|Operations and Management (OAM) Requirements for Multi-Protocol
Label Switched (MPLS) Networks
|T. Nadeau, M. Morrow, G. Swallow, D.
Allan, S. Matsushima
Network Working Group T. Nadeau
Request for Comments: 4377 M. Morrow
Category: Informational G. Swallow
Cisco Systems, Inc.
Operations and Management (OAM) Requirements
for Multi-Protocol Label Switched (MPLS) Networks
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2006).
This document specifies Operations and Management (OAM) requirements
for Multi-Protocol Label Switching (MPLS), as well as for
applications of MPLS, such as pseudo-wire voice and virtual private
network services. These requirements have been gathered from network
operators who have extensive experience deploying MPLS networks.
Table of Contents
1. Introduction ....................................................2
2. Document Conventions ............................................2
3. Motivations .....................................................4
4. Requirements ....................................................4
5. Security Considerations ........................................11
6. References .....................................................12
7. Acknowledgements ...............................................13
This document 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 document
specifies OAM requirements for MPLS, as well as for applications of
Currently, there are no specific mechanisms proposed to address these
requirements. The goal of this document is to identify a commonly
applicable set of requirements for MPLS OAM at this time.
Specifically, a set of requirements that apply to the most common set
of MPLS networks deployed by service provider organizations at the
time this document was written. 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
2. Document Conventions
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 [RFC2119].
Queuing/buffering Latency: The delay caused by packet queuing (value
is variable since it is dependent on the
packet arrival rate, the packet length,
and the link throughput).
Probe-based-detection: Active measurement tool that can measure
the consistency of an LSP [RFC4379].
Defect: Any error condition that prevents a Label
Switched Path (LSP) from functioning
correctly. For example, loss of an
Interior Gateway Protocol (IGP) path will
most likely result in an LSP not being
able to deliver traffic to its
destination. Another example is the
interruption of the path for a TE tunnel.
These may be due to physical circuit
failures or failure of switching nodes to
operate as expected.
operation typically requires agreed upon
definitions of defects (when it is broken
and when it is not) such that both
recovery procedures and service level
specification impact can be specified.
Head-end Label Switching
Router (LSR): The beginning of an LSP. A head-end LSR
is also referred to as an ingress LSR.
Tail-end Label Switching
Router (LSR): The end of an LSP. A tail-end LSR is also
referred to as an egress LSR.
Propagation Latency: The delay added by the propagation of the
packet through the link (fixed value that
depends on the distance of the link and
the propagation speed).
Transmission Latency: The delay added by the transmission of the
packet over the link, i.e., the time it
takes to put the packet over the media
(value that depends on the link throughput
and packet length).
Processing Latency: The delay added by all the operations
related to the switching of labeled
packets (value is node implementation
specific and may be considered fixed and
constant for a given type of equipment).
Node Latency: The delay added by the network element
resulting from of the sum of the
transmission, processing, and
One-hop Delay: The fixed delay experienced by a packet to
reach the next hop resulting from the of
the propagation latency, the transmission
latency, and the processing latency.
Minimum Path Latency: The sum of the one-hop delays experienced
by the packet when traveling from the
ingress to the egress LSR.
Variable Path Latency: The variation in the sum of the delays
experienced by packets transiting the
path, otherwise know as jitter.
ASBR: Autonomous System Border Router
CE: Customer Edge
PE: Provider Edge
SP: Service Provider
ECMP: Equal-Cost Multi-path
LSP: Label Switched Path
LSP Ping: Label Switched Path Ping
LSR: Label Switching Router
OAM: Operations and Management
RSVP: Resource reSerVation Protocol
LDP: Label Distribution Protocol
DoS: Denial of Service
This document was created to provide requirements that could be used
to create consistent and useful OAM functionality that meets
operational requirements of those service providers (SPs) who have
deployed or are deploying MPLS.
The following sections enumerate the OAM requirements gathered from
service providers who have deployed MPLS and services based on MPLS
networks. Each requirement is specified in detail to clarify its
applicability. Although the requirements specified herein are
defined by the IETF, they have been made consistent with requirements
gathered by other standards bodies such as the ITU [Y1710].
