Network Working Group Thomas D. Nadeau
Internet Draft Monique Morrow
Expires: August 2003 George Swallow
Cisco Systems, Inc.
David Allan
Nortel Networks
February 2003
OAM Requirements for MPLS Networks
draft-ietf-mpls-oam-requirements-00.txt
Status of this Memo
This document is an Internet-Draft and is in full
conformance with all provisions of Section 10 of RFC 2026
[RFC2026].
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Abstract
As transport of diverse traffic types such as voice, frame
relay, and ATM over MPLS become more common, the ability to detect,
handle and diagnose control and data plane defects becomes critical.
Detection and specification of how to handle those defects is not
only important because such defects may not only affect the
fundamental operation of an MPLS network, but also because they
may impact SLA commitments for customers of that network.
This Internet draft describes requirements for user and data
plane operations and management (OAM) for Multi-Protocol
Label Switching (MPLS). These requirements have been gathered
from network operators who have extensive experience deploying
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MPLS networks, similarly some of these requirements have
appeared in other documents [Y1710]. This draft specifies OAM
requirements for MPLS, as well as for applications of MPLS such
as pseudowire voice and VPN services. Those interested in specific
issues relating to instrumenting MPLS for OAM purposes are directed
to [FRAMEWORK]
Table of Contents
Introduction 2
Terminology 2
Motivations 3
Requirements 4
Security Considerations 8
Acknowledgments 9
References 9
Authors' Addresses 10
Intellectual Property Rights Notices 11
Full Copyright Statement 11
1. Introduction
This Internet draft describes requirements for user and data
plane operations and management (OAM) for Multi-Protocol
Label Switching (MPLS). These requirements have been gathered
from network operators who have extensive experience deploying
MPLS networks. This draft specifies OAM requirements
for MPLS, as well as for applications of MPLS such as
pseudowire [PWE3FRAME] voice, and VPN services.
No specific mechanisms are proposed to address these
requirements at this time. The goal of this draft is to
identify a commonly applicable set of requirements for MPLS
OAM. Specifically, a set of requirements that apply to
the most common set of MPLS networks deployed by service
provider organizations today. These requirements can then be used
as a base for network management tool development and to guide
the evolution of currently specified tools, as well as the
specification of OAM functions that are intrinsic to protocols
used in MPLS networks.
Comments should be made directly to the MPLS mailing list
at mpls@uu.net.
This memo does not, in its draft form, specify a standard
for the Internet community.
2. Terminology
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CE: Customer Edge
Defect: Any error condition that prevents an LSP
functioning correctly. For example, loss of an
IGP path will most likely also result in an LSP
not being able to deliver traffic to its
destination. Another example is the breakage of
a TE tunnel. These may be due to physical
circuit failures or failure of switching nodes
to operate as expected.
Multi-vendor/multi-provider network operation typically
requires agreed upon definitions of defects (when it is
broken and when it is not) such that both recovery
procedures and SLA impacts can be specified.
ECMP: Equal Cost Multipath
LSP: Label Switch Path
LSR: Label Switch Router
OAM: Operations and Management
PE: Provider Edge
PW: Pseudowire
SLA: Service Level Agreement
VCC: Virtual Circuit Channel
VPC: Virtual Path Connection
3 Motivations
MPLS OAM has been tackled in numerous Internet drafts.
However all existing drafts focus on single provider
solutions or focus on a single aspect of the MPLS architecture
or application of MPLS. For example, the use of RSVP or LDP
signaling and defects may be covered in some deployments,
and a corresponding SNMP MIB module exists to manage this
application; however, the handling of defects and specification
of which types of defects are interesting to operational
networks may not have been created in concert with those for
other applications of MPLS such as L3 VPN. This leads to
inconsistent and inefficient applicability across the MPLS
architecture, and/or requires significant modifications to
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operational procedure and systems in order to provide consistent
and useful OAM functionality. As MPLS matures relationships
between providers has become more complex. Furthermore, the
deployment of multiple concurrent applications
of MPLS is commonplace. This has led to a need to consider
deployments that span arbitrary networking arrangements and
boundaries so that broader and more uniform applicability
to the MPLS architecture for OAM is possible.
