Skip to main content

A Framework for Multi-Protocol Label Switching (MPLS) Operations and Management (OAM)
RFC 4378

Document Type RFC - Informational (February 2006)
Authors Thomas Nadeau , David Allan
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Alex D. Zinin
Send notices to (None)
RFC 4378
Network Working Group                                      D. Allan, Ed.
Request for Comments: 4378                               Nortel Networks
Category: Informational                                   T. Nadeau, Ed.
                                                     Cisco Systems, Inc.
                                                           February 2006

         A Framework for Multi-Protocol Label Switching (MPLS)
                    Operations and Management (OAM)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document is a framework for how data plane protocols can be
   applied to operations and maintenance procedures for Multi-Protocol
   Label Switching (MPLS).  The document is structured to outline how
   Operations and Management (OAM) functionality can be used to assist
   in fault, configuration, accounting, performance, and security
   management, commonly known by the acronym FCAPS.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Fault Management ................................................2
      3.1. Fault Detection ............................................2
      3.2. Diagnosis ..................................................6
      3.3. Availability ...............................................7
   4. Configuration Management ........................................7
   5. Accounting ......................................................7
   6. Performance Management ..........................................7
   7. Security Management .............................................8
   8. Security Considerations .........................................9
   9. Acknowledgements ................................................9
   10. Normative References ...........................................9

Allan & Nadeau               Informational                      [Page 1]
RFC 4378                A Framework for MPLS OAM           February 2006

1.  Introduction

   This memo outlines in broader terms how data plane protocols can
   assist in meeting the Operations and Management (OAM) requirements
   outlined in [RFC4377] and [Y1710] and can apply to the management
   functions of fault, configuration, accounting, performance, and
   security (commonly known as FCAPS) for MPLS networks, as defined in
   [RFC3031].  The approach of the document is to outline functionality,
   the potential mechanisms to provide the function, and the required
   applicability of data plane OAM functions.  Included in the
   discussion are security issues specific to use of tools within a
   provider domain and use for inter-provider Label Switched Paths

2.  Terminology

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

   OAM          Operations and Management

   FCAPS        Fault management, Configuration management,
                Administration management, Performance
                management, and Security management

   FEC          Forwarding Equivalence Class

   ILM          Incoming Label Map

   NHLFE        Next Hop Label Forwarding Entry

   MIB          Management Information Base

   LSR          Label Switching Router

   RTT          Round Trip Time

3.  Fault Management

3.1.  Fault Detection

   Fault detection encompasses the identification of all data plane
   failures between the ingress and egress of an LSP.  This section will
   enumerate common failure scenarios and explain how one might (or
   might not) detect the situation.

Allan & Nadeau               Informational                      [Page 2]
RFC 4378                A Framework for MPLS OAM           February 2006

3.1.1.  Enumeration and Detection of Types of Data Plane Faults

   Lower-layer faults:

      Lower-layer faults are those in the physical or virtual link that
      impact the transport of MPLS labeled packets between adjacent LSRs
      at the specific level of interest.  Some physical links (such as
      SONET/SDH) may have link-layer OAM functionality and detect and
      notify the LSR of link-layer faults directly.  Some physical links
      (such as Ethernet) may not have this capability and require MPLS
      or IP layer heartbeats to detect failures.  However, once
      detected, reaction to these fault notifications is often the same
      as those described in the first case.

   Node failures:

      Node failures are those that impact the forwarding capability of a
      node component, including its entire set of links.  This can be
      due to component failure, power outage, or reset of the control
      processor in an LSR employing a distributed architecture, etc.

   MPLS LSP mis-forwarding:

      Mis-forwarding occurs when there is a loss of synchronization
      between the data and the control planes in one or more nodes.
      This can occur due to hardware failure, software failure, or
      configuration problems.

      It will manifest itself in one of two forms:

      -  packets belonging to a particular LSP are cross-connected into
         an NHLFE for which there is no corresponding ILM at the next
         downstream LSR.  This can occur in cases where the NHLFE entry
         is corrupted.  Therefore, the packet arrives at the next LSR
         with a top label value for which the LSR has no corresponding
         forwarding information, and is typically dropped.  This is a No
         Incoming Label Map (No ILM) condition and can be detected
         directly by the downstream LSR that receives the incorrectly
         labeled packet.

