MPLS Working Group                                     A. D'Alessandro
Internet Draft                                          Telecom Italia
                                                      Deutsche Telekom
Intended status: Informational                                S. Ueno
                                                    NTT Communications

Expires: April 30, 2012                               October 31, 2011

               Temporal and hitless path segment monitoring

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   publication of this document. Please review these documents carefully,
   as they describe your rights and restrictions with respect to this


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

   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.

   This document is a product of a joint Internet Engineering Task Force
   (IETF) / International Telecommunications Union Telecommunications
   Standardization Sector (ITU-T) effort to include an MPLS Transport
   Profile within the IETF MPLS and PWE3 architectures to support the
   capabilities and functionalities of a packet transport network.

Table of Contents

   1. Introduction ................................................ 4
   2. Conventions used in this document............................ 4
      2.1. Terminology ............................................ 5
      2.2. Definitions ............................................ 5
   3. Network objectives for monitoring............................ 5

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   4. Problem statement ........................................... 5
   5. OAM functions for segment monitoring ......................... 9
   6. Further consideration of requirements for enhanced segment
   monitoring .................................................... 10
      6.1. Necessity of on-demand single-layer monitoring.......... 10
      6.2. Necessity of on-demand monitoring independent from proactive
      monitoring ................................................. 11
      6.3. On-demand diagnostic procedures ........................ 12
   7. Conclusion ................................................. 13
   8. Security Considerations..................................... 14
   9. IANA Considerations ........................................ 14
   10. References ................................................ 14
      10.1. Normative References.................................. 14
      10.2. Informative References................................ 15
   11. Acknowledgments ........................................... 15

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1. Introduction

   A packet transport network will enable carriers or service providers
   to use network resources efficiently, reduce operational complexity
   and provide 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 OTN.

   Unlike in SDH or OTN networks, where OAM is an inherent part of every
   frame and frames are also transmitted in idle mode, it is not per se
   possible to constantly monitor the status of individual connections
   in packet networks. Packet-based OAM functions are flexible and
   selectively configurable according to operators' needs.

   According to the MPLS-TP OAM requirements [1], mechanisms MUST be
   available for alerting a service provider of a fault or defect
   affecting the service(s) provided. In addition, to ensure that faults
   or degradations can be localized, operators need a method to analyze
   or investigate the problem. From the fault localization perspective,
   end-to-end monitoring is insufficient. Using end-to-end OAM
   monitoring, when one problem occurs in an MPLS-TP network, the
   operator can detect the fault, but is not able to localize it.

   Thus, a specific segment monitoring function for detailed analysis,
   by focusing on and selecting 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[5],
   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 of the segment monitoring, following up the work done in [5].
   Moreover, this document explains detailed requirements on the new
   temporal and hitless segment monitoring function which are not
   covered in [5].

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 [1].

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  2.1. Terminology

   HSPM Hitless Path Segment Monitoring

   LSP  Label Switched Path

   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 monitoring

   There are two indispensable network objectives for MPLS-TP networks
   as described in section 3.8 of [5].

   (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.

   It is common in transport networks that network objective (1) is
   mandatory and that regarding network objective (2) the monitoring
   shall not change the forwarding behavior.

4. Problem statement

   To monitor, protect, or manage portions of transport paths, such as
   LSPs in MPLS-TP networks, the Sub-Path Maintenance Element (SPME) is
   defined in [2]. 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 header (MPLS
   header)[3] 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.

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   This method has the following general issues, which are fatal in
   terms of cost and operation.

