MPLS-TP Ring Protection
draft-weingarten-mpls-tp-ring-protection-01.txt

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Abstract

This document describes mechanisms to address the requirements for 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
    1.1.  Contributing Authors
2.  Ring Topologies
    2.1.  Interconnected rings
3.  Ring protection schemes
    3.1.  Wrapping
        3.1.1.  Optimization of wrapping
    3.2.  Steering
        3.2.1.  Point-to-Multipoint paths
4.  Conclusions and Recommendations
5.  IANA Considerations
6.  Security Considerations
7.  Acknowledgements
8.  Informative References
§  Authors' Addresses

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). These specifications are based on the requirements that were generated from this joint effort.

The requirements for MPLS-TP [MPLS‑TP Reqs] (Niven-Jenkins, B., Nadeau, T., and C. Pignataro, “Requirements for the Transport Profile of MPLS,” April 2009.) 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

Akira Sakurai (NEC), Rolf Winter (NEC)

2.  Ring Topologies

The MPLS-TP Requirements [MPLS‑TP Reqs] (Niven-Jenkins, B., Nadeau, T., and C. Pignataro, “Requirements for the Transport Profile of MPLS,” April 2009.) 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:

• Configuration of point-to-multipoint paths around a ring are easily accomplished.
• There are always two paths between any two LSRs on a ring that can be easily identified and associated.
• 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 (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

2.1.  Interconnected rings

The Requirements document [MPLS‑TP Reqs] (Niven-Jenkins, B., Nadeau, T., and C. Pignataro, “Requirements for the Transport Profile of MPLS,” April 2009.) states that the ring protection must support a single ring that may be interconnected to other rings. In addition, traffic that traverses a number of rings within a network of interconnected rings must be protected even if the interconnection nodes and links fail.

When interconnecting rings in a network there are two common interconnection schemes:

• Dual-node interconnect – when the interconnected rings are interconnected by two nodes from each ring (see Figure 2 (Dual-node interconnected rings))
• Single-node interconnect – when the connection between the interconnected rings are through a single node (see Figure 3 (Single-node interconnected rings))

The protection schemes presented in Section 3 (Ring protection schemes) are capable of protecting each interconnected ring as a separate entity independent of the other rings in the network. This protects the traffic that traverses the entire network, as each ring will continue to transfer the traffic to the interconnection points, and from there to the next ring.

When the interconnection nodes or links fail, there is the need to protect these connection points. Therefore, it should be noted that in the case of single-node interconnect the interconnection node (LSR-A in Figure 3 (Single-node interconnected rings)) is a single-point of failure and such an interconnection scheme should be avoided. The protection of the dual-node interconnect is essentially a linear-protection situation and should be protected using appropriate protection mechanisms.

                       ____                     ___
                      / LSR\                   /LSR\
                    * \__B_/ *                *\_1_/*
                   *          *              *       *
                  *            *            *         *
               __*              *___      _*_          * ___
             /LSR\              /LSR\****/LSR\          /LSR\
             \_C_/              \_A_/    \_6_/          \_2_/
               *     Ring #1      *        *   Ring #2    *
               *                  *        *              *
              _*_                 *_      _*_            _*_
             /LSR\              /LSR\    /LSR\          /LSR\
             \_D_/              \_F_/****\_5_/          \_3_/
                 *              *           *            *
                  *            *             *          *
                    *   ____  *               *  ____  *
                      */ LSR\*                 */LSR \*
                       \__E_/                   \__4_/


                       *** physical link

                            ____                ___
                           / LSR\              /LSR\
                         * \__B_/ *           *\_1_/*
                        *          *         *       *
                       *            *       *         *
                    __*              *___  *           * ___
                  /LSR\              /LSR\*             /LSR\
                  \_C_/              \_A_/              \_2_/
                    *     Ring #1      * *    Ring #2     *
                    *                  *  *               *
                   _*_                _*_  *  ___        _*_
                  /LSR\              /LSR\  */LSR\      /LSR\
                  \_D_/              \_F_/   \_5_/      \_3_/
                     *              *          *        *
                      *            *           *      *
                        *   ____  *            *___  *
                          */ LSR\*             /LSR\*
                           \__E_/              \_4_/


                       *** physical link

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.

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

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

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] (Pan, P., Swallow, G., and A. Atlas, “Fast Reroute Exensions to RSVP-TE for LSP Tunnels,” May 2005.) 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 LSRs that detect the 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:

• 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.
• For each protected TCME there is a need to define a bypass tunnel that traverses the alternate path around the ring to connect 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.
• 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.
• 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.
• The wrapping also involves greater latency in delivering the packets, as a result of traversing the entire ring.
• 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.

3.1.1.  Optimization of wrapping

It may be possible to optimize the basic performance, in terms of bandwidth utilization, of the Fast-reroute mechanism when protecting particular LSPs. However, it is important to consider the basic criteria for ring protection, i.e. an appreciable minimization of the number of OAM sessions necessarily or the protection resources needed.

One suggestion for optimization is to define the bypass tunnel to wrap only until the furthest egress point of the working LSP. This method reduces the number of links that are traversed twice, in most cases, by not wrapping back to the working path at the far-end of the signal fault. However, the method requires defining a particular bypass tunnel for each LSP and cannot use the optimization of the OAM sessions presented above to protect all LSPs with a single set of bypass LSPs.

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 a possiblity 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 an OAM session on the basis of a Path Segment Tunnel (PST), i.e. between any two nodes of the ring. This would lead to 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:

• Steering can be based on linear protection for the protection of a single ring. For cases of interconnected rings further study is necessary.
• The number of OAM sessions can be greatly minimized, relative to using plain linear protection, by running the OAM sessions on the ring segments for all LSPs that traverse the ring, rather than running a OAM session per LSP. This fulfills the objective presented in [MPLS-TP Reqs].

3.2.1.  Point-to-Multipoint paths

[MPLS‑TP Reqs] (Niven-Jenkins, B., Nadeau, T., and C. Pignataro, “Requirements for the Transport Profile of MPLS,” April 2009.) requires that ring protection must provide protection for unidirectional point-to-multipoint paths through the ring. As was pointed out in section 1, ring topologies provide a ready platform for supporting such data paths.

It is possible to create a protection architecture for p2mp within a MPLS-TP ring topology, based on 1+1 linear protection by monitoring two p2mp uni-directional PST from each ingress node on the ring with egress points at each node. The two PST traverse the ring in opposite directions, i.e. one checks the clockwise path and the second the counter-clockwise path.

The data for a particular p2mp LSP is transmitted on both the working and protecting LSP, using a permanent bridge. While each node detects that there is connectivity from the ingress point, it continues to select the data that is coming from the working path. If a particular node stops receiving the connectivity messages from the working path PST, it identifies that it must switch its selector to read the data from the protecting path.

This architecture has the added advantages that there is no need for the ingress node to identify the existence of the misconnectivity, and there is no need for a return path from the egress points to the ingress.

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.

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

Authors' Addresses