      -  packets belonging to a particular LSP are cross-connected into
         an incorrect NHLFE entry for which there is a corresponding ILM
         at the next downstream LSR, but is associated with a different
         LSP.  This may be detected by the following:

         o  some or all of the misdirected traffic is not routable at
            the egress node, or

Allan & Nadeau               Informational                      [Page 3]
RFC 4378                A Framework for MPLS OAM           February 2006

         o  OAM probing is able to detect the fault by detecting the
            inconsistency between the data path and the control plane

   Discontinuities in the MPLS Encapsulation

      The forwarding path of the FEC carried by an LSP may transit nodes
      or links for which MPLS is not configured.  This may result in a
      number of behaviors that are undesirable and not easily detected.

      -  if exposed, payload is not routable at the LSR, resulting in
         silent discard, OR

      -  the exposed MPLS label was not offered by the LSR, which may
         result in either silent discard or mis-forwarding.

      Alternately, the payload may be routable and packets successfully
      delivered but may bypass associated MPLS instrumentation and

   MTU problems

      MTU problems occur when client traffic cannot be fragmented by
      intermediate LSRs and is dropped somewhere along the path of the
      LSP.  MTU problems should appear as a discrepancy in the traffic
      count between the set of ingress LSRs and the egress LSRs for an
      FEC and will appear in the corresponding MPLS MIB performance
      tables in the transit LSRs as discarded packets.

   TTL Mishandling

      The implementation of TTL handling is inconsistent at penultimate
      hop LSRs.  Tools that rely on consistent TTL processing may
      produce inconsistent results in any given network.


      Congestion occurs when the offered load on any interface exceeds
      the link capacity for sufficient time that the interface buffering
      is exhausted.  Congestion problems will appear as a discrepancy in
      the traffic count between the set of ingress LSRs and the egress
      LSRs for an FEC and will appear in the MPLS MIB performance tables
      in the transit LSRs as discarded packets.

Allan & Nadeau               Informational                      [Page 4]
RFC 4378                A Framework for MPLS OAM           February 2006


      Mis-ordering of LSP traffic occurs when incorrect or inappropriate
      load sharing is implemented within an MPLS network.  Load sharing
      typically takes place when multiple equal-cost paths exist between
      the ingress and egress of an LSP.  In these cases, traffic is
      split among these equal-cost paths using a variety of algorithms.
      One such algorithm relies on splitting traffic between each path
      on a per-packet basis.  When this is done, it is possible for some
      packets along the path to be delayed due to congestion or slower
      links, which may result in packets being received out of order at
      the egress.  Detection and remedy of this situation may be left up
      to client applications that use the LSPs.  For instance, TCP is
      capable of re-ordering packets belonging to a specific flow
      (although this may result in re-transmission of some of the mis-
      ordered packets).

      Detection of mis-ordering can also be determined by sending probe
      traffic along the path and verifying that all probe traffic is
      indeed received in the order it was transmitted.  This will only
      detect truly pathological problems as mis-ordering typically is an
      insufficiently predictable and repeatable problem.

      LSRs do not normally implement mechanisms to detect mis-ordering
      of flows.

   Payload Corruption

      Payload corruption may occur and may be undetected by LSRs.  Such
      errors are typically detected by client payload integrity

3.1.2.  Timeliness

   The design of Service Level Agreements (SLAs) and management support
   systems requires that ample headroom be alloted in terms of their
   processing capabilities in order to process and handle all necessary
   fault conditions within the bounds stipulated in the SLA.  This
   includes planning for event handling using a time budget that takes
   into account the over-all SLA and the time required to address any
   defects that arise.  However, it is possible that some fault
   conditions may surpass this budget due to their catastrophic nature
   (e.g., fibre cut) or due to incorrect planning of the time processing

Allan & Nadeau               Informational                      [Page 5]
RFC 4378                A Framework for MPLS OAM           February 2006

