Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State
RFC 8814

Document Type RFC - Proposed Standard (August 2020; No errata)
Authors Jeff Tantsura  , Uma Chunduri  , Ketan Talaulikar  , Greg Mirsky  , Nikos Triantafillis 
Last updated 2020-08-12
Replaces draft-tantsura-idr-bgp-ls-segment-routing-msd
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Internet Engineering Task Force (IETF)                       J. Tantsura
Request for Comments: 8814                                  Apstra, Inc.
Category: Standards Track                                    U. Chunduri
ISSN: 2070-1721                                   Futurewei Technologies
                                                           K. Talaulikar
                                                           Cisco Systems
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        N. Triantafillis
                                                     Amazon Web Services
                                                             August 2020

 Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol -
                               Link State


   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 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

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
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   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Conventions Used in This Document
       1.1.1.  Terminology
       1.1.2.  Requirements Language
   2.  Advertisement of MSD via BGP-LS
   3.  Node MSD TLV
   4.  Link MSD TLV
   5.  IANA Considerations
   6.  Manageability Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

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 the signaling of
   MSD are defined in [RFC8476] and [RFC8491], respectively.

   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 [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 MSDs
   are known to be useful.  For example, [OSPF-ELC] and [ISIS-ELC]
   define Entropy Readable Label Depth (ERLD), which 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
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