3. Requirements
The following sections enumerate the OAM requirements
gathered from service providers. Each requirement is
further specified in detail to further clarify its
applicability.
3.1 Detection of Broken Label Switch Paths
The ability to detect a broken Label Switch Path (LSP)
should not require manual hop-by-hop troubleshooting of
each LSR used to switch traffic for that LSP. For example,
it is not desirable to manually visit each LSR
along the data plane path used to transport an LSP; instead,
this function should be automated and performed from the
origination of that LSP. Furthermore, the automation of
path liveliness is desired in cases where large amounts of
LSPs might be tested. For example, automated PE-to-PE
LSP testing functionality is desired. The goal is to detect LSP
problems before customers do, and this requires detection of
problems in a "reasonable" amount of time. One useful definition
of reasonable is both predictable and consistent. If the time to
detect defects is specified and tools designed accordingly then
a harmonized operational framework can be established both
within MPLS levels, and with MPLS applications. If the time to
detect is known, then automated responses can be
specified both w.r.t.with regard to resiliency and SLA
reporting. One consequence is that ambiguity in maintenance
procedures MUST be minimized as ambiguity in test results impacts
detection time.
Although ICMP-based ping can be sent through an LSP, the use of
this tool to verify the LSP path liveliness has the potential
for returning erroneous results (both positive and negative)
given the nature of MPLS LSPs. For example, failures can be
may occur where inconsistencies exist between the IP and MPLS
forwarding tables, inconsistencies in the MPLS control and data
plane or problems with the reply path (i.e.: a reverse MPLS
path does not exist). Detection tools should have minimal
dependencies on network components that do not implement the LSP.
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Furthermore, the path liveliness function
MUST have the ability to support equal cost multipath
(ECMP) scenarios within the operator's network. Specifically,
the ability to detect failures on any parallel (i.e.: equal
IGP cost) paths used to load share traffic in order to more
efficiently use the network. It is common to base the algorithm
of how to load share traffic by examining certain fields within
the packet header. Unfortunately, there is no standard for this
algorithm, but it is important that any function be capable
of detecting failures on all operational paths as failure of
any branch may lead to loss of traffic, regardless of load sharing
algorithm. This introduces complexity into ensuring that ECMP
connectivity permutations are exercised, and that defect
detection occurs in a reasonable amount of time. [GUIDELINES]
discusses some of the issues and offers suggestions for ensuring
mutual compatibility of ECMP and maintenance functions (both
detection and diagnostic).
3.2 Diagnosis of a Broken Label Switch Path
The ability to diagnose a broken LSP and to isolate the failed
resource in the path is required. This is particularly true for
misbranching defects which are particularly difficult to specify
recovery actions in an LDP network.
Experience suggests that this is best accomplished via a path
trace function that can return the entire list of LSRs and links
used by a certain LSP (or at least the set of LSRs/links up to the
location of the defect) is required. The tracing capability should
include the ability to trace recursive paths, such as when nested
LSPs are used, or when LSPs enter and exit traffic-engineered
tunnels [TUNTRACE]. This path trace function must also be
capable of diagnosing LSP mis-merging by permitting comparison
of expected vs. actual forwarding behavior at any LSR in the path.
The path trace capability should be capable of being
executed from both the head end Label Switch Router (LSR) and any
mid-point LSR. Additionally, the path trace function MUST have
the ability to support equal cost multipath scenarios as described
above in section 3.1.
3.3 Path characterization
The ability of a path trace function to reveal details of LSR
forwarding operations relevant to OAM functionality. This would
include but not be limited to:
- use of pipe or uniform TTL models by an LSR
- externally visible aspects of load spreading (such as
ECMP), including
type of algorithm used
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examples of how algorithm will spread traffic
- data/control plane OAM capabilities of the LSR
- stack operations performed by the LSR (pushes and pops)
3.4 Service Level Agreement Measurement
Mechanisms are required to measure diverse aspects of Service
Level Agreements:
- availability - in which the service is considered to be
available and the other aspects of performance measurement
listed below have meaning, or unavailable and other aspects
of performance measurement do not.
