Internet Engineering Task Force (IETF)
Request for Comments: 7412
Category: Informational
ISSN: 2070-1721
Y. Weingarten
S. Aldrin
Huawei Technologies
P. Pan
J. Ryoo
G. Mirsky
December 2014

Requirements for MPLS Transport Profile (MPLS-TP)

Shared Mesh Protection


This document presents the basic network objectives for the behavior of Shared Mesh Protection (SMP) that are not based on control-plane support. This document provides an expansion of the basic requirements presented in RFC 5654 ("Requirements of an MPLS Transport Profile") and RFC 6372 ("MPLS Transport Profile (MPLS-TP) Survivability Framework"). This document provides requirements for any mechanism that would be used to implement SMP for MPLS-TP data paths, in networks that delegate protection switch coordination to the data plane.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2014 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 to this document. Code Components extracted from this document must 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 ....................................................3
   2. Terminology and Notation ........................................3
      2.1. Acronyms and Terminology ...................................4
   3. Shared Mesh Protection Reference Model ..........................4
      3.1. Protection or Restoration ..................................5
      3.2. Scope of Document ..........................................5
           3.2.1. Relationship to MPLS ................................5
   4. SMP Architecture ................................................6
      4.1. Coordination of Resources ..................................8
      4.2. Control Plane or Data Plane ................................8
   5. SMP Network Objectives ..........................................9
      5.1. Resource Reservation and Coordination ......................9
           5.1.1. Checking Resource Availability for Multiple
                  Protection Paths ....................................9
      5.2. Multiple Triggers .........................................10
           5.2.1. Soft Preemption ....................................10
           5.2.2. Hard Preemption ....................................10
      5.3. Notification ..............................................11
      5.4. Reversion .................................................11
      5.5. Protection Switching Time .................................11
      5.6. Timers ....................................................12
      5.7. Communication Channel and Fate-Sharing ....................12
   6. Manageability Considerations ...................................13
   7. Security Considerations ........................................13
   8. Normative References ...........................................13
   Acknowledgements ..................................................15
   Contributors ......................................................15
   Authors' Addresses ................................................16

1. Introduction

The MPLS Transport Profile (MPLS-TP) is described in [RFC5921]. [RFC6372] provides a survivability framework for MPLS-TP and is the foundation for this document.

Terminology for recovery of connectivity in networks is provided in [RFC4427] and includes the concept of surviving network faults (survivability) through the use of re-established connections (restoration) and switching of traffic to pre-established backup paths (protection). MPLS provides control-plane tools to support various survivability schemes, some of which are identified in [RFC4426]. In addition, recent efforts in the IETF have started providing for data-plane tools to address aspects of data protection. In particular, [RFC6378] and [RFC7271] define a set of triggers and coordination protocols for 1:1 and 1+1 linear protection of point-to- point paths.

When considering a full-mesh network and the protection of different paths that traverse the mesh, it is possible to provide an acceptable level of protection while conserving the amount of protection resources needed to protect the different data paths. As pointed out in [RFC6372] and [RFC4427], applying 1+1 protection requires that resources are allocated for use by both the working and protection paths. Applying 1:1 protection requires that the same resources are allocated but allows the resources of the protection path to be utilized for preemptible extra traffic. Extending this to 1:n or m:n protection allows the resources of the protection path to be shared in the protection of several working paths. However, 1:n or m:n protection architecture is limited by the restriction that all of the n+1 or m+n paths must have the same endpoints. m:n protection architecture provides m protection paths to protect n working paths, where m or n can be 1.

This document provides requirements for any mechanism that would be used to implement SMP for MPLS-TP data paths, in networks that delegate protection switch coordination to the data plane.

2. Terminology and Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Although this document is not a protocol specification, the use of this language clarifies the instructions to protocol designers producing solutions that satisfy the requirements set out in this document.

The terminology used in this document is based on the terminology defined in the MPLS-TP Survivability Framework document [RFC6372], which in turn is based on [RFC4427].

2.1. Acronyms and Terminology

This document uses the following acronyms:

      LSP  Label Switched Path
      SLA  Service Level Agreement
      SMP  Shared Mesh Protection
      SRLG Shared Risk Link Group

This document defines the following term:

SMP Protection Group: the set of different protection paths that

share a common segment.

