Enhanced path segment monitoring

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Document Type Active Internet-Draft (mpls WG)
Authors Alessandro D'Alessandro  , Loa Andersson  , Manuel Paul  , Satoshi Ueno  , Kaoru Arai  , Yoshinori Koike 
Last updated 2015-07-22
Replaces draft-koike-mpls-tp-temporal-hitless-psm
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Network Working Group                                    A. D'Alessandro
Internet-Draft                                            Telecom Italia
Intended status: Standards Track                            L. Andersson
Expires: January 21, 2016                            Huawei Technologies
                                                                 M. Paul
                                                        Deutsche Telekom
                                                                 S. Ueno
                                                      NTT Communications
                                                                 K. Arai
                                                                Y. Koike
                                                           July 20, 2015

                    Enhanced path segment monitoring


   The MPLS transport profile (MPLS-TP) has been standardized to enable
   carrier-grade packet transport and complement converged packet
   network deployments.  Among the most attractive features of MPLS-TP
   there are OAM functions, which enable network operators or service
   providers to provide various maintenance characteristics, such as
   fault location, survivability, performance monitoring and in-service/
   out of service measurements.

   One of the most important mechanisms which is common for transport
   network operation is fault location.  A segment monitoring function
   of a transport path is effective in terms of extension of the
   maintenance work and indispensable particularly when the OAM function
   is effective only between end points.  However, the current approach
   defined for MPLS-TP for the segment monitoring (SPME) has some
   drawbacks.  This document elaborates on the problem statement for the
   Sub-path Maintenance Elements (SPMEs) which provides monitoring of a
   portion of a set of transport paths (LSPs or MS-PWs).  Based on the
   problems, this document specifies new requirements to consider a new
   improved mechanism of hitless transport path segment monitoring named
   Enhanced Path Segment Monitoring (EPSM).

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute

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   working documents as Internet-Drafts.  The list of current Internet-
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Network objectives for segment monitoring . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   5.  OAM functions supported in segment monitoring . . . . . . . .   8
   6.  Requirements for enhanced segment monitoring  . . . . . . . .   8
     6.1.  Non intrusive segment monitoring  . . . . . . . . . . . .   9
     6.2.  Single and multiple levels monitoring . . . . . . . . . .   9
     6.3.  EPSM and end-to-end proactive monitoring independence . .  10
     6.4.  Arbitrary segment monitoring  . . . . . . . . . . . . . .  10
     6.5.  Fault while EPSM is in place  . . . . . . . . . . . . . .  12
     6.6.  EPSM maintenance points . . . . . . . . . . . . . . . . .  13
   7.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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1.  Introduction

   A packet transport network enables carriers or service providers to
   use network resources efficiently, reduces operational complexity and
   provides carrier-grade network operation.  Appropriate maintenance
   functions, supporting fault location, survivability, performance
   monitoring and preliminary or in-service measurements, are essential
   to ensure quality and reliability of a network.  They are essential
   in transport networks and have evolved along with TDM, ATM, SDH and

   As legacy technologies, also MPLS-TP does not scale when an arbitrary
   number of OAM functions are enabled.

   According to the MPLS-TP OAM requirements RFC 5860 [RFC5860],
   mechanisms MUST be available for alerting a service provider of a
   fault or defect affecting services.  In addition, to ensure that
   faults or degradations can be localized, operators need a method to
   analyze or investigate the problem being end-to-end monitoring
   insufficient.  In fact using end-to-end OAM monitoring, an operator
   is not able to localize a fault or degrade.

   Thus, a specific segment monitoring function for detailed analysis,
   by selecting and focusing on a specific portion of a transport path,
   is indispensable to promptly and accurately localize the fault.

   For MPLS-TP, a path segment monitoring function has been defined to
   perform this task.  However, as noted in the MPLS-TP OAM Framework
   RFC 6371 [RFC6371], the current method for segment monitoring
   function of a transport path has implications that hinder the usage
   in an operator network.

