Network Working Group D. Allan, Ed.
Request for Comments: 4378 Nortel Networks
Category: Informational T. Nadeau, Ed.
Cisco Systems, Inc.
February 2006
A Framework for Multi-Protocol Label Switching (MPLS)
Operations and Management (OAM)
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 Notice
Copyright (C) The Internet Society (2006).
Abstract
This document is a framework for how data plane protocols can be
applied to operations and maintenance procedures for Multi-Protocol
Label Switching (MPLS). The document is structured to outline how
Operations and Management (OAM) functionality can be used to assist
in fault, configuration, accounting, performance, and security
management, commonly known by the acronym FCAPS.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Fault Management ................................................2
3.1. Fault Detection ............................................2
3.2. Diagnosis ..................................................6
3.3. Availability ...............................................7
4. Configuration Management ........................................7
5. Accounting ......................................................7
6. Performance Management ..........................................7
7. Security Management .............................................8
8. Security Considerations .........................................9
9. Acknowledgements ................................................9
10. Normative References ...........................................9
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1. Introduction
This memo outlines in broader terms how data plane protocols can
assist in meeting the Operations and Management (OAM) requirements
outlined in [RFC4377] and [Y1710] and can apply to the management
functions of fault, configuration, accounting, performance, and
security (commonly known as FCAPS) for MPLS networks, as defined in
[RFC3031]. The approach of the document is to outline functionality,
the potential mechanisms to provide the function, and the required
applicability of data plane OAM functions. Included in the
discussion are security issues specific to use of tools within a
provider domain and use for inter-provider Label Switched Paths
(LSPs).
2. Terminology
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 [RFC2119].
OAM Operations and Management
FCAPS Fault management, Configuration management,
Administration management, Performance
management, and Security management
FEC Forwarding Equivalence Class
ILM Incoming Label Map
NHLFE Next Hop Label Forwarding Entry
MIB Management Information Base
LSR Label Switching Router
RTT Round Trip Time
3. Fault Management
3.1. Fault Detection
Fault detection encompasses the identification of all data plane
failures between the ingress and egress of an LSP. This section will
enumerate common failure scenarios and explain how one might (or
might not) detect the situation.
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3.1.1. Enumeration and Detection of Types of Data Plane Faults
Lower-layer faults:
Lower-layer faults are those in the physical or virtual link that
impact the transport of MPLS labeled packets between adjacent LSRs
at the specific level of interest. Some physical links (such as
SONET/SDH) may have link-layer OAM functionality and detect and
notify the LSR of link-layer faults directly. Some physical links
(such as Ethernet) may not have this capability and require MPLS
or IP layer heartbeats to detect failures. However, once
detected, reaction to these fault notifications is often the same
as those described in the first case.
Node failures:
Node failures are those that impact the forwarding capability of a
node component, including its entire set of links. This can be
due to component failure, power outage, or reset of the control
processor in an LSR employing a distributed architecture, etc.
MPLS LSP mis-forwarding:
Mis-forwarding occurs when there is a loss of synchronization
between the data and the control planes in one or more nodes.
This can occur due to hardware failure, software failure, or
configuration problems.
It will manifest itself in one of two forms:
- packets belonging to a particular LSP are cross-connected into
an NHLFE for which there is no corresponding ILM at the next
downstream LSR. This can occur in cases where the NHLFE entry
is corrupted. Therefore, the packet arrives at the next LSR
with a top label value for which the LSR has no corresponding
forwarding information, and is typically dropped. This is a No
Incoming Label Map (No ILM) condition and can be detected
directly by the downstream LSR that receives the incorrectly
labeled packet.
- packets belonging to a particular LSP are cross-connected into
an incorrect NHLFE entry for which there is a corresponding ILM
at the next downstream LSR, but is associated with a different
LSP. This may be detected by the following:
o some or all of the misdirected traffic is not routable at
the egress node, or
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o OAM probing is able to detect the fault by detecting the
inconsistency between the data path and the control plane
state.
Discontinuities in the MPLS Encapsulation
The forwarding path of the FEC carried by an LSP may transit nodes
or links for which MPLS is not configured. This may result in a
number of behaviors that are undesirable and not easily detected.
- if exposed, payload is not routable at the LSR, resulting in
silent discard, OR
- the exposed MPLS label was not offered by the LSR, which may
result in either silent discard or mis-forwarding.
Alternately, the payload may be routable and packets successfully
delivered but may bypass associated MPLS instrumentation and
tools.
MTU problems
MTU problems occur when client traffic cannot be fragmented by
intermediate LSRs and is dropped somewhere along the path of the
LSP. MTU problems should appear as a discrepancy in the traffic
count between the set of ingress LSRs and the egress LSRs for an
FEC and will appear in the corresponding MPLS MIB performance
tables in the transit LSRs as discarded packets.