3. Shared Mesh Protection Reference Model

As described in [RFC6372], SMP supports the sharing of protection resources, while providing protection for multiple working paths that need not have common endpoints and do not share common points of failure. Note that some protection resources may be shared, while some others may not be. An example of data paths that employ SMP is shown in Figure 1. It shows two working paths -- <ABCDE> and <VWXYZ> -- that are protected employing 1:1 linear protection by protection paths <APQRE> and <VPQRZ>, respectively. The two protection paths that traverse segment <PQR> share the protection resources on this segment.

                            \                 /
                             \               /
                              \             /
                              /             \
                             /               \
                            /                 \

Figure 1: Basic SMP Architecture

3.1. Protection or Restoration

   [RFC6372], based upon the definitions in [RFC4427], differentiates
   between "protection" and "restoration", depending on the dynamism of
   the resource allocation.  The same distinction is used in [RFC3945],
   [RFC4426], and [RFC4428].

This document also uses the same distinction between protection and restoration as the distinction stated in [RFC6372].

3.2. Scope of Document

[RFC5654] establishes that MPLS-TP SHOULD support shared protection (Requirement 68) and that MPLS-TP MUST support sharing of protection resources (Requirement 69). This document presents the network objectives and a framework for applying SMP within an MPLS network, without the use of control-plane protocols. Although there are existing control-plane solutions for SMP within MPLS, a data-plane solution is required for networks that do not employ a full control- plane operation for some reason (e.g., service provider preferences or limitations) or require service restoration faster than is achievable with control-plane mechanisms.

The network objectives will also address possible additional restrictions on the behavior of SMP in networks that delegate protection switching for resiliency to the data plane. Definitions of logic and specific protocol messaging are out of scope for this document.

3.2.1. Relationship to MPLS

While some of the restrictions presented by this document originate from the properties of transport networks, nothing prevents the information presented here from being applied to MPLS networks outside the scope of the Transport Profile of MPLS.

4. SMP Architecture

Figure 1 shows a very basic configuration of working and protection paths that may employ SMP. We may consider a slightly more complex configuration, such as the one in Figure 2 in order to illustrate characteristics of a mesh network that implements SMP.

                       \            /    /    \
                        \          M ---/--    \
                         \             /   \    \
                         /|      \     \     \    \
                        / F---G---H    J--K---L    \
                       /                            \

Figure 2: Example of a Larger SMP Architecture

Consider the network presented in Figure 2. There are five working paths:

  • <ABCDE>
  • <MDEN>
  • <FGH>
  • <JKL>
  • <VWXYZ>

Each of these has a corresponding protection path:

  • <APQRE> (p1)
  • <MSTN> (p2)
  • <FPQH> (p3)
  • <JRSL> (p4)
  • <VPQRSTZ> (p5)

The following segments are shared by two or more of the protection paths -- <PQ> is shared by p1, p3, and p5; <QR> is shared by p1 and p5; <RS> is shared by p4 and p5; and <ST> is shared by p2 and p5. In Figure 2, we have the following SMP Protection Groups -- {p1, p3, p5} for <PQ>, {p1, p5} for <QR>, {p4, p5} for <RS>, and {p2, p5} for <ST>.

We assume that the available protection resources for these shared segments are not sufficient to support the complete traffic capacity of the respective working paths that may use the protection paths. We can further observe that with a method of coordinating sharing and preemption, there are no co-routing constraints on shared components at the segment level.

The use of preemption in the network is typically a business or policy decision such that when protection resources are contested, priority can be applied to determine which parties utilize the protection resources.

As opposed to the case of simple linear protection, where the relationship between the working and protection paths is defined and the resources for the protection path are fully dedicated, the protection path in the case of SMP consists of segments that are used for the protection of the related working path and also segments that are shared with other protection paths such that typically the protection resources are oversubscribed to support working paths that do not share common points of failure. What is required is a preemption mechanism to implement business priority when multiple failure scenarios occur. As such, the protection resources may be allocated but would not be utilized until requested and resolved in relation to other members of the SMP Protection Group as part of a protection switchover.