   This document elaborates on the problem statement for the path
   segment monitoring function and proposes to consider a new improved
   method for segment monitoring, following up the work done in RFC 6371
   [RFC6371].  Moreover, this document explains detailed requirements on
   the new temporal and hitless segment monitoring function which are
   not covered in RFC 6371 [RFC6371].

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.1.  Terminology

      EPSM - Enhanced Path Segment Monitoring

      LSP - Label Switched Path

      LSR - Label Switching Router

      ME - Maintenance Entity

      MEG - Maintenance Entity Group

      MEP - Maintenance Entity Group End Point

      MIP - Maintenance Entity Group Intermediate Point

      OTN - Optical Transport Network

      PST - Path Segment Tunnel

      TCM - Tandem connection monitoring

      SDH - Synchronous Digital Hierarchy

      SPME - Sub-path Maintenance Element

2.2.  Definitions


3.  Network objectives for segment monitoring

   There are two required network objectives for MPLS-TP segment
   monitoring as described in section 3.8 of RFC 6371 [RFC6371].

   1.  The monitoring and maintenance of current transport paths has to
       be conducted in-service without traffic disruption.

   2.  Segment monitoring must not modify the forwarding of the segment
       portion of the transport path.

4.  Problem Statement

   Sub-Path Maintenance Element (SPME) is defined in RFC 5921 [RFC5921]
   to monitor, protect, or manage portions of transport paths, such as
   LSPs in MPLS-TP networks.  The SPME is defined between the edges of
   the portion of the transport path that needs to be monitored,
   protected, or managed.  This SPME is created by stacking the shim

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   header (MPLS header) according to RFC 3031 [RFC3031] and is defined
   as the segment where the header is stacked.  OAM messages can be
   initiated at the edge of the SPME and sent to the peer edge of the
   SPME or to a MIP along the SPME by setting the TTL value of the label
   stack entry (LSE) and interface identifier value at the corresponding
   hierarchical LSP level in case of per-node model.

   This method has the following drawbacks, which impact on operation

      (P-1) Lowering the bandwidth efficiency by increasing the overhead
      by shim headers stacking

      (P-2) Increasing network management complexity, as a new sublayer
      and new MEPs and MIPs need to be configured for the SPME

   Problem (P-1) comes from shim headers stacking that increase the

   Problem (P-2) is related to identifiers management issue.  The
   identification of each sub-layer in case of label stacking is
   required for the segment monitoring in a MPLS-TP network.  When SPME
   is applied for on-demand OAM functions in MPLS-TP networks in a
   similar manner to TCM for OTN or Ethernet transport networks, a
   possible rule of differentiating those SPME/TCMs operationally will
   be necessary at least within an administrative domain.  This enforces
   operators to create an additional permanent layer identification
   policy only for temporal path segment monitoring.  Moreover, from the
   perspective of operation, increasing the managed addresses and the
   managed layers is not desirable to keep transport networks as simple
   as possible.  Reducing the managed identifiers and managed sub-layers
   should be the fundamental direction in designing the architecture.

   The analogy for SPME in legacy transport networks is Tandem
   Connection Monitoring (TCM), which is on-demand and does not change
   the transport path.

   Moreover, using currently defined methods, the temporal setting of
   SPMEs also causes the following problems due to label stacking:

      (P-3) Changing the original condition of transport path by
      changing the length of MPLS frames and changing the value of
      exposed label

      (P-4) Disrupting client traffic over a transport path, if the SPME
      is configured on demand.

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   Problem (P-3) has impacts on network objective (2).  The monitoring
   function should monitor the status without changing any conditions of
   the targeted monitored segment or transport path.  Changing the
   settings of the original shim header should not be allowed because
   those changes correspond to creating a new portion of the original
   transport path, which differs from the original data plane
   conditions.  If the conditions of the transport path change, the
   measured value or observed data will also change.  This may make the
   monitoring meaningless because the result of the monitoring would no
   longer reflect the reality of the connection where the original fault
   or degradation occurred.