         ^    --------------
         |    |           ^
         |    |           |----  Time to notify NOC + process/correct
   SLA   |    |           v      defect
   Max - |    -------------
   Time  |    |           ^
         |    |           |-----  Time to diagnose/isolate/correct
         |    |           v
         v    -------------

         Figure 1: Fault Correction Budget

   In figure 1, we represent the overall fault correction time budget by
   the maximum time as specified in an SLA for the service in question.
   This time is then divided into two subsections, the first
   encompassing the total time required to detect a fault and notify an
   operator (or optionally automatically correct the defect).  This
   section may have an explicit maximum time to detect defects arising
   from either the application or a need to do alarm management (i.e.,
   suppression), and this will be reflected in the frequency of OAM
   execution.  The second section indicates the time required to notify
   the operational systems used to diagnose, isolate, and correct the
   defect (if they cannot be corrected automatically).

3.2.  Diagnosis

3.2.1.  Characterization

   Characterization is defined as determining the forwarding path of a
   packet (which may not be necessarily known).  Characterization may be
   performed on a working path through the network.  For example, this
   is done to determine equal-cost multi-paths (ECMP), the MTU of a
   path, or simply to know the path occupied by a specific FEC.
   Characterization will be able to leverage mechanisms used for

3.2.2.  Isolation

   Isolation of a fault can occur in two forms.  In the first case, the
   local failure is detected, and the node where the failure occurred is
   capable of issuing an alarm for such an event.  The node should
   attempt to withdraw the defective resources and/or rectify the
   situation prior to raising an alarm.  Active data plane OAM
   mechanisms may also detect the failure conditions remotely and issue
   their own alarms if the situation is not rectified quickly enough.

   In the second case, the fault has not been detected locally.  In this
   case, the local node cannot raise an alarm, nor can it be expected to

Allan & Nadeau               Informational                      [Page 6]
RFC 4378                A Framework for MPLS OAM           February 2006

   rectify the situation.  In this case, the failure may be detected
   remotely via data plane OAM.  This mechanism should also be able to
   determine the location of the fault, perhaps on the basis of limited
   information such as a customer complaint.  This mechanism may also be
   able to automatically remove the defective resources from the network
   and restore service, but should at least provide a network operator
   with enough information by which they can perform this operation.
   Given that detection of faults is desired to happen as quickly as
   possible, tools which possess the ability to incrementally test LSP
   health should be used to uncover faults.

3.3.  Availability

   Availability is the measure of the percentage of time that a service
   is operating within a specification, often specified by an SLA.

   MPLS has several forwarding modes (depending on the control plane
   used).  As such, more than one model may be defined and more than one
   measurement technique may be required.

4.  Configuration Management

   Data plane OAM can assist in configuration management by providing
   the ability to verify the configuration of an LSP or of applications
   utilizing that LSP.  This would be an ad-hoc data plane probe that
   should verify path integrity (a complete path exists) and that the
   path function is synchronized with the control plane.  As part of the
   payload, the probe would carry relevant control plane information
   that the receiver would be able to compare with the local-control
   plane configuration.

5.  Accounting

   The requirements for accounting in MPLS networks, as specified in
   [RFC4377], do not place any requirements on data plane OAM.

6.  Performance Management

   Performance management permits the information transfer
   characteristics of LSPs to be measured, perhaps in order to be
   compared against an SLA.  This falls into two categories: latency
   (where jitter is considered a variation in latency) and information

   Latency can be measured in two ways: one is to have precisely
   synchronized clocks at the ingress and egress such that time-stamps
   in PDUs flowing from the ingress to the egress can be compared.  The
   other is to use an exchange of PING type PDUs that gives a round trip

Allan & Nadeau               Informational                      [Page 7]
RFC 4378                A Framework for MPLS OAM           February 2006

   time (RTT) measurement, and an estimate of the one-way latency that
   can be inferred with some loss of precision.  Use of load spreading
   techniques, such as ECMP, mean that any individual RTT measurement is
   only representative of the typical RTT for an FEC.