[RFC6372] defines two types of preemption that can be considered for how the resources of SMP Protection Groups are shared: "soft preemption", where traffic of lower-priority paths is degraded; and "hard preemption", where traffic of lower-priority paths is completely blocked. The traffic of lower-priority paths in this document can be viewed as the extra traffic being preempted, as described in [RFC6372]. "Hard preemption" requires the programming of selectors at the ingress of each shared segment to specify the priorities of backup paths, so that traffic of lower-priority paths can be preempted. When any protection mechanism where the protection endpoint may have a choice of protection paths (e.g., m:n or m:1) is deployed, the shared segment selectors require coordination with the protection endpoints as well.

Typical deployment of services that use SMP requires various network planning activities. These include the following:

  • Determining the number of working and protection paths required to achieve resiliency targets for the service.
  • Reviewing network topology to determine which working or protection paths are required to be disjoint from each other, and excluding specified resources such as links, nodes, or shared risk link groups (SRLGs).
  • Determining the size (bandwidth) of the shared resource.

4.1. Coordination of Resources

When a protection switch is triggered, the SMP network performs two operations -- switching data traffic over to a protection path and coordinating the utilization of the associated shared resources. Both operations should occur at the same time, or as close together as possible, to provide fast protection. The resource utilization coordination is dependent upon their availability at each of the shared segments.

When the reserved resources of the shared segments are utilized by a particular protection path, there may not be sufficient resources available for an additional protection path. This then implies that if another working path of the SMP domain triggers a protection switch, the resource utilization coordination may fail. The different working paths in the SMP network are involved in the resource utilization coordination, which is a part of a whole SMP protection switching coordination.

4.2. Control Plane or Data Plane

As stated in both [RFC6372] and [RFC4428], full control of SMP, including both configuration and the coordination of the protection switching, is potentially very complex. Therefore, it is suggested that this be carried out under the control of a dynamic control plane based on Generalized MPLS (GMPLS) [RFC3945]. Implementations for SMP with GMPLS exist, and the general principles of its operation are well known, if not fully documented.

However, there are operators, in particular in the transport sector, that do not operate their MPLS-TP networks under the control of a control plane or for other reasons have delegated executive action for resilience to the data plane, and require the ability to utilize

SMP protection. For such networks, it is imperative that it be possible to perform all required coordination of selectors and endpoints for SMP via data-plane operations.

5. SMP Network Objectives

5.1. Resource Reservation and Coordination

SMP is based on pre-configuration of the working paths and the corresponding protection paths. This configuration may be based on either a control protocol or static configuration by the management system. However, even when the configuration is performed by a control protocol, e.g., GMPLS, the control protocol SHALL NOT be used as the primary mechanism for detecting or reporting network failures, or for initiating or coordinating protection switchover. That is, it SHALL NOT be used as the primary resilience mechanism.

The protection relationship between the working and protection paths SHOULD be configured, and the shared segments of the protection path MUST be identified prior to use of the protection paths. Relative priority for working paths to be used to resolve contention for protection path usage by multiple working paths MAY also be specified ahead of time.

When a protection switch is triggered by any fault condition or operator command, the SMP network MUST perform two operations -- switch data traffic over to a protection path, and coordinate the utilization of the associated shared resources. To provide fast protection, both operations MUST occur at the same time or as close to the same time as possible.

In the case of multiple working paths failing, the shared resource utilization coordination SHALL be between the different working paths in the SMP network.

5.1.1. Checking Resource Availability for Multiple Protection Paths

In a hard-preemption scenario, when an endpoint identifies a protection switching trigger and has more than one potential action (e.g., m:1 protection), it MUST verify that the necessary protection resources are available on the selected protection path. The resources may not be available because they have already been utilized for the protection of, for example, one or more higher- priority working paths.

5.2. Multiple Triggers

If more than one working path is triggering a protection switch such that a protection segment is oversubscribed, there are two different actions that the SMP network can choose -- soft preemption and hard preemption [RFC6372].

5.2.1. Soft Preemption

For networks that support multiplexing packets over the shared segments, the requirement is as follows:

  • All of the protection paths MAY be allowed to share the resources of the shared segments.