   (P-1) Increasing the overhead by the stacking of shim header(s)

   (P-2) Increasing the address management complexity, as new MEPs and
   MIPs need to be configured for the SPME in the old MEG

   Problem (P-1) leads to decreased efficiency as bandwidth is wasted
   only for maintenance purposes. As the size of monitored segments
   increases, the size of the label stack grows. Moreover, if the
   operator wants to monitor the portion of a transport path without
   service disruption, one or more SPMEs have to be set in advance until
   the end of life of a transport path, which is not temporal or on-
   demand. Consuming additional bandwidth permanently for only the
   monitoring purpose should be avoided to maximize the available

   Problem (P-2) is related to an identifier-management issue. The
   identification of each layer in case of LSP label stacking is
   required in terms of strict sub-layer management for the segment
   monitoring in a MPLS-TP network. There is no standardized way to
   identify a layer, however a possible rule of differentiating layers
   will be necessary at least within an administrative domain, if SPME
   is applied for on-demand OAM functions. 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 layer is
   not desirable in terms of simplified operation featured by current
   transport networks. Reducing the managed identifiers and managed
   layers should be the fundamental direction in designing the

   The most familiar example for SPME in transport networks is Tandem
   Connection Monitoring (TCM), which can for example be used for a
   carrier's carrier solution, as shown in Fig. 17 of the framework
   document[2]. However, in this case, the SPMEs have to be pre-
   configured. If this solution is applied to specific segment
   monitoring within one operator domain, all the necessary specific
   segments have to be pre-configured. This setting increases the
   managed objects as well as the necessary bandwidth, shown as Problem
   (P-1) and (P-2). Moreover, as a result of these pre-configurations,
   they impose operators to pre-design the structure of sub-path
   maintenance elements, which is not preferable in terms of operators'
   increased burden. These concerns are summarized in section 3.8 of [5].

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   Furthermore, in reality, all the possible patterns of path segment
   cannot be set in SPME, 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. This restriction is inconvenient when operators have to
   fix issues in an on-demand manner.To avoid these issues, the temporal
   and on-demand setting of the SPME(s) is needed and more efficient for
   monitoring in MPLS-TP transport network operation.

   However, using currently defined methods, the temporal setting of
   SPMEs also causes the following problems due to label stacking, which
   are fatal in terms of intrinsic monitoring and service disruption.

   (P'-1) Changing the condition of the original transport path by
   changing the length of all the MPLS frames and changing label

   (P'-2) Disrupting client traffic over a transport path, if the SPME
   is temporally configured.

   Problem (P'-1) is a fatal problem in terms of intrinsic monitoring.
   As shown in network objective (2), the monitoring function needs to
   monitor the status without changing any conditions of the targeted
   monitored segment or the transport path. If the conditions of the
   transport path change, the measured value or observed data will also
   change. This can 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.

   Another aspect is that 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.

   Figure 1 shows an example of SPME setting. In the figure, X means the
   one label 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 Fig.1, SPME changes the length of all the MPLS
   frames and changes 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 (Delay measurement
   and loss measurement) are sensitive to those changes.

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

   (After SPME settings)
    ---     ---     ---     ---     ---
   |   |   |   |   |   |   |   |   |   |
   |   |   |   |   |   |   |   |   |   |
    ---     ---     ---     ---     ---
    MEP     \                  /    MEP <= transport path
             --210--C--220--            <= SPME
            MEP'          MEP'

                  Figure 1 : An Example of a SPME setting

   Problem (P'-2) was not fully discussed, although the make-before-
   break procedure in the survivability document [4] seemingly supports
   the hitless configuration for monitoring according to the framework
   document [2]. The reality is the hitless configuration of SPME is
   impossible without affecting the conditions of the targeted transport
   path, because the make-before-break procedure is premised on the
   change of the inner label value. This means changing one of the
   settings in MPLS shim header.

   Moreover, this 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. The removal of SPME whose
   segment is monitored could have the same impact (disruption of client
   traffic) as the creation of an SPME on the same LSP.

   Note: (P'-2) will be removed when non-disruptive make-before-break
   (in both with and without C-plane environment) is specified in other
   MPLS-TP documents. However, (P'-2) could be replaced with the
   following issue. ''Non-disruptive MBB, in other words, taking an
   action similar to switching just for monitoring is not an ideal
   operation in transport networks.