TTL Mishandling
The implementation of TTL handling is inconsistent at penultimate
hop LSRs. Tools that rely on consistent TTL processing may
produce inconsistent results in any given network.
Congestion
Congestion occurs when the offered load on any interface exceeds
the link capacity for sufficient time that the interface buffering
is exhausted. Congestion problems will appear as a discrepancy in
the traffic count between the set of ingress LSRs and the egress
LSRs for an FEC and will appear in the MPLS MIB performance tables
in the transit LSRs as discarded packets.
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Mis-ordering
Mis-ordering of LSP traffic occurs when incorrect or inappropriate
load sharing is implemented within an MPLS network. Load sharing
typically takes place when multiple equal-cost paths exist between
the ingress and egress of an LSP. In these cases, traffic is
split among these equal-cost paths using a variety of algorithms.
One such algorithm relies on splitting traffic between each path
on a per-packet basis. When this is done, it is possible for some
packets along the path to be delayed due to congestion or slower
links, which may result in packets being received out of order at
the egress. Detection and remedy of this situation may be left up
to client applications that use the LSPs. For instance, TCP is
capable of re-ordering packets belonging to a specific flow
(although this may result in re-transmission of some of the mis-
ordered packets).
Detection of mis-ordering can also be determined by sending probe
traffic along the path and verifying that all probe traffic is
indeed received in the order it was transmitted. This will only
detect truly pathological problems as mis-ordering typically is an
insufficiently predictable and repeatable problem.
LSRs do not normally implement mechanisms to detect mis-ordering
of flows.
Payload Corruption
Payload corruption may occur and may be undetected by LSRs. Such
errors are typically detected by client payload integrity
mechanisms.
3.1.2. Timeliness
The design of Service Level Agreements (SLAs) and management support
systems requires that ample headroom be alloted in terms of their
processing capabilities in order to process and handle all necessary
fault conditions within the bounds stipulated in the SLA. This
includes planning for event handling using a time budget that takes
into account the over-all SLA and the time required to address any
defects that arise. However, it is possible that some fault
conditions may surpass this budget due to their catastrophic nature
(e.g., fibre cut) or due to incorrect planning of the time processing
budget.
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^ --------------
| | ^
| | |---- Time to notify NOC + process/correct
SLA | | v defect
Max - | -------------
Time | | ^
| | |----- Time to diagnose/isolate/correct
| | v
v -------------
Figure 1: Fault Correction Budget
In figure 1, we represent the overall fault correction time budget by
the maximum time as specified in an SLA for the service in question.
This time is then divided into two subsections, the first
encompassing the total time required to detect a fault and notify an
operator (or optionally automatically correct the defect). This
section may have an explicit maximum time to detect defects arising
from either the application or a need to do alarm management (i.e.,
suppression), and this will be reflected in the frequency of OAM
execution. The second section indicates the time required to notify
the operational systems used to diagnose, isolate, and correct the
defect (if they cannot be corrected automatically).
3.2. Diagnosis
3.2.1. Characterization
Characterization is defined as determining the forwarding path of a
packet (which may not be necessarily known). Characterization may be
performed on a working path through the network. For example, this
is done to determine equal-cost multi-paths (ECMP), the MTU of a
path, or simply to know the path occupied by a specific FEC.
Characterization will be able to leverage mechanisms used for
isolation.
3.2.2. Isolation
Isolation of a fault can occur in two forms. In the first case, the
local failure is detected, and the node where the failure occurred is
capable of issuing an alarm for such an event. The node should
attempt to withdraw the defective resources and/or rectify the
situation prior to raising an alarm. Active data plane OAM
mechanisms may also detect the failure conditions remotely and issue
their own alarms if the situation is not rectified quickly enough.
In the second case, the fault has not been detected locally. In this
case, the local node cannot raise an alarm, nor can it be expected to
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rectify the situation. In this case, the failure may be detected
remotely via data plane OAM. This mechanism should also be able to
determine the location of the fault, perhaps on the basis of limited
information such as a customer complaint. This mechanism may also be
able to automatically remove the defective resources from the network
and restore service, but should at least provide a network operator
with enough information by which they can perform this operation.
Given that detection of faults is desired to happen as quickly as
possible, tools which possess the ability to incrementally test LSP
health should be used to uncover faults.
3.3. Availability
Availability is the measure of the percentage of time that a service
is operating within a specification, often specified by an SLA.
MPLS has several forwarding modes (depending on the control plane
used). As such, more than one model may be defined and more than one
measurement technique may be required.