   To measure information loss, a common practice is to periodically
   read ingress and egress counters (i.e., MIB module counters).  This
   information may also be used for offline correlation.  Another common
   practice is to send explicit probe traffic that traverses the data
   plane path in question.  This probe traffic can also be used to
   measure jitter and delay.

7.  Security Management

   Providing a secure OAM environment is required if MPLS specific
   network mechanisms are to be used successfully.  To this end,
   operators have a number of options when deploying network mechanisms
   including simply filtering OAM messages at the edge of the MPLS
   network.  Malicious users should not be able to use non-MPLS
   interfaces to insert MPLS-specific OAM transactions.  Provider
   initiated OAM transactions should be able to be blocked from leaking
   outside the MPLS cloud.

   Finally, if a provider does wish to allow OAM messages to flow into
   (or through) their networks, for example, in a multi-provider
   deployment, authentication and authorization are required to prevent
   malicious and/or unauthorized access.  Also, given that MPLS networks
   often run IP simultaneously, similar requirements apply to any native
   IP OAM network mechanisms in use.  Therefore, authentication and
   authorization for OAM technologies is something that MUST be
   considered when designing network mechanisms that satisfy the
   framework presented in this document.

   OAM messaging can address some existing security concerns with the
   MPLS architecture.  That is, through rigorous defect handling,
   operator's can offer their customers a greater degree of integrity
   protection that their traffic will not be incorrectly delivered (for
   example, by being able to detect leaking LSP traffic from a VPN).

   Support for inter-provider data plane OAM messaging introduces a
   number of security concerns as, by definition, portions of LSPs will
   not be within a single provider's network the provider has no control
   over who may inject traffic into the LSP, which can be exploited for
   denial of service attacks.  OAM PDUs are not explicitly identified in
   the MPLS header and therefore are not typically inspected by transit
   LSRs.  This creates opportunity for malicious or poorly behaved users
   to disrupt network operations.

Allan & Nadeau               Informational                      [Page 8]
RFC 4378                A Framework for MPLS OAM           February 2006

   Attempts to introduce filtering on target LSP OAM flows may be
   problematic if flows are not visible to intermediate LSRs.  However,
   it may be possible to interdict flows on the return path between
   providers (as faithfulness to the forwarding path is to a return path
   requirement) to mitigate aspects of this vulnerability.

   OAM tools may permit unauthorized or malicious users to extract
   significant amounts of information about network configuration.  This
   would be especially true of IP based tools as, in many network
   configurations, MPLS does not typically extend to untrusted hosts,
   but IP does.  For example, TTL hiding at ingress and egress LSRs will
   prevent external users from using TTL-based mechanisms to probe an
   operator's network.  This suggests that tools used for problem
   diagnosis or which, by design, are capable of extracting significant
   amounts of information will require authentication and authorization
   of the originator.  This may impact the scalability of such tools
   when employed for monitoring instead of diagnosis.

8.  Security Considerations

   This document describes a framework for MPLS Operations and
   Management.  Although this document discusses and addresses some
   security concerns in Section 7, it does not introduce any new
   security concerns.

9.  Acknowledgements

   The editors would like to thank Monique Morrow from Cisco Systems and
   Harmen van Der Linde from AT&T for their valuable review comments on
   this document.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC4377]  Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
              Matsushima, "Operations and Management (OAM) Requirements
              for Multi-Protocol Label Switched (MPLS) Networks", RFC
              4377, February 2006.

   [Y1710]    ITU-T Recommendation Y.1710(2002), "Requirements for OAM
              Functionality for MPLS Networks".

Allan & Nadeau               Informational                      [Page 9]
RFC 4378                A Framework for MPLS OAM           February 2006

Authors' Addresses

   David Allan
   Nortel Networks
   3500 Carling Ave.
   Ottawa, Ontario, CANADA

   Phone: +1-613-763-6362

   Thomas D. Nadeau
   Cisco Systems
   300 Beaver Brook Drive
   Boxborough, MA 01824

   Phone: +1-978-936-1470

Allan & Nadeau               Informational                     [Page 10]
RFC 4378                A Framework for MPLS OAM           February 2006

Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at


   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).

Allan & Nadeau               Informational                     [Page 11]