   The 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

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   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-
   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( and disrupt the in-service customer traffic). From
   the perspective of monitoring in transport network operation, a
   solution avoiding those issues or minimizing their impact is required.
   Another monitoring mechanism is therefore required that supports
   temporal and hitless path segment monitoring. Hereafter it is called
   on-demand hitless path segment monitoring (HPSM).

   Note: The above sentence ''and disrupt the in-service customer
   traffic'' might need to be modified depending on the result of future
   discussion about (P'-2).

5. OAM functions using segment monitoring

   OAM functions in which on-demand HPSM is required are basically
   limited to on-demand monitoring which are defined in OAM framework
   document [5], because those segment monitoring functions are used to
   locate the fault/degraded point or to diagnose the status for
   detailed analyses, especially when a problem occurred. In other words,
   the characteristic of "on-demand" is generally temporal for
   maintenance operation. Conversely, this could be a good reason that
   operations should not be based on pre-configuration and pre-design.

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   Packet loss and packet delay measurements are OAM functions in which
   hitless and temporal segment monitoring are strongly required because
   these functions are supported only between end points of a transport
   path. If a fault or defect occurs, there is no way 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 action to solve the
   problem. Therefore, on-demand HPSM for packet loss and packet delay
   measurements are indispensable for transport network operation.

   Regarding other on-demand monitoring functions path segment
   monitoring is desirable, but not as urgent as for packet loss and
   packet delay measurements.

   Regarding out-of-service on-demand monitoring functions, such as
   diagnostic tests, there seems no need for HPSM. However, specific
   segment monitoring should be applied to the OAM function of
   diagnostic test, because SPME doesn't meet network objective (2) in
   section 3. See section 6.3.


     The solution for temporal and hitless segment monitoring should not
     be limited to label stacking mechanisms based on pre-configuration,
     such as PST/TCM(label stacking), which can cause the issues (P-1)
     and (P-2) described in Section 4.

     The solution for HPSM has to cover both per-node model and per-
     interface model which are specified in [5].

6. Further consideration of requirements for enhanced segment monitoring

  6.1. Necessity of on-demand single-level monitoring

   The new segment monitoring function is supposed to be applied mainly
   for diagnostic purpose on-demand. 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 degradation point on 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

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   A combination of multi-level and simultaneous monitoring is the most
   powerful tool for accurately diagnosing the performance of a
   transport path. However, considering the substantial benefits to
   operators, a strict monitoring function which is required in such as
   a test environment of a laboratory does not seem to be necessary in
   the field. To summarize, on-demand and in-service (hitless) single-
   level segment monitoring is required, on-demand and in-service multi-
   level segment monitoring isdesirable. Figure 2 shows an example of a
   multi-level on-demand segment monitoring.

    ---     ---     ---     ---     ---
   |   |   |   |   |   |   |   |   |   |
   | A |   | B |   | C |   | D |   | E |
    ---     ---     ---     ---     ---
    MEP                             MEP <= ME of a transport path
      +-----------------------------+   <= End-to-end monitoring
            *------------------*        <= segment monitoring level1
              *-------------*           <= segment monitoring level2
                    *-*                 <= segment monitoring level3

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

  6.2. Necessity of on-demand monitoring independent from end-to-end
     proactive monitoring

   As multi-level simultaneous monitoring only for on-demand new path
   segment monitoring was already discussed in section6.1, next we
   consider the necessity of simultaneous monitoring of end-to-end
   current proactive monitoring and new on-demand path segment
   monitoring. Normally, the on-demand path segment monitoring is
   configured in a segment of a maintenance entity of a transport path.
   In this environment, on-demand single-level monitoring should be done
   without disrupting pro-active monitoring of the targeted end-to-end
   transport path.

   If operators have to disable the pro-active monitoring during the on-
   demand  hitless path segment monitoring, the network operation system
   might miss any performance degradation of user traffic. This kind of
   inconvenience should be avoided in the network operations.

   Accordingly, the on-demand single lavel path segment monitoring is
   required without changing or interfering the proactive monitoring of
   the original end-to-end transport path.

