MPLS Working Group A. D'Alessandro
Internet Draft Telecom Italia
M.Paul
Deutsche Telekom
Intended status: Informational S. Ueno
NTT Communications
Y.Koike
NTT
Expires: January 13, 2012 July 11, 2011
Temporal and hitless path segment monitoring
draft-koike-mpls-tp-temporal-hitless-psm-03.txt
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Abstract
The MPLS transport profile (MPLS-TP) is being standardized to enable
carrier-grade packet transport and complement converged packet
network deployments. One of 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 are 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
drawbacks.
This document elaborates on the problem statement for the path
segment monitoring function. Moreover, this document requests to add
network objectives to solve or improve the issues and proposes to
consider a new improved method of 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
4. Problem statement ........................................... 5
5. OAM functions for segment monitoring .........................8
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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........................... 11
7. Conclusion ................................................. 12
8. Security Considerations..................................... 13
9. IANA Considerations ........................................ 13
10. References ................................................ 13
10.1. Normative References..................................... 13
10.2. Informative References................................... 14
11. Acknowledgments ........................................... 14
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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) provides. 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 point.
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. Moreover, this document requests to add
network objectives to solve or improve the issues and proposes to
reconsider the new improved method of the segment monitoring.
2. Conventions used in this document
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 RFC-2119 [1].
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2.1. Terminology
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
None
3. Network objectives for monitoring
There are two indispensable network objectives in the current on-
going MPLS-TP standard 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 was agreed in ITU-T SG15 that Network objective (1) is mandatory
and that regarding Network objective (2) the monitoring shall be
hitless and not change the forwarding behavior.
4. Problem statement
To monitor, protect, or manage a portion of a transport path, such as
LSP 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 LSP 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 a 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.
As the size of monitored segments increases, the size of the label
stack grows. Moreover, if 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 the monitoring purpose should be avoided to maximize the
available bandwidth.
Problem (P-2) is related to an identifier issue, of which the
discussion is still continuing and not so critical at the moment, but
can be quite inefficient if the policy of allocating the identifier
is different in each layer. 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 identifier and managed layer
should be the fundamental direction in designing the architecture.
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).
To avoid these issues, the temporal setting of the SPME(s) only when
necessary seems reasonable and efficient for monitoring in MPLS-TP
transport network operation. Unfortunately, the temporal settings of
SPMEs also cause 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 the MPLS frame (delay measurement and loss
measurement can be sensitive)
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(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.
The monitoring function checks 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.
In addition, 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 is completely different
from the original circumstances.
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. This is not the monitoring of the original transport path but
the monitoring of a different path.
(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
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
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document [2]. The reality is the hitless configuration of SPME is
impossible without affecting the circumstances of the targeted
transport path, because the make-before-break procedure is premised
on the change of the label value. This means changing one of the
settings in MPLS shim header and should not be allowed as explained
in (P'-1) above.
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
traffics) as creation of an SPME on the same LSP.
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
within the monitored segment may disappear 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 way MPLS works requires use of SPMEs to be designed in advance,
hence the utility of SPMEs is basically limited.
To summarize, the problem statement is that the current sub-path
maintenance based on a hierarchical LSP (SPME) is problematic in
terms of increasing bandwidth by label stacking and managing objects
by layer stacking and address management. A 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.
5. OAM functions using segment monitoring
OAM functions in which segment monitoring 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
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locate the fault/degraded point or to diagnose the status for
detailed analyses, especially when a problem occurred.
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, temporal and hitless monitoring for packet loss
and packet delay measurements are indispensable for transport network
operation.
Regarding other on-demand monitoring functions, 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 hitless settings. However,
specific segment monitoring should be applied to the OAM function of
diagnostic test. See section 6.3.
