Segment Routing with the MPLS Data Plane
RFC 8660

Document Type RFC - Proposed Standard (December 2019; No errata)
Authors Ahmed Bashandy  , Clarence Filsfils  , Stefano Previdi  , Bruno Decraene  , Stephane Litkowski  , Rob Shakir 
Last updated 2019-12-06
Replaces draft-filsfils-spring-segment-routing-mpls
Stream IETF
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Stream WG state Submitted to IESG for Publication (wg milestone: Oct 2018 - SR-MPLS sent to IESG... )
Document shepherd Shraddha Hegde
Shepherd write-up Show (last changed 2018-12-03)
IESG IESG state RFC 8660 (Proposed Standard)
Action Holders
Consensus Boilerplate Yes
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Responsible AD Martin Vigoureux
Send notices to Shraddha Hegde <>
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions

Internet Engineering Task Force (IETF)                  A. Bashandy, Ed.
Request for Comments: 8660                                        Arrcus
Category: Standards Track                               C. Filsfils, Ed.
ISSN: 2070-1721                                               S. Previdi
                                                     Cisco Systems, Inc.
                                                             B. Decraene
                                                            S. Litkowski
                                                               R. Shakir
                                                           December 2019

                Segment Routing with the MPLS Data Plane


   Segment Routing (SR) leverages the source-routing paradigm.  A node
   steers a packet through a controlled set of instructions, called
   segments, by prepending the packet with an SR header.  In the MPLS
   data plane, the SR header is instantiated through a label stack.
   This document specifies the forwarding behavior to allow
   instantiating SR over the MPLS data plane (SR-MPLS).

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) 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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
     1.1.  Requirements Language
   2.  MPLS Instantiation of Segment Routing
     2.1.  Multiple Forwarding Behaviors for the Same Prefix
     2.2.  SID Representation in the MPLS Forwarding Plane
     2.3.  Segment Routing Global Block and Local Block
     2.4.  Mapping a SID Index to an MPLS Label
     2.5.  Incoming Label Collision
       2.5.1.  Tiebreaking Rules
       2.5.2.  Redistribution between Routing Protocol Instances
     2.6.  Effect of Incoming Label Collision on Outgoing Label
     2.7.  PUSH, CONTINUE, and NEXT
       2.7.1.  PUSH
       2.7.2.  CONTINUE
       2.7.3.  NEXT
     2.8.  MPLS Label Downloaded to the FIB for Global and Local SIDs
     2.9.  Active Segment
     2.10. Forwarding Behavior for Global SIDs
       2.10.1.  Forwarding for PUSH and CONTINUE of Global SIDs
       2.10.2.  Forwarding for the NEXT Operation for Global SIDs
     2.11. Forwarding Behavior for Local SIDs
       2.11.1.  Forwarding for the PUSH Operation on Local SIDs
       2.11.2.  Forwarding for the CONTINUE Operation for Local SIDs
       2.11.3.  Outgoing Label for the NEXT Operation for Local SIDs
   3.  IANA Considerations
   4.  Manageability Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Examples
     A.1.  IGP Segment Examples
     A.2.  Incoming Label Collision Examples
       A.2.1.  Example 1
       A.2.2.  Example 2
       A.2.3.  Example 3
       A.2.4.  Example 4
       A.2.5.  Example 5
       A.2.6.  Example 6
       A.2.7.  Example 7
       A.2.8.  Example 8
       A.2.9.  Example 9
       A.2.10. Example 10
       A.2.11. Example 11
       A.2.12. Example 12
       A.2.13. Example 13
       A.2.14. Example 14
     A.3.  Examples for the Effect of Incoming Label Collision on an
           Outgoing Label
       A.3.1.  Example 1
       A.3.2.  Example 2
   Authors' Addresses

1.  Introduction

   The Segment Routing architecture [RFC8402] can be directly applied to
   the MPLS architecture with no change in the MPLS forwarding plane.
   This document specifies forwarding-plane behavior to allow Segment
   Routing to operate on top of the MPLS data plane (SR-MPLS).  This
   document does not address control-plane behavior.  Control-plane
   behavior is specified in other documents such as [RFC8665],
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