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    ---     ---     ---     ---     ---
   |   |   |   |   |   |   |   |   |   |
   | 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

  6.3. Necessity of arbitrary segment monitoring

   The main objective of on-demand segment monitoring is to diagnose the
   fault points. One possible diagnostic procedure is to fix one end
   point of a segment at the MEP of a transport path and change
   progressively the length of the segment in order. This example is
   shown in Fig. 4. This approach is considered as a common and
   realistic diagnostic procedure. In this case, one end point of a
   segment can be anchored at MEP at any time.

   Other scenarios are also considered, one shown in Fig. 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
   in which a subset of OAM functions are supported. In this case, if
   one end point and an originator of the diagnostic packet are limited
   to the position of MEP, on-demand segment monitoring will be
   ineffective because all the segments cannot be diagnosed (For example,
   segment monitoring 3 in Fig.5 is not available and it is not possible
   to localize the fault point).

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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 : One possible procedure to localize a fault point by
                  sequential on-demand segment monitoring

   Accordingly, on-demand monitoring of arbitrary segments is mandatory
   in the case in Fig. 5. As a result, on-demand HSPM should be set in
   an arbitrary segment of a transport path and diagnostic packets
   should be inserted from at least any of intermediate maintenance
   points of the original ME.

            ---     ---     ---
    ---    |   |   |   |   |   |    ---
   | 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 : Example where on-demand monitoring has to be configured in
                            arbitrary segments

7. Conclusion

   It is requested that another monitoring mechanism is required to
   support temporal and hitless segment monitoring which meets the two
   network objectives mentioned in Section 3 of this draftthat are
   described also in section 3.8 of [5].

   The enhancements should minimize the issues described in Section 4,,
   i.e., P-1, P-2, P'-1( and P'-2,) to meet those two network objectives.

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   The solution for the temporal and hitless segment monitoring has to
   cover both per-node model and per-interface model which are specified
   in [5]. In addition, the following requirements should be considered
   for an enhanced temporal and hitless path segment monitoring function.

   Note: (P'-2) needs to be reconsidered.- An on-demand and in-service
   ''single-level'' segment monitoring ismandatory. Multi-level segment
   monitoring is optional.

   - ''On-demand and in-service'' single level segment should be done
   without changing or interfering any condition of pro-active
   monitoring of an original ME of a transport path.

   - On-demand and in-service segment monitoring should be able to be
   set in an arbitrary segment of a transport path.

   The followings are specific requirements on each on-demand OAM
   function.Mandatory: Packet Loss Measurement and Packet Delay

   Option: Connectivity verification, Diagnostic Tests (Throughput test),
   Route tracing

8. Security Considerations

   This document does not by itself raise any particular security

9. IANA Considerations

   There are no IANA actions required by this draft.

10. References

  10.1. Normative References

   [1]  Vigoureux, M., Betts, M., Ward, D., "Requirements for OAM in
         MPLS Transport Networks", RFC5860, May 2010

   [2]  Bocci, M., et al., "A Framework for MPLS in Transport Networks",
         RFC5921, July 2010

   [3]  Rosen, E., et al., "MPLS Label Stack Encoding", RFC 3032,
         January 2001

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   [4]  Sprecher, N., Farrel, A. , ''Multiprotocol Label Switching
         Transport Profile Survivability Framework'', draft-ietf-mpls-tp-
         survive-fwk-06.txt(work in progress), June 2010

   [5]  Busi, I., Dave, A. , "Operations, Administration and
         Maintenance Framework for MPLS-based Transport Networks ",
         draft-ietf-mpls-tp-oam-framework-11.txt(work in progress),
         February 2011

  10.2. Informative References


11. Acknowledgments

   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 and Nurit Sprecher for their comments and enhancements to the

   This document was prepared using

Authors' Addresses

   Alessandro D'Alessandro
   Telecom Italia

   Manuel Paul
   Deutsche Telekom

   Satoshi Ueno
   NTT Communications

   Yoshinori Koike

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