Note:
The reason only on-demand OAM functions are discussed at this point
is because the characteristic of "on-demand" is generally temporal
for maintenance operation. Thus, operations should not be based on
pre-configuration. Pre-design and pre-configuration of PST/TCM
(label stacking) for all the possible patterns for the on-demand
(temporal) usage are not reasonable, because these tasks will
increase the operator's burden, although pre-configuration of PST
for pro-active usage may be accepted considering the agreement thus
far. Therefore, the solution for temporal and hitless segment
monitoring does not need to be limited to label stacking mechanisms,
such as PST/TCM(label stacking), which can cause the issues (P-1)
and (P-2) described in Section 4.
The solution for temporal and hitless segment monitoring has to
cover both per-node model and per-interface model which are
specified in [5].
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6. Further consideration of requirements for enhanced segment monitoring
An existing segment monitoring function relates to SPME that
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. From this
perspective, monitoring function related to can be considered
''proactive multi-layer monitoring,'' which has already been determined
by consensus to be mandatory in MPLS-TP.
6.1. Necessity of on-demand single-layer monitoring
In contrast to the ''proactive multi-layer monitoring'' so called
''SPME'', 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-
layer) 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
function.
A combination of multi-layer and simultaneous monitoring is the most
powerful tool for accurately diagnosing the performance of a
transport path. However, considering the balance of estimated
implementation difficulties and the substantial benefits to operators,
a strict monitoring function such as in a test environment in a
laboratory does not seem to be necessary in the field. To summarize,
on-demand and in-service (hitless) single-layer segment monitoring is
required, but the need for on-demand and in-service multi-layer
segment monitoring is not as urgent at the moment. Figure 2 shows an
example of a multi-layer on-demand segment monitoring.
--- --- --- --- ---
| | | | | | | | | |
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
+-----------------------------+ <= End-to-end monitoring
*------------------* <= segment monitoring layer1
*-------------* <= segment monitoring layer2
*-* <= segment monitoring layer3
Figure 2 : An Example of a multi-layer on-demand segment monitoring
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6.2. Necessity of on-demand monitoring independent from proactive
monitoring
As multi-layer simultaneous monitoring only in layers for on-demand
monitoring was already discussed in section6.1, we consider the
necessity of simultaneous proactive and on-demand monitoring.
Normally, on-demand segment monitoring is configured in a segment of
a maintenance entity of a transport path. In this environment, on-
demand single-layer monitoring should be done irrespective of the
status of pro-active monitoring of the targeted end-to-end transport
path.
If operators have to disable the pro-active monitoring during ''on-
demand and in-service'' 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.
From this perspective, the ability for on-demand single layer segment
monitoring is required without changing or interfering the proactive
monitoring of the original end-to-end transport path.
--- --- --- --- ---
| | | | | | | | | |
| A | | B | | C | | D | | E |
--- --- --- --- ---
MEP MEP <= ME of a transport path
+-----------------------------+ <= Proactive E2E monitoring
*------------------* <= On-demand segment monitoring
Figure 3 : Relation between proactive end-to-end monitoring and on-
demand segment monitoring
6.3. On-demand diagnostic procedures
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
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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).
--- --- --- --- ---
| | | | | | | | | |
| 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 and in-service segment
monitoring 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 the two network objectives mentioned in Section
3 are met by MPLS-TP OAM solutions which has been already described
in section 3.8 of [5]. The enhancements should minimize the issues
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described in Section 4,, i.e., P-1, P-2, P'-1 and P'-2, to meet those
two network objectives. In addition, the following requirements
should be considered for an enhanced temporal and hitless path
segment monitoring function.
- An on-demand and in-service ''single-layer'' segment monitoring is
proposed. Multi-layer segment monitoring is optional.
- ''On-demand and in-service'' single layer 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.
8. Security Considerations
This document does not by itself raise any particular security
considerations.
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
[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
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10.2. Informative References
None
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
text.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Alessandro D'Alessandro
Telecom Italia
Email: alessandro.dalessandro@telecomitalia.it
Manuel Paul
Deutsche Telekom
Email: Manuel.Paul@telekom.de
Satoshi Ueno
NTT Communications
Email: satoshi.ueno@ntt.com
Yoshinori Koike
NTT
Email: koike.yoshinori@lab.ntt.co.jp
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