   Figure 1 shows an example of SPME setting.  In the figure, X means
   the label value expected on the tail-end node D of the original
   transport path. "210" and "220" are label allocated for SPME.  The
   label values of the original path are modified as well as the values
   of stacked label.  As shown in Figure 1, SPME changes both the length
   of MPLS frames and label value(s).  This is no longer the monitoring
   of the original transport path but the monitoring of a different
   path.  Particularly, performance monitoring measurement (e.g.  Delay
   measurement and packet loss measurement) are sensitive to those

      (Before SPME settings)
       ---     ---     ---     ---     ---
      |   |   |   |   |   |   |   |   |   |
      |   |   |   |   |   |   |   |   |   |
       ---     ---     ---     ---     ---
       A---100--B--110--C--120--D--130--E  <= transport path
       MEP                             MEP

      (After SPME settings)
       ---     ---     ---     ---     ---
      |   |   |   |   |   |   |   |   |   |
      |   |   |   |   |   |   |   |   |   |
       ---     ---     ---     ---     ---
       A---100--B-----------X---D--130--E  <= transport path
       MEP     \                  /    MEP
                --210--C--220--            <= SPME
               MEP'          MEP'

                  Figure 1: An Example of a SPME setting

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   Problem (P-4) can be avoided if the operator sets SPMEs in advance
   until the end of life of a transport path, which is neither temporal
   nor on demand.  Furthermore SMPEs cannot be set arbitrarly because
   overlapping of path segments is limited to nesting relationship.  As
   a result, possible SPME patterns of portions of an original transport
   path are limited due to the characteristic of SPME shown in Figure 1,
   even if SPMEs are pre- configured.

   Although the make-before-break procedure in the survivability
   document RFC 6372 [RFC6372] seemingly supports the hitless
   configuration for monitoring according to the framework document RFC
   5921 [RFC5921], the reality is that configurating an SPME is
   impossible without violating network objective (2).  These concerns
   are reported in section 3.8 of RFC 6371 [RFC6371].

   Moreover, make-before-break approach might not be effective under the
   static model without a control plane because the make-before-break is
   a restoration application based on the control plane.  So management
   systems should provide support for SPME creation and for coordinated
   traffic switching from original transport path to the SPME.

   Other potential risks are also envisaged.  Setting up a temporal SPME
   will result in the LSRs within the monitoring segment only looking at
   the added (stacked) labels and not at the labels of the original LSP.
   This means that problems stemming from incorrect (or unexpected)
   treatment of labels of the original LSP by the nodes within the
   monitored segment could not be found when setting up SPME.  This
   might include hardware problems during label look-up, mis-
   configuration etc.  Therefore operators have to pay extra attention
   to correctly setting and checking the label values of the original
   LSP in the configuration.  Of course, the inversion of this situation
   is also possible, e.g., incorrect or unexpected treatment of SPME
   labels can result in false detection of a fault where none of the
   problem originally existed.

   The utility of SPMEs is basically limited to inter-carrier or inter-
   domain segment monitoring where they are typically pre-configured or
   pre-instantiated.  SPME instantiates a hierarchical transport path
   (introducing MPLS label stacking) through which OAM packets can be
   sent.  SPME construct monitoring function is particularly important
   mainly for protecting bundles of transport paths and carriers'
   carrier solutions.  SPME is expected to be mainly used for protection
   purpose within one administrative domain.

   To summarize, the problem statement is that the current sub-path
   maintenance based on a hierarchical LSP (SPME) is problematic for
   pre-configuration in terms of increasing bandwidth by label stacking
   and managing objects by layer stacking and address management.  A on-

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   demand/temporal configuration of SPME is one of the possible
   approaches for minimizing the impact of these issues.  However, the
   current method is unfavorable because the temporal configuration for
   monitoring can change the condition of the original monitored
   transport path.  To avoid the drawbacks discussed above, a more
   efficient approach is required for MPLS-TP transport network
   operation to overcome or minimize the impact of the above described
   drawbacks.  A monitoring mechanism, named on-demand Enhanced Path
   Segment Monitoring (EPSM), supporting temporal and hitless path
   segment monitoring is proposed.