- latency - amount of time required for traffic to transit
the network
- packet loss
- jitter - measurement of latency variation
Such measurements can be made independently of the user traffic
or via a hybrid of user traffic measurement and OAM probing.
At least one mechanism
is required to measure the quantity
(i.e.: number of packets) of OAM packets. In addition, the
ability to measure the qualitative aspects of OAM probing must
be available to specifically compute the latency of OAM packets
generated and received at each end of a tested LSP. Latency is
considered in this context as a measurable parameter for SLA reporting.
There is no assumption that bursts of OAM packets are required to
characterize the performance of an LSP, but it is suggested that any
method considered be capable of measuring the latency of an LSP with
minimal impact on network resources.
3.5 Frequency of OAM Execution
The operator MUST be have the flexibility to configure OAM
parameters and the frequency of the execution of any OAM
functions provided that there is some synchronization possible
of tool usage for availability metrics. The motivation for this
is to permit the network to function as a system of harmonious
OAM functions consistent across the entire network.
To elaborate, there are defect conditions (specifically
misbranching or misdirection of traffic) for probe based detection
mechanisms combined with automated network response requires
harmonization of probe insertion rates and probe handling across
the network in order to avoid flapping.
One observation would be that commoditization of MPLS, common
optimized implementation of monitoring tools and the need for inter-
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carrier harmonization of defect and SLA handling will drive
specification of OAM parameters to commonly agreed on values and
such values will have to be harmonized with the surrounding
technologies (e.g. SONET/SDH, ATM etc.) in order to be useful.
This will become particularly important as networks scale
and misconfiguration can result in churn, alarm flapping etc.
3.5 Alarm Suppression and layer coordination
Devices must provide alarm suppression functionality that
prevents the generation of superfluous generation of alarms.
When viewed in conjuction with requirement 3.6 below, this
typically requires fault notification to the LSP egress, that
may have specific time constraints if the client PW independently
implements path continuity testing (for example ATM I.610
Continuity check (CC)[I610]).
This would also be true for LSPs that have client LSPs that are
monitored. MPLS arbitrary hierarchy introduces the opportunity to have
multiple MPLS levels attempt to respond to defects simultaneously.
Mechanisms are required to coordinate network response to defects.
3.6 Support for OAM Interworking for Fault Notification
An LSR supporting OAM functions for pseudo-wire functions that
join one or more networking technologies over MPLS must be
able to translate an MPLS defect into the native technology's
error condition. For example, errors occurring over the MPLS
transport LSP that supports an emulated ATM VC must translate
errors into native ATM OAM AIS cells at the edges of the pseudo-
wire. The mechanism SHOULD consider possible bounded detection
time parameters, e.g., a "hold off" function before reacting as
to harmonize with the client OAM. One goal would be alarm suppression
in the psuedo-wire's client layer. As observed in 3.5, this requires
that the MPLS layer perform detection in a bounded timeframe in
order to initiate alarm suppression prior to the psuedo-wire
client layer independently detecting the defect.
3.7 Error Detection and Recovery.
Mechanisms are needed to detect an error, react to it (ideally
in some form of automated response by the network), recover from
it and alert the network operator prior to the customer informing
the network operator of the error condition. The ideal situation
would be where the network is resilient and can restore service
prior any significant impact on the customer perception of the
service. There are also defects that by virtue of available network
resources or topology that cannot be recovered automatically.
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It is however, sometimes a requirement that the customer be
notified of the defect condition at the same time that the network
operator is made aware of the defect (as in the example of alarm
suppression for PW clients discussed above). In these situations,
the customer network may be capable of processing automated responses
based on notification of a defect condition. It is preferred
that the format of these notifications be made consistent (i.e.:
standardized) as to increase the applicability of such messages.
Depending on the device's capabilities, the device may be programmed
to take automatic corrective actions as a result of detection of
defect conditions. These actions may be user or operator-specified,
or may simply be inherent to the underlying transport technology
(i.e.: MPLS Fast-Reroute, graceful restart or high-availability
functionality).