5.2.2. Hard Preemption

There are networks that require the exclusive use of the protection resources when a protection segment is oversubscribed. Traffic of lower-priority paths is completely blocked. These include networks that support the requirements in [RFC5654], and in particular support Requirement 58. For such networks, the following requirements apply:

  1. Relative priority MAY be assigned to each of the working paths of an SMP domain. If the priority is not assigned, the working paths are assumed to have equal priority.
  1. Resources of the shared segments SHALL be utilized by the protection path according to the highest priority amongst those requesting use of the resources.
  1. If multiple protection paths of equal priority are requesting the shared resources, the resources SHALL be utilized on a first come first served basis. Traffic of the protection paths that request the shared resources late SHALL be preempted. In order to cover the situation where the first come first served principle cannot resolve the contention among multiple equal-priority requests, i.e., when the requests occur simultaneously, tie-breaking rules SHALL be defined in the scope of an SMP domain.
  1. If a higher-priority path requires the protection resources that are being utilized by a lower-priority path, the resources SHALL be utilized by the higher-priority path. Traffic with the lower priority SHALL be preempted.
  1. Once resources of shared segments have been successfully utilized by a protection path, the traffic on that protection path SHALL NOT be interrupted by any protection traffic whose priority is equal to or lower than the protecting path currently in use.
  1. During preemption, shared segment resources MAY be used by both existing traffic (that is being preempted) and higher-priority traffic.

5.3. Notification

When a working path endpoint has a protection switch triggered, it SHOULD attempt to switch the traffic to the protection path and request the coordination of the shared resource utilization. If the necessary shared resources are unavailable, the endpoints of the requesting working path SHALL be notified of protection switchover failure, and switchover will not be completed.

Similarly, if preemption is supported and the resources currently utilized by a particular working path are being preempted, then the endpoints of the affected working path whose traffic is being preempted SHALL be notified that the resources are being preempted. As described in [RFC6372], the event of preemption may be detected by Operations, Administration, and Maintenance (OAM) and reported as a fault or a degradation of traffic delivery.

5.4. Reversion

When the condition that triggered the protection switch is cleared, it is possible to either revert to using the working path resources or continue to utilize the protection resources. Continuing the use of protection resources allows the operator to delay the disruption of service caused by the switchover until periods of lighter traffic. The switchover would need to be performed via an explicit operator command, unless the protection resources are preempted by a higher- priority fault. Hence, both automatic and manual revertive behaviors MUST be supported for hard preemption in an SMP domain. Normally, the network should revert to use of the working path resources in order to clear the protection resources for protection of other path triggers. However, the protocol MUST support non-revertive configurations.

5.5. Protection Switching Time

Protection switching time refers to the transfer time (Tt) defined in [G.808.1] and recovery switching time defined in [RFC4427], and is defined as the interval after a switching trigger is identified until the traffic begins to be transmitted on the protection path. This time does not include the time needed to initiate the protection switching process after a failure occurred, and the time needed to complete preemption of existing traffic on the shared segments as described in Section 4.2. The time needed to initiate the protection switching process, which is known as detection time or correlation time in [RFC4427], is related to the OAM or management process, but the time needed to complete preemption is related to the actions within an SMP domain. Support for a protection switching time of 50 ms is dependent upon the initial switchover to the protection path, but the preemption time SHOULD also be taken into account to minimize total service interruption time.

When triggered, protection switching action SHOULD be initiated immediately to minimize service interruption time.

5.6. Timers

In order to prevent multiple switching actions for a single switching trigger, when there are multiple layers of networks, SMP SHOULD be controlled by a hold-off timer that would allow lower-layer mechanisms to complete their switching actions before invoking SMP protection actions as described in [RFC6372].

In order to prevent an unstable recovering working path from invoking intermittent switching operations, SMP SHOULD employ a Wait-To-Restore timer during any reversion switching, as described in [RFC6372].

5.7. Communication Channel and Fate-Sharing

SMP SHOULD provide a communication channel, along the protection path, between the endpoints of the protection path, to support fast protection switching.