4.1. Detection of Label Switched Path Defects
The ability to detect defects in a broken LSP MUST 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 transited by an LSP; instead, this
function MUST be automated and able to be performed at some operator
specified frequency from the origination point of that LSP. This
implies solutions that are interoperable to allow for such automatic
Furthermore, the automation of path liveliness is desired in cases
where large numbers of LSPs might be tested. For example, automated
ingress LSR to egress LSR testing functionality is desired for some
LSPs. The goal is to detect LSP path defects before customers do,
which requires detection and correction of LSP defects in a manner
that is both predictable and within the constraints of the service
level agreement under which the service is being offered. Simply
put, the sum of the time it takes an OAM tool to detect a defect and
the time needed for an operational support system to react to this
defect, by possibly correcting it or notifying the customer, must
fall within the bounds of the service level agreement in question.
Synchronization of detection time bounds by tools used to detect
broken LSPs is required. Failure to specify defect detection time
bounds may result in an ambiguity in test results. If the time to
detect broken LSPs is known, then automated responses can be
specified with respect and regard to resiliency and service level
specification reporting. Further, if synchronization of detection
time bounds is possible, an operational framework can be established
to guide the design and specification of MPLS applications.
Although an ICMP-based ping [RFC792] can be sent through an LSP as an
IP payload, the use of this tool to verify the defect-free operation
of an LSP has the potential of returning erroneous results (both
positive and negative) for a number of reasons. For example, in some
cases, because the ICMP traffic is based on legally addressable IP
addressing, it is possible for ICMP messages that are originally
transmitted inside of an LSP to "fall out of the LSP" at some point
along the path. In these cases, since ICMP packets are routable, a
falsely positive response may be returned. In other cases, where the
data plane of a specific LSP needs to be tested, it is difficult to
guarantee that traffic based on an ICMP ping header is parsed and
hashed to the same equal-cost multi-paths (ECMP) as the data traffic.
Any detection mechanisms that depend on receiving the status via a
return path SHOULD provide multiple return options with the
expectation that one of them will not be impacted by the original
defect. An example of a case where a false negative might occur
would be a mechanism that requires a functional MPLS return path.
Since MPLS LSPs are unidirectional, it is possible that although the
forward LSP, which is the LSP under test, might be functioning, the
response from the destination LSR might be lost, thus giving the
source LSR the false impression that the forward LSP is defective.
However, if an alternate return path could be specified -- say IP for
example -- then the source could specify this as the return path to
the destination, and in this case, would receive a response
indicating that the return LSP is defective.
The OAM packet MUST follow the customer data path exactly in order to
reflect path liveliness used by customer data. Particular cases of
interest are forwarding mechanisms, such as ECMP scenarios within the
operator's network, whereby flows are load-shared across parallel
paths (i.e., equal IGP cost). Where the customer traffic may be
spread over multiple paths, the ability to detect failures on any of
the path permutations is required. Where the spreading mechanism is
payload specific, payloads need to have forwarding that is common
with the traffic under test. Satisfying these requirements
introduces complexity into ensuring that ECMP connectivity
permutations are exercised and that defect detection occurs in a
reasonable amount of time.
4.2. Diagnosis of a Broken Label Switched Path
The ability to diagnose a broken LSP and to isolate the failed
component (i.e., link or node) in the path is required. For example,
note that specifying recovery actions for mis-branching defects in an
LDP network is a particularly difficult case. Diagnosis of defects
and isolation of the failed component 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). The tracing capability SHOULD include the
ability to trace recursive paths, such as when nested LSPs are used.
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 the head-end Label Switching Router
(LSR) and may permit downstream path components to be traced from an
intermediate mid-point LSR. Additionally, the path trace function
MUST have the ability to support ECMP scenarios described in Section
4.3. Path Characterization
The path characterization function is the ability to reveal details
of LSR forwarding operations. These details can then be compared
during subsequent testing relevant to OAM functionality. This
includes but is not limited to:
- consistent use of pipe or uniform time to live (TTL) models by
an LSR [RFC3443].
- sufficient details that allow the test origin to exercise all
path permutations related to load spreading (e.g., ECMP).
- stack operations performed by the LSR, such as pushes, pops,
and TTL propagation at penultimate hop LSRs.
4.4. Service Level Agreement Measurement
Mechanisms are required to measure the diverse aspects of Service
Level Agreements, which include:
- latency - amount of time required for traffic to transit the
- packet loss
- jitter - measurement of latency variation
- defect free forwarding - the service is considered to be
available, or the service is unavailable and other aspects of
performance measurement do not have meaning.