3.8 The commoditization of MPLS will require common information
modeling of management and control of OAM functionality. This
will be reflected in the the integration of standard MPLS-related
MIBs (e.g. [LSRMIB][TEMIB][LBMIB][FTNMIB]) for fault, statistics
and configuration management. These standard interfaces
provide operators with common programmatic interface access to
operations and management functions and their status.
3.9 Detection of Denial of Service attacks as part of security
management.
4. Security Considerations
LSP mis-merging has security implications beyond that of simply
being a network defect. LSP mis-merging can happen due to a number
of potential sources of failure, some of which (due to MPLS label
stacking) are new to MPLS.
The performance of diagnostic functions and path characterization
involve extracting a significant amount of information about
network construction which the network operator may consider private.
Mechanisms are required to prevent unauthorized use of either those
tools or protocol features.
5. Acknowledgments
The authors wish to acknowledge and thank the following
individuals for their valuable comments to this document:
Adrian Smith, British Telecom; Chou Lan Pok, SBC; Mr.
Ikejiri, NTT Communications and Mr.Kumaki of KDDI.
Hari Rakotoranto, Cisco Systems; Danny McPherson from TCB.
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6. References
[TUNTRACE] Bonica, R., Kompella, K., Meyer, D.,
"Tracing Requirements for Generic Tunnels",
Internet Draft <draft-bonica-tunneltrace-
02.txt>, November 2001.
[LSRMIB] Srinivasan, C., Viswanathan, A. and T.
Nadeau, "MPLS Label Switch Router Management
Information Base Using SMIv2", Internet
Draft <draft-ietf-mpls-lsr-mib-07.txt>,
January 2001.
[TEMIB] Srinivasan, C., Viswanathan, A. and T.
Nadeau, "MPLS Traffic Engineering Management
Information Base Using SMIv2", Internet
Draft <draft-ietf-mpls-te-mib-07.txt>,
August 2001.
[FTNMIB] Nadeau, T., Srinivasan, C., and A.
Viswanathan, "Multiprotocol Label Switching
(MPLS) FEC-To-NHLFE (FTN) Management
Information Base", Internet Draft <draft-
ietf-mpls-ftn-mib-03.txt>, August 2001.
[LBMIB] Dubuc, M., Dharanikota, S., Nadeau, T., J.
Lang, "Link Bundling Management Information
Base Using SMIv2", Internet Draft <draft-
ietf-mpls-bundle-mib-00.txt>, September
2001.
[PWE3FRAME] Pate, P., Xiao, X., White., C., Kompella.,
K., Malis, A., Johnson, T., and T. Nadeau,
"Framework for Pseudo Wire Emulation Edge-to-
Edge (PWE3)", Internet Draft <draft-ietf-
pwe3-framework-00.txt>, September, 2001.
[RFC2026] S. Bradner, "The Internet Standards Process
-- Revision 3", RFC 2026, October 1996.
[Y1710] ITU-T Recommendation Y.1710, "Requirements for
OAM Functionality In MPLS Networks"
[GUIDELINES] Allan, D., "Guidelines for MPLS load
balancing", Internet draft,
<draft-allan-mpls-loadbal-01.txt>, February
2003
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[I610] ITU-T Recommendation I.610, "B-ISDN operations and
maintenance principles and functions", February 1999
[FRAMEWORK] Allan et.al. "A Framework for MPLS OAM", Internet
draft <draft-allan-mpls-oam-frmwk-04.txt>, February 2003
7. Authors' Addresses
Thomas D. Nadeau
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, MA 01824
Phone: 978-244-3051
Email: tnadeau@cisco.com
Monique Jeanne Morrow
Cisco Systems, Inc.
Glatt-Com, 2nd Floor
CH-8301
Switzerland
Voice: (0)1 878-9412
EMail: mmorrow@cisco.com
George Swallow
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA 01824
Voice: 978 244 8143
Email: swallow@cisco.com
David Allan
Nortel Networks
3500 Carling Ave.
Voice: 1-613-763-6362
Ottawa, Ontario, CANADA
Email: dallan@nortelnetworks.com
8. Full Copyright Statement
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