4. Configuration Management
Data plane OAM can assist in configuration management by providing
the ability to verify the configuration of an LSP or of applications
utilizing that LSP. This would be an ad-hoc data plane probe that
should verify path integrity (a complete path exists) and that the
path function is synchronized with the control plane. As part of the
payload, the probe would carry relevant control plane information
that the receiver would be able to compare with the local-control
plane configuration.
5. Accounting
The requirements for accounting in MPLS networks, as specified in
[RFC4377], do not place any requirements on data plane OAM.
6. Performance Management
Performance management permits the information transfer
characteristics of LSPs to be measured, perhaps in order to be
compared against an SLA. This falls into two categories: latency
(where jitter is considered a variation in latency) and information
loss.
Latency can be measured in two ways: one is to have precisely
synchronized clocks at the ingress and egress such that time-stamps
in PDUs flowing from the ingress to the egress can be compared. The
other is to use an exchange of PING type PDUs that gives a round trip
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time (RTT) measurement, and an estimate of the one-way latency that
can be inferred with some loss of precision. Use of load spreading
techniques, such as ECMP, mean that any individual RTT measurement is
only representative of the typical RTT for an FEC.
To measure information loss, a common practice is to periodically
read ingress and egress counters (i.e., MIB module counters). This
information may also be used for offline correlation. Another common
practice is to send explicit probe traffic that traverses the data
plane path in question. This probe traffic can also be used to
measure jitter and delay.
7. Security Management
Providing a secure OAM environment is required if MPLS specific
network mechanisms are to be used successfully. To this end,
operators have a number of options when deploying network mechanisms
including simply filtering OAM messages at the edge of the MPLS
network. Malicious users should not be able to use non-MPLS
interfaces to insert MPLS-specific OAM transactions. Provider
initiated OAM transactions should be able to be blocked from leaking
outside the MPLS cloud.
Finally, if a provider does wish to allow OAM messages to flow into
(or through) their networks, for example, in a multi-provider
deployment, authentication and authorization are required to prevent
malicious and/or unauthorized access. Also, given that MPLS networks
often run IP simultaneously, similar requirements apply to any native
IP OAM network mechanisms in use. Therefore, authentication and
authorization for OAM technologies is something that MUST be
considered when designing network mechanisms that satisfy the
framework presented in this document.
OAM messaging can address some existing security concerns with the
MPLS architecture. That is, through rigorous defect handling,
operator's can offer their customers a greater degree of integrity
protection that their traffic will not be incorrectly delivered (for
example, by being able to detect leaking LSP traffic from a VPN).
Support for inter-provider data plane OAM messaging introduces a
number of security concerns as, by definition, portions of LSPs will
not be within a single provider's network the provider has no control
over who may inject traffic into the LSP, which can be exploited for
denial of service attacks. OAM PDUs are not explicitly identified in
the MPLS header and therefore are not typically inspected by transit
LSRs. This creates opportunity for malicious or poorly behaved users
to disrupt network operations.
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Attempts to introduce filtering on target LSP OAM flows may be
problematic if flows are not visible to intermediate LSRs. However,
it may be possible to interdict flows on the return path between
providers (as faithfulness to the forwarding path is to a return path
requirement) to mitigate aspects of this vulnerability.
OAM tools may permit unauthorized or malicious users to extract
significant amounts of information about network configuration. This
would be especially true of IP based tools as, in many network
configurations, MPLS does not typically extend to untrusted hosts,
but IP does. For example, TTL hiding at ingress and egress LSRs will
prevent external users from using TTL-based mechanisms to probe an
operator's network. This suggests that tools used for problem
diagnosis or which, by design, are capable of extracting significant
amounts of information will require authentication and authorization
of the originator. This may impact the scalability of such tools
when employed for monitoring instead of diagnosis.
8. Security Considerations
This document describes a framework for MPLS Operations and
Management. Although this document discusses and addresses some
security concerns in Section 7, it does not introduce any new
security concerns.
9. Acknowledgements
The editors would like to thank Monique Morrow from Cisco Systems and
Harmen van Der Linde from AT&T for their valuable review comments on
this document.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC4377] Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
Matsushima, "Operations and Management (OAM) Requirements
for Multi-Protocol Label Switched (MPLS) Networks", RFC
4377, February 2006.
[Y1710] ITU-T Recommendation Y.1710(2002), "Requirements for OAM
Functionality for MPLS Networks".
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Authors' Addresses
David Allan
Nortel Networks
3500 Carling Ave.
Ottawa, Ontario, CANADA
Phone: +1-613-763-6362
EMail: dallan@nortel.com
Thomas D. Nadeau
Cisco Systems
300 Beaver Brook Drive
Boxborough, MA 01824
Phone: +1-978-936-1470
EMail: tnadeau@cisco.com
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