5.  OAM functions supported in segment monitoring

   OAM functions that may useful exploited across on-demand EPSM are
   basically limited to on-demand performance monitoring functions which
   are defined in OAM framework document RFC 6371 [RFC6371].  Segment
   performance monitoring is used to evaluate the performance and hence
   the status of transport path segments.  The "on-demand" attribute is
   generally temporal for maintenance operation.

   Packet loss and packet delay are OAM functions strongly required in
   hitless and temporal segment monitoring because these functions are
   supported only between end points of a transport path.  If a defect
   occurs, it might be quite hard to locate the defect or degradation
   point without using the segment monitoring function.  If an operator
   cannot locate or narrow the cause of the fault, it is quite difficult
   to take prompt actions to solve the problem.

   Other on-demand monitoring functions, (e.g. and Delay variation
   measurement) are desirable but not as necessary as the previous
   mentioned functions.

   Regarding out-of-service on-demand performance management functions
   (e.g.  Throughput measurement), there seems no need for EPSM.
   However, OAM functions specifically designed for segment monitoring
   should be developed to satisfy network objective (2) described in
   section 3.

   Finally, the solution for EPSM has to cover both per-node model and
   per- interface model which are specified in RFC 6371 [RFC6371].

6.  Requirements for enhanced segment monitoring

   In the following clauses, mandatory (M) and optional (O) requirements
   are for the new segment monitoring function are listed.

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6.1.  Non intrusive segment monitoring

   One of the major problem of legacy SPME that has been highlighted in
   Sec. 4 is that it does not monitor the original transport path and it
   could distrupt service traffic when set-up on demand.

      (M1) EPSM must not change the original condition of transport path
      (e.g. must not change the lenght of MPLS frames, the exposed label
      values, etc.)

      (M2) EPSM must be set on demand without traffic dispruption

6.2.  Single and multiple levels monitoring

   The new segment monitoring function is supposed to be applied mainly
   for on-demand diagnostic purpose.  We can differentiate this
   monitoring from the proactive segment monitoring as on-demand multi-
   level monitoring.  The most serious problem at the moment is that
   there is no way to localize the degraded portion of a path without
   changing the conditions of the original path.  Therefore, as a first
   step, single layer segment monitoring not affecting the monitored
   path is required for a new on-demand and hitless segment monitoring
   function.  A combination of multi-level and simultaneous segment
   monitoring is the most powerful tool for accurately diagnosing the
   performance of a transport path.  However, on field, a single level
   approach may be enough.

      (M3) Single-level segment monitoring is required

      (O1) Multi-level segment monitoring is desirable

   Figure 2 shows an example of a multi-level on-demand segment

     ---     ---     ---     ---     ---
    |   |   |   |   |   |   |   |   |   |
    | A |   | B |   | C |   | D |   | E |
     ---     ---     ---     ---     ---
     MEP                            MEP <= ME of a transport path
             *------------------*       <=On-demand segm. monit. level 1
               *-------------*          <=On-demand segm. monit. level 2
                     *-*                <=On-demand segm. monit. level 3

       Figure 2: Example of multi-level on-demand segment monitoring

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6.3.  EPSM and end-to-end proactive monitoring independence

   The necessity of simultaneous monitoring of current end-to-end
   proactive monitoring and new on-demand path segment monitoring is
   here below considered.  Normally, the on-demand path segment
   monitoring is configured in a segment of a maintenance entity of a
   transport path.  In such an environment, on-demand single-level
   monitoring should be done without disrupting pro-active monitoring of
   the targeted end- to-end transport path to avoid missing user traffic
   performance monitoring.


