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Versions: 00 01 02 03 04 05 06 draft-ietf-mpls-tp-ring-protection

Network Working Group                                     S. Bryant, Ed.
Internet-Draft                                                     Cisco
Intended status: Informational                        Y. Weingarten, Ed.
Expires: January 8, 2010                                N. Sprecher, Ed.
                                                  Nokia Siemens Networks
                                                            July 7, 2009


                        MPLS-TP Ring Protection
            draft-weingarten-mpls-tp-ring-protection-00.txt

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Abstract

   This document describes mechanisms to address the requirements for



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   protection of ring topologies for Multi-Protocol Label Switching
   Transport Profile (MPLS-TP) Label Switched Paths (LSP) and
   Pseudowires (PW) on multiple layers.  Ring topologies offer the
   possibility of reducing the OAM overhead while providing a simplified
   protection mechanism.  The document analyzes two basic ring
   protection schemes and explains how ring protection can be viewed as
   an application of linear protection.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Contributing Authors  . . . . . . . . . . . . . . . . . . . 3
   2.  Ring Topologies . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Ring protection schemes . . . . . . . . . . . . . . . . . . . . 4
     3.1.  Wrapping  . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  Steering  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Conclusions and Recommendations . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  Informative References  . . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8




























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1.  Introduction

   Multi-Protocol Label Switching Transport Profile (MPLS-TP) is being
   standardized as part of a joint effort between the Internet
   Engineering Task Force (IETF) and the International Telecommunication
   Union Standardization (ITU-T).  The specifications are based on the
   requirements that were generated from this joint effort.

   The requirements for MPLS-TP [MPLS-TP Reqs] inidcates that there is
   requirement to support a network that may include sections that
   constitute a MPLS-TP ring (either logical or physical).  The support
   for ring topologies as stated in the requirements is based on the
   ability to demonstrate that this topology allows the network to
   optimize either the protection or the number of Operations,
   Administration & Maintenance (OAM) entities needed to maintain the
   network.

   This document will examine different proposed mechanisms for
   protection of a ring in the context of MPLS-TP and try and determine
   how they may optimize the protection and the OAM procedures for a
   ring topology.  Finally, we plan to show how the generic protection
   mechanisms can be used to address the requirements in an optimized
   manner.

1.1.  Contributing Authors


2.  Ring Topologies

   The MPLS-TP Requirements [MPLS-TP Reqs] defines a ring as a topology
   in which each LSR is connected to exactly two neighboring LSRs, each
   via a single point-to-point birectional MPLS-TP capable link.  A ring
   provides certain advantages in transport networks, including:

   o  Configuration of point-to-multipoint paths around a ring are
      easily accomplished.

   o  There are always two paths between any two LSRs on a ring that can
      be easily identified and associated.

   o  It is believed that the number of OAM entities needed, in order to
      detect faults and perform recovery actions, may be minimized in a
      ring topology.

   The following figure shows a MPLS-TP ring that is a segment that may
   be traversed by numerous LSPs or PWs.  In particular, the figure
   shows that for all LSP that connect to the ring through LSR-B and
   exit the ring from LSR-F we can define two paths through the ring



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   (the first path along B-A-F, and the second B-C-D-E-F).

                                  ____
                       =========>/ LSR\
                               * \__B_/ *
                              * @      # *
                             * @        # *
                          __* @          # *___
                        /LSR\ @           #/LSR\
                        \_C_/ @           #\_A_/
                          *  @             # *
                          *  @              #*
                         _*_ @              #*_
                        /LSR\@             /LSR\========>
                        \_D_/@             \_F_/
                            * @           @*
                             * @         @*
                              * @@____@@*
                                */ LSR\*
                                 \__E_/


           ===> connected LSP  *** physical link
           ###  logical path   @@@ secondary logical path

                         Figure 1: A MPLS-TP ring


3.  Ring protection schemes

   There are two classic mechanisms that have been proposed in various
   forums to perform recovery of a topological ring network - "wrapping"
   and "steering".  The following sub-sections will examine these two
   mechanisms.

3.1.  Wrapping

   The "easier" recovery architecture is "wrapping".  This mechanism is
   local to the LSRs that are neighbors to the detected fault.  When a
   fault is detected, the neighboring LSR "wrap" all data traffic around
   the ring until arriving at the LSR that is on the opposite side of
   the fault, at which point the traffic continues on the normal working
   path until the egress from the ring segment.








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                                        ____
                              =========>/ LSR\
                                      * \__B_/ *
                                     * @@@@@@@# *
                                    * @       @# *
                                ___* @         @# *___
                               /LSR\ @          @#/LSR\
                               \_C_/ @           #\_A_/
                                 *  @             # *
                                 *  @              XX
                                _*_ @              #*_
                               /LSR\@             /LSR\
                               \_D_/@            @\_F_/
                                   * @          @#*
                                    * @       @@#*
                                     * @@____@##*
                                       */ LSR\*
                                        \__E_/========>

                   ===> connected LSP  *** physical link
                   ###  logical path   @@@ Bypass tunnel

                       Figure 2: Wrapping protection

   In this figure we have a ring with a LSP that enters the ring at
   LSR-B and exits at LSR-E.  The normal working path follows through
   B-A-F-E.  If a signal fault is detected on the link A<-->F, then
   there is the need for configuring a bypass tunnel [FRR] between A &
   F. The traffic will be transmitted over this bypass tunnel from A to
   F, and then will continue on the normal working path from F->E.
   Essentially, in this protection scheme, the traffic will follow the
   path - B-A-B-C-D-E-F-E.