SMP in hard-preemption mode SHOULD include support for communicating information to coordinate the use of the shared protection resources among multiple working paths. The message encoding and communication channel between the nodes of the shared protection resource and the endpoints of the protection path are out of the scope of this document.

Bidirectional protection switching SHOULD be supported in SMP.

6. Manageability Considerations

The network management architecture and requirements for MPLS-TP are specified in [RFC5951]. They derive from the generic specifications described in ITU-T G.7710/Y.1701 [G.7710] for transport technologies. This document does not introduce any new manageability requirements beyond those covered in those documents.

7. Security Considerations

General security considerations for MPLS-TP are covered in [RFC5921]. The security considerations for the generic associated control channel are described in [RFC5586].

Security considerations for any proposed solution should consider exhaustion of resources related to preemption, especially by a malicious actor as a threat vector against which the resources should be protected. Protections should also be considered to prevent a malicious actor from attempting to create an alternate path on which to force traffic from a sensor/device, thereby enabling pervasive monitoring [RFC7258].

8. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
   [RFC3945]  Mannie, E., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Architecture", RFC 3945, October 2004,
   [RFC4426]  Lang, J., Ed., Rajagopalan, B., Ed., and D. Papadimitriou,
              Ed., "Generalized Multi-Protocol Label Switching (GMPLS)
              Recovery Functional Specification", RFC 4426, March 2006,
   [RFC4427]  Mannie, E., Ed., and D. Papadimitriou, Ed., "Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4427,
              March 2006, <>.
   [RFC4428]  Papadimitriou, D., Ed., and E. Mannie, Ed., "Analysis of
              Generalized Multi-Protocol Label Switching (GMPLS)-based
              Recovery Mechanisms (including Protection and
              Restoration)", RFC 4428, March 2006,
   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586, June 2009,
   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
              Sprecher, N., and S. Ueno, "Requirements of an MPLS
              Transport Profile", RFC 5654, September 2009,
   [RFC5921]  Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
              L., and L. Berger, "A Framework for MPLS in Transport
              Networks", RFC 5921, July 2010,
   [RFC5951]  Lam, K., Mansfield, S., and E. Gray, "Network Management
              Requirements for MPLS-based Transport Networks", RFC 5951,
              September 2010, <>.
   [RFC6372]  Sprecher, N., Ed., and A. Farrel, Ed., "MPLS Transport
              Profile (MPLS-TP) Survivability Framework", RFC 6372,
              September 2011, <>.
   [RFC6378]  Weingarten, Y., Ed., Bryant, S., Osborne, E., Sprecher,
              N., and A. Fulignoli, Ed., "MPLS Transport Profile
              (MPLS-TP) Linear Protection", RFC 6378, October 2011,
   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, May 2014,
   [RFC7271]  Ryoo, J., Ed., Gray, E., Ed., van Helvoort, H.,
              D'Alessandro, A., Cheung, T., and E. Osborne, "MPLS
              Transport Profile (MPLS-TP) Linear Protection to Match the
              Operational Expectations of Synchronous Digital Hierarchy,
              Optical Transport Network, and Ethernet Transport Network
              Operators", RFC 7271, June 2014,
   [G.7710]   International Telecommunication Union, "Common equipment
              management function requirements", ITU-T Recommendation
              G.7710/Y.1701, February 2012.
   [G.808.1]  International Telecommunication Union, "Generic Protection
              Switching - Linear trail and subnetwork protection", ITU-T
              Recommendation G.808.1, May 2014.


This document is the outcome of discussions on Shared Mesh Protection for MPLS-TP. The authors would like to thank all contributors to these discussions, and especially Eric Osborne for facilitating them.

We would also like to thank Matt Hartley for working on the English review and Lou Berger for his valuable comments and suggestions on this document.


   David Allan
   Daniel King
   Old Dog Consulting
   Taesik Cheung

Authors' Addresses

   Yaacov Weingarten
   34 Hagefen St.
   Karnei Shomron,  4485500


   Sam Aldrin
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA  95050
   United States

Ping Pan


   Jeong-dong Ryoo
   218 Gajeongno
   Yuseong, Daejeon  305-700
   South Korea

Greg Mirsky