      (M4) EPSM shall be established without changing or interfering
      with the already in place end-to-end pro-active monitoring of
      transport path

     ---     ---     ---     ---     ---
    |   |   |   |   |   |   |   |   |   |
    | A |   | B |   | C |   | D |   | E |
     ---     ---     ---     ---     ---
     MEP                             MEP <= ME of a transport path
       +-----------------------------+   <= Proactive E2E monitoring
             *------------------*        <= On-demand segment monitoring

    Figure 3: Independency between proactive end-to-end monitoring and
                       on-demand segment monitoring

6.4.  Arbitrary segment monitoring

   The main objective of on-demand segment monitoring is to diagnose the
   fault points.  One possible realistic diagnostic procedure is to fix
   one end point of a segment at the MEP of the transport path under
   observation and change progressively the length of the segments.
   This example is shown in Figure 4.

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    ---     ---     ---     ---     ---
   |   |   |   |   |   |   |   |   |   |
   | A |   | B |   | C |   | D |   | E |
    ---     ---     ---     ---     ---
    MEP                             MEP <= ME of a transport path
      +-----------------------------+   <= Proactive E2E monitoring
      *-----*                        <= 1st on-demand segment monitoring
      *-------*                      <= 2nd on-demand segment monitoring
      *------------*                 <= 3rd on-demand segment monitoring
           |                                |
           |                                |
      *-----------------------*      <= 6th on-demand segment monitoring
      *-----------------------------*<= 7th on-demand segment monitoring

    Figure 4: A procedure to localize a defect by consecutive on-demand
                            segments monitoring

   Another possible scenario is depicted in Figure 5.  In this case, the
   operators want to diagnose a transport path from a transit node that
   is located at the middle, because the end nodes(A and E) are located
   at customer sites and consist of cost effective small box supporting
   only a subset of OAM functions.  In that case, if the source entities
   of the diagnostic packets are limited to the position of MEPs, on-
   demand segment monitoring will be ineffective because not all the
   segments can be diagnosed (e.g.  segment monitoring 3 in Figure 5 is
   not available and it is not possible to precisely localize the fault


      (M5) EPSM shall be set in an arbitrary segment of a transport path
      and diagnostic packets should be inserted/terminated at any of
      intermediate maintenance points of the original ME.

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              ---     ---     ---
      ---    |   |   |   |   |   |    ---
     | A |   | B |   | C |   | D |   | E |
      ---     ---     ---     ---     ---
      MEP                             MEP <= ME of a transport path
        +-----------------------------+   <= Proactive E2E monitoring
        *-----*                        <= On-demand segment monitoring 1
              *-----------------------*<= On-demand segment monitoring 2
              *---------*              <= On-demand segment monitoring 3

            Figure 5: HSPM is configured at arbitrary segments

6.5.  Fault while EPSM is in place

   Node or link failures may occur while EPSM is active.  In that case,
   if no resiliency mechanism is set-up on the subtended transport path,
   there is no particular requirement for EPSM while if the trasport
   path is protected, EPSM function should be terminated to avoid
   monitoring a new segment when a protection or restoration path is in
   place.  Therefore

      (M5) EPSM function should avoid monitoring an unintended segment
      when failures occur

   The folowing examples are reported for clarification only and shall
   not be intended to restrict any solution for meeting the requirements
   of EPSM.  A Protection scenario A is shown in figure 6.  In this
   scenario, a working LSP and a protection LSP are set.  EPSM is set
   between A and E.  Considering a fault happens between nodes B and C,
   the EPSM is not affected by protection and continues in the working
   LSP path.  As a result, requirement (M5) is satisfied.

      A - B - C - D - E - F
        \               /
          G - H - I - L

      - e2e LSP: A-B-C-D-E-F
      - working LSP: A-B-C-D-E-F
      - protection LSP: A-B-G-H-I-L-F
      - EPSM: A-E

                      Figure 6: Protection scenario A

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   Differently, figure 7 shows a scenario where only a portion of a
   transport path is protected.  In this case, when a fault happen
   between node B and C along the working sub-path, traffic is switched
   to protection sub-path B-G-H-D.  In the hypotesis that OAM packets
   termination depend only on TTL value of MPLS label header, the target
   node of EPSM changes from E to D due to the difference of hop counts
   between the working path route (ABCDE: 4 hops) and protection path
   route (ABGHDE: 5 hops).  As a result, requirement (M5) is not