   This protection scheme is simple in the sense that there is no need
   for coordination between the different LSR in the ring - only the LSR
   that detect fault must wrap the traffic, either via the bypass tunnel
   (at the near-end) or back to the normal path (at the far-end).

   When applying this scheme to a MPLS-TP ring topology segment there
   are the following considerations:

   o  The OAM should be performed at either the link level (by defining
      a TCME between each adjacent pair of LSR) and/or per LSR (by
      defining a TCME between the LSR that are neighbors of the
      protected LSR) when using node-level protection.

   o  For each protected TCME there is a need to define a bypass tunnel
      that traverses the alternate path around the ring to connect



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      between the two ends of the TCME.  If protecting both the links
      and the nodes, then, for a ring with N nodes, there is a need for
      O(2N) bypass tunnels.

   o  Protection of point-to-multipoint paths is similar to the simple
      protection since the data continues along the original path after
      wrapping around the ring.  The one exception is the case where the
      failed node was one of the egress points for the data.

   o  When wrapping the data is transmitted over some of the links
      twice, once in each direction.  For example, in the figure above
      the traffic is transmitted both B->A and then A->B, later it is
      transmitted E->F and F->E. This means that there is additional
      bandwith needed for this protection.

   o  The wrapping also involves greater latency in delivering the
      packets, as a result of traversing the entire ring.

   o  The resource allocation for the bypass tunnels could be
      problematic, since most of the tunnels will not be used
      simultaneously.  One possibility could be to allocate '0'
      resources and depend on the NMS to allocate the proper resources
      around the ring.

3.2.  Steering

   The second common scheme for ring protection redirects the traffic
   from the ingress point to the alternate route around the ring to the
   egress point.  This is illustrated in Figure 1 above, where if a
   Signal Fault is detected on the working path (B-A-F), then the
   traffic is redirected by B to the secondary path (i.e.  B-C-D-E-F).

   When considering this mechanism it is almost identical to linear 1:1
   protection.  The two paths around the ring act as the working and
   recovery paths.  There is need to communicate to the ingress node the
   need to switch over to the protection path and there is a need to
   coordinate the switchover between the two end-points of the protected
   path.

   There is one aspect that this diverts from the basic linear
   protection scheme - in the number of OAM sessions that would be
   neccesary to detect faults in the protected domain.  Whereas, using
   generic linear protection would neccesitate a separate OAM session
   per LSP that traverses the ring, when using ring protection there is
   apossiblity of taking proper advantage of the realization that we are
   dealing with a ring and reduce the number of OAM sessions.  This is
   done by defining a OAM session on the basis of a Path Segment Tunnel
   (PST), i.e. between any two nodes of the ring.  This would lead to



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   the number of OAM sessions for a ring with N nodes to be O(N*N/2),
   which could be very large.  However, taking into consideration that
   the required support of rings, is for rings with up-to 16 nodes -
   this implies that the number of OAM sessions should be on the order
   of (16*16/2) or 128.

   This form of OAM would allow the ingress LSR to directly detect any
   faulty situations and redirect traffic to the secondary path without
   the need for any additional communication to the LSR.

   The following observations can be drawn from using this protection
   mechanism for MPLS-TP ring topologies:

   o  Steering can be based on linear protection for the protection of a
      single ring.  For cases of interconnected rings further study is
      necessary.

   o  The number of OAM sessions can be greatly minimized, relative to
      using plain linear protection, by running the OAM sessions and the
      protection mechanisms on the ring segments for all LSPs that
      traverse the ring over that specific segment, rather than running
      a OAM session per LSP.  This fulfills the objective presented in
      [MPLS-TP Reqs].

   o  Point-to-multipoint paths through the ring would need further
      consideration.


4.  Conclusions and Recommendations

   In order to fulfill the requirements for protection of ring
   topologies for MPLS-TP networks, according to the conditions stated
   in [MPLS-TP Reqs], the protection should be based on MPLS-TP 1:1
   linear protection.  This mechanism will cover the cases of a single
   fault in a single ring topology.

   When defining the OAM behavior of the ring nodes, they should define
   a segment of all the LSPs that traverse a path within the ring.  The
   OAM should be executed for each ring path, i.e.  PST, to detect
   faults and trigger the protection switching within the ring.


5.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.



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6.  Security Considerations

   This document does not by itself raise any particular security
   considerations.


7.  Acknowledgements

   The authors would like to thank all members of the teams (the Joint
   Working Team, the MPLS Interoperability Design Team in IETF and the
   T-MPLS Ad Hoc Group in ITU-T) involved in the definition and
   specification of MPLS Transport Profile.


8.  Informative References

   [FRR]      Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Exensions to RSVP-TE for LSP Tunnels", RFC 4090, May 2005.

   [MPLS-TP Reqs]
              Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
              and S. Ueno, "Requirements for the Trasport Profile of
              MPLS", ID draft-ietf-mpls-tp-requirements-09, June 2009.

   [MPLS-TP Surv Fwk]
              Sprecher, N. and A. Farrel, "MPLS-TP Survivability
              Framework", ID draft-ietf-mpls-tp-requirements-09,
              April 2009.


Authors' Addresses

   Stewart Bryant (editor)
   Cisco
   United Kingdom

   Email: stbryant@cisco.com


   Yaacov Weingarten (editor)
   Nokia Siemens Networks
   3 Hanagar St. Neve Ne'eman B
   Hod Hasharon,   45241
   Israel

   Phone: +972-9-775 1827
   Email: yaacov.weingarten@nsn.com




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   Nurit Sprecher (editor)
   Nokia Siemens Networks
   3 Hanagar St. Neve Ne'eman B
   Hod Hasharon,   45241
   Israel

   Phone: +972-9-775 1229
   Email: nurit.sprecher@nsn.com











































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