Resilient MPLS Rings

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SPRING                                                       K. Kompella
Internet-Draft                                               A. Deshmukh
Intended status: Standards Track                                R. Torvi
Expires: April 25, 2019                           Juniper Networks, Inc.
                                                        October 22, 2018

                          Resilient MPLS Rings


   This document describes the use of the SPRING MPLS data plane for
   Resilient MPLS Rings.  It describes how to create the bidirectional
   ring LSPs with SPRING, and how protection works.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 25, 2019.

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   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Installing Primary LFIB Entries . . . . . . . . . . . . .   5
     3.2.  Installing Protection LFIB Entries  . . . . . . . . . . .   5
     3.3.  Protection  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Rings are a very common topology in transport networks.  A ring is
   the simplest topology offering link and node resilience.  Rings are
   nearly ubiquitous in access and aggregation networks.  As MPLS
   increases its presence in such networks, and takes on a greater role
   in transport, it is imperative that MPLS handles rings well;
   [I-D.ietf-mpls-rmr] shows how this can be done.
   [I-D.ietf-teas-rsvp-rmr-extension] shows how RSVP-TE [RFC3209] can be
   used to signal RMR ring LSPs.  [I-D.ietf-mpls-ldp-rmr-extensions]
   shows how LDP [RFC5036] can be used to signal RMR LSPs.  This
   document shows how SPRING SID bindings can be used to create RMR
   LSPs, how the basic bidirectional LSPs are set up, and how protection

   While RMR looks at rings potentially with "express links", this
   document focuses on simple rings.  These are most common in access
   networks.  Future revisions will look at more general rings.

1.1.  Definitions

   A (directed) graph G = (V, E) consists of a set of vertices (or
   nodes) V and a set of edges (or links) E.  An edge is an ordered pair
   of nodes (a, b), where a and b are in V.  (In this document, the
   terms node and link will be used instead of vertex and edge.)

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   A ring is a subgraph of G.  A ring consists of a subset of n nodes
   {R_i, 0 <= i < n} of V.  The directed edges {(R_i, R_i+1) and (R_i+1,
   R_i), 0 <= i < n-1} must be a subset of E (note that index arithmetic
   is done modulo n).  We define the direction from node R_i to R_i+1 as
   "clockwise" (CW) and the reverse direction as "anticlockwise" (AC).
   As there may be several rings in a graph, we number each ring with a
   distinct ring ID RID.

                                R0 . . . R1
                              .             .
                           R7                 R2
              Anti-     |  .        Ring       .  |
              Clockwise |  .                   .  | Clockwise
                        v  .      RID = 17     .  v
                           R6                 R3
                              .             .
                                R5 . . . R4

                        Figure 1: Ring with 8 nodes

   The following terminology is used for ring LSPs:

   Ring ID (RID):  A non-zero number that identifies a ring; this is
      unique in some scope of a Service Provider's network.  A node may
      belong to multiple rings.

   Ring node:  A member of a ring.  Note that a device may belong to
      several rings.

   Node index:  A logical numbering of nodes in a ring, from zero upto
      one less than the ring size.  Used purely for exposition in this

   Ring master:  The ring master initiates the ring identification
      process.  Mastership is indicated in the IGP by a two-bit field.

   Ring neighbors:  Nodes whose indices differ by one (modulo ring

   Ring size:  The ring size for a given instantiation is N.  This can
      change as nodes are added or removed, or go up or down.

   Ring links:  Links that connnect ring neighbors.

   Express links:  Links that connnect non-neighboring ring nodes.

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   Ring direction:  A two-bit field in the IGP indicating the direction
      of a link.  The choices are:

      UN: 00  undefined link

      CW: 01  clockwise ring link

      AC: 10  anticlockwise ring link

      EX: 11  express link

   Ring Identification:  The process of discovering ring nodes, ring
      links, link directions, and express links.

   The following notation is used for ring LSPs:

   R_k:  A ring node with index k.  R_k has AC neighbor R_(k-1) and CW
      neighbor R_(k+1).

   NS_k:  Node SID for node R_k.  Note that index arithmetic is done
      modulo the ring size N.

   CAS_k, AAS_k:  Clockwise adjacency SID at R_k, i.e., link R_k, R_k+1
      and anticlockwise adjacency SID R_k, R_k-1 respectively.  Note
      that index arithmetic is done modulo the ring size N.

   CSS_jk:  A clockwise node SID stack, typically with one or two SIDs,
      to be pushed by R_j to reach R_k in a clockwise direction.