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 number of OAM
packets. In addition, the ability to measure the quantitative
aspects of LSPs, such as jitter, delay, latency, and loss, MUST be
available in order to determine whether the traffic for a specific
LSP is traveling within the operator-specified tolerances.
Any method considered SHOULD be capable of measuring the latency of
an LSP with minimal impact on network resources. See Section 2.1 for
definitions of the various quantitative aspects of LSPs.
4.5. Frequency of OAM Execution
The operator MUST have the flexibility to configure OAM parameters to
meet their specific operational requirements.
This includes the frequency of the execution of any OAM functions.
The ability to synchronize OAM operations is required to permit a
consistent measurement of service level agreements. To elaborate,
there are defect conditions, such as mis-branching or misdirection of
traffic, for which probe-based detection mechanisms that incur
significant mismatches in their detection frequency may result in
flapping. This can be addressed either by synchronizing the rate or
having the probes self-identify their probe rate. For example, when
the probing mechanisms are bootstrapping, they might negotiate and
ultimately agree on a probing rate, therefore providing a consistent
probing frequency and avoiding the aforementioned problems.
One observation would be that wide-spread deployment of MPLS, common
implementation of monitoring tools, and the need for inter-carrier
synchronization of defect and service level specification handling
will drive specification of OAM parameters to commonly agreed on
values. Such values will have to be harmonized with the surrounding
technologies (e.g., SONET/SDH, ATM) to be useful. This will become
particularly important as networks scale and mis-configuration can
result in churn, alarm flapping, etc.
4.6. Alarm Suppression, Aggregation, and Layer Coordination
Network elements MUST provide alarm suppression functionality that
prevents the generation of a superfluous generation of alarms by
simply discarding them (or not generating them in the first place),
or by aggregating them together, thereby greatly reducing the number
of notifications emitted. When viewed in conjunction with the
requirement in Section 4.7 below, this typically requires fault
notification to the LSP egress that may have specific time
constraints if the application using the LSP independently implements
path continuity testing (for example, ATM I.610 Continuity check
(CC)[I610]). These constraints apply to LSPs that are monitored.
The nature of MPLS applications allows for the possibility of having
multiple MPLS applications attempt to respond to defects
simultaneously, e.g., layer-3 MPLS VPNs that utilize Traffic
Engineered tunnels where a failure occurs on the LSP carrying the
Traffic Engineered tunnel. This failure would affect the VPN traffic
that uses the tunnel's LSP. Mechanisms are required to coordinate
network responses to defects.
4.7. Support for OAM Inter-working for Fault Notification
An LSR supporting the inter-working of 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 an MPLS transport LSP that supports an emulated ATM VC
MUST translate errors into native ATM OAM Alarm Indication Signal
(AIS) cells at the termination points of the LSP. The mechanism
SHOULD consider possible bounded detection time parameters, e.g., a
"hold off" function before reacting to synchronize with the OAM
One goal would be alarm suppression by the upper layer using the LSP.
As observed in Section 4.5, this requires that MPLS perform detection
in a bounded timeframe in order to initiate alarm suppression prior
to the upper layer independently detecting the defect.
4.8. Error Detection and Recovery
Recovery from a fault by a network element can be facilitated by MPLS
OAM procedures. These procedures will detect a broader range of
defects than that of simple link and node failures. Since MPLS LSPs
may span multiple routing areas and service provider domains, fault
recovery and error detection should be possible in these
configurations as well as in the more simplified single-area/domain
Recovery from faults SHOULD be automatic. It is a requirement that
faults SHOULD be detected (and possibly corrected) by the network
operator prior to customers of the service in question detecting
4.9. Standard Management Interfaces
The wide-spread deployment of MPLS requires common information
modeling of management and control of OAM functionality. Evidence of
this is reflected in the standard IETF MPLS-related MIB modules
(e.g., [RFC3813][RFC3812][RFC3814]) for fault, statistics, and
configuration management. These standard interfaces provide
operators with common programmatic interface access to Operations and
Management functions and their statuses. However, gaps in coverage
of MIB modules to OAM and other features exist; therefore, MIB
modules corresponding to new protocol functions or network tools are
4.10. Detection of Denial of Service Attacks
The ability to detect denial of service (DoS) attacks against the
data or control planes MUST be part of any security management
related to MPLS OAM tools or techniques.
