs into BGP-LS.  The advertisement of
   the node and link attribute information defined in this document
   presents no significant additional risk beyond that associated with
   the existing node and link attribute information already supported in
   [RFC7752].

8.  Contributors

   Siva Sivabalan
   Cisco Systems Inc.
   Canada

   Email: msiva@cisco.com

9.  Acknowledgements

   We like to thank Acee Lindem, Stephane Litkowski, Bruno Decraene and
   Alvaro Retana for their reviews and valuable comments.








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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., Hares, S., and J. Haas, "BGP
              YANG Model for Service Provider Networks", draft-ietf-idr-
              bgp-model-08 (work in progress), February 2020.

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using IS-IS", draft-ietf-
              isis-mpls-elc-11 (work in progress), March 2020.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
              and M. Bocci, "Signaling Entropy Label Capability and
              Entropy Readable Label Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-13 (work in progress), April 2020.




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   [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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <https://www.rfc-editor.org/info/rfc6952>.

   [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>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

Authors' Addresses

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com


   Uma Chunduri
   Futurewei Technologies

   Email: umac.ietf@gmail.com








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   Ketan Talaulikar
   Cisco Systems

   Email: ketant@cisco.com


   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com


   Nikos Triantafillis
   Amazon Web Services

   Email: nikost@amazon.com



































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