   ASS_jk:  An anticlockwise node SID stack, typically with one or two
      SIDs, to be pushed by R_j to reach R_k in an anticlockwise

2.  Motivation

   A ring is the simplest topology that offers resilience.  This is
   perhaps the main reason to lay out fiber in a ring.  Thus, effective
   mechanisms for fast failover on rings are needed.  Furthermore, there
   are large numbers of rings.  Thus, configuration of rings needs to be
   as simple as possible.

   The goals of this document are to present mechanisms for improved
   resilience in ring networks (using ideas that are reminiscent of
   Bidirectional Line Switched Rings), for automatic bring-up of LSPs,
   better bandwidth management and for auto-hierarchy.  These goals are
   achieved using extensions to existing IGP.  This document shows how
   to do this using SPRING techniques, in particular, node SIDs.  Note

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   that in a simple ring topology, there is no need for complex
   algorithms to find loop-free protection paths.

3.  Theory of Operation

   We assume that a ring R has been configured, IGP advertisements have
   been made, and ring discovery is complete ([I-D.ietf-mpls-rmr]).  We
   also assume that node and adjacency SIDs have been distributed.

3.1.  Installing Primary LFIB Entries

   Ring LSPs are not provisioned.  Once a ring node R_i knows its RID,
   its ring links and directions, it kicks off ring LSP computation
   automatically.  In particular, R_j computes clockwise and
   anticlockwise SID stacks CSS_jk and ASS_jk to node R_k.  R_j then
   installs two FIB entries for R_k, CSS_jk and ASS_jk.  It is up to an
   application to choose whether to go clockwise or anticlockwise from
   R_j to R_k.

   R_j also computes CSS_jj and ASS_jj.  Clearly, R_j does not act as
   ingress for its own LSPs.  However, R_j can send OAM messages, for
   example, an MPLS ping or traceroute ([I-D.ietf-mpls-rfc4379bis]),
   using CSS_jj or ASS_jj, to test the entire ring LSP anchored at R_j
   in both directions.

3.2.  Installing Protection LFIB Entries

   At the same time that R_j sets up its primary clockwise and
   anticlockwise SID stacks, it sets up protection for each other node
   R_k.  R_j does this by installing a protection SID stack for the node
   SID to R_k, NS_k.  If the shortest path to R_k is clockwise, then the
   protection SID stack for NS_k is ASS_jk.  Otherwise, it is CSS_jk.

   Similarly, the protection entry for an adjacency SID CAS_j is
   ASS_j,j+1 and for AAS_j is CSS_j,j-1.

3.3.  Protection

   If a node R_j detects a failure from R_j+1 -- either all links to
   R_j+1 fail, or R_j+1 itself fails, R_j switches traffic on all CW
   node and adjacency SIDs to their protection LFIB entries.  This
   switchover can be very fast, as the protection LFIB entries can be
   preprogrammed.  Fast detection and fast switchover lead to minimal
   traffic loss.

   R_j then sends an indication to R_j-1 that the CW direction is not
   working, so that R_j-1 can similarly switch traffic to the AC
   direction.  This can be by an IGP update; other, potentially quicker,

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   mechanisms would be preferable.  These indications propagate AC until
   each traffic source on the ring AC of the failure is aware of the
   failure.  Thus, within a short period, traffic will be flowing on the
   reverse path than that which was chosen, since there is a failure on
   the ring.

4.  Security Considerations

   It is not anticipated that either the notion of MPLS rings or the
   extensions to various protocols to support them will cause new
   security loopholes.  As this document is updated, this section will
   also be updated.

5.  IANA Considerations

   There are no requests as yet to IANA for this document.

6.  References

6.1.  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,

6.2.  Informative References

              Esale, S. and K. Kompella, "LDP Extensions for RMR", 2018.

              Kompella, K., Swallow, G., Pignataro, C., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multi-Protocol Label
              Switched (MPLS) Data Plane Failures", draft-ietf-mpls-
              rfc4379bis-09 (work in progress), October 2016.

              Kompella, K. and L. Contreras, "Resilient MPLS Rings",

              Deshmukh, A. and K. Kompella, "RSVP Extensions for RMR",

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   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <>.

Authors' Addresses

   Kireeti Kompella
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA  94089


   Abhishek Deshmukh
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA  94089


   Ravi Torvi
   Juniper Networks, Inc.
   1133 Innovation Way
   Sunnyvale, CA  94089


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