4.11. Per-LSP Accounting Requirements
In an MPLS network, service providers can measure traffic from an LSR
to the egress of the network using some MPLS related MIBs, for
example. This means that it is reasonable to know how much traffic
is traveling from location to location (i.e., a traffic matrix) by
analyzing the flow of traffic. Therefore, traffic accounting in an
MPLS network can be summarized as the following three items:
(1) Collecting information to design network
For the purpose of optimized network design, a service
provider may offer the traffic information. Optimizing
network design needs this information.
(2) Providing a Service Level Specification
Providers and their customers MAY need to verify high-level
service level specifications, either to continuously optimize
their networks, or to offer guaranteed bandwidth services.
Therefore, traffic accounting to monitor MPLS applications is
(3) Inter-AS environment
Service providers that offer inter-AS services require
accounting of those services.
These three motivations need to satisfy the following:
- In (1) and (2), collection of information on a per-LSP
basis is a minimum level of granularity for collecting
accounting information at both of ingress and egress of an
- In (3), SP's ASBR carry out interconnection functions as an
intermediate LSR. Therefore, identifying a pair of ingress
and egress LSRs using each LSP is needed to determine the
cost of the service that a customer is using.
Accounting on a per-LSP basis encompasses the following set of
(1) At an ingress LSR, accounting of traffic through LSPs that
begin at each egress in question.
(2) At an intermediate LSR, accounting of traffic through LSPs for
each pair of ingress to egress.
(3) At egress LSR, accounting of traffic through LSPs for each
(4) All LSRs containing LSPs that are being measured need to have
a common identifier to distinguish each LSP. The identifier
MUST be unique to each LSP, and its mapping to LSP SHOULD be
provided whether from manual or automatic configuration.
In the case of non-merged LSPs, this can be achieved by simply
reading traffic counters for the label stack associated with the
LSP at any LSR along its path. However, in order to measure
merged LSPs, an LSR MUST have a means to distinguish the source of
each flow so as to disambiguate the statistics.
4.11.2. Location of Accounting
It is not realistic for LSRs to perform the described operations on
all LSPs that exist in a network. At a minimum, per-LSP based
accounting SHOULD be performed on the edges of the network -- at the
edges of both LSPs and the MPLS domain.
5. Security Considerations
Provisions to any of the network mechanisms designed to satisfy the
requirements described herein are required to prevent their
unauthorized use. Likewise, these network mechanisms MUST provide a
means by which an operator can prevent denial of service attacks if
those network mechanisms are used in such an attack.
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 that the network operator MAY consider private.
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
[RFC3812] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Traffic Engineering
(TE) Management Information Base (MIB)", RFC 3812, June
[RFC3813] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Label Switching
Router (LSR) Management Information Base (MIB)", RFC 3813,
[RFC3814] Nadeau, T., Srinivasan, C., and A. Viswanathan,
"Multiprotocol Label Switching (MPLS) Forwarding
Equivalence Class To Next Hop Label Forwarding Entry
(FEC-To-NHLFE) Management Information Base (MIB)", RFC
3814, June 2004.
[Y1710] ITU-T Recommendation Y.1710, "Requirements for OAM
Functionality In MPLS Networks"
[I610] ITU-T Recommendation I.610, "B-ISDN operations and
maintenance principles and functions", February 1999
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
[RFC792] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
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, KDDI. Hari Rakotoranto, Miya Kohno, Cisco Systems; Luyuan
Fang, AT&T; Danny McPherson, TCB; Dr. Ken Nagami, Ikuo Nakagawa,
Intec Netcore, and David Meyer.
Comments should be made directly to the MPLS mailing list
Thomas D. Nadeau
Cisco Systems, Inc.
300 Beaver Brook Road
Boxboro, MA 01719
Monique Jeanne Morrow
Cisco Systems, Inc.
Glatt-Com, 2nd Floor
Phone: (0)1 878-9412
Cisco Systems, Inc.
300 Beaver Brook Road
Boxboro, MA 01719
3500 Carling Ave.
Ottawa, Ontario, CANADA
1-9-1, Higashi-Shinbashi, Minato-ku
Tokyo, 105-7316 Japan
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