          A - B - C - D - E - F
                \     /
                 G - H

      - e2e LSP: A-B-C-D-E-F
      - working sub-path: B-C-D
      - protection sub-path: B-G-H-D
      - EPSM: A-E

                      Figure 7: Protection scenario B

6.6.  EPSM maintenance points

   An intermediate maintenance point supporting the EPSM has to be able
   to generate and inject OAM packets.  Although maintenance points for
   the EPSM do not necessarily have to coincide with MIPs or MEPs in
   terms of the architecture definition,

      (M7) The same identifiers for MIPs and/or MEPs should be applied
      to EPSM maintenance points

7.  Summary

   An enhanced monitoring mechanism is required to support temporal and
   hitless segment monitoring which meets the two network objectives
   described in section 3.8 of RFC 6371 [RFC6371] and reported in
   Section 3 of this document.

   The enhancements should minimize the issues described in Section 4,
   i.e., P-1, P-2, P-3 and P-4.

   The solution for the temporal and hitless segment monitoring has to
   cover both per-node model and per-interface model which are specified
   in RFC 6371 [RFC6371].

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   The temporal and hitless segment monitoring solutions shall support
   on-demand Packet Loss Measurement and Packet Delay Measurement
   functions and optionally other performance monitoring /fault
   management functions (e.g.  Throughput measurement, Delay variation
   measurement, Diagnostic test measurement, etc.).

8.  Security Considerations

   The security considerations defined for RFC 6378 apply to this
   document as well.  As this is simply a re-use of RFC 6378, there are
   no new security considerations.

9.  IANA Considerations

   There are no requests for IANA actions in this document.

   Note to the RFC Editor - this section can be removed before

10.  Acknowledgements

   The author would like to thank all members (including MPLS-TP
   steering committee, the Joint Working Team, the MPLS-TP Ad Hoc Group
   in ITU-T) involved in the definition and specification of MPLS
   Transport Profile.

   The authors would also like to thank Alexander Vainshtein, Dave
   Allan, Fei Zhang, Huub van Helvoort, Italo Busi, Maarten Vissers,
   Malcolm Betts, Nurit Sprecher and Jia He for their comments and
   enhancements to the text.

11.  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,

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,

   [RFC5860]  Vigoureux, M., Ed., Ward, D., Ed., and M. Betts, Ed.,
              "Requirements for Operations, Administration, and
              Maintenance (OAM) in MPLS Transport Networks", RFC 5860,
              DOI 10.17487/RFC5860, May 2010,

D'Alessandro, et al.    Expires January 21, 2016               [Page 14]
Internet-Draft      Enhanced path segment monitoring           July 2015

   [RFC5921]  Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
              L., and L. Berger, "A Framework for MPLS in Transport
              Networks", RFC 5921, DOI 10.17487/RFC5921, July 2010,

   [RFC6371]  Busi, I., Ed. and D. Allan, Ed., "Operations,
              Administration, and Maintenance Framework for MPLS-Based
              Transport Networks", RFC 6371, DOI 10.17487/RFC6371,
              September 2011, <http://www.rfc-editor.org/info/rfc6371>.

   [RFC6372]  Sprecher, N., Ed. and A. Farrel, Ed., "MPLS Transport
              Profile (MPLS-TP) Survivability Framework", RFC 6372,
              DOI 10.17487/RFC6372, September 2011,

Authors' Addresses

   Alessandro D'Alessandro
   Telecom Italia

   Email: alessandro.dalessandro@telecomitalia.it

   Loa Andersson
   Huawei Technologies

   Email: loa@mail01.huawei.com

   Manuel Paul
   Deutsche Telekom

   Email: Manuel.Paul@telekom.de

   Satoshi Ueno
   NTT Communications

   Email: satoshi.ueno@ntt.com

   Kaoru Arai

   Email: arai.kaoru@lab.ntt.co.jp

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   Yoshinori Koike

   Email: koike.yoshinori@lab.ntt.co.jp

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